Precautions for selection of high-voltage inverter
1. The disadvantages of choosing too high a voltage level
Choosing an excessively high voltage level results in excessive investment and a long payback period. As the voltage level increases, the insulation of the motor must be increased to increase the price of the motor. The increase in voltage level increases the number of series connection of power semiconductor devices in the inverter, and the cost rises.
It can be seen that it is extremely uneconomical and unreasonable to adopt voltage levels of 6kV and 10kV for 200-2000kW motor systems.
2. Relationship between inverter capacity and phase number of rectifier
The inverter device put into 6kV power grid must comply with the national regulations on harmonic suppression. This is related to the grid capacity and the rated power of the device.
Short-circuit capacity is within 1000MVA, 1000kW device 12 phases (transformer secondary winding) can be used, if 24 phase power can reach 2000kW, 12 phases basically eliminate the 5th and 7th harmonics with larger amplitude.
After the number of rectified phases exceeds 36, the amplitude of the harmonic current is not significantly reduced, and the manufacturing cost is too high. If the grid short-circuit capacity is 2000MVA, the device allows a larger capacity.
3. Lowering the maximum voltage below 3kV can save a lot of investment
Considering the necessity of voltage level from the characteristics and safety factor of power electronic devices, subject to the power electronic device voltage and the dv / dt limit allowed by the motor, 6kV inverters must use multi-level or multi-device series connection, resulting in complex lines, expensive and reliable Sexuality is poor. For a 6kV inverter using 1700V IGBT, take Robinson's PERFECTHARMONY series 6kV high-voltage inverter as an example. Each phase consists of 5 power units with a rated voltage of 690V connected in series, and there are 60 devices in three phases. If a 3300V device is used, a total of 30 devices in 3 strings are required, which is a huge number. On the other hand, the device current is small, and the current capacity of the device is not fully utilized. Taking 560kW as an example, the 6kV motor current is only about 60A, while the 1700V IGBT current has reached 2400A, and the 3300V device current has reached 1600A. There are large devices that cannot be used. 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 levels of power electronic devices.
For a frequency converter that outputs the same power, 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 and a larger current unit, which means that it should be selected as much as possible under the conditions of the device current. Low voltage level.
4. Isolation transformer problem
In order to isolate, improve the input current and reduce harmonics, all medium voltage "direct frequency conversion" devices are not true direct frequency conversion. The input side is equipped with an input transformer. This configuration will not change in a short time. Since there is a transformer on the input side, the voltage of the inverter and the motor does not have to be the same as the power grid. It is not necessary to use 10kV and 6kV. The voltage below 2500kW can not exceed 3kV. Therefore, there is a problem of the reasonable voltage level of the inverter and the motor.
It is better to choose 380V or 660V voltage grade for frequency conversion speed regulation below 200kW ～ 800kW. It has simple circuit, mature technology, high reliability, small dv / dt and low price. Taking the 560kW motor as an example, the low-voltage inverter of 630kW660V is about 350,000, and the medium-voltage inverter of 6000V with the same capacity is about 900,000. There are several ways to achieve low-low, low-high, high-low and high-low-high. Due to the motor, the price of the transformer is much lower than that of the inverter, even if the motor and transformer are replaced.
5. How to match the original 6kV high voltage motor with the voltage of 3.5kV inverter
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, it is impossible to spend money on replacing the motor. The author proposes a simple plan for reference.
The original 6kV motors of the manufacturer are generally star-connected, and the actual voltage of the phase winding is 3468V, so as long as the winding is changed to a triangle, the other remains unchanged. With a 3.5kV inverter, the voltage of the inverter will be reduced from 6kV to 3.5kV. It can be seen from Table 3 that 4.5kV devices can withstand 3kV withstand voltage without being connected in series. If you use 3 strings of 1.7kV devices. Manufacturing costs will be reduced by 30%. At present, China's 30MW unit's largest motor 2500kW uses 3.5kV voltage is completely reasonable.
6. Preventive measures against harmonic pollution of power grid
From a practical point of view, the 12-phase rectifier formed by the rectifier bridge can eliminate the 5th and 7th harmonics and has basically met the harmonic requirements of the power grid. Therefore, 400kW ～ 800kW can adopt 12-phase rectification, and 1000kW ～ 2500kW can adopt 24-phase rectification.
Shenzhen Zhifu New Energy Co., Ltd. , https://www.sunbeambattery.com