3　Generator Protection

3.0.1　For generators with a voltage of 3kV and above and capacity of 50MW and below，appropriate protections shall be provided to deal with the following faults and abnormal operations:

1　Short-circuit between phases of stator windings；

2　Grounding of stator windings；

3　Inter-turn short-circuit of stator windings；

4　External short-circuit of generators；

5　Overload of stator windings；

6　Overvoltage of stator windings；

7　Overload（negative sequence）of rotor surface layer；

8　Grounding of excitation circuits；

9　Abnormal drop or loss of excitation current；

10　Reverse power.

3.0.2　Operations output by protections may include:

1　Shutdown:to disconnect generator（or generator-transformer unit）circuit breakers and de-excite，shut off main valves of steam turbine generators or guide vanes of water turbine generators.

2　Disconnection and de-excitation:to disconnect generator（or generator-transformer unit）circuit breakers and de-excite，and shed loads of steam（water）turbines.

3　Disconnection:to disconnect generator（or generator-transformer unit）circuit breakers.

4　Minimization of the area affected by faults.

5　Signal:to give out audio and video signals.

3.0.3　Appropriate protections serving as the main protections of generators shall be provided to prevent'single phase-to-phase short-circuit faults of stator windings and leads.These protections shall operate to cause shutdown while satisfying the following regulations:

1　For 1MW and below generators operating independently，overcurrent protection shall be provided at the neutral point side if leads are led out therefrom；if no leads are available at the neutral point side，low-voltage protection shall be provided at the generator terminals.

2　For 1MW and below generators that operate in parallel with other generators or power systems，instantaneous overcurrent protection shall be provided at the generator terminals.Where the sensitivity of instantaneous current protection is not up to the requirements，longitudinal differential protection may be provided；for generators without leads at the neutral point side，low-voltage blocking overcurrent protection may be provided.

3　Generators rated above 1MW shall be provided with longitudinal differential protection.For generator-transformer units with circuit breaker installed between the generator and the transformer，longitudinal differential protection shall be respectively provided for the generator and the transformer；if no circuit breaker is installed between the generator and the transformer，common longitudinal differential protection may be used for generator-transformer units.

3.0.4　Grounding protections for generator stators shall comply with the following regulations:

1　The allowable single phase-to-earth fault current of generator stator windings shall be determined as specified by manufacturers.If no manufacturer specifications are available，Table 3.0.4 may be followed.

2　For generators directly connected to busbars，if the single phase-to-earth fault current（not allowing for the compensation of arc-suppression coil）of stator windings is greater than the allowable value，selective grounding protections shall be provided，which shall operate to cause signaling.However，when the arc-suppression coil is removed out of service or other reasons cause the said fault current to be greater than the allowable value，the protections shall operate to cause shutdown.

The protections shall be connected to the zero-sequence current transformers at the generator terminals.Their settings shall be configured to be higher than the unbalanced current and the generator'steady-state capacitive current resulted from external single phase-to-earth fault，and external short-circuit blocking functions should be provided.

Where there is no grounding protection，grounding monitoring devices shall be installed on the generator voltage busbars，which shall operate to cause signaling.

The protections or grounding monitoring devices shall be capable of monitoring zero-sequence voltage of generators.

3　Generator-transformer units shall be equipped with stator grounding protections with a protected area not less than 90%.The protections shall operate to cause signaling with a time delay，and may operate to cause shutdown depending on the system conditions and generator insulation as well.

The protections shall be capable of monitoring zero-sequence voltage of generators.

3.0.5　The inter-turn short-circuit protection of generator stators shall comply with the following regulations:

1　For generators that employ star connection mode for stator windings，have parallel branches per phase and have outlet terminals of branches at the neutral point，zero-sequence current transverse differential protection or split-phase transverse differential protection shall be equipped.

The transverse differential protection or split-phase transverse differential protection shall instantaneously operate to cause shutdown.Upon occurrence of one point grounding of turbo-generator excitation circuits，it may be transferred to operate to cause shutdown with a short time delay.

2　For generators rated 50MW with star-connected stator windings and only three outlet terminals at the neutral point，inter-turn short-circuit protection may be provided as well.

The inter-turn short-circuit protection shall instantaneously operate to cause shutdown.

3.0.6　Generators shall be provided with protections against external phase-to-phase short-circuit，which'serve as the backup of main protections and the following regulations shall be observed:

1　Overcurrent protections shall be provided for 1MW and below generators which operate in parallel with other generators or power systems.The protections should be configured at the neutral point side of the generators，and the operation currents shall be set to be clear of the maximum load current；for generators without leads at the neutral point，the protections shall be configured at the generator outlets.

2　Low-voltage or composite voltage actuated overcurrent protections should be provided for generators rated above 1MW.The operation current of the current elements may be 1.3-1.4 times the generator ratings；and the operation voltage of the low-voltage elements may be 0.6 times the voltage ratings for steam turbine generators and 0.7 times the voltage ratings for water turbine generators.The operation voltage of the negative sequence voltage elements may be 0.06-0.12 times the voltage ratings.

3　Generators rated 50MW may be equipped with negative sequence overcurrent protection and lowvoltage actuated overcurrent protection.The operation current of the negative sequence current elements may be 0.5-0.6 times of the generator current ratings；the operation current of the current elements and the operation voltages of the low-voltage elements may be determined according to Item 2 above.

4　For generator-transformer units without circuit breakers installed between the generator and the transformer，the generator protections responding to external short-circuit shall be used as backup protections，and no additional protections shall be provided at the transformer low-voltage side；if a circuit breaker is installed between the generator and the transformer，transformer backup protections may be configured according to Section 4.0.5 of this code.Separate protections shall be equipped on auxiliary branches.

5　Low-voltage overcurrent protection with current holding features should be provided for selfexcited shunt-wound generator.

6　Generator backup protections should be provided with two-stage time delay.

3.0.7　Overload protections shall be provided to deal with overload of generator stator windings.The protections should operate to cause signaling with a time delay.

3.0.8　Overvoltage protections to deal with overvoltage of stator windings shall be provided for water turbine generators.The operation voltage may be 1.3-1.5 times of the voltage ratings and the time delay may be 0.5s.The overvoltage protections should operate to cause disconnection and de-excitation.

3.0.9　Generators rated 50MW and with the value A higher than 10 shall be provided with definite time delay negative sequence overload protections to deal with overload of rotor surface layer resulted from unsymmetrical load，open phase operation and unsymmetrical short circuit.The protections shall have operation currents set based on continuously allowable negative sequence currents of generators and higher than unbalanced currents of negative sequence current filters under the maximum load，and shall operate to cause signaling with a time delay.

3.0.10　Grounding protections or grounding detection devices shall be provided to deal with grounding faults of generator excitation circuits，which shall be in accordance with the following regulations:

1　Periodic detection devices intended for detecting one-point grounding faults should be provided for water turbine generators rated 1MW and below；one-point grounding protections shall be provided for water turbine generators rated above 1MW，and shall operate to cause signaling with a time delay，and if conditions permit，to cause shutdown.

2　Grounding detection devices that deals with one-point grounding faults shall be provided for steam turbine generators.The devices may be provided with two-stage settings and should resort to continuous detection.

3.0.11　Loss-of-excitation protection that deals with abnormal drop or complete loss of excitation current'shall be provided，which shall be in compliance with the following regulations:

1　Special loss-of-excitation protection shall be provided for generators which are not allowed to operate under loss-of-excitation conditions or those whose loss-of-excitation will cause significant impact on'systems.

2　Loss-of-excitation protections for steam turbine generators should instantaneously operate to cause'signaling，or to cause disconnection of generators with a time delay if the generator voltage drops to below the allowable value thereafter.

3　Loss-of-excitation protection of water turbine generators should operate to cause disconnection of generators with a time delay.

3.0.12　Reverse power protections shall be provided for gas turbine generators.The protections should operate to cause signaling with a time delay and further to cause disconnection of generators with a time delay.

3.0.13　Excitation transformers of self-excited shunt-wound generators should be provided with instantaneous current protection as main protection and overcurrent protection as backup protection.