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2 CURRENT DIFFERENTIAL PROTECTION PRINCIPLE
Current differential protection is based on Kirchoff’s Law. It generally uses the Merz-Price principle in which the
sum of the currents entering the protected zone should equal the sum of the currents leaving the protected zone.
A difference between these currents is known as differential current. If the differential current exceeds a threshold,
then a protection device may be required to trip. If the differential current is below the threshold then it is expected
to restrain.
Errors caused by mis-match of the current transformers at the different terminals, or saturation of the current
transformers during external faults could lead to false tripping under healthy conditions. For that reason a
restraining quantity is normally applied so that when the magnitude of the current in the system rises, so the level
of differential current necessary to cause a trip rises. The level of restraint applied is called a bias quantity and its
relationship to the operate quantity is called a Biased Differential characteristic.
Current differential protection does not need voltage transformer inputs, but they can be employed to enhance
capacitive charging, current compensation, protection, control, automation, and supervision features of this
product for certain applications.
To provide current differential protection of transmission lines and distribution feeders, it is normal practice to have
similar devices at each terminal with interconnecting communications links to exchange current signal
information between terminals.
When applying numerical current differential to protection of transmission lines and distribution feeders, as well as
communicating details of the local current measurements, the product also communicates timing, status, and
control data to remote terminals. The current, timing, status, and control data are encapsulated into messages
(sometimes referred to as telegrams) which are transmitted frequently and regularly. The timing data is used to
align local and remote current measurements. The control and status data is used for purposes such as
intertripping. Messages are secured by an address field as well as a cyclic redundancy check code (CRC code). The
use of the address field ensures that only the intended receiving device will respond to the message. Corruption of
the data in the messages could potentially cause the product to trip incorrectly. The use of the CRC code together
with other error checking prevents this.
2.1 NUMERICAL CURRENT DIFFERENTIAL PROTECTION
At each terminal in the scheme the power system current input quantities are acquired, converted into numerical
values, filtered, and compared with current input values from the other terminal(s) in the scheme.
For each phase, and at each terminal, the vector sum of the currents entering the protected zone is calculated.
This is known as the Differential current and provides an operating quantity. Also calculated is the scalar sum of
the same currents, of which a proportion is used as a restraining quantity. This is known as the bias current. To
determine whether tripping should occur, the Differential Current is compared with a percentage of the Bias
Current. If it is exceeded then a trip can be initiated.
The Differential and Bias currents are calculated on a per phase basis, and the tripping decision is made on a per
phase basis. However, the Bias current used in the calculation is the same for all three phases and is based on the
highest of the Bias currents calculated for each phase. This is called Maximum Bias and improves discrimination
for single phase faults.
Products are able to protect two to six terminals with a maximum of four junctions. All models have two
communications channels for the multi-end current differential protection function. Where two terminals are
protected with both communications channels, redundancy ensures integrity of the protection in the event of a
single communication link failure. With more than two terminals, the communications channels are ring
connected, which ensures communication if one channel link fails--as communications are in both directions
around the ring.
Line differential protection requires the comparison of power system quantities taken at the different line
terminals. For a meaningful comparison, synchronisation of the current signals is needed so that they are related
to a common time reference. Different methods are used to achieve current signal synchronisation – some
requiring external time reference signals, and some using internal timing signals.
Chapter 6 - Current Differential Protection P54A/B/C/E
98 P54xMED-TM-EN-1