AC Network Research Articles

New Power Quality Measurement Techniques and Indices in DC and AC Networks

Currently, we are living through the implementation of the electrical smart grid, a network that can intelligently integrate the actions of all users connected to it in order to efficiently deliver the sustainable, economical and secure supply of electricity [...]

Concept and Definition of Dynamic Symmetrical Components With Time-Varying Amplitude and Frequency

This paper investigates the dynamic symmetrical component method of internal voltage vector with time-varying amplitude and frequency. With the large-scale integration of power electronic devices into the power grid, when there is an asymmetrical fault in the AC network, the voltages and currents present the characteristics of amplitude and frequency time-varying modulation. Hence, the traditional asymmetrical fault analysis method and concept based on phasor models with constant amplitude and frequency are not adaptive enough. In order to supplement the traditional symmetrical component method and concept, the phasor models and vector models with time-varying amplitude and frequency are established. Then, the asymmetrical three-phase phasors with time-varying amplitude and frequency are decomposed into three groups of symmetrical phasors with time-varying amplitude and frequency. Based on the mathematical and physical characteristics of symmetrical components, the applicable concept and definition are reestablished. Lastly, time-domain simulations corroborate the theoretical analysis.

Power-Balancing Dual-Port Grid-Forming Power Converter Control for Renewable Integration and Hybrid AC/DC Power Systems

In this work, we investigate grid-forming (GFM) control for dc/ac power converters in emerging power systems that contain ac and dc networks, renewable generation, and conventional generation. We propose a change in control paradigm to a universal dual-port GFM control strategy that simultaneously forms the converter ac and dc voltage (i.e., dual-port GFM), unifies standard functions of grid-following (GFL) and GFM (e.g., primary frequency control, maximal power point (MPP) tracking) in a single universal controller, and is backward compatible with conventional machine-based generation. Notably, in contrast to state-of-the-art control architectures that use a mix of grid-forming and grid-following control, dual-port GFM control can be used independently of the converter power source or network configuration. Our main contributions are stability conditions that cover emerging hybrid ac/dc networks as well as machines and converters with and without controlled power source, which only require partial knowledge of the network topology. Finally, a detailed case study is used to illustrate and validate the results.

An Improvement of Power Control Method in Microgrid Based PV-Wind Integration of Renewable Energy Sources

Microgrids are quickly becoming a great success for the future of electricity. The notion of the microgrid combines several microsources without interfering with the functioning of the larger utility grid. The DC and AC networks of this hybrid Microgrid are powered by photovoltaic and wind generators. Both AC and DC Microgrids may couple with energy storage devices. A microgrid powered by a combination of renewable energy sources, such as wind and solar, is shown and controlled in this project. The wind energy conversion machine is a doubly fed induction generator (DFIG), and it is coupled to a battery bank through a DC bus. Solar power is efficiently converted utilising a DC-DC boost converter from a solar photovoltaic (PV) array and then evacuated at the common DC bus of DFIG. With the line side converter's droop characteristics implemented, voltage and frequency may be regulated using an indirect vector control. A battery's energy level is monitored, and the frequency set point is adjusted accordingly to prevent excessive charging or discharging. When wind power is not available, the system can still function. Maximum power point tracking (MPPT) is a feature of the control algorithm used by both wind and solar energy blocks. All conceivable operational scenarios have been accounted for in the system's design, making it fully autonomous. An external power supply is included into the system and may be used to charge the batteries whenever needed. The feasibility of wind and solar energy, imbalanced and nonlinear loads, and a depleted battery are only some of the scenarios simulated in this paper, along with the corresponding simulation findings.

Grid integration of multiple PV inverters with reduced number of interfacing transformers— A dedicated controller for elimination of DC current injection

The injection of dc current offset into ac networks may impacted the lifespan of the distribution system equipment including isolation transformers and measurement devices and in serious event may cause equipment’s malfunction. This paper intents to eliminate dc current offsets in the output currents using a combinational of proportional-integral (PI) and proportional resonance (PR) controls embedded in one inverter unit. Resultant output currents of this method are sinusoidal and clean from dc offset before entering the point of common coupling. This method gives advantages for transformerless option for group of interfacing inverters in the medium-scale solar farm or in arrangement of inverters restricted in a small locale. Moreover, the use of expensive and high-accuracy measurement sensor nor complex transformer can be omitted, whilst indirectly give positive impact to the operational cost of the farm. The simulation verifications proved the usefulness of the proposed method by introducing varying unknown dc offset levels in the phase currents, and a dedicated dc offset suppressor inverter able to successfully eliminate the dc offset to zero. The validity of the proposed method is demonstrated in simulation using MATLAB/Simulink.

Criticality Identification in Power Distribution Networks with Voltage Stability Index based on Apparent Power

—As a network of systems, electrical grid is complex in nature and requires sophisticated control structures to manage the network in stable operating region. These control structures depend on processing of data pertaining to states of the network, derived from voltage levels and power flow studies. Control actions invariably require logical methods of data analysis covering selected regions of the network. This paper investigates the role of a proposed new instability prediction index in characterizing the operating condition of a defined electrical network. The proposed instability calculation method is developed from the defining equations for real and reactive power flows in AC networks to characterize the network under consideration. By using the dependence of power flow on the network branch impedances, an indicator of the nodal voltage stability and weak line detection is developed. Analytical solution constraints for the power flow equations are utilized to derive the criticality of electrical networks in general, irrespective of the topological aspects. Analysis of typical network configurations, like three node, IEEE − 14, 69, 34 and 123 bus systems using the proposed method demonstrates the fast and easy determination of criticality of network operation.

Autonomous power balance in hybrid AC/DC microgrids

A control paradigm is proposed in this paper for decentralized power balance in hybrid AC/DC Microgrids (MGs). In this technique, the AC and DC sub-grids can transact energy from or towards the other side based on their loading conditions. Hybrid MGs use an interlink converter (ILC) as a back-to-back bi-directional converter at the Point of Common Coupling (PCC) between the sub-grids. The main duty of the ILC unit is to maintain the loading balance between the sub-grids. For this purpose, a superimposed AC voltage on the network’s nominal DC voltage is utilized in the DC sub-grid. Also, the AC sub-grid is equipped with the conventional f/P droop control. The frequency of the superimposed AC voltage in the DC sub-grid and the AC network frequency in the AC sub-grid are used for estimating the loading percentage of each side. As a result, the ILC unit acts as a load for one side and concurrently as a source for the other side. The ILC controller adjusts the participation of this unit in a fully decentralized manner. Simulation studies via PLECS test the efficacy of the suggested control system.

Innovative Unit Commitment Modeling Approach Considering Multi-infeed VSC-HVDC Links

This paper introduces an improved optimization model for the unit commitment (UC) problem of AC power systems considering multi-infeed VSC-HVDC links. The proposed formulation is based on the linear modeling of the AC network and point-to-point HVDC links, where power losses are properly considered by piecewise linearization of AC lines, DC links and VSC units. This UC model is featured by a mixed integer linear programming (MILP) framework, thus enabling effective calculations of the optimal hourly generation scheduling for multi-infeed VSC-HVDC power grids. These sophisticated power grid studies are associated with day-ahead electricity markets carried out by power system engineers at control centers. To demonstrate the applicability of the proposed MILP-based UC model, the IEEE RTS 24-bus test system is used to carry out two case studies. The first incorporates three point-to-point HVDC links where an in-depth analysis is carried by comparing its outcomes, with and without HVDC links. The second case study features two interconnected areas having three-infeed HVDC links and several tens of generation units. Both systems are analyzed for a 24-hr planning horizon. It is confirmed that the UC model described in this paper permits to study practical power grids with multi-infeed VSC-HVDC links.

Performance Evaluation and Techno-Economic Analysis of Modular Multilevel STATCOM with Hybrid Storage for Renewable Power Dominated Grids

The increase in renewable power injection into the existing ac network has affected voltage and frequency stability. This creates the need for both active and reactive power support systems at the Common Coupling Point (CCP) of a renewable power generating station. Considering the aforesaid requirements, a distinct configuration using a Modular Multilevel Converter (MMC) with Hybrid Storage System (HSS) has been recommended in this paper. These kinds of systems are termed as “Statcom Embedded with Storage System”(SESS). The SESS helps to pump invariable power into the grid by absorbing fluctuating components of active power generated by the renewable power generating plant. Apart from this, provides ancillary services at CCP like, voltage regulation and compensation for negative sequence currents, and ride through capability during the faults in the ac system are to be provided by SESS. The required control methods to operate the proposed configuration for the aforesaid necessities are discussed in this manuscript. The economic advantage of the proposed configuration in comparison to other multilevel topologies has been identified using a techno-economic study and it is presented. The competency of the recommended configuration for SESS application along with its control is verified through PSCAD/EMTDC simulation and the results show its efficacy.

Optimal wireless rate and power control in the presence of jammers using reinforcement learning

Future wireless networks require high throughput and energy efficiency. This paper studies using Reinforcement Learning (RL) to do transmission rate and power control for maximizing a joint reward function consisting of both throughput and energy consumption. We design the system state to include factors that reflect packet queue length, interference from other nodes, quality of the wireless channel, battery status, etc. The reward function is normalized and does not involve unit conversion. It can be used to train three different types of agents: throughput-critical, energy-critical, and throughput and energy balanced. Using the NS-3 network simulation software, we implement and train these agents in an 802.11ac network with the presence of a jammer. We then test the agents with two jamming nodes interfering with the packets received at the receiver. We compare the performance of our RL optimal policies with the popular Minstrel rate adaptation algorithm: our approach can achieve (i) higher throughput when using the throughput-critical reward function; (ii) lower energy consumption when using the energy-critical reward function; and (iii) higher throughput and slightly higher energy when using the throughput and energy balanced reward function. Although our discussion is focused on 802.11ac networks, our method is readily applicable to other types of wireless networks.

A novel two-phase load compensation method under unbalanced voltages for renewable energy integration

This paper explores new design methods for correcting unbalanced power distribution networks in order to improve the power quality and reliability for sensitive industrial loads. While traditional compensators typically need to be connected to all the three phases of the AC network, this paper proposes a novel three-phase voltage balancing compensator that can operate with a connection to only two of the three phases. The new concept is based on the phasor symmetrical component theory, which can be implemented using either mechanically switched reactors (MSRs), TCRs (thyristor-controlled reactors) or VSCs (Voltage source converters). Three methods are presented to calculate the value of the proposed two-phase compensator, i.e., discrete solution, analytical solution and novel simplified solution. The discrete solution is solved via the exhaustive search method, which was successfully used by StruthersTech to correct the power quality of an unbalanced industrial power system. The analytical solution is based on the nodal voltage of analysis method. The simplified solution derives a practical albeit approximated relationship between the negative sequence source voltage and zero sequence load voltage, thus avoiding the need to solve nonlinear equations. Dynamic simulations are implemented using MATLAB/Simulink Simscape Blockset to validate the effectiveness of the proposed two-phase voltage balancing method.

The Railway Timetable Evaluation Method in Terms of Operational Robustness against Overloads of the Power Supply System

The main aim of this study was to develop a method for assessing the level of robustness of timetabled transport performance in rail transport. When the railway lines are supplied by DC networks, lower voltages are observed, and consequently, current values are often ten times higher than in AC networks. This is an operational problem, as high currents make it easier to overload the supply network. Based on a literature review, the authors show that the problem of running railway traffic when the capacity of the power supply network is limited (by the size of the permitted currents) is not well studied. The authors propose a method based on the Markov approach supplemented by classical theoretical vehicle traffic dynamics to improve the operational robustness of the rail transport system using DC power supply system. Each train run was parameterised in such a way that it is possible to determine the state that the train is in during the run, the transitions between states, and the determination of the probabilities of occurrence of such states. On the other hand, classical vehicle dynamics was used to assess the load generated by the train on the power grid. The proposed method—reduced to a function—was verified using a case study. The method of timetable reconfiguration proposed by the authors increased the operational robustness from 0.9454 to 0.9774.

Employing adversarial robustness techniques for large-scale stochastic optimal power flow

High penetrations of renewables and extreme weather phenomena are driving the need for large-scale stochastic optimization for power grid operations and planning. However, large-scale stochastic optimization remains computationally challenging due to the wide range of potential probabilistic outcomes and the non-convexity of the AC network constraints. Using a minimax formulation rooted in robust optimization, we define a generic methodology to obtain a feasible dispatch that accommodates the probabilistic nature of loads and renewable generation sources. We introduce a method to efficiently solve this minimax formulation, adopting techniques from the literature on adversarial robustness in machine learning to improve scalability and convergence. We demonstrate that our method maintains AC feasibility over a wide range of probabilistic scenarios, and demonstrate the scalability of our method by determining a dispatch for a synthetic 11,000 bus system.

Real-Time Optimization-Based Reference Calculation Integrated Control for MMCs Considering Converter Limitations

The paper addresses a real-time optimization-based reference calculation integrated with a control structure for Modular Multilevel Converters (MMC) operating under normal and constrained situations (where it has reached current and/or voltage limitations, as it may occur during system faults). Firstly, a nonlinear optimization problem has been developed in which it prioritizes to satisfy the AC grid current set-points imposed by the transmission System Operator (TSO). The constrained nonlinear optimization problem is formulated based on the steady-state model of the MMC, whereby the prioritization is achieved through distinct weights defined in the Objective Function’s (OF) terms. The resultant optimization problem, however, is highly nonlinear requiring high computation burden to be solved in real-time. To cope with this issue, this paper applies a Linear Time-Varying (LTV) approximation, which permits to represent the nonlinear dynamics of the system as constant parameters, while a Linear Time-Invariant (LTI) system is used to formulate the optimization constraints. The converter’s current references are determined in real-time by solving a constrained linearized optimization problem at each control time step, which considers the TSO’s demands, the current MMC operating point and its physical limitations. Theoretical analyses comparing the responses of the linear and nonlinear optimization problems are performed to validate the accuracy of the LTV approximation. Finally, the linearized-optimization problem is integrated with the MMC controllers, evaluated under different AC and DC network conditions and compared with conventional control strategies, where it is shown that the presented method can be potentially employed to obtain the MMC current references for distinct network scenarios.

Simulation of the electric field of a polymer bushing in the Ansys Maxwell software environment

The article analyses the software tools for modelling the maps of the electric field strength of insulators. The need to address this issue is dictated by the fact that the uneven electric field on the insulators increases the probability of partial discharges that destroy the insulating layer, so it is very important to optimize the electric field strength on the insulator. The design features of bushings and features of partial discharges in them are considered. The distribution of electric field strength in a polymer pass-through operating in an AC network without contaminants is considered. The results of modelling the electric field of insulators in the software environment Ansys Maxwell, which is based on the simulation of the finite element method, are presented. Points in the construction of a 35 kV bushing polymer insulator have been identified, where the highest level of electric field strength is concentrated. A method of influencing the area under study in the area of the flange connection to the ground plate is proposed. This defines the use of a conductive cover placed between the flange and the place of connection to the polymeric insulating layer, which significantly improves the overall electric field strength distribution on the surface of the bushing polymer insulator and makes it competitive compared to the most commonly used porcelain bushings, by increasing the electrical strength of the structure. According to the results of research, a means of optimizing the electric field of the pass-through polymer insulator is proposed, which allows to increase its electric resistance and reduce the electric field strength in the area of flange contact with the mounting surface, which in turn prevents premature partial discharges in the insulating bodies.

Investigating small-disturbance stability in power systems with grid-following and grid-forming VSCs using hybrid modelling approaches

The mass integration of power electronics-based devices is changing small-disturbance stability issues in power systems from primarily interactions between synchronous generators to include interactions between power electronic devices and the AC network. However, modelling of AC networks for small-disturbance stability assessment in large power systems generally do not include the transmission network dynamics and, therefore, fails to identify these newly emerging power phenomena. A suggested solution is the hybrid modelling approach which models the vicinity around the power electronics with network dynamics while representing the remainder of the network with a steady-state model. This paper identifies the most appropriate selection of dynamic AC network modelling in accordance with different scenarios including different power electronic control schemes, different power electronic operation modes, and different locations of power electronic devices. It is shown that more of the network must be modelled dynamically when VSCs are located in low SCR regions, operating in inverting mode, and using grid following or grid-forming control with inner current control explicitly modelled. Contrastingly, less dynamic detail is required for high SCR regions, rectifying mode, and when modelling grid forming control without explicit representation of the inner current controllers. Investigations are performed using the IEEE 39-bus test system.

Maximum Power Point Tracking Powered Solar PV Grid Integrated System Using DSTATCOM

Abstract: The energy crisis has arisen as a consequence of rapid depletion of fossil fuels and a growing awareness of the need of environmental preservation. As a result, scientists are attempting to develop new methods for harvesting renewable energy sources. Maintaining usage efficiency is essential for conserving resources for future generations. Due to the availability of renewable energy sources, the goal of this study is to explore, assess, and underline the importance of a PV system that is linked to the grid and as a consequence the description of PV systems in connection to grid regulation is examined. The micro grid concept decreases the number of reversals in a single AC or DC grid, making it easier to connect variable renewable AC and DC sources and loads to energy systems. Energy storage systems (ESS) are one way to improve the quality of a power supply while also assuring its stability. The distributed static synchronous compensator (D-STATCOM) is used to adjust if the AC network is unable to supply the power requirement. The suggested model was simulated in MATLAB R2020a simulink programme to see if it was feasible using an Intel core Pentium(R) dual core processor running at 2G.20GHz and 3GB RAM in a 64 bit configuration.

General Nash bargaining based direct P2P energy trading among prosumers under multiple uncertainties

This paper presents a transaction scheme for the direct peer-to-peer (P2P) energy trading problem among prosumers under uncertainties. Prosumers, in the distribution network, coordinate with neighbors to share the idle energy and maximize the cooperative benefit. Most of the existing researches focus on mechanism design with the assumption of accurate forecast. Nevertheless, the P2P energy trading problem is challenging due to multiple uncertainties, caused by the market price, intermittent power output and load demand variation. Although stochastic or robust optimization approaches are introduced and investigated to manage uncertainties, there are limitations for their application, such as large scenario sizes in the stochastic programming, coordination of the conservativeness and economy in the robust optimization. To address these challenges, a hybrid stochastic/robust optimization model is adopted to manage uncertainties which makes trade-offs between computational time and conservatism. The uncertain price is modeled via various scenarios based on forecast values while a robust optimization is developed to take into account the uncertainties of renewable generation and load. Based on the above uncertainties description, the direct P2P energy trading among prosumers is formulated as a general Nash bargaining (GNB) problem which is decomposed into two subproblem: operation cost minimization problem (P1) and bargaining problem (P2). As P1 and P2 are nonconvex problem, linearization techniques including KKT conditions and logarithmic transformation are employed to transform them as mixed-integer linear programming problem. A linear distflow model is formulated to address an OPF problem considering voltage limits of an AC network in a radial structure. Furthermore, a graph theoretic method is proposed to construct the incident matrix about buses and branches in the network constraints which considerably enhances the solution efficiency. Numerical case studies demonstrate that the constructed model can effectively incentivize direct P2P energy trading among prosumers, and multiple uncertainties affect the operation costs and energy transaction decisions. Additionally, the efficiency of the graph theory based modeling approach is proved for solving the energy trading model with OPF constraints.

An energy cooperative system concept of DC grid distribution and PV system for supplying multiple regional AC smart grid connected houses

This paper proposed a suitable control strategy for power sharing management for hybrid AC/DC residential PV systems. This control method is used as an Energy Cooperative Approach (ECA) to manage the power flow between a PV and battery-fed smart grid within a DC micro-grid, supplying some connected houses. In fact, using an ECA which is based on photovoltaic (PV) power transfer between three houses, connected to the AC network; through a DC micro-grid, the photovoltaic energy can be effectively used to satisfy the load demand. Such a process ensures a continuous demand satisfaction, lengthens the storage system life time by minimizing the resort of the battery, and alleviates the stress of the main grid minimizing the consumption of electricity fed by the AC grid. In fact, this strategy increases the system efficiency and the power supply quality. To achieve such purposes, a control algorithm for distributed flexible network residential PV systems was properly developed. The decision criterion for the developed supervisor is based on the instantaneous PV power, the required power, and the battery state of charge in order to manage the power flow under six operating modes. The control strategy was proposed also to regulate DC bus voltage and maintain the power balance within the micro-grid. The simulation results validated the effectiveness and feasibility of the proposed control strategy under various conditions. To highlight this strategy benefits, a comparative study between strategies without and with ECA was performed. The results of this study showed that using the ECA on a typical day, the energy PV contribution to satisfy the loads (Fpv) grows by a rate that varies from 15.5% to 23.45%, the battery contribution part (Fbat) recorded a decrease that varied between 8.77% and 18.42%, while the main grid contribution (Fgrid) was reduced by a rate that varied between 2.41% and 13.1%.

Improved Current Reference Calculation for MMCs Internal Energy Balancing Control

The paper addresses an improved inner current reference calculation to be employed in the control of modular multilevel converters operating during either balanced or unbalanced conditions. The suggested reference calculation is derived based on the AC and DC additive and differential voltage components applied to the upper and lower arms of the converter. In addition, the impacts caused not only by the AC network’s impedances but also by the MMC’s arm impedances are also considered during the derivation of the AC additive current reference expressions. Another issue discussed in this article regards that singular voltage conditions, where the positive-sequence component is equal to the negative one, may occur not only in the AC network but also internally (within the converter’s applied voltages). Several different inner current reference calculation methods are compared and their applicability during the former fault conditions is analyzed. The paper presents a detailed formulation of the inner current reference calculation and applies it to different unbalanced AC grid faults where it is shown that the presented approach can be potentially used to maintain the internal energy of the converter balanced during normal and fault conditions.