Voltage transformer B phase grounded meaning - Database & Sql Blog Articles

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Argument 1: The power system needs to use the secondary voltage of PT to realize the parallel operation of the system at the same time. The first condition of juxtaposition is that the phase sequence of each system must be consistent. To do this, we must first find a common conductor (earth) as the middle phase (B phase) in each of the power systems. Since one phase has been determined (ie, B is the common phase), the other two phases can be determined using the phase sequence table. . There are two ways to ground the PT: 1. The neutral point is directly grounded; 2. The B phase is directly grounded, and then the neutral point is broken through the fuse ground. In fact, in the power plant, the factory PT is generally applied to the first grounding method described above. The phase B grounding is only for the synchronizing device. The synchronizing device (including the synchronous verification relay and the synchronizing table) is connected to both sides. The voltage of the PT is compared to the phase difference, and the two voltages must have a common point to be accurately compared. In addition, simple wiring is also a by-product. Isn't A phase or C phase grounded? In principle, it can be, but it is customary for everyone to be grounded in phase B. Why bother to change the mind, causing unnecessary troubles for installation, commissioning and maintenance? However, for the VV wiring PT (many PT for the same period is VV wiring) must be the intermediate phase grounding, otherwise the protection against high voltage breakdown is too indirect, and the intermediate phase is often B phase. Why is the neutral point broken through the insurance ground? B phase grounding, for the A, C phase, whether it is to prevent high voltage breakdown or lightning protection, they are too far from the ground, so the neutral point plus breakdown insurance to make up.

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Statement 2:

1. The secondary side of the mutual element is grounded to protect the safety of the person and equipment, because if the insulation damage causes the high voltage to enter the low voltage, it may be dangerous to the workers who may be on the secondary circuit, and the insulation level of the secondary circuit. Low, if there is no grounding point, it is easy to penetrate.

Second, the PT for the transformer is generally grounded at the neutral point, and the PT for the generator is generally grounded with the B phase. This is because:

1. Customary problems. In some places, in order to save the PT quantity, the VN.V wiring is used. In order to ensure the safety of the secondary side, there should be a grounding point, and it is customary to always put one A phase of the first side of the two coils according to the C phase, and The common end is in phase B, so the corresponding secondary common point is phase B (in theory, which one is grounded, as long as it corresponds)

2. Simplify the synchronization system. This is mainly for the Y wiring PT. The country may have two PTs, Y wiring and V wiring, in a power plant. If the Y wiring PT is grounded in phase B in the system, the V wiring can be made. Both the Y and Y wirings can be used in the simultaneous system, so that the B-phase grounding of the Y-wire PT can have the following benefits:

A, can save the isolation transformer. B. Simplify the synchronization line to save related equipment.

For power plants, in order to meet different requirements, the secondary side of the voltage transformer has both neutral grounding and phase B grounding. When the two types of grounding voltage transformers are used in the synchronous system, an isolation transformer is generally used to solve the problem of the B-phase coil of the voltage transformer which may be burnt out due to different grounding methods.

3, B phase installed secondary insurance and grounding should be placed behind it, this is to prevent the B phase grounding and a neutral point grounding to form a loop when the PT is broken between the first and second times, so that the B phase secondary coil is shorted. Even burned.

4. Where the secondary side of the PT secondary grounded with B phase is equipped with a breakdown insurance, this is considered in the case of the B phase secondary fuse blowing, even if the high voltage is broken into the low pressure, it can penetrate the breakdown insurance. Make the PT secondary with protective grounding (the breakdown voltage is about 500V).

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Statement three:

First of all, the secondary side of the PT must have a reliable grounding point. At present, the domestic grounding methods are B-phase grounding and neutral point grounding.

Second, the B-phase grounding connection is used in 35KV and below neutral point non-direct grounding systems. In these systems, distance protection is generally not installed. Phase B grounding has less impact on protection and is caused by one system. In the case of a single-phase earth fault, the phase voltage varies with it, and the line voltage triangle does not change. Therefore, the synchronous system cannot use the phase voltage, but must use the line voltage. In order to simplify the secondary circuit, for voltage transformers of 35KV and below, the secondary winding is generally grounded in phase B.

Third, the secondary winding of the secondary winding of the transformer is used for the direct grounding system of neutral point of 110KV and above. Because these systems are generally equipped with distance protection and zero-sequence direction protection, the secondary winding of the voltage transformer is used. The grounding of the point is superior to the protection.

In summary, the use of phase B grounding is not a matter of habit, but it is necessary.

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N-phase grounding is generally used for direct grounding systems with neutral points above 35kV. Reason: Distance protection is required. The distance protection requires zero-phase grounding for the secondary circuit of the voltage transformer, because the neutral line locking device needs to have a neutral line.

The Main Points of the Ministry of Power Industry issued in January 1994, "Power System Relay Protection and Safety Automatic Devices Anti-accident Measures",

8.3 Through the control room zero-phase small bus (N600) Unicom's several sets of voltage transformer secondary circuit, only N600 should be grounded in the control room, the secondary neutral point of each voltage transformer should be disconnected at the grounding point of the switch field In order to ensure reliable grounding, the neutral line of each voltage transformer must not be connected with a switch or contactor that may be disconnected.

8.4 The secondary winding of the voltage transformer that has been grounded at the control room. If necessary, the neutral point of the secondary winding may be grounded through the discharge gap or the zinc oxide valve in the switching field. The breakdown voltage peak value should be greater than 30Imax. Volt, Imax is passed when the grid is grounded

The rms value of the maximum possible ground current of the substation in kiloamperes.