Steam Turbine Governor

 

  Governor Implementation
 The Steam Turbine Governor function is integrated into the DI-TRONICS® IV Control System which is a PLC based system designed to operate a steam turbine driven compressor or generator train.

The integrated steam turbine governor functions are programmed using "C programming language" and reside in the PLC central processing unit. The governor functions operate as a program module which runs in conjunction with the train and process control programming which are programmed using traditional PLC Ladder Logic.

Features
 The following governor features are included.

  • Manual, Semiautomatic and Automatic Start and Stop control
  • Two critical speed avoidance ranges
  • Droop and Isochronous speed control
  • Remote, Auxiliary and Cascade speed control
  • Loss of generator and loss of utility detection
  • Hot standby capability for critical applications
  • Single and dual actuator control
  • Programming configuration using a laptop computer (optional)
  • Operator accessible governor parameters are displayed and modified using the DI-Tronics® IV Control System operator interface
  • Inputs to and outputs from the governor use PLC based I/O cards
  • An optional serial communications card can be mounted in the PLC rack for transmission of operational information regarding the governor and the steam turbine

Manual Operation
 The operator places the system in the Run mode which opens the governor valve to the full open position. The operator then manually opens the T&T (Trip & Throttle) valve to Idle speed to warm-up the unit. After warm-up, the operator manually opens the T&T valve to the full open position. As the turbine speed approaches the Rated speed, the governor takes control with the Governor valve.

Semi Automatic Operation
The operator places the system in the Run mode then manually opens the T&T valve to the full open position. The governor ramps open the governor valve until the unit Idle speed is reached. After the unit warm-up is complete, the operator switches the governor from the Idle mode to the Rated mode. The governor then accelerates the unit from Idle to Rated speed at a predetermined ramp rate. During this acceleration, critical speed avoidance is accomplished by initiating a faster ramp rate in the critical speed ranges. The speed of the turbine is then controlled locally by the operator or remotely by an alternate process signal.

Automatic Operation
 The operator places the system in the Run mode and then manually opens the T&T valve to the full open position. The governor automatically ramps open the governor valve until the unit Idle speed is reached. When the unit warm-up is complete the governor automatically accelerates the unit to Rated speed making sure that the unit accelerates through any critical speeds. The unit speed is then controlled either locally by the operator or remotely by an alternate process control signal.

Note: The steam turbine must be started in accordance with the steam turbine manufacturers recommendations.

Speed Control
Speed control provides the primary method of controlling the speed of the turbine by comparing the
feedback from the power grid or two speed signals to the speed set point. This comparison generates an error term which is used to generate the control output. The control output is low selected with the output of the valve ramp function and the auxiliary loop function to obtain the value for the valve output function.

Remote Speed Control
 A 4-2OmA input from a process controller may be used to remotely set the speed of the turbine.

Critical Speeds
Up to two critical speed ranges may be programmed in the system. When critical speed ranges are selected, the governor forces the speed of the steam turbine to quickly ramp through the critical speed ranges. The governor will not allow the steam turbine to run at constant speed in the critical speed ranges. This function is only provided in the Semi-Automatic and Automatic Modes.

Operator Interface Display
Valve position(s), turbine speed and other parameters are displayed on the DI-Tronics® IV Control System operator interface.

Generator Drive Operating Modes
The governor can be operated in either Droop or Isochronous Mode for electrical generation applications.

Watchdog Circuit
A watchdog circuit is provided to ensure that the PLC program (and its internal steam turbine governor functions) continue to operate as intended.  If the program fails to execute within a predetermined period of time, the machinery is safely sequenced to the shutdown configuration using an external relay based hard-wired shutdown system.

Governor Configuration
The governor can be configured as either a simplex or a redundant (hot stand-by) configuration.

PLC Based Governor Implementation
The steam turbine governor implementation in the PLC has several advantages over the more traditional stand-alone steam turbine governor.
Feature Benefit
Reduced number of components Reduces overall cost
Increases reliability
Common spare parts with the PLC Reduces overall cost
Reduced training requirements Reduces overall cost
Common operator interface with the PLC Reduces overall cost
Reduces time required to learn how to use unit
Less panel space required Reduces overall cost
Reduces size of panel
Load sharing between generators No external hardware required
Reduces overall cost
STGO498
Copyright 1999 Dresser-Rand