bluuebunny/crossref_metadata_2025 huggingface dataset:
Efficient and robust M‐ary differential chaos shift keying scheme with code index modulation
Authors: Tan Yunsheng , Xu Weikai , Hu Wei , Wang Lin
Published: October 2018

In this study, a new chaos modulation scheme which combines M‐ary differential chaos shift keying (DCSK) with code‐index modulation (CIM‐MDCSK) is proposed. In the CIM‐MDCSK scheme, the two Walsh codes that are used to generate two orthogonal chaos components are selected from the candidate codes, whose code indices are determined by information symbols. Thus, the information bits are conveyed not only by M‐ary DCSK signal but also by the indices of the Walsh codes. Theoretical symbol error rate (SER) and bit error rate (BER) of the proposed scheme are derived and confirmed by simulations. Furthermore, by applying the noise‐reduction to reference signals of CIM‐MDCSK, the SER/BER expressions are also derived. Simulation results show that the proposed CIM‐MDCSK outperforms DCSK and M‐ary DCSK based on Hilbert transform. Especially, the CIM‐MDCSK with noise‐reduction shows significant advantages. Moreover, the performance of CIM‐MDCSK scheme is compared with direct sequences spread‐spectrum system with code‐index modulation over flat and frequency selective fading channel. The proposed CIM‐MDCSK can offer good performance over frequency selective fading channel with a low complexity non‐coherent receiver which does not require any channel state information and channel equalisation.
Game framework for optimal allocation of spinning reserve to confront intelligent physical attacks on power system
Authors: Ranjbar Mohammad Hossein , Kheradmandi Morteza , Pirayesh Abolfazl
Published: October 2018

Spinning reserve (SR) serves as an efficient tool to respond to a sudden outage of power system components caused by either natural events or physical attacks. Contrary to natural threats, the intelligent physical attacks are strongly influenced by the measures taken by the system operator to defend the system and exhibit no stochastic behaviour. As a consequence, the probabilistic methods of SR provision and allocation do not prove to be appropriate to confront intelligent attacks. In this study, the interaction between the system operator as the defender and the intelligent attacker is investigated within a game model. In this model, the SR is allocated in such a way that the highest interruption cost that the attacker can impose is minimised. The technical limitations of SR provision together with operational constraints are taken into consideration. The efficacy of the proposed method is examined by conducting case studies of single and multiple‐target attacks on components of IEEE 24‐bus and IEEE 118‐bus test systems. The results show that the proposed scheme produces a considerable reduction in the system interruption cost in comparison with the probabilistic methods.
Optimal location of the U‐turn at a signalised intersection with double left‐turn lanes
Authors: Sun Feng , Sun Li , Ma Dongfan , Wang Yinhai , Yao Ronghan , Zeng Ziqiang
Published: October 2018

The location of the U‐turn median opening at signalised intersections can have important effects on the intersection performance. However, there has been very little prior work on this issue, especially on intersections with double left‐turn lanes. This study analyses the mutual interference of the U‐turn and the left‐turn movements at signalised intersections with double left‐turn lanes. A lane selection model, based on the Wardrop Equilibrium Principles is developed for left‐turn drivers. The saturation flow rate model of the double left‐turn lanes is developed to quantitatively evaluate its utilisation. This study also presents the guidelines for the design of the U‐turn median opening for maximising the saturation flow rate of double left‐turn lanes. The proposed model is also verified by means of simulation, and then a case study is used to compare the intersection performance before and after optimisation. The results of case study show that if the U‐turn locations are varied, the saturation flow rate of the double left‐turn lanes would also vary considerably. The authors show that there can be a major difference of 77.47%, and the saturation flow rate of optimal location can be improved by 7.63% compared with the current location of the U‐turn median opening.
Enhanced orthogonal signal generator for a single‐phase grid‐connected converter
Authors: Taghizadeh Seyedfoad , Hossain M. Jahangir , Lu Junwei
Published: October 2018

This study presents a new orthogonal signal generation (OSG) technique for the control system of a single‐phase grid‐connected voltage‐source converter (VSC). The existing methods mostly suffer from delay, rely on the system parameters, or require multiple inputs. Among them, the second‐order generalised integrator (SOGI) excludes the drawbacks of the other methods and shows better steady‐state and dynamic response. The proposed OSG method here includes the advantages of the SOGI technique while demonstrating superior dynamic response and higher disturbance rejection capability. It is structurally simple without adding complexity to the control system. The proposed method is implemented in a DQ‐frame current controller and its feasibility and reliability are verified through mathematical analysis, simulation, and experimental results.
Impact quantification of satellite outages on air navigation continuity
Authors: Zhai Yawei , Zhan Xingqun , Joerger Mathieu , Pervan Boris
Published: October 2018

Advanced receiver autonomous integrity monitoring (ARAIM) using multiple global navigation satellite system constellations is expected to bring significant navigation performance improvement to civil aviation. However, using multiple constellations, the higher number of satellite outages due, for example, to routine station keeping manoeuvres, could significantly increase continuity risk. In response, this study proposes a new method to rigorously quantify the impact of satellite outages on navigation safety. To achieve this, integrity and continuity risk bounds are derived, which account for all possible outage conditions. In addition, the proposed approach allows to determine whether an exclusion function is needed following an outage event. The method is implemented to analyse horizontal ARAIM (H‐ARAIM) availability performance. Results indicate that dual‐constellation H‐ARAIM can provide high service availability, where both integrity and continuity requirements are met.
Pilot protection scheme for transmission line of a hybrid HVDC system based on polarity characteristics of current and voltage fault components
Authors: Wang Yanting , Zhang Baohui , Cheng Shaojie
Published: October 2018

A hybrid high‐voltage direct current (HVDC) system comprises a line commutated converter (LCC) at the rectifier side and a voltage source converter (VSC) at the inverter side, combining advantages of both LCC and VSC systems. However, the asymmetric converters at both ends of line constrain the direct application of line protections used for conventional HVDC systems. This study proposes a polarity‐based pilot transmission line protection for hybrid HVDC systems. By digitising the current and voltage fault components, their polarity characteristics are extracted to identify the transmission line fault, which may eliminate the influence of the fault component magnitude differences caused by different converter structures and control strategies. Simulation results show that the proposed protection is valid and feasible and is capable of distinguishing fault with large transition resistance.
SSO of DFIG‐based wind farm integrated by a hybrid series compensator
Authors: Zhu Xinyao , Zhou Qian , Zhang Ningyu , Chen Zhe , Zhao Jingbo , Jin Meng
Published: October 2018

Doubly‐fed induction generator (DFIG)‐based wind farms integrated by series compensated transmission lines are confronting with the problem of subsynchronous oscillation (SSO). In this study, the configuration and control stratagem of hybrid series compensator composed of a static synchronous series compensator (SSSC) and a fixed capacitor are proposed, and the SSO of DFIG‐based wind farm integrated by a hybrid series compensator are studied. The results reveal that the SSO risk of the system is lower by using the hybrid series compensator, and the SSO damping is larger with a higher proportion of SSSC in the hybrid series compensator, as well as with a larger compensation impedance or voltage of SSSC. The research is supposed to be helpful for the planning and operating of the power system with large‐scale wind farms integrated.
Performance of the battery energy storage systems based on cascaded H‐bridge multilevel converter
Authors: Gadalla Abuzaid Saeed , Yan Xiangwu , Hasabelrasul Hashim
Published: October 2018

The battery energy storage system (BESS) based on the cascaded multilevel converter, that consists of cascaded H‐bridge converter, is one of the most promising and interesting options, which is taken to compensate the instability of electric power grid when integrated with renewable sources such as photovoltaic (PV) and wind energy. This article describes 14.14 kV, 2 MW, and 1000 Ah BESSs based on a three‐phase cascaded H‐bridge multilevel converter using lithium–ion batteries. Therefore, the article focuses on the performance of the system integrated with both the electric power grid and the local load power applications. Moreover, the control of the system is based on active power control and state‐of‐charge balance control of the individual cell and phase, which depends on superimposing the AC voltage and the zero‐sequence voltage injection. Finally, using MATLAB/Simulink tools, the simulation results obviously verify the controller's performance and efficiency as well.
Benefit analysis of using multi‐port and multi‐function power electronic transformer connecting hybrid AC/DC power grids
Authors: Yang Yanhong , Pei Wei , Deng Wei , Zhang Shizhong
Published: October 2018

This study proposes a generalised configuration of the multi‐port and multi‐function power electronic transformer (MPMF‐PET) based mixed AC/DC power grids, and the equivalent circuit and mathematical model of the MPMF‐PET are established. To illustrate the benefits of using MPMF‐PET to improve the integration and power control ability of a distributed energy resource in an existing distribution network, an iterating evaluation algorithm is proposed for system power flow calculations. Case studies are carried out on a real distribution system modified with MPMF‐PET. Results show that the performance of the hybrid AC/DC distribution system is increased by MPMF‐PET through proper control and optimal operation, which realised the renewable energy in the AC system to power the DC system and increased the renewable energy acceptance.
Contingency‐constrained optimisation method for operating urban power grids with flexible switching stations
Authors: Liu Jia , Cheng Haozhong , Xu Qian , Lan Zhou , Zeng Pingliang , Yao Liangzhong
Published: October 2018

This study presents a multi‐objective operation optimisation model for urban power grids with flexible switching (FS) stations to comprehensively improve system security and reliability. In the proposed model, uncertainties associated with load variation and component failures are considered. Moreover, constraints after a feeder or transformer N  − 1 contingency are preliminarily formulated. The operation problem optimises the apparent power of feeder section loading. The combination of normal boundary intersection and sequential quadratic programming is employed to solve the non‐linear optimisation model. A case study carried out on a test 75‐feeder distribution grid with FS stations demonstrates the effectiveness of the proposed model and solving method. Compared with conventional distribution loadability, the optimal load distribution solution obtained in this study provides a trade‐off between system security and reliability. The coordinated operation optimisation method is suitable for urban smart distribution grids featured by distribution automation and large‐scale interconnections.
Stability analysis of distributed generation grid‐connected inverter
Authors: Zheng Xuemei , Liu Zhuang , Wang Chao , Liu Yidan , Feng Yong
Published: October 2018

With the distributed new energy power generation increased, the power grid system becomes more and more complex, leading to its stability more likely to be affected. Among them, the resonance caused by impedance mismatch is particularly critical. The small‐signal model of phase‐locked loop and current control in the inverter control system have been established by using harmonic linearisation method. Combining the average model of inverter main circuit, the inverter output impedance model has been obtained. Using grid impedance and the inverter output impedance model, the stability analysis method based on impedance is used to analyse the influence of grid impedance on the stability of grid‐connected inverters. Finally, the simulation proves the correctness of the analysis method.
Research on the self‐discipline control strategy of dispersed wind power
Authors: Wang Jing , Duan Jiandong , Chen Lupeng , Sun Qian
Published: October 2018

The voltage violation phenomenon can be very prominent in the condition of dispersed wind power (DWP) widely integrated into network, which affects the safe operation during integration and effective consumption of wind power. In this study, a self‐discipline control strategy of DWP is proposed to solve the voltage violation problem with high penetration. First, the influence of system voltage after the DWP integrated into network is analysed, which raises the feasibility of self‐discipline control strategy. Then, according to the proposed strategy, the object of optimisation model is designed to better the voltage quality of the grid connection point of wind power generator and to maximise the active power outputs of wind power. Considering the non‐linear and multivariable characteristics of the optimisation model, this study used the particle swarm optimisation to obtain the reactive power and active power regulations of wind power. Finally, the proposed algorithm is tested and analysed on the IEEE 33‐node distribution system, and verified by a practical project. The results show that the self‐discipline control strategy can effectively eliminate the voltage violation caused by the DWP integration. Moreover, the wind power can be fully utilised.
Power system stochastic transient stability assessment based on Monte Carlo simulation
Authors: Liu Jiayu , Liu Jun , Zhang Jie , Fang Wanliang , Qu Longchu
Published: October 2018

Electrical power system is a complicated dynamic system with plenty of random disturbances while this character has been aggravated attributed to the rapid development of renewable energy technology. Under this circumstance, traditional power system transient stability analysis methods may not always be appropriate. Thus, stochastic transient stability analysis has been established by introducing stochastic differential equations to traditional transient stability analysis. Previous studies on stochastic transient stability assessment just provided a preliminary result but failed to assess its accuracy and reliability, which are essential mathematical properties. Based on the theory of probability and statistics, the Monte Carlo simulation is utilised to further improve the numerical simulation method for transient stability evaluation. Two stability assessment indices are proposed: instability probability and expectation of critical clearing time (ECCT). Then, a corresponding method to calculate the indices and their confidence level is proposed. A two‐stage adaptive method is introduced to pre‐estimate the sample capacity under a given confidence coefficient, so that the corresponding confidence intervals of the indices can be calculated. The effectiveness of the proposed method is validated on the three‐machine, nine‐bus system, which demonstrates that it would be suitable for future hybrid AC/DC power systems with fluctuating renewable energy generations.
Study on the calculation of shielding failure for dc transmission lines
Authors: Wu Jinbo , Cao Wei , Wan Shuai , Gu Shanqiang , Wang Jian , He Jinliang
Published: October 2018

Lightning is the main cause of transmission line failure, and it is important to improve the lightning protection level of the transmission line for the stability of the power system. Traditional solutions for the shielding failure of transmission lines is to use the electrical geometric model method. However, with the improvement of the voltage level, the physical size increases, and the calculation of lightning protection for ultrahigh‐voltage dc lines has a greater difference with operating experience. Based on the Ningxia‐Shandong dc transmission lines in China, this study utilised the lead development model to describe the flashover of insulators and the breakdown of the gaps. A comprehensive calculation method of shielding failure trip‐out rate considering terrain, earth resistance and weather was proposed. The accuracy was much higher when compared with the traditional calculation and the result can be a reference for other similar calculation for dc lines.
Analysis of DC voltage oscillation mechanism in AC/DC hybrid distribution system
Authors: Li Luyang , Pei Wei , Xiao Hao , Deng Wei , Kong Li
Published: October 2018

DC voltage oscillation occurs more and more frequently in voltage source converter (VSC)‐based AC/DC hybrid distribution system (AC/DC HDS). However, currently, the explanation of the oscillation mechanism is not clear enough. One reason is the traditional state‐space model of the system is too complicated, and the information in the model is too dispersed. This research aims at researching the small‐signal stability of AC/DC HDS and revealing the mechanism of DC voltage oscillation. Firstly, a method to reduce the order of the state‐space model is proposed. A three‐order VSC model with definitude physical meaning is obtained by utilising instantaneous power theory. According to this model, the potential instability factors in VSC are identified. Then, based on this, the interaction behaviour between VSC and DC network is researched by frequency domain analysis. The instability mechanism is ultimately revealed. The analysis result shows that the VSC's phase roll‐off characteristic is the root cause of instability, the inherent oscillation characteristics of DC network has negative interactions with VSC and will narrow the system stability boundary. Finally, the correctness of the theory is verified by electromagnetic transient simulation.
Interpolation algorithm considering simultaneous solution and instantaneous solution for power electronics electromagnetic transient simulation
Authors: Peng Hongying , Liu Min , Mu Qing , Sun Lixiang , Zhang Xing
Published: October 2018

In order to correctly simulate the simultaneous switching of power electronics circuits and to solve the problems of virtual power loss existing in the traditional interpolation algorithm, this article proposes interpolation algorithm considering simultaneous solution and instantaneous solution. After each integration, it searches for the switching events and determines the simultaneous switching. The simultaneous switching events are solved simultaneously. Instantaneous solution is carried out at forced commutation switching instant. The historical terms calculation method in instantaneous solution under different conditions is given, and the exact power loss of forced commutation switch is obtained. After processing all switching events during one time‐step, the half‐step interpolation is performed to eliminate numerical oscillations. The proposed algorithm is applied to the electromagnetic transient programme of advanced digital power system simulator (ADPSS), and the correctness and effectiveness of the algorithm are verified by simulation tests for typical power electronics circuits. The simulation results show that the proposed algorithm has high simulation accuracy and can satisfy requirements of power electronics simulation.
Simplified calculation of maximum ground‐level electric field when two HVDC transmission lines run across at any intersection angle
Authors: Xiao Fengnyu , Yan Jianxing , Zhang Bo , Wang Yanjie
Published: October 2018

When two HVDC transmission lines run across, because of the interaction between the two lines, the maximum ground‐level total electric field is not a simple superposition of the maximum ground‐level total electric field generated when the two lines exist alone. This paper presents an approximate method to use the maximum ground‐level total electric field when only the lower line exists as the maximum ground‐level total electric field when two lines run across. Here, the 3D method of characteristics has been improved to make it suitable for structure of two lines intersected at an angle using a method of coordinate transformation. The 3D method of characteristics is used to calculate the theoretical maximum electric field when the two lines run across. The 2D FEM is used to calculate the approximate maximum electric field when only the lower line exists. The approximate results and theoretical results are compared for line structures of ±800 kV line runs across ±800 kV line, ±660 kV line, and ±500 kV line at any intersection angle. The approximate errors are < ±10%, which can meet the engineering requirement.
Coordinating voltage regulation for an AC–DC hybrid distribution network with multiple SSTs
Authors: Liu Xianglong , Liu Youbo , Liu Junyong , Zhang Xi
Published: October 2018

The increasing penetration of renewable generation and large implementation of the DC loads are challenges that stress the current distribution systems by causing uncertain voltage variations. The AC−DC hybrid distribution network is seen as an innovative solution to address the problem of accommodating highly dispersed sustainable energy. Specifically, the solid‐state transformer (SST), which is capable of enhancing power‐quality performance and controllability of the power distribution, has attracted increasing focus in this decade as a promising advanced converter. It is of great interest to investigate how to operate the multiple SSTs with different modes in a coordinated manner to optimise the distribution network performance. This paper formulates the coordinated operation model for multiple SSTs to enable fast voltage regulation for the hybrid active distribution network equipped with large‐scale penetration of invertor‐based solar power and DC load. A hybrid optimal power flow describing the controlling modes of SSTs and operating interaction between AC−DC subsystems is proposed in this paper. The case studies on a modified IEEE 33‐AC−DC hybrid power distribution system show that the voltage profile can be optimised by using the flexible controlling resource of SSTs, facilitating the timely performance adjustment in the presence of fast fluctuating distributed generator's output.
Research of independent DC electric field sensor with wireless power supply circuit
Authors: When Jianzhong , Yu Zhanqing , Zhang Chao , Mou Ya , Huang Yaosheng , Zhen Zhiming
Published: October 2018

The DC electric field strength is an important electromagnetic environment parameter of the high‐voltage direct current (HVDC) transmission line. Measurement results of the space electric field are dramatically affected by the size and the potential of the sensor. In addition, the battery‐powered solution of electric field sensor (EFS) is not feasible, for the reason that in the high‐voltage electric field environment, the replacement of the battery will bring security risks to the power system and workers. The wireless energy supply solution not only solves the problem of potential isolation of the sensor, but also ensures that the sensor can work continuously. Various wireless power transmission systems (WPTSs) were compared. Microwave wireless power transmission system (MWPTS) was selected as the power supply of EFS. The key boundary conditions of MWPTS were determined. Two microstrip receiving antennas were designed in this article. A good performance antenna with high permittivity substrate was designed, realising 4.26 dB gain with the area of 1 cm2 at 5.8 GHz. The antenna meets the energy requirements of the EFS which is 10 m far from the source.
Bilayer game strategy of regional integrated energy system under multi‐agent incomplete information
Authors: Hao Ran , Ai Qian , Jiang Ziqing
Published: October 2018

In view of the high coupling degree of regional integrated energy system, a bilayer interaction strategy, consisting of energy suppliers, distribution networks, and users, is proposed. Game interaction strategy includes two aspects: scheduling and bidding. The independent system operator (ISO) coordinates all adjustable resources. Depending on the quotation price and multi‐energy load prediction, ISO minimises the total energy cost, which realises the complementary of the multi‐energy in the cooperative game. Under the assumption of incomplete information and bounded rationality, this study designs bidding functions and pay‐as‐bid settlement protocols. On this basis, according to history scheduling data and units’ characteristics, agents for energy suppliers pursue maximum interests. Also, the non‐cooperative bidding process in multi‐energy market is simulated by using Q ‐learning algorithm. Finally, the evolutionary process of the bilayer competitive game model is studied by practical example, and the existence local Nash equilibrium of the strategy is also proven.
Large signal stability analysis for DC microgrid under droop control based on mixed potential theory
Authors: Li Zekun , Pei Wei , Ye Hua , Kong Li
Published: October 2018

Brayton–Moser's mixed potential theory has been used to analyse the large disturbance stability of DC microgrid with constant power load (CPL), such as current‐mode controlled BUCK/BOOST converter with CPL or DC system with multi‐stage LC filters. However, the stability analysis of other topology of DC–DC converter or multiple converters worked under droop control is rarely involved. Based on this issue, this paper first establishes the mixed potential model of a single bidirectional BUCK/BOOST converter with CPL. Then, the paper gives the method for establishing the mixed potential model of a droop‐controlled DC microgrid with multiple bidirectional BUCK/BOOST converters and CPL. Afterwards, the stability criterion of the analytic form of the system under large disturbance is obtained, which shows that the parameters such as system parameters, control parameters, droop coefficients, and CPL power value have the effects on the system's large signal stability. This paper gives and discusses the relationship between system parameters and stability. Thus, instead of calculating and simulating repeatedly, the stability criterion can judge and ensure the stability of the system under large disturbance in the design process. Simulation results demonstrate the correctness of the simple and easily implemented stability criterion.
De‐risking transient stability of AC/DC power systems based on ESS integration
Authors: Yao Liangzhong , Wu Fubao , Qiu Gao , Liu Junyong , Liu Youbo , Cui Hongfen , Sang Bingyu , Ye Jilei
Published: October 2018

Owing to the charging/discharging flexibility, electric storage system (ESS) is widely recognised as a promising technique that can be introduced to enhance transient stability of power systems. Here, a multi‐objective ESS allocating and sizing approach is presented to de‐risk the loss of stability for the AC/DC power system. The approach contains two major steps. Firstly, transient stability risk (TSR) is assessed based on the severity and probability of contingencies which is simulated according to the good point set sampling considering multiple uncertainties and probabilistic fault. In the second step, an allocating and sizing model is proposed for ESS to minimise the operational cost, while the AC/DC system is subject to the TSR. Additionally, strength Pareto evolutionary algorithm (SPEA2) is employed to solve the model, optimising ESS allocation and size as well as output of regular generators. A modified AC/DC test system is used to validate the presented approach. The results indicate that, with the appropriate planning strategy, ESS is able to significantly contribute to TSR improvement as it can assist in transient power balancing after a severe fault.
Design and implementation of the MMC simulation system in the heterogeneous FPGA‐CPU platform
Authors: Li Xiongfei , Mu Qing , Zhang Xing , Xiang Yinxing , Li Qiao , He Guanghui
Published: October 2018

Real‐time simulation of a modular multilevel converter (MMC) plays an important role on the area of large‐scale power electronics research. The authors propose a real‐time MMC simulation system in a heterogeneous computing platform containing the central processing unit (CPU) and field programmable gate array (FPGA). This system decouples the MMC circuits from the grid based on the alternative circuit equivalent model of the MMC bridge arm. Furthermore, the parallel calculation of the equivalent circuits is achieved when the peripheral circuit is realised on the CPU and the MMC arms are realised on FPGA. In addition, a timing optimisation strategy is discussed to reach the timing target requirement. To validate the MMC simulation system, a case study of a grid with 12 MMC bridges and 640 sub‐modules per bridge at 2 μs time step on Xilinx Virtex‐7 XC7VX690T FPGA is simulated in real time. The real‐time simulation results demonstrate high accuracy of the simulation system in comparison to the offline simulation of the original system in the electromagnetic transient program and our design is competitive to recent published works.
Assessment method for identification of electrical connectivity and thermal stability of power grid
Authors: Xu Yao‐Yu , Li Liang‐Shu , Ren Shuang‐Zan , Li Yuan , Liu Ning , Zhang Guan‐Jun
Published: October 2018

Primary equipment failure in substation has a significant impact on the reliability of the power supply, and even threatens the thermal stability of power grid. This study presents a method identification of electrical connectivity and thermal stability assessment of power grid after primary equipment failure. The identification method of electrical connectivity based on incidence matrix provides a way to determine load curtailment after equipment failure. Subsequently, the inject power of substation bus in power grid is modified by load curtailment. Moreover, based on the DC power flow model, the active power flow distribution of power grid is calculated, which is used to evaluate whether the active power of lines exceeds limit. Compared to the traditional N  − 1 assessment method, this method not only has the advantages of fast calculation and strong pertinence but also of convenience for the field application.
Fast frequency response for effective frequency control in power systems with low inertia
Authors: Hong Qiteng , Nedd Marcel , Norris Seán , Abdulhadi Ibrahim , Karimi Mazaher , Terzija Vladimir , Marshall Benjamin , Bell Keith , Booth Campbell
Published: October 2018

The increasing penetration of renewable generation has led to the decrease of power systems’ overall inertia, which introduces significant challenges to frequency stability. In this paper, the potential of using Fast Frequency Response (FFR) to enhance frequency control in power systems with low inertia is investigated in detail. A Generic System Frequency Response (GSFR) model taking into account of the penetration level of Non‐Synchronous Generation (NSG) and FFR has been developed and used to investigate the impact of reduced inertia on frequency control and demonstrate that the amount of reserve power to be scheduled can be significantly reduced with the deployment of FFR. The impact of the different FFR resources' characteristics (e.g. response delay, ramp rate, etc.) on the effectiveness of frequency control is also investigated, based on which the desirable specifications for FFR schemes are summarised. These desirable properties of FFR schemes are taken into account in the design of a wide‐area monitoring and control system termed ‘Enhanced Frequency Control Capability (EFCC)’, which is proposed for the delivery of FFR in the future Great Britain transmission system. The design and operation of the EFCC scheme are presented, along with a case study demonstrating its effectiveness in enhancing the frequency control.
Estimating lower probability bound of power system's capability to fully accommodate variable wind generation
Authors: Liu Bin , Zhai Bingxu , Liu Mengchen , Liu Feng , Lan Haibo
Published: October 2018

As the penetration of wind generation increases, the uncertainty it brings has imposed great challenges to power system operation. To cope with the challenges, tremendous research work has been conducted, among which two aspects are of most importance, that is, making immune operation strategies and accessing the power system's capability to accommodate the variable energy. Driven and inspired by the latter problem, this paper will discuss the power system's capability to accommodate variable wind generation in a probability sense. Wind generation, along with its uncertainty is illustrated by a polyhedron, which contains prediction, risk, and uncertainty information. Then, a three‐level optimisation problem is presented to estimate the lower probability bound of power system's capability to fully accommodate wind generation. After reformulating the inner max‐min problem, or feasibility check problem, into its equivalent mixed‐integer linear program (MILP) form, the bisection algorithm is presented to solve this challenging problem. Modified IEEE systems are adopted to show the effectiveness of the proposed method.
Sensorless control of PMSM for DC micro‐grid flywheel energy storage based on EKF
Authors: Zhang Dan , Jiang Jian Guo
Published: October 2018

As a new type of energy storage system, the flywheel energy storage system has been playing an important role in the field of DC micro‐grid. Permanent magnet synchronous motor (PMSM) is widely used in flywheel energy storage system. In this study, the 2D static magnetic field and transient magnetic field of a permanent magnet synchronous motor are analyzed in Ansoft. The sensorless control strategy of PMSM is applied widely to reduce the system cost and optimize the system. Extended Kalman filter (EKF) is used to estimate the position signal in real time and online, which not only could reduce the complexity of system, but also could improve the stability of system operation. Finally, a simulation model is built in MATLAB/Simulink to verify the correctness of the proposed sensorless control strategy.
Study on the fault ride through critical elements and coordinated control of DFIG's converters
Authors: Tang Bingjie , Chi Yongning , Tian Xinshou , Li Yan
Published: October 2018

The increasing of large‐scale wind power integration capacity brings great challenges to the safe operation and management of power grid. Modern grid codes put forward the requirements of fault ride through (FRT) capability: (i) to remain continuous operation at a given time; (ii) to contribute to support the voltage stability during FRT by providing reactive current. Aiming at the FRT problem of doubly fed induction generator (DFIG), critical elements of the rotor side converter (RSC) and the grid side converter (GSC) to FRT are analysed, and the improving RSC control scheme, adding extra hardware equipment, and improving reactive current control scheme are studied. Matlab/Simulink simulation studies are conducted to verify the effectiveness of the proposed strategies. The results show that when the grid fault is light, the improving RSC control strategy can effectively control the rotor winding overcurrent, when the grid fault is serious, it needs additional hardware equipment; the coordinated control strategy of the RSC and the GSC providing reactive current support is identified as the most advantageous approach because it not only provides enhanced reactive support for the grid, but also avoids over‐speed risk of DFIG based wind turbine shaft.
Electromagnetic transient modelling and simulation of power converters based on a piecewise generalised state–space averaging method
Authors: Wang Lei , Deng Xinchang , Hou Junxian , Wang Tiezhu
Published: October 2018

Common averaging methods are studied for modelling a grid‐connected converter in renewable energy domain to balance accuracy and efficiency of electromagnetic transient simulation. A piecewise generalised state–space averaging (P‐GSSA) method is proposed for converters with large‐scale renewable energy connected to the grid. The piecewise technique is applied to the GSSA model of converters, which combine time segment with similar operating characteristics. In addition, multi‐time scale modelling is successfully achieved for the grid‐connected converters in the renewable energy domain. An example is simulated according to the P‐GSSA model proposed in this paper, and simulation results show that the proposed model can provide an efficient simulation of power converters connected to the grid in the domain of large‐scale renewable energy.
Control of photovoltaic power integration based on multi‐terminal VSC–HVDC system
Authors: Wang Lei , Wu Xiaolong , Hou Junxian , Wang Tiezhu
Published: October 2018

In order to solve the problem of long‐distance transmission of large‐scale photovoltaic power stations, a multi‐terminal voltage source converter‐based high‐voltage direct current (VSC–HVDC) system based on a DC voltage‐active power‐AC voltage control strategy, which is connected with photovoltaic power stations, is designed in this study. In order to verify the feasibility of the method, a model of a five‐terminal parallel system is established by MATLAB/Simulink, which includes two photovoltaic power stations and a passive network. A series of simulations are carried out based on the model. The results show that the active power of the system can be automatically balanced when the output power of photovoltaic power stations changes. When the three‐phase short‐circuit fault occurs in power grid, the photovoltaic power stations can still run in a stable state and have better fault ride‐through ability.
Review of oscillations in VSC‐HVDC systems caused by control interactions
Authors: Yin Congqi , Xie Xiaorong , Xu Shukai , Zou Changyue
Published: October 2018

Owing to the fast control response and distinct characteristics of the voltage source converter (VSC), various types of oscillation incidents related to the VSC‐HVDC systems have been observed around the world. However, the causes of such oscillations are complex due to the interactions among the components of the transmission system and the various control methods of the converter station, and yet not systematically comprehended and summarised. Therefore, this study reviews several typical VSC‐HVDC oscillation incidents and focuses on the root causes of the oscillation phenomenon. The analysis indicates that the oscillations in the VSC‐HVDC system can occur either in subsynchronous frequency or in high‐frequency range, up to 1.2 kHz. The modelling methods, including the conventional detailed model, equivalent detailed model, and average equivalent model, are also affecting the accuracy of describing the oscillations. The control methods, including adding active control scheme, passive filter at oscillation frequency, and temporary power plant disconnections, are also discussed. The advantages and drawbacks of these modelling methods, analysis methods, and control methods are summarised based on the various system conditions. In the end, the future research perspectives and key challenges are provided for further research references.
Calculating for surface electric field of converter valve shield system with fast multipole curved boundary element method
Authors: Shi Yuxin , Wang Zezhong
Published: October 2018

The accurate and fast calculation of surface electric field of DC converter valve shield system have guiding significance in the designing process. However, the electrostatic field analysis is a large‐scale problem with low efficiency using the conventional numerical algorithm. In order to solve this problem, fast multipole curved boundary element method (FMCBEM) was proposed. The FMCBEM is based on the indirect boundary integral equation of electrostatic field. Galerkin curved boundary element method based on coordinate transformation is used to improve the accuracy, then fast multipole method is used to improve the calculation speed and reduce the memory usage. Through simple examples, the accuracy and efficiency of the algorithm were verified. Compared with finite element method, it was found that the accuracy can meet the requirements of engineering. The efficiency was obviously better than the conventional boundary element method. Applying this algorithm, the surface electric fields of different shield systems of ±160 kV converter valve, which the degree of freedoms is more than 220 thousand, were analysed using desktop computer. This algorithm can calculate the large‐scale electric field problem fast and accurately, which is an effective tool for the optimisation design.
Fast single‐end line protection method for the meshed multi‐terminal HVDC grid
Authors: Huang Qiang , Zou Guibin , Xu Chunhua , Zhang Jie , Zhang Shuo
Published: October 2018

In view of the short‐circuit fault characteristics, the protection technique of the dc lines limits the development of the modular multi‐level converter (MMC)‐based HVDC grid in a large extent. This study derives the calculation formulas of the transient voltages of internal and external faults and proposes a single‐end protection scheme based on the transition period (TP) from the abrupt changing point to the first extreme point of the fault voltage. Due to the current‐limiting reactors, the TP of the internal fault is much shorter than that of the external fault, which is used to discriminate internal fault from external fault. A four‐terminal symmetrical bipolar HVDC grid based on MMC is modelled using PSCAD/EMTDC, and simulation results demonstrate the validity of the new proposed dc transmission line protection method.
Suppression measures of commutation failure in MIDC transmission systems
Authors: Qing Wang , Kaipei Liu , Ling Wang
Published: October 2018

With the increasing of direct current (DC) transmission capacity, high‐voltage DC (HVDC) location has become more and more intensive, and commutation failure has become increasingly prominent in multi‐infeed DC (MIDC） transmission systems. To reduce the probability of commutation failure in MIDC systems, this study first analyses the fundamental causes of simultaneous or successive commutation failure, and puts forward the coordinated control strategy of voltage compensation variable slope voltage dependent current order limit, which takes the advantage of the DC current characteristics while fault occurs, and then proposes the influence factors and determination method of compensation coefficient K. Meanwhile add the switch function of K to compensate voltage according to the severity of the fault. Finally, built the actual system using PSCAD/EMTDC, simulation results show that the control strategy can effectively reduce the probability of commutation failure in MIDC transmission systems, and makes all DC systems recovery orderly after fault, meanwhile ensure the good recovery performance which can greatly improve the safety and reliability in MIDC transmission systems.
Adaptive VDCOL control strategy for the recovery of the UHVDC SPC system
Authors: Yao Shujun , Huang Wenerda , Hao Wenbo , Wanhua Guo
Published: October 2018

The separating pole connection (SPC) mode of ultra‐high‐voltage direct current (UHVDC) can significantly improve the power stability, but when the AC bus of one pole is failed and two receiving systems are near, it may cause subsequent commutation failure. In order to restrain subsequent commutation failure and accelerate system's recovery, an adaptive voltage‐dependent current order limiter (VDCOL) control strategy based on granular computing is proposed. According to this VDCOL control strategy, VDCOL, which uses fuzzy rules to separate and granulate a different voltage level, can adjust DC current dynamically in a variety of environments. Also, some modifications on VDCOL characteristics are applied to reduce reactive power consumption during low‐voltage situations. Then based on the MATLAB simulation platform, the simulation systems for 800 kV the UHVDC SPC system are built. Finally, the simulation example of the SPC mode with the adaptive VDCOL control strategy showed that this VDCOL control strategy can reduce the risk of subsequent commutation failure and improve the stability of the AC/DC interconnection system.
Study on the method of fault set for AC/DC system operation considering wind power
Authors: Han Hualing , Huang Ting , Qian Shuangqiu , Xu Xiaoyi , Chen Ning , Zhao Dawei
Published: October 2018

With the rapid development of power system, wind power becomes an important generating way in AC/DC hybrid power grid. Due to the interference between the wind power and the AC network, the risk of DC system continuous commutation failure will be affected. In order to evaluate this effect reasonably, a method for running out of the fault set is presented in this study. Firstly, a distribution factor is proposed to determine the power line transfer after wind power or power line fault, the line with maximum load rate is chosen as the candidate failure lines; Secondly, the η short‐circuit ratio and η effective short‐circuit ratio indexes of the DC system is evaluated after every fault; they are used to judge whether the cascading fault of AC system induces the transient overvoltage of the converter bus to lead to wind turbines to take off the power grid net. Thus, a cascading fault sequence of the AC system is formed, and a cascading fault set that causes the transient overvoltage of the converter bus is completed. Finally, the simulation results calculated by power system department‐bonneville power administration show that when the commutation failure occurs in the DC system, the system runs out immediately, and the validity of the method is verified.
Transient stability analysis of wind‐thermal bundled system based on virtual inertia control
Authors: Shujun Yao , Wanhua Guo , Wenerda Huang , Hongtao Xu , Erman Bian
Published: October 2018

In this study, the transient stability characteristics of the wind‐thermal bundled system is discussed when the wind turbine adopts the additional control of virtual inertia. Firstly, a typical wind‐thermal bundled system model is established, and the operating characteristics and equations of motion of the wind turbine are derived. Then, the transient stability characteristics of wind‐thermal bundled system are analysed by the Extended Area Equality Criterion and a virtual inertia control model is established. Finally, simulations are conducted in the power system analysis software package and conclusions are as follows: When the receiving end is an infinite system, the higher the proportion of wind power is, the better the transient stability of the system is and the application of the additional control of virtual inertia has no significant effect on the transient stability. When the receiving end is a non‐infinite system, the transient stability of the system first increases and then decreases, and there is an optimal ratio. The application of the additional control of virtual inertia can effectively enhance the transient stability and improve the optimal proportion.