Current Differential Protection Principle - GE MiCOM P40 Agile Technical Manual

Single breaker current differential (with distance)
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Chapter 6 - Current Differential Protection
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 the
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.
Some products also perform differential protection on the neutral current. A similar biasing principle is applied, but
the characteristic is generally different.
Products are available to protect two-terminal and three-terminal lines and feeders. Models are therefore available
with one or two communications channels for the Current Differential protection function. Models with two
channels can be used to protect two-terminal or three-terminal feeders. In both of these cases, inherent
communications redundancy assures integrity of the protection in the event of a single communication link failure.
With a single communications channel, the application is restricted to the protection of two terminal lines and the
protection is compromised if the communication channel fails.
The tripping characteristics for two-terminal and three-terminal applications are similar, but two-terminal
applications use two current values for the evaluation of the Bias and Differential currents, whereas three-terminal
100
P543i/P545i
P54x1i-TM-EN-1

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