Control systems for large wind farms and wind turbines

Abstract : With the gradual depletion of energy sources such as coal and oil, humanity is paying increasing attention to the utilization of renewable energy. Wind power is the cheapest and most promising renewable energy source, and it is also a "green energy" that does not pollute the environment. Currently, large-scale wind turbines with a capacity of hundreds of kilowatts have been commercialized abroad, and megawatt-scale wind turbines are about to be commercialized as well.

1. Introduction <br />With the gradual depletion of energy sources such as coal and oil, humanity is increasingly emphasizing the utilization of renewable energy. Wind power is the cheapest and most promising renewable energy source, and it is also a "green energy" that does not pollute the environment. Currently, large-scale wind turbine units with a capacity of hundreds of kilowatts have been commercialized abroad, and megawatt-level wind turbine units are about to be commercialized. The total installed capacity of wind power worldwide has exceeded 10 million kilowatts, with a unit cost of $1,000 per kilowatt and a power generation cost of 5 cents per kilowatt-hour, making it competitive with thermal power generation.
my country possesses abundant wind energy resources, with a theoretical reserve of 1.6 billion kW and an actual exploitable capacity of 250 million kW, indicating enormous development potential. In early 1995, the State Planning Commission, the State Science and Technology Commission, and the State Economic and Trade Commission jointly issued the "Outline for the Development of New Energy and Renewable Energy in China (1996-2010)." In March 1996, the State Planning Commission formulated a wind power development plan driven by domestic production, known as the "Wind Power Development Plan," which pointed the way and created conditions for the localization of wind power generation technology in my country. Simultaneously, my country was one of the earliest countries to utilize wind energy resources for wind power generation and wind-powered water lifting. By the end of 1996, my country had 150,000 small wind turbine generators in operation, with an annual production capacity of 30,000 units, both ranking first in the world.

2. Types of Wind Turbine Generators 2.1 Constant Speed ​​Constant Frequency and Variable Speed ​​Constant Frequency In wind power generation, when a wind turbine generator is connected to the grid, the frequency of the wind power must be consistent with the frequency of the grid, i.e., the frequency must be constant. Constant speed constant frequency means that during wind power generation, the rotational speed of the wind turbine (i.e., the rotational speed of the generator) remains constant, thereby obtaining constant frequency electrical energy. Variable speed constant frequency refers to a method where the rotational speed of the wind turbine varies with the wind speed, and other control methods are used to obtain constant frequency electrical energy.
2.2 Performance Comparison of Two Types of Units Since wind energy is proportional to the cube of wind speed, allowing the windmill to change speed within a certain range when wind speed varies can achieve better utilization of wind energy. The efficiency of a windmill in converting wind energy into mechanical energy can be represented by the power output coefficient CP, which reaches its maximum value at a specific rotor speed ratio λ (the ratio of blade tip speed to wind speed). In constant-speed, constant-frequency units, the windmill speed remains constant, while wind speed frequently changes, making it impossible for CP to remain at its optimal value. Variable-speed, constant-frequency units allow the speeds of the windmill and generator to vary over a wide range without affecting the frequency of the output power. Because the windmill speed is variable, the rotor speed ratio can be appropriately controlled to be at or near its optimal value, thereby maximizing the utilization of wind energy for power generation.
2.3 Characteristics of Constant Speed ​​and Constant Frequency Generators Currently, the most widely used asynchronous generators in wind power systems are constant speed and constant frequency generator sets. To adapt to varying wind speeds, two asynchronous generators with different capacities and pole numbers are typically used. The smaller generator generates power at low wind speeds, while the larger generator generates power at high wind speeds. Simultaneously, a variable pitch system is generally used to adjust the blade angle of attack to regulate the output power. However, this only allows the asynchronous generator to achieve optimal output coefficients at two wind speeds, and it cannot effectively utilize wind energy at different wind speeds.
2.4 Implementation of Variable Speed ​​Constant Frequency Systems Various variable speed constant frequency systems can be used for wind power generation, such as AC-DC-AC converter systems, AC excitation generator systems, brushless doubly-fed generator systems, switched reluctance generator systems, magnetic field modulation generator systems, and synchronous/asynchronous variable speed constant frequency generator systems. Some of these systems achieve variable speed constant frequency by modifying the generator's structure; others combine the generator with power electronic devices and microcomputer control systems. Each has its own characteristics and is applicable to different situations. To fully utilize wind energy at different wind speeds, in-depth research should be conducted on various variable speed constant frequency technologies, and practical variable speed constant frequency technologies suitable for wind power generation should be developed as soon as possible.

3. Control of Constant Speed ​​and Constant Frequency Wind Turbines 3.1 Soft Start and Grid Connection of Wind Turbines During wind turbine startup, the control system continuously monitors wind speed changes. When the 10-minute average wind speed exceeds the starting wind speed, the control system prepares the wind turbine for grid connection: releasing the mechanical brake, retracting the blade tip damping plate, and positioning the rotor in the windward direction. The control system continuously checks the signals from various sensors for normality, such as hydraulic system pressure, wind direction deviation, and grid parameters. If the 10-minute average wind speed still exceeds the starting wind speed, the system checks if the rotor has started rotating and activates the thyristor current-limiting soft starter to quickly start the turbine, controlling the starting current to ensure it does not exceed the maximum limit. During startup, asynchronous wind turbines have very low rotational speeds, resulting in a large slip when connected to the grid. This generates an inrush current equivalent to 5-7 times the generator's rated current, which not only significantly impacts the grid but also affects the lifespan of the wind turbine. Therefore, current-limiting soft-start technology is adopted during the grid connection of wind turbine units to control the starting current. When the generator reaches synchronous speed, the current drops sharply, and the controller issues a command to bypass the thyristor. After the thyristor is bypassed, the current-limiting soft-start controller automatically resets and waits for the next starting signal. This starting process takes about 40 seconds. If it exceeds this time, it is considered a starting failure, and the generator will be disconnected from the grid. The controller determines whether the unit should be restarted based on the detection signal.
Asynchronous wind turbines can also be started without being connected to the grid when the speed is lower than the synchronous speed, and then connected to the grid when the speed approaches or reaches the synchronous speed. This can avoid inrush current and eliminate the need for a thyristor current-limiting soft starter.
3.2 Switching Control Between Large and Small Generators During wind turbine operation, when changes in wind speed cause variations in generator output power, the control system should be able to automatically switch between large and small generators based on these changes, thereby improving the efficiency of the wind turbine. The specific control method is as follows:
(1) Switching from small generator to large generator: During the grid-connected power generation of the small generator, the control system detects its output power. If the instantaneous power exceeds 20% of the rated power of the small generator within 1 second, or the average power within 2 minutes is greater than a certain value, the small generator is switched to the large generator. The switching process is as follows: First, the compensation capacitor is disconnected, then the small generator is disconnected from the grid. After the wind turbine rotates freely to a certain speed, the large generator is softly connected to the grid. If the wind speed suddenly decreases during the switching process, causing the wind turbine speed to decrease instead, the small generator should be softly connected to the grid again to achieve grid-connected operation of the small generator.
(2) Switching from large generator to small generator: Detect the output power of the large generator. If the average power within 2 minutes is less than a certain set value (this value should be less than the rated power of the small generator), or if the instantaneous power within 50 seconds is less than another smaller set value, immediately switch to the small generator. The switching process is as follows: disconnect the compensation capacitor of the large generator, disconnect it from the grid, then softly connect the small generator to the grid, time for 20 seconds, measure the speed of the small generator. If the synchronous speed of the small generator is not reached after 20 seconds, stop the machine, reset the control system, and restart. If the speed has reached the bypass speed of the small generator within 20 seconds, bypass the thyristor soft starter, and then connect the compensation capacitor according to the reactive power of the system.
3.3 Variable Pitch Control Method and Its Improvement After a wind turbine is connected to the grid, the control system adjusts the blade angle of attack via a pitch adjustment mechanism based on wind speed changes to regulate the output power and utilize wind energy more effectively. Below the rated wind speed, the blade angle of attack is near zero degrees, which can be considered equivalent to a fixed-pitch wind turbine, and the generator's output power changes with wind speed. When the wind speed reaches or exceeds the rated wind speed, the variable pitch mechanism activates, adjusting the blade angle of attack to ensure the generator's output power remains within the allowable range.
However, wind in nature is unpredictable. Wind speed is constantly changing, and wind energy has a cubic relationship with wind speed. Even small changes in wind speed will result in large changes in wind energy, causing the output power of wind turbines to fluctuate continuously. For variable pitch wind turbines, when the wind speed exceeds the rated wind speed, the pitch mechanism will adjust the blade pitch to limit the generator's output power. If the wind speed changes significantly and frequently, it will cause the pitch mechanism to operate frequently and dramatically, making it prone to damage. Simultaneously, the blade pitch controlled by the pitch mechanism is a large inertia system with a significant lag time. This lag in pitch adjustment will also cause large fluctuations in the generator's output power, negatively impacting the power grid.
To reduce the adverse effects of pitch control on the power grid, a new power-assisted regulation method—RCC (Rotor Current Control)—can be used in conjunction with the pitch control mechanism to jointly regulate the generator's output power. RCC control must be used on wound-rotor induction generators. Through power electronic devices, the rotor current of the generator is controlled, transforming an ordinary induction generator into a variable slip generator. RCC control is a fast electrical control method used to overcome rapid changes in wind speed. In a variable-pitch wind turbine employing RCC control, the pitch control mechanism is primarily used when wind speeds are slowly increasing or decreasing, adjusting the output power by regulating the blade angle of attack. The RCC control unit is used when wind speeds change rapidly. When wind speed suddenly changes, the RCC unit adjusts the generator's slip, allowing the generator speed to vary within a certain range while maintaining a constant rotor current, thus keeping the generator's output power constant.
3.4 Reactive Power Compensation Control Since asynchronous generators absorb reactive power from the grid, the power factor of wind turbines decreases. Grid-connected wind turbines generally require a power factor of 0.99 or higher, so capacitor banks are necessary for reactive power compensation. Due to the randomness of wind speed variations, the generator's output power varies randomly before reaching rated power; therefore, the connection and disconnection of compensation capacitors need to be controlled. The control system includes four sets of compensation capacitors with different capacities. The computer controls the segmented connection or disconnection of the compensation capacitors based on changes in output reactive power, ensuring a power factor of 0.99 or higher at the half-power point.
3.5 Yaw and Automatic Unmooring Control The yaw control system has three main functions:
(1) Automatic wind alignment during normal operation. When the nacelle deviates from the wind direction by a certain angle, the control system issues a command to adjust the direction to the left or right, and the nacelle begins to adjust to the wind until it reaches the allowable error range, at which point the wind alignment stops automatically.
(2) Automatic unwinding during cable winding. When the nacelle has deflected 2.3 times in the same direction, if the wind speed is less than the wind speed at which the wind turbine starts and there is no power output, the turbine will stop and the control system will make the nacelle rotate 2.3 times in the opposite direction to unwind. If the turbine has power output at this time, the unwinding will not be automatic. If the nacelle continues to deflect 3 times in the same direction, the turbine will stop and the unwinding will be performed. If the automatic unwinding fails due to a fault, the cable twisting mechanical switch will be activated when the cable has twisted 4 times. At this time, a cable twisting fault will be reported, the turbine will stop automatically, and the machine will wait for manual unwinding.
(3) Deviation from wind direction during stall protection. When there is an extremely strong wind, stop the machine, release the blade tip damper, adjust the pitch to the maximum, and yaw 90° away from the wind to protect the wind turbine from damage.
3.6 The shutdown process of the parking control is divided into normal shutdown and emergency shutdown.
(1) Normal shutdown When the controller issues a normal shutdown command, the wind turbine will shut down according to the following procedure:
① Disconnect the compensation capacitor;
② Release the blade tip damping plate;
③ The generator disconnected from the grid;
④ After the generator speed drops to the set value, engage the mechanical brake;
⑤ If a brake failure occurs, retract the propellers and yaw the engine room 900 degrees to the leeward.
(2) Emergency shutdown When an emergency shutdown occurs, the following shutdown operations shall be performed: First, disconnect the compensation capacitor, activate the blade tip damper, and activate the caliper brake after a delay of 0.3 seconds. If the instantaneous power is negative or the generator speed is less than the synchronous speed, disconnect the generator from the grid. If the braking time exceeds 20 seconds and the speed still has not dropped to a certain set value, retract the propeller and yaw the nacelle 90° to the leeward.
If the shutdown is due to external reasons, such as excessively low or high wind speed, or a power grid failure, the wind turbine will automatically enter standby mode after shutdown; if it is due to an internal fault, the controller needs to receive a repair instruction before it can enter standby mode.

4 Control of Variable Speed ​​Constant Frequency Generator Sets 4.1 Control of Synchronous Generator AC-DC-AC System This type of wind turbine set uses a synchronous generator. The frequency, voltage, and power of the generated electrical energy change with the wind speed. This is conducive to maximizing the utilization of wind energy resources. The task of constant frequency and constant voltage grid connection is completed by the AC-DC-AC system.
(1) Control of wind turbines The starting, control and protection functions of wind turbines are basically similar to those of constant speed and constant frequency units. The difference is that these units generally use fixed pitch wind turbines, thus eliminating the need for variable pitch control mechanisms.
(2) Generator Control The generator's output power is controlled by the excitation. When the output power is less than the rated power, it operates with fixed excitation; when the output power exceeds the rated power, the generator's output power is adjusted to operate within the allowable safe range by adjusting the excitation. The excitation adjustment is achieved by the controller adjusting the conduction angle of the thyristors in the excitation system.
(3) Control of AC-DC-AC frequency conversion system. The concept of frequency converter here is different from that of ordinary frequency converter. Ordinary frequency converter is a mains power supply with fixed voltage and frequency (220/380V, 50Hz).
The frequency converter transforms the power source, which has both variable frequency and voltage, into a power source that can adapt to the needs of various electrical appliances. If used in a variable frequency speed control system, the voltage and frequency are constantly changed according to the load requirements. In contrast, the frequency converter here transforms the electrical energy generated by the wind turbine, whose voltage and frequency are constantly changing, into electrical energy with a stable frequency and voltage (220/380V, 50Hz), so as to match the voltage and frequency of the power grid and enable the wind turbine to operate in parallel with the grid.
The so-called "AC-DC-AC" frequency conversion is a type of frequency conversion method. It involves rectifying alternating current (AC) of a certain frequency and voltage into direct current (DC), and then using a microcomputer-controlled power electronic device to invert the DC current back into AC current of a certain frequency and voltage. Its basic principle is shown in Figure 1.
The three-phase alternating current generated by the wind turbine is rectified into direct current by a three-phase full-bridge diode rectifier, and then inverted into three-phase alternating current by six insulated-gate bipolar transistors (IGBTs) under the control of the control and drive circuits and fed into the power grid. The inverter control typically uses SPWM-VVVF, i.e., sinusoidal pulse width modulation variable frequency drive. Variable frequency drives using an AC-DC-AC system have a large capacity, generally exceeding 120% of the generator's rated power.
4.2 Control of Doubly Fed Generators Most current wind turbines employ constant-speed, constant-frequency systems, with generators typically using synchronous or asynchronous induction motors. When a wind turbine supplies power to a constant-frequency grid, speed regulation is unnecessary because the grid frequency forces the rotor speed to change. In this situation, the wind turbine maintains or nearly maintains the same speed at different wind speeds. This leads to decreased efficiency, forced output reduction, and even shutdown, which is clearly undesirable. In contrast, doubly fed generators, whether operating at subsynchronous or supersynchronous speeds, can operate at varying wind speeds. Their speed can be adjusted accordingly to wind speed changes, ensuring the wind turbine always operates at its optimal state, thus improving unit efficiency. Simultaneously, the voltage and frequency of the stator output power can remain constant, regulating the grid's power factor and improving system stability.
(1) Working characteristics of doubly fed motors The structure of a doubly fed motor is similar to that of a wound-rotor induction motor. The stator winding is also excited by a symmetrical three-phase power supply with a fixed frequency. The difference is that the rotor winding is excited by a three-phase power supply with an adjustable frequency. Generally, an AC-AC converter or an AC-DC-AC converter is used to supply low-frequency current.
When the symmetrical three-phase stator windings of a doubly-fed induction generator are powered by a three-phase power supply with a frequency of f1 (f1 = P?n1/60), the rotor speed is n = (ls)n1 (s is the slip, and n1 is the synchronous speed of the fundamental rotating magnetic field in the air gap). To achieve stable electromechanical energy conversion, the stator magnetic field and the rotor magnetic field should remain relatively stationary, i.e., the following should be satisfied:
ωR＝ω1-ω2
Where: ωR is the rotor rotational angular frequency;
ω1 is the angular frequency of the rotating magnetic field generated by the stator current;
ω2 is the angular frequency of the rotating magnetic field generated by the rotor current.
Therefore, the rotor power supply frequency f2 = S?f1 can be obtained. At this time, the rotating magnetic fields of the stator and rotor both rotate at the synchronous speed n1, and the two remain relatively stationary.
Compared to synchronous motors, doubly-fed induction generators (DFIGs) have three adjustable excitation parameters: first, like synchronous motors, the amplitude of the excitation current can be adjusted; second, the frequency of the excitation current can be changed; and third, the phase of the excitation current can be changed. By changing the excitation frequency, the speed can be adjusted. This allows for rapid changes in motor speed during sudden load changes, fully utilizing the rotor's kinetic energy to release and absorb the load, resulting in significantly less disturbance to the power grid compared to conventional motors. Furthermore, by adjusting the amplitude and phase of the rotor excitation current, both active and reactive power can be regulated. Synchronous motors, on the other hand, only have one adjustable parameter: the amplitude of the excitation current. Therefore, adjusting the excitation of a synchronous motor generally only compensates for reactive power. In contrast, the excitation of a DFIG can be adjusted not only for the current amplitude but also for its phase. When the phase of the rotor current changes, the position of the rotor magnetic field generated by the rotor current in the air gap space is displaced, changing the relative position of the DFIG's electromotive force and the grid voltage vector, thus altering the motor's power angle. Therefore, DFIGs can regulate both reactive and active power. Generally, when a motor absorbs reactive power from the grid, its power angle tends to increase, leading to decreased motor stability. However, a doubly-fed induction generator (DFIG) can reduce its power angle by adjusting the phase of its excitation current, thus improving operational stability and allowing it to absorb more reactive power. This overcomes the difficulty of excessively high grid voltage due to reduced load at night. In contrast, asynchronous generators need to absorb reactive excitation current from the grid, and when operating in parallel with the grid, this deteriorates the grid's power factor. Therefore, DFIGs offer superior operating performance compared to both synchronous and asynchronous motors.
(2) Control of Doubly Fed Motors in Wind Power Generation In wind power generation, the unpredictable nature of wind speeds makes their utilization difficult. Therefore, improving wind power generation technology, increasing the efficiency of wind turbine generator sets, and making the most of wind energy resources are of great significance. Any wind turbine generator set includes a wind turbine as the prime mover and a generator that converts mechanical energy into electrical energy. Among them, the efficiency of the wind turbine as the prime mover largely determines the efficiency of the entire wind turbine generator set, and the efficiency of the wind turbine largely depends on whether its load is in an optimal state. No matter how meticulously a wind turbine is designed and constructed, it will lose efficiency if it is under overload or prolonged load. From the aerodynamic curve of the wind turbine, it can be seen that there is an optimal cycle speed ratio λ, which corresponds to an optimal efficiency. Therefore, the best control of the wind turbine generator is to maintain the optimal cycle speed ratio λ. In addition, since the requirements of the power grid for active and reactive power must be considered, the speed of the wind turbine under optimal operating conditions should be derived from its aerodynamic curve and the power command of the power grid. In other words, the rotational speed of the wind turbine should be adjusted promptly according to changes in wind speed and load. It absorbs or releases power by relying on changes in rotor kinetic energy, reducing disturbances to the power grid. The inverter controller controls the power devices in the inverter circuit. This allows for changes in the amplitude, frequency, and phase angle of the doubly-fed generator rotor excitation current, thereby regulating its rotational speed, active power, and reactive power. This improves the unit's efficiency and also helps stabilize the frequency and voltage of the power grid. Figure 2 shows the block diagram of the wind turbine doubly-fed generator control system derived from this control approach.
The entire control system can be divided into three units: a speed regulation unit, an active power regulation unit, and a voltage regulation unit (reactive power regulation). These units respectively receive wind speed and speed, active power, and reactive power commands, and generate a comprehensive signal, which is sent to the excitation control device to change the amplitude, frequency, and phase angle of the excitation current to meet system requirements. Because the doubly-fed induction generator can regulate both active and reactive power, it can generate electricity when there is wind and also act as a compensation device to suppress grid frequency and voltage fluctuations when there is no wind.
(3) Doubly-fed wind turbine generator sets have broad application prospects. In summary, applying doubly-fed motors to wind power generation can solve problems such as the inability to adjust the speed of wind turbines and low unit efficiency. In addition, since doubly-fed motors can adjust both reactive and active power, they can play a role in stabilizing voltage and frequency for the power grid, thus improving the quality of power generation. Compared with synchronous machine AC-DC-AC systems, they also have the advantages of smaller frequency converter capacity (generally 10-20% of the generator's rated capacity) and lighter weight, making them more suitable for use in wind turbine generator sets, while also reducing costs.
The idea of ​​applying doubly-fed induction generators (DFIGs) to wind power generation is not only theoretically sound but also technically feasible. Compared with existing wind power generation technologies, DFIGs have irreplaceable advantages in terms of both economy and reliability, making them highly competitive and conducive to the localization of wind turbine production. Their development prospects are very broad.

5. Computer Monitoring System for Large-Scale Wind Farms <br />The development of wind power generation technology will drive the construction of large-scale wind farms. Large-scale wind farms, composed of large wind turbine generators, can provide renewable green energy to the power grid and also solve the energy supply shortage in remote areas. The operation and management of large-scale wind farms has been put on the agenda. Currently, while importing foreign wind turbine generators, major wind farms in China generally also equip themselves with corresponding monitoring systems. However, each system has its own design philosophy, resulting in incompatibility between wind farm monitoring technologies. If a wind farm has multiple types of wind turbine generators, it will cause great difficulties in the operation and management of the wind farm. Therefore, the State Planning Commission, while implementing the "Ninth Five-Year Plan" for scientific and technological research, also included wind farm monitoring systems in its research plan, aiming to develop a monitoring system suitable for the operation and management of wind farms in China. Based on a survey and analysis of several current domestic wind farm monitoring systems, this paper proposes our overall design concept.
5.1 Communication Methods Currently, wind turbines used in wind farms are mainly large-scale grid-connected units. Each unit has its own control system to collect natural parameters, unit data, and status. Through calculation, analysis, and judgment, it controls a series of control and protection actions, such as starting, stopping, directional adjustment, braking, and starting the oil pump, enabling fully automatic control of a single wind turbine without human intervention. When these high-performance wind turbines are installed in a wind farm, centralized monitoring and management of the operating data, status, protection device operation, and fault types of each wind turbine is crucial. To achieve the above functions, the lower-level computer (unit control unit) control system should be able to communicate the unit's data, status, and fault conditions with the upper-level computer in the central control room through dedicated communication devices and interface circuits. Simultaneously, the upper-level computer should be able to transmit control commands to the lower-level computer, which then executes the corresponding actions, thereby achieving remote monitoring. Based on the actual operation of the wind farm, the communication between the upper and lower-level computers has the following characteristics:
① A single host computer can monitor the operation of multiple wind turbine units, which is a one-to-many communication method;
② The lower-level machine should be able to operate independently and communicate with the upper-level machine;
③ The installation distance between the upper and lower level computers is relatively long, exceeding 500m;
④ The installation distance between the lower-level machines is also quite far, exceeding 100m;
⑤ The communication software between the upper and lower level computers must be coordinated and consistent, and industrial control-specific functions should be developed.
To meet the needs of long-distance communication, the monitoring systems currently introduced in domestic wind farms mainly adopt the following two communication methods:
① Asynchronous serial communication, using RS-422 or RS-485 communication interfaces. Its transmission distance can reach thousands of kilometers, and the transmission speed can reach millions of bits. Because it uses fewer transmission lines, the cost is lower, making it very suitable for wind farm monitoring systems. At the same time, because this communication method has a relatively simple and commonly used communication protocol, it has become the preferred method for long-distance communication.
② Modem method. This method modulates digital signals into analog signals, transmits them over a medium to a distant location, and then uses a demodulator to recover the signal, extract the information, and process it. It's a way to achieve long-distance signal transmission. This method has no distance limitations, allowing information exchange between locations worldwide. It also has strong anti-interference capabilities and high reliability. Although relatively expensive, it is widely used in wind turbine communication.
5.2 Design of Upper and Lower Computer Communication Interfaces (1) Design of Upper Computer Communication Interface In industrial field control applications, industrial control PCs are usually used as upper computers, communicating with lower computers through RS-232 serial ports to form a distributed monitoring system. However, the disadvantage of using RS-232 serial ports for data communication is that they have poor load-bearing capacity and are only used for short-distance (within 15m) communication, which cannot meet the communication requirements of distributed, long-distance wind farm monitoring. Whether using asynchronous serial communication or modulation and demodulation, appropriate improvements and expansions must be made to the PC's RS-232 serial port.
RS-232 uses a single-ended, bipolar power supply. This circuit cannot distinguish between useful signals generated by the driver circuit and externally introduced interference signals, limiting both transmission rate and distance. RS-422, on the other hand, employs balanced drive and differential reception, fundamentally eliminating the signal ground wire. When interference signals appear as common-mode signals, the receiver only receives the differential input voltage, thus ensuring a longer transmission distance and higher transmission rate. Asynchronous communication between the two can be achieved using an RS-232/422 conversion interface board.
(2) Design of the Lower-Level Machine Communication Interface The lower-level machine of the monitoring system refers to the central controller of each wind turbine. Each wind turbine can operate safely and correctly even without the involvement of the upper-level machine. Therefore, relative to the entire monitoring system, the lower-level machine control system is a subsystem, capable of independently handling wind turbine faults under various abnormal operating conditions and ensuring the safe and stable operation of the wind turbines. From the perspective of the overall operation and management of the wind farm, the lower-level controller of each wind turbine should have the function of exchanging data with the upper-level machine, enabling the upper-level machine to understand the operating status of the lower-level machine at any time and perform routine management control, thus facilitating the management of the wind farm. Therefore, the lower-level machine controller must ensure the reliable operation of its respective wind turbine and have a dedicated communication interface for communication with the upper-level machine.
Programmable logic controllers (PLCs) are widely used in industrial control due to their comprehensive functions, high reliability, and ease of programming. Especially in recent years, to meet the requirements of both field control and distributed control, foreign PLC manufacturers have launched communication modules compatible with their PLCs. These modules provide RS232/422 adapters or RS-232 interfaces for data communication with PCs and include dedicated programming software, making software development more convenient. Therefore, using a PLC as the lower-level controller for wind turbine generators can fully meet the requirements of wind turbine generator control and wind farm monitoring.
5.3 Anti-interference measures The main sources of interference for the wind farm monitoring system are:
① Industrial interference: high-voltage AC electric field, electrostatic field, electric arc, thyristor, etc.;
② Natural interference: lightning strikes, various electrostatic discharges, magnetic storms, etc.;
③ High-frequency interference: microwave communications, radio signals, radar, etc.
These interferences enter the control system through direct radiation or conduction via certain electrical circuits, disrupting the stability of the control system's operation. From the perspective of the type of interference, they can be divided into alternating pulse interference and single-pulse interference. Both interfere with the system in the form of electricity or magnetism. Therefore, anti-interference measures should focus on the following aspects:
① Regarding the structure of the chassis and control cabinet: For the host computer, the chassis must effectively prevent electromagnetic interference from spatial radiation, and all circuits and electronic components should be installed inside the chassis as much as possible. Interference from the power supply should also be prevented, so a power supply filtering circuit should be added. At the same time, the chassis and the computer room should have good grounding.
② On communication lines: Signal transmission lines require good signal transmission capabilities, low attenuation, and immunity to external electromagnetic interference, so shielded cables should be used.
③ Communication methods and circuitry: Different communication methods have varying resistance to interference. Generally, the distance between upper and lower level machines in a wind farm does not exceed a few kilometers. In this case, serial asynchronous communication is often used, employing an RS-422A interface circuit. This circuit uses balanced drive and differential reception to fundamentally eliminate signal grounding. This driver is equivalent to two single-ended drive circuits, inputting the same signal and outputting a positive and a negative signal, effectively suppressing common-mode interference. The RS-422A serial communication interface circuit is suitable for point-to-point, point-to-multipoint, and multipoint-to-multipoint bus or star networks. Its transmission and reception are separate, making it very convenient to form a full-duplex network, which is highly suitable for wind farm monitoring systems.
Modulation and demodulation methods are generally suitable for long-distance transmission and multi-station interconnection, and are now also used in wind farm monitoring systems. This communication method is characterized by its balanced differential mode, is half-duplex, and shares the advantages of RS-422A. Communication can be achieved with just one pair of twisted-pair wires, saving on cable investment. However, for short-distance communication, the serial communication method of RS-422A circuits is more economical.
5.4 Development of Monitoring Software Monitoring application software is software developed for specific industrial monitoring purposes. It is used in the monitoring system to monitor, control, and adjust production processes in a timely manner. For a wind farm monitoring system, the first step is to display the overall wind farm and the specific locations of the installed turbines. Then, it is necessary to understand the connection relationships between the turbines and the operating status of each turbine. Therefore, the monitoring software for a wind farm should have the following functions:
① User-friendly control interface. When developing monitoring software, the requirements for wind farm operation and management should be fully considered. A Chinese menu should be used to simplify operation and facilitate wind farm management as much as possible.
② It can display the operating data of each unit, such as the instantaneous power generation, cumulative power generation, power generation hours, wind turbine and motor speed, wind speed, wind direction, etc. of each unit. It can import these data from the lower-level computer into the upper-level computer and display them on the monitor. When necessary, it should also be displayed intuitively in the form of curves or charts.
③ Displays the operating status of each wind turbine, such as start-up, shutdown, directional adjustment, manual/automatic control, and the operation of large/small generators. Understanding the operation of the entire wind farm by monitoring the status of each wind turbine is crucial for the overall management of the wind farm.
④ It should be able to display faults that occur during the operation of each unit in a timely manner. When displaying faults, it should be able to show the type of fault and the time of occurrence so that operators can handle and eliminate the faults in a timely manner, ensuring the safe and continuous operation of the wind turbine units.
⑤ It enables centralized control of wind turbine units. From the central control room, operators can change settings and statuses of lower-level units and control them by simply pressing the corresponding keys labeled with specific functions. Examples include starting, stopping, and adjusting the direction of the turbine. However, such operations require certain access permissions to ensure the safe operation of the entire wind farm.
⑥ System Management. The monitoring software should have the functions of timed printing of operating data, manual real-time printing, and automatic recording of faults, so as to view the historical records of the wind farm's operating status at any time.
The development of monitoring software should, as far as possible, be based on secondary development of existing industrial control software, which can shorten the development cycle. (Continued on page 23) At the same time, modular programming concepts should be adopted in the software development process, which is conducive to software development and overall debugging.

6. Conclusion Wind power generation technology has become increasingly mature and occupies a prominent position in the development of renewable green energy, possessing significant development and utilization value. Especially in remote mountainous areas, pastoral areas, and islands, wind power generation can provide clean energy for the lives and production of local residents, alleviating the tight energy supply situation.

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