CVT vs VT

[Prot123]

It is well known fact that transient performance of CVT is inferior to inductive VTs, however, for higher voltages, because of cost benefit, CVTs are used. My question is for distance protection application does using CVTs impact on the performance of the protection system?

 

[davidbeach]

It depends. Passive ferroresonance suppression performs better than active. High capacitance units perform better than low. System SIR makes a difference. The ability of the relay to recognize what’s happening matters.

 

[scottf]

The CVT data sheets should list the rated transient response performance. Normally it is listed as a % residual voltage after 1 cycle at a given burden.

As Davidbeach mentions, high capacitance can lead to faster/better transient response, but it is a little more complicated than that, since the electromagnetic design of the components in the base tank play a part too.

Some utilities specify higher capacitances assuming that will mean better transient response performance, but that isn't true all of the time. It is possible to have a lower capacitance unit with a metering accuracy rating (for example) have a better transient response rating that the next size up capacitance rating with a relay accuracy rating.

If you are in the ANSI world, transient response ratings will look like (for example) <9% @ ZT (1 cycle). Which means the residual voltage will be <9% after 1 cycle with a burden of 200VA.

In reality, the applied burdens on CVTs are normally well below the rated burdens. The applied burden impacts the stored energy in the CVT circuit, so the lower the applied burden, the faster the transient response. So, for the above example, if you had a connected burden of 50VA, then the transient response would be in the range of <3% of so. It's not linear with burden, but normally you can take a linear approximation and then add a little.

Back to the original question...for CVTs used for distance protection we normally see the transient response spec'd as <5% @ ZZT (1 cycle). My understanding from utilities is that if they buy CVTs with that ratings, they normally don't have issues with distance protection for long lines.