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Current transformer related questions and answers

by:Transmart     2022-04-29

If the secondary load impedance of the current transformer exceeds its allowable secondary load impedance. Why does the accuracy decrease?

The size of the secondary load impedance of the current transformer has a great influence on the accuracy of the transformer. This is because if the secondary load impedance of the current transformer increases a lot beyond the allowable secondary load impedance, the value of the excitation current will increase greatly, and the iron core will enter a saturation state. In this case , a large part of the primary current will be used to provide the excitation current, which will greatly increase the error of the transformer, and its accuracy will decrease.u200bu200b

The current transformer used for differential protection requires good iron core and enlarged iron core section. Why?

When the system is in normal operation or short circuit outside the differential protection range, the current value and phase of the current transformers at both ends of the differential protection are the same, and there should be no current flowing into the differential relay, but in fact the characteristics of the two sets of current transformers It is impossible to be exactly the same, the excitation current will be different, the secondary current will not be equal, and the unbalanced current will flow through the relay. In order to reduce the unbalanced current, the structure of the current transformer must be improved so that it is not saturated, or special silicon steel sheets with low loss are used to make the iron core, and the section of the iron core should be increased.u200bu200b

What are the ways to wire the secondary winding of the current transformer?
According to the different requirements of relay protection and automatic devices, the secondary windings of current transformers usually have the following wiring methods:
⑴Complete (three-phase) star connection;
(2) Incomplete (two-phase) star connection;
⑶Delta connection;
⑷ Three-phase parallel connection to obtain zero-sequence current wiring;
⑸Two phase difference wiring;
⑹One-phase wiring with two current transformers in series;
⑺A phase is connected with two current transformers in parallel.u200bu200b

What is the wiring coefficient of the current transformer? What is the function of the wiring coefficient?

The ratio of the current passing through the relay to the secondary current of the current transformer is called the wiring coefficient of the current transformer, namely Kcu003dIk/I2 where Ik--current flowing into the relay; 12--current current transformer The wiring coefficient is an important parameter in the setting calculation of the relay protection, and the wiring coefficient must be considered in the calculation of the action value of various current protection measuring elements.u200bu200b

What is the reverse charging of the voltage transformer? What is the impact on the protection device?

Charging the uncharged bus through the secondary side of the voltage transformer is called reverse charging. For example, for a 220kV voltage transformer, the transformation ratio is 2200. Even if the primary busbar of a power failure is not grounded, its impedance (including the busbar capacitance and insulation resistance) is relatively large, and it is assumed to be 1MΩ, but the impedance seen from the secondary measurement of the voltage transformer is only 1000000/(2200)2u003d0.2Ω, almost short circuit, so the reverse charging current is large (the reverse charging current is mainly determined by the cable resistance and the leakage reactance of the two voltage transformers), which will cause the secondary side of the voltage transformer to be small during operation. If the switch trips or the fuse blows, the protection device in operation loses voltage, which may cause the protection device to malfunction or refuse to operate.u200bu200b
Installation of zero sequence current transformers
(1) The cable head and the bracket of the zero-sequence current transformer should be reliably isolated by insulation.u200bu200b
(2) When single-phase grounding occurs, the grounding current not only flows in the ground, but also may flow along the cable sheath. To prevent outside
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