GE Multilin DTP-B, Инструкция по эксплуатации

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3. OPERATING PRINCIPLES
GEK-106243E DTP Digital Transformer Protection 3-3
3.2.2 THROUGH CURRENT
Through current is defined as the smallest of the restraint currents for a through current condition (i.e., load flow or
an external fault). This can be understood intuitively as the current that passes through the transformer. For an
internal transformer fault where current flows in on each winding the through current is zero. Through current is
calculated using a special non-linear algorithm which permits compliance with the previous definition. Through
current is computed as a RMS value. This takes into account not only the fundamental value but also all of the
harmonics up to the fifth (higher order harmonics are blocked by the low-pass anti-aliasing filters). The algorithm
used to calculate the RMS value permits a high level of accuracy in the through current measurement.
3.2.3 HARMONIC RESTRAINT
The harmonic restraint principle allows the relay to distinguish between faults and transformer inrush current.
Figure 8 shows that a restraint proportional to the second and fifth harmonics of the differential current is used.
The harmonics are extracted using DFTs. A high rate of sampling guarantees an accurate measurement and
complies with the Nyquist criteria. The total harmonic restraint is the RMS value. This RMS value is equal to the
square root of the sum of the squares of the second and fifth harmonics.
I
h
=
√
(I
2
²
/A
2
²
+ I
5
²
/A
5
²
)
Where:
I
h
Total harmonic restraint current
I
2
Second harmonic current
A
2
Second harmonic percentage restraint
I
5
Fifth harmonic current
A
5
Fifth harmonic percentage restraint
3.2.4 INTERNAL PHASE SHIFT MATCHING
The DTP relay allows internal compensation for the phase shift across the power transformer. This is
accomplished by settings that define the “connection group” of each power transformer winding, as well as the
connections of the current transformers. If this compensation is performed external to the relay, so that currents
arrive “in-phase”, set the compensation setting to EXTERNAL. This is performed under general settings. With this
setting the relay will not filter out the zero-sequence component of current. This must be done externally via
proper CT connections. This setting is especially useful for testing the relay with single phase and three test
sources that supply in-phase currents.
If we want the relay to perform the compensation, we must set the compensation setting to INTERNAL. In this
case, the main CT’s must always be wye connected. When in the INTERNAL mode, filtering of the zero-sequence
component is performed by the relay.