Introduction to DC Distribution Network Topology and Distributed Power Access

The DC distribution network is relative to the AC distribution network. It provides the load with the DC bus. The DC load can be directly powered by the DC bus, and the AC load needs to be powered by the inverter device. If the DC load in the load is proportional Larger, DC power distribution will have a greater advantage. The DC distribution network has low line loss, high reliability, no need for phase frequency control, and strong ability to accept distributed power sources.

1. Background of DC power distribution

In recent years, high-voltage direct current transmission technology has gradually matured and has been widely used in power systems. The development of converters, filters, circuit breakers and other aspects is also relatively mature. The direct current transmission network is more stable than the alternating current transmission network. The construction of large The power grid can fundamentally eliminate the stability problem of the AC grid; therefore, the low-voltage DC power distribution technology has gradually attracted extensive attention from scholars at home and abroad.

After the worldwide energy crisis broke out in the 1970s, problems such as environmental pollution and energy shortages have attracted widespread attention from countries all over the world. Distributed power supplies rely on flexible load changes, high reliability of power supply, small transmission loss and ease of renewable energy. Energy applications and many other advantages have attracted more and more attention at home and abroad. The development of these distributed power sources has played a huge role in promoting the development of DC power distribution systems. The access of distributed power makes the traditional power distribution system expand the function of power generation from a single power supply form. Typical distributed power sources mainly include photovoltaic cells for DC power generation and fans for AC power generation. Photovoltaic power generation generates direct current power. Due to its volatility, it generally needs to go through DC/DC and DC/AC two-stage converters before it can be incorporated into traditional Although wind turbines generate electricity in the form of AC, similar to photovoltaic cells, the AC power generated is usually unstable, and generally requires AC/DC and DC/AC two-stage converters to be incorporated into AC power distribution. net. If these distributed power sources are directly connected to the DC distribution network, the above-mentioned DC/AC inverter link can be omitted, which not only reduces the cost, but also reduces the loss.

In addition, the rapid development of power electronics technology has caused great changes in the way users use electricity. On the one hand, household rotating electrical appliances such as air conditioners, refrigerators, washing machines, etc. have applied power electronic frequency conversion technology. In the AC distribution network, these electrical appliances need to undergo AC/DC/AC two-stage conversion to achieve frequency conversion. For the DC distribution network, only DC/AC conversion is required, thereby omitting the ACIDC rectification link, reducing the loss of the converter and reducing the cost.

2. Research status of DC distribution network at home and abroad

In recent years, high-voltage DC transmission technology has been widely used in power systems, and low-voltage DC power distribution technology has gradually attracted the attention of scholars at home and abroad. DC distribution network has better performance than AC in terms of transmission capacity, controllability, improvement of power supply quality, reduction of line loss, isolation of AC and DC faults, and flexible and convenient access to renewable energy, which can effectively improve power quality, Reduce the use of power electronic converters, reduce power loss and operating costs, coordinate the contradiction between large power grids and distributed power sources, and give full play to the value and benefits of distributed energy. Many foreign countries such as: the United States, Japan, Europe and South Korea have already started the research and introduction of DC power distribution system.

In 2000, Weizhong Tang of General Electric Lighting of the United States and R.H.Lasseter of the School of Electronic and Computer Engineering at the University of Wisconsin-Madison studied industrial low-voltage DC power distribution systems without a central control unit [3]. In 2003, the University of North Carolina took the DC ship power distribution system as an example to discuss the opportunities and challenges when DC power distribution is applied to industrial systems. In 2007, the CPES Center of Virginia Tech in the United States proposed the "Sustainable Building Initiative (SBI)" research plan, which mainly provides electricity for future residences and buildings. With the deepening of research, CPES developed SBI into SBN in 2010. In 2011, the University of North Carolina proposed "The Future Renewable Electric Energy Delivery and Management (FREEDM)" system structure, which is used to build an automatic and flexible power distribution network in the future.

In 2004, Japan's Tokyo Institute of Technology and other institutions proposed the concept of a power distribution system based on a DC microgrid, and realized a 10kW DC power distribution system prototype. In 2007, Osaka University in Japan proposed a DC power distribution system with a low-voltage bipolar structure and studied its basic characteristics. In 2010, losses in DC distribution systems suitable for residential buildings were evaluated and compared with AC systems.

The South Korean government has established a smart microgrid research center with an investment of about 2.72 million U.S. dollars and plans to build a DC microgrid giant grid power supply system before March 2011. The research focuses on DC power distribution, power converters and control and Three aspects of the communication system. The project is implemented by Professor Byung Moon Han of Myongji University in South Korea.

Since 2009, relevant domestic units such as Tsinghua University, North China Electric Power University, and Zhejiang University have gradually carried out related research on DC distribution networks. In 2012, China also established an advanced technology research center for urban power grids based on the Shenzhen Power Supply Bureau. It plans to establish a flexible DC power distribution technology laboratory from 2012 to 2015 and implement research on key technologies related to flexible DC power distribution. The research on DC distribution network in various countries is still in the experimental and exploratory stage. The in-depth research on DC distribution network, especially the research on the DC distribution network connected to the power grid, is close to blank, and the research focus is mostly on the DC microgrid as the core. low-voltage DC distribution network. But it is foreseeable that the DC distribution network will have broad development prospects due to its strong technical and economic advantages.

3. Advantages of DC distribution network for DC power distribution

3.1 Technical characteristics of DC distribution network

The AC power distribution system faces a series of power quality problems such as large line loss, instantaneous voltage drop, voltage fluctuation, grid harmonics, and aggravated three-phase unbalance. It is urgent to change the existing distribution network structure, thus introducing DC power distribution. web concept. The DC distribution network has the following advantages [5]:

3.1.1 The line loss of DC distribution network is small. Considering the active power loss caused by the eddy current of the metal sheath of the AC cable and the reactive power loss of the AC system, when the line voltage of the DC system is twice that of the AC system, the line loss of the DC distribution network is only 15% to 50% of the AC network. Although AC systems can reduce line loss through measures such as reactive power compensation, this will greatly increase the construction cost and complexity of the system. The voltage source converter and DC transformer used in the DC distribution network generally adopt the pulse width modulation (PWM) technology based on the insulated gate bipolar transistor (IGBT), so the on-state loss and switching loss are large, so its power conversion efficiency slightly below the AC transformer. However, the line loss of the DC distribution network is much lower than that of the AC distribution network, and with the development of power electronics technology and devices, the on-state loss and switching loss of the converter are continuously reduced, so the overall efficiency of the DC distribution network is limited. There is a lot of room for improvement, and it may surpass the AC distribution network in the future.

3.1.2 DC distribution network has high power supply reliability. AC power distribution generally adopts a three-phase four-wire or five-wire system, while DC power distribution only has positive and negative poles, and two transmission lines are enough. The reliability of the lines is higher than that of AC lines of the same voltage level. When a fault occurs in the first stage of the DC power distribution system, the other stage can form a loop with the earth without affecting the power transmission of the entire system. When a common single-phase or single-pole instantaneous ground fault occurs, the DC system responds faster and has a shorter recovery time than the AC system, and can eliminate the fault through multiple starts or step-down operations to ensure the normal operation of the system. For the low-voltage DC power distribution system, a multi-bus redundant structure can be used to ensure higher power supply reliability. Due to the access to the power electronic converter, an independent protection area can be formed in the DC power distribution system, and its fault will not spread to the external system. In addition, compared with the AC distribution network, the DC distribution network is more convenient for the access of energy storage devices such as supercapacitors and batteries, thereby improving its power supply reliability and fault ride-through capability.

3.1.3 Does not involve phase, frequency control, reactive power and AC charging current and other issues. When the AC system is running, it is necessary to control the voltage amplitude, frequency and phase, while the DC system only needs to control the voltage amplitude, without involving frequency stability issues, there is no network loss caused by reactive power, and there is no skin effect. loss etc. In addition, the use of cable lines in distribution networks has become a trend. However, the capacitance along the cable is large, so a large capacitance charging current will be generated when transmitting alternating current, which not only reduces the transmission capacity of the line, but also increases the line loss. And adopt direct current power supply, above-mentioned problem can be avoided.

3.1.4 The DC distribution network is convenient for the access of distributed power sources and energy storage devices. The future distribution network should be compatible with the integration of large-scale distributed power sources such as wind energy and solar energy. Photovoltaic cells, etc. emit a random fluctuating direct current, requiring DC/DC, DC/AC converters, and configuring appropriate energy storage devices and complex control systems to achieve AC grid connection; wind power, etc. is a kind of Randomly fluctuating alternating current also requires AC/DC/AC converters, and configuration of appropriate energy storage devices and complex control systems to achieve AC grid connection; various energy storage devices, such as batteries, supercapacitors, as distributed storage The electric vehicle charging station of the energy unit itself works in the form of direct current, and needs two-way DC/AC to connect to the alternating current grid. In the case of a DC distribution network, it is relatively simple to realize interface equipment and control technologies such as distributed power grid-connected power generation and energy storage.

3.1.5 It has the advantages of environmental protection. The "space charge effect" of the DC line makes the corona loss and radio interference smaller than that of the AC line, and the electromagnetic radiation generated is also small, which has the advantage of environmental protection. Two overhead lines with opposite polarities of DC power transmission are usually arranged adjacent to each other. The currents of the two cables have the same magnitude, opposite directions, and are very close to each other. Therefore, the magnetic field generated by them to the outside world can be equivalent to 0 and cancel each other out. The AC transmission system adopts a three-phase system, so the strength and range of the magnetic field generated by it are much larger than that of the DC transmission line, and the harm to the human body and other animals and plants is greater than that of the DC transmission line.

3.2 Structural advantages of DC over AC power distribution system

Nowadays, more and more loads on the user side, such as computers, communications and household appliances, are powered internally by DC voltage. However, the user-side access voltage of the current distribution network system is the AC voltage obtained by stepping down the transformer. Therefore, the power supply of the user-side equipment generally needs to introduce a single-phase rectifier (AC/DC) on the input side to convert the distribution transformer The AC voltage is rectified into a DC voltage, and then converted into the required DC voltage by a DC chopper (DC/DC) to supply power to the load. The current AC power distribution system and the DC power distribution system after the transformation of the AC power distribution system are compared and analyzed.

Transforming the original AC power distribution system into a DC power distribution system has the following advantages:

3.2.1 Make DC electrical equipment such as power electronic communication equipment and lighting loads directly or through a first-level DC/DC connected to the DC bus, eliminating the need for AC/DC links.

3.2.2 If an appropriate DC bus voltage level is selected, DC distributed power sources such as photovoltaic cells can be directly connected to the bus without going through a large number of complicated DC/AC inverters and other devices. For AC distributed power sources such as wind power generation, only a relatively simple rectification link is needed, and then converted into a given DC voltage through DC/DC.

3.2.3 Large-capacity battery packs and supercapacitors can be introduced at the DC bus as system backups. On the one hand, they can be used as power batteries for electric vehicles, and on the other hand, they can also improve the reliability of the power distribution system itself to a certain extent. For the traditional AC power distribution system, the backup power supply of the battery pack needs an inverter device to be connected to the grid, which undoubtedly increases the cost and reduces the efficiency.

By comparing these two different forms of power distribution systems, it is easy to find that the DC power distribution system omits a large number of scattered rectifiers (AC/DC) and grid-connected inverters (DC/AC), which simplifies the distribution network structure and improves Improve system efficiency and reliability, and greatly reduce costs. It makes grid-connection of renewable energy represented by solar energy and wind turbines more convenient and economical, and lays a good foundation for the future development of LED lighting and electric vehicles.

4. The access of distributed power in the DC distribution network of DC power distribution

Through the understanding of the research status and technical characteristics of DC distribution network at home and abroad, it can be known that DC distribution network has better performance than AC distribution network, which can not only improve power supply reliability, reduce line loss, and does not involve phase and frequency Control and reactive power and other issues, but also provide good conditions for the access of distributed power. The popularity of distributed power generation has laid a solid foundation for the development of DC distribution network. The fundamental reason is that the integration of distributed power generation into DC distribution network can save a large number of converter devices. DG generally runs in parallel with the local power distribution network, such as 35kV, 10kV and 380/220V, which have many influences on the topology and voltage level of the DC distribution network. The way the grid is analyzed is elaborated.

The more common distributed power sources mainly include photovoltaic cells, fuel cells, wind turbines, and gas turbines. These distributed power sources use very environmentally friendly and clean power generation methods and have great development prospects.

Moreover, the electric energy generated by these power supplies is direct current or becomes direct current after simple rectification. Therefore, the integration of distributed power into the DC distribution network will save a lot of commutation links. Figure 3 is a typical DC power distribution system compatible with distributed power. These include distributed power generation for direct current: photovoltaic power generation, fuel cells and distributed power generation for alternating current generation: wind power and micro gas turbines. These distributed power sources are connected to the DC bus of the system through their respective converters. The DC bus exchanges energy with the AC main network through the bidirectional DC/AC converter, exchanges energy with the energy storage unit through the bidirectional DC/DC converter, supplies power to the AC load through the DC/AC inverter, and supplies power to the AC load through the DC/DC converter. supply power to DC loads.

5. Topology of DC distribution network for DC power distribution

The future trend of the DC distribution system should include the public distribution network of the medium-voltage distribution network and the low-voltage distribution network on the user side, that is, the medium-voltage DC distribution system and the low-voltage DC distribution system. Among them, the topological structure and voltage level selection of the DC distribution network are important factors of the DC distribution network.

5.1 To determine the topology of the DC distribution network, the following points need to be noted:

5.1.1 The DC power distribution system must be able to operate in parallel with the large power grid, so the grid-connected converter must have the characteristics of bidirectional power flow in order to transmit the excess energy generated by DG to the AC grid;

5.1.2 The DC power distribution system must be able to provide a relatively stable voltage for the load;

5.1.3 The DC power distribution system must have high safety and reliability.

5.2 Topology of MVDC distribution network

Compared with the multi-terminal DC transmission technology, the DC power distribution technology pays more attention to the realization of DC households, which inevitably involves issues such as multi-level DC power distribution, power supply reliability, and power quality. For example, part of the power in the medium-voltage DC distribution network requires After being sent to the low-voltage DC distribution network by a DC step-down device such as a DC transformer, it can be used by users. Therefore, its system structure and engineering realization are relatively more complicated than multi-terminal DC transmission. Its basic topological structure mainly includes three types: ring, radial and power distribution at both ends.

Various power sources and loads such as AC power grid, distributed power supply, energy storage equipment, AC and DC industrial loads, etc., are connected to DC distribution networks of different voltage levels through different types of adapters according to their own requirements. The electric energy generated by various types of AC and DC power sources is converted into DC power of a certain voltage level by VSC and DC/DC converters respectively and delivered to each load terminal through the DC distribution network, and then passed by VSC or DC/DC converters respectively. Converted to AC or DC to power the corresponding AC or DC load. Generally speaking, the VSC connected to the AC power grid has the function of bidirectional energy flow in order to realize the power exchange and power balance between the DC distribution network and the AC power grid; the DC/DC converter connected to the energy storage device involves charging , discharge, is also bidirectional. Generally speaking, the power supply reliability of the ring network and the power distribution network at both ends is relatively high, but the fault identification and protection control cooperation are relatively difficult. The radial network power supply reliability is relatively low, but the fault identification and protection control cooperation are relatively difficult. relatively easy. To select the DC distribution network topology, it is necessary to comprehensively consider the actual project needs such as the reliability of the DC distribution network, the scope of the power supply, and the investment. In general, the mesh structure is mainly used for DC transmission, while the tree structure is mainly used for DC power distribution.

5.3 Topology of LVDC Distribution Network

5.3.1 Classification by Upper Layer Transmission Mode

The low-voltage DC distribution network is the user-side DC distribution network. According to the classification of the upper transmission mode, the topological structure of the distribution network can be divided into two categories. As shown in Figure 7, the DC power distribution system structure of AC transmission and DC transmission.

Since the research on DC distribution network in various countries is still in the stage of experimentation and exploration, the in-depth research on DC distribution network, especially the research on the connection of DC distribution network to the power grid and the development of key equipment is currently close to blank. Therefore, most of the research is now concentrated on the low-voltage DC power distribution system for AC transmission on the upper layer; the DC power distribution system will be the future development direction.

5.3.2 Classification by AC load power supply mode

According to the classification of AC load power supply mode, it can be divided into two types: centralized power supply mode and modular radial power supply mode. Centralized low-voltage DC power distribution system, two AC systems are connected by a DC line. Users are connected to the DC system, and multiple users get power from one converter. Users are not directly connected to the DC system, and each user corresponds to a converter.

5.3.2.1 The centralized power supply mode is a DC power distribution system similar to HVDC transmission. Multiple users get power from one converter. This topology is simple and the converter has high efficiency, but its expansion and redundancy capabilities are poor, which is not suitable for the access of distributed power sources, and the converter has a large capacity and burden weight and reliability will be reduced. It is suitable for the situation where the power supply and the load are relatively concentrated.

5.3.2.2 Modular DC power distribution system. Users are indirectly connected to the DC system, and each user corresponds to a converter as a separate module. This topology has strong scalability and redundancy capabilities, but the efficiency of the converter is low. Applicable to the situation where the power supply and load are relatively dispersed.

The DC power distribution system is conducive to the grid connection of distributed power sources and the power consumption of DC loads. Usually, due to geographical environment factors, the installation of distributed power sources will be relatively scattered; DC users of low-voltage DC power distribution networks can directly obtain power from the DC bus through converters , so users can also be relatively dispersed; in order to be compatible with more distributed power sources and load access systems, the scalability of the system must be fully considered. Therefore, the DC power distribution system is suitable for adopting a radial network structure.

In recent years, high-voltage direct current transmission technology has gradually matured and has been widely used in power systems. The development of converters, filters, circuit breakers and other aspects is also relatively mature. The use of direct current mode to build large power grids can fundamentally eliminate the stability of AC power grids. sexual issues. Therefore, low-voltage DC power distribution technology has gradually attracted extensive attention from scholars at home and abroad. Distributed power generation technology represented by wind energy, solar energy, etc., as an ideal way to comprehensively utilize existing renewable energy, has increasingly attracted the attention of the power industry and research scholars, and has been widely used around the world. With the maturity of DG technology, more and more distributed power sources scattered near the power load are connected to the power distribution system. However, wind power generation and solar power generation are intermittent power sources, and the power generated varies with wind speed and sunlight. And other external factors change, so there are congenital defects such as low stability and efficiency. If it is considered to be connected to the AC power distribution system, this instability may be exacerbated due to the difference in basic characteristics between the distributed generation and the existing AC power distribution network. The research on the DC power distribution structure based on power electronic converters is becoming more and more extensive. The reliability and power quality of the system can be improved by connecting distributed power sources into the DC structure power distribution system and cooperating with hybrid energy storage devices.

According to foreign research, the DC distribution network is significantly better than the AC distribution network in terms of power supply capacity, line loss, power quality, reactive power compensation and suitable range. Therefore, connecting distributed power sources with low investment, low loss, and high reliability into the DC distribution network is bound to become an important development direction in the power field. However, the infiltration of a large number of distributed power sources also brings a lot of uncertainties to the planning and operation of the DC distribution network. Therefore, further analysis and research is needed on the energy storage system planning of the DC power grid with distributed power, so as to minimize the negative impact of distributed power on the distribution network. The future research direction of DC distribution network with distributed power generation is described as follows:

The research on the DC distribution network is only in the qualitative stage, and it only proves that this concept has certain advantages for the AC distribution network in some aspects. But exactly how superior it is, how much advantage it has over the current power distribution system, and how much impact it has on the connected AC grid, all need urgent solutions.

For distributed power generation technologies that use solar energy, wind energy, etc. as primary energy, the active power output is different from traditional power plants, and will fluctuate significantly with changes in natural conditions such as temperature, light, and wind speed, and cannot be monitored. effective conditioning. Therefore, in order to establish a corresponding model, it is necessary to study the law and statistical characteristics of the distributed energy with weather changes.

The mentioned DC power distribution system is only analyzed and simulated for one case of wind power generation, without considering the power compensation scheme of other power generation methods, and this aspect needs further research.