Relation between Power factor & AVR droop setting
kishorpa...@gmail.com
We have 4 x 920 Kw & 1 x 1100 Kw gas generating sets running in
parallel mode. In one of 920 Kw gensets we replaced the AVR two months
back & since then we have noticed that whenever there is a sudden load
change on the engine the PF of this particular engine doesn't respond
& it falls down to 0.6 to 0.65 lagging resulting to PF flctuations in
other engines as all of them are in parallel mode. Under this
circumstences operator needs to ajust the PF by adjusting the
Motorised Potentiometer Pot resistance manully.
I feel that by adjusting the voltage droop from AVR this problem could
be resolved. However, I am not aware of the eexact realtion between
droop & PF. Is anybody aware of that increase in droop will impove
the PF during sudden load change or vice versa?
Please note that we maintain system PF as 0.8 lagging & before
replacemnt of AVR this problem was not there. Hence, somewhere it is
related to AVR fine tuning.
Please let me know your valuable opinions.
Regards.
Kishor
Fred Lotte
<1174713893.2...@e1g2000hsg.googlegroups.com>,
kishorpa...@gmail.com wrote:
There is no specific relation between pf and AVR droop setting.
It sounds like 4 of your machines are regulating pf and not
voltage while the new AVR is regulating voltage. Load goes up,
voltage stays constant, more vars needed, so pf goes down. It
sounds like your machines are sharing watt loading correctly but
one machine is handling most of the change in reactive loading
because it's regulating voltage and not pf. The voltage
regulating machine is picking up the var loading that the others
are rejecting in order to hold constant pf.
I suppose that's it's possible that the new AVR has the droop
circuit connected backwards or the sensing leads are connected
for the wrong phase rotation. I'd approach reversing any
connections with great care and even greater study.
Why do you want to hold constant pf?
Who is regulating the voltage?
I bet your voltage regulation is better now than before.
Normally the load controls the pf. The generator(s) should supply
power at whatever pf the load requires. The generator controls
the voltage. (And the prime mover controls frequency and/or
power.) The AVR droop is set so that paralleled generators share
the vars according to their individual capacities. This is done
by lowering the voltage setpoint as overexcited reactive is
increased.
The pf specification on the machine nameplate is not a limit in
and of itself. It is only a convenient way of indicating where
the capability curve changes from stator temperature limited to
rotor temperature limited. At values of pf between 0 overexcited
and the 'rating', the rotor is the limiting component of the
generator. At values between the 'rating' and 1.0 pf or some
underexcited pf value, the stator is the limiting component. One
caveat is a class of machines with magnetic retaining rings which
may be limited by stator core end heating at some intermediate
value of pf between 'rated' and 1.0pf. The core end heating limit
rules from where it begins all the way to 0 pf underexcited.
Some unit connected machines have a compensation circuit to
compensate for IZ drop of the step up transformer. This is a
droop circuit connected in reverse. As overexcited reactive
increases, the AVR increases voltage to maintain a constant
voltage at some theoretical point beyond the generator terminals.
This is the opposite action to the AVR droop circuit.
--
Fred Lotte
flo...@nospam.stratos.net