How to choose a current transformer?

In power engineering, current transformers convert large currents into small currents and are used for the protection, measurement and measurement of power systems. Today I will explain to you how to choose a current transformer.

The technical conditions for current transformer selection and verification include: primary circuit voltage, primary circuit current, secondary load, secondary circuit current, accuracy level and transient characteristics, relay protection and measurement requirements, dynamic stability multiple, thermal Stability factor, mechanical load and temperature rise. In addition to the above technical conditions, it should also be calibrated according to the use environment: ambient temperature, maximum wind speed, relative humidity, pollution, altitude, earthquake intensity and system wiring method.

The rated primary voltage of the current transformer should not be less than the rated primary voltage of the circuit, which is generally the nominal voltage of the system instead of the highest working voltage.

The rated primary current of the current transformer is an important and difficult point in the selection. Generally, it can be selected according to the rated current or maximum operating current of the primary equipment to which it belongs. The standard values of the rated primary current are: 10A, 12.5A, 15A, 20A, 25A, 30A, 40A, 50A, 60A, 75A and their decimal multiples or decimals. The primary current of the current transformer winding for measurement and metering should be selected strictly according to the loop operating current. In actual use, the transformation ratio difference on each side of the current transformer connected to the main transformer and busbar differential protection should not be greater than 4 times.

As shown in Figure 1, for each circuit of the 35kV bus of a 50MVA booster station, the rated current of the main transformer circuit is 866A, the outlet circuit is 600A, the reactive power compensation circuit is 300A, and the rated current of the station transformer circuit is 10A. Because the rated current of the station transformer loop is very small, the primary current of the protection level winding of the current transformer of this loop is 1/4 times the current of the main transformer side; the primary current of the main transformer loop is selected as 1000A, and the primary current of the protection level of the station transformer loop is optional. It is 250A. Considering the consistency of the transformer ratio, the primary current of the station transformer circuit protection level can be selected as 300A, and the measurement and measurement is generally selected as 50A.

When the high-voltage side of the main transformer is a directly grounded system, the primary rated current of the neutral point current transformer should be 50% to 100% of the rated current of the high-voltage side of the transformer, and meet the specified error limit. The rated primary current of the discharge gap zero-sequence current transformer of the neutral point complete device should be selected according to 100A.

The cores of the current transformers on each side used for transformer differential protection and differential protection of the same busbar should have the same core form.

For P and PR class current transformers, they are only required to meet steady-state non-saturation, while TP class current transformers are required to be non-saturated throughout the entire working cycle.

The above is the current transformer selection compiled by the editor based on specifications and actual engineering experience. Please also pay attention to the reference in practical applications.

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