400 KV STATION AT NELAMANGALA INTRODUCTION It is 400 \ 220 KV station at about 25 kms north west of Bangalore city in BangaloreTumkur road (national high way no.4) established in an 118 acre plot bearing address Sondekoppa road, Nelamanala-562123. Karnataka. It was constructed on TURN-KEY basis by L&T under planning and technical supervision of PGCIL (Power grid corporation of India limited). It is presently owned, operated and maintained by KPTCL (Karnataka Power transmission corporation limited) After establishment of this station there has not been any grid failures of the type described before and thus the purpose has been effectively served.
Station layout: As in the cases of all substations, an earth matt normally made out of about 70mm x 6mm GI flat is constructed at about a meter below the ground level to which all the metallic items which do not carry electricity are connected (equipment grounding).This earth matt design (normally consisting of welded GI flats of rectangular blocks) depend upon levels of fault currents that come into play at time of faults. The size of GI flat, rectangles size and total area of the earth matt is now-a-days decided by a software design in KPTCL. The design also gives out the number of CAST IRON pipe groundings to be provided and connected to the earth matt. Apart from the earth matt groundings certain equipments are also provided individual groundings. Over the entire substation stone gravel of different sizes is spread mainly to reduce the dangerous effects of STEP-POTENTIAL and TOUCH-POTENTIAL, on humans (operating personnel) who may be present in the station yard. These potentials, occur during creation of EPR (Earth potential rise) zones that
are created during phase to ground faults of any feeder. The 118 acre plot not only provides for the existing station and control room for 11 numbers of 400 KV lines and 12 numbers of 220 KV lines, but also has adequate space for future expansions. Out of the 11 numbers of 400 KV lines 5 of the incoming ones are provided with 50 MVA star-connected switchable shunt reactors. In the cases of some unduly lengthy lines (in excess of 150 KM‘s) shooting up of voltages due to their capacitance reactance are found to be very harming. So the capacitance reactance harming effect is offset by purposely introducing an inductive reactance load in the form of shunt or parallel reactors to these lines. {The criteria here is one 50 MVA reactor for all lines exceeding 150 KM‘s in length. } As regards the neutrals (Please note that the word ‗neutral‘ to an electrical engineer is meant to indicate the junction point of a star circuit) some of the reactors are earthed directly while some are earthed through NGR‘s (Neutral Grounding Reactors—Single phase reactors) The reactors were switched on (or switched out) and included as an inductive load whenever a length line like GOOTY BANGALORE line is closed into or taken out of the station bus. That is why they are called switchable reactors. Once the purpose is over the reactors are switched off. This procedure was before synchronization of all 400 KV lines. Now they are used as loads to control the bus voltage and included whenever voltage increase much beyond 400 KV. Strictly speaking all the lines can be both incoming and outgoing lines Called LILO (Line incoming or Line outgoing) lines. The voltage transformation from 400 KV to 220 KV or vice versa is taken care of by transformers totaling to 1000 MVA. The constructional details of the transformers consist of 7 numbers
of 167 MVA single phase AUTO- TRANSFORMERS. (When the HV \ LV transformation ratio is less than two we go for an auto-transformer) Six of them are interconnected to form two numbers of three phase 500 MVA star-star-delta transformers – that is the reason of being called ICT-1 and ICT-2 (Interconnected transformer N0.1 etc.). The seventh one located in between ICT-1 and ICT-2 is so wired as to serve as an alternate transformer for any one of the 6 single phase transformers and is used as such for maintenance purposes. Transfer-buses are constructed both on 400 and 220 KV sides for facilitating the alternate transformer insertion and is actually put into action by operating appropriate isolators.( without physical shifting of the transformers)
The delta (connection established externally) winding is left out for the intended purpose of stability of the ICT‘s only and remains un-tapped. Please read ―Trends in TERTIARY windings of EHV \ UHV transformers‖ by the author. The station auxiliary distribution transformers consist of two numbers of 350 KVA (each used alternatively) transformers fed from an exclusive 11 KV feeder from the near by regular Nelamangala substation. A diesel generator is also used to provide alternate supply. The station is constructed for a never-shut-down situation. There are 2 numbers each of 400 and 220 KV busses to which the 400 and 220 KV lines are connected, of course through necessary control gears of latest type. Any station-equipment like breakers, busses, isolators etc can be taken up for maintenance, repairs or replacements etc. by providing alternate routes for power supplies to the transmission lines. Certain high-cost equipments like breakers are so located that one of the breakers called tie-breaker can be used as alternate breaker for two adjacent lines, ( Three breakers controlling two lines) thus acquiring strange name –ONE & HALF breakers control. All the breakers are of single phase double pole types ie. an interrupter and a capacitor circuit held in porcelain bushings and working in parallel. The interrupter is of double break type ie. there are two female contacts on either ends inside the interrupter with two male contacts striking into these
contacts. The circuit closing or opening is in two stages with a few mili-seconds of time gap between the two actions. The breaking \ making zone held in porcelain bushings is filled with high pressure (here it is 6 kg \ cm²) SF6 gas for arc quenching .The gas is hermitically sealed with a pressure indicator. The movements of the contacts is by AIR-PRESURE-TRIP with a simultaneous SPRING-LOADING. Later the spring loading is used for closing the contacts. For air-pressure operation the air at a particular pressure (here it is 15 kg‘s per cm²) is held in high pressure tanks. Any fall in air pressure is taken care of by an automatically working air compressor. For the purpose of safety of the breakers, fall of gas \ air pressure beyond certain threshold levels results in locking up of the breaker for breaking or closing operations. (LOCK IN : The breaker fails to obey any commands ie neither opens or closes) This interrupter is in parallel with a suitably designed capacitor circuit to raise the potentials of the contacts so that the potential difference between male and female contacts is reduced considerably and so the arcing is very much reduced.. In certain breakers (Triple pole type) a third circuit is inserted to contain the effects of surge currents of some lengthy lines They are called PIR (Pre-inserted resistance) type. Here the initial circuit closing action is through a resister and a little later the interrupter contacts move. (after about 12 milli seconds) But now-a-days due to development of fast acting breakers PIR type breakers are no longer used. Most of the breakers are of auto-reclosure type. In India the reclosing is for single phase faults only. The GOS‘s (Group operating switches ) have an additional grounding switch for the incoming lines. In the latter the interlocking mechanism is such that the grounding switch can be operated only if the isolator is open. As done in all stations load breaking or closing is through breakers and isolators are used for no load closing \ opening. Out of the multi-cored CT‘s (current transformers) and PT‘s (potential transformers) the PT‘s are CVT (capacitor voltage transformer ) based. The CVT‘s are also used to serve as coupling condensers for connecting PLCC equipments. Wave traps prevent telecommunication signals getting into the station side equipments. To reduce corona effects of the 400 KV all the insulators are provided with corona \ grading rings and no conductor or metallic equipment ends in sharp edges but in round surfaces only. The station is having 220, 48 volts (lead-acid batteries) station battery sets energizing DC bus bars of 220, and 48 volt bus bars respectively. As required by rules all the batteries are housed in separate rooms in the ground floor. All the DC operating equipments (all relays) of 220 KV systems are energized by 220 volts DC. The 48 volts DC energies the communication lines emanating from the station. The DC rectifiers (working on boost, float & trickle modes) that converts AC into DC and charge all the batteries. They draw AC power from an AC bus bar energized by the station auxiliary distribution transformers. There are two exclusive 11 KV lines strung from a near by 66 \ 11 KV Nelamangala Sub-station to provide power supply to the distribution transformers. From these AC \ DC busses control cables are run to provide required AC \ DC power supply to various AC \ DC equipments spread out in the station yard The batteries consisting of (as per standard practice in almost all the stations of Karnataka) two-volt cells of various ampere-hour capacities are connected in series to form the required voltage ratings. From the past
mistakes it has been found that the control room staff should always be looking at a clearly visible signs of DC presence or absence. For the purpose a two lamp (each lamp is connected between positive or negative and earth) method of DC indication is provided and exhibited conspicuously. There are two sets of 220 and 48 volts batteries here. Since there are several cells, they are checked by what is called
PILOT CELLS basis. All the cells are numbered and number plates are fixed to each of them. One day the pilot cells are (for example) cell no.1,14, 16, 25, 36 etc. Specific gravity of acid and voltage of each of these cells are checked and maintenance works if necessary are done. The next day another set of pilot cells (cell nos 2,15,17,26,37 etc) are taken up. Thus in a spefic period of say one month all the available cells are subjected to checking and maintenance.
All the activities & all maintenance works are carried out by an executive engineer assisted by a group of AEE‘s and AE‘s It is manned by (in each shift of the day) four teams of an Asst. executive Engineer and assistant engineer working round the clock. The engineers on shift duties in both the control rooms record hourly readings of al the parameters of the station like voltages \ currents \ energies and all the meters of 400 and 220 KV feeders and perform all the routine chores They enter the details into computers which is on line linked to state load dispatch centre. They also prepare daily energy balance sheet of the station. The executive engineer prepares the monthly energy balance sheet taking the energies received at 400 KV side. All the equipments-operation can be locally controlled as well as remote controlled from a control room and SCADA (Supervisory Control and Data Acquisition) operable. An RTU (Remote terminal unit) is also established to convey all the relevant data to the State load dispatch centre and can also be ON LINE controlled from there.
SINGLE LINE DIAGRAM
Necessity of this station : -After establishing a major power generating station at Sharavathi river fall, the power supply was stepped up to 220 KV and 4 numbers of 220 KV lines transferred power to Bangalore. But often the lines were tripping and sometimes a cascading effect resulted in entire grid failure plunging whole \ parts of Karnataka into darkness. This was very often leading to failure of Southern grid even. This grid-failure had to be minimized. Besides the 400 KV power supply from generating stations like GOOTY (Andhra pradesh) was to be integrated into southern grid. All these and further system studies on Southern grid of India by CEA (Central electricity Authority) necessitated a 400 KV station in this zone.
