Parallel Operation of Generators

Parallel operation of generators D.C. generators For compound wound D.C. generators it is usually sufficient to ensure that the voltages of the incoming generator is the same as the bus bar voltage. The equalising connection  joining the junctions between the armatures and their series fields is incorporated in the circuit breaker in such a way that the equalising connection is automatically closed before and opens after, the main contacts. y adjustment of the shunt field regulator the load sharing may be controlled

A.C. alternators To parallel alternators the following conditions are required! ". #ame #ame voltage voltage$che $checke cked d with with the volt voltmet meter er %. #ame frequen frequency$chec cy$checked ked with with the frequenc frequency y meter meter and synchroscop synchroscopee &. #ame #ame phase angl angle$c e$check hecked ed with with synchr synchrosc oscope ope '. #ame phase phase rotation rotation$checke $checked d with rotati rotation on meter. meter. (nly import important ant when connecting shore supply, or after maintenance on switchgear or alternator.

Load Sharing Of Alternators In Parallel )lternators in parallel must always run at the same speed. )fter a machine has been  paralleled and is required to take up its share of the load, this will not be achieved by adjusting the field e*citation current. )lthough the increase in e.m.f. will cause a current to flow in the busbars, and this will show on the machines ammeters, this is a reactive current that lags the e.m.f. by +o + o and produces a reactive -k)r/ but not k0. 1ts only effect is to alter the operating power factor of the alternator. 2ore power may be obtained ob tained at the bus bars from the incoming alternator only by supplying more power to its prime mover. This increase of steam or fuel supply is achieved by altering the governor setting either electrically or manually. )fter adjusting the governor the incoming machine takes up its desired amount of the k0 loading and this is recorded on the machines watt meter. 3owever, if the k0 loading is shared equally between two machines it may be found that the 4oad Current of the incoming machine is more or less than the other machine. This is fue to the incoming machine having a different power factor. This may be corrected by adjusting the e*citation of the incoming alternator. Thus after paralleling an alternator! ". )djust )djust prime prime mover mover governor governor until k0 loading loading is is correct correct %. )djust )djust field field e*citation e*citation current current until current current sharing sharing is correct. correct.

1f the alternators have similar load characteristics, once adjusted, the load will continue to  be shared. 1f the load characteristics of alternators vary, the k0 loading and load current sharing may require readjusting under different load conditions. Load sharing of alternators No1 on load

No1 on load, No2 synchronised and taking 1k!

No1 and No2 sharing load after ad"#sting go$ernor settings, e%citation ad"#sted to pre$ent e%cessi$e $olt drop in No2

No1 and No2 sharing load &ith 'alanced po&er factors 'y ad"#sting e%citation

(he effects of altering (or)#e and *%citation on single phase alternator plant+and 'y e%trapolation a +phase circ#it

efore paralleling, by varying 5b, adjust the e*citation current in the rotor field of 66 until a7b. 0hen in phase and at the same frequency synchronising may take place. 1f there was no e*ternal load on the bus bars the torque on the prime movers of ) and  is only that required by its own alternator and 5a and 5b are adjusted so that 8a and 8b are equal.

5elative to the bus bars 8a and 8b are acting in the same direction with each other making the top bar positive with respect to the bottom bar. -arying the dri$ing tor)#e

1f the driving torque of 66 is reduced -less fuel supplied/ the rotor falls back by an angle say p.f.-b/ giving a resultant e.m.f. of 89 in the closed circuit. The e.m.f. 89 circulates a current 1 which lags behind 89 by angle p.f.-a/. This circulating current 1is more or less in phase with 8a and in opposition to 8b. This means that ) is generating power to motor  and this will compensate for any loss of power in the prime mover of . (nce the power increase in ) equals the power loss of  balance is restored and ) and  continue to run in synchronism. Therefore the power is shared by adjusting the torque ( fuel input.)

Slight loss of po&er in  $is taken up by an increase in power from ). The terminal voltage will not vary and the speed and frequency will stay the same or drop only very slightly. Large loss of po&er in  $with a large circulating current from ) to  the alternator ) will try to drive  as a synchronous motor. The amount of full load power required to drive an alternator as a motor is only % to &: for a turbine and " to "%: for diesel engine. )s the circulating current flows from ) to  the reverse power trip on  will operate after about & to ; seconds. )ll the load now falls on ) which will probably cause the overload trip to operate and 6black out6 . -arying e%citation

Consider ) and  are e*erting the torque required by its alternator and the generated e.m.f. 8a and 8b are equal. There is no circulating current. y reducing 5b the e*citation current in the field of  can be increased and 8b will increase. 89 is the resultant difference -8b $ 8a/ which will give a circulating current 1 through the synchronous impedances of the two alternators. )s the machines are similar the impedance drop in each will be "<%89 so the terminal voltage " 7 8b $ = 89 7 8a > = 89 Therefore increasing the excitation current will increase the terminal voltage

)s p.f.-a/ is almost +o the ?ower circulating from  to ) is very small 89 1 Cos @ p.f.-a/A appro* equals Bero -Cos +o 7 Bero/ *ffect of red#cing *%citation

y increasing 5b the reduction of the field e*citation current of  will reduce the terminal voltage 8a8b terminal oltage  7 8a $ = 89 7 8b > = 89 The circulating current 1 from ) to  will have a large 60attless6 component. 2achine ) now has more of the lagging reactive current and its power factor is reduced. Too large a reduction in e*citation current in  with subsequent increase in load current in ) c ould cause the current overload trip of ) to operate. This could be followed by the low voltage or the overload trip of  operating causing a black out. -oltage reg#lation

The graph demonstrates that e*citation must be increased -generally/ with increasing load to maintain terminal voltage

The worse the power factor the worse the terminal voltage change during load change. oltage regulation 7 D when load removed< Full load terminal voltage )t ". p.f. 7 )C< () )t . p.f 7 )D< () Therefore lower p.f. 7 greater voltage regualtion