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Common problems of relay protection?

time:2020/9/18   source:华天电力  reading:342 time

Why should one-point grounding and two-point grounding protection be installed in the excitation circuit?


The excitation circuit of the generator is grounded at one point, although it will not form a fault current path, which will cause direct harm to the generator, but the possibility of the second point of grounding must be considered, so the signal is sent by the one-point grounding protection to strengthen inspection and monitoring.


Protection Relay Test Set.png


When a two-point ground fault occurs in the excitation circuit of the generator, the generator rotor body will be burned due to the considerable fault current flowing through the fault point. As part of the winding is short-circuited, the current in the field winding increases, which may be burned due to overheating. The winding is short-circuited, causing the air gap magnetic flux to lose balance, causing the unit to vibrate. The steam turbine may also magnetize the shafting and steam turbine.


Therefore, after one point is grounded, two-point grounding protection must be used, so that when two points are grounded, the operation will stop after delay.


Why install negative sequence current protection?


When an asymmetric short circuit or a three-phase asymmetric operation occurs in the power system, there is a negative sequence current in the generator stator winding. This current generates a reverse magnetic field in the generator air gap, which is twice the synchronous speed of the rotor. Therefore, a frequency-doubled current appears in the rotor components. This current causes some parts of the rotor with a high current density to cause partial burns of the rotor. When the guard ring is severely heated, the guard ring may be loosened and cause major damage to the generator; in addition, the 100Hz alternating electromagnetic Torque will act on the rotor shaft and stator base, causing a vibration with a frequency of 100 Hz. In order to prevent the occurrence of the above-mentioned damage to the generator, negative sequence current protection must be installed.


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