Cause Analysis and Countermeasures of VFD (Variable-frequency Drive) Overvoltage Fault

1 Introduction VFD (Variable-frequency Drive) overvoltage fault protection is a protective measure taken after the intermediate DC voltage of VFD (Variable-frequency Drive) reaches a dangerous level. This is a major flaw in VFD (Variable-frequency Drive) design. In the actual operation of VFD (Variable-frequency Drive), there are many reasons for this fault, and there are many measures that can be taken. When dealing with this type of fault, it is necessary to analyze the cause of the fault and take corresponding measures to deal with it. 2 VFD (Variable-frequency Drive) overvoltage hazards VFD (Variable-frequency Drive) overvoltage mainly refers to the overvoltage of the intermediate DC circuit, and the main hazards of the intermediate DC circuit overvoltage are: 1. It causes the magnetic circuit of the motor to saturate. For the motor, too high voltage will inevitably increase the magnetic flux of the motor iron core, which may cause the saturation of the magnetic circuit, and the excitation current is too large, which will cause the temperature rise of the motor to be too high and damage the insulation of the motor. After the voltage of the intermediate DC circuit rises, the pulse amplitude of the VFD (Variable-frequency Drive) output voltage is too large, which has a great impact on the life of the motor insulation. 3 It has a direct impact on the life of the filter capacitor in the intermediate DC circuit. . Therefore, VFD (Variable-frequency Drive) manufacturers generally limit the overvoltage value of the intermediate DC circuit to about DC800V. Once the voltage exceeds the limit value, VFD (Variable-frequency Drive) will trip according to the limit requirements. 3 Causes of VFD (Variable-frequency Drive) overvoltage 3.1 Causes of overvoltage Generally, the causes of overvoltage in the intermediate DC circuit mainly come from the following two aspects: 1 Overvoltage from the input side of the power supply Under normal conditions, the power supply voltage is 380V, the allowable error is -5~10, the peak value of the intermediate DC after three-phase bridge full-wave rectification is 591V, in some cases the power line voltage reaches 450V, and its peak voltage is only 636V, which is not very high. The voltage will not trip the VFD (Variable-frequency Drive) due to overvoltage. The overvoltage on the input side of the power supply mainly refers to the impact overvoltage on the power supply side, such as the overvoltage caused by lightning, the overvoltage formed by the compensation capacitor when closing or disconnecting, etc. The main characteristics are the voltage change rate dv/dt and amplitude Both are big. 2 The overvoltage from the load side mainly refers to when the motor is in the state of regenerative power generation for some reason, that is, the motor is in a state where the actual speed is higher than the synchronous speed determined by the frequency conversion frequency, and the mechanical energy stored in the drive system of the load is converted by the motor The electrical energy is fed back to the intermediate DC circuit of the VFD (Variable-frequency Drive) through the 6 freewheeling diodes of the inverter. At this time, the inverter is in the rectification state. If measures to consume the energy are not taken in the VFD (Variable-frequency Drive), the energy will cause the voltage of the capacitor in the intermediate DC link to rise. When the limit value is reached, it will trip. 3.2 Possible causes of overvoltage from the VFD (Variable-frequency Drive) load side and the main reasons The possible causes of overvoltage from the VFD (Variable-frequency Drive) load side and the main reasons are as follows: 1 VFD (Variable-frequency Drive) deceleration The time parameter setting is relatively small and the VFD (Variable-frequency Drive) deceleration overvoltage self-processing function is not used. When the VFD (Variable-frequency Drive) drags a large inertial load, its deceleration time is set relatively small. During the deceleration process, the VFD (Variable-frequency Drive) output frequency drops faster, and the load inertia is relatively large. The deceleration by its own resistance is relatively slow, so that the speed of the load-driven motor is higher than the corresponding speed of the output frequency of the VFD (Variable-frequency Drive), and the motor is in the state of power generation, while the VFD (Variable-frequency Drive) has no energy processing unit Or its effect is limited, thus causing the VFD (Variable-frequency Drive) intermediate DC circuit voltage to rise, exceeding the protection value, and an overvoltage trip fault will occur. In order to avoid tripping, most VFD (Variable-frequency Drive) specially set the deceleration overvoltage self-processing function. If the DC voltage exceeds the set voltage upper limit during deceleration, the VFD (Variable-frequency Drive) The output frequency will no longer drop, the deceleration will be suspended, and the deceleration will continue after the DC voltage drops below the set value. If the deceleration time is not set properly, and the self-processing function of deceleration overvoltage is not used, this kind of fault may occur. 2 The process requires decelerating to the specified frequency within a limited time or stopping the process to limit the deceleration time of the load. Reasonable setting of relevant parameters cannot slow down this fault, and the system has not taken measures to deal with excess energy, which will inevitably lead to overvoltage Trip failure. 3 When the potential energy load driven by the motor is lowered, the motor will be in the state of regenerative braking. The potential energy load drops too fast, and the excessive feedback energy exceeds the bearing capacity of the intermediate DC circuit and its energy processing unit, and overvoltage faults will also occur . 4 VFD (Variable-frequency Drive) load dump VFD (Variable-frequency Drive) load dump will cause the speed of the load to increase significantly, making the load motor enter the regenerative power generation state, and DC from the load side to the VFD (Variable-frequency Drive) middle Loop feedback energy, concentrated energy feedback in a short period of time, may cause overvoltage faults due to the bearing capacity of the intermediate DC loop and its energy processing unit. 5 This fault may also occur when multiple motors drag the same load, mainly due to the lack of load distribution. Taking two motors driving a load as an example, when the actual speed of one motor is greater than the synchronous speed of the other motor, the motor with high speed is equivalent to the prime mover, and the motor with low speed is in the state of power generation, causing an overvoltage fault . Load distribution control is required during processing. The VFD (Variable-frequency Drive) output characteristic curve can be adjusted softer. 6 VFD (Variable-frequency Drive) intermediate DC circuit capacitor capacity drops. After VFD (Variable-frequency Drive) has been in operation for many years, the intermediate DC circuit capacitor capacity will decrease. To avoid, the adjustment degree of the intermediate DC circuit to the DC voltage is weakened. When the process conditions and setting parameters have not changed, the probability of VFD (Variable-frequency Drive) overvoltage tripping will increase. At this time, it is necessary to adjust the intermediate DC circuit capacitor. Check for capacity drop. 4 Overvoltage Fault Handling Countermeasures For the handling of overvoltage faults, the key is how to deal with the excess energy in the intermediate DC circuit in a timely manner; the second is how to avoid or reduce the excess energy fed to the intermediate DC circuit, so that the degree of overvoltage is limited to the allowable limit within. The following are the main countermeasures: 1. Increase the absorbing device on the power input side to reduce the overvoltage factor. For the power input side, there may be impulse overvoltage, overvoltage caused by lightning, and overvoltage formed by the compensation capacitor when closing or disconnecting. In some cases, it can be solved by connecting surge absorbing devices in parallel or series reactors on the input side. 2 Find a solution from the set parameters of VFD (Variable-frequency Drive) There are two main points in the settable parameters of VFD (Variable-frequency Drive): l deceleration time parameter and VFD (Variable-frequency Drive) deceleration Overvoltage self-processing function. If the load deceleration time is not limited in the process flow, the setting of the VFD (Variable-frequency Drive) deceleration time parameter should not be too short, so that the kinetic energy of the load will be released too quickly. The voltage is limited, and special attention should be paid to the setting of this parameter when the load inertia is large. If the technological process has a limit on the load deceleration time, and the VFD (Variable-frequency Drive) has an overvoltage trip phenomenon within the limited time, it is necessary to set the VFD (Variable-frequency Drive) stall auto-tuning function or first set the VFD (Variable-frequency Drive) -frequency Drive) can be reduced to the frequency value under the condition of no pressure, and then decelerate to zero after a pause, slowing down the speed of frequency reduction. l is the overvoltage multiple of the intermediate DC circuit. 3 Analyze the process flow and find solutions in the process flow. For example, the bag filter system of our factory's aluminum hydroxide fishing project has 8 sets of 50kW feed pumps and 4 sets of 30kW return pumps. In the working process of the bag filter, the filter cake adsorbed on the filter cloth needs to be removed every 20 to 30 minutes. The method of removing the filter cake is to make the pressure on the discharge side of the filter cloth higher than the pressure on the feed side to form a higher pressure. The pressure difference makes the slurry flow back. In the energy storage stage, the feed pump is closed-loop to the flow parameters. In order to maintain a constant flow, the frequency of the VFD (Variable-frequency Drive) has been increasing. ) The load drops suddenly, and the motor enters the regenerative power generation state, causing an overvoltage fault. We analyze that in the later stage of the energy storage stage, it is only necessary to form the pressure required to remove the filter cake in the bag filter, and there is no need to form an excessively high pressure, so that the VFD (Variable-frequency Drive) operates at an excessively high frequency range. For this failure, the internal pressure value of the bag filter can be introduced during the energy storage stage, and the frequency will stop rising when the required pressure is reached. Or the increase in frequency can be stopped during the entire stage of energy storage, which can greatly reduce the feedback of energy from the load side to the intermediate DC circuit in the recirculation stage. This can be done in the DCS distributed control system. For example, when the return pump in the bag filter system backwashes the filter cloth with 2 to 3 bag filters, the cycle unloads, the time is short, the flow rate is large, and air is mixed in the slurry, causing the return pump to run idling and the load to drop suddenly. Make the motor in the regenerative braking condition, resulting in the overvoltage of the VFD (Variable-frequency Drive) intermediate DC circuit, and the VFD (Variable-frequency Drive) protection trip. For this fault, we can start from the process, and in each bag filter A buffer tank is added from the backflow outlet to the backflow tank to change the sudden change of the backflow flow, reduce the impact of flow changes on the VFD (Variable-frequency Drive), and solve the overvoltage problem. 4 Use the method of increasing the bleeder resistor. Generally, the VFD (Variable-frequency Drive) less than 7.5kW is equipped with a control unit and a bleeder resistor in the internal intermediate DC circuit when it leaves the factory. The actual situation is to add a control unit and a discharge resistor to provide a channel for the release of excess energy in the intermediate DC circuit, which is a commonly used method of discharge energy. Its disadvantage is high energy consumption, frequent switching or long-term operation