ELCOME Dear friends of protection and control engineering!
Distribution stations are sprouting up like mushrooms. In order to put freshly installed transformer differential protection systems into operation, the test engineer is always faced with the same question:
"Where can I get the three-phase system for the primary test?"
From the socket!
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:
In case of three-winders, it is recommended to check the two secondary windings separately in order to have the test current under control. If the 400 V test can not be carried out from the socket, a suitable 3-phase test transformer is helpful as well.
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)!
Failure to observe the following points can result in death, serious injury or material damage!
Hazardous voltages may be present when carrying out the tests and checks described in this manual. The safety rules and regulations regarding electrical systems must be strictly observed at all times. The generator must always be shut down when working on the primary system; appropriate grounding and short-circuiting facilities are to be provided at the respective workplaces. When carrying out a primary check on a turbine set, take care to ensure that no overheating of the turbine occurs.
The work described in this manual may only be carried out by qualified personnel, who must be conversant with the relevant safety regulations and safety measures as well as the warnings in the manuals provided by the suppliers of the various components. The contents of this manual must not construed as work instructions. All statements in this manual must be carefully considered in light of the safety rules and regulations. The information presented in this manual does not claim to be complete.