reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

11/3 3kV P COAL PLANT

AUX IL 1AR Y TRANSFORMER 'A'

SWITCH BOARD A'

STACKER/

RECLAIMER

MACHINE 1 CONVEYORS A.

3.3/0.415V COAL PLANT

AUXILIARY TRANSFORMER 'A

SWITCH BOARD ./4:

CONVE YORS

AND

MISCELLANEOUS PLANT

A SIDE

11/3 3kV COAL PLANT

AUXILIARY TRANSFORMER B•

SWITCH BOARD 'A.4

3 3kV

COAL PLANT AUXILIARIES BOARD

3.3/0. 4 15kV COAL PLANT

AUXILIARY TRANSFORMER B

SWITCH• BOARD 'B'

/(5

CONVEYORS

AND

MISCELLANEOUS PLANT

13 . SIDE

upon deflection. The trip switches are connected directly into the conveyor drive motor contactor control circuit to cause the conveyor to stop when initiated. Each conveyor is fitted with a speed detector. Beltdriven speed detectors are prone to slip when dusty or Wet and have been dropped in favour of electronic Pulse or similar types. The conveyor is tripped if the conveyor belt slips or breaks.

8. 1 .5 Stacker/reciaimer machine

The stacker/reclaimer machine is supplied by a reeling-

drum trailing-cable system. These cables carry the power, control and communication supplies necessary for its operation. The machine may be operated by electric motors or electrohydraulic devices.

The 3.3 kV three-phase 50 Hz power supply is stepped down to 415 V by a transformer located on the machine. Supplies for bucket wheel, boom conveyor, travel and boom control drive motors are taken from a motor control panel specially designed with 1P54 protection to prevent coal dust ingress. Drive motors are 415 V three-phase 50 Hz totally-enclosed,

Mechanical plant electrical services

fan cooled squirrel-cage induction motors with pro-

tection to 1P55.

An operator's cabin is located to give a clear view for operational purposes. All control equipment is of dust-tight construction having 1P54 protection. A complete set of controls for both manual and semiautomatic operation is provided. Driver supervision is required at all tii-cti2s 1.0 ullen. ise stacking and reclaiminv. operations and to ,,af:itttuard the machine.

Controls are provided for travel, !tiff and slew motions, and for other service requirements, such as audible alarms, machine access lighting and floodlighting. Deadman-type controls are provided for travel, luff and slew motions, these controls automatically returning to the off position when released by the operator.

To safeguard personnel, boom and elevator conveyors are equipped with trip-wire systems. Emergencystop pushbuttons are located on access platforms, in the operator's cabin and in the electrical equipment enclosure. An emergency trip causes all machine drives to be de-energised.

8.2 Ash and dust handling plant

8.2.1 General description of dust handling plant

Dust is collected in hoppers below the precipitators and boiler economisers (Fig 10.31). Air-jet blowers transfer the dust from the precipitator dust collection hoppers to the precipitator surge hopper, and from the economiser dust collection hoppers to the boiler-grit surge hopper. A boiler-grit pump transfers grit from the boiler-grit surge hopper to the precipitator surge hopper. From there, dust is pumped into a common storage bunker. Aeration fans ensure that dust in the surge hoppers and in the storage bunker is maintained in a fluid state. From the storage bunker, dust is transferred by rotary feeders onto a system of conveyors, which feed it to the disposal area.

The dust-handling plant for a large modern power station is outlined in Fig 10.32.

8.2.2 General description of ash handling plant

The design of the ash handling plant is dependent upon the method of ash disposal. It may be pumped into a disused quarry or transported from the power station for processing into building materials. A typical system, outlined below, includes both facilities.

Ash from the boiler is collected in a hopper directly below the furnace, whence it is removed by high pressure water jets and discharged, via sluiceways, into ash crushers. The crushed ash falls into ash tanks, and is then projected by water-jet pumps supplied by HP sluice pumps through discharge pipelines to ash pits, where it is allowed to settle. After settlement and drainage, the ash is removed by grabbing crane for disposal by road vehicles or by conveyor to a disposal pit.

Chapter 10

The motive water is pumped from the ash pits t o settling lagoons. After settlement, it is recirculated by return water pumps, via the ash water reservoir, t o the HP sluice pumps.

The system for a large modern power station i s illustrated in Fig 10.33.

8.2.3Electrical supplies

The supply requirements of the ash and dust handling plant are met by an integrated distribution system, comprising 3.3 kV and 415 V three-phase 50 Hz switchboards, diversely supplied and interconnected to ensure maximum system availability and to minimis e disruption due to supply faults. The 3.3 kV switch. board feeds the HP sluice pumps and the transformer feeders to the 415 V switchboard. Other requirements, such as movable ash hoppers, trace heating, ash-grab- bing crane, conveyor drives, ash crushers, dust conditioners, rotary feeders and aeration fans are suppli e d from the 415 V board.

8.2.4 Electrical control

With the exception of the ash-grabbing crane and mobile dust hoppers, the ash and dust plant is controlled from a mimic control panel, normally sharing a control room with the coal plant. Local control facilities are provided on the major items of plant to satisfy safety and maintenance requirements.

A sequence control and interlock system is provided to ensure correct start-up and shutdown. This system covers airheater, economiser and precipitator hopper jet-blowers, grit and dust pumps, dust bunker aeration fans, rotary feeders, dust conditioners, conveyors, and other items of plant which must be operated in a specific sequence to ensure the correct functioning of the overall plant.

Direct control is provided of sluice pumps, glandsealing water and make-up pumps.

Instrumentation, discrepancy equipment and alarm equipment is incorporated in the mimic control panel to enable plant status to be monitored.

8.2.5 Mobile ash hoppers

Ash is discharged from the ash pits via mobile ash hoppers onto a conveyor system for disposal. Each hopper is supplied at 415 V three - phase 50 Hz through a reeling-drum type trailing-cable system. Motor control gear, protected to 1P55 weatherproof, is provided on the hopper for travel, traverse and conveyor functions and for an anti-collision system. Drive motors are 415 V three-phase 50 Hz totally-enclosed fan cooled squirrel-cage induction motors.

8.2.6 Ash-grabbing crane

The ash-grabbing crane is generally as described in Section 3 of this chapter, and is supplied at 415 V

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Coal, ash and dust plant

AERATION FANS

DUST

CONDITIONERS

DUST CHUTE

JUNCTION

CONVEYORS HOUSE

4

DUST CONDITIONER House

FIG. 10.31 Dust collection plant

three-phase 50 Hz by a reeling-drum, trailing-cable systern.

8.2.7 Trace heating

The precipitator surge-hopper is equipped with trace heating to prevent condensation within the hopper during shutdown. If not prevented, heavy condensation would solidify the ash and cause blockages.

8.2.8 Local control panels

Local control panels and cable termination boxes are provided of weatherproof, dust-tight construction to IP55 or IP65, depending upon the severity of the environment. Facilities are provided on local control panels for full manual control of the plant item.

8.2.9 Conveyors

Conveyors are driven by 415 V three-phase 50 Hz squirrel-cage induction motors through fluid couplings

to protect conveyors during start-up. The motors are totally-enclosed, fan cooled with protection to 1P55.

Each conveyor is equipped with an emergency tripwire system running the full length of the conveyor alongside the access way. Switches are located at intervals along the trip wire. The trip wire may be a wire rope, arranged to operate trip switches directly, or a low voltage electric cable arranged to operate a trip relay. In the latter system, a trip relay is energised through the cables and the switches between which they are suspended. Upon deflection of a cable, the relay circuit is broken by the associated switches. Since a cable fault or breakage will also cause the trip relay to operate, the system is self-monitoring.

The contacts of the trip switches or relays are connected directly into conveyor drive-motor contactor circuits and cause the conveyors to stop when the trip is operated.

Each conveyor is fitted with a speed detector. Belt - driven speed detectors are prone to slip when dusLY or wet and have been dropped in favour of electronic

FIG. 10.32 (coned)

Dust pumps and grit pumps are controlled by hopper

, ,, ruent measurement, load cells or level probe systems -;:ng used for this purpose.

9 Elecrrostatic precipitators

9.1General description of plant

ln electrostatic precipitator removes dust from flue -'r-kes by charging the particles in an electrostatic field up between discharge and collecting electrodes

..ithing them to be separated from the gas stream. The

,

'I l es are drawn through large chambers containing ' lecting electrodes in the form of large vertical flat

Dust handling plant

is shared between three precipitators, known as 'flows', each flow consisting of approximately sixty gas paths. Figure 10.34 shows the general arrangement of a precipitator having six zones.

A DC voltage of between 20 kV and 30 kV, produced by transformer/rectifier units, is applied between the discharge electrodes and collector plates, the voltage being controlled automatically at a level just less than that which would cause flashover, to provide maximum discharge current (termed corona discharge), and hence maximum operating efficiency.

In order to maintain a free flow of dust, the dust collecting hoppers are electrically-heated and thermally- insulated to maintain a high dust temperature. Compacting of the ash is prevented by electrically-driven aeration fans feeding air into the dust hoppers.

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9.2 Electrical supplies

9.2,1 415 V switchboards

The electrical requirements of the precipitators are

supplied from 415 V three-phase 50 Hz switchboards, separate boards being provided for each precipitator flow. They feed high voltage control cubicles located with the switchgear in the Precipitator Control House. Other feeds go to the collector-plate rapper motors,

discharge-electrode rapper motors, dust-hopper heaters and aeration fans, control house ventilation fans and hopper level equipment. Figure 10.35 shows a typical control house distribution equipment system.

9.2.2 High voltage control cubicles

Each high voltage control cubicle accommodates the automatic control equipment, power thyristors and power distribution equipment associated with one zone of the precipitator gas flow.

Controls and indicators for the precipitators are provided in the Precipitator Control House and the Central Control Room, control normally being carried out from the latter. Automatic and manual control facilities are available.

Particular attention is paid to the design of the power thyristor and diode equipment to ensure high reliability and long service life, having regard for the inability of semi-conducting devices to withstand deviations outside their specified operating conditions. Overload and short-circuit protection are provided by high speed semiconductor fuses. Surge suppression circuits are incorporated to protect thyristors and diodes against supply voltage transients which may occur in service. Thyristors and diodes are conservatively rated in respect of their current carrying ability, junction operat-

ing temperature and heat-sink performance at th e maximum air temperature likely to occur inside th e equipment cubicle during operation.

Automatic voltage control is provided by thyris_ tors. The electrode voltage is continually monitored and varied by the thyristor control circuits, usin g wave-chopping techniques to maintain the highest per. missible precipitator operating voltage and operati ng efficiency. Typically, the high voltage must be abou t 20 kV for corona discharge to occur. Random arcing may occur at about 32 kV so the automatic voltage control must maintain the voltage just below thi s level. Two basic systems of monitoring and controlling the electrode voltage are in use. In the first system electrode voltage, electrode current and AC supply voltage are monitored and compared. When arcing occurs the electrode voltage falls whilst the electrode current and AC supply voltage increase. This situation is recognised by the comparator and, by control of thyristors in the AC supply, the electrode voltage is reduced to the optimum level at which arcing ceases.

In the second system, the arcing rate is monitored and the electrode voltage varied by control of thyristors in the AC supply to maintain the random arcing rate within acceptable limits commensurate with maximum operating efficiency of the precipitator.

9.3 Maintenance interlocking and locking

Maintenance interlocking is provided for each flow to ensure the correct sequence of operation of the LV transformer/rectifier supply cubicle isolating devices,

11V chamber earthing switch and HV enclosure access doors associated with each zone, so that safety of

personnel is assured.

Coded-key interlocks and exchange boxes form the basis of the interlock scheme. The scheme ensures that all zones of a flow are electrically isolated and mehanically shut-off by closing inlet and outlet dampers before access to the flow can be gained. Figure 10.36 , hows a typical maintenance interlocking scheme.

The interlocking scheme shown in Fig 10.36 allows half-zones to be isolated and earthed individually if a fault occurs, and the remainder to be operated norHowever, all zones must be isolated and earthed keys are released from the exchange box to

al ford access into the flow. Similarly, all access points ;nust be closed and secured, and the keys thus released

inserted into the exchange box before HV supplies can be reconnected.

ln addition to the interlocks, access ways which allow cntrY to the flow or HV enclosure are provided with

Fuel oil is required:

•For main boiler start-up and, in some instances, for oil overburn to meet load requirements at coalfired power stations.

•For main boiler firing at oil-fired power stations.

•For on-load firing of auxiliary boilers.

For all these systems, fuel oil is unloaded or fed into storage tanks which form the supply point for the oil feed system to the burners. Electrically-driven pumps are used to feed oil into the storage tanks, to transfer oil between tanks and to supply oil from the storage tanks to the burners.

Since the oil is of high viscosity, its temperature must be raised to render it suitable for pumping and combustion. Storage tanks and oil pipes are heated continuously by steam, electrical trace heating or immersion heaters. On steam heated systems, electricallydriven pumps are used to transfer the condensate.

When oil is delivered by rail or road tanker, it is not possible to drain the tanks completely using the main unloading pumps. Residual oil is gravity fed into an oil drain tank from where it is transferred to the main storage tanks using electrically-driven pumps.

LOCK DESIGNATION

ZONE NUMBER

PRECIPITATOR

LETTER

BOILER NUMBER

TYPICAL KEY AND LOCK

IDENTIFICATION

NOTES

1 NUMBERING OF LOCKS AND KEYS IS SHOWN FOR PRECIPITATOR CASING 4A FOR OTHER CASINGS BOILER NUMBER AND LETTER WILL CHANGE ACCORDINGLY

2 POSITION OF KEYS DURING NORMAL SERVICE (PLANT ENERGISED) IS SHOWN BY THE SYMBOLS

•KEY TRAPPED IN EARTHING SWITCH OR DOOR LOCK

0 KEY TRAPPED IN EXCHANGE BOX

4 KEY 12-FITS LOCKS 12AND 12 KEY 12 FITS LOCK 12 ONLY