Matching CVT capacitance with breaker properties

[ters]

I'm searhing for some basic info on matching 230 kV bus CVT capacitance with a 230 kV dead tank SF6 breaker. I'm not sure what the idea is, looks like certain breakers have a trouble dealing with low capacitive currents, so matching CVT capacitance with breaker properties helps. Any info would be helpful. Thank you. Ters

 

[scottf]

I'm not sure I follow your comments about breakers not being able to deal with low capacitive currents. I work for a CCVT manufacturer and that's a new one on me. By "matching" do you mean having the CCVT capacitance match the internal breaker capacitance, if present?

Typically, where CCVT capacitance ratings come into play with breakers is when applying the CCVT capacitance to the TRV requirements of the breaker. I.e. the breaker may require 5000 pF (for example) within 300 feet of the breaker to be rated 63 kA - 1 second for close-in faults. Breakers can either be bought with external bushing mounted capacitors as needed, or the CCVT capacitance can be applied to the breaker ratings.

 

[ters]

I have to admit that I don't know enough about this subject (CVTs). I work on a project where several CVTs with various capacitance (13,500, 15,000 and 30,000 pf) will be installed but I'm not directly involed in any design decisons. There was some confusion as to which CVT goes where (bus or line). I was told that it would depended on breaker ability to interrupt capacitive currents, which did not sound very logical, I have also never heard of it. Since I was not that familiar with the subject, I decided to do a small web research and google brought me here... Anyway, your explanation makes sense, TVR must be a dermining factor, along with breaker properties. Do you know where I could find some literature on this on the web? Thank you.

 

[scottf]

The best source for TRV issues for your application would be your breaker supplier.

My best guess is that the person that told you to be concerned with capacitive currents is perhaps mistake or has confused some issues.

 

[cuky2000]

Normally the CVT’s capacitance is selected based on the transient performance associated with the protective scheme such as the distance relays. Also the capacitance of the CVT may improve the short-line fault and the transient recovery voltage performance of the circuit breakers installed in the vicinity. However, if TRV is a real concern, grading capacitor installed directly in the circuit breaker by the manufacturer is considered more reliable choise.

To determine if TRV is a concern, a system study is recommended based in recognized standard such as IEEE or IEC. Since you are using 230 kV dead-tank circuit breaker, I presume this project is located in an ANSI region. Therefore, the IEEE Std C37.011, “IEEE Application Guide for Transient Recovery Voltage for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis” may be applicable.

The enclose site also provide good information related with the TRV.

http://www.ewh.ieee.org/soc/pes/switchgear/TRVTutorial/TutorialTRVAlexander-Dufournet.pdf

 

[scottf]

cuky2000-

At least in the US, it is quite common to apply local CCVTs to the breaker's TRV rating. Why would you think that capacitors installed on the breaker's bushings would be more reliable? Do you mean more reliable in terms of a better grade capacitors or more reliable due to proximity to the breaker?

While it's true that the capacitance rating of a CCVT is part of the equation of transient response, most CCVTs are not ordered with transient response considered, unless a particular utility has had problems in the past. The TR issue with CCVTs has been greatly reduced with the advent of the electronic relay (i.e. very low burdens). For utilities buying CCVTs for metering/protection and application to the breaker rating, the breaker's capacitance requirement is almost always the dominating requirement in terms of capacitance.