The so-called inverter overvoltage refers to the inverter voltage exceeds the rated voltage due to various reasons, and is concentrated on the DC voltage of the inverter DC bus. In normal operation, the DC voltage of the inverter is the average value after three-phase full-wave rectification. If calculated by 380V line voltage, the average DC voltage Ud = 1.35U line = 513V.
When the overvoltage occurs, the storage capacitor on the DC bus will be charged. When the voltage rises to around 700V, the inverter overvoltage protection action (depending on the model). There are two main causes of overvoltage: overvoltage and overvoltage. The overvoltage of the power supply means that the DC bus voltage exceeds the rated value because the power supply voltage is too high. The input voltage of most inverters is now up to 460V. Therefore, the overvoltage caused by the power supply is extremely rare.
The main issue discussed in this paper is overvoltage regeneration. There are mainly causes for regenerative overvoltage: When the load of large GD2 (flywheel torque) decelerates, the deceleration time of the inverter is set too short; the motor is affected by external forces (fan, drafting machine) or position energy load (elevator, crane). For these reasons, the actual speed of the motor is higher than the commanded speed of the inverter, that is, the speed of the rotor of the motor exceeds the synchronous speed. At this time, the slip of the motor is negative, and the direction of the rotary magnetic field cut by the rotor winding is opposite to that of the motor. The resulting electromagnetic torque is a braking torque that blocks the direction of rotation. Therefore, the motor is actually in a state of power generation, and the kinetic energy of the load is "regenerated" into electrical energy.
The regenerative energy is charged by the freewheeling diode of the inverter to charge the DC storage capacitor of the inverter, so that the DC bus voltage rises. This is the regenerative overvoltage. Because the torque generated during the process of regenerating the overvoltage is opposite to the original torque and is the braking torque, the process of regenerating the overvoltage is the process of regenerative braking. In other words, the regenerative energy is eliminated and the braking torque is increased. If the regenerative energy is not large, the inverter and the motor will be consumed because the inverter and the motor itself have a 20% regenerative braking capability. If this part of the energy exceeds the consumption capacity of the inverter and the motor, the capacitance of the DC loop will be overcharged, and the overvoltage protection function of the inverter will act to stop the operation. In order to avoid this kind of situation, must dispose this part of energy in time, also improve the braking torque at the same time, this is the purpose of regenerative braking.
Overvoltage protection
Due to the different causes of overvoltage, the countermeasures adopted are also different. For the over-voltage phenomenon that occurs during the parking process, if there is no special requirement for the parking time or location, then it can be solved by extending the inverter deceleration time or free parking. The so-called free parking means that the frequency converter disconnects the main switching device and allows the motor to coast freely.
If there is a certain requirement for the parking time or the parking position, DC braking (DC braking) can be used. The DC braking function is to decelerate the motor to a certain frequency and then direct current into the stator windings of the motor to form a static magnetic field. The rotor winding of the motor cuts this magnetic field to generate a braking torque, which causes the kinetic energy of the load to be consumed as heat in the rotor circuit of the motor. Therefore, this braking is also called dynamic braking. In the process of DC braking, two processes including regenerative braking and energy consumption braking are actually included. The efficiency of this braking method is only 30-60% of regenerative braking, and the braking torque is small. Since the motor is overheated by consuming energy in the motor, the braking time should not be too long. Moreover, the starting frequency of the DC braking, the braking time and the braking voltage are manually set, and cannot be automatically adjusted according to the level of the regenerative voltage. Therefore, the DC braking cannot be used for overvoltage generated in normal operation, and can only be used for Braking when parking.
For deceleration (from high speed to low speed, but not stopped), overvoltage due to excessive GD2 (flywheel torque) of the load can be solved by appropriately extending the deceleration time. In fact, this method also uses the principle of regenerative braking. To increase the deceleration time is only to control the charging rate of the regenerative voltage of the load to the inverter, so that the 20% regenerative braking capability of the inverter itself can be reasonably used. As for the load that the motor is in a regenerative state due to the effect of external force (including bit energy decentralization), because it normally operates in the braking state, the regenerated energy is too high to be consumed by the inverter itself, so it is not possible to use DC braking or How to increase the deceleration time.
Compared with DC braking, regenerative braking has a higher braking torque, and the braking torque can be adjusted according to the braking torque required by the load (ie, the level of regenerative energy) by the braking unit of the inverter. Automatic control. Therefore, regenerative braking is best suited for providing braking torque to the load during normal operation.
Regenerative braking method:
1. Energy consumption type:
In this method, a braking resistor is connected in parallel with the DC link of the frequency converter to control the on/off of a power tube by detecting the DC bus voltage. When the DC bus voltage rises to about 700V, the power tube is turned on, and the regenerative energy is passed to the resistor and consumed as heat energy, thereby preventing the DC voltage from rising. Because regenerative energy cannot be used, it is an energy-consumption type. The same as the energy consumption type, it differs from the DC braking in that the energy is consumed in the braking resistor outside the motor, and the motor does not overheat, so that it can work more frequently.
2. Parallel DC bus absorption type:
Applicable to multi-motor transmission systems (such as drafting machines). In this system, each motor requires one inverter. Multiple inverters share a grid-side converter, and all inverters are connected in a shared manner. DC bus. In such a system, one or several motors normally operate in a braking state. A motor in a braking state is dragged by other motors to generate regenerative energy, which is then absorbed by a parallel-connected DC bus by an electric motor. In the case of incomplete absorption, it is consumed through the shared braking resistor. The regenerative energy here is partially absorbed and used, but it has not been fed back into the grid.
3. Energy feedback type:
The energy feedback type inverter-side converter is reversible. When there is regenerative energy generated, the reversible converter feeds the regenerated energy back to the grid, so that the regenerated energy is fully utilized. However, this method requires high stability of the power supply. Once a sudden power failure occurs, the inverter will subvert the inverter.
Application of Regenerative Braking
A chemical fiber filament drafting line consists of three drafting machines, each driven by three motors. Roller motor power 22KW, 4 pole, with worm reducer, speed ratio of 25:1; two-roller motor power 37KW, 4 pole, worm reducer, speed ratio 16:1; three-roller motor power 45KW, using cylindrical gear speed Speed ​​ratio 6:1. The motors were respectively driven by Huawei TD2000-22KW Sanken IHF37K and 45K inverters. The three frequency converters use proportional control according to the draft ratio and speed ratio. Its working process is as follows: The tow is wound on a roller, two rollers, and three rollers, and the inverter controls the speed of the tow to be drawn at different speeds between the three rollers.
When driving and debugging, the draft ratio is small, the total tow of the tow is lower, and the system is running normally. After a certain period of production, due to process adjustments, the draft ratio and the total denier of the tow are increased. (The draft ratio is determined by the process. In general, it is generally said that the thickness and number of the tow are much higher. The thicker the tow, the greater the draft or the total denier, the greater the towing force of the three rollers on the two rollers and the one roller.) Problems arise. Driving time is not long, one roll frequency inverter frequently displays SC (overvoltage protection),
Two-roll inverters occasionally have this phenomenon. A little longer time, a roll inverter protection shutdown, fault display E006 (overvoltage). Through careful analysis of the fault phenomenon, the following conclusions are drawn: Since the draft ratio between one roller and the second roller accounts for 70% of the total draft, the power of the two-roller and three-roller motors is greater than that of a roller. Roller motor actually works in the power generation state, it must generate enough braking torque to ensure the draft multiple. The two rollers work according to the process conditions between the electric and the braking state, and only the three rollers are in the electric state.
In other words, if a roller drive cannot process the regenerative energy generated by the motor, it cannot generate enough braking torque, and it will be "dragging" by the two rollers. The main reason for being "dragging" is the function of the frequency converter to automatically increase the output frequency to prevent overvoltage tripping (ie "SC" stall prevention function).
In order to reduce the regenerative energy, the inverter will automatically increase the motor rotation speed in an attempt to reduce the regenerative voltage. However, because the regenerative energy is too high, it cannot prevent the occurrence of overvoltage. Therefore, the focus of the problem is to ensure that the one- and two-roll motors have sufficient braking torque. Increasing the capacity of a roll, a two-roll motor and a frequency converter can achieve this goal, but this is obviously not economical. The over-voltage generated by one roller and two rollers can be processed in time to prevent the DC voltage of the inverter from increasing, and sufficient braking torque can be provided.
Since this is not taken into account in system design, it is not possible to use a common DC bus absorption or energy feedback type. After careful argumentation, only the solution of adding a set of external brake units to each of the one-roller and two-roller frequency converters was adopted. Two sets of Huawei TDB-4C01-0300 brake components were selected after calculation. After driving, the braking resistors of the two groups, especially the braking frequency of one roller, are very high, which means that our analysis is correct. The entire system has been operating for nearly a year, and no overvoltage has ever occurred.
This paper describes in detail the various causes of overvoltage generated by the frequency converter and the corresponding preventive measures. Several methods of regenerative braking are discussed. Through application examples, the application of overvoltage protection and regenerative braking are carefully analyzed. The results prove that the regenerative braking function is the most important method to solve the overvoltage phenomenon.