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Does your company (or clients) use grounding resistors anywhere in the power distribution system?
No
Hi Resistance Grounding
Low Resistance Grounding
Both Hi and Low
It depends - Many locations/clients
Doesn't apply to me
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 Post subject: Using Grounding Resistors
PostPosted: Sun Aug 28, 2016 8:33 am 
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One of the forum member's posts prompted this weeks question of the week. It is about grounding, more specifically - high and low resistance grounding. (some may use reactance grounding). Rather than having a solidly grounded system or a delta connected system, some will use a grounding resistor to limit the line-to-ground short circuit current that flows during a fault.

This week's question:
Does your company (or clients) use grounding resistors anywhere in the power distribution system?

No
Hi Resistance Grounding
Low Resistance Grounding
Both Hi and Low
It depends - Many locations/clients
Doesn't apply to me


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 Post subject: Re: Using Grounding Resistors
PostPosted: Sun Aug 28, 2016 4:03 pm 
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Above 10 kV it kind of becomes impossible to make high resistance grounding work because the impedance just doesn't work right. At that point though you can switch to low resistance (typically 400 A) grounding and keep on going up into 35.4 kV power systems.

I've noticed some definition problems though. Some claims are that 100 A or less is "high resistance" while others claim it is 10 A or less. There is a problem with going THAT low. If the current isn't set to about 300% of the ground current during normal operation, it starts to no longer be resistance grounded and starts responding like an ungrounded system. I've seen some GE and iGard systems for 480 V that are down in the range of 1-2 A, way too low.


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 Post subject: Re: Using Grounding Resistors
PostPosted: Sun Aug 28, 2016 6:45 pm 
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There is a break point between low resistance ground LRG and high resistance ground HRG somewhere around 5 to 10 kV. 480 and LV systems use high resistance and typically reduce the current to around 10 amps. I agree, 1 or 2 amps seems quite low. Medium voltage systems have to allow enough current to flow to overcome capacitive current. Often around 400 to 600 Amps with a 10 second rating.

The gray area seems to be in the 5 to 10 kV range. Typically capacitive current may not be that significant but it depends on how much shielded conductor and other variables. I've seen the size go either way in that voltage range.


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 Post subject: Re: Using Grounding Resistors
PostPosted: Mon Aug 29, 2016 6:47 am 
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Joined: Tue Nov 12, 2013 6:31 am
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Location: Port Huron, Michigan
Most, but not all, of our 480V secondary transformers are high resistance grounded. None of our MV transformers are grounded.


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 Post subject: Re: Using Grounding Resistors
PostPosted: Mon Aug 29, 2016 6:53 am 
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Joined: Sat Feb 27, 2010 5:59 pm
Posts: 71
Location: Cincinnati, OH
PaulEngr wrote:
Above 10 kV it kind of becomes impossible to make high resistance grounding work because the impedance just doesn't work right. At that point though you can switch to low resistance (typically 400 A) grounding and keep on going up into 35.4 kV power systems.

I've noticed some definition problems though. Some claims are that 100 A or less is "high resistance" while others claim it is 10 A or less. There is a problem with going THAT low. If the current isn't set to about 300% of the ground current during normal operation, it starts to no longer be resistance grounded and starts responding like an ungrounded system. I've seen some GE and iGard systems for 480 V that are down in the range of 1-2 A, way too low.


Typically above 10 kV the capacitance from phase to ground becomes large enough that high resistance grounding is not possible. This is typically due to the physical size of the system (too many feet of cable or line) or having surge capacitors on motors - more than a few machines above 10 kV with properly sized surge caps will disallow high resistance grounding.

The definition shouldn't be that complicated: In practical terms, once the let-through current (and the resultant heating at the point of the fault) is high enough that a ground fault escalates to a line-line-ground fault in less than a day, start to wonder if it is really high resistance grounding. If anyone claims this is possible anywhere near 100A then I have a bridge for sale.

The resistor let-through current doesn't necessarily need to be 300% of 3Ico - anything greater than 3Ico is theoretically acceptable. Some systems have a consistent capacitance to ground, while others, particularly systems with rectifiers connected, have a variable capacitance to ground. Depending on how heavily the rectifier is loaded the DC bus capacitors are coupled back onto the AC system in varying degrees. If the system has a lot of variable capacitance then sizing the resistor to 300% of the constant 3Ico value may make sense.

I have seen dozens of systems where 1-2 A worked fine at 480V. Some of them were by accident though - the manufacturer shipped the ground resistors with them connected for the minimum tap of 0.9 A. In one paper mill they all worked fine except for one transformer - it would occasionally have N-G voltage excursions, which were measured by the microprocessor HRG package controller. That transformer was feeding an ASD rated 75% of the size of the transformer, and depending on the speed and acceleration rate of the motor the system charging current fluctuated above the 0.9A resistor tap. Changing to a 2 or 3 A tap solved the problem, and the rest of the resistors in the plant were left at the 0.9 A tap.


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 Post subject: Re: Using Grounding Resistors
PostPosted: Mon Aug 29, 2016 7:06 am 
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Joined: Sat Feb 27, 2010 5:59 pm
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Location: Cincinnati, OH
Jim Phillips (brainfiller) wrote:
There is a break point between low resistance ground LRG and high resistance ground HRG somewhere around 5 to 10 kV. 480 and LV systems use high resistance and typically reduce the current to around 10 amps. I agree, 1 or 2 amps seems quite low. Medium voltage systems have to allow enough current to flow to overcome capacitive current. Often around 400 to 600 Amps. I frequently used 600 A resisters with a 10 second rating.

The gray area seems to be in the 5 to 10 kV range. Typically capacitive current may not be that significant but it depends on how much shielded conductor and other variables. I've seen the size go either way in that voltage range.


The LRG resistors should be sized according to the minimum sensitivity of the relaying scheme that will detect the L-G fault. The most common value of 400A works well for a residually connected time overcurrent ground relay using 1200 A phase CTs. If you have zero sequence CTs on the feeders and a transformer neutral CT, 400A is much higher than necessary for a unit substation with incoming transformers. System designs with master subs and unit subs at the same voltage level often need 400A or more since in some locations it is not possible to have a zero sequence CT and residual connections must be used.

Another factor to consider is the bus differential relaying. Some bus differential schemes may not be able to detect 100A (or 400A) of fault current.


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 Post subject: Re: Using Grounding Resistors
PostPosted: Mon Aug 29, 2016 7:28 am 

Joined: Mon Oct 19, 2009 7:42 am
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At my site we use low resistance grounding for our medium voltage distribution system, 22kV and 6.9kV. Resistors sized to limit fault current to 2000A, but recently, new MV substations installed use low resistance grounding to limit fault current to 1000A. Our distribution feeders are underground routed in duct banks and manholes.

A couple docs describing resistance grounding.


Attachments:
The-Grounding-of-Power-Systems-Above-600-Volts-A-Practical-View-Point.pdf [226.03 KiB]
Downloaded 17 times
Resistance_Grouding.pdf [56.88 KiB]
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 Post subject: Re: Using Grounding Resistors
PostPosted: Mon Aug 29, 2016 8:09 am 
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Typically this is Project / owner specific, in the MV world [4160V – 15kV] I have seen the use of low resistance using the 400A for 10 second as indicated in previous post, however some utilities are still providing ungrounded systems at the service points. For LV systems [208-600V] high resistance grounding however not typically for reasons related to fault current [sometimes used when large amount of ground fault current is available] but for continuity of service in facilities where operations try to minimize downtime where downtime equals lost $$. This allows the benefits of an ungrounded type system without the problems they can create, however being cognizant and doing ones due diligence to insure a properly designed / specified and installed system. I have seen these types of systems specified and installed from manufacturing facilities, to hospital to data centers.


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 Post subject: Re: Using Grounding Resistors
PostPosted: Mon Aug 29, 2016 12:11 pm 
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I've successfully used 25 A for 4160 V and 7200 V for years. That's high enough to get above the system capacitance for most mining and milling operations but still below the ratings of the wiring so that it can operate at this level indefinitely (not that you should do that). At 4160 V the resistor is a small box. At 7200 V the resistor is about the size of the transformer in horizontal (length x width) dimensions. Lower current would decrease I*I*R but at some point system capacitance kicks in. Also of note is that at 4160 V with that size resistor if I recall correctly touch currents work out to 94 mA. At 7200 V it's above this but if we trip in 3.5 s or less, we can stay below Dalziel's threshold for fibrillation with a line-to-ground touch fault condition.

At 480 V I've done the calculations and taken measurements and worst case I've seen is around 3-4 A so with a "300% rule" that works out to around 10-15 A as the ideal resistor size. This is small enough that the resistor doesn't take too much space.

400 A seems ot be somewhat of a standard for low resistance systems although nothing is cast in stone. This is low enough that wiring isn't going to be destroyed right away but still requires some fairly quick response (10 s or less). At this point safety concerns sort of don't apply anymore (it will kill you) and my approach switches over to a question of how much fault current can the equipment tolerate and for how long.

One thing is for sure with high resistance grounding...fault finding becomes difficult because you don't just look for the big, black spot.


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 Post subject: Re: Using Grounding Resistors
PostPosted: Mon Aug 29, 2016 3:09 pm 

Joined: Wed Jun 29, 2011 3:33 pm
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High Resistance Grounding (TT systems) are used extensively in the coal mining industry in Australia, earth fault being cleared using earth leakage relays. 11kV distribution throughout the mine is common, higher than this not as much. 11kV restricted to 5 or 10A, EL relays tripping no more than 500mA/500ms. LV is limited to 5A, again tripping no more than 500mA/500ms.

Have only seen low resistance grounding where transmission voltages are distributed from one site to another (eg from switchyard to prep plant or to UG mine area) via aerials.

Helps having the most common type of fault as a relatively low energy fault. Not safe of course. Also, it does mean the ph-e cable insulation has to be rated for ph-ph voltages.


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 Post subject: Re: Using Grounding Resistors
PostPosted: Mon Aug 29, 2016 10:22 pm 

Joined: Sun Feb 14, 2010 9:28 pm
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Location: Queensland
IEEE 142 defines LRG as typically >100A.
HRG is useful in preventing arcing earth faults escalating to 3ph faults, providing sufficient time for fault clearance.
HRG is particularly useful for (1kV – 11kV) MV MCCs also in preventing under-voltages from 3ph faults occurring.
On overhead distribution networks (6.6kV – 38kV) supplying fuse protected distribution transformers, those fuses may be 6A – 63A, so to clear an earth fault in an acceptable time, will require LRG to supply perhaps more than 10 x the fuse rating. If coordinating with line feeder earth fault protection, then more likely on the higher side, so it is easy to see why 400A – 2000A LRGs may be required.


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