What are the characteristics of three-phase variable frequency power rectification?
Variable frequency power supplies are specially designed and manufactured for import and export equipment, and they correspond to European and American power supply systems. They can be used as a power source for imported 60Hz electrical appliances and production lines, and can also be used as a test for production lines of exported electrical products.
What are the characteristics of three-phase variable frequency power rectification?
The input of the three-phase power system is a three-phase three-wire 380V/50Hz AC network, and the output is a three-phase four-wire 0-500V, 60Hz AC, which can be divided into two parts: the main power conversion circuit and the control circuit. In order to improve the adaptability of the three-phase output to the unbalanced load, the main circuit and the control circuit of the three-phase power supply are designed according to three sets of independent single-phase power supplies. The main circuit adopts AC-DC structure, including rectifier, DC filter, inverter, AC filter and transformer. Among them, the AC-DC part is a bridge rectifier, which is slowly started by an AC contactor and an electrolytic capacitor, and is filtered to obtain a stable current. Rectification has the following characteristics:
1. When the rectifier is partially started, an AC contactor is used to provide a "soft" start and reduce the impact on the grid.
The DC-AC inverter part adopts a single-phase full-bridge structure. Three-phase systems have three sets of identical single-phase inverters that share the DC bus that forms the core of the power supply. Inverters use IGBTs as switching elements. Using a higher IGBT switching frequency, the inverter is controlled by sinusoidal pulse width modulation (SPWM), and the stable direct current is converted into a pulse width modulated alternating current output. The fundamental frequency of alternating current is the desired output power. The pulse width modulation wave output from the inverter is filtered by the LC output filter circuit, and the AC sine wave current is output. The outputs of the three single-phase inverter circuits differ from each other by 120 electrical degrees. They are electrically independent from each other on the primary side of the transformer and connected in a star configuration on the secondary side of the transformer in order to output the required three-phase AC power.
To improve electromagnetic compatibility, connect noise filters to the input and output of the power supply.
The control circuit is composed of intelligent central 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 control of output frequency, voltage and waveform. Power supply system control, system fault diagnosis and protection, operation and status and other functions. Among them, the voltage and waveform control of the single-phase output uses three independent single-phase controls, so each phase of the three-phase power supply can be independently used as a single-phase power supply, and is applicable to any single-phase power supply. End load. This improves the load adaptability of the power supply unit.
2. IGBT drive current and protection circuit
The control and protection circuits of the IGBTs are designed for single-phase inverter bridges. The control and protection circuit consists of a printed circuit board equipped with a single-phase inverter bridge and a cooler to form a single-phase inverter unit module. The control circuit takes the integrated control module M57962 of Mitsubishi Corporation of Japan as the core. M57962 is a dedicated control circuit for IGBT modules that can control components up to 400A/1200V. The circuit has a fast optocoupler isolation inside, suitable for high-frequency switching operation around 20 kHz, and has an internal over-current protection function. The control circuit uses +15 V / -10 V dual power supply to improve the interference suppression ability.
The front stage of the drive circuit is a PWM signal processing circuit. After the single-channel PWM signal sent by the control circuit is shaped and inverted by a voltage comparator, two mutually different 180° signals are used as control signals for the upper and lower bridge arms. The signal passes through the dead zone circuit, and its rising edge is delayed by 3-4μs to ensure that the dead zones of the upper bridge arm and the lower bridge arm are not less than 3μs, and then they are sent to the control circuit.
This power supply overcurrent protection uses a dual overcurrent protection scheme that combines online protection with centralized protection for tubes and bridge arms. The online protection with tube is completed by the internal protection circuit M57962. The central protection circuit uses the HL current sensor with extremely fast response speed to detect the intermediate circuit current. If the circuit exceeds the set threshold, the protection circuit blocks the control signals of all IGBTs of the reverse bridge. Surge protectors use a DC bus in parallel with a non-inductive capacitor to absorb voltage spikes during switching. For the voltage spike generated by the large current during the overcurrent protection process, shortening the DC line to reduce the distributed inductance, properly reducing the protection threshold, and increasing the absorption capacity can solve this problem. In addition, the driver board is equipped with two protection devices to prevent overheating of the power supply unit and undervoltage protection of the intermediate circuit.
3. Control circuit
The power supply uses three phases and is independent of the output control and central monitoring system. It consists of three sets of single-phase control circuits and a set of central monitoring circuits. Single-phase control circuit completes frequency, voltage and waveform control. The central monitoring circuit completes the setting of output voltage and frequency, each functional unit of the power supply system, control panel and I/O logic control, error detection and display. The voltage is set as an analog quantity, and the frequency is set as a tens address strobe signal. Setup signals, control and protection logic signals, and control power supply form the signal bus of the system. Three sets of single-phase control circuits, centralized monitoring circuit and power supply circuit are integrated into one.
1) Three-phase control circuit
Waveform control targets a single-phase voltage output and uses a two-loop control scheme with an inner current loop. In the voltage waveform system composed of two control loops, the current loop is the inner loop, the controlled object of this loop is the current Ic of the filter capacitor, the voltage waveform control loop is outside the current loop, and this loop affects the instantaneous output voltage value. The control is carried out so that the output voltage and the filter capacitor current are detected and shaped by the detection circuit, and then sent directly to the waveform loop compared with the standard sine wave and waveform loop, and the PWM control pulse is generated after the double loop adjustment.
The standard sine wave generation uses a typical look-up table method for addressing and table look-up. The standard sine wave data is stored in the EPROM, and the EPROM is controlled according to the output frequency sequence, and the sine digital output of the EPROM is converted into an analog signal by a D/A converter. . The analog quantity has positive polarity and is shifted down symmetrically by the op amp circuit. After the capacitor is blocked, a sinusoidal standard signal is output.
Voltage control is performed by Billy setup with closed loop control. The AC output voltage signal sent by the detection circuit becomes an adjustable DC feedback variable after amplitude adjustment, absolute value exchange and active filter circuit. Compared with the feedback signal, the deviation is sent to the proportional controller, and after being amplified by the controller, it is sent to the standard sine wave amplitude, so that the average value of the output voltage remains constant and the output is stable.
Frequency control is a control via a standard sine wave setting. The storage capacity of one cycle of the standard sine data is 1024 bytes, the frequency of the standard sine wave corresponds to the nominal output frequency at 60 Hz, and the frequency of the EPROM address strobe signal should be 409.6kHz. The crystal oscillator is used to divide the frequency to obtain the signal, so that the output frequency is accurate and stable, and the performance is well guaranteed. The dedicated frequency modulation circuit can be set to a frequency range of 45-60Hz. In the three groups of single-phase control circuits, the standard sinusoidal data stored in EPROM differs from i by 120 electrical degrees.
2) Central monitoring circuit
The circuit consists of a 16-bit 80C196 microcontroller as the core. It uses the 8-channel A/D conversion interface in the CPU to complete the analog quantity detection, uses the external CPU and PIO interrupts to complete the error detection and operation logic, and the control panel to indicate the control. The input and output overvoltage protection and the output overload protection are implemented in the software.
The detection circuit consists of three parts: output voltage detection, output current detection and filter capacitor current detection. In order to improve the control speed of the bushing and ensure the power quality, the sensing element connected with the bushing uses HL magnetic balance sensor, and all detection signals are electrically isolated from the main control circuit.
