ABB RELION REX640, Техническое руководство, Страница: 1710 / 1960

calculates the circulating current compensation term U
ci
(
Equation 373
) as in the
MCC.
U
I
I
U
ci
ci
n
n
=
−
⋅ ⋅
Stability factor
100
(Equation 377)
I
ci
Circulating current
Stability
factor
Stability setting (the recommended value depends on the loop impedance)
If the transformers operating in parallel have different rated currents, the value of
the
Stability factor setting of the regulator should be proportional to the rated
currents, that is, the higher the rated current, the higher the
Stability factor setting
value.
It is possible to find out if the circulating current has been minimized by comparing
the reactive components of the currents measured by the different regulators. The
circulating current is minimized when the reactive components are equal.
The negative reactance method gives satisfactory results only if the phase angle of
the load current is known relatively accurately. If the actual phase angle deviates
from the phase angle setting, a regulating error occurs. However, if there is an
occasional stepwise change in the phase angle of the load, the regulating error can
be suppressed with the logic. This kind of stepwise change can occur, for example,
when a capacitor bank is switched on to compensate a reactive power flow. Another
possibility is to use an automatic setting group change between setting groups in
different loading situations. The setting groups then have different set values for
the load phase angle.
Minimizing Circulating Current MCC
The MCC principle is an optimal solution for controlling parallel transformers of
different ratings or step voltages in substations with varying reactive loads. Since
this control scheme allows the exchange of data between regulators, the circulating
current can be calculated more accurately than with other schemes. However,
a maximum of six regulators can be connected in parallel. To start the parallel
operation, the active operation mode parameter has to be set to "MCC" for all the
regulators of the connection. Furthermore, the signal
CON_STATUS
must indicate
that the transformers are connected to the network. A unit that minimizes the
circulating current must have the active operation mode set to "MCC". Units that
have the active operation mode set to "Manual" do not perform any circulating
current minimization operations themselves.
Control functions 1MRS759142 F
1710 REX640
Technical Manual