The other important factor is to ensure that the voltage produced across the burden resistor/choke adds with the A.V.R.
sensing voltage in the correct direction. If for any reason this is reversed, a rising characteristic will result which will give
an unstable condition for parallel operation and will produce high circulating currents. This can be checked quite simply
by applying a unity p.f. load on each machine separately and measuring the voltage across auxiliary terminals 6 to 2 and
6 to 8 on the two phase sensed A.V.R. and across auxiliary terminals 6 to 7 and 6 to 4 on the three phase sensed A.V.R.
If the circuit is functioning correctly, the voltage across 6 to 2 should be less than the voltage across 6 to 8 and the voltage
across 6 to 7 should be less than the voltage across 6 to 4.
A.V.R.
Stator Connection
Paralleling
Quadrature Droop
Astatic
Type
(Standard phase rotation)
Fig.
C.T. Position
Fig.
C.T. Position
Two
4 Wire & 6 Wire Star
16
W2
Series Star
16
W2
Phase
12 Wire
Series Delta
16
W2
Not available
Parallel Star
16
W2
Sensed
Series Zig-Zag Star
17
W2
Three
4 Wire & 6 Wire Star
17
V2
Series Star
17
V2
Phase
12 Wire
Series Delta
16*
W2
Not available
Parallel Star
17
V2
Sensed
Series Zig-Zag Star
16*
V2
Fig. 19 Droop C.T. Positions
NOTES:
1.
*For these stator connections the C.T. secondary leads S1 and S2 must be reversed for correct machine operation.
2.
Standard phase rotation is U.V.W. when a.c. generator is rotating clockwise viewed at the drive end. For phase
rotation U.V.W. when rotating anti-clockwise refer to factory for details.
General Notes on Paralleling
Main Metering Requirements
For successful synchronization and load sharing between two or more a.c. generators, the following meters and
equipment are essential:
Voltmeter
This can be one voltmeter per machine, or one which is switchable to either. The latter is more accurate for initial voltage
setting to ensure identical voltages. Another voltmeter may be fitted to the main bus-bars for monitoring the machine on
load.
Ammeter
At least one ammeter per machine is required, preferably with phase to phase switching.
Wattmeter
At least one wattmeter per machine is essential to observe the load sharing of the prime movers, i.e. engines.
Frequency Meter
As with the voltmeter, this can be one per machine or preferably one which is switchable to either, and another registering
the bus-bar frequency.
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