To indicate if a follower is on a different tap position compared to the active master,
TAP_DIFF
output is set to TRUE at that specific follower. Even though the follower
transformer is not yet included in parallel connection, that is, its TR_STATUS is
“Independent”, tap difference to master can be indicated. To enable
TAP_DIFF
output usage, valid tap position
TR_TAP_POS
and M/F status TR_STATUS from
master must be routed through OLGAPC to the follower. The
TAP_DIFF
output can
be used as an alarm or to block the closing of the circuit breaker of the follower
transformer.
Out-of-step function
The out-of-step means that the master detects a difference of at least one step
between the tap changer positions in the follower and in the master. The out-of-
step function is usually used in the M/F mode 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's. 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 tap changer follow included control signals should be connected
crosswise. This requires extra logic where dual point command bits have to be
converted, that is, 0=>0, [01]=1=>[10]=2 and [10]=2=>[01]=1.
M/F is the only parallel mode which has an out-of-step functionality. In the MCC
and NRP operation modes, the circulating current is minimized, which often means
different tap positions in the parallel transformers. Moreover, these modes allow
different ratings and step voltages for the parallel transformers. Therefore, it is
reasonable to apply the out-of-step function only to the M/F operation mode.
In an out-of-step situation at startup, follower commands take place only after the
master has given its own control command, which is echoed to the followers. This is
followed by special raise or lower commands to the diverging follower.
By setting the parameter
Inst out of step Cor to "TRUE", an out-of-step situation
at startup sends special raise or lower commands to the diverging follower after
Follower delay time. Inst out of step Cor is by default set to "FALSE".
If two consecutive commands fail to change the position of the follower to the
right direction, the master activates the
PAR_FAIL
output, that is,
PAR_FAIL
is set
to TRUE, and stops the special recovery efforts. However, every time the master
controls its own tap changer later, it sends a controlling pulse to the diverging
follower too. Furthermore, if the master notices a correct position change after a
sent pulse, it restarts the attempts to drive the follower to the same position and
deactivates the
PAR_FAIL
output, that is,
PAR_FAIL
is set to FALSE. However, if
there still are diverging followers, the reset is not indicated. It is indicated only when
no diverging followers exist. Monitoring, and hence the indication of a paralleling
failure, is not possible in blind control.
The followers with a parallel failure can be read from the monitored data FAIL_FLLW.
For example, if only follower 3 is in the parallel failure state, FAIL_FLLW has the
value "T3". If both followers 1 and 2 are in the parallel failure state, FAIL_FLLW has
the value "T1+T2". By default, when no failed followers exist, the value is "No failed
followers".
Control functions
1MRS759142 F
1708
REX640
Technical Manual