of the power transformer. If the phase voltages are measured, the voltage U_AB
is calculated internally in the protection relay. In case bi-directional regulation is
expected, then voltage measurement from primary side is also required. If not
available, calculated values can be used under certain conditions.
The currents from the secondary side of the power transformer (I_x, where x is A, B
or C) have several uses.
• The highest phase current value is used for overcurrent blocking.
• The currents from the secondary side of the power transformer are used for LDC
(average of the connected inputs).
• The currents from the secondary side of the power transformer are used for
calculating the circulating current in the NRP and MCC operation modes.
When reverse regulation takes place, primary side current measurements are
expected and is used in a similar way.
Both voltage U_AB and the phase currents are always measured using the value
of the filtered fundamental frequency component (DFT). Hence, the harmonics are
always suppressed. Moreover, the measured voltage value is continuously average-
filtered with the eight-value-long sliding window. The phase-compensated voltage
U_A is always used in calculations, although it is not connected. This averaged value,
the parameter U
m
in
Equation 384
, is used for control, and its magnitude can be
read from the monitored data U_MEAS.
Similarly, the magnitude of the phase current of the own transformer, I_x, and the
phase angle difference between the internally phase-compensated voltage U_A and
phase current I_x (the angle difference used in
Equation 376
and
Equation 378
) are
also average-filtered by the same fixed-length window. The phase angle value can
be read from the monitored data ANGL_UA_IA. These currents and phase angle
differences are used solely in circulating current calculations.
There are minimum limits for the voltage and current magnitudes, so the magnitude
and phase angle difference values diverge from zero. The voltage magnitude must
exceed three percent of U
n
and the current I_A must exceed two percent of I
n
.
9.10.5.2 Tap changer position inputs
The position value of the tap changer can be brought to OL5ATCC as a resistance
value, a mA signal or as a binary-coded signal. More information on how the
resistance value, the mA signal or a binary-coded interface are implemented can
be found in TPOSYLTC in this manual.
The indicated tap changer position of the own transformer is internally connected
to the TAP_POS input, and the tap changer positions of the parallel transformers
are fed to inputs TRx_DAT via data combiner function blocks OLGAPC. This also
defines the connection identity so that follower 1 is connected to TR1_DAT, follower
2 is connected to TR2_DAT and follower 3 is connected to TR3_DAT. The own
transformer position can be read from the input TAP_POS. The follower tap changer
positions can be read from the input data TR_TAP_POS of OLGAPCx.
The tap changer position value is given in parentheses. For example, (0) indicates
that there is no tap changer position connected or the quality of the tap changer
position value is bad. Typically, if no tap changer position is connected, all the
TPOSYLTC binary inputs are FALSE by default and the value shown is (0). A value
other than zero indicates bad quality. OL5ATCC treats a bad-quality tap changer
position as unconnected tap position information.
Control functions
1MRS759142 F
1698 REX640
Technical Manual
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