NG Resistor and Parallel Sources

[billyzee]

RC Wilson

I started this thread as a result of my involvement with an existing system that is configured precisely as you described in your previous post, ?Most MV systems are 3P3W with each transformer and generator neutral grounded independently. When a transformer or generator is off line it does not contribute any fault current. The fault level varies with the number of sources in parallel.?

I guess now I am suffering the from the problems with these systems that you warn about in the first sentence of your previous post, ?The change in ground fault current as additional sources are paralleled is why most practical systems use either a single ground point or disconnect switches on the generators' neutral resistors.?

There is no way, I?ll get disconnects added to the resistors as you described, so I think I?m stuck with dealing with the variable SLG fault current (400-5800A).

 

[iceworm]

billy -
I've got some experience with 13.8KV distribution, from multiple generation sources. This should be similar to the 4.16KV you are postulating. That's why I asked about your constrainsts. But, I couldn't help with your questions. I've never seen a low impedance grounded system. I don't design them, but I certainly have put in plenty of time doing or verifying the coordination studies.

RC (and others) have some good points.

...My conclusion is that for a system with several resistance-grounded sources the SLG fault current is quite variable depending on the number of parallel sources that are on-line. ...

All of the ones I have been around were HRG, limiting the ground current to maybe 50A. relaying set to interupt is .5S. Never a question of shield damage.

...I may end up starting a different thread associated with exactly which relay type (51N?) is best for protecting a MV cable shield from overheating due to fault currents running in the shields, but I'm going to talk to some vendors first.

Be interesting to see what the vendors say. Have you plotted out the cable damage curves on a T-C chart? You may wish to do so. Add the available SSC. This will tell you exactly what the protective relay curve has to look like.

With a 6ohm resistive ground on multiple sources, I don't see much chance of success. The SSC is outside on the damage curve. Pretty hard to protect the cable when the relay has to trip before the fault occurs.

But none of this is what you asked - sorry I couldn't help

ice

 

[Phil Corso]

BillyZee...
Following is my reply to similar question on another forum, "Why is the MV system-neutral grounded, especially when a 4th or neutral-conductor is never required!?"

ANSWER: If it were possible to insulate electrical equipment with glass, this discussion would hardly be necessary. Early in electrical history ungrounded poly-phase systems were used somewhat optimistically, that is, it was possible to maintain operation with one-phase grounded. That is, until such operation leads to over-voltages (both transient and continuous) resulting in a substantial increase in subsequent damage. (Although, somewhat loosely connected to this discussion it is interesting to note that LV ungrounded systems are mandated by the USA NEC (National Electric Code), but only in specific situations.)
The premise behind system-neutral grounding, at any voltage level, is to mitigate the effects of ground-fault current flow, i.e., burned-insulation, or melted magnetic-iron. An LV system-neutral can certainly be grounded through an impedance to limit fault-current magnitude (of the order of tens of thousands of kVA) but solid-grounding eliminates the need to install additional ground-fault protective devices. The phase over-current protective device is sufficient. Repair or replacement of LV equipment is (relatively speaking) easily handled, both in material and cost!
Any MV system-neutral can be solidly-grounded, but the resultant ground-fault current is much greater than LV systems (of the order of hundreds of thousands of kVA.) But, if damage is kept low, reasonable repair is possible. If, however, current magnitude is very large, damage to any magnetic-structure like stator-iron in a motor or transformer-core, may preclude repair! Thus requiring total replacement... with its attendent impact on curtailing plant operation!
One way to look at it, resistance-grounding of an MV system-neutral (limiting current to say 100-400A) is more of an economic choice. Almost with certainty, the Benefit-to-Cost Ratio justifies its implementation.

Related Topic 1: The 51N Relay.
Its primary fuction is to protect that part of the system from the transformer to the secondary incomng-breaker (although its efficacy to protect a transformer's winding is questionable!) Its secondary function is to provide "backup" for secondary feeder-breakers. Thus, it is selectively coordinated with feeder-breakers. Typically, they are equipped with protective relays having an instantaneous element, such as the 50N.
Operating your system with high ground-fault capability (assuming the ystem isn't a "Spot-Network" arrangement) you wil certainly experience major iron-core damage!

Related Topic 2: Multiple-Grounding of Cable-Shields
Such practice will neagtively impact ground-fault protection reliability, especially if a toroidal-core (zero-sequnce) CT) is used!

Regards, Phil

 

[Julius Right]

Julius,

"I think the direction protection of this "source" has to work if 4800 A is not enough to put into operation the short-circuit protection relay-and to limit ?in a way-the damage."

If a directional protection is located close to Bus Bar transformer breaker, then will open it when the energy will flow from the Bus Bar into transformer cable. However, this can work if the Power Plant will never supply energy to the outside Grid through this transformer.
For a generator case, the directional protection has to work.

 

[billyzee]

Julius.
Thats a hellava picture. I only have one question. Where is that 3200A (400*8) in the lower right going? Is the scenario 8 more generators that aren't shown. Thanks again.