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Power Transformer
© ABB Power Technology 1_114Q07- 1 -
ﻣوﺳوﻋـــــــــﺔ اﻟﮭﻧدﺳـــــــــــﺔ اﻟﻛﮭرﺑﯾــــــــــﺔ Facebook.com/groups/EEE.Arabic Facebook.com/EEE.Arabic
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© ABB Power Technology 1_114Q07- 2 -
General
Power Transformers
© ABB Power Technology 1_114Q07- 3 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
© ABB Power Technology 1_114Q07- 4 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Fundamental considerations
G
© ABB Power Technology 1_114Q07- 5 -
B1
B2
Static device whose function is to transfer electrical energy from one circuit to another whose common link is a magnetic flux.
The current and voltage characteristics of the incoming energy are modified in the output
Construction: Magnetic core made of stacked sheets of ferromagnetic material and two coils (B1, B2) wounded over the former.
Z
Fundamental considerations
G
© ABB Power Technology 1_114Q07- 6 -
B1
B2
Z
Operation: We connect an a.c. generator at B1 and close B2 through an impedance Z.
B1 generates a flux owing to the current. This is a variable one hence it induces a voltage in B2 which in time generates a current in the secondary circuit.
It is only possible with alternating current (a flux variation is needed)
The circuit connected to B1 is named primary and the one connected to B2 is named secondary
The related V and I characteristics are named as well primary and secondary characteristics.
Fundamental relationships
Primary energy Secondary energy
Hence
If the transformer is single phase
And with three phase
At both cases hence
On the other hand it is known that Voltage in a coil is proportional to the generated flux and number of turns
Primary Power Secondary Power U1 I1 U2 I2 √3 U1 I1 √3 U2 I2 U1/ U2 I2 / I1
U1= k1 n1 and U2= k1 n2
From where it is inferred:
© ABB Power Technology 1_114Q07- 7 -
U1 / U2 = n1 / n2 and I2 / I1 = n1 / n2
The ratio n1 / n2 is called Transformation Ratio
Fundamental relationships
We have then:
U1 / U2 = r1 I1 / I2 = 1 / r1
© ABB Power Technology 1_114Q07- 8 -
The former statements are true in an ideal transformer which complies with the following
No reluctance at the magnetic circuit
No resistance at the windings
No histeresys, eddy-current or I2R losses
No leakage flux
Although this is not accomplished in a real case, all formerly said is useful to understand the performance of a transformer and to obtain some approximate values for an elementary calculation
Fundamental relationships
From:
U1 I1 = U2 I2
It is deduced that:
The winding with higher voltage must stand lesser current and conversely the lesser the voltage the higher the current
On the other hand from: I1 n1 = I2 n2
it is deduced that:
© ABB Power Technology 1_114Q07- 9 -
The winding with higher current will be the one with bigger cross section but lesser number of turns
Power transformer function
Transmission of electrical energy is cheaper as far as the transmission voltage is raised
let´s have a power P to transmit with a voltage U and a current I. Losses will be: Pr = R I²
If voltage is raised to nU, current, for the same power, will be reduced to I/n, hence losses in that case will be: Pr = R I² / n²
© ABB Power Technology 1_114Q07- 10 -
They are hence reduced by the square of the ratio of voltage raising.
Or for the same losses the cross section of the wire can be reduced, so is the cost of the line.
On the other hand electrical energy is easier and safer to use when handled at the lowest voltage.
© ABB Power Technology 1_114Q07- 11 -
Power transformer function
The power loss p in a 3-phase transmission line with a resistance R per phase and a current I flowing in each phase is:
At a system voltage U the transmitted active power is:
Equation (2) can be rewritten as:
Inserted in the equation (1) gives:
Equation (4) indicates that the power loss in the line is proportional to the square of the transmitted active power and inversely proportional to the square of the system voltage.
In other words, the power loss will be lower when the system voltage is increased.
Power transformer function
© ABB Power Technology 1_114Q07- 12 -
The main application of transformers is hence the one depicted at the following diagram :
To raise the voltage and to reduce the current going out from generation stations (Step-up substations).
To reduce the voltage on arriving to the points of utilization (StepDown substations).
© ABB Power Technology 1_114Q07- 13 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Main characteristics of a Power Transformer
© ABB Power Technology 1_114Q07- 14 -
When choosing a transformer it is necessary to define:
Type of transformer
Transformation ratio
Insulation Class
Rated output
Connection
Also it is necessary to establish:
Voltage regulation
Type of cooling
Accessories
© ABB Power Technology 1_114Q07- 15 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Constructive types
Primario
Primario
Secundario Secundario
© ABB Power Technology 1_114Q07- 16 -
Core-form single phase
Core form three phase
© ABB Power Technology 1_114Q07- 17 -
Constructive types
Core-form construction for single-phase transformers consists of magnetic steel punchings arranged to provide a single-path magnetic circuit.
High- and low-voltage coils are grouped together on each main or vertical leg of the core.
In general, the mean length of turn for the winding is comparatively short in the coreform design, while the magnetic path is long.
Constructive types
Primario
Primario
Secundario
Shell-form single phase
Secundario
© ABB Power Technology 1_114Q07- 18 -
Shell form three phase
© ABB Power Technology 1_114Q07- 19 -
Constructive types
Shell-form construction for single-phase transformers consists of all windings formed into a single ring, with magnetic punchings assembled so as to encircle each side of the winding ring.
The mean length of turn is usually longer than for a comparable core-form design, while the iron path is shorter.
Constructive types
In the design of a particular transformer many factors such as insulation stress, mechanical stress, heat distribution, weight and cost must be balanced and compromised.
It appears that, for well-balanced design, both core-form and shellform units have their respective fields of applicability determined by kva and kv rating.
In the larger sizes, shell-form construction is quite appropriate; the windings and magnetic iron can be assembled on a steel base structure, with laminations laid in horizontally to link and surround the windings.
© ABB Power Technology 1_114Q07- 20 -
A close-fitting tank member is then dropped over the core and coil assembly and welded to the steel base, completing the tank assembly and also securing the core to the base member.
Winding types
AT
BT
AT
AT BT
BT
Split windings
© ABB Power Technology 1_114Q07- 21 -
Concentric windings
Winding types
AT
AT
BT AT BT AT BT
BT
BT
© ABB Power Technology 1_114Q07- 22 -
Double concentric windings
Superimposed windings
© ABB Power Technology 1_114Q07- 23 -
Single phase vs. Three phase banks
A three-phase power transformation can be accomplished either by using a three-phase transformer unit, or by inter-connecting three single-phase units to form a three-phase bank.
The three-phase unit has advantages of greater efficiency, smaller size, and less cost when compared with a bank having equal kva capacity made up of three single-phase units.
When three single-phase units are used in a bank, it is possible to purchase and install a fourth unit at the same location as an emergency spare.
This requires only 33 percent additional investment to provide replacement capacity, whereas 100 percent additional cost would be required to provide complete spare capacity for a three-phase unit.
However, transformers have a proven reliability higher than most other elements of a power system, and for this reason the provision of immediately available spare capacity is now considered less important than it once was.
© ABB Power Technology 1_114Q07- 24 -
Single phase vs. Three phase banks
Three-phase units are quite generally used in the highest of circuit ratings, with no on-the-spot spare transformer capacity provided.
In these cases parallel or interconnected circuits of the system may provide emergency capacity, or, for small and medium size transformers, portable substations can provide spare capacity on short notice.
If transportation or rigging facilities should not be adequate to handle the required transformer capacity as a single unit, a definite reason of course develops for using three single-phase units.
© ABB Power Technology 1_114Q07- 25 -
Single phase vs. Three phase banks
© ABB Power Technology 1_114Q07- 26 -
Single phase vs. Three phase banks
Transformers vs. Autotransformers
© ABB Power Technology 1_114Q07- 27 -
An autotransformer inherently provides a metallic connection between its low- and high-voltage circuits; this is unlike the conventional two-winding transformer which isolates the two circuits. Unless the potential to ground of each autotransformer circuit is fixed by some means, the low-voltage circuit will be subject to overvoltages originating in the high-voltage circuit. These undesirable effects can be minimized by connecting the neutral of the autotransformer solidly to ground.
Transformers vs. Autotransformers
© ABB Power Technology 1_114Q07- 28 -
The autotransformer has advantages of:
lower cost,
higher efficiency
better regulation
It has disadvantages including:
low reactance which may make it subject to excessive short-circuit currents
the arrangement of taps is more complicated
the low- and high-voltage circuits cannot be isolated
the two circuits must operate with no angular phase displacement unless a zig-zag connection is introduced.
The advantages of lower cost and improved efficiency become less apparent as the transformation ratio increases, so that autotransformers for power purposes are usually used for low transformation ratios, rarely exceeding 2 to 1.
Transformers vs. Autotransformers
© ABB Power Technology 1_114Q07- 29 -
Three-phase autotransformers for power service are usually starstar connected with the neutral grounded, and in most of these cases it is desirable to have a third winding on the core deltaconnected so as to carry the third harmonic component of exciting current.
This winding could be very small in capacity if it were required to carry only harmonic currents, but its size is increased by the requirement that it carry high currents during system ground faults.
A widely used rule sets the delta-winding rating at 35 percent of the autotransformer equivalent two-winding kva rating (not circuit kva rating).
Since it is necessary in most cases to have a delta-connected tertiary winding, it is often advantageous to design this winding so that load can be taken from it. This results in a three-winding autotransformer with terminals to accommodate three external circuits.
© ABB Power Technology 1_114Q07- 30 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Transformation ratio
Primary voltage : The most usual value of the voltage at the point of the network where the transformer is going to be connected
© ABB Power Technology 1_114Q07- 31 -
When this voltage is expected to vary it could be necessary that the transformer is equipped with an on load tap changer
Secondary voltage: The desired value at the secondary network where the transformer will be connected.
© ABB Power Technology 1_114Q07- 32 -
Transformation ratio and voltage drop
The voltage ratio of a transformer is normally specified in no load condition and is directly proportional to the ratio of the number of turns in the windings.
When the transformer is loaded, the voltage on the secondary terminals changes from that in no load condition, depending on
the angle φ between the voltage on the secondary terminals of the transformer U2 and the secondary current I2
the value of the secondary current I2
the short-circuit impedance of the transformer Z and its active and reactive components, r and ±jx respectively
© ABB Power Technology 1_114Q07- 33 -
Transformation ratio and voltage drop
At no load the secondary voltage is U20.
With the load ZL connected, the voltage at the secondary terminals changes to U2.
For example, when a transformer with values for ur=0,01 and ux=0,06 is loaded with rated current with a power factor of 0,8 inductive the voltage on the secondary terminals decreases to 95,5% of the voltage at no load.
© ABB Power Technology 1_114Q07- 34 -
Transformation ratio and voltage drop
Users and installation planners are recommended to take the variation of the secondary voltage during loading into account when specifying the transformer data.
This may be especially important for example in a case where a large motor represents the main load of the transformer.
The highly inductive starting current of the motor may then be considerably higher than the rated current of the transformer.
Consequently there may be a considerable voltage drop through the transformer.
If the feeding power source is weak, this will contribute to an even lower voltage on the secondary side of the transformer.
Short circuit impedance
© ABB Power Technology 1_114Q07- 35 -
Users have sometimes particular requirements regarding the shortcircuit impedance. Such requirements may be determined by:
parallel operation with existing units,
limitation of voltage drop,
limitation of short-circuit currents.
The transformer designer can meet the requirements in different ways:
The size of the core cross-section. A large cross-section gives a low impedance and vice versa,
A tall transformer gives a low impedance and vice versa.
For each transformer there is, however, a smaller range which gives the optimum transformer from an economic point of view, that is the lowest sum of the manufacturing costs and the capitalised value of the losses.
Short circuit impedance
Short-circuit impedance Z is often expressed as uz in p.u. or in % according to the following formulas:
© ABB Power Technology 1_114Q07- 36 -
Formulas (24) and (25) are valid for for single-phase transformers, where Ir and Ur are rated values of current and voltage on either side of the transformer. For 3-phase transformers the nominator must be multiplied with √3. Based on measured short-circuit voltage the value of Zk expressed in ohm can be calculated from the following formula:
for 3-phase transformers. Sr is the rated power of the transformer.
© ABB Power Technology 1_114Q07- 37 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
© ABB Power Technology 1_114Q07- 38 -
Isolation class
Defines the capability of the transformer to withstand overvoltages without loosing its functionality or undue deterioration
Overvoltages could be:
in
Temporary ov
Switching ov
Lightning ov
networks
Isolation class
The standard insulation classes and dielectric tests for power transformers are given in standards. The insulation class of a transformer is determined by the dielectric tests which the unit can withstand, rather than by rated operating voltage.
© ABB Power Technology 1_114Q07- 39 -
The test values are:
Basic impulse level (lightning)
Short time Power frequency overvoltage
© ABB Power Technology 1_114Q07- 40 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Rated output
The rated kva output of a transformer is that load which it can deliver continuously at rated secondary voltage without exceeding a given temperature rise measured under prescribed test conditions.
© ABB Power Technology 1_114Q07- 41 -
The actual test temperature rise may, in a practical case, be somewhat below the established limit because of design and manufacturing tolerances.
The output which a transformer can deliver in service without undue deterioration of the insulation may be more or less than its rated output, depending upon the following design characteristics and operating conditions as they exist at a particular time:
Ambient temperature.
Top-oil rise over ambient temperature.
Hottest-spot rise over top-oil temperature (hottest-spot copper gradient).
Transformer thermal time constant.
Ratio of load loss to no-load loss.
Loading Based on Ambient Temperature
© ABB Power Technology 1_114Q07- 42 -
Air-cooled oil-immersed transformers built to meet established standards will operate continuously with normal life expectancy at rated kva and secondary voltage, providing the ambient air temperature averages no more than 30º C throughout a 24-hour period with maximum air temperature never exceeding 40 C. Water-cooled transformers are built to operate continuously at rated output with ambient water temperatures averaging 25 C and never exceeding 30 C.
When the average temperature of the cooling medium is different from the values above, a modification of the transformer loading may be made according to Table 7.
In cases where the difference between maximum air temperature and average air temperature exceeds 10 C, a new temperature that is 10 C below the maximum should be used in place of the true average. The allowable difference between maximum and average temperature for water-cooled transformers is 5 C.
Loading Based on Capacity Factor
© ABB Power Technology 1_114Q07- 43 -
Transformer capacity factor (operating kva divided by rated kva) averaged throughout a 24-hour period may be well below 100 percent, and when this is true some compensating increase in maximum transformer loading may be made. The percentage increase in maximum loading as a function of capacity factor, based on a normal transformer life expectancy, is given in Table 8.
Loading Based on Short-Time Overloads
© ABB Power Technology 1_114Q07- 44 -
Short-time loads which occur not more than once during any 24-hour period may be in excess of the transformer rating without causing any predictable reduction in transformer life. The permissible load is a function of the average load previous to the period of above-rated loading, according to Table 9. The load increase based on capacity factor and the increase based on short-time overloads can not be applied concurrently; it is necessary to chose one method or the other.
Loading Based on Short-Time Overloads
© ABB Power Technology 1_114Q07- 45 -
Short time loads larger than those shown in Table 9 will cause a decrease inprobable transformer life, but the amount of the decrease is difficult to predict in general terms. Some estimate of the sacrifice in transformer life can be obtained from Table 10(a) which is based on the theoretical conditions and limitations described in Table10(b).
Loading Based on Short-Time Overloads
© ABB Power Technology 1_114Q07- 46 -
These conditions were chosen to give results containing some probable margin, when compared with most conventional transformer designs. For special designs, or for a more detailed check on some particular unit, the hottest-spot copper temperature can be calculated by the method shown in section 19, and the probable sacrifice in transformer life can then be estimated from Table 11.
Loading Based on Measured Oil Temperat. The temperature of the hottest-spot within a power transformer winding influences to a large degree the deterioration rate of insulation. For oilimmersed transformers the hottestspot temperature limits have been set at 105 C maximum and 95 C average through a 24 hour period; normal life expectancy is based on these limits.
The top-oil temperature, together with a suitable temperature increment called either hottest-spot copper rise over top-oil temperature or hottestspot copper gradient, is often used as an indication of hottest-spot temperature.
© ABB Power Technology 1_114Q07- 47 -
Loading Based on Measured Oil Temperat. Allowable top-oil temperature for a particular constant load may be determined by subtracting the hottestspot copper gradient for that load from 95 C. The hottest-spot copper gradient must be known from design information for accurate results, though typical values may be assumed for estimating purposes. If the hottest-spot copper gradient is known for one load condition, it may be estimated for other load conditions by reference to Fig. 18.
A conservative loading guide, based on top-oil temperatures, is given in Fig. 19.
© ABB Power Technology 1_114Q07- 48 -
One or several transformers?
Advantages of several transformers:
Less spare capacity needed
No outage of the entire system in case of transformer failure
Drawbacks:
Higher cost (spare capacity not considered)
Higher short circuit capacity if transformers are coupled ......
© ABB Power Technology 1_114Q07- 49 -
......
......
......
Parallel operation of transformers
When there are several transformers at the substation it is possible to couple them or not.
© ABB Power Technology 1_114Q07- 50 -
On the other hand in transmission networks the transformers are always coupled.
Advantages: No outage when one of the transformers is disconnected (providing there is capacity enough at the network)
Drawbacks: Higher short circuit capacity
Provisions to be taken before coupling:
Same transformer ratio
Matching connection group
Matching phase rotation direction
Short circuit voltage inversely proportional to rated output
Tap changing coordination
© ABB Power Technology 1_114Q07- 51 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Transformer connections
© ABB Power Technology 1_114Q07- 52 -
Star- Star connection (Y-y):
It stands badly secondary imbalances
Neutral connection is possible
Low cost because of reduced number of turns and lower isolation needed (phase-ground voltage)
Bigger cross section of conductors because of higher current which gives more stiffness to windings and better performance on short circuits
Star- Star with tertiary winding:
It solves the problem of imbalances and third harmonic
Tertiary winding can be used for ancillary services
Transformer connections
© ABB Power Technology 1_114Q07- 53 -
Delta- Star connection (D-y):
Very used in distribution systems
It stands well secondary imbalances
Third harmonic is not transmitted to low voltage
Low voltage Neutral connection possible
High cost because of bigger isolation and HV turns so it is not used at high voltages
Star- Delta Connection (Y-d):
Very used at high voltages because of lower isolation and HV nº of turns needed.
Low voltage neutral connection not possible so they are not used at distribution systems.
The delta connection prevents third harmonic flux because third harmonic current circulates inside the delta windings.
Transformer connections
Desequilibrio 2º 3º Armonico Posibilidad neutro Coste aislamiento Coste Cu
© ABB Power Technology 1_114Q07- 54 -
Y-y D-y Y-d Y-z D-d Y-y-3º
M
M
B
B
B
B
B
B
R
B
R
B
M
B
B
B
R
B
B
R
R
B
M
M
M
B
B
B
B
R
© ABB Power Technology 1_114Q07- 55 -
Voltage Control
Power Transformers
© ABB Power Technology 1_114Q07- 56 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Voltage control
The modern load tap changer had its beginning in 1925.
© ABB Power Technology 1_114Q07- 57 -
Since that time the development of more complicated transmission networks has made tap changing under load more and more essential to control the in-phase voltage of power transformers.
Tap-changing-under-load transformers:
equipment
is
applied
to
power
to maintain a constant secondary voltage with a variable primary voltage;
to control the secondary voltage with a fixed primary voltage;.
Various types of tap-changing equipment and circuits are used depending upon the voltage and kva.
Under-load-tap-changers are built for 8, 16, and 32 steps, with the trend in recent years being toward the larger number of steps so as to give a finer degree of regulation.
The usual range of regulation is plus 10 percent and minus 10 percent of the rated line voltage, with plus and minus 71/2 percent and plus and minus 5 percent being second and third, respectively, in popularity.
Voltage control
© ABB Power Technology 1_114Q07- 58 -
Figure illustrates schematically the operation of one type of mechanism for changing taps under load.
Taps from the transformer winding connect to selector switches 1 through 9. The selector switches are connected to load transfer switches R, S, and T.
The connections for the tap changer positions are shown on the sequence chart.
The sequence of switching is so coordinated by the tap changing mechanism that the transfer switches perform all the switching operations, opening before and closing after the selector switches. All arcing is thus restricted to switches R, S, and T, while switches 1 to 9 merely select the transformer tap to which the load is to be transferred.
© ABB Power Technology 1_114Q07- 59 -
Voltage control
When the tap changer is on odd-numbered positions, the preventive auto-transformer is short-circuited.
On all even- numbered positions, the transformer bridges two transformer taps.
In this position, the relatively high reactance of the preventive auto-transformer to circulating currents between adjacent taps prevents damage to the transformer winding, while its relatively low impedance to the load current permits operation on this position to obtain voltages midway between the transformer taps.
preventive
auto-
© ABB Power Technology 1_114Q07- 60 -
Voltage control
The operation in this case of the selector and transfer switches is exactly as described for the former.
But this type also has a reversing switch which reverses the connections to the tapped section of the winding so that the same range and number of positions can be obtained with one-half the number of tap sections, or twice the range can be obtained with the same number of taps.
The reversing switch is a close-before-open switch which operates at the time there is no voltage across its contacts.
© ABB Power Technology 1_114Q07- 61 -
Voltage control
This type of load tap changer is applied to small power transformers and large distribution transformers.
The transfer switches are eliminated, and each selector switch serves as a transfer switch for the tap to which it is connected.
The schematic circuit diagram and operations sequence chart is shown in Fig.
Load Tap changing.
4
6
5
6 5 4
© ABB Power Technology 1_114Q07- 62 -
3 Par
Impar
3
The tap changer is in position 4
The tap selector without current prepares the new position to change (from 3 to 5)
Load Tap changing.
4
6
5
6 5 4
© ABB Power Technology 1_114Q07- 63 -
3 Par
Impar
3
The transition switch begins the new operation connecting in parallel the initial position (4) with the final one (5)
The transition resistances clear away the energy stored at the coil to be transferred preventing overvoltages ( on the turns to be taken off ) either contribute to that the establishment of the current, at the turns to add, is in a smooth way.
Load Tap changing.
The transition switch interrupts the current through the initial tap (4) and pass on it definitely to the final one (5)
4
6
5
6 5 4
© ABB Power Technology 1_114Q07- 64 -
3 Par
Impar
3
© ABB Power Technology 1_114Q07- 65 -
Autotransformers tap changers
© ABB Power Technology 1_114Q07- 66 -
No load tap changer
© ABB Power Technology 1_114Q07- 67 -
On load tap changers
© ABB Power Technology 1_114Q07- 68 -
Three phase on load tap changer
© ABB Power Technology 1_114Q07- 69 -
Three phase on load tap changer
© ABB Power Technology 1_114Q07- 70 -
Single phase on load tap changer
© ABB Power Technology 1_114Q07- 71 -
Transition switch
© ABB Power Technology 1_114Q07- 72 -
Types of Cooling
Power Transformers
© ABB Power Technology 1_114Q07- 73 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Types of cooling
Cooling is needed to transfer the heat produced by losses so as not to damage isolation
Two main types of isolation exist:
Dry Transformers, air refrigerated
Oil immersed Transformers
In Substations the most used is the second one
© ABB Power Technology 1_114Q07- 74 -
Dry type are only used for low output and when fire hazard is a big concern
Types of cooling
© ABB Power Technology 1_114Q07- 75 -
Several types transformers:
of
cooling
are
possible
with
oil
Oil immersed Self cooled (ONAN)
Oil Immersed Self Cooled/Forced-Air Cooled (ONAF)
Oil immersed Forced cooled/Forced air cooled (OFAF)
Oil immersed Water cooled (ONWN-OFWF)
immersed
Oil inmersed Self cooled (ONAN)
© ABB Power Technology 1_114Q07- 76 -
In this type of transformer the insulating oil circulates by natural convection within a tank having either smooth sides, corrugated sides, integral tubular sides, or detachable radiators.
Smooth tanks are used for small distribution transformers but because the losses increase more rapidly than the tank surface area as kva capacity goes up, a smooth tank transformer larger than 50 kva would have to be abnormally large to provide sufficient radiating surface.
Integral tubular-type construction is used up to about 3000 kva and in some cases to larger capacities, though shipping restrictions usually limit this type of construction at the larger ratings.
Above 3000 kva detachable radiators are usually supplied.
Transformers rated 46 kv and below may also be filled with Inerteen fire-proof insulating liquid, instead of with oil.
The ONAN transformer is a basic type, and serves as a standard for rating and pricing other types.
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Oil inmersed Self cooled (ONAN)
Oil-Imm Self-Cooled/Forced-Air Cooled (ONAF)
This type of transformer is basically an ONAN unit with the addition of fans to increase the rate of heat transfer from the cooling surfaces, thereby increasing the permissible transformer output.
The ONAF transformer is applicable in situations that require shorttime peak loads to be carried recurrently, without affecting normal expected transformer life.
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This transformer may be purchased with fans already installed, or it may be purchased with the option of adding fans later.
The higher kva capacity attained by the use of fans may be calculated as follows :
For 2500 kva (OA) and below: kva (FA)=l.l5Xkva(OA).
For 2501 to 9999 kva (OA) single-phase or 11 999 kva (OA) three-phase : kva (FA) = 1.25 X kva (OA).
For 10 000 kva (OA) single-phase and 12 000 kva (OA) three-phase, and above : kva (FA) = 1.333Xkva (OA). (22)
These ratings are standardized, and are based on a hottest-spot copper temperature of 65 degrees C above 30 degrees C average ambient.
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Oil-Imm Self-Cooled/Forced-Air Cooled (ONAF)
Oil imm. forced cooled/Forced air cooled (OFAF)
The rating of an oil-immersed transformer may be increased from its OA rating by the addition of some combination of fans and oil pumps.
Such transformers are normally built in the range 10 000 kva (OA) single-phase or 12 000 kva (OA) three-phase, and above.
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Increased ratings are defined as two steps, 1.333 and 1.667 times the OA rating respectively.
Automatic controls responsive to oil temperature are normally used to start the fans and pumps in a selected sequence as transformer loading increases.
A variation of this is a type of transformer which is intended for use only when both oil pumps and fans are operating, under which condition any load up to full rated kva may be carried.
Some designs are capable of carrying excitation current with no fans or pumps in operation, but this is not universally true. Heat transfer from
oil to air is accomplished in external oil-to-air heat exchangers.
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Oil imm. forced cooled/Forced air cooled (OFAF)
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Oil imm. forced cooled/Forced air cooled (OFAF)
Oil immersed water cooled (ONWN-OFWF) OW-Oil-Immersed Water-Cooled
In this type of water-cooled transformer, the cooling water runs through coils of pipe which are in contact with the insulating oil of the transformer.
The oil flows around the outside of these pipe coils by natural convection, thereby effecting the desired heat transfer to the cooling water. This type has no self-cooled rating.
FOW-Oil-Immersed Forced-Oil-Cooled With Forced-Water
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Cooler-External oil-to-water heat exchangers are used in this type of unit to transfer heat from oil to cooling water.
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Oil immersed water cooled (ONWN-OFWF)
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Cooling systems. Radiators
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Cooling systems. Air-refrigerator
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Cooling systems. Hydro-refrigerator
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Accessories Practical aspects
Power Transformers
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AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
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Accesories
Pressure relief valve
Gas detector relay. Buchholz
Non return valve
Dehydrating breather
Oil expansion tank
Temperature detectors
Thermostat and oil level indicator
Bushing current transformer
Bushings
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Accesories. Pressure relief valve
Accesories. Gas actuated relay (Buchholz) The gas generated in a transformer during a fault is collected in the gas relay.
The gas will displace the liquid in the relay and minor gas generation will cause the closing of the alarm contact. If an extensive amount of gas is generated or the oil level falls, then the alarm contact will close first, followed by the tripping contact.
A heavy liquid flow from the transformer into the liquid conservator (conservator type only), will cause the immediate closing of the tripping contact. If the tripping contacts operate the transformer will be immediately disconnected from the network.
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Accesories. Gas actuated relay (Buchholz)
Operation of some protective equipment such as gas relay or differential relay does not always mean that the transformer is damaged.
The gas relay can operate for example when:
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An air bubble has been left under the transformer cover. An air bubble is colourless and odourless.
A short-circuit current has passed the transformer. No gas bubbles.
However if the gas has colour or smell, the transformer is damaged.
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Accesories. Non return valve. Dehydrating breather
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Accesories. Oil expansion tank
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Accesories. Temperature detectors
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Accesories. Thermostat / Oil level indicator
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Bushing current transformer
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Accesories. Bushings
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Installation. Handling and lifting
Only approved and suitable lifting equipment shall be used.
Use a forklift only on transport pallets or transformer bottom.
Do not apply load to corrugated fins or radiators and their supports.
Use the provided lifting lugs only.
When lifting a transformer with cable boxes on the cover, special care must be taken.
When hydraulic jacks are used, only provided jacking points shall be used, and in such a way that twisting forces on the transformer tank are avoided.
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Installation. Handling and lifting
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Installation. Handling and lifting
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Transport
The transformer is supplied filled with liquid and normally all accessories fitted, except for the largest units. The radiators may be dismantled during transport.
During transport the following should be considered:
Angle of tilting exceeding 10º must be specified in the contract,
Prevention of damage to bushings, corrugated panels or radiators and accessories,
Larger transformers should preferably be positioned with the longitudinal axis in the direction of movement,
Secure against movement by means of e.g. wooden blocks and lashes,
Adapt vehicle speed to the road conditions,
Vehicle capacity shall be adequate for the transport weight of the transformer,
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Transport
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Transport
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Transport
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Transport
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Maintenance
Inspection and maintenance during operation
Inspection during operation shall only be performed after taking safety measures into consideration:
If there is a maximum indicator on the thermometer the maximum temperature should be recorded,
Inspection for contamination, especially on bushings,
Inspection of surface condition,
Dehydrating breather. The silicagel shall be changed when approx. 2/3 of the silica gel has changed from blue to red colour (old type), or from pink to white, respectively. (Conservator type only),
Inspection for liquid leakages.
For personal safety reasons, only a limited amount of maintenance activities should be performed on the transformer when it is in operation.
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Maintenance
Inspection and maintenance during downtime
Before starting maintenance work, the transformer has to be disconnected from the network and earthed. When the disconnectors have been opened, they shall be locked in open position to prevent them inadvertently closing during maintenance work.
Items to be considered are:
Bushing gaskets; if leaks occur, tightening usually will help, if the gasket has lost its elasticity, it must be replaced. The reason for loss of elasticity can be excessive heating or aging,
Cover gaskets, valves and gaskets of the tap changer. If there are leaks, tightening will usually help,
Welded joints. Leaking joints can be repaired only by welding. A skilled welder and a welding instruction are required.,
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Maintenance
Cleaning contaminated methylated spirit),
Cleaning glasses on gas relay, thermometer and liquid level indicator,
Functional inspection of applicable accessories,
Move tap changer through all positions a few times, all types of tap changers,
Liquid sampling from bottom drain valve for larger units as required,
Check drying material in the dehydrating breather. (Conservator type only),
Amend surface treatment defects.
bushings
(cleaning
agent
e.g.
In heavily contaminated installations more frequent inspections may be needed.
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Maintenance. Transformer liquid and insulation
The task of liquid in a transformer is to act as an electrical insulation and to transfer heat from the transformer’s active parts into coolers.
Liquid acts as a good electrical insulation only as long as it is satisfactorily dry and clean.
Humidity balance between the oil and the insulation implies that most of the humidity will gather in the paper insulation.
Testing of liquid in transformers should normally be performed 12 months after filling or refilling, subsequently every six years.
Testing of oil in on load tap changers must be performed according to the tap changer supplier’s recommendations.
To take liquid samples from hermetically sealed transformers is normally not necessary. The liquid in this type of transformers is not in contact with the atmosphere, and less exposed to moisture.
Especially for large transformers, liquid regeneration may be economically motivated. Liquid regeneration implies drying, filtering, de-gassing and possibly addition of inhibitor.
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Maintenance. Bushings and joints
The porcelain insulators of transformer bushings ought to be cleaned during service interruptions as often as necessary. This is particularly important for places exposed to contamination and moisture.
Methylated spirit or easily evaporating cleaning agents can be used for cleaning.
The condition of external conductor and bus bar joints of transformer bushings shall be checked at regular intervals because reduced contact pressure in the joints leads to overheated bushings etc. and may cause the adjacent gasket to be destroyed by the heat.
© ABB Power Technology 1_114Q07- 1 -
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© ABB Power Technology 1_114Q07- 2 -
General
Power Transformers
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AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
© ABB Power Technology 1_114Q07- 4 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Fundamental considerations
G
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B1
B2
Static device whose function is to transfer electrical energy from one circuit to another whose common link is a magnetic flux.
The current and voltage characteristics of the incoming energy are modified in the output
Construction: Magnetic core made of stacked sheets of ferromagnetic material and two coils (B1, B2) wounded over the former.
Z
Fundamental considerations
G
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B1
B2
Z
Operation: We connect an a.c. generator at B1 and close B2 through an impedance Z.
B1 generates a flux owing to the current. This is a variable one hence it induces a voltage in B2 which in time generates a current in the secondary circuit.
It is only possible with alternating current (a flux variation is needed)
The circuit connected to B1 is named primary and the one connected to B2 is named secondary
The related V and I characteristics are named as well primary and secondary characteristics.
Fundamental relationships
Primary energy Secondary energy
Hence
If the transformer is single phase
And with three phase
At both cases hence
On the other hand it is known that Voltage in a coil is proportional to the generated flux and number of turns
Primary Power Secondary Power U1 I1 U2 I2 √3 U1 I1 √3 U2 I2 U1/ U2 I2 / I1
U1= k1 n1 and U2= k1 n2
From where it is inferred:
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U1 / U2 = n1 / n2 and I2 / I1 = n1 / n2
The ratio n1 / n2 is called Transformation Ratio
Fundamental relationships
We have then:
U1 / U2 = r1 I1 / I2 = 1 / r1
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The former statements are true in an ideal transformer which complies with the following
No reluctance at the magnetic circuit
No resistance at the windings
No histeresys, eddy-current or I2R losses
No leakage flux
Although this is not accomplished in a real case, all formerly said is useful to understand the performance of a transformer and to obtain some approximate values for an elementary calculation
Fundamental relationships
From:
U1 I1 = U2 I2
It is deduced that:
The winding with higher voltage must stand lesser current and conversely the lesser the voltage the higher the current
On the other hand from: I1 n1 = I2 n2
it is deduced that:
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The winding with higher current will be the one with bigger cross section but lesser number of turns
Power transformer function
Transmission of electrical energy is cheaper as far as the transmission voltage is raised
let´s have a power P to transmit with a voltage U and a current I. Losses will be: Pr = R I²
If voltage is raised to nU, current, for the same power, will be reduced to I/n, hence losses in that case will be: Pr = R I² / n²
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They are hence reduced by the square of the ratio of voltage raising.
Or for the same losses the cross section of the wire can be reduced, so is the cost of the line.
On the other hand electrical energy is easier and safer to use when handled at the lowest voltage.
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Power transformer function
The power loss p in a 3-phase transmission line with a resistance R per phase and a current I flowing in each phase is:
At a system voltage U the transmitted active power is:
Equation (2) can be rewritten as:
Inserted in the equation (1) gives:
Equation (4) indicates that the power loss in the line is proportional to the square of the transmitted active power and inversely proportional to the square of the system voltage.
In other words, the power loss will be lower when the system voltage is increased.
Power transformer function
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The main application of transformers is hence the one depicted at the following diagram :
To raise the voltage and to reduce the current going out from generation stations (Step-up substations).
To reduce the voltage on arriving to the points of utilization (StepDown substations).
© ABB Power Technology 1_114Q07- 13 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Main characteristics of a Power Transformer
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When choosing a transformer it is necessary to define:
Type of transformer
Transformation ratio
Insulation Class
Rated output
Connection
Also it is necessary to establish:
Voltage regulation
Type of cooling
Accessories
© ABB Power Technology 1_114Q07- 15 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Constructive types
Primario
Primario
Secundario Secundario
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Core-form single phase
Core form three phase
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Constructive types
Core-form construction for single-phase transformers consists of magnetic steel punchings arranged to provide a single-path magnetic circuit.
High- and low-voltage coils are grouped together on each main or vertical leg of the core.
In general, the mean length of turn for the winding is comparatively short in the coreform design, while the magnetic path is long.
Constructive types
Primario
Primario
Secundario
Shell-form single phase
Secundario
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Shell form three phase
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Constructive types
Shell-form construction for single-phase transformers consists of all windings formed into a single ring, with magnetic punchings assembled so as to encircle each side of the winding ring.
The mean length of turn is usually longer than for a comparable core-form design, while the iron path is shorter.
Constructive types
In the design of a particular transformer many factors such as insulation stress, mechanical stress, heat distribution, weight and cost must be balanced and compromised.
It appears that, for well-balanced design, both core-form and shellform units have their respective fields of applicability determined by kva and kv rating.
In the larger sizes, shell-form construction is quite appropriate; the windings and magnetic iron can be assembled on a steel base structure, with laminations laid in horizontally to link and surround the windings.
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A close-fitting tank member is then dropped over the core and coil assembly and welded to the steel base, completing the tank assembly and also securing the core to the base member.
Winding types
AT
BT
AT
AT BT
BT
Split windings
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Concentric windings
Winding types
AT
AT
BT AT BT AT BT
BT
BT
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Double concentric windings
Superimposed windings
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Single phase vs. Three phase banks
A three-phase power transformation can be accomplished either by using a three-phase transformer unit, or by inter-connecting three single-phase units to form a three-phase bank.
The three-phase unit has advantages of greater efficiency, smaller size, and less cost when compared with a bank having equal kva capacity made up of three single-phase units.
When three single-phase units are used in a bank, it is possible to purchase and install a fourth unit at the same location as an emergency spare.
This requires only 33 percent additional investment to provide replacement capacity, whereas 100 percent additional cost would be required to provide complete spare capacity for a three-phase unit.
However, transformers have a proven reliability higher than most other elements of a power system, and for this reason the provision of immediately available spare capacity is now considered less important than it once was.
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Single phase vs. Three phase banks
Three-phase units are quite generally used in the highest of circuit ratings, with no on-the-spot spare transformer capacity provided.
In these cases parallel or interconnected circuits of the system may provide emergency capacity, or, for small and medium size transformers, portable substations can provide spare capacity on short notice.
If transportation or rigging facilities should not be adequate to handle the required transformer capacity as a single unit, a definite reason of course develops for using three single-phase units.
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Single phase vs. Three phase banks
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Single phase vs. Three phase banks
Transformers vs. Autotransformers
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An autotransformer inherently provides a metallic connection between its low- and high-voltage circuits; this is unlike the conventional two-winding transformer which isolates the two circuits. Unless the potential to ground of each autotransformer circuit is fixed by some means, the low-voltage circuit will be subject to overvoltages originating in the high-voltage circuit. These undesirable effects can be minimized by connecting the neutral of the autotransformer solidly to ground.
Transformers vs. Autotransformers
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The autotransformer has advantages of:
lower cost,
higher efficiency
better regulation
It has disadvantages including:
low reactance which may make it subject to excessive short-circuit currents
the arrangement of taps is more complicated
the low- and high-voltage circuits cannot be isolated
the two circuits must operate with no angular phase displacement unless a zig-zag connection is introduced.
The advantages of lower cost and improved efficiency become less apparent as the transformation ratio increases, so that autotransformers for power purposes are usually used for low transformation ratios, rarely exceeding 2 to 1.
Transformers vs. Autotransformers
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Three-phase autotransformers for power service are usually starstar connected with the neutral grounded, and in most of these cases it is desirable to have a third winding on the core deltaconnected so as to carry the third harmonic component of exciting current.
This winding could be very small in capacity if it were required to carry only harmonic currents, but its size is increased by the requirement that it carry high currents during system ground faults.
A widely used rule sets the delta-winding rating at 35 percent of the autotransformer equivalent two-winding kva rating (not circuit kva rating).
Since it is necessary in most cases to have a delta-connected tertiary winding, it is often advantageous to design this winding so that load can be taken from it. This results in a three-winding autotransformer with terminals to accommodate three external circuits.
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AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Transformation ratio
Primary voltage : The most usual value of the voltage at the point of the network where the transformer is going to be connected
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When this voltage is expected to vary it could be necessary that the transformer is equipped with an on load tap changer
Secondary voltage: The desired value at the secondary network where the transformer will be connected.
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Transformation ratio and voltage drop
The voltage ratio of a transformer is normally specified in no load condition and is directly proportional to the ratio of the number of turns in the windings.
When the transformer is loaded, the voltage on the secondary terminals changes from that in no load condition, depending on
the angle φ between the voltage on the secondary terminals of the transformer U2 and the secondary current I2
the value of the secondary current I2
the short-circuit impedance of the transformer Z and its active and reactive components, r and ±jx respectively
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Transformation ratio and voltage drop
At no load the secondary voltage is U20.
With the load ZL connected, the voltage at the secondary terminals changes to U2.
For example, when a transformer with values for ur=0,01 and ux=0,06 is loaded with rated current with a power factor of 0,8 inductive the voltage on the secondary terminals decreases to 95,5% of the voltage at no load.
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Transformation ratio and voltage drop
Users and installation planners are recommended to take the variation of the secondary voltage during loading into account when specifying the transformer data.
This may be especially important for example in a case where a large motor represents the main load of the transformer.
The highly inductive starting current of the motor may then be considerably higher than the rated current of the transformer.
Consequently there may be a considerable voltage drop through the transformer.
If the feeding power source is weak, this will contribute to an even lower voltage on the secondary side of the transformer.
Short circuit impedance
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Users have sometimes particular requirements regarding the shortcircuit impedance. Such requirements may be determined by:
parallel operation with existing units,
limitation of voltage drop,
limitation of short-circuit currents.
The transformer designer can meet the requirements in different ways:
The size of the core cross-section. A large cross-section gives a low impedance and vice versa,
A tall transformer gives a low impedance and vice versa.
For each transformer there is, however, a smaller range which gives the optimum transformer from an economic point of view, that is the lowest sum of the manufacturing costs and the capitalised value of the losses.
Short circuit impedance
Short-circuit impedance Z is often expressed as uz in p.u. or in % according to the following formulas:
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Formulas (24) and (25) are valid for for single-phase transformers, where Ir and Ur are rated values of current and voltage on either side of the transformer. For 3-phase transformers the nominator must be multiplied with √3. Based on measured short-circuit voltage the value of Zk expressed in ohm can be calculated from the following formula:
for 3-phase transformers. Sr is the rated power of the transformer.
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AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
© ABB Power Technology 1_114Q07- 38 -
Isolation class
Defines the capability of the transformer to withstand overvoltages without loosing its functionality or undue deterioration
Overvoltages could be:
in
Temporary ov
Switching ov
Lightning ov
networks
Isolation class
The standard insulation classes and dielectric tests for power transformers are given in standards. The insulation class of a transformer is determined by the dielectric tests which the unit can withstand, rather than by rated operating voltage.
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The test values are:
Basic impulse level (lightning)
Short time Power frequency overvoltage
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AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Rated output
The rated kva output of a transformer is that load which it can deliver continuously at rated secondary voltage without exceeding a given temperature rise measured under prescribed test conditions.
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The actual test temperature rise may, in a practical case, be somewhat below the established limit because of design and manufacturing tolerances.
The output which a transformer can deliver in service without undue deterioration of the insulation may be more or less than its rated output, depending upon the following design characteristics and operating conditions as they exist at a particular time:
Ambient temperature.
Top-oil rise over ambient temperature.
Hottest-spot rise over top-oil temperature (hottest-spot copper gradient).
Transformer thermal time constant.
Ratio of load loss to no-load loss.
Loading Based on Ambient Temperature
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Air-cooled oil-immersed transformers built to meet established standards will operate continuously with normal life expectancy at rated kva and secondary voltage, providing the ambient air temperature averages no more than 30º C throughout a 24-hour period with maximum air temperature never exceeding 40 C. Water-cooled transformers are built to operate continuously at rated output with ambient water temperatures averaging 25 C and never exceeding 30 C.
When the average temperature of the cooling medium is different from the values above, a modification of the transformer loading may be made according to Table 7.
In cases where the difference between maximum air temperature and average air temperature exceeds 10 C, a new temperature that is 10 C below the maximum should be used in place of the true average. The allowable difference between maximum and average temperature for water-cooled transformers is 5 C.
Loading Based on Capacity Factor
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Transformer capacity factor (operating kva divided by rated kva) averaged throughout a 24-hour period may be well below 100 percent, and when this is true some compensating increase in maximum transformer loading may be made. The percentage increase in maximum loading as a function of capacity factor, based on a normal transformer life expectancy, is given in Table 8.
Loading Based on Short-Time Overloads
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Short-time loads which occur not more than once during any 24-hour period may be in excess of the transformer rating without causing any predictable reduction in transformer life. The permissible load is a function of the average load previous to the period of above-rated loading, according to Table 9. The load increase based on capacity factor and the increase based on short-time overloads can not be applied concurrently; it is necessary to chose one method or the other.
Loading Based on Short-Time Overloads
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Short time loads larger than those shown in Table 9 will cause a decrease inprobable transformer life, but the amount of the decrease is difficult to predict in general terms. Some estimate of the sacrifice in transformer life can be obtained from Table 10(a) which is based on the theoretical conditions and limitations described in Table10(b).
Loading Based on Short-Time Overloads
© ABB Power Technology 1_114Q07- 46 -
These conditions were chosen to give results containing some probable margin, when compared with most conventional transformer designs. For special designs, or for a more detailed check on some particular unit, the hottest-spot copper temperature can be calculated by the method shown in section 19, and the probable sacrifice in transformer life can then be estimated from Table 11.
Loading Based on Measured Oil Temperat. The temperature of the hottest-spot within a power transformer winding influences to a large degree the deterioration rate of insulation. For oilimmersed transformers the hottestspot temperature limits have been set at 105 C maximum and 95 C average through a 24 hour period; normal life expectancy is based on these limits.
The top-oil temperature, together with a suitable temperature increment called either hottest-spot copper rise over top-oil temperature or hottestspot copper gradient, is often used as an indication of hottest-spot temperature.
© ABB Power Technology 1_114Q07- 47 -
Loading Based on Measured Oil Temperat. Allowable top-oil temperature for a particular constant load may be determined by subtracting the hottestspot copper gradient for that load from 95 C. The hottest-spot copper gradient must be known from design information for accurate results, though typical values may be assumed for estimating purposes. If the hottest-spot copper gradient is known for one load condition, it may be estimated for other load conditions by reference to Fig. 18.
A conservative loading guide, based on top-oil temperatures, is given in Fig. 19.
© ABB Power Technology 1_114Q07- 48 -
One or several transformers?
Advantages of several transformers:
Less spare capacity needed
No outage of the entire system in case of transformer failure
Drawbacks:
Higher cost (spare capacity not considered)
Higher short circuit capacity if transformers are coupled ......
© ABB Power Technology 1_114Q07- 49 -
......
......
......
Parallel operation of transformers
When there are several transformers at the substation it is possible to couple them or not.
© ABB Power Technology 1_114Q07- 50 -
On the other hand in transmission networks the transformers are always coupled.
Advantages: No outage when one of the transformers is disconnected (providing there is capacity enough at the network)
Drawbacks: Higher short circuit capacity
Provisions to be taken before coupling:
Same transformer ratio
Matching connection group
Matching phase rotation direction
Short circuit voltage inversely proportional to rated output
Tap changing coordination
© ABB Power Technology 1_114Q07- 51 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Transformer connections
© ABB Power Technology 1_114Q07- 52 -
Star- Star connection (Y-y):
It stands badly secondary imbalances
Neutral connection is possible
Low cost because of reduced number of turns and lower isolation needed (phase-ground voltage)
Bigger cross section of conductors because of higher current which gives more stiffness to windings and better performance on short circuits
Star- Star with tertiary winding:
It solves the problem of imbalances and third harmonic
Tertiary winding can be used for ancillary services
Transformer connections
© ABB Power Technology 1_114Q07- 53 -
Delta- Star connection (D-y):
Very used in distribution systems
It stands well secondary imbalances
Third harmonic is not transmitted to low voltage
Low voltage Neutral connection possible
High cost because of bigger isolation and HV turns so it is not used at high voltages
Star- Delta Connection (Y-d):
Very used at high voltages because of lower isolation and HV nº of turns needed.
Low voltage neutral connection not possible so they are not used at distribution systems.
The delta connection prevents third harmonic flux because third harmonic current circulates inside the delta windings.
Transformer connections
Desequilibrio 2º 3º Armonico Posibilidad neutro Coste aislamiento Coste Cu
© ABB Power Technology 1_114Q07- 54 -
Y-y D-y Y-d Y-z D-d Y-y-3º
M
M
B
B
B
B
B
B
R
B
R
B
M
B
B
B
R
B
B
R
R
B
M
M
M
B
B
B
B
R
© ABB Power Technology 1_114Q07- 55 -
Voltage Control
Power Transformers
© ABB Power Technology 1_114Q07- 56 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Voltage control
The modern load tap changer had its beginning in 1925.
© ABB Power Technology 1_114Q07- 57 -
Since that time the development of more complicated transmission networks has made tap changing under load more and more essential to control the in-phase voltage of power transformers.
Tap-changing-under-load transformers:
equipment
is
applied
to
power
to maintain a constant secondary voltage with a variable primary voltage;
to control the secondary voltage with a fixed primary voltage;.
Various types of tap-changing equipment and circuits are used depending upon the voltage and kva.
Under-load-tap-changers are built for 8, 16, and 32 steps, with the trend in recent years being toward the larger number of steps so as to give a finer degree of regulation.
The usual range of regulation is plus 10 percent and minus 10 percent of the rated line voltage, with plus and minus 71/2 percent and plus and minus 5 percent being second and third, respectively, in popularity.
Voltage control
© ABB Power Technology 1_114Q07- 58 -
Figure illustrates schematically the operation of one type of mechanism for changing taps under load.
Taps from the transformer winding connect to selector switches 1 through 9. The selector switches are connected to load transfer switches R, S, and T.
The connections for the tap changer positions are shown on the sequence chart.
The sequence of switching is so coordinated by the tap changing mechanism that the transfer switches perform all the switching operations, opening before and closing after the selector switches. All arcing is thus restricted to switches R, S, and T, while switches 1 to 9 merely select the transformer tap to which the load is to be transferred.
© ABB Power Technology 1_114Q07- 59 -
Voltage control
When the tap changer is on odd-numbered positions, the preventive auto-transformer is short-circuited.
On all even- numbered positions, the transformer bridges two transformer taps.
In this position, the relatively high reactance of the preventive auto-transformer to circulating currents between adjacent taps prevents damage to the transformer winding, while its relatively low impedance to the load current permits operation on this position to obtain voltages midway between the transformer taps.
preventive
auto-
© ABB Power Technology 1_114Q07- 60 -
Voltage control
The operation in this case of the selector and transfer switches is exactly as described for the former.
But this type also has a reversing switch which reverses the connections to the tapped section of the winding so that the same range and number of positions can be obtained with one-half the number of tap sections, or twice the range can be obtained with the same number of taps.
The reversing switch is a close-before-open switch which operates at the time there is no voltage across its contacts.
© ABB Power Technology 1_114Q07- 61 -
Voltage control
This type of load tap changer is applied to small power transformers and large distribution transformers.
The transfer switches are eliminated, and each selector switch serves as a transfer switch for the tap to which it is connected.
The schematic circuit diagram and operations sequence chart is shown in Fig.
Load Tap changing.
4
6
5
6 5 4
© ABB Power Technology 1_114Q07- 62 -
3 Par
Impar
3
The tap changer is in position 4
The tap selector without current prepares the new position to change (from 3 to 5)
Load Tap changing.
4
6
5
6 5 4
© ABB Power Technology 1_114Q07- 63 -
3 Par
Impar
3
The transition switch begins the new operation connecting in parallel the initial position (4) with the final one (5)
The transition resistances clear away the energy stored at the coil to be transferred preventing overvoltages ( on the turns to be taken off ) either contribute to that the establishment of the current, at the turns to add, is in a smooth way.
Load Tap changing.
The transition switch interrupts the current through the initial tap (4) and pass on it definitely to the final one (5)
4
6
5
6 5 4
© ABB Power Technology 1_114Q07- 64 -
3 Par
Impar
3
© ABB Power Technology 1_114Q07- 65 -
Autotransformers tap changers
© ABB Power Technology 1_114Q07- 66 -
No load tap changer
© ABB Power Technology 1_114Q07- 67 -
On load tap changers
© ABB Power Technology 1_114Q07- 68 -
Three phase on load tap changer
© ABB Power Technology 1_114Q07- 69 -
Three phase on load tap changer
© ABB Power Technology 1_114Q07- 70 -
Single phase on load tap changer
© ABB Power Technology 1_114Q07- 71 -
Transition switch
© ABB Power Technology 1_114Q07- 72 -
Types of Cooling
Power Transformers
© ABB Power Technology 1_114Q07- 73 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
Types of cooling
Cooling is needed to transfer the heat produced by losses so as not to damage isolation
Two main types of isolation exist:
Dry Transformers, air refrigerated
Oil immersed Transformers
In Substations the most used is the second one
© ABB Power Technology 1_114Q07- 74 -
Dry type are only used for low output and when fire hazard is a big concern
Types of cooling
© ABB Power Technology 1_114Q07- 75 -
Several types transformers:
of
cooling
are
possible
with
oil
Oil immersed Self cooled (ONAN)
Oil Immersed Self Cooled/Forced-Air Cooled (ONAF)
Oil immersed Forced cooled/Forced air cooled (OFAF)
Oil immersed Water cooled (ONWN-OFWF)
immersed
Oil inmersed Self cooled (ONAN)
© ABB Power Technology 1_114Q07- 76 -
In this type of transformer the insulating oil circulates by natural convection within a tank having either smooth sides, corrugated sides, integral tubular sides, or detachable radiators.
Smooth tanks are used for small distribution transformers but because the losses increase more rapidly than the tank surface area as kva capacity goes up, a smooth tank transformer larger than 50 kva would have to be abnormally large to provide sufficient radiating surface.
Integral tubular-type construction is used up to about 3000 kva and in some cases to larger capacities, though shipping restrictions usually limit this type of construction at the larger ratings.
Above 3000 kva detachable radiators are usually supplied.
Transformers rated 46 kv and below may also be filled with Inerteen fire-proof insulating liquid, instead of with oil.
The ONAN transformer is a basic type, and serves as a standard for rating and pricing other types.
© ABB Power Technology 1_114Q07- 77 -
Oil inmersed Self cooled (ONAN)
Oil-Imm Self-Cooled/Forced-Air Cooled (ONAF)
This type of transformer is basically an ONAN unit with the addition of fans to increase the rate of heat transfer from the cooling surfaces, thereby increasing the permissible transformer output.
The ONAF transformer is applicable in situations that require shorttime peak loads to be carried recurrently, without affecting normal expected transformer life.
© ABB Power Technology 1_114Q07- 78 -
This transformer may be purchased with fans already installed, or it may be purchased with the option of adding fans later.
The higher kva capacity attained by the use of fans may be calculated as follows :
For 2500 kva (OA) and below: kva (FA)=l.l5Xkva(OA).
For 2501 to 9999 kva (OA) single-phase or 11 999 kva (OA) three-phase : kva (FA) = 1.25 X kva (OA).
For 10 000 kva (OA) single-phase and 12 000 kva (OA) three-phase, and above : kva (FA) = 1.333Xkva (OA). (22)
These ratings are standardized, and are based on a hottest-spot copper temperature of 65 degrees C above 30 degrees C average ambient.
© ABB Power Technology 1_114Q07- 79 -
Oil-Imm Self-Cooled/Forced-Air Cooled (ONAF)
Oil imm. forced cooled/Forced air cooled (OFAF)
The rating of an oil-immersed transformer may be increased from its OA rating by the addition of some combination of fans and oil pumps.
Such transformers are normally built in the range 10 000 kva (OA) single-phase or 12 000 kva (OA) three-phase, and above.
© ABB Power Technology 1_114Q07- 80 -
Increased ratings are defined as two steps, 1.333 and 1.667 times the OA rating respectively.
Automatic controls responsive to oil temperature are normally used to start the fans and pumps in a selected sequence as transformer loading increases.
A variation of this is a type of transformer which is intended for use only when both oil pumps and fans are operating, under which condition any load up to full rated kva may be carried.
Some designs are capable of carrying excitation current with no fans or pumps in operation, but this is not universally true. Heat transfer from
oil to air is accomplished in external oil-to-air heat exchangers.
© ABB Power Technology 1_114Q07- 81 -
Oil imm. forced cooled/Forced air cooled (OFAF)
© ABB Power Technology 1_114Q07- 82 -
Oil imm. forced cooled/Forced air cooled (OFAF)
Oil immersed water cooled (ONWN-OFWF) OW-Oil-Immersed Water-Cooled
In this type of water-cooled transformer, the cooling water runs through coils of pipe which are in contact with the insulating oil of the transformer.
The oil flows around the outside of these pipe coils by natural convection, thereby effecting the desired heat transfer to the cooling water. This type has no self-cooled rating.
FOW-Oil-Immersed Forced-Oil-Cooled With Forced-Water
© ABB Power Technology 1_114Q07- 83 -
Cooler-External oil-to-water heat exchangers are used in this type of unit to transfer heat from oil to cooling water.
© ABB Power Technology 1_114Q07- 84 -
Oil immersed water cooled (ONWN-OFWF)
© ABB Power Technology 1_114Q07- 85 -
Cooling systems. Radiators
© ABB Power Technology 1_114Q07- 86 -
Cooling systems. Air-refrigerator
© ABB Power Technology 1_114Q07- 87 -
Cooling systems. Hydro-refrigerator
© ABB Power Technology 1_114Q07- 88 -
Accessories Practical aspects
Power Transformers
© ABB Power Technology 1_114Q07- 89 -
AGENDA
INTRODUCTION
MAIN CHARACTERISTICS OF A POWER TRANSFORMER
Types of transformers
Transformation ratio
Insulation class
Rated output
Transformer connections
VOLTAGE CONTROL
TYPES OF COOLING
ACCESSORIES/ PRACTICAL ASPECTS
© ABB Power Technology 1_114Q07- 90 -
Accesories
Pressure relief valve
Gas detector relay. Buchholz
Non return valve
Dehydrating breather
Oil expansion tank
Temperature detectors
Thermostat and oil level indicator
Bushing current transformer
Bushings
© ABB Power Technology 1_114Q07- 91 -
Accesories. Pressure relief valve
Accesories. Gas actuated relay (Buchholz) The gas generated in a transformer during a fault is collected in the gas relay.
The gas will displace the liquid in the relay and minor gas generation will cause the closing of the alarm contact. If an extensive amount of gas is generated or the oil level falls, then the alarm contact will close first, followed by the tripping contact.
A heavy liquid flow from the transformer into the liquid conservator (conservator type only), will cause the immediate closing of the tripping contact. If the tripping contacts operate the transformer will be immediately disconnected from the network.
© ABB Power Technology 1_114Q07- 92 -
Accesories. Gas actuated relay (Buchholz)
Operation of some protective equipment such as gas relay or differential relay does not always mean that the transformer is damaged.
The gas relay can operate for example when:
© ABB Power Technology 1_114Q07- 93 -
An air bubble has been left under the transformer cover. An air bubble is colourless and odourless.
A short-circuit current has passed the transformer. No gas bubbles.
However if the gas has colour or smell, the transformer is damaged.
© ABB Power Technology 1_114Q07- 94 -
Accesories. Non return valve. Dehydrating breather
© ABB Power Technology 1_114Q07- 95 -
Accesories. Oil expansion tank
© ABB Power Technology 1_114Q07- 96 -
Accesories. Temperature detectors
© ABB Power Technology 1_114Q07- 97 -
Accesories. Thermostat / Oil level indicator
© ABB Power Technology 1_114Q07- 98 -
Bushing current transformer
© ABB Power Technology 1_114Q07- 99 -
Accesories. Bushings
© ABB Power Technology 1_114Q07- 100 -
Installation. Handling and lifting
Only approved and suitable lifting equipment shall be used.
Use a forklift only on transport pallets or transformer bottom.
Do not apply load to corrugated fins or radiators and their supports.
Use the provided lifting lugs only.
When lifting a transformer with cable boxes on the cover, special care must be taken.
When hydraulic jacks are used, only provided jacking points shall be used, and in such a way that twisting forces on the transformer tank are avoided.
© ABB Power Technology 1_114Q07- 101 -
Installation. Handling and lifting
© ABB Power Technology 1_114Q07- 102 -
Installation. Handling and lifting
© ABB Power Technology 1_114Q07- 103 -
Transport
The transformer is supplied filled with liquid and normally all accessories fitted, except for the largest units. The radiators may be dismantled during transport.
During transport the following should be considered:
Angle of tilting exceeding 10º must be specified in the contract,
Prevention of damage to bushings, corrugated panels or radiators and accessories,
Larger transformers should preferably be positioned with the longitudinal axis in the direction of movement,
Secure against movement by means of e.g. wooden blocks and lashes,
Adapt vehicle speed to the road conditions,
Vehicle capacity shall be adequate for the transport weight of the transformer,
© ABB Power Technology 1_114Q07- 104 -
Transport
© ABB Power Technology 1_114Q07- 105 -
Transport
© ABB Power Technology 1_114Q07- 106 -
Transport
© ABB Power Technology 1_114Q07- 107 -
Transport
© ABB Power Technology 1_114Q07- 108 -
Maintenance
Inspection and maintenance during operation
Inspection during operation shall only be performed after taking safety measures into consideration:
If there is a maximum indicator on the thermometer the maximum temperature should be recorded,
Inspection for contamination, especially on bushings,
Inspection of surface condition,
Dehydrating breather. The silicagel shall be changed when approx. 2/3 of the silica gel has changed from blue to red colour (old type), or from pink to white, respectively. (Conservator type only),
Inspection for liquid leakages.
For personal safety reasons, only a limited amount of maintenance activities should be performed on the transformer when it is in operation.
© ABB Power Technology 1_114Q07- 109 -
Maintenance
Inspection and maintenance during downtime
Before starting maintenance work, the transformer has to be disconnected from the network and earthed. When the disconnectors have been opened, they shall be locked in open position to prevent them inadvertently closing during maintenance work.
Items to be considered are:
Bushing gaskets; if leaks occur, tightening usually will help, if the gasket has lost its elasticity, it must be replaced. The reason for loss of elasticity can be excessive heating or aging,
Cover gaskets, valves and gaskets of the tap changer. If there are leaks, tightening will usually help,
Welded joints. Leaking joints can be repaired only by welding. A skilled welder and a welding instruction are required.,
© ABB Power Technology 1_114Q07- 110 -
Maintenance
Cleaning contaminated methylated spirit),
Cleaning glasses on gas relay, thermometer and liquid level indicator,
Functional inspection of applicable accessories,
Move tap changer through all positions a few times, all types of tap changers,
Liquid sampling from bottom drain valve for larger units as required,
Check drying material in the dehydrating breather. (Conservator type only),
Amend surface treatment defects.
bushings
(cleaning
agent
e.g.
In heavily contaminated installations more frequent inspections may be needed.
© ABB Power Technology 1_114Q07- 111 -
Maintenance. Transformer liquid and insulation
The task of liquid in a transformer is to act as an electrical insulation and to transfer heat from the transformer’s active parts into coolers.
Liquid acts as a good electrical insulation only as long as it is satisfactorily dry and clean.
Humidity balance between the oil and the insulation implies that most of the humidity will gather in the paper insulation.
Testing of liquid in transformers should normally be performed 12 months after filling or refilling, subsequently every six years.
Testing of oil in on load tap changers must be performed according to the tap changer supplier’s recommendations.
To take liquid samples from hermetically sealed transformers is normally not necessary. The liquid in this type of transformers is not in contact with the atmosphere, and less exposed to moisture.
Especially for large transformers, liquid regeneration may be economically motivated. Liquid regeneration implies drying, filtering, de-gassing and possibly addition of inhibitor.
© ABB Power Technology 1_114Q07- 112 -
Maintenance. Bushings and joints
The porcelain insulators of transformer bushings ought to be cleaned during service interruptions as often as necessary. This is particularly important for places exposed to contamination and moisture.
Methylated spirit or easily evaporating cleaning agents can be used for cleaning.
The condition of external conductor and bus bar joints of transformer bushings shall be checked at regular intervals because reduced contact pressure in the joints leads to overheated bushings etc. and may cause the adjacent gasket to be destroyed by the heat.
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