The main manifestations of the disturbance of the inverter in the application are: higher harmonics, noise and oscillation, load matching, heat generation and other problems. These disturbances are unavoidable, because the input part of the inverter is a rectifier circuit and the output part is an inverter circuit, which is composed of nonlinear components that play a switching effect, and in the process of breaking the circuit, higher harmonics will occur, and then the voltage waveform and current waveform of the input power supply and output will be distorted. The following analyzes the harmonic problem and proposes corresponding measures.
Inverters with smaller capacity have less influence from higher harmonics. However, when the capacity is large or the number is large, it is necessary to deal with the higher harmonic disturbance caused by the higher harmonic current, otherwise it will affect the equipment and the detection element, and in severe cases, these equipment may malfunction. According to the United Kingdom's ACE report, the sensitivity of various targets to higher harmonics is as follows: the motor has no effect below 10%-20%; The surface voltage distortion is 10%, the current distortion is 10%, and the error is less than 1%. More than 10% of electronic switches will malfunction; Computers make mistakes beyond 5 percent. In view of the above, it is necessary to adopt measures to reduce disturbance and keep it within the permissible limits in industrial sites.
1. Intercept the transmission channel of disturbance
(1) The transmission of disturbance is often transmitted through a common grounding wire. The grounding of the power line and the grounding separation of the control wire is the basic method of blocking this way, the grounding terminal of the power equipment is connected to the grounding wire, and the grounding terminal of the control equipment is connected to the metal shell of the equipment disk.
(2) When the signal line is close to the wire with the source of disturbance, the disturbance will be induced to the signal line, so that the signal will be disturbed, and the wiring separation is effective in eliminating this disturbance. In practical engineering, it is necessary to separate the high-voltage cables, power cables, and control cables from the external cables and computer cables, and separate different bridges. It is also best to route the control line of the inverter in a straight line with its main circuit.
2. Press the higher harmonics
(1) The line reactor is installed on the front side of the inverter, which can press the overvoltage on the power supply side and reduce the current distortion of the inverter to avoid severe disturbance of the main power supply.
This scheme is inexpensive, but the power of the constrained harmonics is limited, and the unacceptable voltage drop loss occurs when the reactance is too large.
(2) Install an LC passive filter in front of the inverter to filter out the higher harmonics, generally filter out the 5th and 7th harmonics, but this method depends completely on the power supply and load, and the flexibility is small.
(3) Set up a special filter to detect the inverter and phase, and generate a current with the same amplitude and opposite phase as the harmonic current, pass into the inverter, and then effectively absorb the harmonic current.
(4) When the adjacent environment of the equipment is disturbed by electromagnetic disturbance, an anti-RF disturbance filter should be installed to reduce the conducted emission of the main power supply, and measures should be taken to shield the motor cable.
(5) When the length of the motor cable is greater than 50m or 80m (unshielded), in order to avoid the instantaneous overvoltage when the motor is started, reduce the leakage current and noise of the motor to the ground, and protect the motor, a reactor is installed between the inverter and the motor.
(6) Add the impedance of the power supply of the inverter. The internal impedance of the general power supply equipment can play a role in buffering the reactive power of the DC filter capacitor of the inverter, the larger the internal impedance, the smaller the harmonic content, and this internal impedance is the short-circuit impedance of the transformer. Therefore, when selecting the power supply of the inverter, it is best to choose a transformer with large short-circuit impedance.
(7) The transformer is selected for multi-phase operation. The general-purpose inverter is a six-pulse rectifier, so the harmonics that occur are larger. If the transformer is selected for multi-phase operation, the phase angle is 30° apart, such as the transformer of Y-Δ AND Δ-Δ combination constitutes the effect of 12 pulses, which can reduce the lower harmonic current and press the harmonic well.
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