Causes and preventive measures for overvoltage generated by the inverter

The frequency converter is often encountered during commissioning and use. After the overvoltage is generated, the inverter will prevent the internal circuit from being damaged, and its overvoltage protection function will operate, causing the inverter to stop running, resulting in the device not working properly. Therefore, measures must be taken to eliminate overvoltage and prevent malfunctions. Since the inverter and the motor are different in application, the cause of the overvoltage is different, so take corresponding countermeasures according to the specific situation.

1. Overvoltage generation and regenerative braking

The overvoltage of the inverter refers to the voltage of the inverter exceeding the rated voltage due to various reasons, and is concentrated on the DC voltage of the DC bus of the inverter. During normal operation, the DC voltage of the inverter is the average value after three-phase full-wave rectification. If calculated with a line voltage of 380V, the average DC voltage Ud=1.35U line = 513V.

When an overvoltage occurs, the storage capacitor on the DC bus will be charged. When the voltage rises to about 700V, the inverter overvoltage protection action (depending on the model). There are two main causes of overvoltage: power supply overvoltage and regenerative 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. Most inverters now have an input voltage of up to 460V, so the overvoltage caused by the power supply is extremely rare.

The main issue discussed in this paper is the regenerative overvoltage. The main reason for generating regenerative overvoltage is as follows: When the large GD2 (flywheel torque) load decelerates, the deceleration time of the inverter is set too short; the motor is affected by external force (fan, drafting machine) or potential energy load (elevator, crane). For these reasons, the actual motor speed is higher than the command speed of the inverter, that is, the motor rotor speed exceeds the synchronous speed. At this time, the slip of the motor 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 generated 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 storage capacitor through the freewheeling diode of the inverter, so that the DC bus voltage rises, which is the regenerative overvoltage. Since the torque generated during the process of regenerating the overvoltage is opposite to the original torque, it is the braking torque, so 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 itself have 20% regenerative braking capacity, and this part of the electric energy will be consumed by the inverter and the motor. If this part of the energy exceeds the consumption capacity of the inverter and the motor, the capacitance of the DC link will be overcharged, and the overvoltage protection function of the inverter will operate to stop the operation. In order to avoid this situation, this part of the energy must be disposed of in time, and the braking torque is also increased, which is the purpose of regenerative braking.

Second, overvoltage prevention measures

Since the causes of overvoltages are different, the countermeasures taken are different. For the overvoltage phenomenon generated during the parking process, if there is no special requirement for the parking time or position, it can be solved by extending the deceleration time of the inverter or free parking. The so-called free stop means that the inverter disconnects the main switching device and allows the motor to coast and stop.

If there is a certain requirement for parking time or parking position, DC braking (DC braking) function can be used. The DC braking function is to decelerate the motor to a certain frequency and then input DC power into the stator winding of the motor to form a static magnetic field. The rotor winding of the motor cuts this magnetic field to generate a braking torque, so that the kinetic energy of the load is converted into electrical energy and is consumed in the form of heat in the rotor circuit of the motor. Therefore, this braking is also called energy braking.

In the process of DC braking, two processes of regenerative braking and energy braking are actually included. This braking method is only 30-60% efficient for regenerative braking and has a low braking torque. Since the motor is overheated by consuming energy in the motor, the braking time should not be too long. Moreover, 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 when parking.

For deceleration (from high speed to low speed, but not stopping), the overvoltage generated by the 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. The deceleration time is only to control the charging speed of the load to the inverter, so that the 20% regenerative braking capability of the inverter itself can be rationally utilized. As for the load that causes the motor to regenerate due to the action of external force (including the potential discharge), since it is normally in the braking state, the regenerative energy is too high to be consumed by the inverter itself, so it is impossible to use DC braking or The method of extending the deceleration time.

Compared with DC braking, regenerative braking has higher braking torque, and the braking torque can be related to the braking torque required by the load (ie, the level of regenerative energy). Automatic control. Regenerative braking is therefore best suited to provide braking torque to the load during normal operation.

Third, the method of regenerative braking

1. Energy consumption type: This method is to connect a braking resistor in parallel with the DC link of the inverter 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 supplied to the resistor to be consumed as heat energy, thereby preventing the DC voltage from rising. Since the regenerative energy is not utilized, it is energy-consuming. The same energy consumption type, it differs from DC braking in that it consumes energy on the braking resistor outside the motor, and the motor does not overheat, so it can work more frequently.

2. Parallel DC bus absorption type: suitable for multi-motor transmission systems (such as drafting machines). In this system, each motor needs one inverter, and multiple inverters share one grid-side converter, all inverters. The department is connected to a common DC bus. In this system, one or several motors are normally working in the braking state, and the motor in the braking state is dragged by other motors to generate regenerative energy, which is then absorbed by the motor in the electric state through the parallel DC bus. In the case of incomplete absorption, it is consumed by the shared braking resistor. The regenerative energy here is partially absorbed but not fed back into the grid.

3. Energy feedback type: The energy feedback type inverter side converter is reversible. When there is regenerative energy, the inverter can return the regenerative energy to the grid, so that the regenerative energy is fully utilized. However, this method requires high stability of the power supply, and once a sudden power failure occurs, inverter subversion will occur.

Fourth, the application of regenerative braking

A chemical fiber filament drawing production line consists of three drafting machines, each driven by three motors. One roller motor power 22KW, 4 pole, worm reducer, speed ratio is 25:1; two-roll motor power 37KW, 4 pole, worm reducer, speed ratio 16:1; three-roll motor power 45KW, cylindrical gear reduction The speed ratio is 6:1. The motors are driven by Huawei TD2000-22KW three-inch IHF37K and 45K inverters. The three inverters are proportionally controlled according to the draft ratio and speed ratio. Its working process is as follows: the tow is wound on one roll, two rolls, three rolls, and the tow is drafted by the frequency converter controlling the different speeds between the three rolls.

When driving, the drafting ratio is small, the total length of the tow is low, and the system is driving normally. After being put into production for a period of time, due to the process adjustment, the draft ratio and the total denier of the tow are increased. (The draft ratio is determined by the process. Generally speaking, the thickness and the number of the tow are higher. The thicker the tow, the larger the drafting ratio or the total denier, the greater the drag force of the three rolls to the two rolls and one roll.) At this time, a problem arises. The driving time is not long, and one-roller frequency converter frequently displays SC (overvoltage prevention), and the two-roller inverter occasionally has this phenomenon. The time is a little longer, one roll inverter protection stops, and the fault shows E006 (overvoltage). Through careful analysis of the fault phenomenon, the following conclusions are drawn: since the drafting ratio between one roller and two rollers accounts for 70% of the total drafting ratio, and the power of the two-roller and the three-roller motor is greater than one roller, The roller motor actually works in the power generation state, and it must generate enough braking torque to ensure the drafting multiple. The two rollers work between the electric and braking states depending on the process conditions, and only three rollers are electrically operated.

That is to say, if a roller inverter cannot process the regenerative energy generated by the motor, it will not generate enough braking torque, and then it will be “drag” by the two rollers. The main reason for being “dragned” is the function of the inverter 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 speed and try to reduce the regenerative voltage. However, because the regenerative energy is too high, the overvoltage cannot be prevented. Therefore, the focus of the problem is that it is necessary to ensure that one roller and two roller motors have sufficient braking torque. Adding one roll, two roll motor and inverter capacity can achieve this purpose, but this is obviously uneconomical. The overvoltage generated by one roller and two rollers is disposed in time, and the DC voltage of the inverter is not increased, and sufficient braking torque can be provided.

Since this is not taken into account in the design of the system, it is not possible to use a common DC bus absorption type or energy feedback type. It has been carefully demonstrated that only one set of external brake units can be added to each of the one-roll and two-roll inverters. Two sets of Huawei TDB-4C01-0300 brake components were selected for calculation. After driving, the braking force of the two groups of braking units, especially the one-roll braking resistor, is very high, indicating that our analysis is correct. The entire system has been in operation for nearly a year and no overvoltage has ever occurred.