Earthing system

EARTHING SYSTEM , HAZARD OF BROKEN EARTHING IN POWER TRANSFORMER

1. EARTHING SYSTEM,HAZARDS OF BROKEN EARTH OF POWER TRANSFORMER BY Prof . J .C . Bhola

2. Types of Earthing i )system earthing ii ) Equipment earthing

3. SYSTEM EARTHING It includes physical metallic connection of Transformer neutral with earth pit or /and with metallic earth mesh.

4. EQUIPMENT EARTHING It includes physical metallic connection of all non-current carrying metallic parts with earth pit /earth mesh.

5. Definition Ground or earth in a mains ( AC power) electrical wiring system is a conductor that provides a low impedance path to the earth to prevent hazardous voltages from appearing on equipment (the terms "ground") and "earth" are used synonymously here). Normally a grounding conductor does not carry current.

6. In  electricity supply  systems, an  earthing  system defines the  electrical potential  of the conductors relative to that of the Earth's conductive surface. The choice of  earthing system  has implications for the  safety  and  electromagnetic compatibility  of the power supply.

7. A  protective earth  (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. It ensures that in the case of an insulation fault (a "short circuit"), a very high current flows, which will trigger an overcurrent protection device ( fuse ,  circuit breaker ) that disconnects the power supply .

8. A  functional earth  connection serves a purpose other than providing protection against  electrical shock . In contrast to a protective earth connection, a functional earth connection may carry a current during the normal operation of a device. Functional earth connections may be required by devices such as  surge suppression  and  electromagnetic interference  filters, some types of  antennas  and various measurement instruments.

9. GROUNDING IN SUB-STATION

10. EARTHING OF EQUIPMENT BODY

11. The main objectives of the earthing to Provide an alternative path for the fault current to flow so that it will not endanger the user to Ensure that all exposed conductive parts do not reach a dangerous potential to Maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment.

12. Objects of E arthing The object of an earthing system in a substation is to provide safety and reliability of equipment protection . Earthing system shall ensure that no human being in the sub station is subject to shock or injury on the occurrence of a short circuit to EARTH i.e. earth fault or lightning on the equipment installed in the substations yard or on the transmission lines radiating from sub-station..

13. Effect of Current Flow Through Human Body Current Level + Shock Hazard   100 m A Threshold of perception 1–5 mA Sensation of pain 5–10 mA Increased pain 10–20 mA Intense pain; unable to release grip 30 mA Breathing affected 40–60 mA Feeling of asphyxiation 75 mA Ventricular fibrillation, irregular heartbeat    

14. Permissible Body current Limits The magnitude and duration of the current conducted through a human body should be less than those that cause “Ventricular fibrillation”. For a 50 kg body weight the allowable body current is 116 mA for 1 sec &367 mA for 0.1 sec. Therefore good earthling system should not allow more than allowable body current even during faults in the s/s or lines.

15. Resistance of the Human body The resistance of the internal body tissues, not including skin, from one hand to both feet, is approximately 300 ohms. Whereas the body resistance including skin ranging from 500-3000 ohms. For voltages above 1 KV and currents above 5A, the human resistance is decreased by damage/ puncture of the skin at the point of contact. Therefore with solid earth system , values of voltages and currents affecting human body shall be within safe limits.

16. Duties of Earthing System It stabilizes circuit potential with respect to ground It should protect the life of personnel moving in the s/s and property It should provide low impedance path to fault current

17. Ground Mat is connected to The natural point of each system through its own independent earth. Equipment framework and other non-current carrying parts. The earth point of lightning arresters,voltage transformers in the substation through their permanent independent earth electrode. Substation fence.

18. What, if neutral of transformer breaks It prevents flow of earth-fault current ,from the location of fault via earth strata to the neutral of transformer through earth-mesh and earth pit.

19. What, if neutral of transformer break’s With the result ,due to non completion of fault current -loop, replica of fault current is not seen by the over-current and earth fault relay . Thereby the O/C &E/F relay does not operates, even if the earth fault exists on the line..

20. What, if neutral of transformer break’s Fault current will try level best to enter all other neutral earth pits , i.e. neutrals of D.C system , earthing of battery charger , trip coil closing coil , C.T. secondary , over current relay & earth fault relay ,earthing points of control panels &those of isolator handles and all types of sub-station structures.

21. A CASE STUDY OF A 33/11 KV TRANSFORMER WITH BROKEN NEUTRAL. Earth resistance values at a 33/11 KV sub-station were 20 ohms, neutral conductor of the transformer was traceable up to some distance and earth pit was found filled with water. Further it was not sure that continuity of earth conductor up to real earth is there or not.

22. What does it indicate It indicates that even though the neutral grounding conductor is visible up to some distance on the ground and the high values of earth resistance indicates that the neutral grounding is not properly connected between transformer neutral and the earth strata, to complete the fault current path loop.

23. EFFECT AT A SUB-STATION During a fault on 11kv line at 33/11 kv sub-station, the faulty feeder did not trip. D.C . fuses at charger blows off. Relay covers fell down due to vibration of panel. Trip-coils of other 33kv VCB located at sub- station got burnt.

24. Step and Touch Potential Step Potential:- A voltage between two feet separated by 1 meter along the earth with ground current flowing during fault condition. Touch Potential :-A voltage between the object touched and the ground point just below the person touching the object when ground currents are flowing.

25. Solutions suggested to obtain ground resistance of 1 ohm or less i)Making a combination of horizontal grid(by using M.S.flat o size 75x4 mm square,burried in the ground at depth of 0.5 meter),and a number of long vertical electrodes penetrating the lower strata of soil of substation,upto moist layer of soil.

26. Solutions suggested to obtain ground resistance of 1 ohm or less ii) Connecting an existing adjacent grounding grid to the newly designed grid i.e. M.S. flat mesh grid & vertical pipe electrodes penetrating up to moist layer of soil. iii) By penetrating long vertical pipe electrodes penetrating the lower strata of soil , upto moist layer, around the s/s fence.

27. The qualities of a good earthing system are: Must be of low electrical resistance Must be of good corrosion resistance Must be able to dissipate high fault current repeatedly

28. EARTHING   The earthing material that is electrically conductive and a fault current will flow to 'earth' through the live conductor, provided. This is to prevent a potentially live conductor from rising above the safe level. All exposed metal parts of an electrical installation or electrical appliance must be earthed.

29. The main objectives of the earthing Provide an alternative path for the fault current to flow so that it will not endanger the user Ensure that all exposed conductive parts do not reach a dangerous potential Maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment.

30. The qualities of a good earthing system Must be of low electrical resistance Must be of good corrosion resistance Must be able to dissipate high fault current repeatedly

31. Neutral Treatment All neutral wires of the same electrical system should have the same electrical potential, because they are all connected together through the system ground. Neutral conductors are usually insulated for the same voltage as the live conductors, with interesting exceptions. The values of over-voltages and fault currents expected in the network are crucial, since they affect Power Quality parameters, namely interruptions and voltage dips.

32. System substations in general tend to have: A high fault level A large earth grid that is walked over by substation staff A number of items of electrical equipment with electrical protection of various clearing times A number of aerial and/or cable feeders Fences with associated prospective touch voltages for people external to the substation Other services ( eg water, communications) connected

33. Types of System Substations   Traction Substations These locations are the supply points for the overhead wiring . relevant. dc protection and voltage regulation. All the sections in this document All the sections in this document may be may be relevant. AC Switching Stations These locations are ac switching stations that have high voltage circuit breakers.

34. Earth grid ,Electrode Standard Electrode The standard electrode is a 3.6 m length of copper tube (14.29 mm outside diameter , 11.03 mm inside diameter). Longer electrodes may be used if there is some difficulty obtaining the required resistance as the soil resistivity is usually found to be lower at a greater depth. The current rating of the earthing electrode is 5 kA for 1 second when tested in free air in an ambient of 15°C to 25°C without exceeding a temperature rise of 350°C

35. Electrode Spacing   The earthing system at a System Substation consists of a minimum of 4 earth electrodes installed around the inside perimeter of the substation and connected together with the earth mesh. The exact spacing of the electrodes will be determined by the final design, which will be based on local conditions, resistivity of the area and space available for electrodes . The spacing between electrodes should be greater than the electrodes ’ length.

36. Although the earth mesh will often result in a low enough resistance without the use of electrodes, a minimum of 4 electrodes are still necessary to ensure the fault level capability i.e. 4 x 5 kA. Electrodes are also required in case of the drying out of the soil at the depth of the earth mesh in long dry spells.

37. Surge Arresters   The connection between the earth side of the high voltage arrester and the earth side of the equipment being protected must be as short as possible (the same applies to the live side of the surge arrester). The resistance connection to remote earth is not critical to the surge arrester operation but it is important to consider the touch potentials during the surge arrester operation.      

38. Earth grid Standard Electrode : The standard electrode is a 3.6 m length of M.S. tube (14.29 mm outside diameter, 11.03 mm inside diameter). Longer electrodes may be used if there is some difficulty obtaining the required resistance as the soil resistivity is usually found to be lower at a greater depth. The current rating of the earthing electrode is 5 kA for 1 second when tested in freeair in an ambient of 15°C to 25°C without exceeding a temperature rise of 350°C.

39. Neutral Grounding Practice Adopted in MP System Solidly / Effectively grounded/ earthed system

40. WHAT IF BREAKAGE IN NEUTRAL GROUNDING OF POWER TRANSFORMERS Neutral of Transformer is solidly connected to make electrical connection with moist layer of under-ground soil.

41. Breakage in neutral grounding may be either due to mechanical breakage of neutral grounding conductor or due to breakage in electrical path of neutral ground with the conducting moist layer of earth i.e. due to high earth resistance value of s/s .

42. Effects of breakage of neutral grounding of Power Transformer Transformer experiences double the overvoltage during faults. Due to neutral floating , transformer will have premature failures Due to neutral floating, over current & earth fault protection of transformer will not operate, due to non-completion of fault current loop, through transformer neutral & winding. Earth resistance value of s/s will rise to high values.

43. TYPES OF EARTHING GROUND. — Grounding of living components of system are termed as GROUND ,such as grounding of Generator, motor, transformer and battery charger and inverter or converter. NEUTRAL ---Equipment/ structure earthling is termed as NEUTRAL, such as connecting of non current carrying parts of equipment i.e. body of transformer ,CT ,PT , Circuit Breaker ,motor , generator etc.

44. Neutral Neutral is a circuit conductor (that carries current in normal operation), which is connected to earth (or ground) generally at the service panel with the main disconnecting switch or breaker. In a poly phase or three-wire (single-phase) AC system , the neutral conductor is intended to have similar voltages to each of the other circuit conductors. By this definition, a circuit must have at least three wires for one to serve as a neutral.

45. Importance of good Earthing : 1) Computers perform better, if they are provided near to zero voltage reference and it is only possible by good earth. 2) For your very Important PLC based machines it is recommended that you need to maintain neutral to earth voltage below 2 volts. Many times it is observed by electrical people that their machine failed and the reason was neglected proper earth or maintaining the existing earth. Sometimes shocks are observed in home appliances i.e. Frieze , cooler, washing m/c etc.

46. 3) It is fact that currents of 3 phases doesn't maintained same but we need to maintain voltages of 3 phases on same level. Proper neutral earthing maintains voltages between 3 phases on balance position. No maintenance of Neutral Earthings has bad effect on motor. i) With 5% Unbalanced Input Voltage between phase to phase - the LOAD will be unbalanced and the capacity of the motor reduces by 40%.

47. ii)Consumption of the three phase motor will increase by 20 - 25%. To avoid frequent tripping of the overload relays and to continue production, setting of motor relay is kept higher than actual required. The motors (particularly smaller motors upto 7.5 H.P.) cannot withstand this high current for long time and in most cases the motor burns out and this high current flows through the relays, contactors, cables etc. resulting in failure of the same.

48. 4) Electrical Shock prevention is one of the biggest requirement. 5) To ground fault current, good earthing is required. 6) To ground lightning energy, earthing must be proper.

49. . Increased volume makes tight contact from Electrode to Earth Stimulating Material and Material to soil. It never wash away in water. It never dries out, as it is "hydrophilic" and absorbs moisture from the soil to remain hydrated and will maintain its density. In direct sunlight the top few inches will become hard and seal itself off, but the rest of the material stays moist and conductive. It is non corrosive, non-toxic and harmless to the soil or surrounding vegetation.

50. Maintenance Free Earthing System Details Earth Electrode: Earth Electrodes are made from GI pipes & Strips of Hot Dipped GI & Copper. The Strip is inserted in Pipe and space around this strip is filled by a special highly conductive and 100% non-corrosive Material . .

51. Earth Stimulating Material : It is ideal earth fill material to get lower earth resistance. It is used as protective and contact material to fill between earth electrode and surrounding soil. It enriches the electrode surrounding with charge carriers for high conductivity. When water is added to it, it absorbs plenty of water about four to six times its dry volume, it becomes a dense paste. This consistency holds its own shape and sticks on to any surface it touches. This consistence helps hold the Earth Electrode in place

52. Limits of earth resistance i )Generating Power Station: 765KV S/S and 400KV S/S - Less than 0.5 Ώ ii)EHV grid s/s up to 220 KV - up to 1.0 Ώ iii)33 kv s/s -- 1.0 to 3.0 Ώ iv)Line / tower footing resistance-up to8 Ώ

53. Earthing system needs to satisfy three demands In general any earthing system needs to satisfy three demands : i) Lightning and short circuit: the earth system must protect the occupants, prevent direct damage such as fire, flashover or explosions due to a direct lightning strike and overheating due to a short-circuit current .

54. ii) Safety: the earthing system must conduct lightning and short-circuit currents without introducing intolerable step-voltage and touch-voltages.

55. iii) Equipment protection and functionality: the earthing system must protect electronics by providing a low impedance path to interconnect equipment. Proper cable routing, zoning and shielding are important aspects and serve the purpose of preventing sources of disturbance from interfering with the operation of electrical equipment.

56. Typical Earth Resistance Values. 1. Power Stations – 0.5 to 1.0 ohms. 2. EHV Sub-Stations –around 1.0 ohm. 3. 33/11 KV S/S – 2 to 3 ohms. 4. Other Installations – 5 to 8 ohms .

57. The ground electrode : The design of the ground electrode - its size, shape and disposition - are critically important, not only for producing a sufficiently low impedance but also for controlling the shape of the electric field on the surface of the ground ..

58. Ground Potential Rise The resistance of the earth electrode system and the current into the soil determine the voltage difference between the system and the ground. For large fault currents this voltage will be very high at the electrode and will decrease with distance from the earth conductor as the volume of earth through which the current is flowing increases. This ground potential rise (GPR) can result in dangerous situations .

59. Before examining the issue further it is necessary to define some terms (see Figure). ‘Touch potential’ is the voltage difference between the earthed structure and a person standing on the ground within touching distance of the structure. ‘Step potential’ is the voltage between the feet of a person (assumed to be 1 meter apart) standing on the ground. The maximum touch and step potentials are limited by various Standards.

60. It is quite possible to achieve a low impedance - under the right soil conditions – with a single earth rod. Note that the ground potential slope is very steep – i.e. the step and touch voltages would be high – so this is not a suitable choice of electrode.

61. Figure 1 shows (right hand side) the effect of adding a guard ring 1 metre outside the perimeter, buried 0.5 metres deep, to the system. Not only does this reduce the impedance and therefore the GPR (because a greater volume of earth is carrying the current), but the shape of the field is also controlled within the guard ring reducing step and touch voltages.

62. As can be seen, the step and touch voltages around the building are much smaller when an earthing ring is used . The top of the ‘potential mountain’ is made wider and flatter by the field shaping characteristics of the ring in the soil. When no ring is used the ‘mountain’ is steeper and higher, especially near a grounding rod or concrete pile, thus possibly leading to dangerous situations .

63. The buried earth electrode ring around the building should be located at a distance of least 1 metre from the exterior wall. It should be deep enough to ensure that it will not be affected by freezing in winter and will not dry out in summer. Where there is no applicable local guidance the depth should be at least 0.5 metres . The earth ring should be made of copper with a cross-sectional area of at least 50 mm2 .

64. The earth electrode ring must be connected to a meshed network under the structure and to the meshed network around the structure if applicable . Connections between the ring and the rest of the earthing system of the building or site must be made at several points .

65. Site-wide earthing system A low impedance path to earth is required to conduct lightning and short circuit currents into the soil. This main earthing system has to be a network that provides a low impedance connection between all objects and a good distributed contact with the soil. It must be able to conduct all possibly occurring currents while avoiding dangerous touch voltages and large currents on cables connecting remote objects.

66. Electromagnetic compatibility Every piece of electrical and electronic equipment produces some electromagnetic radiation. Similarly , every piece of equipment is also sensitive, to a greater or lesser extent, to electromagnetic radiation. If everything is going to work, the cumulative level of radiation in an environment must be rather less than the level that will disrupt the operation of the equipment working in that environment. To achieve this goal , equipment is designed, built and tested to standards to reduce the amount of radiation that is emitted and increase the amount that can be tolerated .

67. EMC is defined in the IEC 61000 series as: ‘ The ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.’

68. Maintaining this compatibility in practice requires great care in the design and implementation of the installation and the earthing system.

69. Improvement in earth resistance values after taking corrective steps Initial earth resistance value….20 ohms. After completing M.S.flat mesh work buried below 0.5 meter ,making the connections of all its raisers with body earth of all equipments & making its interconnection with 2 no. earth pit for Transformer neutral, results of earth resistance were reduced to 4 ohms. After making interconnection with long driven pipe electrode of 150 mm G.I.pipe , results of earth resistance were further reduced to 0.4 ohms.

70. Conclusion By adopting M S flat mesh and earth pits & long driven GI pipe electrodes together have been found useful to reduce the earth resistance of s/s from 20 ohms to 0.4 ohms. It rectified the problem of breakage of neutral grounding of Power Transformer. Mal-operations carried at s/s due to floating neutral were overcome and protection started working in normal manner. Floating neutral condition of Xmer was also rectified.

71. T HA NK YOU