1: Choosing an over-voltage level can lead to unnecessary costs and a longer return on investment. When the voltage is set too high, it increases the motor's insulation requirements, which raises the motor's cost. Additionally, higher voltage levels require more power semiconductor devices connected in series within the inverter, increasing overall system costs. This makes it both economically inefficient and technically unreasonable to use 6kV or 10kV voltage levels for motors ranging from 200 to 2000 kW.
2: The relationship between inverter capacity and the phase number of the rectifier device is crucial. When connecting to a 6kV grid, the inverter must comply with national harmonic suppression regulations, which depend on the grid’s short-circuit capacity and the unit’s rated power. If the short-circuit capacity is below 1000MVA, a 12-phase system (with a dual-winding transformer) can be used for a 1000kW device. For a 2000kW system, a 24-phase configuration can effectively reduce large amplitude harmonics like the 5th and 7th. However, beyond 36 phases, the reduction in harmonic current becomes minimal, while manufacturing costs rise significantly. If the grid short-circuit capacity reaches 2000MVA, larger-capacity devices can be supported.
3: Lowering the maximum voltage below 3kV can significantly reduce investment. It’s important to consider the characteristics and safety factors of power electronics when selecting the voltage level. Due to the voltage limits of power electronics and the dv/dt constraints of motors, a 6kV inverter typically requires multi-level configurations or multiple devices connected in series. This leads to complex wiring, higher costs, and reduced reliability. For example, a 6kV inverter using 1700V IGBTs may have five 690V power modules per phase, totaling 60 devices across three phases. Using 3300V devices would require three strings of 30 units, which is even more costly. Additionally, the device current is relatively low, leading to underutilization of the current capability. For a 560kW motor, the 6kV current is only about 60A, while the 1700V IGBT can handle up to 2400A, and the 3300V device can manage 1600A. Using many small devices in series is not efficient. Even at 2000kW, the current remains around 140A, which is still low.
Foreign medium-voltage inverters come in various voltage levels such as 1.1kV, 2.3kV, 3kV, 4.2kV, and 6kV, mainly determined by the capabilities of power electronics. For inverters delivering the same power, using a higher voltage or more series-connected units tends to increase costs compared to using lower voltages, fewer units, and higher current ratings. Therefore, it is generally more practical to choose a lower voltage level whenever possible.
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