Switchyard and general Significance
Electrical power supervision law ensures furnish of power to every buyer at all times at rated voltage, frequency and specified waveform, at lowest cost and minimum environmental degradation. The switchgear, safety and network automation are integral parts of the modern power supervision law and national economy.
Switchgear In Thermal Plant Ghtp
The modern 3-ph, 50 Hz, Ac interconnected law has any conventional 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 connected electrical loads. The power in electrical form is supplied to varied consumers placed in vast geographical area, instantly, automatically, and safely with required capability at all times. The assistance continuity and high capability of power furnish have come to be very important.
For fulfilling the foresaid purpose, a state of the art, scientifically and technologically advanced 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 output units each having a capacity of 210 Mw. The generating voltage is puny to 15.75 Kv and this voltage as stepped up by two-generator transformer 15.75/220 Kv manufactured by Bhel. A part of 15.75 Kv furnish 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 output from the generator transformer is fed to either of the two bus bars through relays and circuit breakers and these are connected to varied feeders through varied 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 electric current to which many connections may be made. These are mainly convenient means of connecting switches and other equipments into varied arrangements.
At Gndtp there are two 220 Kv bus bars. These are made of aluminium and all the incoming and outgoing supplies are connected through the bus bars.
2. Lightening Arrestors: -
These are equipments designed to protect insulators of power lines and electrical installations from lightening surges by diverting the surge to earth and abruptly restoring the circuit insulation to its general strength 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 through its original winding, a current of nearly 1 amp will appear in its secondary winding.
The original is so connected that the current being passes through it and secondary winding is connected 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 potential coils of indicating and monitoring instruments can read. These are also used to feed the potential coils of relays. The original winding is connected 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 furnish reactive power payment during low loads.
6. Wave Trap: -
Wave trap is used to forestall 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 safety in carrying out work on tool 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 essential that apparatus, which makes a descriptive 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 tool surrounding it but will also cause the flashover in the middle of the phase in other words, it will succeed in short circuit in the factory i.e. Why isolators are used only for disconnecting parts after de-energizing them by opening their respective circuits by use of their circuit breakers.
9. Earthing Switches: -
Earthing switch is used to dismissal the voltage on dead lines to earth. An auxiliary switch to furnish interlock all the time accomplishes it.
10. Circuit Breakers: -
Circuit breakers are mechanical devices designed to close o open caress 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 particular pressure (4to6kgf/cm2). The pressure and gas flow required for arc extinction is obtained by piston action.
In duplicate pressure type Sf6 Cb the gas from high-pressure law is released to into low-pressure law over the arc during 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 furnish voltage 240 V dc
11. Air pressure 20.5 bar
12. Frequency 50 Hz
13. Mass 3800 kg
14. Rated current 1600 A
11. varied Other Equipments
Shunt capacitors are used to furnish 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 furnish 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) law - Earth mat - Earthing spikes - Earthing risers To furnish an earth mat for connecting neutral points, tool body, preserve structures to earth. For safety of personnel and for enabling earth fault protection. To furnish the for discharging the earth currents neutrals, faults, surge arrestors over-head shielding wires etc. With safe step- potential and touch potential.
2. Overhead earth wire shielding or lightning To protect the outdoor substation tool from lightning strokes.
3. Illumination law (lightning) - For switchyards - structure - Roads, etc. To furnish illumination for vigilance, execution and maintenance.
4. Protection law - safety relay panels - operate cables - Circuit breakers - Ct's, Pt's, etc. To furnish alarm or automated tripping of faulty part from wholesome part and also to minimize damage to faulty tool and connected system.
5. Control cabling For protective circuits, operate circuits, metering circuits.
6. Power cables To furnish furnish path to varied auxiliary equipments and machines.
7. Plcc law power line carrier current law - Line trap - Coupling capacitor - Plcc panels For communications, telemetry, tele operate power line carrier protection, etc.
8. Fire fighting law - Sensors, detection law - Water spray law - Fire safety operate Panels To sense the occurrence of fire by sensors and to inaugurate water power spray, to disconnect power furnish to affected region to pin point location of fire by indication in operate room.
9. J Cooling water law - Coolers - Water tanks This law 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 law - Diesel generator sets - Switch gear - Distribution law 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 any ways in witch the switchyard can be connected in the electrical layout of generating station, receiving middle point or a switchgear in a distribution system. The following aspects in general work on 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, safety of personnel.
6. Simplicity.
7. Provision of extension.
8. Protective zones.
Duplicate Bus Bar Arrangement
The duplicate bus bar law provides further flexibility, continuity of furnish 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 preserve 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 preserve 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 preserve insulators. The interrupts enclose a set of fixed and enthralling contact. The enthralling contacts can be drawn apart by means of the operating links of the operating mechanism. The operating mechanism of the circuit breaker gives essential power for opening and closing of contacts of the circuit breaker.
13. Relays
Relay is a gadget by means of which an electric circuit (trip or alarm circuit) is controlled (closed) by change in the other circuit. Relays are automatic. The function of relay in power furnish law is to identify a start out and to inaugurate the execution of Cb or other devices to separate the defective elements with minimum disturbance to the general power furnish system. The must operate at required speed and stability to forestall 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 safety 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 furnish for Dc supervision
8. 80B Dc furnish operate supervision
9. 80A Relay for mains
10. 500/L Triple pole over current definite term
11. 98 Trip circuit supervision 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 definite 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 caress close, the trip circuit is finished 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 enthralling 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 opening operations. The Cb contacts separate.
o Arc is drawn in the middle of the breaker contacts. The arc extinguished in the Cb by convenient 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 electric 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 execution of protective tool so automatically disconnect faulty part from the system.
· To break efficiently short circuits without giving rise to risky conditions.
The switchgear has the responsibility of transmitting the power from the generator to the grid. Any way plant also needs power for its working. It is mandatory not to use more than 10% of the power advanced by the plant. So that all the auxiliaries and lightening law of the plant must be designed in such a way that there consumption is in the limit.
There are two bus bars placed in ducts, which furnish power to the switchgear and are fed by two middle point transformers St-I and St-Ii. The rating of these identical bus bars is 6.6 Kv, 3150 Amps, 50 Hz. These are known as C-I and C-Ii middle point buses respectively. The material of conductor is Aluminium. C-I middle point 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 distinct kinds of load.
The auxiliary voltage for the ac furnish law is 6.6 Kv and 415 V. 6.6 Kv furnish law is in case,granted with high resistance grounded neutral with provision of alarm/ trip. 415V auxiliary furnish law has solidly/high resistance earthed neutral. All the electric 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 furnish system
· Dc furnish system
Ac auxiliary furnish law is used to feed all the Ac auxiliaries installed in the plant.
The Dc furnish law which consists of 220 V Dc, 110 V Dc,
+/- 24 V Dc, 48 V Dc etc. Is used for operate supplies as required for law operate and safety equipment.