Planning and analysis of the demonstration project of the MVDC distribution network in Zhuhai

Abstract

The DC distribution system is an important development direction of the distribution system, which can improve the reliability and the quality of the power supply, and support the new energy, the energy storage, the electric vehicles, and the flexible access of AC and DC loads to grid. To realize the demonstration application of the DC distribution technology, China’s first demonstration project of the medium voltage DC distribution network will be built in Zhuhai, Guangdong Province to support the construction of the city energy internet. First, this paper analyzes the demand of the DC distribution network project, and puts forward the construction content and construction target. Then, it designs and analyzes the electrical connection mode, system operation mode, and startup and shutdown mode of the DC distribution network, and proposes the overall project construction plan. Finally, it conducts the specific project design and analysis, which mainly include the selection of equipment such as inverters, DC transformers and DC circuit breakers, the design and analysis of the DC control and protection system, the design and analysis of the over-voltage protection and the configuration scheme of the lightning arrester, and analysis of the system transient characteristics. The design and analysis of the engineering program is a combination of China’s distribution network engineering practice and technical characteristics, which lays a solid foundation for the advancement of the DC power distribution technology in China, and has reference value and demonstration effect for the design and construction of other projects.

Keywords

MVDC distribution network / DC solid state transformer / DC circuit breaker / modular multilevel converter

Introduction

With the rapid development of distributed power supplies, increasing DC loads, and increasing energy storage applications, the problems of traditional AC power distribution and utilization systems are growing. In addition, as customers continue to increase their demands for power quality and reliability, traditional AC power distribution systems are facing new challenges in terms of power supply stability and economic efficiency. The DC power distribution and utilization system which will be an important develop direction for power distribution systems helps to solve a series of new problems in the development of traditional AC power distribution systems due to its advantages such as flexible power deployment, high system efficiency, large power supply capacity, small line loss, good power quality, reactive power compensation, and its adaptive distributed power supplies, energy storage devices, and flexible DC load connections[¹–⁵].

At present, the relevant technology research, experimental systems and demonstration projects in the field of DC power distribution systems have been gradually carried out. The US has conducted research on DC distribution systems earlier. In 2007, the CPES Center of Virginia Tech University proposed the “Sustainable Building Initiative (SBI)” research plan, mainly providing DC power for future residences and buildings. In 2010, the CPES Center developed SBI again as “Sustainable Building and Nano-grids (SBN)” [⁶]. Based on the SBN, the CPES Center also proposed a hybrid distribution system structure with hierarchically connected AC and DC power distribution [⁷]. In 2011, the University of North Carolina proposed the “The Future Renewable Electric Energy Delivery and Management (FREEDM)” system architecture for the construction of future automated and flexible distribution networks [⁸,⁹]. In 2004, Tokyo Institute of Technology proposed the concept of the distribution system based on the DC microgrid and established a 10 kW DC distribution system model [¹⁰]. In 2006, Osaka University proposed a bipolar DC microgrid system [¹¹,¹²]. Reference [¹³] proposed a DC power distribution system based on the distributed power supply, whose scheme is similar to the bipolar structure proposed by Osaka University. In 2007, Bucharest University of Technology proposed a DC distribution system with alternating power supply [¹⁴]. In 2014, Aalborg University began the construction of an Intelligent DC Microgrid Living Laboratory to study the modeling, design, coordination, control, communication, and management of future low-voltage DC power distribution systems [¹⁵–¹⁷]. In 2016, Aachen University built a 10 kV medium-voltage DC power distribution system [¹⁸,¹⁹], and conducted a pilot study to achieve a breakthrough in the practical application of medium-voltage DC power distribution systems. Since 2009, China’s relevant research institutes have gradually studied the DC distribution system, focusing mainly on the research and development of the DC devices. The research on the overall planning of the DC distribution networks is, therefore, relatively lacking [²⁰,²¹]. In 2013, China issued the National 863 Program of “Research and Application of Key Technologies for Smart DC Power Grids Based on Flexible DC Power Supply” [²²] to complete the construction of a flexible DC power distribution system laboratory and carry out the system control protection prototype test and dynamic simulation test of a flexible DC distribution system. The National 863 Program of “Key Technology for High Density Distributed Energy Access AC/DC Hybrid Microgrid” undertaken by Zhejiang Electric Power Co. Ltd. was formally launched in 2015. It mainly completed the research on the typical system architecture of DC power distribution systems and built the real-time digital simulation research platform and dynamic model laboratory of the DC power distribution with five or more terminals.

In summary, although related technology research, experimental systems and demonstration projects in the field of the DC power distribution system have been gradually conducted, most of them are in a general exploration stage. The planning, design, coordination, control and protection of the DC power distribution systems have not been studied in-depth and systematically, and there is a lack of practical application engineering. To realize the demonstration application of the DC power distribution technology, improve the power supply reliability and power quality of the distribution network, support the flexible access of new energy generation, energy storage, electric vehicle charging equipment and AC and DC loads, and increase the regional distribution network accident backup capacity and load transfer capability, China’s first demonstration project of the medium voltage DC distribution network will be built in Zhuhai, Guangdong Province to support the construction of the City Energy Internet, supported by the “Internet+” Smart Energy (Energy Internet) demonstration project.

Planning and layout of DC distribution network

Demand analysis

The location of substations with voltage classes of 110 kV and above in the Tangjiawan area of Zhuhai is shown in Fig. 1.

The construction of this project will interconnect the 10 kV busbars of Jishan substation and Tangjia substation, which will greatly increase the accident reserve and load transfer capacity and improve the reliability and quality of power supply in the region. This project is of great significance for optimizing the operation of the existing power grid and improving the reliability.

Construction content

The project plans to build a±10 kV/±375 V medium voltage DC distribution system, the structure of which is illustrated in Fig. 2. To achieve power transfer and improve the reliability of the distribution network power supply, the system uses a “star” network topology with three independent AC power supplies. To improve the power quality of the AC distribution network connected to the DC distribution system, the three-terminal AC system and the medium-voltage DC distribution bus are all connected by using a fully-controlled voltage source converter.

Overall construction plan of DC distribution network

Since there is no engineering practice of the DC distribution network and a lack of the engineering construction specifications, this paper designs the engineering scheme of Zhuhai DC distribution network in combination with the design process and method of the AC distribution network and the DC transmission engineering. Specifically, the engineering design includes the main electrical connection, the parameters of the main loop, the control strategy, the main equipment, transient characteristics of system, the protection scheme, and the overvoltage and insulation coordination.

Electrical connection mode

The main electrical connection modes of the medium voltage DC power distribution system include unipolar asymmetric systems, unipolar symmetric systems, and bipolar systems. The electrical connection mode of this project adopts the unipolar symmetric system. To reduce the insulation level of the DC line to the ground, the neutral point of the DC side capacitor is generally grounded to form a pseudo-bipolar system with a positive and a negative polarity. For the MMC-based DC system, because there is no centralized capacitor on the DC side of the MMC converter, this project uses a star reactance on the AC side to connect the transformer valve to construct an artificial neutral point, which is grounded by the grounding resistor. In this way, the symmetric positive and negative polarities of the DC line to ground do appear as depicted in Fig. 3.

System operation mode

The DC power distribution system constructed in this project exchanges energy with the AC system through three converters, which can be powered by the voltage source converters at each end to the DC system, and can also maintain the normal operation with the remaining voltage source converter during one end (or two ends) of the voltage source converter out of operation. The possible operating modes of the system mainly include three-terminal networking operation, two-terminal hand-in-hand operation, double-terminal isolated power supply, single-terminal power supply, and STATCOM.

When the operating conditions of the DC distribution system change, the system should be able to automatically switch to the operating mode under new operating conditions. The switching relationship between the operating modes and the corresponding equipment is demonstrated in Fig. 4.

Startup and shutdown mode

The DC voltage is an indicator of whether the DC system power is balanced. Therefore, the core of the DC power distribution system startup is to establish a stable DC voltage, followed by the converter power control. If there is a power-bidirectionally controllable energy storage device in the system, it can be connected to the system in the next step to stabilize the power fluctuation caused by accessing the distributed power supply. Then the distributed power supply is connected and the AC-DC load is finally connected.

The simulation of the start-up process of the DC distribution system is performed. The charging resistances of Tangjia Station, Jishan Station I, and Jishan Station II are 2 W, and the charging resistance of the DC transformer is 20 W. The AC circuit breaker closes at 0 s, the charging resistors of all MMC converter stations access at the same time, the charging resistors of all MMC converter stations bypass at 0.25 s, and the MMC converter stations unlock. Tangjia Station works in the mode of the DC voltage control, with a target value of±10 kV, while Jishan I Station and Jishan II Station operate in the mode of the power control, with a target value is 50% of the system load. DC transformer connects to the system at 0.4 s and its charging resistor bypasses at 0.9 s. The simulation results are exhibited in Fig. 5.

Design and analysis of specific plan of DC distribution network

Analysis of major equipment selection

Converter selection

Similar to the flexible DC transmission, the technology solutions of VSC mainly include the two-level converter, the three-level NPC converter, and the MMC converter. In this project, both the converters at Tangjia Station, Jishan I Station and Jishan II Station are bidirectional. To ensure a high power supply quality, the VSC based on fully-controlled devices is required. Considering that MMC converters do not require devices to be directly connected in series, they have advantages in terms of AC harmonics, DC harmonics, losses, and reliability. In this project, the AC-DC converters are designed using a modular multilevel topology.

For Tangjia Station and Jishan II Station, because the DC bus is equipped with a DC circuit breaker, a basic MMC converter topology can be used. The power module topology are displayed in Fig. 6.

Since the DC bus of Jishan Station I is not equipped with a DC breaker, its converter should adopt a topology with a DC fault self-clearing capability. The topology with DC fault self-clearing capability includes MMC based on the IGCT cross-clamped (ICC-MMC), the MMC based on the full-bridge sub-module, and half-bridge sub-module mixed. For the ICC-MMC, only a small amount of the ICM clamping module is needed for each bridge arm, and the cost is only slightly increased with the half-bridge MMC as a reference. The power sub-module design, inverter control method, and operation of the MMC are not changed. At the same time, due to the low on-state loss characteristics of the IGCT device, the efficiency of the ICC-MMC solution is significantly higher than that of other blocking MMCs. Therefore, it is recommended that converters of the Jishan Station I that does not have a DC circuit breaker be equipped with ICC-MMC, as shown in Fig. 7.

Taking the transient fault as an example for simulation analysis, the simulation waveforms are shown in Fig. 8, which suggest that after a DC fault, the fault is quickly blocked; after the fault is cleared, the MMC can be quickly put back into operation to restore the DC voltage and power transmission.

Selection of DC transformer

In this project, the high voltage side of the DC transformer is±10 kV port, which is connected to the DC distribution network. The low voltage side has two voltage levels:±375 V and±110 V. According to the structure of the medium-voltage power distribution system described in this project, DC transformer mainly implements voltage conversion and power transmission between the medium-voltage DC distribution bus and the low-voltage DC microgrid. It is planned to adopt the ISOP multi-modular technology topology. The topology is shown in Fig. 9, whose power and voltage levels can be flexibly adjusted to a high degree of modularity.

Selection of DC circuit breaker

In view of the current breaking requirements of the MVDC distribution systems and device breaking capacity, this project selects the hybrid DC circuit breaker based on coupled negative voltage commutation. Its topology is shown in Fig. 10. The main branch is composed of the fast mechanical switch, which has the advantages of low on-state loss, space saving, high reliability, and low operation maintenance costs. The fast mechanical switch adopts the electromagnetic repulsion operating structure and the electromagnetic buffer mechanism, which has the advantages of simple structure, high reliability, and millisecond-level breaking and recovery. The transfer branch is mainly composed of the power electronic switch and the coupling negative voltage circuit in series. The current commutation is realized by the coupling negative voltage circuit. There is no problem of long commutation time in the case of small current, and the controllability is strong. The energy-consuming branch is composed of multiple arresters and is connected in parallel with the transfer branch. The redundancy is more flexible and the reliability is high.

DC control and protection system

DC control system

The DC control system is the core of the entire DC distribution system. The basic requirement of the DC distribution control system is to maintain the reliability of the power supply and power quality of the important load in the AC/DC power distribution system, and to realize the full utilization of the distributed power supply. The DC distribution control system is divided into three levels of control: the optimization layer, the coordination layer, and the local layer. The specific control architecture is presented in Fig. 11.

The optimal control is the top level control. Based on the steady-state power flow calculation of the DC distribution network, the network loss and voltage quality are selected as objective functions, and a multi-objective optimization mathematical model of the DC distribution network operation is constructed, as expressed in Eq. (1).

(1)\min f=\{P_{\mathrm{loss}},I_{\mathrm{vq}}\},|P_{\mathrm{loss}}={\sum _{i=\mathrm{1}}^n{\displaystyle \sum _{j=\mathrm{1},j\ne i}^n(V_i-V_j)}}^{\mathrm{2}}{y}_{ij}{I}_{\mathrm{vq}}={{\displaystyle \sum _{i=\mathrm{1}}^n\left({\displaystyle \frac{V_i-V_{ei}}{V_{ei}}}/\mathrm{\Delta }{V}_{\max }\right)}}^{\mathrm{2}},

where P_loss is the network loss of the DC distribution network, n is the node number of the DC distribution network, V_i is the ith node voltage, V_j is the jth node voltage, y_ij is the DC conductance of the branch between the ith node and the jth node, I_vq is the voltage quality index of the DC distribution network, V_ei is the expected voltage of the ith node, and ΔV_max is the maximum voltage deviation that can be accepted. The total voltage quality of the DC distribution network can be measured by the quadratic sum of ratio of the actual voltage deviation to the ΔV_max for each node.

The above multi-objective optimization model needs to meet the power flow equation constraints, DC voltage constraints, converter station capacity constraints, and energy storage power constraints. The specific constraint equations are

(2)\{\begin{array}{l}{P}_j\le P_{j\max }\hfill \\ \left|{\displaystyle \frac{V_i-V_{\mathrm{e}i}}{V_{\mathrm{e}i}}}\right|\times \mathrm{100}\mathrm{\%}\le \sigma \hfill \\ P_{\mathrm{VSC}k\min }\le P_{\mathrm{VSC}k}\le P_{\mathrm{VSC}k\max }\hfill \\ P_{\mathrm{Emin}k}\le P_{\mathrm{E}k}\le P_{\mathrm{Emax}k}\hfill \end{array},

where P_jis the power flowing through the jth branch, P_jmax is the upper limit of the transmission power for this branch, P_VSCk is the power of the kth VSC, P_VSCkmax and P_VSCkmin are the upper and lower power limits for the kth VSC, P_Ek is the power of the kth energy storage device, and P_Emaxk and P_Emink is the upper and lower power limits for the kth energy storage device.

The above multi-objective optimization mathematical model is solved by using the hybrid particle swarm optimization algorithm, and a series of non-inferior solutions are obtained. The fuzzy control technique is used to make the decision to obtain the optimal commands such as voltage and power of the centralized control system of the DC distribution system. Taking the optimal control of Tangjia Station as an example, the results of network loss and voltage quality are given in Fig. 12. The maximum improvement of line loss is 6.12 kW, and the worst-case voltage deviation is 4.35%.

DC protection system

According to the basic principle of the power system protection area, the protected objects in the DC power distribution system can be divided into several different parts such as AC system, voltage source converter, DC transformer, DC line, and DC bus. The protection area of the DC power distribution system is shown in Fig. 13.

Overvoltage and insulation coordination

Overvoltage protection measures

The internal overvoltage of the converter uses the metal oxide arresters as the main protection and limits the overvoltage caused by extreme conditions by reasonably adjusting control and protection strategies. The external overvoltage adopts the lightning protection line to protect the overhead line and use the arrester or the lightning rod to protect the converter.

Arrester configuration

The arrester configuration mainly considers the main converter and interface devices, which is plotted in Fig. 14.

The “A” type arrester is used to protect the AC bus equipment in the converter, which is located as close as possible to the system side of the connection transformer to limit the overvoltage of the primary and secondary sides of the connection transformer. The “A2” type arrester is used to protect the bridge arm reactor and the secondary side of the connection transformer, and also to protect the earthing branch of the valve side of the connection transformer. The “DB” type arresters are used to protect the DC bus and its related equipment and cooperate with the A2 arrester to protect the converter valve. The “DL” type arresters, installed on the DC line side, are used to protect the DC bus and its related equipment and to maintain the voltage between the two poles.

Analysis of system transient characteristics

According to the location of the fault, the fault is divided into the main converter area fault and the line area fault, of which, the main faults of the DC line include unipolar ground faults and bipolar flashover faults. Bipolar flashover faults are selected for system transient analysis.

Assume that a short-circuit fault occurs on line L4 at a distance of 1.5 km from Jinshan I Station. Because the 10 kV DC line of the DC distribution system is a cable, the fault is a permanent one. After the bipolar fault, the DC voltage rapidly decreases, and the capacitor of the converter sub-module rapidly discharges, resulting in a rapid increase of the DC current in a short time. About 20 ms after the fault occurs, the current of the DC transformer reaches 2 times its rated value, triggering converter’s over-current protection, and then blocking converter. About 400 ms after the fault occurs, the differential current across the DC line L2 reaches the high-value segment of the DC line differential protection, and the protection system issues a protection action command after a delay of 500 ms. About 2 ms after the fault occurs, the three-terminal DC circuit breaker receives the breaking instruction of the protection system, and starts to break the Jinshan Station I port. At the same time, about 960 ms after the fault occurs, the converter of Jinshan Station I triggers the over-current protection of bridge arm and immediately triggers its IGCT clamp sub-module, cutting off the short circuit current contributed by Jinshan Station I to the short circuit point. At 2.155 ms after the fault occurred, the converter of Jinshan Station II triggers the over-current protection of bridge arm and locks the converter; at 2.535 ms after the fault occurs, the converter of Tangjia Station triggers the over-current protection of bridge arm and locks the converter. After receiving the breaking instruction, the three-terminal DC circuit breaker disconnects the Jinshan I port about 4.5 ms after the fault occurs, completing the isolation of the fault current and isolating the fault point. About 60 ms after the fault occurs, the converter of the DC step-down station, Tangjia Station and Jinshan Station II are unlocked, and the system enters the self-recovery stage. The entire process is shown in Fig. 15. The AC side bus of the converter station does not trigger over-current protection and therefore does not disconnect its AC circuit breaker.

Conclusions

The technology solution of the medium-voltage DC distribution network with a high power-supply reliability and a high power supply-quality is proposed in this paper, which can provide support for the development of the City Energy Internet in Zhuhai, Guangdong Province.

(1) First, the demand analysis of the DC distribution network project is conducted, and the construction of a±10 kV/±375 V medium voltage DC distribution network project is proposed.

(2) Then, the electrical connection mode, system operation mode and startup and shutdown mode of the DC distribution network is designed and analyzed, and the overall project construction plan is proposed. The system adopts a star topology, and three voltage source converters connect the three 10 kV buses at Tangjia Station and Jishan Station respectively. The symmetric single-pole connection mode is adopted on the DC side. The possible operating modes of the system mainly include three-terminal networking operation, two-terminal hand-in-hand operation, double-terminal isolated power supply, single-terminal power supply and STATCOM.

(3) Finally, the specific design and analysis of the project is conducted. The converters in the Tangjia Station and the Jinshan Station II adopt a half-bridge modular multilevel converter, and the converter in the Jinshan Station I adopts a modular multilevel converter based on the IGCT cross-clamping. The DC circuit breaker at the outlet of the converter station adopts the hybrid DC circuit breakers, and the DC circuit breaker at the breaking station adopts the three-terminal hybrid DC circuit breakers. The DC distribution control system is divided into three levels of control, the optimization layer, the coordination layer, and the local layer. The protection area in the DC power distribution system can be divided into the AC system, the voltage source converter, the DC transformer, the DC line, and the DC bus. The system over-voltage is protected by the metal oxide arresters. The DB-type pole-to-ground arresters are placed on the DC reactor at the valve side of the three voltage source converters, and the DL-type arresters are placed on the line side of the DC circuit breaker and DC transformer.

The design and analysis of the medium-voltage DC distribution network project in Zhuhai lays a solid foundation for promoting the deployment and operation of the DC distribution technology in China, and has reference value and demonstration effects for the design and construction of other projects.

References

[1] Baran M E, Mahajan N R. DC distribution for industrial systems: opportunities and challenges. Industry Applications IEEE Transactions on, 2003, 39(6): 1596–1601

[2] Riccobono A, Santi E. Comprehensive review of stability criteria for DC power distribution systems. IEEE Transactions on Industry Applications, 2014, 50(5): 3525–3535

[3] Nilsson D, Sannino A. Efficiency analysis of low-and medium-voltage DC distribution systems. In: IEEE Power Engineering Society General Meeting, Denver, CO, USA, 2004, 2315–2321

[4] Starke M R, Li F, Tolbert L M, Ozpineci B. AC vs. DC distribution: maximum transfer capability. In: 2008 IEEE Power and Energy Society General Meeting–Conversion and Delivery of Electrical Energy in the 21st Century, Pittsburgh, PA, USA, 2008

[5] Starke M R, Tolbert L M, Ozpineci B. AC vs. DC distribution: a loss comparison. In: 2008 IEEE/PES Transmission and Distribution Conference and Exposition, Chicago, IL, USA, 2008, 1–7

[6] Boroyevich D, Cvetković I, Dong D, Burgos R, Wang F, Lee F. Future electronic power distribution systems a contemplative view. In: 2010 12th International Conference on Optimization of Electrical and Electronic Equipment, Basov, Romania, 2010, 1369–1380

[7] Bifaretti S, Zanchetta P, Watson A, Tarisciotti L, Clare J C. Advanced power electronic conversion and control system for universal and flexible power management. IEEE Transactions on Smart Grid, 2011, 2(2): 231–243

[8] Wu T F, Sun K H, Kuo C L, Chang C H. Predictive current controlled 5-kW single-phase bidirectional inverter with wide inductance variation for DC-microgrid applications. IEEE Transactions on Power Electronics, 2010, 25(12): 3076–3084

[9] Huang A Q, Crow M L, Heydt G T, Zheng J P, Dale S J. The future renewable electric energy delivery and management (FREEDM) system: the energy internet. Proceedings of the IEEE, 2011, 99(1): 133–148

[10] Ito Y, Zhongqing Y, Akagi H. DC microgrid based distribution power generation system. In: 4th International Power Electronics and Motion Control Conference, Xi’an, China, 2004, 1740–1745

[11] Kakigano H, Miura Y, Ise T, DC micro-grid for super high quality distribution- system configuration and control of distributed generations and energy storage devices. In: 2006 37th IEEE Power Electronics Specialists Conference, Jeju, South Korea, 2006, 1–7

[12] Kakigano H, Miura Y, Ise T. Low-voltage bipolar-type DC microgrid for super high quality distribution. IEEE Transactions on Power Electronics, 2010, 25(12): 3066–3075

[13] Brenna M, Tironi E, Ubezio G. Proposal of a local DC distribution network with distributed energy resources. In: 11th International Conference on Harmonics and Quality of Power, Lake Placid, NY, USA, 2004, 397–402

[14] Magureanu R, Albu M, Priboianu M, Dumitrescu A M. A DC distribution network with alternative sources. In: 2007 Mediterranean Conference on Control & Automation, 2007, 1–4

[15] Diaz N L, Dragičević T, Vasquez J C, Guerrero J M. Intelligent distributed generation and storage units for DC microgrids—a new concept on cooperative control without communications beyond droop control. IEEE Transactions on Smart Grid, 2014, 5(5): 2476–2485

[16] Shafiee Q, Dragičević T, Vasquez J C, Guerrero J M. Hierarchical control for multiple DC-microgrids clusters. IEEE Transactions on Energy Conversion, 2014, 29(4): 922–933

[17] Li C, Chaudhary S K, Dragičević T, Vasquez J C, Guerrero J M. Power flow analysis for DC voltage droop controlled DC microgrids. In: IEEE 11th International Multi-Conference on Systems, Signals & Devices, Barcelona, Spain, 2014, 1–5

[18] Stieneker M, Butz J, Rabiee S, Stagge H, Doncker R W D. Medium-voltage DC research grid Aachen. In: International ETG Congress 2015; Die Energiewende–Blueprints for the new energy age, Bonn, Germany, 2015, 1–7

[19] Mura F, Doncker R W D. Design aspects of a medium-voltage direct current (MVDC) grid for a university campus. In: 8th International Conference on Power Electronics, Jeju, South Korea, 2011, 2359–2366

[20] Jiang D, Zheng H. Research status and developing prospect of DC distribution network. Automation of Electric Power Systems, 2012, 36(8): 98–104 (in Chinese)

[21] Wang D, Mao C, Lu J, Chen X, Zeng J, Zhang J. Technical analysis and design concept of DC distribution system. Automation of Electric Power Systems, 2013, 37(8): 82–88 (in Chinese)

[22] Qiang S, Zhao B, Liu W, Zeng R. An overview of research on smart DC distribution power network. Proceedings of the CSEE, 2013, 33(25): 9–19 (in Chinese)

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag GmbH Germany, part of Springer Nature