Draft Handbook for Maintenance of Distribution

Transcription

1 Draft Handbook for Maintenance of Distribution

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3 MAINTENCE PROCEDURES Proper maintenance of electrical systems helps in minimizing/preventing unwarranted breakdowns of Equipment / lines improving the reliability and generating revenue. Periodical or preventive maintenance is normally followed to keep the equipment continuously in service for desired output. Condition based maintenance is the most accepted and adopted concept in maintenance now a day s which helps in providing advance information about the health of the equipment to take corrective action in advance. As the system is growing, it is getting more and more complex. Due to stringent regulations the utility cannot afford to have breakdowns in the system resulting in interruption of power supply to the consumer. On the other hand equipment are getting older and are more prone to failure. Equipment overhaul and major maintenance are also expensive and need to be planned based on the condition of the equipment rather than on a periodic basis as a routine. SLD OF 33/11KV SUBSTATION MAJOR EQUIPMENTS INSTALLED AT 33/11 KV SUBSTATION 1. POWER TRANSFORMER 2. CIRCUIT BREAKER 3. CURRENT TRANSFORMER 4. POTENTIAL TRANSFORMER 5. LIGHTNING (SURGE) ARRESTERS KV LINE ISOLATORS & EARTH SWITCHES 7. BATTERY BANK & BATTERY CHARGERS KV VCB PANELS KV C&R PANELS KV OUT DOOR BUS KV INDOOR BUS 12. APFC PANELS 13. ACDB ( AC-DISTRIBUTION BOARD) 14. ISOLATORS

4 15. POWER AND CONTROL CABLE 16. CAPACITOR BANK 1. POWER TRANSFORMERS A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Electromagnetic induction produces an electromotive force within a conductor which is exposed to time varying magnetic. It constitutes the single most expensive item in a substation. In order to provide long and trouble free service, it is important that a careful and regular supervision and maintenance of the transformer and its components is carried out. The frequency and extent of such a supervision and maintenance is dependent on the experience, climatic conditions, environment, service conditions, loading pattern etc. All work done on transformers should be recorded in maintenance register / checklist. S. No. RATING POWER TRANSFORMER USED IN DHBVN CURRENT SECONDARY TRANSFORMR CURRENT RATIO 33 KV SIDE PRIMARY CURRENT CURRENT TRANSFORMR RATIO 11 KV SIDE 1 4 MVA Amp Amp /1-1-1 Amp / Amp 2 5 MVA Amp Amp /1-1-1 Amp / Amp MVA Amp Amp /1-1-1 Amp / Amp 4 8 MVA Amp Amp /1-1-1 Amp / Amp 5 10 MVA Amp Amp /1-1-1 Amp / Amp MVA Amp Amp /1-1-1 Amp / Amp Alarms & Trippings provided for Power Transformers being used in Sub Stations under DHBVN S. No. Alarm / Tripping Ratings of Power Transformers 4 MVA 5 MVA 6.3 MVA 8 MVA 10 MVA 12.5 MVA 1 Differential relay 2 Pressure Release Valve (PRV) Oil Temperature Alarm (OTA) Oil Temperature Trip (OTT) Winding Temperature Alarm (WTA) Winding Temperature Trip (WTA) Low Oil Level Alarm (LOLA) 8 Buchholz Alarm (BA) 9 Buchholz Trip (BT) 10 Over Current Relay (O/C) 11 Earth Fault Relay (E/F)

5 General Supervision in 33/11 Power transformer 1. Dirt and Dust: The external transformer surfaces shall be inspected regularly; and when required cleaned of dust, insects and other air borne dirt etc. 2. Rust and Treatment: A regular inspection is to be done on the external surface of the transformer tank and radiators. 3. Mechanical Damage: Checks must be carried out for mechanical damage to the fabrications and associated equipment. Particular attention should be given to vulnerable areas such as radiators. 4. Check out all Joints for Signs of Leakage: All joints, both welded and gasketted, must be checked for signs of oil leakage. If there is any doubt of a leak, the area must be cleaned of oil, using a suitable solvent (methyl alcohol) and sprayed with liquid chalk. 5. Check for Oil Level: All oil levels associated with the equipment including oil conservator and all oil filled bushings shall be checked. Also the oil in the oil seal should be maintained. 6. After completing all the checks ensure that all materials or tools, used for maintenance work, have been removed. All clothes and other debris must be disposed of. The transformer compound should be left in a clean and tidy condition. 7. Silica Gel Breather: In open breathing transformer, the breather plays active role in maintaining- the transformer dry by admitting dry air when transformer breathes. In transformers having air cell or diaphragm, the breather ensures dry air inside the air cell or above the diaphragm. The silica gel inside the breather becomes pink from bottom to top over a period of time. TESTS OF POWER TRANSFORMERS Following test should be carried out before the installation of Power transformer Polarization index (by Digital Meggar) or Insulation resistance (by Analog meggar) a) Connect the one lead of meggar at HV side of Transformer and Other one at LV terminal. b) Take the IR of 60 Sec and 10 minutes. The ratio of both the results should be greater than 1 and less than 5. c) Repeat the same procedure between HV terminal to earth and between LV terminals to earth. d) If analog meggar available then just measure the IR of all three condition mentioned above. It should me 5000MOhm or above for new transformer. Ratio Test. a) Connect the ratio meter if available. b) Connect the one terminal of meter at HV side other at LV side to confirm the ratio. c) If ratio meter is not available then just connect the supply of 440V to HV side and measure the voltage at LV side with respect to the ratio of PTR (33/11 kv). The deviation should not be more than 0.5 %. Magnetic Balance Test. a) This test preferably conducted on LV side of Transformer. b) Connect the supply of 250 volt between (r) phase and neutral (N). c) Measure the voltage between y-n and b-n. The sum should be equal to supply voltage d) Repeat the same procedure between y-n and b-n and conform the results. Vector group test ( for Dyn11) a) Short the R phase of HV side with r phase of LV side

6 b) Supply the voltage (440 V) at HV side. c) Measure the voltage between R-N, N-Y, B-b, B-y, Y-b and Y-y d) Condition for confirming vector group of Dyn11 is 1) RN + NY = RY 2) Bb < By 3) Yy = Yb. Transformer Oil Analysis Dissolved gas Analysis (DGA) IEC gas ratio analysis Dual Triangle method GENERAL LIMITS OF TRANSFORMER OIL TEST 2. CIRCUIT BREAKERS: A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by faults. Circuit breaker can be reset (either manually or automatically) to resume normal operation. OPERATING PRINCIPLE Under fault conditions, current intensity increases multiple times the normal current flow. This current senses by relay and energized tripping coil of breaker to isolate the system. Circuit breaker also used to isolate the section manually Type of circuit breakers used in DHBVN At 33 KV level 1. Oil circuit Breakers 2. Vacuum circuit breaker At 11 KV level 1. Vacuum circuit breakers CIRCUIT BREAKER RATING:The breaking capacity of a circuit breaker depends on (i) Symmetrical breaking capacity (ii) Asymmetrical breaking capacity

7 Symmetrical breaking capacity It is the rms. Value of the ac component of the fault current that the circuit breaker is capable of breaking under specified conditions of recovery voltage. Asymmetrical breaking capacity It is the rms value of the total current comprising of both ac and dc components of the fault current that the circuit breaker can break under specified conditions of recovery voltage. Making Capacity of circuit breakers The rated making current is defined as the peak value of the current (including the dc component) in the first cycle at which a circuit breaker can be closed onto a short circuit. Testing of circuit breakers BREAKER TIMING CONTACT RESISTANCE MEASUREMENT Testing Procedure: De- energize, discharge the equipment which is to be tested as a safety precaution.. Check whether Timer leads and associated clips that are to be connected to the breaker poles are good & tight. Make the test connections on the lines of circuit diagram Switch on the power supply to the timer and issue Close command from the timer. Note down the close timing displayed on the timer After recommended time interval (3 seconds), issue Open command from the timer. Note down the open timing displayed on the timer. Now issue Close-Open (CO) command from the timer. Note down the CO timing displayed on the timer. Confirmation of Test: Breaker opening time should be less than the closing time of the breaker, it should come in the range of ms. while closing time should be ms. CO- time breaker should at the middle of the closing and opening time. Opening time < closing -Opening time < closing time Please refer GTP Details of the Breaker for confirmation of the Timing Results. Inference/Recommendation: If the specimen breaker satisfies the respective condition then the respective breaker is considered to be satisfactory. If any deviation is observed from above mentioned condition then corrective action should be taken with the help of the manufacturer /OEM. 3. CURRENT TRANSFORMERS: A current transformer (CT) is a type of instrument transformer designed to provide a current in its secondary winding proportional to the alternating current flowing in its primary. They are commonly used in metering and protection circuits Principle The current transformer works on the principle of variable flux. In the "ideal" current transformer, secondary current would be exactly equal (when multiplied by the turn s ratio) and opposite of the primary current 4. POTENTIAL TRANSFORMERS

8 Voltage transformers (VT) or potential transformers (PT) are another type of instrument transformer, used for metering and protection in high-voltage circuits. Typically the secondary of a voltage transformer is rated for 110 V at rated primary voltage, to match the input ratings of protective relays. Ratio test is a routine test of potential transformer, ratio test is recommended during overhaul and maintenance to check the expected ratio of potential transformer. Limit level of the tests is +- 3 %. 5. LIGHTNING (SURGE) ARRESTERS: Lightning is one of the most serious causes of over voltage. A lightning rod provides a cone of protection, which has a ground radius approximately, equal to its height above the ground. The typical lightning arrester also known as surge arrester has a high voltage terminal and a ground terminal. LA voltage rating Corresponding to the system voltage RATED SYSTEM VOLTAGE (KV) HIGHEST SYSTEM VOLTAGE (KV) ARRESTER RATING IN KV FOR EFFECTIVE EARTHED SYSTEM / / Surge Arrestors installed in the substation are intended to divert surges to earth and thus protect costly switchyard equipment. Proper insulation coordination is necessary for enhancing life span of costly substation equipment. 6. ISOLATORS & EARTH SWITCHES Disconnectors have main current carrying arms and operating mechanism for connection and disconnection. Being off-line devices, they are normally air break type. Normally horizontal double break, Horizontal center break, Pantograph, Vertical break Disconnectors are in use in EHV substations. The alignment of Disconnectors is very important for smooth operation. The limit switches, the healthiness of auxiliary contacts needs to be checked periodically. The main contacts are to be inspected and made smooth if any pitting marks seen. All moving parts are to be lubricated for smooth operation. The gear mechanism and motor normally do not require any maintenance and manufacturer's' recommendation should be referred for maintenance of gears. Earth Switches: The earth switch is a safety device and smooth operation is to be ensured by proper alignment. The earth blade contacts are to be cleaned properly for proper contact and contact resistance to be measured to ensure healthiness. The earth connection from blade to earth is to be carefully checked. All the joints should be tightened. Flexible copper braid connections are provided and healthiness ensured. All moving parts should be lubricated for smooth operation. 7. BATTERY AND BATTERY CHARGERS: In substation DC supply has equal importance. All protection & indication circuits enable with DC supply only. Substations generally use Lead Acid batteries / Nickel-Cadmium batteries for DC supply. Now a days valve regulated lead acid batteries (maintenance free batteries) are also in use for substation applications.

9 Battery Chargers: Battery charger is to be maintained for keeping the battery always charged and also to supply normal DC load for operation. Rating of Battery bank along with VA capacity used BATTERY CHARGER DETAIL USED IN DHBVN VOLTAGE LEVEL VA CAPACITY OPERATING VOLTAGE 33 KV Volt/ 50 Volt KV VCB CONTROL AND RELAY PANELS A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by faults. Circuit breaker can be reset (either manually or automatically) to resume normal operation KV CONTROL & RELAY PANELS 33 KV relay & control panels are used to operate 33 KV outdoor circuit breakers. It also used to get all the monitoring information like breaker on /off status, spring charge, local remote status, trip circuit healthy information, DC & AC voltage related information etc. 33 KV control panels also include relays for protection & master trip relays also KV OUT DOOR BUS 33 KV outdoor bus is used to connect incoming lines, power transformers, and outgoing lines etc. 33 KV outdoor bus works as the most visual architecture of a grid substation to interconnect various substation equipment KV INDOOR BUS 11 KV indoor bus system includes circuit for LT side of power transformer to 11 KV bus system. 11 KV bus includes 11 KV incomer as the source of power and outgoing feeders as supply source of energy to the system. The size of the bus-bar is important in determining the maximum amount of current that can be safely carried. The bus-bar should be able to carry the expected maximum load current without exceeding the temperature limit. The capacity of bus should also be checked for maximum 12. APFC PANELS (AUTOMATIC POWER FACTOR CONTROLLERS) APFC consists of a number of capacitors that are switched by means of contactors. These contactors are controlled by a controller that measures power factor in an electrical network. Depending on the load and power factor of the network, the power factor controller will switch the necessary blocks of capacitors in steps to make sure the power factor a selected value. Power factor reduce due to high inductive load in system. Presence of this high inductive load power factor reduces. Generally on any supply system power demand is varying, power factor also varies as a function of load requirement. Hence it is difficult to maintain consistent power factor by using fixed compensation i.e. fixed capacitors. Thus under light load condition it may cause leading power factor, which results over voltage, Saturation of transformer, Mal-operation of relays, Penalty by electrical authority.

10 13. ACDB (AC-DISTRIBUTION BOARD):- AC supply is required at grid substation for various purpose. It includes. Supply to ac motor of breakers. Charging of battery banks Switch yard lighting system. Panel lighting system. Sources of supply for grid auxiliaries. Normally a local transformer is used for such type of supply. Also a temporary sources also available at grid level to supply in case of maintenance of local transformer. EARTHING SYSTEM AT SUBSTATION An earthing system in a sub-station is to provide under and around the sub-station a surface which shall be at a uniform potential and near zero or absolute earth potential as possible.this provision ensures that no human being in sub-station is subjected to a shock or injury on occurrence of a short circuit or development of other abnormal conditions in the equipment Installed in the yard. Ground mat is provided below ground level. Earth electrodes are driven into ground at several points and are connected to grounding mat to form earth mesh. All the structures, Transformers & other power equipment are connected to this mesh by galvanized steel strips (40x5 mm). Earth Electrodes is a metal plate, pipe or a conductor electrically connected to earth. Copper, Aluminum, mild steel, galvanized iron are generally used for electrodes.generally 40mm dia, 2.6m long GI/MS rods are used EARTH MAT: It is formed by steel 75/8 flats laid horizontally at a depth of 0.5 m below surface of ground to form a mesh of rectangular formation & are welded together. The spacing of earth rods is determined by required touch & step potential. In addition to earth mat, earth electrodes are used these are also connected to the mesh. Solid bar risers are welded to mesh and brought up near the equipment foundations. BASIS FOR DESIGN OF GROUNDING SYSTEM Magnitude of fault current Transient Over voltage Lightning Protection Application of Protective devices for selective ground fault protection Type of load served viz. Motor, Generator etc. GENERAL GUIDELINES FOR EARTHING An effective grounding system must satisfy the following conditions: Provide a low impedance path to ground for personnel and equipment safety. Withstand and dissipate repeated faults and surge currents Provide ample corrosion allowance to various chemicals to ensure continuous service during life of the equipment being protected. Provide rugged mechanical properties for easy driving of earth electrodes with minimum difficulty.

11 All non-current carrying metal parts associated with installation shall be effectively earthed to a grounding system or mat which will limit the touch and step potential to tolerable values. BASIC REQUIREMENT FOR EARTHING: The current carrying path of earthing system should have enough capacity to carry maximum fault current. The resistance of earth & current path should be low enough so as to prevent voltage rise between N & E. Step & Touch potential should be within the safe limits. Systematic survey of earth resistivity should be carried out. The earth electrodes should be driven in the ground to a sufficient depth. To obtain sufficient low earth resistance, The area of earth mesh is increased. The spacing of buried rods is reduced. Soil is kept wet. The number of earth electrodes are increased. Earth resistance is reduced by means of Nacl (Common salt), CaCl2 (Calcium chloride), Na2CO4 (sodium carbonate), CuSO4 (Copper Sulphate), charcoal, soft coal etc. Method of improving Grounds: Increasing number of electrodes, creating parallel path. Increasing depth of driven rod (2.44 m is economical depth).volume of soil contact increases. Moisture contact at depth also increases. Increasing size of driven rod. Soil treatment with salts. These are effective up to the soil of resistivity 300 ohm meters. After that other methods are used. One of the most suitable substance for chemical treatment of soil of very high resistivity for reducing the earth resistance is Natural Clay, also called Bentonite. It is non corrosive, stable & has a resistivity of 2 Ohm.m at 300% moisture. 33 KV Transmission lines In DHBVN, HVPNL is the main source of supply, all 33 KV lines come to substations via rail or RCC pole structure. Following conductors have been used S. NO. CONDUCTOR TYPE AREA RATED CURRENT UNDER NORMAL CONDITION MVA LOADING (33 KV LEVEL) 1 WOLF LEOPORD DOG RACCOON RABBIT WEASEL ITEMS USED IN 33 KV LINES

12 PCC Pole 11.0M Long. ACSR Conductor 150mm2 33 KV Pin insulators with pin 11 KV Disc insulators 45KN B&S type Disc Fitting B&S type Single Tension Fittings for 130mm2 ACSR Conductor Suspension Fittings for 130mm2/ 150 mm2 ACSR conductor U- Bolt for suspension Fitting PG Clamps for 130mm2 / 150 mm2 ACSR conductor 33 KV Top Brackets / hamper MS X Arm (Single Pole) 100x50x6 mm (2200mm) M.S. Channel (X-Arm) 100x50x6mm 3050mm (for H pole) M.S.Channel 75x40x6 3350mm (for earth wire) M.S. Angle 50x50x6mm-2860mm (Bracing H. Pole) M.S Angle 50x50x6mm-2200mm (Belting for H-pole) Half Clamps Full Clamps Stay Sets 8' long complete with X-Plat 460mm of Angle 65x65x6 Elbow & rod with Disc Stay wire 7/8 SWG G.I. Pipe 40 mm dia 6 mtr. Long for earthing Earthing with Earth rod as per Drawing GSL 8 SWG MS Nuts & Bolts Danger Plates (Enameled) Number Plates (Enameled) Phase plate for each phase set of 3 (on each H-Pole & 4-Pole) Eye Screw Bolts 9 x5/8" Barbed Wire "G.I. Strip 25x6 mm 9 meter. For earthling Knee Bracing - MS Flat 50x6 Catenary wire 7/14 SWG CI pipe 250 mm diameter 14 mm thick for highway crossing S/C 630mm2 33kV XLPE cable End termination kit for S/C 630 sq. mm Cable 33KV Isolators (630 Amps), with clamps ACSR Conductor 150mm2 (Leopard) 33 KV AB cable 3 Core mm2 End termination kit for AB cable 3 Core mm2 Dead end calm p assembly with eye hook Trench (with angle and MS sheet cover) for laying 33 KV XLPE cable in yard

13 MAINTENCE OF 33 KV LINES Normal ground patrolling of lines should be conducted by lines maintenance crew periodically. Periodicity will be decided on the basis of importance of the line, terrain condition, and proximity of the line to habitations, forests, gardens, water borne areas etc. and environmental impact on the line. During ground patrolling of the line, the patrolman should check for the following Location no; type of poles (PCC/Rail pole etc.), location address. Clearances of the line, both to ground and in air. Availability of all poles and their condition. Obstructions in the proximity of the line (within line corridor) like trees, branches, structures etc. location wise should be noted. Observing the condition of insulators, conductors, earth wire, jumpers, clamps, dampers, spacers etc., with binoculars, from ground level/nearest possible elevated level. Special patrolling of the line should be done after momentary tripping of the line on fault, for any defects. Defects noticed shall be rectified immediately if this can be attended while the line is live or at the earliest by availing shutdown, anyway, before the defect develops into a major one causing breakdown. Proper planning of materials and manpower is required in order to carryout rectification under shutdown in minimum time to avoid/minimize interruption to loads. Break down maintenance: When a permanent/semi-permanent fault occurs on the line, the line may break down causing interruption of longer duration. The faults could be snapping of conductors, disconnection of jumpers, cross-arms/ pole twisting or pole collapse with or without damage to foundation. This may be due to various reasons like heavy wind due to storms, weakening of pole due to aging, accidents, thefts etc. BEST PRACTICES FOR PERSONNEL SAFETY Electrical Safety Clearance (Live Metal clearance) Rated Voltage Safety Clearance (meters) Up to 33kV kV 1 MINIMUM CLEARANCE (IN M) BETWEEN LINES WHEN CROSSING EACH OTHER MINIMUM CLEARANCE (IN M) BETWEEN LINES WHEN CROSSING EACH OTHER KV KV 220 KV 400 KV 800 KV LOW AND MEDIUM KV KV KV KV KV

14 GUARDING SYSTEM Guarding means safe guard. This is below the line. In case of conductor snapping, it will touch the guarding wire before laying on ground or will lay on guarding. The guarding is always earthed. The line protection will operate and will switch off the line. Snapped conductor will not remain charged. In absence of guarding, conductor will fall on ground and as no protection is operated, conductor will remain charged. This will cause accidents. Hence the guarding is very essential. Types of Guarding:- P. V. C. Guarding. Lecing guarding. 11 KV LEVEL DISTRIBUTION TRANSFORMER

15 Scope: This operational guideline is intended to carry out the fundamental checks on Transformer for its maintenance and while taking it into service. This requires that the concerned SDO should prepare the action plan and ensure the availability of necessary tools and tackles to carry out this activity. Prerequisites: Ensure that necessary permit has been taken. Feeding Sub-station is responsible for providing outage and PTW. The working party is responsible for ensuring isolation. Ensure that the transformer is de-energized, isolated & grounded. Working party should ensure the availability of proper tools and accessories, safety apparels, spares and consumables required for carrying out the work. UNDERGROUND CABLES: the underground cable network of any electricity distribution utility is a vital component of the LV/HV network of the system. It has a several unique benefits over overhead networks. In particular, they have: Lower transmission losses. Can absorb emergency power loads. Have lower maintenance costs. Emit no electric field and can be engineered to emit a lower magnetic field than an overhead line; Require a narrower band of land to install. They are less susceptible to the impacts of severe weather. The reliability of the underground distribution network directly affects the reliability of the overall system and ultimately the overall customer satisfaction. Directives & Checks Abnormality Causes/ Effect Preventive-Corrective action plan Electrical Stress 1. Flash over due to Phase to phase & Phase to earth clearance at cable termination 2. Termination failure due to Core crossing above cut back area at cable termination Ensure sufficient clearance Ensure core crossing below cut back area at cable termination Mechanical Stress 3. Termination failure due to Contaminated surface of terminations 4. Cable failure due to Cable testing at high voltage(dc) 1. Stress develop due to Improper Bending Radius 2. Stress develop due to Mechanical vibrations 3. Stress develop due to Improper cable supports Ensure cleanliness of terminations Ensure Testing at Recommended Voltage (Refer table2) Ensure Proper Bending radius (Refer table1) Ensure proper depth (Refer table3) Ensure proper span length Thermal Stress 1. Insulation degradation due to overloading Ensure Recommended Loading

16 Abnormality Causes/ Effect Preventive-Corrective action plan 2. Insulation degradation due to Hot spot at joints & Terminations (Refer table4) Ensure proper Tightening/crimping of Lugs & Ferrules 3. Insulation degradation due to Improper Earthing connections Ensure Cable earthing connected Sub-station earthing with External damage Insulation Damaged by external agency Ensure Patrolling of cable routes Water ingress Leakage current through the cable insulation/ cable failure, Corrosion in cable conductor &Armour due ingress of water Ensure Lug seal to be intact at outdoor termination. Minimum permissible Bending radius (Table1) PILC XLPE 1C 3C 1C 3C Upto 1.1 KV 20D 15D 15D 12D 1.1 KV to 11 KV 20D 15D 15D 15D Above 11 KV 25D 20D 20D 15D D Diameter of the cable Table 2: Tests on Cable during Fault Localization, After Repair or when Network Modification is done. S. No Test UG Cable 1 Meggar Phase toground and Phase to Phase to be tested with 2.5/5.0 KV meggar for one minute or till reading stabilized. Minimum IR value should be 50 M ohms. 2 Hi pot Each phase to be 6.5 KV DC for 5 minute with respect to ground with other two phases grounded. The cable must with stand the test voltage & leakage current should not exceed 2mA Depth of the cable (Table-3) Voltage Level Depth of cable LT cable (1.1kv) 0.6 m HV cable (11kv) 0.9 m EHV cable (33kv) 1.2 m EHV cable (66kv) 1.5 m Current Rating (Table4)- 11 KV XLPE UG Cable Nominal Size of Cable Current Carrying Capacity (Amps) S. No. (Sqmm) for 11 kv level In Ground at 30 C In Duct at 30 C In Air at 30 C 1 3x x x x x x

17 Nominal Size of Cable Current Carrying Capacity (Amps) S. No. (Sqmm) for 11 kv level In Ground at 30 C In Duct at 30 C In Air at 30 C 7 1x x HT OVERHEAD LINES Introduction: Overhead lines are the most cost effective and frequently used carriers for electric energy. Exposure to environment accelerates deterioration; if this not detected and repaired quickly, some of it will degenerate into serious problems with time. To increase line loading and increase quality of power supply, frequent inspection programs is required to locate and repair any significant failure at the earliest possible stage. Frequent failure in the Overhead lines can cause extensive damage to the insulator and other electrical & earth joints which not only interrupts electricity supply but also results in large revenue losses. Poor/inadequate maintenance in the areas of pole, span length, non-tension electrical joints, conductor spacing, line stay leads to the instability in the electrical and mechanical parameter which reduce the life of the line. With appropriate on site actions, like proper maintenance procedure, the useful life of the overhead lines can be extended. Scope: This operational guideline is intended to carry out the fundamental Line Survey / Checks for its maintenance and while taking it into service. This requires that the concerned engineer should prepare the action plan and ensure the availability of required materials, tools and tackles to carry out this activity. Prerequisites: Ensure that necessary permit has been taken from feeding sub-station officer who is responsible for providing outage and PTW. The Operating personnel is responsible for ensuring de-energization, isolation and properly grounding the network where the activity has to be carried out. Properly Grounding means All conductor of the isolated working zones, should be shorted with earth by shorting links at both ends. The end of the shorting links which are to be connected with conductor & earth terminal should be fitted with crocodile clip. (Presently all conductor are shorted with safety chain without earth.a thin layer of Aluminium oxide form on conductor as it expose in air which behaves like insulator. Due to small surface contact area by the safety chain &conductor may cause high resistance at the chain contact point causing unusual behavior. Shorting links should Better performance than safety chain) If there be any branch line within the working zones, should be isolated/all conductor to be shorted in similar manner. If there be any DT within the working zones, the G.O (Gang Operated Switch) of all DT s should be OFF to avoid any back feeding from downstream. During fixing the crocodile clip of shorting links to the conductor, it should be careful : Before connection the shorting clip to the conductor, the earthing terminal of the shorting clip should be earth. Before connecting the shorting clip to a conductor, it (conductor) should be earth by earth stick & holding the earth stick on the said conductor, the crocodile clip of shorting links should be fastened with the conductor by

18 wearing safety gloves & then the earth stick to be removed. Repetition of the same process of other too conductors must also be ensured. The Working personnel should ensure the availability of proper tools and accessories, safety apparels, spares and consumables required for carrying out the work. Clearance Minimum clearances from any accessible point LV 11 KV The vertical clearance from the highest point 2.5 m 3.7 m The horizontal clearance from the nearest point 1.2 m 1.2 m Minimum Ground Clearance Across the street 5.8 m 6.1 m Along the street 5.5 m 5.8 m Line erected elsewhere 4.6m 4.6m Line with Insulated Conductor 4 m 4m Line crossing 11 KV & LT line 2.4 m Current carrying capacity of conductor Size of conductor Current currying capacity in Amp Dog (100 sqmm/ 1 SWG) 312 Raccoon (80 sqmm/3 SWG) 297 Rabbit (50 sqmm/ 6SWG) 247 Weasel (30 sqmm/ 8 SWG) 114 Maintenance Activity: General Maintenance Check visually, the condition of Pole base & alignment of pole. If the pole tilted, should be straighten & rammed the pole base with brick bats after digging the earth around the pole base. If any bend occurred at the mid of the pole, has to be changed & pole to be erected as per drawing attached. Check visually the condition of existing stay Check the Condition & size of GI wire. It should be 7/10 SWG. Check the Condition of Guy/Egg Insulator, if any damage or missing observed, should be fix/replace. Check the Condition of tension screw, if any damage observed, it has to be changed otherwise to tighten the turn buckle,if required Check the condition of anchor hook Check visually the Line Hardware Condition of Disc Insulator, if there is any crack, flash mark on the surface of the disc, disc should to be change. Condition of strain clamp of disc insulator, If any crack or flash mark on surface, it has to be change. Condition of Pin insulator, if there be any flash mark on the insulator surface, it has to be change otherwise check the tightness with the spindle. Condition of Pin insulator spindle, if any damage or bend, to be change otherwise check the tightness of the fixing nuts.

19 Condition of Top hamper; if any tilt observed at top hamper, it has to be straight or any damage occurred to the metal part, it is to be change, otherwise check the tightness of fixing nuts bolts. Condition of the V-arm. if any damage occurred on V-arm or on the fixing clamp, it is to be changed otherwise check the tightness of the fixing Nuts, bolts Condition of channel-arm, if any damage occurred on channel-arm or on the fixing clamp, it is to be changed otherwise check the tightness of the fixing Nuts, bolts. All metal part to be painted by Primer &Aluminium paint for increasing the durability of life span. Further all metal parts on the pole of line should be connected to proper earth. Visual inspection of bare conductor -if any damage occurred on the conductor, it is to be replaced the damaged portion by same conductor with sleeve joint & the joint is limited to maximum 2 nos.per span. If any loose, conductor to be re-sagged from section to section. Also maintain spacing between the conductors., Span length should be maximum 40 meter for urban & rural residential area & 60 meter for rural field area if the length larger than above so mid pole to be provided. Line should be sectioned at every 4/5 span apart. Components of an Overhead 11 KV (H.T.) Line of O/H Lines 1. Reinforced cement concrete poles (R.C.C. Poles) 2. Double pole structure 3. Cross arms 4. Line conductors 5. Pin-type insulators 6. Strain insulators 7. Earth wires 8. Angle iron for earth wire 9. Hooks for earth wire 10. Top hamper 11. Stay wire 12. Stay insulators 13. Anchor rod 14. Dead end clamps 15. Parallel grooved clamp (P.G. Clamp) 16. Earthing 17. Lightning arrestors 18. Danger plates 19. Anti climbing device 20. Tie wire HT AERIAL BUNCHED CABLE The AB cable network of any electricity distribution utility is a vital component of the HV network of the system. The reliability of the AB cable distribution network directly affects the reliability of the overall system and ultimately the overall customer satisfaction. Scope The operational guidelines apply to DHBVN& any of Business Associates involved in Testing & Repairing of Cable faults being carrying out for maintenance of electricity distribution system. Prerequisites Ensure that necessary permit has been taken. Sub-divisions are responsible for ensuring isolation of the faulty cable for both ends & providing the PTW. Ensure that the cable is de-energized, isolated & grounded.

20 Patrolling of AB cable feeder may be conducted to identify & removal of unauthorized tappings before starting of the work. Copper Screen of HT ABC must be shorted along with messenger wire at starting & End points and joints with crocodile clamp shorting s. So that all conductive paths may be kept at zero potential/ Earth potential AB cable joints should be made properly. AB cable should not be jointed temporarily. In case of emergency, proper joints must be made the next day. For jointing of Messenger wire, mid span joint sleeve should be used instead of binding wires. The Business Associates should ensure the availability of proper tools and accessories, safety apparels, spares and consumables required for carrying out the work. Laying of AB Cables It may be ensure that AB cable should not be dragged on roads/ footpaths etc. It must be laid by use of rollers & pulleys. In case AB cable is laid through trees/ vegetation area, branches may be trimmed to avoid abrasion on AB cables. Reason of Failure: Design Snags- Induction of high voltage at copper screen. Under rated fault withstand capacity of Cu screen. Manufacturing defects Installation snags - Improper connection between Cu screen & Messenger Mechanical damage during installation. Correctives Measures for Design snags: HT ABC with revised specs. (AL wire screen) Current Rating HT ABC (Standard Sizes of Single core cable) S. No. HT AB Cable Size (SqMM) Max. Continuous Load (Amps.) Cable jointing Process Power cables Cablesare used for the transmission/distribution of electrical power and consist of three major components: Conductors Insulation Protective jacket

21 For cables operating at or above 6,000 volts between conductors, a conductive shield surrounds each insulated conductor to equalize the electrical stress on the cable insulation. The construction and requirements are specified in standards such as IS7098 and IEC 60502, IEC6084. The important steps in the cable jointing of medium voltage XLPE insulated screened armoured cable a. Strip the jointing ends of both the cables to be done i.e. stripping of outer sheath, armour, inner sheath, insulation screen, core insulation and conductor screen. Longer end will be 1050mm and other end 750mm for 33Kv XLPE 3*400 sq. mm cable. Park the tubes 1000mm and 500mm then 1000mm at the longest side of the cable. b. All the conductor cores shall be joint with the help of jointing ferrule/mechanical connector and its crimping by suitable crimping tool. c. Fill up the space between the ferrule and the core insulation and the crimped portion in ferrule with semi conducting clay, so that it forms a smooth and round profile with 2 mm. d. Measure a distance of 20 mm on both sides of the semi-conducting tape. Apply stress grading pad of 30mm width over the core covering 10mm of the semi conducting tape. e. Keeping conducting layer of core, wrap the self-amalgamating insulating tape so that the required insulation thickness is built up. Ensure a tapered profile of the tape towards the semi conducting layer of the insulation, the self-amalgamating tape should be stretched to 2/3 rds of its original width while applying. f. Fill up the gap 5mm between self-amalgamating insulating tape and semi conducting layer of core by stress grading pad of 30mm width. g. Apply semi conducting tape one layer half over lapped about 10mm on one side of metallic shielding to the other end in the same manner. h. Wrap 2 layers of self-amalgamating insulating tape, each half overlapped to cover the semi conducting tape. Stretch the tape 2/3rds of width while applying as shown in figure given below. i. Wrap one layer of copper wire mesh on the core to connect the copper tape from end to another over the tapes. j. Place the black stress control tube centrally over the joint. Starting at the centre shrink the tube towards one end and then towards the other end until fully shrunk. Apply heat evenly throughout the process. When completed adhesive will flow from the ends, wipe off excess. Role of Earthing in Power Distribution Earthing In Electrical System Earthing means making an electrical connection to general mass of earth. Earth forms an intrinsic part of electricity System. Earthing of electrical installation is primarily concerned with ensuring safety. Soil conductivity is an important consideration. Lower the resistivity, easier to get good earth. The ground is more effective conductor than copper wire used in electrical circuit. Earth provides the shortest path for fault current to travel. Short circuits allows electrical current to by-pass its normal route. Under short circuit, current follows the least resistance fault to ground. Reason for Poor Earthing Practices: Inadequate understanding about technical requirement of earthing & Standards. Earthing has no role to play during normal conditions. Lack of compatibility between earthing requirement of electronic equipments & power system.

22 Lack of motivation & importance to engineers for keeping earthing & connections in proper state. Why Earthing Important? Earthing in a substation is extremely important for the safety of operating personnel as well as for proper system operation. It provides safe passage to earth fault current to flow. E/F current flow through the earth and may readily cause operation of fuse or E/F relay. It provides low resistance path to fault current to ensure prompt & consistent operation of protective devices under fault current. It protects the equipment other than live parts from attaining potential. Under fault conditions, frames & enclosures etc may attain high potential w.r.t ground. So these non-current carrying parts should be connected to ground for the safety of the operating personal & other human beings for discharging electrical energy to earth. It protects the equipment from over voltages, HV surges resulting from lightning & other causes. It ensures safety of life from electric shocks by grounding all metallic structures and also avoid hazardous voltage gradients by suitably designing the grounding system. The potential of earthed body does not reach to dangerously high values during fault. It ensure that ground potential rise does not have any dangerous effect on communication system. It plays an important role in increasing the reliability & continuity of supply service and stabilizing the voltage conditions. It has means of discharging feeders & equipments to ground before proceeding with maintenance. Attributes Of Good Earth Connection Lowest soil resistivity. Low electrical resistance to earth. Good corrosion resistance. Ability to carry high current repeatedly. A reliable life of at least 30 years. Low loop impedance path of earthing. Limits of Earthing Resistance Following are values of earth resistance considered appropriate for different electrical installation a) Large HV and EHV s/stns ohms b) Industrial and Distribution s/stns ohms c) Medium voltage systems(415 V) ohms FUSES A switching device, which by the fusion of one or more of its specially designed and proportioned components open the circuit, in which it is inserted and breaks the current, when the same exceeds a

23 given value for a sufficient time. The term fuse covers all the parts forming the complete switching device. Principle of operation The operation of fuse depends upon the heating effect of an electric current. When the electric current is increased in a circuit due to short circuits or over loads, it increases the rate of heat generation, which will increase the temperature of the fuse wire and thereby the rate of heat dissipation increases from the exposed surface of the fuse wire. The final temperature reaches to such stage that the heat generation will be equal to the rate dissipation. If this temperature happens to be above the melting point of the material for fuse wire, fuse must have operated Selection of fuse wire To select the proper fuse wire to be inserted in a circuit two factors viz. (a) maximum current rating of the circuit (b) current rating of the smallest size of wire of accessories is to be seen, the fuse wire inserted should be of size so that when the current with reference to factors (a) and (b) is increased, it should blow out. The type of wire to be selected to use as a fuse wire depends upon the type of load connected to the circuit i.e. steady load and fluctuating loads. The steady load covers the heating loads and the fluctuating load consists of motor, capacitor and transformer loads, all of which take transient over current when they are switched into the circuit. In steady load circuits, the fuse forms only the protection part of the circuit. Therefore, the fuse rating should be equal to or next greater than the ratings of the smallest cable used in the circuit. But if a number of fuse wires are run in parallel to augment the rating of fuse, total rating should not be equal to the product of rating of and strand and number of strands used. In fluctuating load circuits, fuse should allow the short time over current to flow without blowing. For this, it is necessary to select fuses of rated current greater than that of the cable of the circuit. The fuse wire should always be inserted in live wires or phase of the circuit, or otherwise even if the fuse blows out, the faulty circuit will be fade and would cause considerable damage in case of leakage fault. LT UNDERGROUND CABLES The underground cable network of any electricity distribution utility is a vital component of the LV/HV network of the system. The reliability of the underground distribution network directly affects the reliability of the overall system and ultimately the overall customer satisfaction. Scope: The operational guidelines involved in Testing & Repairing of Cable faults being carrying out for maintenance of electricity distribution system. Prerequisites: Ensure that necessary permit should be taken. Site In-charge is responsible for ensuring isolation of the faulty cable for both ends & providing the PTW. Ensure that the cable is de-energized, isolated & grounded. The workman should ensure the availability of proper tools and accessories, safety apparels, spares and consumables required for carrying out the work. Minimum permissible Bending radius PILC PVC XLPE Type of Core 1C 3C 1C 3C 1C 3C Upto 1.1 KV 20D 15D 15D 12D 15D 12D D Diameter of the cable Tests on Cables during fault localization, after repairs or when network modification is done Test 230/440 V

24 Megger Test Depth of the cable LT cable (1.1kv) Phase to ground and phase to phase to be tested with 1KV Megger for 1 Minute/or till reading stabilizes. Minimum IR values should be 50M Ohms. 0.6 m LT Cables Current Rating S No. No. of Cores Size (Sqmm) Current Rating of AL PVC cables in Amp Current Rating of AL XLPE cables in Amp AIR Ground AIR Ground LT OVERHEAD LINES Overhead lines are the most cost effective and frequently used carriers for electric energy. Exposure to environment accelerates deterioration; if the fault is not detected and repaired quickly, some of it will degenerate into serious problems with time. To increase line loading and quality of power supply, frequent inspection program is required, to locate and repair any significant failure at the earliest possible stage. Frequent failure in the Overhead lines can cause extensive damage to the insulator and other electrical & earth joints which not only interrupts electricity supply but also results in large revenue losses. Poor/inadequate maintenance in the areas of pole, span length, non-tension electrical joints, conductor spacing, pole stay leads to the instability in the electrical and mechanical parameter which reduce the life of the line. With appropriate on site actions, like proper maintenance procedure, the useful life of the overhead lines can be extended. Scope This operational guideline is intended to carry out the fundamental Line Survey / Checks for its maintenance and while taking it into service. This requires that the concerned engineer should prepare the action plan and ensure the availability of required materials, tools and tackles to carry out this activity. Prerequisites: Ensure that necessary permit has been taken from SDO/JUNIOR ENGINEER who is responsible for providing outage and PTW. The PTW has to transfer to Working personnel after ensuring de-energization, isolation and properly grounding the network where the activity has to carry out.

25 The Working personnel should ensure the availability of proper tools and accessories, safety apparels, spares and consumables required for carrying out the work. Clearance Minimum clearances from any accessible point 1 The vertical clearance from the highest point 2.5 m 2 The horizontal clearance from the nearest point 1.2 m Minimum Ground Clearance Maintenance Activity General Maintenance 1 Across the street 5.8 m 2 Along the street 5.5 m 3 Line erected elsewhere (Bare) 4.6 m 4 Line erected elsewhere (Insulated) 4 m 5 Line crossing 11 KV & LT line 2.4m Check visually, the condition of Pole base & alignment of pole. If the pole is tilted, should be straighten & rammed the pole base with brick bats after digging the earth around the pole base. If any bend occurred at the mid of the pole, should be changed & new pole to be erected. Check visually the condition of existing stay Check the Condition & size of GI wire. It should be 7/10 SWG. Check the Condition of Guy/Egg Insulator-if any damage observed or is missing, it should be fixed/replaced. Check the Condition of tension screw-if any damage observed, it has to be changed. Tighten the turn buckle if required. Check the condition of anchor hook. Check visually the Line Hardware Condition of Shackle Insulator-if there is any crack, flash mark on the surface of the insulator, it should be changed. Condition of Pin insulator spindle- if any damage or bend happened, it needs to be changed. Check the tightness of the fixing nuts. Condition of Top hamper- if any tilt observed at top hamper or any damage occurred to the metal part, it has to be change. Check the tightness of fixing nuts bolts. Condition of the Double line X-arm. if any damage occurred on X-arm or on the fixing clamp, it has to be changed. Check the tightness of the fixing Nuts, bolts All metal parts to be painted by Primer &Aluminium paint for increasing the durability & life span of the same. Visual inspection of bare conductor -if any damage occurred on the conductor, it to be replaced with the same conductor through sleeve joint & the nos. of joint is limited to maximum 2 per span.