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ELCOME Dear friends of protection and control engineering!

Distribution stations are sprouting up like mushrooms. To put freshly installed transformer differential protection systems into operation, the 400 volt test is the appropriate test method. In our article from 20.08.2017 we had presented you the theory for the implementation of the 400 volt test. In the following article we summarize these again and introduce you to our free Excel tool for calculating the required test current. You can download our tool for free. Let's go!

### What is being tested?

We check the current transformer circuits of transformers and drives at the medium voltage and distribution network level with the so-called "400 volt test". For transformers, for example, this means:

We short circuit the low-voltage side of the transformer by means of a suitable short-circuiting device for all 3 phases. The short circuit location should ideally be outside the protection zone. Now we apply the 400 V rotating field of a low-voltage supply, to the three phases on the hi-voltage side of the transformer. The current flowing through the short-circuit impedance of the transformer serves to examine the magnitudes, pointers, and the differential and bias values ​​of the respective differential protection function.

Since transformers with a very low short-circuit voltage are often used (for example, low-voltage), it is always advisable to calculate the test current in advance and check for suitability. The test current is estimated using the following formula: