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A Case Study of Limiting Earth Potential Rise - Under Difficult Conditions -

B.T.Prakash Kumar Deputy General.Manager,BESCOM,Bangalore

( This Paper was presented at the Fifth international R&D Conference on “Development &Management of Water and Energy Resources” held at Bangalore during February 2005 Organised by CBIP and Governoment Of Karnataka. ) Abstract :-

Considerable growth of power and Telecom sectors in recent years warranted commissioning of New sub-stations and telephone Exchanges and Lines. The decision of location of New Sub station in most of the cases is driven by Energy savings and Economical considerations and less importance is given for the proximity of power and Telecommunication systems. The Interference between Power and Telecommunication system is to be given a serious consideration at the time of selection of New Sub-Station site so as to design suitable Earthing System to limit the Earth potential rise (E.P.R.) within stipulated values . Limiting Earth potential rise (E.P.R.) after completion of the project may results in either expensive re-engineering work or incurring huge compensation for shifting of Telecommunication Exchanges/Lines to a safe distance. In some cases however other local conditions may prevail upon selection of sub-station site near Telecommunication System thus requiring design of Suitable Earthmat to Control E.P.R within tolerable limits. In areas where limited area is available with high resistivity of the soil and fault current, designing an Earthmat becomes very difficult to obtain low resistance to earth and to control potential gradients. This paper describes a case study of reducing EPR in the neighborhood of substation by using Satellite Earthmat.

Introduction :- A safe grounding system in a HV/EHV Sub-Station has two main objectives:- § To provide means to carry electric currents into the Earth under normal and

fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service .

§ To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shocks.

One of the main concern while designing grounding systems is to ensure that no Electrical Hazards exists outside or within the Sub-Station during normal and fault conditions.

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When an earth fault occurs in a power system, some of the fault currents takes the path to the earthing system via the Earth. This current raises the potential of the earthing system with reference to a remote earth for the duration of the fault and is known as Earth potential Rise (EPR). The transfer of potentials between the EPR areas and outside places by conductors of Telecommunication circuits and other metallic structures may cause serious Hazards to consumers, telecommunication installation or personnel .

The power stations and line towers are the locations where EPR may occur and the following factors influence the level and zone of EPR. • Size of the Sub-Station, • Maximum fault current, • Earthing arrangement and value of Earth resistance, • Number and characteristics of power lines.

Magnitude and Duration of EPR

The Magnitude of EPR depends upon the fault current (few hundreds to several thousands of amperes ) and resistance of the Earthing system . The acceptable value of earthing system resistance (Generally between 0.25 Ohm to 10 Ohm ) depends upon the acceptable values of EPR. The Voltage gradients are also affected considerably by factors such as non-homogeneity of the soil, Location of metallic structures, pipes, metal sheath or Armored cable etc.,

S T A T I O N E A R T H M A T

T E L E P H O N E E X C H A N G E

F A U L T P O I N T

R E M O T E E A R T H

V e

E .P .R . W I T H R E F E R E N C E T O R E M O T E E A R T H A N D H A Z A R D Z O N E

P

P

F

V e = E .P .R A T T H E S T A T I O N E A R T H M A T E t = E .P .R A T T H E T E L E P H O N E E X C H A N G E

V f = E .P .R A T T H E F A U L T P O I N T

V f

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Limits of Earth potential rise and Hazard Zone

The following EPR Limits of danger is adopted in our country as per PTCC manual:-. a) Telephone Exchanges - 430 volts b) Telecommunication Lines - 650 volts. The EPR Zone near an Earthing system varies from some tens to thousands of meters depending on soil resistivity, Layout of Earthing system , power network, fault levels and other local conditions. In general the zone of EPR in urban areas is small compared to rural areas. Methods for Limiting EPR :-

At stations where Soil resistivity is high and areas for ground mat is limited it is difficult to limit the EPR to a reasonable value for which the grid resistance is to be around 1 Ohm. To tackle such situations, some possible solutions suggested are : -

1. Making a combination of horizontal grid and a number of long vertical

electrodes penetrating the lower strata of soils.

2. Connecting an existing adjacent ground grid to the newly designed grid.

3. A predominant use of remote ground electrodes and drilled ground wells.

4. Chemical treatment of soils or use of sodium Bentonite clay for back filling.

5. Use of counterpoise Earthing.

6. Connecting overhead ground wires to the grounding system

Considering its effects while designing.

7. Barring access to limited areas where it may be impractical to eliminate the

possibility of excessive potential differences during a fault.

8. Use of concrete encased electrodes.

9. Burying the grid perimeter-grounding conductor outside the fence line.

10 To connect the station grounding grid with another ground electrode.

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EPR problems at Yeslur 66 KV Sub-Station Yeslur in Hassan District, Karnataka – a Case study. The Yeslur village in Karnataka is a hilly region and forest area with coffee estates. Considering the difficulties in obtaining right of way for New HV & LV lines and suitable land with easy access to the main road, it was decided to set up New sub-station at the vacant available land in possession of KPTCL measuring 4417 Sq mtrs., where a Telephone Exchange is situated at about 90 mtrs. from the boundary of the Sub-Station. Soil resistivity Data The soil resistivity measurements made as per Wenners’ Four probe method and the variation in soil resistivity is as shown in the following graph .

2 5 10 15 25 50PROBE DISTANCE IN MTR.

200

400

600

800

1000

1200

1400

1600

SOIL

RES

ISTI

VITY

IN O

HM

-MTR

SR

AVG SR

SOIL RESISTIVITY PROFILE AT YESLUR

An average Soil resistivity of 718 ohm -mtrs (after weather correction ) was taken for the Design of Earthmat. Earthmat Design: - A combination of Horizontal grid and No. of long vertical rods was proposed and the design feature of the Earthmat is as follows. ♦ Area of Earthmat - 4417 Sq. Mts ♦ Design spacing b/w mat conductors - 2.5 mts ♦ Depth of Burial of Earthmat - 0.9 Mtrs ♦ Size of M.S. Flat Earthmat Conductor - 50 X 6 mm ♦ 15 Nos of vertical CI pipes of Length 2.75 mtrs, 100 mm dia and Bentonite Treatment proposed for all CI pipes.

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E.P.R TEST RESULT AND COMMENTS: -

The substation work was completed during March 2004 and the results of the first EPR test conducted on 20-3-2004 indicated an EPR at Earthmat as 1905 Volts and at Telephone Exchange as 494 volts. The soil resistivity profile indicated the high sub-soil resistivity and hence providing deep earth electrodes to reduce Earthmat resistance and thereby EPR is not considered. The bentonite treatment around the peripheral earthflats was given and the results of the EPR test conducted on 23-4-2004 indicated an EPR at Earthmat as 1670 Volts and at Telephone Exchange as 441 volts. Satellite Earthmat The diversion of fault current through the earthmat by providing satellite earthmat nearby is considered to reduce the EPR . A site located at a distance of 1 km from the station among four probable sites is selected for satellite earthmat, after considering the soil resistivity, distance from the main earthmat and right of way for interconnection with main Earthmat.

The design feature of the Satellite Earthmat is as follows. • Average soil resistivity for design - 329 Ohm-mtr. • Area of Earthmat - 3600 Sq. Mts • Design spacing b/w mat conducts - 10 mts • Depth of Burial of Earthmat - 0.9 Mtrs • Size of M.S. Flat Earthmat Conductor - 50 X 6 mm • Vertical Cast– Iron pipes in the periphery - 6 Nos

SATELLITE EARTHMAT

MAIN EARTHMAT

TRANS.

GROUND WIRE

FAULT-2

FAULT-1

Ig

Igw

IsIn

Ig5 Ig4 Ig3 Ig2

Is

If

Im

In = Neutral currentIf = In

Im = Total current flowing in the Earthmat

Ig = Part of the fault current Entering theMain Earthmat through the Earth.

Igw = Part of the fault current Entering the

Is = Part of the fault current Entering the

EARTH POTENTIAL RISE ( E.P.R )

In = Im = If = Ig+Igr+Is

ONLY THE CURRENT Ig CONTRIBUTES TO THE E.P.R AND NOT THE TOTAL CURRENTFLOWING IN THE EARTHMAT (Im) OR NEUTRAL (In)

CASE-1 Fault within the sub-station Ig = Igr = Is =0In = Im and E.P.R is Zero w.r.t remote point

CASE-2 Fault outside the substationE.P.R of Earthmat w.r.t remote point is directly proportional to Ig

Ig1

Main Earthmat through the Overhead

Main Earthmat through the

Ig = Ig1+Ig2+Ig3.........+Ign

Igw

If = Fault current

Ground wire.

Satellite Earthmat .

TRANSFORMER

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A 3x95 Sq.mm aluminum cable was used for interconnection between main and satellite earthmat in view of right of way problem for running overhead line

0 10 30 50 70 90 110 130 155DISTANCE FROM EARTHMAT IN MTRS.

0

500

1000

1500

2000

EAR

TH P

OTE

NTI

AL

RIS

E IN

VO

LTS

EPR

EPR-BEN

EPR-SAT

430 VOLTS

RESULTS OF EARTH POTENTIAL RISE TESTS AT YESLUR

Note:- EPR – Grid alone, EPR-BEN – Grid after bentonite treatment, EPR – SAT– After interconnection with Satellite Earthmat.

The results of the EPR test conducted in the presence of PTCC authorities on 20-10-2004 indicated an EPR at Earthmat as 1062 Volts and at Telephone Exchange as 274 volts. After obtaining necessary PTCC approval the station was commissioned. CONCLUSION

The distance between the telephone exchange and proposed sub-station site is a very important factor for deciding the location of a sub-station. The distance at which EPR would be less than 430 volts depends upon soil resistivity of the area, fault level and the design earthmat features. while deciding the location of new sites for sub-stations in KPTCL grid, as a general guideline, a safe distance of five times the diagonal length of the station area is taken wherever alternative sites are available The case study gives one of the solutions to reduce EPR by providing a satellite earthmat to the main earthmat having high resistivity, restricted area with telecommunication Exchange nearby. References

• ANSI/IEEE Std 80-2000,”IEEE guide for safety in AC Substation Earthing.” • CB &IP Technical Report No. !03 • PTCC Manual 1995 Edition • Asia Pacefic Telecommunity Study Group No IV Report

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