A distributed control scheme for voltage regulation in DC distribution systems

This paper proposes optimal voltage control strategies for distribution systems that have multiple voltage regulator devices. The system must operate reaching desirable voltage profiles for scenarios that consider extreme loading (heavy and light), unbalanced loads and distributed generation connection. The problem is modeled as an optimization problem that ensures the attendance of the constraints related to voltage limits, power balance and control devices' capacity. The decision variables are the taps of the voltage control devices, which play an important role on Volt/Var control. A genetic algorithm implemented through a computer interface between Matlab and OpenDSS packages determines the number of control operations. It is assumed that the distribution system has a centralized control with all necessary communication infrastructure. To confirm the validity of the proposed method, simulations were performed by using the IEEE 34 node test feeder. The results showed that the strategies contributed to reduce the voltage deviation along the feeder, the voltage unbalance and the total feeder losses.

A novel strategy for optimal placement of locally controlled voltage regulators in traditional distribution systems

Distributed energy resources (DERs), including renewable energy resources (RESs) and electric vehicles (EVs), have a significant impact on distribution systems because they can cause bi-directional power flow in the distribution lines. Thus, the voltage regulation and thermal limits of the distribution system to mitigate from the excessive power generation or consumption should be considered. The focus of this study is on a control strategy for DERs in low-voltage DC microgrids to minimize the operating costs and maintain the distribution voltage within the normal range based on intelligent scheduling of the charging and discharging of EVs, and to take advantage of RESs such as photovoltaic (PV) plants. By considering the time-of-use electricity rates, we also propose a 24-h sliding window to mitigate uncertainties in loads and PV plants in which the output is time-varied and the EV arrival cannot be predicted. After obtaining a request from the EV owner, the proposed optimal DER control method satisfies the state-of-charge level for their next journey. We applied the voltage sensitivity factor obtained from a load-flow analysis to effectively maintain voltage profiles for the overall DC distribution system. The performance of the proposed optimal DER control method was evaluated with case studies and by comparison with conventional methods.

Electric Springs has been testified recently to enhance voltage regulation in distribution systems using demand side management. In this paper, a 7-level Multilevel Inverter (MLI) with a resonant switched capacitor Converter based on sinusoidal PWM, is implemented to analyze the performance of an electric spring under voltage variations at PCC. By the proposed MLI based ES, voltage regulation of critical load voltage is studied for voltage sag and swell conditions. Remarkable features of the proposed topology are maintaining voltage balance in input capacitors and reduction of power components. Simulations have been done in MATLAB/Simulink on distribution system with DGs equipped with MLI based ES under line voltage anomalies. Tested results are analyzed with THD% in critical load voltage.

Robust model predictive control based voltage regulation method for a distribution system with renewable energy sources and energy storage systems

With the rising potential for the employment of low- and medium-voltage direct-current (dc) electric power distribution systems, most notably for a more efficient integration of plug-in electric vehicles and such other distributed energy resources as photovoltaic (PV) panels, there is a need for robust ac/dc electronic power converters that can interface such dc distribution systems with the legacy alternating current (ac) power system. Thus, this thesis proposes a new single-stage low-voltage three-phase ac-dc power converter that is simple structurally, en- ables a bidirectional power exchanges between the ac and dc distribution systems, and can handle short-circuit faults at its dc as well as ac sides. The proposed converter consists of three legs, corresponding to the three phases of the host ac grid, each of which hosting two full-bridge submodule (FBSM), in an architecture that can be regarded as a special case of the so-called modular multi-level converter (MMC). Thus, at the dc port each FBSM is connected in parallel with a corresponding capacitor, while the ac voltage of each phase is synthesized by the coordinated sinusoidal pulse-width modulation (SPWM) of the two corresponding FBSMs. This architecture allows the generation of low-distortion ac voltage while it also provides the converter with the very important dc fault current blocking capability since, upon the detection of a short circuit across the converter dc port, the switches of the FBSMs are turned off and disallow the flow of any dc current. The thesis also presents a mathematical model for the converter, for analysis and control design purposes. Thus, the control for the regulation of the overall dc-side voltage, as well as those for the regulation of the dc voltages of the FBSMs are devised based on the aforementioned mathematical model and presented with details. It is further shown that the voltage conversion ratio of the proposed converter is the same as that offered by a conventional voltage-sourced converter (VSC), whereas the VSC is vulnerable to dc- side shorts. The proposed converter can be extended to medium-voltage levels by multi- plying the number of FBSMs in each leg. The effectiveness of the proposed converter and its controls is demonstrated through time-domain simulation studies conducted on a topological model of the converter in PSCAD/EMTDC software environment.

SUMMARYThis paper studies a DC distribution system, which has been supplied by utility AC grid and converter‐based distributed generation (DG) in order to provide an overall solution to power quality problems. The power flow control in DC distribution systems is achieved by network and storage converters, which have DC voltage controllers. A control strategy for these converters has been presented, which is based on DC voltage droop controller and a novel instantaneous power regulation scheme. Furthermore, this paper presents a simple DC bus voltage controller, which can be used for DC system voltage determination. Also, a novel control system has been proposed for DG and load converters. The simulation results, based on PSCAD/EMTDC software, show that DC distribution system including converter‐based DG unit can provide the premium power quality. Copyright © 2011 John Wiley & Sons, Ltd.

This chapter introduces an optimization problem of sizes (capacities) and locations (connection points) of flexible alternative current transmission systems (FACTS) devices for voltage regulation in electric power distribution systems as an application target of particle swarm optimization (PSO). This study focuses on operations of the distribution systems in consideration of penetration of photovoltaic generation (PV) systems. Massive installation of the PV systems brings unexpected significant impacts on voltage regulating operations of the distribution systems because status of the PV systems normally cannot be monitored or controlled from the distribution systems’ operators. Meanwhile, installation of the FACTS is expected as an effective solution for the issues; however, finding appropriate set of sizes and locations of the FACTS devices becomes a complicated optimization problem. In this chapter, first, a sizing and allocation problem of the FACTS devices is formulated. Next, a PSO-based solution method is proposed and then applied for determining quasi-optimal set of the sizes, and the connection points of the FACTS devices. Finally, the usefulness of the authors’ proposal is verified through numerical simulations and discussions on their results.

Distributed generation (DG) may result in voltage fluctuation by changing line flow and reactive power injection, especially DG that generates power from renewable energy resources. To cope with this problem, this paper proposes an optimization process to optimally regulate the system voltage profile to lie close to the desired values by using the adaptive Tabu search (ATS) algorithm. The system voltages will be regulated by using dispatchable DG and voltage control devices, i.e. voltage regulator and capacitor. Moreover, probabilistic load flow calculation by using Monte Carlo simulation is chosen to evaluate the uncertainty of DG powered by renewable energy resources. The number of switching operations of the voltage regulator and capacitor are also accounted for in the optimization constraints, as excessive frequent switching operations can damage these devices. The optimal sizes and locations of dispatchable DGs and capacitors are considered as the optimization variables. The proposed method is demonstrated in an IEEE 34‐bus distribution test system and a modified 21‐bus Provincial Electricity Authority (PEA) system (Thailand). © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

<p>The continued interest in the use of distributed generation in recent years is leading to the growth in number of distributed generators connected to distribution networks. Steady state voltage rise resulting from the connection of these generators can be a major obstacle to their connection at lower voltage levels. The present electric distribution network is designed to keep the customer voltage within tolerance limit. This may require a reduction in connectable generation capacity, under utilization of appropriate generation sites. Thus distribution network operators need a proper voltage regulation method to allow the significant integration of distributed generation systems to existing network. In this work a voltage rise problem in a typical distribution system has been studied. A method for voltage regulation of distribution system with multiple DG system by coordinated operation distributed generator, capacitor and OLTC has been developed. A sensitivity based analysis has been carried out to determine the priority for individual generators in multiple DG environment. The effectiveness of the developed method has been evaluated under various cases through simulation results.</p>

DC distributed systems are highly reliable and efficient means of delivering DC power or adopting renewable energy resources. However, DC distributed systems are prone to instability and dynamic performance degradation due to the negative incremental input impedance of DC-DC converts. In this paper, we propose a generic method to eliminate the impact of the negative input impedance on DC systems by shaping the source output impedance such that its bode-plot is restricted in the area that is contained below the product of the source’s duty ratio and its characteristic impedance. The performance deterioration originates whenever the output impedance of the source exceeds, in magnitude, the input impedance of the load converter due to deficiency in stability margins. Hence, confining the impedance in the proposed region helps decouple the interaction between the converters and preserve their own dynamic performances. The proposed method was proven by analytical analysis, time-based simulation, and practical experiments. All of their outcomes were in agreement, proving the effectiveness of the proposed method in preserving the dynamic performance of distributed systems.

The power flow control in DC distribution systems is achieved by source converters which have DC voltage controllers. This paper proposes a suitable control strategy for source converters which has a DC voltage droop controller and a novel instantaneous power regulation scheme. Furthermore, this paper develops a simple method, named load-generation voltage control (LGVC) scheme, which can be used for DC system voltage determination. Simulation results indicate the effectiveness of the proposed control strategy for source converters and the LGVC scheme.

The rapid increase in the installation of renewable energy sources, particularly solar photovoltaic (PV) sources associated with unbalanced features of distribution systems (DS), disturbs the classic control strategy of voltage regulation devices and causes voltage violation problems. This paper proposes an effective control strategy for voltage regulators in the DS based on the voltage sensitivity using a multi-agent system (MAS) architecture. The features of the unbalanced distribution system (UDS) with the PV and different types and configurations of voltage regulators are considered in the proposed strategy. The novelty of the proposed method lies in realizing both the control optimality of minimizing voltage violations and the flexibility to accommodate changes in the DS topology using an MAS scheme. An advantageous feature of using the MAS scheme is the robust control performance in normal operation and against system failure. Simulation studies have been conducted using IEEE 34-node and 123-node distribution test feeders considering high PV penetration and different sun profiles. The results show that the proposed voltage control strategy can optimally and effectively manage the voltage regulators in the UDS, which decrease their operation stresses and minimize the overall voltage deviation.

In this paper, a novel distributed control strategy achieving (feasible) current sharing and voltage regulation in Direct Current (DC) microgrids is proposed. Firstly, the (convex) optimization problem is formulated, with the microgrid’s steady state equations and/or desired objectives as feasible set. Secondly, we design a controller, the (unforced) dynamics of which represent the continuous time primal-dual dynamics of the considered optimization problem. Then, a passive interconnection between the physical plant and the controller is presented. Furthermore, global asymptotic convergence of the closed-loop system to the desired steady-state is proved and simulations successfully confirm the theoretical results.

The insertion of renewable and distributed energy resources and the use reduction of fossil-fuel-based ones have been in the spotlight of studies of the scientific community in general. Aiming at an efficient connection interface for these resources and at the reduction of distribution losses, the employment of a Low Voltage Direct Current distribution system (LVDC) is considered in this study. For this purpose, the modification of a pristine Low Voltage Alternate Current distribution system (LVAC) to a DC one is proposed without network’s rerouting and with minimal adaptations. Both systems were implemented in the electric power distribution system simulator, OpenDSS (open-source code), and compared to their losses, energy transferred to the loads, voltage drops along the line, as well as number of conductors required for their operation.