Selected Electrical Question And Answer for MEO Class 2 (4)

Consider two AC generators operating in parallel:

            (a)        State how corrections are made to kW load sharing.                         (3m)

(b)               State how the power factor of an individual generator can be

altered.                                                                                                            (3m)

(c)                Explain how parallel operation may be continued if the automatic voltage regulator of one generator should fail.                                          (4m)

Answer :-

(a)       

                        gen 1                            

                                                                           gen 2                         

 

                                                                                                           droop

                           kW1                                         kW2

              Adjusted by governor

             

                                       Total load kW

Sharing and correction of kW power load will depend entirely on the speed regulation of the prime mover. In other words,  generator governor directly controls the load (kilowatts).

Load sharing is achieved by raising the governor acting of the incoming generator while lowering the setting of running machine.

            The balance of power sharing is dictated by the governor droop of each machine.

(b)               The power factor of an individual generator can be altered:

kW1

                                                                                                 kVA1

                                                            gen 1       f1

 

                             kVAR2                                                        kVAR1

 

               excitation                         

               increased                                    f2

                gen 2

                    kVA2                                          kW2

                        unbalanced loading

It can be achieved, within limit the effects of alteration of excitation of any of them, however increasing the excitation of one generator worsen its power factor whilst improving that of the other generators.

The situation of operating generator of different power factor is undesirable because it impose a greater share of load in the generator operating t the lower power factor.

(c)                If the automatic voltage regulator of one generator fails, parallel operation can still be continued by :

1)      a temporary operation is to be operate one machine under AVR control and keep the second machine under hand control.

2)      The busbar voltage will be maintained by the AVR controlled machine and the other, under constant excitation will take approx. constant kVAR load leaving the controlled machine to take care of any change in total kVAR.

3)      From time to time, load sharing can be adjusted by hand control.

4)      This will be fairly satisfactory provided the total load does not vary too frequently or too rapidly.

5)      If one machine is larger than the other, the larger should be chosen for AVR contol.