Megmeet inverter - high-voltage inverter selection considerations

First, the disadvantages

Choosing too high a voltage level results in too high an investment and a long payback period. With the increase of the voltage level, the insulation of the motor must be improved, so that the price of the motor increases. The increase of voltage level increases the number of series connections of power semiconductor devices in the inverter, and the cost rises.

It can be seen that it is extremely uneconomical and unreasonable to use 6kV and 10kV voltage levels for 200-2000kW motor systems.

Second, the relationship between the number of phases

The inverter device put into the 6kV power grid must comply with the national regulations on harmonic suppression. This is related to the capacity of the grid and the power rating of the device.

The short-circuit capacity is within 1000MVA, and the 1000kW device can be 12 phases (transformer secondary side double winding), if the 24-phase power can reach 2000kW, the 12-phase basically eliminates the 5th and 7th harmonics with large amplitudes.

After the number of rectifier phases exceeds 36 phases, the harmonic current amplitude decreases insignificantly, and the manufacturing cost is too high. If the grid short-circuit capacity is 2000MVA, the allowable capacity of the installation is larger.

3. Save investment

Considering the necessity of voltage level from the characteristics and safety factor of power electronic devices, limited by the voltage of power electronic devices and the allowable dv/dt of the motor, the 6kV inverter must be connected in series with multiple levels or multiple devices, resulting in complex lines, expensive and poor reliability. For the 6kV inverter, if 1700VIGBT is used, take the PERFECTHARMONY series of 6kV high-voltage inverter of United States Robin Kang as an example, each phase is connected in series by 5 power units with a rated voltage of 690V, and a total of 60 devices in three phases. If 3300V devices are used, a total of 30 devices in 3 strings are also required, which is a huge number. On the other hand, the device current is small, the current capacity of the device is not fully utilized, taking 560kW as an example, the current of the 6kV motor is only about 60A, and the IGBT current of 1700V has reached 2400A, and the current of the 3300V device is 1600A. Even with a motor power of 2,000 kW, the current is only about 140 A, which is still very small.

Foreign medium voltage inverters have multiple voltage levels: 1.1kV, 2.3kV, 3kV, 4.2kV, 6kV, which are mainly determined by the voltage level of power electronic devices.

For an inverter with the same output, the cost of using a higher voltage or more units in series is greater than the cost of using a lower voltage, a smaller number of units with a larger current, that is to say, the lowest voltage level should be selected as much as possible under the allowable condition of the device current.

Fourth, the issue of isolation

In order to isolate, improve the input current and reduce harmonics, all medium-voltage "direct inverters" are not true direct inverters, and their input side is equipped with an input transformer, which will not change for a short time. Since there is a transformer on the input side, the voltage of the inverter and the motor is not necessary to be the same as the power grid, 10kV and 6kV must be used, and the voltage below the power of 2500kW can not exceed 3kV, so there is a reasonable voltage level of the inverter and the motor.

——380V or 660V voltage level should be selected for frequency conversion speed regulation below 800kW. It has simple lines, mature technology, high reliability, small DV/DT and low price. Taking 560kW motors as an example, there are about 350,000 low-voltage inverters with 630kW and 660V low-voltage inverters, and about 900,000 medium-voltage inverters with the same capacity of 6000V. There are several forms of low-low, low-high, high-low, and high-low-high. Because of the motor, the price of the transformer is much lower than that of the inverter, and even if the motor and transformer are replaced, it is reasonable.

Fifth, how to match

How to match the original 6kV high-voltage motor with the 3.5kV inverter voltage

Since the founding of the People's Republic of China, the traditional 6kV high-voltage motor is the main product that has been put into production, in order to promote the 3.5kV inverter can no longer spend money to replace the motor, the author proposes a simple scheme for reference.

The original 6kV motor in the manufacturer is generally star-wired, and the actual voltage of the phase winding is 3468V, so as long as the winding is reconnected to a triangle, the others remain unchanged. With a 3.5kV inverter, the inverter voltage is reduced from 6kV to 3.5kV, and it can be seen from Table 3 that the 4.5kV device can withstand 3kV withstand voltage without being connected in series. If you use a 1.7kV device for 3 strings, you can do it. Manufacturing costs will drop by 30%. At present, the maximum motor of 30MW unit in China is 2500kW, and the voltage of 3.5kV is completely reasonable.

6. Harmonic pollution

From a practical point of view, the rectifier bridge is composed of 12-phase rectification, which can eliminate the 5th and 7th harmonics, which basically meets the harmonic requirements of the power grid. Therefore, 400kW-800kW can be rectified with 12 phases, and 1000kW-2500kW with 24 phases can also meet the requirements.

Megmeet inverter - high-voltage inverter selection considerations

Megmeet inverter - high-voltage inverter selection considerations

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