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 paper summarizes several wiring methods for current transformers in differential protection of two-winding transformers based on the analysis of wiring principles; at the same time, it 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 there are relatively high requirements for some characteristics of current transformers to meet the requirements of electrical action sensitivity and rapidity.Circuit breaker analyzers, high current testers are professional power test equipment and testing 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.
At 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 due to incorrect tap connections). Therefore, the field transformation ratio check test has become a project that has remained unchanged for many years.
The working principle of a current transformer is roughly the same as that of a transformer, except that the alternating main magnetic flux in the transformer core is generated by the AC voltage at both ends of the primary coil, while the alternating main magnetic flux in the current transformer core is generated by the current in the primary coil. The primary main magnetic flux induces a secondary electromotive force in the secondary coil, producing a secondary current.
From the working principle of the current transformer, it can be known that the transformation ratio of the current transformer is determined by the ratio of the number of turns of the primary coil to the number of turns of the secondary coil. The main reasons affecting the transformation ratio error of the current transformer are: (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 coil, core cross-section, core material, number of turns of the secondary coil, etc., but these are determined by design and manufacturing.
The error test for the transformation ratio change of the current transformer should be completed by the manufacturer during the factory test or carried out in the laboratory. The field test of the current transformer transformation ratio is of an inspection nature, that is, it does not consider the above reasons affecting the transformation ratio error of the current transformer 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 measuring the current ratio can both calculate the turns ratio.
