Selecting Transformers For Hydro Power Plants

Selecting Transformers For H ydro Power Plants

1. INTRODUCTION Electrical energy is a major requirement for the economic and industrial growth of any country. Most of the electrical energy is generated from the combustion of fossil fuels (coal, oil, gas). The fossil fuels are not only limited in nature, but generating electricity with these

fuels

have

serious

environmental,

economical

and

safety

consequences. Energy from the sun, water, wind biomass and the ocean is renewable, biomass clean and environment friends. India has sufficient potential to develop hydro-electric power, but the development rate of large, hydro power plants is still low because of high capital cost, long gestation period, increasing difficulties in implementation detoriating socioeconomic aspects and adverse environmental impacts. On the other hand, small, hydro power plants not only overcome these draw-backs but also provide advantages like continuous power availability on demand, concentrated and predictable source, limited maintenance low running costs, long lasting and robust in construction. The small hydro power plant may be subdivided as micro (upto 100 KW), mini (101 KW to I MW) and small (1001 KW to 5 MW).

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Selecting Transformers For H ydro Power Plants Transformer plays a very important role to transfer electric power from the generating station to the consuming centre. The generating site of consume centre. Generating site of hydro power plants are situated very far away from the consuming centre, necessitating very long transmission systems for which much higher transmission voltage are required to reduce the transmission cost as well as transmission loss. In India, the generating voltage is generally up to 11 KV. The unit system for voltage step up is adopted universally m which each generator is directly connected to the unit step up transformer (called main transformer without any circuit breaker in between. A generator and its unit step up transformer form the unit. Each unit has its own turbine, control panel and auxiliaries. The unit auxiliaries are supplied through unit auxiliary transformer. The station auxiliaries are supplied through another transformer called station service transformer. A general connection diagram of a unit step up transformer is shown in figure. 1. The unit system for voltage set up is available for large hydro power plant. However, other ways for stepping up the voltage are by generator step up transformer, interbus transformer and outgoing feeder transformer. These methods are very much suitable for the small hydro power plants where several different rating generators are operating Govt. Polytechnic, Washim.

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Selecting Transformers For H ydro Power Plants simultaneously.

In generator step up transformer, each generator is

connected to the HV bus through a step up transformer. Here the number of transformers used are equal to the number of generators. In interbus transformer, the transformer is connected between two busbars-one at generating voltage and other at high level transmission voltage. In this case, one or more transformers may be connected in parallel between the LV and HV buses. Hence the number of transformers required are less than the total number of generators. In outgoing feeder transformer, the transformer is connected between the generating voltage bus bar and directly to the transmission lines on the high voltage side. In this method, one HV bus and its switchgear has been saved, and the number of transformers required is less than the total number of generators connected. This method is economical and simpler over the above two types. The single line connection diagrams of these types are shown in figure.2(a), 2(b) and 2(c) respectively. The sum of MVA rating of transformers must be equal to the sum of MVA rating of the generator /generators connected. The auxiliary transformers required for hydro power stations are about 15 per cent of the generator rating. They are therefore, universally of ON type with cooling tubes welded to the main tank. Govt. Polytechnic, Washim.

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Selecting Transformers For H ydro Power Plants However, such transformers should be located outdoors in open cells flanked by fire-barrier walls. It is sometimes convenient to minimize cable connections to generator unit transformers by locating them indoors.

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Selecting Transformers For H ydro Power Plants

2. SELECTION OF POWER TRANSFORMER FOR HYDRO ELECTRIC PLANTS When selecting the transformer for a hydroelectric plant, number of factors, namely operating condition transporting facility available, methods of cooling, insulating level, per cent of impedance, voltage regulating at different power factors, losses at different power factor loads, weights and dimensions, cost etc. are taken into consideration.

a) OPERATING CONDITION The operating condition of the transformer used in hydro power plant is different from that of the thermal and nuclear power plants. In the hydro power plant the atmosphere is humid, and the site is in a very far remote location. Particularly for small hydro power plants, the range of generators are large i.e. from 20 KVA to 15 KVA. Types of generators generally used are synchronomous generator or induction generator. Induction generators are preferred upto 250 KVA (sometimes upto 500 KVA) because of low cost, ruggedness in construction, requires less maintenance and needs no synchronization. This absorbs reactive power from the grid.

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Selecting Transformers For H ydro Power Plants A capacitor bank is connected to improve power factor. If the grid is weak, an induction generator is coupled with a synchronous generator. Synchronous generator is generally preferred for higher rating (above 250 KVA), and this is very suitable for the isolated system. Small capacity induction generator (upto 20 KW) with variable capacitor bank may be used for isolated system if there is no significant inductive load connected in the system.

B) SIZE. The size of transformer depends on the size of the generating units. The KV capacity of the three phase transformer must be the same as that of the generator/generators to which it is connected. If a bank of three single phase transformers. If a bank of three single phase transformers is used (one in each phase), the KVA Capacity of each single phase transformer must be one third (1/3) of the KVA of the generators/generator connected.

C) WEIGHT AND DIMENSION: The weight and dimension of the transformer is very considerable of the hydro power plant where transport facilities are not adequate. A considerable proportion of the weight is due to the oil, and,

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Selecting Transformers For H ydro Power Plants consequently, the larger units are transported dry and filled on the site. These heavy, dry transformers are filled either with dried air or with nitrogen. Sometimes, for the limitations in transport, heavy transformers are transported with few auxiliaries and are fully assembled with various auxiliaries at the site. Due to the large dimension and weight, sometimes, three single transformers are adopted instead of a three-phase large power transformer.

D)

THREE-PHASE

TRANSFORMER

OR

THREE,

SINGLE PHASE TRANSFORMERS: A three phase unit is cheaper, weighs less, occupies less space and is more compact than three, single phase transformer for the same rating, for a large hydroelectric plant, the single, three- phase transformer units have usually been adopted in the past. Now, the use of separate single phase unit in each phase is increasing popularity because, it occupies less space, weighs less making the system more reliable, increases the availability, easy to maintain and provides better transportation/handing facilities. This makes the single phase transformer very much suitable for where space in the tunnel would be limited and the handing of heavy weights would be difficult.

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Selecting Transformers For H ydro Power Plants E) TRANSPORT FACILITIES AVAILABLE: Transformer is selected as per the transport facilities, available between the manufacturer's premises and final site. Dimensions of the transformer should be matchable with the available sizes of wagons, traitors, weight lifting arrangements.

F) METHODS OF COOLING : The methods of cooling transformers differ and affect its rating and size. In self, oil-cooled transformers, the cooling surface is provided by radiators through which oil is circulated by natural convection. The rate of heat dissipation can be increased, and hence the capacity of transformer would increase abut 30 per cent to 40 per cent by direct blowing of air over the radiator surface by means of fans (known as forced air cooling). Similarly, the heat dissipation can be increased in forced oil cooling by circulating oil through the radiators with pumps in addition to blowing air over the radiator surface, which increases the capacity of transformer about 60 per cent to 66 per cent. A tabular form for different types of cooling and its rating is given in

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Selecting Transformers For H ydro Power Plants Table 1. Types of cooling Air Natural ( A.N.)

Description The ambient air is used for cooling. This method is useful for oil less transformer up to 1.5 MVA, but

Oil

Generally used for small transformers below 25 KVA. Air This is widely used for transformer up to 30

Natural

MVA

Oil

Natural (ONAN) Transformers are fitted with panel Type radiators for oil natural circulation. Air This is widely used for transformers between 30 MVA

Natural

forced ( ONAF)

and 60 MVA. The panel type radiators are provided With cooling fans. Fans are switched on during heavy

Oil

Loads. Air This method is used for transformer above 60 MVA.

Forced

Forced (OFAF)

Oil

Forced

The oil is circulated through coolers. The coolers have

Cooling fans to exchange heat term oil to air. water The heat is exchanged from oil to cooler water. Both

Forced ( OFWF)

oil and cooling water are circulated through heal Exchanger.

In hydro power plants, water is used for cooling the oil in the radiators in a separate heat exchanger as large amount of water is available. Self, oil cooled transformers have the largest weight and size compared to forced air-cooled and forced oil-cooled units. The forced, air-

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Selecting Transformers For H ydro Power Plants cooled, transformer reduce weight, area and height by about 10%, 10% and 5% respectively the some parameters by about 30%, 30% and 10% respectively than the natural air-cooled transformers.

G) IMPEDANCE OF TRANSFORMER : The impedance of the transformer increases with the insulation level, and with forced method of cooling and lies between 4.5 per cent to 20 per cent. The impedance of transformers play an important role in their selection particularly when they are operated in parallel where it must have same percentage impedance voltage for efficient working. If transformers are required with lower impedance than other normal value , their cost is more.

Forced cooled transformers are ideal for systems

requiring high impedance. The

percentage

impedance

of

self

oil

cooled

transformers varies between 6 per cent and 14 per cent while that of forced, air cooled is 9 per cent. Higher impedance signifies higher impedance voltage, low short circuit current higher voltage regulation and lower efficiency.

H) VOLTAGE REGULATION The voltage regulation of a transformer increases with an increase in lagging power factor load. The average regulation is 4 per

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Selecting Transformers For H ydro Power Plants cent to 9 per cent for self. Oil cooled transformers. 6 per cent to 11 per cent for forced air cooled transformer and 7.5 per cent to 11 per cent for forced oil cooled transformer at 0.8 power factor lagging. A variation for voltage regulation with changes in load power factor is shown in figure 3.

I) EFFICIENCY: Efficiency of the power transformer of large capacity is as high as 99 per cent to 99.5 per cent. From the efficiency point of view, it is preferable to use a three phase transformer rather than a bank of three single phase transformers. The combined copper losses in the above case will be the same but the core loss will be increasing about 20 per cent. The efficiency of a transformer varies with load and load factor Transformers for power transmission work are commonly arranged with their maximum efficiency point about 50 per cent - 55 per cent load. The maximum efficiency point is that at which the fixed losses are equal to copper losses. Thus, with a maximum efficiency at 50 per cent load, the total losses at this load will be twice of no load losses, while at 100 per cent load, they will be five times the no load losses. For generator step up transformers, the maximum efficiency preferred is flat up to 75 per cent load as shown curve (1) in figure 4.

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Selecting Transformers For H ydro Power Plants J)

INDOOR,

OUTDOOR

AND

SEMI-OUTDOOR

INSTALLATION. Generally,

large

power

transformers

are

located

outdoors for economy of site construction and to achieve optimum cooling conditions. Some shielding of sound is desirable in built-up areas. Outdoors location relative to the turbine house will naturally depend on the route of the overhead lines (or cables) connecting the transformers to the local or remote high voltage switchgear. A semi-outdoor arrangement is often adopted to minimize heavy law voltage connections LV side where the transformers are carried on platforms over the tail race. For

underground

hydro

power

plants,

the

main

transformers can be accommodated either in the special chambers or outdoors near the mouth of the access tunnel. The choice depends largely on the distance from the machines to the possible surface site. If this is about 200 m or more the indoor location is preferable both as regards capital cost and losses . In heavily polluted atmosphere, indoor installation is generally preferred.

K) BUSHING OR CABLE CONNECTIONS :

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Selecting Transformers For H ydro Power Plants The choice of either open bushing or cable box transformer connections for any voltage depends on the particular arrangements of the station. When bare copper connections are to be made, the transformer must have open bushing. When cable connections have to be made, it is not essential to incorporate the sealing boxes with the transformer as separately supported cable sealing ends can be arranged close to the transformer bushing and linked there to by short bare-copper connections in air. This arrangement has particular merit for heavy current cables at generated voltage, and greatly reduce the work involved in disconnecting a faulty transformer to obviate the additional excavation required to give earth clearance from

L) INITIAL COST AND COST DUE TO ENERGY LOSS : The initial cost of transformer varies with the type of construction, class of insulation used and the capacity 9KVA. Larger transformers have less cost per KVA. for selection of a transformers, it is necessary to calculate financial loss due to ohmic losses in addition to the initial cost.

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Selecting Transformers For H ydro Power Plants The financial losses may be evaluated either (a) an approximate cost of Rs.1000/KWH for the life of the transformer. Generally method (a) is preferred for the evaluation of financial loss.

3. CONCLUSION As we know that the operating condition of the transformer used in hydro power plant is different from that of the thermal and nuclear power plants, in hydro power plant the atmosphere is humid,

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Selecting Transformers For H ydro Power Plants and they are situated very for away from the consuming centre, necessitating very long transmission systems for which much higher transmission voltages are required to reduce the transmission cost as wheel as transmission loss. So by selecting proper transformers for Hydro power plants we can, reduce-the transmission losses and the transmission cost by an considerable amount.

REFERENCE 1) BHEL 2) B.L. Theraja

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Selecting Transformers For H ydro Power Plants 3) Electrical India

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