High voltage frequency converter is a commonly used frequency converter product, which uses the on-off action of power semiconductor devices to convert power frequency power supply to another frequency power control device, and is widely used in many fields. What aspects should the user consider in the process of choosing a high voltage inverter? The following Xiaobian will specifically introduce some high-voltage inverter selection methods, hoping to help you.

1, the disadvantages of choosing too high voltage level

Choosing too high voltage level causes too high investment and 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 will increase. With the increase of voltage level, the number of series of power semiconductor devices in frequency converter increases, and the cost increases.

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

2, frequency converter capacity and rectifier device phase number relationship

The input of the frequency converter into the 6kV power grid shall comply with the national regulations on harmonics. This is related to the capacity of the grid and the rated power of the device.

Short circuit capacity within 1000MVA, 1000kW device 12 phase (transformer secondary side double winding) can be, if 24 phase power can reach 2000kW, 12 phase basically eliminates the larger amplitude of the 5th and 7th harmonics.

When the number of rectified phases exceeds 36, the amplitude of harmonic current is not significantly reduced, and the manufacturing cost is too high. If the short-circuit capacity of the grid is 2000MVA, the device allows a larger capacity.

3, the high voltage down to 3kV can save a lot of investment

Considering the necessity of voltage level from the characteristics and safety factor of power electronic devices, the voltage of power electronic devices and the dv/dt allowed by the motor limit, 6kV frequency converter must use multi-level or multi-device series, resulting in complex lines, expensive, poor reliability. For 6kV frequency converter if using 1700VIGBT, to the United States Robin Kang's PERFECTHARMONY series 6kV high voltage frequency converter as an example, each phase by 5 rated voltage 690V power units in series, a total of 60 three-phase devices. If 3300V devices are used, a total of 30 devices in 3 strings are needed, 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 6kV motor current is only about 60A, and the IGBT current of 1700V has reached 2400A, and the current of 3300V devices has reached 1600A, there are large devices can not be used, and it is unreasonable to use a large number of small devices in series. Even if the motor power reaches 2000kW, the current is only about 140A, 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.

Output the same power of the inverter, the cost of using higher voltage or more units in series is greater than the cost of using a lower voltage, a smaller number and a larger current unit, that is to say, under the condition that the device current allows, the low voltage level should be selected as far as possible.

4. Isolation transformer problem

In order to isolate, improve the input current and reduce harmonics, now all the medium voltage "direct inverter" is not a direct inverter, its input side is equipped with an input transformer, this configuration will not change for a short time. Since there is a transformer on the input side, there is no need for the voltage of the inverter and the motor to be the same as the power grid, it is necessary to use 10kV and 6kV, and the voltage below 2500kW can not exceed 3kV, so there is a reasonable voltage level problem of the inverter and the motor.

Frequency conversion speed regulation below 200kW ~ 800kW should choose 380V or 660V voltage level. It has simple line, mature technology, high reliability, small dv/dt and cheap price. Still taking 560kW motor as an example, 630kW660V low-voltage inverter is about 350,000, while the same capacity 6000V medium-voltage inverter is about 900,000. The method of realization has several forms: 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, even if the replacement of the motor and transformer is reasonable.

5, how to match the original 6kV high voltage motor with 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 frequency converter, it is impossible to spend money to replace the motor, the author puts forward a simple scheme for reference. The original 6kV motor of the manufacturer is generally star-shaped wiring, and its phase winding is subjected to the actual voltage of 3468V, so as long as the winding is changed into a triangle other unchanged. With a 3.5kV frequency converter, the frequency converter voltage is reduced from 6kV to 3.5kV, and it can be seen from Table 3 that 4.5kV devices can withstand 3kV voltage without being in series. If you use 1.7kV device 3 strings can be. Manufacturing costs will fall by 30%. At present, the 3.5kV voltage of the large motor 2500kW of the 30MW unit in China is completely reasonable.