1 voltage fluctuations on the inverter
Voltage fluctuation refers to a series of changes or continuous changes in the voltage rms value. Mainly caused by the start and run of the motor with impact load, repeated short-time work load, large motor start, short circuit of power supply system and lightning strike of power supply line.
1.1 The effect of overvoltage on the inverter
The basic component circuit of the general-purpose inverter is two parts: a rectifier circuit and an inverter circuit. The rectifier circuit rectifies the power frequency alternating current into a direct current. The inverter circuit then inverts the direct current into alternating current with adjustable frequency and voltage. The frequency conversion speed regulating devices generally adopt the alternating current and alternating voltage mode. The overvoltage of the inverter generally refers to the overvoltage of the intermediate DC loop. The main hazards are as follows: First, the increase of the grid voltage will increase the magnetic flux of the motor core, which will easily cause saturation of the magnetic circuit, increase the excitation current, and cause the motor temperature. If the grid voltage rises, the pulse voltage of the output voltage of the inverter will be too large, which will have a great influence on the insulation life of the motor. The third is to filter the intermediate DC loop. Capacitor life has a large impact and can even cause capacitors to burst.
1.2 The effect of undervoltage on the inverter
The frequency converter has many power devices such as GTO, IGBT and IGCT. These power devices usually have a certain overload capability. When the grid undervoltage amplitude is small and the duration is short, it has little effect on the normal operation of the power device. When the grid voltage drop is too large and the duration is long, the switching power supply of the inverter cannot start, and the output of the control power supply stops or the output power decreases. It is easy to cause disorder of the inverter control system, and the power device cannot be turned off, causing damage to the inverter.
2 Measures to solve the inverter's ability to resist voltage fluctuation
2.1 Overvoltage fault resolution measures
To solve the influence of grid overvoltage on the inverter, the main idea is to effectively and timely treat the excess energy of the intermediate DC loop of the inverter, and at the same time prevent or reduce the excess energy to be fed to the intermediate DC loop of the inverter, so that the overvoltage generated by the grid is at a certain level. Within the allowed value.
1) Install a surge absorbing device or a series reactor as an absorbing device
The overvoltage of the power grid, the overvoltage caused by lightning, and the compensation capacitor are the main causes of overvoltage at the input end of the inverter when it is closed or disconnected. For such hidden dangers, surge absorbers or series reactors can be installed in the frequency converter for prevention. The surge absorbing device connects the semiconductor surge absorbing elements between the power lines connecting the U, V, and W phases of the inverter and the motor, and between the power lines and the ground. These semiconductor surge absorbing elements have a characteristic of flowing a current and clamping a voltage when a predetermined voltage or more is reached between the terminals. The series reactor can reduce the inrush current and inrush current of the capacitor bank, improve the short circuit impedance, reduce the short circuit capacity, reduce the short circuit current, reduce the overvoltage amplitude caused by operating the capacitor bank, and avoid the overvoltage protection of the power grid. Voltage effective method.
2) Adjust the parameters that the inverter has set
If the load deceleration time is not limited in the process flow, when setting the inverter deceleration time parameter, it is set on the condition that the intermediate circuit overvoltage is not limited. It should not be too short to avoid the load kinetic energy release too fast, especially the frequency conversion. For devices with large load inertia controlled by the device, the deceleration parameters should be increased appropriately; if the production process has certain requirements for the load deceleration time, in order to prevent the inverter from over-voltage tripping within a limited time, the inverter must be set to stall. The setting function can also set the frequency value of the inverter and reduce the speed of the controlled device by slowing down the frequency.
3) Increase the bleeder resistance
The bleeder resistor is a resistor connected in parallel across the energy storage component to provide a path for energy consuming energy to make the circuit safe. This resistor is called a bleeder resistor. It can be a diode, such as a diode in parallel with an inductor (relay line package). The current power is smaller. Generally, the internal intermediate DC loop is designed with the control unit and the bleeder resistor. The high-power inverter provides a channel for the DC loop to release the excess energy. It should be based on the process. It is necessary to increase the bleeder resistance to prevent overvoltage.
4) Increase the inverter circuit
The basic function of the inverter circuit is to convert the DC power output from the intermediate DC circuit into an AC power supply with adjustable frequency and voltage under the control of the control circuit. Add the inverter circuit on the input side of the inverter to make the inverter The excess energy of the intermediate DC link is fed back to the grid. However, the cost is high and the technical requirements are complicated.
5) Add a suitable capacitor to the intermediate DC loop
According to the capacity of the inverter and the estimation of the current and voltage of the DC link in the middle, a suitable capacitor can be added to the DC link in the middle. This capacitor can stabilize the loop voltage and improve the ability of the loop to withstand overvoltage. It can also be used in the design stage. Large-capacity inverters effectively prevent the effects of overvoltages.
6) Reduce the power frequency supply voltage
At present, the common power supply side of the inverter is an uncontrollable rectifier bridge, which is characterized by a high power supply voltage and a voltage generated by the intermediate DC circuit. For example, when the power supply voltage is 380V, the DC link voltage of the inverter reaches 537V. If the inverter is at a position close to the transformer, the input voltage is generally above 400V, which will cause the intermediate DC loop to withstand higher overvoltage. Therefore, under the condition of the condition, the tap changer of the transformer can be used to reduce the overvoltage capability of the inverter by lowering the power supply voltage by placing the low voltage gear.
7) Multiple inverters share DC bus
According to the actual needs, the DC bus loops of multiple inverters can be connected in parallel (the inverter itself is designed with an external DC bus output terminal), so that the current drawn by any inverter from the DC bus is usually It is greater than the excess current fed from the outside at the same time, and the voltage of the shared DC bus can be maintained. Therefore, at least two inverters running at the same time have a common DC bus to balance the DC bus voltage of the inverter, so that the device starts and stops. The impact of the grid is also low, and at the same time the generator is turned off into a generator, and the energy is fed back to the DC bus.
8) Solve the overvoltage problem of the inverter through the function advantages of the control system
The deceleration of the frequency converter and the sudden drop of the load are generally controlled by the controlled system in the process flow. Therefore, the inverter can be controlled by the governed process control system before the deceleration of the inverter and the sudden drop of the load, reducing excessive energy feeding into the intermediate DC loop of the inverter. For example, the uncontrollable rectifier bridge on the input side of the inverter is replaced by a semi-controllable or fully controlled rectifier bridge. The deceleration overvoltage can be controlled. In the process flow deceleration, the intermediate DC voltage control can be met within the required low value range, and the middle is added. The DC loop is capable of withstanding energy and prevents overvoltage. If the production process makes the regular load of the inverter suddenly drop, the control system of the control function of the FOSBORO DCS distribution system can be used to properly increase the frequency of the inverter and reduce the load on the intermediate DC circuit of the inverter before the load drops. Excessive energy is fed in.
2.2 Undervoltage fault resolution measures
1) Set the inverter automatic restart function
The voltage of the equipment with high power is dropped for a short time at the start, which makes it easy for the inverter to stop and stop due to undervoltage. Therefore, setting the automatic restart function of the inverter effectively prevents the influence of undervoltage on the inverter. The design is that after the inverter loses power, the filter capacitor is discharged, and the inverter can automatically reset when the power is lost. Some inverters also have "power frequency switching option", which can be used to set the motor to leave the inverter due to instantaneous stop and other reasons, and continue to operate when it is re-powered, that is, it is used as a instantaneous stop and restart device, so that the inverter The inverter control power supply can be automatically reset after a sudden power loss. At present, there are several measures to achieve instantaneous power failure restart: first, after the equipment controlled by the inverter is completely stopped, then start it automatically; second, use the mechanical brake or DC brake to make the equipment controlled by the inverter. Stop the operation quickly and reduce the free rotation time. The third is to detect the frequency of the induced electromotive force generated by the residual magnetism after the power failure in the general-purpose inverter. The sine wave is converted into a square wave by the optocoupler and the comparator, and the frequency of the square wave is detected. Obtain the running frequency of the motor, and the inverter can restart the equipment controlled by the inverter according to the frequency value and the corresponding voltage.
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