Analysis of Control Components and Performance of Hydropower Turbine Governor

Abstract: The governor is a crucial piece of equipment in hydropower plants, directly impacting the performance and safety stability of the generating unit. Its core control components include controllers and electro-hydraulic conversion elements. The selection and use of these control components should be based on the unit's structure, operational requirements, control principles, and characteristics. In addition to ensuring reliability, sensitivity, and stability, the needs of hydropower station development should also be considered, including the construction of intelligent and digital hydropower stations, to meet and adapt to current technological and developmental requirements. This paper mainly introduces the principles, characteristics, performance analysis and comparison, and applications of various types of controllers and electro-hydraulic conversion elements, providing a reference for the selection and use of turbine governor control components to ensure the safe, stable, reliable, and economical operation of hydropower stations.

Keywords: Performance analysis of control components of hydro turbine governor

1. Introduction

The development of turbine governors has been rapid, with a wide variety of product structures. Their core control components are mainly controllers and electro-hydraulic conversion elements. Commonly used controllers include microcontrollers, industrial control computers (IPCs), programmable logic controllers (PLCs), and programmable computer controllers (PCCs). Electro-hydraulic conversion elements include servo proportional valves, stepper motors, digital valves, and electro-hydraulic converters. Different combinations of these components result in various structural modes for turbine governor control elements. Improper selection and use often lead to numerous problems affecting the unit's performance and normal operation. Some units have undergone multiple governor modifications, resulting in wasted funds and adverse effects. Therefore, correctly selecting governor control elements is crucial. This paper explains the principles, characteristics, applications, and performance analysis of control elements to help select and use reliable, stable, and suitable governor control elements.

2. Controller Section

The controllers for turbine governors fall into several categories: microcontrollers, industrial control computers (IPCs), programmable logic controllers (PLCs), and programmable computer controllers (PCCs). Among them, control systems based on microcontrollers are becoming obsolete due to their poor reliability and high failure rate, and will only be briefly introduced here.

2.1 Microcontroller

Microcontrollers (MCUs) were early products used as control components for speed controllers. Based on the requirements of the speed control system, microcontrollers such as the 8051, 8086, and 8096 were selected as the hardware foundation for designing circuit boards to form the controller. However, due to a lack of professional anti-interference design, poor manufacturing processes, and insufficiently rigorous component selection, they suffered from low reliability, poor anti-interference capabilities, and high failure rates. They have gradually been phased out of the market and are now considered obsolete.

2.2 Industrial Control Computer (IPC)

2.2.1 Definition of Industrial Control Computer

Industrial control computers (IPCs) are computers designed and manufactured specifically for electromagnetic compatibility and industrial applications, taking into account the circuit boards, memory, and chassis of personal computers, and are used in industrial control environments.

2.2.2 Characteristics of Industrial Control Computers

Industrial control computers (ICCs) have high processing speeds, large storage capacities, strong program portability, and the ability to perform multi-task concurrency. They also offer better programming methods, network communication, and human-machine interfaces. However, because their hardware design is still based on the bus architecture of personal computers (PCs), and their software platform must be based on operating systems such as Windows or Linux, their reliability is still an order of magnitude lower than that of controllers designed specifically for industrial environments.

2.2.3 Applications of Industrial Control Computers

Industrial control computers have shown good performance in tests such as static characteristics of speed governors, no-load oscillation, and stationary time of relays. They have been used to some extent in the speed governor control system of hydropower stations, but their development has been somewhat restricted due to reliability issues.

2.3 Programmable Logic Controller (PLC)

2.3.1 Definition of Programmable Controller

A programmable logic controller (PLC) is a digital electronic device specifically designed for industrial applications. It employs a programmable memory to store instructions for performing logical, sequential, timing, counting, and arithmetic operations, and can control various types of equipment or production processes through digital or analog inputs and outputs.

2.3.2 Characteristics of Programmable Logic Controllers

The PLC is designed based on the principles of easy integration with industrial control systems and easy expansion of its functions; it is manufactured using modern large-scale integrated circuit technology and strict production processes, and adopts advanced anti-interference technology internally, resulting in high reliability; it has complete hardware support, rich I/O interface modules, comprehensive functions, and strong applicability; it is easy to maintain and modify; and it has strong network functions and advanced application functions.

2.3.3 Applications of Programmable Logic Controllers

The high reliability of PLCs has been widely verified. Their control methods, control speeds, and advanced functions can be well applied in the speed governor control system of hydropower plants. With their excellent performance, they have immediately become the mainstream direction of speed governor control components and are widely used in the speed governor control systems of hydropower stations of various types and capacities.

2.4 Programmable Computer Controller (PCC)

2.4.1 Definition of Programmable Computer Controller

A programmable computer controller (PCC) is an integration of the standard control functions of a programmable controller and the time-sharing, multi-tasking operating system functions of an industrial computer.

2.4.2 Characteristics of Programmable Computer Controllers

The PCC features a time-sharing multitasking operating system; it can use high-level languages ​​to write complex programs; its unique and innovative pluggable modular hardware and software structure allows for flexible and diverse expansion and combination of the system; its high-speed counting module has high frequency measurement accuracy and reliability; and its CPU has high operating efficiency and large memory capacity.

2.4.3 Applications of Programmable Computer Controllers

The characteristics of PCC provide a strong guarantee for the turbine governor control system. In recent years, programmable computer controllers have been gradually expanded and developed in turbine governor control systems, and have great potential for growth.

2.5 Performance Analysis of Programmable Logic Controllers (PLCs), Programmable Computer Controllers (PCCs), and Industrial Control Computers (IPCs)

Table 1: Controller Comparison Table

Table 1 compares the controllers in terms of features, technical performance, reliability, and applications.

2.5.1. In terms of computing speed and storage capacity, industrial control computers (IPCs) outperform programmable logic controllers (PLCs) and programmable computer controllers (PCCs).

2.5.2 Industrial control computer systems that use all or part of domestically designed and manufactured components suffer from poor reliability due to limitations in hardware design capabilities and manufacturing processes. Programmable logic controllers (PLCs) and programmable computer controllers (PCCs) are superior to industrial control computers in terms of reliability and versatility.

2.5.3 The price of industrial control computers is lower than that of programmable logic controllers (PLCs) and programmable computer controllers (PCCs).

2.5.4 In terms of programming methods, network communication, and human-machine interface, there is no significant difference in performance between programmable logic controllers (PLCs), programmable computer controllers (PCCs), and industrial control computers.

2.5.5 Programmable Logic Controllers (PLCs) are industrial control devices specifically designed to address the harsh environments of industrial sites, and their high reliability has been widely verified.

2.5.6 Practical operating experience shows that programmable microcomputer controllers and programmable computer controllers have become more reliable control components than industrial control computers.

2.5.7 Currently, speed governors with programmable logic controllers (PLCs) and programmable computer controllers (PCCs) as their core control components have become mainstream in China. Many power plants have upgraded their speed governors, which previously used industrial control computers as their core control components, to those using these two technologies. When applied to hydroelectric turbine speed governors, their high reliability and stability immediately made them the mainstream choice for speed governor control components. Currently, no existing speed governor control system offers higher reliability than PLCs and PCCs; therefore, these two should be given priority as the control components for hydroelectric turbine speed governors.

3. Electro-hydraulic conversion element

Based on the electro-hydraulic conversion method, the control elements of the turbine governor can be divided into several types: electro-hydraulic converter, stepper motor, servo proportional valve, and digital valve. Among them, the electro-hydraulic converter has been basically phased out of the market.

3.1 Servo proportional valve

3.1.1 Servo proportional valve definition

A servo proportional valve is a solenoid valve that converts an input electrical signal into a flow output signal.

3.1.2 Characteristics of Servo Proportional Valves

Servo proportional valves combine the advantages of both servo and proportional valves. They offer the high precision and responsiveness of servo valves, while possessing the large electromagnetic operating force, strong resistance to oil contamination, and "reverse centering" characteristics of proportional valves. However, servo proportional valves also suffer from temperature drift and zero drift generated by the controller's analog circuitry, making the control system susceptible to temperature changes. Furthermore, inherent nonlinear factors such as dead zone and hysteresis are difficult to fully compensate for. The electromagnets and torque motors exhibit inherent magnetic leakage, resulting in hysteresis of 2%-8% in the valve's external control characteristics.

3.1.3 Application of Servo Proportional Valves

Servo proportional valves are widely used in various types of turbine governor control systems.

3.2 Stepper Motor

3.2.1 Stepper Motor Definition

A stepper motor rotates a certain angle for each pulse signal input; it is an actuator that converts pulses into displacement.

3.2.2 Characteristics of Stepper Motors

Stepper motors operate in a step-by-step manner, eliminating accumulated errors and offering advantages such as high repeatability and hysteresis-free operation. The angular displacement can be controlled by adjusting the number of pulses, thus achieving accurate positioning. Simultaneously, the speed and acceleration of the motor can be controlled by adjusting the pulse frequency, thereby achieving speed regulation.

3.2.3 Stepper Motor Applications

A stepper motor converts the comprehensive electrical signal output by the control unit into a mechanical displacement signal with a certain operating force and displacement, thereby driving the hydraulic amplification system to regulate the turbine generator set. However, speed governors using stepper motors must retain the pilot valve and main control structure, resulting in a complex turbine governor structure with many machined parts and poor interchangeability. Stepper motor turbine governor control systems are widely used in large and medium-sized mixed-flow, axial-flow, and through-flow turbine generator sets.

3.3 Digital Valves

3.3.1 Definition of Digital Valve

A digital valve has only two operating states: on and off, which are equivalent to the high and low levels (i.e., 1 and 0) of a digital circuit, hence the name digital valve.

3.3.2 Characteristics of Digital Valves

Because digital valves only have two states, on and off, the interface between digital valves and control systems is simple; they have good sealing and no leakage under high oil pressure; and they have a fault lock function when the frequency measurement signal disappears or power is cut off.

3.3.3 Application of Digital Valves

Digital valves, as governor control elements, can adapt well to servo drives of different volumes, avoiding over- and under-adjustment caused by mismatches between the servo drive volume and the governor's main oil pipe diameter in large units, thus achieving precise regulation. Digital valves, applied to microcomputer governors of hydro turbines, can realize the digitization of the entire system, forming a fully digital governor with a very broad application prospect. However, due to limitations such as flow rate and pressure pulsation, the application of digital valves in large governors is somewhat restricted; currently, they can only be used for the regulation and control of non-double-regulating units with relatively low operating power.

3.4 Performance Analysis of Servo Proportional Valves, Stepper Motors, and Digital Valves

Servo proportional valves are indirect digital control systems. They use linear current and voltage of 4-20mA, 0-5V, and -10-10V as drive signals. The controller must use a D/A conversion stage to achieve control through an analog interface. Servo proportional valve speed controllers are a typical indirect control digital hydraulic control system. They achieve digital control through a D/A interface. Traditional servo valves and proportional valves also belong to this category. Servo proportional valves have the following disadvantages: (1) Due to the presence of analog circuits in the controller, temperature drift and zero drift are easily generated. The system is easily affected by temperature changes, making it difficult for the controller to completely compensate for the nonlinear factors of the servo proportional valve itself, such as dead zone. (2) The proportional electromagnets and torque motors used to drive proportional valves and servo valves have inherent magnetic leakage, which causes the external control characteristics of the valve to exhibit hysteresis of 2%-8%. The hysteresis of the proportional valve can be basically eliminated by using closed-loop control methods such as valve core position detection and feedback, but this greatly increases the cost of the valve, makes the structure complex, and reduces reliability. (3) Due to its structural characteristics, the magnetic circuit of an electromagnet can generally only be made of integral magnetic material. Under the action of high frequency signals, the temperature rise caused by iron loss is more serious.

Both stepper motors and digital valves are direct digital control electro-hydraulic conversion elements. Stepper motors operate in a step-by-step manner, eliminating accumulated errors and offering advantages such as high repeatability and hysteresis-free operation. They can control angular displacement by controlling the number of pulses, achieving accurate positioning; and control the motor's speed and acceleration by controlling the pulse frequency, achieving speed regulation. However, stepper speed governors must retain pilot valves and main control structures, resulting in complex structures, numerous machined parts, and poor interchangeability. In contrast, digital valves have a simpler interface with the control system and can form a direct digital control system without intermediate links, offering broad application prospects. Applying digital valves to turbine microcomputer speed governors can achieve complete system digitization, creating a fully digital speed governor. However, due to limitations such as flow rate and pressure pulsation, the application range of digital valves in large speed governors is somewhat limited; currently, they can only be used in the regulation and control of non-double-regulating units with low operating power.

Table 2 Comparison of Electro-hydraulic Conversion Components

Table 2 compares the characteristics, technical performance, reliability, and applications of electro-hydraulic conversion elements. As shown in Table 2 and above, the speed controller control system based on direct digital control is superior to the speed controller control system based on indirect digital control. Digital valves are more suitable for small and medium-sized speed controllers, while stepper motors and servo proportional valves are more suitable for large speed controllers.

4. Conclusion

Power systems impose different requirements on various hydropower stations, such as base load, intermediate load, frequency regulation, peak shaving, emergency backup, or a combination thereof. The actual conditions of each power station's generating units are different, including unit structure, monitoring signals, frequency, head, and operational requirements. These factors all influence the requirements of the governor control components. It is essential to understand the control principles, characteristics, and applications of these components and to verify their performance during testing and actual operation, including static characteristics, start-up, shutdown, no-load oscillation, no-load disturbance, and load shedding. All test data must meet national standards. While some components, such as stepper motors, may have less desirable performance in areas like servo time and load shedding data, which is related to the mechanical and hydraulic system structure, their outstanding characteristics and high reliability still lead to their widespread use. Therefore, the selection and use of turbine governor control components need to be considered from multiple perspectives. While meeting the requirements of reliability, sensitivity, and stability, the needs of hydropower station development must also be taken into account, including the construction of intelligent and digital hydropower stations to meet and adapt to current technological and developmental needs, ensuring the safe, stable, reliable, and economical operation of the hydropower station.

About the Author

Name: Ling Shengjun

Unit: SDIC Gansu Xiaosanxia Hydropower Co., Ltd., Daxia Hydropower Plant, No. 738 Lanbao Road, Baiyin City, Gansu Province, 730900, China

Telephone: 13884229410 E-mail: [email protected]