Circuit Breakers

Circuit Breakers (Theory, Classification, & Applications)

Circuit Breakers (Theory, Classification, & Applications)

Explored By:

Engr. Syed Muhammad Munavvar Hussain

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

Circuit Breakers

D

uring the operation of the power systems, it is often desirable and necessary to switch on or off the various circuits

(e.g.,

transmission

lines,

distributors,

generating plants etc.) under both normal and faulty conditions. Previously this function was performed by a switch and a fuse in series with the circuit. However, such a means of control presents two disadvantages. Firstly, when fuse blows out, it takes quite sometime to replace it and restore the supply to the customers. Secondly, a fuse can not successfully interrupt the heavy fault currents that occur on the modern high voltage power systems and large capacity circuits. Therefore, with the advancement in the power systems, there was a need to develop a more reliable means of control. The circuit breaker was developed to switch on and off the various circuits of a power system.

Devices used for circuit breaking (or making) 1. Fuse and ironclad switches Fuse is an over current switch in the sense that when the current exceeds a preassigned value in a circuit or device, it melts and causes the current interruption. The supply is restored only when a healthy fuse replaces the damaged (melted) one in the line. To permit this without any danger of shock to the operator, fuses are connected on the load side of an ironclad switch.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) 2.

Isolators An isolator is a switch connected after a circuit breaker. When a circuit or a

busbar is taken out of service by tripping the circuit breaker, the isolator is then open circuited and the isolated line is earthed through earth switch so that the trapped line charges are safely conducted to ground. 3. Circuit breakers A circuit breaker is a complex circuit-breaking device with the following duties: Makes or breaks both normal and abnormal currents Appropriately manages the high-energy arc associated with current interruption. The problem has become more acute due to interconnection of power stations resulting in very high fault levels. Effects current interruption only when it is called upon to do so by the relay circuits. In fact they are required to trip for a minimum of the internal fault current and remain inoperative for a maximum of through fault current Rapid and successive automatic breaking and making to aid stable system operation Three pole (3-pole) and single pole (1-pole) auto-reclosing arrangement In addition to the making and breaking capabilities, a circuit breaker is required to do following tasks under the following typical conditions: Short-circuit interruption Interruption of small inductive currents Capacitor switching Interruption of short-line fault Asynchronous switching A circuit breaker is a switching i.e. current interrupting or making device in switchgear. In more proper words, a circuit breaker is defined as a piece of equipment which can do any one of the following tasks: Makes or breaks a circuit either manually or by remote control under normal conditions

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) Breaks a circuit automatically under fault conditions Makes a circuit either manually or by remote control under fault conditions

Figure 1: “A Circuit Breaker with Internal Structure” Thus a circuit breaker is used for incorporating manual as well as automatic control for the switching function. It is easy to percept what manual operation means for the circuit breakers. On the other hand, the automatic control of the circuit breaker is incorporated with the help of relays and is only done in case of fault conditions. The main advantage associated with the use of circuit breaker is that, unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Operating Principle The primary function of the circuit breakers mechanism is to provide the means for opening and closing the contacts. Initially, this seems to be a rather simple and straightforward requirement. However, when one considers the fact that most circuit breakers, once placed into service, will remain in the closed position for long periods of time, and yet on the few occasions when they are called upon to open or close, they must do so reliably, without any delay or sluggishness, then one realizes that the demands on the mechanisms are not as simple as was first thought. A circuit breaker essentially consists of fixed and moving contacts. These contacts are called electrodes. The need for carrying the continuous current and for withstanding

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) a period of arcing makes it necessary to use two sets of contacts in parallel, one is the primary contact and the second is the arcing contact. The primary contact is always made of a high conductive material such as copper and the arcing contact is made of arc resistance material such as tungsten or molybdenum, which has a much lower conductivity than those used for primary contacts. When the circuit breaker opens to interrupt the current, the primary contacts open before the arcing contacts. Under the normal operating conditions, these contacts remain closed and are not open automatically until and unless the system becomes faulty. Of course, the contacts can be opened manually or by remote control when ever desired. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit. When the contacts of a circuit break are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. The production of the arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself. Therefore, the main problem in the circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value. In single phase (1-phase) circuits (i.e., lighting circuits etc.), a switch is located in only one of the two conductors to the load. However in the power circuits, a circuit interrupting device (i.e., circuit breaker) is put in each phase or conductor. These are, sometimes, called 3-pole circuit breakers.

Arc Phenomenon Arc in an ac circuit breaker occurs in two ways: 1.

When the current-carrying contacts are being separated, arcing is possible even when the circuit e.m.f. is considerably below the minimum cold electrode breakdown voltage, because of the ions neutralizing the electronic space charge and thus allowing large currents to flow at relatively low voltage gradients. This way of occurrence of an arc is common to both dc and ac circuit breakers.

2.

The other way of occurrence of an arc happens only in ac circuit breakers. In such case, the arc is extinguished every time the current passes through zero and can restrike only if the transient recovery voltage across the electrodes,

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) already separated and continuing to separate, reaches a sufficiently high value causing breakdown. The function of an ac circuit breaker is to prevent restriking of the arc, which depends upon the following important factors: The nature and pressure of the medium of arc The external ionizing and de-ionizing agents present The voltage across the electrodes and its variation with time The material and configuration of the electrodes The nature and configuration of the arcing chamber

Categories of Arcs Arcs in the circuit breakers are categorized as: a)

High-pressure arcs:

with ambient pressures of 1 atm and above

b)

Vacuum arcs:

with ambient pressures below 10-4 torr

Arc is an essential part of any circuit breaker operation where contact is physically parted. Before they are opened by the protective system, a heavy current flows through the contacts of circuit breaker, when a short circuit occurs.

Figure 2: “Arc Phenomenon in Circuit Breakers” When two current carrying contacts open, an arc bridges the contact gap and prevents an abrupt interruption of the current. At the instant when the contacts begin to

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionize the air or vaporize and ionize the oil. The ionized air or vapors act as conductor and an arc is struck between the contacts. The potential difference between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists. The arc is useful in a way as it provides a low resistance path for the current after contact separation. It prevents current chopping and associated abnormal switching over-voltages in the system. The arc plays an important role in the process of current interruption and therefore must not be regarded as an undesirable phenomenon. It must also be realized that, in the absence of the arc, the current flow would be interrupted instantaneously, and due to the rate of collapse of the associated magnetic field, very high voltage would be induced which severely stress the insulation of the system. On the other hand, the arc provides a gradual, but quick, transition from the current-carrying to the current-breaking states of the contacts. Therefore, it permits the disconnection to take place at zero current without indicating the potentials of dangerous values. The function of an arc-control device in a circuit breaker is therefore clearly to employ the beneficent action of the arc as efficiently as possible.

Significance of Arc Resistance During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller is the current that flows between the contacts. The arc resistance depends upon the following factors: Degree of Ionization Length of the Arc Cross-section of the Arc

a.

Degree of Ionization The arc resistance increases with the decrease in the number of ionized particles

between the contacts.

b.

Length of the Arc The arc resistance increases with the length of the arc i.e., separation of contacts.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

c.

Cross-section of the Arc The arc resistance increases with the decrease in the area of cross-section of the

arc.

Principle of Arc Extinction Prior to discussing the methods of arc extinction, it is essential to scrutinize the factors accountable for the maintenance of arc between the contacts. These are: Potential difference between the contacts Ionized particles between the contacts

i)

Potential Difference between the Contacts When the contacts have small separation, the potential difference between them

is sufficient to maintain the arc. One way to extinguish the arc is to separate the contacts to such a distance that potential difference becomes inadequate to maintain the arc. However this method is impracticable in high voltage systems where a separation of many meters may be required.

ii)

Ionized Particles between the Contacts The ionized particles between the contacts tend to maintain the arc. If the arc

path is de-ionized, the arc extinction will be facilitated. This may be achieved by cooling the arc or bodily removing the ionized particles from the space between the contacts.

Classification of Circuit Breakers There are quite a few ways to classify the circuit breakers. However, the most general way of classification is on the basis of medium used for the arc extinction. Other bases for the classification of circuit breakers are summarized in the flow chart on the next page.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

Circuit Breakers 1. Based on Voltage    

Low Medium High/Extra High Ultra High

2. Based on Location  

Indoor Outdoor

3. Based on External Design  

Dead Tank Live Tank

4. Based on Interrupting Media     

Air Break Air Blast Oil SF6 Vacuum

Figure 3: Classification of Circuit Breakers

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

1.

Based on Voltage On the basis of the voltage levels for which they are used, the circuit breakers are

classified as listed in table (with corresponding voltage ranges of use).

Category

Range of Voltage

Low voltage

Less than 1 kV

Medium voltage

1 kV to 52 kV

High/Extra High voltage

66 kV to 765 kV

Ultra High voltage

Above 765 kV

Table 1: “Classification of circuit breakers on Voltage Basis”

2.

Based on Location Circuit breakers are, based upon where they are located, classified as, indoor and

outdoor types. Medium and low voltage breakers are categorized as Indoor circuit breakers, whereas the circuit breakers which have air as external insulating medium are classified as outdoor circuit breakers.

3.

Based on External Design Outdoor circuit breakers can be identified as either dead tank or live tank type

circuit breakers, from the point of view of their physical structural design. In the dead tank circuit breakers, the switching device is located, with suitable insulator supports, inside a metallic vessel at ground potential and filled with insulating medium. In dead tank circuit breakers, the incoming and outgoing conductors are taken out through suitable insulator bushings, and low voltage type current transformers are located at lower end of both insulator bushings, i.e. at the line side and the load side. In live tank circuit breakers, the interrupter is located I an insulator bushing, at a potential above ground potential. The live tank circuit breakers are cheaper (with no current transformer), and require less mounting space.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

4.

Based on Interrupting Media The interrupting media has been a vital factor in the evolution of circuit breakers. It

dedicates the overall design parameters of the breaker. The choice of air and oil, as the interrupting media, was predominant till late 70s. but today, vacuum and SF6 are the only dominant interrupting technologies, for medium and high voltage segments of circuit breaker design respectively. The medium used for the arc extinction can be: Oil Air Vacuum Sulphur Hexafluoride (SF6) Accordingly, the circuit breakers may be classified into following categories (which will be treated in detail in the present report): Oil Circuit Breakers Air-blast Circuit Breakers Sulphur Hexafluoride (SF6) Circuit Breakers Vacuum Circuit Breakers

1.

Oil Circuit Breakers In such circuit breakers, some insulating oil (i.e., transformer oil) is used as an

arc quenching medium. The contacts are opened under oil and an arc is struck between them. The heat of the arc evaporates the surrounding oil and dissociates it into a substantial volume of hydrogen gas at a high pressure. This large volume of the hydrogen gas pushes the oil away from the arc. In an oil circuit breaker, the arc quenching process is entirely dependent on arc energy generated. The arc drawn across the contacts is contained inside the interrupting pot, and thus the hydrogen gas formed by the vaporized oil (gas) is also contained inside the chamber. As the contacts continue to move, and the moving contact rod separates itself from the orifice of the chamber, an exit similar to a nozzle allows escape of the hydrogen gas trapped inside the interrupting chamber. The escaping high pressure hydrogen gas, having a high thermal conductivity, takes away the heat, thus making the contact gap cool and free from ionization, immediately after current zero. However, the oil breakers have prolonged arcing times, due to insufficient vapor pressure generated at lower interrupting currents. Periodic monitoring of the oil Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) dielectric conduction and its maintenance is important for the effective operation of medium oil circuit breaker. However, maintenance required for oil circuit breakers is of low technology.

Figure 3: Oil Circuit Breaker The arc extinction is facilitated mainly by two processes: 1.

Firstly, the hydrogen gas has high heat conductivity and cools the arc, thus aiding the de-ionization of the medium between the contacts.

2.

Secondly, the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the arcing products from the arc path. This results in extinguishing the arc and as a result the circuit current is interrupted.

Advantages: As an Arc extinguishing medium oil has the following advantages: 1. Oil absorbs the arc energy to produces hydrogen gas during arcing. The hydrogen has excellent cooling properties and helps extinguish the arc. (In addition to hydrogen gas, a small proportion of methane, ethylene, and acetylene are also generated in oil decomposition.) 2. The oil provides insulation for the live exposed contacts from the earthed portions of the container. 3. Oil provides insulation between the contacts after the arc has been extinguished. 4. The oil close to the arc region provides cooling surface.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) Disadvantages: 1. Oil is inflammable and may cause fire hazards. When a defective circuit breaker fails under pressure, it may cause an explosion. 2. The hydrogen generated during arcing, when combined with air, may form an explosive mixture. 3. During arcing, oil decomposes and becomes polluted by carbon particles, which reduces its dielectric strength. Hence, it requires periodic maintenance and replacement.

Types of Oil Circuit Breakers The oil circuit breakers find extensive use in the power systems. They can be classified with the reference to the quantity of oil used. The types are: Bulk Oil Circuit Breakers Minimum Oil Circuit Breakers a)

Bulk Oil Circuit Breakers These circuit breakers use a large quantity of oil. The oil has to serve two purposes: 1.

It extinguishes the arc during opening of contacts.

2.

It insulates the current conducting parts from one another and from the earthed tank. In the bulk oil circuit breakers, the interrupting unit is placed in a tank of oil at

earth potential and the incoming and outgoing conductors are connected through insulator bushings. b)

Minimum Oil Circuit Breakers These circuit breakers use a small quantity of oil. In such circuit breakers, oil is

used only for arc extinction; the current conducting parts insulated by air or porcelain or organic insulating material. In these circuit breakers, the oil requirement can be minimized by placing the interrupting units, in insulating chambers at live potential, on an insulator column.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

2.

Air-Blast Circuit Breakers These circuit breakers employ a high pressure air-blast as an arc quenching

medium. The contacts are opened in a flow of air-blast established by the opening of the blast valve. The air-blast cools the arc and sweeps away the arcing products of the atmosphere. Consequently, the arc is extinguished and flow of current is interrupted. Whenever current at high voltages needs to be interrupted, more breaking units are used, in series. Dry and clean air supply is one of the most essential requirements for the operation of the air-blast circuit breakers.

Figure 4: Air-Blast Circuit Breaker In addition, other gases such as Nitrogen, Carbon dioxide, and Hydrogen can also be used. But air is preferred because of the fact that the Carbon dioxide tends to freeze, and the hydrogen gas is very expensive. This type of circuit breaker has been used earlier for open terminal HV applications, for voltages of 245 kV, and 400 kV up to 765 kV, especially where faster breaker operation was required. This type of breaker has been used for special applications wherein several super thermal power stations located in close vicinity to each other have been inter-connected. These one-cycle breakers from the inter-connecting link which, in the event of a fault in an outgoing feeder of a particular station, isolates the station from the neighboring ones thereby limiting the fault level for the feeder breaker. Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) The interrupting capability of air circuit breaker is usually increased by increasing the normal pressure range. Normally, the pressure level is around 30 to 35 bars. In order to maintain the insulation level and reliability of operation, it is also necessary for the condition of the air to be very dry. Currently, however, SF6 circuit breakers have practically eliminated the use of this technology. Advantages: 1. The risk of fire is eliminated in these circuit breakers. 2. The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations. So the expenditure of oil replacement is avoided in air-blast circuit breakers. 3. The size of these breakers is reduced, as the dielectric strength grows so rapidly that final contact gap for the arc extinction is very small. 4. Due to the rapid growth of the dielectric strength, the arcing time is also very small. It causes less burning of oil. The arc energy is also very small fraction of that in oil circuit breakers. 5. The arc extinction is facilitate by the high pressure air, and is independent of the fault current to be interrupted. Disadvantages: 1. These circuit breakers are very sensitive to the variation s in the rate of rise of restriking voltage. 2. The air-blast is supplied by the compressor plant that needs considerable maintenance.

3.

Sulphur Hexafluoride (SF6) Circuit Breakers In these circuit breakers, Sulpher hexafluoride gas (SF6) is used as the arc

quenching medium. The SF6 is an electronegative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF6 gas and an arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. The SF6 circuit breakers have been found to be very effective for high power and high voltage services.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) The excellent insulating properties of SF6 gas make it possible to design circuit breakers with smaller overall dimensions, shorter contact gaps, which help in the construction of outdoor breakers with fewer interrupters. The behavior of the arc during current zero and immediately afterwards, is of decisive importance for the reliable interruption of the current. In SF6 gas, the diameter of an arc is relative small. As a consequence, the thermal time constant of the arc, approaching current zero, is almost 100 times smaller than in air. The dielectric strength of the break increases rapidly after current zero; so that the breaker is able to control even extreme rates of rise of the transient recovery voltage (TRV), in case of short line faults. These circuit breakers are available for complete range of medium voltage and high voltage application up to 800 kV and above. This medium is most suitable for metal-clad and hybrid HV sub-stations. Although the share of SF6 circuit breakers in medium voltage application is gradually shrinking, this technology is still emerging as the only choice in the HV range.

Applications SF6 circuit breakers find use in systems with voltages ranging in 115 kV to 230 kV, as they are designed for this range with power ratings of 10 MVA to 20 MVA and interrupting time less than 3 cycles. Advantages: 1. Because of the high conductivity of the arc in the SF6 gas, the arc energy is low. (Arc voltage is between 150 and 200V.) 2. Due to the low energy the contact erosion is small. 3. The gaseous medium SF6 possesses excellent dielectric and arc quenching properties. After arc extinction, the dissociated gas molecules recombine almost completely to reform SF6. This means that practically no loss/consumption of the quenching medium occurs. 4. Due to the superior arc quenching property of the SF6 gas, such circuit breakers have very short arcing time. Furthermore, they can interrupt much larger current. 5. These breakers give noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike the air-blast circuit breaker.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) 6. The SF6 gas is not inflammable, so there is no risk of fire in SF6 breakers. 7. Since SF6 breakers are totally enclosed and sealed from the atmosphere, they are particularly suitable where explosion hazards exist, i.e. in coal mines. Disadvantages: 1. These circuit breakers are expensive due to the high cost of SF6 gas. 2. Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose. 3. The SF6 gas has been identified as a greenhouse gas, and safety regulations are being introduced in many countries in order to prevent its release into the atmosphere. Therefore, the HV circuit breaker would be designed to ensure that there is minimum leakage during the service period and that the utilities let out the least amount of SF6 gas into the atmosphere during maintenance. As SF6 gas has an impact 23 times stronger than the CO2 gas on the greenhouse effect, in order to minimize the emission of SF6 gas, N2- SF6 and CF4- SF6 gas mixtures use may be more prominent in future as an alternative to using pure SF6 gas.

4.

Vacuum Circuit Breakers In such circuit breakers, the vacuum is used as the arc quenching medium. The

vacuum circuit breaker takes the advantage of non-sustainability of electric arc in vacuum, and employs the principle of contact separation under vacuum where there is no ionization due to medium. The initial arc caused by field and thermionic emissions during the contacts separation, will die away soon, as there is no further ionization because of vacuum. The degree of vacuum in these circuit breakers is in the range from 10-7 to 10-5 torr. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. When the contacts in the vacuum circuit breakers are opened in vacuum, an arc is produced between the contacts by the ionization of metal vapors of contacts. However, the arc is quickly extinguished because the metallic vapors, electrons and ions produced during arc rapidly condense on the surface of the circuit breaker contacts.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

Figure 5: A Vacuum Circuit Breaker Vacuum circuit breakers are cost-effective in the medium voltage range. Vacuum interrupters are sealed units and maintenance-free for 10,000 normal load operations.

Construction The vacuum circuit breaker consists of fixed contact, moving contact and arc shield mounted inside the vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapors falling on the inside surface of the outer insulating cover.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

Figure 6: Cross Sectional View of Vacuum Circuit Breaker

Applications For a country like Pakistan, where distances are quite long and accessibility to remote areas are difficult, the installation of such outdoor, maintenance-free circuit breakers should prove a definite advantage. Vacuum circuit breakers are being employed for outdoor applications ranging from 22 kV to 66 kV. This technology has been found to be most suitable for medium voltage application though the experimental interrupters for the 72.5 kV and 145 kV have been developed, they were not found to be commercially viable. Advantages: 1. The vacuum circuit breakers are compact in size and have longer lives. 2. Operating energy requirements are low, because the mechanism must move only relatively small masses at moderate speed, over very short distances. 3. Because of the very low voltage across the metal vapor arc, energy is very low. (Arc voltage is between 50 and 100V.) 4. Due to the very low arc energy, the rapid movement of the arc root over the contact and to the fact that most of the metal vapor re-condenses on the contact, contact erosion is extremely small. 5. There is no generation of gases during and after the circuit breaker operation. 6. The outstanding feature of these breakers is that it can break any heavy fault current perfectly just before the contacts reach a definite open position. Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) 7. They can successfully withstand lightning surges. 8. It is now possible to produce cost-effective VI (vacuum interrupter) designs with electrical lives that exceed the required mechanical life of the circuit breakers, and that will even be able to satisfy a recent requirement of extended shortcircuit operating life.

5.

Air Circuit Breakers Air circuit breaker is defined as a circuit breaker, in which the contacts open and

close in air at atmospheric pressure. In general, the use of this type of circuit breakers is restricted to low voltage applications or high security installations where the risk of an oil fire or oil contamination of the environment is too high to be tolerated. Countries following the American practice used air circuit breakers almost exclusively for systems up to 15 kV until the advent of the new vacuum and SF6 technologies. The principles of arc interruption used in an air circuit breaker are rather different from those in any other type of circuit breaker. However, the objective is the same for both categories of the circuit breakers, i.e. to prevent the resumption of arcing after current zero by creating a situation wherein the contact gap will withstand the system recovery voltage; the air circuit breaker does this by creating an arc voltage in excess of the supply voltage. This can be achieved in three ways: Intense cooling of the arc plasma, so that the voltage gradient is very high Lengthening the arc path to increase the arc voltage Splitting up the arc into a number of series arcs Although these circuit breakers are considered obsolete for medium voltage applications, they continue to be preferred choice for high current rating in low voltage applications.

Circuit Breakers Ratings A circuit breaker must operate under all conditions, but its operation becomes critical when there is a fault on the system in which the breaker is used. During fault conditions, a circuit breaker must open the faulty circuit and break the fault current. The ratings of the circuit breakers are also for breaking and making capabilities. There are three ratings for breakers as:

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) Breaking Capacity Making Capacity Short-time Rating

Breaking Capacity Breaking capacity is defined as the r.m.s. current that a circuit breaker is capable of breaking at given recovery voltage and under specified conditions (i.e. power factor, rate of rise of restriking voltage). The breaking capacity is always stated at the r.m.s. value of fault current at the instant of contact separation. When the fault occurs, there is a considerable asymmetry in the fault current due to the presence of a d.c. component. In the Britain, it is a usual practice to take breaking current equal to the symmetrical breaking current. However, in America, the practice is to take breaking current equal to asymmetrical breaking current. Therefore, the American rating given to a circuit breaker is higher than the British rating. It is a common practice to express the breaking capacity in MVA by taking into account the rated the rated breaking current and rated service voltage. Thus if I is the rated breaking current in Amperes and the rated service voltage is V in volts, the breaking capacity for three-phase circuit is: Breaking Capacity = sqrt 3 x V x I x 10-6 MVA However, the agreed international standard of specifying breaking capacity is defined as the rated symmetrical breaking current at a rated voltage. The MVA breaking capacity is illogical in a sense that when the short circuit current is flowing there is only a small voltage across the breaker contacts, while the service voltage appears across the contacts only after the current has been interrupted. Thus MVA rating is the product of two quantities which do not exist simultaneously in the circuit.

Making Capacity It is the peak value of current (including d.c. component) during the first cycle of current wave after the closure of circuit breaker. There is always a possibility of closing or making the circuit breaker under the short circuit conditions. The capacity of a circuit breaker to make current depends upon its ability to withstand and close successfully against the effects of electromagnetic forces. These forces are proportional to the square of maximum instantaneous current

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) on closing. Therefore, making capacity is stated in terms of a peak value of current instead of r.m.s. value. Mathematically stated, making capacity is given as: Making Capacity = 2.55 x symmetrical breaking capacity

Short-time Rating The period for which the circuit breaker is able to carry fault current while remaining closed is known as short-time rating. This rating is needed because sometimes a fault on the system is of a temporary nature and persists for only a second or two after which the fault is automatically cleared. For the sake of continuity of the supply, the breaker should not trip in such situations. This means that the circuit breakers should be able to carry high current safely for some specified period while remaining closed. This means that they should have a specified short-time rating. However, if the fault persists for a duration longer than the specified time limit, the circuit breaker will trip, disconnecting the faulty section. The short-time rating of a circuit breaker depends upon its ability to withstand: The electromagnetic force effects The temperature rise

TYPE TESTS

Sr.# 1

Type test Dielectric tests

Purpose To check characteristics of Circuit breaker for the following tests: I)

Dry, wet, power frequency

II) Lightning impulse voltage III) Switching impulse voltage ( applicable

for 420 kV

CB and above) 2

Radio

To determine RIV on CB pole in both close & open position

Interference voltage test

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) 3

Resistance of

To record circuit resistance during temp. rise test at 200 C

the main circuit 4 5

Temp. rise

To ensure capability of contacts to carry rated normal

test

current within specified temp. rise limits

Short-time

To check the ability of the circuit to carry the maximum rated

withstand

short-circuit withstand current ( 2.5 times the RMS value) at

current & peak

50 Hz in close position during the specified short-term

withstand

duration of 1 or 3 sec.

current 6

Mechanical

To check the characteristics of the breaker for 2000

operation test

operations on each pole ( with multiple circuit breaker with

at ambient

individual drive) OR complete assembled breaker

temp.

(mechanically gang-operated breaker with one common drive)

7

Short-circuit

To check ability of the CB to clear the current on different

current

tests in symmetrical & asymmetrical conditions as per

making/

provisions of IEC

breaking tests 8

Capacitive

To check withstand capability of the CB for no load transient

current

lines, cables , and capacitor banks

charging tests: -Line charging current breaking tests -Cable charging current breaking tests

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

ROUTINE TESTS S.

Routine test

Purpose

Dielectric test on

To check the dielectric’s withstand capability of live terminals

main circuit

to live terminals and to earth in both close & open condition

No. 1

of CB 2

Dielectric test on

To check the dielectric’s withstand capability of auxiliary &

auxiliary &

control circuit of CB subject to short duration (60 sec) voltage

control circuit

withstand test for 2 kV . ( For motor or other devices , they are subjected to a dielectric test as per appropriate specs.)

3

Design

Verifies the CB for compliances in terms of language of name

identification test

plate, identification of aux. Equipment, colour & quality of paint etc.

4

Mechanical

Includes

operating test 1)

5 open-close operations at max. rated and minimum control voltage

2)

5 close-open operation at rated control voltage

3)

recording opening & closing times at the rated operating pressure & voltage

5

Measurement of

To record contact resistance of CB for mechanical operations

resistance of

to be within specified limits

main circuit THE ROUTINE TESTS ARE TO BE COSIDERED AS ACCEPTANCE TESTS

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

Important Terms and Definitions Here are described some vital terms, used in the circuit breakers’ analysis, with simple definitions and a short description.

Arc Voltage Arc voltage is defined as the voltage that appears across the contacts of the circuit breaker during the arcing period (the period in which the arc persists). As soon as the contacts of the circuit breaker are separated, an arc is formed between them. The voltage that appears across the contacts during this period, until the arc is extinct, is called the arc voltage. The value of this voltage is highest at the zero current point. This peak value of the arc voltage helps maintain the current flow in the form of arc.

Restriking Voltage Restriking voltage is the transient voltage that appears across the contacts at or near current zero during the arcing period. The current interruption in the circuit depends upon the high frequency transient voltage, the restriking voltage. If the restriking voltage rises more rapidly than the dielectric strength of the medium between the contacts, the arc will persist for the next half-cycle. On the other hand, if the dielectric strength of the medium builds up more rapidly than the restriking voltage, the arc fails to restrike and the current will be interrupted.

Recovery Voltage Recovery voltage is defined as the normal (50 Hz) voltage that appears across the contacts of the circuit breaker immediately after the final arc extinction. It is approximately equal to the system voltage. When the contacts of the circuit breaker are opened, current drops to zero after every half cycle. At some current zero, the contacts are separated adequately apart and dielectric strength of the medium between the contacts attains high value due to the elimination of ionized particles. At such an instant, the medium between the contacts is strong enough to prevent the breakdown by restriking voltage. Consequently, the final

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) arc extinction takes place and circuit current is interrupted. Immediately after the final current interruption, the voltage that appears across the contacts has a transient part. However, these transient oscillations cave in rapidly due to the damping effect of the system resistance and normal circuit voltage appears across the contacts. The voltage across the contacts is of normal frequency and is identified as recovery voltage.

Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications)

Summary The basic requirements of switching in power system practice are two-fold; to permit apparatus and circuits to be conveniently put into or taken out of service, and to permit appropriate and safe isolation of apparatus and circuits automatically, in a pre-determined time period, when they develop faults. Circuit breakers are preferred over fuses because they are needed to be replaced after every fault in the power system, as they blow out in such case. The isolators are used to isolate a faulty circuit breaker from the rest of the system to aid in repairing it. When a circuit break operates under fault conditions, an arc is struck between the contacts. The arc delays the current interruption and also generates enormous heat which may cause damage to the system or to the circuit breaker itself. Therefore, the arc must be extinguished within the shortest possible time. In the oil circuit breaker, the oil absorbs the arc energy to produces hydrogen gas during arcing; the hydrogen helps extinguish the arc, due to its excellent cooling properties. The oil also provides insulation for the live exposed contacts from the earthed portions of the container. In the air-blast circuit breakers, the risk of fire is eliminated, the arcing products are completely removed by the blast, and the size of these breakers is reduced. Although the air circuit breakers are considered obsolete for medium voltage applications, they continue to be preferred choice for high current rating in low voltage applications. Circuit breakers are classified (on the basis of medium used) as; oil circuit breakers, air-blast circuit breakers, sulphur hexafluoride (SF6) circuit breakers, vacuum circuit breakers. It is important that for the maintenance of SF6 gas equipment like circuit breakers, the technical personnel involved should have a complete understanding of the system. They should not only know how to personnel involved should be a complete understanding of the system. They should not only know how to perform maintenance tasks, but also why and when these should be performed. Condition-based maintenance (CBM) and Reliability center maintenance (RCM) Engr. Syed Muhammad Munavvar Hussain

10/18/2009

Circuit Breakers (Theory, Classification, & Applications) are being proposed to replace periodic maintenance of EHV circuit breakers, in order to minimize the maintenance costs. Vacuum circuit breakers are being employed for outdoor applications ranging from 22 kV to 66 kV. The vacuum circuit breakers are compact in size and have longer lives. Operating energy requirements are also low, as the mechanism must move only relatively small masses at moderate speed, over very short distances. The circuit breaker ratings as making capacity, breaking capacity, and the shorttime rating are to be considered carefully while selecting the type of breaker in a particular application. The type and routine tests of the circuit breakers are stated in tabular form, making it easier for the reader to quickly go through them. The important terms and their definitions are necessary in context with the circuit breaker theory. I stated the terms; arc voltage, restriking voltage, and recovery voltage.

For comments and any sort of questions regarding this article, you can contact me from 10:00 hours to 23:00 hours (Pakistan Standard Time; PST) on:

+92-333-805-3114 Or by e-mail: [email protected] [email protected]

Engr. Syed Muhammad Munavvar Hussain

10/18/2009