Inverter overvoltage fault causes and countermeasures - Database & Sql Blog Articles

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1 Introduction

The overvoltage fault protection of the inverter is the protection measure taken after the intermediate DC voltage of the inverter reaches the dangerous level. This is a major defect in the design of the inverter. There are many reasons for this fault in the actual operation of the inverter. There are also many. In dealing with such faults, it is necessary to analyze the cause of the fault and take corresponding measures to deal with it.
2, the inverter overvoltage hazard Inverter overvoltage mainly refers to the intermediate DC loop overvoltage, the main DC loop overvoltage main hazard lies in:
(1) Causes motor magnetic circuit saturation. For the motor, the excessive voltage of the main motor will inevitably increase the magnetic flux of the motor core, which may cause the magnetic circuit to be saturated, the excitation current is too large, and the temperature rise of the motor is too high from the surface;
(2) Damage to motor insulation. After the intermediate DC link voltage rises, the pulse amplitude of the inverter output voltage is too large, which has a great influence on the insulation life of the motor;
(3) It has a direct impact on the life of the intermediate DC link filter capacitor. If it is serious, it will cause the capacitor to burst. Therefore, the inverter manufacturer generally limits the intermediate DC link overvoltage value to about DC800V. Once the voltage exceeds the limit value, the inverter will trip protection according to the specified requirements.
3, the cause of the inverter overvoltage
3.1, the cause of overvoltage Generally can cause the overvoltage of the intermediate DC loop mainly from the following two aspects:
(1) Overvoltage from the input side of the power supply

Under normal circumstances, the power supply voltage is 380V, the allowable error is -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 the peak voltage is only 636V, not very high, the general supply voltage will not cause the inverter to trip due to overvoltage. The overvoltage on the input side of the power supply mainly refers to the overvoltage of the power supply side, such as the overvoltage caused by lightning, the overvoltage generated when the compensation capacitor is turned on or off, etc. The main features are the voltage change rate dv/dt and the amplitude. They are all very big.
(2) The overvoltage from the load side mainly refers to the state in which the motor is in the regenerative power generation state for some reason, that is, the motor is in the state where the actual speed is higher than the synchronous speed determined by the frequency conversion frequency, and the mechanical energy stored in the load transmission system is The motor is converted into electrical energy and fed back to the intermediate DC link of the frequency converter via the six freewheeling diodes of the inverter. At this time, the inverter is in a rectified state. If the inverter does not take measures to consume these energies, these energies will cause the voltage of the capacitor of the intermediate DC loop to rise. When the limit is reached, the line trips.
3.2. The situation that may cause overvoltage from the load side of the inverter and the main reason and the main cause of overvoltage from the load side of the inverter are as follows:
(1) The inverter deceleration time parameter setting is relatively small and the inverter deceleration overvoltage self-processing function is not used.
When the inverter drags the large inertia load, its deceleration time is set relatively small. During the deceleration process, the inverter output frequency decreases faster, while the load inertia is larger. The speed of the motor is higher than the speed corresponding to the frequency of the inverter output. The motor is in the power generation state, and the inverter has no energy processing unit or its function is limited. As a result, the voltage of the intermediate DC link of the inverter rises and exceeds the protection value. An overvoltage trip fault will occur.
In order to avoid tripping, most inverters have specially set the self-processing function of deceleration and overvoltage. If the DC voltage exceeds the set voltage upper limit during deceleration, the output frequency of the inverter will not drop and the deceleration will be suspended. Continue to decelerate after the DC voltage drops below the set value. If the deceleration time setting is not appropriate and there is no self-processing function that uses the deceleration overvoltage, such a fault may occur.
(2) The process requirements are decelerated to the specified frequency within a limited time or the process is stopped. The deceleration time of the load is limited. Reasonable setting of relevant parameters can not slow down the fault. The system does not take measures to deal with excess energy, which will inevitably lead to Overvoltage trip failure.
(3) When the position of the motor is driven down, the motor will be in the regenerative braking state, the potential energy load will drop too fast, and the excess feedback energy will exceed the capacity of the intermediate DC loop and its energy processing unit. Overvoltage fault It will happen too.
(4) Inverter load sudden drop The load sudden drop of the inverter will increase the speed of the load obviously, so that the load motor enters the regenerative power generation state, and the energy is fed back from the load side to the intermediate DC loop of the inverter. The concentrated feedback of energy in a short time may be The overvoltage fault is caused by the tolerance of the intermediate DC loop and its energy processing unit.
(5) This fault may also occur when multiple motors are dragging the same load, mainly due to no load distribution. Taking two motors to drag 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 low speed is in the power generation state, causing overvoltage fault. . Load distribution control is required for processing. Can adjust the inverter output characteristic curve softly
(6) The capacity of the DC link capacitor in the middle of the inverter decreases. After many years of operation, the capacity of the intermediate DC link capacitor will decrease. The adjustment of the DC voltage of the intermediate DC loop will be weakened. The process conditions and setting parameters have not changed. Underneath, the probability of over-voltage tripping of the inverter will increase. At this time, it is necessary to check the capacity drop of the intermediate DC-loop capacitor.

4. Overvoltage fault handling countermeasures For the treatment of overvoltage faults, the key is how to deal with the excess energy of the intermediate DC loop in time; the second is how to avoid or reduce the excess energy to the intermediate DC loop, so that the degree of overvoltage is limited. Within the limits. Here are the main countermeasures:
(1) Increase the absorption device on the input side of the power supply to reduce the overvoltage factor

In the case where there is an overvoltage at the input side of the power supply, an overvoltage caused by lightning, or an overvoltage generated when the compensation capacitor is turned on or off, a method of parallel surge absorber or series reactor on the input side may be used. Solve it.
(2) Finding solutions from the parameters that have been set in the inverter There are two main points in the parameters that can be set in the inverter:
★ Deceleration time parameter and inverter deceleration overvoltage self-processing function. If the load deceleration time is not limited in the process flow, the setting of the inverter deceleration time parameter should not be too short, so that the load kinetic energy is released too fast. The setting of this parameter should not be limited to the intermediate circuit overvoltage, especially Pay attention to the setting of this parameter when the load inertia is large. If the process has a limitation on the load deceleration time and the inverter has an overvoltage trip during the limited time, set the inverter stall auto-tuning function or set the frequency value that can be reduced when the inverter is not under pressure. Slow down to zero after the delay, slowing down the rate of frequency reduction.
★ is the intermediate DC loop overvoltage multiple.
(3) Analyze the process flow and find solutions in the process flow. For example, the aluminum hydroxide fishing float material bag filter system of our factory has 8 sets of 50kW feed pumps and 4 sets of 30kW return pumps adopt Fuji inverter speed control. 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 difference. This is achieved by flowing the slurry back. In the energy storage stage, the feed pump is closed to the flow parameter. In order to maintain a constant flow rate, the frequency of the frequency converter is always increasing. When the return phase is reached, the feed valve is suddenly closed, the load of the feed pump inverter 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, as long as the pressure required to remove the filter cake is formed in the bag filter, it is not necessary to form an excessively high pressure, and the inverter is operated at an excessively high frequency range. In the energy storage phase, the internal pressure value of the bag filter is introduced to reach the required pressure, that is, the stop frequency rises. Alternatively, the frequency rise can be stopped during the entire phase of energy storage, so that the feedback of the load side energy to the intermediate DC loop during the reflow phase can be greatly reduced. This can be done in the dcs distributed control system.
For example, when the reflux pump in the bag filter system is backwashed by 2 to 3 bag filters to the filter cloth, the material is discharged cyclically, the time is short, the flow rate is large, and the air is mixed in the slurry, causing the return pump to idling and the load to be reduced. The motor is in the regenerative braking condition, which causes the intermediate DC circuit of the inverter to overvoltage and the inverter protection trip. For this fault, it is possible to start from the process, and add a buffer tank at the return outlet of each bag filter to the return tank. Change the sudden change of the flow rate, reduce the influence of the flow change on the inverter, and solve the overvoltage problem.
(4) The method of increasing the bleeder resistance is generally less than 7.5 kW. The internal intermediate DC circuit is equipped with a control unit and a bleeder resistor. The inverter above 7.5 kW needs to be added with the control unit and bleed according to the actual situation. The resistor provides a channel for the excess energy release of the intermediate DC link and is a common method of venting energy. The shortcoming is that the energy consumption is high, and frequent switching or long-term operation may occur, resulting in an increase in resistance temperature and equipment damage.
(5) Method of adding inverter circuit on the input side The best way to handle the energy of the intermediate DC link of the inverter is to add an inverter circuit on the input side to return excess energy to the grid. However, inverter bridges are expensive and technically complex, and are not economical methods. This limits its application in practice and is only used in higher-level situations.
(6) The method of adding appropriate capacitance to the intermediate DC circuit plays an important role in the stability of the voltage and the ability of the circuit to withstand overvoltage. Appropriately increasing the capacitance of the circuit or replacing the capacitor with too long running time and decreasing capacity is an effective method to solve the overvoltage of the inverter. It also includes a method of selecting a larger capacity inverter during the design phase, in order to increase the capacity of the inverter in exchange for an increase in overvoltage capability.
(7) Appropriately reduce the power frequency supply voltage when conditions permit. Currently, the power supply side of the inverter generally adopts an uncontrollable rectifier bridge. The power supply voltage is high, and the intermediate DC link voltage is also high. When the power supply voltage is 380V, 400V, 450V, the DC circuit The voltages are 537V, 565V, 636V. Some inverters are very close to the transformer, and the input voltage of the inverter is up to 400V. It has a great influence on the overvoltage capability of the intermediate DC circuit of the inverter. In this case, if the condition allows, the tap changer of the transformer can be placed at low voltage. The file, by appropriately reducing the power supply voltage, achieves the purpose of relatively improving the overvoltage capability of the inverter.
(8) Method of sharing DC bus with multiple inverters At least two inverters running at the same time share the DC bus, which can solve the problem of overvoltage in the intermediate DC link of the inverter, because any inverter is taken from the DC bus. The current is generally greater than the excess current fed from the outside at the same time, so that the voltage of the shared DC bus can be substantially maintained. The biggest problem with the use of a shared DC bus is the problem of shared DC bus protection, which should be taken into account when using the shared DC bus to solve the overvoltage problem.
(9) Solving the overvoltage problem of the inverter through the function advantages of the control system In many process flows, the deceleration of the inverter and the sudden drop of the load are governed by the control system, and some functions of the control system can be utilized in the deceleration of the inverter. Control the load before the sudden drop, reducing excessive energy feeding into the intermediate DC loop of the inverter. For regular deceleration overvoltage faults, the uncontrolled rectifier bridge on the input side of the inverter can be replaced by a semi-controllable or fully controlled rectifier bridge. Before the deceleration, the intermediate DC voltage is controlled to a lower allowable value, which is relatively larger. The DC loop is capable of withstanding the ability to feed energy to avoid overvoltage faults. For the regular load dumping overvoltage fault, the control function of the DCS distributed system of the control system such as FOXBORO can be utilized to properly increase the frequency of the inverter before the load dips, reducing excessive energy feeding on the load side. DC loop to reduce the overvoltage fault caused by it.
5. Conclusion The intermediate DC overvoltage fault of the inverter is a weak point of the inverter. The key is to distinguish the reason, combined with the parameters of the inverter itself, the status of the control system and the process flow, etc., in order to formulate corresponding countermeasures, as long as it is taken seriously, Voltage failure is not difficult to solve.