A safe and reliable operation of distribution network is increasingly affected by the power generation from new energies,and the defect is more obvious when only a single real-time digital simulation or physical simulation is used to reveal the interaction between distribution network and grid-connected devices.To solve this problem,a novel approach of digital and physical hybrid simulation with the tracking of component in DC voltage is proposed for a grid-connected PV system,which is used to improve the hybrid simulation modeling for grid-connected power generation from new energies and distribution network with distributed generations.By means of establishing a grid-connected PV system model with low voltage ride-through capability,the DC component in output voltage is introduced as a new con-trol object,and an improved inverter control strategy for digital-analog connector is analyzed.In this way,the digital and physical hybrid simulation of grid-connected PV system is realized together with the tracking of component in DC voltage,which can ensure that there is no DC component in the output voltage from the digital-analog connector under a stable operation condition,while the acyclic voltage appearing in grid under a fault condition can be retained.The ef-fectiveness of this model for grid-connected PV generation is verified by simulations conducted in a PSCAD/EMTDC platform.

针对功率参考指令经常发生变化的基于电压源型换流器的多端直流输电(VSC-MTDC)系统的控制问题,提出了一种考虑直流侧响应的VSC-MTDC系统协调控制方法.建立了计及有功平衡的标幺值状态空间模型,其可进行系统的稳定性分析,并能反映直流侧的响应过程;基于该模型,提出了下垂系数和公共电压参考值的修正方法,可同时实现直流网络不平衡功率的合理分配和电压调整,且能够保证系统的稳定性;在PSCAD/EMTDC仿真平台上进行仿真,结果表明了所提方法的有效性.

Digital-analog hybrid simulation technology combining the advantages of real-time digital simulation and physical simulation will be effective for simulation and analysis in future power systems.In order to meet the development need of modern power systems, this paper proposes a architecture of the universal high power digital-analog hybrid simulation system.And it adopts the improved interface algorithm based on ideal transformer model (ITM), the high power interface on the physical side based on power electronic converter and the anti-interference signal interface system, and achieves the 400 V/50 kVA high power digital-analog hybrid real-time simulation system.Digital-analog hybrid simulation experiments have been carried out on the digital-analog hybrid simulation platform, by comparing the results of real-time digital simulation, which verifies the stability and accuracy of the system.

数字物理混合仿真在适应当前电力系统仿真时优势越来越明显,进而对它的准确性和有效性提出了更高的要求.为此,提出了一种数字物理混合仿真相位校正方案,用于改善电压电流信号在数字仿真子系统与物理仿真子系统之间传递过程产生的时间延迟.通过明确数字物理混合仿真的组成部分,解析信号交互过程,对不同类型的相位校正环节传递函数进行伯德图比较,引入相位误差评价标准,使得在一定频率范围内相位校正环节超前角度与谐波次数具有近似线性关系,保证待研究全景仿真对象在稳态工况下工频信号相位零误差,故障工况下各低次谐波误差平方和最小.在PSCAD/EMTDC平台中搭建数字物理混合仿真模型,验证了所提方案的有效性.

In order to solve the problem of DC component elimination in digital analog hybrid simulation system,the DC component of instantaneous compensation strategies based on the content of DC current component and PI control are proposed.The system eliminates the DC component in the steady state.If the DC component is smaller than the set value S,the DC component is to be eliminated;otherwise it is to be retained.The effectiveness of the control strategy is verified by simulation test.The results show that this method can effectively eliminate the DC component under steady state condition,keep the DC component required for transient fault of the system,and ensure the accurate,efficient and stable operation of digital analog hybrid simulation system.This study provides a new method to eliminate the DC component of digital-analog hybrid simulation system.

在分析负荷模型参数影响因素的基础上,基于多元线性回归法提出一种动态负荷模型参数实时选择法.首先,对一段时间内负荷的全部历史扰动实测数据进行负荷模型参数辨识,积累成模型参数数据库,并按照电压振荡幅值将其分为小扰动、一般扰动和大扰动负荷模型参数数据库;然后,根据仿真需要从相应类型的负荷模型参数数据库中用多元线性回归法搜索最匹配系统实时状况的负荷模型参数并分析其拟合精度;最后,用某城市群两个变电站的实测数据验证了所提方法的有效性和准确性.

负荷模型准确度对电力系统稳定仿真分析与控制至关重要.由于负荷特性一直在变化,在历史数据上辨识出的负荷模型参数只在有限范围内有效.目前主要采用分类综合法、多曲线辨识法和主导参数在线辨识法解决负荷参数时变性,但它们一定程度上降低了模型拟合准确度;系统的运行情况和负荷的动态特性一直在变化之中,有限分类很难获得较好的分界面、分类间还可能存在交集.在分析负荷模型参数影响因素后,利用基于可信区间的模糊线性回归提出一种动态负荷参数预测法,在负荷模型参数历史数据库中循环滚动地搜索与系统实时运行状态相匹配的动态负荷参数.这样,随着负荷模型参数数据库的增大,选择出的负荷模型参数会更加符合实际负荷动态特性.通过某大城市两个220kV变电站实测数据验证了所提算法的有效性和准确性.

In order to realize effects of complement each other’s advantages and optimization on simulation of combination of real-time simulation and physical simulation of power system,a method of ideal transformer model decoupling was used to realize power connection technology for all-digital real-time simulation device and physical simulation device and develop power exchange typed digital-analog connector. Meanwhile,a digital-analog hybrid simulation model of large power grid system was built for experiment by using this power connection technology. Comparison analysis on results of digital-physical hybrid simulation test and all-digital simulation test indicated that this technology was of higher simulation precision,stability and good practical value.

The increasing wind power penetration poses significant technical problems to the development of electric power systems. In this article, an integrated control strategy is proposed based on the characteristics of the directly driven permanent magnet synchronous generator wind turbine. The directly driven permanent magnet synchronous generator wind turbine is incorporated with superconducting magnetic energy storage, which can smooth the wind power fluctuations, enhance the low-voltage ride-through capability of the wind generation system, and achieve an uninterruptible power supply to local loads under isolated grid operation. According to the MATLAB/SIMULINK-based (The MathWorks, Natick, Massachusetts, USA) digital computer simulation model, the proposed control strategy is confirmed to be effective in power control of grid operation.

Since real load characteristics are time-varying all along, load model parameters built on some historical data are only valid within limited scenarios. A real-time dynamic load model parameter selection method based on multiple linear regression (MLR) is proposed to find the load model parameters that are best matched with real time system operation condition from load model parameters history database. And along with the database size growing larger, the load model parameter matching accuracy will become higher and higher. The effectiveness and accuracy of the proposed method are verified on field measurement data collected from a substation in a metropolitan area of China.

From points of circulating current and through potential,vector decomposition method for superimposed electro-magnetic induced potential was applied for analyzing mechanism of unbalanced three-phase current of multiple circuit trans-mission lines on the same tower,and it was pointed out the basic reason for causing unbalance three-phase current was asym-metric superimposed induced potential and low resistance circuit. A line and B line of 220 kV Xiangtie transmission line of Guangdong power grid was taken for example for simulating verification. The results indicate that it is able to reduce unbal-anced current of small lines by optimizing phase arrangement of the multiple circuit transmission lines on the same tower and changing circulation path and the former way is more economic and reliable.

储能及其变换器在微网并网时控制微网与电网的能量交换,离网时保障微网内部敏感负荷供电.通过建立储能逆变器并网/离网控制系统仿真模型研究了并网/离网切换控制策略.对于主动离网过程,提出了一种平滑无过渡过程的切换控制方法;对于被动离网过程,提出了一种基于滞环电流控制的切换控制方法;对于并网切换过程,研究了存在初始相位差的切换控制方法.建立了系统仿真模型,并对3种切换过程进行了仿真,仿真结果表明,所提切换控制方法实现了无冲击无缝切换,能够保障微网中敏感负荷的供电质量.

In order to control the cascaded H-bridges (CHB) converter with staircase modulation strategy in a real-time manner, a real-time and closed-loop control algorithm based on artificial neural network (ANN) for three-phase CHB converter is proposed in this paper. It costs little computation time and memory. It has two steps. In the first step, hierarchical particle swarm optimizer with time-varying acceleration coefficient (HPSO-TVAC) algorithm is employed to minimize the total harmonic distortion (THD) and generate the optimal switching angles offline. In the second step, part of optimal switching angles are used to train an ANN and the well-designed ANN can generate optimal switching angles in a real-time manner. Compared with previous real-time algorithm, the proposed algorithm is suitable for a wider range of modulation index and results in a smaller THD and a lower calculation time. Furthermore, the well-designed ANN is embedded into a closed-loop control algorithm for CHB converter with variable direct voltage (DC) sources. Simulation results demonstrate that the proposed closed-loop control algorithm is able to quickly stabilize load voltage and minimize the line current's THD (<5%) when subjecting the DC sources disturbance or load disturbance. In real design stage, a switching angle pulse generation scheme is proposed and experiment results verify its correctness.

Electronic power transformer( EPT) has the potential advantage of improving power system stability and supply flexibility. In this paper,both the steady-state model and transient stability model of EPT are established based on power system analysis software package( PSASP). The power flow calculation methods are studied by considering EPT,and the power flow solution based Newton's alternating method is proposed. With the help of user program interface( UPI) function and user defined( UD) model function provided by PSASP,the flow calculation and transient stability simulation are implemented. Based on 9-bus WSCC system example containing EPTs,the simulation results show the validity of the established model and the good convergence of the proposed algorithm; with the introduction of EPT,not only the power flow can be controlled flexibly,but also the system transient stability can be improved.

Integrating a battery energy storage system (BESS) with a wind farm can smooth power fluctuations from the wind farm. Battery storage capacity (C), maximum charge/discharge power of battery (P) and smoothing time constant (T) for the control system are three most important parameters that influence the level of smoothing (LOS) of output power transmitted to the grid. The economic cost (EC) of a BESS should also be taken into consideration when determining the characteristic parameters of BESS (C, P). In this study, an artificial neural network-based long-term model of evaluated BESS technical performance and EC is established to reflect the relationship between the three parameters (C, P, T) and LOS of output power transmitted to the grid, the EC of BESS. After that, genetic algorithm is used to find optimal parameter combination of C, P and T by optimising the objective function derived from the mathematical model constructed. The simulation results of the example indicate that the parameter combination of C, P and T obtained by the proposed method can better not only meet the technical demand but also achieve maximum economic profit.

The electronic power transformer (EPT) is a novel transmission and transformation device, which consists of a series connection of two voltage-source H-bridge converters and a DC-DC converter with high-frequency isolation transformer. When applied to microgrids, EPT will not only deliver energy to loads from the utility grid but also inject some amount of excess power into the utility grid. Hence, the capability of bidirectional power flow is important for EPT, which depends on the DC-link stage. The traditional synchronous chopper control for a dual active bridge (DAB) converter has some limitations in the application of bidirectional power flow area. This study proposes a novel synchronous chopper control strategy for the DAB converter to implement bi-direction power flow, details the basic principle and steady-state operation and presents the mathematical derivations. A three-phase three-stage circuit configuration of 10 kV/400 V bi-direction EPT system based on the novel control DAB converter is designed, and corresponding control schemes for the system are discussed. The performance of this EPT system is validated by the MATLAB/Simulink-based simulations and the laboratory prototype experiments.

A digital-analog hybrid simulation system consists of two subsystems:a real-time digital simulation system and a physical simulation system, which are connected with an interface device. This paper analyzes the basic design requirement of digital-analog interface, and designs the main circuit structure and control program. Simulations of the design are carried out with the MATLAB, and the results indicate that the interface device can realize voltage or current tracking well, meeting the requirements of digital-analog hybrid simulation system.

The electric power system consists of generator and power grid of both the relationship is inseparable.The generator is the source for providing electricity,supporting grid voltage,adjusting and keeping stability of the system while the grid is to connect customers and all units for forming a stable dynamic system.It is necessary to realize extensive coordination and correct interaction between generating unit and grid for ensuring common safety and stability.Among mass famous blackout accidents in the world,there are examples of inconsistence of generating unit and grid.Modern power grid is tremendous with complicated structure and equipments,meanwhile,it is complex in aspects of voltage and power angle stability,coordination of equipment safety and protection automatic device which definitely comes up with higher requirements for generating unit and grid coordination.This paper summarizes and analyzes affect,role and significance of generating unit and power grid coordination for modern power grid from the angle of global electric power system as far as possible.It states comments,development,theoretical foundation,action principles and application of generating unit and grid coordination in order to provide conference and help to study and improve problems of generating unit and grid coordination.

Parameter identification is the key technology in measurement-based load modeling. A differential evolution algorithm is proposed in this paper to identify dynamic load model’s parameter, which overcomes the slowly convergent speed and oversize population problem existing in the differential evolution algorithm for static load model parameter identification. The proposed differential evolution algorithm shows good robustness, fast convergence, strong global optimization searching capability, and reduces population size and computation burden. Dynamic load model parameter identification for field measurement data from two substations in a metropolitan shows that the accuracy of the proposed algorithm is obviously better than the genetic algorithm and the Levenberg-Marquardt algorithm combined hybrid learning algorithm, and the practicability of the proposed algorithm is validated accordingly.

Nowadays,distribution systems are facing challenges such as: increasing operation efficiency,enhancing service reliability,improving power quality and safe-interconnecting distributed generations,which on the other hand,provide new developing opportunities for distributing technology.A direct current(DC) distribution system is proposed to try to resolve the issues.The basic principle and technology characteristics of DC distribution system are summarized and some of its key technologies and feasibility are analyzed.An electronic power transformer-based multi-level DC distribution system concept is proposed.As a case study,a concrete low voltage DC distribution system for residential application is designed in detailed and is demonstrated by simulation.