Switchyard and general Significance
Electrical energy management theory ensures supply of energy to every buyer at all times at rated voltage, frequency and specified waveform, at lowest cost and minimum environmental degradation. The switchgear, security and network automation are integral parts of the contemporary energy management theory and national economy.
Water Pressure Sensor Circuit
The contemporary 3-ph, 50 Hz, Ac interconnected theory has some accepted and non-conventional power plants, Ehv Ac and Hvdc Transmission system, Back to Back Hvdc coupling stations, Hv Transmission network, substations, Mv and Lv Distribution systems and associated electrical loads. The energy in electrical form is supplied to assorted consumers settled in vast geographical area, instantly, automatically, and safely with required ability at all times. The assistance continuity and high ability of power supply have become very important.
For fulfilling the foresaid purpose, a state of the art, scientifically and technologically industrialized Substation is required. Sub-Station is the load operate center of the thermal plant where power at rated voltage, frequency and waveform is exported/imported as per requirements.
The substation at Ghtp Lehra Mohabat has only one 220 Kv switchyard. There are two production units each having a capacity of 210 Mw. The generating voltage is slight to 15.75 Kv and this voltage as stepped up by two-generator transformer 15.75/220 Kv artificial by Bhel. A part of 15.75 Kv supply is fed to unit auxiliary transformer, which is stepped down to voltage 6.6 Kv which is used to run the major auxiliaries of the plant.
After step up, the 220 Kv production from the generator transformer is fed to either of the two bus bars straight through relays and circuit breakers and these are associated to assorted feeders straight through assorted equipments.
Different Tyes Of Equipments Used In Sub-Stations
1. Bus-Bars: -
Bus bar is a term used for main bar of conductor carrying an galvanic current to which many connections may be made. These are mainly favorable means of connecting switches and other equipments into assorted arrangements.
At Gndtp there are two 220 Kv bus bars. These are made of aluminium and all the incoming and outgoing supplies are associated straight through the bus bars.
2. Lightening Arrestors: -
These are equipments designed to safe insulators of power lines and electrical installations from lightening surges by diverting the surge to earth and promptly restoring the circuit insulation to its general power with respect to earth.
3. Current Transformers: -
The main purpose of current transformer is to step down the current to a level that the indicating and monitoring instruments can read. When rated current flows straight through its customary winding, a current of nearly 1 amp will appear in its secondary winding.
The customary is so associated that the current being passes straight through it and secondary winding is associated to an ammeter. The Ct steps down the current to the level of the ammeter.
4. Potential Transformer: -
These are used to step do the voltage to a level that the inherent coils of indicating and monitoring instruments can read. These are also used to feed the inherent coils of relays. The customary winding is associated to the voltage being measured and the secondary winding to a voltmeter. The Pt steps down the voltage to the level of the voltmeter.
5. Power Transformer: -
These are used to step up down the voltage from one a.c voltage to an additional one a.c voltage level at the same frequency. Shunt reactor in Ehv substations is to supply reactive power payment while low loads.
6. Wave Trap: -
Wave trap is used to prevent high frequency signals from entering other zones.
7. Indicating And Metering Instruments: -
Ammeters, voltmeters, watt meters, Kwh meters, Kvar meters are installed in sub-station to watch over the currents flowing in the circuit and the voltages and the power loads.
8. Isolators: -
One of the cardinal measures for ensuring full security in carrying out work on equipment in electrical installations is to disconnect reliably the unit or the section on which the work is to be done from all other live parts of the installation. To guard against mistakes, it is vital that apparatus, which makes a illustrated break in the circuit such as isolators, should do this.
Isolators do not have arc operate devices therefore cannot be used to interrupt currents at which the arc will be drawn over the contacts. The open arc in these is very dangerous, in that it will not only damage the isolator or the equipment surrounding it but will also cause the flashover in the middle of the phase in other words, it will follow in short circuit in the installation i.e. Why isolators are used only for disconnecting parts after de-energizing them by occasion their respective circuits by use of their circuit breakers.
9. Earthing Switches: -
Earthing switch is used to extraction the voltage on dead lines to earth. An auxiliary switch to supply interlock all the time accomplishes it.
10. Circuit Breakers: -
Circuit breakers are mechanical devices designed to close o open feel or electrical circuit under general or abnormal conditions. Cb is adequate with a strip coil directly attached to relay or other means to operate in abnormal conditions such as over power etc. In Gndtp three types of Cb are used. Sf6 Cb is used to operate 220 Kv in switchyard.
Vcb are used to operate 6.6kv in switchgear & Abcb are used to operate 415 Kv in switchgear.
In sulphar puffer type Sf6 Cb is filled with Sf6 gas at single pressure (4to6kgf/cm2). The pressure and gas flow required for arc extinction is obtained by piston action.
In double pressure type Sf6 Cb the gas from high-pressure theory is released to into low-pressure theory over the arc while the arc quenching process.
Specifications Of Cb Type Elf-Sl4-1
S. No. Specifications Ratings
1. Voltage 245Kv
2. Normal current 3150 A
3. Lightening impulse withstand Voltage 1050V
4. Switching impulse withstand voltage 1050V
5. Short circuit breaking current 40Ka
6. Short circuit withstand current And duration 40Ka 3 sec
7. Line charging breaking current 125A
8. Operating sequence - First pole to clear factor 1.3
9. Supply voltage 220 V dc
10. Auxiliary circuit supply voltage 240 V dc
11. Air pressure 20.5 bar
12. Frequency 50 Hz
13. Mass 3800 kg
14. Rated current 1600 A
11. assorted Other Equipments
Shunt capacitors are used to supply payment to reactive loads of lagging power factor.
Series reactors are used to cut the short circuit current or beginning currents. Neutral grounding resistors are used to limit the earth fault current.
Coupling capacitors are used to supply relationship in the middle of high voltage line and power line carrier equipment.
Operating Voltage 230 V
Rated current 2000 A
Minimum short circuit current in bus bars 40 Kv
Minimum phase to phase clearance 2.5 M
Number of horizontal levels of tubular bus bar/flexible bus bars 2.0 M
Height of tubular bus-bar of first level above ground 6 m
Height of tubular bus-bar of second level above ground 4 m
Tubular aluminium bus bar Al Astmb241 4"Ips (International pipe standard)
Various Subsystems In Substations And Their Function
S.No. System Function
1. Substation Earthing (Grounding) theory - Earth mat - Earthing spikes - Earthing risers To supply an earth mat for connecting neutral points, equipment body, keep structures to earth. For security of personnel and for enabling earth fault protection. To supply the for discharging the earth currents neutrals, faults, surge arrestors over-head shielding wires etc. With safe step- inherent and touch potential.
2. Overhead earth wire shielding or lightning To safe the outdoor substation equipment from lightning strokes.
3. Illumination theory (lightning) - For switchyards - structure - Roads, etc. To supply illumination for vigilance, performance and maintenance.
4. Protection theory - security relay panels - operate cables - Circuit breakers - Ct's, Pt's, etc. To supply alarm or self-acting tripping of faulty part from salutary part and also to minimize damage to faulty equipment and associated system.
5. Control cabling For protective circuits, operate circuits, metering circuits.
6. Power cables To supply supply path to assorted auxiliary equipments and machines.
7. Plcc theory power line carrier current theory - Line trap - Coupling capacitor - Plcc panels For communications, telemetry, tele operate power line carrier protection, etc.
8. Fire fighting theory - Sensors, detection theory - Water spray theory - Fire security operate Panels To sense the occurrence of fire by sensors and to get underway water power spray, to disconnect power supply to affected region to pin point location of fire by indication in operate room.
9. J Cooling water theory - Coolers - Water tanks This theory is required for cooling the valves in Hvdc substation.
10. Dc batteries sets and battery chargers Auxiliary low voltage Dc supply.
11. Auxiliary stand by power theory - Diesel generator sets - Switch gear - Distribution theory For supplying beginning power, stand by power for auxiliaries.
12. Telephone system For internal and external Communication.
12. Bus Bar Arrangement In Switchyards
There are some ways in witch the switchyard can be associated in the electrical layout of generating station, receiving center or a switchgear in a distribution system. The following aspects in general influence the choice of the scheme:
1. Degree of flexibility of operations desired.
2. Importance of load and local conditions.
3. Economic conditions, availability, and cost.
4. Technical conditions.
5. Maintenance, security of personnel.
6. Simplicity.
7. Provision of extension.
8. Protective zones.
Duplicate Bus Bar Arrangement
The double bus bar theory provides added flexibility, continuity of supply and permits periodic maintenance without total shut down. In the event of fault o n one bus the other bus can be used.
While transferring the power to the keep bus, the following steps may be performed:
1. Close tie circuit breaker, i.e. Bus coupler. The two buses are now at the same potential.
2. Close isolators on keep bus beginning from far end.
3. Open isolators o9n main bus beginning from far end.
Each pole of the circuit breaker comprises one or more interrupts or arc extinguishing chambers. The interrupts are mounted on keep insulators. The interrupts enclose a set of fixed and spirited contact. The spirited contacts can be drawn apart by means of the operating links of the operating mechanism. The operating mechanism of the circuit breaker gives vital energy for occasion and closing of contacts of the circuit breaker.
13. Relays
Relay is a gadget by means of which an galvanic circuit (trip or alarm circuit) is controlled (closed) by change in the other circuit. Relays are automatic. The function of relay in power supply theory is to identify a start out and to get underway the performance of Cb or other devices to detach the defective elements with minimum disturbance to the general power supply system. The must operate at required speed and stability to prevent the current and voltage to exceed their limits.
A few relays with their international codes are mentioned below:
S. No. Codes Types
1. 27 Instantaneous under voltage relay
2. 99 Composite motor security relay
3. 64 Earth fault relay
4. 87M Differential relay for motors
5. 50N Earth fault relays with times for time delayed operation
6. 95 Fuse failure relay
7. 80 Auxiliary supply for Dc supervision
8. 80B Dc supply operate supervision
9. 80A Relay for mains
10. 500/L Triple pole over current specific term
11. 98 Trip circuit management relay
12. 87 Triple pole circulating current
13. 63 Auxiliary relay for transformer fault indication
14. 33B Auxiliary relay for stop
15. 25 Check synchronizing relay
16. 52Sc Instantaneous high set over current relay
17. 2B Static time delay relay
18. 51Ryb Single pole (Idmtl) Inverse specific Minimum time over current relay.
The Fault Clearing Process
During abnormal or faulty conditions the relays sense the fault and close the trip circuit of the circuit breaker.
As the relay feel close, the trip circuit is fulfilled, and the operating mechanism of the Cb opens and an arc is drawn in the middle of them. The arc is extinguished at some natural current zero of a.c. Wave.
o Fault Occurs: - As the fault occurs the fault impedance being low, the currents growth and the relay get actuated. The spirited part of the relay moves because of the growth in the operating torque. The relay takes some time to close its contacts.
o Relay contacts close, the trip Cb closes and trip coil is energized.
o The operating mechanism starts operating for the occasion operations. The Cb contacts separate.
o Arc is drawn in the middle of the breaker contacts. The arc extinguished in the Cb by favorable techniques. The current reaches final zero as the arc is extinguished.
Switchgear
The switchgear constitutes all parts or equipments of power plant whose function is to receive and distribute galvanic power. It comprises of the following:
· Assemblies of switching apparatus
· Protective and indicating metering devices
· Interconnecting bus bar systems and relevant accessories
Functions of Switchgear: -
The functions performed by switchgear are listed below: -
· To facilitate redistribution of load, inspection and maintenance of system.
· To localize the effects of faults by performance of protective equipment so automatically disconnect faulty part from the system.
· To break efficiently short circuits without giving rise to perilous conditions.
The switchgear has the responsibility of transmitting the power from the generator to the grid. However plant also needs power for its working. It is mandatory not to use more than 10% of the power industrialized by the plant. So that all the auxiliaries and lightening theory of the plant must be designed in such a way that there consumption is in the limit.
There are two bus bars settled in ducts, which supply power to the switchgear and are fed by two center transformers St-I and St-Ii. The rating of these same bus bars is 6.6 Kv, 3150 Amps, 50 Hz. These are known as C-I and C-Ii center buses respectively. The material of conductor is Aluminium. C-I center bus feeds Us-Ia & Us-2A buses and C-Ii feeds Us-Ib & Us-2B buses. C-I & C-Ii buses are also interlinked. These unit assistance buses feed the separate kinds of load.
The auxiliary voltage for the ac supply theory is 6.6 Kv and 415 V. 6.6 Kv supply theory is provided with high resistance grounded neutral with provision of alarm/ trip. 415V auxiliary supply theory has solidly/high resistance earthed neutral. All the galvanic motors of 150 Kw rating or below are rated at 415 V and motors above 160 Kw are rated at 6.6 Kv.
General Electrical Supplies In The Plant
Electrical Auxiliary System
· Ac Auxiliary supply system
· Dc supply system
Ac auxiliary supply theory is used to feed all the Ac auxiliaries installed in the plant.
The Dc supply theory which consists of 220 V Dc, 110 V Dc,
+/- 24 V Dc, 48 V Dc etc. Is used for operate supplies as required for theory operate and security equipment.