Current Transformer Error
Jul 21, 2026
Reasons for errors
In fact, there must be energy loss during the operation of the current transformer, which causes errors in the current transformer. To generate current in the secondary winding, an excitation current I0 is required to excite and generate induced potential and current. The excitation current is provided by the primary winding. The product I0W1 of the excitation current I0 and the number of turns W1 of the primary winding is called the excitation ampere turn or excitation electromotive force. It means that the first winding turns need to be deducted from the example turns before they are transmitted to the first winding, resulting in errors. The error is caused by providing excitation turns.
The error of current transformers consists of two parts: non ratio difference and phase difference.
It should be noted that the transformer ratio is the percentage of the secondary current error value to the actual primary current, not the percentage of the rated current. This is different from the method of expressing the error of other general measuring instruments as a percentage of fullness (% F.S). There is no concept of full capacity for current transformers, only the measurement range and rated current. For example, a micro transformer with an accuracy level of 0.1 has an allowable ratio difference of+/-0.1% at 100% rated current and+/-0.4% at 5% rated current. Assuming the rated current is used as the full scale, the allowable ratio difference at 100% rated current is expressed as a percentage of full scale, which is+/-0.1% F.S. The allowable ratio difference at 5% rated current is expressed as a percentage of full scale, which is+/- (0.4x5%/100%)%=+/-0.02% F.S
Error compensation of current transformer
Current transformers without compensation have negative specific differences and positive angular differences. The allowable range of error for current transformers at all levels is positive and negative deviation. Therefore, the surplus range of positive and negative deviations can be utilized to improve the accuracy of the transformer.
In order to improve the accuracy of transformers, various compensation methods are generally used. In general, due to the small compensation value, it can be considered that the magnetic field of the iron core should not be abstracted. This can be calculated using error superposition. The compensation methods for current transformers include turn compensation, auxiliary iron core compensation, capacitor compensation, etc.
Turn compensation
The compensation method for the number of turns of micro current transformers is the simplest, as long as the secondary winding is wound Nx fewer than the rated number of turns. The ratio difference before compensation of the current transformer is replicated, and increasing the current of the secondary winding by fewer turns serves as compensation. The compensation amount is as follows:
△f=Nx/(N2-Nx)x100%
The ratio difference of the turns compensation team plays a compensating role, and the compensation amount is independent of the secondary load and current size. The compensation turns are generally only a few turns, and the turn compensation should calculate the error between the maximum current low-end secondary impedance and the minimum current high-end secondary impedance. Du Yu's high-precision micro current transformer can compensate for excessive turns even if it only compensates for one turn. At this point, half turn or fractional turn compensation can be used. However, the number of turns of a current transformer is calculated based on the closed circuit passing through the iron core window, and the number of turns of a current transformer is calculated by smashing one by one, without the situation of half a turn. The use of half turn or fractional turn compensation requires the use of auxiliary terminals, such as dual windings, dual iron cores, etc.
Auxiliary iron core compensation
The auxiliary iron core compensation has a compensating effect on the contrast difference and angle difference, but the manufacturing process of the auxiliary iron core compensation method is relatively complex.
Capacitor compensation
Capacitor compensation can be achieved by directly connecting capacitors in parallel at both ends of the secondary winding of the current transformer. Its contrast difference plays a positive compensation role, and the compensation size is proportional to the X component in the secondary load Z=R+iX, and proportional to the size of the compensation capacitor; It has a negative compensation effect on the cross, and the compensation size is proportional to the R component in the secondary load Z=R+iX, and proportional to the size of the compensation capacitor. Capacitor compensation is an ideal compensation method. In micro precision current transformers, the secondary winding is generally directly connected to the current/voltage conversion of the operation and discharge, and the secondary impedance is basically 0. At this time, the role of capacitor compensation is relatively small. Generally, adding a phase shift circuit in the interpretation of current/voltage conversion can solve the angle difference problem. Users can adjust and calculate the phase shift circuit based on the error data of the voltage reduction in the inspection report of the current transformer that comes with it when it leaves the factory.
There are many compensation methods for current transformers. The compensation of current transformers is an important way to improve the accuracy level of current transformers, and one of the important tasks in designing high-precision current transformers.
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