4　Power Transformer Protection

4.0.1　Power transformers rated 3-110kV，63MVA and below shall be equipped with corresponding protections to cope with the following faults and abnormal operating modes:

1　Single phase-to-phase short-circuit fault between windings and their outgoing leads，and single phase-to-earth short-circuit of systems with neutral solidly grounded or via low resistance；

2　Inter-turn short-circuit of windings；

3　Overcurrent caused by external phase-to-phase short-circuit；

4　Overcurrent and neutral point overvoltage of power grids with neutral points grounded solidly or via low resistance caused by external grounding short-circuit；

5　Overload；

6　Oil level decrease；and

7　High oil temperature，high winding temperature，high oil tank pressure，gas generation or cooling'system fault of transformers.

4.0.2　The 0.4MVA and above workshop oil-immersed transformers，0.8MVA and above oil-immersed transformers and oil-filled tap changers of on-load tap changing transformers shall all be installed with buchholtz protection，which shall operate instantaneously to cause signaling in case of light gas generation or oil level drop in oil tank due to faults，and shall operate to trip the circuit breakers at various sides of transformers in case of excessive gas generation.

Buchholtz protections shall be provided with provisions to cope with malfunction caused by vibration and lead failure of gas relays.

Where there is no circuit breaker or short-circuit switch at the power supply side of a transformer，the protections shall operate to cause signaling and issue remote trip command to open the circuit breaker at the opposite side of the line.

4.0.3　The following protections shall be provided as main protections to cope with short-circuit faults of transformer outgoing leads，bushings and internal transformers，which shall operate instantaneously to open the circuit breakers at various sides of the transformer and be in compliance with the following regulations:

1　Transformers rated 10kV and below with a capacity of 10MVA and below that are operated independently shall be provided with instantaneous current protection.

2　Transformers rated 10kV and above with a capacity of 10MVA and above that are operated independently and transformers with a capacity of 6.3MVA and above that operate in parallel shall be provided with longitudinal differential protection.

3　Longitudinal differential protection may be installed for important transformers that are operated independently with a capacity below 10MVA.

4　Important transformers rated 10kV or transformers having a capacity of 2MVA and above should be provided longitudinal differential protection if the sensitivity of instantaneous current protection does not comply with the relevant requirements.

5　Transformers having a capacity of 0.4MVA and above，a primary voltage of 10kV and below and delta-wye connected windings may be provided instantaneous current protection configured in two-phase three-relay connection.

4.0.4　Longitudinal differential protections of transformers shall comply with the following requirements:

1　They shall not operate in case of the excitation inrush current and the unbalance current caused by external short-circuit.

2　They shall be able to identify open line fault of current circuits，and operate to initiate alarm or trip selectively.

3　The reach of longitudinal differential protection shall cover transformer bushings and outgoing leads.Where outgoing leads are not covered，auxiliary measures shall be taken for fast fault clearing.However，in 63kV or 110kV terminal substations，branch substations，or substations with bypass busbars where transformer circuit breakers are out of service and substituted by bypass circuit breakers，longitudinal differential protections may temporarily resort to the current transformers within transformer bushings，in which case bushing and lead faults may be cleared by backup protections；if required by safe and stable operation of power grids，the longitudinal differential protections shall be transferred to the current transformers of the bypass circuit breakers.

4.0.5　The following protections shall be used as back-up protections to deal with transformer overcurrent caused by external phase-to-phase short circuit，which shall operate to open relevant circuit breakers with a time delay and be in compliance with the following regulations:

1　Overcurrent protection should be provided for step-down transformers.

2　Complex voltage initiated overcurrent protection or low voltage-blocked overcurrent protection'should be used for step-up transformers，system interconnecting transformers，or step-down transformers for which the overcurrent protection fails to meet the sensitivity requirements.

4.0.6　External phase-to-phase short-circuit protection shall comply with he following regulations:

1　For single-supply double-winding or three-winding transformers，phase-to-phase short-circuit protection should be installed at each side，which may be provided with one-zone time delay at the supply'side and two-zone or three-zone time delay at other sides.

2　For double-or triple-supply double-winding or three-winding transformers，the phase-to-phase'short-circuit protection shall be equipped with directional elements according to the selectivity requirements，which should direct to the busbar at the same side.However，the backup protection used to open the circuit breakers at various sides of the transformer shall not be directional.

3　Step-down transformers that have branches at the low voltage side and are connected to different busbar sections operating independently shall be equipped with phase-to-phase short-circuit backup protection at each branch.

4　Where there is no special busbar protection at the LV side of the transformer and the sensitivity of phase-to-phase backup protection at the HV side is insufficient to deal with the phase-to-phase shortcircuit on the LV side busbar，phase-to-phase short-circuit backup protection shall be provided at the LV'side.

4.0.7　The external phase-to-phase short-circuit protection of three-winding transformers may be'simplified according to the following rules:

1　Protections at various sides other than the main power supply side may be used as backup protections only for the neighboring electric power equipment and circuits at the same side.

2　Where a protection is used as backup protection for the neighboring power equipment and circuits at the same side，its sensitivity may be appropriately lowered，provided that it can meet the sensitivity requirements for various short-circuits of the busbar at this side.

4.0.8　In 110kV power grids with neutral points grounded solidly，the transformers with power supply at the LV side that operate with the neutral point grounded solidly shall be equipped with zero sequence current protection to cope with the overcurrent caused by external single phase-to-earth fault，which shall comply with the following regulations:

1　The protection may consist of two zones whose operating currents shall be coordinated with that of the zero sequence overcurrent protection of relevant circuits.Each protection zone shall be provided with two time delays，the short one for minimizing the area exposed to fault or opening the circuit breaker at the faulty side，and the long one for opening the circuit breakers at other sides of the transformer.

2　The zero sequence current protection of double-winding and three-winding transformers shall be connected to the current transformers on the outgoing leads of the neutral point.

4.0.9　In 110kV power grids with neutral points grounded solidly，transformers with power supplies connected at the LV side that may operate with the neutral point being either grounded or not grounded'shall be provided with backup protection to deal with overcurrent caused by external single phase-to-earth fault and voltage rise due to loss of neutral point grounding，which shall comply with the following regulations:

1　Uniform insulation transformers:the zero sequence protection shall consist of zero sequence current protection in accordance with 4.0.8 of this code and zero sequence overvoltage protection shall be added.When the power grid to which the transformer is connected selects to disconnect the grounding connection of the transformer at the neutral point，the zero sequence overvoltage protection shall operate to open the circuit breakers at various sides of the transformer after a time delay of 0.3-0.5s.

2　Graded insulation transformers:a discharge gap shall be established at the neutral point of the transformer for the zero sequence protection.Two sets of zero sequence overcurrent protections shall be installed，one corresponding to the case where the transformer neutral point is grounded solidly and the other to the case where the transformer neutral point is grounded via the discharge gap，and zero sequence overvoltage protection shall be provided.Transformers with neutral point grounded solidly shall be provided with zero sequence current protection in accordance with 4.0.8 of this code；and transformers with neutral point grounded via discharge gap shall be provided with zero sequence current protection and zero'sequence overvoltage protection that respond to clearance discharge.In the event that the power grid to which the transformer is connected loses neutral point grounding and suffers single phase-to-earth fault，the zero sequence current/overvoltage protection shall operate to open the circuit breakers at various sides of the transformer with a time delay of 0.3-0.5s.

4.0.10　Where the neutral point at the LV side of a transformer is grounded via low resistance，threephase overcurrent protection shall be provided at the LV side；and zero sequence overcurrent protection with two time delays shall also be provided at this side，which should be connected to the zero sequence current transformers of the neutral point circuit at the LV side.

4.0.11　Special grounding transformers shall be equipped with main protection in accordance with 4.0.3 of this code and shall be provided with overcurrent protection and zero sequence overcurrent protection as backup protections.

4.0.12　Where transformer neutral point is grounded via arc suppressing coils，zero sequence overcurrent protection or zero sequence overvoltage protection shall be provided at the neutral point，which shall operate to cause signaling.

4.0.13　For transformers having a capacity of 0.4MVA and above，wye-wye connected windings and LV'side neutral point grounded solidly，the following protections shall be selected accordingly to cope with LV'side single phase-to-earth short circuit，which shall operate to cause trip with a time delay.

1　Where the HV side over-current protection is used，the protection should be of three-phase type.

2　To install zero sequence current protection on the neutral line at the LV side.

3　To install three-phase overcurrent protection at the LV side.

4.0.14　Transformers having a capacity of 0.4MVA and above，primary voltage of 10kV and below，deltawye connected windings and LV side neutral point grounded solidly shall make use of HV side overcurrent protections to cope with single phase-to-earth short circuit at the LV side，which shall operate to cause trip with a time delay provided that the sensitivity requirements are met；otherwise，relevant protection shall be provided in accordance with Item 2 and 3 in 4.0.13 of this code，which shall operate to cause trip with a time delay.

4.0.15　Transformers having a capacity of 0.4MVA and above that operate in parallel or transformers operating independently that serve as backup power suppliers for other loads shall be equipped with overload protection.Protections of multiple-winding transformers shall be able to response to overloads at various sides of the transformer.The overload protections shall operate to cause signaling with a time delay.

In substations not attended frequently，the overload protections may operate to cause tripping or disconnect partial loads.

4.0.16　Transformers shall be equipped with protections to cope with high oil temperature，high winding temperature，low oil level，high oil tank pressure，gas generation and cooling system fault，which shall operate to initiate signals or trip.