Some transformer neutral point is directly grounded,and some transformer neutral point is grounded through the gap.The protection of transformer neutral point is mainly to reflect the grounding fault.

Case 1:When a grounding fault occurs in the system,the neutral earthed transformer should be equipped with zero-sequence current protection,which can be composed of two sections,each with two time limits.The short time limit action is to disconnect the busbar or subsection circuit breaker to reduce the scope of the fault,and the long time limit action is to disconnect the circuit breaker on each side of the transformer.

Case 2:When a grounding fault occurs in the system,after the neutral earthed transformer is switched off,the zero sequence voltage rise or resonant overvoltage of the power grid will endanger the neutral insulation of the transformer that is not grounded.Therefore,neutral point ungrounded transformer should be installed with zero sequence voltage protection or gap zero sequence current protection.

(1)Graded insulated transformer

Large transformer is the core equipment of power production,because of its high cost,so in the 110 kV and above neutral point directly grounded power grid,the transformer with graded insulation is mostly used.

In actual operation,some of the neutral points of transformers are directly grounded.But some of the neutral points of the transformer are not grounded.The so-called graded insulation,is the transformer coil near the neutral part of the main insulation,the insulation level is lower than the insulation level of the coil end.

Problems should be paid attention to in the operation of graded insulated transformers:

1,the neutral point of graded insulation transformer must be equipped with arrester and prevent overvoltage gap;

2,if conditions permit,operation mode permits,graded insulated transformer must be grounded operation;

3,if the neutral point of graded insulated transformer is not grounded,the neutral overvoltage protection must be reliable input.

(2)key principles of transformer neutral point configuration

1,in the operation of the double bus,it should be considered when the bus switch trip,ensure that the two separate systems should have at least one transformer neutral point grounding;

2,transformer,low voltage side of the power supply,the neutral point of the transformer shall be directly grounded to prevent the trip of the high voltage side circuit breaker,the transformer becomes the neutral insulation system;

3.The neutral point of the main transformer of the generator-transformer-line group should be kept grounded.

(three)three forms of transformer neutral overvoltage

1.Atmospheric overvoltage

2.Overvoltage caused by single-phase grounding fault

3.Over-voltage caused by incomplete phase switching of circuit breaker(mainly manifested as non-synchronous reclosing,incomplete phase action and wire disconnection of circuit breaker in power grid)

(four)three ways of transformer neutral point clearance protection

Gap,arrester and arrester combined discharge gap can be used in three ways.

The transformer adopts the protection mode of arrester combined with discharge gap.

The discharge gap adopts rod-rod gap,and the arrester is mostly configured as zinc oxide arrester.

The protection division of the parallel gap of the lightning arrester is that the power frequency and operating overvoltage are borne by the gap,while the lightning and transient overvoltage are borne by the lightning arrester.At the same time,the gap is used to limit the power frequency overvoltage and residual voltage that may appear on the lightning arrester with too high amplitude.This way not only protects the neutral point of the transformer,but also plays a role of mutual protection.

(5)transformer neutral point gap overcurrent protection

To prevent overvoltage damage to the transformer neutral point insulation,for the main transformer neutral point is generally installed discharge gap measures,and the use of current transformer neutral point casing or installed in the discharge gap loop independent current transformer,a transformer neutral point discharge gap zero sequence current protection(hereinafter referred to as"clearance over-current protection)"

The principle of transformer neutral gap protection is shown in the figure below.

When is the voltage side neutral discharge gap protection put in,when to quit,why?

Should exit before the neutral point grounding switch is closed,and put in after the neutral point grounding switch is opened.(i.e.discharge gap protection and neutral point cannot be in the input state at the same time)

Because:discharge gap protection is composed of zero-sequence voltage and zero-sequence current in parallel,and the current setting is sensitive,the time is short,and there is no relationship with other protection.In the state of direct grounding,if there is an external fault,there will be zero sequence current flowing in the neutral point CT,which will cause the misoperation of clearance overcurrent protection.

In the clearance grounding state,in the occurrence of grounding fault,after other grounding changes jump,the neutral point zero sequence voltage will rise,so that the clearance zero sequence voltage protection action.After gap breakdown,zero sequence current action,protection ungrounded transformer.