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Protection Riddle No.96 - Busbar differential protection
how differential protection of bus bar works? how outgoing feeders ct is connected to differential protection relay?
Author : george.v.sebastian - From: India
Mon, November 12th, 2012 - 19:21
The Merz-Price principle is applicable to a multi-terminal zone such as a busbar. The principle is a direct application of Kirchhoff's first law. Usually, the circulating current arrangement is used, in which the current transformers and interconnections form an analogue of the busbar and circuit connections. A relay connected across the CT bus wires represents a fault path in the primary system in the analogue and hence is not energised until a fault occurs on the busbar; it then receives an input that, in principle at least, represents the fault current. The scheme may consist of a single relay connected to the bus wires connecting all the current transformers in parallel, one set per circuit, associated with a particular zone, as shown in Figure (a). This will give earth fault protection for the busbar. This arrangement has often been thought to be adequate.
If the current transformers are connected as a balanced group for each phase together with a three-element relay, as shown in Figure (b), additional protection for phase faults can be obtained. The phase and earth fault settings are identical, and this scheme is recommended for its ease of application and good performance.

Protective Relays Application Guide, ALSTOM
Fri, February 22nd, 2013 - 17:16
In simple words,bus bar differential relay calculates the differences between current entering to bus-bar and current leaving bus bar.
There are two kinds of Bus-Bar differential relays
1) High Impedance
2) Low Impedance

In high impedance ,usually a stabilizing resistor is connected in series with the coil of differential relay.This compensate for any mis-match between connected C.Ts, difference in their magnetizing curves,Different winding resistance etc etc.Usually the C.T selected turn ratio of all C.Ts feeding differential relay should be same for high impedance relay which is not necessary for low impedance differential.
To your second query,the C.T of all our goings can be added at single place such that red is added to red ,yellow to yellow and so on.Hence four wire for all outgoings can be taken in to differential relay. 
Author : Talha - From: Pakistan
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