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Chapter 6 - Transformer Differential Protection
CctFail Delay: to set a time delay
If the differential current is larger than the Is-cctfail setting and no trip is issued after the time delay has elapsed,
an alarm is issued indicating a CT problem.
3.8
TRIPPING CHARACTERISTIC STABILITY
Conditions such as CT saturation and transient switching operations can cause incorrect operation of differential
elements. To avoid this a number of techniques are employed in this device. These include :
Maximum and Residual Bias
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Transient Bias
CT saturation detection (to discriminate between internal and external faults)
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No Gap Detection
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External Fault Detection
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Current Transformer Supervision
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Circuitry Fail Alarm
Second Harmonic Blocking
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Fifth Harmonic Blocking
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3.8.1
MAXIMUM BIAS
The differential and bias currents are calculated on a per phase basis eight times per cycle. A comparison between
differential and bias current is used to determine whether to trip or not. The comparison is made on a per phase
basis using the differential current measured on that phase. However a common bias current is used for all three
phases.. To assure stability, the largest bias current calculated on any phase is used to restrain all phases. This is
referred to as the Maximum Bias.
3.8.2
DELAYED BIAS
To provide further stability when external faults are being cleared, the protection checks for the highest value of
bias current calculated during the previous cycle. If that value is higher than the present value, it is used to restrain
the tripping decision. This is referred to as the Residual Bias. The Residual Bias technique maintains through fault
stability for clearance of external faults.
3.8.3
TRANSIENT BIAS
If there is a sudden increase in the mean-bias measurement, an additional bias quantity (caled Transient Bias) can
be introduced into the bias calculation, on a per-phase basis. Transient Bias is only active for external faults. It can
be enabled or disabled with the Transient Bias setting in the DIFF PROTECTION column.
Transient bias, decays exponentially. The transient bias quantity is added to the operating current calculated at
the maximum bias. Therefore, the following differential current thresholds are available:
Differential threshold phase A = Iop at max bias + transient bias_phase A
Differential threshold phase B = Iop at max bias + transient bias_phase B
Differential threshold phase C = Iop at max bias + transient bias_phase C
The transient bias technique uses a time decay constant, stability coefficients, and some differential function
settings to provide a dynamic bias characteristic. The following diagram shows the behaviour during an external
fault. For the device to trip, the fundamental of the differential current should be above the operating current at
max bias + transient bias.
112
P64x
P64x-TM-EN-1.3
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