Search

To improve the filtering capability of frequency-locked loop (FLL), the high-order bandpass filters FLL was proposed recently. This article proposes a high-order FLL in synchronous ( dq ) reference frame where the high-order low-pass filter is adopted. A general analytical model is established for the high-order FLL. Based on the model, it is proven that the conventional high-order FLL exhibits no improvement in terms of filtering ability of the frequency estimation. Based on this analysis, a new frequency estimation loop is proposed to obtain an enhanced filtering capability for the FLL. Additionally, the proposed high-order FLL has the advantage of simple implementation compared to conventional high-order FLLs. A design example is then demonstrated and simulated to show the accuracy of the model and the effectiveness of the proposed FLL. Finally, the proposed high-order FLL is experimentally verified on a testbed using a 32-bit floating-point DSP TMS320F28379D at 200 MHz.

This paper proposes an image-based algorithm for detecting and cleaning the wind turbine abnormal data based on wind power curve (WPC) images. The abnormal data are categorized into three types, negative points, scattered points, and stacked points. The proposed algorithm includes three steps, data pre-cleaning, normal data extraction, and data marking. The negative abnormal points, whose wind speed is greater than cut-in speed and power is below zero, are first filtered in the data pre-cleaning step. The scatter figure of the rest wind power data forms the WPC image and corresponding binary image. In the normal data extraction step, the principle part of the WPC binary image, representing the normal data, is extracted by the mathematical morphology operation (MMO). The optimal parameter setting of MMO is determined by minimizing the dissimilarity between the extracted principle part and the reference WPC image based on Hu moments. In the data mark step, the pixel points of scattered and stacked abnormal data are successively identified. The mapping relationship between the wind power points and image pixel points is built to mark the wind turbine normal and abnormal data. The proposed image-based algorithm is compared with k -means, local outlier factor, combined algorithm based on change point grouping algorithm and quartile algorithm (CA). Numerous experiments based on 33 wind turbines from two wind farms are conducted to validate the effectiveness, efficiency, and universality of the proposed method.

Due to the intermittent and stochastic nature of solar power, solar power interval prediction is of great importance for grid management and power dispatching. A combination interval prediction model based on the lower and upper bound estimation (LUBE) is proposed to efficiently quantify the solar power prediction uncertainty. In the proposed model, the upper and lower bounds are separately predicted by two prediction engines. The extreme learning machine (ELM) is selected as the basic prediction engine. The auto-encoder technique is used to initialize the input weight matrix of ELM for efficient feature learning. A novel biased convex cost function is developed for ELM to predict the interval boundary. The output weight matrix of ELM can be solved via the convex optimization technique instead of the conventional heuristic algorithm. The proposed interval prediction model can be formulated as a bi-level optimization problem. In the lower-level problem, the lower and upper ELMs are trained under different candidate hyper-parameters of the biased cost function. In the upper-level problem, the optimal combination of the lower and upper prediction engines is determined by evaluating the interval prediction performance. Comprehensive experiments based on public data set are conducted to validate the superiority of the proposed interval prediction model.

With the rapid development of renewable energy sources (RESs), adopting power-to-gas to improve the penetration of RESs is promising. This study designs a grid-forming application for the fuel cell/electrolyser (FC/ELZ) system to supply the frequency support for the grid as well as the voltage source. The detailed control methods including the DC, AC voltage controls and the power synchronisation control are designed for the FC/ELZ system so that it can operate independently without any other energy sources. Several simulation results are provided to demonstrate the effectiveness of the designed grid-forming application of the FC/ELZ system.

Owing to the limited operating regions of combined heat and power (CHP) units, the operation of integrated energy systems suffers from low flexibility, low-cost efficiency, renewable curtailment etc. Meanwhile, as the capacity of renewable energies keeps growing and integrating into power systems, various methods, such as installing electric boilers to enable electricity-heat conversion, have been developed to absorb excessive renewables and increase system operation flexibility. To further increase the system operation flexibility, this study explores the possibilities of utilising electrolysers and hydrogen storage tanks to enable electricity-hydrogen-heat conversion. To better visualise the enhanced flexibility, this study presents extra flexibilities from electric boilers, electrolysers and hydrogen tanks as the equivalent operating region expansion for CHP units. In this study, the system is modelled as a mixed-integer optimisation problem which balances the electricity, heat and hydrogen demands in a 24-hour period. A 6-bus test system is used in the case studies to illustrate the effectiveness of implementing electrolysers and hydrogen storage tanks. The optimisation results show the application of hydrogen energy improves the system operation flexibility, reduces wind curtailment, thereby decreasing fuel consumption and carbon emission.

The concept of virtual synchronous machine (VSM) was proposed to deal with the shortcomings of low inertia and damping of traditional control strategies for power electronic converters. But what if all distributed energy resources and controllable loads in a microgrid adopt the VSM control strategy, and will it present better performance than conventional droop control-based microgrid (DMG)? In this paper, the VSM-based microgrid (VSMG) is analyzed. The small-signal modeling of the VSMG is studied at first. Then static stability and dynamic characteristics of the VSMG are analyzed and compared with the DMG in both frequency-domain and time-domain. With the growing scale of microgrids, their modeling and simulation are becoming significant computational burdens. Inspired by the participation factor analysis of the VSMG and the concept of coherency in power systems, the VSMG small-signal model is equivalent to a modified third-order synchronous generator (SG) model in this paper. The equivalencing involves gray-box system identification and is realized by estimating equivalent electrical parameters alternately and iteratively. The equivalent SG (EqSG) model is compared with three representative model order reduction methods to verify its effectiveness. Simulation results confirm the accuracy of the EqSG model substituting detailed VSMG model in time-domain simulations.

As the penetration of renewable energy continues to increase, stochastic and intermittent generation resources gradually replace the conventional generators, bringing significant challenges in stabilizing power system frequency. Thus, aggregating demand-side resources for frequency regulation attracts attentions from both academia and industry. However, in practice, conventional aggregation approaches suffer from random and uncertain behaviors of the users such as opting out control signals. The risk-averse multi-armed bandit learning approach is adopted to learn the behaviors of the users and a novel aggregation strategy is developed for residential heating, ventilation, and air conditioning (HVAC) to provide reliable secondary frequency regulation. Compared with the conventional approach, the simulation results show that the risk-averse multi-armed bandit learning approach performs better in secondary frequency regulation with fewer users being selected and opting out of the control. Besides, the proposed approach is more robust to random and changing behaviors of the users.

Distribution networks with high penetration of distributed generation yield complicated and uncertain power flow, which makes most existing faulted line identification methods not adaptable for industrial applications. Driven by this motivation, a novel single-phase-to-ground (SPTG) faulted line identification method is proposed based on hybrid state estimator (HSE). The first step of the method is to present an HSE for power distribution networks using power flow measurements mixed phasor measurement units. Then, an SPTG fault on a power line is treated as an event that suddenly increases one virtual bus in the monitored network, so as to form the extended bus admittance matrix and augmented HSE based on the specific network topology. In this way, the faulted line identification could be obtained by computing parallel estimated results transversally. Robustness and effectiveness of the proposed HSE and the HSE-based SPTG faulted line identification method are validated by means of a cyber-physical system (a cosimulation platform), where two typical three-phase power distribution networks are considered to simulate with its hybrid measurement system.

With the development of distributed power technology and the promotion of China's photovoltaic poverty alleviation policy, a large number of photovoltaic integrations to rural distribution network have caused serious problems of voltage overrun and line loss surge, which has aroused a great deal of attention for the operation of traditional rural power network. This paper proposes a revised planning optimization solution model for this application scenario and takes into account the use of the reactive power incentive and reward mechanism under the power market to promote the participation of user-side photovoltaic in reactive power regulation, so as to achieve significant cost reduction. A practical numerical sample of a rural distribution in inner mongolia is carried out to verify the proposed solution.

The increasing penetration of distributed generations (DGs) has brought huge challenges to distribution networks operation, such as reverse power flow and overvoltage. Under this background, an optimal active and reactive power dispatch (OARPD) method is proposed in this paper. Considering the three-phase unbalance property, the bus injected model (BIM) of power flow is established while a novel high precision linearized method is proposed. Based on the linearized power flow model, the multi-objective OARPD model which aims to minimize networks losses while maximize the absorption of renewable energy is constructed. Moreover, to ensure the system safe operation and ameliorate the adverse effects caused by single-phase integration of DG, a novel voltage unbalance constraint is established. Finally, examples have been used to demonstrate the validity and effectiveness of the proposed method.

The increasing penetration of photovoltaics (PVs) and fluctuating load exacerbate the risk of voltage and reactive power violations in distribution systems. The conventional regulation strategy is based on the nine-zone diagram control (NDC), which is difficult to quick response and meet the requirement with high precision. Utilizing the delay of NDC, a nonintrusive control strategy of voltage and reactive power for distribution systems is developed in this paper. The proposed control strategy uses the ability of PV to cooperate with traditional devices without changing the original NDC strategy, which makes it easy to implement and extend in the project. First, a time-series control architecture is developed to reduce the number of traditional devices and eliminate voltage violations. Then, by using the sensitivity values of multiple devices, the reactive power output of PVs is calculated, and the voltage reference value in PV inverter control (PVC) horizon can be tracked based on feedback. A case study is carried out on an IEEE 33-node system to illustrate the effectiveness of the proposed method. It is shown that PV can be used as a continuous reactive power source to realize fast and accurate regulation and the proposed method can improve adjustment accuracy and reduce the operation of conventional equipment compared to the NDC.

In classical inverter control, load current (disturbance) decoupling control is often used to improve the control performance. Yet, in a recent state-space control structure for multiresonant controllers, the load current decoupling technique is less studied. Moreover, in the classical control, the load current decoupling only has one control degree of freedom. Therefore, in this paper, a load current decoupling control combed linear quadratic (LQ) regulation is investigated, the proposed control structure that manipulates α and β axes as a unified system is adopted to increase the control degree of freedom. The LQ control synthesizes the state feedback control law including gains of the resonant controllers. Two general criteria, namely $H_{\infty }$ norm and zero dynamic, are proposed as the design guideline for the load current decoupling control. The effectiveness and robustness of the proposed approach are validated through MATLAB/Simulink simulations and experiments with a 5-kVA testbed. The results prove that some favorable performances, for example, the fast recovering time, the small voltage drop, and the low total harmonic distortion, are achieved by the proposed approach compared to the classical dual-loop proportional-resonant (PR) control with load current decoupling and the optimal $H_{\infty }$ control.

Modern distribution network with high penetration of intermittent renewable sources and the so-called prosumers requires more reliable distribution automation (DA) system for safe operation and control. The design of optimal feeder terminal units (FTU) placement is critical and economical for the effective DA application. Previously proposed solutions of optimal FTU placement aiming to ensure the accuracy of state estimation (SE), typically include the following two main shortcomings: 1) only to obtain the optimal FTU placement in quantity, and the analysis of FTU location is not considered yet; 2) few consider the uncertainty of intermittent power injections in the analysis of state estimation. In this paper, a modified methodology of FTU placement is proposed not only aiming to ensure the accuracy of state estimation with the minimum number of meters, but also finding those specific FTU locations to guarantee the power service reliability. Moreover, the uncertainty models of those intermittent power injections are also considered by using probability density function (PDF). The resultant optimization problem is addressed by using the covariance matrix adaptation evolution strategy (CMA-ES). Case studies demonstrate the feasibility and effectiveness of the proposed methodology.

To introduce flexible power regulation into a microgrid, this paper proposes a load-storage integrated control strategy (ICS), consisting of an economic control model (CCM) and an efficient control model (FCM). CCM is adopted to reduce the action cost of controllable loads (CLs) and the operation cost of energy storage and to improve the efficiency of a microgrid when it operates in a grid-connected state. FCM is adopted to achieve rapid, accurate power regulation to improve the stability of a microgrid when it operates in an islanded state. FCM not only can maintain the voltage and frequency of the system alone, but it also can share the regulated power with other distributed generators (DGs) under the proposed mirror droop control strategy. Additionally, a customer satisfaction model (CSM) is introduced to help customers select the control models and parameters of ICS in order to meet different requirements for various customers. A physical prototype, load-storage integrated control unit (ICU), is designed to implement ICS. Both numerical simulations and experimental tests successfully verify that ICS or ICU can provide more economic and more efficient power regulation in contrast to respectively regulating controllable loads and energy storage.

In the DC power distribution system which needs to supply power for the passive network, because of the modular multilevel converter (MMC) connected to the passive network has no power supply, the MMC control mode can't directly be set to the constant voltage when to start MMC. Accordingly, the particular start-up control strategy must be adopted. This paper presents a novel start-up approach to the DC distribution network. Firstly, the process of the start-up is divided into an uncontrolled charging stage and a controllable charging stage. By placing a current limiting resistor in MMC AC-side, designing the AC-side and DC-side circuit breaker closing sequence of the converter station, and setting the reference voltage by the slope, the start-up of the DC distribution network can be achieved no large impact current. Secondly, by setting one end of the converter station as the master station and designing the control mode of connecting the passive network converter station, the local and the remote MMC sub-module capacitors can be charged at the same time, the start-up of the DC distribution network can be completed without the help of another auxiliary power supply or equipment. Finally, the mathematical model is established by Matlab/Simulink to verify the effectiveness of the proposed method. The studies we have performed showed that the designed start-up strategy is simple, easy to implement, reliable and economical.

In this paper, an adaptive active power optimal allocation strategy for minimizing power loss during power transmission in islanded microgrids is proposed. A secondary regulated variable is added to the P-f droop control as a frequency offset to regulate power output reasonably among multiple distributed generation (DG) units. The rate of change of the total power loss during power transmission is calculated. The frequency offset is adaptively generated in turn among multiple DG units according to the rate of change of power loss. Thus the power flowing through different transmission lines is optimized and the power loss is minimized. Simulation results validate the effectiveness of the proposed strategy.

Interval prediction could provide more valuable information compared with point prediction for power system monitoring and scheduling. This paper proposed a new short-term solar power interval prediction model which combines the prediction interval initialization technology with Extreme Learning Machine (ELM). Due to the unknown prediction interval information, the heuristic algorithm is employed to train the ELM. Thus, the initial values of the parameters in the prediction model dramatically impact the training performance. A prediction interval width initialization approach is developed to initialize the initial values of the parameters. Numerical experiments are conducted to prove that the prediction interval width initialization approach can assist different heuristic algorithms in improving their effectiveness and efficiency on the model training.

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of DC renewable energies, DC loads. For the fault ride-through requirements in the DC grid, this article proposes a new distributed high step-down ratio DC transformer, which consists of two parts. The front stage implements the capabilities of buck and fault ride-through. The latter stage implements the functions of voltage controlling and isolation. Related simulations verify the validity and feasibility.

Aiming at the requirement of high reliability and lean planning for urban medium-voltage distribution network, modularization concept, reliability analysis and comprehensive evaluation methods are proposed. Modular distribution network based on station, room, line and cable is explained. The reliability calculation method based on "component-submodule-module", considering multi-outage and inter-module transfer is proposed. The comprehensive evaluation model of modular distribution network is established in multi-dimension. Considering the small gap of evaluation indexes, an improved entropy weight method is used to carry out the comprehensive evaluation, and an example of a high reliability industrial park is analyzed. The results show that modular design and combination are of great significance for realizing flexible and reliable distribution network.

The aim of this paper is to propose a sinusoidal voltage signals tracking control strategy, adaptive for frequency of a three-phase inverter for a battery energy storage system (BESS) applied to an isolated microgrid. To support the normal operation of distributed renewable energy and feed the critical loads under the isolated mode, the BESS generally works in voltage control mode (VCM). The proposed control strategy, which transforms the tracking control problem into a state feedback control problem, includes two terms, one of which is state feedback control used to stabilize system while another employs multiple resonant controllers applied to achieve harmonics compensation control. The optimal linear-quadratic (LQ) control approach in which the root locus method is used for optimization selection of weight matrix Q is adopted to tune the control parameters of multiple resonant controllers. The proposed control strategy performs outstanding voltage control performance to maintain an excellent voltage level such as zero steady-state error and low total harmonic distortion (THD) under the situation of sudden load changes, back power flow of renewable energy such as photovoltaic and nonlinear load in an isolated microgrid. Finally, the validity of the proposed approach is verified through simulations and a three-phase experiment prototype of BESS with TMS320F28335 DSP.

Inverter voltage control is an important task in the operation of a DC/AC microgrid system. To improve the inverter voltage control dynamics, traditional approaches attempt to measure and feedforward the load current, which, however, needs remote measurement with communications in a microgrid system with distributed loads. In this paper, a load current observer (LCO) based control strategy, which does not need remote measurement, is proposed for sinusoidal signals tracking control of a three-phase inverter of the microgrid. With LCO, the load current is estimated precisely, acting as the feedforward of the dual-loop control, which can effectively enlarge the stability margin of the control system and improve the dynamic response to load disturbance. Furthermore, multiple PR regulators are applied in this strategy conducted in a stationary frame to suppress the transient fluctuations and the total harmonic distortion (THD) of the output voltage and achieve faster transient performance compared with traditional dual-loop control in a rotating dq0 frame under instantaneous change of various types of load (i.e., balanced load, unbalanced load, and nonlinear load). The parameters of multiple PR regulators are analyzed and selected through the root locus method and the stability of the whole control system is evaluated and analyzed. Finally, the validity of the proposed approach is verified through simulations and a three-phase prototype test system with a TMS320F28335 DSP.

Low leakage current and high efficiency are two key indexes for transformerless PV grid-connected inverter. The transformerless inverter topologies have superior efficiency thanks to saving transformer, but their semiconductor devices are still on hard-switching state at present. First and foremost, a novel zero-current-transition (ZCT) concept for the single-phase full-bridge transformerless PV grid-connected inverters is presented in this paper. Second, the zero-current turn-off for high-frequency main switches of the inverters and the zero-current turn-on for auxiliary switches added are achieved by introducing two resonant tanks. Furthermore, a family of ZCT transformerless grid-connected inverters with sinusoidal pulse width modulation is deduced. Especially, taking zero-current-transition six-switch full-bridge topology (ZCT-H6-I) as an example, its operation principle, soft-switching conditions, duty cycle constraints, and parameter design procedure of the resonant tank are analyzed in detail. Finally, the validity of the ZCT concept is verified by a ZCT-H6-I prototype rated at 50 kHz, 1 kW.

After the distributed generation is connected to the distribution network, the topology of the unidirectional power supply of the traditional distribution network has been changed, which may lead to the problem of voltage limit and bidirectional power flow. The traditional current protection method is no longer applicable. The fault current characteristics of the inverter interfaced distributed generation under PQ control and the influence of the distributed generation on the traditional current protection technology are analyzed. On this basis, an active distribution network protection scheme based on the area current direction is proposed. The distribution network is partitioned and the fault area is judged by collecting the current fault component direction. The relevant circuit breaker is controlled to complete the isolation of the fault area. Finally, the proposed protection scheme is simulated in PSCAD / EMTDC. The simulation results show the adaptability of the protection scheme to the active distribution network.

To achieve accurate reactive power sharing and voltage frequency and amplitude restoration in low-voltage microgrids, a control strategy combining an improved droop control with distributed secondary power optimization control is proposed. The active and reactive power that each distributed generator (DG) shares is calculated by extracting load information and utilizing a power sharing ratio, and is reset to be the nominal power to recalculate droop gains. The droop control curves are reconstructed according to the nominal active and reactive power and the recalculated droop gains. The reconstructed active power-frequency droop control can regulate active power adaptively and keep frequency at a nominal value. Meanwhile, the reconstructed reactive power voltage droop control can reduce voltage amplitude deviation to a certain extent. A distributed secondary power optimization control is added to the reconstructed reactive power voltage droop control by using average system voltage. The average system voltage is obtained by using a consensus algorithm in a distributed, sparse communication network which is constituted by all controllers of DGs. As a result, accurate reactive power sharing is realized, average system voltage is kept at a nominal value, and all voltage amplitude deviations are further reduced. Due to the absence of a microgrid central controller, the reliability of the strategy is enhanced. Finally, the simulation results validate the proposed method.

It is increasing popular to integrate distributed generation (DG) into distribution systems, the traditional optimal reactive power dispatch approach is no longer able to cope with it. The robust optimization (RO) theory is introduced to deal with the uncertainty of large-scale DGs. Adaptive robust optimal reactive power dispatch (AR-OPRD) is proposed to reduce the conservation of RO. An adaptive function is constructed to derive the relationship between the optimal reactive power output and active power output of an arbitrary DG. A new cutting plane algorithm is introduced to solve AR-ORPD mathematical model efficiently. In this paper, examples have been used to demonstrate the validity and effectiveness of the method.

DC grid enables power system more flexible and efficient, and increases absorptive capacity of the renewable energy, which will be an important part of the future smart grid. However, higher requirement of protection is a significant barrier to more widespread adoption of dc grid. To solve the drawbacks of the existing protection methods, a new protection scheme which delivers fast operation for a two-terminal DC network is presented in this paper. The proposed scheme comprises a communication-assisted fault detection and an isolation method through differences of fault current directions. The scheme uses multiple intelligent electronic devices (IEDs) to exchange digital signals of zone current direction, which can distinguish line and bus fault, then combines with circuit breakers to cut-off DC faults within sub-millisecond timescales. Converter and battery protection are also presented to perfect grid protection. The effectiveness and practicability of the proposed protection strategy is validated through PSCAD.

Operation control is a vital and complex issue for microgrids. The objective of this paper is to explore the practical means of applying decentralized control by using a multi agent system in actual microgrids and devices. This paper presents a hierarchical control framework (HCF) consisting of local reaction control (LRC) level, local decision control (LDC) level, horizontal cooperation control (HCC) level and vertical cooperation control (VCC) level to meet different control requirements of a microgrid. Then, a hybrid multi-agent control model (HAM) is proposed to implement HCF, and the properties, functionalities and operating rules of HAM are described. Furthermore, the paper elaborates on the implementation of HAM based on the IEC 61850 Standard, and proposes some new implementation methods, such as extended information models of IEC 61850 with agent communication language and bidirectional interaction mechanism of generic object oriented substation event (GOOSE) communication. A hardware design and software system are proposed and the results of simulation and laboratory tests verify the effectiveness of the proposed strategies, models and implementations.

Frequency stability of islanded smart distribution networks with peer-to-peer (P2P) controlled distributed generators (DGs) has attracted special attention recently. Using power line communication (PLC) technology, a multi-agent system (MAS)-based, decentralized, under-frequency load shedding (UFLS) scheme is investigated in this study for smart distribution networks with the communication constraint that each agent can only communicate with its neighboring agents. The proposed scheme uses the two-layer nearest neighbor consensus algorithm (NNCA) to overcome the drawback of the leader–follower consensus algorithm and guarantee the implementation of a decentralized UFLS. Based on the global information (i.e., the magnitude of the total active power imbalance) discovered in the first layer of the NNCA, the multi-stage UFLS can be executed by the second layer of the NNCA. Simulation results demonstrate that the proposed scheme can effectively implement the decentralized UFLS while maintaining the frequency stability of an islanded smart distribution network.

Based on power line carrier communication technology, a decentralized multi-agent system (DMAS)-based frequency control strategy is proposed and investigated in this study on an autonomous microgrid with communication constraints, where each agent can only communicate with its neighboring agents. Using the optimized average consensus algorithm, the global information (i.e., total active power deficiency of the microgrid) can be accurately shared in a decentralized way. Depending on the discovered global information, the cooperative frequency control strategy, which involves primary and secondary frequency control and multi-stage load shedding, is executed to achieve a cooperative frequency recovery. Simulation results indicate that the proposed frequency control approach can guarantee the consensus and coordination of the distributed agents and maintain the frequency stability of islanded microgrids even in emergency conditions.

It is well known that many real-world systems can be described by complex networks with the nodes and the edges representing the individuals and their communications, respectively. Based on recent advances in complex networks, this paper aims to provide some new methodologies to study some fundamental problems in smart grids. In particular, it summarises some results for networks properties, distributed control and optimisation, and pinning control in complex networks and tries to reveal how these new technologies can be applied in smart grids.

A combined cooling, heating and power (CCHP) microgrid with distributed cogeneration units and renewable energy sources provides an effective solution to energy-related problems, including increasing energy demand, higher energy costs, energy supply security, and environmental concerns. This paper presents an overall review of the modeling, planning and energy management of the CCHP microgrid. The performance of a CCHP microgrid from the technical, economical and environmental viewpoints are closely dependent on the microgrid’s design and energy management. Accurate modeling is the first and most important step for planning and energy management of the CCHP microgrid, so this paper first presents an review of modeling of the CCHP microgrid. With regard to planning of the CCHP microgrid, several widely accepted evaluation methods and indicators for cogeneration systems are given. Research efforts on the planning methods of the CCHP microgrid are then introduced. Finally, the energy management of the CCHP microgrid is briefly reviewed in terms of cogeneration decoupling, control strategies, emission reduction and problem solving methods.

Ultracapacitors (UCs), with their features of high power density and high current charge-discharge, have become the best choice for dynamic power compensation to improve the stability of microgrids and are increasingly being applied in microgrids. This paper presents the control of an energy storage system (ESS) based on ultracapacitors in the context of grid-connected microgrids. The ESS is composed of DC/AC and DC/DC converters tied by a dc link. An improved dynamic model for the ESS is proposed. Based on the proposed model a Proportional-Integral-Resonant (PIR) DC link voltage controller is proposed to maintain the DC link voltage through the charging-discharging control of ultracapacitors, capable of working properly under all operating conditions. An extra double frequency component is injected into the UC current by a R controller to dynamically compensate for DC instantaneous power and double frequency AC instantaneous power due to unbalanced grid conditions and disturbances. This feature maintains the DC link voltage constant under unbalanced conditions and increases the degrees of freedom of the DC/AC converter and thus facilitates the application of UCs in microgrids. Simulation and experimental results verify the effectiveness of the proposed control strategy.

In this paper, based on the similarity, in structure and principle, between a grid-connected converter for a direct-driven permanent magnet synchronous generator (D-PMSG) and an active power filter (APF), a new D-PMSG-based wind turbine (WT) system configuration that includes not only an auxiliary converter in parallel with the grid-side converter, but also a coordinated control strategy, is proposed to enhance the low voltage ride through (LVRT) capability and improve power quality. During normal operation, the main grid-side converter maintains the DC-link voltage constant, whereas the auxiliary grid-side converter functions as an APF with harmonic suppression and reactive power compensation to improve the power quality. During grid faults, a hierarchical coordinated control scheme for the generator-side converter, main grid-side converter and auxiliary grid-side converter, depending on the grid voltage sags, is presented to enhance the LVRT capability of the direct-driven PMSG WT. The feasibility and the effectiveness of the proposed system’s topology and hierarchical coordinated control strategy were verified using MATLAB/Simulink simulations.

This paper presents an improved control strategy for both the rotor side converter (RSC) and grid side converter (GSC) of a doubly fed induction generator (DFIG)-based wind turbine (WT) system to enhance the low voltage ride through (LVRT) capability. Within the proposed control strategy, the RSC control introduces transient feed-forward compensation terms to mitigate the high frequency harmonic components and reduce the surge in the rotor currents. The proposed GSC control scheme also introduces a compensation term reflecting the instantaneous variation of the output power of the rotor side converter with consideration of the instantaneous power of grid filter impendence to keep the dc-link voltage nearly constant during the grid faults. To provide precise control, non-ideal proportional resonant (PR) controllers for both the RSC and GSC current regulation are employed to further improve dynamic performance. Simulations performed in Matlab/Simulink verify the effectiveness of the proposed control strategy.

The wide application of distributed generations (DGs) and microgrids (MGs) infuses new vitality into the voltage control of the inverter, which represents a traditional power electronic research area. In this paper, a self-synchronized resonance voltage control algorithm is proposed to satisfy the requirement of an inverter's seamless switching from grid-connected (GC) mode to stand-alone (SA) mode, limiting switching over-voltage and improving switching dynamic performance. Meanwhile, a frequency closed-loop control method is proposed to get rid of the frequency static errors introduced by the self-synchronized feedback voltage control. The effectiveness of the proposed algorithm was confirmed by an experimental MG system.

The increasing presence of wind power in power systems will likely drive the integration of large wind farms with electrical networks that are series-compensated to sustain large power flows. This may potentially lead to subsynchronous resonance (SSR) issues. In this paper, a supplementary controller on the grid-side converter (GSC) control loop is designed to mitigate SSR for wind power systems based on doubly fed induction generators (DFIGs) with back-to-back converters. Different supplementary controller feedback signals and modulated-voltage injecting points are proposed and compared based on modal analysis and verified through root locus analysis to identify the optimal feedback signal and the most effective control location for SSR damping. The validity and effectiveness of the proposed supplemental control are demonstrated on the IEEE first benchmark model for computer simulations of SSR by means of time domain simulation analysis using Matlab/Simulink.

A composite space vector hysteresis current controller (CSVHCC) is proposed in this paper to control the three phase power converter. The duration of the modulation period is divided into two parts. One part for the carrier comparison controllers is to minimize errors at low frequency and provide a basic switching sequence. The other part for space vector hysteresis current controller (SVHCC) is to minimize errors at high frequency and its switching sequence is added to the basic switching sequence. Thus the proposed CSVHCC can make coordination between accuracy and dynamic response. The simulation and experiment shows the validity of the proposed CSVHCC.

To ensure stable and optimal operation of DC microgrid, a three-level based hierarchical control strategy is proposed which consists of primary, secondary and tertiary controls. Usually, the primary control is performed locally without communication, the higher levels, however, can be either centralized or distributed. Considering the risks brought by centralized control, such as surviving central node or link failures, this paper proposes a distributed secondary control strategy for DC microgrid which is designed by Discrete Average Consensus Algorithm (DACA). In this paper, the controller embedded the distributed strategy will communicate with their neighbors through Low Bandwidth Communication to obtain average voltage of the whole system or power flow of grid-connected converter. Through the proposed novel strategy, the secondary control objectives such as reference power control, equal load sharing, voltage regulation will be realized in distributed means. Furthermore, the influence of communication delay to the strategy is studied at length, and the effectiveness and robustness of the proposed strategy are verified through detailed simulations.

Because the microgrid's varied operation states and the application of inverter micro sources, failure characteristics may be quite different when the microgrid in different states. Microgrid feeder protection configuration is a problem to be solved. To solve this problem, the application of the adaptive current protection policies in the microgrid feeder protection was studied. System model was established and simulated. The emulational results showed that when the microgrid operation state changed, the protection equipment could adjust setting values of current in time according to the adaptive current fast-tripping protection strategy. The protection strategy is simple and does not require communication devices. It offers a solution for solving the problem that the microgrid feeder protection setting value is difficult to determine.

The Voltage Source Converter-HVDC (VSC-HVDC) system applied to wind power generation can solve large scale wind farm grid-connection and long distance transmission problems. However, the low voltage ride through (LVRT) of the VSC-HVDC connected wind farm is a key technology issue that must be solved, and it is currently lacking an economic and effective solution. In this paper, a LVRT coordinated control strategy is proposed for the VSC-HVDC-based wind power system. In this strategy, the operation and control of VSC-HVDC and wind farm during the grid fault period is improved. The VSC-HVDC system not only provides reactive power support to the grid, but also effectively maintains the power balance and DC voltage stability by reducing wind-farm power output, without increasing the equipment investment. Correspondingly, to eliminate the influence on permanent magnet synchronous generator (PMSG)-based wind turbine (WT) systems, a hierarchical control strategy is designed. The speed and validity of the proposed LVRT coordinated control strategy and hierarchical control strategy were verified by MATLAB/Simulink simulations.

in this paper, a configuration with a dual-port power converter bridging two sides of the transformer is proposed for power quality enhancement in distribution systems. The proposed configuration employs a nine-switch power converter instead of the traditional back-to-back power converter with 12 switches. The high voltage side port works as a double resonance injection hybrid active power filter to enhance the injection ability of harmonic currents, while the low voltage side port works as a traditional shunt active filter, thus the harmonic and reactive currents to the transformer can be reduced while the stability of DC-link voltage is enhanced. The simulation shows the validity of the proposed configuration.

A microgrid with low-voltage ride-through capability is designed. The designed microgrid avoids operating in unplanned islanded mode during an asymmetric ground fault which occurs in the low voltage distribution network and supports fault recovery for distribution network. Furthermore, compared with the traditional microgrid topology, the proposed microgrid topology also saves a lot of power electronic devices. The simulation results with PSCAD/EMTDC show that the microgrid can keep the distributed generations and loads operate normally when an asymmetric ground fault occurs in the low voltage distribution grid. It can also increase the active power output according to the requirement of the distribution network to support the distribution network fault recovery.

Reasonably selecting the capacity of different types of DGs is the key element to guarantee that microgrid system can operate economically and reliably. Firstly a grid-connected hybrid wind/solar/storage micro-grid system is constructed and the output models of various distributed generations are built. On the basis of correction of wind speed and atmospheric density, the power of wind turbine is predicted by linear interpolation. The output of the photovoltaic generation system is calculated by using the Hay, Davies, Klucher and Reindl model(HDKR model). In order to determine the amount of energy that can be absorbed by or withdrawn from the battery bank each time step, this paper uses the Kinetic battery model. Then considering tax collection of greenhouse gas emission and reliability of the system, an optimal configuration design method is proposed, taking minimizing the whole life cycle cost of the system as the optimization objective. Finally, based on the weather conditions in Yantai city, a simulation analysis is performed, the optimal configuration scheme meeting the load is given. According to Chinese meteorological conditions, a sensitivity analysis of the wind speed and solar resource is given and optimal results are obtained on different conditions. The results are conducive to Chinese planning and design work of the micro-grid.

The Future Renewable Electric Energy Delivery and Management (FREEDM) System smoothly integrate and manage a variety of controllable loads, DRER (distributed renewable energy resource) and DESD (distributed energy storage device) through the revolutionary solid state transformer (SST). The distributed grid intelligence integrated with the SST facilitates the optimal power dispatch for a residential distribution system. This paper presents a mathematical formulation to determine the optimal power dispatch strategy in order to minimize the operational cost as well as the emission pollution. In this paper, a multi-objective immune particle swarm optimization (MIPSO) based computational intelligence approach is adopted to find the Pareto solution. Several numerical simulation studies demonstrate the effectiveness and accuracy of the proposed power dispatch strategy in a residential distribution system. More analysis indicates that the fitting degree between load profile and output of renewable energy generators (such as PV cells) is an identifier, which would be applied as a reference to make power exchange schedule between micro grid and utility grid.

Advanced techniques in light-emitting-diode (LED) light sources allow LED lighting to appear in more and more applications. Besides providing the constant driving current source, LED ballast is usually isolated for safety specifications and dimmed for energy saving. Normally, LED ballast is designed with transformer for isolation and external circuits for dimming, increasing the challenges and complexity of the ballast design. To overcome the above limitations, this paper proposed a non-electrical dimming method based on a contactless transformer, taking full use of the transformer parasitical parameters to transfer energy by resonance. By adjusting the transformer air gap mechanically, the parameters in the resonance tank will change sensitively, which can be used for dimming control without additional components and control. In some extreme conditions, the proposed ballast provides a total environmentally-shield secondary enclosure with contactless transformer. Analysis, design example and prototype verification are given for the LED ballast and dimming control.

VSC-HVDC power transmission system that connected long-distance high-capacity wind farm to the grid will be an important direction of the wind power technology development. The grid access solutions designed according to traditional AC grid structure could not fully reflect the advantages of the VSC-HVDC transmission. Firstly the typical grid solutions of VSC-HVDC based wind power system is analyzed briefly. And then, several new topology applied the wind farm internal DC bus and the Variable Speed DC (VSDC) power system that is suitable for those topologies are proposed. The operating control of VSDC is discussed and maximum wind power tracing is realized by simulation in the end.

Wind power is one of the common distributed generation in microgrid. According to the requirements of microgrid, permanent magnet synchronous wind generator (PMSWG) for wind energy distributed generation was designed in this paper and it has simple structure, easy control and low cost characteristics, non-controllable rectifier and grid-connected PWM convertor is adopted as the AC grid-connected circuit for the distributed generation. Researching on its operating and control strategy, this paper has realized the d/q power decoupling control of the wind energy distributed power and proposed a novel maximum wind power tracking strategy based on the principle of direct-control grid-connected power which can directly realize grid power control through grid current the only one parameter. The experimental system was established and the experiment results have proved correctness and feasibility of the proposed wind energy distributed generation and its control method, it is a suitable wind energy distributed generation for microgrid.

Power quality monitoring is the necessary condition that makes power supply in good working order. This paper presents a kind of new system that can monitor regional power quality. The system adopts the standard of PQDIF as the power quality data norm, expert system to analyse the status of regional power quality and the communication way of GPRS to transmit data. First of all, through this system the power quality manager can check the power quality status of each monitoring station by network from time to time, so we can analyse in time and reflect overall power quality level. Secondly, through the monitoring track record and statistic of power quality accident, the power supply and consumers both can define respective power quality administration obligation, so this system can also provide evidence which effectively settles the dissension because of power quality. Also it can monitor actual power quality of customers and is useful to establish a green electric power system.

With increasing wind power in power systems, impact of subsynchronous resonance (SSR) on wind generator is of interest. This study presents the models of a double-fed induction generator (DFIG)-based wind turbine in a series-compensated network for SSR study. For DFIG (induction machine), the induction generator effect (IGE) is the most important factor of SSR. The control scheme for IGE is proposed in this study and the impact of the control parameters is also studied in detail. Eigenvalue analysis is conducted to study the impact of wind speeds, series compensation levels and control scheme on IGE phenomena. Time-domain simulations are performed in Matlab/Simulink to confirm the analysis.