ABB RELION REX640 Technical Manual page 1559

1MRS759142 C
REX640
Technical Manual
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.
The out-of-step function is triggered when the master detects a difference of at least
one step between the tap changer positions in the follower and in the master. The
master then sends special raise or lower commands to the diverging follower. 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".
Negative Reactance Principle NRP
This parallel control scheme is suitable for power transformers with different ratings
and step voltages. Since no communication between the regulators is needed, this
principle can be applied even when the parallel transformers are located at different
substations. To start the parallel operation, the active operation mode has to be set to
"NRP" for all the regulators of the connection. The active operation mode can be
changed via function block inputs or by setting it either locally or remotely.
When applying this principle, each regulator has a phase angle setting φ
parameter Load phase angle) towards which it tries to regulate the current. The setting
value is chosen according to the expected power factor of the load (positive setting
value equals inductive load). When the actual phase angle of the load current is the
same as the setting and the transformers and their tap changer positions are identical,
the currents of the transformers are in the same phase as the total load current. If the
tap changer positions are different, the circulating current flows and the currents of
different transformers either lag or lead the load current.
circulating current is the reactive component which separates the measured current
vector from the expected angle value.
Section 9
Control functions
(setting
Load
Figure 814 shows that the
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