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

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1. The Disadvantages of Using a High Voltage Level

Choosing an excessively high voltage level leads to unnecessary investment and a longer return on investment period. 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 connected in series within the inverter, increasing both cost and complexity. This makes it economically unwise and technically inefficient to use 6kV or 10kV voltage levels for motors ranging from 200 kW to 2000 kW.

2. Relationship Between Inverter Capacity and Rectifier Devices

When connecting an inverter to a 6kV grid, it must comply with national regulations regarding harmonic suppression. This depends on the grid capacity and the rated power of the system. If the short-circuit capacity is below 1000MVA, a 12-phase configuration (with double windings on the transformer secondary side) is suitable for up to 1000kW. For systems up to 2000kW, a 24-phase setup can significantly reduce the amplitude of 5th and 7th harmonics.

Once the number of rectifier phases exceeds 36, the reduction in harmonic current becomes minimal, and the manufacturing costs increase substantially. If the grid has a short-circuit capacity of 2000MVA, the system can support larger capacities.

3. Cost Savings by Lowering Maximum Voltage Below 3kV

The selection of voltage levels must consider the characteristics and safety margins of power electronic devices. A 6kV inverter requires multiple devices connected in series, leading to complex wiring, high costs, and reduced reliability. For example, using 1700V IGBTs in a 6kV inverter would require five 690V modules per phase, totaling 60 devices. Using 3300V devices would require three strings of 30 units each, which is even more expensive.

Additionally, the current capability of these devices is not fully utilized. For a 560kW motor, the current is only around 60A, while a 1700V IGBT can handle 2400A, and a 3300V device can manage 1600A. It’s inefficient to use many small devices in series. Even for a 2000kW motor, the current remains low at around 140A.

Medium-voltage inverters from abroad typically have multiple voltage levels such as 1.1kV, 2.3kV, 3kV, 4.2kV, and 6kV, all determined by the voltage capabilities of power electronics.

For inverters delivering the same power, using a lower voltage with fewer, larger current units is more cost-effective than using higher voltages with more devices. Therefore, it’s better to choose the lowest possible voltage level that meets the system requirements.

4. Isolation Transformer Considerations

To isolate the system, improve input current quality, and reduce harmonics, most medium-voltage "direct conversion" systems actually include an input transformer. This setup is unlikely to change soon. Because of the transformer on the input side, the inverter and motor voltages may not match the grid voltage. Therefore, for systems under 2500kW, it's reasonable to limit the voltage to below 3kV.

For motors with a power range of 200kW to 800kW, using 380V or 660V inverters is ideal. These options offer simpler wiring, mature technology, high reliability, and lower dv/dt. For example, a 630kW 660V low-voltage inverter costs about $350,000, while a similar 3.5kV medium-voltage inverter costs around $900,000. The cost of a transformer is much lower than that of an inverter, making it a more economical choice.

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

Traditionally, 6kV high-voltage motors have been used since the founding of the country. To integrate them with a 3.5kV inverter without replacing the motor, a simple solution is proposed. Most 6kV motors are connected in a star configuration, with each winding experiencing 3468V. By changing the winding to a delta connection, the voltage seen by the inverter can be reduced to 3.5kV.

According to Table 3, a 4.5kV device can withstand 3kV without being connected in series. Using three 1.7kV devices can reduce manufacturing costs by 30%. China’s 30MW units currently operate at 3.5kV for motors up to 2500kW.

6. Harmonic Pollution Control Measures

In practice, a 12-phase rectifier bridge can eliminate 5th and 7th harmonics, meeting most grid standards. Systems between 400kW and 800kW can use this method, and even 1000kW to 2500kW systems can meet the requirements. This approach ensures cleaner power and reduces the risk of grid instability.