High-voltage inverter selection considerations - Solutions - Huaqiang Electronic Network

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1. The Disadvantages of Choosing a High Voltage Level

Opting for an excessively high voltage level can lead to over-investment and a prolonged return on investment. As the voltage increases, the motor’s insulation must also be upgraded, which raises the overall cost. Additionally, higher voltage levels require more power semiconductor devices to be connected in series within the inverter, increasing both cost and complexity. This makes it economically inefficient and impractical to use 6kV or 10kV systems for motors ranging from 200 kW to 2000 kW.

2. Relationship Between Inverter Capacity and Number of Rectifier Devices

When connecting an inverter to a 6kV grid, it must comply with national regulations regarding harmonic suppression, which depends on the grid capacity and the unit's rated power. If the short-circuit capacity is below 1000 MVA, a 12-phase system (with a double-winding transformer on the secondary side) can be used for a 1000 kW device. For a 2000 kW system, a 24-phase setup can effectively suppress the 5th and 7th harmonics. However, beyond 36 phases, the reduction in harmonic current becomes minimal, while the manufacturing cost rises significantly. If the grid has a short-circuit capacity of 2000 MVA, larger device capacities are allowed.

3. Benefits of Lowering the Maximum Voltage Below 3kV

The choice of voltage level is influenced by the characteristics and safety margins of power electronic devices. A 6kV inverter requires multiple power units connected in series, leading to complex wiring, high costs, and reduced reliability. For example, a 6kV inverter using 1700V IGBTs may require five 690V modules per phase, totaling 60 devices. Using 3300V devices would require three strings of 30 modules each, which is even more expensive. Moreover, the current rating of these devices is not fully utilized—such as a 560kW motor operating at only 60A, while the IGBTs can handle up to 2400A. It is inefficient to use many small devices in series. Even for a 2000kW motor, the current remains relatively low at around 140A.

4. Isolation Transformer Considerations

Although called "direct conversion," most medium-voltage inverters include an input transformer to isolate the system, improve input current quality, and reduce harmonics. This configuration is unlikely to change soon. Because of the presence of a transformer, the inverter and motor voltages may differ from the grid voltage. Therefore, it is unnecessary to use 10kV or 6kV for systems below 2500kW. A reasonable voltage level should be selected based on the inverter and motor specifications.

5. Matching a 6kV Motor with a 3.5kV Inverter

Many traditional 6kV high-voltage motors have been in operation since the early days of industrialization. To integrate a 3.5kV inverter, replacing the motor is not always feasible. One simple solution is to reconfigure the motor’s winding from star to delta, reducing the effective voltage to 3468V. This allows the 3.5kV inverter to operate efficiently. Using 1.7kV devices in three strings can further reduce costs by up to 30%. China’s 30MW units currently support up to 2500kW at 3.5kV.

6. Harmonic Pollution Control Measures

In practice, a 12-phase rectification system can eliminate the 5th and 7th harmonics, meeting most grid requirements. Systems between 400 kW and 800 kW can use this approach, and even those up to 2500 kW can comply with standards. This ensures stable and clean power supply without excessive harmonic distortion.