1. Introduction
Wind turbine generator sets mainly consist of two components: the main control system and the pitch control system. The main control system primarily manages the operation of the entire wind turbine, while the pitch control system is specifically designed for precise control of the blades under different operating conditions, enabling normal blade movement and emergency blade retraction. A complete pitch control system includes key components such as a drive and controller (some pitch control systems only have a drive, not a controller), a pitch motor, and a backup power supply. Each pitch control system has its unique structural features; therefore, a thorough understanding of its structural components is essential for a better understanding of pitch control systems.
2. Structural Analysis of the Dankong Pitch Control System
Founded in Denmark in 1933, Danish company AB has been designing wind turbine generators since the early days of the wind power industry. Its overall design of pitch control systems is at a world-class level, especially its research on pitch controllers and drives.
The system configuration of the Dankong pitch control system is shown in Figure 1. It adopts a three-cabinet structure, with the main controller placed in shaft control cabinet one, which serves as the main control cabinet for the pitch system. This control system provides identical control for all three blades, using servo drives to control AC motors that rotate the blades, achieving precise control over the speed and position of each blade. In the event of a loss of external 400V power, a supercapacitor acts as a backup power source to power the servo drives, enabling emergency pitch retraction and ensuring the safety of the wind turbine.
Figure 1. Diagram of the Danco pitch control system
The pitch control unit in bay one uses the Advanced Wind Turbine Controller (AWC). The controller's main function in the entire pitch system is communication with the main control system and with the pitch drive. In addition, a watchdog relay in the drive is connected in series in the pitch safety chain. If one or more of the following conditions occur: software thread timeout, power failure of the controller's external/internal (24VDC) power supply, program download failure, or controller hardware failure, the pitch safety chain will break. At this time, the pitch system itself will immediately execute the corresponding action: all three blades will simultaneously retract urgently until the 91-degree limit switch is triggered.
The pitch drive uses an Integrated Motor Drive (IMD). Because the pitch controller primarily connects to the main controller and the drive, the pitch drive has a very high degree of integration. All the signals the pitch control system needs to collect, such as motor encoder signals, redundant encoder signals, motor temperature signals, motor braking signals, pitch safety chain signals, braking resistor signals, and charger signals, are integrated into the drive to ensure safe operation of the control system. When the system generates an alarm or fault signal, the drive can respond promptly and accurately, controlling the corresponding devices to safely retract the pitch or shut down the wind turbine. The entire pitch system uses three pitch cabinets, each containing one drive. These cabinets operate independently, each controlling a single blade to perform its corresponding action, demonstrating excellent independence.
When the external 400V power supply to the system is lost, the pitch system requires a backup power source to complete emergency pitch recovery. The Danco control pitch system uses a supercapacitor bank as this backup power source. The supercapacitor bank is composed of two sets of five 9.6F 90V capacitors connected in series, and then the two sets of series-connected capacitors are connected in parallel. This combination method significantly increases the energy stored in the supercapacitor bank, ensuring sufficient energy for the emergency pitch recovery of the pitch system.
The primary purpose of a pitch control system is to control the speed and position of the wind turbine blades, ensuring the turbine generates electricity when wind speeds are suitable and safely retracts the blades when wind speeds are too high. To drive the blades, pitch motors are commonly used. The Dankong pitch control system employs a synchronous motor. Synchronous motors, like induction motors, are widely used AC motors. Their characteristic is that during steady-state operation, the rotor speed and the grid frequency remain constant. If the grid frequency is constant, the synchronous motor speed remains constant in steady state, independent of the load. When the system requires blade rotation, the driver sends commands to control the pitch motor, bringing the blades to a predetermined position to fulfill commands from the main controller or provide a normal response to emergencies.
The safety chain of the Dankong pitch control system is a major feature of its system. The pitch safety chain is connected in series with the main control system's safety chain via slip rings, serving as part of the main control system's safety chain and becoming a safety constraint at the pitch position. The Dankong pitch system's safety chain is independent of the pitch system's software system. It consists of eleven normally closed contacts connected in series: pitch controller fault output, pitch system hardware fault, emergency stop buttons on three axes, maintenance switches on three axes, and inverter fault signals on three axes. These contacts are then connected to the main control system via slip rings. This is the entire hardware composition of the pitch safety chain. Its most significant feature is the use of safety relays to connect all these signals in series. These safety relays are not infallible relays, but rather relays that can effectively respond to faults. They have a forced-guided energization structure, ensuring system safety even in the event of contact fusion. When the emergency stop is released, it prevents sudden restart of the system; in the event of a failure in the system's safety circuit, it can safely retract the blades, ensuring the safety of the wind turbine.
The adoption of redundancy technology is another major feature of the Dankong pitch control system. Redundancy refers to the use of two or more identical, relatively independent configurations connected together to increase system reliability. By providing redundancy for all critical components required for system operation, a fault-tolerant system or system structure is achieved. When a system failure occurs, the redundant components step in and take over the work of the failed component, thereby reducing the system's downtime. Redundancy technology is applied in many structures of the system, such as the motor encoders and redundant encoders for measuring blade position and speed, the PT100 temperature sensors and KTY temperature sensors for measuring motor temperature, the external 400V power supply providing power to the pitch system, and the supercapacitor bank as a backup power source. This effectively reduces the system's failure rate and improves its stability.
3. Summary
This article primarily introduces the structural composition of the Dankong pitch control system, with a focused analysis of several key components. Furthermore, it highlights two key features of the Dankong pitch control system: the application of safety chains and redundancy technology. It is hoped that this content will be helpful in your learning process regarding pitch systems.
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