SST is the fundamental component of the innovative smart micro-grid future renewable electric energy delivery and managementl(FREEDM) proposed by NC State University. In this paper, the operating mode, system function and topology of SST are analyzed from the FREEDM's overall system objectives. A three phase SST model based on Chinese power system parameters is built. The control strategies of input AC-DC rectifier, DC-DC converter and output DC-AC inverter are illustrated respectively, and SVPWM control technique is adopted to strengthen SST's performance. Simulation results demonstrate that the SST can exchange energy with grid smoothly while operating under a unit power factor condition. The low voltage wave is a perfect sine curve. The SST has good stability when heavy disturbances occur. This paper will play a constructive role in promoting the FREEDM research in China.

SST is the fundamental component of the innovative smart micro-grid future renewable electric energy delivery and managementl(FREEDM) proposed by NC State University. In this paper, the operating mode, system function and topology of SST are analyzed from the FREEDM's overall system objectives. A three phase SST model based on Chinese power system parameters is built. The control strategies of input AC-DC rectifier, DC-DC converter and output DC-AC inverter are illustrated respectively, and SVPWM control technique is adopted to strengthen SST's performance. Simulation results demonstrate that the SST can exchange energy with grid smoothly while operating under a unit power factor condition. The low voltage wave is a perfect sine curve. The SST has good stability when heavy disturbances occur. This paper will play a constructive role in promoting the FREEDM research in China.

By virtue of its remarkably low voltage stress，low harmonics and low power loss，the three-level inverter is increasingly applied in renewable energy systems such as the solar and wind power system.Anyway，the manifest feature of the new energy generating system is power fluctuation.Proceeding from the stochastic power fluctuation，the conventional three-level inverter is evaluated in terms of its topological power loss first.Then，some new schemes of hardware and control strategies are proposed for a three-level inverter applied in a solar inverter.Finally，a prototype is built to verify the improvement.Test results show the proposed schemes can improve the efficiency of the three-level inverter，especially the efficiency in the low and medium power range.

By virtue of its remarkably low voltage stress，low harmonics and low power loss，the three-level inverter is increasingly applied in renewable energy systems such as the solar and wind power system.Anyway，the manifest feature of the new energy generating system is power fluctuation.Proceeding from the stochastic power fluctuation，the conventional three-level inverter is evaluated in terms of its topological power loss first.Then，some new schemes of hardware and control strategies are proposed for a three-level inverter applied in a solar inverter.Finally，a prototype is built to verify the improvement.Test results show the proposed schemes can improve the efficiency of the three-level inverter，especially the efficiency in the low and medium power range.

在三相并网型太阳能、风力发电系统和交－直－交变频调速传动系统当中，PWM逆变器作为直流侧的典型负载，其直流侧的静、动态行为，对于其前端变流器的建模、控制方法及静动态性能研究和系统设计都具有重要作用。该文通过研究逆变器与四象限变流器(4QC)拓扑结构的统一性，将4QC的状态空间平均(SSA)模型经过移植得到逆变器的SSA模型，进而提出逆变器直流侧的等效数学模型和近似简化等效模型；通过理论分析和仿真研究揭示PWM逆变器的直流侧等效模型与逆变器及交流侧电路参数之间的定量关系，并给出等效模型的参数设计公式。仿真与实验研究结果证明了所建模型及理论分析的正确性与可行性。

在三相并网型太阳能、风力发电系统和交－直－交变频调速传动系统当中，PWM逆变器作为直流侧的典型负载，其直流侧的静、动态行为，对于其前端变流器的建模、控制方法及静动态性能研究和系统设计都具有重要作用。该文通过研究逆变器与四象限变流器(4QC)拓扑结构的统一性，将4QC的状态空间平均(SSA)模型经过移植得到逆变器的SSA模型，进而提出逆变器直流侧的等效数学模型和近似简化等效模型；通过理论分析和仿真研究揭示PWM逆变器的直流侧等效模型与逆变器及交流侧电路参数之间的定量关系，并给出等效模型的参数设计公式。仿真与实验研究结果证明了所建模型及理论分析的正确性与可行性。

当采用大量电力电子设备的微网孤岛运行时，使用P/f下垂控制存在负荷快速分配与频率稳定控制之间的矛盾，为此，提出改进的P/?和Q/V下垂控制策略，将电压和相角偏差作为前馈量加入功率控制环节，实现电压和相角的二次调整，提高潮流变化时系统的动态适应性。仿真分析表明，所提出的控制策略不仅可实现负荷快速分配，而且相比P/f下垂控制具有更小的频率偏差，为高压微网孤岛运行时频率稳定控制提供了一种新方法。

当采用大量电力电子设备的微网孤岛运行时，使用P/f下垂控制存在负荷快速分配与频率稳定控制之间的矛盾，为此，提出改进的P/?和Q/V下垂控制策略，将电压和相角偏差作为前馈量加入功率控制环节，实现电压和相角的二次调整，提高潮流变化时系统的动态适应性。仿真分析表明，所提出的控制策略不仅可实现负荷快速分配，而且相比P/f下垂控制具有更小的频率偏差，为高压微网孤岛运行时频率稳定控制提供了一种新方法。

在微电网多逆变器并联系统中，由于逆变器的输出阻抗以及与公共连接点的线路阻抗存在差异，应用传统下垂控制法会导致逆变器间的环流较大及功率均分精度较低。在分析多逆变器并联系统中传统下垂控制法及逆变器输出阻抗对系统性能的影响基础上，通过引入感性虚拟阻抗，提出一种适合微网多逆变器并联的电压电流双环下垂控制策略。虚拟阻抗的引入使输出阻抗仅由滤波电感值决定，减少了逆变器输出电阻的影响；考虑线路阻抗的影响，提出一种新型改进下垂控制算法，通过对下垂系数进行修正，减弱了线路阻抗差异对并联均流的影响，提高了多逆变器并联性能。仿真与实验结果表明了该控制策略的正确性和有效性。

在微电网多逆变器并联系统中，由于逆变器的输出阻抗以及与公共连接点的线路阻抗存在差异，应用传统下垂控制法会导致逆变器间的环流较大及功率均分精度较低。在分析多逆变器并联系统中传统下垂控制法及逆变器输出阻抗对系统性能的影响基础上，通过引入感性虚拟阻抗，提出一种适合微网多逆变器并联的电压电流双环下垂控制策略。虚拟阻抗的引入使输出阻抗仅由滤波电感值决定，减少了逆变器输出电阻的影响；考虑线路阻抗的影响，提出一种新型改进下垂控制算法，通过对下垂系数进行修正，减弱了线路阻抗差异对并联均流的影响，提高了多逆变器并联性能。仿真与实验结果表明了该控制策略的正确性和有效性。

多逆变器并联系统中大多采用基于相量的均流控制策略，这种均流方式由于精度低，且缺乏对瞬时环流冲击的抑制能力，已逐渐被瞬时均流方式所取代。针对逆变器并联系统建立瞬时环流模型，提出一种在瞬时环流的反馈通路中引入虚拟阻抗的瞬时均流方法。通过合理配置该虚拟阻抗值可获得简洁的动态环流调节方程，从而实现良好的瞬时均流调节特性。分析表明，引入虚拟阻抗的瞬时均流方法具有很好的均流效果，且虚拟阻抗的引入并不影响并联系统的稳定性及输出特性。仿真与实验结果验证了所提出方法的优越性。

多逆变器并联系统中大多采用基于相量的均流控制策略，这种均流方式由于精度低，且缺乏对瞬时环流冲击的抑制能力，已逐渐被瞬时均流方式所取代。针对逆变器并联系统建立瞬时环流模型，提出一种在瞬时环流的反馈通路中引入虚拟阻抗的瞬时均流方法。通过合理配置该虚拟阻抗值可获得简洁的动态环流调节方程，从而实现良好的瞬时均流调节特性。分析表明，引入虚拟阻抗的瞬时均流方法具有很好的均流效果，且虚拟阻抗的引入并不影响并联系统的稳定性及输出特性。仿真与实验结果验证了所提出方法的优越性。

A multiple feedback loop control scheme for micro source inverters in Microgrid is developed in this paper. Based on the analysis of the inverter output impedance influenced by the line impedance parameters and the controller parameters, the voltage and the current controllers in the inner-loop are designed. The PI voltage regulator is applied to make the load voltage stable. The current loop uses the proportional regulator to improve the dynamic performance. After the output impedance is designed as an inductance impedance, the power controller in the outer-loop is produced based on the droop characteristics to achieve the wireless communication between paralleled inverters in Microgrid. With the implementation of the multiple feedback loop controller, the Microgrid can export high-quality power under the grid-connected mode. When the utility fault occurs, the Microgrid runs from grid-connected mode to islanding mode, and the designed controller will regulate the active and reactive power flow and enable power sharing between the paralleled micro sources in the Microgrid. Also, the controller can make the conversion between different modes seamless. The simulation results show the validity of the proposed scheme.

A multiple feedback loop control scheme for micro source inverters in Microgrid is developed in this paper. Based on the analysis of the inverter output impedance influenced by the line impedance parameters and the controller parameters, the voltage and the current controllers in the inner-loop are designed. The PI voltage regulator is applied to make the load voltage stable. The current loop uses the proportional regulator to improve the dynamic performance. After the output impedance is designed as an inductance impedance, the power controller in the outer-loop is produced based on the droop characteristics to achieve the wireless communication between paralleled inverters in Microgrid. With the implementation of the multiple feedback loop controller, the Microgrid can export high-quality power under the grid-connected mode. When the utility fault occurs, the Microgrid runs from grid-connected mode to islanding mode, and the designed controller will regulate the active and reactive power flow and enable power sharing between the paralleled micro sources in the Microgrid. Also, the controller can make the conversion between different modes seamless. The simulation results show the validity of the proposed scheme.

The application of droop control in microgrids is analyzed in theory. The rationality and superiority of the method used in high-voltage microgrids based on solid state transformer（SST） are given. The relationship between droop coefficient and system dynamic performance is studied， and the influence of droop coefficient on the stable range of frequency is analyzed. Droop control is achieved through high-voltage converter of SST， and the relationship between droop coefficient and capacity of SST is given as well. A circular high-voltage microgrid is established， the effect of proposed droop control method on stabilizing frequency and voltage in grid-connect mode， islanding mode and the switching period between two modes is simulated and analyzed. Results show that the voltage and frequency of microgid has good stabilizing performance in different modes.

The application of droop control in microgrids is analyzed in theory. The rationality and superiority of the method used in high-voltage microgrids based on solid state transformer（SST） are given. The relationship between droop coefficient and system dynamic performance is studied， and the influence of droop coefficient on the stable range of frequency is analyzed. Droop control is achieved through high-voltage converter of SST， and the relationship between droop coefficient and capacity of SST is given as well. A circular high-voltage microgrid is established， the effect of proposed droop control method on stabilizing frequency and voltage in grid-connect mode， islanding mode and the switching period between two modes is simulated and analyzed. Results show that the voltage and frequency of microgid has good stabilizing performance in different modes.

To improve the applicability of conventional P-f and Q-V droop control methods of inverters in middle-voltage and low-voltage microgrids, this paper puts forward a novel control method of using virtual inductance and artificial output inductance in combination. A multiple feedback loop control strategy is proposed, the state-space equations are derived, and a complete small-signal model is established. Based on this, the matching principle of choosing virtual inductance and artificial output inductance is proposed. The inductance value is reduced significantly according to this principle, so problems of system stability and power loss are overcome. Simulation and analysis under linear and nonlinear load show that the proposed control strategy and parameter selecting method can not only enhance the output inductance of inverters, thus reduce the interference by line parameters, and also provide accurate load sharing, and inhibit circulating current effectively. This paper provides a new strategy for using conventional droop control in middle-voltage and low-voltage network.

To improve the applicability of conventional P-f and Q-V droop control methods of inverters in middle-voltage and low-voltage microgrids, this paper puts forward a novel control method of using virtual inductance and artificial output inductance in combination. A multiple feedback loop control strategy is proposed, the state-space equations are derived, and a complete small-signal model is established. Based on this, the matching principle of choosing virtual inductance and artificial output inductance is proposed. The inductance value is reduced significantly according to this principle, so problems of system stability and power loss are overcome. Simulation and analysis under linear and nonlinear load show that the proposed control strategy and parameter selecting method can not only enhance the output inductance of inverters, thus reduce the interference by line parameters, and also provide accurate load sharing, and inhibit circulating current effectively. This paper provides a new strategy for using conventional droop control in middle-voltage and low-voltage network.

This paper presents an architecture for a future electric power distribution system that is suitable for plug-andplay of distributed renewable energy and distributed energy storage devices. Motivated by the success of the (information) Internet, the architecture described in this paper was proposed by the NSF FREEDM Systems Center, Raleigh, NC, as a roadmap for a future automated and flexible electric power distribution system. In the envisioned "Energy Internet," a system that enables flexible energy sharing is proposed for consumers in a residential distribution system. The key technologies required to achieve such a vision are presented in this paper as a result of the research partnership of the FREEDM Systems Center.

The smart grid features ubiquitous interconnections of power equipments to enable two-way flows of electricity and information for various intelligent power management applications, such as accurate relay protection and timely demand response. To fulfill such pervasive equipment interconnects, a full-fledged communication infrastructure is of great importance in the smart grid. There have been extensive works on disparate layouts of communication infrastructures in the smart grid by surveying feasible wired or wireless communication technologies, such as power line communications and cellular networks. Nevertheless, towards an operable, cost-efficient and backward-compatible communication solution, more comprehensive and practical understandings are still urgently needed regarding communication requirements, applicable protocols, and system performance. Through such comprehensive understandings, we are prone to answer a fundamental question, how to design, implement and integrate communication infrastructures with power systems. In this paper, we address this issue in a case study of a smart grid demonstration project, the Future Renewable Electric Energy Delivery and Management (FREEDM) systems. By investigating communication scenarios, we first clarify communication requirements implied in FREEDM use cases. Then, we adopt a predominant protocol framework, Distributed Network Protocol 3.0 over TCP/IP (DNP3 over TCP/IP), to practically establish connections between electric devices for data exchanges in a small-scale FREEDM system setting, Green Hub. Within the real-setting testbed, we measure the message delivery performance of the DNP3-based communication infrastructure. Our results reveal that diverse timing requirements of message deliveries are arguably primary concerns in a way that dominates viabilities of protocols or schemes in the communication infrastructure of the smart grid. Accordingly, although DNP3 over TCP/IP is widely considered as a smart grid communication solution, it cannot satisfy communication requirements in some time-critical scenarios, such as relay protections, which claim a further optimization on the protocol efficiency of DNP3.