Synchronizing Equipment
This can be in the form of a synchronizing meter or a system of lamps connected across the a.c. generators to be
paralleled. The synchronizing meter gives a direct indication when the machines are in synchronism. The lamps are
connected across like phases, i.e. U-U. V-V, W-W, such that the machines are in phase when the lights are dimmed. It
is important to rate the lamps to at least twice the machine voltage and in most instances it would be necessary to
connect two or three in series.
Protection Devices
In addition to essential protection devices, i.e. overload circuit breaker, fuses, etc. it is recommended that the breakers
have a shunt release coil working in conjunction with a reverse power relay trip. This ensures that, should one of the sets
malfunction for any reason, i.e. engine shutdown, voltage drop, etc. the power transferred from the other set, which is
now trying to 'motor' the failed plant, will then de-energize the contact breaker and disconnect the sets.
Setting Up Procedure for Parallel Operation
The droop equipment should be tested and set correctly and the machines can be set for synchronization as follows:
1.
The speed of each set must be adjusted to the nominal frequency of the system, i.e. 50 Hz or 60 Hz.
2.
The voltage of each machine can now be set identically on the hand voltage trimmers or A.V.R. range control.
3.
The machine should only be paralleled when the difference in frequency between the two sets is small enough to
enable the breaker to be closed when the in-phase condition is observed. This is achieved by adjusting the speed of
the incoming set to match the set on the bus-bars and the easiest method of speed adjustment is with a governor
motor control and a Raise/Lower speed control button.
Care must be taken to ensure that the machines are exactly in phase before paralleling is attempted, as the
forces set up by out of phase paralleling can create severe mechanical and electrical stresses in the sets.
4.
As load is applied any discrepancy between the kW meters can be eliminated by slight adjustment of the speed on
one of the machines.
5.
With the kW meters sharing load correctly, any discrepancy between the ammeters can be eliminated with slight
adjustment of the voltage on one of the machines.
NOTE: Current discrepancies are due to circulating currents which flow between the sets when there is a difference in
excitation levels between the a.c. generators. High circulating currents may be due to a reversal in one of the
droop transformers or incorrect adjustment of the droop setting on the burden resistor or choke. In general,
when paralleling identical machines, the amount of droop should be approximately equal on all sets.
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