Voltage Unbalance Research Articles

Virtual Inertia Based SVPN Sequence Control Scheme for Voltage Unbalance Mitigation in Coordination with DSM on a Stand Alone Microgrid

There is a growing proliferation of renewable sources of energy and single-phase loads, which are unevenly distributed across distribution networks. This imbalanced distribution intensifies unfavorable conditions within the network, leading to detrimental impacts on power quality, stability, and reliability. The objective of this paper is to demonstrate the collaborative functioning of a space vector-based positive and negative (SVPN) sequence controller and demand-side management to maintain voltage unbalance factor in the accepted threshold of 2%, as specified by the IEC standard 61000-3-13 in a typical IEEE 33 bus network. The presented approach for managing thermostatically controlled devices also proves to be highly effective in ensuring the thermal comfort of end users. Also, the absence of rotational masses in converter-interfaced microgrids leads to a reduction in system inertia. Any disturbance in the system leads to significant issues, such as an increase in the rate of change of frequency (RoCoF) and poor frequency response. Therefore, virtual synchronous generator is integrated with the SVPN controller to improve the frequency response of the system. The coordinated control scheme of the islanded microgrid has undergone assessment using the MATLAB/Simulink platform. Small signal modeling technique has been applied to the converters based on VSG control, incorporating positive and negative sequence controllers to address the escalating dynamic challenges in power electronics-based microgrids. This enhanced stability analysis facilitates handling the ever-increasing dynamic issues encountered in such microgrids.

ОБРАТИМЫЙ ПРЕОБРАЗОВАТЕЛЬ ЭНЕРГИИ С МАХОВИЧНЫМ НАКОПИТЕЛЕМ В СИСТЕМАХ ЭЛЕКТРОСНАБЖЕНИЯ

The problem of voltage unbalance and the need to power factor correction remain a relevant objective for electric power engineering. A variable load profile reduces the operating efficiency of electric power plants and leads to an overestimation of the power of transformer substations and the wire size of supply lines. Thus residential buildings power equipment remains underloaded for a significant part of the time. The decrease of power-frequency characteristic is becoming increasingly noticeable. This situation is not a problem yet, but in the future it may complicate the task of frequency stabilization. Finding solutions to existing and predicted problems is an important topic of research. To solve the problems posed, the work proposes the concept of a reversible energy converter, that is an electrotechnical complex with a flywheel energy storage and a two-section frequency converter based on two reversible voltage converters (active front end). The planning area of application of a reversible energy converter is transformer substations of residential and public buildings of urban power supply systems. The reversible energy converter is designed to solve such problems as load leveling, reactive power compensation (power factor correction), balancing voltages and power-frequency characteristic control. The work presents a connection scheme a reversible energy converter to the supply system and provides an expression for the phase load voltage.

Analysis of voltage unbalance and mitigation of circulating power in bilateral co-phase traction system

The primary aim of this paper is to analyse and mitigate the problems inherent in adopting bilateral co-phase traction network on Indian Railways. The reason for adopting bilateral co-phase traction network is to enhance regenerative braking energy (RBE) utilization in the railway traction network. There are two inherent problems in adopting bilateral co-phase traction configuration. First, it may lead to increase in the Voltage Unbalance factor (VUF) as single-phase loading by trains is effectively doubled across the same phases. Second, and more importantly, it may lead to flow of circulating power, under no load conditions, where power shall flow from leading voltage traction sub-station to lagging voltage traction sub-station. This may offset any advantage due to improved RBE utilization. This paper demonstrates through simulation, using realistic data from Indian Railways, that VUF stays within limits for bi-lateral traction configuration. It is important to mention that improved RBE utilization also plays a role in reducing VUF when there is no feedback of RBE to the grid. For the second problem, this paper takes inspiration from a control scheme existing in literature to adapt it to the proposed scenario. This scheme shows that it is possible to limit the power angle within 0.5˚ and thereby minimising circulating power.

Analysis of Voltage Profiles, Unbalance Voltages and Phase Balance Optimization of Single-Phase Customer and Photovoltaic Connection at Real Low Voltage Feeder

Abstract Voltage unbalance is common issue encountered in low voltage distribution networks, caused by uneven allocation of single-phase customers among phases. This paper analyses part of real low voltage distribution network in Bosnia and Herzegovina. The impact of single-phase customers and single-phase connected micro photovoltaic power plants (MPPP-s) were analysed. To reduce unbalance and improve voltage profiles, Phase Balance Optimization toolbox in DIgSILENT PowerFactory software was performed in six scenarios with different distribution of customers and different percentage of penetration of MPPS-s. The aim was to find method with least number of changes in customer and photovoltaic phase connection that fits within defined limits of voltage variations and voltage unbalance of European standard for power quality (EN 50160). Conclusion is that MPPP-s cause voltage increases in the network, as well as an increase in voltage unbalance, but these effects can be mitigated by proper distribution of customer loads and MPPS-s among phases.

Modified instantaneous power theory control of dynamic voltage restorer powered by photovoltaic system

<span lang="EN-US">Power quality issues have become widespread due to nonlinear electronic converters and loads. Nonlinear components in the power system hamper the power quality, network efficiency and voltage regulation. The power quality issues like sag, swell, and voltage unbalance can be reduced by employing a dynamic voltage restorer (DVR). DVR provides the compensating power from a DC source to minimize the effect of sag, swell, and voltage unbalance. This article presents a power system with DVR using a photovoltaic system (PV) as a DC source instead of a conventional battery source. This article proposes a modified instantaneous power (PQ) control technique to generate the reference signal of load voltage to assure the DVR's superior performance. This modified technique uses an anti-aliasing filter to generate a reference signal. This proposed technique is tested under extreme power quality issues of 80% sag, 20% sag, 120% swell, 170% swell, and voltage unbalance. The DVR is modeled and simulated using MATLAB/Simulink. A comparison is made between the traditional and modified PQ methods from the obtained results. The analyzed results under severe power quality issues suggest that the proposed P-Q control technique is a preferable option for the DVR with PV as a DC power supply.</span>

Control Strategy of Three-level Active Neutral Point Clamped Grid Connected Inverters in Unbalanced Power Grid

Traditional phase-locked loops in three-level active neutral point-clamped inverter grid-connected systems for photovoltaic power generation exhibit a deficiency in the decoupling of positive and negative sequences under unbalanced power grid conditions. To address this issue, this study proposes the implementation of the fourth-order generalized integrator into the phase-locked loop to facilitate positive and negative sequence decoupling. Initially, a 3L-ANPC inverter grid connection system is established. However, under unbalanced grid voltage conditions, the PLL effect of the Decoupled Double Synchronous Reference Frame Phase-Locked Loop is found to be suboptimal, leading to considerable fluctuations and distortions in the output frequency signal. To address these complications, we propose an optimized phase-locked loop structure. In this improved design, the PLL first utilizes the FOGI to filter out multiple harmonics in the grid voltage prior to decoupling the positive and negative sequence voltage. This process effectively mitigates the effect on the PLL, thereby enabling rapid and precise tracking of the grid voltage. Simulation analysis is conducted by using Matlab/Simulink to substantiate the effectiveness of the proposed PLL, demonstrating its ability to reliably lock onto the frequency and phase of the fundamental voltage under conditions of unbalanced power grid voltage.

A New Virtual Oscillator-Based Grid-Forming Controller with Decoupled Control Over Individual Phases and Improved Performance of Unbalanced Fault Ride-Through

Virtual Oscillator (VO) control is the latest and promising control technique for grid-forming inverters. VO Controllers (VOCs) can simultaneously provide time-domain synchronization and steady-state droop functionalities. In recent developments, VOCs have been integrated into system-level control architecture to provide additional functionalities such as capacitor voltage regulation and overcurrent protection. This article has further refined the existing VO-based system-level grid-forming control architecture with two essential and specific research contributions. The first contribution is to improve the decoupling in control over individual phases, which is achieved by combining Symmetrical Component based VOC (S-VOC) with nested voltage and current controls. The proposed phase decoupling control feature improves the grid-forming performance of existing VOCs in the presence of unbalanced grid voltages. The second contribution is to improve the fault ride-through performance under unbalanced faults by modifying the feedback of the existing VOC. Systematic development of the proposed grid-forming control architecture with analytical reasoning is presented. The proposed controller is successfully validated using simulations and experiments.

Research on a Three-Phase Energy Mutual-Aid Strategy for a Grid-Connected Inverter Based on Constructed Negative Sequence Current Control

With the increased grid-connected capacity of a single-phase distributed power supply, three-phase power unbalance is more likely to occur in a power grid. Three-phase power unbalance can further lead to three-phase voltage unbalance, which can have adverse effects on power quality and power supply reliability. Therefore, there is a need to build a three-phase power transmission channel to realize power exchanging among phases. In this paper, a novel grid-connected inverter control strategy for three-phase power exchanging is proposed based on constructed negative sequence current control. A completed negative sequence current control loop is added to a conventional three-bridge inverter to realize the decoupling control of three-phase grid current, and then three-phase power exchanging is realized. On this basis, this paper further puts forward a strategy for three-phase power exchanging aimed at three-phase voltage balance. Correspondingly, the three-phase grid current is controlled according to the feedback of the three-phase voltage. Then, three-phase voltage balance is achieved by three-phase power exchanging. The simulation and experimental results show that the proposed strategy is suitable for a three-phase unbalanced power grid, which can realize three-phase power exchanging, and further, can achieve three-phase voltage balance. The proposed strategy can help to improve power quality and supply reliability.

State-of-charge balancing strategy of battery energy storage units with a voltage balance function for a Bipolar DC mircrogrid

For an islanded bipolar DC microgrid, a special problem of making the better compromise between a state-of-charge (SOC) balance among multiple battery energy storage units (MBESUs) in positive and negative polar, and bus voltage balance, should be considered. In order to solve this problem, three kinds of the simplified load equivalent circuits on the different operation status of voltage balancer (VB) are established; based on these equivalent circuits, the SOC unbalance mechanism is analyzed and the rules of SOC deviation and voltage unbalance factor are summarized, which are influenced by micro-source and load changing; then a SOC automatic balancing strategy among MBESUs with a voltage balance function is proposed in this paper. The proposed controlling strategy takes into consideration the energy flow relationship under various working conditions; with the aim of controlling the equilibrium time and energy flow direction of SOC, creates a SOC balancing power bridge with the SOC deviation as the main control variable; adopts the adaptive virtual resistance algorithm for both positive and negative BESUs according to the variation rule of the SOC deviation. When the SOC deviation is significant, according to the net power difference of the system, the values of the virtual resistances can be adaptively adjusted to alter the operation mode of BESUs and the VB control strategy is also adjusted so as to ensure the stable running of the system and accelerate the SOC balancing speed via maximizing the balancing power by VB. In order to decrease the voltage unbalance on SOC balancing, apart from adding bus voltage recovery control to the positive and negative batteries, the virtual resistances are related to the SOC deviation when the SOC deviation is slight. The simulation results demonstrate that the control strategy proposed in this paper can quickly realize SOC balancing when the SOC deviation is significant, has superior equilibrium performance as load and micro-source power varying when the SOC deviation is slight, and is also feasible in decreasing the bus voltage unbalance.

Stator winding fault detection of induction motors using fast Fourier transform on rotor slot harmonics and least square analysis of the Park's vectors

AbstractAn efficient method for the detection of the stator winding faults (SWFs) of induction motors was dealt based on Park's vector of rotor slot harmonics (RSHs). The induced harmonic voltages of the RSHs in the stator windings are the supreme signatures for fault detection, which can be monitored in the motor currents. Using such a signature, unbalanced supply voltage and stator windings fault situations could be segregated straightforwardly. On the other hand, despite some merits, Park's vector analysis‐based methods suffer from low sensitivity and unreliability in the case of low‐severity faults. To have the advantages of both methods while tackling the shortcomings, Park's vector approach and the RSH signatures are combined in the proposed method. The RSHs are extracted using the fast Fourier transform method and the least squared approach is employed for pattern recognition. Different experimental results under various scenarios, even in the case of low severity faults, confirm that the locus of Park's vector is an elliptical shape in SWFs, while it is shapeless in other cases, such as healthy and unbalanced supply voltage cases.

A New 5L SC-Based Boosting Inverter with Reduced Spike Current

The drawbacks of conventional multi-level inverters (MLIs) include insufficient boost capacity, unbalanced capacitor voltage, and complicated regulation. This research presents a 5-level (5-L) switched capacitor (SC) boost inverter for single-phase applications as a solution to the aforementioned problems. The 5-L inverter is well-suited for renewable energy generation since it can produce five output levels with two voltage gains and maintain a natural balance in capacitor voltages with only seven switches. However, the SC inverter’s large current spikes could harm capacitors, switches, and DC sources. The suggested inverter’s charging loop incorporates an inductor to mitigate this issue by smoothing out voltage fluctuations and current spikes. The simulation and experiment findings presented here prove that the proposed system works and performs as expected.

Modelling some rod set imperfections of a quadrupole mass filter.

The problem of modeling the mass peak shape of a quadrupole mass filter (QMF) with round rods is considered. A number of factors leading to the degradation of the mass peak shape are studied, namely, displacement of the electrodes with respect to their original position, changes in the diameter of the electrodes, and asymmetry of the supply potentials. Decomposition of the rod set field on multipole fields allows to obtain an analytical representation of the ion motion equations. Simulations have shown that a parallel shift of one or two electrodes leads to a shift of the peak along the mass number axis and dips at the peak apex. Unbalance of supply voltages does not significantly distort the peak shape, but it shifts the peak along the mass axis and creates a jump of the quadrupole offset potential proportional to the relative unbalance of X and Y rods potentials.

Analysis of Novel Energy-Efficient Converters to Ensure the Required Quality of Electrical Energy

This article proposes several solutions for the use of novel AC voltage regulators as electrical energy quality conditioners and for the use of a half-bridge voltage inverter circuit as an active filter. This study was carried out with a real object, and more attention was paid to it. Structural models of electrical energy quality assurance systems, the calculation of control system elements and experimental results are presented. In particular, the use of a half-bridge voltage inverter circuit was considered as a replacement for the passive filter of the battery charger and rectifier device. AC voltage regulators are also used as compensators for higher-current harmonics, namely active filters and reactive power, voltage drop, voltage unbalance and flicker effect compensators. Block diagrams of power quality conditioners are presented, control algorithms are developed and the results of the current high-frequency harmonics compensation, reactive power and signal balancing are presented. The results of an active filter experiment based on the NRT 160.220 charge-rectifier device circuit showed a reduction in ripple of up to 1% with smaller dimensions compared to a passive filter. The control characteristics and external characteristics of the regulators are removed. The dependences of the current THD factor and the power factor are presented depending on the modulation depth for AC voltage regulator circuits used as power quality conditioners.

Analysis of the Mechanism and Control of the Unbalanced Operation of Three-Phase Four-Wire Inverters

In this paper, a solution is proposed to the problem of the unequal phase imbalance of output voltage caused by a three-phase, four-wire, split capacitor inverter when the load is unbalanced. First, the triple-loop control strategy was used to solve the unequal amplitude problem. This method used the feedforward + feedback composite control strategy on the inductor current inner-loop and voltage mid-loop to decrease the disturbance of the power and load. And the Root Mean Square (RMS) of voltage on the outer-loop completed the control of amplitude for the three-phase voltage. Second, to solve the imbalanced phase problem, the imbalance operation mechanism of the three-phase four-wire inverter was analyzed. It is known from the analysis that the phase imbalance is related to the DC-side splitting capacitance. The function relations between the DC-side capacitance and phase angle between each phase was simulated by MATLAB. But, it was too complicated to calculate the magnitude of the capacitance value through the functional relationship. In order to simplify the design of the DC-side splitting capacitor, the relations among the imbalanced current, the voltage fluctuations of the DC-side capacitor and the harmonics of load voltage were analyzed. In addition, by following the requirement of the national standard about the harmonics of load voltage, a DC-side capacitor design was mentioned to decrease the influence of imbalanced phase. Finally, simulation and experimental results show that the three-phase load voltage is stable, the THD value is less than 3%, and three-phase voltage unbalance is less than 2%, thus verifying the effectiveness of the proposed DC-side split capacitor design and control strategy.

Development of a Novel Control Scheme to Achieve the Minimum Unbalance Factor and Real Power Fluctuations under Asymmetrical Faults

The increasing share of converter-based renewable energy sources in the power system has forced the system operators to demand voltage support from the converters in case of faults. In the case of symmetric faults, all the phases have equal voltage support, but in the case of asymmetric faults, selective voltage support is required. The grid codes define the voltage support required in the case of symmetric/asymmetric faults, which is the reactive current injection in the respective sequence proportional to its voltage dip, but studies confirm that it does not result in a minimum unbalance factor in the case of asymmetric faults. The unbalance factor is an indication of the level of imbalance voltage among the phases. Moreover, it also results in fluctuated active power injection in the case of asymmetric faults, which causes dc link voltage fluctuations, and the power reversal may also occur due to such fluctuations, which leads to higher protection costs for the dc link. In order to (1) enhance the uniformity of voltage among different phases in the case of asymmetric faults and (2) minimize the real power fluctuations in such conditions, a novel control scheme is presented in this paper. It optimally distributes the negative sequence current phasor into its active and reactive components to achieve the minimum voltage unbalance factor. It also confirms the minimum real power fluctuations by adjusting the positive and negative sequence current phasors. The proposed scheme also ensures the current limit of the converter. The proposed scheme is developed in Matlab/Simulink and tested under different faulty conditions. The results confirm the better performance of the proposed scheme against the grid code recommendation under different faulty conditions.

Study of the variation of operation of a low voltage electric network due to the integration of distributed energy resources—Steady state condition

Distributed energy resources (DERs) can significantly impact distribution networks, necessitating characterization studies to understand the technical consequences in various operating conditions. Existing literature primarily applies deterministic or probabilistic techniques for modeling and simulating networks and DERs, but lacks studies that integrate multiple DERs simultaneously. Additionally, assessing various indicators is necessary to determine the influence of DERs on network operation. Furthermore, resilience approaches often overlook DER integration as a disruptive event. To address these gaps, this work employs deterministic, probabilistic, and harmonic distortion analyses to characterize the impact of photovoltaic systems, storage units, and electric vehicles on a reference low voltage network. The study investigates how these DERs influence network operation at different time intervals throughout the day. By examining parameters and indicators (PaI) such as RMS values of voltages and currents, voltage unbalance, and harmonic distortions, the individual and collective effects of DERs are estimated. An adapted resilience evaluation technique groups the PaI values to generate a representative population of a single R indicator for each hour, allowing for quantification of network operation variations. Data for the analysis is obtained through PowerFactory-Python co-simulation, encompassing twenty operating scenarios.

Optimal scheduling of a microgrid with power quality constraints based on demand side management under grid-connected and islanding operations

Power quality (PQ) issues are direct consequences of integrating power electronics components and non-linear loads into microgrids. These issues can also be triggered by an unbalanced loading in the microgrid. Certainly, they affect the daily operation scheduling of the microgrid. This paper proposes an optimal harmonic power flow (OHPF) framework for the daily optimal scheduling of a grid-connected microgrid, which is constructed by combining the optimization formulation and harmonic power flow (HPF). Within the framework, PQ is evaluated by observing three indices including voltage magnitude, voltage total harmonic distortion (THDV), and voltage unbalance factor (VUF). A non-iterative mitigation scheme based on demand side management (DSM) is proposed to avoid PQ indices violations and is integrated into the optimization formulation part of the OHPF as a set of load constraints. The proposed constraints allow a flexible combination of different DSM actions including load shedding and interphase load transfer (ILT). Furthermore, the constraints to limit voltage magnitude, voltage THD, and VUF are integrated into the formulation in the form of penalty functions. The software implementation of the proposed OHPF involves Julia-based JuMP.jl and Gurobi solver for optimization part and OpenDSS for harmonic load flow part. The effectiveness of the proposed framework is tested using a modified IEEE 37-bus feeder for different load fluctuations, which allow different combinations of PQ indices violations. The assessment involves both normal and intentional islanding conditions. The results demonstrate that, under different random events, the proposed framework can avoid the violation of PQ indices limits in most cases without adding an excessive computational burden to the original optimization and harmonic load flow algorithms.

Linearized power flow calculation of bipolar DC distribution network with multiple flexible equipment

The bipolar direct current distribution network (BDCDN) has the advantages of high reliability, large transmission, and high efficiency. However, voltage unbalance is a crucial power quality problem of BDCDN. On one hand, flexible equipment, such as DC electric spring, DC power flow controller, and DC transformer, can effectively suppress unbalanced voltage. On the other hand, the diversity of topology and control methods of multiple types of flexible equipment complicates power flow calculation. In this study, a linear power flow algorithm is proposed. First, the structural characteristics of a BDCDN with multiple types of flexible equipment are analyzed. Second, a linear ZIP load model based on Taylor expansion is derived. The steady-state model of multiple types of flexible equipment is established by analyzing the topological structure and control mode. Third, a network modeling and power flow calculation method of BDCDN with multiple types of flexible equipment are proposed. Finally, the modified IEEE14-node case and IEEE33-node case are used to verify the validity and correctness of the proposed algorithm. The effectiveness of flexible equipment on the power quality issue is demonstrated through comparison studies of power flow distributions.

Algorithm for passive reactive power compensation of an unbalanced three-phase four-wire system using capacitors ensuring minimum line losses

Currently in three-phase distribution networks, especially low-voltage networks, there are some inefficient powers. These are defined as reactive power, unbalanced power owing to linear loads, power owing to supply unbalanced voltage, and harmonic power. This has given rise to the concept of power quality. To improve power quality, devices called active filters have been developed. These devices eliminate all the inefficient power. However, for most processes, the elimination of reactive power needs to be from a price/performance perspective because the use of active filters is very expensive. Therefore, the use of capacitor banks in any of their versions (single-phase, three-phase, scalable battery, SVC, etc.) is the most economical and sufficient solution. In this study, a calculation algorithm is proposed to obtain compensators for the inductive reactive power of the load, consisting only of single-phase capacitor banks. These capacitors are designed to minimise losses in the supply line, which are lower than those obtained using the minimum loss line (MLL) strategy. The resulting compensator consists of three, two, or one capacitor, depending on the load characteristics.

Modified VOC Using Three Symmetrical Components for Grid-Supporting Operation During Unbalanced Grid Voltages and Grid-Forming Operation in Hybrid Single-Phase/Three-Phase Microgrid

Virtual Oscillator (VO) control is the most advanced time-domain based control strategy for grid-supporting and grid-forming inverters. VO Controllers (VOCs) provide better dynamic performance than droop controllers and virtual synchronous machine controllers. However, the existing VOCs can only provide synchronization with the positive sequence voltages of a connected electrical network. As a result, the existing VOCs cannot operate in an electrical network with unbalanced voltages. This paper has introduced the Instantaneous Symmetrical component based VOC (S-VOC), which can provide continuous synchronization simultaneously with the positive, negative, and zero sequence voltages of a connected electrical network. The proposed S-VOC enables two very important and relevant functionalities for grid-supporting and grid-forming inverters. The functionalities are the grid-supporting operation in the presence of unbalanced grid voltages and the accurate load-sharing in grid-forming mode when single-phase and three-phase sources are integrated into the same hybrid microgrid architecture. At the same time, the proposed S-VOC preserves the superior dynamic performance of the existing VOC. Furthermore, the proposed S-VOC can replace the existing VOC without any extra sensor. The systematic design procedure and mathematical analysis of the proposed S-VOC are presented in this article. Simulation studies and hardware experiments are conducted for validation.