Plant Assignments POB
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What is the energy transfer process that occurs in the production of electricity in a hydro-electric generator?
Potential energy of water stored in lakes -> kinetic energy when water flows down the pipes to the power stations -> Mechanical energy as the moving water makes the turbine spin -> electrical energy because the turbine is connected via a shaft to magnets which spin around inside coils of the conductor.
What two factors dictate the amount of power a hydro-electric generating unit can output?
- · Water pressure acting on the turbine
- · Volume of water available
What is purpose of the “Headworks” at a hydro-electric generating station?
The Headworks is a collective name for all the devices that control the flow of water at a dam.
What components typically make up the “Intake” portion of the “Headworks”?
- · Trashrack
- · Intake Maintenance Gate (or Penstock Inlet Valve)
- · Intake Operating Gate
- · Bubbles
- · Surge Tower
- · Penstock
- · Scroll Case
- · Stay Vanes
- · Wicket Gates
Explain the purpose and function of the trashrack.
Coarse screens that keep out debris such as floating logs, plants, leaves, and other trash. Located upstream of the stop logs at the face of the intake structure.
Explain the purpose and function of the Intake Maintenance Gate.
The Intake Maintenance Gate is used when maintenance and operations of the Intake Operating Gate is required with the penstock dry.
Explain the purpose and function of the Intake Operating Gate.
The Intake Operating Gate is used to control the flow of water into the penstock during normal operation. When the unit is operational the gate must remain more than 90% open.
In small penstocks, a Penstock Inlet Valve (PIV) is used instead.
Explain the purpose and function of the bubbler system.
The bubbler system prevents freezing by creating turbulence as a result of rising bubbles. It consists of a supply of compressed air feeding into a perforated tube located some distance below the surface of the water.
Explain the purpose and function of the surge tower.
A Surge Tower is fitted on each penstock and used to absorb any pressure surges that may occur if the control gates or Turbine isolation valves are closed quickly.
Explain the purpose and function of the penstock.
Conducts water from the reservoir to the powerhouse.
Explain the purpose and function of the scroll case.
The scroll case is a large snail-shell shaped spiral which winds once around the turbine inlet. The shape provides even water pressure across the turbine from all sides.
Explain the purpose and function of the stay vanes.
Stay Vanes are permanently welded in place at the end of the scroll case to direct water at the correct flow angle to the wicket gates.
Explain the purpose and function of the wicket gates.
Wicket Gates control the amount of water flowing through the Turbine. Opening or closing the wicket gates adjusts the power of the generator. The governor controls the wicket gate hydraulics.
Explain the purpose and function of the draft tube.
The Draft Tube is the passage that carries the water discharged from the turbine runner to the tailrace. The Draft Tube is made of reinforced concrete walls. The Draft Tube recovers as much of the water’s remaining energy as possible by reducing the velocity of the water. Creates negative pressure or "suction head", increasing the pressure differential for the turbine which increases efficiency.
How are Intake Maintenance Gates operated? Where is it installed in relation to the Intake Operating Gate?
It is usually lowered into place upstream of the Intake Operating Gate using a Gantry Crane on top of the intake structure.
How are Intake Operating Gates operated? Where is it installed in relation to the penstock?
The Intake Operating Gate is raised and lowered using a hydraulic hoist system. It is installed before the penstock.
Where is the surge tower typically located? What is a key installation criteria for the Surge Tower in relation to the forebay?
A surge tower is typically located between the penstock and the powerhouse.
Why is the scroll case snail shaped?
This shape provides even pressure across the turbine from all sides.
How are Draft Tube Maintenance Gates operated? Where are they installed in relation to the draft tube?
A Draft Tube Maintenance Gate is installed using a tailrace gantry crane. It is installed at the bottom of the Draft Tube before the Tailrace.
What is the tailrace?
The tailrace is the section of riverbed below the powerhouse and immediately in front of and downstream of the turbine discharge.
What are the two main types of turbines? Describe the principle of operation of each.
- Impulse turbine: derive all of their rotational energy from the velocity of the water. There is no pressure difference between the outlet and inlet sides of the turbine blades.
- Reaction turbine: In addition to the energy from the velocity of the water, they derive kinetic energy from the pressure differential. Pressure drop across the blades.
Which type of reaction type runner is used for medium head plants? Describe how it uses water to create rotational energy on the turbine shaft?
- Francis (reaction type) turbine runner.
- Water enters the runner from the scroll case around the perimeter of the runner, then it enters through the wicket gates at the sides and leaves at the bottom. The water then goes through the turbine “buckets” which cause the runner to spin and then exit through the draft tube.
Which type of reaction type runner is used for low head plants? Describe how it uses water to create rotational energy on the turbine shaft?
Propeller (reaction type) turbine runner. Guide vanes on the stay ring give the water a circular motion as it enters the runner.
Which type of impulse type runner is used for high head plants? Describe how it uses water to create rotational energy on the turbine shaft?
Pelton (impulse type) turbine runner. A pelton wheel has several nozzles at the bottom of the penstock and transfers its kinetic energy by striking the bowl shaped buckets of the runner with a jet of high velocity water. The water then discharges to atmosphere inside the runner housing.
What is the purpose of the Turbine Guide Bearing?
Turbine Guide Bearings keep the rotating shaft in position. They are lubricated by oil or water.
What is the basic function of a hydro-electric generator? Discuss the energy transfer process.
Converts mechanical energy (rotating turbine shaft) to electrical energy.
What type of machines does BC Hydro use for the majority of their energy production and why?
BC Hydro uses large three-phase Alternating Current synchronous machines because of their ability to produce both active and reactive power.
In principle, how is an alternating current waveform generated by a hydro-electric generator?
A hydro-electric generator takes the rotational energy from the turbine shaft (mechanical energy), to spin a set of field windings carrying direct current, thus creating a rotating magnetic field that will induce an alternating current in the stator. (electrical energy)
Because the machines are three phase synchronous generators, three phase windings are required to establish the 3 phase alternating voltage waveform. In this case the phase windings are spaced 120o apart which generates three separate voltages
What is the purpose of the DC being applied to the field coils on the rotor?
To produce a magnetic field about the rotor. As the rotor turns, this induces a voltage in the stator.
What is the air gap?
The physical separation between the rotor and stator. The magnetic field strength across the gap increases with a smaller air gap.
What are the two types of rotors? Which are used on hydro-electric generators and why?
- · Cylindrical rotors
- · Salient pole rotors
Salient pole rotors are used on hydro-electric generators because they run on a vertical axis.
What formula represents the relationship between speed and frequency on a hydro-electric generating unit?
- RPM = 120 x F/P
- · RPM is equal to speed of rotation of the rotor
- · F is the frequency
- · P is the number of poles on the rotor
How fast does a 2 pole machine spin in a 60 Hz system?
RPM = 120 x (60Hz)/2 pole = 3600 RPM
In which component, the rotor or the stator, is the alternating current waveform induced?
To get a 3 phase alternating current, the stator windings are spaced ___ degrees apart.
What is the purpose of bearings in a hydro-electric generator?
Thrust bearings support the static loading of the unit on vertical units or the shaft and unit on horizontal units.
What pump is used to supply lubricating oil between the thrust pad and collar during startups to prevent damage?
A lift pump.
What is the purpose of the raw water system?
- Delivers water from the reservoir the rest of the generating station for:
- · Cooling water systems
- · Powerhouse water systems
- · Fire water systems
What is the purpose of the cooling water header?
Cooling water header takes water from the raw water header to use for cooling components.
Name the components that require cooling water from the cooling water header. What is the purpose of delivering cooling water to each of these components?
- · Generator air coolers, cooling of the stator windings
- · Generator Bearing oil coolers, keeps the bearing oil cool
- · Turbine upper and lower seals, provides cooling and lubrication to reduce wear of the sealing surfaces that seal turbine shaft and headcover of turbine.
- · Generator transformers, provides cooling water to generator transformers
- · Turbine glands, cools the seal for turbine glands
- turbine bearing oil coolers
- · Powerhouse heat pumps, water that leaves generator air coolers is used for powerhouse heat pumps.
How is the cooling water header pressure regulated?
Pressure regulating valve that reduces the pressure to 50 psi.
How is cooling water monitored at a hydro-electric generating station?
Each unit has a cooling water monitor cubicle. Various cooling water functions are monitored with alarms generated both locally and at the control center.
What is the purpose unit circuit breaker?
The unit circuit breaker is used to synchronize the generating unit to the rest of the Bulk Electric System.
What is the purpose of the step-up transformer?
The step-up transformer is used to step up the rated terminal voltage of the generating unit to the Bulk Electric System voltage.
What is the purpose of the generator isolation disconnect?
The generator isolation disconnect is used when only isolation of the generating unit is required.
What is the purpose of the iso-phase bus?
The iso-phase bus is used to connect the output of the stator is the disconnect. Each phase is enclosed by individual metal housing and the bus is cooled by fans. Without adequate cooling, unit output might be reduced.
What is the purpose of the synchronizing and protection VTs?
One is used for synchronizing and one for protection.
What is the purpose of the surge arresters?
Protects equipment from overvoltage by diverting the high voltages to ground.
What sources can generating stations get there AC station service from?
- · Dedicated station service transformers connected to station buses.
- · Tertiary windings of main transformers.
What are the standard AC bus voltages in BC Hydro?
120/208V and 347/600V
Why do substations have DC station service?
- DC power is required for:
- · Normal operation of control, alarm, and protection circuits
- · Switches and lights on control panels and some operating circuits
- · Emergency lighting, pumps, and fans
- · Uninterruptible power supplies for vital computer aided controls and communication equipment.
Should the AC system fail where will the substation get its back up power from?
Back up AC: · Line feeding in from another station. Stand-by generator powered by internal combustion engine
DC station battery. DC energy can be stored in batteries that are kept charged by connecting them across the rectifier output. When AC fails, the battery provides the power.
Where are the service transformers located in a station with a generator bus?
Connected to the generator bus. Low voltage side of transformer connected to 600V station service bus.
Where are the service transformers located in a station without a generator bus?
Connected to transmission bus. A service transformer steps the power system voltage on the transmission bus from a high voltage down to 600V.
What is the purpose of the generator cooling equipment?
Dissipate heat away from the generator, rotor, and bearings so the ability of the unit to produce power is not compromised.
What is the purpose of the thrust and guide bearings in a hydro-electric generator?
Thrust bearings support the static loading (weight) of the unit as well as the rotational forces.
Guide bearings keep the rotating shaft in position while the main weight is supported elsewhere. The shaft is held steady by the guide bearings.
What is the purpose of the high pressure lift pump? When does it start and when does it stop during both startups and shutdowns of hydro-electric units?
The high pressure lift pump forces a wedge of oil between the thrust collar and the thrust bearing.
- On startup, the pump is started before the wicket gates are opened and it continues to run until the unit reaches 90% speed. In some cases until the bearing oil reaches normal operating temperature.
- On shutdown, the lift pump automatically starts up when rotor speed drops to 90%. It continues to run until rotor speed comes to a complete stop. In some cases it runs until the bearing oil has cooled.
A lift pump will always start when creep is detected.
What is the purpose of the braking system? When does the braking system come on during a shutdown of a hydro-electric unit?
- A braking system brings the unit to a complete stop a few minutes after the generator is shut down to prevent the stator and rotor from touching eachother due to low lubrication because of low rotation speed.
- · Generator circuit breaker must be open
- · Turbine wicket gates must be closed
- · Unit must have decelerated to a pre-set speed (maybe 25% speed)
Explain the basic operation of a steam based thermal generator.
- · Converts the chemical energy from fuel into electrical energy
- · Boils water to produce steam which passes through a turbine to turn the rotor of the generator
Explain the basic operation of a combustion turbine thermal generator.
- · Fuel is burnt inside cylinder
- · Expanding gases impinge on blades of turbine, causing it to rotate.
What is the purpose of the compressor in a combustion turbine?
Pressurizes a working fluid. Transfers energy from the rotating blades to a continuously flowing gas.
What is the purpose of the combustion chamber in a combustion turbine?
The purpose of the combustor is to increase the energy stored in the compressor exhaust by raising its temperature.
What is meant by flame detection, why is flame detection important, and how is loss of flame detected?
- The presence of absence of flame can be detected by a photo-electric device or by monitoring exhaust gas temperature.
- Flame detectors sense the presence of absence of flame in the combustion chambers.
Why is vibration a serious concern in a thermal generator?
Vibration causes excessive wear on the thermal generator
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