Effect Of Line Impedance Research Articles

Lyapunov-based distributed secondary frequency and voltage control for distributed energy resources in islanded microgrids with expected dynamic performance improvement

Microgrids (MGs) can effectively integrate the high penetration distributed energy resources (DERs) for the energy and environmental crisis. Still, fluctuations in intermittent DERs may lead to severe frequency and voltage deviations or even the instability of islanded MGs. Secondary control has successfully compensated for the frequency and voltage deviations. Research on secondary control mainly focused on steady-state operating objectives, i.e., frequency and voltage recovery and power sharing. Still, improving the dynamic responses of secondary control is crucial for system-stable operation, especially in the presence of synchronous DERs. This is because these units can cause undesired oscillatory modes, leading to system instability. In addition, because of the line impedance effect, accurate reactive power sharing is usually ignored when voltage recovery is considered. This will lead to an overload of MGs. To improve the dynamics of secondary control while trading off voltage regulation and reactive power sharing, we propose a Lyapunov-Function (LF)–based distributed secondary control strategy. In the proposed LF-based control strategy, the frequency, voltage, and power information are introduced into feedback control based on the Lyapunov stability theorem. Therefore, the improved frequency and voltage deviations and accurate power sharing can be guaranteed when the Lyapunov function asymptotically attenuates to zero. The inherent conflict between voltage regulation and reactive power sharing is addressed by converging the average voltage to the rated reference. Besides, the improved dynamic performance of secondary control is achieved by considering global power variations, which are obtained by distribution-level phasor measurement units. Global power variations can establish control actions to dampen system oscillations and accelerate system restoration. Furthermore, the controller design method based on the Lyapunov stability theorem can naturally promise the stability of the proposed secondary frequency and voltage controllers. Numerical simulations on the IEEE 34-bus system validate that the proposed control strategy can ensure the significantly improved dynamic performance of secondary control while achieving steady-state operation objectives.

Modeling and Control of a Two-Bus System With Grid-Forming and Grid-Following Converters

The utilization of power converters in a distribution network could give rise to stability problems, especially when the penetration is high. The existing literature studies the modeling and stability of converters with specific control or synchronization methods. However, the types or the control schemes of converters are normally different in an actual grid, and moreover, the line impedance plays an important role on the network stability. In this regard, this article aims at studying the modeling and stability issues of a two-bus electric network with both grid-forming (GFM) and grid-following converters. The main purpose is to provide design guidelines and solution of two-bus system with converters for stable operation. The interactions between the converters in various scenarios will be investigated by considering the effect of line impedance. More importantly, a filter-based stabilized control strategy will be proposed for the GFM converters to mitigate the oscillation in the network. Simulation and experimental results are provided to validate the effectiveness of the theoretical analysis and control strategy.

An Optimal Droop Control Method of DC Micro-grid with Local Loads

An optimal droop control optimization method is proposed of the DC microgrid. First, a modeling of the DC micro-grid is made based on the idea of model equivalence. And then, the traditional particle swarm optimization (PSO) is improved by introducing adaptive learning factors and inertial weights. Finally, based on the improved PSO algorithm, the droop parameters are optimized to optimize the system performance. The simulation results show that even under the dual effects of line impedance and local load, the system can still achieve a high accuracy of current sharing and has a significant suppression effect on the bus voltage deviation.

Power quality control based on voltage sag/swell, unbalancing, frequency, THD and power factor using artificial neural network in PV integrated AC microgrid

Abstract In the present scenario of microgrid system, conversion of electrical energy has initiated a challenge to maintain the power quality within a satisfactory range. It can be influenced by the voltage deviation, sag/swell, unbalancing, frequency, total harmonic distortion (THD) and power factor as per nature of local loads and the condition of distributed energy resources (DERs). The relationship between power quality and the set of these variables are non-linear in nature. The existing literature show that the above mentioned parameters are not considered simultaneously for the assessment and controlling of power quality in PV based AC microgrid. To minimize the effect of these variables, a novel artificial neural network (ANN) based control approach has been proposed which can control the power quality as per IEEE/IEC standards. The proposed method has shown fast, smooth and stable operation while the performance of the same is verified with that of the proportional–integral (PI) and fuzzy-PI controllers using Matlab-Simulink software. The small size microgrid model is tested with the effect of line impedance and communication delay for the assessment of power quality parameters. This model is extended to a large size realistic microgrid structure for the feasibility of control methodology. The realistic microgrid structure is verified under the analysis of line impedance, communication delay, demand response and off-nominal conditions. The proposed control methodology is validated in a realistic microgrid structure and simulation results are presented to show the performance of proposed controller under different test conditions to identify an ANN library.

Decentralised non‐linear I–V droop control to improve current sharing and voltage restoration in DCNG clusters

This study proposes a new decentralised non-linear I–V droop control to achieve current sharing and voltage restoration for paralleled DC nanogrids (DCNGs) in a cluster. The performance of the conventional droop methods in current sharing among parallel converters degrades due to line impedances. The proposed method improves current sharing among the DCNGs with parallel-connected converters by considering the effect of line impedance, which is an important issue in practical applications. By the proposed method, desired current sharing from the light-load to heavy-load condition is achieved. Since only local information (the output voltage and output current of the DCNGs) is used, the proposed method is fully decentralised, and no communication infrastructure is required, which improves the reliability and stability of the overall system. Furthermore, Lyapunov stability theory is applied to investigate the stability of the proposed method. Additionally, the plug-and-play feature is achieved as an important desired functionality in the DCNGs as well as the proposed method provides scalability and modularity for the DCNGs in a cluster. In the end, theoretical analysis and experimental results validate the effectiveness of the proposed control framework for different scenarios.

Direct Digital Control of Single-Phase Grid-Connected Inverters With <italic>LCL</italic> Filter Based on Inductance Estimation Model

Soft magnetic powder cores with their high saturation flux density and low core loss are excellent alternatives for filter inductors in inverter-based applications. However, their nonlinear current-dependent inductance characteristics pose a challenge for control of grid-connected inverter with LCL filter. In this paper, the variable inductance conundrum is discussed and a modified direct digital control method based on a variable-structure inductance estimation model that takes into consideration wide nonlinear variation in both inverter- and grid-side inductances is proposed. The proposed method is shown to have better grid-voltage harmonic rejection and improved stability margins. However, investigation of stability which is conventionally based on nominal values of filter inductors cannot predict instabilities over the entire range of inductance variation. Hence, a parametric approach to conventional stability methods with a parameter space defined by variation in actual and estimated inductance is explored in this paper. The effect of line impedance on stability is also investigated with impedance-based stability criterion by considering line inductance as an additional dimension in the parameter space. A pattern in stability margins is observed due to inductance variation, with inductance at its minimum being most vulnerable to instability. Experimental results measured from a 5 kW single-phase grid-connected inverter with various LCL filters have verified the feasibility of the proposed control method. The experimental results also match the analytical results with reasonable accuracy.

Control Power Sharing of Parallel Inverters in Microgrid with Consideration of Line Impedance Effect

This paper presents a power sharing control method for use between paralleled three-phase inverters in an islanded microgrid. In this study, the mismatch of power sharing when the line impedances have significant differences for inverters connected to a microgrid has been solved, the accuracy of power sharing in an islanded microgrid is improved, the voltage droop slope is tuned to compensate for the mismatch in the voltage drops across line impedances by using communication links. The method will ensure in accurate power sharing even if the communication is interrupted. If the load changes while the communication is interrupted, the accuracy of power sharing is reduced but the proposed method is better than the conventional droop control method. In addition, the accuracy of power sharing base on the proposed method is not affected by the time delay in the communication channel and local loads at the output of the inverters. The control model has been simulated in Matlab/Simulink with two or three inverters are connected in parallel. Simulation results demonstrate the accuracy of the proposed control method. Futhermore, in order to validate the theoretical analysis and simulation results, an experimental setup was built in the laboratory. Results obtained from the experimental setup verify the effectiveness of the proposed method.

Load Sharing in Medium Voltage Islanded Microgrids With Advanced Angle Droop Control

Demand for converter-fed distributed energy resources are rapidly growing in microgrids and these energy sources are mainly responsible for sharing loads in islanded mode of operation. In high voltage lines, sharing real and reactive load power requirements can be achieved by conventional droop control methods, nevertheless strong coupling between active and reactive power in medium voltage lines with low X/R ratio degrades rating-based load sharing among converters. A transformation matrix which incorporates the line impedance effect in droop parameter calculation by modifying droop coefficients can be used to overcome this drawback. However, inclusion of power decoupling term in reference voltage calculation, has found to be more powerful to overcome coupling effects in load power sharing. By utilizing lesser steady state frequency deviation of angle droop in comparison with frequency droop, this paper adopts angle droop in real power loop. Angle droop concept along with power decoupling factor has an added advantage of improved reactive power sharing with surplus accuracy in real power distribution, which in turn, allows harmonic power distribution based on converter rating under different load conditions. Moreover, discrete state feedback control together with third harmonic injection-based pulse width modulation scheme provides additional robustness to individual converters.

A New Virtual Harmonic Impedance Scheme for Harmonic Power Sharing in an Islanded Microgrid

This paper proposes a control strategy for harmonic power sharing in an islanded microgrid, without affecting the voltage quality at the load bus. The harmonic power distribution in an islanded microgrid depends on the line impedance. The proposed control strategy employs negative virtual harmonic impedance to compensate for the effect of line impedance on harmonic power delivery. The influence of mismatched line impedances on nonlinear load distribution is minimized by virtually varying the effective line impedance at harmonic frequencies. The fundamental active and reactive power sharing is implemented using the droop technique and the proposed method operates independently, without affecting the linear load sharing. Furthermore, a new droop method, called $ {Z_{h}} - H $ droop, is also proposed to estimate the virtual harmonic impedance value adaptively. The proposed concept of negative virtual harmonic impedance and $ {Z_{h}} - H $ droop is validated using simulation and digital signal processor-based experimental studies.

Designing robust controller to improve current-sharing for parallel-connected inverter-based DGs considering line impedance impact in microgrid networks

Microgrids are designed to supply electrical and heat loads for small communities. An important research area related to microgrids is proper controller strategy for ensuring the current and power sharing of each generating unit under different load and system conditions. In this paper, considering effect of line impedance, a new approach based on robust control design method, H∞ loop-shaping design (HLSD) procedure, was presented for parallel-connected inverters of DGs in an islanded microgrid. Changes in the parameters of lines were considered uncertainties, based on which the system model was built. The designed controller adjusted the output voltage of each inverter by controlling duty cycles in order to change the output current delivered by each inverter. So, the controller controlled the duty cycles as control inputs to the system so that the system robustness to uncertainties was maximized and the voltage error was minimized, which in turn would cause reduction of the circulating current. For resistive, inductive, and non-linear loads, time responses of the system were achieved. Results of simulations indicated that the designed robust controller, in comparison with gain scheduling controller, was capable of guaranteeing robust stability and performance under parameter uncertainty and different loads. Also, effectiveness of the proposed controller in minimizing the circulating current among parallel-connected inverters was demonstrated.

Improved Instantaneous Average Current-Sharing Control Scheme for Parallel-Connected Inverter Considering Line Impedance Impact in Microgrid Networks

A new control scheme for parallel-connected inverters taking into account the effect of line impedance is presented. The system presented here consists of two single-phase inverters connected in parallel. The control technique is based on instantaneous average current-sharing control that requires interconnections among inverters for information sharing. A generalized model of a single-phase parallel-connected inverter system is derived. The model incorporates the detail of the control loops that use a proportional-resonant controller, but not the switching action. The voltage- and current-controller design and parameters selection process are discussed. Adaptive gain scheduling is introduced to the controller to improve the current and power sharing for a condition, where the line impedance is different among the inverters. The simulation results show that the adaptive gain-scheduling approaches introduced improve the performance of conventional controller in terms of current and power sharing between inverters under difference line impedance condition. The experiments validate the proposed system performance.

Control of parallel inverters in distributed AC power systems with consideration of line impedance effect

The authors have developed a new control technique which allows paralleled inverters to share linear or nonlinear load in a distributed AC power supply system. This technique does not require control interconnections and automatically compensates for inverter parameter variations and line impedance imbalances. Experimental results are provided in the paper to prove the concept.

Some unexplored fields in electrical engineering

In the present scenario of microgrid system, conversion of electrical energy has initiated a challenge to maintain the power quality within a satisfactory range. It can be influenced by the voltage deviation, sag/swell, unbalancing, frequency, total harmonic distortion (THD) and power factor as per nature of local loads and the condition of distributed energy resources (DERs). The relationship between power quality and the set of these variables are non-linear in nature. The existing literature show that the above mentioned parameters are not considered simultaneously for the assessment and controlling of power quality in PV based AC microgrid. To minimize the effect of these variables, a novel artificial neural network (ANN) based control approach has been proposed which can control the power quality as per IEEE/IEC standards. The proposed method has shown fast, smooth and stable operation while the performance of the same is verified with that of the proportional–integral (PI) and fuzzy-PI controllers using Matlab-Simulink software. The small size microgrid model is tested with the effect of line impedance and communication delay for the assessment of power quality parameters. This model is extended to a large size realistic microgrid structure for the feasibility of control methodology. The realistic microgrid structure is verified under the analysis of line impedance, communication delay, demand response and off-nominal conditions. The proposed control methodology is validated in a realistic microgrid structure and simulation results are presented to show the performance of proposed controller under different test conditions to identify an ANN library.