1. Introduction With the development of science and technology, higher requirements have been placed on the power supply quality and generation cost of power plants. The original pure hydraulic regulating system of the old units can no longer meet the requirements of unit coordinated control (CCS) and automatic generation control (AGC) in terms of controllability and control functions. Furthermore, it suffers from drawbacks such as easy jamming of regulating system components, high lag rate, poor regulating quality, and inability to achieve valve management. Advanced digital electro-hydraulic regulating systems (DEH) can flexibly configure various control strategies, meeting the requirements of modern steam turbine control systems. In terms of system safety and reliability, they have also met the requirements of power plants. Therefore, in our province, Qingzhen Power Plant Units #7 and #8, and Panxian Power Plant Unit #2 have already been upgraded from pure hydraulic regulating systems to high-pressure fire-resistant digital electro-hydraulic regulating systems, and many other power plants are also scheduled to undergo such upgrades. This article briefly analyzes the various upgrade schemes currently used in China, points out the advantages and disadvantages of each scheme, and discusses several issues for reference by power plants when upgrading their steam turbine regulating systems. 2. A Brief Introduction to Current Retrofit Schemes in China The retrofit schemes currently used in China include the following: a: Synchronizer control b: Electro-hydraulic coexistence (including joint control and switching control) c: Turbine oil pure electric control (including retaining the cam valve mechanism and removing the cam valve mechanism) d: Fire-resistant oil pure electric control Each is briefly described below: 2.1 Synchronizer Control Retrofit Scheme The original hydraulic regulating system remains unchanged; only the synchronizer and starting valve are modified. The DEH control signal interfaces with the hydraulic regulating system through the original synchronizer motor to achieve closed-loop control of the unit. The original synchronizer is driven by a general electric motor, which has poor control characteristics and is difficult to interface with the CCS automatic system. Furthermore, such motors generally have defects such as high speed, easy coasting, unstable speed, low control accuracy, and easy arcing and burnout of control contacts. The retrofit uses a high-performance electric motor or an advanced electric actuator, which has good control performance and is easy to interface with the CCS for coordinated control. The synchronizer can be directly controlled by the CCS system or a separate PI controller can be made to form a cascade regulation system with the original hydraulic system to achieve speed increase/decrease and load control. 2.2 Electro-hydraulic Coexistence Control Retrofit Scheme The original hydraulic system is fully retained, and an additional electro-hydraulic control system is added, allowing the two systems to coexist and switch operation. This retrofit scheme includes the following two types: a: Joint Control Retrofit Scheme The DEH control signal interfaces with the hydraulic system through the electro-hydraulic converter and synchronizer motor to achieve closed-loop control of the unit. The electro-hydraulic converter and the secondary pulsating oil circuit form an electro-hydraulic amplifier, replacing the hydraulic amplifier, receiving the DEH control signal, and completing the control of the hydraulic motor. An oil circuit is connected in parallel to the pulsating oil circuit of the original hydraulic system and connected to the electro-hydraulic converter, allowing the DEH to control the unit by controlling the discharge (or inflow) of the pulsating oil through the electro-hydraulic converter. The electro-hydraulic converter and synchronizer can be used in a transfer manner to achieve joint control: the electro-hydraulic converter regulates dynamic loads, and the synchronizer handles slowly changing loads. In steady state, the electro-hydraulic converter is at zero position. In steady state, the electro-hydraulic converter can be seamlessly disconnected, switching to synchronizer control mode. Electro-hydraulic and hydraulic control are controlled in a small selection (or large selection) mode: if the synchronizer is placed in the highest (or lowest) position, the hydraulic control is eliminated, and the hydraulic amplifier is completely deactivated, with the electro-hydraulic amplifier completing the full electro-hydraulic control. If the synchronizer decreases (or increases) to a certain value, it can still revert to synchronizer control. b: Switching control modification scheme DEH control signal interfaces with the hydraulic system through the electro-hydraulic converter and synchronizer motor to achieve closed-loop control of the unit. The synchronizer achieves electro-hydraulic tracking through a simulated pulsating oil circuit, and DEH can control the switching valve to achieve bumpless switching. In the electro-hydraulic position, the throttling control of the oil outlet by the electro-hydraulic converter controlled by DEH replaces the oil outlet controlled by the governor slide valve, thereby achieving unit control. In order to enable mutual tracking between the electro-hydraulic and hydraulic control and achieve bumpless switching, a simulated pulsating oil circuit and a tracking and switching valve control circuit are added. 2.3 Turbine oil pure electro-hydraulic control modification scheme The hydraulic regulator is cancelled and a digital regulator is adopted, while the actuator and protection system are basically retained. This modification scheme includes the following two options: a) Retaining the cam-driven steam distribution mechanism control modification scheme: The DEH control signal is transmitted through an electro-hydraulic interface formed by an electro-hydraulic converter and a hydraulic actuator to achieve closed-loop control of the unit. The electro-hydraulic converter is tightly integrated with the hydraulic actuator slide valve and hydraulic actuator piston. The pulsating oil of the hydraulic actuator is directly controlled by the electro-hydraulic converter, forming an electro-hydraulic servo hydraulic actuator. The DEH servo unit, the electro-hydraulic servo hydraulic actuator, and the hydraulic actuator stroke sensor LVDT form a position follow-up system. The speed measurement, synchronizer setpoint, governor slide valve, intermediate slide valve, and hydraulic actuator feedback slide valve in the original hydraulic regulation system are all excluded from the system. This scheme retains the cam-driven steam distribution mechanism, achieving fixed-mode valve management, with the management mode being a hybrid regulation mode. b: The cam-removal steam distribution mechanism control modification scheme removes the cam, camshaft, crude oil actuator, and all hydraulic adjustment components, retaining only the safety system components. The hydraulic actuator slide valve and electro-hydraulic converter are assembled together, and the cylinder (piston) is fixed on the camshaft seat. The cylinder is a push-type cylinder, replacing the cam to push the lever to open the regulating valve. The cylinder has only one pulsating oil pipe and one drain pipe, which is well sealed and can strictly prevent oil leakage, avoiding potential fires. The high-pressure regulating valve is a one-valve-one-cylinder type, while the medium-pressure regulating valve is still a one-cylinder-to-four-valve type, which can realize variable valve management function. The control function of this scheme is exactly the same as that of the high-pressure fire-resistant oil pure electric control scheme. However, it eliminates the investment, maintenance, and operating costs of setting up a separate oil source. 2.4 Fire-resistant oil pure electric control modification scheme Except for the valves, the regulating system is basically completely modified. This scheme can use high-pressure fire-resistant oil as the working medium or medium-pressure fire-resistant oil as the working medium. Currently, the high-pressure fire-resistant oil scheme is more commonly used in China. This plan requires the installation of an independent fire-resistant oil source, the removal of all regulating components in the original hydraulic regulating system, replacement of the regulating valve control seat, and placement of a fire-resistant oil actuator on the regulating valve control seat to directly drive the regulating valve stem. 3. Comparison of Various Retrofit Schemes Retrofitted Units: Liaoning Fuxin Power Plant 100MW, Anhui Wuhu Power Plant 125MW, Anhui Tongling Power Plant 125MW, Sanshui Power Plant 50MW, Jilin Changshan Thermal Power Plant 200MW, Yunnan Yangzonghai Power Plant 200MW, Yunnan Xiaolongtan Power Plant 100MW, Jiangsu Changshu Power Plant 300MW, etc. (No data available), Guizhou Qingzhen Power Plant 200MW, Guizhou Panxian Power Plant 200MW, etc. The control functions of most of the above-mentioned DEH devices can be achieved by any type of DEH, especially the most important functions achieved by DEH retrofitting: automatic wide-range speed-up closed-loop control, power closed-loop control, CCS control, and AGC control. As mentioned above, the advantages of the synchronizer control retrofit scheme are mainly reflected in its small workload and low cost. It can also achieve CCS control and AGC control. However, since the hydraulic regulation system is not modified, its shortcomings cannot be eliminated. It is suitable for small and medium-sized units with good original hydraulic regulation system operation. The advantages of the electro-hydraulic coexistence control retrofit scheme are mainly reflected in the unified system oil usage, which facilitates management; it has hydraulic regulation tracking backup, which can appropriately reduce the redundancy requirements of electronic equipment, and can even be configured with a single CPU and a single I/O, thereby reducing system investment. For units equipped with hydraulic regulation and already in operation, especially those with base load, it is a relatively ideal control scheme. While gaining the corresponding advantages, it inevitably brings limitations in other aspects. The existence of hydraulic regulation backup increases the mechanical and hydraulic links, which affects the system's lag rate and other performance aspects; unified oil usage poses a certain risk of oil quality interference. System adjustment is also relatively troublesome. In terms of system tracking, the hydraulic regulation tracks the electro-regulator through the synchronizer. Therefore, if the tracking dead zone is set too small, frequent starting of the synchronizer motor will cause the motor to be damaged quickly. If the tracking dead zone is set too large, it will lead to large switching errors and large output fluctuations. This solution is suitable for small and medium-sized units where the amplification section of the original system is functioning well. The turbine oil pure electric control retrofit solution, due to its lower operating pressure, limits the lifting force. For large-capacity units, this would result in excessively large hydraulic actuators and pipelines. Calculations show that for the same thrust hydraulic actuator, when the operating oil pressure drops from 12.8MPa to 4MPa, the actuator diameter needs to be increased to 1.79 times its original diameter. The solution eliminates the hydraulic regulator and adopts a digital regulator, removing the cam-type steam distribution mechanism to achieve variable valve management functions, with control functions identical to high-pressure fire-resistant oil systems. An independent oil source can be used for regulation, avoiding the risk of oil quality interference associated with using a unified oil source. With increasing national environmental awareness and improved environmental policies, the environmental issues arising from the slightly toxic characteristics of fire-resistant oil are becoming increasingly prominent, making the turbine oil pure electric control retrofit solution increasingly important. This solution offers excellent performance and price, making it suitable for medium and large-sized units. The fire-resistant oil pure electric control system, due to its significantly increased working pressure and effectively guaranteed lifting force, can achieve variable valve management for various capacity units. This minimizes hydraulic links and improves dynamic regulation quality. Furthermore, it allows for full-cycle and partial steam intake selection at different stages of unit startup and operation, enabling more economical unit operation (except for main-pipe units). In addition, the independent control oil and the use of fire-resistant oil minimize the impact of oil quality and fire hazards. However, the use of fire-resistant oil supply and regeneration devices significantly increases system complexity, system price, commissioning and maintenance workload, spare parts quantity, and related costs, making it difficult for small and medium-capacity units to afford. Moreover, the environmental impact of the slightly toxic nature of fire-resistant oil is increasingly attracting attention. This solution is suitable for large units with sufficient funds. 4. Discussion of Several Related Issues 4.1 Selection of Control Oil Source In the DEH control system, there are two options for the control oil source: fire-resistant oil and turbine oil. The use of high-pressure fire-resistant oil stems from the following: As turbine unit capacity increases and steam parameters rise, the turbine rotor time constant decreases. To control the speed overshoot during load shedding within acceptable limits, the shut-off time of the hydraulic actuator needs to be reduced. Furthermore, larger unit capacity and higher parameters result in greater forces acting on the regulating valves, necessitating increased hydraulic actuator lifting force. To meet the requirements of lifting force and shut-off time without overly large actuators, the hydraulic oil pressure needs to be increased. However, increased oil pressure increases the risk of leakage, and spraying onto high-temperature components could cause a fire. Therefore, high-pressure fire-resistant oil is employed. Fire-resistant oil is a chemically synthesized triaryl phosphate ester liquid. It is slightly toxic, does not decompose spontaneously, and is environmentally harmful. Waste liquid cannot be simply buried and must be centrally treated at the manufacturing plant. High-temperature environments accelerate its degradation during use, leading to increased acid value and particulate matter. Elevated acid values ​​can corrode hydraulic components, and particulate contamination can cause jamming and wear of these components, which are major malfunctions in hydraulic systems. Due to the small size of hydraulic actuators, the volume of components, and the small dynamic and static clearances, high-quality anti-repellent fuel oil is required. High oil pressure can easily cause ruptures at welded joints, pipe walls, and the accumulator liner in pressure oil pipelines. Because of these drawbacks of anti-repellent fuel oil, especially its environmental pollution, foreign turbine manufacturers also offer pure electric control systems using turbine oil as the regulating oil. For example, Hitachi's units below 600MW in Japan all use a pure electric control scheme using turbine oil. Currently, in China, units using turbine oil as the regulating oil source can control the speed overshoot during load shedding within acceptable limits. Furthermore, modern turbine structural design and oil piping technology have greatly reduced the fire hazard of oil systems. In recent years, approximately 30% of the pure electric control systems in imported large-capacity turbines use turbine oil as the working medium in their hydraulic systems. For example, ABB provides such systems, but with specific and detailed design requirements. Recently, during the upgrading of the regulating system in operating units, some electronic control systems have adopted turbine oil as the hydraulic medium. When using turbine oil as the hydraulic medium, due to the lower operating pressure, larger component dimensions, and larger clearances, the requirements for controlling the solid particulate contamination of the oil can be appropriately reduced. When using high-pressure fire-resistant oil, the particulate contamination requirement is generally 14/11 (according to ETSI data). When using turbine oil, the particulate contamination requirement is 16/13, with a limit of 17/14 (according to ABB data). However, because turbine oil can become contaminated with water during operation, it is necessary to strengthen the water removal process and make necessary improvements to the turbine's steam sealing system and enhance maintenance management to reduce the amount of water entering the oil when using turbine oil as the hydraulic medium. In summary, both fire-resistant oil and turbine oil have their advantages and disadvantages as regulating oil sources, but from a long-term and environmental perspective, using turbine oil is more suitable for small and medium-capacity units. Regardless of the oil source used, oil cleanliness must be ensured. Oil quality issues causing jamming in electro-hydraulic servo valves or converters will directly affect the normal operation of the unit. 4.2 Discussion on Valve Management Functions Valve management functions include variable valve management and fixed valve management. Generally, valve management refers to variable valve management. The steam turbine's steam intake needs to be adjusted according to changes in electrical load. There are two adjustment methods: throttling regulation (full circumferential steam intake) and nozzle regulation (partial steam intake). In the former, the regulating valves rise and fall simultaneously, which can evenly heat the turbine, resulting in a more uniform circumferential temperature and lower thermal stress during unit startup and heating. However, the throttling loss is large, making it uneconomical. In the latter, multiple regulating valves can be used for partial steam intake. These valves open sequentially, with the next valve opening only after the previous one is fully open. This method can lead to uneven heating during unit startup and higher thermal stress, but it results in less throttling loss and better economy during partial load operation. Therefore, the ideal control method is to use throttling regulation to control startup, and then use nozzle regulation to control load changes after the unit has heated up. For example, the 300MW unit at Anshun Power Plant uses single-valve control (throttling regulation) during startup, switching to sequential valve control (nozzle regulation) when the load rises to 30% ECR. Generally, large and medium-sized domestic units have four high-pressure regulating valves. Valves #1 and #2 control the steam intake at symmetrical positions of the high-pressure cylinder, accounting for approximately 70% of the intake flow, and both increase and decrease are throttling regulation. After valves #1 and #2 are fully open, valve #3 opens, controlling approximately 30% of the flow. Valve #4 is the overload valve, controlled when the unit's operating parameters do not meet design values ​​and require rated load or exceed the rated load. This opening procedure is fixed by a mechanical cam-type steam distribution mechanism. It is neither purely throttling regulation nor purely nozzle regulation, but a hybrid approach that combines both, which can be called "fixed valve management." "Variable valve management" is a control function introduced by West House in the United States, typically referring to seamless switching between high-pressure steam intake throttling regulation and nozzle regulation, enabling sequential valve control and single-valve control of the regulating valves. The following is a comparison of the two steam distribution methods, "variable valve management" and "fixed valve management," under several operating conditions of the unit: a: Start-up phase: During speed-up: "Variable valve management" uses 4 valves for full-circumference steam supply, while "fixed valve management" uses 2 valves for symmetrical steam supply. The heat uniformity of full-circumference steam supply is slightly better than that of symmetrical steam supply, while the throttling loss of symmetrical steam supply is lower than that of full-circumference steam supply. During load increase: Sliding pressure is mostly used for load increase. Both "variable valve management" and "fixed valve management" use 4 valves fully open, resulting in identical temperature uniformity for rotor heating. b: Full load condition: Both "variable valve management" and "fixed valve management" use valves #1, #2, and #3, which are identical. c: Variable load condition: To ensure stable boiler combustion, the unit load cannot be less than 70% in most cases. Therefore, only valve #3 participates in the regulation of variable load control, and the "variable valve management" and "fixed valve management" methods are the same. In fact, the sequence valve curve in the DEH system valve management program is translated into computer software from the cam-based steam distribution mechanism program of the "fixed valve management" method. From the comparison of the above operating conditions, it can be seen that "variable valve management" is superior to "fixed valve management" only in the initial stage of unit startup; other operating conditions are similar. Currently, many people in China believe that "variable valve management" can adjust the overlap of each valve to 0, thus improving unit efficiency. This understanding contains certain misconceptions for the following reasons: a: The primary task of the steam distribution mechanism (or valve management law) is to obtain reasonable flow characteristics by rationally arranging the lift of each regulating valve. This characteristic should ensure that the steam flow changes proportionally with the total valve position signal, and the change process should be continuous and stable. Appropriate overlap is necessary to obtain the above characteristics. b: Due to the nonlinearity of valve flow characteristics, the overlap should be the pressure overlap determined based on the flow characteristics, not the stroke overlap. c: Within the overlap range, the throttling loss is the sum of the throttling losses of the two valves. However, within the overlap range, the throttling loss of a nearly closed regulating valve is very small, and the flow of a newly opened regulating valve is very small, so its throttling loss is also very small. The combined result of both is that the loss is only greater than that of an ideal nozzle adjustment with an infinite number of valves, but still smaller than the maximum throttling loss per valve. The maximum throttling loss per valve occurs at the middle flow rate of that valve. Therefore, zero overlap can only save throttling losses within the overlap range, and this throttling loss is very small, having little effect on improving unit efficiency. d: At the middle flow rate of each valve, the throttling loss should be the same as in the case of zero overlap. Therefore, it can be considered that the cam-type steam distribution mechanism originally designed by the turbine manufacturer is basically reasonable and does not require major modifications. As for specific units, if the overlap is not properly adjusted, the roller clearance can be finely adjusted to improve it. In summary, fixed valve management (cam-type steam distribution mechanism) is a very good management method that can adapt to the start-up and operation requirements of small and medium-sized units. As for some defects in the existing cam-type steam distribution mechanism, such as the excessive opening speed of the control valve when it is first opened, and the self-locking phenomenon in some parts, these can be solved by locally modifying the cam profile. Of course, variable valve management also has its advantages. For example, in the 300MW unit of Anshun Power Plant in our province, the vibration of shaft #1 was significant when sequential valve control was activated. After adjusting the DEH system configuration software to switch to single-valve control throttling regulation, the vibration improved. Later, changing the opening sequence of valves #3 and #4 significantly reduced the vibration. Fixed valve management would require shutting down the unit and modifying the cam profile, incurring significant time and financial costs. Variable valve management can also achieve a 3-valve fully open sliding pressure operation mode, preventing the steam parameters from dropping too low during lower load operation, and maintaining a higher level than the 4-valve fully open sliding pressure operation mode, thus reducing the unit's heat rate and improving thermal efficiency. 4.3 Discussion on the Application of DEH Functions Due to the flexibility of its electrical circuits, DEH can easily adapt to various turbine operating requirements, especially with the application of computer technology and flexible configuration software, making the electrical circuits even more adaptable. Therefore, it is evident that the DEH system can achieve far more functions than hydraulic regulation systems. However, how are these functions actually applied in the field? There are two scenarios where functions have poor application performance: The first is that the function is functional, but its actual usage is limited. This is primarily due to a mismatch between people's perceptions and the function itself. Take the valve testing function, for example. Regularly closing and reopening valves during unit operation to determine their operational status is crucial for ensuring the normal operation of the turbine protection system. However, valve closure and reopening can disrupt turbine operation and temporarily affect the unit's load. Turbine manufacturers require valve tests to be conducted below 70-80% of rated load, ensuring the load impact during testing does not exceed 5%. Weekly testing under these conditions is easily achievable, but people are generally reluctant to conduct tests due to concerns about valve malfunction. The purpose of designing the valve testing function in the electronic control system is to identify valve malfunctions early. Discovering valve malfunctions during testing demonstrates the necessity of regular valve testing. Even if individual valves malfunction, the unit has sufficient time to safely shut down. Furthermore, valve malfunctions can sometimes be addressed during operation. In short, valve malfunctions are not caused by the valve testing function itself, but rather discovered through this function. Similar situations exist in load limiting functions. Another situation arises when the original design functions of the DEH system do not match the actual characteristics or operating requirements of the turbine itself. For example, the turbine's automatic start-up function (ATC) requires rapid acceleration, speed increase, and grid connection during warm-up, bringing the load to the initial load corresponding to the high-pressure cylinder wall temperature before warming up the turbine at a certain load increase rate. However, the DEH system's ATC program requires the unit to warm up at a certain temperature rise rate based on stress calculations, resulting in excessively long start-up times and excessive alternating stress on the unit. Other functions, such as thermal stress limiting and life management functions, are also not used by power plants for similar reasons. This shows that DEH functions cannot be truly applied when they do not fully meet actual operating requirements. Merely implementing control functions is insufficient; only functions that truly bring real benefits to turbine operation can be truly applied. 5. Conclusion In summary, there are several possible modification schemes for the turbine regulating system, including synchronizer control, electro-hydraulic coexistence (including combined control and switching control), turbine oil pure electric control (including retaining the cam steam distribution mechanism and removing the cam steam distribution mechanism), and high-pressure fire-resistant oil pure electric control. The regulating oil sources include high-pressure fire-resistant oil and turbine oil. Each modification scheme has its advantages and disadvantages. When modifying a unit, the current status of the unit should be considered, taking into account the original control method, unit capacity, and whether the site conditions meet the layout requirements of the newly added equipment. The functions of the DEH (Regulating Heating System) after modification should be determined based on the actual situation of the unit. Simultaneously, the modification funds, input-output ratio, and modification period should also be considered to determine which scheme to use for modification.