and
Line drop V Ris, the I
CT_n1
used in the equation should be the sum of the rated
currents of all the transformers operating in parallel. Here, I
injected
is also defined as
the average of the connected secondary currents (I_A, I_B and I_C). The calculated
line drop compensation value can be read from the monitored data LDC.
Master/Follower principle M/F
The Master/Follower (M/F) operation principle is suitable for power transformers
with identical ratings and step voltages. One voltage regulator (master) measures
and controls and the other regulators (followers) follow the master, that is, all
the tap changers connected in parallel are synchronized. This parallel operation is
obtained by connecting the
FLLWx_CTL
output of the master to the corresponding
input
TAPCHG_FLLW
of the followers via a horizontal GOOSE communication.
The values for the
FLLWx_CTL
command are 1=Lower follower x and 2=Raise
follower x. Consequently, the values for the
TAPCHG_FLLW
command are 1=Lower
and 2=Raise.
If several regulators are to act as masters (one at a time), their outputs also have
to be routed to the inputs of other regulators. To start the parallel operation, the
master regulator is set to the "Auto master" mode and the followers to the "Auto
follower" mode. To implement this setting, a group changing has to be planned.
To keep all the tap changers in the same position, the master needs to know the tap
positions of the followers. This way, the circulating current is kept at its minimum.
The position values of the followers can be brought to the master either via the
horizontal GOOSE communication or TPOSYLTC.
If it is not possible to use horizontal communication between the IEDs and the
position information cannot be wired from the parallel transformers, the M/F
principle can still be used to regulate two or an unlimited number of transformers in
parallel. Since the master cannot detect the tap positions of parallel transformers,
it just activates the lowering and raising outputs for all the followers when it
controls its own tap changer. This is called blind control. In this case, a number
of parallel transformers are regulated as one unit. The tap position inputs 1…3
(
TR1_TAP_POS
..
TR3_TAP_POS
) must be left unconnected for the master to know
that the tap positions of the followers are unknown. The time delay between
successive commands can be set by the
Follower delay time setting. The default
value is six seconds.
When a disconnected transformer is taken into use and the tap position is
unknown, the follower should be manually controlled to the same position as the
master. This can also take place in the master/follower mode. First, the master
gives a control command to its own transformer, that is, it is echoed to the
followers (the follower tap positions have to be connected). Thereafter, successive
control commands to the followers take place until the master and followers have
the same tap positions.
Out-of-step function
The out-of-step function is usually used in the M/F modes only. The out-of-step
function means that the master is able to detect the position values of the followers
and control them to the same position as the master is. In this case, the master
assumes that the followers also have either
Raise block tap higher than Lower block
tap or Lower block tap higher than Raise block tap because this defines what is
the given command pulse for a follower. If the master has
Raise block tap higher
than
Lower block tap and the follower has Lower block tap higher than Raise block
tap, the corresponding
TAPCHG_FLLW
included control signals should be connected
1MRS757644 H
Control functions
620 series
Technical Manual
1055
