Why would accuracy decrease if the secondary load impedance of a current transformer exceeds its allowable secondary load impedance??

Dry-type Composite Current TransformerThe size of the secondary load impedance significantly affects the accuracy of transformers. This is because, if the secondary load impedance of a current transformer increases greatly beyond the permissible limit, the magnitude of the excitation current will greatly increase, causing the core to enter saturation. In this situation, a large portion of the primary current will be used to provide the excitation current, resulting in a substantial increase in the transformer's error and a subsequent decline in accuracy.

Why do current transformers used for differential protection require a good core and an increased core cross-section?

During normal system operation or external short-circuit within the differential protection range, the current transformer values and phases at both ends of the differential protection should be identical, indicating no current should flow into the differential relay. However, the characteristics of these two current transformers cannot be Identical in all respects, the excitation current differs; the secondary current will not be equal, and a current will flow through the relay. Unbalanced Current. To reduce unbalanced current, it is necessary to improve the structure of current transformers to prevent saturation, or to use low-loss special silicon steel sheets to manufacture the cores, and to increase the cross-sectional area of the cores.

What are the different wiring methods for the secondary winding of current transformers?

According to the different requirements of relay protection and automatic equipment, the secondary winding of current transformers usually has the following wiring methods: 

⑴Complete (three-phase) star wiring. 

⑵Incomplete (Two-Way) Star Wiring

⑶Triangle wiring 

⑷Three-phase wye connection for zero-sequence current wiring.

⑸Two-wire difference connection 

⑹A wiring configuration using two current transformers connected in series. 

⑺A wiring configuration using two current transformers in parallel.

What is the meaning of the connection coefficient of a current transformer??What is the role of the coefficient of contact??

The ratio of the current through the relay to the secondary current of the current transformer is called the connection coefficient of the current transformer.Kc=Ik／I2 In Equation 

Ik--Current in the flow relay;

12--Secondary current of the current transformer for leakage current.

The wiring factor is a critical parameter in relay protection setting calculation, and it must be considered in the calculation of the operating values for various current protection measuring elements.

What is reverse charging of a voltage transformer??What impact does it have on the protective device??

Recharging the bus bar without power through the secondary side of a voltage transformer is called reverse charging.220kVVoltage Transformer, ratio of2200A busbar that experiences a power outage, even if not grounded, has its impedance...(Bus Capacitors and Insulation Resistance)Although large, assumed to be1MΩHowever, the impedance observed from the secondary measurement of the voltage transformer was only1000000／(2200)2=0．2ΩThe circuit was almost short-circuited, hence the reverse charging current was relatively high.(The reverse charging current is primarily determined by the cable resistance and the leakage reactance of the two voltage transformers.)An open or blown fuse on the secondary side of the operating voltage transformer could cause the protective device to lose voltage during operation, potentially leading to improper operation or failure to activate the protective device.

Installation of Zero Sequence Current Transformers

（1The bracket applications for cable heads and zero-sequence current transformers utilize insulating materials for reliable isolation.

（2When a single-phase to ground fault occurs, the grounding current not only flows through the earth but may also travel along the cable sheath. To prevent misoperation of the equipment during out-of-region single-phase grounding faults, the grounding wire at the cable end should pass through a zero-sequence current transformer before grounding. (The grounding wire returns through the zero-sequence current transformer and then grounds)