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Alternative measurement of the ratio error

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Update time : 2024-09-30 09:38:00
For low-leakage reactance current transformers, the following indirect test will lead to results which are very close to the results obtained in the direct test.  

Nevertheless, routine tests for ratio error determination shall always be performed as a direct test, as this method gives the highest evidence of the “low-leakage reactance property” of a core, including magnetic homogeneity of the iron core. On the other hand, the alternative method is suitable for on-site measurements, and for monitoring purposes. 

In this case, it shall be noted that this method never considers the influence of current flow in the neighborhood of the current transformer. 

For the determination of the ratio error, the simplified equivalent circuit diagram shown in Figure 2E.1 is used: 

A substantially sinusoidal voltage is applied to the secondary terminals S1 – S2 of the current transformer. The test voltage across the terminals  Us Test and the current  Is Test are measured. The injected voltage should generate an e.m.f. across the main inductivity with the same amplitude as during operation with a certain current and the actual burden. The e.m.f. can be calculated from the test results by subtracting the voltage drop across the winding resistance Rct from the test voltage Us Test across the S1 – S2 terminals. This subtraction has to be done in the complex plane. The measured current Is Test is equal the error current. 

The ratio error can be expressed as: 

with: 

the ratio error can be expressed as: 

To determine the ratio error for a certain secondary current  Is the following test procedure is proposed: 

Calculation of the secondary voltage across S1 – S2: 

Measurement of the secondary winding resistance R (value at the actual temperature) 

Calculation of the corresponding e.m.f.    

Injection of 

into the secondary terminals S1 – S2 

Measurement of the voltage Up Test across P1 - P2 

Calculation of the turns ratio  

Calculation of the corresponding Ip

The ratio error can be calculated as: 
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