Process Description
The Combined Cycle Power Plant consists of the following five sections:
- Boiler feedwater system (deaeration and pumping)
- Natural gas fired gas turbine with generator
- Heat recovery steam generator (HRSG)
- Steam turbine with generator
- Cooling tower
Polished makeup water from battery limits is fed to Deaerator D-101 to offset the loss of boiler feedwater elsewhere in the plant. The Deaerator also receives warm steam turbine condensate from the HRSG E-201. The Deaerator allows release of any non-condensables in the feed waters through an overhead vent to atmosphere. Deaerated water is collected in the base of D-101 and serves as a reserve to handle fluctuations of water inventories elsewhere in the plant. Boiler feedwater is pumped out of the Deaerator by BFW Pumps P-101A/B and mainly supplies water to the HRSG E-201 to generate high pressure steam in Steam Drum D-201. Boiler feedwater is also used for desuperheating steam in the HRSG and in the Steam Turbine’s Intermediate Pressure (IP) header as needed. Normally, no boiler feedwater is used for desuperheating.
The Gas Turbine JT-401 generates electric power in Generator G-401 by expanding hot, high pressure gas through the expander section to turn the shaft of the Gas Turbine. High pressure gas for the expander is attained by an air compressor on the same shaft. The air naturally heats of due to compression to 318 PSIG21.9 BARG.
Extra high temperature of the air is attained by combusting natural gas fuel with the hot compressed air in the combustor assembly between the compressor and expander sections. The combustion gas cools significantly as it passes through the expander section. The exhaust gas is normally 1100 DEG F593 DEG C and is routed to the HRSG to generate steam. A bypass stack is provided on the transfer duct to the HRSG in case the HRSG must be taken out of service while the Gas Turbine is running.
The HRSG E-201 generates high pressure steam using the waste heat from the Gas Turbine exhaust. Saturated steam is produced in the Steam Drum D-201 and is superheated in the 1st and 3rd coils of the HRSG. IP Steam from the Steam Turbine JT-301 is reheated in the 2nd coil and returned to the Steam Turbine. Water from the Steam Drum is boiled (evaporated) in the 4th coil of the HRSG. High pressure boiler feed water from P-101A/B is preheated in the 5th, 6th and 7th coils of the HRSG. Steam Turbine condensate is preheated in the 8th coil of the HRSG before being returned back to the Deaerator D-101.
A stack is attached to the HRSG to route cooled flue gas safely to atmosphere. The entrance duct to the HRSG is outfitted with Duct Burner B-201 to provide supplemental heat to the Gas Turbine exhaust for additional steam generation and superheating in the HRSG. Normally, the Duct Burner is not in service.
Superheated steam is produced by the HRSG at 1,044 DEG F562 DEG C and is distributed from the High Pressure (HP) Steam Header to Steam Turbine JT-301. The header is maintained at 1,800 PSIG124.1 BARG by control of the steam flow to the Steam Turbine. In case the Steam Turbine is shut down, a bypass line is provided (not shown on schematic 2) to route HP Steam to the Condenser E-301 on a temporary basis.
The Steam Turbine JT-301 consists of three stages: High Pressure (HP), Intermediate Pressure (IP) and Low Pressure (LP). Steam exits the HP Stage at 323 PSIG22.3 BARG and 633 DEG F334 DEG C and is routed to the 2nd coil of the HRSG to be reheated to 995 DEG F535 DEG C before entering the IP Stage. The steam exiting the IP Stage is then introduced to 2 LP Stages arranged counter-opposed on the common shaft of the Steam Turbine. The shaft drives the Generator G-301 to produce additional electric power.
Steam exhausting from the 2 LP Stages of the Steam Turbine is directly routed into Surface Condenser E-301 which is cooled with circulating cooling water from Cooling Tower CT-501. The low temperature of the surface condenser results in all the steam condensing at a low (vacuum) pressure. To prevent case non-condensables from accumulating in the steam side of the Surface Condenser, a vacuum system (not shown on schematic 3) is provided. The condensed steam is collected in Hotwell D-301 and pumped by Condensate Pumps P-301A/B to the 8th coil of the HRSG to be reheated before returning to Deaerator D-101.
Cooling water from the basin of Cooling Tower CT-501 is circulated by Cooling Water Pumps P-501A/B to the Surface Condenser E-301. The warm cooling water is returned to the top of the Cooling Tower to be cooled with air flowing up through the Cooling Tower. The air flow through the Cooling Tower is produced by a fan at the top. Cooled water falls into the basin. Makeup water is continually added to the basin to replace water which is evaporated in the air flowing through the Cooling Tower.
Boiler Feedwater System
Polished makeup water from battery limits is sent to the top of the stripping section of Deaerator D-101 where it combines with warm steam condensate returned from the HRSG E-201. The stripping section of D-101 is filled with structured packing to ensure good mixing of any added deaeration steam and water. Intermediate Pressure (IP) steam from the IP header on Steam Turbine JT-301 can be injected to the base of the stripping section of D-101 to ensure the Deaerator produces a small flow of steam from the top of D-101 through restriction orifice RO-101. Normally, the returning condensate is warm enough to generate enough top steam by flashing so that no stripping steam is required.
Stripping steam is added at the base of the stripping section and warming steam, if required, is sparged into to the water in the reserve base of D-101. A flow of steam through RO-101 ensures any dissolved oxygen and other gases are removed from the makeup water and recirculated steam condensate and vented from the system. Oxygen is particularly undesirable in high pressure boiler service because it leads to corrosion of the steam generating equipment.
The Deaerator normally operates slightly above atmospheric pressure. Water low in dissolved oxygen falls from the stripping section into the bottom section of the Deaerator which serves as a reserve volume of boiler water for the system.
Boiler Feed Water Pumps P-101A/B are high head, multi-stage centrifugal pumps and take suction from the bottom of D-101. Normally only one pump is in operation. The pumps supply deaerated boiler feedwater to the users in the plant as follows:
- Economizer coils of HRSG E-201 for supply of Steam Drum D-201
- Desuperheater J-201 between the superheating coils on HRSG E-201
- Desuperheater J-202 on the bypass line from steam header of HRSG E-201 to Steam Turbine Condenser E-301
- Desuperheater J-301 on the line from IP header of Steam Turbine JT-301 to the IP reheater coil of HRSG E-201
To protect the pumps against being run blocked-in, a minimum flow line from the discharge line of the pumps back to D-101 is provided. In case the Deaerator is overfilled, a manual drain line to the offsite water storage tank is provided off the discharge line from the pumps.
Gas Turbine Section
The Gas Turbine uses compressed air and fuel gas to drive the expander of the Gas Turbine which turns an electric power generator. The hot exhaust gas from the gas turbine is sent to the HRSG. Electric power produced by the generator is delivered to a power grid for distribution to electric power users.
Ambient air is pulled into the Intake Plenum of Gas Turbine JT-401. A screen on the inlet of the plenum prevents larger objects from being pulled into the plenum. A filter is installed in the plenum to remove any smaller objects from reaching the Gas Turbine rotating equipment.
The filtered air is compressed in the Air Compressor section of JT-401. The Intake Plenum and the Air Compressor are specially designed to ensure even distribution of the air throughout the intake to the compressor and into the Combustor Assembly. The Air Compressor is an axial type and is fitted with inlet guide vanes which are essentially louvers that can be used to restrict the air flow into the compressor if needed. The angle of the guide vanes are positioned by actuator ZV-401. Normally the guide vanes are nearly fully open (85%) but can be adjusted during startup and shutdown and off-design ambient temperature operation to give optimal air flow to the Combustor Assembly.
Hot air from the discharge of the Air Compressor enters the Combustor Assembly at roughly 318 PSIG21.9 BARG. This assembly consists of a specially designed series of burners arranged radially. There are 10 burners. Each burner set consists of two primary burners and one secondary burner. The fraction of fuel gas distributed between the primary and secondary burners is automatically controlled depending on load. This design permits efficient combustion of fuel while minimizing nitrous oxide (NOx) emissions.
The Combustor Assembly is designed to distribute air to the burners for combustion and to use a portion of the air to cool critical parts of the Gas Turbine near the burners. In order to avoid extremely high combustion temperatures which would cause mechanical problems in the Combustor Assembly and in the Expander, the Gas Turbine operates with a fairly large excess of air for combustion. A significant fraction of this excess air bypasses the burners and then remixes with hot gas from the burners along the outlet assembly of the burner sets prior to entering the Expander.
High pressure natural gas flow is regulated by the speed control system with control valve SV-401 and is distributed into the 10 burners of the Combustor Assembly. The split of fuel between the primary and the secondary burners of the burner sets in the Combustor Assembly is assumed to be ideal on the simulator. An automatic spark system and flame sensors ensure combustion is safely maintained. Only two of the burner sets are outfitted with spark plugs and every other set is outfitted with a flame sensor. Crossfire tubes connect each burner set laterally to ensure quick light-off of the other eight burners from an adjacent burner. The Gas Turbine is designed for operation only on natural gas; therefore, the burner sets do not include injection nozzles to handle liquid fuels. Also, the staged combustion system does not require the use of water injection to attain low NOx performance.
The high pressure, hot combustion gases from the outlets of the 10 burner sets are distributed evenly along the outside of the Expander inlet. As hot gas flows through the wheels of the Expander it turns the shaft which drives the Air Compressor and the Generator. As the gas expands and does work on the wheels of the Expander it cools, leaving the Expander at around 1,100 DEG F593 DEG C and near-atmospheric pressure. The warm exhaust gas is collected in the Exhaust Plenum and routed to the Heat Recovery Steam Generator (HRSG) prior to being exhausted to atmosphere. Eight temperature sensors are radially distributed at the Expander’s exhaust to detect any maldistribution of heat in the Combustor Assembly.
The shaft of Gas Turbine JT-401 is connected to Reduction Gear Box B-401 which in turn is connected to the shaft of Generator G-401. The Reduction Gear Box changes the shaft speed of the Gas Turbine by one-half. Generator G-401 produces electric power which is sent to a region-wide electric power grid. A breaker switch is provided to allow connection and disconnection of Generator G-401 to/from the power grid.
Starter Engine J-401 is a diesel engine that provides power during startup to turn the air compressor section of the Gas Turbine prior to starting fuel gas to the combustor assembly. Clutch C-401 allows engagement and disengagement of the shaft of the Start Engine J-401 with the shaft of Gas Turbine JT-401. In normal operation J-401 is stopped and C-401 is disengaged.
Heat Recovery Steam Generator
Hot exhaust from Gas Turbine JT-401 is routed to the HRSG E-201 via a large insulated duct. Prior to entering the HRSG, the Bypass Stack S-201 is connected to the duct to permit direct exhausting of the Gas Turbine to the atmosphere. Large motor-controlled dampers, MOV-211 and MOV-212, control the routing of Gas Turbine exhaust to the HRSG or to the Bypass Stack. Controller XIC-211 ensures that there is always an open path for the Gas Turbine exhaust while switching the line-up of the dampers.
The Gas Turbine exhaust gas enters a plenum in the HRSG which is outfitted with a specially design Duct Burner B-201 to permit supplemental heating of Gas Turbine exhaust if needed. Normally, no fuel gas is fired in the Duct Burner.
Gas turbine exhaust continues into the body of the HRSG E-201 which is outfitted five heating sections:
- HP Steam superheating consisting of 2 separate sections
- IP Steam reheating for Steam Turbine JT-301
- HP Steam generation (evaporator)
- HP boiler feedwater heating (economizer) consisting of 3 separate sections
- Steam condensate reheating
The cooled gas turbine exhaust is routed to a stack at the end of the HRSG to vent the exhaust safely to the atmosphere.
Cold condensate from Condensate Pumps P-301A/B flows into the condensate reheat coil of E-201 to recover heat from the gas turbine exhaust before it is discharged to the stack. The warmed condensate is sent to the Deaerator D-101.
Warm boiler feed water from BFW Pumps P-101A/B flows into the three economizer coils of E-201 to recover heat from the gas turbine exhaust before it passes to the condensate reheating coil. These coils provide a large surface area to absorb most of the available heat from the gas turbine exhaust. The large area is needed because of the low temperature difference between the cooler gas turbine exhaust leaving the evaporator section and the boiler feedwater. Preheated boiler feed water from the third coil enters the Steam Drum D-201.
Water from the Steam Drum D-201 circulates through the evaporator coil of E-201 by natural circulation. The coil is connected to a mud drum located near the bottom of the plenum of E-201. Relatively cooler water from the Steam Drum circulates down one-half of the coil to the mud drum. As heat is picked up from the gas turbine exhaust, the water partially vaporizes by the time it reaches the mud drum. Additional heating and vaporization occurs in the riser coils of the evaporator, resulting in a natural circulation of water through the evaporator. The riser coils return a mixture of water and steam to the Steam Drum which is fitted with separators to disengage the steam from the risers and route it to the top of the Steam Drum. Separated water combines with boiler feed water and is circulated back down to the mud drum. To avoid accumulation of solids in the mud drum over time, it is continuously drained. The relatively small blowdown flow from the mud drum is sent to battery limits for disposal.
Steam produced by the Steam Drum flows to the superheater coils of the HRSG. The superheater consists of two coils. Boiler feed water is injected into the Spray Desuperheater J-201 to control the final superheat temperature of the steam. The second superheater coil is designed to withstand the hot temperatures of the gas turbine exhaust from battery limits and acts as a thermal shield for the downstream coils.
The IP Steam reheat coil is located between the two HP Steam superheating coils. Warm IP steam flows from the exhaust of the HP Stage of Steam Turbine JT-301 into the reheat coil and is returned back to the IP Stage of the Steam Turbine. Reheating the IP Steam results in higher power output from the Steam Turbine.
Superheated steam from the last HP Steam superheating coil is sent to the HP Steam Header for distribution to the Steam Turbine J-301. The HP Steam can also bypass the Steam Turbine under transient conditions. Any HP Steam bypassing the Steam Turbine is first desuperheated in Bypass Desuperheater J-202 before continuing to the Surface Condenser E-301. Desuperheating of the bypass steam is required to avoid extreme thermal stresses and performance issues when high temperature steam is directly passed to the Surface Condenser. In case of severe upsets, HP Steam can be directly vented to the atmosphere if the pressure of the HP Steam Header becomes too high.
Steam Turbine Process
Superheated high pressure (HP) steam from the HP Steam Header of the HRSG enters the Steam Turbine JT-301 under control of the turbine control system which regulates the inlet throttle valve SV-301. JT-301 is a three-stage turbine; the first stage exhausts intermediate pressure (IP) steam which is routed to IP Desuperheater J-301 prior to passing through the IP reheating coil of HRSG E-201. Increasing the temperature of the IP steam before using it in the second stage increases the power availability of the steam and makes the power generation cycle more efficient. Reheated IP steam from the HRSG is readmitted to JT-301 and passes through the IP stage of the steam turbine and then to the low pressure (LP) stages within the same casing. Steam exiting the LP stages is exhausted directly to the shell side of Surface Condenser E-301 which uses cooling water to condense the exhausted steam from JT-301.
E-301 normally condenses all the steam from JT-301. The condensate from E-301 drains directly into the Hotwell D-301. The pressure of E-301 is essentially determined by the vapor pressure of the condensate leaving E-301 so it normally operates at vacuum conditions. To ensure vacuum conditions are maintained, the Vacuum Ejector EJ-302 pulls a small flow of low pressure vapor from E-301 using IP Steam taken off from JT-301. The motive steam and vapor from EJ-302 are condensed in Vacuum Condenser E-302 using cooling water. The condensate from E-302 drains into the Hotwell D-301.
During transient operation at startup, shutdown and upsets, the HP Steam Header will bypass some or all of the flow produced by the HRSG around the Steam Turbine JT-301 directly to the Surface Condenser E-301. The steam is desuperheated before entering E-301 so as to minimize thermal stresses on the Condenser and to avoid heat transfer problems due to very high steam temperatures.
At startup or in case of leaks of air into E-301 or in case of non-condensables in the HP steam, the pressure in E-301 may build due to pocketing of the non-condensable vapor. If this occurs, the Startup Ejector EJ-301 can be placed in service to exhaust the vapor directly to atmosphere via a vent. Refer to Schematic #15 in the “Process Schematics” section below for a diagram of EJ-301.
The Hotwell D-301 is a vertical drum directly connected to outlet of Surface Condenser E-301 and collects steam condensate from E-301 and E-302. The condensate is pumped by Condensate Return Pumps P-301A/B to the condensate reheat coil of the HRSG E-201 and then to the Deaerator D-101 in the BFW treatment section of the plant. Either pump can be set to auto-start in case of high level in D-301.
Cooling Tower, CT-501
Water/Air Contacting in Cells
Warm cooling water is returned from the Surface Condenser, E-301, and Ejector Condenser, E-302, via the return header and is routed to the top of the Cooling Tower, CT-501, via a hand valve controlled by HIC-503. The warm water is split equally and sent to opposite sides of the Cooling Tower and distributed evenly into the two opposing cells.
Within each cell, falling warm water is contacted with air drawn into the cell by a motor-driven fan located at the top of the tower. Air is drawn in along the outside of each cell the contacting method is known as “cross-flow”. In a cross-flow cooling tower, adjustable louvers are installed to allow control of the air flow into the tower. A hand controller, HIC-508, allows adjustment of the louvers’ position. Only one cell is simulated.
Intimate mixing of the warm water and ambient air is accomplished by engineered packing material (also known as “fill”) installed inside the framework of the cell structure of the Cooling Tower.
Heat Transfer
As warm water contacts ambient air, heat is transferred from the warm water to the cooling tower at the surfaces of the fill material. The more surface area, the more heat transfer will occur until the temperature of the air and the water are the same. In practice, the equilibration of the air and water temperatures does not normally occur in a cooling tower operating at normal loads.
After the water completes its fall through the cell and its fill, it is collected in a basin at ground level for recirculation to the plant’s heat exchangers.
Evaporation of Water
In addition to heat transfer, some of the cooling water will evaporate into the air. Evaporation results in a loss of heat that can produces temperatures below the ambient air temperature. The rate of evaporation of cooling water into the air will depend on the temperature of the air and its moisture content. At the same temperature, air with lower moisture content can evaporate more water than air with higher moisture content.
The moisture content of ambient air is represented by its dewpoint. The dewpoint is the temperature at which the moisture in ambient air will begin to produce dew if it is cooled any further.
The maximum evaporative capacity of ambient air is essentially the difference between the saturated moisture content of the air leaving the cooling tower and the actual moisture content of the air supplying the cooling tower. So, a cooling tower whose air leaves at 80 DEG F will be able to theoretically increase its moisture content by 3.45 – 0.83 = 2.62% with ambient air at a dewpoint of 40 DEG F but only increase it by 3.45 – 2.46 = 0.99% with ambient air at a dewpoint of 70 DEG F. Makeup water must be added to the cooling system to offset evaporation losses. Water can also be lost due to carryover of water mist in the air drawn by the fan and by leaks.
Cold Weather Operation
In cold weather, the Cooling Tower may overcool the water at low process rates even with the louvers at their minimum opening. To raise the cooling water supply temperature in this condition, a portion of the return water can be routed directly to the basin as needed with the manual valve controlled by HIC-502.
Cooling Water Pumps, P-501A/B
The Cooling Water Pumps, P-501A/B, are identical electric motor-driven centrifugal pumps with a design capacity of 43,000 GPM at 55 PSIG. Normally only one pump is in operation. The pumps take suction from the basin of the Cooling Tower, CT-501, and deliver cooling water to Surface Condenser, E-301, and Ejector Condenser, E302, via the cooling water supply header. The flow of the cooling water to E-301 is adjustable using hand controller HIC-305 and the flow to E-302 is adjusted with HIC307.
Note that in high capacity cooling systems, there is normally more than one cooling water pump in operation to improve service reliability and for purchase cost and capacity capability reasons. On the simulator, only one pump is in service. This improves training since the loss of or trouble with the operating pump will have immediate and stronger consequences on the operation of the Surface Condenser and Steam Turbine compared to the loss of one pump out of two or three operating pumps.
The cooling water supply header pressure is controlled to 50 PSIG by returning a portion of the cooling water back to the Cooling Tower, CT-501. Keeping the supply pressure constant minimizes cooling water flow disturbances to the process heat exchangers. Also, a line is provided off the supply header for sending blowdown water to disposal facilities as needed.
Cooling Water Filter Circuit
The cooling water filter circuit consists of the Cooling Water Filter Pump, P-502, and the Cooling Water Filter, F-501. The circuit circulates water from the basin of the Cooling Tower, CT-501, through the filter to remove any solids that are suspended in the cooling water. The filtered water is returned to the basin. This ensures that fouling of the Cooling Tower and the process heat exchangers is minimized. P-502 normally circulates 250 GPM of water. The pressure drop across F-501 is around 5 PSIG when clean.
Instrumentation
Deaerator
Polished water from battery limits is controlled by LIC-101 to maintain the level in the base of Deaerator D-101. The flow of polished water is indicated on FI-101. The pressure and temperature of the polished water are indicated on PI-101 and TI-101, respectively.
Steam flow from the IP header of Steam Turbine J-301 to the sparger of the stripping section of the Deaerator is controlled by FIC-104. The flow of IP steam to the sparger in the base of the Deaerator is controlled by FIC-105.
The pressure of the Deaerator is indicated by PI-103. Steam flow from the overhead of the Deaerator is indicated on FI-106 before passing through restriction orifice RO-101.
The flow of steam condensate returning from the HRSG is indicated on FI-107. The temperature of the returning condensate is indicated on TI-107.
The level of water in the base of the Deaerator is also indicated on LAH-102 which is used by Interlock I-101 (see below) to protect the Boiler Feedwater Pumps against operating when there is excessively low water level.
Boiler Feedwater Pumps
Switch HS-101A operates the motor of Boiler Feedwater Pump P-101A and switch HS-101B operates P-101B. These switches are locked in the STOP if interlock I-101 is tripped as indicated on switch XA-101. XA-101 is an indicate-only switch.
FIC-103 controls the flow of boiler feedwater to the boilers at battery limits. FIC-102 controls the amount of boiler feedwater passing through pumps P-101A/B to a minimum flow in case the demand for water to the boilers is low. This helps protect the high-head pumps against damage at low-flow conditions due to high temperatures and cavitation/vibration. The pressure of the discharge header of the pumps is indicated on PI-102.
Interlock I-101
Interlock I-101 protects the Boiler Feedwater Pumps P-101A/B from running in case of low level in the base of the Deaerator. I-101 activates if the level of LAL-102 is less than 10%. I-101 will remain active anytime LAL-102 is less than 10% and will stop and lock the motors of P-101A/B in the STOP state. I-101 will automatically reset when higher than 10%. However, P-101A/B must be manually restarted after the interlock resets. The interlock status is indicated on XA-101.
Gas Turbine Controls and Instruments
The temperature of ambient air for combustion in Gas Turbine JT-401 is measured by TI-402 at the Intake Plenum entrance. The pressure drop across the filter in the Intake Plenum is measured by PDI-402. The position of the inlet guide vane of the Air Compressor of JT-401 is adjusted by ZIC-401. The discharge pressure of air leaving the Air Compressor is measured by PI-402.
The natural gas flow to JT-401 is indicated on FI-401. The supply pressure of natural gas is indicated on PI-401 and its temperature is indicated on TI-401. Speed controller SIC-401 regulates the opening of natural gas control valve SV-401.
The speed of the shaft of JT-401 is measured by SI-401. This instrument is also used by the speed control system for JT-401. The speed of JT-401, expressed as % of design speed (3,600,300 RPM), is indicated on SIC-401. SIC-401 normally operates in cascade mode when Generator G-401 is connected to the electric power grid. This control mode is entered by placing droop control switch HS-403 into the DROOP state. Droop control is explained in the next section. SIC-401 can be taken out of droop/cascade control and placed in automatic or manual mode. Automatic mode is used only at startup when the generator is not connected to the grid. In this case, SIC-401 directly controls the shaft speed. In manual mode, SIC-401 is used to manually adjust the fuel flow to JT-401. Manual mode of SIC-401 is available any time the Gas Turbine is not tripped. Manual mode is entered any time the droop control switch HS-403 is changed from the DROOP to the OFF state.
For startup, switch HS-406 is used to start and stop the Starter Engine J-401. SIC-405 is used to control the speed of the motor and the Gas Turbine at startup. The starter engine is connected to the shaft of the gas turbine by clutch C-401 which is engaged and disengaged using switch HS-405. This switch should only be switched to the ON state (engaged) when the Gas Turbine is not rotating to avoid mechanical damage to the unit. The clutch can be disengaged at any time.
The ignition system for the Gas Turbine is started by placing switch HS-404 into the ON state. HS-404 should only be turned off when the Gas Turbine is shut down. XAL-401 indicates the lowest burner intensity reading from the Gas Turbine monitoring system (see details of the monitoring system in the section about interlocks). TAH-403 indicates the highest exhaust temperature from the monitoring system. The oxygen content of the Gas Turbine exhaust is indicated on AI-403. The pressure of the Gas Turbine exhaust plenum is indicated on PI-403.
The shaft speed of Generator G-401 is indicated on SI-402. The power output of G-401 is indicated on JI-420.
Generator G-401 is provided with a synchroscope in order to visually see the difference of the frequency and phase between electricity produced by G-401 and the electric grid at startup. Generator G-401 is connected to the electric power grid using switch HS-422. SI-420 indicates the frequency of electricity at the terminals of G-401. SI-421 indicates the frequency of electricity of the electric grid after the breaker switch. SI-422 indicates the phase difference between electricity generated at the terminals of G-401 and the electric grid. Before connecting the Generator to the grid with breaker switch HS-422, the frequencies of the Generator must be the same and the phase difference must be nearly zero. Otherwise, the Generator may suffer major damage when the breaker switch is closed.
The synchroscope also includes an auto-synch controller for connecting the Generator with the grid. XI-420 will indicate when it is permissible to activate the auto-synch controller. The permissive signal is given under the following conditions:
- The breaker switch HS-422 is OPEN
- The absolute value of the frequency difference between the Generator (SI-420) and the electrical grid (SI-421) has been less than 0.5 Hz for more than 20 seconds continuously
Placing auto-synch switch HS-420 into the ON state will activate the auto-synch controller XIC-420 (not shown on schematic 7). While auto-synching, XIC-420 will lock speed controller SIC-401 in manual mode and adjust its output until the phase difference as indicated on SI-422 is within 2 degrees of 0 and the frequency difference has been less 0.01 Hz for more than 20 seconds continuously. If closure of the breaker switch HS-422 cannot be made within 10 minutes, the auto-synch controller will stop (HS-420 will be placed back in the OFF state) and SIC-401 will remain in manual mode. Note that the auto-synch controller may not successfully work if there are large fluctuations in the fuel gas supply or in the grid frequency during synchronization.
Gas Turbine Droop Control
When any synchronous electric generator is connected to a large grid in parallel with many other synchronous machines such as generators and electric motors, a single generator cannot easily or reliably control the frequency of the electric power of the grid because it is only generating a small fraction of the total power being consumed from the grid. In this case, the generator will run at the grid speed or frequency. Therefore, the speed of the power turbine that drives the generator cannot be controlled when the generator is connected to a large grid.
The grid frequency dynamically depends directly on the balance of power generation and consumption across the grid. If generators are producing more power than the power consumers on the grid, the grid frequency will increase, causing all synchronous motors connected to the grid to speed up. As they speed up, they will consume more power until the power consumption comes into balance with power generation. In order for many generators to supply electricity to a large grid, they cooperatively adjust their power output using what is known as droop control.
Droop control simply proportions a generator’s power output to the deviation between of the actual grid frequency and its setpoint frequency (6050 Hz). If the actual grid frequency is at the setpoint, the generator will put out its design power. When the grid frequency is higher than the setpoint, the generator will decrease its power output in proportion to the deviation. Each generator system with droop control is configured with a characteristic droop control constant, expressed as % of setpoint speed. For JT-401/G-401 this constant is 4%. At 4% overspeed of the grid (i.e. 62.452 Hz) the droop controller will adjust the power output to the minimum stable power operation for JT-401/G-401.
When SIC-401 is placed into droop mode using switch HS-403, the PV of SIC-401 is computed as follows:
PV = [SI-421.PV * 60 + (SIC-401.OP – 25.0) * 3.3333] * 100/3,600
PV = [SI-421.PV * 72 + (SIC-401.OP – 25.0) * 3.3333] * 100/3,600
The setpoint of SIC-401 is locked at 104.0 when in droop mode. Any deviation of the grid frequency (SI-421.PV) will cause SIC-401 to move its output such that the PV is restored back to the setpoint of 104.0. The integral action of controller SIC-401 will cause the output (and power) to move gradually.