Zero-sequence current transformer is mainly used as the primary current of the zero-sequence current transformer when there is a fault in the system circuit. Generally, the secondary current is very small, usually 1A—-3A, and some are even smaller, and then the secondary current drives the relay and its protection equipment to produce actions to achieve the purpose of protecting the system.

   The zero sequence current transformer transformation ratio is the current ratio of the zero sequence transformer, that is, the ratio of the rated primary current to the rated secondary current.

  Common zero sequence current transformer transformation ratio

The application of zero-sequence current transformer generally uses a smaller transformation ratio, such as: 50/5; 75/5; 100/5; 150/5; 200/5; 20/1; 50/1; 100/1 ; 150/1; 200/1, because only when a ground fault occurs, the zero-sequence current transformer can output. People will not make the protection action when the ground current is too large. (Don’t worry about avoiding the load current) But because the primary winding is a power cable with only one turn, so, 50/5; the secondary rated number of turns of a 10/1 zero-sequence current transformer is only 10 turns, so 50/5 , 10/1 zero-sequence current transformer has poor load characteristics. When the actual load impedance and the capacity of the zero-sequence current transformer are inconsistent, there will be a large error, and the error will increase when the current is lower than the rated current, so If possible, try to use larger changes first.

  Zero sequence current transformer transformation ratio selection

First, determine the installation location of the zero sequence CT, know the zero sequence impedance of the equipment on both sides of the CT, and then calculate the maximum zero sequence current value through the CT according to the voltage level and operation mode, and then select the CT transformation ratio according to this value .

   The choice of primary current mainly depends on how much zero sequence current can be achieved when the system fails? The national standard for secondary current selection has 1A, 2A, and 5A. Considering that the zero-sequence current transformer generally has a small transformation ratio, try to choose 1A to improve the load capacity. However, some comprehensive protection settings are set to 1A or 5A using the menu selection. At this time, the secondary rated current of the zero-sequence current transformer is subject to the secondary rated current value of the main CT.

   For example, if the calculated zero-sequence current is 2000A, then 5P20 or 10P20 of 100/5 or 100/1 can be selected, which means that the error does not exceed 5% or 10% at 20 times the rated current to meet the protection requirements.

  The purpose of selecting the variable ratio is to balance economy and reliability. If the CT ratio is smaller, the CT will be saturated in the event of a failure, and there is the possibility of rejecting the operation. If the ratio is larger, it will be more expensive.


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