A novel power flow algorithm for AC microgrids based on time domain iteration

Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes. Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality. When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units. However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted. Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing. Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method. Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods. The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control. Finally, the future research trends on islanded microgrids are also discussed in this paper.

With high penetration of power electronics-based distributed generation (DG) units, the decentralized control strategy based on droop control with virtual impedance has been widely studied in recent years. But the conventional virtual impedance cannot properly deal with the massively increasing of nonlinear loads. Therefore, an improved virtual impedance method based on second-order general-integrator (SOGI) is presented. Besides, a further value correction of reactive power in droop control is also proposed in this paper. The proposed approaches have been applied and verified in a parallel system formed by two DG units connected with nonlinear loads. The form of SOGI and derivative formula of corresponding virtual impedance in the system has been analyzed in detail. The obtained experimental results show the proposed approaches can achieve less output voltage and current distortion of DG units with nonlinear loads, and also realize good performance of power sharing.

The world is transitioning to utilise distributed generation (DG) to reduce social, economic and environmental effects. As DG penetration increases, determining the associated impacts on distribution networks is vital. This paper proposes a concept around sphere of influence of single and multiple DG units through line current analysis and integrated Sankey and Geospatial visualisation approaches. With the increasing integration of renewable energy sources and the need for grid stability, it becomes imperative to analyse the sphere of influence of DGs in geographic context. The main contribution of the paper is to present the visualisation techniques to determine the sphere of influence of DGs within power networks by considering single and multiple DGs. The proposed study combines the power of Sankey visualisation, a flow diagram technique, with geospatial data to gain insights into the DG sphere. The advantage of the proposed method is it acts as a visual lens to determine DG clusters, flow of power, coherency analysis, voltage support, resource allocation, operational planning, and grid resilience assessment. The proposed method is tested using 33 bus distribution network and visualisation on a representative geospatial map is carried out using Python library and Tableau.

Power losses and voltage deviations in distribution power networks (DPNs) are high since they carry more power demand than transmission power networks. Also, voltage deviation beyond the allowable range causes voltage stability problems in the DPN. The power loss (PL) in the DPN should be kept at the minimum level for the economic operation of the electric grid. Integrating distributed generation (DG) in appropriate sites of the power networks can minimize the power losses and voltage drops. An integrated optimization approach is proposed in this paper, by combining an analytical and metaheuristic algorithm to optimize the placement and sizing of multiple DGs. The active power loss sensitivity (APLS) index is an analytical mathematical computation approach used to identify the optimal bus locations for DG placement. The modified ant lion optimization (MALO) algorithm is applied to optimize the ratings of the DG systems. The MALO algorithm is proposed by adopting the Lévy flights (LF) pattern in the random walk process (RWP). LF representation of RWPs enhances the exploration phase of the ALO algorithm and helps to obtain the near-optimal solution. The proposed integrated approach optimizes multiple units of photovoltaic (PV) and wind turbine (WT) units to minimize the multi-objective function, including AP loss and voltage deviation (VD) minimizations. The effectiveness of the proposed integrated approach is validated on the IEEE 69-bus, 85-bus, and 118-bus radial DPNs. Besides, the simulation study is extended for ant lion optimization (ALO), BAT, and artificial bee colony (ABC) algorithms-based techniques. The integrated approach has reduced the total AP loss of the IEEE 69-bus and 85-bus radial DPN from 225 kW to 70.51 kW and 316.12 kW to 162.80 kW, respectively, for the optimized three PV DG units allocation. Likewise, the total AP loss of the 118-bus radial DPN is cut down from 1296.3 kW to 432.3 kW after the optimized five PV DG units allocation. Meanwhile, the total AP LOSS of the 69-bus, 85-bus, and 118-bus radial DPNs is reduced to 4.78 kW, 53.87 kW, and 112.2 kW, respectively, after the optimized WT DG allocation. Additionally, the optimized inclusion of multiple DG units significantly minimized the VD of the DPNs. The minimum VD of the 69-bus, 85-bus, and 118-bus test systems is reduced from 0.0908 p.u., 0.1297 p.u., and 0.1312 p.u. to 0.0174 p.u., 0.0384 p.u., and 0.0201 p.u., respectively, for the multiple PV unit allocations. Similarly, the minimum VDs of the 69-bus, 85-bus, and 118-bus radial DPNs are minimized to 0.0048 p.u., 0.0190 p.u., and 0.0093 p.u., respectively, following the multiple WT DG unit allocations. The simulation findings of the APLS-MALO integrated approach are related to the various optimization techniques. The comparative study reveals that the proposed integrated approach gives a more effective and efficient solution than ALO, BAT, ABC, and other optimization techniques. Finally, the simulation findings of the APLS-MALO integrated technique are verified via the calculation of conventional statistical metrics and the conduction of a non-parametric Wilcoxon test.

This paper describes a control method for distributed generation (DG) units to implement active power sharing and frequency recovery simultaneously in an islanded microgrid. Conventional active power–frequency ( P – f ) droop control is used for the DG controller, and the frequency deviation is recovered by the DG itself via self-frequency recovery control, without requiring secondary frequency control. Because the electrical distance (impedance) from each DG unit to a point where the load demand changes differs among DG units, the instantaneous frequency deviations may differ between DG units. These differences are fed into the integrators of the self-frequency recovery control and may result in errors in active power sharing. To solve this problem and share active power more accurately, a compensation control method is developed for active power sharing, which considers the droop coefficients of each of the DG units. Simulation results show that the proposed control method is effective.

Distributed generation (DG) units has become an essential part of the smart grid due to increased penetration of renewable energy sources. These units help to improve power quality according to their position and control. Grids could face unexpected faults which affects power quality most especially voltage profile and power losses. This paper presents the potential application of (DG) units as a solution for improving voltage profile and mitigating power losses in loop distribution systems in case of faults for different positions of DG unit. Load flow analysis of a 66/20 kV distribution line network is performed to calculate the various values of voltage and power flow at each bus for different faults in each position of DG unit. Simulation of this distribution substation network with DG units have been developed in the MATLAB/Simulink Environment. The objective of this study is enhancement of voltage profile at various buses and the improvement in power flows in the distribution network with reduction in branch losses in the presence of fault.

The primary function of microgrid (MG) is to share the power accurately among different distributed generation (DG) units according to the load demands. The droop control methods are normally used for appropriate power sharing. However, these control techniques suffer badly due to the feeder impedances and uncontrollable power coupling issues. Many techniques have been proposed to share power accurately, most of them rely on communication link. In this work a localized/communication free signal generating mechanism is proposed. The mechanism injects the transient reactive power in frequency droop characteristics by observing the changes in microgrid. This injection disturbs the actual active power sharing which is helpful for the computation of reactive power sharing errors. These errors are minimized by utilizing the proportional integral (PI) controller. The PI controller modifies the y-intercept and the slop of Q-E (Reactive Power –Voltage) droop characteristic for the correction of reactive power sharing error. Moreover, the parameters of droop and PI controller of inner control loop are optimized by Particle Swarm Optimization (PSO) to obtain high precision and reliability. The proposed technique has the following salient features:1. Accurately shares the reactive power without the information of feeder line impedances2. This accurate reactive power sharing scheme has been started locally without the need of communication signal from the central controller. Thus, there is no delay in monitoring the notable changes in microgrid caused by DGs. 3. The careful selection of PI & droop controller parameters through PSO makes the system more stable as compared to the conventional methods in terms of transients and steady state responses.

Driven by environmental concerns and fluctuations of electric energy cost, the integration of distributed generators (DGs) into distribution networks close to the loads, has gained its importance and emerged as complementary to the conventional central power plants. However, the proliferation of distributed generators (DGs) has altered a distribution network from a passive system to an active one. Hence, modifications in the existing distribution power flow methods are needed to account for a DG unit, which can operate either as a PV or PQ bus. Among these methods, the Sequential Power Flow (SPF) method can readily accommodate for PV buses as long as the DG models in the sequence-component frame can be formulated. However, the convergence of SPF is heavily dependent on the choice of initial conditions. Moreover, when the resistance-to-reactance ratio (or R/X) is high, SPF will also diverge. This paper presents a new robust three-phase distribution power flow, which incorporates a homotopic method into the SPF method to overcome these problems. An IEEE benchmark system is used to test the validity and robustness of the proposed algorithm

Voltage regulation in the distribution networks becomes more challenging when the distribution generation (DG) units are introduced. The reasons behind that are: the reverse power flow which is introduced by the DG units, in addition to the probabilistic nature of the generated power due to renewable-based DG units. In this paper, a fuzzy algorithm is proposed to provide an adaptive reference of the OLTC controller, such that the OLTC, which feeds a multiple feeders, can mitigate the effect of the high penetration of the DG units. The motivations behind using the fuzzy logic are: 1) it can map nonlinear relations behind its inputs and output; and 2) it can provide a smooth transition, which lead to a more relaxed tap operation compared to the conventional logic control. The proposed fuzzy algorithm is tested using a radial structure distribution network which is modeled using SIMULINK/MATLAB.

A single-phase distributed generation (DG) sources embedded in three-phase microgrids develop with a fast-paced trend, it is important to make use of suitable power sharing strategies among multiple DGs and utilizing the power generation of these units to the full capacity. This paper presents an innovative sliding mode-based power control strategy for microgrids. The multi-bus microgrid consists of three-phase DG units that are two photovoltaic (PV) array, and three single-phase DG units including PV, battery and fuel cell (FC). The dynamic modeling of all DGs is based on voltage source inverter (VSI). One of the three-phase DGs is responsible for frequency and voltage control, and the other one for current control. The single-phase DGs are controlled based on the three-phase DGs. Finally, the voltage and power control operations are implemented in a per-unit system. The proposed control strategy has a fast response and the ability to trace a reference signal with a low steady-state error compared with the PI controller; moreover, it provides the accurate active and reactive power sharing among energy units under various loading and fault conditions along with robustness against the microgrid parameters. Additionally, the ability to maintain the dc-link voltage and frequency constant is another feature of this controller.

Increased non-linear residential loads in today's distribution system is a concern due to the harmonics related power quality issues. The situation gets worsened by the harmonic resonance introduced by the installation of power factor correction (PFC) capacitor banks in the distribution network. At the same time, more and more renewable energy-based distributed generation (DG) units are being installed in the residential area. These DG systems can be used as an effective way to mitigate the harmonic related power quality problems introduced by the nonlinear residential loads. In literature, very limited work has been done to identify harmonic compensation priorities that should be assigned to different DGs operating at different locations of the distribution system for improved compensation performance. This issue is addressed in this paper. A selective harmonic compensation scheme based on modal analysis is developed to assign compensation priorities on DGs operating at different distribution system nodes for improved compensation performance. A modeled residential distribution system containing distribution components such as distribution line, PFC capacitors, transformers, and household appliances along with DG units is used to verify the improvement of compensation performance. Experimental verification of the proposed method is also provided.

Control strategies of a microgrid which includes both synchronous generators and converter-based distribution generation (DG) units must be designed such that effective operation of the microgrid is achieved. The main objective of this paper is to develop a high performance control strategy for an islanded medium voltage (MV) microgrid consisting of inverter and non-inverter interfaced DG units. A new control method for the synchronous generator in an islanded microgrid is proposed based on a virtual droop scheme. The proposed strategy can effectively manage the real and reactive powers of the microgrid among the inverter and non-inverter based DG units. The steady state and dynamic responses of the MV microgrid is significantly improved by using the proposed power management scheme. The performance of the proposed control strategy is verified by using digital time-domain simulation study in the PSCAD/EMTDC software environment.

This paper introduces a smart coordinated allocation of distributed generation (DG) units, shunt-connected capacitors (SC), and static VAR compensators (SVC) for power loss minimization. The problem is formulated as a mixed-integer non-linear programming and is solved using a BONMIN solver embedded in General Algebraic Modeling System (GAMS). The proposed method’s evaluation was performed on the IEEE 33-bus test system, where three cases were discussed. GAMS/BONMIN was used to find optimal locations and power outputs of DGs in the first case. Simulation results, in this case, showed that losses were reduced by 48.42% with one DG unit, 58.58% with two DG units, and 65.44% with three optimally allocated DG units. In the second case, the proposed method was used to coordinate DGs and SCs, while in the third case, coordinated allocation of DG and SVC was investigated. The results showed that SC/SVC coordination further reduces active power losses. With three SCs allocated, the reduction in the active power losses was 93.5% compared to the base case, and 82.18% compared to losses in the case where only 3 DGs are connected, while in the case of three SVC allocated, it was 93.85% compared to the base case, and 82.21% compared to the case where 3 DGs are connected. The second and third cases also showed that the coordination of SC/SVC leads to an improved voltage profile, where the bus voltages are close to the acceptable range, and voltage fluctuation is significantly reduced. The comparison between different non-linear programming solvers embedded in GAMS for solving optimal power flow was also presented.

Accommodation of distributed generation (DG) units in the distribution power network (DPN) reduces the power losses (PL), improves the voltage profile (VP), and enhances the stability. The size and site for distribution generations have to be optimized to avail favorable results. Otherwise, the DPN may experience greater power losses, higher voltage deviation, and voltage instability issues. This article implements an optimization technique using a hunter-prey optimizer (HPO) algorithm to optimize single and multiple (two) DG units in the radial DPN to minimize total real power losses (RPL) and total voltage deviation (TVD). The effectiveness of the HPO algorithm is assessed on the IEEE benchmark 69-bus radial DPN and a real-world Cairo-59 bus RDS. The simulation outcome after the optimized inclusion of DGs shown significant RPL reduction and considerable voltage enhancement. Furthermore, the optimized results of HPO algorithm were compared to the different algorithms and the comparison proved that the HPO can provide a more promising and authentic outcome than other algorithms.

The integration of small-scale renewable energy has demonstrated a considerable impact on distribution systems, mitigating power losses and enhancing system reliability. This article proposes a methodology for identifying the optimal locations of distributed generation units along a long medium- voltage electrical network, in order to, minimize energy losses, improve voltage levels, and facilitate the planning of the electrical distribution system. Utilizing existing distribution system data in ArcGIS, load flow analysis was conducted using the unbalanced Newton-Raphson method within the CYME simulation software.Through the Power Loss Index method and the Flower Pollination Algorithm, this study identified the optimal locations and power levels for distributed generation units within the feeder. The results of various study scenarios indicate that placing one or multiple distributed generation units along the long feeder reduces total power losses and improves voltage level. In particular, the incorporation of distributed generation units led to a 39.62% reduction in active energy losses and a 41.88% reduction in reactive energy losses. Furthermore, the voltage profile was improved compared to the base case.