Power Supply Installation PDF

2. POWER SUPPLY FOR TRACTION 2.0 Supply System The single phase 50 Hz power for the electric traction is obtained from 220/132/110/66 kV Extra High Voltage 3 phase grid system through step down single phase transformers. For this purpose duplicate feeders comprising of only 2 phases are...

2. POWER SUPPLY FOR TRACTION 2.0 Supply System The single phase 50 Hz power for the electric traction is obtained from 220/132/110/66 kV Extra High Voltage 3 phase grid system through step down single phase transformers. For this purpose duplicate feeders comprising of only 2 phases are run from the nearest sub-station of the Supply Authority to the traction sub- station. The brief description of the system is given in Chapter 2 of Volume I. The 25 kV single phase conventional system as adopted on Indian Railways has been described in that chapter. A schematic diagram of the traction sub-station and feeding post indicating the general feeding arrangement in indicated at Fig 1.01. 2.1 Liaison with Power Supply Authorities For ensuring continuity and reliability of power supply for traction it is important that effective liaison is maintained between the officials of Railway and Supply Authorities. Broadly action on the following lines be taken a) A system of periodic meetings at different levels at mutually agreed intervals needs to be evolved. b) Reliability of supply involves also the maintenance of traction voltage between 25 kV and 27.5kV at the feeding posts and frequency between 48.5 Hz and 51.5Hz. the serious repercussions on Railway traffic if the above limits are not adhered to should be constantly impressed upon the Supply Authorities. c) The traction load should be treated as essential load and should not be disconnected or reduced to meet supply system exigencies. This principle has been accepted by most Supply Authorities and where this has not been done, constant efforts should be made at the high level periodic meetings to get this principle accepted. d) Since the cumulative effect of frequency power supply interruptions, even though of short duration at a time, can be very serious to Railway working. A periodical review of ‘such interruptions should be made at the Divisional level and the cause of each interruption ascertained as far as possible. The results as possible. The results of the review should be furnished to CEE to keep him fully informed of power supply position. This subject should also form an important item for discussion at the periodic meetings with the Supply Authorities. e) Power Supply for electric traction should be governed by a specific Agreement entered into by the Railway with the concerned Supply Authority before the supply is actually taken. Where this has not been done already, urgent action should be taken to have it finalized without delay. 10  f) When grid supply to any traction sub-station fails and consequent emergency working has to be resorted to by extending the feed from adjacent sub-stations, the maximum demand at these sub-stations may go up. Most Supply Authorities have agreed to ignore such temporary increase in maximum demand for billing purpose. Where this has not yet agreed to, efforts should be continued to persuade the Supply Authorities to accept this principle. g) The present methodology of measuring maximum demand at each individual sub-stations for the purpose of billing has been reviewed by the Central Electricity Authority. It has been that Railways should be charged for traction power on the basis of simultaneous maximum demand recorded in contiguous sub-stations of the SEB serviced by the same grid transformers. Modalities to implement the decision would have to be mutually settled between SEB’s and Railways, with cost of the equipment borne by Railways. 2.2 Tariff for Traction a) In Electric Traction the energy cost forms a substantial portion of the total operating and maintenance cost. The tariffs charged by various state Electricity Boards vary from a simple flat rate for the energy (charged by some states like MSEB & GEB) to a very complex tariff structure covering a variety of parameters (as indicated in the tariff charged by MPEB). The implications of the various parameters should be studied carefully to keep the energy cost to the minimum possible level. b) Contract Demand for each sub-station should be stipulated in relation to the expected actual Maximum Demand in such a manner that in fructuous payments by way of minimum guarantee on the one hand and penal charges for exceeding the contract demand on the other, are avoided. Notice period for altering Contract Demand should be kept as low as possible in the agreement. Preferably 4 to 6 weeks. c) In the tariff charged for electric traction, following are some of the parameters that should be given careful consideration with a view to keeping down the energy bill to the minimum: i) Maximum demand charge Rs/kVA/month. Normally one feeder is ‘ON’ for feeding the traction load. If two sets of trivector meters are provided the higher of the two should be the MD to be charged. Caution may be excercised to ensure that addition of both is not taken as MD in billing. ii) Energy charge Paisa/kWh. iii) Fuel Adjustment Charge (FAC) accounting for the variations in cost of fuel and calorific value compared to stipulated basis 11  figures. This charge should be realistic and should be periodically verified with the Supply Authorities. iv) Penalty for low power factor. The penal charge is prescribed as an extra amount leviable in Rs/kVA of Maximum Demand if power factor falls below a specified value. SEBs usually insist on consumer providing PF correcting equipment and do not permit power factor lower than a prescribed value. v) Billing Demand is usually a certain percentage of contract demand or the actual MD whichever is higher. vi) Excess over Contract Demand and Corresponding units of energy are usually charged at higher tariff (excluding FAC). Even if the excess MD is for a short period of just 15 min. proportionate units for the entire month are charged at penal rate. One of the SEBs does computation of excess energy as under: Excess Energy = TU (1-CD/MD) Where TU = Total Energy MD = Maximum Demand CD = Contract Demand The Contract demand therefore, has to be carefully determined and reviewed periodically and if necessary modified to avoid penal charges. vii) Minimum Guarantee Usually, the agreement with SEB stipulate a percentage return of 15 to 20% on the capital cost invested by the SEB for giving the connection, as minimum guarantee. This is generally met by the pattern of energy consumption in traction. However, minimum guarantee in some case is specified in terms of guaranteed average load factor (say 30%). This ties up the Contract demand with the units consumed. If a few heavy trains operate in a section raising the Maximum Demand high, the average load factors may not reach 30 % unless adequate frequency of passenger trains also forms part of the traffic pattern. Here, if contract demand is too high, 30% load factor is difficult to achieve while if contract demand is too low, exceeding it and attracting penal changes becomes a possibility. Careful balance between the two conflicting requirements is, therefore, to be struck. 12  viii) Harmonic Voltage Distortion The consumer is required to carry Harmonic Analysis under full load conditions. It is stipulated that the individual harmonic distortion (Vn) and total harmonic voltage distortion (Vt) at the point of supply shall not exceed 1% and 3% respectively. V - RMS value of fundamental voltage. Vn - RMS value of harmonic voltage of order “n”, expressed as percentage of RMS value of fundamental and shall be calculated using the following expression. √ (V22 + V32 + V42 +…….+Vn2) Vt = ------------------------------------------ x 100 V d) The tariff charged for traction should be reviewed periodically with the SEB. It should be ensured that the rates do not exceed those charged in EHV tariff of the SEB applicable to other consumers. 2.3 Monthly Meter Readings a) In earlier Railway Electrification installations, only one set of meters owned by the Supply Authority has been installed to meters the traction load. In later installations, a second set of meter is to be provided on the sub-station switchboard at Railways cost. Where only one set of meters belonging to the supply authority is installed yearly testing of the meter should be carried out. If its accuracy is the doubt at any point of time, the Railway is entitled to ask for testing and certification of the meter. Where a second set of meters has been provided at Railways cost, the figures for billing purpose should ordinarily be based on the average readings of the two sets of meters, unless specifically provided for otherwise the Agreement. The exact procedure covering these aspects should be embodied in the Agreement with the Supply Authorities. b) The monthly meter reading should be taken on an agreed date each month jointly by representatives of the Supply Authority and the Railway. The meter card as well as the printomaxigraph chart reading showing maximum demand for the month should be initiated by representatives of both parties. Only readings jointly recorded as above should be accepted for billing purposes. c) When visiting the grid sub-stations for taking meter readings, the supervisory official concerned will also obtain additional information such as daily maximum demand for traction, power factor, load factor, variation of voltage, changes in the system of interconnection, which 13  have a bearing on power supply for traction. Suggestions for suitable changes in the Supply Authority’s network may be made at appropriate level and if necessary concrete proposals initiated for making power supply 100% reliable. 2.4 Scrutiny of Bills a) The Supply Authorities bills should be carefully scrutinized in the Divisional office with reference to the Agreement and the tariff. A time schedule should be laid down jointly with the Accounts Department for scrutiny and passing of the bill so as to take advantage of the rebate admissible, if any, for prompt payment. Penal charges levied, if any should be carefully scrutinized and appropriate remedial measures taken to prevent recurrence. If the minimum charge payable is in excess of the amount warranted by the actual energy consumption this fact should be promptly brought to the notice of CEE as well as the operating Department to take special steps to arrange for movement of additional traffic to the extent possible in the affected section, including diversion from other routes. b) Detailed instructions should be issued locally, jointly with the Accounts Department listing the items to be checked prior to passing the bills from the Supply Authorities. An illustrative list given below i) Arithmetical accuracy. ii) Meter readings shown on the bill tally with those received earlier from the subordinates. iii) The tariff applied is in terms of the agreement. iv) The method of computation of the maximum demand for billing purposes is in accordance with the agreement and that temporary increase in maximum demand on account of emergency feeding has not been taken into account where this principle has been accepted. v) The time allowed for payment is in accordance with the agreement. vi) There is no duplication in billing. vii) The payee as provided for in the agreement is clearly indicated. The full particulars of the payee should be advised to the Accounts Branch to enable that Branch to issue cheques accordingly. viii) Each new bill should be analysed and compared with earlier bill and the reasons for any significant departures investigated. In case of any dispute/discrepancy, the payment be made “under protest”. 14 2.5 Power Factor Improvement a) Provision of power factor improvement capacitors at 25 kV bus of traction sub-stations should be planned giving priority to sub-stations (i) which feed large marshalling yards and (ii) where penalty for low power factor and /or exceeding maximum demand has been stipulated in the tariff. b) The average monthly power factor is calculated as ration of kWh and kVAh over a month. Care should be taken to make sure that it does not go ‘leading’ while P.F. correcting equipment is used and is kept near unity. Switched capacitor be used where load variations are wide. c) The Guidelines issued by RDSO in respect of selection of the kVAr rating should be kept in view at the time of planning. 2.6 Shut-Downs of traction Supply to be Pre-Planned a) At all grid sub-stations and traction sub-stations owned by the railways, duplicate EHV feeders are available. Most of the sub-stations also have two sets of traction power transformers and associated switchgear. Maintenance of equipment and transmission lines should not, therefore, necessitate total shut-down of EHV and 25 kV supply at a sub-station. It should be arranged with the Supply Authorities that on the rare occasions when shut-down becomes inescapable, notice should be given well in advance to Sr. DEE/DEE (TRD) stating the reasons for the shut-down and the anticipated duration. Such shut- downs should be arranged by Sr.DEE/DEE (TRD) in consultation with the Operating Department which may have to re-schedule trains and take a other measures as necessary. b) A double circuit set of transmission lines from the Grid Sub-station are run to give supply to traction sub-station. Therefore, maintenance of the transmission line does not necessitate total shut-down of the system. However, all such shut-downs should be planned well in advance giving the reasons to the shut-down and anticipated duration. 2.7 Operating Instructions for Grid Sub-stations Detailed operating instructions mutually agreed to between the Supply Authorities and the Railway should be made out for each sub-station as well as traction sub-station owned by the Railway and should be issued to TPC as well as operators at grid stations. These instructions should contain the following details: a) Procedure for carrying out switching operations at the sub-station. b) Procedure for interchange of message of pre-planned or emergency shut-downs. 15  c) Procedure to be followed in case of failure of supply and information to be conveyed by grid sub-station operator regarding duration of failure and anticipated time of restoration to enable emergency works to be introduced. d) Records to be maintained by grid sub-station operators and TPC regarding emergency feed arrangements. e) List of office and residential telephone numbers of important grid and railway officials to be contacted in an emergency. f) Mutual assistance to be rendered for transmission of important messages in the event of telephone failures at the grid sub-stations or RCC. 2.8 Statistical Data regarding Energy Consumptions In the divisional office, a register should be maintained to record month-wise the following particulars in regards to energy consumption at each supply point :- a) Energy consumption (kWh) b) Maximum demand (kVA) c) Average power factor (kWh/kVAh) d) Monthly average load factor (percent) e) Payment for energy. f) Payment for maximum demand g) Payment towards meter rent h) Payment of fuel surcharge, if any i) Payment for P.F. surcharge/penal charge j) Payment of covering the minimum guarantee load, if any k) Other payment, if any l) Total amount of bill under all heads. m) Average total cost per kWh A consolidated statement giving the above details for all supply points should be furnished by Sr. DEE(TrD) to CEE each month by a stipulated date. CEE will in turn furnish a monthly statement in the prescribed proforma to the Railway Board and Research Design and Organisation (RDSO) -------------------- 16