Reactive Power Requirements Research Articles

Nonlinear Control of Multilevel Inverter for Grid Connected Photovoltaic System with Power Quality Improvement

The problem of interfacing a PV Generator (PVG) with a three phase grid is addressed in the presence of nonlinear loads. The connection of the PVG to the grid is performed in two stages with both a DC/DC converter and a multilevel inverter. This latter is based on a Neutral Point Converter (NPC) topology operating simultaneously as an interfacing system and as an active power filter. The main objectives of the proposed control system are threefold: (i) transferring the extracted power from PVG to the grid; (ii) cancelling the load current harmonics and compensating the reactive power requirement of non-linear loads; (iii) balancing capacitor voltages and regulating the DC bus voltage. To this end, two nonlinear closed loop controls are developed on the basis of the averaged PV system model. The control system includes an AC current controller designed by the Backstepping control technique, and a DC bus voltage controller which is designed by the sliding mode control technique. The formal analysis of the control system performances is carried out on the basis of Lyapunov's stability and Averaging Theory. Simulations show that the proposed control system achieved the listed objectives under a widely varying load and solar irradiance profile.

Definition and on-field validation of a microgrid energy management system to manage load and renewables uncertainties and system operator requirements

The present paper proposes an Energy Management System (EMS) to be used in grid connected microgrids. To do this, first a suitable model (for optimization purposes) of all the components that typically appear in a microgrid is presented, then, four possible electric network models are detailed and finally the overall architecture of the optimization problem of the EMS is set up. Moreover, as the optimization of the energy production/consumption of a microgrid relies on the thermal and electric load and on the renewables forecasting, an online empirical correction of forecasted data is proposed, highlighting its positive impact on the overall operational cost of the microgrid. Another aspect which is accounted regards the possibility of allowing the EMS to act as a power plant controller in compliance with the Distribution System Operator (DSO) requirements in terms of reactive power management and voltage control (as requested by the majority of grid codes and national regulations). So, the proposed algorithm structure splits the optimization problem into two sub problems: the first one basically dictates the active power production of the dispatchable units minimizing an economic objective function, while the second accounts for the satisfaction of the DSO requirements. The experimental validation of the proposed EMS is performed on the University of Genoa Smart Polygeneration Microgrid (SPM), where the proposed EMS is currently running. The considered test cases highlight the effectiveness of the proposed EMS in economically operating the microgrid (compared to simpler EMS previously installed in the SPM) and in satisfying the reactive power or voltage regulation requirements provided by the Italian technical requirements.

Wind turbine manufacturers observation regarding reactive power support and control requirements

With the increase of power generated by wind power plants, network operators have placed steady-state and dynamic reactive power requirements at their grid connection point in several EU member states. This has forced wind turbine manufacturers to invest in upgrade of their technology and power plant owners and developers to invest in external compensation devices, adding to overall project costs. With the introduction of EU regulation Network Code Requirements for Generators (NC RfG) in 2016, it is expected that many countries (national implementation) will implement higher than currently required reactive power requirements accordingly to the new EU framework. The purpose of this study is to discuss the reactive power requirements and to investigate the reactive power support actually provided by wind power plants to date. SCADA data from a number of operational sites are analysed with regards to their actual reactive production utilising different control modes and to compare the given reactive power support with the requirements defined by the limits within the NC RfG. At the end of this study, a derived proposal for steady-state reactive power needs and dynamic power needs are discussed, therefore a minimum technical reactive power provision required is derived based on the current utilisation.

EFFECT of DG Optimizing on Overload Transmission Line Stability

Background: Typical steady state studies always treat the peak power demands as the worst case conditions. Periods of light load area also critical in the assessment of the possible state of a power system. While heavy load conditions are generally associated with overload, low voltage and generation deficiency, light load conditions may give rise to over-voltage and undesirable reactive power requirements at generation side. Method: This paper focus on study the effect of DG Optimizing on overload Transmission line Stability. The system dispatch constraints should be taken into account to compensate for varying DG generation output and to enhance the operational performance of power systems. Findings: This dispatching operation depends on the change of DG generation and the dispatching strategy. The impact of DG generation uncertainty is limited with the generation dispatching operation and should not be neglected in system analysis. Application: distributed generators based wind turbine to investigate the effect of DG on Transmission line stability.

Dynamic Power Management of Autonomous Wind Diesel Hybrid System Using Flywheel Storage During Transition from Wind-Only Mode of Operation

The paper investigates the active and reactive power flow in an autonomous wind diesel hybrid power system working with flywheel energy storage system (FESS)in the post high wind mode of operation. The power system operating mode transition from a stage involving only wind generator for meeting the power generation requirements to one which needs both wind generator and diesel generator (DG) for the same is met using a permanent magnet synchronous motor(PMSM) based flywheel energy storage system. This involves the stage where the DG changes from stalled stage to generation stage and needs a proper mechanism to meet the transient stage for meeting the active and reactive power requirements. This paper investigates the application of FESS for meeting the above stated transient stage including the analysis of active and reactive power flow, both from the source and to the load /sink. The wind generator described in the paper is of asynchronous type and the diesel generator is a synchronous salient pole machine. The mathematical analyses of power generating modules are discussed and the system power balance is validated using Matlab /Simulink environment.

An evaluation of options to mitigate voltage rise due to increasing PV penetration in distribution networks

Australia and most other countries are adopting renewable energy generation as the dominant means of reducing dependence on fossil fuels. This has been made more feasible by the exponential take-up of solar photovoltaic (PV) systems and their concurrent production scale-up and cost decline. Rooftop solar PV, combined with battery storage, seems likely to be the dominant means of providing household electricity needs. In response to the technical challenges from rooftop PV, network utilities have implemented various low cost options to cope with PV’s impact on network voltages. However, if we want this clean energy technology to fully utilise the available roof space and eventually meet residential electricity needs, additional hardware, control and commercial options will need to be adopted by both network utilities and their customers to overcome the technical barriers, especially voltage rise. This paper presents the authors’ evaluations of options to mitigate voltage rise, including operating solar inverters with reactive power absorption (var absorbing), dependent only on solar power output or operating the solar inverters in a volt–var response mode (voltage droop control) where the inverter adjusts its reactive power (Q ) in response to changes in its terminal voltage – Q (V ). This paper also considers the fulltime Q (V ) option, where an inverter’s reactive power capacity is independent of solar conditions – statcom mode. The network utility option of using line drop compensation (LDC – used on long rural MV feeders) on urban MV feeders during daylight hours is assessed to lessen voltage rise on LV feeders with low net loading or reverse power flow due to high solar PV generation. The paper concludes that a combination of solar inverters performing fast fulltime voltage droop control outside a voltage deadband (statcom mode) and HV/MV substation transformers with slow acting daytime LDC mitigates voltage rise, whilst limiting feeder reactive power requirements.

Multi Level Inverter Based STATCOM for Grid Connected Wind Energy Conversion System

The sustainable energy resources, like wind energy, for electrical energy generation is augmented due to environmental problems and the scarcity of conventional energy sources, leading to integration of large number of wind generators in to grid. Integration of large scale of wind generators in to grid presents challenges such as voltage stability, reactive power management, frequency control, grid stability and power quality. In this proposed scheme, Multi Level Inverter based Static Compensator with a battery energy storage system employs Hysteresis Current Controller to diminish the effects of power quality issues. The proposed scheme for the grid connected wind energy conversion system is simulated using MATLAB/SIMULINK. The efficacy of the proposed control scheme is, it takes care of the reactive power requirement of the load and the induction generator, thus improving the source side power factor and also there will be a discernible reduction in the Total Harmonic Distortion. Using Multi Level inverter based STATCOM, lower source current distortions and lower switching frequency shall be obtained, when compared to conventional two level inverter based STATCOM.

Particle Swarm Optimization Application for Optimal Location of Multiple Distributed Generators in Power Distribution Network

Distributed generators (DGs) play a vital role in present power distribution networks. The integration of distributed generators in distribution systems require optimal placement and sizing of distributed generators to yield minimum power losses and improved voltage profile. Often single DG placement may not be sufficient for power distribution system and multiple DGs may be required to be integrated to power distribution network. Concerning this, optimal location of multiple DGs in power distribution systems is very important. This paper presents a Particle Swarm Optimization (PSO) based algorithm for optimal location of multiple DGs into the power distribution network for power loss minimization. The proposed algorithm has two major steps; first step is the finding of optimal location and active power injections of multiple DGs and later is the computation of optimal reactive power injection of DGs. A bio-inspired particle swarm optimization algorithm is used in the first step to locate multiple DGs optimally and to obtain optimal active power injections of DGs. In the second step, reactive power injections of DGs are selected based on the reactive power requirement of the area fed by the DG. The proposed method is successfully tested on IEEE 13 bus system and its performance has been benchmarked with the Improved Analytical method.

ANFIS-PI Controller based Coordinated Control Scheme of Variable Speed PMSG based WECS to Improve LVRT Capability of Wind Farm Comprising Fixed Speed SCIG based WECS

Objective: This paper proposes a novel ANFIS-PI controller based coordinated control scheme of VS-PMSG based WECS, located in close proximity of FS-SCIG based WECS; for improving LVRT capability of FSIG based WF. Methods/Statistical Analysis: Conventional PI controller based coordinated control scheme is simple and gives a good performance. But, with changing parameters of the grid, especially at the time of grid disturbance; the conventional PI controller cannot effectively control the system. To perform this online returning of parameters, a novel ANFIS-PI controller based coordinated control scheme is proposed here; which dynamically changes the controller parameters in accordance with the change in power system impedance during grid disturbance. Findings: MATLAB simulations are performed to check the effectiveness of the proposed ANFIS-PI controller based coordinated control scheme on a typical arrangement of two MW-size WFs connected to an infinite bus. It has been observed that at the time of grid disturbance, the reactive power requirement of the FSSCIG based WECS is effectively controlled by the ANFIS-PI controller based coordinated control scheme of VS-PMSG based WECS. Simulation results exhibit the improvement in LVRT capability of the whole wind farm, comprising VS-PMSG based WECS with FS-SCIG based WECS. Results of the ANFIS-PI controller based coordinated control scheme are compared with the conventional PI controller based coordinated control scheme. Wherein, results with ANFIS-PI controller are proven better than the results with conventional PI controller. The proposed approach for improving LVRT capability of FS-SCIG based WECS seems to be a cost effective, as it need not have any additional installation of FACTS devices. Application/ Improvements: The largely installed FS-SCIG based WECS could be made LVRT capable by the proposed method. The prototype model of the proposed simulation work is the future scope of research.

Deadband Control of Doubly-Fed Induction Generator Around Synchronous Speed

Semiconductor devices in power electronic converters of the doubly-fed induction generator (DFIG) are susceptible to significant junction temperature variations when operating around synchronous speed, thereby reducing the lifetime of the converters. This is due to the fact that the frequency of the rotor current in a DFIG is determined by the stator flux frequency and rotor speed, and hence will lead to low rotor current frequency when operating closer to the synchronous speed, and ultimately result in significant thermal stress on semiconductor devices. In this paper, a multimode operation control strategy is proposed for the DFIG to prevent operating around the synchronous speed (within a predefined deadband); thus, the proposed control strategy can avoid the thermal stress problem. The proposed strategy engages the existing crowbar scheme for DFIG-based wind energy conversion system to intentionally alter the operating mode of the generator between DFIG and induction generator (IG). Smooth transition between the two operating modes can be achieved with the supplementary control strategies. Unity power factor can also be maintained in both operating modes by using the grid side converter as a static synchronous compensator (STATCOM) to fulfill the reactive power requirement of the DFIG in IG mode.

Guaranteed Performance Control of DFIG Variable-Speed Wind Turbines

This brief presents a novel power control strategy for variable-speed wind turbines equipped with doubly fed induction generators (DFIGs). The control objective is to optimize the extracted power from wind while regulating the stator reactive power to meet grid requirements. First, in order to optimize the extracted power, an adaptive control technique is designed to drive the electromagnetic torque to follow its reference generated by the maximum power point tracking algorithm. Subsequently, aiming at satisfying reactive power requirements on the grid side, an adaptive reactive power controller is proposed to manipulate the stator reactive power to follow a given desired reactive power determined by the grid. Compared with most existing studies, we are capable of quantifying and further guaranteeing the system performance on both transient and steady-state stages. All signals in the closed-loop system are proved to be bounded via standard Lyapunov synthesis. Finally, the effectiveness of the proposed scheme is validated on a 1.5-MW DFIG-based wind turbine using the FAST (Fatigue, Aerodynamics, Structures, and Turbulence) simulator developed by the National Renewable Energy Laboratory.

Three‐phase grid‐tied single‐stage solar energy conversion system using LLMS control algorithm

In this study, a three-phase grid-tied single-stage solar energy conversion system (SECS) is implemented using a leaky least mean square (LLMS) based control algorithm. The SECS extracts the solar energy from the photovoltaic cells and converts it into electrical energy. The SECS makes use of voltage source inverter to invert the extracted DC power into the three-phase AC power. Apart from SECS, this system configures a three-phase grid, ripple filters and connected linear or non-linear loads. The main objective of this system is to eliminate power quality issues such as harmonics, voltage fluctuations and flickers, unbalancing in loads, reactive power requirement and so on. Moreover, this system operates at unity power factor and zero voltage regulation modes. In this study, SECS employs a single-stage topology controlled by perturb & observe approach of maximum power point tracking technique and for extracting fundamental component of load currents, a LLMS-based adaptive neural network control method is used. This system is modelled, designed and simulated in MATLAB using simpower system toolbox and experimental validation is carried out on a developed prototype in the laboratory.

A smart distribution system reconfiguration algorithm with optimal active power scheduling considering various types of distributed generators

This paper presents an efficient way of solving the distribution system reconfiguration (DSR) problem in electrical power systems with consideration of different types of distributed generators (DGs). The objective of a DSR is to minimize the system power loss while satisfying the system constraints and keeping the topology of the system radial. In this paper, a new DSR algorithm based on a modified particle swarm optimization (PSO) is proposed to incorporate DGs with the constant voltage control mode. The proposed method is very efficient because it avoids an extra iteration loop for computing the reactive power at PV buses in order to keep the voltage at a specified magnitude. Furthermore, if the reactive power requirement is not met in between the extreme limits, the proposed algorithm strictly searches for the best possible tie switch combination to simultaneously reduce the power loss and ensure that the DGs operate in PV mode within acceptable reactive power limit. The proposed algorithm also integrates hourly DSR with optimal DG active power scheduling considering the DG type, generation limit constraints, and the allowable DG penetration level. The validity and the effectiveness of the proposed method has been tested using standard IEEE 33‐bus and 69‐bus distribution networks with various case studies. Test results show that the proposed method is robust and delivers a minimal average power loss compared with different methods, and it efficiently models DGs in DSR, demonstrating that the presence of DGs can further reduce the system loss. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Reduced-rating Hybrid Active Power Filter Comprising Zero-sequence Transformer and Three-phase Three-wire Active Power Filter for Three-phase Four-wire Distribution System

A three-phase four-wire active power filter is proposed in this article that consists of a three-phase three-wire active power filter, zero-sequence transformer, and single-phase active power filter. The reference current for tracking the desired source current is based on the neural network (constraint anti-Hebbian rule algorithm). The three-phase three-wire active power filter compensates current harmonics and reactive power requirement of the non-linear load. A zero-sequence transformer and single-phase active power filter are used to mitigate the neutral current. The proposed hybrid scheme (three-phase four-wire active power filter) significantly reduces the volt–ampere rating of the active power filter and improves the performance under different utility voltage conditions. The simulation of the proposed three-phase four-wire active active power filter is carried out for different voltage conditions. Further, the MATLAB-simulated (The MathWorks, Natick, Massachusetts, USA) response of the three-phase four-wire active active power filter has been validated on a prototype of the three-phase four-wire active-active power filter. The experimental result obtained from the prototype verifies the effectiveness and viability of the proposed scheme.

Enhancement of transient stability of distribution system with SCIG and DFIG based wind farms using STATCOM

This study proposes optimal rating and location of static synchronous compensator (STATCOM) for enhancing transient voltage stability of a real distribution network with wind power generation. The test network consists of fixed and variable speed wind turbines connected to a rural load centre. To ensure reliable and secure operation of the grid, the wind farm (WF) has to comply with the grid code for its low-voltage ride through operation. Loads play significant role in voltage stability analysis. Induction motors and general loads are modelled according to real-time data available with the system operator. In this study, dynamic reactive power requirement of WFs is considered for determination of rating of the STATCOM. Reactive power margin (Q margin) of load bus is used as an index to identify the location of the STATCOM. Extensive simulation has been carried out in DIgSILENT to show the validity of the proposed approach.

Oil drilling rig diesel power-plant fuel efficiency improvement potentials through rule-based generator scheduling and utilization of battery energy storage system

This paper presents the development of a rule-based energy management control strategy suitable for isolated diesel power-plants equipped with a battery energy storage system for peak load shaving. The proposed control strategy includes the generator scheduling strategy and peak load leveling scheme based on current microgrid active and reactive power requirements. In order to investigate the potentials for fuel expenditure reduction, 30days-worth of microgrid power flow data has been collected on an isolated land-based oil drilling rig powered by a diesel generator power-plant, characterized by highly-variable active and reactive load profiles due to intermittent engagements and disengagements of high-power electric machinery such as top-drive, draw-works and mud-pump motors. The analysis has indicated that by avoiding the low-power operation of individual generators and by providing the peak power requirements (peak shaving) from a dedicated energy storage system, the power-plant fuel efficiency may be notably improved. An averaged power flow simulation model has been built, comprising the proposed rule-based power flow control strategy and the averaged model of a suitably sized battery energy storage system equipped with grid-tied power converter and state-of-charge control system. The effectiveness of the proposed rule-based strategy has been evaluated by means of computer simulation analysis based on drilling rig microgrid active and reactive power data recorded during the 30day period. The analysis has indicated that fuel consumption of thus modified drilling rig diesel generator power-plant can be reduced by about 12% compared to current practice in the field relying on human operator-based decision making, which would also result in proportional reduction of carbon-dioxide emissions. Finally, the analysis has also shown that the return-of-investment period for the considered battery energy storage system might be between one and two years depending on the power-plant utilization (duty) ratio.

Multiobjective optimization in combinatorial wind farms system integration and resistive SFCL using analytical hierarchy process

This paper presents a positive approach for low voltage ride-through (LVRT) improvement of the permanent magnet synchronous generator (PMSG) based on a large wind power plant (WPP) of 50 MW. The proposed method utilizes the conventional current control strategy to provide a reactive power requirement and retain the active power production during and after the fault for the grid codes compliance. Besides that, a resistive superconducting fault current limiter (RSFCL) as an additional self-healing support is applied outside the WPP to further increase the rated active power of the installation, thereby enhance the dc-link voltage smoothness, as well as the LVRT capability of the 50 MW WPP. This is achieved by limiting the exceed fault current and diminishing the voltage dip level, leading to increase the voltage safety margin of the LVRT curve. Furthermore, the effect of the installed RSFCL on the extreme load reduction is effectively demonstrated. A large WPP has a complicated structure using several components, and the inclusion of RSFCL composes this layout more problematic for optimal performance of the system. Hence, the most-widely decision-making technique based on the analytic hierarchy process (AHP) is proposed for the optimal design of the combinatorial RSFCL and 50 MW WPP to compute the three-dimensional alignment in Pareto front at the end of the optimization run. The numerical simulations verify effectiveness of the proposed approach, using the Pareto optimality concept.

Coordinated Volt/Var Control in Distribution Systems with Distributed Generations Based on Joint Active and Reactive Powers Dispatch

One of the most significant control schemes in optimal operation of distribution networks is Volt/Var control (VVC). Owing to the radial structure of distribution systems and distribution lines with a small X/R ratio, the active power scheduling affects the VVC issue. A Distribution System Operator (DSO) procures its active and reactive power requirements from Distributed Generations (DGs) along with the wholesale electricity market. This paper proposes a new operational scheduling method based on a joint day-ahead active/reactive power market at the distribution level. To this end, based on the capability curve, a generic reactive power cost model for DGs is developed. The joint active/reactive power dispatch model presented in this paper motivates DGs to actively participate not only in the energy markets, but also in the VVC scheme through a competitive market. The proposed method which will be performed in an offline manner aims to optimally determine (i) the scheduled active and reactive power values of generation units; (ii) reactive power values of switched capacitor banks; and (iii) tap positions of transformers for the next day. The joint active/reactive power dispatch model for daily VVC is modeled in GAMS and solved with the DICOPT solver. Finally, the plausibility of the proposed scheduling framework is examined on a typical 22-bus distribution test network over a 24-h period.

A Novel Equivalent Capacitance Model of DFIG to Study its Reactive Power Control Capabilities and its ability to Stabilize SEIG

The penetration of wind generation in modern day energy system is increasing day by day. Wind is a variable parameter in nature which has a significant effect on the generator behavior. Since most of the existing installed generators are self-excited induction generators (SEIG), they have a negative effect on the system during varying wind speeds and varying load. The primary parameter found responsible is reactive power requirement for self-excitation. So, instead of replacing these generator sets, an alternative reactive power source connected in coordination to machine can well handle these situations. In this context a new promising method of stabilizing SEIG with doubly fed generator (DFIG) is developed in this paper. A new technique of representing DFIG in terms of its equivalent capacitance is developed, to study reactive its power handling capabilities. The potential of the developed model in stabilizing the SEIG during varying wind speeds and varying load conditions is simulated and analyzed. The developed model is independent of complex d-q axis model and simple to understand the behavior of machine. The result shows that this method of supplying reactive power requirement of SEIG is satisfactory in maintaining voltage build up of SEIG. The results obtained are well validated by power balance.

Integration of Induction Generator Based Distributed Generation in Power Distribution Networks Using a Discrete Particle Swarm Optimization Algorithm

An induction generator always absorbs lagging reactive power from the power network. In most previous power system optimization studies involving induction generator based distributed generation, this generator reactive power demand is calculated using an approximate empirical formula. In addition, shunt compensation capacitors have not been considered as part of the network optimization study. In this article, the use of the per phase equivalent circuit of the induction generator is proposed to compute its reactive power requirement, thus providing a more accurate estimation. A discrete particle swarm optimization algorithm is then employed to address the problem of simultaneous integration of the induction generators and shunt compensation capacitors. The proposed algorithm has the advantage of being able to cope with a mixed search space optimization problem of integer, discrete, and continuous variables. The study is carried out on a standard 69-bus benchmark distribution network. Results show that the use of the approximate empirical formula leads to an underestimation of the machine reactive power demand. The inclusion of shunt compensation capacitors in the optimization process results in lower network reactive power flows with improved network power losses, improved voltage profile, and increased levels of distributed generation integration.