Earthing of power trafos 1

[Andiri]

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.

 

[rbulsara]

Earth is earth but not all neutrals are earth! Your LV neutral is not same as earth when earthed through a NER.

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.

 

[RalphChristie]

Distribution Standard of Eskom (Local South African electricity provider) re MV and LV reticulation earthing:
(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

 

[Andiri]

Thanks Ralph. I will work my way through it all.
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.

 

[alehman]

I agree with Rbulsara's comments regading the NER.

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.

 

[RalphChristie]

I've missed the grounding through the NER. (Neutral earth resistor) In this case the LV and MV earthpoints can be on the same electrode.(Like rbulsara and alehman pointed out)

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

 

[GamezBeCJ]

Earth or ground is your lowest (or "zero") voltage reference. Like rbulsara said, neutral is a diferent story, you might have in some cases a Neutral to Ground voltage, but the Earth reference is the same for everything. Think about it in this way, if you split the ground of two systems it means you expect to have different "zero" references for the voltages, which is not a common thing.

Carlos Gamez, P.E.
Industrial Consultant
Transformer Specialist

 

[mc5w]

Here in the U.S. the practice is to interconnect the primary and secondary neutrals but the primary line has to have 4 grounds per mile for the neutral or overhead static wire. With 4 primary grounds per mile about 1/2 the neutral current in a primary feeder flows back through the earth.

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