Circulating Current Control - GE KVGC 202 Technical Manual

KVCG202/EN M/H11 Technical Manual
KVGC202
3 x IP x (RLcos φ + XLsin φ + XTsin φ)
Vr =
VT_ratio
where Cosϕ = power factor of the load
Figure 20: Low Power Factor with Negative Reactance Control and LDC 2
4.7.4

Circulating current control

An alternative method of parallel control of transformers are the circulating current control
schemes. These offer the advantage of achieving a fully stable operating scheme whilst
retaining both resistive and reactive components of line drop compensation. These
schemes are preferred where a large variation in system power factor is envisaged.
Where the paralleled transformers are not of similar electrical characteristics then it is
necessary to include interposing CTs to provide suitable coupling between transformers.
Circulating current control is obtained by separating the IL and Ic components fed into the
LDC circuits. This is obtained by interconnection via pilot wires between the relays in a
parallel group. The average of the two currents, IL+Ic and IL-Ic seen by the VRRs, IL, is
circulated through the pilot wires. The remaining currents +Ic and –Ic are then circulated
through the tertiary windings of the circulating current transformers of the VRRs. These
extracted Ic currents are then used to derive a variable compensating voltage Vc which is
set to offset the adverse effects of IcXL as previously described.
Precise values of Vc are determined during commissioning procedures to give stable
control of two or more transformers in a parallel group. An approximate setting is given
by:
3 x IP x XT
Vc =
VT_ratio
where XT = reactance of the transformer.
As can be seen from the above equations the KVGC is set in terms of the volt drop that
will occur when rated current is applied to the relay. The relay then applies a level of
compensation proportional to the level of circulating current it measures. For example, a
setting of Vc = 20 V will produce a compensation voltage equal to 20 *Ic/Irated Volts.
Figure 21 shows two similar parallel transformers where transformer T1 has tapped up
before T2. Both VRR1 and VRR2 now see the circulating current as an additional load
current. In this example transformer T1 is on a higher tap than transformer T2. This will
force circulating current to flow from T1 into T2. The current measured by the relay on T1
will therefore be IL + Ic, and the current seen by the relay on T2 will be IL - Ic. By
connecting pilot wires between the relays currents +Ic and -Ic are extracted by the
circulating current control circuit which derives a compensation voltage +Vc and –Vc.
Figure 22 shows how +Vccvoltage is applied as a compensation voltage to the regulated
voltage to increase this voltage so that the VRR will tend to tap down and vice versa for
the other VRR. Using this method runaway is avoided even if LDC is not required and