Contents

1. NPL Power Limited 1.1 Single line diagram

2. The Switch Yard2.1 The general overview2.2 Transmission lines2.3 Bus bar schemes2.4 Isolator2.5 SF6 circuit breaker2.6 Instrumentation2.7 Step up Transformer

3. Engine hall3.1 Generator 3.2 AVR3.3 Diesel engine3.4 Steam Turbine3.5 Governor control

4. HRSG4.1 The process overview4.2 Auxiliary boiler4.3 Actuators

5. Protection schemes5.1 Transmission lines protection5.2 Bus bar protection5.3 Transformer Protection5.4 Generator Protection5.5 Surge Arrestor

6. Sensors applications6.1 Mass flow meters6.2 Speed measurement6.3 Temperature measurement6.4 Vibration measurement6.5 Oil mist detector6.6 Pressure sensors6.7 Position sensors

7. Control operations7.1 Control room operations7.2 AVR operation modes7.3 Synchronization7.4 Power factor control7.5 Soft start

7.6 AC motor speed control8. Workshop machines9. Fuel oil treatment

1 NPL Nishat power limited

Nishat power limited is an independent 200 MW diesel power plant. It is located along BS link canal near Jambar Kalan, 66 km Multan road. NPL has 12 generators, 11 of which are of capacity 17 MW each and are coupled with Wartsila diesel engines. One generator run by steam turbine is of capacity 14 MW. Steam is generated by heat recovery from the exhaust of the 11 diesel engines. NPL is the owner of the plant and operation is handled by Wartsila, a Finland based multinational company, handling power plant operations all over the world.

1.1 Single line diagram

There are three main aspects of power, Transmission, Generation and Distribution. We find all three of those blocks here at the power plant. The most efficient tool for getting an idea of these blocks is to look at the single line diagram of the plant. A single line diagram, as the name suggests, is a single phase representation of the system in order to provide a quick understanding of a plant’s electrical layout. Complete with the ratings of different components it can provide a good idea about the capacity of the plant and it also serves as a useful tool in electrical fault analysis. Single line diagram of NPL power plant is as shown in figure 1 attached.

The single line diagram includes these important components of the power plant.

3 outgoing lines to Pattoki, CP mill and Chunian2 HV lines, bus bars, per phase (132 KV 50 Hz 1600 A)4 transformers (65 MVA 132/15 KV no load 136KV) 11 generators with diesel engines, capacity 17 MW each1 steam turbine capacity 14 MWMV line rated at 15KV 50 Hz 3600 A4 auxiliary transformers (2500 KVA 15000/400V no load 415V)1 small transformer for admin building (400 KVA 15000/400V no load 415V)LV line at 400V1 backup generatorAlso there are several isolators and circuit breakers shown along the cables.

We discuss all of these components, their ratings, working principle and protection in detail in the subsequent sections.

2 Switch Yard

For an electrical engineer one of the most important parts of the power plant is the switch yard. The switch yard deals with dangerous high voltages so the ground is covered with gravel and stones for insulation, and the entire switch yard is barred. These stones provide a high surface area for heat dissipation and also such ground has the advantage that it does not allow stagnant water to accumulate in case of rain.

2.1 General overview

The switch yard has four step-up transformers (65 MVA). As shown in the single line diagram they step up the voltage to 132 KV from 15 KV. It is followed by a VCT and a CT for measurement of voltage and current. Then a SF6 breaker, an isolator and a bus bar. This is better described in the flow diagram below.

In the coming sub sections we look at the ratings and working principles of these components. Their function in the switch yard would be discussed in detail. Protection of these components is a vast field and it would be dealt with separately in section 5.

2.2 Transmission lines

The three transmission lines NPL connects with are supplying power to the Pattoki grid, Century mill and Chunian grid. The MV transmission lines and the HV transmission lines within the switch yard are what NPL is actually responsible for. The MV lines carry very high current up to 900 A so the diameter of a single wire to carry all that current would become very large and hence the current from stator winding to the step up transformer is divided in 5 parts and carried in 5 wires, instead of 1 very thick wire. The HV lines carry considerable less current so are thinner. All the transmission lines are made of shielded copper conductors in order to reduce the effect of stray capacitances.

2.3 Bus Bar Schemes

There are many different bus bar schemes, each of them with their own advantages and disadvantages. In this section a brief description of these schemes is provided and then we look at the scheme utilized at NPL for HV and MV bars.

Some of the common bus bar schemes are

1. Single bus bar (fig 1.1)

Advantage: it’s the cheapest and the simplest model.

Disadvantage: if the bus bar needs maintenance the plant has to be completely shut down

2. Single bus bar with sections (fig 1.2)

Advantage: its price and simplicity is comparable to that of single bus bar but the additional edge is that if one of the bus bar needs maintenance the whole plant would not shut down.

transformer CVT CT SF6 breaker isolator bus bar

Figure 2 switch yard summary

Fig 1.1

Disadvantage: in case of bus bar maintenance at least one portion of the plant would be deprived of any power source and transmission would have to be stopped partially.

3. Double bus bar with single breaker (fig 1.3)

Advantage: relatively simple relaying. Any bus bar can be isolated without interrupting the supply.

Disadvantage: requires one additional circuit breaker compared to single bus bar. Also every incoming and outgoing line has to be interrupted for circuit breaker maintenance.

Based on the merits and demerits of those schemes as stated above NPL has used “Double bus bar with single breaker” scheme for the HV bus bar and “Single bus bar with sections” for MV bus bar. The HV bus is rated for use at Voltage: 132KV Current: 1600A Frequency: 50Hz. The rated short time current 1s: 40KA. Whereas the LV bus is rated for use at Voltage: 15KV Current: 3600A Frequency: 50Hz. The rated short time current 1s: 40KA.

2.4 Isolator/Disconnector

An isolator is a device which can be used to switch from one terminal to the other while the line is off power. Unlike the circuit breaker it cannot change its state while the line is carrying current. This is because it has got no material to quench the arc which would be produced by disconnecting two conductors while high current is flowing. The type of isolators used here are called two column rotary Disconnector and it is manual operated. The ratings of these isolators are as given below

Rated voltage: 145 kVRated current: 1250 ARated short time current 1s: 40 kARated peak withstand current: 100 kA

2.5 SF6 Circuit breaker

A circuit breaker, as the name suggests, is a component used to open the circuit on receiving a trigger. It is used for protecting the circuit in case of some fault. It is just an operating mechanism for breaking the circuit, it does not sense any fault in the circuit by itself. When circuit breakers are used to break circuits at high voltages the unwanted phenomena of arcing occurs. At the moment a breaker just opens the contact, there could potentially be a huge voltage difference between two ends of the contact that the air gap sees, it ionizes and starts the process of arcing. SF6 stands for Sulfur Hexafluoride and it is used in the circuit breaker as a medium to quench the arc. The qualities of SF6 that make it an ideal material for use in circuit breakers are

1. High dielectric strength (despite a very high potential difference across it, it would insulate the two ends of the contacts)

Fig 1.3

Fig 1.2

2. Excellent thermal stability (arc quenching can generate a lot of heat but due to thermal stability SF6 can tolerate high temperature without being decomposed or changing any of its properties too significantly)

3. Good thermal conductivity (it can cool down and dissipate heat much quickly)

The operating mechanism of the breaker [6] can be best described by dividing the breaker into two parts, the pole mechanism and the operating mechanism. The pole mechanism is contained inside the bushing. It contains the contacts, both arcing contacts and the auxiliary contacts. It also contains the SF6 gas in two separate chambers, compression section and the auto puffer section. The operating mechanism contains a motor and two energizing coils. These coils get control signal and depending on those signals the closing and opening operations take place. The operating mechanism is contained inside a metal box attached to the bottom of the poles. The operating mechanism is connected to the pole mechanism by a pull rod system.

For breaking the circuit the motor would separate the auxiliary contacts and this would automatically cause the gas from compression section to be released at the time when arcing contacts separate. However if the fault current is very large like in a short circuit, the heat from the arcing would cause extra pressure to develop in the auto puffer section and gas from that section too would be released and would quench the arc. In this way the extra arcing is taken care of without putting in any extra effort. The heat produced from the arcing itself functions to quench the arc. On closing the contact, the motor pushes the piston such that both the SF6 chambers get filled with gas and are ready for next breaking operation.

The ratings of the circuit breaker used in the switch yard is

Rated Voltage: 145 kVRated current: 2000ARated Frequency: 50 HzWithstand voltage 1min at 50Hz: 275 kVRated breaking current: 40kAMaking current/peak: 100kANumber of auxiliary contacts: 9NO + 9NC

2.6 Instrumentation transformers

Current and voltage measurement:

This is done through voltage and current transformers. Since the voltage and current are too large for digital measurement, the transformers step it down. The voltage is stepped down from 132kv to 110v and the current is stepped down to 5A. The PLC (programmable logic control) just takes the input from PT and CT and calculates the phase difference between them. The values of voltages and currents are accordingly scaled and are then used to calculate the real and reactive power which is then stored and displayed.

Voltage measurement:

For measuring voltage at the higher voltage of the transformer a CVT (capacitive voltage transformer) is used. The voltage transformer is always used in parallel to the transmission line. Its secondary end is kept at high resistance to limit current flow. The ratings of this transformer are as follows

Max system voltage: 132kVRated frequency: 50HzPrimary rated voltage: 132/√3kVSecondary rated voltage: 110//√3V

Rated burden and accuracy:-winding 1: cl. 0, 5 / 30VA-winding 2: cl. 3P / 75VA

If simple turn ratio from 132kV to 110V is to be calculated it would be a very large number. Hence the conversion is not done simply by having a primary and a secondary winding according to this ratio. The Capacitive voltage transformers are used where the higher voltage is greater than 66kV. In this type of transformation we have two units, the capacitive voltage divider (CVD) and the electromagnetic unit (EMU). The capacitive voltage divider consists of a number of capacitors in series connecting phase to the ground. These capacitors act as potential divider for the AC voltage at the primary. A tapping is taken from the CVD and send to the EMU where through electromagnetic induction a voltage is induced at the secondary of the transformer. The CVD is inside a porcelain bushing and it terminates into a cylindrical unit at the bottom of the bushing which contains the EMU.

Current measurement:

A current transformer is installed at the secondary end of the step-up transformer. It is always installed in series with the line. It secondary terminals are shorted so that high voltages do not develop. The ratings of the current transformer being used at NPL are

Max system voltage: 145kVRated voltage: 145kVRated frequency: 50HzPrimary rated current: 600 x 1200ASecondary rated current: 1ARated burden and accuracies:-core1: cl. 0.5 Fs 5/ 15 VA-core2: 5P20/ 20VA -core3: 5P20/ 20VA -core4: 5P20/ 20VA

It contains a coil wound around the transmission line. The coil does not touch the line, the link is purely inductive. The current passing through the line induces a current to flow through the coil as well. The coil is calibrated such that it scales the current through the line to a max of 1A through the coil.

Both the instrumentation transformers are oil filled for cooling and insulation purposes. Significance of oil would become clear in the next section of power transformer.

2.7 Step-up transformer

NPL uses 4 step-up power transformers rated at 65 MVA. Their function is to step up the 15kV produced by the generators to 132 kV so that it can be connected to WAPDA’s grid. The reason for stepping up voltage is to reduce the line losses during transmission. The important readings from the transformer datasheet are mentioned below

Rated power (ONAN/ONAF): 50/65 MVARated frequency: 50HzNo load ratio: 136(+/-) 8x1.25% / 15Oil temperature rise (max value): 50˚Winding temperature rise: 55˚Winding Connection: YNd11

Winding connection YNd11 means that the transformer has a star with neutral higher voltage end and delta medium voltage end. The phase difference between input and output is 30˚ lagging represented by the “11” in the winding connection specification. The transformer is solidly earthed through a wire to the ground. The reason for solid earthing is that in the case of an earth fault large current would flow which could be easily detected by protection devices.

Important aspects of transformer construction are its core and windings. The core is laminated to reduce eddy current losses which are induced due to magnetic field present in the core. For the higher current low voltage side, the winding is helical whereas for the lower current, higher voltage side the winding would be disk type to withstand a short circuit fault current. Cellulose paper is used to insulate the windings from the core. For electrical insulation oil is being used.

Mineral oil serves two very important purposes in the transformer tank. First it provides electrical insulation. Oil has a very high dielectric strength compared to water or air. So even though there are very high voltages at small distances from each other, the oil does not break down (ionize) and provide a path for current to flow. Secondly oil is used as a cooling medium in the transformer tank [9]. Life expectancy is halved for every 8 degree rise in temperature. There are several different cooling techniques which are conventionally written as ONAN, ONAF, OF etc. ONAN stands for oil natural, air natural cooling. Natural oil cooling occurs through convention flow of oil inside the tank without any external aid. For forced oil cooling a motor and pump are installed inside the tank which keep the oil in motion. However in the transformer that NPL uses only ONAN and ONAF methods are available. The surface area for radiation is increased by installing radiators on all sides of the transformers. Oil flows through these radiators and passes the heat to the air outside the tank. Forced cooling is achieved through use of DC fans installed underneath the radiators. When the temperature of the oil goes above a certain threshold e.g. 73˚ the fans are switched on automatically and the cooling method is switched to ONAF. There are a total of 15 fans for forcing the hot air away from the radiators.

The transformer has an oil storage tank called the conservator tank, attached to the main tank. The conservator has the function to make sure that the transformer tank is always completely filled with oil. The pipe connecting conservator to the main tank passes through a Buchholz relay whose function we can see in the transformer protection section. The transformer breathes as it intakes air from the surroundings. The air passes through a breather into the conservator tank. Breather contains silica gel which absorbs water from the air before letting it in. water can be damaging for insulation inside the transformer. Silica gel changes its color as it absorbs moisture. The silica gel particles used at NPL change from golden to white when they have absorbed their capacity of moisture and this indicates that they need to be replaced.

At the secondary end of the transformer there are 17 taps available from the windings and any one of them can be connected to the outgoing lines. A motor unit is present to switch from one tap to the other depending on the voltage at the grid lines. As load on the grids increase, the voltage drops, the tap of the winding has to be changed accordingly. This system is called on load tap changing (OLTC). The OLTC has its own oil chamber in the conservator and its own breather. The OLTC at the transformer used at NPL allows the voltage to be changed from 122.4kV up to 149.6kV at the same input voltage of 15kV.

The transformer has 15 CT inside it for function as protection transformers, instrument transformers and for thermal imaging.

3 Engine Hall

NPL has two engine halls. Each of them has 6 Generators. One has 5 diesel engines and a steam turbine while the other has 6 engines. Many auxiliary units also function in the engine hall. In this section we

would look at the various important components of the engine hall, their ratings, functions and working principle.

3.1Generators

Some important specifications from the technical datasheet of these generators are:

Manufacturer: Converteam Ltd. Voltage and frequency: 15000V 50HzOutput: 21345 kVARated Power factor: 0.8 lagPoles: 12Speed: 500 rpmCoolant: airFlow: 9m3/secAir gap: 9.5mmExciter air gap: 2mmExcitation for normal voltage onO.C cold: 3.7AMachine rated load, hot: 10.9APMG air gap: 2mmPMG ceiling output: 214V, 6.6A, 91.7Hz

The rated powers for transformers and generators are always in kVA instead of kW like in the case of motors.

The type of excitation used here for the rotor is called brushless excitation. The current is provided to the rotor in the way explained below.

The generator can be thought of as divided in three sections, permanent magnet generator (PMG), the exciter and the alternator. In the PMG a permanent magnet is mounted on the rotor shaft and as the rotor first moves, 3 phase alternating current is induced in the stator winding of the PMG. This 3 phase current has frequency of 91.7 Hz and through calculation it tells that the permanent magnet has 22 poles. The 3 phase current is rectified in an automatic voltage regulator (AVR) and sent to the stator of the exciter. Due to the direct current on the stator of exciter a 3 phase alternating current is induced on the rotor windings. This current is then rectified by a diode rectifier mounted on the rotor. The DC thus resulting is fed to the rotor of the alternator.

Once the steady state of generator is achieved, the input from PMG is no longer required to provide excitation current for the exciter; instead the alternator is then used to supply power for excitation current and the whole process becomes cyclic.

Generator cooling is done through air. An air exhaust is provided at the top the generator. Also air is filtered for dust and moisture before it is allowed to enter. The air gap between rotor and stator is so small that even film of dust cannot be allowed to develop, and any moisture would be a great threat to the insulation.

Generator earthing is resistive. A generator neutral resistance (GNR) is inserted before connecting the earth to the ground. The purpose is to limit the earth current in case of some fault because earth fault of generator can lead to very large currents and cause severe damage to the body of the generator by inducing unwanted vibrations in the rotor due to unbalanced current.

3.2 AVR

Automated voltage regulator is placed next to the generator in a panel along with other controllers of the generator. The functions and modes of operation for AVR are discussed in the control section of the report. Listing a few functions

1) Provide excitation current by rectification.2) Synchronize generator with the MV bus.3) Control the reactive power and hence the power factor of the generator.

3.3 Diesel engines

The diesel engines used here are Wartsila 18V46. 18 represents the number of engine heads, V is for the V shaped piston and 46cm is the diameter of the piston used here. These engines use heavy furnace oil (HFO) or diesel as fuel. The operation of generator and engine under steady state can be briefly described by the flow diagram (fig. 4) below.

Each cylinder head has one air inlet and one exhaust outlet, 2 valves for inlet and 2 valves for exhaust, and one Injector pump. There are safety valves as well in the cylinder heads which open when the temperature reaches dangerously high level. The diesel engine uses 4 stroke mechanisms in the internal combustion engine to rotate the crankshaft. The 4 steps involved in each cycle of piston movement are listed below

Steps

1) As the piston goes down, it pulls in air from the inlet and the inlet valves open (inlet stage)2) Once the cylinder head is filled with air, and the piston has reached bottom dead center, the piston goes up

and compresses air. Inlet valves and exhaust valves are both closed. (compression stage)3) Injector throws in fuel at very high pressure when the piston has reached top dead center. The signal to

inject fuel is mechanically triggered by the piston. Combustion occurs and piston is pushed down due to force of expansion. (expansion stage)

4) Piston again moves up and this time exhaust valves are open. Products of combustion go out through exhaust (exhaust stage)

5) Repeat step 1

Top dead center -> piston level when maximum compression occurs

Figure 4 coupling of engine and generator

Bottom dead center -> piston level when maximum intake of air occurs

In the mechanism described above we are only getting a linear vertical motion of the piston. Through gears attached at the bottom end of the piston this linear motion is converted into the rotary motion of the crankshaft. This crankshaft is coupled to the rotor of the generator through another gear system. A small motor is attached at the point of coupling which may sometime be used to rotate the stator for cleaning and debugging purposes.

There is a certain firing order which is followed for the fuel injection in the cylinder heads. It depends on the gearing at the time of manufacture. Pistons in every cylinder head are not moving at the same phase. They reach top dead center at different times and firing order is decided keeping that in mind.

Engine cooling is done through water as there is no danger of electric short circuits in the engine. LTHT pumps (low temperature high temperature pumps) are used to pump cooling water throughout the engine cylinder heads. Air filters are used before the air inlet in the cylinder heads. These are used to cool and purify the incoming air. Cool air has higher efficiency compared to hot air because it is denser and in the same volume provides more expansion force, or in other words, uses less fuel to provide the same expansion.

To maintain a certain level of viscosity and to purify the lube oil, separators are used inside the engine hall. They make sure that only the lube oil with the required viscosity should enter the engine. The speed of the motors involved in those separators is controlled by VFD whose working principle would be discussed later.

Another important auxiliary system in the engine hall is the starting air tanks. They hold the starting air at high pressure which is required to provide some initial rotation to the crankshaft when the engines are not operating. 6 compressors are used for starting air and they keep the air under a pressure of around 30 bars. Other auxiliary systems include heaters to maintain the temperature of the furnace around 100˚c and temperature of lube oil around 65˚c.

3.4 Steam turbine

A steam has exactly the same function as the diesel engine but the execution is completely different. The source of mechanical force in this case is the high pressure steam which is used to rotate the turbine. The rpm of the turbine is greater than the diesel engine. It moves at around 5000rpm however it needs to be coupled with the generator’s rotor that should move at 1500 rpm. This coupling is done through speed reducing gears. The steam that has passed through the turbine is carried to a condenser unit where it is cooled down to form water again so that it can be reused in the boiler.

3.5 Governor/fuel control

The entry of fuel in the diesel engine and steam in the turbine are adjusted by actuators controlled by Fuel control also called governor control in case of steam turbine. The more steam or fuel that enters the turbine or engine respectively would influence the frequency of the generated AC voltage before synchronization. And after synchronization the same parameter would influence the amount of real power generated by the system. The governor set point also would be in terms of real power for example 16600W and it would then adjust the fuel or steam entry such as to achieve that much real power. In the control section of the report this would be discussed further.

4 Heat recovery steam generation (HRSG)

Almost 200 grams of HFO are consumed for generating 1kWh. The steam turbine is an innovative way to make the whole process more energy efficient. The exhaust of diesel engines is very hot. Water is used as a coolant, while cooling the exhaust at 400 deg centigrade; the coolant water is converted to steam. The kinetic energy of the steam from all 11 generators combined is used to rotate the prime mover of the 12th generator which is the steam turbine. This whole process is called heat recovery steam generation. This increases the efficiency of the plant from 46% to 51%

4.1 The process overview

The input water for the process comes from the reverse osmosis (RO) plant and the condensed water from the engine hall. RO plant treats the water and makes sure that it is pure, free of corrosive and of correct PH. Boiler is the container in which water is heated by exhaust gases to form steam. These are some of the technical specifications about the boiler:

Super heater Evaporator Economizer DeaeratorWater volume (L) 600 2250 750 1600Capacity (kW) 625 3000 327 1000Max working pressure (bars)

18.5/15 18.5/15 18.5/15 18.5/9

Max working temperature (˚c)

382/327 211/201 211/165 211/180

Min working temperature (˚c)

20 20 20 20

A boiler consists of 6 parts

1) LP evaporator2) Economizer3) HP evaporator4) Super heater5) Steam drum6) Deaerator

Some of the parts of the boiler listed above can be seen in figure 5. The four chambers of the boiler can be seen in the figure as well. LP evaporator stands for low pressure evaporator. It contains steam with very high water content and low temperature. This steam is passed to the feed water tank where it is heated and sent to the chemical dosing unit where mineral contents and PH of the water are readjusted for boiler protection. Then the feed water tank sends the steam to a pumping unit which transfers it to the economizer section of the boiler. The economizer section stores some of the steam in the steam drum where it is kept at high pressure and temperature. From the steam drum steam is moved to the HP evaporator for further heating and eventually in the super heater section the steam is perfectly dry with minimum water contents

Fig 5 boiler system

and the required temperature and pressure. From the super heater, the steam is carried to the turbine passing through further actuators along the way. The motors shown as circles on the right of the boilers are used to pump steam from one chamber to the other.

The deaerator is not mentioned in the figure. It serves the function of removing air from the system, particularly oxygen. It consists of chemicals that can absorb oxygen from the steam.

4.2 Auxiliary boiler

Apart from the 11 main boilers NPL also has an auxiliary boiler located right under the feed water tank. It is a small boiler of capacity only 77 m3. The purpose of this boiler is to generate steam for heating purposes when the plant is inactive. Furnace and lube oil need to be kept at their respective temperatures i.e. 100˚c and 65˚c before the engine could start and if there is no power from the generators then this steam from auxiliary boiler has to be used. The source of heat is a small diesel combustion chamber within the boiler.

4.3 Actuators

In case we don’t want the exhaust to enter the boiler we have actuators that close the passage of exhaust going into the boiler and open an alternate path which take the exhaust straight to the chimney. In this way the boiler is bypassed. In this section we discuss the control and working mechanism of these actuators.

There are many kinds of actuators but those which allow the maximum control are electric actuators. They are usually connected to a valve controlling flow of a fluid, in this case the exhaust gases. The important components of the actuators are:

1) Motor: any motor can be used for opening or closing the valve. In case of boiler exhaust, servo motor is employed due to its high locking ability.

2) Limit and torque sensors: They are sensors used to sense when the limit of opening and closing has been reached. These sensors transmit position of the valve as a voltage signal.

3) Gearing: locks the motor in its position and also controls the speed4) Valve attachment: Connect actuator strongly to the valve.5) Hand wheel: for manually changing the valve position.6) Control circuitry: contains feedback loops for maintaining the position of the valves. There are

switches that increase or decrease the angle of the servo accordingly.

Actuators are also used at many other locations all over the plant. They adjust the amount of steam entering the turbine, the amount of water leaving the RO plant, the amount of HFO flowing in the fuel treatment house etc.

5 Protection Schemes

The protection used must have certain attributes which would make it more efficient. First of all whatever protection is used it must be very reliable. It must only trigger an action when the device is actually under some threat, not during some abnormal operating conditions. For reliability it is a good idea to have back up protection as well. Another important quality is that the protection must be very selective. It should be able to identify exactly where the fault has occurred and then trigger the required action to isolate the faulty part without disrupting rest of the system. Another important attribute of good protection is that it finds the right balance between accuracy and speed. Both bear an inverse relation. In order to be accurate the device needs more information that can only come with time but with more time the device is more likely to be

damaged. Lastly the protection must exhibit the correct amount of sensitivity. It must not trip at every little diversion but still be good enough to respond to the tiniest of faults.

Keeping these attributes in mind many protection schemes have been designed and each of them makes use of a component called relay. NPL is one of the modern IPPs in Pakistan and all the protection here is done by numerical relays instead of electromechanical relays. Numerical relays are microprocessor based and it is just one device that performs all the required protection for several different faults. Almost all of these schemes use protection transformers, both current and voltage as well. Like the instrumentation transformers described earlier, these transformers step down voltages and current so that they can be handled by electronic devices. However there is a very purpose based difference in the working of these two types of transformers. The instrumentation transformers are not designed to measure fault currents, so their secondary output saturate very near the rated voltage or current at the primary end. Whereas the protection transformers saturate at 5 or 6 times the rated current or voltage so that information about even the high currents or voltages is received accurately.

A very brief description of some different protection schemes is provided here.

Over current protection:

Simple over current protection can be provided by feeding the secondary output of a current transformer to a relay. The current transformer would provide a representation of the current flowing through the line to be protected. (See figure 6). The plug settings of the relay can be used to set a threshold for the current above which the relay would trigger some action. The option of time settings brings two further varieties in the over current protection. These are called definite time over current relay and inverse time over current relay. The time setting allows user to program a time after detection, that a relay should wait before sending the trip signal.

In the definite time over current relay, the relay waits for a fixed time set by the user irrespective of the magnitude of the fault current. The inverse time over current relay is based on the concern that very large current should be quickly handled but with smaller fault current the relay can afford to wait for some time. So in this type of relay the time setting is done such that the waiting time is inversely dependent on the magnitude of fault current. The exact relation can have some variety which makes the relay either inverse definite minimum time (IDMT), very inverse or extremely inverse type relay.

Sometimes it is required that over current is detected in only one direction, and in the other direction the relay restrains from tripping. This type of relay makes use of the phase difference between the voltage and current to decide the direction of current and it requires both CT and PT for complete information. One of those signals usually the current signal is skewed using variable capacitors and resistors to add a little phase offset. Now when the phase difference between the voltage and current becomes equal to that phase offset, we know that in reality the voltages and current are in sync and the current is about to change direction. The electromechanical system in relay is such that maximum torque is produced at that instant and hence the phase offset is also called the maximum torque angle and it defines the boundary between trips and restrain region.

Distance protection:

Fig 6 over current relay

It deals with monitoring the impedance of the system under protection, while taking the data from one point. The impedance of a system would be a function of distance from that point. The protection is programmed such that it would create a trip signal whenever the impedance of the zone falls below a certain threshold. For doing so it requires input from CT and PT at that point. We only need to provide those input to a numerical relay and the micro processor would calculate the impedance, but in case of electromechanical relays, the voltage signal would provide the restraining torque while current signal would provide the operating or tripping torque. In case of low impedance the operating torque would exceed the restraining torque and trip signal would be generated.

Differential protection:

Another protection scheme with very precise zone is the differential protection. Differential protection is very well illustrated in figure 7.

Whenever a fault occurs in the system, the current leaving and entering it are not the same. In case of transformers, the ratio of input current to output is fixed for normal operation but random when a fault occurs. So this information about the current entering and leaving the system is utilized in this sort of protection scheme. The ratio of current transformers is selected such that the current at their secondary ends

Fig 7 differential protection

are equal during normal operation. The difference of these currents called the spill current is used to operate the relay.

A further improvement to this scheme is the percentage differential protection. There is always some error spill current which is high for larger currents. The secondary current of the current transformers are used for a restraining torque, which means that for larger currents, the excess spill error current would be compensated by the excess restraining torque.

5.1 Transmission line protection

Line protection numerical relays are used. It can be set to perform the following protections using similar schemes as explained above:

1) High impedance differential protection

It is only of secondary importance here as transmission lines are too long for efficient differential protection.

2) Distance protection (primary protection)

This is used to detect phase to phase or phase to earth shunt faults, which may occur due to bad weather and loss of insulation. This type of protection also is used to detect power swings.

3) Current protection

This includes instantaneous over current detection as well as sensitive directional protection for the lines. The feature for detecting thermal overload is also present. Current protection also includes setting for over power and under power protection. It also detects if there is some fault in the breaker by looking at the current.

4) Voltage protection

It includes loss of voltage check and detection of over voltage on the line.

5) Frequency protection

It provides under frequency, over frequency and unstable frequency detection.

A line trap is installed just before the HV lines leave the plant. The function of the line trap is to act as a low pass filter and filter out all the high frequency components of the signal entering the grid.

Earth switches in form of Disconnector are also installed before the line. This is required because when the supply from NPL is disconnected from grid then it could have some static residual charges which would be dangerous for anyone who comes into contact with the lines. A residual voltage transformer does the same job. It provides a passage for all the residual charges to ground so that any accident can be avoided.

4.2 Bus bar protection

Numerical relay device is used for bus bar protection. It offers the following protections for the bus bar.

1) Differential Protection (primary protection)

The entering and leaving ends of the bus bar are very close so differential protection becomes the most suitable scheme. It can detect both shunt and series fault in the bar. Almost all the CTs along a line are connected to this equipment and then it performs zone selection and takes actions accordingly in case of a fault.

2) Current protection

It provides only instantaneous over current protection. Directional protection is not required for the bus bar. It also includes settings for breaker fault detection.

5.3 Transformer protection

The common electrical faults that might occur in the transformer are due to loss of insulation. They include phase faults like phase to phase or phase to earth fault. For protection from such faults, the following two schemes are used by the numerical relay attached to the transformer.

1) Percentage Differential protection (primary protection)2) Over current protection3) Distance protection

Over fluxing can be detected by the relay through calculating the voltage to frequency ratio. That ratio must always remain in a certain tolerable range when fluxing in the transformer is operating normally.

When the transformer first starts, it pulls in a large amount of current called inrush current due to saturation of its magnetic core. It is quite likely that the inrush current would be greater than the threshold or trip level of the over current relay. For preventing this from happening the numerical relay then calculates the magnitude of current’s 2nd harmonic and if it is large, the relay restrains from tripping.

There is one more electrical fault in the transformer which cannot be detected by the numerical relay. This is the inter turn fault and it causes heavy currents to flow within the windings. This can be detected using the Buchholz relay.

Buchholz relay has two mechanisms to detect faults. It has a float whose level changes with level of oil in the relay. Second mechanism is a flap which detects the flow of oil from conservator to the main tank. There are 4 types of fault that this kind of Buchholz relay would be able to detect.

1) Small inter turn faults will burn oil and small bubbles will be made that will flow and be trapped in the relay. This would push down the oil level in the relay and the float would trigger an alarm

2) Heavy inter turn faults would burn the oil so quickly that it generates a fast flow of oil from conservator to the tank through the relay. The flap would then trigger a signal to trip the transformer.

3) Oil leakage would have the same effect as the above fault. Again the flow of oil would be too fast.4) Penetration of air inside the tank would result in that air being trapped in the relay and again the float

would trigger the alarm.

Finally overload protection is provided in the transformer through thermal imaging of the temperature of its core windings. The overload condition of the transformer can be judged through the temperature of its windings. However temperature of the winding cannot be directly measured as bringing any electrical equipment near the winding would cause interference. Thus the idea of thermal imaging is used. In this method a current transformer heats an element at its secondary end such that the temperature of that element is a replica or an image of the actual temperature of the winding. The temperature of that element

is then recorded and sent to a numerical relay which decides whether the transformer is in overload condition or not.

5.4 Generator protection

The generator consists of two parts, rotor and the stator. Both have their possible faults but there is one relay which deals with all those protection. Just the data from the respective CT and PT is fed to that relay and control of the breaker is provided to it. The protections provided by the numerical relay are: [15]

1) Current protection

This includes instantaneous over current protection for shunt faults in the stator windings. These faults could be phase to phase or phase to earth. Also it provides Reverse power protection through directional over current relays. If the generator is provided with reverse power due to some synchronization fault then it would start to act like a motor resulting in catastrophic damage to the engine which is coupled to it. This protection also monitors the excitation current. Unbalanced load is checked by over current protection of neutral wire.

2) Differential protection (primary protection)

Differential protection is provided to the stator windings and it basically provides protection from shunt faults and ground faults.

3) Voltage protection

Checks for over voltage at the stator windings and also checks for residual voltage when the generator shuts down.

4) Frequency Protection

It provides under frequency, over frequency and unstable frequency detection.

5) Rotor speeding protection

It provides protection to the rotor in case of over speeding or under speeding.

5.5 Surge Arrestors

Surge arrestors are used in the switch yard right next to the transformers and their task is to prevent any surge in voltage from damaging the equipments present there. They are constructed from porcelain insulated bushings, and their wires for connection to phase and to ground is kept as small as possible because under surge conditions they are likely to carry very large currents. Surge arrestors are installed in parallel to the line. It is made from a non linear resistive element which has a very high resistance at normal operating voltages of the phase so current in order of mA passes through it. But at high voltages such as those induced by lightning its resistance becomes so small that almost all the current passes through it in orders of kA.

6 Sensor Applications

Before moving to control it is important to understand how many of the important parameters in the plant are measured. Most of these sensors send analog signals to the PLC (programmable logic control) cards which then convert it to digital and process it.

6.1 Mass flow meters

The function of mass flow meter is to provide the mass of fluid flowing per unit time through a pipe or container. There are many ways to measure mass flows and several of them are being used at NPL. Some of the common methods are: [16]

1) Rotameters: Contains a vertical oriented glass or plastic tube with a freely moving float inside it. As a fluid moves through the tube, the float rises and eventually reaches a steady state height where equilibrium is reached between upward and downward forces. Upward forces are differential pressure and buoyancy factors, while downwards is gravity. The height of the float is a function of mass flow and can be calibrated accordingly. Rotameters are being used in RO plant of the NPL to measure the flow of water that is being treated per unit time.

2) Calorimetric flow meter: This scheme uses two heat sensors isolated from each other. One of the sensors is heated. The flow of the fluid has a cooling effect on the sensor which is being heated. The temperature difference between the two sensors then depends on how much the heated sensor has been cooled. Hence the temperature difference is a function of flow rate and can be calculated by a control unit. Such meters are being used in the engine hall to measure flow of furnace.

3) Electromagnetic flow meter (hygrometer): it has high power consumption and can be used only for conducting liquids like water. Energized coils on the pipe produce magnetic fields and EMF is induced in the fluid and that EMF is related to the flow rate. EMF is measured by electrodes mounted in the pipe. This method of measuring flow of water is being used in NPL at many of the pipes carrying water to the boiler.

4) Positive Displacement flow meter: this is one of the modern methods and a very accurate one. This method measures flow rate by rotation of a rotor caused by the fluid. The rpm is calibrated and linked to the volume of the fluid. The number of rotation is counted by an electronic pulse transmitter and converted to volume and flow rate.

5) Coriolis flow meter: The biggest advantage is that it measures the mass flow directly and not through some indirect method like temperature measurement or pressure measurement. The fluid runs through a pipe which is in state of forced vibrations. Due to Coriolis Effect on fluid, the tubes will deform and an additional component would be added to the vibrations. This would cause phase difference in the oscillations along the length of the tube and this can be measured using sensors. From these additional oscillations the mass flow can be measured. Such flow meters are used in the engine halls to measure flow rate of furnace. A remote logic unit makes the necessary calculations and the flow rate is displayed on a meter.

6.3 Speed measurement:

In a power plant we would need to measure speed of rotation in the range from 500 rpm to 20000 rpm. Two different techniques suit best to these two speed levels.

1) Magnetic pick up.

Magnetic induction is not a very fast process so it can be used to measure speeds around 500 rpm accurately but not more than that. The crankshaft of diesel engine is usually rotating at this speed. The

sensor consists of a coil and a permanent magnet. As the teeth of the shaft approaches the sensor, the permanent magnet links with the rotor and magnet field lines pass through the coil. As the teeth moves away there is change in linkage and flux through the coil and an EMF is generated in the coil. So the output of this sensor is a sine wave voltage signal whose frequency is related to the speed of the shaft.

2) Proximity Sensor.

Unlike magnetic pick up this sensor requires power and is used to accurately measure higher speeds like that of turbine or the turbo charger. Sensor consists of a voltage being generated due to Hall Effect. Hall Effect sensors consist of a current carrying conductor and are used to sense magnetic fields. When a magnetic field is present, and the electrons are moving through the conductor, they experience a force and change their path. This causes a voltage to develop across the conductor perpendicular to the flow of electrons. As the teeth of the rotor comes close to the sensor, the permanent magnet links its magnetic field to it and due to Hall Effect a voltage is generated. Voltage goes back to zero as the teeth moves away. In this way a square shaped signal is received whose frequency is proportional to the speed of rotor.

3.4 Temperature measurement

Temperature is one of the key indicators of load conditions and proper functioning of the system. In a power plant temperature of ranging from room temperature up to 700˚c would need to be measured and mostly two types of temperature measuring devices are used.

1) Thermocouple:

It consists of two junctions made of alloys and due to temperature difference between the two junctions a voltage is generated between the two terminals. That voltage is small in mV range but is accurate enough to give information about the temperature. There are many types of thermocouples, most common being J and K type. They have different temperature ranges and junction alloys. NPL uses K type thermocouple which

has chromel and alumel alloys.

2) RTD ( Resistance temperature detector)

Unlike thermocouples they require a power source. These sensors are used to measure temperature by correlating the resistance of RTD elements with temperature. The element is usually platinum, copper or nickel. They are used because of unique, repeatable and predictable resistance versus temperature characteristics.

RTD are more suitable for temperatures below 500˚c so thermocouples are used for measuring temperature of exhaust gases of the engine whose temperature may go up to 650˚c. Thermocouple gives a reading in millivolts which can easily be corrupted due to high voltages in the electrical devices, so RTD are used for measuring temperature of transformer oil, and windings of generator and transformer. In transformers the temperature of winding is not taken directly but through thermal imaging of winding as described before.

6.5 Vibration measurement

The air gap in generator is very small so any vibration needs to be closely monitored. Also vibrations are a good indicator of current imbalance which may be due to some phase fault or earth fault. For rotor contact less sensing is required and that is done through the same magnetic pick up technique used for speed. For

speed we are concerned with frequency of signal and for vibration we would have to look for variations in amplitude and signal form.

6.6 Oil mist detectors

Oil mist detectors are an application of optical sensors used in the plant. OMDs are installed in a little chamber above the diesel engine. It takes a little sample of lube oil from all the cylinder heads and then checks it for mist. If there is friction in the moving parts of engine, the lube oil burns and mist is produced in it. Hence OMD is a protective device which detects friction in the engine. An opt coupler, receiver and transmitter set, detects mist in the air and trips the generator.

6.7 Pressure sensors

Pressure switches and measurement devices make use of small piezoelectric disk shaped sensors, placed under a diaphragm. The fluid is made to hit the diaphragm and based on its deformation we get a voltage signal which is a representation of pressure. The conventional manometers can also be used to measure pressure where their use is convenient.

6.8 Position sensors

For position sensors NPL uses reed switches. These are devices consisting of a contact that opens or closes in presence of a magnetic field. So a stationary reed switch can be used to detect the position of a moving object if that object has a magnet mounted on it. To accurately measure the actual position of the object with respect to some reference, a linear voltage differential transformer (LVDT) is used. See figure 8 to understand how this type of sensor works. The moving object links primary end of the transformer to the secondary ends. There are two secondary ends shown in the figure. The respective secondary voltage developed on each end would tell about the relative position of the object.

7 Control operations

Control room uses programmable logic cards to interpret meaning out of the voltage signals received from the sensors and communicate it to the outside world. One of the basic functions of control room is to acquire data and display information. The important information which is displayed on the panels:

1) Generator control panel

Information available

Name of information measurement

Range available Sample values from a running generator

Phase current L1 0 to 2000 A 760 A

Figure 8 LVDT working

Phase current L2 0 to 2000 A 760 APhase current L3 0 to 2000 A 760 APower factor 0.5 lagging to 0.5 leading 0.9 leadingFrequency 45 to 55 Hz 49.5HzVoltage 0 to 20 KV 13.9 KVEngine speed 0 to 650 rpm 480 rpmTurbo speed 0 to 30000 rpm 22000 rpmActive power 0 to 20 MW 16 MWReactive power 4 Mvar used to 15 Mvar supplied 7.5 Mvar supplied

Power Monitor unit (displays)P 16700 KWQ 7934 KvarF 49.77 Hzᶲ 0.90

Generator protection relay (displays)IL1 727 AIL2 697 AIL3 711 AIO2 0.015 A

Generator differential current relay (displays)IL1 738 AIL2 704 AIL3 716 A

2) Turbine instrument control panel

Information

Label RangeControl oil pressure 0 to 30 barsLube oil pressure 0 to 4 barsTurbine speed 0 to 10000rpm

Turbine speed ranges:2000 to 3000 rpm (critical speed)3000 to 6200 rpm (normal running speed)6200 to 6300 rpm (over speed block)

Displays

1) Human machine interference2) Vibration monitor

3) Generator control panel (steam turbine)

Information Range Power factor 0.5 lagging to 0.5 leadingReactive power 0 to 20 MvarActive power 0 to 20 MWExcitation field voltage 0 to 200 VGeneration frequency 45 to 55 HzExciter field current 0 to 15 AGenerator ammeter 0 to 800 AGenerator voltage 0 to 20 KV

Displays

1) Reactive Mvarh2) Active MWh

The manual control operations available are as follows.

1) Generator control panel

Push buttons

Label Option 1 Option 2Engine Start StopBreaker Closed OpenEngine Shutdown ResetBreaker trip ResetLamp test

Knobs

Label Option 1 Option 2 Option 3Generating Auto Set control manual

Engine control Speed droop KW isochGenerator control Voltage droop Pf VdcFuel Decrease IncreaseExcitation Decrease IncreaseSynchronizing 0 Select/startSelection of speed measurement

Engine Turbo A Turbo B

2) Turbine instrument control panel

Push buttons

1) Turbine emergency stop2) Audible alarm mute

Knobs

Label Option 1 Option 2 Option 3Turbine speed control

Lower N Raise

Over speed Off TestControl mode Local Remote

3) Generator control panel (steam turbine)

Knobs

Label Option1 Option2 Option3 Option4Generator voltage Lower N RaiseExcitation control Off OnAmmeter selection Off L1 VTurbine speed control Lower N RaiseVoltmeter selection Off U-V V-W W-U

Push buttons

1) Lamp test2) Auto sync initiate3) Fail to sync reset

The systems which are being controlled are:

1) Generator set2) Electrical3) Power

4) Lube oil5) Emissions6) Air system

7) Fuel oil8) Heat recovery9) Oily water

10) Water supply11) Fire fight12) Start air

13) Automation14) Temp/press15) Ambient

16) Deaeration17) Cooling18) Exhaust gases

7.1 Control room operations

For the systems mentioned above, the control room engineer must always be alert and looking for any slight indication of malfunctioning. The temperatures must always be kept under control and if something trips then it must be identified and the appropriate action should be taken. Also one of the duties of control room operator is to isolate any part of the plant for repair or maintenance purposes. The isolation must be made without interrupting rest of the plant and breakers must be opened or closed keeping synchronization in mind. Also while transferring loads care must be taken not to exceed ratings for any components. A sample procedure for isolating a transformer and connecting it back to the grid is as follows. Consider the transformer arrangement in figure 9.

T1 and T2 are auxiliary transformers. LV bus tie is normally open and all the other breakers are normally closed. Suppose the transformer T1 is to be isolated for maintenance the procedure would be to:

1) Synchronize the LV bus tie and close it. This would transfer the load of T1 to T2.

2) Open LV CB1. Now the transformer is at no load condition.3) Open MV CB1.4) Close earth switch of transformer so that it is safe to handle.

In this way T1 is isolated.

Now suppose the maintenance is complete and we wish to restore the transformer T1, the procedure would be as follows:1) Open the earth switch2) Close MV CB13) Synchronize LV CB1 and close it. Now the transformer is supplying to load.4) Open bus tie LV so that the transformer has its entire load.

7.2 AVR operation modes

The major electrical control available for the plant is the AVR. It has 4 modes of operation [17] and it can shift from one mode to another on operator’s control or smartly by itself looking at the phase of generation. In all of these 4 modes it is controlling some important variable of the power generated by the plant through an open or close loop.

1) Automatic Voltage regulator

In this mode it regulates the terminal voltage of the synchronous generator for synchronization. See figure 10.1.

Sensors: CT and PTActuator: Generator

Fig 9 transformer arrangement

Fig 10.1

Control variable: Terminal voltageController: AVR PID control

2) Manual control

In this mode it regulates the field current of the exciter. See figure 10.2

Sensors: CTActuator: GeneratorControl variable: Field currentController: AVR PI control

3) PF or Var regulator

In this mode the AVR must maintain a certain PF by adjusting the value of reactive power. See figure 10.3

Sensors: CT and PTActuator: GeneratorControl variable: Power factor and reactive powerController: AVR PID control

4) Open loop

In this mode the AVR just once provides a fixed field current and then does not regulate it. See figure 10.4.

Sensors: NoneActuator: GeneratorControl variable: Field currentController: none

7.3 Synchronization

When the generator is isolated, its frequency is controlled through the rpm of its rotor, which in fact is controlled by the fuel intake. Fuel intake is adjusted by governor set points. The voltage at the terminal is controlled by the excitation current. These two parameters can be controlled independent of one another. For synchronization the terminals of generator and the MV bus bar should have the same voltage, similar frequency, same phase sequence, and 0 phase difference at the time of connection. Synchronization of generator with MV bus bar is one of the functions of AVR. The setup is illustrated in figure 11. [18]

For Synchronization AVR acquires the following inputs

1) Line voltage using a PT2) Generator voltage using a PT3) Generator stator current using a CT

From these inputs it calculates the following information

Fig 10.2

Fig 10.3

Fig 10.4

Figure 11

1) The slip for the machine. This would be the difference between f setting and frequency of the bus.2) The voltage difference between MV bus and generator frequency3) The phase difference between MV bus voltage and generate terminal voltage.

The program then works to minimize the slip. For that it needs to adjust the frequency at which governor runs the turbine. So it provides the governor a frequency bias so that governor can change speed of turbine. For minimizing the voltage difference, it would need to change the terminal voltage of generator by adjusting the excitation current. Once these objectives are achieved within a certain limit, the AVR issues a close command to the syncronocheck relay. The syncronocheck relay then waits for the moment when the generator and bus voltage cross the zero point together. At that point it closes the circuit breaker and the generator is connected to the bus. This also deactivates the synchronize command and the AVR switches to some other mode.

F bias signal is always zero when either of these conditions is true

1) AVR is not operating in synchronizing mode2) The frequency of bus is not between 45 and 66 Hz

7.4 Power factor control

The technique for power factor control is called voltage droop compensation (VDC) [19]. Real power is controlled by the governor set points, so in order to get the required power factor AVR adjusts the reactive power by changing the excitation current. When the generator is in sync with the bus, its terminal voltage is then fixed with the bus voltage and would not change by changing the excitation current. The only thing that would change with the excitation current now is reactive power as shown in figure 12.

The operating voltage would always stay at V bus but the reactive power can be changed from Q1 to Qsp by changing the excitation current. Changing excitation current would in effect change the no load voltage from V1to Vsp. So in order to compensate for the droop we change the no load voltage.

All the AVR are connected to each other through RS 485 bus and they communicate with each other the reactive power being supplied. Each AVR or unit reads those values and so every AVR knows the power factor at which all generators are running.

3.5 Soft start

When the generator is to be started from 0 voltages at its terminal, the AVR takes its terminal voltage up to the bus voltage in the manner shown in fig 13. This is known as soft start. The reason for providing some hold time is to minimize the in rush current by allowing time for it to induce a back EMF in the system at a low voltage.

7.6 AC motor speed control

When an induction motor starts, it pulls in a lot of inrush current while doing very little useful work. This means a very poor efficiency and wastage of energy. The reason for in rush current is that at the start no

Fig 12

Fig 13

Back EMF is generated and the terminal voltage sees only the winding resistance. One way of starting such a motor is to provide it with variable voltage and frequency, starting with a low value and slowly ramp it up. In this way inrush current would develop only for a small voltage and hence be limited.

Under steady state AC motor drives vary their frequency based on a control signal to adjust the speed of the motor. One such system could be the LTHT pumps of the diesel engine. It may be fitted with a temperature sensor and a control unit which makes the motor pump cooling water depending on the temperature of the engine. This way is more efficient than pumping water at full speed all the time. Such AC motor drive based control systems are also used in Lube oil separators, RO plant water transfer unit and FT house feeder units. Modern AC drives also contain over load protection for the motor and it can trip the motor in such a case.

8 Workshop Machines

Workshop is the place of electrical and mechanical repair and maintenance of components from the plant equipment. Workshop has a small instrument air compressor of its own for generating instrument air at 7 bars used for drying parts that have been washed and for cleaning parts by blowing dust off them. Some other machine available in workshop and their function is as follows:

Sand blasting machine: for cleaning large machine parts without using water.

Lathe Machine: for small operation like turning, facing, drilling etc

Bench drill machine: Common drill machine for drilling holes in metal objects.

Hydraulic press: for inserting bearings in its housing at high pressure.

Valve grinding machine: for tuning the shape of the valve and smoothing its edges.

9 Fuel treatments

The fuel is first unloaded from the containers at the decanting area. Fuel is pulled in by pumps and carried to the pipes. There are heaters available in case the furnace is too cold to move. Sludge from the container is first separated here and is carried in a separate pipeline. The pipe carries the furnace to a large storage tank. NPL has three of those. From storage tank, fuel is transferred to buffer tank through pumps installed in a transfer unit at the FT house. From the buffer tank, furnace goes to a separator unit in the FT house where it is purified to remove water, sludge and any solid particles. The technique used to purify is centrifugation which separates fluids based on their specific gravities or viscosities. Water and sludge being denser than furnace settle at the bottom of the centrifuge tank. The furnace at day tank is then ready to be used by the engine. It is transferred to the engine hall using pumps at the feeder unit of the FT house. The motors in feeder unit are controlled by VFDs which adjust the speed of pumping based on furnace tank oil level and the engine fuel requirement. Inside the engine hall is the booster unit which pumps furnace to the fuel injectors and heaters that maintain the temperature of oil near to a 100˚c.