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Some Discussions on Overvoltage Obstacles of Inverter

Mar 31, 2021

Some Discussions on Overvoltage Obstacles of Inverter

Inverter over-voltage barrier protection is a safeguard method adopted after the central DC voltage of the inverter reaches a critical level. This is a major defect in the inverter plan. There are many reasons for this obstacle in the actual operation of the inverter, so it can be adopted. There are also many. When dealing with such obstacles, it is necessary to analyze and understand the reasons for the obstacles, and adopt corresponding methods to deal with them in a targeted manner.

2 The hazards of inverter overvoltage

    The overvoltage of the frequency converter refers to the voltage passing through the central DC circuit. The important hazards of the central DC circuit passing voltage are:

(1) Cause the motor magnetic circuit to saturate. For the motor, the main voltage is too high, which will definitely increase the magnetic flux of the motor core, which may cause the magnetic circuit to saturate, and the excitation current is too large, which will cause the motor temperature to rise too high;

(2) Infringe on the motor. After the central DC link voltage is raised, the pulse amplitude of the output voltage of the inverter is too large, which has a great impact on the end life of the motor;

(3) It has a direct impact on the life of the central DC loop filter capacitor, and it will cause the capacitor to burst when it is tight. Therefore, inverter manufacturers usually limit the central DC return voltage to around DC800V. Once the voltage exceeds the limit, the inverter will trip and protect according to the limit.

3 Causes of inverter overvoltage

3.1 Causes of overvoltage

    The main reason that can cause the central DC return voltage is mainly from the following two aspects:

(1) Overvoltage from the input side of the power supply

    The power supply voltage in the normal environment is 380V, and the allowed deviation is -5%~+10%. After three-phase bridge full-wave rectification, the peak value of the central DC is 591V. In some environments, the power line voltage reaches 450V, and its peak voltage is only 636V is not very high, and the normal power 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 on the power supply side, such as the overvoltage caused by lightning, the overvoltage formed when the compensation capacitor is switched on or off, etc. The important feature is the voltage change rate dv/dt and amplitude The values are great.

(2) Overvoltage from the load side

    Important means that when the motor is in a state of regenerative power generation, that is, the motor is in a state where the actual speed is higher than the synchronous speed determined by the variable repetition rate, the machine stored in the load transmission system can be converted into electrical energy by the motor. The 6 freewheeling diodes of the converter feed back into the central DC loop of the frequency converter. At this time, the inverter is in a rectifying state. If the inverter does not adopt a method to lose this energy, this energy will cause the voltage of the capacitor in the central DC circuit to rise. Tripping occurs when the limit is reached.

3.2 The environment and important reasons that may cause overvoltage from the load side of the inverter

    The environment and important reasons that may cause overvoltage from the load side of the inverter are as follows:

(1) The inverter acceleration time parameter setting is relatively small and the inverter acceleration overvoltage self-processing function is not used.

When the inverter is dragging a large inertia load, its acceleration time is set relatively small. During the acceleration process, the output frequency of the inverter drops faster, while the load inertia is relatively large, and the acceleration due to its own resistance is relatively slow, which makes the load drag The speed of the motor is higher than the speed corresponding to the frequency output by the frequency converter. The motor is in the state of generating electricity, and the frequency converter has no energy processing unit or its function is limited, which causes the central DC circuit voltage of the frequency converter to rise and exceed the guard value. There will be an overvoltage trip barrier.

    In order to prohibit tripping, most inverters are specially configured with acceleration overvoltage self-processing function. If the DC voltage exceeds the set voltage upper limit during the acceleration process, the output frequency of the inverter will no longer drop and the acceleration will be temporarily delayed. Continue to accelerate after the DC voltage drops below the set value. If the acceleration time is set appropriately and the self-processing effect of the acceleration overvoltage is not used, such obstacles may appear.

(2) The process requires acceleration to the rule frequency or interruption of operation within a limited time

The technological process limits the acceleration time of the load. Just setting the relevant parameters can't alleviate this obstacle. The system [industrial electrical appliances network-cnelc] system also did not adopt the method to deal with excess energy, which will definitely cause overvoltage trip obstacles.

(3) When the potential energy load driven by the motor is lowered

When the motor will be in the state of regenerative braking

The potential energy load falls too fast, and the excessive feedback energy exceeds the ability of the central DC circuit and its energy processing unit to suffer, and overvoltage obstacles will also occur.

(4) Inverter load dump

A sudden drop in the load of the inverter will obviously increase the speed of the load, causing the load motor to enter the state of regenerative power generation. Energy will be fed back to the central DC circuit of the inverter from the load side. The accumulation of energy in a short period of time will probably affect the central DC circuit and its energy. The ability of the processing unit to suffer an overvoltage barrier.

(5) This obstacle may also occur when multiple motors drive a single load, mainly because there is no load dispatch. Taking two motors to drive 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 generating electricity, causing overvoltage obstacles. . Load distribution control needs to be added during processing. The output characteristic arc of the inverter can be adjusted softer

(6) Capacitance drop of the central DC circuit of the inverter

After the inverter has been in operation for many years, the capacitance drop of the central DC circuit will not be prohibited. The regulation of the DC voltage by the central DC circuit will be weakened. Under the environment where the process status and setting parameters have not changed, the frequency of inverter over-voltage trip will increase. At this time, it is necessary to check the environment in which the capacity of the central DC circuit capacitor falls.

4 Overvoltage obstacle treatment countermeasures

    The key to dealing with overvoltage obstacles is how to deal with the excess energy of the central DC circuit regularly; the other is how to prohibit or reduce the delivery of excess energy to the central DC circuit so that the degree of overvoltage is limited to the permitted limit. The following are important countermeasures:

(1) Increase the suction device on the power input side to reduce the overvoltage factor

To deal with the environment where there is overvoltage on the input side of the power supply, overvoltage caused by lightning, and overvoltage generated by the compensation capacitor during closing or disconnection, a parallel surge absorption device or series reactor can be used on the input side Wait for the essentials to be handled.

(2) Look for the essentials from the set parameters of the inverter

There are two important points in the settable parameters of the inverter:

l The acceleration time parameter and the self-processing function of the inverter acceleration over-voltage. In the process flow, if the load acceleration time is not limited, the acceleration time parameter setting of the inverter should not be too short, so that the kinetic energy of the load is released too quickly. The parameter setting should be limited to the voltage that does not cause the central return path, especially Pay attention to the setting of this parameter when the load inertia is large. If the process has a limit on the load acceleration time, and the inverter has the appearance of an overvoltage trip within the limited time, it is necessary to set the inverter stall auto-tuning function or first set the frequency value that the inverter can be reduced to under the environment of no external pressure , Accelerate to zero after being suspended, and slow down the rate of frequency reduction.

l is the multiple of the central DC return voltage.

(3) Analyze the technological process and find the essentials in the technological process

    For example, the bag filter system of the aluminum hydroxide fishing project of our factory has 8 50kW feed pumps and 4 30kW reflux pumps using Fuji inverter speed control. In the bag filter labor process, it is necessary to adsorb every 20~30min. The filter cake on the filter cloth is removed. The key to 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 to make the slurry flow backwards. In the energy storage stage, the feed pump is closed in the flow parameter. In order to maintain a constant flow, the frequency of the inverter is always increasing. At the reflux stage, the feed valve suddenly closes, the load of the feed pump inverter drops suddenly, and the motor enters regenerative power generation. State, causing overvoltage barriers. 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, there is no need to form too high pressure, so that the inverter can run in a high frequency range. This obstacle can be dealt with. In the energy storage stage, the internal pressure value of the bag filter is introduced, and the increase in frequency is interrupted when the required pressure is reached. Or the frequency increase can be interrupted during the entire stage of energy storage, so that the energy feedback from the load side to the central DC circuit in the return phase can be greatly reduced. This can be done in the DCS distributed control system.