This paper proposes a precise power flow algorithm for islanded hybrid AC/DC microgrids (HMGs). In our analysis, we have considered a multi-grounded unbalanced bipolar DC microgrid and a ...multi-grounded four-wire AC microgrid connected through one or more interlinking converters (ICs). The proposed method is based on the implicit Z BUS method, presenting fast convergence and robustness regardless of the R / X ratio of the lines. It can be applied in all network configurations including highly meshed distribution systems. Numerical simulations are conducted in a 12-Bus and a 47-Bus islanded unbalanced HMG to verify the validity of the proposed power flow approach considering several distributed generator (DG) operational modes. A case study in a large 1024-Bus islanded HMG further highlights the outstanding accuracy and computational performance of the proposed approach against other existing power flow methods.
The growing penetration of converter-interfaced distributed renewable energy sources (CI-DRES) has posed several challenges into the electric power systems, e.g., the instability caused by the ...intermittent nature of the primary sources, power quality issues, etc. Several algorithms have been proposed to mitigate the CI-DRES power fluctuations and reduce high active power ramp-rates (RRs) at the point of common coupling (PCC) with the grid using energy storage systems (ESS). However, these algorithms present some drawbacks. In this article, a new ramp-rate limitation (RRL) control method is proposed to address existing gaps in the technical literature. This algorithm is performed considering the connection of a supercapacitor (SC) at the dc-link of a DRES converter. The relationship between the SC voltage and the degree to which the RRL is achieved is established, aiming to reduce the SC voltage fluctuations and increase the SC life time. The RRL control is validated in a real experimental testbed and compared to state-of-the-art approaches. This control is also modeled in Matlab/Simulink in order to perform techno-economic investigations on the influence that several parameters (SC size, SC charging/discharging cycles, cost, etc.) have on the achieved RRL at the DRES PCC.
This paper proposes a comprehensive load flow algorithm for balanced and unbalanced distribution systems considering the effect of a multi-grounded neutral conductor. The method is based on the ...implicit Z BUS method presenting fast convergence and robustness regardless of the R/X ratio of the lines. Its distinct feature is its ability to handle all network configurations, including highly meshed distribution systems that operate in either islanded or grid-connected mode. Furthermore, an algorithm is proposed for overcoming the limitation of implicit Z BUS method to treat PV nodes. Additionally, the concept of virtual impedance is incorporated into the proposed algorithm to accurately simulate the behavior of distributed generators (DGs) during the islanded operation. Numerical simulations are conducted in 33-Bus and 38-Bus balanced networks as well as in 25-Bus and 30-Bus unbalanced networks to verify the validity of the proposed load flow algorithm.
The integration of photovoltaics (PVs) in low-voltage (LV) grids is expected to rise within the following years posing technical challenges to the reliable operation of the electrical system. To ...tackle these challenges, distributed energy storage systems (ESSs) coupled with PVs at prosumer side arise as a promising solution. Therefore, during the last years several control schemes have been developed to manage ESSs. To overcome disadvantages of conventional control strategies, a new localized control strategy is proposed in this paper. The proposed control aims to improve voltage profile along distribution feeders, by mitigating the peaks of the net injected/absorbed power at prosumers' point of common coupling. Additionally, the new control aims to maximize prosumers' self-consumption. To achieve peak mitigation at the net power profile, an exponential droop is introduced that charges/discharges ESSs with different rates based on the residual power between PV generation and load demand. Using this droop, ESS charging/discharging power is exponentially increased as the residual power becomes greater. To maximize the self-consumption of the installation, an optimization procedure is developed that properly adjusts the exponential droop parameters to prosumer's generation and consumption profile. The effectiveness of the control strategy is validated by both simulation and experimental results.
•A coordinated reactive power control (RPC) is proposed.•Distributed generation (DG) units and loads participate in the RPC.•Time-delays are introduced to prioritize the response of DG units.•On-load ...tap changer acts supplementary to the RPC method.•Dynamic and long-term simulations are performed to evaluate the proposed method.
The main objective of this paper is the effective voltage regulation in radial medium-voltage (MV) distribution networks with high distributed generation (DG) penetration, ensuring near-minimum active power losses. For this purpose, a new control strategy with low computational complexity is proposed. The method exploits the reactive power capability of DG units to mitigate overvoltages in coordination with the on-load tap changer of the high-/medium-voltage transformer to achieve power losses reduction. This is attained by introducing a time delay allocation method based on the graph theory to prioritise the response of DG units. The control scheme is further enhanced by the active participation of MV loads in the voltage regulation process, contributing to the reactive power control of DG units. To evaluate the performance of the proposed control strategy, time-domain and time-series simulations are performed in radial MV distribution networks. The former demonstrates the robustness and fast response of the proposed control scheme, while the latter highlights its improved power system performance over existing centralised as well as decentralised control methods.
During the last two decades, the use of residential photovoltaic (PV) systems has been widely promoted by governments through various support mechanisms such as feed-in-tariffs, net-metering, ...net-billing, etc. These support schemes have developed a secure investment environment, increasing the penetration level of PVs in low-voltage distribution grids. Nonetheless, increased PV integration may introduce several technical problems regarding the secure operation of distribution grids. Battery energy storage (BES) systems can mitigate such challenges, but the high capital cost is one of the most important limiting factors towards the widespread use of these systems. In fact, the financial viability of integrated PV and BES systems under different support schemes remains an open issue. In this study, the profitability of PV and BES systems is evaluated through an advanced techno-economic model, that provides the optimal size of PV-BES system in terms of net present value, based on the electricity production and consumption profile of the installation, PV and BES systems costs, and electricity charges. The proposed model may be a useful tool for prosumers, grid operators and policymakers, to assess the impact of various incentive policy schemes and different BES operation strategies on the economic viability of PV-BES systems.
The advent of distributed renewable energy sources (DRESs) has led to the progressive transformation of traditional distribution networks to active components of the power system. This ...transformation, however, may jeopardize the reliable grid operation due to the advent of new technical problems, such as network overloading, over-/under-voltage events, abnormal frequency deviation and dynamic instability. In this challenging scenery, the installation of a modern measuring infrastructure has created new sources of data and information that facilitate the provision of ancillary services (ASs) via measurement-based analysis. The ACTIVATE (ancillary services in active distribution networks based on monitoring and control techniques) project aims to design innovative AS solutions for power system operators. These solutions aim to tackle the technical issues emerged by the ever-increasing DRES penetration and their volatile nature. In this context, in ACTIVATE, a holistic system is proposed comprising centralized and decentralized control features to enhance the overall network performance. Additionally, a network monitoring system is designed to support a number of online and offline dynamic analysis applications by exploiting measurements obtained at the transmission, primary and secondary distribution network. This paper presents a validation of the overall system, which is performed by using simulation and power-hardware-in-the-loop results in combined transmission and distribution network models.
•Comprehensive step voltage regulator model.•Introduction of fictitious impedances overcomes divergence issues of power flow.•Generic applicability in all configurations of step voltage ...regulators.•Step voltage regulator taps are modelled as equivalent current sources.•Computation time of power flow is reduced.
This paper presents a comprehensive three-bus equivalent circuit model of three-phase step voltage regulators. The proposed model can be efficiently integrated in the Z-bus power flow method and can accurately simulate any configuration of step voltage regulators. In contrast to the conventional step voltage regulator models that include the tap variables inside the YBUS matrix of the network, the proposed model simulates them in the form of current sources, outside the YBUS matrix. As a result, the re-factorization of the YBUS matrix is avoided after every tap change reducing significantly the computational burden of the power flow. Furthermore, possible convergence issues caused by the low impedance of step voltage regulators are addressed by introducing fictitious impedances, without, however, affecting the accuracy of the model. The results of the proposed step voltage regulator model are compared against well-known commercial softwares such as Simulink and OpenDSS using the IEEE 4-Bus and an 8-Bus network. According to the simulations, the proposed model outputs almost identical results with Simulink and OpenDSS confirming its high accuracy. Furthermore, the proposed 3-bus equivalent model is compared against a recently published conventional step voltage regulator model in the IEEE 8500-Node test feeder. Simulation results indicate that the proposed step voltage regulator model produces as accurate results as the conventional one, while its computation time is significantly lower. More specifically, in the large IEEE 8500-node network consisting of four SVRs, the proposed model can reduce the computation time of power flow around one minute for every tap variation. Therefore, the proposed step voltage regulator model can constitute an efficient simulation tool in applications where subsequent tap variations are required.
In this paper, a two-stage approach is introduced to efficiently solve the optimal voltage regulation problem in radial medium-voltage (MV) distribution networks. The proposed method uses the ...available reactive power of distributed generation units and the on-load tap changer (OLTC) of the high-voltage/MV transformer to regulate network voltages, while also minimizing two conflicting objectives, namely the network energy losses and the frequency of tap changes. This bi-objective optimal voltage regulation problem is addressed in two distinct stages. In the first stage, the network is linearized and a simplified optimal voltage regulation problem is solved to determine the candidate OLTC operating plans (COOPs). For each COOP, a new reactive power allocation method is employed in the second stage to regulate the network voltages and minimize network losses. This method follows a rule-based approach, also allowing its implementation under real-field conditions. The final outcome of this two-stage process is the Pareto-front which can be used by the distribution system operators to select the most preferable solution for the real-time network operation. The proposed methodology is characterized by reduced computational complexity compared to the application of conventional optimization techniques. Time-domain and time-series simulations on a radial MV distribution network are employed to thoroughly evaluate the performance of the proposed method.
The intermittent and volatile nature of renewable energy sources (RESs) has introduced new technical challenges that affect the secure and reliable grid operation. These challenges can be tackled at ...the RES level by reducing power fluctuations with the use of power smoothing (PS) techniques. Several PS methods have been proposed in the literature to smooth RES output exploiting battery energy storage systems (BESSs). However, a comprehensive comparative evaluation of PS methods is missing. Moreover, the effect of the long-term PS operation on the BESS life is usually ignored in such analyses. This article proposes a methodology for the systematic evaluation of well-established PS techniques, comparing their effectiveness on the PS of photovoltaic output based on various signal metrics. In addition, an accurate aging model for lithium-ion batteries is employed to investigate the impact of PS on the BESS lifetime, highlighting the main parameters that influence capacity degradation.
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