Capacitor Bank Testing Question 2

[FEinTX]

Hello,

We are receiving a new substation at a wind farm equipped with capacitor banks. At a previous job, they used Joslyn zero voltage switches to engage the cap banks and during final testing a digital O-scope was used to confirm the timing was correct. 2 of the 6 switches had to be recalibrated as a result of this testing as there were significant voltage spikes.

I am at a new site now and they are using a different method. They have Southern States pre-insertion closing resistor switches which use a resistor as a shunt to limit spikes. I am new to this type device, and they are telling me there is no need to capture closing waveforms as this is a very simple design unlike the zero-voltage switches from Joslyn.

Question: Am I being fed a line? Seems to me you'd still need to verify the resistor is engaging prior to the switch closing and providing a smooth transition. Any thoughts are greatly appreciated.

FEinTX

 

[magoo2]

Old oil breakers used to have closing resistors. The resistors close in and are then bypassed. So you've got a damping effect from the resistors right away.

With the Southern States switch, it would operate much the same way. I'd be tempted to try it on an energizing operation of the cap bank and see if it anything bad happens. It shouldn't, but it is a mechanical device.

 

[MxGxK]

I don't think you are being fed a line. MEPPI and S&C also make pre-insertion resistor/inductor switching devices for cap banks. I believe they are designed such that it is mechanically impossible to close without first inserting the resistor/inductor. I remember seeing an early version of the SS Cap Switching device. I don't think it had the fault current withstand/interrupting rating necessary for our application. From what I remember, It was a variation of their LLS2000 load switching device.
I saw at least one JOSLYN VBU synchronized with a system of lights. One light was connected across each vacuum bottle and they had to turn on/off at the same time (a small tolerance was allowed while moving the closing mechanism). It would have been nice to see the waveform. However, they seemed to close well. It was tripping the bank that caused the VBU its problems.

 

[DanDel]

There may be no reason to think that you are 'being fed a line', but there's also no reason why you shouldn't check the waveforms as part of the normal acceptance testing, if for nothing else but just to have a baseline for how much transient limitation they provide.

 

[ePOWEReng]

I agree with DanDel that checking the waveform would be a good idea and provide interesting information as well as model verification. But the checking the waveform for the pre-insertion resistors is not as critical as with a zero-crossing breaker. The zero-crossing breakers have to be tuned because they are trying to close at a very precise moment in time (as the voltage across the breaker is close to zero) the electronic circuitry in the breaker needs to "learn" the exact mechanical closing time before it can do this effectively. With the pre-insertion resistors timing is unimportant the damping of the transient is due to a resistor that is temporarily switched into the circuit before being by-passed