Control system for bypass steam turbines

A reheat steam turbine for operation with a boiler supplying steam under pressure comprising:

• a high-pressure (HP) turbine section, at least one lower-pressure (LP) turbine section, steam conduit means connecting the HP and LP sections, means reheating the steam between the HP and LP turbine sections, at least one control valve for controlling the flow of steam to the HP section, an intercept valve for controlling the flow of reheated steam to the LP section, an HP bypass for passing steam around the HP turbine section, an HP bypass valve for controlling the flow of steam in the HP bypass, an LP bypass for passing steam around the LP turbine section, an LP bypass valve for controlling the flow of steam in the LP bypass, a control loop for controlling the flow of steam to the turbine to maintain preset turbine speed and load and for supplying a control valve position signal and a turbine demand signal, means for supplying a signal representative of boiler steam pressure, means for generating a combined flow reference (CFR) signal representative of total boiler steam flow, means for generating an actual load demand (ALD) signal as the product of the boiler pressure signal and the turbine demand signal, an HP bypass control loop having means for generating a first preselected reference signal as a function of the CFR signal and means for positioning the HP bypass valve to maintain equilibrium between the boiler pressure signal and the first preselected reference signal, means supplying a signal representative of reheated steam pressure, an LP bypass control loop having means for generating a second preselected reference signal as a function of the CFR signal and means for positioning the LP bypass valve to maintain equilibrium between the reheated steam pressure signal and the second preselected reference signal, a steam flow limiting means disposed within said LP bypass control loop to limit the maximum steam flow in said LP bypass, and an intercept valve control loop having means for amplifying the ALD signal by a factor proportional to the inverse of reheated steam pressure to supply an amplified ALD signal and means to position the intercept valve in accord with the amplifier signal.

In combination with a reheat steam turbine operating in conjunction with a boiler supplying steam under pressure, the turbine of the type having a high-pressure (HP) section, at least one lower pressure (LP) section, a steam conduit interconnecting the HP section to the LP section, a steam conduit interconnecting the HP section to the LP section through a steam reheater, at least one admission control valve for regulating the flow of steam to the HP section, and an intercept valve for regulating the flow of steam to the LP section, a comprehensive control system comprising:

• an HP bypass subsystem for passing steam around said high-pressure section, said bypass subsystem including an HP bypass valve for regulating steam flow therein and means providing an HP bypass valve position signal;
• an LP bypass subsystem for passing steam around said lower pressure section, said bypass subsystem including an LP bypass valve for regulating steam flow therein;
• a load and speed control loop for controlling the flow of steam to said turbine to maintain preset turbine speed and load, said control loop providing an admission control valve position signal;
• means providing a combined flow reference (CFR) signal representing the sum of products of (1) boiler pressure and said admission control valve position signal and (2) boiler pressure and said HP bypass valve position signal;
• an HP bypass control loop for operating said HP bypass valve to control boiler steam pressure in accord with a first reference signal determined from said CFR signal;
• an LP bypass control loop for operating said LP bypass valve to control reheater steam pressure in accord with a second reference signal determined from said CFR signal.