Alarm system for 480V ungrounded systems 5

[jburn]

I work in a large industrial plant, which was built in the 1940's. We have 35, 6900/480 Volt substations. All of the transformers are delta-delta, and the 480 Volt systems are all ungrounded. For years, we have had our shift electricians check each station once a day for any grounds. Most of our stations now have a set of three volt meters, which are connected to a 3 Phase PT, and are reference to ground. When there is no ground present all three meters read about 277 Volts. When any one phase goes to ground, that meter goes to zero, and the others go to 480. I am looking for an inexpensive relay which will monitor this circuit, and operate when ever there is a ground on the substation. My plan is to take the relay output signal from all of the relays back to a central utilities control room, along with transformer temperature and substation room temperature for alarming and data collection. I have looked at the Cutler-Hammer and the GE high resistive grounding system, but there is no way I could get that many dollars approved. Could anyone recommend a device I could use to monitor my substations for grounds. Thanks

 

[peterb]

One relatively inexpensive way to provide the alarm would use a meter relay with high and low setpoints. The meter would have to be scaled 480V full scale (or use a 120V meter with a PT).
The 480V system operates normally at around 277V each phase to ground. When a ground fault occurs on one phase, the faulted phase voltage to ground drops (to a minimum of 0 V) and the unfaulted phase voltages to ground increase (to a maximum of 480V).
By installing the meter relay on any one phase to ground, you will sense either a high or low voltage when a ground fault occurs; the high & low voltage alarm contacts can be wired to provide the alarm you need.
Determine high & low settings based on measurements of normal system voltage, but these should probably be in the range say +/- 15% of 277V (318 V high set, 235V low set).
An over/undervoltage relay can be substituted for the meter relay, if preferred.

 

[busbar]

jburn, please, please, please reconsider hi-r grounding mods to your plant’s ungrounded systems. The problems (false economy; nee, insanity) of LV ungrounded systems have been noted in print since at least 1955. Read through recent, detailed archives on this at thread238-3370

It’s not that expensive to do. Subject to reasonable engineering review, for <1MVA 480V systems you can use a set of three 4:1 10-30kVBIL PTs (even machine-tool CPTs may do) with grounded-wye primary and *broken-delta* (see ANSI C57.105) secondary connections. Place a resistor across the secondary and something like an ITE-59G relay for remote alarm initiation. The resistor should be sized to limit phase-to-ground fault current to something like 1 AMP/xfmrMVA. (For existing installations, this can be measured in the field.) The relay needs to be able to pick up at ~0.01-0.15 p.u. voltage, (~1-18 volts with 4:1 PTs ) but be able to withstand 1.73 p.u. voltage (208V) continuously and be desensitized for third harmonics to avoid false operation.

The reduction in personal antacid intake will likely offset the engineering and installation costs of such a scheme. The resultant minimization of process downtime becomes gravy. Be nice to your low-voltage insulation!

 

[REDDOG]

How about a ground detector with 3 relays with 277 volt coils connected in wye (same as the voltmeters). Connect the NC contacts in parallel and in series with the alarm. When any relay drops out it will sound the alarm.

 

[busbar]

Reddog, if you think about it, aren’t there times where those relays will need to be able to handle at least 480V continuously? That is, in a sustained phase-to-ground fault, there would be 0 volts on one relay coil but what about those on the other two phases? [This is referred to as neutral shift.] “Ice-cube” relays (and probably others) acquire an unforgettable aroma in short order under this condition. Also, all three coils risk some potentially fierce transient overvoltage during arcing-ground faults. [Read through the Beeman chapter 6 reference in the linked thread.] The broken-delta/resistor arrangement will effectively damp these.

Once measured a steady phase-to-ground 1800+ volts with a “1000V” Fluke meter on a so-called 480V roadway lighting circuit. . . a high pucker-factor situation, I’d say.

 

[REDDOG]

Try Erickson Electrical Equipment Company for ground detector.

http://www.ericksonelectric.com/catalog/catalog.htm

 

[busbar]

Might be worth a read:

John P. Nelson and Pankaj K. Sen, High-Resistance Grounding of Low-Voltage Systems: A Standard for the Petroleum and Chemical Industry, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 4, JULY/AUGUST 1999

 

[don01]

hi
not withstanding the much trumpeted benefits of the high resistance grounding idea (not intended to be derogatory in any way) you may want to consider some of the new generation volt meters on the market. Same standard din hole 96x96 but with multidrop or data retransmit features. This could all be run back to the control room for the panel operator to see.

Cutler & hammer is one brand that springs to mind but I'm sure there are lots of them on the market.

 

[jbartos]

Suggestion: There are some very small very inexpensive solid state relays (SSRs) available. All what is needed is to fine tune their input switching circuits by potentiometers. One may also spend a small amount on a multimeter. When those SSRs are so inexpensive, one could possibly design a redundant scheme in case of some malfunctions. Output of those solid state relays, could be used for an alarm and tripping. However, if there is some inexpensive, high-resistance grounding scheme over grounding trasformers, this one would be a better solution since it would prevent overvoltages, overvoltage oscillations, and spikes. This investment in the high resitance grounding scheme should pay for itself handsomely.

 

[busbar]

Online manufacturer's publication on LV Hi-R grounding:

http://www.geindustrial.com/industrialsystems/switchgear/notes/GEI-72116.pdf

Note that the 'pulse contactor' and associated controls are only needed for troubleshooting convenience.

Although not in great detail, the material discusses a neutral connection derived with a separate wye-delta bank, or connection of a grounding resistor directly to the X0 bushing of a 480Y-secondary power transformer.

 

[jbartos]

Suggestions:
1. Small inexpensive solid state relays (~5US$ each) could be used to implement the ground fault detection scheme, e.g. Omron, Siemens, etc.
2. Combination of resistors and light bulbs or LEDs could be used for monitoring. The analog nature of this scheme indicates a little fault and a big fault (when bulbs are bright). You probably could afford this one.

 

[peterb]

In reading jbartos' latest post, I am forced to ask whether these are appropriate suggestions for implementation in a real-world industrial system. Connecting a $5 relay (can you even find one that is rated for 480V?) to a 480 V distribution system doesn't sound like a great idea - at least state that the circuit must be fused, and consider that the relays will be continuously energized (what is the expected life of this device under these conditions?).
I still consider that the meter relay is the most appropriate solution, see my post of May 7 & don01's of May 11. This provides quantitative data as well as an alarm.

 

[Dan76]

jburn, your system sounds alot like the electrical system of a Navy ship, 480V Delta Ungrounded. We had ground detectors, one type was &quot;Active&quot; and used 500VDC to test basically an installed meggar you could use on hot cables.
It gave oyu a value but didn't tell you which phase.
The other was a simple 3 light system (probably like your voltmeters) when one light was dark there was a ground on that phase unfortunately it wasn't a quantatative test.
I'll try to find some drawings to send if You will provide your Email address.
The system with the relays sounds good perhaps set them to test periodically with a manual test if you suspect grounds earlier.
Good luck, Dan76

 

[busbar]

The all-time classic description is detailed in a chapter on system grounding in Beeman’s Industrial Power Systems Handbook

Low-voltage ungrounded and high-resistance grounded systems have been extensively covered in IEEE/Industry Applications in the last ~25 years.

Here is a wonderful 1955 description of the perils on LV ungrounded systems--

&quot;Chapter 6 --- System Grounding

About midafternoon one day in a West Coast manufacturing plant, normal operations became suddenly disrupted. The first evidence of trouble came in the form of a motor failure on the 480-volt system, then another, and still another in close succession. An nspection of switchboard voltmeters (measuring line-to-line volts) and ammeters indicated no unusual conditions. System equipment continued to fail. A test voltmeter was rigged up having a full-scale calibration of 1200 volts. Upon connecting it phase-to-ground, the pointer went off scale. A phase-to-ground potential on a 480-volt system of more than 1200 volts existed!

At once the incoming service transformers were suspected of internal breakdown between high- and low-voltage windings. As the last of these transformers was isolated and individually tested, it became evident that they were not at fault. System equipment continued to fail, and the situation was desperate.

A frantic group went into a huddle and decided that the only way out was to trip the main incoming service breaker which would deenergize the entire system. At this point one of the workmen noticed a small wisp of smoke coming from a motor-starting autotransformer and, upon approaching, could hear a buzzing noise inside. This circuit was switched clear of the system, and the overvoltages disappeared. During the two-hour period that this arcing fault existed, between 40 and 50 motor windings had failed.

Finally it was found that the autotransformer enclosing case had been hashed in and was practically in contact with the coil. The spot where arcing had taken place was evident although not badly burned. An attempt was made to show the plant engineer what had been the trouble. A solid connection was made between the frame and the burned spot on the coil. Much to the bewilderment of the operating men and according to the expectations of the plant engineer, no more than the 73 per cent increase in the voltage to ground on the other two phases occurred. The main ingredient of the overvoltage (discontinuous conduction) had been omitted.

This is an actual case of severe prolonged experience of overvoltage of repetitive restrike origin on a 480-volt ungrounded system. This story is spectacular because of the magnitude of the disturbance and consequential damage. Similar occurrences of lesser extent are not uncommon, however, and there is evidence that they are more frequent than realized. It is a characteristic of ungrounded systems that they are subject to relatively severe transient overvoltages. This trouble can be avoided by proper grounding of the system, and other important benefits are also obtained.&quot;

pp. 337-338 L. J. Carpenter and L G. Levoy, Jr.
Industrial Power Systems Handbook
DONALD BEEMAN, Editor
Manager Industrial Power Engineering / Industrial Engineering Section General Electric Company / Schenectady, New York
McGRAW-HILL BOOK COMPANY, INC. 1955

 

[nbucska]

Instead of three meters I would use a resistor STAR and meter -- or relay coil -- from its center to GND.

You could even add a simple circuit to set any sensitivity or speed/delay you want.

<nbucska@pcperipherals.com>

 

[electricpete]

Great story busbar. That is truly a classic story worth remembering. (I vote you a 2nd star).

 

[BJC]

busbar - good story about Beemans book.
If anyone ever gets a chance to get the General Electric &quot;Industrial Power Systems Data Book&quot; grab it ( or write me- I'll take it).
These were the basis of Beemans book. They were published up to the mid 60s.
Some of the information in them can't be found in Beemans book. It's hard to find in other places as well. The paper on DC systems and short circuits in DC systems is very good.
I read somewhere that GE gave a group permission to copy it as it is no longer published. Any one know who's doing this??

Besides the motor story somewhere in there is ( I think ) a write up of power factor using the glass of beer. Low power factor = 1/2 glass of beer and 1/2 glass of foam. High Power factor = 1 whole glass of beer.

 

[jbartos]

Suggestion: See more related postings and D. Beeman Handbook excerpts in this Forum, e.g.
1. Multiple Resistor Grounding
2. Arcing Ground Faults
3. High Resistance Grounding
Just type Beeman in Search box.