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EPRO PR9268/300-000 front-end

Operating characteristic The restraint current in the case of a three-winding power transformer is derived from the currents of two windings and not three. In the interest of the best possible through-fault stability, the two largest currents of the three windings are used for this purpose. The restraint current is either defined by the equation I I xI x H
1 2 cos
for -90° <  < 90° or it is zero I H
0 for 90° <  < 270° The angle
 (, ) I I 1 2 The following vector diagram of the current on primary and secondary sides and of the differential current measured for a transformer on load was assumed.

According to the equation for the restrain current, IH for throughfaults ( = 0) becomes : I II H
1 2 and for internal faults a) fed from one side (I2 = 0) : IH = 0 b) fed from both sides ( = 180°) : IH = 0 High through-fault currents can cause c.t. saturation and for this reason, the gradient of the characteristic is switched to infinity for IH/IN > b. When measuring the operating characteristic, it should be noted that the gradient of the characteristic is only switched to infinity, if apart from IH, I1 and I2 are also higher than b.

Increased basic setting g-High 

The increased basic setting g-High has been provided as a means of preventing false tripping under particular operating conditions. It is activated by an external signal. Situations occur during normal system operation which cause a higher differential current, e.g.
increased magnetising current as a consequence of a higher system voltage (switching operations, following load shedding, generator regulator faults etc.)
large variation of current ratio (tap-changer at one end of its range) Providing special conditions of this kind are detected by a voltage relay or a saturation relay, the corresponding signal can be used to switch the differential function from "g" to "g-High". The recommended setting is g-High = 0.75 IN. The reset ratio following a trip remains unchanged at 0.8 g

Operating characteristic of the transformer differential protection for high through-fault currents This characteristic, however, would scarcely be able to detect faults in the protected zone at through currents as low as the load current. Therefore if one of the windings is conducting a current which is less than the setting of "b", i.e.

Operating characteristic of the transformer differential protection for low through-fault currents This characteristic provides high sensitivity for the detection of faults in the protection zone. Example: Internal fault and rated current flowing through the transformer: