How to calculate EPR, touch potential 7 design earthing system?

[musaUKPN]

I am involved in design of a new supply for a commercial building. The building will have supplies from two different sources each coming from different 132/11kV utility substations.

Supply-1 will be fed @ 400volts 3-phase from utility source-1 via dedicated 1MVA 11kV delta / 0.433 kV Y transformer (Z=4.5%). The transformer will be installed inside the building along with 11kV switches and is owned by utility but it will supply our building only. The utility solidly grounds the Y connected secondary and provide 3-phase + neutral supply to the building. They also provide earth terminal if certain conditions are fulfilled. Supply-1 is for the main building use (facilities, office, parking, etc) except for the two shops on the ground floor of the building which are fed via supply-2. The expected peak load for supply-1 will be 810kVA.

Supply-2 will be @ 400volts 3-phase from utility source-2 via a utility distribution transformer. The utility distribution transformer is in another nearby building around 300 meters away and it supplies multiple customers. The shops in our building will be supplied via two 3-phase+ PE (combined neutral & earth) service cables (each rated 100A). These two services will be PME (protective multiple earth). The expected peak load for supply-2 will be 130kVA (65kVA each service). The utility also solidly grounds their distribution transformer and provides PME supplies to all customers.

The rough value for the utility and dedicated transformer earthing is less than 1 ohm. The earth loop impedance limit for the two PME services is less than 0.35 ohm. The utility provided 3-phase & SLG fault values (max & min) are given. I have attached schematics for your understanding with all values.

My questions are:
1. How can we calculate Earth potential rise (EPR)?
2. How we calculate touch potential?
3. What are the best ways to arrange earthing/ bonding at above site? Shall I consider single earthing for both the supplies or two separate earths?

Your comments will be highly appreciated.

Please let me know if more information/ data is required.

 

[cuky2000]

The max. EPR may be calculated as follow:

. EPR_max = I_earth*R_earth

Assumming all earth fault current (SLG)is injected into the earth the maximum EPR will be estimated as follow:

Low Voltage (433V): EPR_max={[1MVA/SQRT(3)*.433kV]/4.5%}*1Ohm ~3kV.

If the transformers are connected in parallel, the EPR will be larger accordingly with the total MVA and voltage.

To calculate the touch potentials, additional data such as ground grid configuration, clearing time, etc is required. However, there is a good chance that the concrete slab and rebar act as an equipotential surface resulting in a safe conditions. In this case the calculation may be a trivial exercise.

A touch potential measurement may be considered in lieu of the calculation to prove that the facility is safe.

 

[jghrist]

Assumming all earth fault current (SLG)is injected into the earth the maximum EPR will be estimated as follow:

If there is an earth wire between the utility transformer secondary neutral and the building ground, then very little of the 400V SLG fault current will be injected into the earth. It will essentially all flow through the PE.

The real EPR may come from a ground fault on the utility 11 kV side. This will depend on the earthing of the utility 11 kV line, the utility 11 kV SLG fault current, and the utility ground at the transformer. Some of the SLG fault current will flow into the earth through the building ground system and cause EPR and maybe touch potentials. The calculation of EPR and touch potentials will be very complicated and is not normally done for commercial buildings.

Cuky is right that the building concrete slab will probably act as an equipotential surface and practically eliminate touch potentials.

 

[musaUKPN]

@cuky2000:

"Low Voltage (433V): EPRmax={[1MVA/SQRT(3)*.433kV]/4.5%}*1Ohm ~3kV"

Can you please elaborate this?

As per my understanding above expression doesn't result in 3kV but in 30kV??
EPRmax={[1000kVA/(1.732*0.433kV)]*[100/4.5]*1 ohm}=30 kV

My question was to calculate the worst EPR. Will above expression give the worst EPR?? jghrist pointed out the worst EPR will be due to 11kV fault at or near the dedicated 1MVA substation. If so how will we calculate it? I have already given the 11kV SLG fault max & min currents on the drawing attached earlier.

The other question was that there are supplies from two different 132/11kV utility sources. Can we combine the earthing for these two supplies and also bound it to our building? If we combine the earthing and use PME for these supplies; are there any potential hazards that should be investigated further?

Another solution could be to segregate the earthing for these two supplies.

That is we will use the PME system for supply-2 (shops supplies) and any fault current will return to the distribution transformer which is not in our building.

We will then use separate earths for the utility owned dedicated substation providing supply-1 and foir all of our supplies inside building (except for the shops).

I would greatly appreciate if anyone can help me explaining what earthing arrangement to be adopted and why?

musa UKPN

 

[Marmite]

If these supplies are going into a single building then you should bond the neutral/earth terminals of both incoming supplies together, and to the building PME bonding. It doesn't matter that they are derived from separate HV sources. The purpose of PME bonding within the building is to avoid touch potentials developing.
You don't have enough information to calculate the earth potential rise for an 11kV fault, and you shouldn't need to. It would be the responsibility of the utility to ensure that the EPR at their substation remains within safe limits. In an urban system fed via 11kV underground cables it would be extremely unlikely to be a problem.
Regards
Marmite

 

[Kiribanda]

cuky2000,

I am little confused with 30 kV EPR. How could EPR be 30,000 V on a 400 V network? Your explanation is much appreciated.

 

[7anoter4]

I think cuky used the metallic short-circuit current instead of grounding fault current in order to calculate the EPR. If the current flow through the grounding resistance one has to complete the impedance with 3 times the grounding resistance in zero sequence impedance.
If ztrf~xtrf then:
Xtrf=Vlv^2/Strf*uk%/100=0.008437 ohm
where Vlv=0.433 kV Strf=1 MVA uk=4.5%
If x1=x2=xo=Xtrf for this transformer then the homopolar impedance[zero sequence impedance] will be:
zo=SQRT((3*Xtrf)^2+(3*Rearth)^2)=30.00001 ohm and the Iground will be:
Iground=3*Vlv/sqrt(3)/zo=0.024999 kA
and EPR=Iground*Rearth=0.02499*10=0.2499 kV
As jghrist said if there is a grounding conductor from the substation along the supply cable then
the grounding [or PE conductor] impedance for 300 ft could be 0.76 ohm
[4/0 awg copper and 100 ohm.m earth resistivity, for instance]
Then Sf [ the fault current division factor] will be 0.76/(10+.76)= 0.07
Iground will be 0.025*.07=0.00177 kA and EPR=0.0177 kV.

 

[MAS2000]

7anoter4 much appreciated response. Your calculations match mine.
I will ask for more explanation for: "Then Sf [ the fault current division factor] will be 0.76/(10+.76)= 0.07"

How 10+.76 is used in calculations?

Also if you could comment on the presence of a PME supply + TN-c-s supply in the same building. All i want to know if we bond the building earth + the dedicated substation earth (HV & LV); and at the same time have a PME supply for the shops (remeber the earthing for the PME supply is in another building 300m away from ours; what are hazards.

Suppose a fault on the 11kV network (SLG fault current approx 9kA); the volts at PME supplies will go up. As the PME earth is in another building; will it not cause a touch/ transfer voltage hazard there?

M.A.Sh.
Elect. Engr.

 

[7anoter4]

Let's say ZPE is the module of the PE conductor impedance. Then, from the splitting point where ground resistance is connected with PE there are mainly two impedances connected parallel. So 1/Ztot=1/ZPE+1/Rearth and Ztot=ZPE*Rearth/(ZPE+Rearth)
Now as Itot*Ztot=Iground*Rearth then Iground=Itot*Ztot/Rearth=Itot*ZPE/(ZPE+Rearth).Sf=ZPE/(ZPE+Rearth)
Correctly it has to be done in complex where ZPE=RPE+j*XPE .RPE is small so ZPE~XPE.The "correct" calculation
result is Sf=0.075 but I think 0.07 is still a good approximation.
In connection with your second quire I think PME is the same as TN-C-S only the first name appears in IEE Regulation.
It is not a very good solution but could be tolerable if the Utility will agree with. See:

http://www.tlc-direct.co.uk/Book/5.5.1.htm