To know that in the case of high-power DC power supply, the switching frequency is difficult to increase, why? Two important indicators to measure the switching device are the passing current, the other is the withstand voltage, the cost of high-power switching devices is very high, and due to technical limitations, the power of a single switching device can not be too large. Pen & Fun & Pavilion www.biquge.info

Thyristor is the most powerful switching device, but the frequency is too low, and the power of the new generation of high-frequency switching devices of MOSFET and IGBT is very limited.

Zhang Zheng is well aware of this, he participated in the design of the high-voltage DC transmission project from Sanxia hydropower to Jianghai, each converter valve is composed of hundreds of thousands of IGBT in parallel after series connection, pressure and current sharing is very difficult to achieve, each IGBT drive signal is made of optical fiber, as long as there is even a hair gap, pressure and current sharing failure, then all the switches in a valve will be burned instantly, and it will cause a terrible chain reaction.

This kind of converter valve can only be achieved by a few companies in the world, and the process requirements for pressure and flow equalization are too high.

Even so, the switching frequency of the entire valve is around 1000Hz.

The restriction of high power on the switching frequency is very large, a single switching device can not meet the requirements, the frequency is slightly increased, and the difficulty of voltage and current sharing is doubled.

The high-voltage DC generator developed this time requires 200kV DC output, the highest peak current reaches 800mA, and the maximum short-term power exceeds 160kW.

It must be soft switched, pressure and current must be equalized, and the weight must be light enough, and the frequency must be increased, which is a series of factors that increase the difficulty, otherwise how can a group of experienced professors be at a loss.

Zhang Zheng couldn't wait to ask Liu Chen for advice, and there were too many doubts in his heart.

If the switching frequency is 10k, I roughly calculate that the fluctuation will be more than 5%. ”

"10k? That's not enough, this control mode has a switching frequency of at least 100kHz or more. The complete elimination of switching losses is aimed at high frequency, and this structure is not required for the frequency order of 10K. ”

"100kHz?" Zhang Zheng's eyes widened, with such a high frequency, the weight of the overall equipment must be greatly reduced. It's just that......

"With such a high frequency, IGBT can not be reached, and only MOSFET structure switching devices can be used, and the capacity of a single chip can be very small, which has to consider the complex problem of voltage and current sharing. Zhang Zheng grimaced. He thinks the ideal situation is to increase the frequency to 30k to achieve balance.

"This is simple, the withstand voltage of the MOSFET is no problem, that is, the passing current is small, and we use this topology to switch the switch when the current is zero, do you think of it? That is the natural current sharing condition, as long as several switches are connected in parallel together, the problem of small capacity can be solved, and there is no need to add additional auxiliary circuit current sharing. ”

Zhang Zheng almost jumped up, waved his fist excitedly, and shouted, "That's right, the problem of switching capacity has been solved." ”

After thinking about it, I don't think it's quite right. "At such a high frequency, it's not easy to control, right? And you need to detect the resonant current crossing point, the controller can't respond, will the stability be too bad?"

"The hardware circuit implementation is a bit cumbersome, but it's not impossible. Liu Chen laughs that when he develops devices with this structure, he usually puts the frequency close to 200kHz.

"How, exactly, can you tell me about that?"

"It's not a few words that can be explained, Professor Zhang. Liu Chen said with a smile, he would not be stupid enough to tell the person he met for the second time about the skills he mastered, even if the other party was a humble and polite professor.

Zhang Zheng also understood. This kind of advanced key technology, if he asks, people will tell him immediately, unless he is a fool. After pondering for a moment, he said: "Can this structure be used to achieve a step-up to 200kV DC without overshoot within 1ms, and there are strict requirements for weight, and the stability also requires more than five years of operation, and the space EMC can not be large." ”

Liu Chen's eyes lit up when he heard it, so many demanding requirements, the application occasion is self-evident. I roughly guessed what kind of project this was, and thought about it carefully: "This kind of UHVDC equipment has high insulation requirements, especially after the frequency is very high, how many withstand voltage level devices can be provided by using fast rectification diodes?"

This can be taken into account, Zhang Zheng admired in his heart, the young man in front of him has absolutely rich experience in engineering development, and said: "Before we all considered relatively low-frequency working conditions, according to the frequency you said, this diode is afraid that the withstand voltage is not high, about 20kV withstand voltage is no problem, in this way, the boost side of the step-up transformer needs to be divided into more than a dozen windings, which is probably not stable, right?"

"Why do you want to divide the windings?" Liu Chen looked at him strangely, "It's enough for you to provide this kind of withstand voltage level." ”

Zhang Zheng's face turned red, his voice was much quieter, he was a little inferior when he was criticized by people for a while, "Regardless of the winding, how to deal with the pressure problem?"

"This is simple, you can use a double inverter structure, you can also build a simple rectifier circuit into a voltage equalization bridge, there are many ways to solve it, the transformer is divided into three windings at most to be unstable, two is the best, the magnetic circuit design is a positive and negative structure just to offset EMC. ”

Eloquently, to put it mildly.

"Equalization rectifier bridge? Do you know this?" Zhang Zheng was completely impressed, so that the withstand voltage of the device was also solved, and the EMC was effectively controlled.

"A little understanding, a little understanding, in fact, there is nothing difficult, it is written in the book "Power Electronics Technology". ”

Zhang Zheng desperately recalled that he had organized the publication of a volume of this book, and as a textbook for many schools, how could he not know about the Rectifier Bridge? This is a very profound technique.

Liu Chen drew a rectifier bridge on paper and handed it to Zhang Zheng, "Isn't it very simple, just add two capacitors between each rectifier circuit, and add one capacitor inside the bridge, and the voltage will be equalized, and it can also be filtered, but in turn, the value of the capacitor is very particular, and high-precision customized capacity must be used." ”

Zhang Zheng almost cried when he saw it, why didn't he think of it, the simpler the structure, the more practical, and excitedly grabbed the piece of paper tightly in his hand, lest the wind would blow and run away.

He resisted the urge to ask about the value of the capacitance, which was a secret for the researchers.

Nuo Min has been standing on the side to play the role of a little maid, pouring milk tea and taking paper and pencil or something, although she can't understand a word about the topic that these two people are talking about, but she feels so profound, so powerful.

Watching Liu Chen draw while talking, and the big professor looked like a primary school student, he was almost adored to death, and the little stars in his eyes kept bubbling and rising. (To be continued.) )