Use of Reclosers for Capacitor Bank Switching 1

[CKent]

Hello!

Does anyone have any experience on the use or application of Three-Phase Vacuum Reclosers for capacitor bank switching? What are the key points that need to be address to be able to do this. I read an article about CEMIG (Brazil) being able to use such 3P vacuum reclosers for their 15kV capacitor applications.

CK

 

[chris8410]

Hello CK,
As you may be aware, capacitor bank switching can be a source of severe voltage transients. Transient (inrush) currents can be also severe. These transient voltages and currents occur primarily when a capacitor bank is energized. Voltage spikes over 2pu can be often be generated, especially if re-striking occurs in the breaker. Back-to-back switching of capacitor banks is even more potentially severe.

To minimize the voltage transients during switching, pre-insertion resistors and inductors are very commonly used in combination with the capacitor circuit switcher (ie breaker).

The 90 degree phase shift between the voltage and capacitor current also presents a challange. As the breaker contacts are seperating, the contacts will normally interrupt the current at a zero-crossing. For a purley capacitive load, the system voltage is a maximum during the zero-crossing of the current. The presence of maximum voltage during the current zero-crossing makes it much more difficult for the interruption device to successfully break the circuit. Hence re-strikes can be common during cap bank switching.

Cooper Power Systems and Josyn Electric have excellent technical articles in PDF format on their web sites. There are many papers that discuss the issues relating to cwitching devices.

Also, a very good reference text is "Electrical Transients in Power Systems", by Allan Greenwood. Mr. Greenwood was at the forefront of vacuum switching develpoement in the early 1970's, while working for GE. His text book is an outstanding referance book, a true invaluable classic.

Regards,
Chris

 

[jghrist]

Unless you are also using the recloser as overcurrent protection, I would think a vacuum switch would serve the same purpose at less cost.

Are these large substation capacitor banks or line capacitors?

 

[bacon4life]

We have vacuum switches on some of our pole mounted banks. The last batch we got was preassembled units from Cooper that included the capacitors and vacuum switches on one rack.

 

[CKent]

Jghrist, you got the whole point...however, the application would really be for substation capacitor banks, at 13.8 kV, 7.2MVAR...thus, we need overcurrent/fault protection capability...actually we are avoiding having to retrofit a breaker into the existing calor-emag switchgear and thus hoping to just use a vacuum recloser to do the job...this would take away the work of retrofitting and based on our costing, would be cheaper than buying a breaker for the purpose...

hope to hear some experiences or studies related to this...

CK

 

[Robert789]

I'm thinking that a call to the manufacturer of the recloser you intend to use may be in order. 330 Amps or so at 90 degrees leading may not be exactly what they designed the unit for.

Make sure you look at the source impedance when considering switching a bank this large. We tried switching a 2400 KVAR bank at a rural substation a few summers ago. I received several calls from customers who saw their lights get brighter each morning and dimmer each evening at about the same time. I then calculated that the switching was causing a voltage rise/drop of about 4.5% To stop the phone calls, we broke it into four banks of 600 KVAR each and switch them in sequence at 120 second intervals to allow the regulators time to smooth the transition.

 

[apowerengr]

Robert789 - I'd be curious if you placed switching reactors in between the 600kVAr banks - manages inrush from energized banks trying to maintain bus voltage into the just-energized banks.

Chris8410 makes a great point - the manufacturer's have guidance on this application. Cooper's text states roughly 2 amps for capacitor or cable-energization switching due to the extremely X/R ratio. I even discussed this with S&C regarding the use of an omni-rupter for manual disconnection of a cap-bank at 12.47kV - limitation was 600kVAr maximum (30 amps) on a 600 amp rated load-break switch.

 

[Robert789]

I don't believe that we have reactors on the 600 KVAR banks. We do have them on some multiple sets of 1800 and 2400 KVAR banks at other stations.

 

[Rocketir]

I have seen them used on a rural circuit with a small (600 kVAR) bank. We used a Nova recloser and operated on voltage. This was due to a large quarry type of customer causing large voltage flucuations on the primary. We would close instananeously during voltage dips.