Unit 5 Substation Equipment and Distribution Lines PDF

Substation Equipment and Distribution Lines...

Substation Equipment and Distribution Lines Unit 5 Substation Learning Objectives Equipment and Distribution Lines After studying this unit, you should be able to: describe the main equipment required for the construction of a 66-33/11kV substation; classify the distribution line equipment; describe the main equipment for overhead lines; discuss the important features of underground power cables in the distribution system network; enunciate the general operation and maintenance practices for substation equipment, distribution lines and capacitors; explain hotline maintenance techniques and tools; and explain the effect of HT/LT ratio on line losses and voltage. 49 Operation and Maintenance 5.1 INTRODUCTION In Unit 4, you have studied about the power distribution system and its components. You have also learnt about distribution system planning and the general O & M objectives and practices. You will agree that the smooth operation of the power distribution system depends on how well it is maintained. This includes the operation and maintenance of all its components. We begin this unit with a discussion of the substation equipment and distribution lines so that you know the standards prescribed for the equipment. Adhering to these standards would ensure the smooth operation of the equipment. We next discuss the operation and maintenance of equipment used in the 66-33/11 kV substations, 11/0.4 kV substations, overhead lines, underground cables and capacitors. Finally, we take up hot line maintenance tools and techniques and the impact of LT/HT ratio on losses. In the next unit, we deal with the O & M of distribution transformers separately. 5.2 66 - 33/11 kV SUBSTATION EQUIPMENT Equipment in a substation can broadly be categorized as follows:  structures;  power transformers;  bus-bars;  circuit breakers (33 kV and 11 kV);  isolators or isolating switches (33 kV and 11 kV);  earthing switches;  insulators;  power and control cables;  control panel;  lightning protection  surge arrestors;  instrument transformers (current and power transformers, i.e., CTs and PTs);  earthing arrangements;  reactive compensation;  DC supply arrangement;  auxiliary supply transformer; and  fire-fighting system. The design of the substation equipment must comply with the requirement of 50 relevant Indian Standards.  Substation We now briefly describe each one of these. Equipment and Distribution  Structures Lines Structures are required to provide entry from the overhead line to the substation and to extend out required number of feeders. The numbers of structures should be kept to a minimum as large number of structures would not only be uneconomical but give an ugly look to the substation and may prove to be obstructions in extending bus-bar, lines, etc. The main structures required for 66/11 or 33/11 kV substations are:  incoming and outgoing gantries;  support structures for breaker, isolators, fuses, insulators, CTs and PTs; and  bus-bars. Switchyard structures can be made of fabricated steel, RCC or PSCC, Rail or RS Joist.  Power Transformers You have learnt about the underlying principle and design of a power transformer in Unit 4. The general operation and maintenance practices of power transformers are similar to those of distribution transformers, which are discussed in detail in Unit 6.  Bus-bars A bus-bar in electrical power distribution refers to thick strips of copper or aluminum that conduct electricity within the substation (Fig. 5.1). 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 temperature under short circuit conditions. Different types of bus-bars, namely, single bus-bar, single bus-bar with bus sectionalizer, main and transfer bus, double bus-bar, double bus-bar with double breaker scheme and mesh scheme are used in a substation in accordance with the safety and reliability considerations.  Circuit Breakers Fig. 5.1: Bus-bars A circuit breaker is a switching device built ruggedly to enable it to interrupt/ make not only the load current but also the much larger fault current, which may occur on a circuit. A circuit breaker contains both fixed contacts and moving contacts. The purpose of circuit breakers is to eliminate a short-circuit that occurs on a line. Circuit breakers are found at the arrivals and departures of all lines incident on a substation. When the circuit breaker is closed these contacts are held together. The mode of action of all circuit breakers consists in the breaking of the fault current by the separation of the moving contacts away from the fixed ones. An arc is immediately established on 51 Operation and Maintenance separation of the contacts. Interruption of the current occurs after the arc at these contacts is extinguished and current becomes zero. Elements of a Circuit Breaker Circuit breakers contain the following elements, irrespective of the medium for arc quenching and insulation:  main contact at system voltage;  insulation, such as porcelain, oil or gas, between the main contacts and ground potential;  operating and supervisory accessories, of which tripping facilities are most important. A wide variety of closing and tripping arrangements (using relays with variable time delay) and a number of operating mechanisms (based on solenoids, charged springs or pneumatic arrangements) are available now-a-days. The types of breakers used in a distribution system are:  air break type;  oil break type;  vacuum type; and  SF6gas breaker. (a) (b) Fig. 5.2: Circuit Breakers: a) Oil Break Type Breaker; and b) SF 6 Gas Breaker The rated voltage of circuit breakers for 66 kV level is 72.5 kV ,33 kV level is 36 kV, and for 11 kV, it is 12 kV. The short circuit current rating is31.5 kA 52 for 66 kV breakers, 25 kA for 33 kV breakers and 16kA for 11 kV breakers. The 11 kV switchgear is generally metal enclosed indoor type.  Substation  Isolators Equipment and Distribution Isolators are mechanical switching devices capable of opening or closing Lines a circuit  when a negligible current is broken or made, or  only a small charging current is to be interrupted, or  when no significant voltage difference exists across the terminals of each pole. Fig. 5.3: Isolators Isolators are capable of carrying current under normal conditions and short circuit currents for a specified time. In open position, the isolator should provide an isolating distance between the terminals. The standard value of rated duration of short time current capacity withstand for isolator and earthing switch is normally 1 second. A value of 3 seconds is also sometimes specified. For 33 kV, horizontal type isolating switches are used. The rated normal current is 630 A at 36 kV. For 11 kV, both horizontal and vertical mounting isolating switches of 400 Amps at 12 kV are used.  Earthing Switches Earthing switches are provided at various locations to facilitate maintenance. Main blades and earth blades are interlocked with both electrical and mechanical means. The earthing switch has to be capable of withstanding short circuit current for short duration as applicable to the isolator.  Insulators An electrical insulator resists the flow of electricity. Application of voltage difference across a good insulator results in negligible electrical current. Adequate insulation is of prime importance for obvious reasons of reliability of supply, safety of personnel and equipment, etc. The insulators in use at substations are post insulators of pedestal type or solid core station type. The station design should be such that the number of insulators is kept at a minimum at the same time ensuring security of supply. In the areas where the problem of insulator pollution is expected (such as near the sea or thermal station, railway station, industrial area, etc.) special insulators with higher leakage resistance 53 should be used. Operation and Maintenance  Power and Control Cables Power and control cables of adequate current carrying capacity and voltage rating are provided at the substation. Power cables are used for 66 kV, 33kV,11 kV or LT system to carry load current. The control cables are required for operating and protection system connections. The cables are segregated by running in separate trenches or on separate racks.  Control Panels Control panels installed within the control building of a switchyard provide mounting for mimic bus, relays, meters, indicating instruments, indicating lights, control switches, test switches and other control devices. The panel contains compartments for incoming lines, outgoing lines, bus-bars with provision for sectionalizing, relays, measuring instruments, etc. The panel is provided with:  suitable over-current and earth fault relays to protect the equipment against short circuit and earth faults; and  measuring instruments such as ammeter, voltmeter and energy meter for 66, 33kV and 11 kV systems.  Lightning ProtectionSurge Arrestors Large over voltages that develop suddenly on electric transmission and distribution system are referred to as “surges” or “transients”. These are caused by lightning strikes or by circuit switching operations. Surge arrestor is a protective device for limiting surge voltages on equipment by discharging or bypassing surge current. The surge arrestor which responds to over-voltages without any time delay is installed for protection of 66, 33 kV switchgear, transformers, associated equipment and 11 kV and 33 and 66 kV lines. The rated voltage of arrestors for 33 kV should be 30 kV for use on 33 kV systems and with nominal discharge current rating of 10 kA. The rated voltage of lightning arrestors should be 9 kV (r.m.s.) for effectively earthed Fig. 5.4: Surge Arrestors 11 kV system (coefficient of earth not exceeding 80 % as per IS: 4004) with all the transformer neutrals directly earthed. The nominal discharge current rating should be 5 kA.  Instrument Transformers (Current and Voltage Transformers) The substations have current and voltage transformers designed to isolate electrically the high voltage primary circuit from the low voltage secondary circuit and, thus, provide a safe means of supply for indicating instruments, meters and relays. Current Transformer (CT) Current transformers are used in power installations for 54 supplying the current circuits of indicating instruments  Substation (ammeter, wattmeter, etc.), meters (energy meter, etc.) and Equipment and protective relays. These transformers are designed to provide a Distribution standard secondary current output of 1 or 5 A, when rated current Lines flows through the primary. A fundamental characteristic of CT is its transformation ratio, expressed as the ratio of the rated primary to NOTE rated secondary current. Current transformers have two inherent errors: the current ratio and phase displacement. These two A current errors serve as a basis on which current transformers are classified transformer is an for accuracy. instrument transformer in which the current ratio is within the specified limit. The primary winding is connected in series with the load and carries the load current to be measured. The secondary winding is connected to the measuring instrument or relay, which together with the winding impedance of the transformer and lead resistance constitute the burden (a) (b) (b) of the transformer. Fig. 5.5: a) Current Transformers; and b) Voltage Transformer Voltage Transformer or Potential Transformer (PT) These instrument transformers are used for supplying the voltage NOTE circuit of indicating instruments, integrating meters, other measuring apparatus and protective relays or trip coils. These may be of single Voltage transformer phase or three phase design and of the dry or oil immersed types. A is an instrument voltage transformer or PT is rated in terms of the maximum burden transformer in which the secondary (VA output) it will deliver without exceeding specified limits of error. On voltage is the other hand, a power transformer is rated by the secondary output it substantially will deliver without exceeding specified temperature rise. All voltage proportional to the transformers are designed for a standard secondary voltage of 110 V primary voltage and or 110 / 3 V. phase angle near to zero for an  Earthing Arrangements appropriate direction of connection. Earthing has to be provided for  safety of personnel,  prevention of and minimizing damage to equipment as a result of flow of heavy fault currents, and  improved reliability of power supply. The basic grounding system is in the form of an earth mat with risers. 55 Operation and Maintenance Risers of MS flat are generally provided. Earth mat is provided within the substation area. The earth rods are connected to the station earth mat. The earthing must be designed so as to keep the earth resistance as low as possible. Earthing practices have been discussed in Unit 6 of the course BEE-002.  Reactive Compensation Reactive compensation (as indicated by system studies of the network) has to be provided. It is always a good idea to ensure a power factor correction for transformers, since even when they are operating on low load (e.g., during the night) they absorb reactive power, which must be compensated to avoid unnecessary loadings and losses. You can recall this aspect from Appendix 1 to Unit 4. Shunt capacitors (Fig. 5.6) are connected on the secondary side (11 kV side) of the 33/11 kV and 66/11kV power transformers. The capacitors are generally of automatic switched type. The automatic system of the capacitor bank has the task of switching in the necessary capacitance according to the load requirements at each given moment.  Station Battery/DC Supply Arrangement Fig. 5.6: Shunt Capacitors Station batteries supply energy to operate protection equipment such as breakers and other control, alarm and indicating equipment (Fig. 5.7). The station batteries are a source for operating DC control system equipment during system disturbances and outages. During normal conditions the rectifier provides the required DC supply. However, to take care of rectifier failure, a storage battery of adequate capacity is provided to meet the DC requirements. Normally, in a 33/11 kV substation, the DC system is of 30 cells consisting of 15 lead acid storage batteries or Nickel-Cadmium batteries. The battery is connected in parallel with a constant voltage charger and critical load circuits. The charger maintains the required voltage at battery terminal and supplies the normally connected loads. This sustains the battery in fully charged condition. The correct size battery charger has to be selected for the intended application.  Auxiliary Supply Transformer Fig. 5.7: Battery Bank An Auxiliary Supply Transformer of adequate capacity is required to be provided for internal use for lighting loads, battery charging, oil filtration plant, etc. The supply should be reliable. In a substation it is normally provided from a station transformer connected on 66,33 or 11 kV bus bar.  Fire Fighting System In view of the presence of oil filled equipment in a substation, it is important that proper attention is given to isolation, limitation and extinguishing of fire so as to avoid damage to costly equipment and reduce chances of serious dislocation of power supply as well as ensure safety of personnel. The layout of the substation itself should be such that 56 the fire should not spread to other equipment as far as possible. Fire  Substation extinguishers of the following type must be provided: Equipment and  Carbon dioxide extinguisher, and Distribution Lines  Dry chemical powder extinguisher. Carbon dioxide (CO2 type) extinguisher and Dry chemical powder type extinguisher should conform to IS: 2878 and IS:2171, respectively. For oil fire, foam type extinguishers are used (see Unit 7, BEE-002 also). The fire fighting equipment should be maintained and kept in top condition for instant use as per IS: 1948-1961 “Fire Fighting Equipment and its Maintenance including Construction and Installation of Fire Proof Doors- Fire Safety of Buildings (General)”. So far we have described the equipment in a 66-33kV/11kV substation. You may like to review the information before studying further. SAQ 1: Equipment at 66-33/11kV substation List the equipment being used in your utility for the construction of 33/11 kV substation along with their typical ratings. ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… 5.3 11/0.4 kV SUBSTATION EQUIPMENT The main equipment at an 11/0.4 kV distribution substation comprises:  distribution transformers;  transformer mounting structure;  protection system;  earthing system;  lightning arrestors;  LT distribution box; and  reactive compensation. We shall be discussing the distribution transformers in detail in the next unit. Here, we briefly describe the remaining components.  Transformer Mounting Structure Transformers can be mounted outdoors (Figs. 5.8 and 5.9) in one of the 57 Operation and Maintenance following ways: Plinth mounting, H-pole mounting and direct mounting. We describe these mountings, in brief. (a) (b) Fig. 5.8: Transformer Mountings: a) Plinth Mounting; and b) H-Pole Mounting Plinth mounting: The transformer is mounted on a plinth made of concrete. The plinth has to be higher than the surroundings. The method can be used for all sizes of transformers. Where the distribution substations are plinth mounted, they are efficiently protected by fencing so as to prevent access to the apparatus by unauthorized persons. H-pole mounting: The transformer can be mounted on cross- arms, fixed between two poles, which are rigidly fastened to the poles. The transformer has two base channels, which rest on the transformer mounting structure. Direct mounting: The transformer is clamped directly to the pole by suitable clamps and bolts. This method is used for transformers up to 25 kVA only.  Protection System The HT side of all transformers is normally protected by drop out expulsion type fuse. Three 11 kV drop out fuse units comprising a set are installed on mounting cross-arm. The fuse element is soldered on both ends between woven wires, which are sufficiently strong to withstand tension when fixed to the terminals on both ends. The element is housed in an insulated tube of paper or insulating material. Horn gap fuses are also used on distribution transformers on HT side. The fuse wire is fixed between arcing horns. The advantage is Fig.5.9: Direct Mounting that ordinary fuse wire of rated capacity can be used for replacement while for drop out fuses, fuse elements are required to be stocked for 58 replacement.  Substation  Earthing Equipment and Distribution Pipe earthing or rod earthing is provided for the distribution substation. Lines Three electrodes forming an equilateral triangle are provided so that adequate earth buffer is available.  Lightning Arrestors 11 kV lightning arrestors 9 (kV) of outdoor type are used for diverting the lightning surges to earth resistance of earth. The lightning arrestor should be installed on the HT side and its lead should be kept at a minimum.  LT Distribution Box For transformers of 100 kVA and above, sheet metal LT distribution box consisting of LT breaker and fuse cut-outs is provided from where distribution feeders are to be taken out. The size of the box has to be suitable for accommodating MCCB, fuse cut-outs, cable connectors, bus-bars, etc.  Reactive Compensation The load incident on the distribution system is mostly inductive, requiring large reactive power. The best method is to compensate the reactive power at the load end itself but it is difficult to implement in practice. Hence, providing compensation on the distribution system is essential. So wherever the power factor is low, reactive compensation may be provided on the distribution transformers. The shunt capacitor supplies constant reactive power at its location, independent of the load. Fixed or automatic switched type capacitors of adequate rating are to be provided on the LT bus of the distribution transformers. In the switched capacitor system, the capacitors are switched on and off along with the load to avoid over-voltage during low load operation. SAQ 2: Equipment at distribution substation List the equipment and their typical ratings, being used in the distribution substations of your utility. Are all the protection equipment listed above being used? ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… 59 Operation and Maintenance 5.4 DISTRIBUTION LINE EQUIPMENT The distribution lines can be either overhead or underground. These are usually overhead, though for higher load densities in cities or metropolitan areas, these are underground. The choice between overhead and underground depends upon a number of widely differing factors such as the importance of service continuity, improvement in appearance of the area, feasibility in congested areas, comparative annual maintenance cost, capital cost and useful life of the system. 5.4.1 Overhead Lines An overhead power line is intended for transmission of electric power by a bare or covered overhead conductor supported by insulators, generally mounted on cross-arms near the top of poles. The overhead line may be 66, 33, 11 kV or LT line. The basic equipment used for the line remains the same. The main equipment required for an overhead line is as follows:  supports,  cross-arms,  insulators, (a)  earthing knob,  earthing coil,  strain hardware set,  conductors,  line accessories, (b) Fig. 5.10: a) Overhead  guard wires, and Lines Mounted on Cross-arm  LT line spacers. Poles; b) Close-up We describe each one of these, in brief.  Supports A support is a column of wood, concrete, steel or some other material supporting overhead conductors by means of arms or brackets. The supports used for overhead line construction vary in design and the purpose they have to perform. The different types of supports for overhead lines are: wood poles, concrete poles, steel poles and lattice type towers. Wood poles: Chemically treated wood poles are used for distribution lines. The advantage of using wood poles is that they are low in cost. However, they are susceptible to decay. The specifications for wood poles are covered by IS:876 and IS:5978. According to this standard, 60 the timber suitable for poles has been classified into three groups  Substation depending upon its strength. For example, IS 6056 for jointed wood Equipment and poles for overhead lines specifies that sal, deodar, chir, kail, wood be Distribution used. Jointed wood poles with wire bound lap joint are considerably Lines less expensive and found to be very suitable for LT and HT lines in rural areas. Concrete poles: Concrete poles are more expensive than wood poles but cheaper than steel tubular poles. Concrete poles are of three types:  Pre-cast cement concrete poles (PCC) made of cement concrete;  Reinforced cement concrete poles (RCC);  Pre-stressed cement concrete poles (PSCC). The low maintenance, competitive price and aesthetic appearance of PCC poles makes them superior to steel or wood for use in electric lines. Ease and speed of installation means faster project completion and lower installation cost. RCC poles have an extremely long life and need little maintenance but they are bulky in size and comparatively heavy. They have shattering tendency when hit by a vehicle. PSCC poles take care of these shortcomings to some extent. However, the handling, transportation and erection of these poles is more difficult because of their heavy weight. Steel poles: The steel poles are of the following types:  Steel tubular poles whose specifications are covered by IS:2713-1967. Due to their light weight, high strength to weight ratio and long life, they possess distinct advantages over other types of poles. The use of a pole cap at the top, concrete muff in the ground and regular coating of paint prolongs their life.  Old and second hand rails and Rolled Steel (RS) joists are frequently used as supports for overhead lines. The portion embedded in the ground should be protected by concrete muff and the remaining portion by regular paint unless galvanised steel is used. Lattice type supports: These are fabricated from narrow base steel structures. They are light in weight and economical and can be assembled at site if bolted construction is used. Normally both welded and bolted types are used.  Cross-arms The shape and length of the cross-arms depend upon the desired configuration of conductors. The following types of cross-arms and brackets are used:  V cross-arms for tangent locations with clamps; 61 Operation and Maintenance  double channel cross-arms for tension or cut point locations where double poles are used; and  top clamps. Cross-arms of hand wood (sisso, sal), or creosoted soft wood (chir) or fibre glass are used sometimes. Steel cross-arms are stronger and last much longer. MS angle iron and channel iron sections are generally used for this purpose. Smaller sections are used for communication circuits.  Insulators You have learnt that an electrical insulator resists the flow of electricity. Application of a voltage difference across a good insulator results in negligible electrical current. Insulators made of glazed porcelain, tough glass and polymers are used for supporting the conductors. Porcelain insulators prevent the electrical current from energizing the power pole. The principal types of insulator are described below:  Pin insulators are manufactured for voltages up to 33 kV and are cheaper than the other types. IS:1445 and 731 cover detailed specifications for these. The pins for the insulators are fixed in the holes provided in the cross-arms and pole top brackets. The insulators are mounted over the pins and tightened. The cost of pin insulators increases very rapidly as the working voltage is increased. For high voltages these insulators are uneconomical. Moreover, Fig. 5.11: Pin Type Insulator replacements are expensive.  Disc insulators are made of glazed porcelain or tough glass. They are used as insulators on high voltage lines for suspension and dead ending. The line conductor is suspended below the point of support by means of the insulator or a string of insulators. A disc insulator consists of a single disc-shaped piece of porcelain, grooved on the under-surface to increase the surface leakage path between the metal cap at the top and the metal pin underneath. The cap is recessed so Fig. 5.12: Disc Type as to take the pin of another unit, and in this way a string of any Insulator 11 kV required number of units can be built up. The cap is secured to the insulator by means of cement. Disc insulators are “ball and socket” or “tongue and clevis” type. A suspension clamp is used to support the conductor, if suspension configuration of the line is chosen.  Shackle insulators are used for distribution lines dead ending and supporting conductors laid in vertical formation. IS:1445-1977 covers shackle insulators for voltages below 1000 V. The two standard sizes listed in this specification are 90 mm dia x 75 mm height and 115 mm dia x 100 mm height. A shackle insulator is supported by either two straps and two MS bolts or one U clamp or D strap and two MS bolts as per IS:7935.  Stay insulator/Guy strain insulators of egg type porcelain 62 are used for insulating stay wire, guard wires, etc. wherever it is not  Substation proposed to earth them. As per IS: 5300, two strength sizes (ultimate Equipment and tensile strength) are used: 44 kN and 88 kN, respectively, for LT and Distribution HT lines. Lines  Stays/Guys and staying arrangement: Guys of stranded steel wire are used on all terminal, angle and other such poles where the conductors have a tendency to pull the pole away from its true vertical position. The guys are fastened to the poles near the load centre point with the help of pole clamps. The other end of the guy/stay is secured to a stay rod embedded in the ground. The stay rod should be located as far away as possible.  Earthing Knob The earthing knob is used for supporting the neutral-cum-earth wire used for earthing of metal parts of supporting structures of low-tension lines, i.e., 400/230 V lines. The knob is generally made of cast iron 52x42 mm and its electrical resistance is not to exceed 200 mega ohms. Moreover, the breaking strength at the neck of the knob is not to be less than 11,500 kg when force is applied.  Earthing Coil Two types of earthing arrangements are used. One is with GI pipe and the other is with GI wire. In case of GI pipe earthing, 40 mm dia and 2500 mm long pipe is used for earthing of supports and fittings. GI wire is used for earthing of lines. Generally 8 SWG wire with 115 turns, 50 mm dia and 1500 mm length is used.  Strain Hardware Set The conductor is strung between sections through a strain hardware set. It is fixed with the last disc of the string of disc insulator. It is made from malleable iron or aluminium alloy. Alloy hardware is preferable as the losses are less.  Conductors Conductor represents 30  50% of the installed cost of the line. All aluminium conductors (AAC), all aluminum alloy conductors (AAAC) and aluminium conductor steel reinforced (ACSR) are generally used. Technical specifications of conductors are covered in IS: 398. These conductors are of standard construction and the ultimate tensile strength of the whole conductor is based on the total strand strength.  Line Accessories This is the associated equipment required for fastening the conductors to supports and taking off the power or supply points such as joints material, clamps and compounds. For lines up to 33 kV, the following fittings are used:  Conductor dead-end fittings  LT conductor dead-end grips, 63 Operation and Maintenance  guy grips dead-end,  service grips,  full tension splices,  distribution ties,  side ties,  spool ties,  tee connectors,  lashing rods, and  line guards. Preformed fittings made of aluminium alloy are used as it saves cost, labour and time. It also eliminates chances of error of judgment. No tools are required. These fittings are fast and simple to apply and assure uniformity of application every time.  Joints should conform to IE Rule 75. For conductors up to 50mm2, crimped joints are made with simple hand crimping tools and for higher sizes, compression type or hydraulic type crimping tools are used. Joints are of the following types:  uni-joints/ compression joints,  twisting joints,  two part compression joints, and  dead-end joints.  Insulator ties secure the conductor to the insulator. In general, the tie wire should be the same kind of wire as the line wire, i.e., for tying aluminium conductors on insulators, aluminium wire should be used. The tie should be made of soft annealed wire so that it is not brittle and does not injure the line conductor.  Taps and jumpers are made by various accessories, which are not subjected to mechanical tension. Tapping should be taken off only at a point of line support.  Guard Wires Guard wires are to be used at all points where a line crosses a street, road or railway line, other power lines, telecommunication lines, canals, rivers, along the road and public places. As per IE Rule 88, guard wires of galvanized steel of minimum 4 mm dia having breaking strength not less than 635 kg should be used.  LT Line Spacers Very often clashing of LT conductors in the mid span takes place due to sag, wind and longer spans. This results in faults and interruptions. Spacers are provided to overcome this problem. You may like to revise the information given in this section before 64 studying further.  Substation Equipment and SAQ 3: Overhead lines Distribution Lines List the equipment being used for construction of overhead distribution lines in your utility. Describe the types of supports and insulators being used for construction of lines. ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… 5.4.2 Underground Power Cables Due to the fast growth in load densities in major towns and cities 66kv 33 kV, 11 kV and LT underground cables are being used to meet the ever growing demand of electric power. The underground cable system has attained considerable importance in distribution networks. This is because in towns and cities, almost all roads are already occupied by LT, HT overhead lines, telephone lines, street lights, advertising boards, etc., on either side of the roads. Further, high-rise buildings make it difficult to go for overhead systems for sub-transmission or distribution. Moreover, the overhead system with bare conductors is prone to frequent breakdowns causing interruption in power supply. Uninterrupted power supply can be maintained by employing underground cable ring system. The underground cabling system is particularly important for metropolitan cities, city centres, airports and defense services. Underground distribution costs are between 2 to 10 times that of the overhead system. Yet, it is preferred due to elimination of outages caused by abnormal weather conditions such as snow, rain, storms, lightning, fires, stress, accidents, etc. Moreover, this system is environment-friendly and has a long life. In addition, improved cable technology has reduced the maintenance cost of the underground system compared to the overhead system. We summarise the main reasons for underground cable systems in Box 5.1. Box 5.1: Reasons for Having Underground Cables The right of way for erecting overhead systems is no longer available; It is possible to extend the supply from source to load centres on any route profile; Fairly uninterrupted and reliable power supply can be maintained; and Aesthetic beauty of the town/city as a whole can be ensured. 65 Operation and Maintenance We now describe, in brief, the selection criteria, sizing, jointing and terminating of underground cables.  Selection of Cable The following factors influence the selection of cable:  load;  system voltage;  type of insulation;  short circuit rating;  mode of installation; and  economy and safety. For the same conductor size, the maximum continuous current carrying capacity depends on the depth of laying, ground temperature, silicon oil resistivity, ambient temperature, proximity of other cables, type of ducts used. Paper Insulated Lead Covered, PVC and XLPE cables are being used. Depending upon the voltage at which the power is transmitted or distributed, the cables are designed as follows: 1. EHV Cables 66 kV and above 2. Medium and HV Cables 3.3 kV to 33 KV 3. LT Cables up to 1100 V  Sizing of Cables The sizing of cables depends on the following factors:  current carrying capacity,  short circuit current,  voltage drop, and  losses.  Jointing and Terminations Cables are laid in lengths supplied over reels. Cable extensions are made through joints and terminated at the ends to connect them to the system for use. Since the cable consists of many items right from the conductors to the outer sheath, all joints are to be made as straight through joints so that each joint has the same features/characteristics of the original cable. Straight jointing is ensured by providing:  core continuity;  stress controlling screens;  insulation;  continuity of earth potential parts of the cable by clamping and running 66 the earth lead;  Substation  mechanical protection by installing the brass or aluminum covers; and Equipment and  Finishing over the mechanical protective cover. Distribution Lines Cable ends are terminated by providing:  stress control screens;  the earthing clamp lead, etc.;  insulation;  lugs; and  rain sheds. While making joints and terminations, it is essential to know the size and type of the cable in order to select appropriate kits for joints and terminations. The kits contain the accessories required along with instruction sheets for step-by-step procedure for making joints and terminations. The cable and end terminations should be prepared as per the dimensional drawing and procedure given in the instruction sheet. Types of Joints and Terminations The joint is considered to be the weakest link in the system but the overall reliability of a distribution system depends on it. Therefore, jointing accessories and techniques have an important and critical role despite their comparative low value in the overall investment. The following types of joints and terminations are used:  cast iron moulded,  epoxy resin type,  heat shrinkable,  cold shrinkable, and  ‘push on’ type. The heat shrinkable, cold shrinkable and ‘push on’ type joints and terminations do not need any setting time and can be taken into service immediately. SAQ 4: Underground cables List the reasons for using underground cables. State the selection criteria, sizing, jointing and terminating of underground cables. ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… So far, we have discussed the construction of the substation equipment and distribution lines. In the next two sections, you will learn about the general O&M practices for these components of the power distribution system. 67 Operation and Maintenance 5.5 O&M PRACTICES FOR SUBSTATION EQUIPMENT AND DISTRIBUTION LINES Planned maintenance schedules for various components of the power distribution systems are carefully drawn up by the power distribution utility even before the installation work is completed. During plant shut-downs for overall maintenance and before re-energisation, the sub-transmission and distribution plants are subjected to certain inspection and testing procedures. This also applies to the cable route that has been de-energised for a long period of time. Such planned shut down of the plant to be tested and network reconfiguration ensure continuity of supply to consumers while the testing takes place. The power distribution utility must formulate such planned outage schemes at different times of the year (depending upon the load demands) for different maintenance periods in such a fashion that consumer supply is least affected. NOTE This also involves putting in place a system for handling customer complaints Source: Special report about power supply breakdowns. on CEA website “Guidelines for Project Customer Relationship Management System Management and Performance Evaluation A trouble call management facility should be provided to attend to the power of Sub-transmission and supply interruptions promptly and to improve the reliability of power supply as Distribution Project”. well as minimise the down time. It should also attend to fuse off calls promptly as well as the complaints of the customer on quality of supply. A computer based facility provided in the substation/complaint attending centre would certainly improve this aspect of O&M. We now describe some general maintenance practices for the substation equipment and distribution lines. 5.5.1 General Maintenance Practices There are two aspects of general maintenance: ❖ Firstly, replacement of the parts that are worn out during the normal operation must be carried out from time to time. ❖ Secondly, preventive maintenance should be carried out for detecting deterioration and mal-operation of the system components. In the daily operation of the substation it is the duty of the attendant to inspect the equipment externally and remedy any abnormality that does not require disconnection of the apparatus. During this inspection, a watch is required to be kept for deposits of dust and dirt on the equipment, heating of contacts, joint or some part, low oil level and oil leakages, etc. Checks should also be made to ensure that  the locks and doors of the switch house are in good condition, 68  no leaks have developed in the roof,  Substation  the ventilating and heating systems are operating normally, Equipment and Distribution  the prescribed safety aids are in place and in good order, Lines  the earthing connections remain unbroken,  the packing of the cables entering or leaving a cable trench or tunnel within the premises are intact,  the ventilating louvers are not damaged, and  the access roads leading to the oil filled apparatus are unobstructed, and will allow the approach of the fire engines in the event of an oil fire during an emergency. On-line inspection and testing is normally limited to visual, external and physical examination in order to ensure that the plant is in a safe condition. Infra-red detectors must be used periodically for inspection of overhead lines and open terminal, substation bus-bars for hot spots caused by faulty terminals. In addition, live line washing techniques are also available for cleaning overhead lines or open terminal substation insulators. Purified water with a high resistance value is used in a fine spray fitted from well-earthed nozzle. Functional testing and trip schemes require special switching arrangements initially to reconfigure the power system network. Switchgear site tests during operational maintenance stage vary from utility to utility depending upon the quality of upkeep of the equipment and environmental conditions of the site. These generally involve the following checks and tests:  General checks include inspection and checking of  the tightness of terminal connection, piping junctions and bolted joints;  painting and corrosion protection;  cleanliness;  cracking and chipping of bushings;  foundation bolts; and  Lubrication of contacts and moving parts of the circuit breakers.  Electrical circuit checks include checking of  insulation check;  dielectric strength of the insulating oil;  level of the oil;  quality of SF6gas/ insulating medium such as humidity content, filling pressure or density except for sealed apparatus;  leakage of oil, etc.  Mechanical tests include  inspection of operating circuits (hydraulic, pneumatic, spring charged) and consumption during operation; 69 Operation and Maintenance  verification of correct rated operating sequence (recharging, etc).  Time checks include checking and adjustment of  track alignment and interlocking mechanism;  closing and opening times;  operation and control of auxiliary circuits;  recharging time of operating mechanism after specified sequence; and  other specific operations.  Electric tests include  dielectric tests; and  testing of the resistance of main circuit. If the substation is constantly attended, the rounds of switchgear are usually planned for each shift so that all the equipment will be looked at least once a day. Equipment is also inspected immediately after a trip out. Substation switchgear requires regular cleaning in accordance with its design, type of insulation, the degree of pollution of the atmosphere or ambient air, etc. The frequency of cleaning depends upon the type of layout of the apparatus and insulators. However, cleaning must be done during each preventive maintenance activity. Even though the vacuum switchgear does not require elaborate maintenance like the oil insulated switchgear, it is still necessary to make periodic routine inspection. The absence of ionized gas and carbon during interruption removes the major source of insulation contamination. 5.5.2 Maintenance of Lines Pre-monsoon inspection of all 66 ,33 kVand 11kV lines should be completed between January and March every year after obtaining due approvals for pre- arranged shut downs for the entire programme. The staff responsible for the pre-monsoon inspection should carry all the necessary equipment such as ropes, petroleum jelly, cotton waste and sufficient O&M materials like insulators, discs, nuts for the pins, binding wire, etc. In the routine maintenance practices, all the tree clearances are done and all the minor defects like damaged insulators, improper pin binding, loose jumpering and loose stays are rectified during the inspection itself. All the insulators are cleaned, all AB switches are lubricated and defective blades replaced. The defects that may take considerable time for rectification are noted down and attended within the next one week. Examples are insertion of poles, replacement of damaged conductors, replacement of damaged supports, etc. Periodical patrolling of 66, 33 kV and 11 kV lines has to be done on a monthly 70 basis. The  Substation patrolling is also done and suspected defects rectified, whenever the line trips Equipment and on fault. One of the major precautions to be kept in mind by the maintenance Distribution staff is to take the permit to work or line clear to work on distribution lines. Lines Procedure for Permit to Work (Line Clear) A line clear or a permit to work (PTW) on any electrical equipment or line is issued by an authorised person to another authorised person. If there are more than one gangs working under the same supervisor, each gang takes sub-line clears from the supervisor who has taken the line clear. In case, the line clear has to be issued for the supervisor, s/he takes self line clear. In this case also, all the precautions that are to be followed in issue and return of line clear are followed. Line clear books are very important records. Pages in these books are serially numbered and no paper from this book is used for any other purpose. If any page is to be destroyed, the custodian specifically mentions the reasons for doing so. It is attested by his/her dated signature. The line clear books are reviewed periodically by the Competent Authority. Line clear can be issued/received over telephone. It is desirable that the issuer/receiver recognise each other’s voice. The requisition for line clear and the line clear issue messages are repeated by both the parties to ensure that line clears are issued/received on the equipment on which it is intended. A secret code number is followed in such cases. You may like to revisit Units 6 and 7 of the course BEE-002 for the details. 5.5.3 Operation and Maintenance of Capacitors A routine check of the capacitor performance is made by measuring current with the help of Ammeter/Tong tester once in two months and the record is maintained. If any reduction in current /failure of capacitor is noticed, supplier/ manufacturers must be contacted immediately and replacement of capacitor initiated. The status of the capacitor is determined by the voltage at the highest voltage bus available at the substation. It is subject to the maximum permissible voltage at the bus on which the capacitor bank is connected and the loading factor. The loading factor is the ratio of the total MVA load on the bus at which the capacitor is installed to the MVAR rating of the capacitor. Accordingly, the switching on/off of the capacitor bank is done as per Table 5.1. Table 5.1: Voltage of Highest Level at the Substation Voltage of Highest Level at the Substation (kV) System Voltage Above Between Below For 220 kV level 230 230 - 220 220- 215 215- 205 205 For 132 kV level 140 140 - 130 132 - 128 128 - 122 122 For 66 kV level 70 70 - 68 68 - 65 65 - 60 60 For 33 kV level 35 35 - 34 34 - 32 32 - 30 30 71 Operation and Maintenance The loading factor and the status of capacitor switch are given in Table 5.2. Table 5.2: Loading Factor and the Status of Capacitor Switch Loading Factor Status of Capacitor Switch Above 2 Off Status-Quo On On On Between 1 to 2 Off Off Status-Quo On On Below 1 Off Off Off Status-Quo On LV bus voltage is controlled by changing transformer taps. Notwithstanding the above, if the voltage at the bus on which capacitor is connected is 1.1 per unit or higher, the capacitor is switched off. 5.5.4 Hot Line Maintenance Work performed on transmission and distribution lines while they are energized and in service is called hot line maintenance. Hot line tools are all types of tools mounted on insulated poles used to maintain energized high voltage lines and other safety equipment. Insulated disconnect stick, wire-holding stick, auxiliary arm, cross-arm mount, pole mount, wire tong, saddles, flexible line hose and hoist link stick are some of the hot line tools in use. When working with energized power lines, linemen must use protection to eliminate any contact with the energized line. Some distribution-level voltages can be worked using rubber gloves. The limit of how high a voltage can be worked using rubber gloves varies from company to company according to different safety standards and local laws. You may like to refer to Units 6 and 7, Block 2 (BEE-002) for more information. Fig. 5.13: Hot Line Maintenance 72  Substation Voltages higher than those (which can be worked using gloves) are worked Equipment and with special sticks known as hot-line tools, with which power lines can be Distribution safely handled from a distance. Linemen must also wear special rubber Lines insulating gear when working with live wires to protect against any accidental contact with the wire. The buckets from which linemen sometimes work are also insulated using rubber. For high voltage and extra-high voltage transmission lines, specially trained personnel use so-called “live-line” techniques to allow hands-on contact with energized equipment. In this case, the worker is electrically connected to the high voltage line so that he is at the same electrical potential. The lineman wears special conductive clothing which is connected to the live power line, at an instant such that the line and the lineman are at the same potential allowing the lineman to handle the wire safely. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are the objects of live-line maintenance practices. SAQ 5: Hot line maintenance a) At which line voltages do personnel in your company carry out the maintenance work using i) rubber gloves, and ii) hot-line tools? b) Explain the live-line maintenance technique. ………………………………………………………………………………… ………………………………………………………………………………….. ………………………………………………………………………………….. 5.6 LENGTH OF LT LINES, HT:LT RATIO AND IMPACT ON LOSSES AND VOLTAGE The ratio of primary line length to its concerned secondary distribution line length is one of the important factors that influence the performance of distribution system. Over the years, large scale expansion of the urban system and rural electrification programme in the country has resulted in considerable expansion of Low Tension (LT) distribution network. The size of the distribution transformers has been constantly increasing to meet the increasing demand due to load growth. As a result, the length of LT lines/circuits is also increasing resulting in high losses in LT lines, excessive voltage drops, frequent faults on LT network and higher rate of failure of distribution transformers. This has also resulted in very large length of LT lines as compared to High Tension (HT) lines resulting in high LT/ HT ratios. The ratio of LT to HT lines in our country has been of the order of 3. This results in high losses and low voltages at the consumer end. 73 Operation and Maintenance 5.6.1 Impact of Increasing HT Lines Increasing HT lines can help in reducing both line losses and voltage drops. Reduction in Line Losses In the low voltage distribution system, supply at low voltages with long LT lines using smaller conductor sizes causes high line losses. However, the loss in HV system for the distribution of the same power is less than 1% of the LV system. Hence, with HV system the total energy losses are considerably reduced. Reduction in Voltage Drops The voltage drop in LV lines is very high as the lines are long and have smaller conductor sizes. In HV distribution systems, the voltage drop for the distribution of same quantum of power is less than 1% as against that in low voltage distribution system. This ensures proper voltage profile at the consumer end. All other parameters, like load factor, power factor, etc., remaining the same, the percentage losses in a system having higher LT/HT ratio will be higher than in a system having lower LT/HT ratio. A ratio of 1 to 1.2 would be very beneficial for power distribution. As this measure is a must to improve efficiency and voltage regulation of distribution, additional capital investment should not come in the way. With this discussion on the impact of increasing HT lines on reduction in line losses and voltage drops, we now end the unit and summarise its contents. 5.7 SUMMARY  In the overall power development scenario, the Transmission and Distribution system constitutes the essential link between power generating sources and the ultimate consumers and substations and lines have to be erected for providing quality power supply.  The main equipment used in a substation comprises structures, transformers, bus-bars, circuit breakers, isolators, earthing switches, lightning arrestors, substation batteries, fire extinguishing equipment, etc. Overhead distribution lines and underground cables (in urban areas) carry power to the end-user.  There are two aspects of general maintenance: replacement of parts that are worn out from time to time and preventive maintenance for detecting deterioration and mal-operation of the system components. Periodic checks and tests should be carried as per specified procedures, which may vary from utility to utility depending upon the site conditions.  Special hot line maintenance techniques and tools are required for carrying out maintenance on live lines.  Due to increasing LT lines in the distribution system, losses, excessive 74 voltage drops and frequent faults have resulted in the LT network leading to  Substation a higher rate of failure of distribution transformers. The high LT/HT ratios Equipment and result in high losses and low voltages at the consumer end. An LT/HT ratio Distribution of 1 or 1.2 is preferable. Lines 5.8 TERMINAL QUESTIONS 1. Describe the equipment required for the construction of a 66-33/11 kV substation. 2. Describe the equipment required for the construction of a 11/0.4 kV distribution substation. 3. What equipment is required for the construction of an overhead distribution line? 4. Distinguish between current and voltage transformers. 5. List the different types of underground cables in use today. What criteria are used for the selection of these cables? 6. State the precautions that need to be taken in jointing and terminating underground cables. 7. Give reasons why underground cabling is being opted for in urban areas. What are its advantages? 8. Explain hot line maintenance techniques and tools. 9. Explain the impact of LT/HT ratio on losses and voltage. 75 Operation and Maintenance 76