Thermal Plants

Thermal Plants




SWITCHYARD and GENERAL SIGNIFICANCE

Electrical energy management system ensures supply of energy to every consumer at all times at rated voltage, frequency and stated 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 traditional and non-traditional strength plants, EHV AC and HVDC Transmission system, Back to Back HVDC coupling stations, HV Transmission network, substations, MV and LV dispensing systems and connected electrical loads. The energy in electrical form is supplied to various consumers located in great geographical area, immediately, automatically, and safely with required quality at all times. The service continuity and high quality of strength supply have become very important.

For fulfilling the foresaid purpose, a state of the art, scientifically and technologically progressive SUBSTATION is required. Sub-stop is the load control center of the thermal plant where strength 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 by relays and circuit breakers and these are connected to various feeders by 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 functional method 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 by the bus bars.

2. LIGHTENING ARRESTORS: –

These are equipments designed to protect insulators of strength lines and electrical installations from lightening surges by diverting the surge to earth and immediately 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 suggesting and monitoring instruments can read. When rated current flows by its dominant winding, a current of nearly 1 amp will appear in its secondary winding.

The dominant is so connected that the current being passes by it and secondary winding is connected to an ammeter. The CT steps down the current to the level of the ammeter.

4. possible TRANSFORMER: –

These are used to step do the voltage to a level that the possible coils of suggesting and monitoring instruments can read. These are also used to satisfy the possible coils of relays. The dominant 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. strength 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 strength compensation during low loads.

6. WAVE TRAP: –

Wave trap is used to prevent high frequency signals from entering other zones.

7. suggesting AND METERING INSTRUMENTS: –

Ammeters, voltmeters, watt meters, KWH meters, KVAR meters are installed in sub-stop to watch over the currents flowing in the circuit and the voltages and the strength loads.

8. ISOLATORS: –

One of the cardinal measures for ensuring complete safety in carrying out work on equipment in electrical installations is to disconnect dependably 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 consequently 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 consequence 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 release 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 wire directly attached to relay or other method to function in abnormal conditions such as over strength 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 kind 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 kind SF6 CB the gas from high-pressure system is released to into low-pressure system over the arc during the arc quenching course of action.

SPECIFICATIONS OF CB kind ELF-SL4-1

S. NO. SPECIFICATIONS RATINGS

1. Voltage 245KV

2. Normal current 3150 A

3. Lightening impulse resist Voltage 1050V

4. Switching impulse resist voltage 1050V

5. Short circuit breaking current 40KA

6. Short circuit resist current And duration 40KA 3 sec

7. Line charging breaking current 125A

8. Operating ordern – 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 strength 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 strength 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, sustain 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- possible and touch possible.

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. strength cables To provide supply path to various auxiliary equipments and machines.

7. PLCC system strength line carrier current system – Line trap – Coupling capacitor – PLCC panels For communications, telemetry, tele control strength 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 begin water strength spray, to disconnect strength supply to affected vicinity 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 strength System – Diesel generator sets – Switch gear – dispensing system For supplying starting strength, stand by strength 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 stop, receiving stop or a switchgear in a dispensing 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.

6. Simplicity.

7. Provision of extension.

8. Protective zones.

DUPLICATE BUS BAR ARRANGEMENT

The duplicate bus bar system provides additional flexibility, continuity of supply and permits regular maintenance without total shut down. In the event of fault o n one bus the other bus can be used.

While transferring the strength to the save bus, the following steps may be performed:

1. Close tie circuit breaker, i.e. bus coupler. The two buses are now at the same possible.

2. Close isolators on save 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 sustain insulators. The interrupts enclose a set of fixed and moving contact. The moving contacts can be drawn apart by method 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.

13. RELAYS

Relay is a device by method 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 strength supply system is to recognize a start out and to begin the operation of CB or other devices to isolate the defective elements with minimum disturbance to the normal strength supply system. The must function at required speed and stability to prevent the current and voltage to go beyond 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 course of action

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 wire 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 appropriate techniques. The current reaches final zero as the arc is extinguished.

SWITCHGEAR

The switchgear consists of all parts or equipments of strength plant whose function is to receive and spread electric strength. It comprises of the following:

· Assemblies of switching apparatus

· Protective and suggesting metering devices

· Interconnecting bus bar systems and applicable accessories

roles of Switchgear: –

The roles performed by switchgear are listed below: –

· To ease 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 strength from the generator to the grid. However plant also needs strength for its working. It is mandatory not to use more than 10% of the strength 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 strength to the switchgear and are fed by two stop 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 stop buses respectively. The material of conductor is Aluminium. C-I stop 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 satisfy 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 satisfy 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.




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