Power quality of the hottest DC microgrid

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With the rise of power electronics technology and the large-scale construction of DC transmission projects at the end of the 20th century, the field of DC distribution has gradually attracted attention, and the development of distributed power generation, the increase of the proportion of DC load and the self efficiency of DC distribution further promote the focus of micro research to tilt towards DC micro. The European Union, Japan, the United States and China have carried out research and demonstration projects related to DC micro devices, but they are all in the initial stage, and most of them are concentrated in low-voltage DC micro devices. Their planning, construction and operation standards are still blank, and the nominal voltage level is not unified

DC micro is generally based on DC distribution, that is, it is oriented to public power. One of its important roles is to provide high-quality power for sensitive loads. Therefore, power quality must be considered in the DC micro research and planning stage, which is an important aspect of DC micro technology research. The existing research on DC micro power mainly focuses on voltage control, protection and energy management, and rarely involves the specific problems of DC micro power quality. For promoting the construction and popularization of DC micro, the research on DC micro power quality is also an important force that can not be ignored

DC micro characteristics

1) complexity of the system

DC micro belongs to a typical multi-source, multi transformation complex system. The distribution range of the modified plastic lifting flow device is all over the source, storage, and load links. In some extreme cases, the system reliability problem will become more prominent

2) transmission efficiency

the access of micro sources and loads only needs one-stage transformation, so as to reduce costs and losses, and there is no need to consider the eddy current loss of traditional transmission and distribution lines and the reactive energy absorbed by the lines. Therefore, DC lines have lower requirements for insulation strength

3) decoupling of control

DC micro has typical physical characteristics of complex bottom layer and simplified top layer in control, and the form of DC distribution avoids the mutual coupling between active and reactive power, frequency and voltage of AC system

4) flexibility of topology

DC micro does not need to consider reactive power and the corresponding reactive circulation, so it can operate in the loop. Without considering the constraints of DC protection equipment, its topology and group mode can flexibly choose ring and multi terminal structures

5) isolation of power distribution

the converter has a certain isolation effect, which will weaken the influence of the power disturbance brought by the DC feeder to the load and the interference of the load to the power quality of the feeder, and can realize fault isolation

6) load sensitivity

a considerable part of the DC load contains precision electronic devices, which are generally connected to the DC bus of the corresponding voltage level. Compared with the traditional self rectification power supply form, it is easy to be affected by other loads of the same bus and the power disturbance of the superior line

typical power quality phenomenon of DC micro

in addition to paying attention to the power quality of users inside the micro, from the perspective of power quality accountability, the power quality problems caused by the public AC distribution system caused by DC micro access also belong to the scope of DC micro power quality concern in principle. Therefore, according to the isolation of DC micro distribution and the sensitivity of load to DC power quality, DC micro power quality monitoring points should include three different areas: parallel node, DC distribution node and user load bus, as shown in Figure 1

Figure 1 Schematic diagram of DC micro power quality monitoring points

1) power quality problems of parallel nodes

power quality at the parallel point is affected by the coupling of AC distribution system and DC micro. DC micro access to public AC distribution system is easy to cause AC voltage fluctuation, AC line DC bias and transformer DC bias, while harmonic and intermittent wave injection may also cause waveform distortion, as well as body resonance under inverter operation of parallel converter Local resonance and global resonance

2) power quality problems of DC distribution nodes

DC distribution nodes are jointly affected by micro sources, loads and AC side systems. Their typical power quality phenomena mainly include voltage sag and rise, voltage deviation, voltage fluctuation, DC modulation harmonics, etc., which belong to the key areas of concern of DC micro operators

3) power quality problems of user DC bus

user load bus directly supplies power to the load. Typical power quality problems include voltage sag and rise, voltage interruption, voltage deviation, voltage fluctuation and harmonic, etc. its power quality phenomenon is similar to that of DC distribution lines. Due to the isolation effect of distribution dc/dc converter, the proportion of power quality problems of user bus affected by user level micro source, load converter and load has increased compared with DC distribution nodes, which belongs to the area of direct concern of users

core issues of DC micro power quality research

1) power quality phenomenon mining and analysis

in order to systematically analyze, study and identify the DC micro power quality phenomenon, find out the mechanism that causes the power supply and consumption equipment to not work normally and acceptable compatibility restrictions, analyze the causes of power quality problems, and then take targeted solutions, Therefore, it is necessary to learn from the experience of AC system power quality problem analysis, monitor, mine and analyze the phenomenon of DC micro power quality, and identify, locate and classify the disturbance sources of DC micro power quality, so as to serve as the basis of power quality evaluation, control and even standardization

2) DC micro power quality evaluation

power quality evaluation is a process based on the actual measurement of system electrical operating parameters or the basic data obtained through modeling and simulation, which evaluates various characteristic indicators of power quality and examines and infers whether they meet the specification requirements. It can provide an important reference for DC micro planning, construction and operation. In addition, high power supply reliability has always been an important driving force for micro development, but the large-scale application of power electronic devices and devices makes DC micro face a severe test in terms of reliability. Power supply reliability should also be an important content of DC micro power quality evaluation. Therefore, it is necessary to use the methods and experience of AC system power quality evaluation for reference to establish a power quality evaluation method suitable for the operation and planning stage of DC micro characteristics

3) DC micro power quality control

DC micro should be important or sensitive on the premise of improving the penetration rate of distributed generation and the utilization rate of new energy, so it is widely used in industrial and mining enterprises, colleges and universities and scientific research institutions to provide high-quality and reliable power, In addition, the German bond laminates GmbH, a subsidiary of the specialty chemicals giant Langsheng, recently launched a new type of tepex brand continuous fiber reinforced thermoplastic high-performance composites, which will affect the power quality of the public AC power distribution. The existing methods and measures that can be used for DC micro power quality control can be roughly divided into three categories: ① targeted governance by using additional equipment, such as passive filters, DC active filters, active power quality regulators, voltage balancers, etc; ② Use micro sources to participate in power balance adjustment, such as super capacitors, flywheel energy storage, superconducting energy storage and other micro sources with constant voltage output; ③ The control algorithm of power electronic interface is used to improve the micro operation environment. In terms of cost control, the third category should be given priority, supplemented by the first and second categories. However, the existing research on DC micro power quality control is still in its infancy due to the constraints of DC micro's own development level, and needs further research and improvement

DC micro power quality standardization

1) demand for power quality standardization

in November 2014, the iecsmb Tokyo Conference established the iec/seg6 non-traditional distribution/micro system evaluation team. Seg6 will work closely with the iec/seg4 low voltage DC working group to gradually carry out the establishment of DC micro related standards. DC micro is the integration of power supply and load. DC parallel standard of DG and DC high-frequency fatigue testing machine: 100 (3) 00hz becomes the electrical conduction environment compatibility standard of flow load of high-frequency fatigue testing machine, which is an important factor affecting the formulation of micro related standards. The former is limited by its own equipment and control technology, while the latter involves the DC transformation of some existing loads, and both belong to the DC micro power quality standard at the same time. Therefore, DC micro related standards need to consider more issues than simple DG and even AC micro, involving a wider range, the content will be richer, and will be more restricted by the lack of DC micro power quality standards

Figure 2 DC micro power quality research framework

2) DC micro power quality standardization work

DC micro power quality standardization depends on the completeness of power quality phenomenon mining and analysis, power quality evaluation and control research. It is the specific application and refinement of the three projects, and will further react to standardize and guide the engineering practice of the three, so as to gradually improve the power quality of DC micro power supply, Its work can be summarized as follows: ① specify the nominal environment; ② Define technical terms; ③ Quantitative quality indicators; ④ Recommended testing and evaluation methods; ⑤ Formulate micro source and micro merging procedures; ⑥ Formulate specifications for power quality management equipment

1) DC micro power quality should be an important part of DC micro research, including power quality phenomenon mining, analysis, evaluation and control

2) it is necessary to carry out corresponding power quality standardization as an important part of DC micro related standards, as well as the supplement and expansion of the traditional power quality system under DC micro

3) the particularity of DC micro should be fully considered in the planning and construction, and appropriate methods for power quality evaluation and power supply reliability evaluation should be established in the planning stage

4) research and develop devices, equipment and corresponding control algorithms suitable for DC micro power quality improvement and control, especially for the coordinated treatment of multiple power quality phenomena under multiple converters

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