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Registered Member #3824
Joined: Sun Apr 10 2011, 08:29PM
Location: Edmonton, Alberta, Canada
Posts: 54
Hi, just a question regarding instrument transformers. Say for example you have a CT with a ratio of 2000:5 (for a 2000A service). If this service was say at 600V, would the potential difference between the first and last CT winding not be (600*400) 240000V? It obviously is not as CTs are used in every industrial application but then how is the voltage kept to a reasonable level? Last CT I saw had a BIL rating of 10kV.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
CT is short for Current Transformer.
Look at my attached picture. 2000A passing through the transformer is one winding, therefore you will have 5A on the secondary side. Depending on which load resistance (CR1) you use, you can select what output voltages (V1) you want, simply by using Ohms law.
The voltage rating you have seen are likely the isolation voltage of the entire units enclosure.
Registered Member #3824
Joined: Sun Apr 10 2011, 08:29PM
Location: Edmonton, Alberta, Canada
Posts: 54
The thing I do not understand is how the voltage across the CT windings (V2) is kept to a minimum. Like above, if the ratio is 400:1, and a primary voltage of 600V is used, then theoretically V2 should be 600*400 should it not? What is done to limit the voltage of the secondary?
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Ed -
A current transformer winding is always "terminated" with a relatively low value burden resistor, which limits the maximum voltage present across the winding. If this resistor were not present, a current transformer could develop destructively high "open circuit" voltage.
Here's a brief writeup on current transformer theory of operation:
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
LD LANGER wrote ...
The thing I do not understand is how the voltage across the CT windings (V2) is kept to a minimum. Like above, if the ratio is 400:1, and a primary voltage of 600V is used, then theoretically V2 should be 600*400 should it not? What is done to limit the voltage of the secondary?
What is the voltage across the primary winding in Mads's picture? Not 600 volts! That's the voltage between either end of primary and some reference (neutral, earth, etc.). Not part of the CT circuit, so it doesn't matter.
As others have said, the secondary voltage is determined by the 1/N current times the burden resistor. Then the primary voltage drop due to CT circuit is the 1/N voltage. For example, with your 2000:5 CT at full rated current, with 1 ohm burden. Secondary voltage = I * R = 5 volts. Primary voltage = Vs/400 = 12.5 mV (extracting 25 watts of power from the primary circuit).
With no burden resistor, the secondary voltage is limited by core saturation. Maximum is on the order of some volts per turn, for the size CT you are thinking of, in the form of spikes each time the core flux reverses. Can be a serious electric shock hazard, and/or can damage insulation. Still, the primary circuit voltage you are thinking of doesn't matter. The CT could be on a 1000 amp 10 volt circuit that heats a crystal-growing furnace, like at a summer job I had once.
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CT voltage
