Thermoelectricity Boiler and Turbine Control Scheme
Ⅰ.Boiler-Turbine coordination control
The control system adopts Direct EnergyBalance, according to electromechanical boiler’s load-bearing capacity andauxiliary engine’s operational aspect, to rapidly, accurately and stablyrespond to automatic power generation control system (AGC) or load instructionfrom power plant’s operators to work efficiently. At the same time, the systemconsiders operation limiting conditions like auxiliary engine’s bug and device’sdisorder, etc. in a highly adapting way to make the load-bearing capacity reachits groove to meet the demands of continuous and safe operation.
1.1 The system should provide sliding pressuremode of operation to meet the demands of three kinds of loads as follows.
a.Valve opening fixation/sliding pressure operation
Turbine valve maintains in a fixed position andthe vapor pressure rises with increase of loads. When it comes to 91% loads,the pressure reaches rated value, the system comes to the mode of fixedpressure operation and if increase more loads, master turbine valve should beopened.
b.Valve opening fixation with±10%regulation
Inthe process of pressure’s increase and loads’ rise to 91%, turbine valve isallowed to adjust within the range of ±10%, so as to respond to fluctuation of load andimprove stability of frequency.
Atthe time of under load operating mode (not more than 25% loads), turbine valveshould be adjusted to meet the loads’ demand, the vapor pressure maintains in alower constant value, and once loads’ demands increase, it comes to the mode ofsliding pressure operation. When the pressure rises and loads increase, theturbine valve generally remains stationary besides the ±10%regulating variable as a response to fluctuation of load and improvement offrequency stability. When loads come to 91%, unit operation shifts into themode of fixed pressure operation. The system design provides operators withapproaches when they choose needed modes of operations.
1.2 The control system can automaticallyoperate in any of the three ways as follows.
AdoptDirect Energy Balance, organically establish proper relationship between theboiler and the turbine, and in the meanwhile, respond to unit load instruction.
Turbineresponds to the variations of unit load instruction or operators’ instructionby manual operation, while boiler responds to variations of steam flow anddeviations of vapor pressure caused by turbine. Vapor pressure deviation can beused to revise load instruction.
Boilerresponds to the variations of unit load instruction or operators’ instructionby manual operation, while turbine responds to variations of vapor pressure.
Thesystem design provides operators with approaches when they choose needed modesof operations. When modes of operations change, the system won’t be perturbedby it at all. Besides, when unit meets with restricted operating mode, controlsystem may placidly shift mode of operation into any proper mode automatically.If the boiler is under restrictions when responding to load demands, the systemshould shift into mode of turbine following, and be back to mode of operationwhen limitations are cancelled.
Ifthe turbine is under restrictions when responding to load demands, the systemshould shift into mode of boiler following until it backs to mode of operation.When it can’t realize the mode of operation chosen by operators, the systemwill alarm to operators.
Choosingany modes of automatic control requires turbine speed governor, fuel and feedwater subsystem in automatic operation status. If any relevant subsystem out ofautomatic control, coordination control can be shifted into maximum automaticmode and adapted with subsystems which are in automatic operation.
1.3 Coordination control theorem
DEB (DEB is the abbreviation of Direct EnergyBalance) coordination control, actually is also a special “coordination controlbased on boiler following.”
Unit’s rate of power is regulated by turbinesides, and the load response is fast. The ram sides adopt cascade stage controland inner loop PI takes turbine first stage pressure as feedback. Take energybalance signal Ps*P1/Pt as the boiler sides’ load feed forward instruction,take Heat signal Pl+dPd/dt as feedback and control boiler’s energy inputdirectly according to turbine’s energy demands. The system doesn’t needfeedback control of the machine front pressure and cancels machine frontpressure closed loop adjustment which must be possessed by all companies’ CCSsystems worldwide.
The attached picture is DEB coordinationcontrol’s SAMA chart, and one outstanding point of DEB coordination control isto use Energy Balance Signal Ps×P1/Pt orenergy instruction signal. Pressure ratio Pi/Pt linear stands for turbine’svalid valve location. This valid valve location is more direct, simple andeffective than turbine governing valve shift and provides accurate measurementfor practical governing valve’s opening. DEB system access (Ps×P1/Pt)is an energy balance signal suitable for any unit’s operation by fixed pressureor sliding pressure. In steady state, it is P1—steam flow into the turbine;in active state, it is steam flow into the turbine when the turbine adapts the variationsof load demands.
Next is to use heat quantity signal. Drumboiler’s DEB control system, whether the middle coal pulverizing system or the directfiring pulverizing system, all adopt heat signal to measure fuel quantity. Certainly,direct current boiler can also adopt heat signal, it is just a littlecomplicated and its micro hot spot heat signal is:
In the formula: Q— Heat signal;
DMETSI— Micro hot spot metering steam flow;
PS1— Micro hot spot vapor pressure;
Kl, K2—Scale factors.
In engineering application, if the variation ofmicro hot spot vapor pressure is not wide and dynamic deviation is small, termsof K1 and PS1 can usually be neglected in heat signal.
The features of heat signal are:
Heat signal measures the boiler’s generalenergy input and calculates general heat output inside the boiler from allburning fuels (coal, petrol).
Heat signal identifies the variation of theshared conditions of coal quality and combustion, like combustion heat value,moisture and grey, etc. The system can fast eliminate any fuel inputperturbations without waiting to affect combustion ratio instruction’svariation.
Heat signal measures the boiler’s energy inputand puts boiler’s energy storage, so it is suitable for static state as well asactive state with instantaneity.
As the feedback of fuel control system, Heatsignal only reflects the boiler’s inside perturbation (fuel variation), but notouter perturbation.
Above only discusses the “advantages” of theadoption of heat signal. In practical application, difficult problems lie in “adjusting”.More experiments should be done to confirm its parameter. In the meanwhile,because of the irrealizable ideal differential, the differential of minus steamflow needs to be introduced to improve input under the disturbance of load(outer perturbation) and H·H·, etc.All these bring difficulties to practical well use of heat signal.
Ⅱ. Unitload instruction
2.1 Unit load instruction consists of signalsprocessed by AGC signals input, or frequency, power rate, vapor pressure,turbine valve opening, unit’s operation condition and required limit, etc.Operators can realize following functions on the picture of CRT keyboard andload managerial control:
a.Mode selection by manual/automatic operation: unit load control responds to AGCload demand instruction in automatic way, and to load instruction input byoperators in manual way.
b.Manual regulation on unit load instruction
c.Regulation on limit value of high and low load
d.Setting on rate of load’s change
e.Indication on load’s direction change (speeds up or slows down)
f.Indication on limit value of high and low load
g.Master vapor pressure deviation indication
h.Setting and indication on master vapor pressure set point
i.Load instruction and master generated output indication
j.Choice and indication of boiler following, turbine following and coordinationcontrol modes
k.Indication on load block increase, load block decrease and auxiliary engine bugrun back.
1.Choice and indication of sliding pressure and fixed pressure’s operation modes
2.2 The control system realizes the followingfunctions stably:
a.Frequency coordination: turbine’s revolving speed control is used to stabilizesystem frequency. Unit load instruction follows practically measured generatorload automatically to avoid perturbation.
b.Limit: unit’s biggest load instruction can adapt itself to boiler’s biggestoutput and turbine’s load-bearing capacity. Functions of fuel/ air’s precedence/hysteresis and cross-limiting control are provided. When the controlledcapacity or allowed output comes to maximum and minimum limits, control signalsof block increase and decrease should be sent.
c. Auxiliary engine bug run back:
RUNBACK function is provided when boiler feedpump, primary air fan, pressure fan and draught fan, etc. are in outputstoppage operating mode, and generator cuts off water. Each kind of RUNBACKowns individual maximum allowed load and unloaded rate to adapt dynamic propertiesof various devices. Operators can obtain information of RUNBACK operating modeby CRT, and all RUNBACK accomplish automatically. When RUNBACK happens, thecontrol system shifts into operation modes of boiler following or turbinefollowing and remains this mode until operators choose a new one. RB logicpossesses inspecting and estimating functions on instruction’s execution andemergency trip will be done when the logic fails.
2.3 Connector to AGC is provided to distantcontrol unit load.
Signals sent from DCS to AGC at least are:
a.Analog quantity signal:
Active power and reactive power (KW) of each generator
Load maximum set point
Load minimum set point
Set point of load rate of change
Load set point
b.Switching value signal:
Unit AGC standby mode
Unit in AGC mode
Unit output limit and RUNBACK
Signalsfrom AGC to DCS
AGC load instruction
AGC may be sent?
Ⅲ. Turbine control
3.1 De-aerator water level and pressure control
De-aerator water level control is realized byadjusting the de-aerator water level control valve and condensation waterrecycling control valve. In order to adjust better de-aerator water level, thesignals between these two valves should be in proportion.
When in HH water level, de-aerator water levelcontrol valve closes and condensation water recycling control valve opens untilde-aerator water level is lower than maximum.
De-aerator water level control valve closes,while condensation water recycling control valve opens at the instant ofturbine’s tripping operation. After delay of a adjustable time, regulatingsystem comes to order and water level control valve opens as required.
Within the time of start, auxiliary steamcontrol valve is open; maintain de-aerator pressure at a beforehand set point,and when turbine’s tripping operation, a comparative high set point damped withtime function appears, in case of cavitations from booster and feed pump causedby de-aerator’s flash. Under the normal operation, set point follows thede-aerator pressure.
3.2 De-aerator’s cascade tank water levelcontrol
In De-aerator’s cascade tank water levelcontrol system, water level measured value and set constant conduct PIDoperation, and the result control chemical make-up water regulating valve’sopening to maintain the constant of cascade tank water level.
When cascade tank water level reaches themaximum, all control valves should be closed; when it is below certain point,all control valves should be open.
3.3 High-pressure and low-pressure heater waterlevel control
Complete high-pressure and low-pressure heaterwater level control system is provided. Dewatering control valve’s opening iscontrolled to keep the normal value of high-pressure and low-pressure waterlevel.
a. Keep high-pressure water level at a setpoint by cascaded drain valve when at normal.
b. When heater water level of this rank goes uptwo ranks, close cascaded drain valve of previous rank and open malfunctiondrain valve of previous rank to keep heater water level of previous rank at aset point. When high-pressure and low-pressure water level rises, alarm will begiven; when high-pressure and low-pressure water level goes up two ranks,emergency dewatering valve will be open while high-pressure steam extractioncheck valve and electrically operated valve will be closed.