Abstract Overvoltage fault protection of frequency converter is a protective measure taken when the intermediate DC voltage of the frequency converter reaches a dangerous level. This is a major defect in the design of frequency converter. There are many reasons for this fault in the actual operation of frequency converter, and there are also many measures that can be taken. When dealing with this type of fault, it is necessary to analyze the cause of the fault clearly and take corresponding measures to deal with it. Keywords Frequency converter Motor Rectifier Frequency converter overvoltage mainly refers to the overvoltage of its intermediate DC circuit. The main hazards of intermediate DC circuit overvoltage are: (1) It causes the magnetic circuit saturation of the motor. For the motor, the voltage is too high, which will inevitably increase the magnetic flux of the motor core, which may lead to magnetic circuit saturation, excessive excitation current, and thus excessive motor temperature rise; (2) It damages the motor insulation. After the intermediate DC circuit voltage rises, the pulse amplitude of the frequency converter output voltage is too large, which has a great impact on the insulation life of the motor; (3) It has a direct impact on the life of the intermediate DC circuit filter capacitor, which may cause the capacitor to burst in severe cases. Therefore, inverter manufacturers generally limit the overvoltage value of the intermediate DC circuit to about DC 800V. Once the voltage exceeds the limit, the inverter will trip according to the limit requirements. I. Causes of inverter overvoltage 1. Causes of overvoltage The causes of overvoltage in the intermediate DC circuit are mainly from the following two aspects: (1) Overvoltage from the power input side Under normal circumstances, the power supply voltage is 380V, with an allowable error of -5% to +10%. After three-phase bridge full-wave rectification, the peak value of the intermediate DC is 591V. In some cases, the power line voltage reaches 450V, and its peak voltage is only 636V, which is not very high. Generally, the power supply voltage will not cause the inverter to trip due to overvoltage. Overvoltage on the power input side mainly refers to the impulse overvoltage on the power supply side, such as overvoltage caused by lightning, overvoltage formed when the compensation capacitor is closed or opened, etc. The main characteristics are that the voltage change rate dv/dt and the amplitude are very large. (2) Overvoltage from the load side mainly refers to the situation where the motor is in regenerative power generation state due to 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. The mechanical energy stored in the load's transmission system is converted into electrical energy by the motor and fed back to the intermediate DC circuit of the frequency converter through the six freewheeling diodes of the inverter. At this time, the inverter is in rectification state. If the frequency converter does not take measures to consume these energies, these energies will cause the voltage of the capacitor in the intermediate DC circuit to rise. When the limit is reached, the circuit breaker will trip. 2. Situations and main causes of overvoltage from the load side of the frequency converter The situations and main causes of overvoltage from the load side of the frequency converter are as follows: (1) The frequency converter deceleration time parameter is set relatively small and the frequency converter deceleration overvoltage self-processing function is not used. When the frequency converter drives a large inertia load, its deceleration time is set relatively short. During the deceleration process, the frequency output of the frequency converter drops relatively quickly, while the load inertia is relatively large and the deceleration due to its own resistance is relatively slow. This causes the speed of the motor driven by the load to be higher than the speed corresponding to the frequency output of the frequency converter. The motor is in the generator state, and the frequency converter has no energy processing unit or its function is limited. As a result, the voltage of the DC circuit in the middle of the frequency converter rises and exceeds the protection value, which will cause an overvoltage trip fault. Most frequency converters have a self-processing function for deceleration overvoltage in order to avoid tripping. If the DC voltage exceeds the set voltage limit during deceleration, the frequency output of the frequency converter will no longer decrease and the deceleration will be temporarily suspended. 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 for deceleration overvoltage is not used, such a fault may occur. (2) The process requires deceleration to the specified frequency or stop operation within a limited time. The process flow limits the deceleration time of the load. Even if the relevant parameters are set reasonably, this fault cannot be mitigated. The system has not taken measures to process the excess energy, which will inevitably lead to an overvoltage trip fault. (3) When the potential energy load driven by the motor is released, the motor will be in a regenerative braking state. If the potential energy load drops too quickly, the excessive feedback energy will exceed the capacity of the intermediate DC circuit and its energy processing unit, and an overvoltage fault will occur. (4) Sudden load drop of the inverter will cause the speed of the load to increase significantly, causing the load motor to enter a regenerative braking state and feed energy back to the intermediate DC circuit of the inverter from the load side. The concentrated feedback of energy in a short period of time may cause an overvoltage fault due to the capacity of the intermediate DC circuit and its energy processing unit. (5) This fault may also occur when multiple motors drive the same load, mainly due to the lack of load distribution. Taking two motors driving one load as an example, when the actual speed of one motor is greater than the synchronous speed of the other motor, the motor with the higher speed is equivalent to the prime mover, and the motor with the lower speed is in a generating state, causing an overvoltage fault. Load distribution control is required to handle this. The output characteristic curve of the inverter can be adjusted to be softer (6) The capacitance of the intermediate DC circuit of the inverter will decrease. After the inverter has been running for many years, the capacitance of the intermediate DC circuit will inevitably decrease. The regulation of DC voltage by the intermediate DC circuit will be weakened. Under the condition that the process conditions and setting parameters have not changed, the probability of inverter overvoltage tripping will increase. At this time, it is necessary to check the capacitance of the intermediate DC circuit. II. Countermeasures for handling overvoltage faults The key to handling overvoltage faults is: First, how to deal with excess energy in the intermediate DC circuit in a timely manner; Second, how to avoid or reduce the feeding of excess energy to the intermediate DC circuit so that the degree of overvoltage is limited to the allowable limit. The main countermeasures are as follows: (1) Add an absorption device on the power input side to reduce overvoltage factors If there is a possibility of impulse overvoltage, overvoltage caused by lightning, or overvoltage formed when the compensation capacitor is closed or opened on the power input side, the surge absorption device or series reactor can be connected in parallel on the input side to solve the problem. (2) Find solutions from the inverter's set parameters. There are two main settable parameters for the inverter: ★ Deceleration time parameter and inverter deceleration overvoltage self-handling function. If the load deceleration time is not limited in the process flow, the inverter deceleration time parameter should not be set too short, causing the load kinetic energy to be released too quickly. This parameter should be set to avoid causing overvoltage in the intermediate circuit, paying particular attention to the setting when the load inertia is large. If the process flow limits the load deceleration time, and the inverter trips due to overvoltage within the limited time, the inverter stall self-tuning function should be set, or the inverter should first be set to the frequency value it can reduce to under overvoltage conditions, then slow down to zero, thus reducing the frequency reduction rate. ★ This is the overvoltage multiple of the intermediate DC circuit. (3) Analyze the process flow and find solutions within it. For example, in our aluminum hydroxide floating matter removal project, the bag filter system has eight 50kW feed pumps and four 30kW return pumps, all speed-regulated by Fuji inverters. During the bag filter's operation, the filter cake adsorbed on the filter cloth needs to be removed every 20-30 minutes. The method for 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, creating a higher pressure difference that causes the slurry to flow back. In the energy storage stage, the feed pump operates in a closed loop based on the flow parameters. To maintain a constant flow rate, the inverter frequency keeps increasing. During the return stage, the feed valve suddenly closes, causing a sudden drop in the load on the feed pump inverter, and the motor enters regenerative power generation mode, leading to an overvoltage fault. We analyzed that in the later stages of the energy storage stage, it is sufficient to create pressure within the bag filter that meets the requirements for removing the filter cake. There is no need to create excessively high pressure, causing the inverter to operate at excessively high frequencies. To address this fault, the internal pressure value of the bag filter can be introduced during the energy storage stage, and the frequency increase will stop once the required pressure is reached. Alternatively, the frequency increase can be stopped throughout the energy storage phase, thus significantly reducing the feedback of energy from the load side to the intermediate DC circuit during the return flow phase. This is achievable in a DCS distributed control system. For example, in a bag filter system, when 2-3 bag filters are backwashing the filter cloth, the return pump may run dry due to short-duration, high-flow-rate unloading of the slurry containing air. This sudden load reduction puts the motor in regenerative braking mode, leading to overvoltage in the intermediate DC circuit of the frequency converter and causing the frequency converter protection to trip. To address this fault, a process-related solution can be implemented: adding a buffer tank between the return outlet and the return trough of each bag filter to alter the sudden change in return flow rate, reducing the impact of flow rate variations on the frequency converter and resolving the overvoltage problem. (4) Using the method of adding a bleed resistor: Generally, inverters with a power supply of less than 7.5kW are equipped with a control unit and a bleed resistor in the internal intermediate DC circuit when they leave the factory. Inverters with a power supply of more than 7.5kW need to add a control unit and a bleed resistor according to the actual situation. This provides a channel for releasing excess energy in the intermediate DC circuit and is a commonly used method for releasing energy. Its disadvantage is that the energy consumption is high and frequent switching or long-term operation may occur, which will cause the resistor temperature to rise and the equipment to be damaged. (5) Adding an inverter circuit on the input side: The best way to deal with the energy in the intermediate DC circuit of the inverter is to add an inverter circuit on the input side, which can feed the excess energy back to the grid. However, the inverter bridge is expensive and the technical requirements are complex, so it is not a more economical method. This limits its application in practice and it is only used in higher-level situations. (6) Adding an appropriate capacitor to the intermediate DC circuit: The capacitor in the intermediate DC circuit plays a very important role in stabilizing the voltage and improving the circuit's ability to withstand overvoltage. Appropriately increasing the capacitance of the circuit or replacing the capacitor that has been running for too long and whose capacitance has decreased in time is an effective way to solve the overvoltage problem of the inverter. This also includes the method of selecting a larger capacity inverter during the design phase, which is to increase the inverter capacity in exchange for improved overvoltage capability. (7) Appropriately reduce the power frequency voltage when conditions permit. Currently, the power supply side of the inverter generally uses an uncontrolled rectifier bridge. The power supply voltage is high, and the intermediate DC circuit voltage is also high. When the power supply voltage is 380V, 400V, and 450V, the DC circuit voltage is 537V, 565V, and 636V, respectively. Some inverters are very close to the transformer, and the inverter input voltage is as high as 400V or more, which has a great impact on the overvoltage capability of the inverter's intermediate DC circuit. In this case, if conditions permit, the transformer tap changer can be placed in the low voltage position, and the overvoltage capability of the inverter can be relatively improved by appropriately reducing the power supply voltage. (8) Method of sharing a DC bus for multiple frequency converters: Sharing a DC bus for at least two frequency converters operating simultaneously can effectively solve the problem of overvoltage in the intermediate DC circuit of the frequency converter. This is because the current drawn from the DC bus by any frequency converter is generally greater than the excess current fed in from the outside at the same time, thus maintaining the voltage of the shared DC bus. The biggest problem with using a shared DC bus is the protection of the shared DC bus. This should be noted when using a shared DC bus to solve the overvoltage problem. (9) Solving the overvoltage problem of frequency converters through the functional advantages of the control system: In many process flows, the deceleration of the frequency converter and the sudden drop of the load are controlled by the control system. Some functions of the control system can be used to control the frequency converter before the deceleration and the sudden drop of the load, reducing excessive energy fed into the intermediate DC circuit of the frequency converter. For example, for regular deceleration overvoltage faults, the uncontrolled rectifier bridge on the input side of the frequency converter can be replaced with a semi-controlled or fully controlled rectifier bridge. Before deceleration, the intermediate DC voltage can be controlled at a lower allowable value, which relatively increases the ability of the intermediate DC circuit to withstand the fed-in energy and avoids overvoltage faults. For recurring load drop overvoltage faults, the control functions of a distributed control system like FOXBORO's DCS can be utilized to appropriately increase the inverter frequency before the load drop, reducing excessive energy feed into the intermediate DC circuit from the load side and thus mitigating the resulting overvoltage fault. III. Conclusion Intermediate DC overvoltage faults in inverters are a weakness. The key is to identify the cause and consider the inverter's parameters, control system status, and process flow to develop appropriate countermeasures. With careful attention, this overvoltage fault is not difficult to resolve.