EMVT Vs CVT

Bad Acronym Alert!

It is the question of feasibility & costing of insulation of EM VTs for voltages higher than 110kV. This is where a CVT helps, as it is a Capacitor-Potential-Divider.

As far as accuracy is concernde, by Standards, CVTs must comply with the accuracy classes as pecified for EMVTs as well.

First of all Thanks for your answer...

But now-a-days EMVTs are using in powerplants of rating 400Kv(suppliers: CGL) only for metering purpose( class of accuracy : 0.2 )

i dont know the exact reason but i heared that some Drift problem is there in CVT as far as accuracy is concerned & precise accuracy is not available with CVT.

can you explain me in this regard...?

1. For CVT you must have 2 capacitors - 2 variables to affect ratio.
2. Capacitance depends on dimensions and permittivity.
3. Dimensions and capacitance are very different in HV side and earth side capacitors, making them track to keep ratio over time and temperature change is difficult.
4. Water has permittivity 80 compared to ~4 for solid dielectric - any tiny penetration has big effect.
5. Resistive leakage currents are unstable, but can effect ratio.
6. CVT usually have to finish up with an EMVT for safety isolation -which adds its own errors.
   

In EMVT if primary has 10000 turns and secondary 100 turns there is very little that can alter that ratio. Leakage inductance affects effective ratio, but is not very variable. Magnetizing current, via leakage reactance, affects reading but is fairly stable and can even be monitored in service. So the accuracy arguments favour EMVT. Also in EMVT, it is possible to earth core, wind LV winding over earthed end of primary, add earthed screens and insulation over LV winding - all for personnel safety in event of breakdown in HV side.

In capacitor VT, consider safety effect of breakdown of HV capacitor...... do you want to be measuring LV circuit if that happens??