Power Supply Installation - 75-100 PDF

DESIGN ASPECTS OF TRACTION SUBSTATION 4.O SPACING AND LOCATION 4.01 The sub-station spacings largely depends upon the permissible voltage drop at the farthest end, which in turn depends upon various factors such as the traffic to be moved, anticipated traffic in the future and gradients of the se...

DESIGN ASPECTS OF TRACTION SUBSTATION 4.O SPACING AND LOCATION 4.01 The sub-station spacings largely depends upon the permissible voltage drop at the farthest end, which in turn depends upon various factors such as the traffic to be moved, anticipated traffic in the future and gradients of the section to be electrified. The voltage drop at the farthest end is calculated both for normal and extended feed condtions on the basis of given combination of trains on UP & DOWN tracks, loads and specified speeds, track parameters of the section on the assumed length of the feed zone. The calculations are repeated for different assumed lengths of feed zone and it is ensured that the voltage at the farthest end is within the permissible limits. 4.02 Alternatively, the power requirement is calculated on the basis of average specific energy consumption of goods and passenger trains Following figures of specific energy consumption are taken for level or lightly graded sections. (a) Goods train - 11 kwh. 1000 GTKM (b) Pass train - 19 kwh. 1000 GTKM For Medium or heavily graded sectins, the specific enery consumption figures have to be based on trials. The power requirement is calculated by the formula as given below :- Q x (2Lo) x W 60 1 P = X X KVA 1000 H cosø Where, 2 X Lo = Sub-Stn. covering area for double track line(km) W = Weight of a train in tones Q = Energy consumption rate (kwh/1000 GTKM) H = Headway in minutes during peak time (assumed) COSø = P.F. (lagging) Conversely, using the above formula ‘Lo’ can be calculated for a given rating of traction transformer. Typical calculation based on this formula is shown below - 72 SPECIFIC ENERGY CONSUMPTION Considering a double track section and specific energy consumption 11 kWH / 1000 GTKM for Goods Train in the Loaded Directions and 19 kwh/1000 GTKM for passenger trains in the empty direction W = (4570 + 2 X 120) t For 4500t Loaded Train W = (1100 + 113)t For 22 Coaches Passenger Train H = 18 Minute Headway 2Lo = 45 KM Hourly maximum output – Q x (2Lo) x W 60 1 P = X X = kVA 1000 H cos∅ 11 x 45 x 4810 + 19 x 45 x 1213 60 1 = X X kVA 1000 18 0.8 = 14242 KVA MAKING AN ALLOWANCE OF 25% FOR EXTENDED FEED CONDITIONS POWER REQUIREMENT = 14242 x 1.25 kVA = 17803 kVA. 4.0.3 In Planning the requirement of traction sub-station and its location on any section for track electrification, the factor to be kept in mind may be summerised as given below :- Availability of adequate and reliable power supply lines. The transmission lines should be as close as possible to the Rly lines. Willingness of electric supply authorities to extend their HV transmission lines to feed the railway traction loads. Settlement of tariff rates. Traffic to be handled in the section. Gradients of the section. Anticipated traffic in the future. Single or double line section. 73  Characteristics of the locomotive and speed etc. Allowable permissible voltage drop at the farthest end. Strength of the system to permit the voltage and current unbalance caused by the traction single phase loads. Suitability of standard equipments. The load is within the standard ratings of the transformer and other equipment under normal and extended feed conditions. Availability of reasonably good levelled land as near to Rly track as possible. Location should be away from the dumping yards. Location of sub-station should not be less than 3 km from the airport. Provision of siding track for loading and unloading of heavy equipments. Location should be close to main Rly. station where inspection staff can reach the spot in the shortest time 4.0.4 On an average the spacing between the successive sub-station as adopted in earlier electrification schemes was about 50 to 80 km, but with the interlocution of heavy haul trains and increased passenger and goods traffic the spacing has been reduced to 40 to 60 km. only. On high density routes it may reduce further by converting existing SP into TSS and SSP into SP. 4.1 TRACTION POWER SUPPLY SYSTEM 4.1.1 Before going into details of design aspects of various substation equipments, we may briefly discuss the power supply system adopted for feeding the traction substations. 4.1.2 Indian Rlys. purchase electric power from various state electricity boards and as well as from other electric utilities through their regional grids at different voltage, normally 220/132/110/66 kV. The incoming supply is stepped down to 25 kv. a.c. with the help of step down transformer. The primary winding of the transformer is connected across two phases of the three phase effectively earthed system and one terminal of the 25 kV. secondary winding is connected to the overhead equipment (OHE) and other terminal of the 25 kV. secondary winding is solidly earthed and connected to the running rails. The load current flows through the OHE to the locomotive and return through the rails and earth to the traction sub-station. The substations are provided as close to the railway traction as possible at intervals varying from 40 to 60 km depending upon the traffic density and track conditions. In the initial stages of the AC electrification schemes, traction 74 substations were owned and maintained by electric supply authorities. But later on in the late sixties Indian Railways started purchasing bulk power at 220 or 132 or 110 or 66 kV. at a single paint and run their own transmission lines and installed, operated and maintained their own substations. 4.1.3 In addition to two transformer circuit breakers, which are provided each on primary and secondary side of the traction transformer, the output from the transformer is fed to the overhead equipment on one side of the substation through a feeder circuit breakers and two interrupters provided at each line. The transformer breaker acts as a back up to the feeder breaker. The feeder breaker perform the usual duties of breaking the circuit under the normal and abnormal conditions according to situation. The interrupter is also a type of circuit breaker, but it is non-automatic i.e. it is not called upon to trip under fault conditions. It is capable to carry the normal rated current and through fault currents. It performs the duty of breaking the load current and is also called a load switch. All the breakers and interrupters are outdoor type and remote controlled from Central Control Room. generally situated at the Railways Divisional Headquarters. 4.1.4 Typical layout of traction substation is shown in figure 1.01 Each traction substation is provided with two transformers. Only one transformer feeds the traction over head equipment on either side of traction substation through the two feeder circuit breakers. For protection, in all six circuit breakers are provided at each traction substation out of which two are installed on the primary side and two on the secondary side of the transformer. These breakers are known as transformer breakers and act as back up protection to the feeder circuit breakers. Two feeder circuit breakers control the supply to the overhead equipment. In the event of any fault on the OHE, the feeder circuit breaker will trip and clear the fault. The interrupter, load switch controls supply for each track. 4.1.5 Approximately midway between two adjacent substations, a dead zone known as ‘neutral section’ or phase break is provided to separate two different phases. The section between the substation and the neutral section is called sector which is further subdivided into subsectors by a set of interrupters located at subsectioning posts situated at intervals of 10 to 15 km. To reduce the voltage drop along the line, both the lines in a double track section are paralleled at each subsectioning post and sectioning post with the help of a paralleling interrptor at each post. At each sectioning post, a bridging interuptor with an under voltage relay is provided at each line which enables the extension of feed from a substation to the section fed by an adjacent substation, in case of an emergency caused by failure of the adjacent substation. 4.2 LAYOUT OF SUB-STATION Once the locations of the traction substation, is decided based on the 75 considerations enlisted in para 4.0.3 the next step to be followed is the design of the substation. The layout of the traction substation is influenced by the type and orientation of transmission lines with respect to tracks to be fed i.e. whether the feeding transmission lines are parallel to the track or at right angle to the track. Based on these various combinations of transmission lines and tracks, ten different types of substation layouts have been standardised by RDSO. These layout plans have been developed after giving due consideration to the following points. i) Physical and Electrical clearances between different equipments. ii) Phase to phase clearances. iii) Phase to ground clearances iv) Sectional Clearances. A typical layout plan and cross sectional view of 220/132/110/66 kV. traction substation showing position of all the equipments is shown in the Fig. 1.01. 4.3 TRACTION POWER TRANSFORMER 4.3.1 Traction power transformer is the most important and costly equipment of the substation Therefore, utmost care is taken while designing and selecting the parameters of the traction power transformers In the initial stages of the electrification, the transformers used were of 7.3 and 10 MVA ratings, but later on transformers of 13.5 MVA were used in most of the electrification projects. The readings of the transformer are standardised on the basis of average spacing between the traction substations, loads to be hauled and gradients of the section to be electrified 4.3.2 With the introduction of heavy haul trains and increased passenger and goods traffic the transformer of 13.5 MVA rating are not adequate to meet the load requirements. Therefore in the ongoing electrification schemes transformers with 20 MVA are being used and on high traffic density routes transformers of 30 MVA rating are being tried. 4.3.3 The duties performed by the traction power transformers are very much different from the conventional distribution transformers. The traction transformers are subjected to peaky loads, rapid load variations and frequent short circuits. Therefore windings of these transformers are specially reinforced to withstand the high stresses developed due to the following service conditions. i) Repeated short circuits. ii) Frequent load variations. iii) Frequent variation in supply voltage. iv) Magnetizing in rush current due to repeated switching ‘ON’ of the 76  transformers from ‘OFF’ position. v) Overloading of the transformers as specified. 4.3.4 The traction transformers experience frequent short circuits due to various reasons such as bird dropping a wire across an insulator, insulator failure, flashover of insulators, switching surges, accidents, activities of miscreants i.e. theft of wires entering of loco in dead section, flashover of wire under overline structure where clearances are restricted and mechanical failure of OHE fittings etc. To limit the magnitude of fault current the percentage impedance for traction transformer has been specified as (12+0.5)%. It shall not be less than 11.5% and not more than 12.5% at any tap position. 4.3.5 Technical particulars of 20 MVA, traction power transformer are as under :- i) Type ONAN cooled, single phase step down power transformer, double limb wound core type for outdoor installation. The transformers are designed to keep provision of forced cooling at a future date without requiring any modification. ii) Windings Concentric, Disc/interleaned for primary and secondary windings. Both windings are uniformly insulated. iii) Rated frequency 50Hz (±3%) iv) Rated primary voltage 220 kV, 132 kV, 110 kV, or 66 kV (As the case may be) v) Nominal secondary 25 kV Voltage vi) Rated current Primary 151.5 A (for 132 KVø Secondary 740.7A) vii) Percentage impedance (12 ± 0.5)% Voltage viii) Rated MVA at rated 20 MVA Secondary voltage ix) Overload capacity 50% for 15 minutes & 100% for 5 Minutes x) Tapping off-circuit +10% to -15% on low voltage side in steps of 5% xi) No-load losses 12.5 kW 77  xii) Load losses 135.0 kW xiii) Current density Less than 2 Amp. per Sq.mm xiv) Temperature-rise limits a) Windings – 50ºC (Temp. rise measured by resistance method) b) Insulation oil – 40ºC (Temp. rise measured by the thermometer method) c) For current – 35ºC (Temp. rise measured by carrying parts in air thermometer method) 4.4 CIRCUIT BREAKERS 4.4.1 Next to transformer, the other important equipment at any substation is the circuit breaker. Circuit breakers play an important role in the control and performance of a power supply system. From consideration of cost aspect also the circuit breakers constitute as a major item. Power circuit breakers are designed not only to carry the rated normal currents continuously but isolate the faulty section of the system under all normal and abnormal condition, and shall also be capable of interrupting load currents, capacitive and small inductive currents. They must be capable of clearing terminal and short line fault and shall also be capable to operate reliably under all ambient temperatures, under severe polluted conditions and at high attitudes. Voltages induced in the system due to switching operations shall be minimum. 4.4.2 Different types of circuit breakers use different types of quenching medium like oil, compressed air, SF6 gas and vaccum bottles. Till recently Indian Railways were using oil circuit breakers for control of traction power supply system. In the initial stages of a.c. electrification these breakers were imported but later on these were procured from reputed indigenous manufacturers. The maintenance cost of oil circuit breakers particularly when used as feeder circuit breaker, various from Rs.3000/- to Rs.40,000/- per annum depending upon the number of trippings. Experience has shown that at some of the traction. substation oil 78 circuit breakers are just not able to meet the required duty due to very high cost of maintenance. Just to have a idea about the number of trippings a statement showing month wise number of trippings for the year 1985 for one of the traction substation (N Rly) are shown below :- Statement showing monthwise No. of trippings for year 1985 Northern Railway (Chanakyapuri Substation) Month Feeder No. CB-61 Feeder No CB-62 January 67 95 February 44 12 March 59 20 April 71 27 May 99 41 June 139 53 July 108 56 August 81 48 September 86 45 October 104 29 November 96 32 December 84 48 Total : 1038 507 4.4.3 To minimise the rising maintenance cost and to keep the down time of the breaker to bare minimum, Indian Railways have adopted switchgear based on modern technology i.e. SF6 gas and vaccum. In all future electrification schemes, so far as 25 kV feeder circuit breakers are concerned, will be either of SF6 gas type or vacuum type. 4.4.4 Rated short circuit breaking current (for HV breakers) Rated short circuit breaking current depends on the three phase short circuit level of the system. Short circuit levels at present for the different voltages varies between 1000 MVA to 10,000 MVA depending on the proximity of the generating station. Based on the short circuit levels the rated circuit breaking current values are as under. 79  item Nominal System Voltage 66kV 110kV 132kV 220kV Max fault MVA 3500MVA 5000MVA 5000MVA 10000MVA Fault current 30.62kA 26.24kV 21.86kA 26.24kA Rated short circuit breaking current as assigned to the breaker kA 31.5kA 31.5kA 25kA 31.5kA The method of calculating the fault current both for primary and secondary side of the substation is shown below - 220 kV bus incoming (fault level - 10000 MVA) For 20 MVA 220/25 kV power transformer FAULT CURRENT = FAULT MVA 10000 = = 26.24 kA. on primary side 3 X VOLT IN kV 3 X 220 IN CASE OF SINGLE TRANSFORMER - SYSTEM REACTANCE UPTO 220 kV BUSBAR (AT 100 MVA ASSUMED BASE) CHOSEN BASE IN MVA X 100 100 X 100 Xs = = = 1% FALUT LEVEL 10000 THE CORRESPONDING PERCENTAGE REACTANCE OF TRANSFORMER AT 100 MVA BASE (Assuming short circuit Impedence of tranformer of 20 MVA to be 12%) 220 kV BUS 100 X 12 3 1% Xt = = 60% 20 3 60% 25 kV BUS THEREFORE TOTAL REACTANCE = 1 % + 60 % = 61 % FAULT LEVEL ON 25 kV BUS JUST OPPOSITE THE SUB-STATION(S/S) = 100 x 100 = 163.93 MVA 61 163.93 FAULT CURRENT JUST OPPOSITE THE S/S = = 6.557 kA 25 80  INCASE OF TWO TRANSFORMERS RUNNING IN PARALLEL - TOTAL REACTANCE = 1% + 30% = 31% 220 KV BUS 3 1% 100 X 100 FAULT LEVEL = = 322.58 MVA 60% 31 3 3 60% 25 KV BUS 322.58 FAULT CURRENT = = 12.90 KA. 25 It will be seen from the above that though the fault current is less but rated short circuit braking current value assigned to the breaker is higher. This is because IEC, IS and BS specification have standardised the rated short circuit breaking current are as given below :- 6.0 kA, 8.0 kA, 12.5 kA, 16 kA, 20 kA, 25 kA, 31.5 kA, 40 kA, 50 kA, 63 kA, 80kA, and 100 kA. Therefore, as a part of standardisation, the recommeded values of rated short circuit breaking current have been adopted Recommended values of rated short circuit current and normal rated current are shown as under :- CO-ORDINATION TABLE OF RATED VALUES FOR CIRCUIT BREAKERS Rated Rated Short Rated Normal Current Voltage circuit Breaking (Amps) (kV) Current (kA) 12.5 800 1250 16 800 1250 72.5 20 1250 1600 2000 31.5 1250 1600 2000 12.5 800 1250 20 1250 1600 2000 123 25 1250 1600 2000 40 1600 2000 12.5 800 1250 20 1250 1600 2000 145 25 1250 1600 2000 31.5 1250 1600 2000 40 1600 2000 50 2000 81  20 1250 1600 2000 245 31.5 1250 1600 2000 40 1600 2000 50 1600 2000 Value of rated short circuit braking current (kA) 6.0, 8.0, 10.0, 12.5, 16.0, 20.0, 25.0, 31.5, 40.0, 50.0, 63.0, 80.0 and 100 kA Value of rated normal current (Amps) - 400, 630, 800, 1250, 1600, 2000, 2500, 3150 and 4000 Amps 4.4.5 Normal rated current As indicated in para 4.3.5 the normal rated current of the primary winding of 20 MVA transformer is 151.5 Amps only for 132 kV system Since these tranformers are designed to deliver 100% overload for 5 minutes, therefore Primary current is likely to reach the value of 303 Amps. But the assigned value of normal rated current for HV circuit Breker is 1250 Amps for 66 kV. 110 kV, 132 and 220 kV system. This is again because IEC, IS and BS standards have standerdised the normal rated current values to match the value of rated short circuit breaking current. The IEC and IE recom mendations are as shown below :- 630A, 800A, 1250A, 1600A, 2000A, 2500A, 3150A & 4000A. 4.4.6 Thus it can be concluded that while deciding the ratings of the breakers used on the primary side, the rated short circuit breaking current is of importance and deciding factor and not the rated normal current which is generally much less. But for the breakers on the secondary side the situation is just the reverse while the deciding factor for the selection of the circuit breaker is the normal rated current and not the fault current. 4.4.7 The salient technical particulars as adopted for 220 KV and 25 KV breakers are as indicated below :- S.N. Pariculars Nominal system Voltage 200 kV 25 kV i) No. of poles 2 or 3 as reqd. 1 ii) Nominal system voltage 220 kV 25 kV iii) Highest system voltage 245 kV 52 kV class iv) Rated one minute power 460 kV (rms) 105 kV frequency withstand voltage v) Rated impulse (1.2/50 1050 kV (peak) 250kV (peak) micro-second) withstand voltage vi) Rated normal current 1250 A 1600 A 82  vii) Rated short circuit 31.5 kA 20 kA breaking current viii) Rated breaking capacity (Symmetrical) a) Two Pole 7717.5 MVA 450 MVA b) Three pole 13366.7 MVA ix) Rated making current 78.8 kA 50 kA x) Rated operating sequence 0-0.3 sec 0-0.3 sec CO-3 sec. - CO 30 sec -CO xi) Total breaking time Not more than 80 ms. Not more than 65 ms xii) Rated short time 31.5 kA for 20 kA for withstand current one second 3 seconds 4.5 25 KV INTERRUPTORS 4.5.1 The interruptors ar non-automatic circuit breakers which are provided at feeding posts and switching stations. Inter rupters ar not required to clear the fault, except for the one which is provided at the sectioning post as bridging interruptor. This interruptor is called upon to clear the fault under extended feed conditions. The breaking capacity of the interruptor has been specified as 4000 Amps at a recovery voltage of 27.5 kV and a short time current with stand capacity of 4000 Amps. for 3 seconds. But now with the use of higher rating traction transformers, the breaking capacity and normal rated current has been incrased to 8 kA and 800 Amps respectively in the RDSO’s new specification. 4.6 ISOLATING SWITCHES 4.6.1 When carrying, out inspection or repair on sub-station equipments, it is essential to disconnect reliably the unit or the section, on which the work is to be done, from all other live parts on the installation in order to ensure complete safety of the working staff to guard against mistakes, it is desirable that this should be done by an apparatus which makes a visible break in the circuit. Such an apparatus is called the isolating switch or isolator. Isolators are used to open or close the circuit either when negligible current is flowing or when no current is flowing through the circuit. These are also called as off load switches. The location of the isolating switch is decided in the sub-station on the basis of scheme of bus bar connections. Generally on either side of the circuit breakers, isolators are provided for attending to maintenance work etc. 4.6.2 Two types of isolating switches have been used in traction substation. On the primary side i.e. HV side. Isolators used are either of two pole or three pole according to number of poles. From constructional point of view these may be divided as. 83  i) Three post, centre post rotating double break type and ii) Two post single break type. These isolators are of horizontal break type on the secondary side i.e. 25 KV side, the isolators used are of vertical break type. The rated normal current of these isolators is fixed to match the rated current of circuit breakers and bus bars etc. The standard values of rated normal current as specified in the IEC and IS specifications are the same as shown in para 5.5 for breakers. 4.7 CURRENT & POTENTIAL TRANSFORMERS 4.7.1 Bushing type CTs have been provided on primary and secondary side of the traction transformers and are exclusively meant for differential protection. Separately mounted 132 KV and 25 KV CT and 25 KV PTs are of conventional type. 4.8 BUS BARS 4.8.1 Two types of bus bar have been used for traction substation viz. strung type bus bar on the HV side and rigid type bus bar on LV side i.e. 25 KV side. The strung bus used in the earlier electrification schemes was consisting of All Aluminium ‘spider’ conductor of size 19/3 99 mm. The capacity of this conductor is adequate only to meet the maximum fault current of above 17KA, but due to increase in the fault/level of grid system, the value of fault current has exceeded 25KA. Therefore, to meet the higher requirement of fault current in the system, use of ‘Zebra’ Acsr conductor has been adopted in the ongoing electrification schemes, (size 61/3.18, dia 28.62 mm). This conductor is capable to withstand fault currents of the order of 31.5 KA. The tension in the strung bus is kept between 500 to 900 kgf. To keep corona losses within limits, the minimum diameter of conductors (strung bus) and jumpers shall not be less than 28 mm in case of 220 KV system. 4.8.2 Similarly the rigid type bus used in earlier electrification schemes was of Aluminium Alloy of 36 mm O.D. (36/28 mm). This bus was capable to carry the normal rated current of 960 Amps. But with the use of 20 MVA, transformers the requirement of rated normal current has increased to 1500 Amps (for short duration). Therefore, the size of the rigid bus bar has also been increased to 50 mm O.D. (50/39 mm). This bus is capable to carry continuos current of 1530 Amps. The rigid type bus is supported on the support insulators provided at a distance not exceeding 3 m. The minimum height of the 25 KV bus bar has been kept as 3800 mm. Chances of failure of rigid bus are very remote, but its installation is costlier than the strung bus. 4.9 LIGHTNING ARRESTERS 4.9.1 Lightning arrestors ar also called surge diverters. The primary purpose of a lightning arrester is to protect the system from getting damaged by the over-voltages caused due to lightning strokes and switching surges. Lightning arresters absorb the energy and reduce the over voltage in the system. The ideal arrester is one which draws negligible current at operating voltage by offering very high impedance and negligible impedance 84 during flow of current. The breakdown voltage of LA should be kept much lower than that of the other equipments in the substation, but should not be so low as to cause a power frequency flashover due due to variation of the supply voltage or normal switching surges. While selecting a lighting arrester the following points are generally considered. i) Maximum line to line voltage ii) The rated discharge current iii) The power frequency flashover voltage iv) Impulse flashover voltage v) Residual discharge voltage 4.9.2 Types of lightning arresters 4.9.2.1 There are four general types of lightning arresters, namely :- The rod gap type, Expulsion type, Conventional value type and Zinc oxide gapless lightning arresters. The rod gap and expulsion type lightning arrester do not provide the required level of protection. The conventional value type LAs have become complicated due to use of large number of components forming ionising system, trigger systems grading net work, interrupting and are stretching system. Zinc oxide gapless lighting arrecters are of new generation and these LA’s have the following advantages over the types of LAs i) Matching and controlled protective level ii) Faster response due to elimination of series gap iii) Energy is absorbed and hence over voltages are reduced. iv) Light and rugged construction. v) Improved thermal characteristics. vi) Better performance under polluted conditions. vii) Better pressure relief performance. viii) Better sealing arrangement. 4.9.3 The Location of lighting arrester and precaustions to be taken in its installation. 4.9.3.1 The lighting arrester shall be installed very close to the apparatus to be protected. A practical rule is that the distance should not be more than 10m. In case of big transformers LA is installed immediately after the busing. In case of the incoming Transmission line is more than 4 km. long then LA’s shall also be provided at the entry of the S/S. The connections should be solid and direct. Earth connections should be of ample cross section to carry the rated discharge current & the earthing terminal of the arrester and that of the other electrical equipments shall be connected together to the main earthing bus. 4.10 CLEARANCES To ensure satisfactory and reliable performance of any electrical net work, it is essential to provide adequate electrical clearances Electrical clearance is defined as the minimum distance required between live parts and earthed material (earth clearance) or 85 between live parts and different potentials (phase clearance) in order to prevent flashovers. Safety clearance also called as sectional clearance is defined as the minimum distance required between unscreened live conductor and the limits of a work section. Safety clearances are required for safety of personnel in inspection, operation and maintenance. Minimum electrical clearances for outdoor switchgear as stipulated in IS 3072. Min clearance Minimum from any point Impulse Minimum where they may be Voltage clearance withstand ciearanee reauired to stand rating between to the nearest level to earth phases unscreened conductors air(section safety clearance) kV (rms) kV (Peak) (mm) (mm) (mm) 72.5 325 630 750 3230 350 675 810 3270 123 450 920 1065 3520 550 1150 1350 3750 145 550 1150 1350 3750 650 1380 1600 3980 245 875 1800 2000 4400 900 1900 2300 4550 1050 2300 2700 4900 Figures underlined have been adopted in RDSO’s specifications. 4.11 EARTHING 4.11.1 Earthing system is of utmost importance for the purpose of protection in both electricity supply and utilisation. The primary need of earthing is that in the event of fault sufficient current shall flow through the fault path so as to operated the protective gear and preventing dangerous potential rise on parts of electrical equipments that are not alive. It is therefor, essential that earthing system shall have sufficient cross section and low resistance to provide a path for the traction current. Earthing also provide the return path for the traction current, ensures that non current carrying parts such as equipment frames, fencing and structures are always at ground potential even after the failure of the insulation. Earthing also helps to reduce the effect of induced voltage in adjacent communication circuits. 86  4.11.2 Types of earthings There are two types of earthing namely i) Equipment earthing ii) System earthing Equipment Earthing Equipment earthing is for the safety of operating personel, public and property. In this earthing all the non-current carrying metallic parts, such as frame of circuit breakers, interruptors, transformers, potential and current transformer, steel structures, fencing panels and uprights are connected to main earthing bus also known as earth grid by means of two separate and direct connectors. During an earth fault in the equipment heavy leakage current flows to earth resulting in potential rise almost that of live conductor and at that time if any person comes in contact with the frames or carrying maintenance will get severe shock which may prove to be fatal. Therefore, it is very essential to maintain a very low earth resistance value for all the metal parts so as to enable circuit breaking device to trip at pre-determined value. 4.11.3 System earthing In system earthing one leg of the secondary bushing on 25KV side of each traction power tranformer is solidly earthed by connecting it to the earthing ring by means of two 75mm X 8mm or 80mm X 12mm M.S. flats. Further the earthing ring shall be connected to a buried rail in the ground by the side of the track by means of four 75mm X 8mm M.S. flats (for two track section). One of the designated terminals of the secondary of each potential, current and auxiliary supply transformers is also connected to the earthing ring by means of duplicate 50mm X 6mm MS flats. 4.11.4 Earth resistance At each power supply installations, an earthing ring or bus comprising the required number of earth electrodes, also called earthing stations; inter connected by means of MS flat is provided. The combined resistance of system earth is not allowed to exceed the following limits. Traction Sub-station - 0.5 ohms switching station - 2.0 ohms Booster Transformer Station - 2.0 ohms Auxiliary supply transformer station - 10.0 ohms 87 4.11.5 Design of grounding system 4.11.5.1 The design of the grounding earthing system depends on the following considerations Magnitude of fault current Duration of fault current Thermal stability limits of material Mechanical strength Corrosivity The short time current carrying capacity of a conductor can be determined from the following formula :- A = (12.15 X 10-3) X 1 X √t for wedded joints and A = (15.7 X 10-3) X 1 X √t for bolted joint where A is the cross-sectional area in mm2 & 1 current in Amps and ‘t’ is fault clearance time. To compensable the loss due to corrosion, the main earthing ring size shall be increased by 100% and size of inner conductors by 50%. 4.11.5.2 The minimum number of pipe earth electrodes to be provided at traction substation depends upon the soil resistivity. However, the approximate number of electrodes can be determined from the following equation. Fault Current Number of electrodes = 500 The distance between the two earth electrodes shall not be less than twice the length of the electrode. The earth electrode used at TSS is of 40 mm dia galvanised iron pipe of four meter length. 4.11.6 Shielding wire/earth screen An overhead earth wire is provided in the switch yard of the sub-station connecting the principal gantry masts with 7/9 SWG or 19/2.5 G.I. wire for protection against direct lightening strokes. The shielding conductor is strung about 3.5 metres above the strung bus (for 132 KV’s) so that all the conductors and equipments lies within the protection angle of 88  Fig 4.11 450 as shown in Fig 4.11. An angle of 60 deg. may be used where more than one wire is used. Instead of shielding wire some of the electricity authorities are using spikes which serve the same purpose. These spikes are provided on the gantry towers/masts. 4.12 INSULATION CO-ORDINATION OF SUBSTATION EQUIPMENTS 4.12.1 For each system voltage basic impulse, insulation level has been fixed by most of the National and International standards. The major substation equipments, viz. trans formers, and potential transformers are manufactured for the same insulation level. In general, four levels of insulation in a station are recognised, the bus section is the highest, the post insulators, breakers, switches etc. next lower, the transformers next lower and the lighting arrester is the lowest. 89  i 4.12.2 BIL of various equipments used in the Traction substation as adopted by Indian Railways are as given below One minute Name of the 1.250 micro SN wet power second impulse equipment frequency withstand withstand voltage test voltage KV (rms) 1. Traction Power Transformer (a) 220 KV --- (b) 132 KV 275 650 (c) 25 KV 105 250 2. Circuit Breakers (a) 220 KV 460 1050 (b) 132 KV 275 650 (c) 25 KV 105 250 3. Isolators (a) 220 KV 460 1050 (b) 132 KV 275 650 (c) 25 KV 105 250 4. 132 KV CT275 650 5. 25 KV CT 95 250 6. 25 KVPT 105 250 7. 25 KV interruptor 105 250 8. Lightening arrestor (a) 25 KV Continued Op volt35 KV F/o 125 KVP max. (b) 132 KV -do-95 KV F/o 350 KVP max. (c) 220 KV -do-160 KV F/o 550 KVP max. 4.13 SCHEME OF PROTECTION 4.13.1 The scheme of protection provided at each traction sub-station can be broadly divided into the following two categories. i) Protection of Traction Power Transformer. ii) Protection of the overhead equipment. 90 4.13.2 Protection of traction power transformer The protection used for the transformer follows conventional methods and comprises the following :- (a) Protection against internal faults by means of high speed differntial ralay with necessary restraining features to prevent operation due to in rush magnetising current when the transformer is charged. (b) Back-up protection of internal earth faults by means of instataneous restricted earth leakage relays provided separately on primary and the secondary sides. (c) Protection against over current by means of non-directional relays with inverse definite minimum time lag characterstics, provided on one of the phases on the primary side of the transformer and on the un-earthed leg on the secondary side of the transformer. The relay on the HV side is also provided with an instantaneous over current element. (d) Protection against internal faults by means of a buchholz relay (e) Protection against low oil level (f) Protection against high oil temperature (g) Protection against high winding temperature. (h) Protection agianst high voltage surges by means of LA. (i) Protection against direct lightening stroke by means of shielding wire and spikes. (j) Provision of adjustable arcing horns. 4.13.3 Protection for overhead equipement The scheme of protection of the overhead equipement as adopted comprises the following relays :- (a) A ‘Mho” relay for an impedance of 20-25 ohms and a phase angle of 75 0 for protection against the earth faults. This relay works on the principle of discrimination between the phase angle of the fault impendance and the working impendance of the system. This is used for protection against distance earth faults. (b) Instantaneous over current protection. This relay provides primary protection to the OHE on earth faults in the vicinity of the feeding post. The current setting of the relay may correspond to about 200% of the continuous current rating of the traction transformer. (c) Wrong phase coupling relay The ‘Mho’ relay with a maximum torque angle of 750 as not adequate for protection against wrong phase coupling of the two different phases at the neutral section or at the feeding post during extended feed conditon. Therefore, an additional MHO relay with a maximum torque set at 125 0 is provided. 91 (d) High speed inter tripping relay. In the event of failure of traction sub-station, 25 KV supply is extended from the adjacent sub-station by closing the bridging interruptor at sectoining post Under such emergency feed conditons, wrong phase coupling may be caused at the overlap oppostite the failed S. S. by the pantograph of the locomotive, rasulting in the tripping of 25 KV CB at any one of two S/S through wrong phase coupling relay (Mho). This may result in the formation of an arc at the overlap due to which the OHE may be damaged. The damage due to arc can be minimised by tripping the feeder circuit breaker at the other sub-station also. This is achieved by an inter tripping arrangement through the remote control equipment. (e) Auto reclosing of feeder circuit breakers single shot auto-reclosing scheme for 25 KV feeder circuit breakers at the traction sub-station has been adopted to facilitate reclosing of the 25 KV feeder circuit breaker authomatically once after a preset time delay after tripping of the circuit breaker on OHE fault. This feature will help in quick restoration of traction power supply to OHE if the fault is of transient nature. It will also help in checking the continuance of arc in the event of the pantograph of a moving locomotive passing the overlap opposite the feeding post. (f) Panto Flash Over protection relay : Panto Flash Over relay is provided for protection of OHE from flash over at insulated over lap in front of Traction sub - station, when pantograph passes from live OHE to dead OHE across the over lap. This relay opens the closed feeder circuit breaker to prevent melting down of OHE. Relay can be bypassed either locally or Remotely. One relay monitors one Line. Principle and Operation : When ever one of section of insulating over-lap (IOL) is tripped on intermittent fault, and electric train enters from live to dead section of the FP-IOL, there shall be a heavy flash over, particularly when the Panto leaves the IOL which may damage the Panto. The extent of damage is dependent upon the intensity of current drawn by locomitive. to indentify such situtation and trip the feeder circuit breaker connected to the live side of the overlap. Refer Fig. PFRS the single line diagram of typical traction substration (TSS). One side of the insulated overlap, A or B can become dead while other section is live due to the tripping of respective feeder CB on fault or manual tripping. This 92  condition can take place on normal condition or at feed extended condition. The potential transformer PT1 to PT2 connected at either side of IOL indicates OHE healthiness. During dead connection the PT secondary voltage is considerably low. If voltage is appeared on PT secondary and respective feeder CBs is in open condition then this voltage is due to the bridging of Panto. The relay continuously monitor the status of both PTs, CB at TSS and SSP / SP and depend upon logic give trip command to respective CBs. The relay can by pass manually or either remotely from RC. Logic Chart Sr. No. FCB1 SCB1 PCB1 PCB2 FCB2 SCB2 PCB3 PCB4 PT1 PT2 Trip1 Trip2 1 0 0 0 0 * * * * 1 1 1 0 2 * * * * 0 0 0 0 1 1 0 1 1-Breaker / Interrrupter Close / PT Normal 0 - Breaker / Interrrupter Close / PT no voltage *- Don’t care N - Normal Feed Note : Same Logic shall hold good for other lines also with another Logic 2 Realy. g) Delta - I protection Relay : Delta I protection Relay is provided for clearance of high resistance earth fault of OHE. The protection functions include Vectorial Delta - I protection & Distrubance Recorder. The realy continuously monitoring the increments in the fundamental and the third harmonic currents. Operation : Digital Integrated Vectorial Delta - I Relay : The Delta - I Relay continuously monitors the OHE Current and Voltage through CT and PT inputs. The high speed Micro - controller of the unit simultaneously samples these current and voltage signal using two separate A/D converter for zero phase difference error. The micro-controller performs a power full digital algorithm on the digitized current and voltage samples to find line Impedence (Z,R and X), the fundamental, second harmonic and third harmonic of the current, Vectorial values of current and voltage as required for the operation of the realy. The tuned band-pass characterstics provide stable and excellent filter response, rejecting noise signals. 93 The relay continuously monitors the vectorial ∆-I, the moment ∆-I current cross the set value, relay start Trip Time. During the Trip Time if X components of complex impedence is with in the set blinder limit then relay provides trip command. The relay allows power transformer changing also starting of multiple locomotives (EMUs) on the same section without unwanted tripping. For this, the 2nd and 3rd harmonic components of the load current are monitored, and appropriate restraint of the Relay element is done. if 2nd harmonics component of current increases more than 15% then relay blocks the tropping operation. If a 3 rd harmonics component increases more than 15% of fundamental then relay internally de-sensitized the - I setting by factor of 0% to 100% (programmable). The De-sensitized V. Delta - I can be claculated by following formula. De-sensitized V. Delta I setting = (V. Delta - I setting * De - sensitivity) + V. Dealta I setting 100 The range of measured / computed parameters as well as internal digital flgas status are dynamically displayed on a pront panel dot - matrix 16 x 2 LCD display. Fig. P.F.R.S. 94  4.14 INSULATORS 4.14.1 Provision of adequate insulation in a substation is of primary importance from the point of view of reliability of power supply system and safety of the working personnel. However, the substation design should be such that the quantity of insulators used is a minimum commensurate with the security of supply. The creepage distance of the insulators to be used in the traction substation depends on the degree of pollution level. But as the pollution level, vary from place to place, therefore insulators to be used at the traction sustation have been provided with a creepage distance of 25 mm/ KV so that these insulators are able to withstand the service conditons even under heavily polluted condition including coastal areas. 4.14.2 For strung bus, standard 10” disc insulators of 7000 Kgf. strength have been used for system voltage upto 132 KV and for 220 KV traction substation 11” disc insulators of higher strength have been specified. But for rigid bus, on the primary side of the transformer post type stack insulators have been used and for 25 KV side only solid core type insulators have been adopted for all applications. Due to development of solid core insulators even for higher system voltage and their added advantages over conventional post type stack insulators the same are being used now for all applications such as bus bar support insulators and isolators. Insulators used for the constructions of circuit breakers, interruptors, PTs, CTs and bushings of power transformer are of hollow porcelain housing. ------------------------------------------- 95  ANNEXURE I SULFUR HEXAFLUORIDE(SF6) GAS The SF6 Gas in a pure state in inert, exhibits exceptional, thermal stability and has excellent arc quenching properties as well as exceptional high insulating properties. It is one of the most stable compounds. inert, nonflammable, nontoxic and odourless. the SF6 gas remains gas without liquification down to - 30 C at the maximum pressure of the puffer type breakers. The density of SF6 Gas is about five times that of air and heat dissipation in it is also much more than in the air. At the atmospheric pressure, the dielectric strength is about 2.4 times that of air and at about 3 kg/cm2 it is same as that of oil. Table No.A gives physical properties. There is some decomposition of the gas after long periods of arching however, such decomposition is very little and has no effect up on dielectric strength and interrupting capability. The solid arc-product formed by arcing is metallic fluoride which appears in the form of a fine grey powder. This arc generated powder had high dielectric strength under dry conditions as existing in the breaker. A good quality absorbent is used in the apparatus to remove most of gaseous decomposed by products. So the level of this gaseous By-prodouct is kept very very low. Certain impurities such as air result in the dilution of the SF6 but it is not worth while bothering to measure the dilution of SF6 Gas at the field as long as the process recommended is followed. 96 Physical Properties Of Sf6 Gas 1 Molecular Weight 146.07 2 Melting Pointing -50.7C 3 Sublimation Temp -63.8C 4 Critical Pressure (45.547+0.003)C 5 Critical Pressure 38.55kgf/cm2 6 Critical Density 0.730g/cm3 7 Dielectric Constant 1.002 at 25C 1atm 8 Thermal Conductivity 3.36*10-5 at 300C 9 Specific heat ratio 1.07 10 Density at 200C 6.25gm/lit at 0kg/cm2-g at 1 “ 12.3 at 5 “ 38.2 at10 “ 75.6 at15 “ 119.0 11 Solubility in oil 0.297 cc/cc in H2O 0.001 cc/cc of H2O 0.0035+0.01gm/gm at 300C 12 Cp 23.22cal/mole0C --------------------------------------------------- 97