SOURCE EARTH 1

[lyledunn]

In Ireland we have a 11Kv / 400v delta /star connected trfmr as the most widely used means of public supply to buildings. Up until now i thought that I fully understood the reason for source earth on the lower voltage side. This source earth is usually effected by a driven earth electrode which should have a resistance of no more than 10 ohms and is connected to the star or neutral point.There are a number of reasons why this earth may be required and I think that I am reasonably ok with them. However, I have just read in guidance with respect to a British Standard on earthing that the prime reason for this source earth is "to preserve the security of the supply network by limiting the potential of the live conductors (with respect to earth) to a value consistent with their insulation". Now I understand that this connection will effectively limit line to earth voltages to around 240v but if the actual line voltage is 400v, why the need to limit the line / earth voltages for the purpose of insulation requirements? If a source earth is not effected is there a possibility of voltages greater than line voltages appearing between line and earth?
Many thanks.

Regards,

Lyledunn

 

[jbartos]

Suggestion: Yes, arcing transients can build up the nominal voltage to higher voltages and damage insulation.

 

[busbar]

It’s a big set of tradeoffs—trying to make it work while not killing someone in the process. The quality/’quantity’ of the earth connection is a bit incidental except where lightning or higher-voltage switching transient exist, and typically these are short lived, except for something like a sustained intertwinding fault. Lightning and switching surges on the 11kV side of the transformer are capacitively/electrostatically coupled to 400V transformer windings. Through grounding, insulation stresses are limited to fairly well-understood mechanisms.

“Grounding/Earthing” in low-voltage systems has two important aspects. Besides the electrical connection to earth through electrodes, bonding interconnects equipment enclosures and the transformer-secondary neutral point to limit voltage rise between exposed conductive surfaces and permit large enough current flow to operate overcurrent devices—ideally reducing damage to nearby electrical (and non-electrical) components.

 

[lyledunn]

busbar,
Many thanks. My own understanding of source earth on the lv side is simply to establish a reference point and to ensure that N/E voltage is maintained at or about earth potential. The source earth has litlle function in carrying fault current unless it is part of a TT system. I also accept the bonding aspect although this is usually referenced to the building earth thus minimising fault voltages between conductive elements that are bonded to it.
What intrigues me is the notion of capacitive coupling on the HV side impressing itself on the lv side and how star point earthing would limit the voltage to earth on the lv side.I would be grateful for any further information sources on this aspect.


Regards,

Lyledunn

 

[busbar]

As I follow it, having an ungrounded secondary on a distribution transformer can be viewed as having a primary-secondary capacitance with another capacitance between the ungrounded secondary winding and “ground”. This forms a capacitive voltage divider, allowing the secondary winding to potentially float well above ground, and subject to transient overvoltage and eventual breakdown from insulation stress in most everything secondary connected.

See an old account at 64.146.180.232/dat/beemaIPSH6z.doc. A not-too-huge resistor in parallel with secondary-to-ground capacitance is usually capable of damping the destructive overvoltage. Fortunately, intentionally ungrounded low-voltage systems are not too popular these days, except for maybe a remote single-transformer/single-motor installation for something like an agricultural water pump, so far as the North-American continent is concerned. In most cases, neiengineering.com/papers/papers.html, it is lunacy not to install at least a high-resistance-grounded system, with singificant, well-established warnings half-a-decade old.

 

[jbartos]

Suggestion: The ungrounded transformer secondary power distribution is nowadays used for dedicated loads only. The reason for this is that such power distribution is so fine tuned that it does not pay to have any kind of system grounding there. E.g. a large motor load, one circuit or two alternating large motors on one transformer secondary; usually with relatively short feeders.
Visit

http://www.wmrc.com/businessbriefing/pdf/hosp_eng_2003/reference/ref18.pdf

etc. for ungrounded systems in medical applications.

 

[dwraith]

The majority of British (and Irish) engineers would probably share your ignorance of the problem as LV systems other than solidly grounded are virtually unheard of in the UK and Ireland. American electrical engineers are more familiar with the problem as I believe that ungrounded LV systems were standard in the US up to the 1950's and many still exist. Beeman (American textbook) has a very good section on the problem of arcing ground faults. If you can't borrow a copy try searching for "arcing ground faults" on the net. The esteemed jbartos and busbar, being Americans (I guess) probably don't understand your problem.

 

[jbartos]

Suggestion: I am familiar with Beeman Handbook and system grounding. The ungrounded dedicated circuit appears to be an exception not the rule. I indicated this in my above posting that the ungrounded circuits or power distribution should be used for dedicated loads only. The dedicated circuit with its load can take advantage of the protection system they may have. Therefore, the grounding may be viewed as redundant. Obviously, it would not cause any harm.

 

[jbartos]

Suggestion: The ungrounded dedicated circuit to its load has some advantages, namely:
1. I will allow one fault, i.e. one phase insulation fault, which will be detected and service as soon as possible. This is especially convenient where more expensive processes are involved. How are more expensive processes treated in Ireland? Are Irish ignorant to process costs?

 

[busbar]

dwraith, I mean no personal offense, but “don’t understand” exactly what?

 

[lyledunn]

jbartos, indeed not. Irish engineers would be well acquainted with appropriate supplies for loads where first fault situations could cause problems. I am bound to say that it usually would not be ungrounded but grounded through an impedance and hooked up via insulation monitoring.
However, this is straying slightly.The kernel of my original question was why the need for the source earth for insulation protection because this was the PRIME reason for providing a source earth as stated in some guidance to standards.I thank busbar for providing some good explanation with respect to this.

Regards,

Lyledunn

 

[busbar]

Here is a ~50-year old account of the significant damage that can occur with ‘floating’ distribution-transformer secondary windings.

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 inspection 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.

Chapter 6 — System Grounding 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

 

[peebee]

The IEEE Red Book offers some nice insights into the overvoltage issues. In particular, "Because of the capacitance coupling to ground, the ungrounded system is subject to dangerous overvoltages (five times normal or more) as a result of an intermittent contact ground fault (arcing fault) or a high inductive reactance connected from one line to ground."

Side note: I believe (but cannot confirm) that this 5x factor is where a lot of BIL specs come from, that is, 75kV BIL (5x) or 90kV BIL (6x) on 15kV systems.

 

[ruggedscot]

Working with ungrounded systems you start to see some major concens with this mode of operation. Under normal operational conditions it works pretty much as expected but when a fault occurs and specifically an arcing fault develops then problems arise with insulation breaking down due to the excessive voltages produced. Arcing currents when coupled with the complex capacitive inductive components present in a circuit develop and sustain these over voltages. As the circuit includes generally three phases and such the voltages experienced cover the majority of the installation, leading to breakdowns at any weak points in the system. It also presents a very real and significant risk to any individual adjacent. Including a resistor of a few hunderd ohms between the neutral and ground provides a reference to ground. Monitoring equipment can then detect any ground faults and allow either circuit protection to operate or inform personel to take action.
One of the problems that I have seen involves delta circuits where there is no earth provided. Such systems are earth free natrually, to provide an earthing arrangement a star wound transformer was installed to give a neutral point. This solved the earthing issue.