High Resistance Grounding in 15kV Above Application? 4

[nightfox1925]

I have read and, in some projects, applied HRG system on 480VAC and 4.16kV Application. I haven't heard of any 13.8kV and above application. The IEEE states the ampere limitation of the HRG system but does not address limitation on voltage level application. Publications from postglover states that these systems are limited only up to 5kV but did not provide any standard reference limiting them to do so. Hence, at what voltage level this can be applied and if there are limitations, what are the reference standard? article?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[davidbeach]

Distribution systems are generally solidly grounded if fed from a wye transformer, occasionally low resistance grounded. This makes it easier for the ground fault to draw enough current to ensure rapid clearing. The use of fuses on distribution taps requires fault currents high enough to blow the fuse.

Solidly grounded systems also permit the use of line-to-neutral rated lightning arrestors whereas resistance grounded systems require line-to-line rated lightning arrestors. The line-to-neutral rated arrestors provide much better protection than line-to-line would.

Utility generators, on the other hand, are almost universally high resistance grounded to protect the stator against damage from internal ground faults. The high side of the GSU provides the solid ground for the transmission system. The generator grounding resistor is usually a 240V resistor on the secondary of a distribution transformer in the neutral. That way a resistor of an ohm or less can provide primary current limitation in the 5-10A range and only need be isolated for 120V to ground. A resistor directly connected between the neutral and ground would have to be in the several k-ohm range and isolated for line-to-neutral voltage of the generator.

 

[nightfox1925]

Thanks david. But at what voltage is the utility generator being operated? Was there step-down transformer as part of the HRG system. Is this the only application so far where HRG was used in systems 5kV and above?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[ScottyUK]

nightfox,

HRG is used on generators because the low fault current protects the hugely valuable stator core from major damage in the event of an earth fault. The vast majority of large generators connect to a step up transformer located physically close to the generator, usually also to auxiliary and excitation transformers fed from the generator main terminals. As a result the area protected by the HRG system is relatively small and well-defined so many of the problems found with applying HRG to a distribution network don't occur or at least are insignificant.

Most large generators operate in the approximate range 11kV up to about 25kV or so. Above 25kV insulation becomes a problem because of the amount of unproductive space required in the stator slots simply to insulate the conductors.

Generator HRG systems almost universally use a single phase distribution transformer loaded with a physically robust low ohmic value resistor.


----------------------------------

I don't have an attitude problem. You have a perception problem...

 

[rcw retired EE]

As David stated, the primary use of high impedance grounded systems above 5 kV is for larger generators (11 kV- 28 kV) with a grounding transformer / low voltage resistor combination.

We have put resistors on systems up to 7.2 kV, limiting the current to about 400A, low enough to minimize ground fault damage to motor stators but high enough for secure fault detection and relay coordination.

At 11 kV and higher we switch to the transformer/resistor and limit the fault current to about 10 amps to protect the generator stators. We would solidly ground the system if it only had distribution and no generators.

 

[RalphChristie]

nightfox1925

The highest voltage I have seen using high-resistance grounding was 11kV. I have once come across it in an underground mine. This was a flame-proof mine, and I think this was the reasoning for using such a grounding-system.

Another reasons for using high-resistance grounding in the mining-industry on MV-systems, is to reduce step and touch potentials. We use it on the trailing cables of our earth-moving equipment, rated 6.6kV. (open-pit mining)

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376

 

[nightfox1925]

Thank you gentlement for all your inputs. If there are other applications above 5kV in your experience, feel free to share it here.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[Runsor]

I guess this discussion is about North America, but anyway:
In Europe ungrounded networks and Petersen coil grounded i.e. compensated networks does exist at least in Scandinavia and AFAIK in France and Italy.
In fact in Norway, Sweden and Finland all utility distribution networks at 10 kV and at 20 kV are ungrounded or compensated. Earth fault protection is done with cable CTs and sensitive directional residual overcurrent function. (These networks are driven readially and thus directional residual function gives complete selectivity.)

 

[TestBeforeTouch]

The very large industrial plant I am at has been using HRG at 13.8KV for approximately 40 years. The current is limited to 10 amps. We make many types of plastic and continuity of service is critical so that the plastic does not solidify in the pipes. The plant uses a primary selective system so that when a fault occurs we switch to the alternate feed and then take the faulted feeder out of service with no disruption to the business.

 

[slavag]

Hi Ransor.
Sorry for Q, what is AFAIK in France and Italy?
Are you know, what is a distribution networl in Spain and German?
Regards.
Slava.

 

[lz5pl]

Just to share my experience also:
Here in Bulgaria we use low-resistor neutral grounding - typically limiting earth-fault current to 300 A - for almost all distribution grid. With privatization of distribution utilities new owner of one of them - Austrian company EVN - is going to change it's grid to compensated grounding - this grid is 10 and 20 kV.
Of course mining grids are isolated neutral systems for obvious reasons.
The highest voltage level I have seen with isolated neutral is Albanian grid - including 35 kV system. I work there two years ago on a rehabilitation project and was surprised to see that they prefer to keep it isolated even with new equipment.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[Don348]

By way of sharing experience.

I am familiar with several high-resistance grounded systems at 10 or 15 A up to 35 kV. In addition I am familiar with 72 kV resistance-grounded systems with let-through current of 200 A.

I my experience neutral-grounding resistor manufacturers (for example Powerohm) are quite capable of producing NGR's rated for line-to-neutral voltage on systems up to 72 kV.

The advantage of a directly connected resistor is limiting in-rush current and limiting current on rectified faults like a fault on a VFD.

 

[Yuma]

In their generators, Brown Boveri used to limit the fault to 5 Amps, with a directly connected resistor -i.e., no distribution transformer.

Some other makers used to limit the fault in their generators to values between some hundreds to 1000 Amps, but in our company we don't consider this kind of grounding to be safe for the generator, and in some of this old generators, we have been changing the grounding to either 5 or 10 Amps. With this fault level, a relay is perfectly capable of identifying a stator-earth-fault in the 95% of the armature winding (100% stator-earth-fault detection normally requires a different technology), so we see no reason to allow a higher fault level.

 

[davidbeach]

100% stator-earth-fault detection normally requires a different technology

Say what?? Any decent modern generator relay can provide 100% stator earth fault protection.

 

[PowerfulStuff]

Hi All,
One other really good reason to use solid grounding at higher voltages is that transformers can have graded insulation i.e. not much insulation at the neutral end of the winding and progressively increasing towards the line end of the winding. This generates a substantial saving and reduces transformer impedance. I don't make transformers but have seen 33kV as the turning point - above this it is worthwhile, below this not.

Martin

 

[Don348]

"One other really good reason to use solid grounding at higher voltages is that transformers can have graded insulation"

It is my understanding that graded transformer insulation applies only to single phase transformers. The only time it would apply on a three-phase system is when three single-phase transformers are ganged together to form a three-phase transformer.

Is that correct?

 

[davidbeach]

The wye winding of a three phase transformer can have graded insulation. The delta winding of a three phase transformer, or any single phase transformer used to form a delta must have full insulation for the whole winding.

 

[ScottyUK]

Graded insulation is frequently encountered on large transmission system transformers where star connected auto-wound types are common. At 275kV and 400kV (UK voltages) insulation is thick and takes up a disproportionate amount of the winding aperture. Reducing the amount of unproductive space dedicated to insulation results in a worthwhile saving in core steel and ultimately in cost.


----------------------------------

I don't have an attitude problem. You have a perception problem...

 

[cuky2000]

I believe the rational of limiting the HRG to low and MV is a combination of economic reasons associated with the cost of higher insulation level, requirement of more sensitive protection system to detect the fault, limit fault current below 25 A, tripping and safety practices.

On systems above 5kV, low resistance grounding (LRG) is perhaps more common than HRG because:
• LG fault currents > 25A.
• The system capacitive charging current > 25A.
• Let-through currents in the neutral grounding resistor > 10A.

For generator applications, the rational is associated with the fact that the zero sequence impedance is lower than the positive and negative impedances. This result in having a single line to ground fault usually higher than the three phase fault. Some of the usual generator grounding standard practice is as follow:
• Solid grounded for small unit in commercial and industrial applications.
• LRG for 10-50 MVA with resistor rating from 100A to 1200 A (typ. 400A)
• HRG for > 100 MVA with resistor connected grounding transformer. (This scheme reduce the size of resistors)

 

[ScottyUK]

The main reason for HRG on utility sized generators is to protect the hugely valuable stator core from catastrophic damage in the event of an earth fault. A core wreck could leave a generating unit out of service for upwards of a year while a new core was fabricated: generator and turbine shops have grossly extended lead times at present due to lack of world capacity to meet demand and raw materials are at a premium. Reduction in the winding bracing requirements is a secondary benefit which could be overcome.


----------------------------------

I don't have an attitude problem. You have a perception problem...