Through Fault Stability; Restricted Earth Fault Types; Low Impedance Ref Principle - GE P642 Technical Manual

Chapter 8 - Restricted Earth Fault Protection
For solidly earthed systems, the operating current for the transformer differential protection is still significant for
faults over most of the winding. For this reason, independent REF protection may not have been previously
considered, especially where an additional device would have been needed. But with this product, it can be
applied without extra cost.
2.3

THROUGH FAULT STABILITY

In an ideal world, the CTs either side of a differentially protected system would be identical with identical
characteristics to avoid creating a differential current. However, in reality CTs can never be identical, therefore a
certain amount of differential current is inevitable. As the through-fault current in the primary increases, the
discrepancies introduced by imperfectly matched CTs is magnified, causing the differential current to build up.
Eventually, the value of the differential current reaches the pickup current threshold, causing the protection
element to trip. In such cases, the differential scheme is said to have lost stability. To specify a differential scheme's
ability to restrain from tripping on external faults, we define a parameter called 'through-fault stability limit', which
is the maximum through-fault current a system can handle without losing stability.
2.4

RESTRICTED EARTH FAULT TYPES

There are two different types of Restricted Earth Fault; Low Impedance REF (also known as Biased REF) and High
Impedance REF. Each method compensates for the effect of through-fault errors in a different manner.
With Low Impedance REF, the through-fault current is measured and this is used to alter the sensitivity of the REF
element accordingly by applying a bias characteristic. So the higher the through fault current, the higher the
differential current must be for the device to issue a trip signal, Often a transient bias component is added to
improve stability during external faults.
Low impedance protection used to be considered less secure than high impedance protection. This is no longer
true as numerical IEDs apply sophisticated algorithms to match the performance of high-impedance schemes.
Some advantages of using Low Impedance REF are listed below:
There is no need for dedicated CTs. As a result CT cost is substantially reduced.
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The wiring is simpler as it does not require an external resistor or Metrosil.
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Common phase current inputs can be used.
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It provides internal CT ratio mismatch compensation. It can match CT ratios up to 1:40 resulting flexibility in
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substation design and reduced cost.
Advanced algorithms make the protection secure.
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With High Impedance REF, there is no bias characteristic, and the trip threshold is set to a constant level. However,
the High Impedance differential technique ensures that the impedance of the circuit is sufficiently high such that
the differential voltage under external fault conditions is lower than the voltage needed to drive differential current
through the device. This ensures stability against external fault conditions so the device will operate only for faults
occurring inside the protected zone.
High Impedance REF protection responds to a voltage across the differential junction points. During external faults,
even with severe saturation of some of the CTs, the voltage does not rise above certain level, because the other
CTs will provide a lower-impedance path compared with the device input impedance. The principle has been used
for more than half a century. Some advantages of using High Impedance REF are listed below:
It provides a simple proven algorithm, which is fast, robust and secure.
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It is less sensitive to CT saturation.
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2.4.1

LOW IMPEDANCE REF PRINCIPLE

Low Impedance REF can be used for either delta windings or star windings in both solidly grounded and resistance
grounded systems. The connection to a modern IED is as follows:
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P64x
P64x-TM-EN-1.3