Factor ct orss That That Af Affect fect Substation ubs tation Ground Groun d Gri Gridd De Design si gn
Jerry Johnson POWE POWER R Eng Engin inee eers rs,, Inc Inc.. Hailey Hail ey,, Idaho, Idaho , USA USA (208) 788-3456
Presented at the POWER POWER Engin Engineers eers Substatio n Conference
September 1999 1999
Table of Contents ABSTRACT....................................................................................................................... 1
1.0 INTRODUCTION........................................................................ .............................. 1
2.0 BASIC GROUNDING CONSIDERATIONS.......................................................... 1
3.0 SOIL RESISTIVITY ................................................................................................. 3
4.0 FAULT CLEARING TIMES............................................................... ..................... 6
4.1 EFFECTS ON THE HUMAN BODY ............................................................................. 6 4.2 HIGH SPEED FAULT CLEARING .............................................................................. 6 4.3 EFFECTS ON TOLERABLE STEP AND TOUCH POTENTIALS....................................... 7 5.0 FAULT CURRENT ................................................................................................... 8
5.1 CALCULATION OF SF .............................................................................................. 9 5.2 DECREMENT FACTOR DF ............................................... ....................................... 10 6.0 SUMMARY .............................................................................................................. 11
REFERENCES................................... ............................................................................. 11
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Factors that Affect Substation Ground Grid Design Abstract This paper discusses three major factors that influence substation grounding system design: 1) Soil Resistivity, 2) Fault Clearing Time and 3) Ground Fault Current. Each must be considered along with short-comings of the analysis software and alternative designs to develop a safe and the most economic solution.
1.0 Introduction As available fault currents increase on today’s electrical power grid, interest in substation grounding system design also increases. Personnel safety is primary, but the economics are also a key factor. Engineers do not wish to “over design” grounding systems, but they do want to design systems that protect personnel and equipment while providing an optimized economic solution.
This paper discusses how each of these three factors can affect the design of a substation grounding system.
2.0 Basic Grounding Considerations Line-to-Ground faults occurring in or near a substation cause a current to flow from the energized line through the buried ground grid in the station back to the source. This current flow causes the grid to “rise in potential” above remote areas that are considered to be at zero potential. The flow of current also causes the potential (voltage) or Ground Potential Rise (GPR) to vary at different points in the substation. This can produce a potential difference or Step Voltage between the feet of an individual standing on the surface of the soil. This current flow can also cause a potential difference between metal structures and various points on the surface of the soil. This difference results in a Touch
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Voltage between the hands of an individual touching a structure and the individual’s feet on the surface of the soil as illustrated in Figure 1.
Figure 1: Basic Shock Situations
The human body can withstand considerable voltage for a short period of time. For a worker to be safe within a substation, this value must be below the level that will cause the heart to fibrillate. The values of maximum allowable step and touch potentials for a person weighing 50kg or 110lbs is defined as:
Estep =
(1000 + 6Cs ρ s)0.116
Etouch =
t s
(1000 + 1.5Cs ρ s)0.116 t s
Eq 1 [1].
Eq 2 [1].
Where: Cs = the reduction factor for a high resistivity layer of crushed rock. ρs = the resistivity of the surface material in Ω-m ts = the duration of the shock current in seconds. Usually the ground grid design will require a layer of high resistivity crushed rock placed on the surface of the substation to act as an insulator between a person’s feet and the substation grid and to raise the tolerable voltages. When the station grid is designed, this feature can be used to decrease the amount of buried conductor used or to increase the
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safety margin in the design. However, the cost of the crushed rock layer should be considered since high resistivity rock may be difficult to find in certain areas.
3.0 Soil Resistivity The potential differences within a station result from the ground current flowing from the grid conductors into the surrounding soil (earth).
All soils have some resistance to
electric current flow which is measured as “resistivity”. The electrical resistivity of the various layers of soil have a great influence on the resulting step and touch potentials within the substation. The Canadian Electrical Association conducted an extensive study of various types of soil under a variety of conditions [2]. The study found that the resistivity of the soil varied with soil type, density, moisture content, temperature and state (frozen or unfrozen). Figure 2 shows how a clay soil was found to vary with temperature and state for three different moisture contents.
Figure 2: Soil Resistivity Variation with Moisture and Temperature
As Figure 2 inicates, the resistivity rises linearly on the log scale as the temperature drops; but after passing the frozen state it rises rapidly with the falling temperatures. The percent moisture also has a strong influence on the resistivity. The higher the moisture content, the higher the resistivity becomes when the soil is in the frozen state and lower
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in the unfrozen state. It is important for the engineer/designer to determine moisture content and temperature the soil can experience in the area of the substation when the station grid is being designed. They also need to keep in mind the frost depth and whether the ground rods penetrate unfrozen soil year round. Figure 3 shows the ground grid that was used in the analysis. The following parameters were used: Station Size Conductor Burial Depth Conductor Size Grid Mesh Size Fault Current Fault Clearing Time Soil Resistivity Insulating Layer
180 ft by 80 ft 18 inches 4/0 AWG Copper 20 ft by 20 ft 10,000 Amperes 0.25 seconds 25 Ω-m 4” of 3000 Ω-m Crushed Rock
180 ft
80 ft
Figure 3: Substation Grid Layout
Analysis of the grid was done using Safe Engineering Services, CDEGS Program [3] using a uniform soil model of 25 Ω-m and 100 Ω-m. The values were taken from Figure 2, Curve 2 for temperatures of 5 °C (41°F) and -5°C (23°F). The results of the case runs show that for the 25 Ω-m case, the station meets IEEE 80 standards for both step and touch potentials as illustrated in Figures 4 and 5. However, for the 100 Ω-m case, allowable touch voltages are exceeded throughout the station grid as shown in Figure 6.
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Figure 4: Step Potential Plot for 25
-m Soil Case
Figure 5: Touch Potential Plot for 25
-m Soil Case
Figure 6: Touch Potential Plot for 100
-m Soil Case
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Possible solutions would be to add additional conductor and/or ground rods to lower the potential differences to allowable values. Alternative means to achieve safety include equipment platforms electrically connected to the equipment or 6” buried mesh around the equipment.
4.0 Fault Clearing Times 4.1 Effects on the Human Body The effects of an electrical current passing through vital parts of the human body depend on the magnitude, duration and frequency of the current. The most serious consequence from exposure is ventricular fibrillation, resulting in stoppage of blood circulation. The human body is very susceptible to the effects of current at power frequencies (50Hz and 60Hz). Currents of approximately 0.1mA can be lethal. The most common effects of electrical shock on the body are perception, muscular contraction, unconsciousness, heart fibrillation, respiratory nerve blockage and burning [1].
4.2 High Speed Fault Clearing Another means to reduce the dangerous fault circumstances is to modify the fault clearing time. High speed fault clearing has two main advantages: 1. The probability of shock is significantly reduced by a fast clearing time in contrast to faults that persist for several minutes. 2. Experience as well as tests show that the chance for serious injury or death is reduced if the duration of current through the body is very brief. The allowable current values may be based on the primary relaying or protective devices, or in some cases, that of the backup relaying.
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IEEE 80 states that a good case can be made for using the primary protective device clearing time to calculate the maximum allowable step and touch potentials. This is due to the low combined probability that the primary relay malfunctions will coincide with all other adverse factors that are needed for an accident [1].
4.3 Effects on Tolerable Step and Touch Potentials Equations 1 and 2 show the relationship of the fault clearing times to the allowable values of step and touch potentials. Since t s is in the denominator of each equation, the smaller the clearing time the larger the allowable values of step and touch. Using fault clearing values of 0.25sec. and 1.0sec. in the previous 25 Ω-m case analyzed, shows that the allowable touch potential is exceeded for the 1.0 sec. case along the perimeter of the station as shown in Figure 7.
Figure 7: Touch Potential Plot for 25
-m Soil Case 1.0 second Clearing Time
To design the station to meet backup relaying contingencies, additional ground conductor (smaller grid spacing) would be required for the substation to meet IEEE 80 standards for touch potential.
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5.0 Fault Current The determination of the maximum grid current to be used in substation grounding design has been receiving a lot of attention. For many years, engineers have been using the maximum line to ground fault values in their designs. This approach is being looked at more closely. Computer analysis and other techniques can be used to determine the value for the maximum grid current. This can result in a more cost-effective substation ground grid that still meets IEEE 80 standards [1]. Another reason for determining a more accurate value of the grid current is the increasing magnitudes of fault currents. These increasing currents have a direct relationship to increasing the GPR making it difficult and expensive to protect communication circuits [4]. The symmetrical grid current that flows between the ground grid and the surrounding earth is defined as: Ig = Sf If
Eq 3 [1].
Where: Ig = symmetrical grid current in Amps. Sf = current division factor relating the magnitude of fault current to that of the grid current. If = rms value of the symmetrical ground fault current in Amps. The design value of the maximum grid current is then defined as: IG = CpDf Ig
Eq 4 [1].
Where: IG = maximum grid current in Amps. Cp = projection factor for the increase in fault current during the station life-span. Cp = 1 is for zero future growth. Df =Decrement factor based on the fault clearing time, t s.
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5.1 Calculation of S f Computer programs are available to calculate the current division factor S f. These programs accurately calculate the actual maximum ground fault current flowing in the grid, but they require a considerable amount of data to be input into the program. In many cases, an approximation can be used to estimate the value of S f. In some situations this approximation value is sufficient for designing the grounding system. Southern Company Services and Georgia Power Company developed a set of curves, which can be used to approximate S f [4]. The information required to use these graphs is the number of transmission and distribution lines at the substation and the substation grid resistance. A typical graph is shown in Figure 8.
Figure 8: Percent Grid Current Versus Substation Grid Resistance
For the case run previously, if one transmission line serves the station and two distribution feeders exit the station and the calculated grid resistance is 0.3 Ω (from the output of the CDEGS Program), Figure 8 can be used to determine S f . This value is approximately 75% or 0.75. Using this value of S f , the symmetrical grid current Ig is equal to 0.75*10,000 or 7,500Amps.
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5.2 Decrement Factor D f The maximum grid current I G, is the maximum asymmetrical ac current that will flow between the station grid and the surrounding soil. This current is defined by Equation 4 and includes the symmetrical current Ig as well as a correction factor for the dc component, the transient and subtransient ac components. Both the ac transient and subtransient as well as the dc component decay exponentially. The design of the station ground grid must take into account the asymmetrical current; therefore a Decrement Factor, Df is derived to take into account these asymmetrical components. The decrement factor can be computed by using Equation 5.
Df = 1 +
Ta tf
(1 − e −
tf
Ta
)
Eq 5 [1].
Where: tf = fault clearing time in seconds. Ta = equivalent system subtransient time constant in seconds. Typical values of the decrement factor are provided by IEEE 80 for an assumed X/R ratio of 20 and are shown in Table 1. Table 1: Typical Decrement Factors Fault Duration (sec.) Cycles (60Hz ac) 0.008 0.5 0.1 5 0.25 15 0.5 or longer 30 or more
Decrement Factor 1.65 1.25 1.10 1.0
Using the typical value of the decrement factor for the fault clearing time of 0.25 sec., a growth projection factor of 15% and data previously calculated yield I G as: IG = (1.15)(1.10)(7,500) IG = 9,487A This value would be used in the final design of the substation grid instead of the 10,000 Ampere value used previously.
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6.0 Summary The soil’s electrical resistivity can vary with the temperature and moisture content. These variables need to be taken into account in the design and analysis to ensure the station grid meets IEEE 80 standards for the varying soil conditions.
The human body can withstand exposure to high current levels for only a short period of time. High-speed fault clearing is essential to minimize the exposure time to the levels of fault currents available in most substations. Quick clearing of faults also allows higher permissible touch and step voltages. The maximum grid current must also be considered. Much of the station’s ground fault current may be carried out of the station by overhead static wires or system neutral wires. Calculation of the current division factor is important, so that the station’s grid is not over designed. Many factors are involved in the design of a substation ground grid. These factors determine the extent and amount of ground conductor required for the substation grid design. When these factors are optimized, an economic station ground grid that meets or exceeds IEEE standard can be designed.
References 1. “IEEE Guide for Safety in Substation Grounding”, IEEE Standard 80-1986 , Institute of Electrical and Electronic Engineers, Inc. New York, 1986. 2. “Earth Resistivities of Canadian Soil”, Research Report 143 T 250, Canadian Electrical Association, July 1988. 3. CDEGS User Manual, Safe Engineering Services, Montreal Canada, 1998. 4. Garrett, D. L., Myers, J. G. and Patel, S. G., “Determination of Maximum Substation Grounding System Fault Currents Using Graphical Analysis ”, IEEE Transactions on Power Delivery, Vol. PWRD-2, No. 3, pp. 725-732, July 1987.
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