ABB PSPG-E7 SteamTMax Precise control of main and reheat ST May 8, 2014 Slide 1
Challenge m S m att T in T out Live steam and reheated steam temperatures are controlled process variables critical in steam generation control They need to be regulated with high precision in order to prevent unnecessary material stress (thermal stress) in thick-walled components, especially in steam generators and turbines Traditionally, the live steam (and reheated steam) temperature control was done by means of a PI-PI cascade arrangement or some other simple control structure May 8, 2014 Slide 2
solution ABB s State Controller with observer Evaporator 1st Superheater 2nd Superheater Reheater Feed water control Temperature control Temperature control Temperature control Setpoint Controller G May 8, 2014 Slide 3
Classic temperature control Attemperator Superheater m S m m att T in T out M PI - Classic PI-PI cascade control: PI The primary controller (master) calculates the required inlet temperature necessary to obtain the desired outlet temperature The subordinate controller (slave) regulates the inlet temperature by means of the spray attemperator This configuration delivers good performance in superheaters with low-order and fast transient behavior. However, performance is moderate in superheaters with wider warm-up span, higher-order behavior and/or substantial dead time - T SP May 8, 2014 Slide 4
Internal structure Attemperator m S T in T 1 T out T 2 T 3 m W Observer K 1 K 2 K 3 K 4 T diff T ^ 1 T ^ 2 T ^ 3 ^ T 4 - T SP R 0 R 1 R 2 R 3 R PI 4,5 M State Space Controller Model of the plant: The observer calculates the theoretical internal temperatures in the superheater, making them available to the controller to be used as interstate variables Non-linear load-dependent behavior: The K and R vectors are dynamically calculated to provide optimum performance at any load operation point Performance: Thanks to its predictive capability and ultra-fast integration component, the state controller delivers improved performance without sacrificing robustness. May 8, 2014 Slide 5
Melody function block set point and process variables plant characteristics controller configuration SCO parameterization SCO incl. PI-PI-cascade backup system internal control variables Dynamic parameter calculation based on pole positioning (Ackermann) PI-PI cascade parameterization binary logic and feedback values correcting variable (Y sp ) May 8, 2014 Slide 6
Integration and commissioning Attemperator Superheater m S m att T in State Space Controller with Observer T out M Linearization PI - PI - T SP The SCOT is normally implemented parallel to a simpler control: In existing systems (e.g. SCOT as optimization solution) it runs parallel to the existing control structure In new implementations or complete control logic replacement (e.g. new power units or full retrofits) the integrated PI-cascade backup system is used This simplifies commissioning and avoids disturbances to the process during controller parameterization and fine tuning procedures Switch-over between controllers takes place smoothly thanks to the built-in tracking mode logic May 8, 2014 Slide 7
Integration and commissioning An accurate identification of the plant is a key factor to achieve maximum performance of the controller ABB s plant identification tool for superheaters is designed to simplify the identification process and to deliver high accurate plant characteristics May 8, 2014 Slide 8
Increased efficiency as a result of higher temperatures T out PI-PI-cascade State Controller with Observer (SCOT) Physical Temperature Limit DCS max Temperature Outlet Temperature T Set Point Temperature SP Time Increasing the live steam and reheated steam operation temperature results in an immediate increment of unit efficiency With a higher set point, however, the margin between the operation point and max. admissible temperature is reduced This is no problem thanks to the improved performance that the state controller delivers May 8, 2014 Slide 9
Benefits Attemperator m S m W M T in Observer R 0 R 1 R 2 R 3 State Space Controller T out T1 T 2 T 3 T diff K 1 K 2 K 3 K 4 T^ 1 T^ 2 T^ 3 T ^ 4 - R PI 4,5 T SP Fatigue of thick-walled components (especially in the turbine and steam generator) is greatly reduced Load change rates can be increased without risk of exceeding temperatures limits. As a result, the power unit is better suited to rapidly respond to load demand changes Improved precision (no overshoot) and speed while correcting temperature deviations minimizes actuator work and helps reducing unit oscillations caused by back coupling with other control loops in the system Effective for any superheaters; highly effective for superheaters with wide warmup span (e.g. >70 K for coal, >100 K for gas/oil fired power plants) and/or a significant dead time and slow transient behavior May 8, 2014 Slide 10