This paper discusses the digital simulation methods with parallel computing of large-scale AC/DC power systems. Firstly, we introduce four power system simulators with parallel computing capability, and analyze their characteristics and applicability. Secondly, we compare three equivalent methods of power system including: the equivalent method based on frequency response, the Ward equivalent method and the dynamic equivalent method based on physical equivalent； and analyze their advantages and disadvantages. Then, we study the parallel computing method of large-scale AC/DC power system based on PSCAD/EMTDC simulator in detail, from three aspects of model transformation and correspondence, element layout and connection, subsystem division. Meanwhile, we comprehensively analyze the parallel computing simulation method of large-scale AC/DC power system based on RT-LAB, as well as the system's initialization in detail.
For the multi-terminal MMC （modular multilevel converter）-HVDC system and DC grid, the traditional electromagnetic transient simulation programs based on serial structure have been unable to meet the demand of the actual calculation, so it is necessary to adopt parallel computing technology. As electromagnetic transient simulation software for power system, PSCAD/EMTDC is widely used in the world, and the latest version has full support for parallel computing. Through large model split and multi-threaded operation, this software can solve the problems of cannot being simulated or low simulation efficiency due to too large model, which can provide the possibility to achieve the fast simulations of multi-terminal MMC-HVDC system and DC grid. We analyze the operation mechanism and function and of PSCAD/EMTDC in detail, introduce and research the parallel computing function in its new version. Then we construct the model for the simulation test and discuss the technique of parallel computing. The simulation results show that the parallel computing function can greatly reduce the simulation time of large scale power system and effectively improve the efficiency of simulation analysis.
Aiming at the dynamic characteristics of HVDC and its control model in electromechanical transient simulation unable to correspond accurately with the ones of actual systems, this paper proposes a HVDC modeling method based on DC voltage fault response characteristics. Firstly, we analyze the differences between the electromechanical transient model and electromagnetic transient model of DC system, and identify the key factor that influences HVDC dynamic characteristics. Secondly, we present the key idea which utilizes the characteristics of HVDC electromagnetic transient model under fault to improve the electromechanical transient model. Finally, based on the detailed simulation in PSCAD/EMTDC, we extract the fault response characteristics of DC voltage, and introduce the response characteristics into the electromechanical transient model of DC converter according to AC system fault type and converter bus voltage variation. Based on the method, a user-defined HVDC model has been built in PSS/E, and the simulation results verify the practicality of the modeling method.
The traditional backup protection has some drawbacks, including difficult protection setting, long time delay and complicated coordination. Given this background, this paper proposes a fault identification and wide-area backup protection strategy based on limited phasor measurement units （PMUs）. Firstly, the whole power network is divided into several backup protection zones （BPZs） based on the placement of PMUs. Once a fault occurs, the electrical quantities are utilized to identify the faulted BPZ, so as to quickly narrow down the scope of suspected fault sections. Then, we present fault identification method and wide-area backup protection strategy based on the analytical model. When the main protection fails to clear the fault, this analytical model can comprehensively use the alarm information of main protection, circuit breaker action alarm, backup protection startup information and power direction relays to point to the information, adopt tabu search （TS） algorithm to seek the optimal fault hypothesis, and determine the malfunction devices through analyzing the action logics of protection and circuit breaker. Based on the identified malfunctions, the corresponding backup protection strategy is initiated to prevent the fault from spreading. The simulation results show that the proposed method is feasible and has strong fault tolerance ability.
For the situations of complicated and multiple faults with missing and/or false alarms, it is still a challenging task to develop an accurate approach for fault diagnosis of high-voltage （HV） lines and operating behavior evaluation of protective devices. Given this background, this paper presents a rule-network based approach for this purpose by employing fault recorder data. Firstly, the fault recorder data are pre-processed by using the well-developed wavelet transform to extract the fault characteristics of HV lines from analogue electrical information, and the events of sudden current change and protection action are obtained. On this basis, we construct a rule-network based model for fault diagnosis of HV lines with considering the temporal characteristics of alarm messages, determine the sets of fault hypotheses as well as the expected states of protective relays and circuit breaker, and determine the fault cause and evaluate the performance of protection and circuit breaker with using the causal and temporal relationships between them. In addition, we further analyze the fault recorder to determine the type, characteristics and location of fault. Finally, we adopt actual power system fault cases to demonstrate the proposed method.
With the fast development of smart distribution systems, more and more distributed generation （DG） sources are connected, so traditional fault diagnosis approaches for distribution systems with single power source are no longer applicable. Given this background, this paper proposes a fault diagnosis method based on fault transient components and genetic algorithm for distribution systems with DGs. Considering that the DG connections make the transient zero-sequence current direction complicated at fault point, an improved coding method for feeder terminal units is presented by specifying the upstream and downstream power sources for each switch, and the priority level of a faulted line to be diagnosed is also taken into account. We adopt a wavelet based method to extract fault transient components, which is well applicable for the widely-implemented non-effective grounding in middle-voltage distribution systems in China, and can solve, to some extent, the problem of inconspicuous steady fault current with single-phase ground faults. Finally, we demonstrate the proposed method with a sample system, and analyze its fault tolerance performance for distorted and missing fault information.
Investment plan selection is vital in the preparation stage of photovoltaic （PV） power generation project and the multi-criteria decision making （MCDM） methods play the important roles in the process of project plan selection. But the present MCDM methods have not considered the fuzzy nature of decision making and the conflict between the criteria, which decreases the evaluation quality of power generation project. Therefore, this paper introduces the fuzzy set theory （FST） and VIKOR method to construct the decision framework of PV project plan selection. We use fuzzy numbers instead of numerical values to decrease the loss of decision information； and adopt VIKOR method to determine compromise solutions for the problem that the decision results cannot be accepted by all the decision makers when several conflict indicators exist. The proposed framework is divided into 3 parts: the determination of alternative evaluation scheme, the determination of criteria weight based on expert group order relation, and the investment plan selection based on fuzzy VIKOR. These three parts are closely related and influence each other. Finally, taking a 100 MW ground grid-connected PV power station project as example, we verify the effectiveness and reasonableness of the proposed framework.
To solve the problem of intermittent power connected to grid, this paper proposes the flexibility evaluation model of multi-time scale power system with considering wind power uncertainty and load fluctuation. This paper firstly introduces the wind power uncertainty, and describes the concept and characteristic of flexibility. From the view of the output change capacity of the wind farm in power grid, according to the change of load, we enumerate three kinds of time scales, and propose the flexibility evaluation model for the system response to the maximum change rate of wind farm output in power grid based on different time scales. Then, we explain the mathematical model of specific flexibility evaluation, and use interior point method to solve the model. Finally, taking Gansu power system as an example, the proposed flexibility indices is illustrated by a specific example.
With the development of smart grid and the deregulation of electricity market, the demand response (DR) has been widely used in the operating mode of power market. Many countries have introduced the realtime pricing (RTP) strategies of DR to improve the efficiency of power system and the ability of power market. The careful analysis of the users response to the RTP is significant for the implementation of DR strategies. Therefore, we propose a regression model to learn the price elasticity of electricity demand (PED) based on the PED model and simulate the responsive behaviors of users. Experimental results show that comparing with the content PED obtained by tradition questionnaire, the learned PED can effectively meet the demand, and the regression model can overcome the problems both in time and space and achieve the analysis of users responsive behaviors efficiently, which can provide decision support for the realtime pricing strategy.
This paper applies phasor measurement unit （PMU） to measure the real-time data including the voltage and power flow of any （non PV） node and adjacent busbar, and constructs a network equivalent model. Meanwhile, we present the on-line voltage stability index with comprehensive consideration of the active power and reactive power of load node based on this model. And we suggest that the on-line voltage stability indexes of active power and reactive power can be used as active power or reactive power margin for the characterization of load node respectively, which can be the supplement to the on-line voltage stability index of node （system）. If the on-line voltage stability index of load note approaches to 1, it shows that the power system draws near voltage collapse. Simulation results of EPRI-36 bus system show that the proposed method can effectively judge the system voltage stability, and is suitable for the monitoring and prediction of on-line voltage stability. Compared with other indexes, the rationality and superiority of the proposed model and index are reflected. Finally, we briefly explain the feasibility of the proposed method through the voltage stability margin evaluation of load node in Shandong 500 kV main network.
This paper focuses on the optimal planning issue of integrated energy system （IES） to improve the energy efficiency of IES. Firstly, we construct a two-layer optimization model for IES planning with power as core, which includes the optimization models for IES dispatching and IES planning. And the operation indicators of system optimization are taken as the basis of system planning optimization. The objective functions of this model are under the economic optimum criterion and the environmental optimum criterion respectively, and it also emphasizes on the operation constraints of IES on periods of heating and cooling. Secondly, we propose particle swarm optimization algorithm and tabu search algorithm to solve the two-layer optimization model respectively. Finally, the two-layer model and the algorithms are verified through a case study to be useful for the optimal planning schemes of IES under the economic and environmental criterion respectively. There is contradiction between the planning scheme under economic criterions and that under environment criterion. The former one is inclined to energy storages to take advantage of time-of-use electricity price, while the latter one is inclined to energy supply devices with high efficiency.
To solve the problems existing in the practical application of half wavelength AC power transmission （HWACT） technology, this paper carries out the experimental research on HWACT. We choose 1 000 kV UHVAC transmission line and 750 kV transmission line as real lines for the experimental research, and adopt technology economic analysis method to propose two 1 000 kV HWACT test schemes and four 750 kV HWACT test schemes. Based on the feasibility analysis of the test schemes, we present the recommended field test scheme of HWACT. Finally, according to the simulation analysis on the recommended test scheme, we obtain the suppression measures for the over voltage and secondary arc current of HWACT.
It is necessary to calculate the main circuit steadystate parameters at the beginning of the planning and design of HVDC projects. Firstly, this paper simplifies UHVDC transmission system with hierarchical connection mode into two singlelayer systems, and concludes the method of main circuit steadystate parameters calculation for singlelayer system. Then we analyze two ways to increase the reactive power of converters during low power transmission and present the corresponding reactive power validations. If the DC current increases due to its reactive power requirement in one system, the DC current through the hierarchical systems after reactive power validation will be different in low power condition. Aiming at this problem, we propose calculation between layers as a solution. The lowest DC voltage across the converters of hierarchical systems is selected to be the control voltage between layers, the other control parameters of the converters are adjusted according to this voltage. We analyze the changing characteristics of extinction angle, ideal noload DC voltage, as well as the reactive power consumption of converter after adjustment with using Matlab. At last, we analyze the steadystate operating characteristics with an example, while verifying the reasonability and effectiveness of the proposed algorithm.
This paper firstly introduces the general situation of Jiangsu Power Grid and electric power input from external regions. According to the operation status of electric power input from external regions, we discuss the problems caused by inputting electric power to Jiangsu Power Grid from views of total amount, regulating characteristics, electricity balance, etc. Based on the brief analysis of the main influence factors of electric power accommodation from external regions, we analyze the accommodation capacity of the existing electric power input and the planning schemes for the future input electric power increase. From the analysis it can be found that, the planned grid structure of Jiangsu Power Grid in 2020 can be satisfied to receive 55 GW electricity from outside system; but the dynamic reactive power compensation device with a certain capacity should be configured near the UHV placement, and the 500 kV grid construction should be improvec near the placement and the layout of accident reserve capacity should be optimized in the province. Meanwhile, the planned main grid structure of Jiangsu Power Grid in 2020 can meet the requirements of the safe and stable operation during N-2 fault or double-level locking fault in a DC convertor station.
As a kind of important means for DC active power modulation, frequency limit controller （FLC） can improve the frequency stability level of DC/AC hybrid system. However, DC active power modulation will change the reactive power consumption of converter station, thus affecting the voltage characteristics of system. In allusion to Chuxiong-Suidong UHVDC sending system under islanded operation, this paper analyzes the influence mechanism of DC active power modulation by FLC on the voltage recovery characteristics of rectifier station after the short circuit fault of AC system. On this basis, we propose the optimization control strategy for FLC based on the change rate of reactive power, which can improve the voltage stability of sending system. The simulation results show that this strategy can inhibit frequency fluctuation, while improve the voltage recovery characteristics of islanded system.
The technical improvement project of pole Ⅱ lower converter valve in ± 800 kV HVDC
Southern Hami south converter station is the first technical improvement project of certain foreign brand HVDC valve completed in China. This paper firstly introduces the technical defects and technical promotion programs of this foreign brand valve, and then discusses the designs and main innovation points of project commission programs of subsystem, station system and whole system. Site test analysis results show that the running temperature of DC grading resistor of improved thyristor valve is significantly reduced, and thyristor state detection mechanism is improved. There is no protection misoperation, fault trip or commutation failure in the test, and the performance parameters of improved thyristor valve meet the requirements of engineering design.
Reliability, real-time performance and bidirection are the requirements of the communication system of intelligent distribution network. Based on the analysis of current intelligent distribution network communication system and the latest IEC 61850 standard, this paper constructs the model for the traffic and communication needs of the intelligent distribution network. We build the simulation model of the communication network in intelligent distribution network on the OMNeT++, which is an open source platform of multi protocol network simulation software. In the background of the data flow which adds a large amount of distributed energy and electric vehicle, we study the communication process between the process-level IED and the station-level distribution substation, and compare the transmission delay of IED and distribution substation under different bandwidth and traffic background.
According to the stochastic volatility characteristics of distributed generation and load in active distribution network, this paper proposes the multi-time scale optimization strategy for active distribution network based on soft normally open point （SNOP）. With considering the operating characteristics of SNOP, we control the output power of SNOP through the global optimization strategy of distribution network on long-time scale, and realize the dynamic adjustment on the output reference value of SNOP through voltage fluctuation hysteresis control on short-time scale, which can maintain the line voltage stable, and enhance the absorptive capacity of the distributed generation. In the improved IEEE-33 bus system, we analyze the global optimization on long-time scale and the dynamic adjustment on short-time scale, as well as discuss the installation position, capacity and number of SNOP. The results show that the multi-time scale optimization strategy of active distribution network based on SNOP is feasible and effective, and UPFC-type SNOP has a smaller capacity requirement than that of B2B-type.
To make full use of the advantages of hybrid energy storage system （HESS）, and improve the economic and reliability of microgrid operation, this paper proposes the HESS wavelet packet-fuzzy control strategy. Considering the real-time price of exchange power under grid connected operation and the lack of electricity under island operation, the grid economic evaluation index for grid connected operation and the loss of power supply probability for island operation are established based on smoothing the output power fluctuations of renewable energy. We obtain the initial charging and discharging instructions of HESS through the wavelet packet decomposition of intermittent micro power supply, use super capacitors to stabilize instantaneous power fluctuations, apply the unbalanced power to modify the instructions of storage battery charging and discharging, and then obtain the final instructions of storage battery charging and discharging through fuzzy control. Finally, the efficiency of the proposed control strategy is demonstrated by a 'Wind-PV-Coal-Energy Storage' microgrid.
Domestic main transformer was exposed in early time and its conserver was likely to be stroke by lightning. Through the analysis on the causes of the protection distance of metal oxide arrester （MOA）, this paper constructs the transient model of transformer during lightning stroke with using electromagnetic transient simulation software ATP/EMTP, and analyzes the influences of lightning current amplitude and MOA's position on the overvoltage of substation devices combined with engineering examples. The results show that, the main transformer is the mainly influenced device in the platform, and the MOA's position has a great effect on the lightning overvoltage of the transformer, during the lightning stroke on the conserve in main transform. Therefore, this paper further analyzes the protection distance of MOA, whose results show that MOA should be installed as close as possible to the main transformer during the design of offshore substation. At last, through the parameter fitting, we obtain the curves of MOA's protection distance with lightning current amplitude and that with earth resistance, which can a reference for the MOA's installment in offshore substation.
In order to simplify the calculation, generally, the model of cable joints is established with its axial symmetry during the temperature field simulation. But this model is different from the actual laying situation, as a result, the simulation is not accurate. We simulate the temperature fields of the cables under the axial symmetric model and the actual laying conditions respectively with using finite element software Ansys. Through the comparison and analysis, it can be found that a certain radius of the annular soil can be used to simulate the actual soil conditions. The radius is applied to the temperature field simulation of cable joints. Based on the proposed method of using a certain radius of the annular soil to simulate the actual cable laying soil conditions, we can make the temperature field simulation of cable joints more accurate under the axial symmetry model.
