PIC 16F877 is the most complete microcontroller in PIC 16F87X series chips produced by American Microchip company. Online debugging and online programming can be realized. It has 8 channels of 10 bit A/ D converter, 8k х 14 bit FLASH program memory, 368 х 8 bit ram, 256 х 8 bit EEPROM, 14 interrupt sources and 3 timers/counters. As many as 65438+ are integrated on the chip.
2 Self-shunt microcomputer excitation regulator basic working principle [4]
Figure 1 is the principle wiring diagram of self-shunt excitation system. The generator excitation power is taken from the generator end, stepped down by the excitation transformer LB and rectified by the silicon controlled rectifier KZL to excite the generator. According to the voltage and current signals collected by the voltage transformer TV and the current transformer TA installed at the generator outlet, the automatic excitation regulator controls the phase-shifting pulse that triggers the thyristor of the three-phase fully-controlled rectifier bridge according to the preset regulation criterion, so as to adjust the excitation current of the generator, realize automatic voltage stabilization when a single machine runs, and realize automatic reactive power regulation when it is connected to the grid, thus improving the stability of the power system.
The line voltage UAC and the phase current IB of the generator are transmitted by the voltage transformer and the current transformer, respectively, and then the phase detection circuit generates a square wave pulse with a voltage period and a square wave pulse signal with a voltage-current phase difference, which is sent to the PIC 16F877 microcontroller. By measuring the width of these two pulses with PIC counter, we can get the phase difference count value, that is, the power factor angle of the power grid [1]. Then the corresponding power factor is obtained by looking up the table, and the active power and reactive power are further calculated.
The control unit is PIC 16F877 single chip microcomputer. Because PIC 16F877 single chip microcomputer has internal A/D conversion function, it does not need external A/D module, which reduces external devices, reduces cost and enhances anti-interference ability. PIC microcontroller carries out control calculation and logical judgment according to the real-time data of generator running state variables collected from input channels, and obtains the control quantity. In the SCR rectifier circuit, the control circuit is required to output a series of pulses to the SCR control electrode according to the phase of AC power supply, so as to realize the smooth conduction and natural commutation of the SCR. The function of "synchronous and digital trigger control circuit" is to convert the SCR control angle calculated by computer CPU and expressed by digital quantity into trigger pulse. The trigger pulse is amplified by the power amplifier circuit to trigger the thyristor, thus controlling the excitation current.
3. 1 CPU control module
The CPU control module is the control core of the excitation regulator, which adopts PIC 16F877 single chip microcomputer produced by American Microchip company. PIC 16F877 has a unique RISC (Reduced Instruction Set) structure, and Harvard bus structure with data bus and instruction bus separated, which makes the instruction only have the characteristics of single word length, and allows the number of instruction codes to be larger than 8 bits of data. Compared with the traditional 8-bit single chip microcomputer with CISC structure, it can realize the code compression of 2∶ 1, and the speed is increased by 4 times. PIC 16F877 has 8-way 10-bit A/ D converter, 8kх 14-bit FLASH program memory, 368х 8-bit ram, 256х 8-bit EEPROM, 14 interrupt sources and 3 timers/counters. In addition, there is a low-power sleep mode and an on-chip watchdog circuit, which is easy to realize low-power design and anti-interference design.
3.2 Data acquisition module
The excitation regulator of PIC single-chip microcomputer collects four analog signals that reflect the running status of the generator, namely the generator terminal voltage UAC and stator current IB, excitation voltage UL and excitation current IL. After shaping and filtering, these four analog signals are sent to the corresponding four-chip sample-and-hold LF398. Under the synchronous control signal generated by the RE 1 pin of PIC 16F877 microcontroller, the sample-and-hold device completes the synchronous sampling of four signals, and connects these four signals to four 10 bits A/d of RA0, RA 1 RA2 and Ra3 respectively. The analog reference voltage of the analog input can be set in the register. The A/D conversion result of PIC 16F877 is stored in two 8-bit registers ADRESH and ADRESL. Because the A/D module of PIC is a voltage input, the voltage signal can be directly input to the loop input, while the current signal should be connected to an appropriate resistor at the input terminal and converted into voltage before input. When the input voltage of A/D module exceeds its maximum input voltage of 5V, it may damage the equipment. Therefore, the A/D input terminal is grounded with a 5V regulator, so that when the input voltage is higher than the maximum allowable voltage, it can be stabilized in the normal range with the regulator.
Switch value, shutdown, start-stop excitation, manual, automatic, power increase and power decrease are connected to port B of PIC 16F877 after photoelectric isolation.
3.3 display module
In order to save pins, the static display of digital tube is realized by using the powerful I/O expansion function of PIC 16F877 single chip, SPI mode of MSSP module and shift register chip 74HC595.
3.4 Communication module
At present, RS-485 is used for communication between most excitation systems and the main engine. RS-485 is a half-duplex communication protocol, which can only form a master-slave communication network, and the communication mode is imperative. This mechanism makes it difficult to build a real-time measurement and control network in a large complex industrial site. CAN bus has the functions of point-to-point, point-to-multipoint and global broadcast data transmission, and has the advantages of high reliability, strong anti-electromagnetic interference ability, fast transmission rate, long communication distance, simple use and maintenance, and convenient network expansion. Considering that the excitation system is used in strong electromagnetic interference environment, this system uses CAN bus for communication. PIC 16F877 chip does not integrate CAN function module, but it CAN communicate with PC with intelligent adapter through its SPI mode and CAN controller MCP25 10 chip.
Software design of microcomputer excitation regulator
PIC microcomputer excitation regulator software adopts PIC 16F877 assembly language and C language mixed programming, with friendly man-machine interface and simple operation. In addition, with the modular design idea and main program as the core, subroutines of each functional module are designed, so that a large number of functions can be realized in subroutines and the software design structure is simplified. Subroutine modules mainly include system initialization and power-on self-check module, PID adjustment module, operation mode tracking module, over-excitation and under-excitation control module, start-stop module, communication module and so on. The flow chart of the main program of the system is shown in Figure 4.
The system provides three different operation modes, namely constant voltage regulation, constant excitation current regulation and constant reactive power regulation. You can switch different operation modes and set a given value through the keyboard. In addition, the system also has an operation mode tracking module, that is, the output of the standby operation mode tracks the output of the current operation mode, so that there is no disturbance when switching operation modes.
Because the excitation system has inertia and lag control objects, which require high control accuracy and fast response, this design adopts an improved PID regulation mode, that is, the integral separation algorithm is used to eliminate the integral saturation income and reduce overshoot, and the variable gain method is used to increase the proportional effect in the dynamic response and reduce the proportional effect in the steady-state process, so as to eliminate the big deviation, speed up the transition process and make the excitation regulator have ideal regulation characteristics.
In order to improve the reliability of the whole system, in addition to the power-on self-test, error detection, fault-tolerant processing and software watchdog are also carried out in each calculation cycle.