CT Secondary injection
CT Secondary injection
(OP)
It is proposed to carry out secondary injection with the relay still connected to the CTs (not using test terminals but injecting directly onto the loop). Has anyone done such a test before, i doubt it is meanful?






RE: CT Secondary injection
If you separate the loop remotely from the CT for current injection and voltage measurements, then you will have voltage drop that appears in your measurements which is not related to the CT. At first glance you'd think you could subtract out a constant-resistance / linearly-increasing voltage from the result to isolate the CT portion of the voltage, but that doens't account for phase angle. I don't think you'd have much hope of getting a meaningful curve although you could probably tell the knee pretty well and if comparing 3 CT's on similar circuits and they look similar, that's a good sign as well.
Of course it wouldn't hurt anything to inject current remotely if you measure voltage locally at the ct.. then you would get the true ct characteristics and presence of remainder of circuit doesn't affect the measurement.
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(2B)+(2B)' ?
RE: CT Secondary injection
If your tester can produce the required current (which could be a problem) I personally don't see problem with ct being in the circuit (other than perhaps higher probability of accidentally opening energized ct circuit).
There are probably others that can provide better answers than me.
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(2B)+(2B)' ?
RE: CT Secondary injection
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(2B)+(2B)' ?
RE: CT Secondary injection
Of course firstly be sure that CT is not energized primary !
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RE: CT Secondary injection
RE: CT Secondary injection
I would imagine that would give qualitatively useful results, but not directly comparable to direct testing of a single ct.
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(2B)+(2B)' ?
RE: CT Secondary injection
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If we learn from our mistakes I'm getting a great education!
RE: CT Secondary injection
Let us hope that you are not carrying out such tricks on energized circuits...
rasevskii
RE: CT Secondary injection
To amke matters worse- we have interposing star-delta ct between the main ct and the relay ( this is for the trans diff protection)
We even have more complicated arrangements on the feeder cts which means during injection there is likelihood of multiple current paths into the other cts .
LZ5PL says he/she has done the test with the cts on before - but is this arrangement a standard test? What do we achieve that can't be accoumplished by perfoming stand alone test of the ct followed by the relay , ir & loop resistance?
RE: CT Secondary injection
Well, you don´t achieve anything by leaving the CT connected to the relay if it is the CT itself that you are testing, as said (mag curves, IR, etc.)in fact you cannot possibly do a mag curve if the relay is connected.
Do a primary injection at the CT with all the circuits in service connection, and see that the correct secondary current can be measured at links, test blocks, and the relay itself. If this is a trafo diff protection with aux CTs and something was modified or changed from the original, then a 3-phase injection has to be done onto a short circuit at the other side of the trafo, so that operating currents can be simulated (about 10% of nominal current is enough) using the generator itself (temporary excitation supply regulatable from zero upwards), or a temporary LV generator used to supply the SC current instead.
The above would be tests that were done at the original commissioning only.
In fact, if the CTs were changed as you said, it will likely be that the trafo diff protection will trip due a wrong connection or wrong polarity somewhere. Therefore the above 3-phase test to avoid unpleasant surprises...
How big a unit is this and what sort of relays do you have?
Voltage levels? MVA?
How old is the installation, roughly?
rasevskii
RE: CT Secondary injection
That said, I personally would be very reluctant to do this on a CT on a live circuit. There are too many hazards involved.
Also be advised that circuit burden may cause unusual readings. Under conditions where the applied voltage is low in relation to the actual CT saturation voltage, the current flow will be through the current circuit, with the CT essentially acting as a very high impedance.
In other words, an amp applied at the terminals should return an amp at the metering and relaying. However, if circuit burden produces a high voltage (near saturation voltage), a significant portion of current will flow through the CT windings, resulting in part of your injected current NOT showing up at the end devices.
old field guy
RE: CT Secondary injection