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SPM Power Generation Series
SPM-5100 Steam Turbine with Generator
Click to view schematic display A Click to view schematic display B Click to view schematic display C Click to view schematic display D
       
     
Click to view schematic display E      
        

Process Description

Simtronics’ Steam Turbine with Generator program represents a typical steam turbine used for electric power generation. The scope of the program includes the condenser, the vacuum system and the hotwell. The steam turbine drives an electric generator connected to a large, regional electric power grid. Simtronics’ Steam 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 Steam Turbine with Generator simulator. These include normal, startup, shutdown, and emergency shutdown procedures.

Steam Turbine with Generator
The Steam Turbine with Generator program represents a typical gas turbine/power generator unit found in a power plant. High pressure steam generated in the power plant is used to drive the Steam Turbine which turns an electric power generator. The exhaust from the steam turbine is condensed with cooling water and is returned to the boiler feed water preparation section of the power plant. Electric power produced by the generator is delivered to a power grid for distribution to electric power users.

Superheated high pressure (HP) steam from battery limits 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 Coils E-303 of an HRSG. 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 E-303 is readmitted to JT-301 and passes through the IP stage of the steam turbine and then to the low pressure (LP) stage within the same casing. Steam exiting the LP stage 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 medium pressure steam from battery limits as the motive fluid. 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.

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.

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 Deaerator in the BFW treatment section at battery limits for reuse as boiler feed water. Either pump can be set to auto-start in case of high level in D-301. 


Instrumentation

Steam Turbine Controls and Instruments
The HP steam flow to JT-301 is indicated on FI-301. The supply pressure of HP steam is indicated on PI-301 and its temperature is indicated on TI-301. HP steam passes through trip valve XV-301 which is controlled by HIC-301. HP steam flow can also pass through a smaller warm-up line at startup. HIC-302 controls the warm-up valve HV-302 on this line. Speed controller SIC-301 regulates the opening of speed control valve SV-301 inside the turbine casing. SV-301 controls the flow of HP steam to the first stage of JT-301.

The speed of the shaft of JT-301 is measured by SI-301. This instrument is also used by the speed control system for JT-301. The speed of JT-301, expressed as % of design speed (3,600 RPM), is indicated on SIC-301. SIC-301 normally operates in cascade mode when Generator G-301 is connected to the electric power grid. This control mode is entered by placing droop control switch HS-303 into the DROOP state. Droop control is explained in the next section. SIC-301 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-301 directly controls the shaft speed. In manual mode, SIC-301 is used to manually adjust the HP steam flow to JT-301. Manual mode of SIC-301 is available any time the Steam Turbine is not tripped. Manual mode is entered any time the droop control switch HS-303 is changed from the DROOP to the OFF state.

The pressure of IP steam exhaust from JT-301 is indicated on PI-302 and its temperature is indicated on TI-302. The IP steam is reheated in E-303 using HRSG flue gas. The flow of flue gas to E-303 is controlled by HIC-311. The outlet temperature of reheated steam is controlled by TIC-303 which controls the rate of injection of HP boiler feedwater through TV-303 to the IP steam line upstream of E-303. The outlet pressure of steam from E-303 is indicated and controlled by PIC-303. An IP steam admission valve PV-303 is provided on JT-301. Normally this valve is wide open at design operation. However, PIC-303 will regulate this valve to make sure the IP steam does not drop below 300 PSIG at the outlet of E-303. Too low an IP pressure can lead to condensate in the exhaust from the 1st stage turbine of JT-301 which will quickly erode the turbine blades.

The pressure of cooling water to Surface Condenser E-301 is indicated on PI-305 and its temperature is indicated on TI-305. The flow of cooling water to E-301 is controlled by HIC-305 which adjusts the valve opening of HV-305. The temperature of steam condensate leaving E-301 is indicated on TI-304 and its pressure is indicated on PI-304. The temperature of cooling water leaving E-301 is indicated on TI-306.

The shaft speed of Generator G-301 is indicated on SI-302. The power output of G-301 is indicated on JI-320.

Generator G-301 is provided with a synchroscope in order to visually see the difference of the frequency and phase between electricity produced by G-301 and the electric grid at startup. Generator G-301 is connected to the electric power grid using switch HS-322. SI-320 indicates the frequency of electricity at the terminals of G-301. SI-321 indicates the frequency of electricity of the electric grid after the breaker switch. SI-322 indicates the phase difference between electricity generated at the terminals of G-301 and the electric grid. Before connecting the Generator to the grid with breaker switch HS-322, 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-301/G-301 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-301/G-301.

When SIC-301 is placed into droop mode using switch HS-303, the PV of SIC-301 is computed as follows:

PV = [SI-321.PV * 60 + (SIC-301.OP - 13.0) * 2.542373] * 100/3,600

The setpoint of SIC-301 is locked at 104.0 when in droop mode. Any deviation of the grid frequency (SI-321.PV) will cause SIC-301 to move its output such that the PV is restored back to the setpoint of 104.0. The integral action of controller SIC-301 will cause the output (and power) to move gradually.

Vacuum System Controls and Instruments
The flow of medium pressure (MP) steam to the vacuum system is indicated on FI-307. The pressure of MP steam is indicated on PI-307.

The flow of steam to Startup Ejector EJ-301 is controlled by HIC-306 which adjusts the opening of valve HV-306. The flow of steam to Startup Ejector EJ-301 is controlled by HIC-306 which adjusts the opening of valve HV-306.

The flow of cooling water to Ejector Condenser E-302 is controlled by HIC-308 which adjusts the valve opening of HV-308.

Hotwell Controls and Instruments
The level of condensate in Hotwell D-301 is indicated and controlled by LIC-301 which adjusts the position of LV-301 on the discharge of the Condensate Return Pumps P-301A/B. The flow of condensate taken to the Deaerator is indicated on FI-309.

The motors of Condensate Return Pumps P-301A/B are operated by switches HS-301A and HS-301B, respectively. LAH-301 also indicates the level of condensate in D-301 and its high alarm signal (80% setpoint) is used to auto-start P-301A or P-301B by selecting the AUTO state of switch HS-302A for P-301A or by selecting the AUTO state of HS-302B for P-301B. Normally one pump is in service with its auto-start switch in the MAN state and the other pump is on standby with its auto-start switch in the AUTO position.

Interlock I-301
Interlock I-301 protects the Steam Turbine from overspeed and mechanical problems. I-301 activates on any of the following inputs:

HS-301 serves as both a trip and reset switch. All other trip inputs must be cleared in order to reset I-301.

When I-301 trips, the following actions occur:

Interlock I-302
Interlock I-302 protects the Generator from damage when disconnected from the grid and from mechanical problems. I-302 activates on any of the following inputs:

HS-322’s signal is a one-shot signal to allow a reset of I-302 when restarting. HS-302 serves as both a trip and reset switch. Trip input XA-321 must be cleared in order to reset I-302.

When I-302 trips, the following actions occur: