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2.2.3 Matching of CT Secondary Currents
The currents supplied to the differential elements must be matched in phase displacement and
amplitude under through-load and through-fault conditions.
Generally, it is difficult to completely match the incoming current with the outgoing current for
the relay input because the CT ratios at the primary, secondary and tertiary sides of a transformer
are not matched in terms of the CT ratio, phase angle and cancelling of zero-sequence current.
GRT100 provides the following matching method:
Matching of phase
angle / Zero-sequence
current elimination
Ip
Is
Ip/N1=i1
Primary
CT ratio: N1
Is/N2=i2
Secondary
CT ratio: N2
Matching of phase
angle / Zero-sequence
current elimination
Matching of CT ratio Matching of CT ratio
Differential relay calculation
Transformer
GRT100
Kct1
×
i1 Kct2
×
i2
Figure 2.2.2.1 Matching Method
GRT100 supports selectable two matching methods, α-method (Alpha) and β-method (Beta). The
method is selected by the scheme switch [Phase matching].
Phase matching is performed by setting according to the hands of a clock and the transformer
connections described in IEC60076-1. For details of the setting, refer to 2.2.5.
2.2.3.1 α-method phase matching
This method corrects the phase angle by using each winding current calculated as follows:
- Current substructed zero-sequence current from each phase current in Star- winding side of
transformer
- Phase-to-phase Current in Delta-winding side of transformer
The followings show calculation formula and current vectors in an example of a transformer
Yd11.
Ipb Ipc
Isb
Ip1
Is1
Isc
Isa
Ipa
3
2
1
pcIpbIpaI
pI
&&&
&
−−
=
,
3
1
scIsaI
sI
&&
&
−
=
(1)