NOTES:
1.
Depending on machine type, one or more main stator cables are connected to each output terminal, insert
droop C.T. in one cable only. See Fig. 17 for C.T. position and wiring diagram.
2.
Connect to auxiliary terminals 2 - 8 (after removing link) on two phase sensed machines and 4 - 7 (after
removing link) on three phase sensed machines.
Operation
As stated earlier paralleling systems fitted to the generator can only ensure that the excitation system maintains the output
voltage to the correct level to give satisfactory kVAr load sharing and minimize circulating currents to acceptable levels.
As far as kW load sharing is concerned, this is entirely due to the engine throttle/governor setting.
For both two phase and three phase sensed machines the operation is the same in that any circulating current between
machines produces a voltage across the burden-resistor or choke which directly adds or subtracts from the sensing
voltage fed into the A.V.R. This makes the excitation system sensitive to circulating currents and ensures correct sharing
of the kVAr load. The larger the droop voltage is set, the more flexible becomes the excitation system to reduce
circulating currents and ensure kVAr load sharing. In most cases a 5% droop on the output voltage at full load, zero p.f.
lag. is satisfactory, this setting impacts to worsen the voltage regulation by approx. 1% at full load, unity p.f. and approx.
3% at full load, 0.8 p.f. When a unity power factor load (kW) is applied the voltage produced across the burden
resistor/choke adds vectorally at right angles to the sensing voltage and has a minimal effect. When the machines are run
individually, the droop circuit can be switched out by short circuiting auxiliary terminals 2 to 8 on the two phase sensed
A.V.R. and 4 to 7 on the three phase sensed A.V.R. to obtain the close regulating characteristics of the machine.
Setting up the droop circuit can be difficult as in most cases only unity p.f. load is available. A simple way of setting up
the droop circuit under these conditions is to measure the voltage across the resistor/choke when load is applied, this
should be in line with the graph Fig. 18 to obtain approx. 5% droop at full load zero p.f. lag.
36
