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:
Your calculation tool!
To check whether the required test currents and power are ok, you can use our free tool to calculate the required 400 Volt currents. The values of the short-circuit voltages, voltages and currents can be taken from the rating plate of the transformer to be tested. These are entered in the calculation tool shown below and the results for the required test currents can then be read below. From the finally determined test currents and the given test voltage (400 volts) you can then calculate the required power of your test transformer or the low voltage supply by multiplying this two values with root (3). The figure shows an example of a typical low-voltage transformer.
As a registered member of our platform, you can download the 400 Volt-Test Excel-Tool for free in our download area.
After all current transformer and differential protection circuits are ok, the 400 volt test is complete.
This article is an excerpt from our free e-book: "The Tool Box". Please download your freecopy of the "Tool Box" and find out everything about voltage and current transformer commissioning (just click on the figure below)!
