Defects of pseudo-sine wave grid-connected inverter and corresponding countermeasures

In the process of converting DC to AC, people first made "square wave" inverters, then triangle wave, trapezoidal wave, and pseudo-sine wave inverters. At present, many photovoltaic companies in our country use SPWM pulse width modulation pseudo-sine wave technology controlled by single-chip microcomputers imported from abroad (or developed by themselves). Some companies call this type of inverter a "pure sine wave" inverter. But is this inverter really a pure "sine wave" inverter?

Not only do people not understand what different functions so many AC inverters have, or what disadvantages and advantages they have in different fields, but most of our technicians also do not understand why so many types of inverters need to be continuously upgraded. Why can't they meet the requirements of grid connection even in the era of "pure sine wave"? Some people accuse the technology of not meeting the technical specifications of large-scale grid connection? More directly, most inverters produced by most photovoltaic grid-connected manufacturers have too many sinusoidal waveform "burrs". The reasons for the excessive "burrs" are of great practical significance. Even if the grid-connected inverter has an inverter efficiency of 97%, the actual contribution rate to the power grid and power-consuming equipment may not reach 80%. What is the reason for this?

The author analyzes from the following aspects, hoping that both technical and non-technical personnel can understand the practical significance of "sine wave" and help our electrical appliance supporting enterprises get out of the misunderstanding of thinking.

1. The significance of sinusoidal alternating current in reality: Everyone is familiar with sinusoidal waves. When it comes to sinusoidal waves, it is easy to think of a set of beautiful curves that change regularly. This is exactly the set of curves in which the magnitude and direction of the current in the power grid are constantly changing. This set of curves corresponds to the "uniform" rotation of the generator. This set of beautiful sinusoidal curves is transmitted to the motor through the power grid, which also brings about the "uniform" rotation of the motor. Therefore, although the sine wave is not "uniform rotation", the cause and result of its generation are accompanied by mechanical "uniform rotation". This can be seen from the sinusoidal wave function i=Imsinωt. The angular change "ω" representing uniform rotation is a fixed value, that is, the angular velocity is a constant, which represents "uniform rotation", so the sine wave and "uniform rotation" have an "inevitable" connection. It can be thought that the vertical component value of the rotational force or centrifugal force received by an object when it is performing uniform rotation is the sine wave function value, and the horizontal component value received is the cosine wave function value. Therefore, it can be said that the sine wave not only represents the appearance of "the object rotates at a uniform speed", but also represents the effect of the uniformly changing "rotational force" or "centrifugal force". If "electricity" is regarded as a kind of "force" (here it is different from the word "electric power"), then this electric "force" is the "rotational force" or "centrifugal force" whose direction changes all the time. The existence of this rotational force causes the uniform rotation of the machine. But if it is assumed that two rotating machines rotate at the same time, but the rotation directions are opposite, or the magnetic poles are opposite at the same time, their "phase difference" is 180°. If these two machines rotate "grid-connected" or the electricity they generate is "connected" together, they will cancel each other's forces, and the contribution rate to the rotation of the machine is not 1+1=2, but 1-1=0. Therefore, it can be said that the "sine wave" represents the propagation direction of the electric "force", and its direction cannot be ignored. Therefore, the state of the sine wave at a certain time cannot be ignored either, because it is directly related to the success or failure of grid connection.

Second, the real metaphor of dragon boat racing and "being in the same boat". We often see the TV footage of the dragon boat racing in Jiangnan on May 5th on TV. Many people row at the same time and in the same pace. There are too many skills here. It is not that a hundred strong men can win by trying their best, but it focuses more on whether the pace of the companions is consistent. From a technical analysis, it depends on whether the phase of the action is consistent. If there are several people with different paces, it will affect the rowing speed. That is to say, the rower does not depend on how much strength he exerts. The greater the strength he exerts, if it is in the opposite direction of everyone's force, not only will he not contribute, but there will be a greater reaction. Therefore, only when the pace is consistent, that is, the "phase of action" is the same, will it contribute more to the result of the dragon boat race. Similarly, "being in the same boat" does not mean that two people can achieve "mutual assistance" when they sit on the same boat. "Being in the same boat" does not necessarily mean "mutual assistance". It requires continuous running-in and technical progress of the "people in the same boat". Similarly, in all kinds of "electricity" grid connection, we should not only pay attention to the power and power conversion efficiency of the inverter, but also pay attention to whether the waveform of the sine wave is perfect and the phase is consistent, because the waveform represents the direction of the "force" of "electricity". The addition of "electricity" in various states does not necessarily play a "good" role, but may sometimes cancel each other out. Only the perfect sine waveform and consistent phase are required for power grid connection. This is exactly the basic knowledge of grid "connection" that we must understand in this era.

3. The actual defects of the current imitation sine wave inverter: Although some imported inverters and some inverter manufacturers call their products "pure sine wave" inverters, they are actually amplified by SPWM pulse width modulation waveforms, and there are many "burrs" in the waveform detection. Therefore, many technicians hope to make the waveform "smooth" by filtering. Even in order to make the waveform look more like a "pure sine wave", the sine wave waveform is bolded to indicate, but these methods have no practical significance for the inverter effect of the inverter. In reality, compared with square wave inverters, there are many improvements in motor rotation, but the inverter waveform with many "burrs" objectively indicates that the force exerted on the motor is not a uniformly changing "rotational force", but changes up and down in the direction of the rotational force. The performance of the motor rotation is that it constantly generates "vibration" while rotating, and more energy is consumed in the vibration of the motor and affects the life of the motor. Therefore, the SPWM waveform inverter currently introduced from abroad is not an ideal product, and it cannot be blindly introduced and "foreign products" are regarded as "supreme".

Fourth, the improvement of the inverter: In fact, no matter it is a square wave inverter, a triangle wave inverter or an SPWM wave inverter, there is a "waveform generating circuit" in the main circuit, and then the amplifier circuit amplifies the waveform, so what kind of "wave" this waveform generates depends on what type of "wave" the amplifier circuit amplifies. The waveform generating circuit is like a seed, and the seed you sow determines the external performance of the "waveform". Therefore, manufacturers should break through the influence of the idea that "SPWM waveform" is the current "advanced circuit" and confirm that "suitable" is the best. They should not only dare to learn from foreign countries, but also dare to have their own technological innovations, and design "inverters" that are truly suitable for my country's grid-connected conditions and energy structure. If you are struggling to find the answer, the author has several solutions, and you can communicate with the author (contact mailbox: fengyian7707@163.com).

5. Requirements for grid-connected inverters: Grid connection requires "same waveform, same or slightly higher voltage, same frequency, same phase". In fact, as long as the waveform change law is completely synchronized, it is "same frequency and same phase". If the power conversion efficiency reaches more than 97%, it also depends on whether the waveform output to the grid has clutter and "harmonics". For example, a sine waveform with too many "burrs" actually outputs "harmonics" to the grid, which the grid cannot accept. Although power is output to the grid, some of it will cause internal friction with the grid. Too many SPWM "burr" inverters connected to the grid will make the grid composition more complicated and lead to a decline in the benefits of the original power companies. As the "Dragon Boat Race" principle introduced earlier, it is necessary to find the seeds of "pure sine waves" to make the inverter truly a "pure" and "positive" sine wave inverter without "burrs". Our technical workers can make their own contributions in the construction of electric energy.