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Phase-locked loop (PLL) is commonly utilised for AC power systems to detect phase and frequency. With the increasing use of small-scale distributed power generation, the technique becomes widely available for grid interconnection of renewable power source into single-phase AC distribution network. Through comprehensive analysis and design, this study proposed a new approach that includes dual independent control loops to enhance the transfer delay-based PLL capability in terms of speed and accuracy. The effectiveness and advantages of the proposed PLL structure are demonstrated by numerical simulation and verified by experimental test.

This article presents a GPS-based decentralized control strategy for hybrid ac/dc microgrids. The proposed method uses V-I droop characteristics for the distributed energy resources (DERs) in each of ac and dc subgrids. In particular, the output voltage of each DER is adjusted as a piecewise linear function of the per-unit (pu) output current. In addition, a droop control method is proposed for the interlink converter (IC) to attain global power sharing among the dc and ac DERs. The IC controller changes the deviation of the ac bus voltage as a function of the deviation of the dc bus voltage. This way, the power flow between ac and dc subgrids is adjusted such that the ac and dc DERs experience the same pu voltage deviation and hence deliver the same pu current and active power. In contrast with the existing control approaches, the proposed method does not require frequency or power calculations but directly uses voltage and current measurements in the control loops. So the delay associated with the feedback signals is minimized, which avails accurate load sharing during transients. Experimental results show the proposed method eliminates the transient overshoots of DERs' output powers by offering a fast and overdamped dynamic response.

This paper presents a support vector machine (SVM)-based fault detection algorithm for islanded meshed microgrids (MGs). Considering the low level of fault current in islanded MGs, the fault detection is carried out based on the magnitude of the voltage rather than the current. Accordingly, the rms voltages of the DERs are assigned as the feature vector for fault detection SVM. Upon the occurrence of a fault, the fault location is identified by means of an interharmonic injection scheme. To identify the faulty zone, the DER which is electrically closest to the fault injects interharmonic currents into the grid and the zone with highest current is considered as the faulty zone. Then, a second DER located in the same zone inject interharmonic currents at different frequencies. The resulting interharmonic voltages are measured by each DER to form a feature vector. A multi class SVM uses this feature vector to identify the faulty line within that zone. The proposed algorithm is tested on a nine bus meshed MG. Test results show that the proposed method is capable of accurate detection of fault location even with high fault impedances.

In this paper, a decentralized secondary control strategy for microgrids, with fast dynamic response is proposed. This high bandwidth approach is realized by applying a finite control set, model predictive control (FCS-MPC) at the primary control level of the voltage source converters (VSCs) control. At the upper control level, a novel decentralized secondary control structure is proposed to regulate the islanded microgrid voltage and frequency subsequent to load change, with no need of any communication infrastructure. The proposed control strategy, restores the microgrid frequency and voltage to the nominal value while maintaining accurate power-sharing of the droop mechanism. Experimental results are also provided to verify the effectiveness of the proposed approach.

The conventional control scheme of hybrid microgrids (MGs) uses active power-frequency and reactive power-voltage droop control methods to realize proportional active power sharing in ac and dc MGs, respectively. In order to equate the percentage loadings of dc and ac subgrids, the power transfer through the interlink converter is controlled such that the per-unit frequency deviation of ac MG is equated to the per-unit voltage deviation of dc MG. The main drawback of the mentioned strategy is the poor dynamic response caused by the slow dynamics of the conventional droop method as well as the delay associated with frequency measurement in the interlink converter (IC) controller. To enhance the dynamic response, a control scheme based on voltage-current droop characteristics is proposed in this paper. In this method, the d and q axis V-I droop control schemes are adopted for proportional active and reactive power sharing among ac Distributed Energy Resources (DERs), and the dc V-I droop control method is used for coordination of dc DERs. Furthermore, a novel control strategy based on d-axis voltage/dc voltage droop characteristics is proposed for the IC to realize global power sharing with a fast dynamic response. Experimental results are presented to showcase the efficacy of the proposed scheme.

In this paper, a distributed control scheme is proposed for voltage control and power management of coupled microgrids (MGs). The control scheme is implemented in a two-level structure. In the primary level, a voltage–current droop controller is adopted in the $dq$ reference frame to facilitate proportional load sharing among the distributed energy resources (DERs) with fast dynamic response. A novel distributed secondary control scheme is introduced to regulate the power exchange among MGs and restore the average voltage to the nominal value. The secondary controller is comprised of a network of control agents interconnected through low bandwidth communication links. Each control agent alters the offset of $d$ - and $q$ -axis droop characteristics to change the current output of the DERs within each cluster until the current exchanges among the clusters are regulated at the set points and the average voltage is restored to the nominal value. If the load of an MG cluster is increased above its total generation capacity, the secondary controller alters the current exchange among the clusters to prevent overload. Simulation and experimental results are presented to demonstrate the efficacy of the proposed method.

This paper proposes a new decentralized power management and load sharing method for a photovoltaic based islanded microgrid consisting of various photovoltaic (PV) units, battery units and hybrid PV/battery units. Unlike the previous methods in the literature, there is no need to communication among the units and the proposed method is not limited to the systems with separate PV and battery units or systems with only one hybrid unit. The proposed method takes into account the available PV power and battery conditions of the units to share the load among them. To cover all possible conditions of the microgrid, the operation of each unit is divided into five states and modified active power-frequency droop functions are used according to operating states. The frequency level is used as trigger for switching between the states. Efficacy of the proposed method under different load, PV generation, and battery conditions is validated experimentally in a microgrid lab prototype consisting of three units.

© 2019 IEEE. This paper proposes a distributed control strategy for coordination of distributed energy resources (DERs) in low-voltage resistive microgrids. The proposed framework consists of two level structures; primary and secondary control. Unlike the existing distributed control solutions, the proposed method is based upon the practical assumption of network impedance being resistive. The primary control level consists of a V-I droop mechanism, where GPS timing is used to synchronize the control agents. A consensus-based distributed secondary control method is introduced to improve the voltage regulation and load sharing accuracy of the V-I droop method. In the proposed approach, the d-axis component of the voltage is altered so as to regulate the average microgrid voltage to the rated value while guarantying proper sharing of active power among the DERs. Additionally, the q-axis component of voltage is adjusted to perform proper current and, accordingly reactive power sharing. The proposed control methodology accounts for the distribution line impedances. It features a plug-and-play environment; prior system knowledge is not required, and an arbitrary DER can enter the microgrid without any need for additional synchronization. An ac microgrid is prototyped to experimentally demonstrate the efficacy of the proposed method.

Coordinated control of distributed energy resources (DER) is essential for the operation of islanded microgrids (MGs). Conventionally, such coordination is achieved by drooping the frequency of the reference voltage versus active (or reactive) power. The conventional droop method ensures synchronized operation and even power sharing without any communication link. However, that method produces unwanted frequency fluctuations, which degrade the power quality. In order to improve the power quality of islanded MGs, a novel decentralized control method is proposed in this paper. In this method, the GPS timing technology is utilized to synchronize the DERs to a common reference frame, rotating at a nominal frequency. In addition, an adaptive Q–f droop controller is introduced as a backup to ensure stable operation during GPS signal interruptions. In the context of the common reference frame, even sharing of active ( $i_{d}$ ) and reactive ( $i_{q}$ ) components of the current are achieved based on $v_{d}\,\hbox{-}\,i_{d}$ and $v_{q}\,\hbox{-}\,i_{q}$ droop characteristics. The method has been tested on a laboratory-scale MG. Experimental results demonstrate the efficacy of the proposed method in terms of dynamics, power quality, and robustness with respect to GPS interruptions.

Unbalanced load currents not only give rise to unbalanced voltages but also adversely affect the performance of the conventional current-limiting mechanisms. The latter might result in overcurrent stress on the distributed energy resources (DERs) or current harmonics. In this paper, a novel decentralized control method is proposed to improve the power quality and protect DERs from overload. The proposed controller makes use of the model predictive control (MPC) technique to minimize the voltage unbalance, improve current limiting, and prevent active power overload. The MPC is combined with the ${\rm V}$ - ${\rm I}$ droop method to realize coordinated operation with fast dynamic response. The proposed method is tested on the CIGRE benchmark microgrid. Simulation results demonstrate that the proposed method improves power quality but also allows for operation close to the maximum load capacity without imposing DERs to overload.

Distribution network protection is becoming more sophisticated in the wake of ever-changing landscape of power systems driven by the vast renewable energy integration mostly sited behind the meters, growing uncertainty and volatility subsequent to smart demand response and renewable energy integrations, further fusion of power electronics-interfaced equipments, and more constrained distribution branches as the result of load growth and limited investments. In the opposite side, the massive deployment of smart meters, proliferation of advanced measuring devices such as phasor measurement units, emerging electric and not-electric sensors, and IoT-enabled data gathering platforms continually expand/nourish the databases; they hence offer unprecedented opportunities to take the advantage of data-driven techniques. Machine learning (ML) as a principal class of artificial intelligence is the perfect match solution to this need and has newly revoked many researchers’ interests to tackle the problems excluding their exact/detailed models. This paper discusses applications of ML techniques in protection and dynamic security assurance of active distribution network, microgrids, and power electronics-based systems.

Microgrids’ performance and stability mostly depend on power-flow control strategy. In order to allow for coordinated control while maintaining reliable operation, decentralized control methods based on $P$ and $Q$ droop characteristics have been utilized. Inherently, the power droop control methods have slow dynamics. In this paper, a novel control method based on V-I characteristics is introduced to exploit the flexibility and fast dynamics of the inverter-based distributed energy resources. In the proposed method, the direct and quadrature axis voltage components are drooped with the corresponding currents according to a piecewise linear droop function. Eigenvalue analysis of a sample microgrid shows that the proposed method features faster dynamics and improved damping compared to the conventional droop scheme. Simulation results are presented to verify the efficacy of the proposed method.

This paper presents a systematic methodology for the design and the tuning of the current controller in LCL grid-connected converters for wind turbine applications. The design target is formulated as a minimization of the current loop dominant time constant, which is in accordance with standard design guidelines for wind turbine controllers (fast time response and high stability margins). The proposed approach is derived from the impedance/admittance stability formulation, which, on one hand, has been proved to be suitable for the controller design when the active damping is implemented and, on the other hand, has also been proved to be very suitable for system-level studies in applications with a high penetration of renewable energy resources. The tuning methodology is as follows: first, the physical system is modeled in terms of the converter admittance and its equivalent grid impedance; then, a sensitivity transfer function is derived, from which the closed-loop eigenvalues can be calculated; finally, the set of control gains that minimize the dominant time constant are obtained by direct search optimization. A case study that models the target system in a low-power scale is provided, and experimental verification validates the theoretical analysis. More specifically, it has been found that the solution that solves the minimization of the current controller time constant (wind turbine controller target) also corresponds to a highly damped electrical response (robustness provided by the active damping).

© 1986-2012 IEEE. This paper proposes a novel hierarchical control strategy for improvement of load sharing and power quality in ac microgrids. This control framework is composed of a droop-based controller at the primary level, and a combination of distributed power sharing and voltage conditioning schemes at the secondary level. The controllers in the primary level use GPS timing technology to synchronize the local reference angles. The voltage reference of each distributed generation is adjusted according to a voltage-current (V-I) droop characteristic to enable proper current and power sharing with a fast dynamic response. The droop coefficient, which acts as a virtual resistance, is adaptively changed as a function of the peak current. This strategy not only simplifies the control design but also improves the current sharing accuracy at high loading conditions. The distributed power sharing scheme uses consensus protocol to ensure proportional sharing of average power. The voltage conditioning scheme produces compensation signals at fundamental and dominant harmonics to improve the voltage quality at a sensitive load bus. Experimental results are presented to validate the efficacy of the proposed method.

In this article, a new bi-static method for the detection and determination of magnitude and location of winding radial deformation in power transformers has been proposed. In this method, which is based on confocal microwave imaging, a ultra-wideband transceiver is utilized to emit a short pulse toward the transformer winding and determine its reflection at several points along a linear path. The measured signals are then processed to obtain a 2D image of the winding. The effectiveness of this algorithm for radial deformation is demonstrated through four different experiments. The resultant image provides satisfactory information of the magnitude and position of the radial deformation in transformer.

© 2016 IEEE. This paper proposes a new distributed control method for coordination of distributed energy resources (DERs) in low-voltage resistive microgrids. The proposed framework consists of two level structure; primary and secondary control. Unlike the existing distributed control methods, the proposed method is based upon the practical assumption of resistive network impedance. In this context, a V-I droop mechanism is adopted in the primary control level, where GPS timing is used to synchronize the control agents. A new distributed secondary control method based on consensus protocol is introduced to improve the voltage regulation and load sharing accuracy of the V-I droop method. In this method, the d-axis components of the voltage is altered so as to regulate the average microgrid voltage to the rated value while guarantying proper sharing of active power among the DERs. Additionally, the q-axis component of voltage is adjusted to perform proper current and, accordingly reactive power sharing. The proposed control methodology accounts for the distribution line impedances. It features a plug-and-play environment; prior system knowledge is not required, and an arbitrary DER can enter the microgrid without any need for additional synchronization mechanisms. An AC microgrid is prototyped to experimentally demonstrate the efficacy of the proposed method.

This paper presents a systematic methodology for design and tuning of the controller of LCL grid-connected converters. The proposed approach is derived from the impedance/admittance stability formulation , which eases the controller modelling of LCL grid-connected systems. After system modelling, a modified sensitivity function is defined to analyze the closed loop dynamics. For practical development, the control objectives are linked to main requirements in wind turbine applications: 1) the current control time constant should be minimized and 2) active damping actions should effectively mitigate the LCL resonance. After the theoretical development, an insightful case study is provided. It is shown that the tuning problem can be solved approximately by root-loci inspection and an optimum solution can be found by direct search. The theoretical approach is experimentally verified by a lab-scale prototype that uses the same parameters of the case study.

© 2016 IEEE. This paper proposes a novel control method for accurate sharing of load current among the Distributed Energy Resources (DER) and high power quality operating in islanded ac microgrids. This control scheme is based on hierarchical structure comprising of decentralized primary controllers and a centralized secondary controller. The controllers in the primary level use GPS timing technology to synchronize their local time with a common time reference. In this context, proportional current sharing is achieved by adjusting the reference voltage of each DER unit according to a voltage-current (V-I) droop characteristic. The droop coefficient, which acts as a virtual resistance, is adaptively changed as a function of peak current. This strategy not only simplifies the control design but also enables faster dynamics and higher accuracy of current sharing especially at high loading conditions. The secondary controller produces compensation signals at fundamental and dominant harmonics to improve the voltage quality at a sensitive load bus. Experimental results are presented to validate the efficacy of the proposed method.

In this paper, a novel method for the detection and determination of the dimension and position of the winding radial deformation is presented. In this method, which is based on synthetic aperture radar imaging, an ultrawide-band transceiver is utilized to transmit a short pulse toward the transformer winding and measure its reflection at several points along a linear path. The measured signals, which bear the geometrical details of the winding, are further processed to take a 2-D image of the transformer. By analyzing this image, the occurrence of the radial deformation as well as its dimension and position can be detected. Experimental results for five different radial deformations are presented to verify the proposed method.

Single phase loads, which constitute the majority of low voltage consumers, give rise to unbalanced load currents in microgrids. As a consequence the voltage unbalance might rise beyond the permissible range. The voltage unbalance can be reduced by injecting a negative sequence (NS) compensation signal into the DERs reference voltage. However, such compensation might cause circulating currents between the DERs and also excessive current in the heavily loaded phases of some DERs. In this paper, a novel NS compensation method is proposed to reduce the circulating currents and eliminate the excess current in inverter-based microgrids. In this method, the NS voltages of the DERs are adjusted according to again scheduled NS droop method. The compensating signal is then computed by solving an optimization problem, which aims at tracking the NS voltage with minimum compensating action. Simulation results are proposed to verify the efficacy of the method.

This paper presents a comprehensive modeling of a three-phase cage induction machine used as a self-excited squirrel-cage induction generator (SEIG), and discusses the regulation of the voltage and frequency of a self-excited SEIG based on the action of the static synchronous Compensator (STATCOM). The STATCOM with the proposed controller consists of a three-phase voltage-sourced inverter and a DC voltage. The compensator can provide the active and reactive powers and regulate AC system bus voltage and the frequency, but also may enhance the load stability. Moreover, a feed forward control method for the STATCOM is introduced and applied for controlling the SEIG's terminal voltage using a two-degree of freedom RST controller. Simulation results for the steady-state operating condition and transient operating conditions for the system subjected to a wind reference step change, and a step load change are presented to demonstrate the effectiveness of the proposed controller.

In this paper a new controller design method for inverter-based distributed energy resources in standalone microgrids is proposed. The objective of the design is to exploit the flexibility and fast dynamics of the voltage inverter. The proposed control method is composed of droop, voltage and current controllers. The droop controller is based on V-I droop characteristics, and exhibits significantly faster dynamics compared to the conventional droop method. The voltage and current controllers utilize the structured H∞ PI regulation strategy to track the reference voltage and current. The PI controller parameters are tuned using Hinfstruct optimization function from the MATLAB® Robust Control Toolbox. The performance of the closed-loop system is analyzed to illustrate the capabilities of the new technique. Simulation results are presented to prove validity and verify the efficiency of the proposed control.

In this paper a single phase Active Power Filter(APF) based on least compensation current control method is presented. Despite it's simple implementation this technique shows quicker dynamic, better stability and higher reliability compared with existing control methods. Simulation results using MATLAB are presented to verify theoretical predictions and demonstrate effectiveness of the proposed control method.