Motors are the main power-consuming equipment in industrial production. The application of high-voltage and high-power motors is more prominent, and most of these devices have great potential for energy conservation. Therefore, it is necessary and urgency to vigorously develop high-voltage and high-power frequency control technology.
At present, with the rapid development of modern power electronics and microelectronics technologies, high-voltage, high-power inverter devices are continuously maturing. High-voltage problems that have been difficult to solve have been solved in recent years through device serialization or cell series connection. . Its application areas and scope are also becoming more and more widespread, which provides technical prerequisites for industrial and mining enterprises to use energy (especially electric energy) efficiently and reasonably.
2. Main circuit analysis of several common high-voltage frequency converters
(1) Multi-unit series voltage source high-voltage inverter
Multi-unit series voltage source high-voltage frequency converters use low-voltage single-phase inverters in series to compensate for the lack of voltage capability of power device IGBTs. The so-called multiplexing means that each phase consists of several low-voltage power units connected in series. Each power unit is powered by a multi-winding phase-shifting isolation transformer, controlled by a high-speed microprocessor and driven by optical fiber isolation. However, it has the following disadvantages:
a) There are too many power units and power devices to use. The 6kV system uses 150 power devices (90 diodes, 60 IGBTs). The volume of the device is too large, the weight is large, and the installation location and infrastructure investment are problematic.
b) Too many high-voltage cables are required, the internal resistance of the system increases intangibly, the wiring is too much, and the fault points increase accordingly;
c) When a unit is damaged, the unit can be bypassed, but at this time the voltage at the center point of the output voltage imbalance is floating, causing voltage and current to be unbalanced, and the harmonics are accordingly increased. When the unit is barely operated, it will eventually lead to a motor. Damage
d) The output voltage waveform is good at rated load, and below 25Hz, the distortion is prominent;
d) The output voltage waveform is good at rated load, and below 25Hz, the distortion is prominent;
e) As there are transformers in the system, it is not easy to realize the improvement of system efficiency; In phase-shifting transformers, 6kV three-phase 6-windings × 3 (12 windings × 3 at 10kV ×) delta-side delta connection, with unbalanced three-phase voltage ( In fact, when the three-phase voltage is impossible to be absolutely balanced, the generated internal circulation will inevitably cause an increase in the internal resistance and a loss of the current, which will also increase the copper loss of the transformer. At this time, coupled with the inherent loss of the transformer core, the efficiency of the transformer will be reduced, which will affect the efficiency of the entire high-voltage inverter. The more significant this condition is at running below the rated load. At 10kV, the transformer has nearly 400 connectors and nearly 100 cables. Efficiency up to 96% at rated load but less than 90% at light load.
(2) Neutral clamped three-level PWM inverter
This series of inverters uses the traditional voltage-type inverter structure. Neutral point clamped three-level PWM inverter inverter part of the traditional three-level approach, so the output waveform will inevitably produce relatively large harmonic components, which is inherent in the three-level inverter mode . Therefore, the output LC filter must be configured on the output side of the frequency converter for use with ordinary squirrel-cage motors. Also due to harmonics, the power factor, efficiency, and even life of the motor are affected. Only at the rated operating point can the best working condition be achieved. However, as the speed decreases, the power factor and efficiency will decrease accordingly. .
Multi-level + multiple high-voltage frequency converters. The idea of ​​multi-level + multiplexed high-voltage frequency converters is to solve the problem of limited pressure of high-voltage IGBTs, but this method not only increases the complexity of the system, but also reduces the multiple redundant performance and three-level structure. Simple advantages. Therefore, this kind of inverter is actually not desirable.
The performance and price advantage of this type of inverter is not so great. Instead of adopting both multi-level and multi-level technologies, it is not as good as using the aforementioned high-voltage IGBT multi-inverter or three-level inverter.
(3) Current source high voltage frequency converter
The current source high-voltage frequency converter directly connected in series with the power device is a large inductor connected in series in the line, and then the SCR (or GTO, SGCT, etc.) power devices with relatively slow switching speeds are directly connected in series.
Although this method uses less power devices and controls current easily, it does not really solve the series problem of high-voltage power devices. Because even if the power device fails, the di/dt is limited due to the current limiting effect of the large inductance, although the power device is not easy to damage, but the problem is caused by serious pollution to the power grid and low power factor. And the current source high-voltage frequency converter is sensitive to the change of the voltage of the grid and the load of the electrical machinery, cannot make the real general-purpose product.
The current source high voltage frequency converter is the earliest product, but wherever the voltage type frequency converter arrives, it is forced to withdraw because it is obviously at a disadvantage economically and technically.
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