Zero-sequence current transformer outdoor processing scheme, principle and application

For split core current transformers installed in outdoor overhead lines or cable trenches, the cut surface of the transformer is exposed to the air and will be exposed to rain or soaked in water in the cable trench, which will cause rust and affect product performance. Water accumulation in the secondary terminal will also cause short circuit, so measures need to be taken to prevent rain or water accumulation. There are currently two solutions that can be used, which are briefly introduced as follows:

The first solution is to bury the secondary wire directly into the resin casting body (the wire uses a three-proof silicone wire) to prevent short circuits in the secondary terminal, and add a waterproof silicone ring to the cut surface for sealing and waterproofing. This solution has good protection for the secondary end and the cut surface, and can be widely used in all rain and water immersion occasions. In addition, because the base cannot be fixed in the cable trench, the base installation method is not used. The inner hole with a rubber plug is fixed on the cable. This installation method is used.

The second solution is the "open current transformer + silicone rubber sleeve" solution, which is also used by customers for installation on low-voltage outdoor lines. However, this method can only prevent light rain and cannot prevent immersion in water, so it cannot be used for installation in places where water may accumulate, such as cable trenches. This solution is limited by the specifications of the silicone rubber sleeve, and the inner diameter of the transformer can only be 50mm.

1. Principle of zero-sequence current transformer

In a three-phase four-wire system, the vector sum of the three-phase current is equal to zero, that is, Ia+Ib+Ic=0. If a current transformer is connected to the three-phase four-wire, the induced current is zero. When an electric shock or leakage fault occurs, there is leakage current flowing through the circuit. At this time, the vector sum of the three-phase current passing through the current transformer is not equal to zero, and its vector sum is: Ia+Ib+Ic=I (leakage current). In this way, there is an induced voltage in the secondary coil of the transformer, and this voltage is applied to the electronic amplifier circuit of the detection part. The transformer connected to the protection is called a zero-sequence current transformer. The vector sum of the three-phase current is not equal to zero, and the current generated is the zero-sequence current.

2. Application of zero-sequence current transformer

The zero-sequence current transformer is a single-turn through-type current transformer, but its primary winding is the three-phase conductor of the protected system (the three-phase conductor passes through the transformer ring core together), and the secondary winding reflects the zero-sequence current of the primary system. Zero-sequence current transformers are generally used in conjunction with small current grounding line selection devices, microcomputer detuning devices, etc. When zero-sequence grounding current is generated in the power system, it is used in conjunction with relay protection devices or signals to activate the device components to achieve protection or monitoring.

In the 10kV power supply and distribution system, zero-sequence current transformers are generally divided into high-voltage zero-sequence and low-voltage zero-sequence:

1. High-voltage zero-sequence

When a ground fault occurs on the lower side of the 10kV circuit breaker, the zero-sequence CT can detect the grounding current and trip the circuit breaker. However, when the circuit breaker is equipped with comprehensive protection (the internal software has the function of calculating zero-sequence current), the zero-sequence current can be calculated from the current of the three-phase current transformer, causing the circuit breaker to trip.

2. Transformer low-voltage zero sequence

Install at a position where all N-line currents at the neutral point of the transformer can be detected. If the three-phase load of the three-phase transformer is balanced, the total current passing through the neutral point should be 0. When an asymmetric load occurs during operation, current will pass through the neutral point, which generally does not cause damage to the equipment, but in extreme cases or when the deviation is large, voltage drift will occur, that is, the voltage is unstable and even burns out the electrical equipment or the transformer itself. Therefore, the equipment can be effectively protected by monitoring the neutral point current.

3. Selection of zero-sequence current transformer

1. Installation method

From the aspects of maintenance and installation, try to use an open-and-close zero-sequence current transformer.

2. Product structure

Select according to the actual use. Generally, most of them use cable type, and busbar type is used in very few occasions, such as zero-sequence protection of generator outlet.

3. Selection of zero-sequence current transformer ratio

For 10kV system, high-voltage zero-sequence can be selected according to the calculated protection setting. For example, when the protection setting is 80A, the protection action can be selected as 100/1 or 100/5.

Generally, transformers with a capacity of more than 500kVA are equipped with transformer low-voltage zero-sequence. The calculated current is obtained according to the installed capacity of a single transformer/0.4/1.732.

4. Selection of the inner diameter of the zero-sequence current transformer

When selecting the inner diameter of the zero-sequence current transformer, the inner diameter of the transformer is required to be 20-30mm larger than the outer diameter of the cable to facilitate installation and maintenance.