Energy feedback is essentially active inversion. The method for implementing energy feedback in general-purpose frequency converters involves connecting a three-phase inverter in anti-parallel at the uncontrolled rectifier stage before the general-purpose frequency converter to feed regenerated energy back to the power grid. The main circuit of the energy feedback device consists primarily of an inverter bridge composed of thyristors, IGBTs, and IPM modules, along with some peripheral circuits.
The output of the inverter bridge is connected to the input terminals R, S, and T of the frequency converter via three choke reactors. The input is connected to the positive DC terminal of the general-purpose frequency converter via an isolation diode to ensure unidirectional energy flow in the "frequency converter - active inverter bridge - power grid" direction. The choke reactors are used to balance voltage difference, limit current, and filter, playing a crucial role in the regenerative energy feedback to the power grid.
The system works as follows: when the motor is running, the active inverter is not working, and all inverter switching transistors are blocked and in the off state; when the motor is in regenerative power generation state, energy is fed back to the grid by the motor, at which time the active inverter needs to be started.
The activation of the active inverter during energy feedback is controlled by the magnitude of the DC-side voltage Ud of the frequency converter. This is based on the fact that when the motor is in motoring mode, the DC-side voltage of the frequency converter remains basically constant. When the motor is in regenerative braking mode, the regenerative energy of the AC motor charges the energy storage capacitor in the intermediate DC link of the frequency converter, causing the DC bus voltage to rise. By detecting the magnitude of Ud, the state of the motor can be determined, thereby controlling the active inverter to achieve energy feedback.
When the DC bus voltage exceeds the peak value of the grid line voltage due to energy feedback from the motor to the DC side, the rectifier bridge of the general-purpose inverter is turned off due to reverse voltage; when the DC bus voltage continues to rise and exceeds the starting voltage of the active inverter, the inverter starts to work and feeds energy back to the grid from the DC side; when the DC bus voltage drops to the shutdown voltage of the inverter, the active inverter is turned off.
By using an active inverter method to feed the regenerative energy generated during the deceleration and braking of the motor back to the power grid, the general-purpose frequency converter can overcome the inefficiency caused by the braking resistor method of the traditional general-purpose frequency converter and its inability to meet the requirements of rapid braking and frequent forward/reverse rotation, so that the general-purpose frequency converter can operate in four quadrants.
1) Energy feedback control system
A complete energy feedback control system should meet the control conditions of phase, voltage, and current. That is, the feedback process must be synchronized with the phase of the power grid, and the active inverter 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 devices
In the past, the main circuit of energy feedback devices was mostly composed of thyristors and IGBTs. In recent years, some new energy feedback devices have also used intelligent modules such as IPMs, which have simplified the system structure of energy feedback devices.
(1) Thyristor energy feedback device: The main circuit of energy feedback is composed of thyristor devices. This is also a relatively early energy feedback device, which is used not only in frequency converters, but also in the braking of some DC reversible speed regulation systems.
① General-purpose frequency converter forward operation state: When the motor is in motoring mode, the frequency converter's rectifier is working, while the thyristor devices in the energy feedback device are not triggered and are in the off state, so the rectifier is in forward operation. The controllable inverter section of the inverter is triggered to work, while the uncontrollable reverse rectifier section is in the off state, so the inverter is in forward operation.
② Reverse operation state of general frequency inverter: When the motor is in the generating state, the rectifier of the frequency inverter is in the off state, and the thyristor devices in the energy feedback device are triggered to work. The controllable inverter section of the inverter is still triggered to work, while the uncontrollable reverse rectifier section is in the working state, and the inverter is in reverse operation.
(2) IGBT energy feedback device: The main energy feedback circuit is composed of IGBT devices. This type of energy feedback device is most commonly used in general-purpose frequency converters. The freewheeling diode integrated with the IGBT device 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 device.
① General-purpose inverter forward operation: When the motor is in motoring mode, the inverter's rectifier is working, but the IGBT devices in the energy feedback device are not triggered and are in the off state, so the rectifier is operating in the forward direction. When the inverter's IGBT devices are triggered, the uncontrolled reverse rectification section is in the off state, so the inverter is operating in the forward direction.
② Reverse operation state of general frequency converter: When the motor is in generating state, the rectifier of the frequency converter is in the off state, and the IGBT devices in the energy feedback device are triggered to work. The IGBT devices in the inverter are still triggered to work, the uncontrolled reverse rectification section is in working state, and the inverter is in reverse operation.
