1. Background and significance of the topic
Microcomputer excitation controllers entered practical use abroad in the 1980s. During this period, Toshiba of Japan, General Electric of Canada, ABB of Switzerland, Mitsubishi of Japan, ELIN of Austria, SIEMENS of Germany, GEC of the UK, and Rolls-Roy, among others, successively produced microcomputer excitation regulators. These large companies all possessed strong research and development capabilities. The computer systems used in their excitation controllers were based on high-speed microprocessors and consisted of self-developed dedicated control boards, featuring compact structures and high reliability. Many of these products have been used in large-scale hydropower, thermal power, and nuclear power plants in my country. These microcomputer excitation controllers mostly employ PID control, with relatively complete control and limiting functions, and the overall manufacturing level of the devices is high.
Currently, several domestic units are conducting research and development on microcomputer excitation controllers. Among them, the products developed by Nanjing Institute of Automation, Tsinghua University, and Huazhong University of Science and Technology are already in use. However, their functions are simple, and their reliability and manufacturing processes are far behind those of foreign countries. Furthermore, several large motor manufacturers in the same industry as our company do not have their own digital voltage regulators or use simple voltage regulators composed of single-chip microcomputers.
In 2002, our company collaborated with Hunan University to develop a regulator for excitation control of dual-winding generators with a power output of tens of kilowatts. This regulator was composed of an 8098 microcontroller, but its functions were simple, limited to voltage regulation only, and could not be used with other synchronous generators. This is because a modern digital voltage regulator should possess the following basic functions:
a) Automatic Voltage Regulation (AVR) – with a high steady-state voltage regulation rate.
b) Stability – that is, the PID parameters should be able to be flexibly set for different generator parameters.
c) Reactive power/power factor control – used for generator and power system grid connection.
d) Parallel compensation – used for two or more generators operating in parallel or connected to the grid.
e) Protection functions – such as overexcitation limit, magnetic field overvoltage, overcurrent, generator overvoltage, exciter rotating diode failure, etc.
f) Communication function – It should be able to support RS-485 or CAN communication.
In summary, none of the products previously developed by our company met the requirements, and because the technology was controlled by our partners, they were of little significance to our company's technological progress, and their technical level lagged significantly behind that of foreign counterparts. Therefore, in order to improve the control level of synchronous generators and the company's overall technical capabilities, the development of a new type of digital automatic voltage regulator is urgently needed.
2. Overall structure and working principle of the excitation controller
As mentioned earlier, the excitation controller designed in this paper uses the most mature PID control algorithm to control the constant generator terminal voltage. The overall structure of the excitation control system is shown in Figure 1.
Figure 1. Overall structural block diagram of the excitation control system
In the self-excited excitation control system, the excitation power supply is taken from the generator terminal voltage, and the generator operates normally.
Previously, since excitation current could not be provided, an external excitation power supply was required for generator start-up. Generally, to improve the reliability of the excitation power supply, two power sources were selected: AC power from the plant and DC battery power. The AC power was stepped down and rectified before being supplied to the excitation winding for start-up. When the program determines that the generator terminal voltage has reached the rated voltage, this value can be modified online, and a control signal is automatically issued to disconnect the contactor, cut off the excitation power supply, and enter normal voltage adjustment and boosting.
When the generator is operating normally, the excitation power is provided by the excitation transformer connected to the generator terminals, through a three-phase full-range power supply.
The rectified current from the control bridge is supplied to the generator's excitation current. The control unit is responsible for collecting the power data and sending it to the DSP chip. After real-time calculation, the data is sent to the controller, which processes it through the control algorithm and outputs the control quantity, namely the firing angle α of the three-phase full-control bridge. The magnitude of the generator's excitation current is controlled by changing the firing angle.
The device collects analog quantities including generator terminal voltage, system voltage (grid voltage), stator current, excitation voltage, and excitation current. The AC signals from each voltage transformer and current transformer, along with the excitation voltage and current, are isolated before being converted into digital signals via the analog input channel. These digital signals are then filtered by the main control system and processed using the root mean square algorithm to calculate the effective values of the generator terminal voltage, system voltage, stator current, active power, reactive power, power factor, and the average values of the excitation current and excitation voltage. These status feedback signal data are used by the controller for calculation and analysis. Simultaneously, the A-phase voltage is converted into a synchronization signal via a synchronization square wave conversion circuit for frequency detection and synchronization pulse triggering. To ensure real-time control and regulation, the program first calculates the latest analog quantity acquired in the calculation module, and then calculates the phase-shifting trigger angle of the three-phase fully controlled bridge according to the control algorithm. This trigger angle is then converted into a timer count value. When the timer value is reached, the motor control PWM module on the DSP chip (hereinafter referred to as the PWM module) generates a control pulse. This pulse, after isolation and power amplification, triggers the three-phase fully controlled bridge to control the magnitude of the excitation current. When the measured value of the generator terminal voltage is lower than the given value, the excitation current is increased to raise the terminal voltage; conversely, the excitation current is decreased, thereby achieving the purpose of controlling and regulating the generator terminal voltage and reactive power. The controller will also perform logical judgments based on the operation and status signals input from the field to realize the excitation regulation, limiting, protection, detection, and fault diagnosis functions required for various operating modes.
3. Excitation Controller Design Task Analysis
The excitation controller, as an important auxiliary control device for synchronous generator control, consists of two parts: the excitation power unit and the excitation controller unit. This research project, based on the needs and development trends of excitation controllers, fully utilizes the rich peripheral resources of the selected microcontroller chip to complete the hardware and software design of each module of the excitation controller. This digitally integrates multiple functions of the excitation controller into a single unit, resulting in comprehensive functionality, a simple structure, and improved reliability of the excitation control system. Based on the research of the functions and structures of existing domestic and international excitation control devices, the main design tasks of this excitation controller are determined to include:
(1) Signal Acquisition Unit: To control the generator, the current state of the generator and power system must first be obtained. These state quantities are characterized by a series of signals. The task of the signal acquisition unit is to quickly and accurately acquire external signals to provide parameters for the control algorithm. The acquisition of analog signals needs to ensure the accuracy and speed of data sampling and reduce the influence of interference factors such as harmonics. The detection of switching signals requires attention to the strong and weak current isolation between the high-voltage circuit and the control circuit, and attention to enhancing anti-interference capabilities to prevent signal malfunctions.
(2) Synchronization signal capture unit: The frequency of the power system changes constantly. The excitation controller needs to track the frequency changes of the power system through the synchronization capture unit. This is beneficial to improving the accuracy of signal AC sampling and provides a time base and timing starting point for the generation of control trigger pulses.
(3) Main Control Unit: Utilizing the abundant on-chip resources and powerful control processing capabilities of the DSP, the core of the control unit is constructed, I/O space is configured, and good peripheral interface circuits are provided to facilitate system hardware expansion. The control program is written to complete functions such as voltage regulation, analog quantity acquisition and calculation, frequency measurement, and excitation limiting.
(4) Pulse Trigger Unit: The results calculated by the PID control algorithm need to be converted into control pulses to drive the power devices to execute and realize the required control functions. This part requires the hardware and software design of the phase-shifting pulse generation, shaping and modulation, and power amplification sections.
4. Overall Hardware Design of the Excitation Controller
To achieve the above functions, the excitation controller needs to have basic units such as electrical quantity measurement, regulation calculation, synchronization signal detection, and pulse phase-shifting amplification. Based on the basic requirements of the excitation controller described above, the excitation controller also consists of the following basic units: main control unit, analog input channel, digital input/output channel, synchronization frequency measurement unit, and pulse amplification unit. The overall hardware structure block diagram of the excitation controller is shown in Figure 2.
Figure 2. Overall hardware structure diagram of the excitation controller
The excitation controller designed in this paper first calculates the generator frequency using the voltage signal captured by the DSP chip's capture unit, determining the sampling period for the AD conversion. It performs high-speed AC sampling of analog quantities such as generator terminal voltage, system voltage, and stator current, and DC sampling of excitation current and excitation voltage. After sampling, the root mean square algorithm is used to calculate the generator terminal voltage, system voltage, stator current, active power, reactive power, and power factor. This state feedback data is used by the PID control device for calculation and analysis. When the synchronization signal arrives, the DSP chip's timer and PWM module generate a phase-shift trigger pulse. This pulse triggers the three-phase rectifier bridge through the power amplification unit, achieving the purpose of controlling and regulating the generator voltage or reactive power by controlling the generator rotor excitation current. Simultaneously, the excitation controller will also perform logical judgments based on the on-site input operation and status signals to realize the excitation regulation and limiting functions required for various operating modes.
5. Overall Software Design of the Excitation Controller
Hardware design forms the foundation of excitation control, while software is its soul. Based on the hardware, the main functions of the excitation controller are all implemented in software. The software design of the excitation system follows a structured, modular, top-down, and progressively refined programming approach. The excitation control application program consists of a main program and an interrupt routine. The interrupt routine performs the main functions of excitation control, such as frequency detection calculation, data exchange sampling calculation, and phase-shift triggering. The main program initializes the main controller and peripheral expansion circuits, preparing for interrupt responses. The main program flowchart is shown in Figure 3.
The enhanced extended interrupt handling capabilities of DSP chips greatly facilitate and benefit program design. The flexible application of interrupts can easily solve many problems that are difficult to address using other methods. To complete the control functions of the excitation controller, the following programming is required: a generator terminal frequency capture program, a data exchange sampling program, a pulse phase-shift trigger program, a constant generator terminal voltage control program, and various excitation limit programs.
Figure 3. Flowchart of the main program of the excitation control system software
6. Summary
The excitation system is a crucial component of a synchronous generator, directly influencing its operating characteristics and significantly impacting the safe and stable operation of the power system. Besides the inherent characteristics of the generator itself, the superior quality of a synchronous generator largely depends on its excitation system. A high-performance digital automatic excitation system can greatly improve the performance of a synchronous generator and enhance its competitiveness in the market.
