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Join Date: Nov 2016
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CVT

05/12/2017 9:36 AM

Dear all

what is the difference between voltage transformer and capacitor voltage transformer, and

Can we use one instead of the other?

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#1

Re: CVT

05/12/2017 9:51 AM

To question #2: NO.

To question #1: a voltage transformer is based on differences in the number of turns on a core (could be an air winding for HVT), between the primary coil and secondary coil. The voltage changes based on the primary to secondary winding turns number.

A capacitor voltage transformer - -A capacitor voltage transformer (CVT or CCVT), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay.

It includes both capacitors (two in series), and a transformer in a tuned circuit. The arrangement is designed to provide a large step-down of voltage from a HV transmission line to an instrument level for reading.

Frankly, I am a bit surprised you didn't already know how to Google that. You need to learn what Wikipedia is, and why you should look at it often.

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#3
In reply to #1

Re: CVT

05/12/2017 7:49 PM

That is true, but then I wouldn't have learned the above. Thank you J.S.

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#4
In reply to #1

Re: CVT

05/15/2017 11:00 AM

Actually, the answer to question #2 is "It depends." There are functions for which a CVT and a VT can be used interchangeably, and others for which it is preferable not to do so. CVTs usually do not have a high a VA capacity of their secondaries. They can carry modern digital relays and metering which don't demand a high VA input. However, if you are using the transformer for all sorts of bus metering, synchronization, incandescent voltage indicating lights, and other loads, then an inductive VT with higher capacity is needed. Also, there can be issues with line distance relaying due to transient responses of CVTs, where an inductive VT may be better suited.

So depending on what the OP needs to do with the secondary voltage, he could use either, or only one of the two.

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#2

Re: CVT

05/12/2017 1:15 PM

As is true in most things related to power engineering, voltage, current, function, and/or economics plays a major part. For PTs, the dividing line is voltage, above which it is uneconomic to build a purely transformer-based PT, so a capacitive voltage divider is used to drop the voltage down to a point where a lower voltage transformer is practical for the output side (aka CVT).

Functionality rears its ugly head when discussing the difference between a CVT and a CCVT. The extra "C" is for "coupling", as in PLCC (Power Line Carrier Control/Communication) system. A CCVT is used to tap-off the 80-500kHz signal that is superimposed on the 50/60Hz power line frequency. Basically the CCVT acts as a resonant LC trap to direct the carrier/signal frequencies off to the PLCC equipment.

So the answer is: each has its own special features, and you need to know how the equipment is going to be used before the question can be answered.

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#5
In reply to #2

Re: CVT

05/15/2017 11:07 AM

I beg to differ. CVT and CCVT are alternative terms for the same thing. A capacitive VT is also doing its transformation by capacitive coupling to the HV source. The carrier accessories are only additional items that are either supplied standardly or added as options during manufacture as desired by the customer. You will find that CVT and CCVT are used interchangeably throughout the industry.

To your first point, there are also inductive type VTs available at the higher voltages, where higher VA capacity is needed. These days with digital relays and meters, and their lower burden, CVTs usually can provide adequate VA to supply them accurately. However, if the burden is higher (such as for bus metering and relaying), inductive VTs are often still used at transmission level voltages (at least to 345kV, as I have seen).

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