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DEH system

key word:Heat exchange element

Category:


Product description

1. Characteristics of reheat steam turbine

1.1. Due to the existence of intermediate reheat volume, the power of medium and low pressure cylinder lags behind, which reduces the adaptability of the unit to load.

1.2. The large reheat volume time constant (8 ~ 12s) of large steam turbine results in the change of unit components and control law

1.2.1. In case of load rejection of the unit, even if the main steam valve and regulating steam valve are closed immediately, the steam stored in reheat volume can make the unit over speed by more than 40%. Therefore, the medium pressure main steam valve and regulating steam valve should be added, and the two inlet valves should be closed at the same time during load rejection of the unit.

1.2.2. The control requirements of reheat steam turbine are as follows: the high pressure regulating valve is used to control the load or speed, and it is required to be adjustable in the whole process; The middle pressure regulating steam valve can be adjusted from start-up to about 1 / 3 load, and fully opened above 1 / 3 load, without regulation.

 

2. Digital electro hydraulic control system (DEH) of large steam turbine

2.1. Digital electro hydraulic control system of imported 300MW Unit

2.1.1 composition of DEH control system of 300MW unit:

Figure 9-1 shows the digital electro-hydraulic control system of the imported 300MW unit, which is composed of two parts: the digital system with computer as the main body and the hydraulic servo system with high-pressure fire-resistant oil. The output of the digital system, after conversion and amplification, is controlled by the hydraulic servo system.

2.1.1.1. Electronic controller: it mainly includes digital computer, mixed digital analog plug-in, interface, power supply and other equipment, which is used for giving, receiving feedback signal, logic operation and sending command for control.

2.1.1.2. Operation and monitoring system: it is mainly image station (DAS), including operation panel, independent computer, display and printer, etc., which is used by operators to provide operation information, conduct man-machine dialogue, operation and supervision, etc.

2.1.1.3. Oil system: high pressure oil and lubricating oil system. The high-pressure oil is fire-resistant oil, which provides control and power oil for control and protection system, and receives instructions from electronic controller and operation panel; Lubricating oil provides lubricating and cooling oil for bearings.

2.1.1.4. Conversion, amplification and actuator: including servo system, manual standby control system, and unilateral hydraulic motor with additional quick closing, isolation and check device, which is responsible for controlling high and medium pressure main steam valves and regulating steam valves.

2.1.1.5. Protection system: it mainly includes relay system and solenoid valve, which is used to protect and block the unit. Protection system (OPC) is used to close the regulating steam valves of HP and IP cylinders when the unit is overspeed; The protection system (ET5) is used to shut down all main steam valves and regulating steam valves immediately when the parameters seriously exceed the standard and endanger the safety of the unit. In addition, there are mechanical overspeed protection and manual trip system.

In addition, measurement and conversion elements for unit control, automatic start stop and supervision services are also essential.

2.1.2. Features and functions of DEH control system of 300MW unit:

The DEH system of Xinhua company is introducing the DEH - Ⅲ system of Westwood company, using digital computer and new technology of high-pressure fire-resistant oil, which has many new features:

2.1.2.1. There are two main control computers A and B, one working and one hot standby, with self diagnosis function and fault-tolerant technology to realize undisturbed switching. In addition, machine C is used for automatic program control.

2.1.2.2. The system has multiple control modes, multiple operation modes and multiple manual simulation systems, which can realize automatic, manual tracking and undisturbed switching.

2.1.2.3. The system has the flexibility to change the control mode, speed change rate and valve pipe mode on line.

2.1.2.4. Through monitoring and numerical calculation, the automatic start-up and stop, automatic grid connection and loading of the unit, as well as complete fault protection and valve quick closing functions can be realized.

2.1.2.5. The main control parameters of the system have perfect and reliable discrimination function, and the selection method of "two out of three" is adopted; Das provides necessary state variables for ATC; CRT image station has the functions of monitoring, displaying, tabulating and printing the system itself and the unit.

Because of these characteristics, the DEH system has strong function and high precision. Under the rated parameters, the control deviation of speed is ± 2 r/min, the power deviation is  ± 2MW。

2.2. Digital electro hydraulic control system of GEC company in UK (omitted)

2.3. Digital electro hydraulic control system of Alstom (omitted)

 

3. Working principle of DEH Control System

Figure 9-2 shows the schematic diagram of DEH system of imported 300MW steam turbine, and the output in the figure is the speed( φ), External disturbance is load change R, internal disturbance is steam pressure p, transmission speed λn and power λp given.

3.1. Three kinds of feedback signals: set three feedback signals: pressure PT of regulating stage, power P of unit and speed n.

3.2. Servo system: a servo system composed of servo amplifier, electro-hydraulic servo valve, oil motive and its linear displacement transmitter (LVDT), which undertakes power amplification, electro-hydraulic conversion and valve position change, steam intake change, and performs control tasks on the unit.

3.3. Cascade PI control system: cascade PI control system, and the adjustment operation is completed by some digital areas. The system consists of internal circuit and outer circuit, which promotes the speed of the regulation process, while the outer circuit ensures that the output is strictly equal to the initial value. The proportional link in PI regulation enlarges the adjustment deviation signal rapidly; The integral link ensures the elimination of the static difference of the system, and it is a kind of non differential control system.

The direction of "switch" K1 and K2 in the system can be adjusted according to the cascade PI or single-stage PII or P12 regulation mode. When the system is disturbed, the flow rate of the steam turbine changes, first of all, the change of the regulating stage pressure, which responds quickly. The power circuit of the generator has a slow response. When the unit participates in frequency regulation, the speed depends on the frequency of the grid. In the case of large power grid capacity, the feedback of the speed loop is generally small and the influence is weak.

3.4. Speed setting and power setting: when the system is working, the speed given represents the required target speed and the power given represents the target load. However, when the unit participates in frequency modulation, the deviation between the speed given and the speed feedback signal reflects the magnitude and direction of the external disturbance. If the power given value is not changed, the load value maintained by the control system is different. The power given value corrected by the speed deviation is the load value maintained by the system.

3.5. Operation mode of DEH system: it can be operated in frequency modulation mode and basic load mode.

A: When the system is in frequency modulation mode, if the load of the power grid increases, the frequency will decrease and the unit speed will decrease. After comparison with the given value of the speed, the output is positive deviation. After the signal corrected by PI regulator, the servo amplifier is input, and then the power of generator increases by opening the large regulating steam valve through the electro-hydraulic servo valve and oil motive. At this time, the system has two balance modes:

(1) The given power value is added to adapt to the load required by the grid. The frequency of the grid rises, the speed deviation is zero, and the frequency of the grid remains unchanged;

(2) The power is still constant, and the load value increased at this time is balanced by the increase of the speed deviation. The deviation of speed then represents the increase of power, which is the balance obtained at the cost of damaging the power grid frequency. The increase of load by increasing the given value of speed will make the dynamic quality of the unit deteriorate when the load is dumped, and even the danger of overspeed will occur. Therefore, the correct way is to make the speed given as the rated value, and to adapt to the increase of external load by increasing the power given value.

B: When the unit is under basic load, the deviation signal of rotation speed is not connected to the system, or the deviation is multiplied by a small percentage, so that it is insensitive to the change of external load, that is, the so-called "dead zone". Therefore, the control system will control the unit according to the given value of its own power to maintain the basic load operation.

3.6. The system has strong anti internal disturbance capability:

When the DEH system is subject to internal disturbance, for example, if the main steam parameter is reduced, the output power will decrease. Because of the positive deviation between power given and power feedback output, the regulating valve is required to open large to make the output power equal to the given value of power, and the system reaches balance. Therefore, the system has strong anti internal disturbance capability.

The internal circuit of the system has two circuits: regulating stage pressure and power feedback. When disturbed, the pressure loop of regulating stage reacts fastest. Through the action of PI2, the opening of regulating steam valve is changed rapidly; The power feedback circuit needs to change the opening of the regulating valve through PI1 and PI2, and the adjustment process is slow. The inner loop only plays the role of coarse adjustment, and the outer circuit plays a fine-tuning role. Because both internal circuits have the ability of quick response to external and internal disturbances, cascade P-I control is the best operation mode, while single-stage P11 or P12 control mode has poor regulation quality, so it should be used as standby operation mode.

3.7. Adjustment parameters setting:

The setting of adjusting parameters in the system should ensure the dynamic quality of the system. In order to avoid the instability of the system due to the strong integration, the upper and lower limits of the output shall be set to swing around 1.

3.8. Dynamic holding of DEH system during load rejection

The external disturbance, internal disturbance or the change of given value all lead to the system action. The change of the given value (changing the target speed or load) is manual operation and the action is generally slow. The internal disturbance is small.

The disturbance of external load is the biggest, the most serious is the rated load rejection of the unit. At this time, all steam entering the turbine will make the unit speed up, which is in danger of overspeed. There are two ways to operate the regulating system:

3.8.1. Power given is not cut off at the same time: in this case, the negative deviation of the output of the speed loop is the closing signal, and the positive deviation of the power circuit output is the door opening signal, which is called "reverse adjustment". Only when the door closing signal of the speed deviation overcomes the power deviation, the system can be stabilized. The results showed that the dynamic quality of the system was deteriorated, and the steady speed was higher than the rated speed, and the value was the speed change rate;

3.8.2. When load rejection, the power given value is cut off at the same time. In this case, the power circuit has no deviation output. The system quickly closes the regulating steam valve based on the negative deviation signal output by the speed loop. Its dynamic holding performance is the best, and the stable speed is equal to the rated speed.

3.9. protection system of DEH system:

DEH system also has over speed protection (OPC) and electric overspeed protection (ETS) and mechanical overspeed protection system. Multiple protection is implemented. In case of emergency, any system action can close the regulating steam valve or simultaneously close the main steam valve and regulating valve to ensure the safety of the unit.

 

4. Basic functions of DEH system

The overall functions of DEH system can be summarized as follows: automatic start and stop of steam turbine, automatic control of steam turbine, automatic protection of steam turbine, operation monitoring of steam turbine, etc.

4.1. automatic start and stop of steam turbine:

The parameters of large steam turbine are high, change greatly, shape is complex, large size, heat transfer is uneven, and the working condition of start-up and stop process changes dramatically. There are many monitoring items and complex operation in the start and stop process. In order to achieve safety, economy and speed, it is very important to realize the self start and stop of the unit. The computer program control with microprocessor as the core can make full use of the computer's operation, processing and logic judgment ability, and through continuously detecting the state parameters of the unit and calculating the thermal stress of the rotor and other parts, the unit can realize the self start and stop of the unit at the highest speed and the shortest time within the allowable range of thermal stress.

DEH system is equipped with two startup modes, cold and hot.

4.1.1. when cold start-up, it is controlled by high-pressure main steam valve and regulating valve, and the medium pressure main steam valve and regulating steam valve are fully open. The turning speed is 2900r/min from turning to the speed, which is controlled by the high-pressure main steam valve; After the speed reaches 2900r/min, switch to the high-pressure regulating steam valve to control the speed rise, and connect the grid to the initial load; After the system is transferred to the load control circuit, the load is controlled by the high pressure regulating steam valve.

4.1.2. when hot start-up, use joint start mode, and medium pressure regulating steam valve shall participate in control. During startup, the medium pressure main steam valve is fully open, the high-pressure main steam valve is fully closed, and the high-pressure regulating steam valve is kept fully open. First, the middle regulating valve enters the steam and controls the speed. When the speed rises to 2600r/min, the opening of the middle control valve remains unchanged, and then switches to the main steam valve to control the speed. When the speed rises to 2900r/min, switch to the high-speed regulating valve control. After the switch is successful, the high-pressure main steam valve is fully open, and the high-pressure regulating valve controls the speed rise, grid connection and initial load; Then the load is controlled by the high and middle regulating valves. After reaching the rated load of 35%, the middle control valve is fully opened, and the high pressure valve continues to load up until the end of the start-up.

4.2. Automatic load control of steam turbine:

Load control of steam turbine: when cold start, it is controlled by high pressure regulating steam valve, and in hot state, it is controlled by high and medium pressure regulating valve to 35% rated load. The medium pressure regulating steam valve is fully open and the load is controlled by high pressure regulating steam valve.

DEH system has the following automatic control modes in load control stage:

4.2.1. operator automatic control mode (OA): in this mode, the operator inputs the target load and load change rate through the operating panel, and DEH controller completes the operation and processing of the adjustment variables; Finally, the automatic control of load is realized.

4.2.2 remote control mode (REMOTE): in this mode, the load command (target load and load change rate) of DEH is changed through remote control interface by the load management center (LMCC) or load dispatching center (ADS) of CCS, and the load of the unit is controlled through DEH system.

4.2.3. plant level computer control (PLANT COMP): the plant level computer sends out the command of target load and load change rate, and automatically controls the load of the unit through DEH.

4.2.4. automatic turbine program control mode (ATC): in the load control stage, the automatic turbine control mode can also be combined with the above three automatic control modes.

4.2.4.1. operator automatic and automatic turbine control joint mode (OA-ATC): in this mode, the operator inputs the target load and load change rate of the unit through the operating panel, and the automatic turbine program control selects the minimum rate from the following rate as the actual load change rate of the unit:

① The optimal rate determined by the calculation of rotor stress by ATC software package;

② The rate of load change limited by the turbine generator;

③ Load change rate input by the operating panel;

④ Allowable load change rate of the power plant.

When the unit load reaches the target load set by the operator, ATC program automatically changes to the operator automatic (OA) mode.

4.2.4.2. ATC-CCS, ATC-ADS or ATC plan comp combination mode composed of ATC-CCS, ATC-ADS or ATC-PLANT COMP respectively with the coordination of machine and furnace or power plant level computer: in these modes, load instructions are from CCS, ads or plant comp, and the load and load change rate selected by the operator are replaced by coordinated control, remote control source or power plant level computer instruction; ATC monitors the corresponding rate. If the required rate is higher than the allowable speed of the unit, ATC will turn to the "hold" mode.

*In the normal operation stage of the unit, only one of the above control modes is accepted as the current control mode.

*In addition, DEH system also has TPC and RUNBACK control mode to protect the main and auxiliary equipment in case of abnormal operation of the unit. If the main steam pressure is reduced, the load of the unit is reduced by closing the regulating valve to maintain the stability of the main steam pressure; External load return control is to consider auxiliary machine failure. If single feed pump or one-sided fan fails, DEH system will close the regulating valve at a certain rate to reduce the load to 50% rated load or predetermined value. After the fault is eliminated, these control circuits will automatically exit and return to normal control.

4.3. Operation monitoring of steam turbine generator set

DEH monitoring system is used to monitor the status of the unit and DEH device itself during start-up and operation. Besides the measurement and data acquisition (DAS), ATC software mainly realizes the contents including operation status button indication, status display, CRT screen, report printing, etc; The monitoring of DEH includes important channels, internal program operation status and power supply. CRT screen display includes important parameters, operation curve, trend diagram fault display and picture copy of unit and DEH system, as well as over limit alarm and accident recall.

4.4. Automatic protection of turbine generator set

The main contents of DEH system automatic protection are as follows:

4.4.1. Overspeed protection system (OPC): the system consists of three parts: quick closing function of medium pressure regulating valve (CIV), load drop prediction function (LDA) and overspeed control function

4.4.1.1. The quick closing function of medium pressure regulating valve refers to the sudden power drop of generator caused by some reason, and quickly close the medium pressure regulating valve, and then open after delay of 0.3-1s, so as to maintain the stability of power system in the moment of partial load rejection.

4.4.1.2. Load drop prediction function refers to the protection system to quickly close the high and medium pressure regulating valves when the generator oil switch trips and the turbine still has more than 30% load, so as to avoid shutdown caused by the action of severe overspeed and emergency trip system.

4.4.1.3. Overspeed control function refers to the closing of high and medium pressure regulating steam when the unit speed exceeds 103% n0, and changing the load control to speed control. After the speed drops, open the medium pressure regulating valve. If the speed is no longer rising, open the high-pressure regulating steam valve to keep the unit running without load and can be connected to the grid quickly. The overspeed protection function is realized by making the OPC solenoid control valve act and releasing the oil pressure of OPC main pipe by the overspeed protection controller.

4.4.2. Mechanical overspeed and manual tripping system: when the speed exceeds 110% n0, or the operator requests to stop for some reason, the oil pressure of the trip master pipe will be released through mechanical overspeed or manual tripping system, and all main steam valves and regulating valves shall be closed to realize the emergency shutdown.

4.4.3. Emergency trip control system (ETS): when the important parameters endangering the safety of the unit exceed the specified value, DEH cooperates with emergency trip system (ETS) to make AST solenoid valve lose power, release pressure oil in the main pipe of automatic shutdown and trip, and drain the pressure oil of oil motive, and make all steam valves close quickly and implement emergency shutdown.

Both overspeed protection and emergency trip protection are realized by software and hardware. In order to avoid the protection system from misoperation or rejection and ensure reliable shutdown, hardware protection adopts three sets of identical equipment, which can only work after three out of two processing are carried out on the output part.

In addition, DEH protection system can cooperate with ETS system to conduct 103% overspeed test, 110% overspeed test, emergency shutdown overspeed test and periodic test of each solenoid valve in order to ensure that the system can always be in good standby state.

 

5. Speed control and load control of DEH system

DEH system control

According to the actuator: high pressure main steam valve (TV) control, high pressure regulating steam valve (GV) control, medium pressure main steam valve control and medium pressure regulating steam valve (1V) control;

According to operation mode: it can be divided into digital control, analog control and manual control;

According to control objectives: it can be divided into speed control and load control. When the unit starts and stops, the speed is controlled, and the load is controlled when the unit is connected to the grid.

 

5.1. High pressure main steam valve (TV) control:

The high pressure main steam valve control is used for emergency shutdown when starting up speed and unit tripping. During cold start-up, the speed of TV rises to 2900 r/min from the turning gear; During hot start-up, when the maximum speed (2600 r/min) of TV controlled by I V rises to 2900 r/min, the speed control changes from TV to GV, and then TV remains fully open until the unit is connected to the grid and operated with load, which is always controlled by GV.

5.1.1. Working mode of high pressure main steam valve:

5.1.1.1. When the control system operates in automatic mode and the unit speed is controlled by TV, the digital system has two automatic working modes:

5.1.1.1.1. Operator automatic mode (OA): this mode is controlled by the operator inputting the target speed and speed up through the operation keyboard.

5.1.1.1.2. Automatic steam turbine program control mode (ATC): the target speed request value of each stage is given by ATC software package, and the speed change rate of each stage is controlled according to the rotor stress calculation.

5.1.1.2. When the TV control unit speed rises to 2900r / min, the TV to GV switch is implemented. After the switch is completed, there is an open bias signal to keep the high pressure main steam valve fully open.

5.1.1.3. TV manual mode: in this mode, the digital system does not participate in the unit control, only in the tracking state, and only controls the unit speed through the analog system.

5.1.2. Speed control principle of high pressure main steam valve:

Figure 9-3 shows the speed control schematic diagram of high pressure main steam valve. It is composed of control mode selection, set point forming loop comparator (COMP), GO-HOLD, set point counter (REF COUNTER) and PI corrector.

The whole process of TV speed automatic control is: in each control cycle, on the one hand, the state parameters of the controlled object are continuously detected and data processed, and the control process flag is established; On the other hand, it is state identification, which continuously calculates the control variables according to the process selection control strategy to implement closed-loop control of the speed. The main contents are as follows

5.1.2.1. Control mode selection: TV automatic control has two modes: OA and ATC. During operation, the operator selects one of the modes, and the set value should be set to zero before the unit is locked (ASL = 1).

5.1.2.2. There are two main state logics in TV speed control stage

5.1.2.2.1. Control logic of TV: the logic has four status signs: TV control, TV light on, TV to GV switching in progress and TV to GV switching end (TROCM). Only when the "TV light on" logic is set, can the unit start to speed up under the control of TV.

5.1.2.2.2. Carry out a hold (GO-HOLD) logic: when the unit is in OA mode, whether the input target speed and rising rate are accepted by the DEH system depends on the set logic state. When the "GO" key is set, that is, GO = 1, the DEH system executes the set value of the valve control circuit, and adjusts the opening of the steam valve according to the rising rate selected by the operator to achieve the target speed; When the logic status key "HOLD" is set, the system only keeps the original state.

5.1.2.3. Set value forming loop: the loop is composed of COMP, REF COUNTER and some status marks.

The task is to change the speed from one steady state to another with the thermal stress within the allowable range.

The working principle is: at the input end of the comparator, if the set value is less than the target request value, it will output a raise signal to make the given rate of the counter tend to the target request value. Until the set value is equal to the target request value, the set value forms a loop, and vice versa. In OA mode, only after the "GO" (HOLD) flag is established, the actual set value will gradually approach the target request value.

5.1.2.3. Control loop: it compares the output of the loop formed by the set value with the actual speed feedback signal, and outputs the corresponding control quantity after correction by PI regulator.

 

5.2. High pressure control valve (GV) control

5.2.1. Tasks of high pressure regulating valve control:

5.2.1.1. During the start-up, the speed starts from 2900r / min, TV switches to GV, and GV controls the speed of the unit.

5.2.1.2. Grid connection: according to the frequency of power grid, when the unit speed rises to the synchronous speed (3000 r/min), the operator selects automatic synchronous control or manual step control mode to control the unit speed until grid connection; Maintain no load and initial load (about 3% ~ 10% rated load).

5.2.1.3. After the unit is connected to the grid with initial load, GV controls the unit to increase the load. If hot start, before the load reaches 35% of the rated load, the medium pressure control valve also participates in the load regulation.

5.2.1.4. In case of abnormal conditions (such as ASL, OPC action, etc.), the regulating steam valve shall be closed quickly and shut down in an emergency to ensure the safety of the unit.

5.2.2. Working mode of high pressure regulating steam valve:

5.2.2.1. High pressure regulating steam valve automatic (AUTO) mode: it is a control mode with computer participation (i.e. digital system operation), and can work in six ways, specifically:

5.2.2.1.1. Automatic steam turbine program control (ATC);

5.2.2.1.2. Automatic synchronization control: this mode accepts the signal from the automatic synchronization system and is only used when the excitation is not put into operation;

5.2.2.1.3. POS mode: the control of this mode comes from the monitoring program, and is used in internal failure, external application interruption and task priority arrangement;

5.2.2.1.4. Operator automatic mode (OA);

5.2.2.1.5. Remote control: this mode is used for coordinated control (CCS) and automatic dispatching system control (ADS);

5.2.2.1.6. PLANT COMP: this mode is only used when there is a plant level upper computer in the power plant.

One of these modes can be selected according to the operation needs, and the two modes are not allowed to run at the same time. In addition, DEH can also accept the runback command, main steam pressure signal and unit trip set value zero signal to realize the automatic protection of the unit.

5.2.2.2. Manual mode of high pressure regulating steam valve: in this mode, the computer does not participate in the work, but the operator issues instructions and controls them through the simulation system.

A tracking system is set between automatic and manual control to realize undisturbed switching when the working mode changes.

5.2.3. Automatic speed control of high pressure regulating valve:

The speed control of high pressure regulating valve includes three stages: switching from TV control to GV control, increasing the speed to synchronous speed and realizing synchronous grid connection.

Before the unit turns to speed control, according to the high-pressure regulating valve control (GC) logic, the GV control status flag is first established, so that the DEH system can select the control strategy according to the unit status. When the system is in the automatic mode, there are two ways to select the GC state (GC = 1). The operator presses the corresponding key to set the GC trigger and press the operator automatic (OA) or automatic turbine program control (ATC) mode to work. When the unit trips (ASL), the GC is reset.

The speed control principle of GV is shown in Figure 9-4. In addition to adding automatic synchronizer to control the signal and limiter of speed up and speed down, the working principle of GV is similar to that of figure 9-3 (TV). The main problem is to control the speed to make it automatic synchronization. Therefore, it is necessary to establish the flag logic of synchronizer control, so that after entering the automatic synchronization control mode, the DEH system can accept the input signal from the automatic synchronization device and convert it into the speed setting value, so as to realize the speed of steam turbine equal to the speed corresponding to the grid frequency. When the frequency, phase angle and voltage of the generator are consistent with the grid, the unit will be automatically connected to the grid and enter the load control stage.

5.2.4. Automatic load control of high pressure regulating valve

The load control mode and process of GV are complex

5.2.4.1. Working mode selection:

The manual or automatic control of GV load control is realized by manual / automatic switch.

As the unit can only work in one mode at any time, DEH system is equipped with "working mode" selection logic (logic is that corresponding status flag is set inside the controller). When using, just press the function key on the operation panel to interrupt the controller and realize the corresponding working mode. These signs are:

5.2.4.1.1. Operator automatic mode (OA): after the unit is connected to the grid, the manual / automatic switch is switched to the "automatic" position, DEH enters OA mode, and the operator can change the unit load by inputting the target load and its change rate. The downward mode of this mode is to reverse to manual operation mode (AUTO), and upward is to upgrade to other automatic modes (such as ATC, REMOTE, PLANT COMP, etc.).

5.2.4.1.2. Automatic steam turbine program control mode (ATC): according to different initial conditions, there are two ways to enter this mode:

5.2.4.1.2.1. When the system is in OA mode and the set target value is inconsistent with the existing set value, press the "ATC start" key, and the system will enter OA-ATC joint mode;

5.2.4.1.2.2. When the system is in remote control mode (REMOTE = 1) or power plant computer control mode (PLANT COMP), press "ATC start" key, the system can also enter ATC joint working mode.

Before pressing the "ATC start" key, the "ATC monitoring" key must be put into operation in advance.

5.2.4.1.3. Remote control mode (REMOTE): this mode receives signals from CCS and ADS, so it can be used for coordinated control of turbine and boiler or economic dispatch of load.

5.2.4.1.4. PLANT COMP: when it is under load operation (BR = 1) and remote control has been cut off (REMOTE = 1), press the "plant level computer control" key to enter the plant computer control mode.

DEH system is connected with power plant computer through data link and its interface

A. The power plant computer collects data from the public data area of DEH system through data link, and can also obtain information from ATC.

B. The power plant computer changes the load setting value of DEH system through the data link to control the unit.

5.2.4.2. Set value formation logic

During the load control of DEH system, the target load and load change rate under the specified working mode are converted into the set value that can be accepted by the unit, which is carried out through the logic of GV control set value.

The set value is generated by the "set value counter", which is controlled by the three signals of RAISE, LOWER and RATE. There are two sources of increasing or decreasing signal: one is the output of comparator; The other is that it comes directly from various external signals, and these signals depend on the working mode of DEH.

5.2.4.2.1. When the DEH system is in OA mode, the set value of the target load is compared with the existing set value through the comparator to generate an increasing or decreasing signal, which makes the set value counter change the set value at a given rate. Its principle and method are similar to TV control.

5.2.4.2.2. When the DEH system is in OA-ATC mode, the target load value is still set by the operator on the operation panel, and the set value counter is controlled by the comparator, and the speed of the counter is determined by ATC software according to the rotor thermal stress calculation.

5.2.4.2.3. When the DEH system is in other working modes, the external signal directly enters the counter to change the load setting value.

5.2.4.2.4. Signals of protection type

A. Low main steam pressure control signal: the signal comes from the main steam pressure controller (TPC). When the main steam pressure is lower than a specified value, turn down the TV and reduce the load of the unit to avoid the sharp drop of the main steam pressure.

B. External load return request signal (RUNBACK): in OA mode, DEH receives three contact inputs from the power plant and generates three RUNBACK load reduction modes respectively. Each mode corresponds to a certain load change rate and minimum load value. They are:

RUNBACK No. Load change rate%/min Min. load% rated power
1 200 20
2 100 20
3 50 20

C. Power set value cut-off signal: the signal is sent to a set value set to zero when the unit trip condition (ASL = 1) is met, so as to improve the dynamic characteristics of the system and ensure the safety of the unit.

5.2.4.3. Load control of high pressure regulating valve

This circuit is the most used one in unit operation. It forms the output load command of the loop according to the set value, which is corrected by the speed, power and the first stage pressure, and then selected by the limit value. It is input into the valve management program as the flow request value under the current operation mode, selects the valve management mode, changes the regulation mode, and controls the unit load.

Figure 9-5 is the schematic diagram of GV load control. 

 

5.3. Control of intermediate pressure control valve (IV)

The reheated steam of 300MW unit enters the intermediate pressure cylinder through the intermediate pressure main steam valve and the intermediate pressure regulating steam valve. The medium pressure main steam valve is switch type and does not participate in the control of the unit.

The middle pressure regulating valve is a regulating valve. Its tasks in DEH control system are as follows:

A. When the unit starts up in hot state and the bypass system is put into operation, the medium pressure control valve participates in the control; After the bypass system is cut off, the regulating valve is fully opened.

B. Under abnormal working conditions (such as unit load rejection), the medium pressure regulating steam valve is closed quickly to prevent the unit from overspeed.

5.3.1. Working mode of medium pressure regulating valve

5.3.1.1. When the unit is started in hot state, the bypass system is put into operation (BPON), and the DEH system is in automatic mode (AUTO), the intermediate pressure regulating valve participates in the speed and load control of the system

5.3.1.1.1. After the unit is turned, the target speed and rising rate are given by the operator (OA) or automatic turbine program control (ATC), and the turning speed is automatically increased to 2600 r/min.

5.3.1.1.2. When the speed reaches 2600 r/min, the DEH system automatically completes the switching from the medium pressure control valve (IV) to the high pressure main steam valve (TV), and the TV controls the unit to continue to increase the speed until the grid is connected and the load is increased.

5.3.1.1.3. When the TV control speed rises to 2900r / min, the DEH system is ready to switch from (TV) to (GV), first turn off GV, and then stop turning off when the speed drops; Close IV again until the speed drops; Finally, turn on TV. When the speed is increased to 2600 r/min by gradually opening IV, the opening of IV will remain unchanged, and then GV will control the unit speed up, grid connection and initial load.

5.3.1.1.4. After the unit is connected to the grid with initial load, IV shall also accept the flow request signal controlled by GV and adjust its opening. When the unit rises to 35% of rated load, the bypass system will be automatically cut off and IV will be fully opened until the end of start-up.

5.3.1.2. When the unit is cold start, bypass system is not put into operation and DEH is in automatic mode, a full open bias signal is output to make the medium pressure regulating valve fully open, and it does not participate in the speed and load control of the unit.

5.3.1.3. When the manual mode (AUTO) is started, the manual signal of the operator operating (IV) is also fed back to the (IV) automatic control loop as the tracking signal of manual to automatic undisturbed switching.

5.3.1.4. When the unit trips (ASL), the protection system closes the medium pressure regulating valve, and the corresponding automatic circuit of the medium pressure regulating valve also outputs the zero signal of closing the valve.

5.3.2. Speed control principle of medium pressure regulating valve

5.3.2.1. The speed control of medium pressure regulating valve is the process from turning speed to switching speed (2600 r/min) during hot start of the unit. Figure 9-6 (a) is the schematic diagram of IV speed control. Similar to TV control, it is also composed of control mode selection, setting value formation, comparator, PI corrector and high limit value processing.

 

When the manual / automatic key switch on the operation panel is in the "AUTO" position, after power on reset, the DEH system will automatically enter the operator automatic (OA) mode, press the "GO" key and establish the "GO" sign before starting the set value to form a loop, allowing the operator to set the target speed and speed up through the key panel to speed up the unit. If it is converted to automatic program control (ATC) mode, the "ATC monitoring" should be on-line first, and then the "ATC start" key should be pressed. After the system enters ATC mode, the target speed is given by ATC software package, and the speed up is determined by calculating the rotor stress of the unit after the software package scans the state of the unit.

5.3.2.2. The task of setting value to form a loop is to control the thermal stress of the computer group and ensure that the thermal stress is within the allowable range when the speed or load changes from one state to another. Its working principle is: at the input end of the comparator, if the actual setting value is less than the target request value, a RAISE signal is output to make the counter tend to the target request value at a given rate; On the contrary, if the actual set value is greater than the target request value, a LOWER signal will be output, and the set value loop will not stop working until the set value is equal to the target request value.

5.3.2.3. The actual working process of setting value forming loop is to approximate a step input signal with a series of approximate linearization processing of stepped slope input. The time width t of the step reflects the scanning cycle of DEH system for control task, and the amplitude of the step is the speed-up given by the operator, which reflects the allowable variable speed value or variable load value of unit stress, When the scanning period of the control task is much smaller than the time constant of the object, the gradient can be approximated by a straight line.

5.3.2.4. After linearization, the set value is compared with the speed feedback signal of the unit. After PI correction, the speed control signal of the unit is output, and then sent to the high limit value for processing. The limit value is larger during IV control. When IV is switched to TV, the IV opening at the conversion time is regarded as its high limit value and remains unchanged. Changes are allowed only if:

When the speed deviation is too large, beyond a specified range, as a supplementary control means, the IV can be turned down.

When the reheat steam pressure changes, the opening of the intermediate pressure regulating steam valve shall be adjusted according to the change value of the pressure.

During the conversion from IV to GV, there is a short time to turn down the process, that is, after the GV has been turned down, IV needs to be turned down, and when the speed drops, IV will be opened to the upper limit valve position again, so as to ensure the stability of the unit speed during the conversion process.

According to the regulation law of the medium pressure control valve, the unit should be fully opened when the load reaches 35% of the rated load.