VIEWS: 112 PAGES: 22 POSTED ON: 11/15/2011
ELECTRICAL EARTHING SYSTEM CONTENTS • 1. What is Earthing • 2. Why Earthing required • 3. Touch Voltage • 4. Step Voltage • 5. Soil Model • 6. Design Input CONTENTS • 7 . Touch Voltage Calculation • 8 . Step Voltage Calculation • 9. Ground Current Calculation • 10 Types of Earthing • 11 What is Earthing • The process of earthing is to connect all the objects which could become charged to the general mass of the earth, to provide a path for electrical current and to hold the parts as close as possible to earth potential. • For tying the electrical supply system to earth, supply transformer neutral conductor (Y point of 3 phase Supply) is connected to earth using an earthing electrode or the metal sheath and armouring of a buried conductor. Why Earthing required • The purpose of grounding system is to provide a low impedance electrical contact between the neutral of an electrical system and earth • 1. To provide safety during normal or fault condition. • 2. To stabilize the voltage during transient conditions and therefore to minimize the probability of a flashover during transients. • 3. To dissipate lightening strokes • Due to impedance of the system, the potential of the grounding system may become different than the potential at various points on earth during abnormal operating condition or fault condition. • Due to this potential difference a touch and step voltages are subjected by the persons which generate an electrical current through his body. This flow of current is a source of danger. • Electrical body current below 116/(sqrt(t) mili amps, where t is shock duration time • Vtouch, allowable = 0.116(1.5 + 1000)/ (sqrt(t) • Vstep, allowable = 0.116(6.0 + 1000)/ (sqrt(t) Touch Voltage • A person touching a grounding structure which has a potential that is different from that of earth at which the person is standing. In this case the person is subjected to a voltage which generate an electrical current through his body. The voltage to which the human body is subjected is called the touch voltage. • Safe Electrical body current < 116/sqrt(t) mili Amps • Vtouch, allowable = 0.116(1.5Cs + 1000)/ sqrt(t) Volt • Where • = Soil resistivity (- M) • t = duration of electric shock in seconds • For a regular ground mat: - • Vtouch, max = KmKi Ie/L Volt • Ie : - Total Current injected in to grid • Ki : - Asymmetry Factor • Ks, Km : - Geometric Factor • L : - Total conductor Length Step Voltage • A person working on the surface of the earth will experience a voltage between his feet. This voltage will generate electric body current. In this case the voltage to which the person is subjected is called the “step voltage”. • Safe Electrical body current < 116/sqrt(t) mili Amps • Vstep, allowable = 0.116(6.0 Cs + 1000)/ sqrt(t) Volt • Vstep, max = KsKiIe/L Volt • The aim of design of grounding system is to meet safety criterion. It means value of step and touch voltages must be below allowable values. The maximum touch voltage occurs at a point located approx. above the centre of the outer mesh of the ground mat. • Maximum step voltage occurs at the periphery of the ground mat near the centre. Soil Model • 1. Determination of soil model :- • there is two methods of soil resistivity measurement: - • 1. Wenner Method • 2. Driven rod method • From soil resistivity data (taken from the above method) closed curve has been plotted jointing all the resistivity points. The area inside the curve is measured and equivalent circle of the same area is drawn. The radius of the circle will be average resistivity of the site and the same is considered for design of grounding system. • From available power plant data, • Average soil resistivity = 195.2 M at a depth of 2.5 Meter • Hence 200 M resistivity is considered for design. Factor affecting Soil resistivity • Following Factors affect the soil resistivity: - • 1. Voltage gradient does not affect the soil resistivity unless it exceeds certain critical values( touch and step). Because grounding system is designed on the basis of the touch and step voltages. • 2. the effect of current flowing from the electrode in to the surrounding soil is to cause significant drying and thus increase the effective soil resistivity. • Current is not to exceed 200A/M2 for one second • 3. The resistivity of soil rises abruptly whenever the moisture content accounts for less than 15 % of soil weight. • 0.08 to 0.15 meter in depth are very useful incorporating the evaporation of moisture and this limiting the drying of top soil layers during prolonged dry whether periods • Depending upon the type of soil, temperature affects resistivity to a greater or lesser extent. Design Input • 1. System Voltage • 11KV, 40 KA for 1 sec. (Max 100 Amp ground fault current) • 6.6 KV, 40 KA for 1 sec. (Max 100 Amp ground fault current) • 415 volt , 50 KA for 1 sec ( solidly grounded) • 2. Soil Resistivity = 200 M Size of ground Conductor • A = I{(terr * 104)/(TCAP * ln(1+(Tm-Ta)/(k0+Ta))}1/2 • Where ;- • A = Conductor X- section in mm2 • I = Fault current in KA = 50 • Tm = Max. allowable temperature • Ta = Ambient Temp. • r = Thermal co-efficient of resistivity at reference temp. Tr • Tr = Reference temp. Tr = 20 0C • K0 = (1/r) – Tr • Tc = fault current duration • TCAP – Thermal capacity factor in J/cm3/‟c • r = 20 = 0.00423 • 20 = 13.8*10-6 M • K0 = (1/r) – Tr = 1/0.00423 – 20 = 216.4 • TCAP = 4.184* Sh*Sw where • Sh = Specific Heat in Cal/gm/‟C = 0.114 • Sw = Specific weight gm/cm3 = 7.86 • TCAP = 4.184 * 0.114* 7.86= 3.75 W/cm3/‟C • A = I{(terr * 104)/(TCAP * ln(1+(Tm-Ta)/(k0+Ta))}1/2 • = 584.0 mm2 • Taking 15% allowance for corrosion • So minimum X – sectional area of the conductor = 1.15 *584 = 671.6 mm2 • Hence conductor size selected = 75 *10 mm2 Design of Grid • Total area covered = 150 * 80 M2 • Max spacing of ground conductor = 12 meter( selected) • Estimated length of conductor = 80*14+50*8 = 2320 Meter Ground Resistance Calculation • Considering 20 nos. 40 mm dia, 3 meter long MS rod will be used as ground electrode. As per Sehawarg‟s Formula • Rg =(R1R2-R212)/(R1+R2-2R12) • R1 = (1/ l1)*(ln(2l1/h‟) + K1(l1/sqrt(A) – K2)) = 0915 • R2 = (a/2n l2)*(ln(8l2/d2) - 1 +2 K1(l2/sqrt(A)*(sqrt(n) – 1)2 ) = 3.33 • R12 = (a/ l1)*(ln(2l1/l2) + K1(l1/sqrt(A) – K2 + 1)) = 0.857 • Ground Resistance Rg = (R1R2-R212)/(R1+R2-2R12) = 0.912 • Where • R1 = resistance of grid conductor • R2 = resistance of all ground rods • R12 = mutual resistance between group of grid conductors and group of ground rods • 1 = Soil resistivity encountered by grid conductor = 200M • a = apparent Soil resistivity as seen by ground rod = 200M • l1 = Total length of conductor = 2320 meter • l2 = length of ground rod = 3 meter • n = number of ground rod = 20 • d1 = eqt. Dia of grid conductor = 0.0309 meter • d2 = Dia of grid Rod = 0.04 meter • h = depth of buried conductor = 0.6 meter • h‟ = sqrt(d1h) = 0.136 meter • A = Total Area = 80*150 = 1200M2 • K1 = 1.33 • K2 = 5.21 Touch Voltage Calculation • Touch voltage (Permissible) • Vtouch, allowable = 0.116(1.5Cs + 1000)/ sqrt(t) Volt = 213.2 Volt • Where • Cs = reduce factor = 1 • t = duration of shock • Ig = portion of fault current passing through ground = 1282 Amp • Mess Voltage Em based on estimated length • Em = KmKi Ig/L Volt • = 203.9 volt • Em < Etouch • Hence OK Step Voltage Calculation • Step Voltage : - • Step voltage permissible • Vstep, allowable = 0.116(6.0 Cs + 1000)/ sqrt(t) Volt = 360.9 Volt • Step voltage based on estimated length • Es = KsKiIg/L1 Volt = 106 Volt • L1 = L + 0.115 Lr • Es < Estep • Hence OK Ground Current Calculation • Calculation of ground Current • Conductor resistance • Rc = c*(path length / cross sectional area) • c = 13.8 * 10-8 M (at 20 „C) = 15.66* 10-8 M (at 50 „C) • Path length (Max) = 150+80 = 230 Meter • X – Sectional Area = 750*10-8 M2 • Rc = 0.048 • No. of parallel path in one direction = 14 • Rc eqt = 0.048/14 = 0.00343 • The ground fault current will have 14 parallel paths 1) Through Ground and 2) through ground conductor. • The current passing through each path will depend upon the resistance of each path • Current through ground Ig = 50KA *Rc eqt/(Rg + Rc eqt) = 187.34 Amps • And rest of current will pass through ground conductor Types of Earthing • 1. Resonant Grounding • 2. Solid Grounding or effective grounding • 3. Resistance Grounding • 4. Reactance Grounding • 5. Zig – Zag Transformer Grounding Resonant Grounding • If the value of inductive reactance is such that the fault current balances exactly the charging current, then the grounding is known as resonant grounding or ground fault neutralizer or Peterson coil. • Inductance connected between neutral and earth – L = 1/(3ω2C) • The use of the resonant grounding will reduce the line interruption due to transient line to ground faults which will not be possible with other form of grounding. • Ground Fault neutralizer should not be used where • 1) Fully graded insulation transformers are used as the neutral of such transformers are not sufficiently well insulated • 2) Auto transformer having a ratio greater than 0.95 to 1 are used Solid Grounding • In this grounding system the voltage of the healthy phases in case of a L-G fault does not exceed 80% of the L-L voltage and is much less as compared to other form of grounding, the equipment for all voltage classes are less expensive. • On system 115 KV and above additional saving are possible because of the transformers with the insulation graded towards neutral are less costly. Resistance Grounding • Resistance grounding is normally used where the charging current is small( ie.-Generators, low voltage short length overhead line) • Resistance Required = KV⁄(√3*I) Ω • For 6.6 KV system = 38.1 Ω • For 11 KV system = 63.5 Ω • Advantages:- • 1. Reduce arcing ground hazardous • 2. permits ready relaying of ground faults • 3. Help in improving the stability of the system during L-G faults by replacing the power dropped, as a result of low voltage , with an approximately equal power loss in the resistor, thus reducing the advance in phase of the conductor. Reactance grounded System • A reactance grounded system is one in which the neutral is grounded through impedance which is highly reactive. • System is solidly grounded or reactance grounded depends upon the ratio of X0/X1 • For reactance grounded system X0/X1 >3.0 • For solidly grounded system X0/X1 < 3.0 • Reactance grounding is used for synchronous motors and capacitors and also for circuits having large charging currents Earthing Transformers • If a neutral point is required which otherwise is not available (delta connection and bus bar point etc.) a zig-zag transformer is used. • This type of transformer do not have secondary winding. These transformers are of short time rating usually 10 second to 10 minutes.