High Voltage Current Transformer

[D!NNy]

Question1:
Can someone explain me what is the purpose of using the tap changer on the Current transformer.

Here is ABB TG type current transformer shown in the link below:
http://www05.abb.com/global/scot/sc...instrument transformers presentation_base.pdf

Refer to Slide 15, 21 and 25 is this same as that multi turn ratio current transformer in low voltages.

Are there multiple cores available in the CT one for each amapcity???
If so the terminations should be changed on the secondary side of the CT? why on the primary side of the transformers?

Question 2:
How is DC current measured in the high voltages?

Thanks in advance

 

[dereckbc]

The tap changer is for you to select what Ratio you want to use. Example 600:1 400:1.

DC current can be measured in several different ways but the most common method is a SHUNT. A Shunt is nothing more than a precision power resistor. They typically have a 0 to 50 mv scale at full deflection. For example a 500 amp shunt would be a .0001 Ohm's @ 25 watts.

 

[GoldDigger]

The tap changer is for you to select what Ratio you want to use. Example 600:1 400:1.

DC current can be measured in several different ways but the most common method is a SHUNT. A Shunt is nothing more than a precision power resistor. They typically have a 0 to 50 mv scale at full deflection. For example a 500 amp shunt would be a .0001 Ohm's @ 25 watts.

At high DC voltages, when there is no convenient (or safe) place to put a shunt, you can also use a Hall-effect sensor to measure the current indirectly by measuring the magnetic field it produces. But this is far less accurate and stable than a shunt.

It is the method that is used in clamp-on DC ammeters.

You can use a shunt even in an ungrounded DC system, but you must use a floating-input voltmeter that can accurately measure just the difference in voltage between the two ends of the shunt.

For a current transformer which is just a toroid through which the primary passes, the only way to change the ratio (short of changing the number of turns
of wire that you run through the middle of the CT) is to change taps on the secondary.
For a current transformer which has internal primary and secondary windings, changing both the ratio and the maximum current the transformer can handle can only be done on the primary side.

 

[Besoeker]

DC current can be measured in several different ways but the most common method is a SHUNT.

We usually use Hall effect transducers. Have done so for decades. You get an isolated output at around 0-10V.
Prior to such units becoming commercially available we did use shunts. This was a ball ache on systems that were typically 700Vdc or above. Even just for driving a meter.
We designed isolators to get from shunt voltages to reasonable analogue voltages that could be fed into electronics at low voltage.
No requirement for them now.

 

[D!NNy]

The tap changer is for you to select what Ratio you want to use. Example 600:1 400:1.

I did understand that this is multi turn ratio CT (MTRCT). But my understanding for any CT primary is always current carrying conductor and that is fixed. And for the secondary side in order to increase the ampacity range the tap position has to be changed ex: lets X1,X2,X3 are CT ratios for 400A:5, 600A:5, 800A:5 respectively and X4 for the second conductor (return or neutral?). and i thought these all are prewired to the junction in the low voltage side. refer to page 15. For instance if i want to increase the CT range i just need to rewire the CT inputs to the metering unit from X2 to X3.

Correct me if i am wrong!


[/QUOTE]
DC current can be measured in several different ways but the most common method is a SHUNT. A Shunt is nothing more than a precision power resistor. They typically have a 0 to 50 mv scale at full deflection. For example a 500 amp shunt would be a .0001 Ohm's @ 25 watts.[/QUOTE]

 

[mivey]

Question1:
Can someone explain me what is the purpose of using the tap changer on the Current transformer.

To give different primary winding ratios (like we do when we wrap the primary through a donut more than once). The additional passes through the windings can be used in parallel or series. The series connection is for lower currents and creates additional primary loops. For higher currents you connect them in parallel for one pass through the CT.

You can find these HV CTs with a 2:1 or a 4:2:1 primary ratio. The one you pictured is a 4:2:1 CT (you can tell because there are three outer current return bus bars between the P2 and P1 side).

Are there multiple cores available in the CT one for each ampacity???

Multiple cores are available (different donuts but I think they will all have the same number of primary loops). These are useful for when you need different accuracies and saturation criteria from the same device, like for metering and relaying.