505E Digital Governor for Extraction Steam Turbines

The Woodward 505E is an advanced, 32-bit microprocessor-based digital governor designed specifically to control single extraction, extraction/admission, or admission steam turbines. It integrates turbine control and an Operator Control Panel (OCP) into a single, robust package. It is a highly flexible, field-configurable solution whose core design philosophy is guided by menu-driven software. This allows field engineers to program the controller for specific generator or mechanical drive applications, enabling a single hardware design to serve a wide array of control needs, significantly reducing both cost and delivery time.

The 505E can operate as a stand-alone unit or in conjunction with a plant's Distributed Control System (DCS). It features a comprehensive front panel with a two-line (24-character each) display and a 30-key touchpad, allowing complete turbine control functionality—including configuration, online adjustments, and operation—without requiring additional control panels.

Detailed Core Features

The 505E's feature set is extensive, designed to meet the complex demands of modern steam turbine control.

1. Dual-Parameter Control & Valve Management:

• The 505E controls two parameters by managing the High-Pressure (HP) valve and the Low-Pressure (LP) valve. The most typical combination is speed (or load) and extraction/admission pressure (or flow).

• However, its application is far broader. Through configuration, it can also control or limit parameters such as: turbine inlet pressure/flow, exhaust (back) pressure/flow, first-stage pressure, generator power output, plant import/export power levels, compressor discharge pressure/flow, unit/plant frequency, process temperature, or any other turbine-related process parameter.

2. Multi-Mode Speed Control:

• Speed Control: When the generator breaker is open, the PID controller maintains turbine speed at the setpoint, independent of load changes.

• Frequency Control: When the generator breaker is closed but the utility tie breaker is open (i.e., islanded operation), the governor controls unit frequency.

• Unit Load Control (Droop): When both the generator and utility tie breakers are closed (i.e., paralleled with the grid), the governor controls generator load based on the speed setpoint and a droop feedback signal. The droop function allows the unit to share load stably with other non-isochronous units or a large grid.

3. Extraction/Admission (Extr/Adm) Control:

• This is a dedicated PID controller used to maintain extraction or admission pressure/flow at its setpoint. It is the core function for controlling extraction/admission turbines.

4. Auxiliary (AUX) Control:

• Controller Mode: When enabled, it takes over the LSS bus to directly control a parameter like inlet pressure, exhaust pressure, or generator power.

• Limiter Mode: Always active, it does not actively control but limits the LSS bus signal, thereby constraining a monitored parameter (e.g., power) to below its setpoint.

• This is an extremely versatile control channel, configurable in two modes:

5. Cascade (CAS) Control:

• This controller compares its process variable (e.g., inlet pressure) to a setpoint, and its output directly resets the speed controller's setpoint. This is a cascaded control strategy used for processes that are regulated by changing turbine speed or load.

6. Ratio/Limiter: This is the intelligence of the 505E for extraction/admission applications.

• Ratio Function: Automatically calculates the coordinated movement of the HP and LP valves based on programmed turbine performance parameters (steam map). Its purpose is to minimize interaction between the two controlled processes when speed/load demand or extraction/admission demand changes, achieving "decoupling."

• Limiter Function: Ensures valve commands always remain within the turbine's safe operating boundaries (defined by the steam map). When an operating limit is reached (e.g., a valve fully open or closed), the limiter logic, based on the preset priority (Speed Priority or Extr/Adm Priority), decides which parameter to sacrifice to protect the other.

7. Automatic Start Sequence:

• The 505E offers three start modes: Manual, Semi-Automatic, and Automatic.

• It can also be configured for an Auto Start Sequence, which automatically manages the run-up from low idle to high idle to rated speed based on downtime (hot or cold start), including programmable warm-up times and acceleration rates.

• The Critical Speed Avoidance function allows for two speed bands to be defined; the setpoint will ramp quickly through these bands to avoid prolonged operation within them, protecting the turbine.

8. Valve Limiters:

• HP and LP valve limiters are used to manually restrict the maximum or minimum opening of the valves during start-up, shutdown, and troubleshooting, adding operational safety and flexibility.

9. Communication Capabilities:

• The 505E is equipped with two Modbus communication ports (supporting RS-232, RS-422, RS-485, and ASCII or RTU protocols) for data exchange and remote control with plant DCS, PLCs, or CRT-based operator stations.

• All critical setpoints, process variables, statuses, and alarms can be read from or written to via Modbus.

10. Protection & Alarm:

• Provides both Emergency Shutdown and Controlled Shutdown procedures.

• Built-in Overspeed Test function for testing electrical and mechanical overspeed protection devices.

• First-Out Alarm indication records and displays the first cause of a trip among up to five shutdown inputs, facilitating diagnostics.

• Comprehensive input signal failure detection (e.g., loss of speed sensor or analog input signal).

Principles of Operation

The control principle of the 505E is based on classic closed-loop PID control algorithms, combined with specialized coordination logic for extraction turbines.

1. Signal Flow & Architecture:

• The core of the control is a Low Signal Select (LSS) bus. The outputs of the Speed PID and the Auxiliary PID (in controller mode) compete for access to this bus. The lowest value signal wins control and is sent to the Ratio/Limiter.

• The Ratio/Limiter receives the signal from the LSS bus (representing speed/load demand) and the signal from the Extr/Adm PID (representing extraction/admission demand).

• The Ratio logic uses these two demand signals, along with pre-defined turbine characteristics (K coefficients, steam map), to calculate two outputs: an HP valve demand signal and an LP valve demand signal. The calculation is typically linear (e.g., HP = K1*S + K2*P + K3).

• The calculated HP and LP signals are then compared with the HP/LP valve limiter signals (HP is Low-Signal-Selected with its limiter, LP is High- or Low-Signal-Selected depending on application type) to produce the final control command sent to the actuators.

2. Ratio/Limiter Operating Modes:

• Decoupled Inlet (HP): Only the HP valve moves to control inlet pressure; both valves move to control extraction pressure.

• Decoupled Exhaust (LP): Only the LP valve moves to control exhaust pressure; both valves move to control extraction pressure.

• Decoupled HP & LP: The HP valve controls one parameter alone (e.g., inlet pressure), and the LP valve controls another parameter alone (e.g., exhaust pressure). In this mode, speed/load is no longer controlled but becomes a dependent variable.

• Coupled HP & LP Mode: This is the most common mode. HP and LP valves move in coordination to control both speed and extraction pressure with minimal interaction. A change in either demand causes both valves to reposition.

• Decoupled Modes: Used when controlling two independent parameters (e.g., inlet pressure and extraction pressure).

3. Priority Logic:

• Because the turbine only has two control valves, when a physical limit is reached (e.g., a valve is fully open), only one parameter can be prioritized. The 505E allows the operator to select or automatically switch priority.

• Speed Priority: Upon reaching a limit, maintain speed/load and sacrifice extraction/admission control.

• Extraction/Admission Priority: Upon reaching a limit, maintain extraction/admission pressure/flow and sacrifice speed/load control.

4. Droop Control Principle:

• In Unit Load Control mode, the speed PID setpoint automatically increases as load increases. Droop is defined as: (No-Load Speed - Full-Load Speed) / Rated Speed × 100%.

• For example, a 5% droop means the setpoint will rise by 5% of rated speed as load increases from 0% to 100%. This creates a stable speed-load characteristic, allowing multiple units with droop to share changes in grid load proportionally. Droop feedback can come from a generator power (kW) signal or from valve position signals.

5. Startup Logic:

• Taking the Automatic Start mode as an example: After the operator opens the Trip & Throttle (T&T) Valve and presses "RUN", the 505E raises the HP Valve Limiter at a pre-set rate while the speed setpoint ramps up at the "Rate to Min" speed to the minimum governing speed. When actual speed matches the setpoint, the Speed PID takes over control to maintain speed. The operator can then enable other functions like extraction control.

Summary

The Woodward 505E Digital Governor is a powerful, highly integrated, and flexibly configurable control platform. It utilizes advanced microprocessor technology, sophisticated control algorithms (PID, Ratio, Decoupling, Priority), and comprehensive I/O interfaces to achieve precise, reliable, and automated control of extraction/admission steam turbines. Its core value lies in using a single hardware platform that can be software-configured to meet a vast range of application requirements, from simple speed control to complex multi-parameter coordinated control. Coupled with comprehensive protection, communication, and user interface features, it is an ideal control solution for modern industrial steam turbine drives.

Reference：http://woodwardtj.com/UploadPhotos/201711061837412136.pdf