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UNSW Australia Earthing Trevor Blackburn School of Electrical Engineering 1 INTRODUCTION Functions & benefits of earthing (grounding): Limitation of touch, step and transferred potentials to prevent electric shock Equipotential bonding of exposed metal conductors to prevent electric shock and static charge buildup Promote rapid and reliable operation of electrical protection and to limit earth fault damage Limitation of over-voltages (eg lightning and switching) on equipment to prevent damage to insulation and to electronic components TO EARTH OR NOT TO EARTH? Unearthed system Earthed system Electrical systems: with earthing Electrical supply substations (utility operated) Transmission and distribution SWER systems HV utilization substations (owner operated) Industrial sites Mining sites Commercial sites Domestic sites From Hpls;s;s (Taken from From Ausgrid Standard NS116) Electrical earthing Electrical supply substation Earth grid or electrode_supply earth reference HV utilization substations Industrial and mining sites HV earth system_extended grid and/or electrodes LV earth systems_local busbar or wiring interconnection Commercial sites LV earth system mainly busbar & wiring interconnection Domestic sites Totally LV earth systems: mainly wiring interconnection Design requirements of earthing Traditional approach has been prescriptive and regulatory This approach is now changing to an acceptable risk-based probabilistic process for utility systems and for large consumers with their own operations: based on the approach given in ENA EG-0 (Power System Earthing Guide) Applies to: major substations (AS2067), T and D networks, power stations and large industrial systems Does not apply to: LV earthing on customer premises, DC systems, railways, mining installations and equipment, ships and off-shore installations Australian Standards AS 1824.1, Insulation co-ordination – Definitions, principles and rules AS 1824.2, Insulation co-ordination (phase to earth and phase to phase, above 1 kV) – Application guide AS 2067, Power installations exceeding 1 kV a.c. Australian/New Zealand Standards AS/NZS 1768, Lightning protection AS/NZS 3000, Electrical Installations AS/NZS 3835 Earth potential rise – Protection of telecom network users, personnel and plant AS/NZS 3931, Risk Analysis of Technological System – Application Guide AS/NZS 4360, Risk management AS/NZS 60479.1, Effects of current on human beings and livestock, Part 1: General Other Documents ENA C(b)1, Guidelines for design and maintenance of overhead distrib and trans lines ENA EG1, Substation earthing guide IEEE Std 80 „IEEE Guide for Safety in AC Substation Grounding‟. Risk of death in various activities: (from AS1768) Electric shock: design constraints on LV earthing 2 METHODS OF POWER SUPPLY SYSTEM EARTHING General requirements of an earthing system: low impedance path (resistance and reactance) to the earth of the local supply system and thence to earth at the main substation Equipment/appliance items needing earthing must be connected to earth by a low resistance/impedance to provide equipotential bonding between all items. Avoid earth loops. earth potential rise (EPR) associated with any conductor carrying fault current must be limited to safe levels earth conductors must capable of handling all prospective fault currents without thermal or mechanical damage METHODS OF EARTHING Unearthed system Solidly earthed system Resistance earthed system Reactance earthed system Use of an earthing transformer METHODS OF EARTHING Unearthed system Solidly earthed system Resistance earthed system Reactance earthed system Use of an earthing transformer Advantages: Solidly earthed systems High fault current and fast protection operation Better personnel and equipment safety Earth fault current is easy to detect Unearthed systems Line to earth fault will not interrupt supply thus improves production line reliability Low earth fault current (limited by the line capacitance to earth) will not cause damage Disadvantages: Solidly earthed systems Line to earth fault causes loss of supply Line to ground fault current may be high enough to cause significant damage but be undetectable by protection (HIAF) Unearthed systems Line-line fault current may be low and may not trip protection May have prolonged arc faults Line-line voltage imposed on phase insulation with earth fault Overvoltages may cause increased insulation stress if there is an earth fault Finding earth faults may be difficult – requires monitoring Regular maintenance is required 3 PERSONNEL PROTECTION REQUIREMENTS FOR EARTH SYSTEM DESIGN STANDARDS AS/NZS 60479.1:2002 : Effects of current on human beings and livestock - General aspects Uses Biegelmiers (IEC) stepwise graphical methods Not easy to apply for step and touch potentials In Nth America: IEEE Std.80 – 1986 Uses Dalziel‟s “Electrocution equation” Much simpler to incorporate in earthing design From AS60479.1 HUMAN BODY ELECTRICAL IMPEDANCE (REF. AS/NZS 60479.1:2002) Dalziel’s “Electrocution equation”: Defines a specific energy (I2t) as the determining factor for potential electrocution (fibrillation). Uses the I2t value to determine a fibrillating current level Values are based on extensive tests carried out on humans (his students!) and animals. Comparison of the IEC and IEEE approaches IEEE 4 SUBSTATION EARTH: DESIGN REQUIREMENTS EARTH POTENTIAL RISE (EPR [GPR]) Hazardous situations which can occur in substations when there is an earth surface potential rise (EPR) in the vicinity of an earth electrode due to fault current flowing to earth through that earth electrode. Potential rise around an earth electrode taking fault current: (from IEEE 80) HAZARDOUS POTENTIALS NEAR EARTHS Step potential : voltage difference between a person's feet when spaced 1m apart (radially w.r. to the earth). Touch potential: voltage difference between exposed metal object, connected directly to earth electrode, and ground surface potential where feet are placed (usually distance of 1m is used). Grid (mesh) potential: maximum possible touch voltage in an earth grid area. Transferred potential: voltage difference between earth surface potential and exposed metal object connected to remote earth (effectively at true earth potential of zero volts). VT: Touch VS: Step Vtrans: Transferred Potential hazards due to Earth Potential Rise from a fault (From AS1768). CALCULATED GROUND SURFACE POTENTIAL IN AN EARTH GRID POTENTIAL PLOTS AND PERMISSIBLE LEVELS FOR AN EARTH GRID 5 EARTHING CONDUCTOR - Current carrying capacity ADIABATIC SHORT CIRCUIT HEATING From IEEE Std. 80 6 SYSTEMS OF EARTHING IN LOW-VOLTAGE INSTALLATIONS Direct earthing system: system relies on current flow through the ground thus requires low earth resistivity and good earth electrode. Not always possible. MEN system is preferred as it utilizes supply utility‟s neutral to provide an additional earth return path. MEN system: earth connections to neutral at consumer‟s installation and along route to supply substation neutral provides the return path while in direct earth system the metallic path is provided by water pipes, cable sheaths or by special earthing connections if provided balancing of load to utilize phase current cancellation in return neutral to minimize voltage drop neutral conductor must be earthed at the substation and at other locations as necessary to ensure that total impedance between neutral and earth does not exceed 10 ohms conductors used to earth neutral conductor of distribution system must have a cross-section area of at least 20% that of the smallest size of neutral used in system CMEN system: Common Multiple Earthed Neutral extension of MEN system high voltage and low voltage equipment are bonded (via a neutral conductor) to a single common earth impedance to ground of this interconnected system of earthing is very low, typically 1 ohm or less. 7 TYPES OF EARTHING SYSTEMS IN CONSUMER’S INSTALLATIONS TN-C TN-S TN systems TT systems IT systems TN-C-S In practice, only TT and TN systems are commonly used. 1st letter (I or T) gives relationship of supply to earth T (terra): direct connection of one point of supply system to earth I (insulation): all live parts of supply isolated from earth or one point connected to earth through an impedance 2nd letter (T or N) gives relationship of exposed conductive parts of the general installation to earth T (terra): direct connection of exposed conductive parts to earth, independent of earthing of supply system N (neutral): direct connection of exposed conductive parts to earthed point of supply (neutral point). TT system: one point directly earthed, exposed conductive parts connected to earth via separate earth electrode, no direct connection between live parts and earth, exposed conductive parts connected to earth TN systems: source side directly earthed, exposed conductive parts connected to that point by the protective conductor (PE) TN-S system: separate neutral (N) and PE throughout TN-C system: N and PE combined into a single conductor throughout TN-C-S system: N and PE combined into a single conductor in a part of the system, separated in another part TN-S TN-C-S TN-C IT system: no direct connection between live parts and earth, exposed conductive parts connected to earth 8 EARTH RESISTANCE OF BURIED ELECTRODES Earth resistance determined by: shape of electrode(s) extent of electrode(s) electrical resistivity of the soil Resistance to ground for various electrode configurations 9 EQUIVALENT HEMISPHERE MODEL OF AN EARTH ELECTRODE: Use for touch, step and transferred potential calculations The most common form of earth electrode is a vertical driven rod or horizontal pipe or a complex distributed mesh in the ground. These are not simple for analytic calculation of touch and step potentials An approximate estimation used is to determine an equivalent hemisphere and then used for potential distribution calculations Example: V(r) = ground potential w.r.t earth electrode 10 MEASUREMENT OF EARTH RESISTANCE Three-electrode method Fall-of-potential method potential probe remote current probe STAKELESS TEST METHOD FOR MULTIPLE ELECTRODES (FROM FLUKE MANUAL) 11 EARTH ELECTRICAL RESISTIVITY not a good conductor compared to metals. Extremely variable depending upon physical nature of soil/ground material and chemical composition heavily influenced by moisture content, temperature and dissolved salts Can be improved by use of electrolytes at the earth stake (eg bentonite) Rule of thumb resistivities of some materials Mud (compressed coal): Wet soil: Moist soil: Dry soil: Rock: 1 Ωm, 10 Ωm, 100 Ωm, 1000 Ωm, 10000 Ωm. Typical general values are 100 - 300 Ωm 12 ELECTRIC SHOCK EFFECTS IN SUBSTATIONS r = soil resistivity body resistance foot contact resistance Equivalent circuits for touch and step potentials Assume bare feet or conducting footwear: Tolerable touch and step potentials