Thermal PlantsMohamed Bedawy
SWITCHYARD and GENERAL SIGNIFICANCE
Electrical energy management system ensures supply of energy to every consumer at all times at rated voltage, frequency and specified waveform, at lowest cost and minimum environmental degradation. The switchgear, protection and network automation are integral parts of the modern energy management system and national economy.
The modern 3-ph, 50 Hz, AC interconnected system has several 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 energy in electrical form is supplied to various consumers located in vast geographical area, instantly, automatically, and safely with required quality at all times. The service continuity and high quality of power supply have become very important.
For fulfilling the foresaid purpose, a state of the art, scientifically and technologically advanced SUBSTATION is required. Sub-Station is the load control 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 limited 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 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 output from the generator transformer is fed to either of the two bus bars through relays and circuit breakers and these are connected to various feeders through various 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 various 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 instantly restoring the circuit insulation to its normal 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 primary winding, a current of nearly 1 amp will appear in its secondary winding.
The primary 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 primary 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 another a.c voltage level at the same frequency. Shunt reactor in EHV substations is to provide reactive power compensation during 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 safety 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 necessary that apparatus, which makes a visible break in the circuit such as isolators, should do this.
Isolators do not have arc control devices therefore cannot be used to interrupt currents at which the arc will be drawn across 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 between the phase in other words, it will result in short circuit in the installation 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 discharge the voltage on dead lines to earth. An auxiliary switch to provide interlock always accomplishes it.
10. CIRCUIT BREAKERS: –
Circuit breakers are mechanical devices designed to close o open contact or electrical circuit under normal or abnormal conditions. CB is equipped 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 control 220 KV in switchyard.
VCB are used to control 6.6kv in switchgear & ABCB are used to control 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 system is released to into low-pressure system 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 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. VARIOUS OTHER EQUIPMENTS
Shunt capacitors are used to provide compensation to reactive loads of lagging power factor.
Series reactors are used to reduce the short circuit current or starting currents. Neutral grounding resistors are used to limit the earth fault current.
Coupling capacitors are used to provide connection between 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) System – Earth mat – Earthing spikes – Earthing risers To provide an earth mat for connecting neutral points, equipment body, support structures to earth. For safety of personnel and for enabling earth fault protection. To provide 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 equipment from lightning strokes.
3. Illumination system (lightning) – For switchyards – Buildings – Roads, etc. To provide illumination for vigilance, operation and maintenance.
4. Protection system – Protection relay panels – Control cables – Circuit breakers – CT’s, PT’s, etc. To provide alarm or automatic tripping of faulty part from healthy part and also to minimize damage to faulty equipment and associated system.
5. Control cabling For protective circuits, control circuits, metering circuits.
6. Power cables To provide supply path to various auxiliary equipments and machines.
7. PLCC system power line carrier current system – Line trap – Coupling capacitor – PLCC panels For communications, telemetry, tele control power line carrier protection, etc.
8. Fire fighting system – Sensors, detection system – Water spray system – Fire protection control Panels To sense the occurrence of fire by sensors and to initiate water power spray, to disconnect power supply to affected region to pin point location of fire by indication in control room.
9. J Cooling water system – Coolers – Water tanks This system 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 System – Diesel generator sets – Switch gear – Distribution system For supplying starting power, stand by power for auxiliaries.
12. Telephone system For internal and external Communication.
12. BUS BAR ARRANGEMENT IN SWITCHYARDS
There are several ways in witch the switchyard can be connected in the electrical layout of generating station, receiving station or a switchgear in a distribution system. The following aspects in general affect the selection 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.
7. Provision of extension.
8. Protective zones.
DUPLICATE BUS BAR ARRANGEMENT
The duplicate bus bar system provides additional 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 reserve 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 reserve bus starting from far end.
3. Open isolators o9n main bus starting from far end.
Each pole of the circuit breaker comprises one or more interrupts or arc extinguishing chambers. The interrupts are mounted on support insulators. The interrupts enclose a set of fixed and moving contact. The moving contacts can be drawn apart by means of the operating links of the operating mechanism. The operating mechanism of the circuit breaker gives necessary energy for opening and closing of contacts of the circuit breaker.
Relay is a device 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 supply system is to recognize a start out and to initiate the operation of CB or other devices to isolate the defective elements with minimum disturbance to the normal 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 protection 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 control 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 contact close, the trip circuit is closed and the operating mechanism of the CB opens and an arc is drawn between 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 increase and the relay get actuated. The moving part of the relay moves because of the increase 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 between the breaker contacts. The arc extinguished in the CB by suitable techniques. The current reaches final zero as the arc is extinguished.
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 operation of protective equipment so automatically disconnect faulty part from the system.
· To break efficiently short circuits without giving rise to dangerous 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 developed by the plant. So that all the auxiliaries and lightening system 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 supply power to the switchgear and are fed by two station 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 station buses respectively. The material of conductor is Aluminium. C-I station 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 service buses feed the different kinds of load.
The auxiliary voltage for the ac supply system is 6.6 KV and 415 V. 6.6 KV supply system is provided with high resistance grounded neutral with provision of alarm/ trip. 415V auxiliary supply system 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 supply system
· DC supply system
AC auxiliary supply system is used to feed all the AC auxiliaries installed in the plant.
The DC supply system which consists of 220 V DC, 110 V DC,
+/- 24 V DC, 48 V DC etc. is used for control supplies as required for system control and protection equipment.