The key to creating green power

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The key to deriving green power based on existing power conversion technology is to solve the harmonic pollution problems on the grid side and the load.

Network side processing

　　Dynamic harmonic compensation can be performed on the existing pollution on the grid side to limit the harmonic components within the permitted range. The national standard GB/T14549-93 "Power Quality Public Grid Harmonics" has been officially implemented since March 1, 1994 and should be implemented resolutely.

　　When developing new power supply devices, grid-side compensation measures are adopted to make them integrated. The power factor and grid current sinusoidal index that meet green standards can be formulated according to the ratio of device capacity to grid capacity. This work can be completed by the authority that formulates power supply standards for reference by technical quality supervision departments.

　　Since the mid-1980s, grid-side power factor correction and dynamic harmonic compensation technology have entered the development process of devices implemented by turn-off power electronic devices. Large-scale foreign compensation devices have entered the field and played a role in the economic operation of power grids. International and domestic research in this field is also very active. In the early 1990s, Professor Cai Xuansan of Tsinghua University and others systematically summarized and concluded active correction and hybrid correction technologies. In order to re-understand and solve the problems in this field, Dr. Liu Jinjun of Xi'an Jiaotong University, with the support of his mentor Professor Wang Zhaoan, redefined the dynamic power factor in theory and applied this theory to guide the development of a new generation of compensation devices. In recent years, Professor Masada of Japan has devoted himself to using superconducting technology to solve the problem of peak and valley power storage in power systems while dealing with grid-side harmonic pollution. A group of scholars represented by Professor Fred.C.Lee of the United States is committed to combining the development of power topology and soft switching technology to integrate grid-side correction technology: At present, there is a frontier accumulation in the integrated measures of grid-side correction for AC/DC and DC/DC conversion, and the integration of grid-side correction for various other power conversions is still to be studied in the future. Since 1996, scholars from various countries have published papers on this topic in IEEE Trans.onPE, which has become a hot topic. Power supply products already have dedicated grid-side compensation control chips.

　　In addition to compensating and correcting harmonics below the 19th order, the organic combination of soft switching technology also suppresses the interference of high-frequency noise caused by high frequency to the local machine, network equipment and communication systems.

　　The above achievements directly promote various existing power electronic power supplies to move towards the "green" direction and form a new product series with grid compensation function.

　　The VVVF inverter product series is active in the field of power saving applications of fans and pumps. In the field of large-scale and wide-ranging applications ranging from a few kW to hundreds of kW, the development and research of integrated grid-side compensation products has not yet been carried out. In fact, as a power source with relatively high power, if the inverter is not integrated with grid-side compensation, the impact on the power grid will be more serious than that of general power sources, which will have a negative effect on the effect of power saving. So, what is the reason that restricts the upgrading of the integrated grid-side compensation of inverter products? The main obstacle is not the technology itself, but the reality that the added value of inverter products is far lower than that of power products with similar topology (inverters, etc.) with the same capacity in the previous stage of market operation, making it difficult for manufacturers to reasonably raise prices and make profits when upgrading their products. Therefore, should the country introduce certain macro-adjustment policies to promote the upgrading of integrated grid-side compensation of inverter products of domestic or joint venture enterprises, and thereby occupy a sufficient share of the domestic market in advance?

Load side processing

　　The load problem should be analyzed specifically. Electrical appliances such as lighting and electric heating have no special requirements for power supply waveforms, and various waveforms including DC power supply are allowed; inductive loads such as motors require sine wave power supply, and unsuitable square wave forced power supply will cause strong harmonic noise, motor heating, and reduce motor life. Medical instruments often contain thyristor circuits and high pulse currents. When sharing a power supply with other electrical appliances, more stringent requirements should be placed on the power supply's adaptability capacity and power supply waveform.

　　Due to the multiplicity of load properties and ranges at the power supply point, the majority of them need to provide sinusoidal power supply. Therefore, the results of AC/DC/AC, DC/AC, and AC/AC conversion require that the sinusoidal distortion meet the needs of users. The conventional method is to use LC filtering (single or multi-stage) for the output stage. Many power supply product manuals mark the distortion THD (Total Harmonic Distortion) as 2% (linear load), which only refers to resistive loads, and avoids describing the THD indicators of loads of other properties, which actually avoids the description of load adaptability. This reflects that the conventional filtering method of obtaining high-sinusoidal power supply is still relatively fragile. Once the load changes from resistive to resistive-inductive or resistive-capacitive, or even thyristor load, or on this basis, a large load range is required, the output voltage waveform of the device will obviously deviate from the marked THD value. This can introduce the concept of soft and hard THD values. Given that scholars and researchers in product development departments have not yet accumulated research on the deviation of THD values under different load conditions, product manufacturers' markings are limited to resistive loads.

　　The ambiguity of THD index labeling makes some users inconvenient when choosing inverter power supply. It seems that the index reflecting the performance of AC power supply should add an additional item - THD hardness, which is used to describe the degree of deviation of the power supply waveform from the sine wave under different loads.

　　Different from conventional filtering, CTA (Comparing-Tracking-Amplifying) power conversion technology has THD hard characteristics in a wider load range and non-pure resistive load conditions. Therefore, CTA power supply is one of the best choices when complex loads have high sinusoidal requirements.