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This protection function is mostly used to protect induction and synchronous motors against phase-to-phase faults. This function requires two sets of CT’s, one at beginning of the motor feeder, and the other at the star point. Differential protection may be considered the first line of protection for internal phase to phase or phase to ground faults. In the event of such faults, the quick response of the differential element may limit the damage that may have otherwise occurred to the motor.
The differential protection function can only be used if both sides of each stator phase are brought out of the motor for external connection such that the phase current going into and out of each phase can be measured. The differential element subtracts the current coming out of each phase from the current going into each phase and compares the result or difference with the differential pickup level. If this difference is equal to or greater then the pickup level a trip will occur. GE Multilin motor protective relays support both three and six CT configurations. For three CT configuration both sides of each of the motors stator phases are being passed through a single CT. This is known as the core balance method and is the most desirable owing to it’s sensitivity and noise immunity.
If six CTs are used in a summing configuration, during motor starting, the values from the two CTs on each phase may not be equal as the CTs are not perfectly identical and asymmetrical currents may cause the CTs on each phase to have different outputs. To prevent nuisance tripping in this configuration, the differential level may have to be set less sensitive, or the differential time delay may have to be extended to ride through the problem period during motor starting. The running differential delay can then be fine tuned to an application such that it responds very fast and is sensitive to low differential current levels.
Biased Differential protection method allows for different ratios for system/line and neutral CT’s. This method has a dual slope characteristic. To prevent a maloperation caused by unbalances between CTs during external faults. CT unbalances arise as a result CT accuracy errors or CT saturation.
Ground Fault Protection
Damage to a phase conductor’s insulation and internal shorts due to moisture within the motor are common causes of ground faults. A strategy that is typically used to limit the level of the ground fault current is to connect an impedance between the neutral point of the motor and ground. This impedance can be in the form of a resistor or grounding transformer sized to ensure that the maximum ground fault current is limited to a level that will reduce the chances of damage to the motor.
There are several ways by which a ground fault can be detected. The most desirable method is to use the zero sequence CT approach, which is considered the best method of ground fault detection methods due to its sensitivity and inherent noise immunity. All phase conductors are passed through the window of a single CT referred to as a zero sequence CT. Under normal circumstances, the three phase currents will sum to zero resulting in an output of zero from the zero sequence CT’s secondary. If one of the motor’s phases were shorted to ground, the sum of the phase currents would no longer equal zero causing a current to flow in the secondary of the zero sequence CT. This current would be detected by the motor relay as a ground fault.
If the cables are too large to fit through the zero sequence CT’s window or the trench is too narrow to fit the zero sequence CT, the residual ground fault configuration can be used. This configuration is inherently less sensitive then that of the zero sequence configuration, owing to the fact that the CTs are not perfectly matched. During the motor start, the motor’s phase currents typically rise to magnitudes greater than 6 times the motors full load current. The slight mismatch of the CTs combined with the relatively large phase current magnitudes produce a false residual current, which will be seen by the relay. This current can be misinterpreted by the motor relay as a ground fault unless the ground fault element’s pickup is set high enough to disregard this error.