Earthing of power trafos
Earthing of power trafos
(OP)
Doing a recent installation of a 2,5MVA 6,6kV/525V delta/wye trafo in a sub, the question came up again. Earth is earth, or am I wrong? There is an NER installed on the LV side. Should the earth conductor running with the MV supply cable be connected to the same common-earth bar in the sub as the LV earths to the MCC, the earth mats and the bottom side of the NER? Some have told me that the MV and LV earth systems should be separate (ie separate earth bars too). Which way to earth?
Making the smoke come out is easy; getting it back in is a bugger.






RE: Earthing of power trafos
The earthing conductor runnig with the MV (primary) side should be connected to the transformer body, or the earth terminal in the primary terminal box. The transformer body (enclosure) must be connceted directly to earth. You do not want to insert restance in 'earthing' circuit for the primary.
There is a thought in some countries to keep the primary side gorund and/or the tranformer tank grounding electrode seprate from the LV earthing electrode. Their logic is to safeguard rise in voltage on lv side earth or neutral in case of a primary side earth fault
The 'bottom' side of the NER is earth and must be connceted to a earting electrode and can be connceted to your substation 'earth' bar.
RE: Earthing of power trafos
(reference SCSASAAL9)
4.2.2 Transformer installations
Distribution equipment associated with trsf installations that is either ground-mounted or pole-mounted and fed by underground cable or overhead line, shall be installed, connected and earthed in accordance with the following requirements:
a) Star point of trsf LV winding shall be earthed,
b) MV surge arresters, trsf tank and other metalwork shall be bonded to the MV earth electrode,
c) a combined MV/LV earth electrode may be employed only where the electrode resistance to earth does not exceed 1 ohm,
d) where seperate MV and LV earths are used:
1) MV and LV earth electrodes shall be seperated by not less than 5m,
2) a Neutral surge arrester shall be installed between the LV neutral terminal and the transformer tank and
3) care shall be exercised to ensure that there is no metalic or other low impedance conducting path between the MV and LV earths,
e) a Consumer's bare earth continunuity conductor in contact with the earth and of sufficient length might sometimes produce a low low enough earth resistance value to exclude the need for an LV earth electrode. However, this shall be confirmed by measurement.
The combination of MV and LV earths into a single electrode was the preferred earthing scheme at Eskom until the early 1980s. At this time, a new Code of Practice was published which exposed a hazard of shock that would be presented to customers in the event of power frequency current flow through the combined earth electrode. Today, a combined electrode shall only be installed where the electrode resistance to true earth is no more than 1ohm.
The 1ohm resistance criterion is specified in order to limit to a safe level the 50Hz voltage rise of the LV neutral in the event of a fault between MV phase and the transformer tank or core. For such a fault, the condition of raised potential on the LV neutral will remain until operation of MV earth fault protection.
It is important to notice that the 1ohm criterion is derived to limit the power frequency voltage rise of the LV neutral.
The hazard of high transfer potentials on the customer neutral in event of a power frequency current discharge through the MV/LV earth is largely averted by the use of seperate MV and LV earths. In avoiding one hazard, however, the seperation of the MV and LV earth electrodes introduces a hazard of its own.
The hazard from seperate electrodes is realized during high frequency discharges by the MV surge arresters. During these times, the potential of the transformer tank is raised to the voltage drop across the MV lead inductance and the MV earth electrode impedance. High tank potentials stress the insulation between the LV windings and the transformer tank and core. Over time continued insulation stress may lead to transformer failure.
Regards
Ralph
RE: Earthing of power trafos
I am still open to hear what others have to say as well.
Making the smoke come out is easy; getting it back in is a bugger.
RE: Earthing of power trafos
If this is a Y-Y transf with the primary solidly earthed, I can think of no reason not to connect the bottom of the NER to the same grid. If you have two separate grids for primary and secondary (presumably in close proximity), there will be some small, but unpredictable varying amount of resistance effectively coupling them (the earth). I don't see the benefit to that - and it makes the secondary fault current unpredictable.
RE: Earthing of power trafos
The Eskom standard is applied to a solidly grounded LV-winding.
Don't be confused by the term electrode. In the Eskom standard the following definition apply:
electrode (earth electrode)One or more conductive parts that are embedded in the earth for the purpose of making effective electrical contact with the general mass of earth
Read it in conjuction with the standard I've provided.
Regards
Ralph
RE: Earthing of power trafos
Carlos Gamez, P.E.
Industrial Consultant
Transformer Specialist
RE: Earthing of power trafos
In some instances the connection between the primary and secondary neutral is done with a lightning arrestor and the secondary neutral grounding electrode is kept well away from the primary rods. One of them is if the primary neutral has less then 4 ground per mile. The other is to keep tingle voltages out of the milking parlor when the primary line is single phase or when a 3 phase primary line has significant neutral current because there is both a 3 phase and single phase transformer present. ---- Cows do not like the Cow Voltmeter Method particularly when being milked.
What happens in the rural case is that about 1/2 of the primary neutral current tries to return through the earth. For long distances the earth has very low resistance because 1/3 of the voltage drop of a ground rod happens in the first 3 feet (slightly less than 1 meter) of soil around the rod. There are similar proportions for other types of electrodes such as the Ufer that uses metal in the bottom of a concrete foundation.
Mike Cole, mc5w@earthlink.net