Bus Duct Capacitive Reactance?

[mayanees]

I have a situation where I've measured transient voltages upon energization of a 700' section of 480V bus duct.
The busduct feeds a Data Center. Transients have blown filter capacitors, so the filter was removed and the line was energized while it still had a 150 kVa transformer and a 125 kVA system connected. Voltages were measured as high as 2100 Volts. I thought perhaps ferroresonance for this.
But when the loads were removed and the bus was energized, a 1900 Volt, 2ms spike was detected. My thoughts got the 540' section of Eaton Pow-R-Way III sandwich bus and the two (2) 150', 1000-amp busduct sections off of it.
Comments welcomed.
John M

 

[mayanees]

someone walked in and I didn't get to edit!

(repeated: moderator please delete the first if appropriate)
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I have a situation where I've measured transient voltages upon energization of a 700' section of 480V bus duct.
The busduct feeds a Data Center. Transients have blown SCIF filters, so the filter was removed and the line was energized while it still had a 150 kVa transformer and a 125 kVA UPS system connected, but there was little to no load. Transient Voltages were measured as high as 2100 Volts, 2 ms duration. I thought perhaps ferroresonance for this.
But when the loads were removed and the bus was energized, a 1900 Volt, 2ms spike was detected. My thoughts are that the 540' section of Eaton Pow-R-Way III sandwich bus and the two (2) 150', 1000-amp busduct sections off of it are acting lika a capacitor and that this is a characterisitc capacitive transient voltage response.
Comments welcomed.
John M

 

[SG-1]

I supect the spikes are being caused by the device used to energize the bus duct. Switching is a major cause of transients.

You might consider adding some transient overvoltage protection.

 

[mayanees]

Thanks SG for the comment.
I think it's resolved, with Eaton agreeing that by the nature of the design of the busway there is inherent capacitance. Since the system is unloaded the capacative reactance becomes significant, with no other loads to dampen it.
They sell Surge Protection Devices (TVSS Plug-ins, Clipper Power Systems Visor Series) for the busway. I will recommend one at each piece of critical equipment, of which there are two.
John M

 

[mull982]

Thanks SG for the comment.
I think it's resolved, with Eaton agreeing that by the nature of the design of the busway there is inherent capacitance. Since the system is unloaded the capacative reactance becomes significant, with no other loads to dampen it.
They sell Surge Protection Devices (TVSS Plug-ins, Clipper Power Systems Visor Series) for the busway. I will recommend one at each piece of critical equipment, of which there are two.
John M

Can you explain the theory of how this capacitive reactance causes voltage transients upon energization? I'm assuing your talking about capacitince coupling to ground?

 

[mayanees]

mull,

The best thing to do for an explanation is to google capacitive switching transients.

The explanation is rooted in a capacitor's inability to change voltage rapidly. The phenomena is experienced on energization and de-energization.

This isn't a capacitive coupling to ground, but an actual line capacitance associated with 800 feet of sandwiched plates that make up the busway. When the busway was unloaded there was a little less than one amp of current flow, which I believe was the equivalent of about 750 VARS of capacitance. The Dranetz measurement showed the 1900 Volt, 2ms spike occurred upon de-energization of the unloaded busway, with the amperage dropping from about 0.9 amps to 0.

John M