ABB RELION REX640 Technical Manual page 1561

1MRS759142 C
REX640
Technical Manual
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.
φ
2
GUID-41312EFD-CE4B-472E-B96C-F6771C2D36D7 V1 EN
Figure 815: The circulating current between two parallel transformers
In this case, the circulating current can be calculated with the equation
φ
1
I
TR1
I
TR1
2 x Ici
I
TR2
Ici = circulating current
Section 9
Control functions
U_A
x sin(φ )
1
I
TR2
x sin(φ )
2
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