Errors of Daily Current Transformers

In the power system, in order to ensure the normal power supply and maintain the safety of valuable equipment, there is a set of relay maintenance lines composed of various relay control equipment. When a fault occurs in the power system, these maintenance devices will act to cut off the faulty line. If it is an occasional fault, it can also be automatically closed to ensure normal power supply.

Ⅰ. Maintenance of current transformers

1. The maintenance current transformer is to change the current on the line into a certain current. Supply power to maintenance installations such as relays. When a short circuit or other fault occurs on the line, the current on the line increases sharply, and the current through the maintenance and installation of the current transformer supply relay also increases sharply, which makes the maintenance and installation of the relay act and cut off the faulty line.

2. The accuracy level of the maintenance current transformer is represented by 5P and 10P, which is also equivalent to its allowable error of 5% and 10%.

It can be seen that the working conditions of maintenance current transformers are completely different from those of measurement current transformers. The current transformer for measurement is used to measure the current and power on the line when the line is normally powered, while the current transformer for maintenance is only at a current several times or even dozens of times larger than the normal power supply current when the line fails, it can work effectively.

Ⅱ. There are three main requirements for maintenance current transformers:

1. Insulation must be fixed to ensure safety;

2. There must be a large enough accurate limit coefficient;

3. Sufficient thermal stability and dynamic stability are required.

When the current transformer is in normal operation, the primary current and the secondary current basically rise or fall according to the proportional relationship of formula 1. However, when a fault occurs and the primary current increases sharply, due to the saturation of the iron core, when the secondary current increases to a certain level with the primary current, it will no longer increase in a certain proportion. The exact limit factor is used to describe this performance of the current transformer.

The maintenance current transformer can meet the primary rated accuracy limit primary current required by the accuracy class 5P or 10P under the rated load. The so-called accurate limit coefficient is the ratio of the rated accurate limit primary current to the rated primary current. The specification values for the exact limit factor are 5, 10, 15, 20, 30.

It can be seen that the so-called accurate limit coefficient means that after the current has increased sharply to several times the rated current, the current transformer error has reached 5% or 10%. If the primary current is increased, the error will be larger, and the secondary current will no longer be proportionally increased.

Therefore, regarding the maintenance current transformer, it is necessary to have a large enough accurate limit coefficient, so that when the line fails, the primary current increases sharply, and the secondary current can be reflected to the relay maintenance and installation, so as to ensure the timely and reliable action of the relay maintenance, cut off the fault line. The ratio difference between the maintenance current transformers under the rated primary current and rated secondary current load shall not exceed ±1% or ±3%.

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