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SPM Power Generation Series
SPM-5200 Gas Turbine with Generator
 
Click to view schematic display A Click to view schematic display B Click to view schematic display C  
        

Process Description

Simtronics’ Gas Turbine with Generator program represents a typical gas turbine used for electric power generation. Unlike real industrial units, the simulated gas turbine is managed by the operator instead of a highly automated control system. This allows the operator to better understand the basic principles of operation of a gas turbine. Additionally, the gas turbine drives an electric generator connected to a large, regional electric power grid. Simtronics’ Gas Turbine with Generator simulator requires the operator to manually synchronize the gas turbine/generator set with the electric grid before connecting to it. This allows the operator to understand the concept of synchronization and its importance in keeping the generator from being damaged during connection to the electric power grid, since most real industrial systems use an auto-synching device/system to connect a generator to a power grid.

A full range of operations can be learned and practiced on the Gas Turbine with Generator simulator. These include normal, startup, shutdown, and emergency shutdown procedures.

Gas Turbine with Generator
The Gas Turbine with Generator program represents a typical gas turbine/power generator unit found in a power plant. 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 a Gas Turbine with Generator. 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 316 PSIG. 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 F 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.

Instrumentation

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 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.

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 (60 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

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.

Monitoring System
The monitoring system protects the Gas Turbine and Generator from conditions that would damage them. This includes:

There are separate interlock circuits for the Gas Turbine and the Generator that use the monitoring systems. I-401 protects the Gas Turbine and I-402 protects the Generator.

For the Gas Turbine, flame intensities are measure on indicators XA-401A/B/C/D/E. The lowest of these flame intensities is selected by indicator XAL-401. When XAL-401 gets below 10%, it will activate a trip alarm which is used by interlock I-401.

Also, exhaust temperatures from the Gas Turbine are indicated on TI-403A/B/C/D/E/F/G/H. The highest of these temperatures is selected by indicator TAH-403. When TAH-403 exceeds 1,200 DEG F, it will activate a trip alarm which is used by interlock I-401.

SAH-401 indicates the Gas Turbine shaft speed independently of SI-401 used for the speed control system. When SAH-401 exceeds 4,000 RPM it will activate a trip alarm which is used by interlock I-401.

Any problems with the mechanical systems of JT-401 (not simulated in detail) will be alarmed on XI-411. If the problem becomes worse or another serious mechanical problem with JT-401 occurs, it will cause XA-411 to alarm. XA-411 is used by interlock I-401.

Any problems with the mechanical systems of G-401 (not simulated in detail) will be alarmed on XI-421. If the problem becomes worse or another serious mechanical problem with G-401 occurs, it will cause XA-421 to alarm. XA-421 is used by interlock I-402.

Interlock I-40
Interlock I-401 protects the Gas Turbine from flame-out, overheating, overspeed and mechanical problems. I-401 activates on any of the following inputs:

XAL-401’s signal is a one-shot signal to allow a reset of I-401 when there is no fuel to the unit. HS-401 serves as both a trip and reset switch. All other trip inputs except for XAL-401 must be cleared in order to reset I-401.

When I-401 trips, the following actions occur:

Interlock I-402
Interlock I-402 protects the Generator from damage when disconnected from the grid and from mechanical problems. I-402 activates on any of the following inputs:

HS-422’s signal is a one-shot signal to allow a reset of I-402 when restarting. HS-402 serves as both a trip and reset switch. Trip input XA-421 must be cleared in order to reset I-402.

When I-402 trips, the following actions occur: