*att_powermea@hotmail.com

Safety analysis for Grounding Potential Rise of Two Neighbouring Substations: Case Study of Metropolitan Electricity Authority’s System

A. Phayomhom*, N. Chirataweewoot and S. Intharaha S. Sirisumrannukul

Metropolitan Electricity King Mongkut’s University of Authority Technology North Bangkok

Thailand T. Kasirawat A. Puttarach

Provincial Electricity Authority Chiang Mai University Thailand

SUMMARY

This paper presents construction procedures for small air insulated substation (temporary substation) in the existing substation compound to be operated during renovation of the existing air insulated substation (AIS) to be gas insulated substation (GIS). During the time of isolating ground grid of two distribution substations, the effect of the auxiliary grounding system (de-energized electrical power site) of the existing AIS substation will exist. This creates ground potential rise (GPR) to be steep between the ground grids of two neighbouring substations. It is found that the percentage of GPR ratio between the disconnecting auxiliary grounding system and the main ground grid in uniform or homogenous soil is constant while the percentage of GPR ratio are different in case of two-layer soils, i.e. The GPR ratio is proportional to the bottom soil resistivity while it is invertly proportional to the upper soil’s. This implies that only a risky case can be considered in substation design, although the condition of soil varies by season (rainy, winter or summer). The ground grid design for the Pathumwan (PM) substation of Metropolitan Electricity Authority (MEA) is examined with the main objective to assess its grounding grid system condition in terms of ground potential rise, maximum touch voltage and step voltage. These three parameters are analyzed to ensure that they satisfy the safety criteria defined in the IEEE Std 80-2000 with five scenarios classified by 25 kA in MEA Distribution System Improvement and Expansion Plan (years 2012-2016). It is found that safety criteria should not be ignored in the meantime of ground grid isolation because the auxiliary grounding system of the existing substation can create steep ground potential rise and therefore the voltage difference can harm personels working nearby and cause damage to equipment in the vicinity of faults, particularly when the ground grid of the two neighbouring substations are not connected. Modelling and simulation are carried out on the Current Distribution Electromagnetic interference Grounding and Soil structure (CDEGS) program. KEYWORDS

Ground potential rise, Substation, Step voltage, Touch voltage

21, rue d’Artois, F-75008 PARIS B3-207 CIGRE 2012 http : //www.cigre.org

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1. INTRODUCTION This paper proposes a safety grounding design system of two neighboring substations in Metropolitan Electricity Authority (MEA). The methodology is illustrated by the case of Pathumwan (PM) substation, a 69 kV outdoor air-insulated substation that has been operating for more than 30 years. To enhance better security and reliability of the power system and also taking into account of harmonizing with its surrounding, this substation will be replaced with an indoor gas insulated substation (GIS). All high voltage GIS equipment will be installed in metal–clad with SF6 insolated and the supply voltage need to be upgraded from 69 kV to 115 kV in 2015. In the meantime, a small AIS substation (temporary substation) is temporarily required to cover for the existing substation. The existing outdoor substation will then be removed and replaced with a new indoor substation. Some parts of the outdoor substation, however, can still be used as spare parts. The small AIS substation will be put into operation for approximately 1 or 2 years before the new indoor substation can be put in place [1]. Prior to the removal of the existing substation, a new small AIS substation has to be constructed to temporarily cover for the existing substation. After that the existing substation can be removed and the construction of the new GIS substation can be initiated. However, based on previous practice, during the construction of the new GIS substation, the ground grid of the small AIS substation and that of the existing substation are not joined together, this often leads to the damage of equipment and injury or even loss of life of personnel. If this is the case, you are recommended to adapt the finding of this paper as a guideline to reduce the effect. Additional study in this paper also covers the GPR ratio between the auxiliary grounding system and the main ground grid which is the case generally found in substations construction. The GPR ratio exhibits interesting finding related to soil resistivity, ground grid distance, etc, which is worth to know in ground grid design. 2. SAFETY CRITERIA [2] In the process of designing the ground grid system, safety criteria is considered firstly to specify the required safety level, then the maximum touch and step voltage are calculated in comparision with the safety criteria to assure whether it is safe to work on the area. 2.1 Touch Voltage Criteria Touch potential is normally the voltage between the energized object and the feet of a person in contact with the object. In this case, it is the difference in voltage between the grounded structure a person in contact and the point where the person stands on his feet and undergoing ground potential rise (GPR). 2.2 Step Voltage Criteria Step potential is the voltage between the feet of a person standing with two feet (approx. 1 foot) apart near an energized grounded object. It is equal to the difference in voltage, given by the voltage distribution curve, between two points at different distances from the "energized grounded object ". For example, during a stroke of lightning, a person could be at risk of injury owing to step voltage across his or her feet by standing near the grounding point of the lightning arrestor. 3. MAXIMUM OF MESH AND STEP VOLTAGE To calculate both maximum touch and step voltage, apparent resistivity factor is required and it can be obtained by applying the Wenner arrangement method.

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4. PROCESS OF SAFETY ANALYSIS The safety analysis is carried out using the CDEGS program via adhering to the steps as follows: Step 1: Measure the resistance (Ω) of the soil located within the interested substation area by using the

Wenner arrangement method. Step 2: Input the obtained resistance value from step 1 into the Rural Electric Safety Accreditation

Program Module (RESAP) by using steepest method to get the soil characteristic such as soil resistivity (Ω.M) and the thickness of the layer soil.

Step 3: Input the obtained resistance value from step 1 into the CDEGS program using the MALT module to achieve the safety criteria.

Step 4: Design the ground grid system corresponding to each studied ground grid configuration for the substation.

Step 5: Obtain the maximum touch and step voltage by running the simulation program for each ground grid configuration, then justify the result by comparing the achieved maximum touch and step voltage with the safety criteria – neglect the one should either its maximum touch or step voltage exceed that specified in the safety criteria, otherwise, retain it. The configuration that fails may subject to further improvement until both its maximum touch and step voltage are well within the limit of the safety criteria.

5. EFFECTS OF NEARBY AUXILIARY GROUNDING SYSTEM OF SUB STATION Many a time, the small AIS substation is under construction while the existing substation is still in operation and not yet removed. There are two grounding systems for each substation that is not connected each other. The ground grid of the main substation is called main ground grid whereas that of the small AIS substation is called auxiliary ground grid. During the time of disconnecting of ground grid, the small AIS substation is de-energized, its auxiliary grounding system however exposes to the risk of high GPR caused by the main substation which is still in operation. The GPR’s steepness is located between the main and auxiliary ground grid. 6. CASE STUDY Fig. 1 shows the installation of a typical grounding system for Pathumwan (PM) substation together with its grid dimension. The cross section of the ground grid conductor used is 240 mm2 and its ground rod is 2.4 m long and 15.875 mm in diameter. The ground grid is 0.5 m buried below the ground surface level, and all ground rods are exothermic welded to the main ground grid. The auxiliary ground grid, the cross section of the ground grid conductor is 95 mm2; 0.5 m below the ground surface, ground rod is 3.0 m long; 15.875 mm in diameter; exothermic welded to the ground grid.

Fig.1. Typical installation of grounding system in MEA

6.1 Ground Grid Simulation Model

The ground grid system of the PM substation modelled by CDEGS program as shown in Fig. 2.

Fig.2. Top view of ground grid model of PM substation

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6.2 Soil Resistivity Result The soil layer characteristics of the PM substation were analyzed by a built-in module called Rural Electric Safety Accreditation Program module (RESAP) in the CDEGS program, the result is shown in logarithm scale in Fig. 3. Given the soil resistivity model in Fig. 3, the resistivity of the PM substation is shown in Table I. The resistivity of the top and bottom layers is 22.2588 and 1.019092m⋅Ω respectively. The resistivity of top layer is more than that of the bottom layer (deeper layer) due to a number of factors, such as: moisture content, chemical composition, salts dissolved in the water, and grain size [3].

Fig.3. Soil resistivity model Table I Summary of soil resistivity of PM substation

1ρ : resistivity of top layer soil,

2ρ : resistivity of bottom layer soil The safety criteria for a person of 50 kg body weight of PM substation is analyzed by MALT and the result is shown in Table II. Let’s select 1,000Ω .m as the surface layer resistivity, the touch and step voltage is then 804.90 and 2,352 volt respectively. Although there are a number of suitable ground grid configurations can be applied, only five combinations of existing and temporary ground grid configuration will be analized as given in Table III.

Layer

Resistivity ( m⋅Ω )

Thickness ( m )

Reflection Resistivity Contrast ratio

coefficient (p.u.)

Top 22.2588 1.831156 -1.0000 0.22259E-18 Bottom 1.019092 infinity -0.91244 0.45784E-01

Table II Safety criteria for 50 kg body weight

User defined extra foot resistance: 500Ω , Body resistance: 1,000Ω .

Surface layer

resistivity m)( ⋅Ω

Fault clearing time

0.1 sec

Foot

resistance: 1 Foot

)(Ω Touch voltage

(V)

Step voltage

(V) None 371.50 618.60 69.6 500 583.3 1,465.7 1,534.3

1,000 804.9 2,352.0 3,066.7 1,500 1,026.4 3,238.3 4,599.1 2,000 1,248.0 4,124.5 6,131.5

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Table III Difference combination of ground grid configuration

Case 1:

rod length of existing grid = 2.4 m

fault at existing substation

(existing grid )

Case 2:

rod length of temporary grid = 3.0 m fault at small AIS substation

(temporary grid )

Case 3:

rod Length of existing grid = 2.4 m

rod length of temporary grid = 3.0 m

fault at temporary or existing substation

(existing grid with connected temporary grid)

Case 4:

rod length of existing grid = 2.4 m

rod length of temporary grid = 3.0 m

fault at small AIS substation

(existing grid disconnects with temporary grid)

Case5:

rod length of existing grid = 2.4 m rod length of temporary grid = 3.0 m fault at existing substation

(existing grid disconnects with temporary grid)

existing grid: existing ground grid of the existing outdoor substation temporary grid: temporary ground grid of the small AIS substation The three voltage performance indices are listed in Table IV. The data in Table IV are graphically displayed in Figs 4 to 5.

Table IV. GPR , touch and step voltages for all five cases

Case Voltage level (V) Type of voltage

GPR Touch Step 1 1,166.6 1,082 × 313 2 774.9 694 171.5 3 542.72 451 118.7 4 770.6 662 171.5 5 1,161.4 1,054 × 312

Legend: touch (Max.) : 804.9 volt step (Max.) : 2,352 volt : acceptable (within the limit) ×××× : Not acceptable (out of the limit) The study of all 5 cases found that: for 2 unconnected ground grids, and one of which is a substation that is under operation. When fault occurs, its touch voltage may violate the safety limit stated in safety criteria as in case 5. Case 5 is the same as case 4 except their fault location, case 4 faults at the small AIS station while case 5 faults at the main substation.

In this case, 0.45% (1,166.6 volt to 1,161.4 volt) for maximum GPR, 2.59% (1,082 volt to 1,054 volt) for maximum touch voltage and 62.08% (313 volt to 312 volt) for maximum step voltage are decreased because the total resistance of case 5 is less than that of case 1. The 3-dimension GPR and 2-Dimension spot touch voltage of case 5 is shown in Fig. 4 and 5 respectively. The areas that are colored in orange and red are unsafe.

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Fig.4. 3-Dimension ground potential rise of case 5 Fig.5. 2-Dimension spot touch voltage of case 5 From all 5 cases, we can conclude that, steep GPR found in the boundary of existing and small AIS substation in cases 4 and 5. This will be the cause of ground potential difference (GPD). If the value of GPR is high, the touch voltage within the substation area will also be high. Anyway, despite the GPD, it is still safe as long as the maximum touch voltage does not exceed safety criteria. For the procedure of constructing the substation, case 2 should be chosen for the design. Because there is only ground grid of a small AIS substation, it is safe for the first step. After that, the design which consists of two neighbouring substations is processed for the next step of substation building. Despite the safety value of maximum touch voltage in case 2, the rate of safety is increased when there is the connection with ground grid system of nearby substation. If there is a separation with ground grid, the status of main return will occur and it will create GPD. Hence, the value of maximum touch voltage within safety criteria should be considered before commencing the construction in the next step.

6.3 Nearby Distribution Substation Building The above discussed unsafe condition owing to GPR in the case of two grounding grids exist close together can also be applied for a newly constructing distribution substation which happens to be close to another substation’s mesh of ground grid. Safety consideration requires that the new distribution and existing substation’s ground grid are interconnected, so that the de-energized electrical power site’s (the constructing site’s) ground grid can act as an auxiliary grounding system of the existing substation. However, if the effect of the existing is to be taken into account for a grounding design so as to reduce the performance requirements of the substation grounding system, the copper conductors must be connected in a reliable manner to the substation grid [4]. The essence of the study case mentioned above is the case of two substations, the grounding systems of which are isolated from each other, thus create the effect of the auxiliary grounding system of one to another. To make it simple, the case will further be divided into 4 cases : case A to D. According to the appearance of 3 auxiliary grounding system of the substation’s status, the steep GPR will create high GPD and high GPD will subsequently cause the high touch voltage. However, it still depends on ground grid whether it is designed to deal with such a situation or not, in other words, although high GPD results in high touch voltage, but it may still fall within the limit of the safety criteria. The 4 cases represent different ground grid configuration as follows : For cases A to D, all of the cross section of the ground grid conductor is 95 mm2 and the ground rod is 3.0 m long and 15.875 mm in diameter. All the grid conductors are 0.5 m deep buried in the top layer soil. The figure of an installation of ground rod will be spread out. The dimension of ground grid which presents the status of return will be categorized into 3 sizes : small (15 mx15 m), medium (30 mx30 m), and large (45 mx45 m). The main one is of medium size (30 mx30 m). Furthermore, the value of soil resistivity is chosen to be 1, 50 and 100, this applies to both top and bottom layer soil. Thus, the short circuit current of 25 kA is specified.

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Table V Configuration of the auxiliary grounding system of the substation

Table VI GPR, touch voltage and step voltage for different configurations

Case Configuration A

Main 30mx30m B

Main 30mx30m, Auxiliary 15mx15m

C

Main 30mx30m, Auxiliary 30mx30m

D

Main 30mx30m, Auxiliary 45mx45m

Legend : M: main ground grid system Au: auxiliary grounding system of the existing substation

The interesting findings are as follows: Case A: The value of GPR, touch and step voltage will increase if either the soil resitivity of top or bottom layer or both increases. In addition, the concept can be well applied to the site the soil of which is homogeneous. Case B: This case includes the effect of the auxiliary grounding system of the existing substation status (return ground grid). The resulting GPR, touch and step voltage are thesame as Case A : they increase if either the soil resitivity of top or bottom layer or both increases, with the exception that, the GPR of return ground grid, when the resistivity of top layer soil is higher than the bottom’s, the GPR will decrease (sheaded area) when the resitivity increases. In the case of the homogeneous soil, the resulting

Case Type

of voltage

1ρ ( m⋅Ω )

Voltage level (V)

2ρ ( m⋅Ω )

1 50 100

A

GPR 1 334 4,812.7 6,338 50 419.41 16,700 29,245 100 422.02 18,417 33,400

Touch 1 279 2,762 3,163 50 364 13,941 24,011 100 367 15,470 27,881

Step 1 122.7 497 536 50 246 6,133 9,356 100 250 7,628 12,265

B

GPR

M 1 327.71 4,636.7 6,132.1

50 418.02 16,386 28,529 100 418.48 18,136 32,771

Au 1 172.11 3,932.5 5,400.5 50 170.73 8,605.7 16,359 100 169.02 8,831.6 17,211

Touch 1 254 373 2,397 50 373 12,761 21,554 100 376 14,229 25,431

Step 1 135.7 258 594 50 258 6,787 10,320 100 259 8,263 13,574

C

GPR

M 1 314.77 4,310.2 5,733.1 50 407.17 15,738 27,129 100 409.91 17,544 31,477

Au 1 142.51 3,578.9 4,990.6 50 139.83 7,125.7 13,637 100 139.8 7,322.9 14,251

Touch

1 233 1,374 1,499 50 395 11,636 18,032 100 440 13,957 23,271

Step

1 141.4 523 523 50 262 7,072 10,762 100 266 8,586 14,143

D

GPR

M 1 304.76 4,014.2 5,366.7 50 400.41 15,238 26,052 100 405.32 17,081 30,476

Au 1 115.18 3,176.8 4,519.2 50 113.37 5,759.1 11,054 100 114.18 5,935 11,518

Touch

1 253 1,217 1,314 50 419 12,647 19,838 100 427 15,043 25,293

Step

1 152.4 596 621 50 270 7,621 11,783 100 278 9,177 15,241

GPR of main and return ground grid, touch and step voltage are directly proportional with the soil resistivity.

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Case C and D, the GPR value of main and auxiliary ground grid, touch and step voltage have the same characteristic as case in B. Now, let’s consider the normalized ratio expressed in percent (%) between GPR of auxiliary grounding system to the main ground grid of Case B, C and D, and putting in the distance between each ground grid as a new parameter of interest. The result is listed in Table VII. For all cases, if the soil is homogeneous, the ratio is the same (diagonally shaded), no matter how much the resistivity is. This implies that the ground grid design can still be applicable in case of homogeneous soil without the consideration of its ground resistivity even the gournd resistivity changed by season. The higher the ratio, it means the GPR of auxiliary grounding system approaches that of the main grid’s. This is a safety condition, becuse the GPD between the two grids is small and hence small value of touch and step voltage. In case of two layer soil, the ratio is proportional to the resistivity of bottom layer, whereas it is inversely proportional to the resistivity of the top layer soil. Table VII GPR ratio between auxiliary and main ground grid configuration

Case Distance

(m)

1ρ ( m⋅Ω )

GPR (%)

2ρ ( m⋅Ω )

1 50 100

B

3 1 52.52 84.81 88.07 50 40.84 52.52 57.34 100 40.39 48.70 52.52

18 1 30.21 62.69 68.90 50 23.91 30.21 33.37 100 23.76 28.03 30.21

33 1 21.75 51.05 58.28 50 17.23 21.75 24.06 100 17.12 20.19 21.75

C

3 1 45.27 83.03 87.05 50 34.34 45.28 50.27 100 34.11 41.74 45.27

18

1 26.37 58.96 65.81 50 20.82 26.37 29.23 100 20.69 24.43 26.37

33

1 19.51 47.92 55.51 50 15.45 19.51 21.61 100 15.35 18.10 19.51

D

3 1 37.79 79.14 84.21 50 28.31 37.79 42.43 100 28.17 34.75 37.79

18 1 22.98 55.12 62.58 50 18.11 23.04 25.56 100 18.00 21.26 22.98

33 1 17.52 44.98 52.82 50 13.85 17.52 19.43 100 13.77 16.24 17.52

From Table VII, the value of top and bottom layer resistivity at any point where the status of main and auxiliary of case B occurs. We can note that the GPR ratio of case B is higher than that of case C and D for the same distance, because of the higher total grid resistance of case B. It is therefore evident that, case C and D constutes unsafe condition and should be subjected to further consideration against the safety criteria. In addition, the study of case B is analyzed by varying the distance between the main and auxiliary ground grid. In the previous case, the inter-distance is fixed at 3 m. In this study, the GPR ratios (Table VII) of the inter-distance of 18 m (6 times) and 33 m (11 times) are obtained. It is found that the greater the distance, the lower the GPR ratio. For instance, in case B, for a given resistivity, the GPR ratio of the inter- distance of 18m compares to that of 33m will be 30.21% and 21.75% respectively.We know that low GPR ratio results in high touch and step voltage, and finally, constutes an unsafe contition. We have little to discuss about the results of case C and D, for they have the same characteristic as of case B. Compare the last 3 study cases B, C and D, with the case of PM substation distributes the electricity to small substation as illustrated in case 4. The old inter-distance between the main and the auxiliary grid was 4.4 m and later had been proposed to change to 74.4 m or 144.4 m if applicable. The GPR ratio of

Table VII was implemented for PM substation, and found that : the GPR ratio decreases from the original 21.84% (4.4m) to 5.53% (74.4 m) and 3.18% (144.4 m) repectively. The study result is shown in Table VIII. However, when the resulting touch voltags were checked to see review. However, if the

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two neighbouring ground grids are connected, it is safe for the people if working g around. Care should be given in obtaining the accurate soil resistivity from measurement in field test.they violated against the safety criteria in Table II. It is found that the touch voltage's for both distance are still within the safety limit, with the exception that, case 5 was unsafe from the begining, so it remains unsafe when its inter-distance increases. In addition, case 3 is analyzed by varying the inter-distance, it will remain safe because the two neighbouring gronding grids are interconnected and the total resistance of 74.4 m (0.01777Ω ) and 144.4 m (0.015734Ω ) grounding system are less than 4.40 m (0.021709Ω ). Case 5 is not safe for the given configuration, however, this can be mistigated by reducing its construction time interval by increasing the number of workers. Also, it can be alleviated by dropping crushed rock#2 with the resistivity of 3,000 (m⋅Ω ) for approximately 10-20 cm in thickness.

ACKNOWLEDGMENT The first author was financially supported by MEA Thailand. He would like to express his deepest gratitude to late Assoc. Prof. Dr. Jamnarn Hokierti, Kasertsart University, Thailand and Mr. Praditpong Suksirithaworngule, ABB, Thailand, for teaching him the essential knowledge of power system. The author would like to express his sincere thanks to Provincial Electricity Authority (PEA) for CDEGS program and MEA for the technical data used in this research work. High appreciation is given to Mr. Chotepong Pongsriwat, PEA, Northern Region1, Chiang Mai, Thailand for his constructive comments. The author is deeply indebted to Power System Planning Department for research time and strong support in this work. BIBLIOGRAPHY [1] A. Phayomhom, S. Sirisumrannukul and T. Kasirawat. “Safety Design of Ground Grid in

Distribution Substation: Case Study of Metropolitan Electricity Authority’s System” (International Journal of GMSARN ,vol. 4, no. 2, June 2010, pages. 64 - 74)

[2] IEEE Guide for Safety in AC Substation Grounding. (IEEE Standard 80-2000, January 2000) [3] BS. Code of Practice for Earthing (BS Standard 7430:1998, November 1998, pages 8 - 10) [4] Safe Engineering Services & Technologies Ltd., “Grounding & Electromagnetic Field”

(Technical Semina. Chapter 2 Fundamental Grounding Concepts, 1996, pages 2_2 -1 _ 15)

Table VIII GPR , touch and step voltages for PM substation cases 3 to 5 with varied distance

Cases Distance

(m)

Voltage level (V) Type of voltage GPR

Touch Main Return

Auxiliary /Main (%)

3 4.40 542.72 451.00 74.40 444.26 381.25 144.40 393.34 340.68

4 4.40 770.6 168.27 21.84 662.00 74.40 774.91 42.887 5.53 700.00 144.40 774.92 24.673 3.18 727.00

5 4.40 1,161.40 168.36 14.50 1,054.00× 74.40 1,166.60 42.89 3.68 1,097.55× 144.40 1,166.6 24.673 2.11 1,122.18×

7.CONCLUSION The ground grid design for the PM substation is examined with the main objective to assess its grounding system condition in terms of ground potential rise, touch voltage and step voltage. These three parameters are analyzed to ensure that they satisfy the safety criteria defined in the IEEE Std 80-2000 with five scenarios classified by 25 kA in MEA Distribution System Improvement and Expansion Plan No.11 (years 2012-2016). The study found that, if two unconnected ground grids happen to exist closed together, they may expose the workers to unsafe condition as the touch and step voltages violate the safety criteria. In such a case, the original design should be subjected for safety