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Principles, Functions, and Classification of Zero-Sequence Current Transformers

Principle and function of current transformers

The basic principle of zero sequence current protection is based on Kirchhoff's current law: the algebraic sum of the complex currents flowing into any node in the circuit is equal to zero. In normal operation of the circuit and electrical equipment, the vector sum of the phase currents is equal to zero. Therefore, there is no signal output from the secondary winding of the zero sequence CT current transformer and the executing element does not act.

When a ground fault occurs, the vector sum of the phase currents is not equal to zero. The fault current causes a magnetic flux in the toroidal core of the zero sequence CT current transformer, and the induced voltage on the secondary side of the zero sequence current transformer causes the executing element to operate, driving the power switching device to switch to another power source, achieving the purpose of ground fault protection.

  • Function: in case of electric shock or leakage fault in the circuit, the protection device will act and cut off the power supply.

  • Usage: a zero sequence current transformer can be installed on each phase of a three-phase line, or the three-phase wires can be passed through a zero sequence CT current transformer together. A zero sequence current transformer can also be installed on the neutral wire N to detect the vector sum of the three-phase currents.

Selection of zero sequence current transformers

How to choose a zero sequence CT current transformer?

Zero sequence current transformers are generally used in high-voltage systems, an independent system with a large grounding current. When a grounding fault occurs in a certain line of the system, the circuit breaker on the power supply side of the faulty line is triggered. This fault signal is detected by the zero sequence current transformer and sent to the protection device to trip the circuit breaker.

Methods to select zero sequence CT current transformer

  • Select based on the maximum grounding current of the system to ensure that the protection device can operate correctly and the zero sequence current transformer will not saturate magnetically;

  • Select based on the cable size and type.

Classification of zero sequence current transformers

When the line in the power system is operating normally or there is a three-phase short circuit, the vector sum of the three-phase currents is equal to zero. Since the cable line is a primary winding, when a transformer is added to the line, the induced current is zero. When a single-phase ground fault occurs in the circuit, the vector sum of the three-phase current passing through the transformer is not zero, and the ground capacitive current passes through the core, the secondary side of the transformer outputs a signal to operate the executing element. Therefore, the current that is not equal to zero generated when the three-phase current passes through the transformer under fault conditions is called zero sequence current, and the transformer connected to it is called a zero sequence CT current transformer.

The zero sequence current transformer is a device that uses the characteristic that the zero sequence current value is greater than the non-fault current value when a single-phase ground fault occurs in a small current system, and uses a current transformer to extract the zero sequence signal to make the relay operate or send a signal. It is mainly used in power systems to generate zero sequence current and to cooperate with relay protection devices or signal devices to achieve protection and monitoring purposes. It is divided into two categories:

  • For measuring zero sequence current when a single-phase ground occurs in a small current system, and it is matched with a microcomputer selection device for grounding;

  • A zero sequence current transformer for large current grounding systems (also known as low-resistance grounding) protection, which can be used with protection devices to achieve relay protection.

In summary, the zero sequence current transformers have the characteristics of reasonable structure, beautiful appearance, good insulation performance, good linearity, high sensitivity, reliable operation, easy installation, and high measurement accuracy. Users can choose the appropriate model according to the operating mode of the system.

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