Group Presentation 03

8/3/2019 Group Presentation 03

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Design of Optimum Earthing System

to Substation Using a Software

Presented by GroupC

D V G Chamara

L Neelawala

D H S Piyumika

S L Prabashwara

G W V Priyantha

E A K Sandakalum

T M Weerakkody


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Associated Journals

N M Tabatabaei and S R Moetezaeei,Design of Grounding Systems in

Substations by ETAP Intelligent Software,

International Journal on Technical andPhysical Problems of Engineering, Vol 2,No. 4, P 45-49, March 2010

E U Ubeku and F Odiase, SubstationEarthing Grid Safety Analysis, International

journal of Electrical and Power Engineering,Vol 3, No. 5, P262-267, 2009


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Abstract

Journal 1:In a properly designed ground system,under faulty conditions voltage

gradient should not exceed the limitsthat can be tolerated by the humanbody.

maximum tolerable potential

deference should be carefullydesigned. Genetic algorithm wasdeveloped to find the optimum

grounding system to ensure safety ofeo le even at severe fault.


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Abstract (contd..)

Journal 2:

The study presents a user interactiveprogram to design a substation

earthing grid system. Standard IEEE80 and 2000 are used to write theprogram for simulation of touch, step

and mesh potentials to avoid trial anderror process to ensure accuracy andsafety.


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Objective of Earthing Design

To provide a direct path for electric currentsto flow to the ground

under normal and fault conditions

without exceeding any operating and

equipment limits, or adversely affectingthe continuity of service

To ensure that persons in the vicinity ofgrounded facilities are not exposed to the

danger of electrical shockmaintaining step and touch voltages attolerable values


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Key Factors of EarthingDesign Ground Potential Rise (GPR)

1. Ground Potential Rise (GPR) is a phenomenon that occurswhen large amounts of electric current enter to the earth.

2. typically caused when substations or high-voltage towersfault, or when lightning strikes occur (fault current).

3. The voltages produced by a Ground Potential Rise or EarthPotential Rise event can be hazardous to both personneland equipment.


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Key Factors of EarthingDesign Touch and Step Potential


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Touch PotentialThe potential difference between the ground

potential rise (GPR) and the surface potential at

the point where a person is standing while atthe same time having a hand in contact with agrounded structure

Key Factors of EarthingDesign

Step voltage

The difference in surface potential

experienced by a person bridging a distanceof 1m with the feet without contacting any

grounded object


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Algorithm for program


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Algorithm for program


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Design Procedure

Step 1:Determine the soil resistivity profile and the soil model needed(that is, uniform or two-layer model).

Step 2:Determine the conductor size. The fault current should be themaximum expected future fault current, and the time, tc, shouldreflect the maximum possible clearing time

Amm2 conductor cross section in mm2

I rms current in kA

Tm maximum allowable temperature in C

Ta ambient temperature in C

Tr reference temperature for material constants inC

o thermal coefficient of resistivity at 0 C in 1/C

r thermal coefficient of resistivity at referencetemperature Trin 1/C

r resistivity of the ground conductor at referencetemperature

Ko 1/oor (1/r)Trin C

tc duration of current flow in s

TCAP thermal capacity per unit volume


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Step 3:Determine The tolerable touch and step voltages. The choice oftime, ts, is based on the judgment of the design engineer

Estep= (RB+6Cs) IB

Etouch= (RB+1.5Cs) IB

Design Procedure

Tolerable body current limit

IB= rms value of the current through the body in mA

ts= duration of the current exposure in s (from 0.03 3

s)

k= empirical constant related to electric shock energy

For a 50kg person, k50=116

For a 70kg person, k70=157

IB =ts

k

Effect of a thin layer of surface material

Corrective factor Cs used to compute effective foot resistance in thepresence of a finite thickness of surface material


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Design Procedure

Step 4:The preliminary design should include a conductor loopsurrounding the entire grounded area, plus adequate crossconductors to provide convenient access for equipmentgrounds, etc.The initial estimates of conductor spacing and ground rodlocations should be based on the current IG and the area beinggrounded.

70 m

70 m


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Design ProcedureStep 5:

Estimate the preliminary resistance of the grounding system.For the final design, more accurate estimates of the resistance maybe desired. Computer analysis based on modeling the componentsof the grounding system in detail can compute the resistance with ahigh degree of accuracy, assuming the soil model is chosencorrectly.

Step 6:Determine the grid current.

To prevent over design of the grounding system, only that portion ofthe total fault current, that flows through the grid to remote earthshould be used in designing the grid. The current should, however,reflect the worst fault type and location, the decrement factor, andany future system expansion.

Step 7:If the GPR of the preliminary design is below the tolerable touchvoltage, no further analysis is necessary. Only additional conductorrequired to provide access to equipment grounds is necessary.

GPR= If,max.Rg

This Should be less than Etouch voltage


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Design Procedure

Step 9:If the computed mesh voltage is below the tolerable touchvoltage, the design may be complete .If the computed mesh voltage is greater than the tolerabletouch voltage, the preliminary design should be revised

Step 10:

If both the computed touch and step voltages are belowthe tolerable voltages, the design needs only therefinements required to provide access to equipmentgrounds. If not, the preliminary design must be revised.

Step 11:If either the step or touch tolerable limits are exceeded,revision of the grid design is required.These revisions may include smaller conductor spacings,additional ground rods, etc.


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Design Procedure

Step 12:After satisfying the step and touch voltage requirements, additional

grid and ground rods may be required.The additional grid conductors may be required if the grid design

does not include conductors near equipment to be grounded.Additional ground rods may be required at the base of surge

arresters, transformer neutrals, etc.The final design should also be reviewed to eliminate hazards due totransferred potential and hazards associated with special areas ofconcern.

DISADVANTAGE OF MANUAL


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DISADVANTAGE OF MANUAL

CALCULATION

1 ITERRATIVE PROCEDURE

2 TIME CONSUMING

3. LACK OF MUTUAL EFFECTCONSIDERATION

4. FREQUENCY RESPONSE IS NOT INCONSIDARATION

5. NONLINEERITY AND CLIMATE CHANGESARE NOT IN CONSIDERATION


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Sample StimulationResults

Case-01 :


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Case-02:


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Case-03:


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Case 04: Designing of Earth SystemNetwork with the same interval

Case 05 : Design of Earth System Network

with optimum conductors land base networkoptimization single variable

Case 06 : Designing of Earth SystemNetwork with comprehensive multivariateoptimization (based on optimizing thenumber of conductors and earth rodnetworks


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Advantages over ManualProcess Associated complex equations can be solved

more accurately. Mutual effects of adjacent grounding objects

can be taken into the account.

Facility to simulate under various soil layermodels with seasonal effects. Simulate under fast transient and power

frequencies. 3D Geometry of the grounding system can be

modeled Ability to design cost optimum safe grounding

system.


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Future Milestones

Substation Grounding is a complexprocess due to number of involvedphenomena. One of major problem iswhen lightning occur, non-linear andionization phenomena appear in thesoil. Therefore, we should investigateand develop a model to evaluate

performance of soil under impulsetransient. Thereafter this model can beincorporate to existing software.


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Conclusion

Substation earthing grid design is a , Trial andError Procedure and Large No of parametersinvolved in the calculation. Therefore,Accurate computation of earthing behavior

under steady and transient conditions can onlybe achieved through advance computer basesoftware only.It has Ability to implement cost optimize design

where necessary. And also Advance featureslike stimulation for different cases can beeasily analyzed and modeled. So we highlyrecommended a advanced computer softwaresuch as MATLAB, ETAP, for the Grounding

Substation Design.


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Thank You

Documents

Group Presentation 03