Shunt and series reactive compensation using capacitors has been a widely recognized and powerful method to combat the problems of voltage drops, power losses and voltage flicker in power distribution networks. The importance of compensation schemes has gone up in recent years due to the increased awareness on energy conservation and quality of supply on the part of the Power Utility as well as power consumers. This article (in two parts) amplifies on the advantages that accrue from using shunt and series capacitor compensation. It also tries to answer the twin questions of how much to compensate and where to locate the compensation capacitors.
Why Use Shunt Capacitor compensation in Distribution systems ?
Fig. 1 represents an a.c. generator supplying a load through a line of series impedance (R+jX) ohms. Fig. 2(a) shows the phasor diagram when the line is delivering a complex power of (P+jQ) VA and Fig. 2 (b) shows the phasor diagram when the line is delivering a complex power of (P+jO) VA i.e. with the load fully compensated. A thorough examination of these phasor diagrams will reveal the following facts.
Current in the line, generator and intervening transformers ,if any, is higher by a factor of
in the case of uncompensated load compared to compensated load. This results in a power loss, which is higher by a factor of () 2 compared to the minimum power loss attainable in the system.
The loading on generator, transformers, line etc is decided by the current flow. The higher current flow in the case of uncompensated load necessitated by the reactive demand results in a tie up of capacity in this equipment by a factor of . i.e. compensating the load to UPF will release a capacity of (load VA rating X ) in all these equipment.
The sending-end voltage to be maintained for a specified receiving-end voltage is higher in the case of uncompensated load. The line has bad regulation with uncompensated load.
The sending-end power factor is less in the case of an uncompensated one. This due to the higher reactive absorption taking place in the line reactance.
The excitation requirements on the generator is severe in the case of uncompensated load. Under this condition, the generator is required to maintain a higher terminal voltage with a greater current flowing in the armature at a lower lagging power factor compared to the situation with the same load fully compensated. It is entirely possible that the required excitation is much beyond the maximum excitation current capacity of the machine and in that case further voltage drop at receiving-end will take place due to the inability of the generator to maintain the required sending-end voltage. It is also clear that the increased excitation requirement results in considerable increase in losses in the excitation system.
It is abundantly clear from the above that compensating a lagging load by using shunt capacitors will result in Lesser power loss everywhere upto the location of capacitor and hence a more efficient system Releasing of tied-up capacity in all the system equipments thereby enabling a postponement of the capital intensive capacity enhancement programmes to a later date.Increased life of eqipments due to optimum loading on them Lesser voltage drops in the system and better regulation Less strain on the excitation system of generators and lesser excitation losses. Increase in the ability of the generators to meet the system peak demand thanks to the released capacity and lesser power losses.
Shunt capacitive compensation delivers maximum benefit when employed right across the load. And employing compensation in HT & LT distribution network is the closest one can get to the load in a power network. However, various considerations like ease of operation and control, economy achievable by lumping shunt compensation at EHV stations etc will tend to shift a portion of shunt compensation to EHV & HV substations. Power utilities in most countries employ about 60% capacitors on feeders, 30% capacitors on the substation buses and the remaining 10% on the transmission system. Application of capacitors on the LT side is not usually resorted to by the utilities.
Just as a lagging system power factor is detrimental to the system on various counts, a leading system pf is also undesirable. It tends to result in over-voltages, higher losses, lesser capacity utilisation, and reduced stability margin in the generators. The reduced stability margin makes a leading power factor operation of the system much more undesirable than the lagging p.f operation. This fact has to be given due to consideration in designing shunt compensation in view of changing reactive load levels in a power network.
Shunt compensation is successful in reducing voltage drop and power loss problems in the network under steady load conditions. But the voltage dips produced by DOL starting of large motors, motors driving sharply fluctuating or periodically varying loads, arc furnaces, welding units etc can not be improved by shunt capacitors since it would require a rapidly varying compensation level. The voltage dips, especially in the case of a low short circuit capacity system can result in annoying lamp-flicker, dropping out of motor contactors due to U/V pick up, stalling of loaded motors etc and fixed or switched shunt capacitors are powerless against these voltage dips. But Thyristor controlled Static Var compensators with a fast response will be able to alleviate the voltage dip problem effectively.
Why Use Series Capacitor Compensation in Distribution Systems ?
Shunt compensation essentially reduces the current flow everywhere upto the point where capacitors are located and all other advantages follow from this fact.But series compensation acts directly on the series reactance of the line. It reduces the transfer reactance between supply point and the load and thereby reduces the voltage drop. Series capacitor can be thought of as a voltage regulator, which adds a voltage proportional to the load current and there by improves the load voltage.
Series compensation is employed in EHV lines to 1) improve the power transfer capability 2) improve voltage regulation 3) improve the load sharing between parallel lines. Economic factors along with the possible occurrence of sub-synchronous resonance in the system will decide the extent of compensation employed.
Series capacitors, with their inherent ability to add a voltage proportional to load current, will be the ideal solution for handling the voltage dip problem brought about by motor starting, arc furnaces, welders etc. And, usually the application of series compensation in distribution system is limited to this due to the complex protection required for the capacitors and the consequent high cost. Also, some problems like self-excitation of motors during starting, ferroresonance, steady hunting of synchronous motors etc discourages wide spread use of series compensation in distribution systems.
Shunt Capacitor Installation Types
The capacitor installation types and types of control for switched capacitor are best understood by considering a long feeder supplying a concentrated load at feeder end. This is usually a valid approximation for some of the city feeders, which emanate from substations, located 4 to 8 Kms away from the heart of the city. Ref Figs 3 & 4.
Absolute minimum power loss in this case will result when the concentrated load is compensated to upf by locating capacitors across the load or nearby on the feeder. But the optimum value of compensation can be arrived at only by considering a cost benefit analysis.
The reactive demand of the load varies over a day and a typical reactive demand curve for a day is given in Fig 5.
It is evident from Fig 5 that it will require a continuously variable capacitor to keep the compensation at economically optimum level throughout the day. However, this can only be approximated by switched capacitor banks. Usually one fixed capacitor and two or three switched units will be employed to match the compensation to the reactive demand of the load over a day. The value of fixed capacitor is decided by minimum reactive demand as shown in Fig 5.
Automatic control of switching is required for capacitors located at the load end or on the feeder. Automatic switching is done usually by a time switch or voltage controlled switch as shown in Fig 5. The time switch is used to switch on the capacitor bank required to meet the day time reactive load and another capacitor bank switched on by a low voltage signal during evening peak along with the other two banks will maintain the required compensation during night peak hours.
Economic Justification for Use of Capacitors
The increase in benefits for 1 kVAR of additional compensation decrease rapidly as the system power factor reaches close to unity. This fact prompts an economic analysis to arrive at the optimum compensation level. Different economic criteria can be used for this purpose. The annual financial benefit obtained by using capacitors can be compared against the annual equivalent of the total cost involved in the capacitor installation. The decision also can be based on the number of years it will take to recover the cost involved in the Capacitor installation. A more sophisticated method would be able to calculate the present value of future benefits and compare it against the present cost of capacitor installation.
When reactive power is provided only by generators, each system component (generators, transformers, transmission and distribution lines, switch gear and protective equipment etc) has to be increased in size accordingly. Capacitors reduce losses and loading in all these equipments , thereby effecting savings through powerloss reduction and increase in generator, line and substation capacity for additional load. Depending on the initial power factor, capacitor installations can release at least 30% additional capacity in generators, lines and transformers. Also they can increase the distribution feeder load capability by about 30% in the case of feeders which were limited by voltage drop considerations earlier. Improvement in system voltage profile will usually result in increased power consumption thereby enhancing the revenue from energy sales.
Thus, the following benefits are to be considered in an economic analysis of compensation requirements.
Benefits due to released generation capacity.Benefits due to released transmission capacity.Benefits due to released distribution substation capacity.Benefits due to reduced energy loss.Benefits due to reduced voltage drop.Benefits due to released feeder capacity.Financial Benefits due to voltage improvement.
Which are the benefits to be considered in capacitor application in distribution system? Capacitors in distribution system will indeed release generation and transmission capacities. But when an individual distribution feeder compensation is in question, the value of released capacities in generation and transmission system are likely to be too small to warrant inclusion in economic analysis. Moreover, due to the tighty inter-connected nature of the system, the exact benefit due to capacity release in these areas is quite difficult to compute. Capacity release in generation and transmission system is probably more relevant in compensation studies at transmission and sub- transmission levels and hence are left out from the economic analysis of capacitor application in distribution systems.
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