One three-phase principle of variable frequency power supply
The power supply system input is three-phase three-wire 380V/50Hz AC mains, output is three-phase four-wire 0-500V,60Hz AC, can be divided into power conversion main circuit and control circuit two parts. In order to improve the adaptability of three-phase output to unbalanced load, the main circuit and control circuit of three-phase power supply are designed according to three sets of independent single-phase power supply. The main circuit adopts AC and AC structure, including rectifier, DC filter, inverter, AC filter and transformer. Among them, the AC-direct part is a bridge rectifier, which is soft opened to the electrolytic capacitor filter through the AC contactor to obtain a smooth current. Rectifier has the following characteristics:
1 When the rectifier part is started, AC contactor is used to achieve “soft” start and reduce the impact on the power grid.
The DC-AC inverter part adopts single-phase full-bridge structure, and the three-phase system has three sets of identical single-phase inverters, which share a DC bus and are the core of the power supply. The inverter uses IGBT as the switching element. The inverter is controlled by sinusoidal pulse width modulation (SPWM) based on the high switching frequency of IGBT. The steady DC is converted to the AC output of pulse width modulation. The fundamental frequency of the AC is the required output frequency of power supply. After the PWM wave output by the inverter is filtered by the output LC filter circuit, the sine-wave AC is output. Three sets of single-phase inverter circuit output difference of 120 electrical Angle, in the transformer side of the original electrical independence from each other, in the transformer sub-side connected into a star, output the required three-phase AC.
In order to improve the performance of electromagnetic compatibility, the input and output terminals of the power supply are connected with anti-interference filters.
The control circuit is composed of intelligent centralized monitoring, single-phase voltage and waveform control, IGBT inverter control, output detection, fault detection and protection, operation display interface, control power supply and other parts, to complete the power system output frequency, voltage, waveform control, system fault diagnosis and protection and operation and status and other functions. Among them, the voltage and waveform control for single-phase output, using three sets of independent single-phase controller, so that any phase of the three-phase power supply can be used as a single-phase power supply independently, and can adapt to any unbalanced load, thus greatly improving the load adaptability of the power supply.
2, IGBT drive and protection circuit
The drive and protection circuit of IGBT is designed for single-phase inverter bridge, and the drive protection circuit is made into a circuit board, which is equipped with single-phase inverter bridge and radiator into single-phase inverter unit module. The drive circuit is composed of the integrated drive module M57962 from Mitsubishi Corporation of Japan. The M57962 is a dedicated driver circuit for IGBT modules that can drive up to 400A/1200V components. The circuit has a fast optocoupler isolation, suitable for high frequency switching operation of about 20kHz, and has an overcurrent protection function inside. The drive circuit uses +15V/-10V dual power supply to improve anti-interference ability.
The front stage of the drive circuit is a PWM signal processing circuit, which turns the single PWM signal sent by the control circuit into a 180° signal after the voltage comparator shaping and inverting, which is used as the control signal of the IGBT element of the upper and lower bridge arm. The signal passes through the dead zone circuit and its rising edge is delayed by 3-4μs to ensure that the upper and lower bridge arm conduction has a dead zone of not less than 3μs before it is sent to the drive circuit.
The power supply overcurrent protection adopts the dual overcurrent protection scheme which combines single tube online protection and bridge arm centralized protection. The single-tube online protection is completed by the M57962 internal protection circuit, and the centralized protection circuit uses the HL current sensor with extremely fast response speed to detect the DC bus current. When the circuit exceeds the set threshold, the protection circuit acts to block the driving signal of all IGBTs of the reverse bridge. Over voltage protection adopts DC busbar parallel non-inductive capacitor to absorb voltage peak in switching process. For voltage peak caused by high current turn-off in over-current protection process, it can be solved by shortening DC wiring, reducing distributed inductance, appropriately reducing protection threshold and increasing absorption capacity. In addition, the drive board is also provided with the power device overheating DC bus undervoltage two protection.
3. Control circuit
The power supply adopts three-phase output independent control and centralized monitoring system control scheme. It consists of three sets of single-phase control circuit and one set of centralized monitoring circuit. Single-phase control circuit to complete the frequency, voltage, waveform control. Centralized monitoring circuit to complete the output voltage, frequency set, power system function units, operation panel and I/O logic control, fault detection and display. The voltage to the analog quantity, the frequency to the ten address to select the communication number, the given signal, the control and protection logic signal and the control power supply constitute the signal bus of the system, the three sets of single-phase control circuit, the centralized monitoring circuit and the power circuit into one.
1) Three-phase control circuit
Waveform control adopts double loop control scheme with current inner loop for single phase voltage output. In the voltage waveform system composed of two control rings, the current ring is the inner ring, and the controlled object of the ring is the current Ic of the filter capacitor. The voltage waveform control ring is located outside the current ring, which controls the instantaneous value of the output voltage, so that the output voltage and filter capacitor current are directly sent to the waveform ring after being checked and shaped by the detection circuit, and the PWM control pulse is generated after being adjusted by the double loop compared with the standard sine wave.
The standard sine wave is generated by A typical counting and addressing method. The standard sine data is stored in EPROM, and the EPROM output sine digital quantity is changed into analog quantity by D/A converter. The analog is positive, symmetrically moved down through the op-amp circuit, and the capacitor is separated straight, and the standard sinusoidal signal is output.
Voltage control adopts closed-loop Billy regulation, detects the AC output voltage signal sent by the circuit, and turns it into adjustable feedback direct flow after amplitude adjustment, absolute value exchange and active filter circuit. Compared with the feedback signal, the deviation is sent to the proportional regulator, amplified by the regulator and sent to the standard sine wave amplitude, so that the average value of the output voltage remains constant and the output stability is achieved.
Frequency control is open loop controlled by standard sine wave setting. The standard sinusoidal data weekly storage capacity is 1024 bytes, the standard sinusoidal frequency is the same as the rated output frequency of 60Hz, and the EPROM address selection communication frequency is 409.6KHz. This paper adopts crystal frequency division to obtain the signal, so that the accuracy and stability of the output frequency are well guaranteed. Frequency modulation with a special circuit, the frequency range can be adjusted by 45-60Hz. In the three sets of single-phase control circuit, the standard sinusoidal data stored in EPROM is different by 120 electrical angles. Because the EPROM address is the same group of signals, the three standard sinusoidal waves are different by 120 electrical angles, so as to realize the phase control of the output voltage.
2) Centralized monitoring circuit
The circuit is composed of A 16-bit MCU 80C196 as the core, using the CPU internal 8-channel A/D conversion interface to complete the analog quantity detection, using the CPU external interrupt and PIO to complete the fault detection and operation logic, panel display control, input and output over-voltage protection and output overload protection are completed by software.
The detection circuit includes three parts: output voltage detection, output current detection and filter capacitance current detection. In order to improve the control speed of the waveform ring and ensure the quality of the power supply, the magnetic balance HL sensor is used in the detection element related to the waveform ring. All detection signals are electrically isolated from the main control circuit.