Chapter 1: Introduction
Ghorashal Power Station (GPS) is the largest power station in Bangladesh. It has started its operation in 1974. The total capacity of the GPS is 950 MW which is generated by six units of gas fired steam turbines
- The type of power plant of GPS is thermal power plant. It is a great opportunity to do internship at GPS. During the time of internship we have achieved practical knowledge of generation, operation, substation, instrumentation and control section of GPS. Here we relate the theoretical knowledge with the practical activities in GPS. In this report we will discuss what we have observed and learnt in GPS.
1.1. Objective of the Internship
The main goal of this internship is to achieve practical knowledge and experience about power station. In this internship report, we focused on generation process, various protection systems, control system, substation, and maintenance of the equipment such as generator, transformer and transmission of electricity at GPS.
1.2. Company Profile
GPS is the largest power station in Bangladesh. It is situated on the Eastern bank of river Shitalakhya at North East of Dhaka under Palash, Narsingdi district. In this power station, natural gas (CH4) is used as fuel from Titas Gas Transmission and Distribution Company.
The profile of GPS is given below:
Name : Ghorashal Power Station
Year of establishment : 1974
Corporate office : Bangladesh Power Development Board (BPDB)
|Status||: Public Limited Company|
|Area||: 0.43 km2|
|Company Name||: Technopromexport, Russia|
|Main Work||: Power generation|
|Number of Generation Units||: 6 (Steam turbines)|
|Installed Capacity||: 950 MW|
|De-rated Capacity||: 450 MW|
|Regular employee||: 1200|
Above information are collected from the mentors of GPS.
1.3. Company Mission
The mission of GPS is to deliver quality electricity at reasonable and affordable price with professional service and generate more than GPS is generating at present. Target of GPS is to increase power generation capacity to 1300 MW by December, 2016. The upcoming project at GPS is a Combined Cycle Power Plant with a capacity of 365 MW.
1.4. Production Capacity
GPS has 6 units. 2 units with 55 MW capacity each and 4 units with 210 MW capacities each, all those units are steam turbine power plants. All the units have been installed by a company named Technopromexport, Russia. The description of the power generating units of GPS is shown in table 1.
Table 1: Capacities of different units of GPS.
|Unit no||Installed Capacity(MW)||De-rated Capacity(MW)||Present Status|
|Unit-6||210||Under tendering process|
|for repair since 2010|
|Total = 950 MW||Total = 450 MW|
1.5. Future Plan
GPS installed generation capacity is 950 MW. A plan has been taken to increase the total established generation capacity. In order to achieve this target, GPS has decided to set up a new power plant project named GPS 365 MW Combined Cycle Power Plant Project. To install GPS 365 MW Combined Cycle Power plant, Bangladesh Power Development Board (BPDB) signed turnkey EPC (Engineering procurement construction) contract with China National Technical Import & Export Corporation and China National Machinery Import and Export Corporation. This contract was signed on 25th May, 2013. It is estimated that the construction of GPS 365 MW Combined Cycle Power Plant Project will be completed in 900 days by turnkey EPC contractor. Still it is under construction. The Power Development Board (PDB), the Ministry of power, Energy and Mineral Resources declared that the project will be finished by December 2018 . After the construction the generation capacity will increase and it will add to the national grid.
1.6. Scope and Methodology
This internship report is based on the knowledge we gathered during the internship period where we had experienced and reviewed the basic processes of GPS. It covers the generation, operation, substation, instrumentation and control section of GPS. The data we used here are collected during the internship period. The discussions with the superintendent engineer was very helpful.
Chapter 2: Water Treatment Plant
Water is used to generate steam in all thermal power plants, where steam is used in turbine as prime mover. Another application of water is to condensate the steam into water. The condensation is used to increase the efficiency of the thermal power plant. Water treatment plant is very important for production of steam and cooling of used steam. The source of the raw water is Shitalakhya river. This raw water is purified and softened by mechanical and chemical processes. Then it is supplied to the system. In this chapter we will discuss about the process of water treatment. The water treatment process is shown in figure 2.1.The main objective of water treatment plant is to produce mineral free water which is called demi water. It is needed for managing three problems. These are:
- Corrosion and
- i. Scaling
Scaling occurs under a certain temperature and chemistry when the dissolved mineral salts form solid deposits. In GPS, when river water flows in the pipe, layers are created on the metal surfaces. As a result pipe width decreases and increases the energy which is used in pumping the water through the pipes.
Corrosion means destruction of metal because of chemical reaction. Corrosion occurs when metal ion is combined with oxygen and water. GPS uses river water which contains large amount of metal. As a result water and oxygen mixed with iron and creates rust. This rust damages the water pipes. When corrosion occurs in GPS, the water pipe line is gradually damage the water flow. In long term pipe will be damaged for corrosion which may create permanent damage of equipments.
Erosion is a mechanical process. It occurs for water flow, friction or other mechanical processes. In GPS, the raw water flow into the pipe this water contain large amount of minerals which create a friction with this pipe. This friction causes damage to the equipment.
The flow diagram of water treatment process is shown in figure 2.1.
Figure 2.1: Flow diagram of water treatment process in GPS.
2.2. Water Treatment Procedure
Water is collected from the river, Shitalakhya. This raw water has two types of substances:
- Dissolved solid and
- Un-dissolved solid.
Dissolved solids are different types of ions and minerals. Un-dissolved solids are clay, mud etc. By following several steps, the solids can be removed. GPS mainly follows these steps.
Here, the raw water is mixed with Al2SO4. The sludge is deposited in a tank. This sludge is removed from the tank and the clarified water is stored in coagulated tank. By using clarification, 70% of un-dissolved solid from water is removed. The chemical reaction is
Al2SO4+Clay (H2O) →Al(OH)3 (sludge)+H2SO3
Clarifier tank and clarifier system of GPS is shown in
figure 2.2 and
The clarified water is used for filtering. After clarification, the water contains a lot of small or big particles which are harmful and destructive to the plant. The filter removes any solid particle like sand, silt etc.
Demineralization is the process of removing mineral salts from water by using ion exchange process. Demineralized water is also known as deionized water with removed mineral ions. Mineral ions such as cations of sodium, calcium, iron, copper, etc and anions such as chloride, sulphate, nitrate, etc are common ions present in water.
This deionized water contains positive ion like Mg++, Fe+++, Al+. When it is passing through cation exchanger resin, these cations are caught by the following reactions. R-H2+Ca(HCO3)2→ R-Ca+2H2CO3
This water also contains negative ion like Cl–, SO42-, NO3-. When passing this water through anion exchanger resin, these anions are caught by the following reactions.
R-(OH)2+H2SO4 → R-SO4+2H2O
This demi water is stored in demi tank. Then this water is used for producing steam.
2.3. Electrolyzer Plant
Electrolyzer is used for electro-chemical decomposition of water to obtain hydrogen and oxygen. In GPS, hydrogen is used for generator cooling and oxygen is released to the atmosphere. Electrolyzer has these headers: the upper one is used for removal of gases and electrolyte and the lower one serves for returning cooled electrolyte to cells. All elements of electrolyzer are assembled as a pack and fastened with four tie bolts. Electrolyzer of GPS is shown in figure 2.3.
2.3.2. Raw Materials of Electrolyzer plant:
- Demineralised water: This is used for producing steam.
- Potassium hydroxide: This is used for increasing conductivity
- Potassium dichromate: This is used to prevent leakage current.
- Mixing ratio of potassium hydroxide and potassium dichromate is 10:1.
2.3.3. Different Equipments of Electrolyzer Plant:
Separator columns are intended for separation of gases from electrolyte and for cooling and circulation of electrolyte.
Pressure regulator is a vertical vessel used for maintaining equal pressure of hydrogen and oxygen in electrolyzer. Pressure regulator vessel is shown in figure 2.4.
Surge tank serves for keeping a stock of demi-water feeding into electrolyzer. Demi-water is used to restock the electrolyzer cell.
Alkali tank serves for preparation of electrolyte. It is equipped with electrolyte pumps.
Water-locks are devices serving for release of oxygen to the atmosphere and at the same time for prevention of air into the electrolyzing plant system.
Receivers are vessels introduced for storing a stock of hydrogen and supplying of hydrogen into generator cooling system. The receiver of hydrogen gas is shown in figure 2.5.
Chapter 3: Steam Generating Plant
Boiler or steam generator is a device which generates steam at predetermined temperature and pressure. It may be used in steam engine or turbine. In GPS, the main purpose of boiler section is to produce steam which is used to run turbine. Boiler is designed to transmit heat produced by burning of fuel to water and thus generate steam. To produce steam, the boiler has some requirements. The primary requirements of the boiler are:
- Water must be safely contained.
- Steam must be delivered at rated pressure, temperature.
- Maximum heat from external combustion source should be utilized.
In GPS, there are six water tube boilers to generate steam for the power plant. The water is treated in the water treatment plant and the treated water is passed to the boiler and converts into steam. The steam is collected into the steam drum which is a part of the boiler. Table 2 shows the different boiler capacity and their efficiency.
Table 2: Capacity and efficiency of boiler.
|Unit 1, 2||Unit 3, 4, 5, 6|
|Maximum capacity||170 ton/hr||670 ton/hr|
3.2. Different Types of Boiler Equipments
Furnace is a chamber which is situated inside the boiler. Here, with the presence of air the natural gas is burned to produce heated gas or flue gas. The ratio of gas and air is 1:14. In GPS, every furnace chamber has nine furnaces. Inside the chamber the temperature is 15000C to 18000C. The treated water from the feed water tank enters into the furance through tubesand flue gas produced inside the furnace. The flue gas releases heat to the water as a result water becomes saturated steam. Saturated steam temperature is about 350°C. At the beginning of the firing the burner needs small amount of natural gas and air. This small amount of gas is known as ignition gas which is supplied by ignition pipe or line. Draft fan is used to supply the air into the furnace. Finally the saturated steam from the furnace goes to the boiler drum.
3.2.2. Boiler Drum
Boiler drum is used as a reservoir of steam and water. In the boiler drum the level of water is measured by a level transmitter. When the water level crosses the limit then the plant will trip. So it is very important to control the level of the water in boiler drum. This is done by an automatic system. From the boiler drum the saturated steam is transferred into super heater. At GPS, we observed boiler drum which is shown in figure 3.1.
3.2.3. Super Heater
It is an integral part of boiler and is placed in the path of hot flue gases from the furnace and the saturated steam is converted into a super heated steam. When steam has no water it is called super heated steam. The super heated steam’s main purpose is to increase the temperature of saturated steam without raising its pressure. In GPS, temperature of the super heated steam inside the super heater is about 540°C. There are three super heaters inside the boiler section of every steam power plant of GPS. There are bundles of tubes inside the super heater which carries the saturated steam and the flue gas passes around these tubes. The flue gas releases heat when it passes around the tubes and the saturated steam receive the heat and becomes dry.
Function of economizer is to recover some of the heat which is carried away by the flue gas. The recovered heat is utilized to raise the temperature of feed water. Then the feed water at raised temperature is supplied to the boiler. If the feed water at raised temperature is supplied to the boiler, it needs less heat to convert the water into steam. Recovery heat from flue gas raise feed water temperature up to 345°C. Economizer is used to save the consumption of fuel.
Deaerator is a device which is used to remove air and other dissolved gases from the feed water to steam-generating boilers. The metal piping and other metallic equipment is damaging because of dissolved oxygen in boiler. It increases the efficiency and optimum thermodynamic utilization. So deaerator is used in GPS to keep safe of the equipments.
3.2.6. Flue Gas Stack (chimney)
A chimney is a structure which provides ventilation for hot flue gases or smoke from a boiler to outside atmosphere. Furnace produces flue gas. This flue gas is used to create the steam for rotating the turbine. The flue gas passes through several numbers of equipment and finally goes into the nature through chimney. In GPS, one chimney is common for two units for passing flue gases. So, there are three chimneys for six units. In GPS, we observe flue gas stack of unit-5 and unit-6. The figure 3.2 shows the flue gas stack of GPS.
Condenser is a mechanical device which converts the steam into water. A condenser is a part of steam generator where exhaust steam is condensed and converted into water. Condenser creates a very low pressure at the exhaust of turbine, which permits expansion of the steam in the prime mover to a very low pressure. The condenser of GPS of unit-6 is shows in figure 3.3. This helps in the conversion of heat energy of steam into mechanical energy in the prime mover. The condensed water is supplied to the feed water tank, which can be used as feed water to the boiler.
3.2.8. Safety Valve
The safety valve is a device which permits the steam into the boiler to escape to atmosphere when the steam pressure in the boiler exceeds a certain specified limit. So the safety valve prevents the building up of excessive pressure in the boiler.
3.2.9. Forced Draught (FD) Fan and Induced Draught (ID) Fan
Forced draft fan is used to collect air from nature. In the steam power plant there are two forced draft fans which are used to collect air. In the forced draft fan, there are inlet vane actuators which control air collection. When generator load increases then it increases air collection from atmosphere. Dampers are used to control the quantity of air admitted to the furnace. Induced draught fan is located near or at the base of the chimney. It sucks hot gases from boiler side and discharge to the stack. The speed of forced draught and induced draught fan is 750 rpm.
3.3. Boiler Protection
3.3.1. Gas Control System
To produce heat in GPS, natural gas is used as a fuel for ignition and combustion in the boiler. The source of gas is Titas Gas Transmission and Distribution Company Limited Bangladesh. Normal pressure of gas is 6 kgf-cm-2. But the pressure of natural gas can be reduced from 6 kgf-cm-2 to 0.9 kgf-cm-2 before entering into the burner. The mixture of gas and air are burned into the boiler to produce heat. The pressure of the gas is strictly maintained pressure. If the gas level excesses the limit, power station may be tripped. So gas control system is very important in power station.
3.3.2. Temperature Control
The temperature is controlled in GPS by using a sensor which is called resistance temperature detector. It is a temperature sensor called resistance thermometer that detects any change in electrical resistance of some materials with changing temperature. In GPS, platinum material is used as resistance. It is used for accuracy and long-term stability. Normally temperature of the super heated steam inside the super heater is about 550°C. If main steam temperature is above 565°C or below 450°C the resistance temperature detector sensor pass a signal to the control room. Resistance is measured by applying a constant current and measuring the voltage drop across the resistor.
3.3.3. Lube Oil Pump
Lube oil pump is a one kind of pump which controls the lube oil. It is mainly used for pumping lube oil. It pumps more lube oil into the system if the pressure of the lube oil falls. The lube oil pump needs to operate over a wide range of temperature and liquid viscose conditions. The regulation system do not work if the lube oil temperature is below 17°C. Every machine is operated by a rated temperature, pressure etc.
which are fixed by the manufacturer. Normally operator follows this rating to operate the machine. This is called regulation system. To increase efficency and life time operators follow the ratings.
3.3.4. Furnace Pressure Control
At GPS, furnace pressure is controlled by FD and ID fans. FD fan is a fan or blower that is installed near boiler to supply air to the boiler furnace and maintain 1.6 kgf-cm2 positive pressures. ID fan is a fan which is located near the base of chimney. In GPS, it is used to suck hot gases from the boiler side and maintain 0.9 kgf-cm2 negative pressure from the furnace to the outlet of the ID fan.
3.3.5. Drum Level Feed Water Control
The boiler of GPS uses differential pressure transmitter which represents the level control measurement and probe type sensor which gives level alarms and low and high shutdown. The drum level must be controlled within specific limits and it is specified by the boiler controller. Drum contains both water and steam. From drum, water goes to furnace through water tube and absorbs heat and turns into steam. The steam goes to drum as saturated steam. The minimum level of drum is -160 mm and the maximum level of drum is +200 mm.
Chapter 4: Steam Turbine
Steam turbine power plant uses steam to move the turbine. In GPS, steam is used to move turbine so it is a steam turbine power plant. Steam turbine is the device which is used to convert thermal energy to mechanical energy. The turbine mainly consists of rotor and stator blades. The steam expands from high pressure to lower pressure in blades and this steam applies force across these blades, as a result turbine rotor begins to rotate. So the thermal energy transforms into mechanical energy. Turbine rotor blades of GPS are shown in figure 4.1.
4.2. Sections of a Steam Turbine
The steam turbines used in GPS have three different sections. The size and characteristics of the blades of the turbines in these sections are different from each other. These are,
- High pressure cylinder (HPC),
- Intermediate pressure cylinder (IPC),(iii) Low pressure cylinder (LPC).4.2.1. High Pressure CylinderFrom the super heater, steam with 130 kg/cm2 pressure and 540oC temperature first enters the high pressure turbine and then hits the high pressure turbine blades. The high pressure turbine has twelve stages with moving and fixed blades. After completing twelve stages, the temperature of steam falls to 332oC and pressure 28 kg/cm2. Steam again goes to super heater to gain heat. High pressure cylinder rotor of unit-4 is shown in figure 4.2.
4.2.2. Intermediate Pressure Cylinder
From the re-heater steam goes to the intermediate pressure cylinder. This cylinder has eleven stages. Here, steam pressure is 25 kg/cm2 and the temperature is 540ºC. If the temperature falls below 440o C or raise above 565o C then the intermediate pressure cylinder (IPC) valve will trip and the steam will go to the boiler for gaining more heat. After completing eleven stages, the temperature of steam falls to 171oC and pressure 1.34 kg/cm2. Intermediate pressure cylinder rotor of unit-4 is shown in figure 4.3.
4.2.3. Low Pressure Cylinder
From the intermediate pressure cylinder, steam enters the low pressure cylinder and continues its expansion. Here, the pressure is about 1.34 kg/cm2 and the temperature is about 171oC. In low pressure cylinder blades are larger than the previous two sections but the energy of steam is less than the previous two sections. LPC is divided into two sections and each section contains 3 or 4 stages at GPS. Low pressure cylinder of unit-4 in GPS is shown in figure 4.4. In LPC, steam temperature at 171ºC and pressure 1.34 kg/cm2 flows from the turbine. The low pressure turbine is coupled with the generator at GPS. In low pressure cylinder the steam losses their all energy and goes to the condenser.
4.3. Different Types of Turbine Oil Pumps
4.3.1. Main oil pump
During running condition main oil pump is coupled with turbine rotor for turbine regulation and lubrication system. The main oil pump rotates at 3 rpm and oil pressure is 20 kg/cm2. Steam turbine is installed to capture heat energy from water and convert this to electrical power which is called turbine regulation.
4.3.2. Starting oil pump
Starting oil pump is used to supply oil. It is used at the starting time of unit. The power of electric motor is 200 KW and voltage is 6.6 KV. At the time of starting turbine, starting oil pump supply oil for turbine regulation and lubrication system.
4.3.3. Stand by oil pump
During the turbine shut down condition stand by oil pump supplies oil to the turbine lubrication system.
Stand by oil pump rotates at 1450 rpm. The power of electric motor is 22 KW and voltage is 6.6 KV.
4.3.4. Emergency oil pump
Emergency oil pump is used if AC supply is not available and oil pressure is below the required pressure for any reason. The power of electric motor is 13 KW and voltage is 6.6 KV and the rotation rate is 1500 rpm.
4.3.5. Lubrication oil pump:
Lubrication oil pump is used to protect the turbine bearing by lubricating and cooling the bearings. If main oil pump and stand by oil pump are unable to supply oil, lubrication oil pump is used to supply oil. Lubrication oil pump is also used for cooling and lubricating generator bearing. Lube oil pump is shown in figure 4.5. The emergency oil pump works when the AC supply is not available and lubrication pump works only when DC supply is available. But Lubrication oil pump works in AC supply when the oil pressure of the main oil pump and stand by oil pump oil is below the required pressure for any reason.
4.4. Different Types of Turbine Equipments
Steam turbine operation is to be maintained at a constant speed. To control the flow of the steam into the turbine, governor is used to control speed of turbine blades. In governor the steam pressure is 28.5 kg/cm2 and the temperature is 336ºC. If blades cross the required speed, governor sends a signal to steam control valves to open or close.
4.4.2. Turbine Bearings
In GPS, in total seven bearings are used. These seven bearings are of two types. One is journal bearing and the other is thrust bearing. Journal bearing are used to support the weight of the turbine rotor. The bearing metal is divided into six pads which are self-aligned. Journal bearing is the main bearing of steam turbine. The thrust bearing is located on the main shaft of the turbine. The axial thrust of the turbine is absorbed by the thrust bearing.
Steam condenser is a device in which the exhaust steam from steam turbine is condensed. The main purpose of a steam condenser in turbine is to maintain a low back pressure on the exhaust side of the steam turbine. In every unit of GPS, two condensers are used. One is left condenser and the other is right condenser. The working procedure of both condensers is same. Turbine condenser is shown in figure 4.6.
A nozzle is a device which is used to rotate the turbine at a low speed at the starting and after tripping. The speed of turbine motor is 750 rpm and the turbine rotates at the speed of 3 to 4 rpm by the help of shaft turning gear at GPS. Nozzle reduces the possibility of damage of turbine at sudden change at starting time.
4.5. Turbine Protection
4.5.1. Turbine Pressure and Temperature Control
In GPS, turbine operates at a certain pressure. To get better output, turbine must be operated at rated pressure. In GPS, two types of pressures are monitored in turbine. These are above atmospheric pressure and below atmospheric pressure. Temperature sensors are located at the main steam path, re-heat line, turbine extraction lines and on low pressure turbine exhaust to control the steam temperature. To determine steam temperature in a turbine, thermocouples are used in a steam line. The thermocouples generate electrical signals and pass to the control room for observation.
4.5.2. Lube Oil Protection
In GPS, lube oil is used for the purpose of cooling and lubrication of turbine and generator bearing. It acts as a cleaning agent. If lube oil temperature is over 60ºC or under 40ºC it passes a signal and the alarm alerts the operator about the situation and allows time to take proper action. The regulation system does not work if lube oil temperature is below 17ºC. So lube oil temperature is very important to get good performance of turbine. The lube oil pipe of GPS turbine is shown in figure 4.7.
4.5.3. Speed Regulation Protection
Before switching off the turbine, it is very important to move turbine blade slowly for 72 hours to prevent rotor damage. To save the turbine from the damage, turbines need to run slowly.
4.5.4. Vibration Protection
For machinery equipments, vibration is very harmful. The action of equipment moving back and forth or from side to side or from up and down per unit time is called vibration rate. In GPS, the allowable vibration rate is 1.2 mm/s. When turbine crosses this vibration rate turbine should be tripped.
Chapter 5: Generator
Generator converts mechanical energy to electrical energy. AC generator is also known as alternator. Depending on power generating capacity, there are two types of generator in GPS. These are 55 MW generators and 210 MW generators. Unit 1 and 2 have 55 MW generators and unit 3-6 have 210 MW generators. The ratings of 55 MW and 210 MW generators are shown in table 3 and table 4. Unit 5 generator is shown in figure 5.1.
Table 3: Ratings of 55 MW Generator.
|Number of Phase||3|
|Phase connection||Y (star)|
|Rotor voltage||400 V|
|Rotor current||1445 A|
|Stator voltage||11.5 KV|
|Stator current||2761 A|
Table 4: Ratings of 210 MW Generator.
|Number of phase||3|
|Phase connection||Y (star)|
|Rotor voltage||430 V|
|Rotor current||1950 A|
|Stator voltage||15.75 KV|
|Stator current||7698 A|
5.2. Main Components of the Generator
The generator consists of different components. The major components are discussed below:
The stator is the stationary part of a generator. It contains either an electromagnet or a permanent magnet. In GPS, we saw electromagnet type stator. This electromagnet stator contains wire winding. A moving magnetic field which is generated by the rotor around the stator induced a voltage difference between the windings of the stator and it produces alternating current as output of the generator. The stator of unit-4 generator is shown in figure 5.2.
The rotor is the rotating part of a generator. In this generator, rotor is an electromagnet. Coil energizes the rotor and makes it an electromagnet. The coil that energies the rotor is known as the field coil. At GPS,
the rotor is used as the field exciter and the rotor is driven by the generator prime mover. The rotor of unit-4 generator is shown in figure 5.3.
5.2.3. Carbon Brush
Carbon brush supplies power to the slip ring. Carbon brush always remains in contract with the slip ring and provide power to the rotor. For two slip rings in the generator, there are two carbon brushes in the generator. The combination of slip rings and carbon brush provide power to the rotor and make the rotor an electromagnet. Graphite or carbon powder is commonly used for manufacturing of carbon brush. Carbon brush is shown in figure 5.4.
5.2.4. Slip Rings
A slip ring is used in any electromechanical system that requires rotation while transmitting power. Slip ring allows the transmission of electrical power from a stationary part to a rotating structure. The Slip ring is connected to the rotor to allow the supply of the power to the rotor.
5.2.5. Armature Winding
The armature winding is a part of a stator. It is the main current-carrying winding in which the electromotive force or counter electromotive force of rotation is induced. The current in the armature winding is known as the armature current.
5.3. Excitation System of the Generator
Exciter or Excitation is very important for the generator. Excitation is the process by which magnetic field is created. DC or direct current magnetic field is the excitation system for generators. DC power supply is needed to create this magnetic field.
5.3.1. AC or DC Excitation
All the six units of GPS use DC excitation system. DC power is used for DC excitation. For large generators, high amount of DC power is needed. To give this amount of power to the generator GPS uses AC power. In GPS, first AC power is stepped down by a transformer then it is rectified to DC power by bridge rectifier and then this DC power is delivered to the rotor. The DC current is transmitted to the rotor through slip rings.
5.4. Synchronization System of Generator
Synchronization of generator means the generator is working parallel with the grid. Synchronization is basically the process to synchronize a three phase synchronous or AC generator to another generator or to a power grid. Synchronization process includes synchronization of voltage, frequency and phase.
5.4.1. Voltage Synchronization
The transformer output voltage has to be equal to grid’s voltage for transmission. To obtain this objective voltage is normally 10.5 KV for unit 1 and 2 and 15.75 KV for unit 3, 4, 5 and 6.
5.4.2. Frequency Synchronization
The generator frequency has to be equal to the grid frequency. Our national grid frequency is normally 50 Hz. For this reason to synchronize with the national grid frequency, the generator rotates at a speed of 3000 rpm. Rotation speed of the generator depends on the number of poles.
N= Rotation speed of the rotor per minute
f = frequency
P = Pole
We know, frequency = 50 Hz and Pole = 2 So,
Rotation speed = 3000 rpm
5.4.3. Phase Synchronization
Phase of the generator must be synchronized with the phase of the grid. For this process generator’s phase angle and phase sequence have to synchronize with the gird. If the grid sequence is R-Y-B then the generator sequence must be also R-Y-B.
5.5. Generator Protection System
Different electrical and mechanical faults can affect the performance of the generator. To protect the generators from those faults different electrical and mechanical protections are required. Immediate shutdown or alarming occurs if any fault is found.
5.5.1. Over Current / Under Voltage Protection
In grid system more than one generators supply the load and if for any reason one or more generators suddenly trip, other generators try to supply the load and each of these generators will experience a sudden increase in current and thus decrease in voltage. In GPS, automatic voltage regulator is connected to the system which tries to restore the voltage. Under voltage relay is also used for the under voltage protection. In GPS, over current relay is used for over current protection. To construct this relay only current coil is needed, there is no need of voltage coil.
5.5.2. Over Speed Protection
Over speed protection is generally provided for prime mover driven generators. All generators in GPS are prime mover driven generator. Prime mover is the part that rotates the rotor of the generator. In GPS, a steam turbine works as a prime mover. If the prime mover speed increases then the generator frequency increases. The speed governor normally controls the speed of the rotor. If any abnormality exists, it trips the circuit breaker of the generator.
5.5.3. Stator Earth Fault Protection
This fault occurs when the stator of the generator becomes shorted with the ground. Generally the stator of a generator is very close to the ground. So, this fault can occur. Stator earth faults can also be caused by the degradation of insulation in the windings. If this fault occurs, the normal low neutral voltage will rise as high as line-to-neutral voltage. This fault may cause the serious damage. This fault can be detected by measuring secondary voltage of neutral grounding transformer.
5.5.4. Rotor Earth Fault Protection
Rotor field winding of the generator is electrically isolated from the ground. Rotor earth fault may be caused due to insulation failure of winding. Two methods are available to detect this type of fault. The first method is that one resistor is connected to the field winding and exciter. The resistor is centrally tapped and grounded with a voltage sensitive relay. If any earth fault occurs, the relay senses and closes the path. In second method, one voltage sensitive relay is connected to the field and exciter circuit. Other terminal of the voltage sensitive relay is grounded. A capacitor and secondary of one auxiliary transformer is installed between ground and relay. When earth fault occurs, a secondary voltage will appear across the secondary winding of the auxiliary transformer. The relay will be operated by sensing the voltage. In GPS, these two methods are used in all six units.
5.5.5. Frequency Protection
The generator needs to be operated within a certain frequency range. If generator operates over or under this limit, vibration occurs in the generator which can damage the rotor. This occurs because of sudden load change in the grid. This relay operates, if the frequency falls below 48 Hz or goes over 52 Hz.
5.5.6. Temperature Protection
Temperature protection is needed to protect the generator from overheating. Overheating is caused due to overload in grid system or failure in cooling system. It could cause serious damage to the generator. To protect the stator against overheating, embedded resistance temperature detector or thermocouples are used. In GPS, thermocouple is used to detect the temperature. When it starts to overheat, the temperature change is detected and the relay operates an alarm.
5.5.7. Over Voltage Protection
Over voltage occurs due to sudden loss in the load on the generator. If suddenly load decreases then the voltage start to rise and causes over voltage. Here, prime mover speed does not change with the load change. The over voltage protection is provided by two over voltage relays. They are instantaneous relay and IDMT (Inverse Definite Minimum Time) relay. Instantaneous relay operates for 130 to 150% of the rated voltage and IDMT relay operates for 110% of rated voltage .
5.6. Generator Cooling System
Generator deals with very high power. So the temperature of generator rises while running. Too much heat affects the performance and lifetime of the generator. In GPS, there are cooling systems installed for the generator which absorb the temperature of the generator while it is running.
5.6.1. Water Cooling
Water flow is used in generator for cooling purpose. This type of cooling system is used for stator cooling. Stator is surrounded by the water. The water absorbs heat and then carries the heat away. Water flows continuously by a motor to dissipate the heat.
5.6.2. Hydrogen Cooling
Hydrogen cooling system is used for rotor cooling. In this system hydrogen gas is used to dissipate the heat from the rotor. As hydrogen gas is lighter than air, it does less interruption with the rotating speed of the rotor than air. For this cooling system hydrogen gas is supplied by a hydrogen plant. Figure 5.5 shows the hydrogen plant of GPS. In this hydrogen plant, water is electrolyzed into hydrogen and oxygen.
Chapter 6: Substation
Substation is an essential part for transmission and distribution of electricity. The generated electricity in any power plant is transmitted and distributed though substation. GPS has a large substation. Unit 1 and 2 generates electricity at 11.5 KV and unit 3, 4, 5 and 6 generates electricity at 15.75 KV. The electricity of 11.5 KV is transformed to 132 KV and 15.75 KV to 230 KV by step-up transformer. There are various types of equipments in the substation which are used for the protection, transmission and distribution system of GPS. Single line diagram is given below in figure 6.1.
6.2. Major Components of Substation in GPSHere, in single line diagram we can observe that from generator 2.5 MVA transformer is connected which supplies power to the generator for excitation. From generator two 125 MVA transformer is connected which transforms the voltage from 15.75 KV to 230 KV and supply the power to the 1230 KV grid line. There is a 32 MVA reserve transformer which is installed in the substation. It takes power from the grid and supplies to the auxiliary equipment of the substation at 0.4 KV voltage when the generator is in off mode. The auto transformer is installed between 230 KV grid line and 132 KV grid line. This transformer supplies power to one grid line from another when there is excess power in other grid line. Substation of GPS is shown in figure 6.2.
6.2.1. Bus Bar
Bus bar is a copper wire which carries a large amount of current in a very high voltage. In the substation of GPS, there are 132 KV and 230 KV bus bars. Bus bar is shown in figure 6.3.
6.2.2. Bus Bar ArrangementIn GPS, we have seen two types of Bus bar arrangements. They are single bus bar arrangement and double bus bar arrangement. In single bus bar arrangement, all the incoming and outgoing lines are connected to it. The arrangement of 11 KV is single bus bar arrangement. It is simple in design and cost effective. In double bus bar arrangement, two identical bus bars are connected in such a way that any incoming and outgoing line can be taken from any of these two identical bus. These lines are connected in parallel.
6.2.3. Circuit Breaker
A circuit breaker is an electrical device which works as a switch and operates automatically when any abnormality founds in voltage and current flow. It is a protective device. It protects the whole electrical system from abnormal condition like overload, short circuit etc. If any abnormality occurs, the circuit breaker automatically disconnects the circuit from the system. There are three types of circuit breakers in GPS.
- Oil Circuit Breaker
In oil circuit breaker, oil is used as an arc quenching medium. Mineral oil has a good insulating property. The fixed contacts and the moving contacts are immerged in the oil. When circuit breaker disconnects, arc creates and this arc is quenched by oil vaporization. GPS has some oil circuit breakers.
- Air Blast Circuit Breaker
In air blast circuit breaker, high pressure air is used as an arc quenching medium. When arc creates for sudden break of circuit, high pressure air flows to the arc between fixed contacts and the moving contacts to cool down the arc. This operation is very safe and has a very low maintenance problem. GPS has a lot of air blast circuit breakers. Air blast circuit breaker is shown in figure 6.4 .
- SF6 Circuit Breaker
Sulphur hexafluoride (SF6) gas is used as the arc quenching medium in SF6 circuit breakers. SF6 circuit breakers have very good arc quenching property. At GPS, the weight of the SF6 gas is 26 kg
Department of Electrical and Electronic Engineering, East West University Page 54
and the total weight of the device is 3530 Kg. The temperature range of the SF6 circuit breaker is – 25°C to +55°C. The SF6 circuit breakers in GPS are made by Siemens, Germany. SF6 circuit breaker is shown in figure 6.5.
Isolator is a mechanical switch which is used to separate a circuit under no current or off load condition from the power system. Normally Isolators are installed along with circuit breaker. If a circuit breaker trips then an isolator is used to separate circuit breaker from the system. If isolator opens under on load condition then arc creates and it could be harmful for the system and the things near the isolator.
6.2.5. Bus Coupler
Bus coupler is a device which arranges coupling of the two or more buses. During maintenance work in the bus, a part of the bus is isolated. At that moment the system can get electricity from other bus through bus coupler.
6.2.6. Lightning Arrester
Lightning arrester is used at GPS substation to protect the equipments of substation from the lighting surge. A lightning arrester is a device to protect the equipments of the system from the sky lightning or a surge voltage. Lightning occurs when clouds are highly charged with respect to ground. It has a high voltage terminal and a ground terminal. There are various types of lightning arrester. In GPS, polymer metal oxide and zinc oxide arresters are mainly used. Lightning arrester is shown in figure 6.6.
6.2.7. Earthing Switch
Earthing switch is very essential for a substation. It is a switch between the line conductor and earth. Even when any line is off, there are still some charges which are trapped in the line because of their own capacity. This trapped charge is very harmful for human. Before maintenance of that equipment the charge need to be neutralized. Earthing switch discharges the trapped charge to earth and keep the equipment safe for human.
A transformer is a static device which is used to transfer the power from one voltage to another voltage without changing the frequency. In GPS, there are power transformer, auxiliary transformer, coupling transformer, instrument transformer like current transformer and potential transformer. Power transformer, auxiliary transformer and coupling transformer are used to step up or step down the voltage. Current transformer and potential transformer are used for measuring and protection purpose .
6.2.9. Power Transformer
The power transformer of the substation in GPS is used to step-up the voltage. Unit 1 and 2 generate 11.5 KV which is transformed by step-up power transformer to 132 KV and goes to the national grid. Power transformer is shown in figure 6.7.
6.2.10. Auxiliary Transformer
Auxiliary transformer is used to provide power to the auxiliary equipment of a power station. This transformer is directly connected to the generator output. It is a three-winding transformer. It has one primary and two separate secondary windings. Primary winding takes voltage according to the generator voltage rating. The voltage of the secondary windings is generally 11 KV or 6.6 KV. Auxiliary transformer is shown below in figure 6.8 and the rating of auxiliary transformer is shown in table 5.