What is a high impedance differential relay?

High-impedance bus differential relays are applied to the paralleled output of all CTs from each phase connected to a common bus, as shown in Fig. 11. As the name implies, the high-impedance bus differential relay presents a very high impedance to the flow of current.

What is the difference between low impedance and high impedance relay?

The basic principle of the high impedance protection is the same as the low impedance principal, it sums up all currents measured by the CT’s surrounding the bus. The biggest difference is how the high impedance principle handles the problem of CT saturation on external faults.

What is a bus differential relay?

Bus protection relays Bus differential, which is the most sensitive and reliable method for protecting a substation bus, is installed at transmission and distribution substations and switchyards. CTs are connected so that relays respond to the difference between incoming and outgoing current.

What happens in a differential relay when the saturation is too high?

The effect of CT saturation on transformer differential protection is double-edged. Although, the percentage restraint reduce the effect of the unbalanced differential current, in the case of an external faults, the resulting differential current which may be of very high magnitude can lead to a relay male-operation!

Why are high impedance relays used on buses?

High impedance bus differential relays are used on most of the busses in North America. The popularity can be explained by the good performance of this scheme in relation to CT saturation and the low cost, if used on a simple bus system.

What kind of differential relay does a bus use?

The most common bus differential relays use either high- impedance or low-impedance differential elements. A high- impedance differential relay forms an operating quantity by connecting zone CTs in parallel with a resistive element. A high-impedance differential relay can be microprocessor- based or electromechanical, and this paper makes no

How does a high impedance differential scheme work?

A high impedance differential relay can have a current setting that monitors the current through the 87 element. In this case, the setting must be higher than 0.016Arms (peak * 0.707 = 0.022 * 0.707), otherwise the relay will trip for a fault outside the zone of protection as per the measurements in the example.

Why does high impedance busbar differential protection not work?

High impedance busbar differential protection will not work properly if the CTs are not sized correctly for the connected circuit. We calculated that the voltage across the non-saturated CT, when another CT saturates, is 83.33V. Any CT with a saturation voltage greater than 83.33V should work correctly in our example.