Voltage Unbalance Compensation Research Articles

Simultaneous Voltage Regulation and Unbalance Compensation in Distribution Systems With an Information-Driven Learning Approach

Simultaneous Voltage Regulation and Unbalance Compensation in Distribution Systems With an Information-Driven Learning Approach

Remote Multinodal Voltage Unbalance Compensation in Islanded AC Microgrids

Remote Multinodal Voltage Unbalance Compensation in Islanded AC Microgrids

A novel control scheme for load current components sharing improvement, and unbalanced and harmonic voltage compensation, in islanded provisional-microgrids

Microgrids (MGs) are the building blocks of future smart distribution grids, bringing several benefits to both utilities and consumers. In recent years a new class of MGs has been introduced, called provisional-microgrid (P-MG), consisting of master-MG and slave-MG. Slave-MG cannot operate isolated from master-MG. However, power quality is of the major challenges in MG investigations. In this study, a novel hierarchical and distributed control scheme is proposed for the improvement of unbalanced and harmonic load current components sharing among the distributed generation (DG) units as well as voltage quality (VQ) enhancement of sensitive load bus (SLB) in provisional-microgrid. Compared to the previous works, the proposed control strategy takes the instantaneous free capacity of DGs and multi-voltage-quality level (MVQL) indices of different buses into account for both load current sharing and voltage quality improvement functions. Furthermore, it considers the nature of variable power generation-renewable energy resources (VPG-RERs) and does not need any information about the loads or lines impedances. The efficiency of the proposed control scheme is evaluated by simulation of a typical islanded P-MG under several cases. Simulations are carried out in MATLAB/Simulink environment, and their results demonstrate the appropriate speed and accuracy of the presented method.

Control of a transformerless static synchronous series compensator for unbalanced voltage compensation

AbstractThis paper explores a transformerless static synchronous series compensator (TL‐SSSC) for mitigating voltage distortion in a power distribution line with renewable generations. A novel deadbeat current control scheme with time delay compensation is proposed which models the a‐b‐c three phase line with TL‐SSSC plus its LC filters and predicts the compensating voltages to be injected by the TL‐SSSC. Without resolving measured voltages and currents into positive and negative sequences, the scheme is computationally efficient and effective in mitigating voltage distortion due to faults occurring at different locations along the distribution line. It also eliminates the problem of inaccurate compensation voltage caused by inverter filter choking voltage drop. The paper also analyses the power controllable ranges for a TL‐SSSC under different unbalanced voltage ratios. The proposed TL‐SSSC and control scheme is validated using Typhoon Hardware‐In –Loop (HIL) device. Experimental results on mitigation unbalanced voltages of a 11kV transmission line confirm the effectiveness and high performance.

Optimal Allocation and Assessment of Distributed Generation Units with Multi-Objective Planning in Unbalanced Distribution Networks

—Real-world distribution networks are generally unbalanced due to unbalanced loads and asymmetric line parameters. Proper allocation of distributed generation (DG) units in unbalanced radial distribution networks to extract their maximum potential benefits is still quite difficult. Converter-coupled DG units have individual phase real and reactive power regulating ability and provide voltage unbalance compensation. This article proposes a new technique for the suitable allocation and capacity of converter-based multiple DG units considering the unbalanced operation of distribution networks. The objective function contains power loss minimization, multiphase voltage stability augmentation, and reduction of voltage unbalances among different phases in the system. A complex multi-objective optimization problem has been synthesized with the help of a Fast and Flexible Radial Power Flow (FFRPF) and solved by employing a Weighted aggregation (WA) based particle swarm optimization (PSO) approach. The performance of the proposed technique is validated on 19-bus, 25-bus, and 34-bus unbalanced radial distribution networks that consider various power factor modes of the converter-coupled DG units. The analysis of the simulation results indicates a significant enhancement in the power delivery efficiency in all the unbalanced test systems

Comprehensive review and a novel technique on voltage unbalance compensation

AbstractVoltage unbalance has gained more attention due to a rapid increase in single‐phase distributed energy resources (DERs) penetration compound with existing unbalance characteristics of low voltage distribution network. The purposes of this article are to summarise various techniques on voltage unbalance compensation from three different strategies: (1) techniques based on passive approach and advanced power electronics; (2) Techniques based on three‐phase DERs; (3) techniques based on single‐phase DERs. In addition, a novel voltage unbalance compensation technique that applies a voltage sensitivity analysis along with a symmetrical component concept to meet a desired voltage unbalance factor (VUF) is introduced in this article. Coordination among single‐phase DERs that contributes in the voltage unbalance compensation is also considered. In order to evaluate the effectiveness of this proposed technique, its results are compared with the results that are obtained using a current unbalance compensation strategy. The results indicate that the novel technique proposed in this study provides more stable voltage unbalance compensation while successfully achieving the specified VUF thresholds.

A Distributed Control Strategy for Unbalanced Voltage Compensation in Islanded AC Microgrids Without Continuous Communication

Due to the unbalanced load connection in the sensitive load bus (SLB), the ac microgrid may suffer from voltage unbalance in the system. To overcome this issue, this article presents a new distributed control scheme to achieve the unbalanced voltage compensation for islanded ac microgrid with limited noncontinuous communication among the distributed generation (DG) units. In this article, a two-layer control framework, including the cyber layer and the primary control layer, is employed. The proposed control strategy that is applied in the cyber layer implements an event-triggered communication to effectively compensate for the unbalanced voltage in the SLB. In contrast with the centralized control strategy, the proposed control strategy is fully distributed, and the information exchange among the DG units at only the event-triggered times. As a result, the communication burdens can be measurably reduced while the control performance is not affected. The stability is analyzed with the Lyapunov function in this article. Finally, several experimental case studies are provided to verify the effectiveness of the proposed scheme.

Coordinated Control of Three Single–Phase BESS Inverters Using Local Measurements to Mitigate Voltage Unbalance

This article proposes a control algorithm for single-phase battery inverters to provide voltage unbalance compensation in distribution networks with high penetration of photovoltaic panels and electric vehicles. A typical distribution system is studied to quantify the impact of single-phase loads and generating units on voltage unbalance levels, and identify the conditions that lead to the highest voltage unbalance. Voltage unbalance compensation is then performed by regulating active and reactive power exchange between three single-phase battery energy storage units and the power grid. The proposed control strategy consists of an upper level control system to coordinate the three units by sending active and reactive power reference signals, and a Battery Energy Storage System (BESS) control system to regulate active and reactive power exchange between each single-phase inverter and the grid. Simulation results show that the effectiveness of this approach depends on the location where the energy storage units are installed. The positive impact of the proposed methodology on balancing the power flow and reducing the transformer zero-sequence current is also demonstrated. Finally, Hardware-In-The-Loop (HIL) experiments are carried out to provide validation of the control algorithm in real-time.

Dynamic Voltage Regulation and Unbalance Compensation in a Low-Voltage Distribution Network Using Energy Storage System

Modern distribution grids may suffer problems of voltage distortion, especially along radial low-voltage feeders with a high penetration of intermittent, unbalanced and distorted loads and generation sources. It is a challenge to develop an effective voltage-regulation method using a straightforward implementation. This paper proposes a novel method for local voltage control and balancing using a shunt-connected energy storage system. The compensation principles are explained, and a complete controller design is proposed. The algorithm is designed to be implemented in power electronic converters that provide the interface between the storage and the grid. The original contribution is the development of a low-level control method, which includes voltage balancing and a method to minimize the compensator current and is to be implemented in power electronic converters that provide the interface between the storage and the grid. The calculation of active and reactive compensator currents is explained with relation to the estimated grid impedance. The efficacy of the designed controller is verified by laboratory experiments. It is shown that voltage regulation using the proposed method is achieved with less apparent power compared to a system where only reactive power is used. The controller presents a very good dynamic response to rapid voltage variations, such as unbalanced voltage dips. The applicability and constraints of the method are discussed with respect to the present state of the art in low-voltage-grid voltage regulation.

Virtual model predictive control for virtual synchronous generators to achieve coordinated voltage unbalance compensation in islanded micro grids

For virtual synchronous generators (VSGs) in islanded micro grids, to achieve voltage unbalance compensation, it is often necessary to deal with the positive and negative sequence components of voltage and current in parallel. The complex structure and cumbersome parameter tuning highlight the shortcomings of linear controllers. To simplify the unbalance compensation scheme, a virtual model predictive control (VMPC) for VSG is proposed in this paper. Based on the theoretical mechanism of unbalance problem and the unbalanced power sharing principle, a negative sequence virtual impedance (NSVI) is embedded in the inverter model to construct the virtual model. Then the predictive control is implemented based on this virtual model, and the voltage unbalance compensation behavior of VSG is controlled. Based on the VMPC method and the NSVI regulation method, the coordinated voltage unbalance compensation scheme is developed for parallel operation of VSGs. Experimental results show that the proposed VMPC method can effectively help reduce the burden on the processor when performing voltage unbalance compensation control algorithm. And the performance of the coordinated voltage unbalance compensation scheme is verified in parallel VSGs with the same or different capacities.

A New Unbalanced Voltage Compensation Method Based on HOPF Oscillator for Three-Phase DC/AC Inverters With Unbalanced Loads

In islanded microgrids, distributed generators (DGs) are employed as distributed compensators to improve the power quality on the load side. Due to the access of unbalanced loads, the low-voltage microgrid will naturally exhibit three-phase unbalance, which may cause additional power losses and deteriorate power quality. Therefore, this paper proposes a novel unbalanced-voltage compensation control method based on the Hopf oscillator, which introduces the negative-sequence voltage and current into the traditional Hopf oscillator to achieve the negative sequence voltage droop characteristics. It can not only realize the negative sequence droop control of a single inverter, but it is also capable to support the reactive power automatic distribution between multi-parallel grid-forming inverters. Compared with the traditional unbalanced voltage compensation method based on filtered negative sequence reactive power, the proposed unbalanced voltage compensation method can slightly improve the dynamic response. Experimental results verify the effectiveness of the proposed control strategy.

Collaborative Voltage Unbalance Compensation in Islanded AC Microgrids With Grid-Forming Inverters

The presence of unbalanced loads in power systems creates voltage imbalances that perturb the operation of sensitive loads, such as induction motors, power electronics converters, and adjustable speed drives. This article presents a novel control scheme that achieves accurate voltage balance at any node of an islanded ac microgrid with seamless integration with the existing hierarchical control of the microgrid. A master–slave architecture is adopted for the unbalance compensation due to its low network traffic requirements and its modular conception. The unbalance compensation follows a collaborative principle where current injections are computed as a function of the current rating of each inverter, thus avoiding the overcurrent stress of the unbalance compensation by a single inverter. This article presents the stability analysis and the design guidelines for the control parameters. The performance of the control proposal has been successfully tested in a laboratory microgrid. Selected experimental results are presented.

Simultaneous voltage unbalance compensation and neutral‐to‐ground voltage minimization for an islanded mini‐grid using model predictive control

AbstractThis paper presents a model predictive control (MPC) and its application on a three‐phase four‐leg inverter in an islanded mini‐grid to compensate for unbalanced load conditions while minimizing the neutral‐to‐ground voltage (NGV). The proposed approach uses the novel discrete‐time model of the four‐leg inverter and LC filter to predict the control variables. These predictions are evaluated by using a multiobjective cost function. The proposed method is based on the minimization of the cost function to select the best vector state of the power converter to be applied in the next sampling instance. The cost function in this study has two objectives: the adequately tracked low voltage reference and minimization of the NGV, simultaneously. The stability analysis is also defined to investigate the stable dynamics of the proposed MPC. The effectiveness of the suggested method is verified through simulations considering the single/three‐phase and balanced/unbalanced loads. The distinguished results of the proposed work indicate that the proposed MPC algorithm provides high power quality under unbalanced and nonlinear load conditions with high stability. The robustness of the proposed controller is also studied with both inductive and capacitive filter parameter variations.

Voltage Unbalance Compensation in Distribution Feeders Using Soft Open Points

Soft open points (SOPs) are power electronic devices that may replace conventional normally-open points in distribution networks. They can be used for active power flow control, reactive power compensation, fault isolation, and service restoration through network reconfiguration with enhanced operation flexibility and grid resiliency. Due to unbalanced loading conditions, the voltage unbalance issue, as a common problem in distribution networks, has negative impacts on distribution network operation. In this paper, a control strategy of voltage unbalance compensation for feeders using SOPs is proposed. With the power flow control, three-phase current is regulated simultaneously to mitigate the unbalanced voltage between neighboring feeders where SOPs are installed. Feeder voltage unbalance and current unbalance among three phases are compensated with the injection of negative-sequence and zero-sequence current from SOPs. Especially in response to power outages, three-phase voltage of isolated loads is regulated to be balanced by the control of SOPs connected to the feeders under faults, even if the loads are unbalanced. A MATLAB/Simulink model of the IEEE 13-bus test feeder with an SOP across feeder ends is implemented, and experimental tests on a hardware-in-the-loop platform are implemented to validate the effectiveness of the proposed control strategy.

Adaptive voltage unbalance compensation based control strategy for islanded microgrid

In order to compensate voltage unbalance at the point of common coupling (PCC) for the islanded microgrid, a novel strategy based on PCC negative‐sequence voltage feedforward is proposed. Through the PCC negative‐sequence voltage feedforward, a virtual voltage source is added to the DG terminal at the negative‐sequence. Then, the effective impedance between the distributed generation (DG) unit and the PCC can be reduced proportionally to the compensation coefficient, thereby reducing the unbalanced voltage caused by the unbalanced load. On this basis, an adaptive mechanism of the compensation coefficient is proposed to achieve compensation effort equalization among parallel DGs. Different from the widely adopted virtual impedance method, the additional unbalanced voltage droop (caused by the virtual impedance) can be avoided. In addition, the H∞ repetitive controller is designed to obtain good voltage control performance for the parallel operation. Finally, microgrid simulations are provided to demonstrate the effectiveness of the proposed control strategy. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

A Secondary Control Method for Voltage Unbalance Compensation and Accurate Load Sharing in Networked Microgrids

Interconnection of microgrids (MGs) provides the opportunity of increasing loadability and enhancing reliability by allowing power exchange among individual MGs. Control of networked MGs requires coordination of distributed energy resources (DERs) within each MG such that the sum of the MG's load and exported power is shared proportionally among DERs while maintaining power quality. This paper proposes a new hierarchical control scheme for accurate load sharing and power quality enhancement in networked MGs with unbalanced load currents. The proposed scheme is comprised of local DER controllers, MG control units, and Networked MG control center. The MG control units adjust the power exchange at the value specified by the control center and regulate the voltage at the point of common coupling (PCC) by means of a secondary control signal. At the DER level, droop control is adopted to enable load sharing with fast dynamic response. The droop controller output is combined with the secondary control signal and a correction voltage which eliminates the effect of line impedances on the current sharing accuracy. This way, accurate sharing of individual phase currents is realized without requirement of data exchange among DERs. Simulation and experimental results showcase the effectiveness of the proposed scheme.

Unbalanced Voltage Compensation with Optimal Voltage Controlled Regulators and Load Ratio Control Transformer

Penetration of equipment such as photovoltaic power generations (PV), heat pump water heaters (HP), and electric vehicles (EV) introduces voltage unbalance issues in distribution systems. Controlling PV and energy storage system (ESS) outputs or coordinated EV charging are investigated for voltage unbalance compensation. However, some issues exist, such as dependency on installed capacity and fairness among consumers. Therefore, the ideal way to mitigate unbalanced voltages is to use grid-side equipment mainly. This paper proposes a voltage unbalance compensation based on optimal tap operation scheduling of three-phase individual controlled step voltage regulators (3ϕSVR) and load ratio control transformer (LRT). In the formulation of the optimization problem, multiple voltage unbalance metrics are comprehensively included. In addition, voltage deviations, network losses, and coordinated tap operations, which are typical issues in distribution systems, are considered. In order to investigate the mutual influence among voltage unbalance and other typical issues, various optimization problems are formulated, and then they are compared by numerical simulations. The results show that the proper operation of 3ϕSVRs and LRT effectively mitigates voltage unbalance. Furthermore, the results also show that voltage unbalances and other typical issues can be improved simultaneously with appropriate formulations.

Simulation of the static synchronous compensator for the system voltage unbalance and high current harmonics compensation from single phase non-linear load

This paper presents the STATCOM model in PSIM, which compensate grid voltage unbalance and current harmonics from the non-linear, two-phase load. Grid voltage unbalance level and harmonics current of non-linear load given from measurement. The model of STATCOM is a 6-level voltage inverter. System voltage, and harmonics currents are controlled. All modes are realized and plots are plotted.

A voltage unbalance compensation strategy based on virtual impedance for DGs in island microgrid without sequence component separation

The increasing application of unbalanced loads may cause power quality issues in microgrid. In this paper, a voltage unbalance compensation strategy for distributed generators (DGs) in island microgrid is proposed to compensate the unbalanced bus voltage and improve the distribution effect of load current. The control system of the DGs mainly consists of active and reactive power droop controllers, a virtual impedance loop, voltage controllers. As the proposed voltage controller has two well decoupled control branches to independently control fundamental positive and negative sequence voltages, sequence component detection is not necessary for the implementation of virtual impedance. Then the positive sequence virtual impedance can be introduced to improve the power distribution effect of the DGs, and the negative sequence virtual impedance is introduced to compensate the voltage drop of the negative sequence current on the line impedance and to reduce the negative sequence circulating current. The design approach of the control system is discussed in detail. Finally, the validity and effectiveness of the proposed method are verified by simulation and experimental results.

Voltage Unbalance Compensation Based on Optimal Current Injection by SSTs

Solid-state transformers (SSTs) have a great ability to improve power quality problems. In this article, an effective strategy is proposed to improve voltage unbalance conditions by these transformers. A searching optimization algorithm is presented to estimate the optimal required negative-sequence current vector. Injection of this current vector into a medium-voltage network provides maximum compensation in the voltage unbalance factor. In this method, the amplitude of the negative-sequence current vector is determined based on the current limitation of the SST and its phase angle is optimized to maximum improve the voltage unbalance conditions. Parametric analysis proves that the proposed injection current vector is unique. This negative-sequence current vector is obtained based on a simple process that is presented in the form of a flowchart. Independence from the extraction of positive and negative sequences of the measured currents is one of the main advantages of the proposed control method. Simulation and experimental results by using a laboratory three-stage SST approve the proposed optimization method. These results show that SSTs can acceptably decrease grid unbalanced voltage by an optimal injection current.