We are working with a system that incorporates a neutral resistive grounding system and our drive manufacturer is taking heartburn to this. They claim that the float associated with the potential to ground during a ground fault condition can be detrimental to their drives. I was wondering if anyone had any experience with these power distribution systems, and/or specifically with ground faults and drive systems?
Neutral Resistive Grounding
- Thread starter Jasonc24
- Start date
On ground faults and variable speed motor drives, once I was called to evaluate a problem at a sawmill. A new wood-fired boiler and generator were being installed by a contractor. A 30 horsepower VFD had failed 3 times. After the first failure, the drive manufacturer said it was failing because of a poor earth ground. The owners, at great expense, installed a 2/0 ground wire from their well casing 500 feet away, the only place to find an adequate ground, because the soil consisted of years of built-up wood bark and chips.
After the second drive failed, the contractor said it must be because of a poor power supply and voltage surges on the line. Again at great expense, the owner installed an isolation transformer and power conditioner. When the third drive failed, the owner fired the contractor and called me. I asked around about how the drives had failed. Lots of smoke, blown fuses, arcing, and heat. I thought that this did not seem like a grounding problem. It sounded like a short circuit. I started to check the wiring from the motor with a megohmmeter. I first removed the motor terminal cover and saw a burned spot. This was a SEW brand motor, and it had screw terminals closely spaced. Two of the phase wire lugs were touching. I fixed the terminals, then started up the FOURTH drive. It worked great.
Be skeptical when someone insists that unknown drive problems are caused by "poor grounding". It is possible of course, but check everything else first.
Why do you have resistive grounding? I hope, for your worker's safety, that you do not have a Delta-Delta supply transformer with no hard ground point.
After the second drive failed, the contractor said it must be because of a poor power supply and voltage surges on the line. Again at great expense, the owner installed an isolation transformer and power conditioner. When the third drive failed, the owner fired the contractor and called me. I asked around about how the drives had failed. Lots of smoke, blown fuses, arcing, and heat. I thought that this did not seem like a grounding problem. It sounded like a short circuit. I started to check the wiring from the motor with a megohmmeter. I first removed the motor terminal cover and saw a burned spot. This was a SEW brand motor, and it had screw terminals closely spaced. Two of the phase wire lugs were touching. I fixed the terminals, then started up the FOURTH drive. It worked great.
Be skeptical when someone insists that unknown drive problems are caused by "poor grounding". It is possible of course, but check everything else first.
Why do you have resistive grounding? I hope, for your worker's safety, that you do not have a Delta-Delta supply transformer with no hard ground point.
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DickDV
Member
There are two types of resistance grounding on wye-source power supplies. The first is usually called low resistance grounding and serves only to limit ground fault current to reduce collateral damage. The ground fault current is set to be high enough to clear circuit breakers and fuses. This type of resistance grounding is not hazardous and is not distructive to AC drive systems.
The second type of system is high resistance grounding. This resistance from the center of the wye secondary of the substation transformer to ground is high enough that, with one phase leg grounded, the power supply continues to work. It is, in fact, similar to floating delta source systems (no neutral). These power networks are proliferating around the country and are hazardous to personnel and damaging to drives and other electronic three phase loads.
I just went thru a huge struggle trying to get two 1.5hp 460V drives to operate properly in a customer's facility. The problem was that the proper input fuses for these small drives is 10 amps and the facility (floating network) had more than 10amps of phase to ground leakage. The leakage current would find a path back to the power source thru the virtual neutral in the inverter and would blow the fuses. The solution was to install a drive isolation transformer (delta primary-wye secondary) and create a balanced grounded power supply. The leakage currents will have to find their way back to the source some other way.
Even tho the transformer KVA was small in this case, it created huge ill will between the customer and me and, frankly, made an otherwise good job into a massive financial loss (unless my time is worthless, of course!!).
Several years ago I was nearly killed by a floating network which was running with one leg grounded. I was inspecting an old analog DC drive on this network and with the disconnect open and all meters showing zero, I climbed up on the machine holding on with one hand and reaching inside an enclosure to get some wiring drawings with the other hand. As my hand passed by the stud on the armature ammeter, an arc nailed me in the knuckle and I got 700VDC from arm to arm. I'm lucky to be alive to tell about it even tho I spent three months healing up. The out-of-balance network had the scr section of the DC drive floating 700VDC above ground and I got nailed. I will not start up or service a DC drive today without an isolation transformer ahead of it.
The second type of system is high resistance grounding. This resistance from the center of the wye secondary of the substation transformer to ground is high enough that, with one phase leg grounded, the power supply continues to work. It is, in fact, similar to floating delta source systems (no neutral). These power networks are proliferating around the country and are hazardous to personnel and damaging to drives and other electronic three phase loads.
I just went thru a huge struggle trying to get two 1.5hp 460V drives to operate properly in a customer's facility. The problem was that the proper input fuses for these small drives is 10 amps and the facility (floating network) had more than 10amps of phase to ground leakage. The leakage current would find a path back to the power source thru the virtual neutral in the inverter and would blow the fuses. The solution was to install a drive isolation transformer (delta primary-wye secondary) and create a balanced grounded power supply. The leakage currents will have to find their way back to the source some other way.
Even tho the transformer KVA was small in this case, it created huge ill will between the customer and me and, frankly, made an otherwise good job into a massive financial loss (unless my time is worthless, of course!!).
Several years ago I was nearly killed by a floating network which was running with one leg grounded. I was inspecting an old analog DC drive on this network and with the disconnect open and all meters showing zero, I climbed up on the machine holding on with one hand and reaching inside an enclosure to get some wiring drawings with the other hand. As my hand passed by the stud on the armature ammeter, an arc nailed me in the knuckle and I got 700VDC from arm to arm. I'm lucky to be alive to tell about it even tho I spent three months healing up. The out-of-balance network had the scr section of the DC drive floating 700VDC above ground and I got nailed. I will not start up or service a DC drive today without an isolation transformer ahead of it.
Jiri Toman
Member
- Join Date
- Jun 2002
- Posts
- 498
Dick,
You couldn't have described it better. I have had similar experience with an electrically driven injection molding machine.
We are talking here large servo drives. Had to provide big *** isolation transformer with WYE on the secondary just like you have outlined. Failure to do that resulted in many fault trips.
Machine just would not operate without a fault trip for any
length of time.
The key is to properly terminate the neutral from the secondary of the transformer to the drive.
One more point. Facilities with high resistance grounding system do have lights or other indicators to show if a phase is grounded.
Eliminating the ground short is of course something you should do first. Sometimes though it is quite a job finding it.
You couldn't have described it better. I have had similar experience with an electrically driven injection molding machine.
We are talking here large servo drives. Had to provide big *** isolation transformer with WYE on the secondary just like you have outlined. Failure to do that resulted in many fault trips.
Machine just would not operate without a fault trip for any
length of time.
The key is to properly terminate the neutral from the secondary of the transformer to the drive.
One more point. Facilities with high resistance grounding system do have lights or other indicators to show if a phase is grounded.
Eliminating the ground short is of course something you should do first. Sometimes though it is quite a job finding it.
Bruce99
Member
Grounding
Ouch! getting zapped sucks..I know... A ground fault sensor is requiured by code on the feeder suppling the loads on the MCC. Ground lights on the MCC are also required. The sensor fault trip setting must be below the tap setting on the resistor, or it will never trip. There are a lot of people that forget this. Some dont even look at the taps on the resistor. When the ground lights show a ground, it becomes a priority to repair, for safety reasons. Production in some cases is stopped. If a ground occures on another phase, you now have a phase to phase fault.
Ouch! getting zapped sucks..I know... A ground fault sensor is requiured by code on the feeder suppling the loads on the MCC. Ground lights on the MCC are also required. The sensor fault trip setting must be below the tap setting on the resistor, or it will never trip. There are a lot of people that forget this. Some dont even look at the taps on the resistor. When the ground lights show a ground, it becomes a priority to repair, for safety reasons. Production in some cases is stopped. If a ground occures on another phase, you now have a phase to phase fault.
Leadfoot
Member
One of the things that hurts drives with ungrounded input power is called COMMON MODE. You have the proper voltages line to line. But your line to ground floats up and down.
An engineer I used to work with described it like a jugler on a trampolean. the juggler juggles 3 balls, representing 3 phase power, and then he jumps up and down on the trampolean, representing neutral to ground.
The balls all hit a high spot about the same distance to ground. When the juggler is bouncing on the trampolean, at the point of the highest bounce, the ball that is currently the highest one juggled, is substancially higher than normal. If this "BOUNCE" happens to 3 phase power, the voltages can be much higher in potential than the insulation of the drive and motors are capable of handling. POP goes the fuse if you are lucky. Unfortuantely, these voltages manifest as spikes or needles that punch holes in the insulation. BANG, there goes the drive as it seems to always be the weak link. Most modern processor based controls are extremely sensitive to common mode. That is another reason why proper grounding is necessary. If you have a drive blow for what ever reason, ALWAYS, ALWAYS, ALWAYS check the motor. It too has a problem. Drives generally DO NOT fail, they are blown up from something outside causing them to BLOW.
I agree with DickDV about isolation transformers in front of drives. I have gotten into the habit of always checking the line to ground voltages when I go into a new facility where I do not know how well their power grid is protected. Especially OLD ones with rusty cabinets. I have seen sparking of the cabinet to the conduit once too often.
Several times the expression "AT A GREAT COST" has been used to properly protect and ground the equipment. That cost is nothing compared to the $$$ that will be paid to the family of the person that got killed due to poor grounding of electrical equipment. Not to mention having OSHA crawling up your anal orfice because of it.
An engineer I used to work with described it like a jugler on a trampolean. the juggler juggles 3 balls, representing 3 phase power, and then he jumps up and down on the trampolean, representing neutral to ground.
The balls all hit a high spot about the same distance to ground. When the juggler is bouncing on the trampolean, at the point of the highest bounce, the ball that is currently the highest one juggled, is substancially higher than normal. If this "BOUNCE" happens to 3 phase power, the voltages can be much higher in potential than the insulation of the drive and motors are capable of handling. POP goes the fuse if you are lucky. Unfortuantely, these voltages manifest as spikes or needles that punch holes in the insulation. BANG, there goes the drive as it seems to always be the weak link. Most modern processor based controls are extremely sensitive to common mode. That is another reason why proper grounding is necessary. If you have a drive blow for what ever reason, ALWAYS, ALWAYS, ALWAYS check the motor. It too has a problem. Drives generally DO NOT fail, they are blown up from something outside causing them to BLOW.
I agree with DickDV about isolation transformers in front of drives. I have gotten into the habit of always checking the line to ground voltages when I go into a new facility where I do not know how well their power grid is protected. Especially OLD ones with rusty cabinets. I have seen sparking of the cabinet to the conduit once too often.
Several times the expression "AT A GREAT COST" has been used to properly protect and ground the equipment. That cost is nothing compared to the $$$ that will be paid to the family of the person that got killed due to poor grounding of electrical equipment. Not to mention having OSHA crawling up your anal orfice because of it.
Leadfoot,
I will put my grounding designs up against yours any day. I beleive in a good grounding system. However I do not have any control over what owners of sawmills install in their facilities.
DickDv,
Those resistor non-grounds and delta floating ground systems should have to be maintained by the man who made the decision to install them. I beleive that these systems are bleed-over designs from the power intergrid engineers, who use only these types on the main substations. It kind of makes sense in that environment, where ony trained linemen work on the systems, and keeping the power on is the main goal. A tree falling on a line may not trip the breaker if only one phase is grounded. I think this idea has been carried over into the industrial plants, where it should never have been used.
In an industrial plant, where you have a lot of people working, some not familar with electrical principles, an ungrounded or high-resistance ground, is very dangerous. If a breaker doesn't trip on ground fault current, then there is not much protection.
Let us all work to eliminate such systems from our plants. The place to start is to investigate the hazards of resistive grounding and wye-delta ungrounded transformers, and then make everyone at your site aware of them.
I will put my grounding designs up against yours any day. I beleive in a good grounding system. However I do not have any control over what owners of sawmills install in their facilities.
DickDv,
Those resistor non-grounds and delta floating ground systems should have to be maintained by the man who made the decision to install them. I beleive that these systems are bleed-over designs from the power intergrid engineers, who use only these types on the main substations. It kind of makes sense in that environment, where ony trained linemen work on the systems, and keeping the power on is the main goal. A tree falling on a line may not trip the breaker if only one phase is grounded. I think this idea has been carried over into the industrial plants, where it should never have been used.
In an industrial plant, where you have a lot of people working, some not familar with electrical principles, an ungrounded or high-resistance ground, is very dangerous. If a breaker doesn't trip on ground fault current, then there is not much protection.
Let us all work to eliminate such systems from our plants. The place to start is to investigate the hazards of resistive grounding and wye-delta ungrounded transformers, and then make everyone at your site aware of them.
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Leadfoot
Member
Lancie1 said:
I was not challenging your designs. I don't design them, more often I end up trying to get them correctly installed.
I was stating an opinion that the cost of good grounding is nothing compared to the results of poor grounding.
When owners of small business balk at the $$$ I drag out a four letter work and use it repeatedly.
That four letter work is ......SAFE.......... I use several forms of it too, UN SAFE and SAFETY are 2 others I repeatedly direct at them.
I also write down what I see is wrong and how to correct it on my work order so I can cover my backside.
Lancie1 said:
I do believe you said it much better than I did.
BTW, how do I get in touch with you next time I have a customer that needs a good ground system?
Jiri Toman
Member
- Join Date
- Jun 2002
- Posts
- 498
I think that you guys are missing something. High resistance grounding is a preferred method in modern plant power distribution systems, as opposed to ungrounded or grounded.
Here is an excerpt from IEEE:
IEEE Standard 242-1986 Recommended Practice for the Protection and Coordination of Industrial and Commercial Power Systems 242-1986 section 7.2.5 offer the following perspective:
"Ungrounded systems offer no advantage over high-resistance grounded systems in terms of continuity of service and have the disadvantages of transient overvoltages, locating the first fault and burndowns from a second ground fault. For these reasons, they are being used less frequently today than high-resistance grounded systems"
[font=Arial, Helvetica, sans-serif]There are many benefits from grounding the electrical distribution system including:[/font]
[font=Arial, Helvetica, sans-serif]A solidly grounded system is one in which the neutral points have been intentionally connected to earth ground with a conductor having no intentional impedance and this partially reduces the problem of transient over-voltages found on the ungrounded system.[/font]
[font=Arial, Helvetica, sans-serif]While solidly grounded systems are an improvement over ungrounded systems, and speed the location of faults, they lack the current limiting ability of resistance grounding and the extra protection this provides. The destructive nature of arcing ground faults in solidly grounded systems is well known and documented and are caused by the energy dissipated in the fault. A measure of this energy can be obtained from the estimate of Kilowatt-cycles dissipated in the arc:[/font]
[font=Arial, Helvetica, sans-serif]Kilowatt cycles = V x I x Time/1000.[/font]
[font=Arial, Helvetica, sans-serif]In the same IEEE Standard as reference above, section 7.2.2 states that:[/font]
[font=Arial, Helvetica, sans-serif]"one disadvantage of the solidly grounded 480v system involves the high magnitude of ground-fault currents that can occur, and the destructive nature of arcing ground faults."[/font]
[font=Arial, Helvetica, sans-serif]Since the vast majority of arcing faults start their life as single-phase faults, the key to reducing their impact is to use technology that either significantly reduces the fault current level thereby reducing the magnitude of the arc hazard and/or using technology that prevents transient overvoltages that can lead to single-phase faults escalating into arcing faults.[/font]
[font=Arial, Helvetica, sans-serif]The answer in both cases is high resistance grounding, as recognized in the Canadian Electrical Code section 10-1100, and the National Electrical Code section 250-36. [/font]
[font=Arial, Helvetica, sans-serif]High resistance grounding of the neutral limits the ground fault current to a very low level (typically from 1 to10 amps) and this is achieved by connecting a current limiting resistor between the neutral of the transformer secondary and the earth ground and is used on low voltage systems of 600 volts or less, under 3000 amp. By limiting the ground fault current, the fault can be tolerated on the system until it can be located, and then isolated or removed at a convenient time. [/font]
Here is an excerpt from IEEE:
IEEE Standard 242-1986 Recommended Practice for the Protection and Coordination of Industrial and Commercial Power Systems 242-1986 section 7.2.5 offer the following perspective:
"Ungrounded systems offer no advantage over high-resistance grounded systems in terms of continuity of service and have the disadvantages of transient overvoltages, locating the first fault and burndowns from a second ground fault. For these reasons, they are being used less frequently today than high-resistance grounded systems"
[font=Arial, Helvetica, sans-serif]There are many benefits from grounding the electrical distribution system including:[/font]
- [font=Arial, Helvetica, sans-serif]Reduced magnitude of transient over-voltages[/font]
- [font=Arial, Helvetica, sans-serif]Simplified ground fault location[/font]
- [font=Arial, Helvetica, sans-serif]Improved system and equipment fault protection[/font]
- [font=Arial, Helvetica, sans-serif]Reduced maintenance time and expense [/font]
- [font=Arial, Helvetica, sans-serif]Greater safety for personnel [/font]
- [font=Arial, Helvetica, sans-serif]Improved lightning protection[/font]
- [font=Arial, Helvetica, sans-serif]Reduction in frequency of faults.[/font]
[font=Arial, Helvetica, sans-serif]A solidly grounded system is one in which the neutral points have been intentionally connected to earth ground with a conductor having no intentional impedance and this partially reduces the problem of transient over-voltages found on the ungrounded system.[/font]
[font=Arial, Helvetica, sans-serif]While solidly grounded systems are an improvement over ungrounded systems, and speed the location of faults, they lack the current limiting ability of resistance grounding and the extra protection this provides. The destructive nature of arcing ground faults in solidly grounded systems is well known and documented and are caused by the energy dissipated in the fault. A measure of this energy can be obtained from the estimate of Kilowatt-cycles dissipated in the arc:[/font]
[font=Arial, Helvetica, sans-serif]Kilowatt cycles = V x I x Time/1000.[/font]
[font=Arial, Helvetica, sans-serif]In the same IEEE Standard as reference above, section 7.2.2 states that:[/font]
[font=Arial, Helvetica, sans-serif]"one disadvantage of the solidly grounded 480v system involves the high magnitude of ground-fault currents that can occur, and the destructive nature of arcing ground faults."[/font]
[font=Arial, Helvetica, sans-serif]Since the vast majority of arcing faults start their life as single-phase faults, the key to reducing their impact is to use technology that either significantly reduces the fault current level thereby reducing the magnitude of the arc hazard and/or using technology that prevents transient overvoltages that can lead to single-phase faults escalating into arcing faults.[/font]
[font=Arial, Helvetica, sans-serif]The answer in both cases is high resistance grounding, as recognized in the Canadian Electrical Code section 10-1100, and the National Electrical Code section 250-36. [/font]
[font=Arial, Helvetica, sans-serif]High resistance grounding of the neutral limits the ground fault current to a very low level (typically from 1 to10 amps) and this is achieved by connecting a current limiting resistor between the neutral of the transformer secondary and the earth ground and is used on low voltage systems of 600 volts or less, under 3000 amp. By limiting the ground fault current, the fault can be tolerated on the system until it can be located, and then isolated or removed at a convenient time. [/font]
DickDV
Member
Note that last statement "By limiting the ground fault current, the fault can be tolerated on the system UNTIL IT CAN BE LOCATED AND THEN ISOLATED OR REMOVED AT A CONVENIENT TIME".Jiri Toman said:
With today's short staffed maintenance departments and uncaring managements, just when do you suppose that "convenient time" might be. Probably five minutes after the lawyers show up with a wrongful death suit!!!
The benefits mentioned by the IEEE can all be had by using low resistance grounding to limit ground fault current except the part about continued operation. That's as far as I will accept. No floating and no high-resistance grounding is good enough.
Leadfoot
Member
Jiri,
I am familiar with what you have posted. I have to deal with ungrounded plant power all the time. I am always questioned why. The information you have posted is basically what I pass along. The high resistance method is what will help control common mode problems as it gives each phase a REFERENCE to ground. I hate open deltas with a passion.
When it comes to the actual ground, as in earth, there is once again many ideas of just what is ground. I like many here have definate things I like to see.
On most drives, they want the drive chasis tied to the ground that is earthed. The power can still be "ungrounded" as in floating neutral. If the methods you posted are used, and mostly they are, there is no great danger to the equipment or personnel. Ground fault detection equipment is a mis-understood safety device. Too many times I have seen it disabled because it was "BROKE". Usually it turned out to be an actual ground fault and nobody wanted to take the time to find it and fix it. You have not begun to trouble shoot unstable drives until you discover that you have low impedance, 500k or less to ground on 2 phases. If they have enough distance between the actual conduction paths, the shields between them conduct the circulating currents.
If a plant does not use the high resistance method an isolation transformer is a must to prevent excessive and stray voltages from recking havoc with things.
I spent 10 years in Uncle Sam's Navy and they had some strict rules that were not to be deviated from with regards to grounds. I tend to get anal about proper grounding because of this. Like I said, I do not design them, just try to keep them correct.
One challenge I have had to face was convincing tool pushers on drilling rigs that the rig ground system was there for a purpose. Too many times I was there rebuilding the electronics after a lightning strike. When they would connect the grounds per the engineers design, there were little or no ill effects after the strike.
GROUND is a subject that has many opinions. Earth ground and equipment grounds are not necessarily the same potential. As is the common in modern electronic equipment.
The very first Step 7 PLC I saw was profii bus connected from the control house to the dog house on the drill floor. For those of you that have not done profii bus, the shield MUST be continuous from both communications modules. Each communications module basically earths the shield. I immediately told my Engineering manager we need to check out the rigs lighting protection. Would you like the lightning coming down your shield to your PLC? Imagine my dismay that the cable was neatly dressed in the cable tray with several 90 degree turns within the first 10 feet.
It is things like I just mentioned that illict a lot of discussion. I join forums like this one to gather many other opinions to help me do a better job. When I feel I can contribute I get busy typing.
Unfortunatly, DickDV has hit the nail with his comment about short staff.
However, I disagree with his comment about the uncaring management.
Management definately cares........ about the bottom line.
Fixing the ground problem costs too much time and money and if it does not affect the bottom line it aint a problem. Of course they will change their tune when some one gets hurt.
I am familiar with what you have posted. I have to deal with ungrounded plant power all the time. I am always questioned why. The information you have posted is basically what I pass along. The high resistance method is what will help control common mode problems as it gives each phase a REFERENCE to ground. I hate open deltas with a passion.
When it comes to the actual ground, as in earth, there is once again many ideas of just what is ground. I like many here have definate things I like to see.
On most drives, they want the drive chasis tied to the ground that is earthed. The power can still be "ungrounded" as in floating neutral. If the methods you posted are used, and mostly they are, there is no great danger to the equipment or personnel. Ground fault detection equipment is a mis-understood safety device. Too many times I have seen it disabled because it was "BROKE". Usually it turned out to be an actual ground fault and nobody wanted to take the time to find it and fix it. You have not begun to trouble shoot unstable drives until you discover that you have low impedance, 500k or less to ground on 2 phases. If they have enough distance between the actual conduction paths, the shields between them conduct the circulating currents.
If a plant does not use the high resistance method an isolation transformer is a must to prevent excessive and stray voltages from recking havoc with things.
I spent 10 years in Uncle Sam's Navy and they had some strict rules that were not to be deviated from with regards to grounds. I tend to get anal about proper grounding because of this. Like I said, I do not design them, just try to keep them correct.
One challenge I have had to face was convincing tool pushers on drilling rigs that the rig ground system was there for a purpose. Too many times I was there rebuilding the electronics after a lightning strike. When they would connect the grounds per the engineers design, there were little or no ill effects after the strike.
GROUND is a subject that has many opinions. Earth ground and equipment grounds are not necessarily the same potential. As is the common in modern electronic equipment.
The very first Step 7 PLC I saw was profii bus connected from the control house to the dog house on the drill floor. For those of you that have not done profii bus, the shield MUST be continuous from both communications modules. Each communications module basically earths the shield. I immediately told my Engineering manager we need to check out the rigs lighting protection. Would you like the lightning coming down your shield to your PLC? Imagine my dismay that the cable was neatly dressed in the cable tray with several 90 degree turns within the first 10 feet.
It is things like I just mentioned that illict a lot of discussion. I join forums like this one to gather many other opinions to help me do a better job. When I feel I can contribute I get busy typing.
Unfortunatly, DickDV has hit the nail with his comment about short staff.
However, I disagree with his comment about the uncaring management.
Management definately cares........ about the bottom line.
Fixing the ground problem costs too much time and money and if it does not affect the bottom line it aint a problem. Of course they will change their tune when some one gets hurt.
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Neutral earth resistors are quite common on HV generator set systems. Ground the star point of one generator and then close to bus. Then synchronise and close other generators. If the earth switch opens, open all generator circuit breakers also. No good having floating voltages - particularly HV. Makes things go bang in the night.
The other common method is to use earthing transformers. Earthing transformer circuit breaker opens, open all generators for the same reason.
Normally with gensets if a tree falls across a line, the protection is set fairly high on gensets to make them "work" and "blow" the tree clear. If protection settings have been calculated properly this works very well. If the fault does not clear or is close to the power station, generally the feeder protection will trip.
The same philosophy is applied to distribution networks as well. This is a fairly simplistic comment on this type of system as the calculations are quite complex and, I feel, out of bounds here. Quite often the protection engineers get it wrong or finish up arguing between themselves over calculations and settings. See it all the time.
Here in Ozz I have to work with several systems that are set up with neutral earth switches and earthing transformers. They are all HV systems and can be found in lots of places. Some of these systems are very new and some very old. For example, Sydney airport, Brisbane Hospital, Christmas Island Power Station, Royal Australian Air Force power stations etc.
They are more common than one would think and often in the most unlikely places.
By the way, I am not an engineer but a controls systems specialist and take advice on these sorts of things from those who should know and quite often do not. If I get involved in that side of the design I employ a young guy (28) who really knows what he is about and invariably finishes up arguing his case with distribution type engineers and winning the case. He is very good and has an excellent business working around protection systems in mines.
Unregistered
Guest
U
i maintain a span type railroad bridge that uses 4 allen bradley 1336e drives the input power from the utility is delta 480, so we installed a wye type resistive neutral grid that corrected the grounding requirements for our vfd's, it has been in service for 9 years with no problems at all.
Jiri,
High-resistance grounding has been around a long time, and it benefits and disadvantages are well-known. I agree that it is better than an ungrounded system. I note that the IEEE main focus is how good it is in protecting equipment. My main concern is in protecting people, as they cannot be replaced with any amount of money or lawsuits, but equipment can be. I want the best personnel protection, and I think the IEEE agrees that is a hard-grounded system with maximum ground fault currents to open breakers as quickly as possible. This is after all the main reason for having a grounding system. The low ground fault current is hyped as a benefit of resistance grounding, when I view it the exact opposite, a great safety problem to be avoided at all costs.
The only way I could see that a high ground resistance migh be more safe is if you happen to be standing next to an equipment box when it arcs to ground and blows open. The more likely scenario with high-resistance grounding is that one phase faults to ground, the only way anyone knows this is because one of the phase indicator lights in the electrical equipment room goes out, but the plant production must be maintained at all costs, so finding the fault is delayed to "a more convenient time", usually the annual plant shut-down at the end of the year. Meanwhile, you are standing next to that same box when the SECOND phase goes to ground, and now you have a phase-to-phase fault, and the box blows and burns. How has high-resistance grounding helped here? Oh, I know! It kept the plant running a few more days so that production goals could be met....
If used correctly with substation-type equipment, ground fault relays ,trip circuits, grounded phase indicators, and a super maintainence program, then high-resistance grounding can be effective. But it only works until the first detector device fails, and nobody thinks it important enough to repair, or a fault occurs, but it is not convenient to find the fault. Finding ground faults is a major headache if you don't have a hard ground. Someday every circuit will have a ground fault interrupter, and maybe then high-resistance grounding will be safer.
I have seen high-resistance electrical systems in plants run for years with one phase grounded, a booby trap waiting to be sprung. I talked to one contract electrician who got a severe jolt from a 480 volt circuit where the "C" phase was grounded. By his own admission he got sloppy while troubleshooting a tank heater. Unknown to him, one of the heater leads had gone to ground in the conduit. Also, the fused disconnect switch handle was broken, so that when you turned it off, the switch did not actually open, and the power to the heater was still on. He opened up the box and put his "wiggy" tester leads across C phase and the edge of the box (instead of checking between all 3 phases). He read less than 5 volts, so he assumed it was safe. He grabbed one of the heater leads to see if it was loose. The back of his hand brushed across one of the other (ungrounded) terminals, and he woke up later lying in the floor. With a hard ground, the breaker or fuses would have tripped right away when the ground fault first occurred, not when the electricain created the second ground fault with his hand.
For people safety, hard grounds are best. For equipment protection, which in manufacturing plants should always be secondary to people protection, an ungrounded delta system is best, but only if properly maintained, and high-resistance grounding is a trade-off or compromise between the first two. Unfortunately there are engineers in Ivory Towers (IEEE) who are willing to write what plant owners want to hear.
High-resistance grounding has been around a long time, and it benefits and disadvantages are well-known. I agree that it is better than an ungrounded system. I note that the IEEE main focus is how good it is in protecting equipment. My main concern is in protecting people, as they cannot be replaced with any amount of money or lawsuits, but equipment can be. I want the best personnel protection, and I think the IEEE agrees that is a hard-grounded system with maximum ground fault currents to open breakers as quickly as possible. This is after all the main reason for having a grounding system. The low ground fault current is hyped as a benefit of resistance grounding, when I view it the exact opposite, a great safety problem to be avoided at all costs.
The only way I could see that a high ground resistance migh be more safe is if you happen to be standing next to an equipment box when it arcs to ground and blows open. The more likely scenario with high-resistance grounding is that one phase faults to ground, the only way anyone knows this is because one of the phase indicator lights in the electrical equipment room goes out, but the plant production must be maintained at all costs, so finding the fault is delayed to "a more convenient time", usually the annual plant shut-down at the end of the year. Meanwhile, you are standing next to that same box when the SECOND phase goes to ground, and now you have a phase-to-phase fault, and the box blows and burns. How has high-resistance grounding helped here? Oh, I know! It kept the plant running a few more days so that production goals could be met....
If used correctly with substation-type equipment, ground fault relays ,trip circuits, grounded phase indicators, and a super maintainence program, then high-resistance grounding can be effective. But it only works until the first detector device fails, and nobody thinks it important enough to repair, or a fault occurs, but it is not convenient to find the fault. Finding ground faults is a major headache if you don't have a hard ground. Someday every circuit will have a ground fault interrupter, and maybe then high-resistance grounding will be safer.
I have seen high-resistance electrical systems in plants run for years with one phase grounded, a booby trap waiting to be sprung. I talked to one contract electrician who got a severe jolt from a 480 volt circuit where the "C" phase was grounded. By his own admission he got sloppy while troubleshooting a tank heater. Unknown to him, one of the heater leads had gone to ground in the conduit. Also, the fused disconnect switch handle was broken, so that when you turned it off, the switch did not actually open, and the power to the heater was still on. He opened up the box and put his "wiggy" tester leads across C phase and the edge of the box (instead of checking between all 3 phases). He read less than 5 volts, so he assumed it was safe. He grabbed one of the heater leads to see if it was loose. The back of his hand brushed across one of the other (ungrounded) terminals, and he woke up later lying in the floor. With a hard ground, the breaker or fuses would have tripped right away when the ground fault first occurred, not when the electricain created the second ground fault with his hand.
For people safety, hard grounds are best. For equipment protection, which in manufacturing plants should always be secondary to people protection, an ungrounded delta system is best, but only if properly maintained, and high-resistance grounding is a trade-off or compromise between the first two. Unfortunately there are engineers in Ivory Towers (IEEE) who are willing to write what plant owners want to hear.
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