Active Power Balance Research Articles

Multi‐slack power flow for islanded microgrids with radial and meshed topologies

This study proposes a new power flow formulation for islanded microgrids. The proposed power flow is based on the effect of the superposition principle and the solution of a small non-linear subproblem to determine the frequency of the microgrid and the voltage magnitude in the angular reference node in each iteration. These variables are determined by considering the following equations in the non-linear subproblem: active and reactive power balance equations and specified phase at the angular reference node. The application of the superposition principle allowed to obtain two versions of the proposed technique: one for radial networks - based on the current summation method - and another for meshed networks - based on the Gauss-Zbus method. Therefore, the iterative framework prosed in this study expands in a simple and integrated way the two most commonly used power flow methods in conventional distribution networks for islanded microgrids. The tests in microgrids with 33, 310 and 1438 nodes showed that the proposed approach has the same accuracy as Newton-Raphson algorithm, but with significantly lower computational cost in large scale microgrids. In addition, the proposed method for island microgrid showed good accuracy and convergence for the most common load models applied in power flow studies of islanded microgrids.

Multiterminal Medium Voltage DC Distribution Network Hierarchical Control

In this research study, a multiterminal voltage source converter (VSC) medium voltage DC (MVDC) distribution network hierarchical control scheme is proposed for renewable energy (RE) integration in a co-simulation environment of MATLAB and PSCAD/EMTDC. A DC optimal power flow (DC OPF) secondary controller is created in MATLAB. In PSCAD/EMTDC, the main circuit containing the adaptive DC voltage droop with a dead band and virtual synchronous generator (VSG) based primary controller for the VSCs is implemented. The simulation of the MVDC network under the proposed hierarchical control scheme is investigated considering variations in wind and solar photovoltaic (PV) power. The network is also connected to the standard IEEE-39 bus system and the hierarchical scheme tested by assessing the effect of tripping as well as restoration of the REs. The results show that during random variations in active power such as increasing wind and PV power generation, a sudden reduction or tripping of wind and PV power, the primary controller ensures accurate active power sharing amongst the droop-based VSCs as well as regulates DC voltage deviations within the set range of 0.98–1.02 pu with an enhanced dynamic response. The DC OPF secondary control optimizes the system’s losses by 38% regularly giving optimal droop settings to the primary controllers to ensure proper active power balance and DC voltage stability. This study demonstrates that the hierarchical control strategy is effective for RE integration in the MVDC distribution network.

Carrier-Phase-Shifted Rotation Pulse-Width-Modulation Scheme for Dynamic Active Power Balance of Modules in Cascaded H-Bridge STATCOMs

In the medium-voltage power distribution system, cascaded H-bridge (CHB) static synchronous compensators (STATCOMs) are widely utilized to solve power quality issues by injecting the controlled reactive current into the system. The carrier-phase-shifted (CPS) pulse-width-modulation (PWM) scheme is preferred for CHB-STATCOMs, because it can minimize the compensating current distortion and realize the relative active power balance among the H-bridge modules. This paper reveals the influence of the carrier phase difference on the module active power balance, and proposes a carrier rotation technique with a CPS-PWM scheme to address this drawback. The rotation rules were analyzed, and the rotation time was especially designed to enhance the robustness of the system. With the proposed method, the natural dynamic active power balance of each module could be achieved, and the capacitor voltage could maintain balance without the individual capacitor voltage control or the auxiliary circuits in theory. The experimental results acquired from a downscaled CHB-STATCOM prototype demonstrated the feasibility of the proposed CPS rotation PWM scheme.

The Performance of the BTB-VSC for Active Power Balancing, Reactive Power Compensation and Current Harmonic Filtering in the Interconnected Systems

Nowadays, the use of power converters to control active and reactive power in AC–AC grid-connected systems has increased. With respect to indirect AC–AC converters, the tendency is to enable the back-to-back (BTB) voltage source converter (VSC) as an active power filter (APF) to compensate current harmonics. Most of the reported works use the BTB-VSC as an auxiliary topology that, combined with other topologies, is capable of active power regulation, reactive power compensation and current harmonic filtering. With the analysis presented in this work, the framework of the dynamics associated with the control loops is established and it is demonstrated that BTB-VSC can perform the three tasks for which, in the reviewed literature, at least two different topologies are reported. The proposed analysis works to support the performance criteria of the BTB-VSC when it executes the three control actions simultaneously and the total current harmonic distortion is reduced from 27.21% to 6.16% with the selected control strategy.

Research on Frequency Control of Islanded Microgrid with Multiple Distributed Power Sources

At present, some achievements have been made in the research on the energy management of microgrid operation. However, the research is mainly on the operation of grid-connected microgrid, while the research on the energy management of islanded microgrid is still relatively few. Frequency is one of the characteristics that affects the reliability and power quality of the microgrid. The essence of controlling frequency stability is to maintain source-load balance and redistribution of active power. Therefore, this paper proposes a frequency control strategy based on dynamically cutting machine to reduce load by analyzing the use priority of different distributed power supply and the division of load importance degree, and combining the influence degree of different frequency variation range on microgrid. To coordinate and control distributed power supply, energy storage device, and load in different frequency change areas, this paper proposes different control strategies. The seed strategies of the control strategy are discussed one by one. Experimental results show that the frequency control strategy can significantly improve the frequency stability of the power supply system and reduce the operating cost of islanded microgrid.

Power management control strategy for hybrid energy storage system in a grid‐independent hybrid renewable energy system: a hardware‐in‐loop real‐time verification

This study proposes a novel control strategy for a hybrid energy storage system (HESS), as a part of the grid-independent hybrid renewable energy system (HRES) which comprises diverse renewable energy resources and HESS - combination of battery energy storage system (BESS) and supercapacitor energy storage system (SCESS). The proposed control strategy is implemented into two parts: in the first part, HESS controller is designed and implemented to maintain the active power balance among different constituents of HRES and regulate DC-link voltage ( V DC ) under surplus power mode and deficit power mode. The low-frequency components of imbalance power are diverted to BESS while the high-frequency components are diverted to SCESS while maintaining the state of charge (SoC) constraints of HESS thereby reducing stress on BESS. In the second part, inverter controller is designed and implemented to maintain AC voltage and frequency within limits in the events of perturbation. The proposed HESS along with its novel control strategy, as implemented in the HRES, is a new proposition in this study. The detailed model of HRES is simulated in MATLAB/Simulink and the results prove the efficacy of the control strategy. Further, hardware-in-loop real-time simulation studies using OPAL-RT real-time simulator demonstrate the feasibility of hardware implementation of HRES with the proposed control strategy.

Current and power quality multi-objective control of virtual synchronous generators under unbalanced grid conditions

In recent years, with the large-scale application of distributed power sources in the power grid, the power grid is moving toward low inertia and low damping. The virtual synchronous generator (VSG) has become a hot topic for scholars since it can simulate the moment of inertia, damping and frequency modulation of synchronous generators. Due to its unbalanced load distribution and the random variation of power loads, grid voltage is asymmetrical. Under grid voltage asymmetry, a VSG experiences output current imbalance and power oscillation. The grid current imbalance further reduces the transformer’s three-phase voltage, which makes it unequal, cyclic, and very easy to cause electricity accidents. Aiming at this problem, this paper proposes three modes of current balance, reactive power balance and active power balance without changing the characteristics of VSGs. The output current balance and power are constant when the grid voltage is asymmetrical. The effectiveness and feasibility of the control strategy are verified by simulation and experimental results, which provides an effective scheme for balanced and stable operation of the grid.

Grid-independent PV system hybridization with fuel cell-battery/supercapacitor: Optimum sizing and comparative techno-economic analysis

In this paper, a comparative techno-economic assessment of two different hybrid energy storage system configurations viz. fuel cell-battery energy storage system (FC-BESS) and fuel cell-supercapacitor energy storage system (FC-SCESS), incorporated in a grid-independent photovoltaic (PV) based power system for a residential building is performed. Firstly, load for the residential building is calculated and accordingly optimum sizing of various constituents of the PV system is obtained using HOMER. Subsequently, the economic analysis for both configurations is performed in terms of cost of energy, net present cost, initial cost, and operating and maintenance cost. The simulation results of HOMER suggest that for grid-independent PV based power system, FC-SCESS configuration is better from the economic point of view. As HOMER’s inability to perform detailed technical analysis, MATLAB/Simulink models of both configurations are built and comparative analysis is performed in terms of different technical aspects such as active power balance, DC link voltage (VDC) regulation etc. From MATLAB results, it is observed that FC-SCESS configuration performs better by regulating the VDC more effectively and thereby helps maintain the active power balance between various constituents of the power system. Furthermore, feasibility of hardware implementation of each configuration is verified using hardware-in-loop real-time digital simulator.

Residential Community Load Management Based on Optimal Design of Standalone HRES With Model Predictive Control

Microgrids being an important entity in the distribution system, and to get their full advantages by incorporating maximum distributed generation, standalone hybrid renewable energy systems (HRESs), being environmentally-safe and economically-efficient, are considered as the promising solution to electrify remote areas where the grid power is not available. In this work, a techno-economic investigation with an optimal design of HRES is presented to fulfill the domestic electricity need for a residential area of the Sherani district in the Province of Baluchistan, Pakistan. Nine case studies based on PV/wind/diesel/battery are analyzed based on net present cost (NPC), cost of energy (COE), and emission to decide the feasible solution. HOMER tool is utilized to accomplish modeling and simulation for economic analysis and optimal sizing. Simulation results demonstrated that HRES with PV-wind-battery is the most viable option for the specified area, and the optimal sizing of components are also obtained with $ 28,620 NPC and $ 0.311/kWh COE which shows 81.65 % reduction in cost and 100 % preserving in toxic emission while fulfilling 100 % energy demand with 67.3 % of excess energy. Furthermore, MATLAB/Simulink modeling for the optimally designed system is built for technical analysis while its effectiveness is proved by keeping dc and ac buses voltage constant, safe operating range of battery state of charge (SOC) with active power balance between HRES components, as well as efficient ac voltage quality, regardless of generation disturbances and load fluctuations. The output signal has total harmonic distortion (THD) of 0.30 % as compared to 5.44 % with the conventional control scheme. The novelty lies in the sequential application of both HOMER and MATLAB simulations of the proposed HRES model and validation of the proposition for the studied area; by using and implementing model predictive control (MPC) of a reconfigurable inverter.

Algorithm for optimization of power system short-term mode in conditions of partial uncertainty of initial information taking into account the frequency change

Many existing methods and algorithms for optimization of short-term modes of power systems provide in calculations the introduction of a slack bus station, which ensures the balance of active power and, accordingly, the permissible frequency. In cases where the real load deviates from the planned one, determined by forecasting, the power system mode may turn out to be not optimal, and sometimes even not acceptable. This factor is especially noticeable in conditions of partial uncertainty of initial information about loads of nodes. To overcome this problem, planning of power system mode should be carried out taking into account the frequency change and, accordingly, the regulatory capabilities of all stations. This paper proposes an algorithm for optimization of modes of power systems in terms of partial uncertainty of initial information about loads, taking into account the frequency change. On the basis of computational experiments using the proposed algorithm, it is shown that taking into account the frequency change when planning modes of power systems with partially undefined loads of nodes can give a significant economic effect.

Dispatching of a Wind Farm Incorporated With Dual-Battery Energy Storage System Using Model Predictive Control

The battery energy storage system (BESS) integrated with a wind farm is an efficient way to smooth wind power fluctuations and improve wind farm dispatchability. The presented study proposed a model predictive control (MPC)-based power dispatch strategy for a wind farm incorporated with dual-battery energy storage system (DBESS). The state space model of the active power balance of the wind farm incorporated with DBESS was established to analyze the operation mode of the DBESS. The MPC was then applied to acquire the control inputs of the two batteries, thereby enabling both batteries to respectively be in a state of charging and discharging to track the alternating dispatch order. Actual wind power data was applied in the simulation, wherein the results indicate that the proposed MPC-based dispatching strategy of the DBESS-incorporated wind farm effectively tracks the dispatching order using the output of the wind/battery storage hybrid system, thereby improve the dispatchability of the wind farm while simultaneously reducing battery switching times and extending the lifetime of battery.

Guest Editorial: Medpower 2018 Selected Papers

Guest Editorial: Medpower 2018 Selected Papers

Enhanced Active Power Balancing Capability of Grid-Connected Solar PV Fed Cascaded H-Bridge Converter

Cascaded H-bridge converter is a promising solution for the next generation large-scale photovoltaic (PV) system having both active and reactive power exchange capacity. However, one challenge with this converter is unbalanced power generation among the clusters or cells due to variation in radiation level, temperature, dusting on PV cells etc. In order to inject balanced power to the grid, in this paper, a controlled exchange of reactive power is proposed so that the converter can work with unbalanced power generation within the clusters. The magnitude of reactive power exchange is derived and necessary conditions to prevent overmodulation are discussed. In addition, the magnitude of zero voltage is derived taking into account both active and reactive power exchange with the grid. By exchanging reactive power, the converter can also be used during low-voltage ride-through condition. This paper introduces a novel representation of unbalanced power generation using the barycentric coordinate system which helps to identify regions in the power plane which require the exchange of reactive power. The proposed method is verified through computer simulation and in a laboratory prototype.

Optimal static and dynamic transmission network expansion planning

In this paper, a novel approach is proposed to solve the problem of static and dynamic Transmission expansion planning (TEP) in electrical power systems. The proposed method is the execution of an Adaptive salp swarm optimization algorithm (ASSO). The ASSO is a meta-heuristic algorithm, which depends on the swarming behavior and population of salps. In the proposed approach, ASSO is the charge of acquiring the best candidate solution for the expansion planning and locate the optimal solution for both the static and the dynamic transmission network expansion in perspective of the minimum error objective function. The objective is formulated to minimize the cost of investment in transmission systems by satisfying the constraints such as active power balance, expansion parameter, and load shedding restrictions. The proposed ASSO technique ensures the system with less complexity, reduced computational time and hence the efficiency of the system is raised. Finally, the proposed model is executed in MATLAB/Simulink working stage and the execution is compared with the existing techniques.

Cold load pickup model parameters based on measurements in distribution systems

The electric power demand after an outage can be significantly different from normal values. A major cause of this is the loss of statistic diversity in load devices that interact with some kind of storage. Examples are heating and cooling appliances as well as some applications of pumps and devices with integrated batteries. This phenomenon is known as cold load pickup (CLPU). Changes in load after an outage are of high relevance for investigations of power system restoration, especially active power control and balancing of generation and load. Besides the very fast inrush effects relevant for fast acting protection and short-term stability, long-term dynamics, thermal overload of equipment and active power balancing in the presence of uncontrolled distributed generation consider a time frame of minutes to hours. To predict CLPU from physical models, a lot of knowledge is required. Since this information is rarely available in the real-world applications, the authors investigate typical ranges of CLPU behaviour. The authors analyse measurement data from reconnections after real outages of medium voltage feeders in Germany and provide parameters for the exponential decay model of CLPU. The relevance of the application of the presented model for long-term stability studies is demonstrated by an example.

Modeling and Investigation of Self-Excited Reluctance Generators for Wind Applications

In the last few years, wind energy conversion systems have been increasingly installed worldwide. The self-excited reluctance generators (SERGs) are found to be potential candidates as isolated wind turbine generators, especially for low- and medium-power applications. This article focuses on the characteristics of SERG for wind turbine generation system. A simplified dynamic model of the generation system is developed, discretized by backward Euler formula, and simulated. With this model, the process of self-excitation and operation characteristic of SERG are investigated. Considering the varying generated voltage at different speeds, variable capacitances allow suitable regulation of the voltage. The variation of capacitance with speed is obtained by the active and reactive power (PQ) balances method. Combining the power speed characteristics of wind turbine and SERG, the PQ balances method is also used to determine the “optimal capacitor and resistor combination” that achieves the highest usage of wind power. This article exhibits the performance of SERG for wind applications, and most importantly, makes the prediction of the highest energy conversion efficiency of the system.

Adaptive Control of Voltage Source Converter Based Scheme for Power Quality Improved Grid-Interactive Solar PV–Battery System

This article presents an implementation of an adaptive learning based backpropagation (AL-BP) control technique for an improved power quality grid-interactive solar photovoltaic (PV)–battery supported system catering the balanced and unbalanced three-phase four-wire (3P-4W) nonlinear loads and single-phase loads of various nature simultaneously. Moreover, the source currents remain balanced and sinusoidal for all operating and loading conditions, such as in highly unbalanced loading conditions where a load on one of the phases is disconnected. An AL-BP control technique is used to control the voltage at point of common coupling and to improve power quality under various loading conditions through mitigation of harmonic currents, reactive power compensation, active power balancing, and neutral current compensation in the 3P-4W system. It also improves the system power factor in highly nonlinear and unbalanced loading conditions. The AL-BP control scheme has the capability to self-tune the arbitrary nonlinear systems, such as a grid-interactive critically unbalanced 3P-4W system feeding highly nonlinear loading systems. Therefore, the proposed control technique is found capable of dealing with the above-mentioned large unknown nonlinearity of the 3P-4W system. This scheme significantly improves the steady-state and dynamic performances of the 3P-4W grid-interactive solar PV–battery supported system. The proposed AL-BP control technique has been implemented using a DSP processor in real time in order to validate the claims.

Active and Reactive Power Joint Balancing for Analyzing Short-Term Voltage Instability Caused by Induction Motor

Short-term voltage instability has a sensational effect once it occurs with massive loss of load, possibly area instability, and voltage collapse. This paper analyzes the short-term voltage instability caused by induction motor from the viewpoint of active and reactive power joint balancing. The analysis method is based on (1) the reactive power balancing between system supply and induction motor demand, and (2) the active power balancing between air-gap power and mechanical power, which is expressed by the region of rotor acceleration and deceleration in the Q-V plane. With the region of rotor acceleration and deceleration in the Q-V plane and the reactive power balancing, the movement direction of the operating point can be visually observed in the Q-V plane, thereby achieving a clear comprehension of physical properties behind the short-term voltage instability phenomenon. Furthermore, the instability mechanisms of two kinds of grid-connected induction motor operation conditions after a large disturbance are discussed to explain the basic theory of the analysis method and to provide examples of its application. Time-domain simulations are presented for a single-load infinite-bus system to validate the analyses.

Steady-state analysis of DC converter using Galerkin’s method

PurposeThe purpose of this paper is to present the Galerkin method for analysis of steady-state processes in periodically time-varying circuits.Design/methodology/approachA converter circuit working on a time-varying load is often controlled by different signals. In the case of incommensurable frequencies, one can find a steady-state process only via calculation of a transient process. As the obtained results will not be periodical, one must repeat this procedure to calculate the steady-state process on a different time interval. The proposed methodology is based on the expansion of ordinary differential equations with one time variable into a domain of two independent variables of time. In this case, the steady-state process will be periodical. This process is calculated by the use of the Galerkin method with bases and weight functions in the form of the double Fourier series.FindingsExpansion of differential equations and use of the Galerkin method enable discovery of the steady-state processes in converter circuits. Steady-state processes in the circuits of buck and boost converters are calculated and results are compared with numerical and generalized state-space averaging methods.Originality/valueThe Galerkin method is used to find a steady-state process in a converter circuit with a time-varying load. Processes in such a load depend on two incommensurable signals. The state-space averaging method is generalized for extended differential equations. A balance of active power for extended equations is shown.

Control of a new stand-alone wind turbine-based variable speed permanent magnet synchronous generator using quasi-Z-source inverter

In this paper a new variable speed permanent magnet synchronous generator (PMSG)-based stand-alone wind energy conversion system (SWECS) is proposed. The interface between the PMSG and the isolated load is accomplished by a quasi-Z-source inverter (qZSI) with battery storage system. The battery-assisted qZSI can balance the stochastic fluctuations of the wind power injected to the load and improve the voltage and frequency control. In addition to the battery storage, a dump load is used in the proposed SWECS to better maintain the active power balance and stability of the dc-link voltage during over-generation condition as well as sudden load changes. The proposed control system is able to provide an uninterrupted and reliable supply to sensitive loads under various power generation scenarios of the SWECS and sudden changes in the load demand. Moreover, the proposed controller provides maximum power point tracking (MPPT) which is essential for optimum operation of the SWECS. The validity of the proposed system is proved by simulation results carried out using MATLAB/SIMULINK.