I Hazards of Multi-point Grounding in CT and PT Secondary Circuits

Multi-point grounding of the neutral (N) wire in secondary circuits refers to the presence of two or more grounding points connected to the substation grounding grid on the neutral wire of the secondary circuits of voltage and current transformer windings with three-phase four-wire wiring, as well as more than one grounding point on the neutral wire of the voltage secondary circuit from the transformer outdoor terminal box to the electric energy meter.

In power systems, secondary circuits play a crucial role in ensuring the safe operation of the system. Under normal system operation, to ensure personal and equipment safety, the Safety Regulations for Electric Power Operation Sites stipulates that one reliable grounding point must be provided for one electrical connection of a transformer secondary circuit. Meanwhile, to ensure the correct operation of relay protection and automatic devices, single-point grounding of the circuit is required. However, the secondary circuits of substations are connected to a large number of devices and extend over a wide range. Multi-point grounding often occurs in the secondary circuit of a single electrical connection due to human wiring errors or inevitable natural factors such as insulation aging. Moreover, most of the system's secondary circuits are outdoors, leading to a high probability of insulation damage. Multi-point grounding can cause incorrect operation of protection devices and frequent large-scale power outages.

 

1. The metering winding is grounded at the outdoor terminal box, and its metering secondary circuit is connected to the control room via a 4-core cable. Additional grounding points exist in the voltage circuit at the measurement and control unit or electric energy meter panel in the control room, thus forming multi-point grounding.

 

2. The N wire of the voltage transformer is directly connected to the zero-phase small bus N600 in the switchyard, and the N-phase voltage of the electric energy meter is also directly taken from the control room.

 

3. The windings of a multi-winding voltage transformer share a neutral wire (N wire) at the PT outdoor terminal box, while each winding is grounded at its respective control room. The windings form multi-point grounding through the common neutral wire at the terminal box.

II Analysis of Multi-point Grounding in Secondary Circuits

 

Unbalanced three-phase PT secondary voltage drop with positive and negative ratio and angular errors

When the secondary circuit of the PT metering winding has a single grounding point only, the ratio errors of the three-phase PT secondary voltage drop are usually negative and basically equal in magnitude; even if a positive ratio error occurs, its value is close to zero. However, in the case of multi-point grounding, the ratio and angular errors of the three-phase PT secondary voltage drop will be both positive and negative, and the three-phase voltage drop values are unbalanced (usually with a large difference in values).

Large zero-sequence current on the N wire at the PT terminal

A relatively large zero-sequence current will appear on the N wire at the PT outdoor terminal box when multi-point grounding exists in the PT metering secondary circuit.

 

The grounding grid of any switchyard is not an ideal equipotential surface. If the voltage transformer secondary circuit is not connected to the grounding grid at a single point, ground current will be generated during operation due to the inconsistent potentials between different grounding points. In the case of multi-point grounding, if a grounding point exists on the electric energy meter side, the voltage of each phase of the electric energy meter will be superimposed with a ground voltage on the basis of the normal voltage, resulting in changes in the voltage amplitude and phase angle at the electric energy meter. At this time, the measured PT secondary circuit voltage drop will show a large difference between the three-phase voltage drops, and the ratio and angular errors of the three-phase voltage drops will be both positive and negative.

Multi-point grounding in the PT secondary circuit will destroy the original three-phase symmetry. In this case, the secondary voltage drop results will inevitably show phenomena such as positive ratio error for one phase, negative ratio error for another phase, positive angular error for one phase, and negative angular error for another phase. When the potential of the grounding point in the control room fluctuates, the phase and amplitude of the negative-sequence and zero-sequence voltages will change, and the finally displayed phase and amplitude of the three-phase voltage will change accordingly. The performance in the secondary voltage drop test results is the occurrence of both positive and negative ratio and angular errors.

III Residual Current Test Method

The Residual Current and Voltage-Current Phase Tester uses the current shunting method to judge and locate multi-point ground faults in the tested system, and can detect various types of multi-point ground faults. It can accurately detect faults regardless of the presence of AC current, and is suitable for residual current measurement in single-phase 2-wire/three-phase 3-wire/three-phase 4-wire AC circuits.

△ Residual Current and Voltage-Current Phase Tester

Operation Method

First turn off the constant current source and connect each part between the grounding points.

Turn on the constant current source and adjust its output current to vary between 0 and 0.2A, then record the current I1 of the clamp ammeter and the current I2 of the ammeter.

As shown in the figure above: I1 = I2 + I3

If I1 is consistently less than I2 during the test, then I3 ≠ 0, and it can be determined that two or more grounding points exist in the substation.

If I1 remains almost equal to I2 in the test results, then I3 = 0, and it can be determined that the substation has a single grounding point.

Functional Features

Multi-point ground fault analysis: Judge whether a multi-point ground fault exists in the secondary circuit grounding system by analyzing the current of other grounding loops.

Current display: Display the signal current value, the shunt signal value of the grounding wire, and the shunt current value of other grounding loops.

Waveform display: Display the signal current waveform of the tested loop when detecting the tested grounding wire.

Direction display: Display the direction of the multi-point ground fault point if the device detects such a fault in the tested loop.

High-precision ammeter: Can be equipped with different types of AC/DC clamp meters to achieve high-precision AC/DC clamp meter functions.

The Residual Current and Voltage-Current Phase Tester has no impact on the system during use. The applied DC current signal acts on the grounding wire, with a maximum signal of only 150mA, generating an almost zero pressure difference on the tester and having no effect on PT and CT measurements. The adopted DC current detection method can effectively filter the power frequency current signal in the tested system, with strong anti-interference ability, enabling faster and more effective detection.