Application and Requirements of Zero Sequence Current Transformers

Zero-sequence CT current transformer, also known as a protection-type CT current transformer, has the requirement that the error under defined conditions shall not exceed the specified range.

The function of this type of current transformers is to cut off power during system fault short-circuit currents. During application, due to the large short-circuit current, the zero-sequence transformer core will be saturated, causing the excitation current of the zero-sequence CT current transformer to increase, leading to an increase in transmission errors. When there is a DC component in the short-circuit current or residual magnetism in the zero-sequence transformer core, this error will be greatly increased.

In response to the saturation of the zero-sequence current transformers during construction, correct selection of the current transformers is necessary. There are three types of current transformers to consider:

Use P-class current transformers, which do not consider transient saturation but consider the stable short-circuit current that passes through the current transformers. An appropriate parameter is adopted by the protection and monitoring system to prevent transient saturation of the CT current transformer. This type of current transformers includes P-class PR and PX, and the accurate limiting coefficient of the CT current transformer is considered based on the compound error of the first-order symmetrical current or the excitation inflection point. This type of current transformers is currently widely used.
The TP-class CT current transformer (including TPS, TPX, TPY, and TPZ types) considers maintaining error not exceeding the specified range when the short-circuit current has periodic and non-periodic components under defined transient operating cycles. This type of current transformers increases the size and cost of the CT current transformer significantly. It is usually used in ultra-high voltage systems or large-capacity generators.
The TPE-class CT current transformer considers peak errors under both steady-state and transient operating conditions and outputs low-level analog or digital signals. Its features include low power consumption, fast response, long transmission distance, non-saturation, large dynamic range, wide frequency, and no harmonic resonance. This type of CT current transformer includes photoelectric transformers, hollow coil transformers, etc. This type of current transformers is mostly used in intelligent power stations.

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