Active Power Injection Research Articles

A simple dead-time compensation strategy for grid-connected voltage-sourced converters semiconductor switches

This paper presents a simple strategy to compensate the distorted currents synthesized by a grid-connected voltage source converter due to dead-time, turn-on and turn-off time delays of the semiconductor switches. The algorithm consider only the polarity of the fundamental component of the currents flowing through the converter terminals and the values of the time delays and voltage drops supplied by the manufacturers to the semiconductors devices. The presented compensation belongs to the group classified as average value compensation technique methodology since it does not change the pulse pattern of the converter's semiconductor switches. A simplified mathematical description of the effects caused by these unwanted time delays is presented and used to derive a correction factor to be added, in real time, to the converter output controller in order to compensate for its terminal voltages. The asymptotic stability and robustness of the proposed methodology is investigated redrawing the converter current controllers, designed in dq-reference frame, as proportional-resonant ones, in the abc coordinates, and adding the effect of the compensating signal in the feedback loop using the concept of describing function. In addition, the minimum value of the DC bus voltage necessary is also evaluated to ensure the operation of the converter in the linear modulation region when the compensation algorithm is active. Experimental results are presented to validate the theoretical analysis and to demonstrate the effectiveness of the proposed strategy for three different operation conditions of a grid-connected converter: (i) active power injection; (ii) active power consumption and (iii) reactive power support.

Active and Reactive Power Injection Strategies for Three-Phase Four-Wire Inverters During Symmetrical/Asymmetrical Voltage Sags

Electric grid codes are expected to change in near future to accommodate an increased number of distributed generation units in the distribution power system without impairing its power quality. It is desired that the generators remain connected during voltage sags and provide ancillary services, such as voltage and reactive power control, ensuring the operational stability of the power system. This paper explores how the existing strategies for active and reactive power injection impact the operation of grid-tied inverters in terms of required power, current flowing, and reduction of active power delivery during the voltage sags. Such inputs are relevant to properly size converters for operation under fault events. In addition, this paper contributes to devise: 1) constant peak current control, 2) constant active current control, and 3) constant average active power control strategies for three-phase four-wire grid-tied inverters considering the natural (abc ) reference frame. The design and implementation of the investigated power injection strategies are discussed, and their effectiveness and technical viability are analyzed through dynamic computational simulations under symmetrical/asymmetrical voltage sags, and variation of the short-circuit ratio.

Dual‐mode control of utility interactive microgrid

This article presents dual-mode control of a single-stage utility interactive microgrid based on a photovoltaic array and battery energy storage with improved power quality. For protection reasons, grid tied solar inverters are mandatory to shut down at loss of the grid. However, proposed multitasking microgrid is developed to provide energy to critical loads, even at disappearance of grid. The microgrid is integrated the BES and a PV array at DC link of voltage source converter of it to provide the harvested energy to local loads. In islanding, the amplitude and waveform of load voltages are maintained sinusoidal by the voltage controller. The transfer of modes (islanded to grid connected and vice-versa) is implemented using the synchronisation controller as per the accessibility of the grid. The instantaneous characteristic-based adaptive filter is utilised to control the microgrid in the grid-integrated mode. Moreover, a PV feed-forward term is incorporated in current control for injection of active power to the utility as well as to improve the dynamic response of microgrid. The phase detection controller estimates phase, frequency of utility and load voltages accurately and capability of disturbance rejection is improved. The effectiveness of PV-BES-based microgrid is validated experimentally.

Static voltage stability improvement with battery energy storage considering optimal control of active and reactive power injection

Voltage stability analysis and improvement remain a major concern of power system operators due to the recurrent risk of voltage collapse. Many approaches have been used to analyze voltage stability but an approach that can directly indicate the closeness of power system to voltage collapse can be used to optimally plan for the improvement of the power system voltage stability condition when compensation devices are to be deployed. In this study, optimal active and reactive power compensation was performed on a continuously loaded power system, using the battery energy storage system (BESS). In order to achieve this, a voltage stability evaluation model which contains information concerning the active and reactive power flow along the transmission line was adopted. The BESS is considered as a combination of storage units and voltage source converter (VSC) in order to facilitate independent control of both the active and reactive injection to the grid. The optimal control of the output of the BESS minimizes its installed capacity and consequently, reduced the cost as shown by simulation on the IEEE 14 bus system.

Full-Observable Three-Phase State Estimation Algorithm Applied to Electric Distribution Grids

This paper proposes the development of a three-phase state estimation algorithm, which ensures complete observability for the electric network and a low investment cost for application in typical electric power distribution systems, which usually exhibit low levels of supervision facilities and measurement redundancy. Using the customers´ energy bills to calculate average demands, a three-phase load flow algorithm is run to generate pseudo-measurements of voltage magnitudes, active and reactive power injections, as well as current injections which are used to ensure the electrical network is full-observable, even with measurements available at only one point, the substation-feeder coupling point. The estimation process begins with a load flow solution for the customers´ average demand and uses an adjustment mechanism to track the real-time operating state to calculate the pseudo-measurements successively. Besides estimating the real-time operation state the proposed methodology also generates nontechnical losses estimation for each operation state. The effectiveness of the state estimation procedure is demonstrated by simulation results obtained for the IEEE 13-bus test network and for a real urban feeder.

Technical and non‐technical losses calculation in distribution grids using a defined equivalent operational impedance

This study presents an approach to calculate average technical losses (TLoss) and non-technical losses (NTLoss) in distribution grids using an equivalent operational impedance (EOI) calculated from a load flow solution for the transformers’ average loads, obtained from customers’ electric bills. If transformers are supervised, i.e. have available active and reactive power injections measurements or estimates from a state estimator program, the transformers’ average technical and non-technical losses can be calculated by running a load flow for the grid supplied by the transformer and using the definition of EOI presented in this study. For unsupervised transformers, i.e. no measurements or estimates are available, pseudo measurements of average active and reactive power injections can be obtained by a procedure that uses active and reactive power measurements at the feeder substation. The use of this methodology by electric utilities allows the calculation of technical and non-technical losses more accurately, as well as the inclusion of non-technical losses in the distribution networks operation planning. Results using the IEEE 13-bus test system and an urban real distribution grid are presented to demonstrate the effectiveness of the proposed methodology.

Scalable Single-Phase Multi-Functional Inverter for Integration of Rooftop Solar-PV to Low-Voltage Ideal and Weak Utility Grid

Integration of rooftop solar-PV (RTSPV) systems and extensive use of nonlinear loads in the low-voltage distribution system (LVDS) leads to poor power quality (PQ). Therefore, it is necessary to address the issues leading to poor PQ at the point of common coupling of the LVDS. In this article, a multi-band hysteresis current control (MB-HCC) for the multi-functional inverter (MFI) is proposed which improves the efficiency of the MFI and also enhances the PQ of the LVDS. The MB-HCC uses simple switching logic and outperforms in its multi-functional tasks such as active power injection and power conditioning. MB-HCC offers better efficiency over variable double-band HCC (VDB-HCC) as it operates at a lower switching frequency. The performance of the proposed system is simulated by using MATLAB/Simulink and validated by OPAL-RT based real-time simulation studies. During the variation of solar irradiation, the proposed MFI has an average efficiency of 98.5% under the ideal grid and 97.34% under the distorted grid. Moreover, the percentage of Total Harmonic Distortion under ideal and distorted grid conditions is brought down to below 5%, and also, reactive power compensation maintains unity power factor operation complying with the IEEE-519-2014 and 1547 standards. These results substantiate the hypothesis of scalability of the single-phase MB-HCC-based MFI for an LVDS contributing to economy and ecology.

VSC-MTDC Operation Adjustments for Damping Inter-Area Oscillations

Application of voltage source converter based multi-terminal high-voltage direct current (VSC-MTDC) grids operation adjustment for damping inter-area oscillations are addressed and analyzed in this paper. The new operation adjustment method is proposed to meet short-term solution requirements and accommodate critical operation conditions where the conventional damping stabilizer by using active power modulation cannot provide sufficient damping. Based on the sensitivities with respect to the active power injections of VSCs and the interior point algorithm, an operation optimization method is presented to obtain a proper VSC-MTDC operation scheme to achieve better damping performance for an ac/VSC-MTDC system. For some critical operation conditions in which an inter-area oscillation has been detected, an operation rescheduling method is proposed as an emergency oscillation control. Time-domain simulations are performed on a New England system with a five-terminal MTDC grid. A comparative study with a conventional damping stabilizer is also included in the case study. Both the theoretical analysis and the time-domain simulations have verified the effectiveness and superiority of the two proposed methods.

Multifunctional Hybrid Structure of SVC and Capacitive Grid-Connected Inverter (SVC//CGCI) for Active Power Injection and Nonactive Power Compensation

In this paper, the structure, coordinate control method, and parameter design of a hybrid system are proposed for active power injection and nonactive power (reactive, harmonic, and unbalance power) compensation. The proposed hybrid system consists of a static var compensator (SVC) in parallel with a capacitive-coupling grid-connected inverter (CGCI) (SVC//CGCI). In SVC//CGCI, the SVC part is used to dynamic compensate the reactive power and unbalance power, while the low rating CGCI part is used to inject active power, provide harmonic power and a fixed amount of capacitive reactive power. Compared with conventional inductive-coupling grid-connected inverter (IGCI), the CGCI can provide active, reactive and harmonic power with low rating of active inverter part. The cost of the SVC part is much lower than that of active inverter part, thus the reduction of inverter rating can lead to a decrease in the total cost of the SVC//CGCI. Therefore, the SVC//CGCI can be a cost effective solution for active power injection and nonactive power compensation. Finally, simulation and experimental results are provided to show the advantages and validity of the proposed SVC//CGCI in compared with the conventional IGCI and SVC//IGCI.

Coordination strategies of distributed energy resources including FESS, DEG, FC and WTG in load frequency control (LFC) scheme of hybrid isolated micro-grid

This paper presents a novel load frequency control (LFC) model for a stand-alone hybrid micro-grid in the presence of renewable energy resources. A perilous fact in operation of isolated micro-grid is to deal with a low-inertia system owing to the unpredictable structure and the intermittent fluctuation of RESs. In comparison to the conventional power system, the rate of change of frequency (RoCoF) is high in isolated micro-gird. Therefore, the need for fast frequency response provision delivered by existing distributed energy resources, which are inverter-connected technologies, arises. Some distributed energy resources (DERs) can be considered as potential reserves for active power injection in the load frequency control scheme. The simulation results depict that renewable energy resources like diesel engine generator (DEG), Fuel Cell (FC), Flywheel Energy Storage System (FESS) and Wind Turbine Generator (WTG) have the capability to improve frequency excursion during various operating conditions if comprehensive small-signal dynamic models for RESs are introduced in isolated micro-grid and proper contribution of them in load frequency control studies is considered.

Dynamic performance comparison involving grid‐connected PV systems operating with active power‐line conditioning and subjected to sudden solar irradiation changes

This study presents a comparative performance analysis involving single- and double-stage photovoltaic (PV) systems, which are connected to a three-phase four-wire electrical system by means of a four-leg inverter topology. The double-stage PV system is composed of a step-up DC/DC converter and a four-leg grid-connected inverter, while in the single-stage PV system the PV array is directly connected to the inverter DC bus. In addition to the active power injection into the grid, both PV systems have two attractive features, described as follows: (i) power quality improvement due to their capability to perform active power-line conditioning, such as load harmonic current suppression, reactive power compensation, and load unbalance compensation; (ii) dynamic improvement in the occurrence of abrupt solar irradiation variations, which is achieved through the implementation of a feed-forward control loop (FFCL) acting directly in the inverter DC-bus voltage control loop, resulting in speeding up of the dynamics of both the DC-bus voltage and the injected grid-inverter currents. The referred comparative analysis addresses the static and dynamic performances considering the PV systems operating under the action of the FFCL, as well as performing active power-line conditioning. Simulation and experimental results are presented to evaluate and validate the proposed study.

DSTATCOM with Improved LMS based IRP theory

In this paper, grid connected DSTATCOM with three phase three wire has been proposed for harmonics elimination, reactive power compensation and active power injection into the grid. An improved instantaneous reactive power theory based on the least mean square is computed. The least mean square (LMS) algorithm is combined with instantaneous reactive power theory (IRPT) to improve its dynamics performance and eliminates the use of low pass filter requirement for computation of reference current. The harmonics component of the active power is estimated from LMS algorithm generated load harmonics currents. The performance characteristic of DSTATCOM is estimated with control scheme using computer simulation. The technique is based on the instantaneous transformation of instantaneous signals and taking advantage of the subsequent calculation of power from supply side to the loads. The neural network based method to estimate the harmonic online, DSTATCOM can compensate the harmonics. The computer simulation is carried out with MATLAB Simulink power system block sets.

Centralized OPF in Unbalanced Multi-Phase Neutral Equipped Distribution Networks Hosting ZIP Loads

The Optimal Power Flow (OPF) model for low voltage active Distribution Networks (DNs), which are equipped with neutral conductors, requires an explicit representation of both phases and neutral conductors in its formulation to obtain complete information about the state variables related to these conductors. In this regard, a centralized OPF relaxation based on semi-definite programming is presented in this paper for neutral-equipped DNs hosting ZIP loads and neutral-ground impedance, and contain a significant level of unbalance. The major restriction in the development of an OPF model for these networks is the coupled power injection across the conductors which is successfully handled by deriving the explicit active and reactive power injections for each conductor through a network admittance matrix-based approach. The shortcomings of existing voltage magnitude-based technique for the modelling of ZIP loads are comprehensively reported and a novel complex voltage variable-based approach is proposed which successfully incorporates ZIP loads in the developed multi-phase OPF relaxation. For the handling of constant current load, a modelling approach based on the first-order-Taylor series is introduced as well. Furthermore, the impact of the application of Kron reduction approach on the global optimal solution of single- and multiple-point grounded DNs is discussed in detail. Three metrics, eigenvalue ratio, power mismatch and cumulative normalized constraint violation, are utilized to evaluate the exactness of proposed relaxation. Simulations, carried out on several medium and low voltage DNs, show that the proposed relaxation is numerically exact under several combinations of ZIP load parameters and a reasonable range of grounding impedance value for both time-varying and extreme system loading scenarios irrespective of the degree of unbalance in a network.

A Convex Approximation for Optimal DER Scheduling on Unbal-anced Power Distribution Networks

The increase of solar photovoltaic penetration poses several challenges for distribution network operation, mainly because such high penetration might cause reliability problems like protection malfunctioning, accelerated decay of voltage regulators and voltage violations. Some control strategies avoid such drawbacks at the cost of not fully exploiting the available energy. Existing solutions based on mathematical programming solve a 3-phase ACOPF to optimally exploit the available energy, however, this might increase all reliability problems above if done carelessly. As a solution to optimally exploit DERs (like local photovoltaic and storage systems) without compromising the network reliability, this paper presents a novel algorithm to solve the 3-phase ACOPF as a sequence of convex Quadratically Constrained Quadratic Programs. Results show that this solution has a lower voltage unbalance and computation time than its non-linear counterpart, furthermore, it converges to a primal feasible point for the non-linear formulation without major sacrifices on optimal DER active power injections.

Modeling and Simulation of the Anticipated Effects of the Synchronous Condenser on an Electric-Power Network with Participating Wind Plants

Installing a synchronous condenser (SC) onto an electricity grid can assist in the areas of reactive power needs, short-circuit strength, and, consequently, system inertia and guarantees better dynamic voltage recovery. This paper summarizes the practical potential of the synchronous condenser coordinated in an electric-power network with participating wind plants to supply reactive power compensation and injection of active power at their point of common coupling; it provides a systematic assessment method for simulating and analyzing the anticipated effects of the synchronous condenser on a power network with participating wind plants. A 33-kV power line has been used as a case study. The results indicate that the effect of the adopted synchronous condenser solution model in the MATLAB/Simulink environment provides reactive power, enhances voltage stability, and minimizes power losses, while the wind power plants provide active power support with given practical grid rules.

Voltage Control in Active Distribution Networks Under Uncertainty in the System Model: A Robust Optimization Approach

Within the context of ancillary services for active distribution networks (ADNs), application of intelligent control techniques is required in order to achieve specific operation objectives. Despite their differences, most control mechanisms proposed in the literature rely on the assumption that the distribution network operator has an accurate and up-to-date model of the network topology and a complete knowledge of the line parameters, i.e., a correct network admittance matrix Y. However, this assumption does not always hold in reality due to both an incomplete knowledge of the grid asset and/or a physical change of the line parameters. In this paper, we consider the problem of optimal voltage control in ADNs under uncertain, but bounded, line parameters with no assumptions on the parameters’ uncertainty distribution. In particular, availability of a monitoring infrastructure is assumed and the goal is to control the active and reactive power injections of a number of distributed generators connected to the network buses in coordination with the transformers on-load tap changers. The optimal control problem is formulated as a mixed-integer linear problem by means of sensitivity coefficients and a robust optimization framework is used in order to account for the uncertainties in the network admittance matrix. In order to estimate the benefits of the proposed method, the evaluation of the algorithm is carried out by using both the IEEE 13- and the IEEE 34-nodes test feeder.

An Efficient Zoning Method for Voltage Control in Distribution Networks Based on Sensitivity Coefficients

Zoning method based on sensitivity coefficients is very efficient. But only reactive power injection is taken into account in most researches. It’s more reasonable to consider active and reactive power injection influences simultaneously. Then, AP clustering method is adopted to select the zones and critical buses in each zone. The results gained by AP method are more appropriate in comparison with the results obtained by hierarchical clustering approach.

Modified affine arithmetic based continuation power flow analysis for voltage stability assessment under uncertainty

Continuation power flow (CPF) analysis has been used in the literature to determine the voltage collapse point from active power versus voltage curves (PV curves) for steady-state voltage stability assessment. Affine arithmetic-based (AA) CPF analysis to determine PV curve bounds under uncertainty in power generation was introduced in the literature to overcome the problem of large computational time with Monte Carlo (MC) simulations, by getting a faster solution with a reasonably good accuracy. However, AA operations lead to more noise terms and hence overestimation of bounds. In the present work, a modified AA (modAA)-based CPF analysis is proposed to determine PV curve bounds by considering uncertainties associated with active and reactive power injections at all buses in the system. The proposed method reduces the overestimation caused by the AA operations and gives more accurate solution bounds. The proposed modAA-based CPF analysis is tested on 5-bus test case, IEEE 57, European 1354 and Polish 2383-bus systems. The simulation results with the proposed method are compared with MC simulations and AA-based CPF analysis to show the efficacy of the proposed method.

An Improved Hybrid Modulation Method for the Single-Phase H6 Inverter With Reactive Power Compensation

Transformerless photovoltaic (PV) inverters are more widely adopted due to high efficiency, low cost, and light weight, etc. Many novel topologies and their corresponding modulation methods have been proposed, verified, and put into use, solely focusing on active power injection without leakage current issues. However, some new grid codes require PV inverters to have the ability of injecting reactive power into the utility for grid support. In order to map the challenge, an improved hybrid modulation method is proposed and evaluated for one nonisolated H6 topology as an example. With only simple modification for switching patterns and phase shift for current reference, the variable reactive power generation ability with zero crossing distortion is achieved, while the common mode voltage is also eliminated. The operation modes with the improved hybrid modulation approach are presented in detail and some design considerations are also provided. Extensive results from a 4-kVA prototype along with the SMPLIS simulation verify the proposed hybrid modulation method.

Control Design For Shunt Active Power Filter Based On p-q Theory In Photovoltaic Grid-Connected System

<span>This paper presents the control for Shunt Active Power (SAPF) filter in photovoltaic (PV) systems connected to the grid. The proposed configuration of the system consists of a photovoltaic array that connected to the grid through the three-phase inverter topology that also serves as an active filter. Photovoltaic is coupled in parallel with the direct curret (DC) side of the active filter. With this configuration, can be obtained three advantages, namely the elimination of harmonic currents caused by nonlinear load, reactive power injection, and injection of active power generated photovoltaic. The p-q Theory is used to calculate the harmonic reference current to be used to control the active filter coupled fotovoltaic in generating anti-harmonic currents. The results show that system can reduce harmonic distortion from THD </span><span>27.22% </span><span>to be THD 1.05%, whereas when the active power from photovoltaic injected, the THD become 2.01%. Power sharing can also be seen from this study.</span>