Source: Electricity Forum
Some transformer neutral points are directly grounded, and some transformer neutral points are grounded through gaps, and the protection of transformer neutral points mainly reflects grounding faults.
Situation 1: When the system has a grounding fault, the transformer grounded at the neutral point should be equipped with zero sequence current protection, which can be composed of two sections, each with two time limits, the short-term action is used to disconnect the bus connection or the segmented circuit breaker, so as to reduce the scope of fault influence, and the long-term action is to disconnect the circuit breaker on each side of the transformer.
Situation 2: When a grounding fault occurs in the system, after the transformer grounded at the neutral point jumps away, the zero sequence voltage of the power grid rises or the resonant overvoltage will endanger the neutral insulation of the transformer that is not grounded at the neutral point. Therefore, transformers with non-grounded neutral points should be equipped with zero-sequence voltage protection or gap zero-sequence current protection.
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Graded insulated transformers
Large transformers are the core equipment of power production, and due to their high cost, they are mostly used in power grids with direct grounding at the neutral point of 110 kV and above.
In actual operation, the neutral point of some transformers is directly grounded. However, there are also some transformers that operate without grounding at the neutral point. The so-called graded insulation is the main insulation of the part of the coil of the transformer near the neutral point, and its insulation level is lower than that of the end of the coil.
Problems that should be paid attention to in the operation of graded insulated transformers:
1. The neutral point of the graded insulated transformer must be equipped with a lightning arrester to prevent overvoltage clearance;
2. If the conditions are permitted, the operation mode is permitted, and the graded insulated transformer must be grounded at the neutral point;
3. If the neutral point of the graded insulated transformer is not grounded, the neutral point overvoltage protection must be reliably invested.
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The key principle of transformer neutral point configuration
1. When the double bus is running, it should be considered that when the bus tie switch trips, it should be ensured that at least one transformer neutral point of the separated system should be grounded;
2. When there is power supply on the medium and low voltage side of the transformer, the neutral point of the transformer must be directly grounded to prevent the circuit breaker on the high voltage side from tripping, and the transformer becomes a neutral insulation system;
3. The neutral point of the main transformer of the generator-transformer-line group should be kept grounded.
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There are three forms of neutral point overvoltage in transformers
1. Atmospheric overvoltage
2. Overvoltage caused by single-phase grounding fault
3. Overvoltage caused by non-full-phase opening and closing of circuit breaker (mainly manifested as non-synchronous reclosing, non-full-phase action, wire disconnection, etc.) of the circuit breaker of the power grid)
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Three ways of neutral point gap protection in transformers
There are three ways to use clearance, lightning arrester and lightning arrester combined discharge gap.
The transformer mostly adopts the protection mode of lightning arrester combined discharge gap.
The discharge gap adopts rod-rod clearance, and the arrester is mostly configured as a zinc oxide arrester.
The protection division of the parallel gap of the arrester is that the power frequency and operating overvoltage are borne by the gap, and the lightning and transient overvoltage are borne by the arrester, and at the same time, the gap is used to limit the power frequency overvoltage and excessive residual voltage of excessive amplitude that may occur on the arrester. This method not only protects the neutral point of the transformer, but also plays the role of mutual protection.
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Transformer neutral point gap overcurrent protection
In order to prevent overvoltage from damaging the neutral point insulation of the transformer, the measures of installing a discharge gap are generally adopted for the neutral point of the main transformer, and the neutral point bushing current transformer or an independent current transformer is installed in the discharge gap circuit to constitute the zero sequence overcurrent protection of the neutral discharge gap of the transformer (referred to as "gap overcurrent" protection)
The principle of neutral point gap protection of transformer is shown in the figure below.
When is the neutral discharge gap protection on the high voltage side of the transformer put in, when is it withdrawn, and why?
It should be withdrawn before the neutral point grounding switch is closed, and put into it after the neutral point grounding switch is opened. (i.e., the gap discharge protection and the neutral point cannot be in the input state at the same time)
Because: the discharge gap protection is composed of zero sequence voltage and zero sequence current in parallel, and the current value is relatively sensitive, the time is short, and there is no relationship with other protection. In the case of direct grounding, if an external fault is encountered, there will be zero sequence current flowing through the neutral point CT, which will cause the gap overcurrent protection to malfunction.
When passing through the gap grounding state, when a grounding fault occurs, the neutral zero sequence voltage will increase after the other grounding transformers jump away, so that the gap zero sequence voltage protection action. After the gap breakdown, the zero sequence current acts to protect the safety of the ungrounded transformer.
There are three wiring methods for transformer neutral gap overcurrent protection:
(1) Wiring diagram of common current transformer
The action values of the two current relays are different, and the properties of the two ground currents are different. The zero-sequence overcurrent is mainly the power frequency quantity.
The interstitial overcurrent has the characteristics of gap and segment development, and the intermittent time and current amplitude are random, and contain a large number of harmonic components.
(2) The wiring diagram of the current transformer is independent of each other
The two sets of protected current transformers are independent of each other, that is, the AC circuit is separated and connected to each other at the correct position. This option is reasonable, but it is expensive.
(3) Schematic diagram of integrated wiring
In order to reduce the cost, the neutral point CT of the main transformer is used for the zero sequence overcurrent, and the integrated wiring of a separate CT is used for the gap overcurrent.
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Transformer zero sequence voltage protection
The neutral discharge gap is a relatively rough equipment, because its discharge voltage is affected by meteorological conditions, adjustment accuracy and the number of continuous discharges. For this purpose, zero-sequence voltage protection should also be installed as a backup protection in the event of a rejection of the discharge gap.