NATIONAL POWER AUTHORITYREPORT ON PUPILAGE TRAINING AT THE GENERATION DEPARTMENT-KINGTOM POWER STATION FROM 1 ST SEPTEMBER TO 31 ST DECEMBER 2010 AS PART FULFILMENT FOR THE CONFIRMATION TO ENGINEER SUBMITTED TO: GENERATION DEPARTMENT- KINGTOM PREPARED AND SUBMITTED BY: MOMODU MANSARAY
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At the switch gear room the brand and type of switches are mainly the Yorkshire –
F6 switch gear. Their current rating is 630A and 1250A for feeder and generator
breaker and 2500A Bus-Bar rating. The other type found in the switch gear room is
the Merlin Gerlin type of fuses also rated at 630A.They are gas operated switch
gears and can be charged automatically or manually .Some are remotely operated
from the control room whilst others are manually operated down in the switch gear
room.
Though electrical power generation is now from different generating sources like
the GTG, Bumbuna and Niigata most the generated electrical power has to gothrough the 11 KV bus bar systems at the Kingtom power station. This is achieved
by means of synchronizations. I.e. voltage, Phase angle and frequency must match.
Also there are other supply points from the Bumbuna which does not pass through
the 11KV system arrangement at the kingtom power station. One such supply goes
straight from the 161KV transformer sited at the Bumbuna Freetown Site at
Kingtom to Congo Cross 2 through the JICA newly constructed overhead lines and
the other feed is known as Mandalay2
Though the following machines were not operational,
Sulzer 4&5;
Mirless 1 & 2;
Mitsubishi and
Caterpillars
Their alternator systems and the method of excitation is hereby explained below
ALTERNATOR SYSTEM
SULZER MACHINES:
Excitation by commutation- brush type alternator (magnetized by
flashing).
1. After rated speed (synchronous) is achieved, the field switch in the control
5. This rotating diode then rectifies the AC voltage into DC voltage that is
conducted into the main rotor field coils along a common shaft
6. The energized rotating rotor (current carrying conductor) develops fluxes that
cut into the main stator winding
7. A voltage (about 11kV) is produced in the main stator winding
8. Part of the 11kV is passed through a step down transformer (excitation
transformer) to produce 50V ac (maximum output) that is sent to the AVR
while the bulk of the power is sent to the main bus bar. The AVR isolate the
PMG from the system when about 40% of the output voltage is produced
The supplied AC power to the AVR then continues the excitation process
endlessly. Since Mirrlees 2 output voltage is 6.6kV, an inter bus transformer
(6.6kV/11kV) is installed between the AC output and the 11kV switchgear
room
EXCITATION SYSTEM
The exciter rotor is a three-phase AC with six leads (three to a +ve plate and three
to a –ve plate) for diode connection. The diodes are connected to form a full wavebridge rectifier. These rotating diodes are mounted on an alluminium heat sink.
A varistor (non-linear resistor) is connected across to DC terminals from the rectifier
to prevent surges generated in the main field coil (winding) from damaging the
diodes. The principle is also applicable to the Caterpillar, Mirrlees 2 (brushless
alternator) and Mitsubishi engines respectively. The Sulzer engine has a
commutator (instead of rotating diode) slip ring, variable resistor (instead of
varistor)
EXPERIENCE GAINED:
During the training I participated in the following electrical maintenance work.
• repair work on the overhead crane in the power house
• the drain pit pump motor control system
• installation of an AVR on a 60KVA Gen. set
• refurbishment of one YSF6 cubicle switch gear
• Work and function test on the automatic transfer switch at President Lodge
after Lighting strike
1st-22nd October
MECHANICAL MAINTENANCE:
This section is responsible to work on all breakdown of machinery that is
mechanical. They either fabricate these parts by the use of the lathe machines
where necessary or put in requisition for the supply of the worn out parts to stores.
Where these parts are not available in the stores, then an order is placed with the
full knowledge of the Management of the generation department to the
Management of the authority
This section is now a shadow of it former status as the only machines which are now
operational are the Niigata machines which are very new ones and far fromdeveloping major mechanical problem as they are seldom used. Even if mechanical
faults may occurs these machines are still under the warranty period.
Maintenance methods and practices is the key to the proper functioning of this
section.
In the event of a less strategic maintenance work, skilled personnel undertake such
maintenance while highly strategic jobs require the services of expatriates or
consultants even though the bulk of the work is being carried out by local
personnel.
Basically, corrective and preventive maintenances were carried out at the Kingtom
Power Station.
Corrective maintenance
This is an unplanned maintenance as it is carried out on an emergency basis.
Breakdown maintenance is carried out without proper planning and hence a
predetermined downtime of the plant could not be determined. The unavailability of
readily needed spare parts to bring the plant into operation at the shortest possible
time could be a major constraint.
Plant defect corrective maintenance is carried out on the plant to arrest defects that
might not sometimes necessarily lead to the shutting down of the plant as it does
not have any performance reduction effect on it and pose no threat to its smooth
running. The execution of such maintenance job can be deferred to coincide with
the plant shutdown to reduce engine down time. If however the defect threatens
the smooth running of the engine, it is shut down to arrest the problem
immediately.
Planned maintenance jobs (major overhauls) are executed following a long term
planning of activities to be undertaken without the plant necessarily breaking down.
Such maintenance is guided by the recommendations of the manufacturer that
would include the necessary spares, maintenance activities, and duration for the
successful completion of the job. When necessary, the services of expatriates are
sought for which detailed report is submitted by the consulting firm at the end of
the job.
Routine maintenance practices rely on recommended set of job activities that
should be executed within a stipulated time frame set by the manufacturer. The
operational performance, the working environment and the hours run of the plantare taken into consideration and the time interval adjusted to suit the prevailing
circumstance. Samples of maintenance work order are also submitted to give a clue
on few of the maintenance activities carried out under this type of maintenance.
With this type of maintenance practice in place, breakdowns are minimized, engine
•• V-block (16 cylinders for Mitsubishi and 12 cylinders for Mirrlees )
•• Medium speed (750 rpm for Mitsubishi and 600 rpm for Mirrlees )
•• Only intake and exhaust valves present
•• Both air and water cooled: water to a greater extent and air to a lesser
extent. The air cooling is an added benefit from the scavenged air.
••
Air starting
•• Designed to consume both heavy and light fuel
CATERPILLAR ENGINE
•• Four-stroke
•• V-block with 16 cylinders
•• High speed (1500 rpm )
•• Electric starting ( with batteries )
•• Consume light fuel only
Working cycles
The working cycle of an engine may be four-stroke or two-stroke; and the enginemay be single-acting or double-acting. These cycles are mechanical sequences of
events for the functioning of the machine. The working cycle (induction,
compression, expansion, and exhaust) is basically the same on both two-stroke and
four-stroke engines but the individual phases take place at different times and at
• Air starting system; comprising of starting air, air bottles, intake and
exhaust systems
• Fuel system; comprising the heavy fuel oil, light fuel oil or both
• Lubricating system; comprising the system oil and sump oil
Cooling system
The purpose of the cooling system is to dissipate to the atmosphere the heat which
builds up in engine components such as pistons, cylinder block and cylinder head
and in the engine oil. The cooling system is necessary because the heat resistance
of the materials used in the engine and of the engine oil is limited. As a result someconsiderable percentage of the thermal energy obtained from the combustion
process is lost. Effective cooling permits higher engine performance as a result of
improved filling.
The cooling system at the power station can be divided into two:
• The Sulzer and Mirrlees engines use the same cooling system
• The Mitsubishi and Caterpillar engines use the radiator type cooling system
For the Sulzer and Mirrlees engines, water via electrically driven pumps fitted with
strainer baskets is drawn from the sea and then delivered to four Alfa Laval made
parallel) installed outside the power station. Heat exchanging between the seawater
and hot water coming from a fresh water heat exchanger via a collecting tank takes
place. The hot seawater after the exchange process discharges into the sea at the
opposite end. The seawater forms part of the tertiary cooling loop (open circuit) and
the hot fresh water coming from a pair of fresh water heat exchangers installed at
the basement of the power station (which are smaller in size as compared to the
ones installed outside the power station) via a hot water collecting tank/pit formpart of the secondary cooling loop. The hot fresh water that goes through the
seawater heat exchangers becomes hot as a result of heat exchanging with fresh
water that directly cools the engine components. This fresh water forms the primary
Air starting is almost universal for large engines (requiring a storage tank plus a
compressor). The compressed air is piped to the storage tank. At the power station,
all the base load engines require air for starting. The air is being produced from
electrically driven air compressors and is stored in steel air bottles fitted with airpressure gauges to indicate pressure level. The engine starting air pressure is
maintained at a minimum pressure of 20 bars. The steel air bottles are however
interlinked through pipe networks to allow for flexibility in the use of a compressor
for starting various engines. Other components include the charged air cooler,
turbocharger, etc.
Fuel system
The task of the fuel supply system is to deliver to the fuel injection system with
sufficient fuel in all operating conditions. The fuel system comprises of both light
and heavy fuel oil.
The light fuel oil (diesel) is stored in the main diesel storage tank from where it is
pumped into the day tanks of the different engines via a transfer pump. From the
day tanks, delivery into the injectors is made possible with the use of a booster unit.
This unit increases the fuel pressure delivered to the injectors and consists of
duplex fuel filters which could be switched over from one filter to the other during
running for the purposes of cleaning. A change over 3-way switch is provided to
regulate the type of fuel required.
The heavy fuel oil also has its main storage tank. It is the main fuel oil that is used
by the base load engines and it is highly viscous containing impurities like water,
metallic components etc. which need to be considerably minimized to reduce the
risk of them entering the fuel line thereby creating problems to the smooth
operation of the engine. The fuel is transferred onto the regulating tank via transfer
pumps 1 and 2. The heavy fuel oil is heated partly by steam from a heat recovery
boiler and partly by electric heaters to reduce its viscosity. It then flows into the
various separators/purifiers for the removal of impurities and metallic compounds
and then sent to the various engines daily service tanks. From the service tank, it is
transferred to the booster unit; the engines are supplied with fuel through the help
of the transfer and booster pump capable of increasing fuel pressure to anacceptable pressure and circulating enough fuel more than that consumed by the
engine. The surplus fuel flows back into the day tank. Owing to the difference in
mass density and the time taken in the day tanks, the rest of the water that
bypassed the separator settle at the bottom of the day tanks. For this, the day
tanks are provided with drain pipes to drain off the water periodically. All the
wasted fuel as a result of overflowing of the regulating tank, fuel pumps and pipe
leaks etc. is collected at the basement and then pumped to an oily water separator
tank for recycling. The fuel flow lines are wrapped with special lagging material to
help reduce heat loss along the flow lines to the environment. A diesel fired
auxiliary boiler is provided in the system to serve as a standby in case of a failure in
the engine’s heat recovery boiler as in the case with the two Sulzer engines.
Lubricating system
The main task of the lubricating system is to prevent friction between the moving
parts of the engine. This is done by supplying them with an adequate flow of
lubricating oil. This oil has the task of cooling engine components which cannot
dissipate their heat directly to the cooling system and the outside air. In addition,
the engine oil makes a seal between sliding-contact components (for instance
piston and cylinder wall), and cleans the interior of the engine by flushing out
deposits and combustion residues. The lubricating
oil system consists of lubricating oil pumps which are normally driven by electric
motors. The oil delivered by the pumps passes through filters and coolers,
circulated with sea-water, and then branches into a low-pressure and a medium-
pressure system. The pressure in the low-pressure system is regulated by a valve
and supplies lubricating oil to the main bearings, thrust bearing, camshaft,
camshaft drive wheels, fuel pumps, chain drive and the rotating shafts in the control
stand. It also provides oil for cooling the crosshead guides.
The medium-pressure system lubricates the crossheads, lower connecting-rod
bearings, and the slippers. It also provides oil for the control-oil system and for the
turbocharger lubrication, if main engine oil is used for this purpose.
The cylinder lubricating is independent of the rest of the lubricating system. Theamount of cylinder oil necessary for the engine depends mainly upon the type of oil
used, the quality of the fuel and the loading of the engine.
Every section within the generation department is important .The stores sectionperforms the function of receiving and issuing goods. Simple as it appears but it
entails much more. At the generation department stores operations are divided
amongst the various sections as follows:
Receiving and issuing of all Fuel oil and Lubricating oil store
Machine spare part store
Electrical components store
All other stores materials other than those mentioned above are requested from the
main stores at head office or the other stores at Blackhall Road and Falconbridge.