Applying 240V on Secondary of 200/5A CT

Which side do you want to apply the voltage, the high current side or the relay circuit side? The phrases "...apply 240V on Secondary..." and "...primary injection test..." don't belong in the same sentence.

If you want to test the complete circuit by forcing current through the high current (primary) side you use a special test set capable of providing high current into essentially a short circuit, not by applying 240V line voltage onto the relaying (secondary) side of the CT.

Get a qualified relaying expert to help you before you release the magic smoke inside your relaying circuitry.

You can't, it is almost a short circuit to 240 VAC on the secondary, because the magentic core will saturate and the the secondary will present no reactance to the applied voltage.

The CT is a current sensing device therefore simply applying 240V on the secondary winding cannot provide what you are looking for.

The CT voltage rating is determined by the insulation class of the winding and the clearance/distance from device terminal to terminal.

It is possible to use an autotransformer with a properly sized current limiting resistor in series with the secondary CT winding so that you have complete control of the current.

You must continuously monitor the current so that you do not exceed the CT rating at any time during the test.

Caution: Do not apply any voltage to the primary of the CT unless the secondary terminals are shorted together. Doing so will result in excessively high voltage and risk of meltdown and/or an explosion and/or personal injury.

I agree with the post. You can use the CT to feed 200A in another circuit to use as primary injection current source.

Just a caution - What is the CT secondary voltage rating (for example, 5P20, 30VA CT can withstand 600V across its secondary terminals without the CT core saturating). If the CT is metering class, the voltage withstand value will be much much less. Please watch on that and hopefully there will not be much problem

* if the conductor on 200A side is generously sized

* the length is kept short

* the CT/relay and connected lead burden is all small.

I once connected the secondary of a CT to a variac to test the trip point of a circuit breaker. I put a heavy gauge wire through the core (primary side) and connected it to both sides of the circuit breaker under test. I monitored the current in this loop of wire with a clamp-on ammeter and increased the variac slowly to determine the trip point.

http://cr4.globalspec.com/comment/972229

I have one spare ct....I want to make it loading transformer to use as primary current source to test feeder protection relay.... 1. I will connect variac to spare ct secondary in order to get flow of 200a primary current (primary connected to feeder outgoing busbar with breaker open circuited) 2. Having 5A at relay terminal I can simulate protection function for testing Regards, T.D.

Tigerdon,

Thank you for the clarifying response. I understand your question better now, which makes an intelligent, helpful answer much easier, thank you.

A transformer, CT or otherwise is just that, a transformer, & yes it will go both ways like any other transformer. You want to drive current in the PRIMARY BUSS by loading the secondary circuit for the purpose of testing your over-current sensors.

NOT going to happen with a CT.

Due to the physical design features that make a CT excellent and precise in normal mode, it is inherently quite useless in the reverse direction.

HOWEVER here is what you are up against:

1. Turns ratio logic error.

You have probably assumed that a 200:5 CT has a turns ratio of 40:1 (200 / 5).

in reverse, the turns ratio appears to be 1:40 & sounds pretty sweet right?

Not so fast. A turns ratio refers to the VOLTAGE difference between the primary & secondary coils. The CURRENT developed from that voltage difference is entirely dependent on several things, the two main ones are that the secondary must be a CLOSED LOOP for any current to be able to flow, and the current depends on the RESISTANCE or IMPEDENCE of that closed loop. The second is that the voltage developed is actually quite a lot smaller than you might imagine from the current rating values.

2. Current limits by design in the secondary.

The actual voltages developed in the secondary of a CT range from zero to about 10 volts (unless you open the secondary loop under load). The reason that this is actually a VARYING CURRENT device on the secondary side is that the other 2 normal variables have been "locked in' so to speak. The RESISTANCE is not variable since it is comprised almost entirely of the resistance of the copper in a closed loop (circuit). To accomplish current control in a closed loop, the conductor is very tiny & lengthy, being wrapped many times around the stacked iron plate core. The primary VOLTAGE is locked in to the Grid and not normally a variable thereby. A mosquito on an elephants ear is not likely to push the elephant's blood pressure around.

3. Primary Turns

The average CT has a 1/4 turn primary at best. It would be a 1 turn primary if the buss made a loop around the core. That means that in reverse mode, it has very limited efficiency to transfer energy TO the primary side. The secondary side is wrapped hundreds of times around the core to get 10 volts in the normal direction.

Wish I had better news for you on this, but you will need a loading transformer with design features that favor your stated purpose. I would compare this to hoping a DC motor will run efficiently on a 3 phase AC power source. Yes they are both motors, but no it will not work without a lot of added equipment and controls.

I'm not sure if you're doing what I described. If you intend to test a breaker, you need a very low resistance circuit through the CT and through the breaker. There will be high current and low voltage in this loop. I would not have it connected to anything else. Please be careful.

There is ittle correction in schematic 1) primary loop cable shall be connected to breaker outgoing terminal busbar which is having protection ct connected to feeder protection relay (for testing purpose feeder circuit breaker kept in open position) Regards, T.D.

You can if you have a fan to blow the smoke and sparks away.

More seasonal nonsense? I suspect these posters have all been at the sherry.

I'm sure the ct is PS class and you must be trying to check the leakage current at knee point of ct. Go fo it!!!!!

I do this for a living at an electric utility.

Yes you can apply 240 volts to the secondary side of a CT transformer, I do it often enough in testing that the meter & secondary wiring loop is good when there has been a period of no customer load as a quick validation.

You MUST however make certain temporary modifications to the secondary loop circuit or the clean-up after the flash and fire is pretty ugly.

It is quite normal to find the secondary loop return current line bonded to neutral or ground at ONE point in the loop in order to trigger protective devices if the CT secondary is compromised with a foreign voltage (which you are proposing.) Therefore it is absolutely required that you construct a temporary circuit, ISOLATING as needed, and utilize a LOAD DEVICE to create and control the current, while respecting the known laws of electricity and circuitry.

NOTE: the secondary of a CT is designed to be shorted at all times when there is the possibility of ANY current through the Primary side!! This is because by E = IR, if R is only the copper wire (not a variable), and I is forced to be zero by an open loop, that leaves the voltage E to be the only remaining variable. Mathematically it rises to infinity, but actually gets to around 50,000 volts due to protective design limits in the newer versions.

As to your desire to use the CT as a a loading transformer, it's special design does not readily lend itself to that purpose by itself. As described above it would be necessary to provide a load producing / controlling device to the circuit anyway.

Why don't you use one of the thousands of available test methods & devices designed for that test?