On a 400/10/5A ring CT, (the 10 being the test winding) can someone answer 1. how the test winding interacts with the main winding, 2. why the test winding is left open circuit.
If the current transformer is used to fire a trip or alarm some other action, it may not be convenient to stage a full load current fault on the primary side to test the trip is correctly set-up. This is especially true of things where solid bus bars and large currents are expected.
A scaled test current can be used on a winding with more than one turn, to get the core to the same magnetic state as if more primary current was flowing, without actually doing so. This is the function of a test winding(s)
If 10A is injected into the test winding, then the voltage drop per turn will equalize such that the 5A load side sees more or less 5A from the test winding plus or minus the normal voltages due to any current flowing in the primary (scaled by 400/5.)
So if for example the primary current was 400A there would normally be a secondary current of 5A in the load for the metering side.
If however you added further to that 10A in to the test winding, if the the test current was in phase with the main current it would add, so the secondary would see double the normal reading i.e. 10A, (5+5) or if the phase was wrong then in the total anti-phase case it would read nothing at all (5-5), and for any intermediate phase it would see the vector sum ( √ ((A+ B.sin∅) 2+(A-B.cos∅)2 ) or something very like that anyways...
Of course if the primary current was zero, or very near, then the test current and the load current would be in the ratio 2:1 and the primary current could be ignored. This is the simple common test case, with the main circuit off.
Note that the voltage drop in the primary side would be the same as if the full 400A was flowing, even though the voltage was generated by 10A in the test winding.
When the transformer test winding is not loaded, it sees a voltage of half that across the metering load, but this is not normally used for anything
If neither the metering load or the test winding is terminated but there is primary current then the voltage will rise until something gives way, either winding insulation on big devices or magnetic saturation on small ones..
Hope this helps, but come back if it does not make sense.
Mike.
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