PI Manufacturing AC generators - Manual (Page 20)

Generally 5% droop at full load current zero p.f. is sufficient to
ensure kVAr sharing.
If the droop accessory has been supplied with the generator it
will have been tested to ensure correct polarity and set to a
nominal level of droop. The final level of droop will be set
during generating set commissioning.
The following setting procedure will be found to be helpful.
6.2.1.1 SETTING PROCEDURE
Depending upon available load the following settings should
be used - all are based on rated current level.
0.8 P.F. LOAD (at full load current)
SET DROOP TO 3%
Zero P.F. LOAD (at full load current) SET DROOP TO 5%
Setting the droop with low power factor load is the most
accurate. Run each generator as a single unit at rated
frequency or rated frequency + 4% depending upon type of
governor and nominal voltage. Apply available load to rated
current of the generator. Adjust 'DROOP' control
potentiometer to give droop in line with above table.
Clockwise rotation increases amount of droop. Refer to Fig
9a, 9b, 9c or 9d for potentiometer locations.
Note 1)
Reverse polarity of the C.T. will raise the generator voltage
with load. The polarities S1-S2 shown on the wiring diagrams
are correct for clockwise rotation of the generator looking at
the drive end. Reversed rotation requires S1-S2 to be
reversed.
Note 2)
The most important aspect is to set all generators equal. The
precise level of droop is less critical.
Note 3)
A generator operated as a single unit with a droop circuit set
at rated load 0.8 power factor is unable to maintain the usual
+/-0.5% regulation. A shorting switch can be connected
across S1-S2 to restore regulation for single running.
Important ! LOSS OF FUEL to an engine can cause its
generator to motor with consequent damage
to the generator windings. Reverse power
relays should be fitted to trip main circuit
breaker.
LOSS OF EXCITATION to the generator can
result in large current oscillations with
consequent damage to generator windings.
Excitation loss detection equipment should
be fitted on trip main circuit breaker.
6.2.2 ASTATIC CONTROL
The 'droop' current transformer can be used in a connection
arrangement which enables the normal regulation of the
generator to be maintained when operating in parallel. This
feature is only supplied from the factory as a fitted droop kit,
however, if requested at the time of order, the diagrams inside
the back cover of this book will give the necessary site
connections. The end user is required to provide a shorting
switch for the droop current transformer secondary.
Should the generator be required to be converted from
standard droop to 'astatic' control, diagrams are available on
request.
The setting procedure is exactly the same as for DROOP.
(Subsection 6.2.1.1)
Important ! When using this connection arrangement a
shorting switch is required across each C.T.
burden (terminals S1 and S2.)The switch
must be closed a) when a generating set is
not running and b) when a generating set is
selected for single running.
6.3 MANUAL VOLTAGE REGULATOR (MVR) -
MX341 and MX321 AVR
This accessory is provided as an 'emergency' excitation
system, in the event of an AVR failure.
Powered from the PMG output the unit is manually set, but
automatically controls the excitation current, independent of
generator voltage or frequency.
The unit is provided with 'MANUAL', 'OFF', 'AUTO' switching
facility.
'MANUAL'
- position connects the exciter field to the MVR output.
Generator output is then controlled by the operator adjusting
the excitation current.
'OFF'
- disconnects the exciter field from both MVR and the normal
AVR.
'AUTO'
- connects the exciter field to the normal AVR and the
generator output is controlled at the pre-set voltage under
AVR control.
switching mode of operation should be carried out with the
generator set stationary to avoid voltage surges on the
connected load, although neither the MVR nor AVR will be
damaged should the switching be carried out with the set
running.
6.4 OVERVOLTAGE DE-EXCITATION BREAKER
MX321 AVR
This accessory provides positive interruption of the excitation
power in the event of overvoltage due to loss of sensing or
internal AVR faults including the output power device.
With the MX321 AVR this accessory is supplied loose for
fitting in the control panel.
Important ! When the circuit breaker is supplied loose,
the AVR is fitted with a link on terminals K1-
K2 to enable operation of the AVR. When
connecting the circuit breaker this link must
be removed.