Setting Guidelines For Shunt Reactor Biased Differential Protection; Figure 57: Busbar Biased Differential Protection - GE P642 Technical Manual

P64x
Note:
CT supervision should be used to prevent maloperation if there is an open circuited CT secondary. The CTS feature can be
used to desensitize the biased differential protection. To do this, raise the differential current pickup setting Is1 to the value of
Is-CTS.
V03120

Figure 57: Busbar biased differential protection

5.6

SETTING GUIDELINES FOR SHUNT REACTOR BIASED DIFFERENTIAL PROTECTION

Shunt reactors are commonly used in the power system to compensate for the capacitive reactance of long
transmission lines. Shunt reactors are inductive loads that are used to absorb reactive power to reduce the over
voltages generated by line capacitance.
One of the main difficulties with shunt reactor protection is that the protection IED may maloperate during iron-
core reactor energization and de-energization. A shunt reactor is typically switched in and out regularly depending
on the power system load. The iron-core shunt reactor energization current contains a DC offset with a long time
constant (up to 1 second) and low frequency components. These current waveforms cause a certain level of flux in
the CT magnetic core. During normal reactor operation the current is generally the nominal current, which is not
high enough to reduce the flux level in the CT. In the next switch in operation, the flux may either increase or
decrease depending on the point o on the wave when energization occurs. The regular switching in and out of the
reactor causes CT saturation; therefore, we recommend that the current transformers on both sides of the reactor
have similar excitation characteristics to reduce the risk of maloperations. A high impedance differential scheme is
generally better than a low impedance differential scheme, because it is not affected by CT saturation.
As per IEEE C37.109-2006 the properties of shunt reactors can be summarized as follows:
● Dry air-air core type reactors: no magnetizing inrush during energization as there is no iron core. The peak
current during energization might be as high as 2 Ö2I
are normally used up to 34.5 kV and often installed on the transformer tertiary winding.
Oil-immersed type reactors: the gapped iron-core type might experience severe energizing inrush. The
●
coreless type experiences less severe magnetizing inrush.
Consider the following shunt reactor:
P64x-TM-EN-1.3
Idiff/Inom
4
3.8
3.6
3.4
3.2
3
2.8
2.6
Is-CTS
2.4
2.2
2
1.8
1.6
1.4
Is1
1.2
1
0.8
0.6
0.4
Is-cctfail > 10%
0.2
0
0 0.5 1 1.5
P64x
Chapter 6 - Transformer Differential Protection
K2 = 80%
2 2.5 3 3.5 4
cctfail CT error
due to transient offset. Air core type reactors
nominal
4.5 5
Ibias/Inom
Is-CTS
133