Parallel Operation of AC Generators

Parallel Operation of AC Generators. General: The KVA load comprises kW load (active load) & kVAr Load (Reactance Load) & are related by the power factor. The case of two AC Generators running in parallel with each other may be must conveniently studied in two sections viz, kW load sharing and kVAR load sharing. KW Load Sharing. The control of the active load i.e.kW is achieved through the torque applied to the shaft by appropriate adjustment of the governor setting, whereas the kVAr load sharing is controlled by excitation of AC Generator only. The prime movers therefore must be suitably governed to ensure correct sharing of the kW load. This is inherent in most engine governors which allow the speed to droop by approximately 4 % as load increases from no-load to full load. The accuracy of the kW load sharing is largely dependant upon the closeness to which the speed droop characteristics of prime movers are matched and upon the stability with which the governor can hold the speed constant any load. The speed setting should be adjusted to obtain the best possible sharing at full load, as unequal sharing at lower part loads has no harmful effect. The AC Generator cannot compensate for KW load sharing. kVAr Load Sharing. The AC Generator should employ suitable means for ensuring satisfactory sharing of reactive (kVAR) load. One of the methods is to produce drooping characteristics of voltage with respect to increasing reactive load. The voltage setting should be adjusted to obtain the best possible kvar sharing. Regulation v/s Load Sharing. The Actual amount of droop required depends on the conflicting requirement of accurate load sharing and accurate regulation. The greater the droop the more accurate the load sharing but less accurate the regulation. Reading kW & kVAr Load Sharing. Since the sharing of the KW & Kvar are two entirely independent operations a considerable simplification in procedure can be secured by fitting a kVAr meter in preference to the power factor meter on the switch board. If kVAr meters are not fitted it is necessary always to adjust the governors to give correct kW sharing before checking the correctness of kVAr sharing as indicated by equal power factors.

Earthing of Neutrals. When the AC Generator operating in parallel have slightly different line to neutral voltage waveforms, these harmonic voltages which do not balance out will cause circulation of harmonic currents, if their neutrals are interconnected. The triple frequency current flows through the neutral causing additional heating of the AC Generator. This can be avoided by connecting the star point of only one AC Generator to the neutral bus which is earthed. It is usual to select the largest rating running machine for this duty. However care must be taken to ensure that his machine is not overloaded as this AC Generator will take all the unbalance in the load. Points to be checked before Attempting Parallel operation. 1. AC Generators under consideration must be suitable for parallel operation. 2. If reactance load sharing is by the method of quadrature current compensation, then all the AC Generators under consideration must have some means of obtaining drooping characteristics of voltage with respect to reactive load. 3. The prime movers should be suitable for parallel operation and the governors should be able to provide speed droop characteristics. 4. The system should be cleared by concerned AC Generator manufacturer/s for check on electro-mechanical oscillations. 5. The control panel must be designed for parallel operation with due consideration. a. Synchronizing facility. b. Capacity of the bus bar. c. Metering and protection. d. Earthing only one of the neutrals of the AC generators (i.e. Neutral of the larger size of the Ac Generator is normally earthed) Conditions for Synchronizing. 1. The phase Sequence of the incoming AC Generator has to be same as that of the AC generator / system with which it is to be paralleled. 2. The voltage of the incoming AC Generator should be equal to that of the AC generator / System, with which it is to be paralleled. 3. The frequency of the incoming AC Generator should be equal to that of the AC generator / System, with which it is to be paralleled. 4. Synchronizing should be done at an instant when the voltage of the incoming AC Generator is in phase with those of the AC Generator / system with which it is to be paralleled.

Adjustments after Synchronizing. The prime mover governor of the incoming set is adjusted as to increase its speed, until the kW load is being shared correctly. It may be necessary partly to reduce the speed setting on the other sets to maintain correct system frequency as well as correct kW sharing. Increasing the voltage setting of the incoming AC Generator, to obtain correct kVAr sharing as indicated by equal power factors. The bus bar Voltage can be adjusted if necessary, without upsetting the kVAR sharing by equal readjustment of all the voltage setting.

Parallel operation of Generating Sets with GRID Parallel running of generating sets with Grid has several advantages and disadvantages. It involves extensive metering protection and special accessories which are not required for solo operation. Generally if the set capacity is below 500 kW, it is not worth considering grid paralleling. Mains paralleling should be restored to be the reasons are very compelling and after the implications are fully understood. Advantages. The cost of generation reduces if the sets are run at near full load region. Full utilization of the generating set capability can be achieved, thereby reducing the consumption from the main supply. Following points may otherwise form restrictions in optimum use of generating sets. 1.1 1.2 1.3 1.4

Load pattern not steady but fluctuating over a wide range. Limitation of the generating set in meeting overloads. Limitation of the generating set in handling transient loads. Limitation of distribution system, in that the individual sections cannot be matched with the capacity of the DG Sets. 1.5 Critical loads can be fed without interruptions. Disadvantages. 1.0 Cost of protection, metering & special equipment may be prohibitive, making the proposal uneconomical. 2.0 If paralleling with grid is by simple droop setting on prime mover & generator, fluctuations of grid voltage & frequency may defeat the very aim of optimum utilization of generating set capacity & further may cause number of nuisance tripping. 3.0 In the absence of proper precautions by way of protection, the following dangerous situations may arise.

4.0 In case prime mover fails to supply power, the AC generator connected to grid may work as synchronous motor and damage the prime mover. 5.0 Power may be fed back to the grid and cause accidents & losses in grid circuit opened for maintenance / other purposes. 6.0 Failure of grid supply may result in shock loading which may result in crank shaft failure. 7.0 Re closure of breakers at electric supply authority’s end, particularly auto re closing after self clearing faults, and may lead to out of phase synchronizing. This Gives rise to heavy circulating currents, which may damage the generator. Requirements: Depending on the scheme envisages the following provisions are required to be made. 1. Facility for droop setting on prime mover. 2. Facility for droop setting on generator. 3. Motorized governor for speed adjustment. 4. Pot for voltage adjustment. 5. Under frequency & over frequency relay. 6. Under voltage & over voltage Relay. 7. Over current & earth fault relay. 8. Electrically operated circuit breakers. 9. Check synchronizing relay. 10. Automatic load sharing ( both active & reactive and smooth load transferring unit) 11. Power comparator monitory controller. 12. Null Power monitor. 13. Communication network to and from electricity supply authority. 14. Reverse power & reverse kVAr relays. 15. Under & over excitation limiters.

Conclusions: While it is logical to evaluate any scheme from economical and technical angles. It is very important to consider restriction and requirement of electricity supply authority, before finalising the scheme. Some of the electricity authorities do not permit parallel operation of captive units with the grid.