High resistance grounding and MOVs.

[birkbeck5275]

Hello!

I have been tasked with converting an entire factory from a solidly grounded to high resistive grounded 480V system (HRG).
We will be utilizing a Post Glover SmartPulse system and have already evaluated the existing distribution to be a 3P/3W with no L-N (277V) loads.
My question concerns MOVs.
Rockwell VFDs and Servo drives (PowerFlex 40, 525, 535, 700, 700S, 753, 755 and Kinetix 6000, 5700) instruction manuals for all of these drives state that if connected to an ungrounded system, the MOVs must all be disconnected. This is a considerable task since the production line has over a hundred devices, but we are scheduling and budgeting for it.
Other than 480V VFD and servos - can anyone think of other 480V equipment which would have a sensitivity to HRG?

 

[jim dungar]

An HRG is not an ungrounded system.
Your only real concern should be the amount of current you will have on your ground. Your HRG needs to be able to handle this amount without alarming. There are ways to measure this current on existing current.

 

[ron]

HRG is grounded, just through a resistor.
I have run into UPS systems that need rework when installed on a HRG, due to how they reference ground.

 

[MrJLH]

Not some much on the equipment side, but need to consider a charging current system test.

 

[David Castor]

Any type of surge suppression would need to be evaluated. They would have to have a Max Continuous Operating Voltage (MCOV) of over 480 V. We would generally use about 110% of 480 V and pick the next higher voltage rating. With the high resistance grounding, during a ground fault, the voltage to ground on the unfaulted phases would be 480 V until the fault is removed.

 

[jim dungar]

Any type of surge suppression would need to be evaluated. They would have to have a Max Continuous Operating Voltage (MCOV) of over 480 V. We would generally use about 110% of 480 V and pick the next higher voltage rating. With the high resistance grounding, during a ground fault, the voltage to ground on the unfaulted phases would be 480 V until the fault is removed.

How? One advantage of HRG is the stabilization of the L-N voltage during a fault versus an ungrounded system.

 

[David Castor]

If one 480 V phase is tied to ground, the phase-to-ground voltage for the other two phases will be 480 V. For any type of resistance grounding, arresters should be rated for full phase-phase voltage. High resistance grounding behaves in a very similar manner to ungrounded systems but it will reduce severe overvoltages due to repetitive restrikes.

 

[jim dungar]

If one 480 V phase is tied to ground, the phase-to-ground voltage for the other two phases will be 480 V. For any type of resistance grounding, arresters should be rated for full phase-phase voltage. High resistance grounding behaves in a very similar manner to ungrounded systems but it will reduce severe overvoltages due to repetitive restrikes.

In an HRG system you cannot tie one leg to ground without creating a fault.
This basically a wye system.

 

[winnie]

An HRG system is used when you want the voltage stability of a grounded system, blended with the continuity of service during a fault found in an ungrounded system. A single fault in an HRG system should permit operation until the fault can be safely addressed and repaired.

This means that in normal operation you essentially have a wye system, but during fault conditions you will end up with a corner grounded system which might remain in that state for an extended period of time.

-Jonathan

 

[David Castor]

In an HRG system you cannot tie one leg to ground without creating a fault.

True, but for low voltage HRG systems, nothing will normally trip, so this fault condition will remain until someone traces down the fault location, or there is a second ground fault in another phase.

 

[jim dungar]

True, but for low voltage HRG systems, nothing will normally trip, so this fault condition will remain until someone traces down the fault location, or there is a second ground fault in another phase.

Yes, most low voltage systems are designed with resistors that limit the fault current to 5A.

The performance, not tripping a large OCPD, of a HRG system during a fault is not significantly different than that of a normal wye with a line to ground fault that has high resistance. This fault behavior is why GF protection is required on 480Y/277V systems.

 

[SAL568]

Can we use the rebar that is protruding from a column of the same building as grounding electrode?