Causes and preventive measures of inverter overvoltage

Overvoltage Generation and Regenerative Braking

The so-called inverter overvoltage refers to the inverter voltage exceeding the rated voltage due to various reasons, which is concentrated on the DC voltage of the inverter DC bus. During normal operation, the DC voltage of the inverter is the average value after three-phase full-wave rectification. If calculated with a 380V line voltage, the average DC voltage Ud=1.35Uline=513V.
When overvoltage occurs, the energy storage capacitor on the DC bus will be charged. When the voltage rises to about 700V (depending on the model), the inverter overvoltage protection will be activated. There are two main reasons for overvoltage: power supply overvoltage and regenerative overvoltage. Power supply overvoltage refers to the DC bus voltage exceeding the rated value due to excessive power supply voltage. Now the input voltage of most inverters can reach up to 460V, so overvoltage caused by power supply is extremely rare.

The main issue discussed in this article is regenerative overvoltage. The main reasons for regenerative overvoltage are as follows: when a large GD2 (flywheel torque) load is decelerating, the inverter deceleration time is set too short; the motor is affected by external forces (fans, drafting machines) or potential loads (elevators, cranes) are lowered. Due to these reasons, the actual motor speed is higher than the inverter's command speed, that is, the motor rotor speed exceeds the synchronous speed. At this time, the motor slip rate is negative, and the direction of the rotor winding cutting the rotating magnetic field is opposite to that of the motor state. The electromagnetic torque it generates is a braking torque that hinders the direction of rotation. Therefore, the motor is actually in a power generation state, and the kinetic energy of the load is "regenerated" into electrical energy.

The regenerative energy charges the inverter DC energy storage capacitor through the inverter freewheeling diode, causing the DC bus voltage to rise, which is called regenerative overvoltage. Since the torque generated during the regenerative overvoltage process is opposite to the original torque and is a braking torque, the regenerative overvoltage process is also a regenerative braking process. In other words, the regenerative energy is eliminated, and the braking torque is increased. If the regenerative energy is not large, this part of the electric energy will be consumed by the inverter and the motor because the inverter and the motor themselves have a 20% regenerative braking capacity. If this part of the energy exceeds the consumption capacity of the inverter and the motor, the capacitor of the DC circuit will be overcharged, and the overvoltage protection function of the inverter will be activated, causing the operation to stop. To avoid this situation, this part of the energy must be disposed of in a timely manner, and the braking torque is also increased, which is the purpose of regenerative braking.

Overvoltage prevention measures

Because the causes of overvoltage are different, the countermeasures are also different. For the overvoltage phenomenon generated during the parking process, if there is no special requirement for the parking time or position, the solution can be to extend the inverter deceleration time or free stop. The so-called free stop means that the inverter disconnects the main switch device and allows the motor to slide freely to stop.

If there are certain requirements for parking time or parking position, then the DC braking (DC braking) function can be used. The DC braking function is to decelerate the motor to a certain frequency, and then pass DC power into the motor stator winding to form a static magnetic field. The motor rotor winding cuts this magnetic field to generate a braking torque, so that the kinetic energy of the load is converted into electrical energy and consumed in the motor rotor circuit in the form of heat. Therefore, this type of braking is also called energy consumption braking. The DC braking process actually includes two processes: regenerative braking and energy consumption braking. The efficiency of this braking method is only 30-60% of regenerative braking, and the braking torque is small. Since consuming energy in the motor will cause the motor to overheat, the braking time should not be too long. In addition, the DC braking start frequency, braking time and braking voltage are all manually set, and cannot be automatically adjusted according to the level of the regenerative voltage. Therefore, DC braking cannot be used for overvoltage generated during normal operation, and can only be used for braking during parking.

For overvoltage caused by excessive GD2 (flywheel torque) of the load during deceleration (from high speed to low speed, but not stopping), the method of appropriately extending the deceleration time can be used to solve it. In fact, this method also uses the principle of regenerative braking. Extending the deceleration time only controls the charging speed of the regenerative voltage of the load to the inverter, so that the 20% regenerative braking capacity of the inverter itself can be reasonably utilized. As for those loads that put the motor in a regenerative state due to external forces (including potential energy decentralization), because they are normally operating in a braking state, the regenerative energy is too high to be consumed by the inverter itself, so it is impossible to use DC braking or extend the deceleration time.

Compared with DC braking, regenerative braking has higher braking torque, and the magnitude of the braking torque can be automatically controlled by the inverter's braking unit according to the braking torque required by the load (i.e. the level of regenerative energy). Therefore, regenerative braking is most suitable for providing braking torque for the load during normal operation.

Regenerative braking method:

1. Energy consumption type:
This method is to connect a braking resistor in parallel in the DC circuit of the inverter, and control the on and 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 into the resistor, which is consumed in the form of heat energy, thereby preventing the DC voltage from rising. Since the regenerative energy cannot be used, it belongs to the energy consumption type. As an energy consumption type, it is different from DC braking in that the energy is consumed in the braking resistor outside the motor, and the motor will not overheat, so it can work more frequently.

2. Parallel DC bus absorption type:
Applicable to multi-motor drive systems (such as drafting machines). In this system, each motor requires a frequency converter, and multiple frequency converters share a grid-side converter. All inverters are connected to a common DC bus. In this system, one or more motors usually work normally in a braking state. The motors in the braking state are dragged by other motors to generate regenerative energy, which is then absorbed by the motors in the electric state through the parallel DC bus. If it cannot be completely absorbed, it is consumed through a shared braking resistor. The regenerative energy here is partially absorbed and utilized, but not fed back to the grid.

3. Energy feedback type:
The grid-side converter of the energy feedback type inverter is reversible. When regenerative energy is generated, the reversible converter feeds the regenerative energy back to the grid, so that the regenerative energy can be fully utilized. However, this method has high requirements on the stability of the power supply. Once there is a sudden power outage, the inverter will be subverted.

Application of regenerative braking

A chemical fiber filament drawing production line consists of three drawing machines, each driven by three motors. The power of the first roller motor is 22KW, 4 poles, and uses a worm reducer with a speed ratio of 25:1; the power of the second roller motor is 37KW, 4 poles, a worm reducer, and a speed ratio of 16:1; the power of the third roller motor is 45KW, and uses a cylindrical gear reducer with a speed ratio of 6:1. The motors are driven by Huawei TD2000-22KW Sanken IHF37K, 45K inverters. The three inverters use proportional control according to the drawing ratio and speed ratio. Its working process is as follows: the tow is wound on the first roller, the second roller, and the third roller, and the inverter controls the different speeds between the three rollers to draw the tow.

During the commissioning, the system started normally because of the small draft ratio and low total denier of the tow. After a period of production, due to process adjustments, the draft ratio and total denier of the tow were increased (the draft ratio is determined by the process, and the total denier is generally speaking, the thickness and number of the tow. The higher the total denier, the thicker the tow. The larger the draft multiple or total denier, the greater the drag of the three rollers on the second roller and the first roller.) At this time, a problem occurred. Not long after the start-up, the inverter of the first roller frequently displayed SC (overvoltage prevention).

The two-roller inverter occasionally has this phenomenon. After a while, the one-roller inverter stops for protection, and the fault display is E006 (overvoltage). Through a careful analysis of the fault phenomenon, the following conclusions are drawn: Since the draft ratio between the first and second rollers accounts for 70% of the total draft multiple, and the motor power of the second and third rollers is greater than that of the first roller, the motor of the first roller actually works in the power generation state, and it must generate sufficient braking torque to ensure the draft multiple. The second roller works between the electric and braking states according to the process conditions, and only the third roller is in the electric state.

That is to say, if the inverter of the first roller cannot process the regenerative energy generated by the motor, it cannot generate sufficient braking torque and will be "dragged away" by the second roller. The main reason for being "dragged away" is that the inverter automatically increases the output frequency to prevent overvoltage tripping (i.e., the "SC" stall prevention function).

In order to reduce the regenerative energy, the inverter will automatically increase the motor speed and try to reduce the regenerative voltage. However, due to the high regenerative energy, it cannot prevent the occurrence of overvoltage. Therefore, the focus of the problem is to ensure that the motors of the first and second rollers have sufficient braking torque. Increasing the capacity of the motors of the first and second rollers and the inverter can achieve this goal, but this is obviously not economical. However, timely handling of the overvoltage generated by the first and second rollers and preventing the DC voltage of the inverter from increasing can also provide sufficient braking torque.

Since this point was not taken into account during system design, it was impossible to adopt the method of shared DC bus absorption or energy feedback. After careful discussion, the only solution was to add an external brake unit to each of the first and second roller inverters. After calculation, two sets of Huawei TDB-4C01-0300 brake components were selected. After starting, the operating frequency of the two sets of brake unit resistors, especially the first roller brake resistor, was very high, indicating that our analysis was correct. The entire system has been running for nearly a year, and no overvoltage has occurred.

This article explains in detail the various reasons for the overvoltage generated by the inverter and the corresponding prevention measures, discusses several methods of regenerative braking, and carefully analyzes the prevention of overvoltage and the application of regenerative braking through application examples. The results show that the regenerative braking function is the most important method to solve the overvoltage phenomenon.