Based on the equivalent circuit diagram of current transformers, this paper discusses the principles and characteristics of two methods for checking the transformation ratio of current transformers (current method and voltage method), and recommends a simple and reliable field test method for checking the transformation ratio of current transformers—the voltage method. From an application perspective, this article summarizes several wiring methods for current transformers in differential protection of two-winding transformers based on the analysis of wiring principles; it also introduces the wiring method for current transformers in differential protection of two-winding transformers when the polarity of the current transformer is unknown.
Current transformers play an important role in electrical systems, so certain characteristics of current transformers are required to be relatively high to meet the requirements of sensitivity and speed in electrical operations.Circuit breaker analyzer, high current tester are professional power test equipment and instruments. Hubei Yitiancheng has been committed to the research, development, production, and sales of circuit breaker analyzers and high current testers for many years.
1. Based on the equivalent circuit diagram of current transformers, this paper discusses the principles and characteristics of two methods for checking the transformation ratio of current transformers (current method and voltage method), and recommends a simple and reliable field test method for checking the transformation ratio of current transformers—the voltage method.
During the handover of current transformers and after replacement of windings, the field transformation ratio check test is always listed as an important test item. Although the accuracy of the current transformer ratio should be guaranteed by the manufacturing department, due to various reasons, errors can occasionally be detected during field tests (mostly incorrect tap connections). Therefore, the field transformation ratio check test has become a long-standing item.
The working principle of a current transformer is roughly similar to that of a transformer, with the difference that the alternating main magnetic flux in the transformer core is generated by the AC voltage across the primary winding, while the alternating main magnetic flux in the current transformer core is generated by the current in the primary winding. The primary main magnetic flux induces a secondary electromotive force in the secondary winding, producing a secondary current.
From the working principle of current transformers, it can be known that the transformation ratio is determined by the ratio of the number of turns in the primary winding to that in the secondary winding. The main factors affecting the transformation ratio error of current transformers include: (1) The magnitude of the current: the ratio error and phase error increase as the secondary current decreases; (2) The magnitude of the secondary load: the ratio error and phase error decrease as the secondary load decreases; (3) The power factor of the secondary load: as the power factor of the secondary load increases, the ratio error decreases while the phase error increases; (4) The influence of power frequency; (5) Other factors. Internal parameters of the current transformer may also cause transformation ratio errors, such as the internal impedance of the secondary winding, core cross-section, core material, number of turns in the secondary winding, etc., but these are determined by design and manufacturing.
The error test for the transformation ratio of current transformers should be completed by the manufacturer during factory tests or conducted in the laboratory. The field transformation ratio test of current transformers is of an inspection nature, that is, it does not consider the above-mentioned causes of transformation ratio errors but focuses on checking the turns ratio. According to electrical principles, the turns ratio is equal to the reciprocal of the voltage ratio or current ratio. Therefore, measuring the voltage ratio and the current ratio can both calculate the turns ratio.
