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ELECTRICAL EARTHING. Special Note:. The approach here is based on standard IEEE 80 (Safety in AC substation grounding) These discussions are for illustration only Grounding practices are subject to local regulations/codes which will take precedence. Objectives of grounding:.
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Special Note: • The approach here is based on standard IEEE 80 (Safety in AC substation grounding) • These discussions are for illustration only • Grounding practices are subject to local regulations/codes which will take precedence
Objectives of grounding: • Provides an electrical supply system with a reference to the groundmass (system grounding) • Protective grounding of electrical equipment enclosures • Makes them safe to persons who may come into contact with them • Enables the flow of fault current in the event of a failure • Provides a low impedance path for accumulated static charges and surges (lightning protection grounding) • Helps in mitigating the generation and propagation of noise (grounding of shields and signal reference planes)
Earthing System Shall satisfy Safety, Functional requirements of electrical installation Shall ensure • Protection against indirect contact • Proper functioning of electrical protective devices • Protective and functional requirements are met under expected conditions • Earth fault, earth leakage currents can be carried safely • Adequate strength appropriate to external influences • Adequate value of earthing resistance
Benefits (1) • Fault damage now minimal • Reduces fire hazard (especially in mines) • Lower outage times • Less lost production, less lost revenue • Touch potentials kept within safe limits • Protects human life
Benefits (2) • Low Fault Currents reduce possibility of igniting gases • Minimizes explosion hazard • Lower Magnetic or thermal stresses imposed on plant during fault • Transient overvoltages limited • Prevents stressing of insulation, breaker restrikes
System grounding • Provides reference for the entire power system to groundmass • Establishes a path for current to ground during insulation failure • Provides protection against equipment damage due to faults • Provides protection against high voltage transients • Enables detection by circuit protective devices for isolation • Reduces maintenance time and expenditure
Fault in Ungrounded system • Single phase ground fault does not result in flow of high fault currents • Voltage to ground of other two phases rises by 73% - Causes insulation stress • Fault current = Vector sum of capacitive currents in the distributed capacitive reactance of other two phases
Fault in Ungrounded system • Intermittent first fault causes transient voltages up to 6 to 8 times nominal system voltage • Such high transient voltage can initiate a second fault at weakest insulation point • Uncleared arcing fault can cause extensive damage to system • Early detection of first fault therefore of paramount importance
Ungrounded System Neutral connection on Generator/ Transformer is not connected to earth at all Grounding methods 1
Grounding methods 2 Solid grounding • Neutral connection on Generator / Transformer is connected to earth by a solid Conductor • Cost Reductiuons due to avoidance of sensitive relays and grounding device, Grading of insulation towards neutral end. • But Circulation of third harmonic currents between neutrals
Grounding methods 3 • Resistance grounding • Neutral connection on Generator / Transformer is connected to earth (0V) through a fixed resistance to limit the earth fault current • Mainly used below 33 KV • Full line to line insulation required towards neutral
Grounding methods 4 • Reactance grounding • Neutral connection on Generator / transformer is connected to earth (0V) through a fixed reactance to limit the earth fault current • Can be cheaper compared to resistance
Grounding methods 5 • Petersen Coil grounding (arc suppression) • Neutral connection on transformer is connected to earth (0V) through a variable reactance to neutralise the capacitive earth fault current. Results in arc extinction
Grounding methods 6 • NEC grounding (with and without resistance) • In HV delta systems no earth connection is available. A 3 phase neutral grounding compensator is connected to allow earth fault currents to flow - allowing detection of these faults
Protective grounding • Protects personnel against shocks • Personnel do not experience dangerous high voltages when contacting enclosure accidentally connected to live parts • Provides a low impedance path for accumulated static charges and surges (lightning protection grounding) • Helps in mitigating the generation and propagation of noise (grounding of shields and signal reference planes)
Importance for Earthing • An Electrical equipment is considered dead when • At or about zero potential • Disconnected/ Isolated from live system • Disconnection alone not adequate • Can retain stored charge • Can acquire a static charge • Can accidentally be made alive • Nearby live conductors may induce voltage
Importance of Earthing • !Ensure earthing before working on electrical equipment • Earthing • Connect apparatus electrically to general mass of earth in such a manner as will ensure at all times an immediate safe discharge of electrical energy • Connect to earthed metal earth bar or spike with good metallic conductor • Earthing by • Closing of earthing links • Attaching of fixed earthing devices • Affixing of portable earthing straps
Importance of Earthing • Ensure before applying earth • Earthing connection is mechanically, electrically in good condition • No broken strands • Clamps should be rigid and without defect • Applied properly in intimate contact with conductors and earth-bar/ spike • Earthing cable tails as short as possible • Connect to earth first when installing earthing, disconnect earth last while removing earthing
Hazards of improper Earthing • Electrocution • Burns from arcing • Electric shock leading to falls
Bonding • Connecting of various grounding systems and non current carrying parts • To achieve potential equalization between different accessible conducting surfaces • Potential difference between different accessible conducting surfaces, different grounding systems hazardous
Touch Voltage Permissible Touch Voltage Voltage at any point of contact with uninsulated metal work • Within 2.5 mtrs from ground surface and • Any point on ground surface within horizontal distance of 1.25 mtrs from vertical projection of point of contact with uninsulated metal work
Step Voltage • Difference in surface potential experienced by a person bridging a distance of 1 mtr with his feet apart, without contacting any other earthed object • Shall not exceed twice the value of Touch voltage
Touch potentials (Reb =1 ) Person touches transformer tank now live
Touch potentials (Reb = 10 ) Lower touch potential
Transferred Potential Make Allowances for • Transferred potential during design, installation • Voltage drop in conductors (where voltage rise in earthing system is transferred by metal work to remote location) • Otherwise regard Transferred potential as earthing system voltage rise
Transferred Earth Potential Rise Earthing system shall be designed to prevent • Transfer of earth grid potentials to a remote earth • Transfer of a remote earth potential into a station • Breakdown of cable over-sheaths due to voltage differences
Substation grounding practices-1: • Grounding design approach depends upon: • Voltage class of the system • Type of installation (Utility or consumer) • LV Systems: • Usually solidly grounded • Metallic contact between ground of consumer and system neutral • Special cases such as SELV power supplies may be of ungrounded type
Substation grounding practices-2: • MV Systems: • Self contained systems (E.g., Turbo generators): High resistance grounding • Industrial systems: Low resistance • Critical systems: Tuned grounding OR Ungrounded (for small systems) • Utilities: Solid grounding • HV/EHV Systems: Solid grounding
A typical ground electrode: Materials • Copper • Copper clad steel • Galvanized steel • Copper clad stainless steel
A typical chemical electrode: • Sometimes called a leach electrode • Chemical mixtures are added to lower resistance of soil • Needs regular maintenance
Area of ground grid: • HV (outdoor) substations use a buried ground grid for dissipation of fault current to the soil • Lower grid resistance to earth is preferable • It reduces the ground potential rise with ref. to remote earth • Also reduces mesh voltage. Lower mesh voltage is safer because it lowers touch/step voltages • Grid of larger area has lower ground resistance and results in lower grid voltage and mesh voltage • Further reduction by using vertical electrodes welded to the grid
Soil resistivity: • Soil resistivity should be based on an average of measurements done in the substation area • Simple designs assume uniform soil • Non uniform soil involves complex design steps and requires computer programs • Lower resistivity results in lower ground resistance of the grid
Other issues to be taken note of: • Soil layers may differ in resistivity • Two layer soil model to be considered if variations are found to be high at different depths • Transferred potential due to buried services going to/ from the substation (water mains, cable metallic sheath) • Introduce non-conducting sections where feasible to avoid transfer of Ground potential
Special attention to be paid to: • Operating handles of equipment: • Most fatalities occur due to high touch potential at these points • Should be made safe by special arrangements • Substation fence: • Decision regarding keeping isolated vs connecting to substation ground • Surge arrestors: • Short, direct and low impedance connections • GIS extensions
Effective substation grounding - 1: • Size ground conductors adequately • Use proper bonding and jointing in ground conductors. Poor joints cause high temperatures during a fault to ground • Select appropriate ground electrode system • Pay attention to soil resistivity. Carry out soil improvement if resistivity is too high. Use chemical electrodes if situation warrants
Effective substation grounding - 2: • Pay attention to step and touch potentials • Building foundations can also be used as grounding electrodes • Integrate fence of outdoor substations as far as possible. Avoid transferred potential from services entering or leaving a substation • Pay special attention to operating handles
Effective substation grounding - 3: • Cable trays to be properly grounded • Surge arrestors to be grounded using short low impedance connections • Carry out temporary grounding for safety when personnel have to work on parts which are normally live
