international journal of electrical, electronics and data ... of grounding system as per ieee...

International Journal Of Electrical, Electronics And Data Communication, ISSN: 2320-2084 Volume-3, Issue-11, Nov.-2015

Effect Of Grid Parameter Variation On The Performance Of Grounding System

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EFFECT OF GRID PARAMETER VARIATION ON THE PERFORMANCE OF GROUNDING SYSTEM

1ANUP KUMAR, 2PRASANNA D. BHARADWAJ

1M.Tech Student, Electrical Engineering Department, Bharati Vidyapeeth College of Engineering, Pune, India

2Associate Professor, Electrical Engineering Department, Bharati Vidyapeeth College of Engineering, Pune, India Email: [email protected], [email protected]

Abstract - Grounding system is very important for a substation in the sense of stability, safety and proper functionality of power system. There are various parameters which affect the grounding system performance such as grid depth, area of grid, horizontal conductors spacing, number of vertical ground rods, soil resistivity, level of fault current etc. This paper describes the performance of grounding grid in uniform soil by varying various design parameters, and calculating the effect using MATLAB tool. Grounding grid performance case study has been done on a 33 kV switchyard of hydropower station having uniform soil of resistivity using IEEE Std. 80-2000. Keywords - Grounding Grid, Grid Depth, Soil Resistivity, Grounding Grid Resistance, Touch Voltage, Step Voltage. I. INTRODUCTION Substation is the important part of the power system because electrical power from generating station to the consumer load end passes through number of substation [6]. Thus power system satisfactory operation depends on the reliable operation of substation and substation reliable, satisfactory and safe operation depends ultimately on grounding system design which ensures safe and proper functioning of protection and control equipment in case of any disturbance or fault in substation and its nearby electrical network [9]. A grounding system consists of buried horizontal conductors having vertical grounding rods, cables connecting buried grounding grid to metallic parts of substation equipment and structures, connections to earthed system neutrals and the insulating surface covering the equipment and other material in substation also [1]. Main objective of a grounding system [2]:

1) To give path for currents generated due to lightning and fault for stabilizing the potential 2) To ensure the safety for a human present near to the grounding facilities 3) To ensure safe functioning of electrical equipment

Therefore design of grounding system should be such that it should be safe under normal and fault condition and also be cost effective. Hence it becomes necessary to analyze grid performance for variations of grid design parameters which are described in this paper by taking an earth mat case study of 33 kV switchyard of hydropower station having uniform soil model. In this paper section II gives the important equations for finding required parameters of grounding system as per IEEE std.80-2000. While section III describes the grounding system performance for variation of some important grid design parameters and section IV includes conclusion.

II. REQUIRED GRID DESIGN PARAMETERS CALCULATION 2.1. Step and Touch Voltage Criteria The maximum allowable step and touch voltages are the important criteria that must be fulfill for designing safe ground grid. The calculated actual step and actual touch voltages of the grounding grid design should be below the maximum tolerable values from safety point of view [1]. For human weighing 50 and 70 Kg, the allowable step and touch voltages as per IEEE std.80-2000 is given as follows:

s

ssstep tCU 116.06100050 (1)

s

ssstep tCU 157.06100070 (2)

s

sstouch tCU 116.05.1100050 (3)

s

sstouch tCU 157.05.1100070 (4)

Where Ustep50 or 70 = Allowable step voltage for 50 kg or 70 kg weight person in volts Utouch50 or 70 = Allowable touch voltage for 50 kg or 70 kg weight person in volts ts= Fault clearing time s= Resistivity of surface layer in .m Cs= Surface layer De-rating factor 2.2. Earth Grid Conductor Cross Section Calculation

a

m

rrc

fmm

TKTK

TCAPt

IA

0

0

4

ln

10

2

(5)

Where Amm

2 = Conductor cross section in mm2



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If = RMS value of fault current in kA tc = Duration of clearing fault current in seconds Tr = Reference temperature for material constants in 0C r = Thermal coefficient of resistivity at reference temperature Tr in 1/0C r = Ground conductor resistivity at reference temperature Tr in -cm TCAP = Thermal capacity per unit volume in J/(cm3. 0C) 0 = Thermal coefficient of resistivity at 0 0C in 1/ 0C K0 = (1/0) or (1/r) at reference temperature Tr in 0C Tm = Max. Allowable temperature Ta= Ambient temperature 2.3. Calculation of Spacing Factor for Computing Actual Mesh and Step Voltages Spacing factor for mesh voltage (Km)

128ln

482

16ln

21 22

nKK

dh

DdhD

hdDK

h

iim

(6)

Spacing factor for step voltage (Ks)

25.0111

211 n

S DhDhK

(7)

Where, D = Spacing between parallel conductors in meters h = Depth of buried earth grid conductors in meters d = Diameter of grid conductors in meters Kii = 1 for grids with rods throughout the grid Kh = Corrective weighting factor emphasizing the grid depth effects n = Geometric factor 2.4. Actual Step and Touch Potential Calculation Actual mesh voltage is calculated by given formula as:

M

Gimm L

IKKE

(8) Actual step voltage is calculated by given formula as:

S

GiSS L

IKKE

(9) For Earth grids having vertical ground rods throughout the grid, LM is as follows:

Ryx

rCM L

LL

LLL

2222.155.1 (10)

The effective buried conductor length LS is given by below equation as follows:

RCS LLL 85.075.0 (11)

Where, = Soil resistivity in .m

IG = Maximum grid current in amperes LC = Earth grid conductors total length in meters LR = Ground rods total length in meters Lr = Ground rod length in meters Km= Spacing factor to compute mesh voltage Ki= Grid geometry correction factor 2.5. Grounding Grid Resistance Calculation Large substations usually have resistance about 1 Ω or less to minimize GPR. The following given below formula is used for calculation of grounding resistance:

1 1 1120 1 20/g

T

RL A h A

(12)

Where, Rg = Substation ground resistance in Ω ρ = Soil resistivity in Ω-m A = Area covered by buried grid conductors in m2 h = Depth of the grid in meters. LT =Total length of conductors buried in soil in meters 2.6. Maximum Current Entering into Earth Grid and Ground Potential Rise (GPR) Calculation Maximum current in earth grid (IG) is given as follows:

ffpgfpG SIDCIDCI 03 (13) And Ground Potential Rise (GPR) is as follows:

gG RIGPR (14) Where, Cp = Corrective projection factor by considering future growth Df = Decrement factor for the duration of the fault I0 = Zero-sequence fault current in amperes Ig = RMS symmetrical grid current in amperes Rg = Substation ground resistance in ohms Sf = Split factor Fault current division factor or split factor (Sf ) used in above equation can be calculated by below equation

03 II

S gf (15)

III. GROUNDING GRID PERFORMANCE ANALYSIS USING MATLAB AS PER IEEE STD.80-2000 IN UNIFORM SOIL Grounding grid performance analysis has been done by varying some important factors that affects grounding grid design using MATLAB as per IEEE Std.80-2000 in uniform soil on earth grid of 33 kV switchyard of hydropower station in Kolhapur district of Maharashtra.



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Fig.1. Design procedure for designing safe grounding grid [1]

Table 1: Available designed ground grid parameters

Table 2: Designed earth grid result

3.1. Effect of Conductor Spacing Variation on Actual Mesh and Step Voltage For a fixed grid area and all other fixed parameters as mentioned in table-1 except grid conductor spacing, graphs in figure-2 show that increase in spacing between adjacent horizontal conductors causes increase in mesh voltage and decreases in step voltage. However it can be observed that decrease in step voltage is more than increase in mesh voltage. For fixed grid area, smaller the distance between horizontal conductors more the number of conductors required. Hence grid designer should carefully take decision for horizontal conductor separation to keep actual mesh and step voltage within tolerable limit.

2(a)

2(b)

Fig.2. Effect of conductor spacing variation on 2(a) Actual mesh voltage, 2(b) Actual step voltage

3.2. Effect of Grid Depth Variation on Actual Mesh and Step Voltages As grid burial depth increases the actual step and mesh voltage decreases for all fixed design parameters given in table-1 except grid burial depth. But actual step voltage decreases more rapidly than



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actual mesh voltage. From graphs in figure 3(a) it can be observed that after certain grid depth there is increase in actual mesh voltage because actual mesh voltage equation is non linear in nature because of logarithmic function in equation. So designer should take care of it for finding optimum depth at which actual mesh and step voltage are within tolerable limit for designing safe ground grid.

3(a)

3(b)

Fig.3. Effect of grid depth variation on 3(a) Actual mesh voltage, 3(b) Actual step voltage

3.3. Effect of Number of Ground Rods Variation on Actual Mesh and Step Voltages Ground rods reduce the earth grid resistance and ground potential rise. Increase in number of ground rods causes decrease in the actual mesh and step voltages which can be observed by graphs in figure 4(a) and 4(b) for all other fixed design parameters given in table-1. Whenever there is need of design modification the ground rods numbers are varied to maintain the actual step and touch voltage within tolerable limit.

4(a)

4(b)

Fig.4. Effect of No. of ground rods variation on 4(a) Actual mesh voltage, 4(b) Actual step voltage

3.4. Effect of Grid Area Variation on Actual Mesh Voltage and Actual Step Voltage and GPR For other fixed design parameters of table-1, graphs in figure 5(a), 5(b) and 5(c) show that there is decrease in mesh voltage, step voltage and GPR with increase in grid area. As the grid area increases, conductor’s length buried in the soil increases and hence grid resistance decreases which leads to decrease in actual step and touch voltages also.

5(a)



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5(b)

5(c)

Fig.5. Effect of grid area variation on 5(a) Actual mesh voltage, 5(b) Actual step voltage, 5(c) GPR

3.5. Effect of Soil Resistivity Variation Effect of soil resistivity on grounding system has been shown graphically, Graphs in figure 6(a), 6(b), 6(c), 6(d) respectively show that grounding grid resistance, actual mesh voltage, actual touch voltage and GPR increases with increase in soil resistivity.

6(a)

6(b)

6(c)

6(d)

Fig.6. Effect of soil resistivity variation on 6(a) Grid resistance, 6(b) Actual mesh voltage, 6(c) Actual step voltage, 6(d) GPR.

3.6. Effect of Surface Material Thickness Variation on Tolerable Step Voltage and Tolerable Touch Voltage Effect of surface material thickness can be observed from graphs in figure 7(a) and 7(b) which show that



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tolerable step and tolerable touch voltage increases rapidly upto thickness 0.3 meters and after that increase in thickness has very negligible effect on tolerable step and tolerable touch voltage.

7(a)

7(b)

Fig.7. Effect of surface material thickness variation on 7(a) Tolerable step voltage, 7(b) Tolerable touch voltage

3.7. Effect of Fault Current Level Variation on Actual Mesh Voltage, Actual Step Voltage, GPR and Tolerable Step Voltage and Touch Voltage For other fixed design parameters of table-1, graphs in figure 8(a), 8(b), 8(c) show that there is increase in actual mesh voltage, actual step voltage and GPR with increase in fault current level. Graphs in figure 8(d) show that there is no change in Tolerable touch and step voltage with increase in fault current level. Since from equation 1 to 4, it can be said that the tolerable step and tolerable touch voltage does not depend on fault current. Actually equation 1 to 4, show that tolerable step and tolerable touch voltage is inversely proportional to the square root of fault clearing time.

8(a)

8(b)

8(c)

8(d)

Fig.8. Effect of fault current level variation on 8(a) Actual mesh voltage, 8(b) Actual step voltage, 8(c) GPR,

8(d) Tolerable voltage limit



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CONCLUSIONS Analyzing grounding system performance by varying grid design parameters can help designer to design safe and economic grounding system. Sometime the grounding grid may be overdesigned which incurred higher cost. So by analyzing grounding grid performance at various design parameters, we can find optimum point at which grid will be safe for human and also cost effective. In present study, for designing earth grid of 33kV switchyard of a hydropower station, the available grid area is fixed whose length and width are 15 meter and 13 meter respectively. Performance of grounding grid in uniform soil by varying various design parameters is calculated using MATLAB tool. By parametric analysis it is found that, safe and cost effective design is highly dependent of the soil resistivity. Precise measurement of the soil resistivity at site is highly recommended. Also from parametric analysis in MATLAB at different design parameters it is found that, 1.4 meters horizontal conductor spacing, grid depth as 0.6 m and surface material thickness as 0.3 m are suitable for designing safe and cost effective grounding grid system at the study project switchyard location. It is also found that actual mesh and step voltage increases as fault current level increases. Therefore at increased fault current level if the actual mesh and step voltage exceeds the tolerable limit then it becomes necessary to modify the design by increasing the number of horizontal grid conductors and ground rods. The number of horizontal grid conductors can be increased by decreasing the

spacing between horizontal grid conductors. For an established working substation, it is not possible to modify the existing grid design, so in that case auxiliary earth mat has to be designed nearby existing main earth mat of substation and the existing main earth mat is connected to newly design auxiliary earth mat to make the grounding system safe for human being. REFERENCES [1] ANSI/IEEE Std,80-2000,”Guide for Safety in AC Substation

Grounding”,IEEE, New York. [2] Manual on,” Earthing of A C Power Systems,” Publication

No 302, C.B.I.P. New Delhi, Oct. 2007. [3] Andrew Ackerman, P.K. Sen, Clifton Oertli, “Designing Safe

and Reliable Grounding in AC Substations With Poor Soil Resistivity: An Interpretation of IEEE Std.80”, IEEE transactions on industry application,Vol.49, NO.4, July/August 2013.

[4] IEEE std 81-1983," IEEE guide for Measuring earth Resistivity, Ground impedance, and earth surface potentials for a ground system"

[5] I.S.3043-1987,Indian Standard Code of Practice for Earthing. [6] Kaustubh A. Vyas and J.G. Jamnani, “Optimal Design and

Development of Software for Design of Substation Grounding System”, Institute of technology, Nirma University, Ahmedabad, 08-10 December 2011.

[7] M.G. Unde and B.E. Kushare, “Grounding grid performance of substation in two layer soil-A parametric analysis”, International Journal of Engineering Sciences and Emerging Technologies, Feb 2012, Volume 1, Issue 2.

[8] F. Dawalibi, D. Mukhedkar, "Influence of Ground Rods on Grounding Grids", IEEE Transactions on PAS, Vol. PAS-98,No. 6, November/December 1979, pp. 2089-2098.

[9] Methodology and Technology for Power System Grounding by Jinliang He, Rong Zeng, Bo Zhang, Department of Electrical Engineering, Tsinghua University, China. Published on 2013 John Wiley & Sons Singapore Pte. Ltd.

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