Plant Assignments POB CD
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What is the IEEE definition of what a governor system is? In essence what does this mean for hydro-electric unit operation?
Governor: The combination of devices and mechanisms which detect speed deviation and convert it into a change in servomotor position. It includes the speed sensing elements, the governor control actuator, the hydraulic pressure supply system, and the turbine control servomotor.
What are the three main subsystems of a governor system?
What is the purpose of each?
- · Control: mechanical, electric, or electronic components that monitor the speed and power output of the system, and initiate changes to the water-flow when required
- · Hydraulic: hydraulic valves and servomotors that effect the changes to the water-flow by moving the wicket gates
- · Hydraulic oil system: oil storage and delivery system that ensures adequate pressure in the hydraulic subsystem, so that it can perform its operations.
What is the primary function of the governor when the unit is not connected to the power system?
To control the speed of the generator by regulating the flow of water which drives the turbine. When the unit is not connected, this will result directly in a change in unit RPM.
What is the primary function of the governor when the unit is connected to the power system?
- When the unit is connected to the system, a change in water flow will result in a change of electrical power output not RPM.
- The governor regulates the frequency and power output of the unit.
What is the purpose of the Pilot Servomotor?
· Pilot Servomotor – A “differential area plunger” rises when a gate opening is requested and lowers when a gate closing is requested. The raising and lowering of the Pilot Servomotor operates the Distributing Pilot Valve Plunger via the Upper Floating Lever.
What is the purpose of the Distributing Pilot Valve Plunger?
- Controls the position of the Valve Servomotor by working in conjunction with the Distribution Pilot Valve Bushing, either porting high pressure oil to, or trapped oil away from, the upper side of the Valve Servomotor.
- The Distribution Valve Plunger/Valve Servomotor combination represents the second stage in the hydraulic amplification process.
What is the purpose of the Wicket Gate Servomotor?
Wicket Gate Servomotor – One or two (or four) large hydraulic cylinders which adjust the shift ring to open or close the wicket gates.
On what principle does the governor system operate?
A governor works on the principle of hydraulic amplification. “A small force on a small area will create a proportionally larger force on a larger area”.
What are the main governor types and what are the typical ways speed of the hydro-electric turbine is detected?
- · Mechanical – detects speed using a flyball on top of the generator shaft or driven by a flexible cable
- · Electric-Mechanical – A synchronous motor drives the flyball unit. The motor is driven be a Permanent Magnetic Generator (PMG) on top of the unit or by the generator VT secondaries.
- · Analog Electronic – PMG (permanent magnet generator) or using a speed signal generator (SSG)
- · Digital Electronic – voltage sensings (from the unit’s voltage transformer) or speed signal generator (SSG) for speed sensing.
What is the Gate Limit?
The Gate Limit setting establishes a maximum Wicket Gate Position between 0 and 100%.
What is meant by Gate Position and what are the two ways it can be controlled?
Gate Position is the relative amount the wicket gate is open
Frequency Setter (15F): When the unit is not synchonized, speed can be varied by adjusting the 15F which opens and closes the wicket gates
Power Setter 65P: When the unit is synchronized, MW output of the generator increases or decreases by adjusting 65P which opens and closes the wicket gates.
What conditions put a governor into Actuator Lock?
Governor control failure
- · Loss of a PT source which prevents MW feedback to governor Control.
- · Failure of the speed signal (SSG) when the unit is over 25% speed, sensed by 65SF. ·
- Loss of power on the redundant power supply, sensed by 27BX. ·
- Removal of any governor electronic modules, sensed by 65MI ·
- Activation of the governor test switch (43GT)
What is meant by Actuator Lock?
Actuator Lock allows the use of the generator, but all automatic governing action is lost.
- Not possible to raise or lower MW
- governor will not respond to changes in turbine speed
- Gates can be lowered using Gate Limit or Shutdown Solenoid
- cannot reset remotely
What is meant by Lock Free?
Actuator Lock Free is when the Gate Limit is lowered to the gate position. The lock free limit switch closes to establish "Actuator Lock Free" status
- The wicket gates follow Gate Limit
- Still no governor action but unit can be raised or lowered using Gate Limit controls
- Allows control centres to control the load
In a hydro-electric generator what is the purpose of the exciter?
The exciter controls the generator’s terminal voltage by controlling the current applied to the field windinngs which determines the strength of the magnetic field on the rotor.
What three factors affect the voltage induced in the stator of a hydro-electric generator?
- · Strength of the magnetic field
- · Rate of change of the magnetic field
- · Length of the conductor exposed to the changing field
What is the most practical method of controlling the voltage on an AC hydro-electric generator? How is this achieved?
- Changing the strength of the magnetic field.
- Achieved by controlling the current flow in the windings of the electromagnets which create the magnetic field of the generator. (exciter)
What is the secondary purpose of the exciter and what components are used to provide this response?
A secondary purpose is to counteract system disturbances by briefly increasing or decreasing the generator terminal voltage. The exciter can respond much faster than the governor because of hydraulic response time.
In general, how does a static exciter work?
Static exciters convert AC from the generator to a DC current which is fed to the field windings.
What is the purpose of the Exciter Transformer?
Transforms the generator’s terminal (output) voltage to the input voltage required by the exciter.
What is meant by the term Field Flashing? What are the two options for Field Flashing and which is typically used?
- Field Flashing provides DC current required for starting because the generator may not retain sufficient residual magnetism. The field is briefly energized by connecting the field windings to a DC source.
- AC source: an existing AC electrical supply, such as station service which is converted to DC.
- DC Source: Station batteries. Blocking diodes required.
What is the purpose of the Field Breaker?
Field Breaker (41) – Provides a means of interrupting the field circuit. NC and opens to isolate the exciter from the generator field.
What is the purpose of the Brushgear?
Provides the rotating connection to the field windings.
The Automatic Voltage Regulator has two control modes, a 70V and a 90V. Which one is manual and what does this mean to the operator? If the unit was in automatic what would it mean to the operator?
- Varies the resistance in the field of the main exciter.
- 70V is manual.
- 90V is automatic. The exciter tries to maintain a setpoint.
Fully explain what a Capability Curve is (include one with your answer).
- There is a minimum and maximum excitation before the machine becomes unstable.
- The curve actually has three areas:
- · The field heating limit (rotor poles) when maximum excitation current is put through the field poles
- · The armature (stator) heating limit when maximum current is pushed through the stator windings
- · The armature (stator) end turn heating limit when minimum excitation current is put through the rotor poles
- 1. Out of Step Conditions – this is where the unit goes past its stability limit and is in danger of slipping a pole due to the weak magnetic field bonding the rotor to the stator (represented by the
- bottom third of the curve)
- 2. Stator Heating Limits – this is where the stator is carrying high levels of current due to the load on the unit which may be causing heating damage to the stator windings (represented by the middle third of the curve)
- 3. Field Winding Limits – this is where the field current is extremely high and heating damage to the field windings may occur (represented by the top third of the curve)
What additional exciter facilities are there and what purpose do they serve?
- Current Limiter – Limits the initial excitation current during generator startup
- - Reduces exciter output during disturbances
- Minimum Excitation Limiter – Regulates the minimum exciter level to prevent machine from becoming under-excited which could endanger synchronism with the power system.
- Volts/Hz Limiter – Limits excitation as the frequency decreases on shutdown
- - Protects the stator and transformer from overvoltage at 60Hz by tripping the unit if the voltage is too high for too long.
- Power System Stabilizer – Allows the exciter to aid in damping power system oscillations by changing the field current.
What are the three quantities that must be matched for synchronizing to take place? How is each accomplished for a hydro-electric generator to be synchronized?
- - Voltage magnitudes
- - Frequency of the voltages
- - Phase angle between the voltages
With respect to synchronizing generators to the system, what is meant by the “incoming” source?
Generator is “incoming” source and the system is the “running” source.
What is the consequence of not matching the voltage magnitude variable?
If the voltage magnitudes are not closely matched, a sudden rise of MVAR will flow across the circuit breaker. This could cause a change in area voltage or instability for the incoming generator or other generators.
What is the consequence of not matching the frequency variable?
If the frequencies are not matched, a sudden change in MW flow will appear across the circuit breaker.
When synchronizing, the generator runs slightly faster so it is producing MW when it syncs, not motoring.
What is the consequence of not matching the phase angle variable?
If the phase angle variable is not matched, a large MW flow increase will appear. This is hard on the generator as the rotor gets snapped into a position relative to the power system.
What is a synchroscope and how is it used?
- A synchroscope is a simple piece of equipment used to monitor the three synchronizing variables.
- A synchroscope inputs voltage waveforms from both sides of the open circuit breaker. If the voltage waveforms are at the same frequency, the synchoscope does not rotate.
What is a synch-check relay and why isn’t it used for generator synchronizing?
- A synchro-check relay electrically determines if the difference in phase angle falls within allowable limits. Synch-check relays do not provide indication of the voltage magnitude, frequency, or phase angle so are not used for generator synchronizing.
What is the difference between a generating station auto synchronizer and a transmission station synchronizer?
Generating station auto synchronizers send pulses to generator’s exciter and governor control systems to change the voltage and frequency of the unit, transmission station synchronizer’s do not adjust any parameters.
What is AGC? What does AGC use as an indicator that generation and load are not in balance?
Automatic Generation Control. AGC systems simultaneously control many governors to balance generation to load. AGC uses ACE (Area Control Error) to balance generation to load.
AGC makes adjustments to the load reference set-points of the governors for all the Balancing Authority.
What are the three control modes for AGC? Describe each.
- - Constant Frequency Control – the AGC system monitors only frequency. This mode only responds to frequency error. Only used when tie-lines to other Balancing Authorities are open.
- - Constant Net Interchange Control – AGC system only monitors tie-lines and responds to interchange errors. Only used when we have no frequency indication.
- - Tie-Line Bias Control – AGC system responds to both frequency and tie-line errors flow. Uses ACE.
Which mode of AGC is typical if the BC Hydro Balancing Authority is connected to its adjacent Balancing Authorities which good sources of tie-line telemetry and frequency?
What mode does a unit have to be in to be started from either FVO or SIO?
All systems must be operational and in the “automatic” and “remote” modes.
What is the typical auto start for a large Francis type turbine, including the pre-start conditions?
- · Penstock full and Intake Gate open 100%
- · Governor pressure and all unit auxiliaries normal
- · Automatic Voltage Regulator, Lift Pump, Unit Brakes, Gate Locks in Auto control
- · Unit control in “auto”
- · Unit in “remote” control
- · All unit relay protection and speed switches operational
- Auto Start:
- Iniate an auto-start via SCADA.
- · Field breaker, if open, closes
- · Brake Solenoid de-energized to release brakes
- · Cooling Water Valve opens
- · Lift pump starts forcing oil into the thrust bearing
- · When all brake pads clear, the Cooling Water Valve is 100% open and the Lift Pump has reached its operating oil pressure, the Governor Shutdown Control Supervision will pick up
- · Gate Lock solenoid picks up and releases the Gate Locks
- · Governor Shutdown Solenoid picks up after Gate Locks are clear
- · Wicket Gates will open after Gate Limit rises to 20% "breakaway" position
- · At pre-determined speed, Speed-No-Load Solenoid picks up to limit the wicket gate opening to speed-no-load position
- · At a pre-determined speed when sufficient oil film has been established at the thrust bearing, the Lift Pump will shut down.
How long does a typical hydro-electric unit take to come to a complete stop following an auto stop? What should be done if an auto stop incomplete sequence alarm comes in?
- A typical hydro-electric unit takes 10 minutes to come to complete stop following an auto stop.
- If an auto stop fails, a decision must be made on whether to start the unit again or call out staff to investigate.
All of BC Hydro hydro-electric generators are capable of running in generate mode. What other mode of operation are some generators capable of running in? Describe this mode of
Synch-condense: unit operates as a motor rather than a generator so the unit is using MW from the system rather than generating them.
What is the basic process for putting a shutdown unit into generate mode?
- · successful start sequence and synchronization to the grid
- · Set the governor output to produce MW
What is meant by the term “prohibited zones”? Explain by defining the various types of “prohibited zones”.
“Prohibited zones” are rough load zones or zones where extreme cavitation occurs.
Rough load zones: turbine is subject to extreme vibration and there is risk to the generator/turbine
What is cavitation?
Cavitation is the formation of empty cavities in a liquid by high forces and the immediate implosion of them. Occurs when a liquid is subject to rapid changes in pressure causing the formation of cavities in the lower pressure regions of the liquid. The cavitation causes wear and pitting of the turbine.
What is meant by the “torque angle” of a hydro-electric generator?
The torque angle of a synchronous machine is defined as the angular separation between the rotor and stator’s rotating magnetic fields.
- The rotating magnetic field of the stator is
- primarily due to the 3 phase system to which the generator is attached while the rotating magnetic field of the generator is controlled by the generator operators.
What is the 65P of a hydro-electric unit used for?
Governor load reference setpoint (65P). Increasing 65P opens the wicket gates. Produces more MW because the torque angle increases.
What is meant by the term governor droop? What is meant by the term isochronous? Do we normally operate units in isochronous? If not, why?
- Governor droop: when added to a governor, forces generators to respond to frequency disturbances in proportion to their size.
- Isochronous: a governor that strives to maintain its target frequency for all load levels.
- We don’t operate governors in isochronous for interconnected power systems because they tend to be unstable and enter into speed oscillations.
- Correct but watch the last part of your answer. What would happen if we had one unit on isochronous in the system and the rest in droop
- In a restored system, additional units can be paralleled to an isochronous generator as long as only one generator is in isochronous mode so they don’t compete. As the system grows, the usefulness of isochronous control diminishes because no single unit can regulate a large power system.
What effect does varying the excitation have on the strength of the field on the rotor? What does varying the field do the MVars the unit is producing or absorbing? What net effect does this have on the terminal voltage of the machine and the station bus voltage?
- Varying the excitation increases or decreases the strength of the field on the rotor.
- If excitation current is raised: produces VARS
- If excitation current is lowered: absorb VARS
- If VARS are produced, the terminal voltage of the machine and the station bus voltage increases.
- If VARS are absorbed, the terminal voltage of the machine and the station bus voltage decreases.
What is meant by the term “synchronous condense” mode?
When a unit is acting as a motor. The torque angle is negative.
What is the basic process for putting a shutdown unit into synchronous condense mode?
- · Unit is unloaded to less than 25% gate position
- sync condense initiated
- • The wicket gates are closed to the SNL position (approx 0 MW)
- • The reverse power relay is blocked
- • The wicket gates are closed completely without tripping the unit offline (the unit immediately starts to draw a large number of MWs from the system)
- • Cooling water valve to the upper and lower seals opens
- • Air admission valves are opened to depress the draft tube water level below the height of the turbine runner allowing the unit to
- · spin in air (reducing the amount of MWs drawn from the system)
What is the reason for admitting compressed air into the draft tube during synchronous condense mode operation?
To depress the water below the height of the runner because motoring the unit with the with the unit submerged requires a large amount of power.
Do the MVars have to be taken off a generator before it can be put in synchronous condense mode?
MVARs do not have to be reduced off the unit. The unit will remain connected to the system and have the largest operating region once in synch condense.
What is the only thermal plant operated remotely from FVO or SIO?
Rupert gas (RPG)
What are the two main reasons RPG is run?
- • The Prince Rupert area has been subjected to an outage (loss of 2L101) and RPG is required to be blackstart to supply the area load
- • 2L101 or another facility is scheduled out of service which requires the operation of the plant at some loading
Can RPG units be run in synch condense? If so, what is the procedure to transfer RPG units into condense? What is the procedure for transferring RPG units from condense into generate?
- · Unit is synchronized as a generator
- · Change unit from generator to condense mode
- · The turbine shuts down automatically with Generator is still connected as a synchronous condenser
- Condense to Generate:
- · Unit must be shut down directly from sync condense
- · 15 minute delay to start the unit back up
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