Voltage Regulation Scheme Research Articles

Voltage Sag Mitigation in DER-Connected Distribution Networks by STATCOM

This article proposes a voltage regulation scheme for distributed energy resource-connected distribution networks based on static synchronous compensator to compensate for both short-term and long-term voltage sags. The proposed voltage regulation scheme provides a fast and precise control on the nodal voltage, with no need to load shedding, capacitor switching, transformers, or power management schemes. The principles supporting the proposed voltage regulation scheme are discussed and the effectiveness of the proposed control is demonstrated through time-domain simulation of a two-unit test distribution network in the PSCAD/EMTDC software environment.

Distributed voltage regulation using permissioned blockchains and extended contract net protocol

Distributed voltage regulation schemes within active distribution networks (ADNs) are receiving plenty of attention because they reduce latency bottlenecks. However, previous schemes often employ a trusted moderator to govern agent communication, thereby exposing potential trust issues that may affect agent performance. Blockchain technology can avoid the need for intermediaries, since it is a fully decentralized network that is governed by agents using distributed consensus mechanisms. Thus, this paper proposes a blockchain-based system for voltage regulation in ADNs that provides a competitive marketplace wherein agents can rationally bid for voltage regulation services. The proposed system utilizes a smart contract that i) enables asynchronous negotiation between agents to mitigate multiple violations simultaneously using the extended contract net protocol (ECNP), and ii) maintains a credit score that reflects the trustworthiness of an agent in resolving voltage violations. The proposed system also develops a bidding and pricing scheme for voltage regulation services that considers both active and reactive power contributions. A permissioned blockchain implementation is used to ensure that only known agents are permitted to access the system. The proposed system is implemented on the Hyperledger Fabric platform, where simulations executed on a 69 bus system demonstrate the system’s ability to mitigate multiple voltage violations simultaneously. As demonstrated by the simulation results, the highlight of the main contribution of the paper is the ability of the proposed system to mitigate multiple voltage violations simultaneously via the ECNP, thereby utilizing less bidding cycles to resolve voltage violations as compared to the original contract net protocol.

A Distributed P and Q Provision-Based Voltage Regulation Scheme by Incentivized EV Fleet Charging for Resistive Distribution Networks

The capability of onboard bidirectional chargers to deliver both real power and reactive power makes electric vehicles (EVs) suitable candidates for the voltage regulation in the distribution network considering the higher <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R/X$ </tex-math></inline-formula> ratio of distribution lines. This article proposes a distributed voltage regulation scheme for dominantly resistive distribution networks through coordinated EV charging/discharging. In the optimal planning stage, the maximal profit of each EV user for the regulation provision is guaranteed, while the driving need can be satisfied. An optimal charging model is proposed to plan the total available <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> , and a parameter “involvement level” is introduced for the fair pricing of participating EV users. In the regulation stage, an adaptive voltage sensitivity coefficient is developed to estimate the amount of required power for voltage regulation. A novel parallel consensus sharing strategy is designed so that the local power dispatch decision can be made by each EV through information sharing between neighboring buses. The backup support from the local energy storage system (ESS) can be activated under the emergency scenario due to communication failure and insufficient EV resources. Simulation results demonstrate that the proposed scheme can effectively and robustly regulate the voltage and is scalable to different network topologies.

Distributed Voltage Regulation for Low-Voltage and High-PV-Penetration Networks With Battery Energy Storage Systems Subject to Communication Delay

The increasing penetration level of photovoltaic (PV) systems in low-voltage networks causes voltage regulation issues. This brief proposes a new voltage regulation strategy utilizing distributed battery energy storage systems (BESSs) while incorporating the inevitable communication delays. The proposed strategy ensures that the voltage regulation burden is appropriately shared among all involved BESSs in the network. The effects of communication delays on the performance of the proposed strategy are analyzed and an admissible delay bound is established. Case studies are conducted to verify the effectiveness of the proposed strategy.

Adaptive Control for Three-Phase Power Converters With Disturbance Rejection Performance

This paper presents voltage regulation and current tracking control strategies for three phase two-level grid-connected power converters. By using power-invariant Park's transformation, an averaged mathematical model of power converters is obtained in dq synchronous reference frame. Then a novel control strategy using adaptive control and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> technique is proposed to regulate the dc-link output voltage as well as track a desired current reference for three-phase power rectifiers. More specifically, an efficient adaptive controller is established in the external loop for regulating dc-link output voltage in the presence of external disturbances. A set of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> controllers are designed in the internal loop to force the input currents track their desired values. Finally, simulation results obtained from the proposed control method are presented, analyzed, and compared with that of sliding mode control, and the superiority of the proposed control laws is verified.

Automatic Voltage Stabilization Using IGBT Based on Load Tap Changer With Fault Consideration

In this work, a voltage regulation scheme to stabilize voltage variation in the consumer side is proposed. The voltage regulation scheme uses IGBT-based onload tap changer (OLTC) and nonlinear hysteresis control algorithm. The proposed switching algorithm establishes the commutation of the taps without redundant short circuit operation of the taps. The experimental results of the designed system show accurate voltage regulation at the consumer side and high ability of the designed system to fix voltage sag and voltage swell in maximum of one cycle period. The paper considers the consequences of fault in the drive circuit of the IGBT tap changer and discusses its reliability. The analysis and the experimental results of the faults in the drive circuit show that the loss of one drive signal results in a large circulating DC current in the primary circuit and may result in fault current of value more than three times the operated current, which leads to unavailability of the system.

Detection of Cyber Attacks on Voltage Regulation in Distribution Systems Using Machine Learning

Several wired and wireless advanced communication technologies have been used for coordinated voltage regulation schemes in distribution systems. These technologies have been employed to both receive voltage measurements from field sensors and transmit control settings to voltage regulating devices (VRDs). Communication networks for voltage regulation can be susceptible to data falsification attacks, which can lead to voltage instability. In this context, an attacker can alter multiple field measurements in a coordinated manner to disturb voltage control algorithms. This paper proposes a machine learning-based two-stage approach to detect, locate, and distinguish coordinated data falsification attacks on control systems of coordinated voltage regulation schemes in distribution systems with distributed generators. In the first stage (regression), historical voltage measurements along with current meteorological data (solar irradiance and ambient temperature) are provided to random forest regressor to forecast voltage magnitudes of a given current state. In the second stage, a logistic regression compares the forecasted voltage with the measured voltage (used to set VRDs) to detect, locate, and distinguish coordinated data falsification attacks in real-time. The proposed approach is validated through several case studies on a 240-node real distribution system (based in the USA) and the standard IEEE 123-node benchmark distribution system. The results show that the proposed approach can detect low margin attacks (as low as 1% of actual measurements) with up to 99% accuracy. All of the developed source codes of the proposed solution are publicly available at Github. https://github.com/nbhusal/Data-Attack-on-Voltage-Regulation.

Collaborative Mid-Point Voltage Regulation in Low-Switching-Frequency MPC for Three-Level NPC Inverters Fed Dual Three-Phase PMSM Drives

In this paper, the control strategy is studied for the neutral-point-clamped three-level (NPC-3L) dual three-phase permanent-magnet synchronous machine (PMSM) drives at low-switching-frequency operation. The finite-control-set (FCS) model predictive control (MPC) scheme is investigated as the basic MPC scheme, where the average switching frequency is established as the cost function. With the high sampling frequency operation, the discretization with both forward and backward Euler methods is utilized so that the two sets of three-phase windings are controlled separately. To further decrease the switching frequency while maintaining the current and voltage control performance, a collaborative mid-point voltage regulation is proposed. The collaborative regulation strategy fully employs the additional control degree of freedom in the dual three-phase drives by setting the two NPC-3L inverters to select small vectors with opposite polarities. Therefore, the situation is avoided where the upper or the lower capacitor in the DC link provides load currents for the two sets of windings simultaneously. Simulation results of a 2.45MW dual three-phase drive and experimental results on a laboratory prototype have been presented to verify the validity of the proposed collaborative mid-point voltage regulation strategy.

Particle swarm optimization algorithm-based PI inverter controller for a grid-connected PV system.

The lack of control in voltage overshoot, transient response, and steady state error are major issues that are frequently encountered in a grid-connected photovoltaic (PV) system, resulting in poor power quality performance and damages to the overall power system. This paper presents the performance of a control strategy for an inverter in a three-phase grid-connected PV system. The system consists of a PV panel, a boost converter, a DC link, an inverter, and a resistor-inductor (RL) filter and is connected to the utility grid through a voltage source inverter. The main objective of the proposed strategy is to improve the power quality performance of the three-phase grid-connected inverter system by optimising the proportional-integral (PI) controller. Such a strategy aims to reduce the DC link input voltage fluctuation, decrease the harmonics, and stabilise the output current, voltage, frequency, and power flow. The particle swarm optimisation (PSO) technique was implemented to tune the PI controller parameters by minimising the error of the voltage regulator and current controller schemes in the inverter system. The system model and control strategies were implemented using MATLAB/Simulink environment (Version 2020A) Simscape-Power system toolbox. Results show that the proposed strategy outperformed other reported research works with total harmonic distortion (THD) at a grid voltage and current of 0.29% and 2.72%, respectively, and a transient response time of 0.1853s. Compared to conventional systems, the PI controller with PSO-based optimization provides less voltage overshoot by 11.1% while reducing the time to reach equilibrium state by 32.6%. The consideration of additional input parameters and the optimization of input parameters were identified to be the two main factors that contribute to the significant improvements in power quality control. Therefore, the proposed strategy effectively enhances the power quality of the utility grid, and such an enhancement contributes to the efficient and smooth integration of the PV system.

Fast-Turnaround Design and Modeling Techniques for a Fast-Transient Digital Low-Dropout Regulator With 3 mV Ripples

This article presents a low-complexity digital low-dropout regulator (DLDO) design featuring a novel voltage regulation and transient enhancement scheme, and a charge-redistribution-based ripple suppression (CRRS) technique that smooths output by redistribution of excessive energy via a small capacitor. Also proposed is a fast-turnaround general DLDO design approach comprising a hybrid discrete-continuous DLDO model and techniques for modeling nonlinear performance enhancement techniques, with which a DLDO's performances for given specifications can be evaluated in the early design stage to shorten the development time. A DLDO test chip fabricated using 110 nm CMOS technology shows a fast settling time of 67 and 250 ns for light-to-heavy (i.e., 2.5 and 50 mA) and heavy-to-light transient, and an output voltage ripple as low as 3 mV with proposed CRRS. The core occupies only 0.022 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , thanks to the resource-sharing architecture of a proposed digital controller and hardware simplicity. With an input voltage of 1 V and output voltage of 0.85 V, proposed design achieves a peak current efficiency of 99.6% with a 100 MHz operating clock frequency.

A Decoupling Rolling Multi-Period Power and Voltage Optimization Strategy in Active Distribution Networks

With the increasing penetration of distributed photovoltaics (PVs) in active distribution networks (ADNs), the risk of voltage violations caused by PV uncertainties is significantly exacerbated. Since the conventional voltage regulation strategy is limited by its discrete devices and delay, ADN operators allow PVs to participate in voltage optimization by controlling their power outputs and cooperating with traditional regulation devices. This paper proposes a decoupling rolling multi-period reactive power and voltage optimization strategy considering the strong time coupling between different devices. The mixed-integer voltage optimization model is first decomposed into a long-period master problem for on-load tap changer (OLTC) and multiple short-period subproblems for PV power by Benders decomposition algorithm. Then, based on the high-precision PV and load forecasts, the model predictive control (MPC) method is utilized to modify the independent subproblems into a series of subproblems that roll with the time window, achieving a smooth transition from the current state to the ideal state. The estimated voltage variation in the prediction horizon of MPC is calculated by a simplified discrete equation for OLTC tap and a linearized sensitivity matrix between power and voltage for fast computation. The feasibility of the proposed optimization strategy is demonstrated by performing simulations on a distribution test system.

Market-Based Volt-Var Optimization and Its Applications on Bottom-Up Load Modeling Method

Due to the visibility and controllability features of the latest distribution networks, there are potentials to model customer loads through a component-based approach more accurately. The electricity markets started in recent years enable trading and exchanges of electricity among traditional power stations and prosumers. It provides a possibility to improve the optimality of classical volt-var optimization (VVO) schemes. In this paper, a tri-level bottom-up load model and a market-based volt-var optimization (mVVO) model are proposed. The energy-to-voltage (ETV) parameter is proposed in our load model as a substitute for the widely used load-to-voltage (LTV) parameter to reflect long-term load characteristics. The mVVO model using ETV parameters aims to combine the optimal voltage regulation strategies and electricity purchase strategies from wholesale and retail markets for maximum cost saving. The case studies based on real load data of Hong Kong show that by coordinating the voltage regulation methods with market behaviors, utilities can reach a higher level of cost saving, in addition to energy saving.

A Novel Command-Filtered Adaptive Backstepping Control Strategy with Prescribed Performance for Photovoltaic Grid-Connected Systems

With the aim of solving the power fluctuation and bus voltage instability problems caused by external environment variations in the photovoltaic grid-connected system, a prescribed performance-based adaptive backstepping controller is proposed for the system to regulate the bus voltage and the inverter current. First, the mathematical model of the grid-connected inverter is established, in which the uncertain system parameters are estimated via a designed projection-based adaptive law. Then, the command-filtered backstepping sliding mode control method is applied to the system for power regulation. In order to achieve favorable tracking performance, the prescribed performance technique is introduced in the voltage regulation strategy by constraining the compensated voltage tracking error within a certain range from a novel point of view. Finally, the simulation is carried out considering the variations of environmental situations, and the obtained results demonstrate the sound performance of the prescribed performance-based control strategy with respect to the photovoltaic grid-connected system.

Improved DC-Link Voltage Regulation Strategy for Grid-Connected Converters

In this article, an improved dc-link voltage regulation strategy is proposed for grid-connected converters applied in dc microgrids. For the inner loop of the grid-connected converter, a voltage modulated direct power control is employed to obtain two second-order linear time-invariant systems, which guarantees that the closed-loop system is globally exponentially stable. For the outer loop, a sliding mode control strategy with a load current sensor is employed to maintain a constant dc-link voltage even in the presence of constant power loads at the dc-side, which adversely affect the system stability. Furthermore, an observer for the dc-link current is designed to remove the dc current sensor at the same time improving the reliability and decreasing the cost. From both simulation and experimental results obtained from a 15-kVA prototype setup, the proposed method is demonstrated to improve the transient performance of the system and has robustness properties to handle parameter mismatches compared with the input-output linearization method.

An Integrated Solid-State Transformer With High-Frequency Isolation for EV Fast-Charging Applications

In this article, an integrated solid-state transformer (I-SST) with high-frequency (HF) isolation for electric vehicle (EV) fast-charging applications is proposed. In the approach proposed in this article, a three-phase I-SST is used as HF galvanic isolation for the system with respect to the medium-voltage grid. A direct multipulse ac–dc rectifier is shown to eliminate low-order harmonics in the input current over a wide range of output dc voltage variation. The proposed approach has the following advantages: no dc link, hence no bulky electrolytic capacitors, galvanic isolation is via a three-phase HF transformer, high-power density, and simple control scheme for regulating output voltage. The approach is easily scalable to several hundreds of kilowatts and is ideal for high-power level-3 EV fast-charging stations. A simplified output voltage regulation scheme for the proposed topology is discussed. Regulating the output voltage enables the circuit to be used as a direct topology for level-3 charge stations with ac bus architecture. Experimental results on a scale down laboratory prototype at 208 V, 1.5 kW are discussed. 1

Design Considerations for High-Voltage Insulated Gate Drive Power Supply for 10-kV SiC MOSFET Applied in Medium-Voltage Converter

High-performance gate drive power supply (GDPS) plays a crucial role in ensuring the reliability and safety of the gate driver for power semiconductor devices. This article focuses on the design of a high-voltage-insulated GDPS for the 10-kV silicon carbide MOSFET in medium-voltage (MV) application. Design considerations, including insulation scheme, high-voltage-insulated transformer design, and load voltage regulation scheme, are proposed. In addition, the performance of the secondary-side-regulated (SSR) GDPS and that of the primary-side-regulated (PSR) GDPS are compared for several aspects, including interwinding capacitance, load voltage regulation rate, conversion efficiency, and hardware complexity. Finally, an SSR GDPS and a PSR GDPS, with an insulation voltage of 20 kV, are built in the lab. The test results demonstrate that the PSR GDPS is more preferable because of lower interwinding capacitance, lower load voltage regulation rate, higher conversion efficiency, and simpler control circuit.

A Dual-Active-Bridge With Half-Bridge Submodules DC Solid-State Transformer for DC Distribution Networks

High-frequency dc solid-state transformer (DCSST) plays an important role in DC bus connection, voltage conversion, electrical isolation, and bidirectional power transmission in dc distribution networks. In various DSSCT topologies, the dual-active-bridge (DAB) scheme becomes a typical solution for DCSST due to its various advantages. However, the DAB scheme still has drawbacks during practical applications. In this article, a DAB with half-bridge submodules (HSDAB) is proposed. By replacing the switches in front bridge arm with half-bridge submodules (SMs), the proposed HSDAB scheme maintains the advantages of conventional DAB and obtains bidirectional dc fault-handling capability, reducing the application of large dc capacitors and dc circuit breaker; therefore, the volume and construction cost of DCSST can be effectively reduced. Besides, a high-frequency-link (HFL) voltage regulation strategy for HSDAB is also proposed in this article. The HFL voltages can be adjusted to match the HFL transformer voltage ratio during the whole operating range, reducing the current stress, HFL reactive power, and power loss of DCSST. Moreover, the proposed HSDAB scheme can also be applied as a basic circuit to establish multiple modular architecture DCSST, flexibly connecting low-voltage dc, medium-voltage dc, and high-voltage dc buses in the dc distribution network. The topology, operation principle, control structure, corresponding bidirectional dc fault handling, and HFL voltage regulation strategy of the proposed HSDAB scheme are investigated. The experimental results verify the correctness and effectiveness of the analysis and proposed scheme.

A Multiple Modular Isolated DC/DC Converter With Bidirectional Fault Handling and Efficient Energy Conversion for DC Distribution Network

The isolated bidirectional dc/dc converter (IBdc) plays an important role in dc buses connection, voltage conversion, electrical isolation, and bidirectional power transmission for dc distribution networks. In this article, a multiple modular IBdc scheme based on hybrid dual-active-bridge (HDAB) for medium-voltage and high-voltage dc distribution network is proposed. In proposed HDAB, the switches in upper position of full-bridge are replaced by the half-bridge submodules, and the proposed HDAB maintains the advantages of conventional DAB and achieves bidirectional fault handling capability, avoiding the application of large dc-link capacitors and dc circuit breakers, thus reducing the volume and construction cost. Besides, a high-frequency-link (HFL) voltage regulation strategy is also proposed in this article, adjusting the HFL voltages in HDAB to match HFL transformer voltage ratio adaptively, thus improving the efficiency and applicability of IBdc under the fluctuation of power flow and dc bus voltage. The topology, operation principle, control structure, and corresponding bidirectional fault handling and HFL voltage regulation strategy of the proposed IBdc scheme are investigated. The experimental results verify correctness and effectiveness of proposed scheme and control strategy, promoting the adaptability and practicality for both IBdc and dc distribution network.

A two-layer dynamic voltage regulation strategy for DC distribution networks with distributed energy storages

In this article, a two-layer bus voltage regulation strategy with distributed energy storages (DESs), is proposed for dc distribution networks. The developed droop principle based on the power coupling characteristics of ac/dc interface is derived as the basis of the primary control. Besides, the state of charge (SoC) of DES unit is considered inherently in the primary regulation, which can effectively contribute to the long-term performance and life span of DES unit. Then, the cost characteristics of DES device and its connecting converter are analyzed in detail, and the secondary voltage regulation is accordingly designed based on the developed dynamic equal incremental cost principle. The proposed two-layer voltage regulation can achieve the economic power dispatch during the multi-bus voltage regulation without deteriorating the voltage quality, which can obtain a trade-off between the voltage performance improvement and the DESs economic operating. The performance of the proposed control strategy is validated with different operating conditions in the case studies.

Operation of Distribution Network Considering Compressed Air Energy Storage Unit and Its Reactive Power Support Capability

Energy storage is playing an increasingly important role in power system operation due to its ability to shave the peak and fill the valley. Advanced adiabatic compressed-air energy storage (AA-CAES) is a clean and scalable energy storage technology and has attracted wide attention recently. This paper proposes a multi-state operation model of AA-CAES capturing the dynamic change of internal physical status. A stand-by state is introduced, at which AA-CAES works like a condenser that produces only reactive power. It brings two benefits. First, the facility is started up and shut down less frequently, which helps extend its service time. Second, it can provide continuous reactive power support all day long, unlike on-load tap changers or shunt capacitors whose voltage regulation strategy is discrete and suffers from a very low switching frequency. Taking AA-CAES into account, the optimal operation of a distribution network is formulated as a bi-objective dynamic optimal power flow problem. Nash bargaining is employed to extract a single Pareto optimal solution that fairly compromises two objectives. To solve the operation problem in a tractable manner, the bargaining objective and power flow constraints are convexified, giving rise to a mixed-integer second-order cone program. A feasibility recovery algorithm is developed to find a physically meaningful solution whenever the convex relaxation on power flow is inexact. Case studies on the IEEE 33-bus system demonstrate the effectiveness and advantages of the proposed method.