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. By adjustment of the shunt field regulator the load sharing may be controlled

A.C. alternators

Load Sharing Of Alternators In Parallel

Alternators in parallel must always run at the same speed. After 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 excitation current. Although 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 90o and produces a reactive (kVAr) but not kW. Its only effect is to alter the operating power factor of the alternator.

More power may be obtained 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.

After adjusting the governor the incoming machine takes up its desired amount of the kW loading and this is recorded on the machines watt meter. However, if the kW loading is shared equally between two machines it may be found that the Load 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 excitation of the incoming alternator.

If the alternators have similar load characteristics, once adjusted, the load will continue to be shared. If the load characteristics of alternators vary, the kW loading and load current sharing may require readjusting under different load conditions.

Load sharing of alternators

No1 on load

No1 on load

No1 on load, No2 synchronised and taking 100kW

No1 on load, No2 taking 100 kW

No1 and No2 sharing load after adjusting governor settings, excitation adjusted to prevent excessive volt drop in No2

No1 , No2 sharing load

No1 and No2 sharing load with balanced power factors by adjusting excitation

No1 , No2 sharing load with balanced power factors

The effects of altering Torque and Excitation on single phase alternator plant-and by extrapolation a 3-phase circuit

Schematic showing two alternators attached to bus bars

Before paralleling, by varying Rb, adjust the excitation current in the rotor field of 'B' until Va=Vb. When in phase and at the same frequency synchronising may take place.

If there was no external load on the bus bars the torque on the prime movers of A and B is only that required by its own alternator and Ra and Rb are adjusted so that Ea and Eb are equal.

Vector diagram for two alternators in parallel

Relative to the bus bars Ea and Eb are acting in the same direction with each other making the top bar positive with respect to the bottom bar.

Varying the driving torque

Vector diagram for two alternators in parallel, one lagging

If the driving torque of 'B' is reduced (less fuel supplied) the rotor falls back by an angle say p.f.(b) giving a resultant e.m.f. of Ez in the closed circuit.

The e.m.f. Ez circulates a current I which lags behind Ez by angle p.f.(a).
This circulating current Iis more or less in phase with Ea and in opposition to Eb.
This means that A is generating power to motor B and this will compensate for any loss of power in the prime mover of B.
Once the power increase in A equals the power loss of B balance is restored and A and B continue to run in synchronism.

Therefore the power is shared by adjusting the torque ( fuel input.)

Slight loss of power in B-is taken up by an increase in power from A. The terminal voltage will not vary and the speed and frequency will stay the same or drop only very slightly.

Large loss of power in B-with a large circulating current from A to B the alternator A will try to drive B as a synchronous motor. The amount of full load power required to drive an alternator as a motor is only 2 to 3% for a turbine and 10 to 12% for diesel engine.
As the circulating current flows from A to B the reverse power trip on B will operate after about 3 to 5 seconds.
All the load now falls on A which will probably cause the overload trip to operate and 'black out' .

Varying excitation

Vector diagram for two alternators in parallel, one reduced excitation

Consider A and B are exerting the torque required by its alternator and the generated e.m.f. Ea and Eb are equal. There is no circulating current.
By reducing Rb the excitation current in the field of B can be increased and Eb will increase. Ez is the resultant difference (Eb - Ea) which will give a circulating current I through the synchronous impedances of the two alternators. As the machines are similar the impedance drop in each will be 1/2Ez so the terminal voltage

V1 = Eb - Ez = Ea + Ez

Therefore increasing the excitation current will increase the terminal voltage

As p.f.(a) is almost 90o the Power circulating from B to A is very small

Ez I Cos [ p.f.(a)] approx equals Zero (Cos 90o = Zero)

Effect of reducing Excitation

By increasing Rb the reduction of the field excitation current of B will reduce the terminal voltage

Ea>Eb terminal Voltage V = Ea - Ez = Eb + Ez

The circulating current I from A to B will have a large 'Wattless' component. Machine A now has more of the lagging reactive current and its power factor is reduced. Too large a reduction in excitation current in B with subsequent increase in load current in A could cause the current overload trip of A to operate. This could be followed by the low voltage or the overload trip of B operating causing a black out.

Voltage regulation

 Graph showing effects on voltage of changing power factor

The graph demonstrates that excitation must be increased (generally) with increasing load to maintain terminal voltage

 Graph showing change in terminal voltage worsens with reducing power factor

The worse the power factor the worse the terminal voltage change during load change.

Voltage regulation = DV when load removed/ Full load terminal voltage

At 1.0 p.f. = AC/ OA

At 0.8 p.f = AD/ OA

Therefore lower p.f. = greater voltage regualtion