In the power and electrical industry, frequency converters are mainly used for energy conservation and improving production processes. As energy-saving and speed regulating devices for motors, they are widely used in metallurgy, power, water supply, petroleum, chemical, coal and other fields. The essence of the energy feedback unit of the frequency converter is active inversion. The implementation method of the energy feedback unit of the general frequency converter is to feed the regenerated energy into the grid by anti parallel three-phase inverters at the uncontrolled rectifier of the front stage of the general frequency converter. The main circuit of the energy feedback unit is mainly composed of an inverter bridge consisting of thyristors, IGBTs, IPM modules, and some peripheral circuits.
The output end of the inverter bridge is connected to the input terminals R, S, and T of the frequency converter through three choke reactors, and the input end is connected to the positive terminal of the DC side of the universal frequency converter through an isolation diode to ensure the unidirectional flow of energy in the direction of "frequency converter active inverter bridge grid". The function of a choke reactor is to balance voltage difference, limit current, and filter, playing a key role in the feedback of regenerative energy to the power grid.
The working process of the system is: when the motor is running, the active inverter device does not work, and the inverter switch tubes are all blocked and in the off state; When the motor is in a regenerative power generation state, energy is fed back to the grid by the motor, and the active inverter device needs to be started to operate.
The activation of the active inverter device during energy feedback is controlled by the magnitude of the DC side voltage Ud of the frequency converter. The basis is that when the motor is in an electric state, the DC side voltage of the frequency converter remains basically constant. When the motor is in a generating braking state, the regenerative energy of the AC motor charges the energy storage capacitor in the middle DC link of the frequency converter, causing the DC bus voltage to rise. As long as the magnitude of Ud is detected, the state of the motor can be determined, and the active inverter device can be controlled to achieve energy feedback.
When the energy is fed back to the DC side by the motor, causing the DC bus voltage to exceed the peak voltage of the power grid line, the rectifier bridge of the universal frequency converter will shut down due to the reverse voltage; When the DC bus voltage continues to rise and exceeds the starting active inverter working voltage, the inverter starts working, feeding energy back to the grid from the DC side; When the DC bus voltage drops to the operating voltage of the inverter, the active inverter is turned off.
By using an active inverter to feedback the regenerative energy generated during motor deceleration and braking to the power grid, a universal frequency converter can overcome the low efficiency and difficulty in meeting the requirements of fast braking and frequent forward/reverse rotation caused by the traditional use of braking resistors, allowing the universal frequency converter to operate in four quadrants.
1) Energy feedback control system
A complete energy feedback control system should meet the control conditions of phase, voltage, current, etc., which requires that the feedback process must be synchronized with the grid phase, and the active inverter device should only be started when the DC bus voltage exceeds a certain value; The system should be able to control the magnitude of the feedback current, thereby controlling the braking torque of the motor and achieving precise braking.
2) Two types of universal frequency converter energy feedback units
Earlier, the main circuit of energy feedback units was mostly composed of thyristors and IGBTs. In recent years, some new types of energy feedback units have also used intelligent modules such as IPM to simplify the system structure of energy feedback units.
(1) Thyristor energy feedback unit:
The energy feedback main circuit is composed of thyristor devices, which is also an early energy feedback unit. It is not only used in frequency converters, but also in the braking of some DC reversible speed control systems.
① Forward working state of universal frequency converter: When the motor is in electric state, the rectifier of the frequency converter is working, while the thyristor device in the energy feedback unit is not triggered and is in the cut-off state, and the rectifier is working in the forward direction. The controllable inverter part of the inverter is triggered to work, the uncontrollable reverse rectification part is in the cut-off state, and the inverter is in forward operation.
② Reverse working state of universal frequency converter: When the motor is in the generating state, the rectifier of the frequency converter is in the cut-off state, and the thyristor devices in the energy feedback unit are triggered to work. The controllable inverter part of the inverter is still triggered to work, the uncontrollable reverse rectification part is in working state, and the inverter is working in reverse.
(2) IGBT energy feedback unit:
The energy feedback main circuit is composed of IGBT devices, which are most commonly used in general frequency converters. The freewheeling diode integrated with IGBT devices cannot be used as a rectifier device due to the limitation of the isolation diode connected to the DC side. Its cost should be higher than that of the thyristor energy feedback unit.
① Fonctionnement direct du convertisseur de fréquence universel : lorsque le moteur est en marche, le redresseur du convertisseur fonctionne, tandis que le transistor IGBT de l’unité de rétroaction d’énergie est bloqué. Le redresseur fonctionne alors en sens direct. Les transistors IGBT de l’onduleur sont activés et la partie de redressement inverse non commandée est bloquée. L’onduleur fonctionne alors en sens direct.
2. Fonctionnement inverse du convertisseur de fréquence universel : Lorsque le moteur est en mode générateur, le redresseur du convertisseur de fréquence est bloqué et les transistors IGBT de l’unité de rétroaction d’énergie sont activés. Les transistors IGBT de l’onduleur restent activés et la partie redresseuse non contrôlée fonctionne, ce qui provoque un fonctionnement inverse de l’onduleur.