Line requirements and grounding system for current transformers

- May 26, 2019-

In the process of operation, the current transformer is mainly based on the electromagnetic induction principle to effectively convert the primary side large current into the secondary side small current to measure the instrument. The current transformer consists of a closed core and a winding. It has a small number of turns on the primary side winding, which is in the line of the current that needs to be measured.

The current transformer often has all the current flowing through the line, and the number of turns on the secondary side is relatively large, which is connected in series in the measuring instrument and the protection circuit. The impedance of the current transformer in the series and the protection circuit is small, and the current transformer is The working state is close to a short circuit. When the current mutual inductance works, its secondary side circuit is always closed, so the measuring instrument converts the primary side large current into the secondary side small current to measure, and the secondary side cannot open the circuit.

How current transformers work

Current transformers have large currents in power generation, substation, transmission, distribution, and power lines, ranging from a few amps to tens of thousands of amps. In order to facilitate measurement, protection and control, it is necessary to convert to a relatively uniform current. In addition, the voltage on the line is generally high. Direct measurement is very dangerous. The current transformer acts as a current transformer and electrical isolation.

The ratio of the current of the primary winding current I1 of the current transformer to the secondary winding I2 is called the actual current ratio K. The ratio of the current when the current transformer operates at the rated current is called the current transformer rated current ratio, expressed as Kn. For zero-sequence CT installation of zero-sequence current protection, it must comply with relevant process standards. For IT grounding systems, grounding currents may not only flow back along the conductor surface of the failed cable, but may also flow back along the conductor surface of the non-faulty cable due to a single-phase ground fault, so the cable head must be installed during installation. The zero-sequence CT is grounded so that the fault current and the non-fault phase capacitor current can pass through the grounding point, which can prevent the protection device from malfunctioning when the fault occurs outside the zone, and can ensure the reliable operation of the device when the fault