What are the principles for using current transformers?

I will explain from several dimensions: winding division, load matching, transient characteristics, circuit configuration, and operation and maintenance management:

Metering-grade windings must ensure that the composite error and phase error under rated current meet the metering accuracy requirements, and their core magnetic flux density is designed to be relatively low. Protection-grade windings focus on transient saturation characteristics and inflection point voltage under short-time high current. The core structures and excitation parameters of the two are different; it is strictly forbidden to connect metering windings to protection circuits, nor can protection windings be used for precise metering. For multi-tap, multi-winding current transformers, the physical wiring of each circuit must be separated to avoid electromagnetic coupling interference.

First, it is necessary to distinguish between rated impedance and actual operating impedance. The total impedance of the secondary circuit, including meters, cables, adapter terminals, and connectors, must not only be less than the rated load impedance, but also the cable voltage drop must be calculated based on the installation distance. For long-distance wiring, large-section secondary cables should be selected, and the circuit contact resistance should be controlled. Secondly, different accuracy classes of current transformers (CTs) correspond to different load ranges. Precision metering CTs need to operate within 25% to 75% of their rated load; excessively low loads will increase phase angle error and ratio error. Furthermore, when multiple instruments are connected in series, the overall load must be verified according to the impedance superposition principle; blindly connecting external devices in series is prohibited.

When the system experiences near-end short circuits, reclosing, or automatic transfer switching, the appropriate TPS/TPY/TPZ type protective current transformer must be selected based on the short-circuit current multiple and short-circuit duration. Ordinary P-class CTs cannot suppress transient saturation, leading to maloperation or failure to operate. Simultaneously, the rated accuracy limit coefficient of the transformer must be checked to ensure that the core does not deeply saturate under the system's maximum short-circuit current. For high-voltage systems, dynamic stability and thermal stability current parameters must also be checked to match the primary side short-circuit impact energy.

Grounding locations have specific requirements: For outdoor equipment and high-voltage switchgear, the secondary grounding point of the CT must be uniformly set at a single-point grounding point on the control room protection panel/metering panel. Repeated grounding on the cabinet or terminal box side is strictly prohibited to prevent interference introduced by ground potential differences. For differential protection and bus differential protection circuits, in addition to conventional grounding, the grounding method of the secondary circuits of the CTs on both sides must be consistent to avoid circulating current. Furthermore, disconnectable isolating switches and fuses are strictly prohibited in the secondary circuits. If additional test terminals are required, dedicated test terminal blocks with short-circuit function must be selected to ensure that the circuit remains short-circuited during meter removal and testing.

Firstly, for current transformer selection, the normal operating load current should ideally fall within 40%~100% of the CT's rated primary current. When the load is consistently below 20% of the rated current, it is recommended to select a low-load characteristic optimized CT to improve accuracy under light loads. For circuits with extremely fluctuating loads, wide-range current transformers should be prioritized. Second, in parallel operation and multi-circuit parallel scenarios, CTs in the same group must ensure consistent transformation ratio, accuracy, and excitation characteristics to prevent unbalanced current. Third, environmental adaptability is crucial. In outdoor, high-humidity, dusty, and corrosive environments, selection must be based on protection and insulation levels, and secondary leads must be protected to prevent latent faults caused by insulation degradation.

LVZW-35 current transformer is a sensor used to measure large current in 35kV system. It is mainly used in substations, power systems and electrical equipment. The transformer adopts magnetic core type, which is characterized by high linearity, strong anti-interference ability, small size and simple structure. It is a current transformer with very high cost performance.

Technical parameter :

1.Rated voltage：40.5kV
2.Primary current：50-1200A
3.Secondary current：5/1
4.Measurement level：0.2/0.2S/0.5/0.5S
5.Protection level：5P/10P
6.Installation method: vertical installation;
7.Applicable range: used with 35kV circuit breakers and transformers.
8.Product advantages：Safe and reliable, high measurement accuracy, wide measurement range, small size, light weight, good dynamic performance, low power consumption, convenient equipment standardization, easy to realize automatic monitoring and control.

Shaanxi West Power Tongzhong Electrical Co., Ltd.
Contact: Ms.Grace Liu (International Sales Manager)

Email:xdtz04@westpowerelectric.com

Mobile: +86 18091765882(WhatsApp/Wechat/facebook/telegram)