Effect Of Circulating Current On Ldc - GE KVGC 202 Technical Manual

Technical Manual
KVGC202
1. Master-follower
2. Circulating current detection
3. Negative reactance compounding
4.7.2.2

Effect of circulating current on LDC

Consider two similar transformers connected in parallel as shown in Figure 11. The
busbar voltage as seen by both VRR's is Vbus. The LDC settings are selected such that
Vr = IL.R
Vxl = IL.X
Where R is the resistive component of the line and X is the reactive component of the line
and IL is at unity power factor.
Figure 12 shows the voltage seen by the relays with transformers T1 and T2 on the same
tap position.
If the system now requires a raise voltage tap change and T1 operates before T2, then a
circulating current Ic which is almost purely reactive is created as previously described.
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.
If these currents are applied to relays that are set up for line drop compensation then the
circulating current will constitute an error signal.
Figure 13 shows the relay that sees IL - Ic (i.e. T2 which is on too low a tap and would
require a raise voltage signal). The circulating current is reactive and is therefore shown
leading the load current by 90° (leading because it is negative Ic). This current
component will provide resistive and reactive compensation which is likewise leading the
Vr and Vxl load current compensation by 90°. The relay is trying to regulate to a remote
voltage shown by Vrem. However, the circulating current has caused the relay to be
presented with a voltage equal to Vreg. This voltage is much higher than Vrem and if Ic
is large enough to take the regulated voltage outside the deadband setting on the relay
then the VRR will initiate a lower voltage tap command. This is incorrect as the voltage
on this transformer is already too low. Should this occur then the tap disparity is
increased and Ic gets larger causing T2 to continue tapping until the lower tap limit is
reached and T2 is locked out.
Likewise, in Figure 14 transformer T1 sees a current IL + Ic because it is on too high a
tap. The net effect of the circulating current in this case is to present a voltage to the
relay, Vreg, which is lower than Vrem. If Ic is large enough to take the regulated voltage
outside the deadband setting on the relay then the VRR will initiate another raise voltage
tap command. This will further increase the tap disparity and hence accelerate the
situation until the upper and lower tap limits are reached on both T1 and T2 respectively.
For this condition both transformers are locked out and the system voltage can no longer
be regulated.
KVCG202/EN M/H11