Active Power Injection Research Articles

Harmonic current prediction by impedance modeling of grid-tied inverters: A 1.4 MW PV plant case study

Grid-tied inverters are the main responsible for the successful integration of photovoltaic (PV) plants in the power grid. Two important challenges are the integration of power plants based-inverters and their contribution to the power system quality. This work analyzes the features of inverter impedance models when used in harmonic integration studies. It is aimed to estimate the harmonic current contribution as a function of the background harmonic voltages components. Time domain simulations based on detailed and average models are compared with the impedance model developed in frequency domain. In grids with harmonic voltages, impedance models can predict the current distortion for all active power injection scenarios. Furthermore, measurements in a 1.4MW PV plant connected in a distributed grid are used to validate the simulation based on impedance models during different power injections and harmonic voltage profiles. Results reinforce that impedance models can represent with relatively accuracy the harmonic current emitted by the PV plants at the point of common coupling (PCC). Lastly, a stress test is performed to show how a variation in the harmonic voltage phase angle impacts the PV plant harmonic emission.

Modified Synchronous Vector Control Design of Multilevel Inverters for AC Grid Applications

This article presents a modified synchronous vector control design of a five-level cascaded H-Bridge inverter for integration of distributed generation (DG) resources to the power grid. The presented control strategy explores the limitations in the conventional synchronous vector control algorithm of grid-connected converter (GCC). The dynamic reference point (DRP) approach is considered to improve the performance of GCC. Compared with conventional control algorithm, DRP based control algorithm exhibits several specific advantages, such as increased reliability of the grid-interfaced converter, more active and reactive power injection capacity of the converter, and reduced cost of the converter. The objectives of control technique of the DG interface system are also employed to enhance the standard of the power grid. The optimized space vector pulse-width modulation (OSVPWM) control technique is used for grid integration. The simulation and experimental results are presented in order to validate the proposed functionalities of the grid-interfaced DG system. The effectiveness of the proposed control scheme is validated with better utilization of DG system, faster dynamics with reduced total harmonic distortion, better power factor, compensated load reactive power, and fault withstanding capability.

Novel Dynamic Voltage Support Capability of Photovoltaic Systems for Improvement of Short-Term Voltage Stability in Power Systems

The large integration of photovoltaic (PV) power generation systems into power systems causes deterioration in power system stability. In our previous work, we showed that reactive power control using the inverters of PV systems, known as dynamic voltage support (DVS) capability, is a promising approach to improve the short-term voltage stability in power systems. In this paper, we propose a novel DVS capability as a function of PV inverters. In contrast to the conventional DVS capability, the proposed method uses both active and reactive power injection to improve the short-term voltage stability. Numerical examples show that the proposed DVS capability further improves the short-term voltage stability compared with the conventional DVS capability. Furthermore, the proposed method can alleviate a frequency drop after a fault caused by interruption in PV systems.

Active Power Injection Control for Power Converters Connected to the Grid Through an L Filter

ABSTRACTThis paper presents the design, implementation, and evaluation of a model-based controller for a general scheme of transformerless power converters connected to the grid through an L filter. The proposed controller is aimed at active power injection, and consists of a damping term over the current error in addition to a filter parameter estimation scheme. The latter provides estimates of the filter internal resistance and inductance values. It involves the fundamental and quadrature components of the grid voltage to asymptotically compensate for current harmonic peturbations. As a result, the closed-loop system fulfills the active power injection objective. The proposed controller is tested in an experimental setup and its performance is compared to that of the Proportional-Resonant controller. Experimental results are presented for a single-phase H-bridge inverter; however, the control scheme can be used in several other H-bridge based inverter topologies such as H5 and HERIC.

Dynamic Performance Improvement of a Grid-Tied PV System Using a Feed-Forward Control Loop Acting on the NPC Inverter Currents

This paper presents a three-phase grid-connected photovoltaic (PV) system, which is implemented using the neutral-point-clamped (NPC) inverter. A current feed-forward control loop (FFCL) is proposed to improve the PV system dynamic behavior, due to the PV array being constantly subjected to sudden solar irradiance change, which causes voltage oscillations in the dc bus and, hence, interferes in proper PV system operation. As the current FFCL acts to speed up the calculation of the inverter current references, the dynamic response of the currents injected into the grid is improved. As a consequence, the dynamic behavior of the dc-bus voltage is also enhanced, reducing both settling time and overshoot. Besides the injection of active power into the grid, the PV system is also controlled to perform active power-line conditioning, so that load harmonic currents are suppressed, as well as load reactive power is compensated. However, the NPC inverter must be properly designed to guarantee that its power rating will not be exceeded, since both fundamental and nonfundamental current components flow through the grid-tied inverter. Extensive experimental results based on a digital signal processor are presented in order to evaluate the effectiveness, as well as the static and dynamic performance of the PV system.

Experimental Verification of a Battery Energy Storage System for Integration with Photovoltaic Generators

This paper presents the experimental verification of a 2 kW battery energy storage system (BESS). The BESS comprises a full-bridge bidirectional isolated dc-dc converter and a PWM converter that is intended for integration with a photovoltaic (PV) generator, resulting in leveling of the intermittent output power from the PV generator at the utility side. A phase-shift controller is also employed to manage the charging and discharging operations of the BESS based on PV output power and battery voltage. Moreover, a current controller that uses the d-q synchronous reference frame is proposed to regulate the dc voltage at the high-voltage side (HVS) to ensure that the voltage ratio of the HVS with low-voltage side (LVS) is equivalent to the transformer turns ratio. The proposed controllers allow fast response to changes in real power requirements and results in unity power factor current injection at the utility side. In addition, the efficient active power injection is achieved as the switching losses are minimized. The peak efficiency of the bidirectional isolated dc-dc converter is measured up to 95.4% during battery charging and 95.1% for battery discharging.

Distribution Voltage Regulation Through Active Power Curtailment With PV Inverters and Solar Generation Forecasts

Distribution voltage profiles are subjected to overvoltage limit violations from high penetration of grid-connected photovoltaic (PV) systems. Such voltage rises seen at the point of PV interconnection can be mitigated by adaptively changing the active and/or reactive power injection from the PV inverter. This work proposes a local voltage regulation technique that utilizes very short-term (15 s) PV power forecasts to circumvent imminent upper voltage limit violation or an overvoltage scenario. To provide these PV generation forecasts, a hybrid forecasting method is formulated based on Kalman filter theory, which applies physical PV generation modeling using high-resolution (15 s) data from on-site measurements. The proposed algorithm employs an active power curtailment based on these PV power forecasts, when the reactive power estimate given by a droop-based method cannot provide the desired voltage regulation within predefined power factor limits. The curtailment threshold values are calculated in such a way that this voltage regulation technique can reduce possible voltage limit violations. The effectiveness of the proposed method is demonstrated with case studies developed on a standard test feeder with realistic load and PV generation profiles.

Lyapunov Function and Sliding Mode Control Approach for the Solar-PV Grid Interface System

This paper deals with control of a solar-photovoltaic (PV) power-generating system interfaced with the grid. A sliding mode control approach is used for achieving maximum power tracking control of a solar-PV array. The Lyapunov function-based control approach is designed and modeled for the dc–ac inverter to serve the functions of an active power injection to the grid, balanced grid currents at unity power factor and load currents harmonics compensation. The proposed approaches eliminate the need of adjustment of system parameters under changing loads and generation scenario. The effectiveness of the proposed control strategies is established using its stability analyses. The performance of the solar-PV power-generating system with the proposed control algorithms is demonstrated using simulation and experimental studies under various operating conditions.

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.

Design and Implementation of a Low Cost Grid-Connected 5 kVA Photovoltaic System with Load Compensation Capability

Design and implementation of a low cost grid-connected 5kVA solar photovoltaic (PV) system is proposed in this paper. Since the inverter is a major component of the PV system, the B4 inverter used in this paper reduces the total cost of the PV system. In order to eliminate the massive transformer, the PV system is connected to the grid through IGBT switches. In addition to injection of active power into the grid, the B4 inverter can compensate reactive power and reduce harmonics of the nonlinear loads. A TMS320F28335 DSP processor is used for effective control of the B4 inverter. Various features of this processor enable the implementation of the necessary control algorithms. As a first step, the PV system is simulated and evaluated in Matlab/Simulink. In the second step, hardware circuits are designed and implemented based on the simulation results. The operation of the PV system has been evaluated under balanced, unbalanced, linear and nonlinear loads which proves its accuracy and efficiency.

The Formulation of a Power Flow Using <inline-formula> <tex-math notation="LaTeX">$d-q$</tex-math> </inline-formula> Reference Frame Components—Part I: Balanced <inline-formula> <tex-math notation="LaTeX">$3\phi$ </tex-math> </inline-formula> Systems

This paper develops a new approach for formulating the power flow for power systems. The developed approach is based on expressing the active and reactive power injections at each bus in a power system using the d-q-axis components of bus voltages and admittance matrix. The new power flow formulation produces a scaled Jacobian matrix, which can offer a fast convergence to the solution. The proposed power flow formulation can also model bus type conversions without affecting its accuracy or fast convergence. In addition, the d-q-axis power flow (DQPF) offers a simplified and reliable representation of photovoltaic buses that have distributed generation units (DGUs). The DQPF is implemented with a step-by-step procedure for performance evaluation on several power systems operated at different conditions. Performance results demonstrate fast convergence, reduced computations, and minor sensitivity to the number of buses, loading conditions, and levels of DGU penetration. Furthermore, performance results show that DQPF power flow can attain solutions in less iterations than Newton-Raphson, Fast Decoupled, and Iwamoto power flow methods.

Quantification and Mitigation of Unfairness in Active Power Curtailment of Rooftop Photovoltaic Systems Using Sensitivity Based Coordinated Control

With increasing photovoltaic (PV) penetration in low voltage networks (LVNs), voltage regulation is a challenge. Active power curtailment (APC) is one possible solution for mitigating over voltages resulting from active power injection in LVNs. There is an inherent unfairness in the APC scheme. When generation is high and consumption is low, the voltages at the end of the feeder tend to be the highest. This results in high curtailment of active power output of the inverters located at the end of the feeder and low or even no curtailment for the inverts located closer to the transformer. A secondary voltage controller has been implemented to mitigate this unfairness in APC based voltage support schemes. The focus of this work is to quantify this unfairness and develop methods that enable residential PV owners serviced by the same feeder to participate equally in voltage regulation in the LVN.

A Multitasking Control Algorithm for Grid-Connected Inverters in Distributed Generation Applications Using Adaptive Noise Cancellation Filters

This paper proposes a multitasking control algorithm for grid-connected inverters (GCIs) in distributed generation (DG) applications. A single-phase H-bridge voltage source inverter is used as a power electronic interface between the DG system and the grid. The proposed control algorithm operates the GCI in current control mode to achieve desired active power injection to the grid. In addition to active power injection, the proposed control algorithm has current harmonics mitigation and reactive power compensation capabilities for power quality enhancement. The control algorithm utilizes only the remaining capacity of the GCI for power quality improvement. The proposed control algorithm employs a current decomposition structure based on multiple adaptive noise cancellation (ANC) filters for extraction of harmonic and reactive currents of the local loads. The extracted harmonics are used in estimating compensating currents to be injected by the GCI. A single-phase phase-locked loop (PLL) using ANC filters has been developed to synchronize the GCI at fundamental frequency, which makes the proposed control algorithm adaptive to grid frequency fluctuations and immune to grid voltage distortions. The effectiveness of the proposed control is illustrated through computer simulation and experimental results. The influence of PLL dynamics on GCI performance has also been studied using results.

Grid‐tied photovoltaic system based on PSO MPPT technique with active power line conditioning

This study presents a single-phase grid-tied photovoltaic (PV) system based on a global maximum power point tracking (MPPT) technique, which is performed by means of the particle swarm optimisation (PSO) method. The PSO-based MPPT technique is employed to solve problems related to mismatching phenomena, such as partial shading, in which the PV arrays are commonly submitted. Considering the search of the global maximum power point under partial shading, the effectiveness of the PSO-based MPPT technique is highlighted when compared with the well-known perturb and observe MPPT technique, since both the mentioned MPPT techniques are used to determine the dc-bus voltage reference to ensure a proper grid-tied inverter operation. A current generator algorithm based on a synchronous reference frame is proposed, which operates in conjunction with a dc-bus controller and MPPT algorithms, computing the reference current of the grid-tied inverter. In addition, the current generator controls the energy processed by the PV system to avoid over power rating of the grid-tied inverter, since the active power injection into the grid, reactive power compensation and harmonic currents suppression are carried out simultaneously. The performance and feasibility of the grid-tied PV system are evaluated by means of simulation and experimental results.

Probabilistic congestion management using EVs in a smart grid with intermittent renewable generation

This paper presents a probabilistic model to reduce the probability of line congestions and voltage violations in a smart grid located in a radial distribution network. Renewable distributed resources and a high penetration of Electric Vehicles (EVs) are considered. The uncertain parameters taken into account are: power demand, power generated by wind and solar photovoltaic generators, and the behavior of the EVs that do not participate in the congestion procedure. A probabilistic power flow based on the point estimate method is firstly used to compute the distribution functions of the line flows and node voltages. Next, a congestion management strategy is proposed in order to keep the line flows and node voltages within the appropriate range at a given confidence level. The strategy is based on the sensitivity distribution factors: Power Transfer Distribution Factors (PTDFs) and Voltage Distribution Factors (VDFs). The control variables to carry out the corrective actions are the active and reactive power injections from a subset of EVs or their charging points. It is assumed that these controlled EVs are able to inject power into the network using Vehicle to Grid (V2G) capabilities. To illustrate the method, a distribution grid based on a modified version of the IEEE-37 Node Test Feeder is tested.

Control of Single-Phase Power Converters Connected to Low-Voltage Distorted Power Systems With Variable Compensation Objectives

This paper presents a flexible control technique for power electronics converters, which can function as an active power filter, as a local power supply interface, or perform both functions simultaneously. Thus, it can compensate for current disturbances while simultaneously injecting active power into the electrical grid, transforming the power converter into a multifunctional device. The main objective is to use all the capacity available in the electronic power converter to maximize the benefits when it is installed in the electricity grid. This objective is achieved by using the orthogonal current decomposition of the conservative power theory. Each current component is weighted by compensation coefficients ( $k_i $ ), which are adjusted instantaneously and independently, in any percentage, by means of load conformity factors ( $\lambda _i $ ), thus providing online flexibility with respect to the objectives of compensation and injection of active power. Finally, simulated and experimental results are presented to validate the effectiveness and performance of the proposed approach.

Real-Time Monitoring of Points of Common Coupling in Distribution Systems Through State Estimation and Geometric Tests

This paper presents an extended state estimation method which allows the identification of unexpected power injections at multiple points of common coupling (PCCs) of power distribution grids. The developed state estimator makes use of a three-phase network modeling where active and reactive power injection levels at PCCs are estimated and checked in real time with the values expected by the system operator. The estimation algorithm applies a sparse tableau formulation in which equality constraints are employed to model power injection assumptions at PCCs. The identification of unexpected power injections is then performed using a methodology based on normalized Lagrange multipliers and geometric tests. Simulations conducted on a modified version of the IEEE 34-node test feeder illustrate the accuracy and applicability of the proposed approach, fostering discussions regarding the future of real-time monitoring on power distribution systems.

Frequency-based control of islanded microgrid with renewable energy sources and energy storage

When a microgrid is mainly supplied by renewable energy sources (RESs), the frequency deviations may deteriorate significantly the power quality delivered to the loads. This paper proposes a frequency-based control strategy, ensuring the frequency among the strict limits imposed by the Standard EN 50160. The frequency of the microgrid common AC bus is determined by the energy storage converter, implementing a proposed droop curve among the state of charge (SoC) of the battery and the frequency. Therefore, the information of the SoC becomes known to every distributed energy resource (DER) of the microgrid and determines the active power injection of the converter-interfaced DERs. The active power injection of the rotating generators remains unaffected, while any mismatch among the power generation and consumption is absorbed by the energy storage system. Finally, in case of a solid short-circuit within the microgrid, the energy storage system detects the severe voltage decrease and injects a large current in order to clear the fault by activating the protection device closer to the fault. The proposed control methodology is applied in a microgrid with PVs, wind generators and a battery, while its effectiveness is evaluated by detailed simulation tests.

Voltage sag compensation strategy for unified power quality conditioner with simultaneous reactive power injection

Unified power quality conditioner (UPQC) holds the capability of solving power quality problems, especially shows good performance in the voltage sag compensation. In this paper, a compensation strategy based on simultaneous reactive power injection for UPQC (namely UPQC-SRI) is proposed to address the issue of voltage sag. The proposed UPQC-SRI determines the injection angle of compensation voltage with consideration of optimal configuration of UPQC current-carrying. Moreover, the compensation strategy also considers the current-carrying limit of UPQC, and then the zero active power injection region of UPQC-SRI (also called UPQC-SRI region) is obtained. Under the conditions which exceed the UPQC-SRI region, the limit value of shunt current is determined by this proposed strategy. Finally, the proposed strategy and the corresponding algorithm are verified under the PSCAD/EMTDC platform. The result indicates the proposed UPQC-SRI strategy in this paper can provide more persistent voltage sag compensation than the previous strategies for the sensitive load.

Real-Time Remedial Action Against Aperiodic Small Signal Rotor Angle Instability

This paper presents a method that in real-time determines remedial actions, which restore stable operation with respect to aperiodic small signal rotor angle stability (ASSRAS) when insecure or unstable operation has been detected. An ASSRAS assessment method is used to monitor the stability boundary for each generator in real-time. The ASSRAS boundary represents the condition when a generator reaches the maximum steady state active power injection. The proposed control method exploits analytically derived expressions for the ASSRAS boundary and other characteristic curves in the injection impedance plane to determine an active power redispatch among selected generators to restore stable and secure operation. Since the method is purely based on analytically derived expression, the computation of the remedial actions is fast and well suited for real-time operation. The method was tested on the IEEE 14-bus and the Nordic32 test systems where results show that the method can efficiently determine the required active power redispatch to avoid an imminent instability.