Transformer Differential Protection Principles; Through Fault Stability; Bias Current Compensation - GE P642 Technical Manual

Chapter 6 - Transformer Differential Protection
2

TRANSFORMER DIFFERENTIAL PROTECTION PRINCIPLES

Transformer Differential Protection (87T) uses the well-known current differential principle where current entering
the protected equipment is compared with the current leaving the protected equipment. If there is no fault, the
current entering the transformer will be equal to the current leaving the transformer multiplied by the inverse of
the turns ratio. If there is a fault in the transformer zone, the currents will not be equal, which results in a
differential current. This differential current is proportional to the fault current for internal faults, but approaches
zero for any other operating conditions. The IED trips the circuit breakers protecting the transformer when it
detects a minimum level of differential current.
The differential scheme creates a well-defined protection zone between the CT sets protecting the transformer.
Any fault within the differential protection zone is called an internal fault, while any fault outside the differential
protection zone is called an external fault. The protection should operate only for internal faults and be sensitive to
low fault currents. It should also restrain on the highest prospective external faults, providing the CTS accurately
reproduce the primary currents. This is difficult to achieve in practice because the CTs never have identical
saturation characteristics. This will result in a differential current, which could cause undesirable operation.
An external fault, which causes a high current to flow through the transformer, is called a through fault. The
through-fault current will usually be high enough to saturate the CTs. The differences in the saturation
characteristics of the CTs will cause a differential current, which could cause the device to trip unless it is
restrained. The term used to specify the IED's ability to cope with these imperfections is called Through Fault
Stability.
CT saturation is not the only cause of undesirable differential current. Other aspects which need to be considered
by the transformer differential element to avoid maloperation are:
● Phase shift between the transformer primary and secondary currents depending on the vector group
Transformation ratio
●
The zero-sequence current, which flows in the grounded star transformer winding or the grounding
●
transformer within the differential protection zone
Tap changer operation to adjust the voltage
●
Magnetising inrush current that flows immediately after the transformer energisation or during a voltage
●
recovery after the clearance of an external fault or when a second transformer is paralleled with the
already energised transformer
● Over excitation of the transformer
2.1

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.
2.2

BIAS CURRENT COMPENSATION

To prevent maloperation, compensation is needed for the protection to remain sensitive to internal faults but to
ignore through faults. This is achieved by applying a proportion of the scalar sum of all the currents entering and
exiting the zone. This scalar sum is called bias current.
The bias characteristic changes the operating point of the IED depending on the fault current. At low through-fault
currents, the CT performance is more reliable so a low bias current is needed. Less differential current is then
needed to trip the circuit breakers, allowing greater sensitivity to internal faults. At high through-fault currents, the
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