Differential protection is a unit scheme that compares the current on the primary side of a transformer with that on the secondary side. Where a difference exists (other than that due to the voltage ratio) it is assumed that the transformer has developed a fault and the plant is automatically disconnected by tripping the relevant circuit breakers. The principle of operation is made possible by virtue of the fact that large transformers are very efficient and hence under normal operation power-in equals power-out. Differential protection detects faults on all of the plant and equipment within the protected zone, including inter-turn short circuits.
Principle of Operation The operating principle employed by transformer differential protection is the Merz-Price circulating current system as shown below. Under normal conditions I1and I2 are equal and opposite such that the resultant current through the relay is zero. An internal fault produces an unbalance or 'spill' current that is detected by the relay, leading to operation.
Design Objectives An ideal scheme is required to be:
Extremely stable under through fault conditions
Very fast to operate for an internal fault
Design Considerations
A number of factors have to be taken into account in designing a scheme to meet these objectives. These include:
The matching of CT ratios
Current imbalance produced by tap changing
Dealing with zero sequence currents
Phase shift through the transformer
Magnetising inrush current
about slope
Biased differential element
Each relay, contains a biased
differential element per phase with
a characteristic as shown in Figure
2. The minimum differential current
required for operation is adjustable
between 10% and 50% of rated
current. The KBCH has a dual slope
bias characteristic (Figure 2).
The initial slope of 20%, from zero
to rated current, ensures sensitivity
to faults whilst allowing for up to
15% mismatch when the power
transformer is at the limit of its tap
range,
in addition to current transformer
ratio errors. At currents above
rated, extra errors may be
gradually introduced as a result of
CT saturation. The bias slope is
therefore increased to 80% to
compensate for this.
Biased Differential Relays
Re: Biased Differential Relays
