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Protection Riddle No.51 - CT Primary Injection
During CT's primary injection testing why we cannot inject 80%-90% of primary current in to CT? ( Say CT ratio: 2500/1A, during primary injection test, why we cannot inject 80% (2000A) into primary of CT and do the testing? ). Please specify any BS/IEE standard on this issue.
Author : Rohana - From: Sri Lanka
 
#1
Mon, October 5th, 2009 - 18:28
The Current transformer primary injection test consists of injecting a test current into the CT primary, to enable measurements to be made of the secondary quantities as verification of the correct installation of the CT, the connection wiring and the equipments.
The test themselves, which are often termed "shots", are required to verify the following:

- Ratio shot: a shot to prove the ratio and phase-to-neutral wiring of a CT.
- Polarity shot: shots to prove the polarity and phase to phase wiring of a group of three (R,Y,B) CTs. This test only gives the relative polarity of one CT to another, whereas the flick test ascertains absolute polarity.
- Matching shot: a shot to prove the polarity of one group of CTs with reference to another group.
- Operating shot: a shot to prove the wiring to, and the operation (setting) of, the connected device. This shot may be subdivided into operation from a single CT or operation from more than one CT.
- Stability shot: a shot to prove that unit protection stabilities on through fault.

As you see, except operation shot test, the magnitude of injection current is not important and because of injection set rated power limitation, we can apply a fraction of rated primary current to subjected test.
A typical primary injection set delivers up to 1000 A from a source of 15 V. Secondary currents will usually be in the milliampere range. Older ammeters posses a high burden on the lower ranges, and this can have an impact on the test results. Such ammeters should not be used. In operation shot, the magnitude of current is often insufficient to cause operation, owing to the limited output of the primary injection set, and thus indication that the current is in the operate direction will suffice.
 
Author : Hamid - From: Iran
 
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