Abstract: Modern steam turbine control systems have evolved from traditional basic control strategies, taking into account the needs of grid control, heating network control, and boiler-turbine coordination control. The Digital Electro-Hydraulic Control System (DEH) is a typical form of modern steam turbine control system. This paper focuses on a system modification scheme based on the traditional electric control functions of steam turbines, with the key being the modification of the hydraulic system to achieve all control functions of a purely electric control system.
Keywords: DCS, DEH, TSI, steam turbine
1. Introduction
The steam turbine control system consists of a regulating system and a safety system.
The regulating system is designed to ensure the stable operation of the turbine unit and to obtain the static characteristics required for operation; the safety system is designed to protect the unit's safety when dangerous operating conditions occur.
The primary purpose of a steam turbine is to drive generators to produce electricity and supply it to users. Steam turbines for power generation are classified into condensing and intermediate reheat condensing types. Some types of steam turbines, in addition to driving generators, also extract steam from one or more stages to supply heat to users. These turbines are called combined heat and power (CHP) units or heating units. Power generation steam turbines have a speed regulation system, or simply regulation system, to maintain the unit at a constant speed to ensure a stable frequency of the supplied electricity. CHP steam turbines, in addition to their speed regulation system, also have a pressure regulation system to maintain stable extraction steam pressure for heating. Speed regulation and extraction steam pressure regulation are the basic control strategies of steam turbines.
There are two operating modes for steam turbines used in power generation: single-unit operation and multi-unit parallel operation. Multi-unit parallel operation forms part of a power grid, also known as grid-connected operation.
When a generator unit operates as a standalone unit, all changes in user electrical load are borne by that unit. The speed control system must be able to adapt to these load changes and stably regulate power to maintain constant speed operation. When the unit operates in grid-connected mode, the user load is shared by all units in the grid. The resulting changes in grid frequency are regulated by secondary adjustments made by certain designated units, called frequency regulating units, based on the shared responsibility of the speed control systems of all units. Since the electrical load is borne by multiple units, load distribution and load adjustment issues arise. Load adjustments by a single unit can cause changes in the grid frequency, leading to a redistribution of load among the units in the grid. Secondary frequency adjustments, load adjustments, and load distribution among units are all based on the characteristics of the speed control system. Modern large steam turbine generator units almost invariably operate in grid-connected mode. Grid dispatch management classifies the units in the grid into base load units, peak-shaving units, frequency regulating units, and peak load units. The regulating characteristics of these units require the addition of corresponding regulating functions to the speed control system, which makes the control strategies of modern large-scale unit control systems more complex and diverse.
Most modern large-scale power plants adopt a unit-based structure, meaning that the steam turbine generator unit and boiler system form a complete system. The control of the steam turbine generator is closely related to the control of the boiler and must be closely coordinated. Therefore, the control strategy of modern large-scale steam turbine generator units also includes the requirement for coordinated control of the boiler and the large auxiliary equipment system. For combined heat and power (CHP) units, the steam turbine control strategy should also include the control of the heating network.
Modern steam turbine control systems have evolved from traditional basic control strategies, taking into account the needs of power grid control, heating network control, and coordinated boiler-turbine control. The Digital Electro-Hydraulic Control System (DEH) is a typical form of modern steam turbine control system.
The safety system is an integral part of the turbine control system. The composition of various turbine safety systems is roughly the same, mainly including overspeed protection systems, emergency shutdown systems, brake activation systems, and various testing systems. The shutdown system interfaces with the shutdown signals from the turbine supervisory instrumentation system (TSI) and other equipment.
2. Classification of Steam Turbine Regulation Systems
The traditional regulating system for steam turbines is a hydraulic regulating system, which consists of measuring elements, a setpoint mechanism, an amplifying element, and an actuator. Depending on the measuring element, hydraulic regulating systems are further divided into two main categories: mechanical-hydraulic and purely hydraulic.
The speed measuring element in a mechanical-hydraulic regulating system is a mechanical centrifugal governor. The regulating systems used by Harbin Turbine Works and Beijing Heavy Electric Machinery Works are mechanical-hydraulic, with spring-plate type high-speed centrifugal governors, a typical form found in the Leningrad Metalworks of the former Soviet Union. Fully hydraulic regulating systems use hydraulic centrifugal governors as the speed measuring element; the hydraulic systems of Dongfang Turbine Works and Shanghai Turbine Works in my country belong to this type. Dongfang Turbine Works uses radial drilling pumps, and its hydraulic system is still of the Leningrad Metalworks type. Shanghai Turbine Works uses rotary damping, and its hydraulic system is derived from Westinghouse Electric Machinery of the United States.
In addition, there is a fully hydraulic system for small steam engines, characterized by the use of a radial drilling pump for both the speed regulating pump and the main oil pump.
3. Basic Components of a Regulation System
As mentioned above, the regulating system consists of components such as a speed measuring element, a setpoint amplifier, and an actuator. Based on the different functions of these components, the regulating system can be divided into two parts: a controller and an actuator. The main task of the controller is to perform control strategy calculations, while the task of the actuator is to drive and position the regulating mechanism according to the calculation results of the controller.
The hydraulic speed control system controller consists of a governor, synchronizer, amplifier, and signal distributor, among other components. Its control strategy is differential speed regulation. The governor and synchronizer provide speed deviation signals, which are amplified by the hydraulic amplifier to form a master valve position signal. This signal is then transmitted to the signal distributor to control the various actuators, i.e., hydraulic actuators. The hydraulic actuators drive and position the turbine's regulating mechanism, i.e., the regulating valves. In stand-alone operation, the synchronizer adjusts the turbine speed; after grid connection, the synchronizer adjusts the load allocated to the unit.
To adapt to the complex control strategies of modern steam turbine control systems, digital electro-hydraulic control systems have emerged. Digital electronic controllers, also known as pure electronic controllers or fully electronic controllers, are abbreviated as DEH. The DEH controller consists of a microcomputer system, and the actuators are hydraulic actuation systems composed of multiple hydraulic actuators.
Depending on the working fluid used in the hydraulic actuator, DEH is further divided into low-pressure turbine oil type and high-pressure fire-resistant oil type.
In addition, there is a transitional DEH type called the electro-hydraulic coexistence type. Its control strategy is calculated using a computer controller, while a hydraulic controller is retained as a backup.
4. Basic Principles of ESC Retrofit Scheme
The main task of the electro-hydraulic control system (EDS) retrofit is to transform the turbine's hydraulic regulating system into an actuator of the electro-hydraulic control system, and then equip it with a computer controller to form a complete electro-hydraulic control system. The key to the EDS retrofit is the modification of the hydraulic system.
The main requirements for hydraulic system retrofitting are: first, to adopt the simplest possible solution to implement the computer controller interface and achieve the required control strategy, based on the condition and characteristics of the original hydraulic system of the unit.
The basic renovation plans can be summarized into three types.
1) Introducing an electro-hydraulic amplifier into an intermediate stage of the hydraulic controller to interface with the computer controller and achieve fully electronic control. This approach is called an electro-hydraulic amplifier type pure electronic controller. The hydraulic system can be retained intact as a backup.
2) Convert the hydraulic actuator into an electro-hydraulic hydraulic actuator to interface with the computer controller and achieve fully electronic control. This solution is called an electro-hydraulic hydraulic actuator type pure electronic control. All components before the hydraulic actuator can be removed, and hydraulic backup is no longer required.
3) High-pressure fire-resistant oil pure electric control: all components of the original hydraulic control system are completely removed, and the hydraulic actuator system needs to be redesigned. The first two options partially retain the original hydraulic system, or are entirely electric control systems modified from the original hydraulic control system. Both are low-pressure turbine oil pure electric control systems, and the modification effect is highly dependent on the design of the modification scheme. The third option is a completely new design and has no relation to the original system.
Principle of Low-Pressure Turbine Oil Pure Electric Control Retrofit Scheme
The following example illustrates the pure electric control (PEC) of a low-pressure turbine oil amplifier type:
5. Description of key renovation points
The secondary pulsating oil drain port from the intermediate slide valve to the governor slide valve is blocked. The governor slide valve and the original speed control can be retained but not used. However, the additional overspeed protection pipeline should be blocked.
If the unit has an electro-hydraulic converter, switching valve, and tracking slide valve, these should be removed, and the oil pressure under the intermediate relief valve should be connected to the overspeed limit slide valve, emergency trip slide valve, and starting valve. Cover plates should be installed at the original installation positions of these components.
The intermediate slide valve then operates in two positions, accepting control from the starting valve, emergency shut-off valve, and overspeed limit valve, enabling each hydraulic actuator to establish opening conditions and achieve rapid closing and shut-off.
For Harbin Turbine and Beizhong Heavy Machinery systems, the Xuzhou micro-displacement valve should be removed or its secondary pulsating oil output line should be disconnected to prevent malfunctions from affecting normal system operation. The overspeed limiting valve should be retained. If the original system lacks an overspeed limiting valve, an OPC solenoid valve should be added to achieve the overspeed limiting function.
Ø Retain all hydraulic actuators, valve levers, and cam valve mechanism, and convert all hydraulic actuators into electro-hydraulic hydraulic actuators.
The electro-hydraulic actuator consists of a DDV valve, an actuator slide valve, an actuator piston, and a dual redundant LVDT. It is controlled by a PI servo board, forming a displacement closed-loop feedback circuit, making the actuator stroke proportional to the DEH master valve position signal. The original hydraulic feedback mechanism, feedback slide valve, feedback lever, etc., of the actuator are removed.
The DDV valve, along with the adjustable flow valve, is mounted on a hydraulic manifold. Each hydraulic manifold can be installed in the original feedback valve position of the hydraulic actuator, utilizing the existing three-stage pulsating hydraulic circuit. The electrical circuitry of each manifold can be concentrated near the original intermediate spool valve, connecting the hydraulic manifold to the hydraulic actuator using the existing three-stage pulsating oil pipeline. The adjustable flow valve adjusts the mechanical offset of the hydraulic actuator, causing the DDV valve to operate in a slightly open pressure port position, allowing the hydraulic actuator to close naturally when the DDV valve is de-energized.
Ø Setup—An external oil filter supplies filtered hydraulic oil to each hydraulic manifold. The filter is a dual-switchable type, equipped with a differential pressure monitor and a switching valve for online filter element replacement. Filtration accuracy is 25μm.
Ø The DDV valve adopts D634
Hydraulic safety systems, starting operating systems, and various testing systems are not included in the scope of modification. If the user has special requirements, alternative solutions can be provided.
The DEH controller can be configured with control strategies according to the pure ESC control function.
6. Conclusion
The innovation of this modification scheme and paper lies in the following: most of the original regulating system components are decommissioned, retaining only the hydraulic actuator slide valve, hydraulic actuator piston, and subsequent steam distribution components. These components have the lowest failure rate in the original system, thus eliminating the main defects of the original system. The electro-hydraulic motor has very high sensitivity, exceeding the original maximum sensitivity per degree, and maintains the same sensitivity across the entire stroke range. The retained cam steam distribution mechanism is a hybrid regulating method, and its control is the same as that of the two-roof valve management. Since the function of the electronic control system depends on the DEH controller, this scheme can realize all the control functions of a pure electronic control system.
References:
[1] Wang Changli, Luo An. Design and Application Examples of Distributed Control Systems (DCS). Beijing: Electronic Industry Press, August 2004.
[2] RW Lewis. Programming industrial control systems using IEC 1131-3 IEE
[3] Li Yongbo, Sun Yu. Analysis and Prospect of Current DCS Technology. Microcomputer Information, No. 7, 2004.
[4] Deng Qingsong: Functional Application and Experiment of DEH-III Digital Electro-hydraulic Control System in 300MW Units; Power Technology (4)
