Turkish Chamber of Electrical Engineers (TMMOB) Grounding Measurements in Electrical Installations and Evaluation of Measurement Results Note: This study was prepared by Electrical Engineer Taner İRİZ and Electrical and Electronics Engineer Ali Fuat AYDIN for use in the trainings of the Chamber of Electrical Engineers. Permission must be obtained from the authors if this work is used outside of the Chamber or if the text is modified. 1. When measuring both soil resistivity and radiating resistance in electrical installations, all electrodes are conceptually considered to be hemispheres. Hemispherical electrodes, which are not used in practice, facilitate calculations in grounding measurement theory. In the case of homogeneous soil resistivity (ρ constant), the resistance of a hemispherical electrode with radius r to the ground can be simply calculated using the relation ρ r2 π, R yk = R. The potential at a location at a distance x from the center of this electrode is calculated using the relation yk = IRφ yk = ρI x2 π, where I is the current flowing through the electrode. In principle, we will use the resistance and potential formulas from the previous slide, creating the appropriate conditions for all calculations. Example: For a vertical rod, ETTY gives the relation Rç ρ= L2 π ln L4 D. Here L is the length of the rod and D is its diameter. To calculate the hemispherical equivalent of the rod, the equality ρ r2 π = ρ L2 π ln L4 D is considered. r = L L4 D ln L=20 cm, D=1 cm. For a stake, r=4.6 cm is found. 3 • SOIL RESISTIVITY MEASUREMENTS • SOIL SPREAD RESISTANCE MEASUREMENTS • EVALUATION OF MEASUREMENTS 4 SOIL RESISTIVITY MEASUREMENTS 5 Until recently, soil resistivity measurement concerned a limited group of electrical engineers. 1. Those involved in cathodic protection (the description in TS 4363 is related to cathodic protection.) 2. Those involved in the grounding design of large substations. With the new regulations published in recent years, soil resistivity measurement has begun to concern a wider segment of the population. 6 Article 10/c-5.i.1 of the Regulation on the Preparation of Electrical Internal Installation Projects dated December 3, 2003, requires the determination of soil resistivity before starting projects. Furthermore, the draft Lightning Protection Regulation also recommends measuring soil resistivity during the design phase of lightning protection systems. Article 16 of the Draft Lightning Protection Regulation states that "in most geographical regions, and especially in areas where temperature and precipitation show unusual seasonal variations, changes in soil resistivity should be taken into account by measuring the depth profile of specific resistivity under different weather conditions." 7 P2 C1 P1 C2 8 φ 1P = ρI 2 π 1 C1P1 1 C2P1 C1 P2 φ 2P = ρI 2 π 1 C1P2 1 C2P2 C2 P1 = φU P1 φ P2 = ρI 2 π 1 C1P1 1 C2P1 1 C1P2 + 1 C2P2 9 (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - = R = U I ρ 2 π 1 C1P1 1 C2P1 1 C1P2 + 1 C2P2 = k 2 π 1 C1P1 1 C2P1 1 C1P2 + 1 C2P2 kρ = U I Here ρ (Ω.m) is the resistivity of the soil, I (A) is the current applied to the ground, U (V) is the voltage between the P1 and P2 terminals, and k is a geometric factor. The k factor depends on the distances between the measurement stakes. 10 (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - 1 C1P1 + 1 C2P2 1 C2P1 + 1 C1P2 Measurement stakes can be placed as desired, provided that each created measurement system has its own unique geometric factor. Example x x C1 x P1 P2 x C2 0U = In this case, ρ cannot be measured. 11 (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) ’ Various classical methods such as Wenner, Schlumberger, dipole-dipole, single electrode-dipole, half Wenner and half Schlumberger can be used in soil resistivity measurement. All the traditional methods mentioned above are applied by driving 4 measurement stakes into the ground at different intervals along a straight line. While specially developed measuring devices are used for measurements made at small intervals, the voltmeter-ammeter method is used for measurements made at large intervals. A +I current with a frequency of 100-150 Hz is sent to the ground from the C1 terminal of the measuring device. This current returns as -I from the C2 terminal. These currents create a potential difference of U at the P1 and P2 terminals. Measuring devices directly give the U/I ratio in Ω. New generation measuring devices determine the k factor in addition to the U/I ratio and directly display it. It can also give a resistivity of 12. WENNER METHOD 13 Rö C1 P1 P2 C2 I I a a a 14 Rö C1 P1 P2 C2 I a a a φ 1P = ρI 2 π 1 a 1 a2 φ 2P = ρI 2 π 1 a2 = φU P1 φ P2 = ρI 2 π 1 a 1 a2 + 1 a2 1 a 1 a 15 (cid:247) ł (cid:246) (cid:231) Ł (cid:230) - (cid:247) ł (cid:246) (cid:231) Ł (cid:230) - (cid:247) ł (cid:246) (cid:231) Ł (cid:230) - - - = R = U I ρ 2 π 1 a 1 a2 + 1 a2 1 a =(cid:247) ρ 2 π 1 a π= aR2ρ 16 ł (cid:246) (cid:231) Ł (cid:230) - - SCHLUMBERGER METHOD 17 Rö C1 P1 P2 C2 I I r O Δr 18 Rö C1 P1 P2 C2 r Δr φ 1P = ρI 2 π 1 r Δ 2 r 1 r Δ 2 + r φ 2P = ρI 2 π 1 r Δ 2 + r 1 r Δ 2 r = φU 1P φ 2P = ρI 2 π r 1 r Δ 2 1 r Δ 2 + r 1 r Δ 2 + r + r 1 r Δ 2 19 (cid:247) (cid:247) (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) (cid:231) (cid:231) Ł (cid:230) - - (cid:247) (cid:247) (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) (cid:231) (cid:231) Ł (cid:230) - - - - - (cid:247) (cid:247) (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) (cid:231) (cid:231) Ł (cid:230) - - = R ρ 2 π P1P2 £ C1C2 10 rΔ = R ρ 2 r2 π 1 r2 Δ 2 r Δ 2 r4 + r2 Δ r Δ + r2r 2 r 2 Δ 4 r r 5, provided that ρ π= 2 r r Δ R 20 (cid:247) (cid:247) (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) (cid:231) (cid:231) Ł (cid:230) - - (cid:247) (cid:247) (cid:247) (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) (cid:231) (cid:231) (cid:231) Ł (cid:230) - (cid:247) ł (cid:246) (cid:231) Ł (cid:230) £ DIPOLE-DIPOLE METHOD 21 Rö C1 P1 P2 C2 I nx x I x C1 C2 P1 P2 22 Rö C1 P1 P2 C2 I x I nx 1 nx 1 + x)2n( + 1 + (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - - 2 n + = R ρ 2 π 2 nn2 2 n2n3 + x)2n)(1n(n + 2 + nn + n2 ρ = π xR)2n)(1n(n + + 24 (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - - - - SINGLE ELECTRODE-DIPOL METHOD 25 Rö C1 P1 P2 C2 I I nx x 26 ¥ φ 1P = ρI 2 π 0 1 nx φ 2P = ρI 2 π 0 1 + x)1n( = φU 1P φ 2P = ρI 2 π 1 nx + 1 + x)1n( = R ρ 2 π + n1n + x)1n(n xR)1n(n2ρ = π + 27 (cid:247) ł (cid:246) (cid:231) Ł (cid:230) - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - The expressions in the previous slides were derived by assuming the medium has a homogeneous character (r is constant) and the measurement stakes are hemispheres. However, in reality, the earth is not homogeneous. In this respect, the calculated resistivity is called apparent resistivity (AP). AP depends on the geological structure within the earth and the resistivity of this structure. Based on this definition, AP and medium resistivity can only be related if the medium is homogeneous and semi-infinite. They can be equal. In layered cases (which is often the case), the resistivity of each layer is different. 28 Various problems are encountered depending on the number of layers. The semi-infinite single-layer problem is the simplest form, but it often does not meet our needs. The 2-layer model can be a good choice for finding the soil resistivity of locations where MV substations are located. In large substations, the 3-layer model should be preferred. Although the n-layer problem was solved by Stefanescu, it is not often used in electrical engineering practice. The n-layer problem is generally a subject of interest to geophysicists. 29 Air Earth ρ constant h 8 ρ1 ρ2 Air Earth Air Earth h1 h2 8 ρ1 ρ2 ρ3 h1 h2 h3 hn ρ1 ρ2 ρ3 . . . . . ρn Air Earth 30 2-LAYER MODEL The difference in resistivities of two layers The ratio of the total to the reflection factor is defined as the reflection factor and is denoted by K. If K = 2ρρ + 2ρ1ρ1ρ2=ρ1, then K=0; if the lower layer is a perfect insulator, K=1, and if the upper layer is a perfect insulator, K=-1. In this case, -1<K<Condition 1 is met. -1<K<ρ2 when 0<ρ1 (the top layer is more resistant than the bottom layer) 0<K<1 when ρ1<ρ2 (the lower layer is more resistant than the upper layer) 31 - Air I Earth ρ 32 Air I Earth h ρ1 ρ2 33 IMAGE METHOD h h I KI hρ Air Earth ρ1 ρ1 ρ2 34 ¥ fi 2h h 2h KI I KI Air Earth ρ1 ρ1 ρ1 ρ2 35 KI I KI K2I 2h h 2h 2h Air Earth ρ1 ρ1 ρ1 ρ1 ρ2 36 K2I 2h KI Earth I KI K2I a P 2h C 2h 2h = φ P Iρ 1 2 π 1 a + 2 =1n 2 a n K ( + ) 2 nh2 ρ1 Rö C1 P1 P2 C2 I a I a a ρ g ρ 1 += 41 = 1n + 1 n K 2 nh2 a 4 = 1n n K + 4 2 nh2 a ρ g = ρ 1 ,K(f a h ) 37 ¥ ¥ (cid:247) (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) (cid:231) Ł (cid:230) (cid:229) ¥ (cid:229) (cid:229) ¥ ¥ (cid:247) ł (cid:246) (cid:231) Ł (cid:230) - (cid:247) ł (cid:246) (cid:231) Ł (cid:230) ρg/ρ1 ordinate, By assigning values to K from -1 to +1 with a difference of 0.1, where a/h is the abscissa, and using a logarithmic scale, families of curves f(K, a/h) can be drawn. These curves are called theoretical resistivity curves for two layers in the Wenner array. K, ρ1, and h are calculated by superimposing the ρg=f(a) curve obtained in the field with the theoretical ρg/ρ1=f(K, a/h) curve. ρ2 can be determined from the expression K1 = ρ2 + K1. 38-39 The radius of the circle whose area is equal to the area of the grounding network is defined as the equivalent radius. The effect of the soil beyond the equivalent radius depth on the grounding resistance can be ignored. To determine whether the soil is homogeneous at the location where the grounding project will be carried out, it is necessary to increase the electrode spacing in the Wenner array to the equivalent radius size. The relation 40 ρ=2πaR is valid if the electrode length L is very small compared to the electrode spacing a (LAssuming >r, rx (cid:231) Ł (cid:230) - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - - @ - = R = U I ρ 2 π 1 r 1 x + 1 y 1 z = Rö ρ r2 π 1 r x + r y r z = R ö 1R g r ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - R ö = R g 1 r x + r y r z =(cid:247) 1 r x 1 x + r y r z = 0 r + 1 y 1 z = 0 1 x + 1 x + 1 y 1 z =(cid:247) 0 = 1 y 1 z 64 (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - (cid:222) - - (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - - - - + 1 x 1 y = 1 z = z xy + yx 2 z = 2 x + 2 y xy2 cos θ P2 z y C2 r θ x T (C1,P1) 2 xy + yx cos θ = = 2 x + 2 y xy2 cos θ + ξ 2 1 2 ξ 1 1 ++ ξ 2 2 ξ ξ= x y 65 - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - (cid:247) (cid:247) ł (cid:246) (cid:231) (cid:231) Ł (cid:230) - cosθ 1 0.875 θ=29 º b a 1 £ ξ 618.1 0.618 1 1.618 ξ a x 0.5x 29º 0.618x b a' 66 £ Although the .8 method is applied in single electrodes and small installations, in large installations, the resistance curve is extracted, and the middle part of the curve is determined. The slope of the section is determined, and accordingly, the voltage stake distance required to measure the actual resistance is determined. Meanwhile, the distance of the current stake from the center of the installation should not be less than 5 times the center diameter. In large and asymmetrical installations, the 4-point method, the intersecting lines method, and the slope method can be applied. In very large switchgear installations, current and voltage stakes are placed on opposite sides. In such places, voltmeter-ammeter or wattmeter-ammeter methods should be preferred. In very large installations, the angle method can also be used due to the parallel nature of the cable connections. In this case, the angle between x and y cannot be less than 60º. 67 68 69 EVALUATION OF MEASUREMENT RESULTS 70 Network with star point grounded through resistor; Z = R I''k1 L1 L2 L3 71 Star points of 154 / 34.5 kV transformers above 25 MVA in our national network; - In substations with overhead line connections, grounding is done with a 60 Ω resistor, and in substations with cable connections, it is done with a 20 Ω resistor. In this case, the phase-to-ground short-circuit current in 34.5 kV HV networks fed by overhead lines is limited to I1k = 3/34500 * 60 A, and the phase-to-ground short-circuit current in 34.5 kV networks with cable connections is limited to I1k = 3/34500 * 20 A. 72 @ ¢ ¢ @ ¢ ¢ Example: 154 kV 50 MVA 34.5 kV 2000 MVA 60 Ω 3 x PIGEON 10 km 3 x SWALLOW 1 km 3 x 95 mm2 XLPE 200 m 1000 kVA 34.5 kV 0.4 kV 154/34.5 kV Since the star point on the secondary side is grounded with a 60Ω resistor, I''k1=300A is limited. c b a V 1000 9 8 7 6 5 4 3 i m 2UTp 1 100 9 8 7 6 5 i l i r e g a m n u k o D 4 3 3 4 5 6 7 8 9 0,1 2 3 87654 9 1 2 Current duration t 3 87654 9 10 s Maximum permissible touch voltages for limited current durations in HV a) Time-dependent touch voltage in animals b) Touch voltage in old VDE 0141 c) Newly adopted curve 74 Fault duration tF Grounding voltage UE On the external walls and fences of installations Inside installations Indoor (internal type) installation Outdoor (external type) installation M4.1 or M4.2 tF>5 s tH 5 s UE 4UTp M1 or M2 M3 Proof that UE>4UTp UE 4UTp UT UTp is true M1 or M2 M3 M4.2 M3 M4.2 Proof that UE>4UTp UE UTp is true 75 £ £ £ £ £ 76 LV OVERHEAD LINES PHASE-TO-GROUND FAULT (PHASE BREAK) RT RB Rh A L1 L2 L3 PEN (N) RB: Operating grounding resistance RE: Contact resistance of L3 phase to ground U0: Effective value of rated AC voltage relative to ground 77 Equivalent circuit: 0 RB U0 RT RH A RE Ih 78 If RT and RH resistances are neglected U 0 + R U 0 + R IR HB I h U R R = = B B = E B R R UB V50 According to Article 3.7 R R B UR 0B U 0 50 U 0 50 U 0 + R R R R R 50 1 + R + E E E B B E 1 B 1 50 B B E R U 0 50 50 B R + R E B U 0 50 R R E B 50 ETTY p.17 R R B E U 0 50 79 £ £ ‡ fi ‡ - ‡ fi - ‡ fi ‡ - £ 380/220 V on mains U0=220 V R R B E 50 220 50 R R B E 50 170 R R B E 294.0 R R E B 1 294.0 R R E B 4.3 R E R.4.3 B 80 £ fi £ fi - £ ‡ fi ‡ fi ‡ L3 0 2 X 220V250X The spreading resistance of a cylindrical ground electrode with length L and diameter d is given as R ρ= L π ln L2 D under Figure T-7 on page 88 of ETTY. d (mm) 5.58 6.60 7.41 8.34 9.36 10.50 11.79 Rose Lily Iris Pansy Popy Aster Pholox ln 40.2 d 9.57 9.40 9.29 9.17 9.05 8.93 8.82 1 π ln 40.2 d 3.05 2.99 2.95 2.92 2.88 2.84 2.81 Avg.3 82 L=1 m = RE ρ 1. π ln 1.2 0086 ,0 = 73.1 ρ L=10 m = RE ρ 10. π ln 10.2 0086 ,0 = 25.0 ρ L=40 m RE 3 ρ’ L RE ρ075,0 83 ' L (m) ρ (Ωm) RE (Ω) 1 10 40 100 100 100 173 25 7,5 Therefore, the minimum contact resistance imposed by the regulation occurs at approximately 40 m conductor length. 84 R ‡ E R4.3 B RE ρ075.0 R minE ρ075.0 R.4.3ρ075.0 B ρ 4.3 075.0 ρ ‡ R B BR.45 RB (Ω) 0.1 1 2 )m(Ωρ 5.4ρ ‡ ρ ‡ ρ ‡ 45 90 85 @ ‡ ‡ (cid:222) ‡ NEUTRAL BREAK RB1 PP P+ΔP L1 L2 L3 PEN (N) RB2 In P Δ= U 0P =Δ U neutral = R nh P Δ = U r l nh P Δ U If U neutral = 0 86 RB1 L1 L2 L3 PEN (N) RB2 87 Equivalent circuit: U0 RB1 I U 2 0 P Δ RB2 = I U 0 R 1B + R 2B + 2 U 0 P Δ = ( RP Δ UP Δ 0 + R 2B 1B ) + 2 U 0 U 2B = UPR Δ 2B 0 ) + + R 2B 1B ( RP Δ fi=Δ U0P 2B 2 U 0 = 0 88 COMBINATION OF GROUNDING INSTALLATIONS IN HV-LV SYSTEMS 89 Article 11 a) In the event of a ground fault in a high-voltage installation, the neutral or PEN conductor of the low-voltage system may be connected to the grounding installations of the high-voltage installation system provided that the following conditions are met. - If no dangerous touch voltages occur in the low-voltage network or in the installed consumption facilities (Table 13) - If the voltage withstand capability (at operating frequency) of the low-voltage devices in the consumption facilities does not exceed the permitted values in Table 13 as a result of a potential rise at the low-voltage star point, 90 b) If a high-voltage installation supplies low-voltage consumers within its grounding area; all operating and protective groundings within the HV grounding installations must be combined. c) Supplying low-voltage installations outside the area of the high-voltage grounding installation: - If the high-voltage grounding installation in question is connected to a global grounding system, - or if the conditions in Table 13 are met in the LV network, the construction of a common grounding installation is recommended. 91 AG System Type Failure Time TT TN t £ 5 st > 5 s PEN grounded only at TM PEN grounded at multiple points Common grounding conditions Touch Voltage Strain Voltage Not applicable UE £ UE £ 1200 V 250 V UE £ UTp UE £ 2.UTp Not applicable 92 THANK YOU 93
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