Distributed Generation Inverters Research Articles

New triple-action controller for inverter power quality improvement

Harmonic distortion is a crucial power quality (PQ) issue especially in microgrids. Distributed energy resources employ either synchronous generator or voltage source inverter to interface microsource with loads. This paper presents the analysis of the proportional resonant (PR) controller along with a selective harmonic compensator (SHC) for a stand-alone distributed generation inverter. Furthermore, a new improved triple-action controller (TAC) design is proposed and implemented. The proposed TAC consists of a conventional PR along with SHC, in addition to, a feed-forward current controller to achieve seamless sinusoidal voltage. This mitigate not only the total harmonic distortion (THD) but also individual harmonics to a level below the stated in IEEE/IEC standards. Moreover, optimize the TAC parameters to ensure its best performance. A comparative study of harmonic effects for various load-types are introduced. The voltage regulation and THD mitigation improved the power factor and efficiency, hence, it improved the PQ of the system.

Optimal Voltage-Support Control for Distributed Generation Inverters in RL Grid-Faulty Networks

During grid faults, the stability and reliability of the network are compromised, and the risk of a widespread disconnection of distributed generation power facilities is increased. Distributed generation inverters must support the power system to prevent this issue. Voltage support depends substantially on the currents injected into the grid and the equivalent grid impedance. This article considers these two aspects and proposes an optimal voltage-support strategy in RL grids. The control algorithm guarantees safe operation of the inverter during voltage sags by calculating the appropriate reference currents according to the equivalent impedance and the voltage sag characteristics, avoiding active power oscillations, and limiting the injected current to the maximum allowed by the inverter. Consequently, the grid can be better supported since the voltage at the point of common coupling is improved and the voltage support objectives are achieved. The proposed control strategy is validated through experimental tests in different grid scenarios. Throughout this article, it is assumed that the grid impedance is known, but the proposed solution requires calculating the grid impedance angle.

Resynchronization of Distributed Generation Based on the Universal Droop Controller for Seamless Transfer Between Operation Modes

Distributed generation (DG) inverters can operate in the islanded mode and the grid-connected mode. When these DG inverters operate in the islanded mode, they often form a microgrid. The magnitude and phase of the main grid voltage deviate from the microgrid voltage. If the microgrid is connected to the main grid under such condition, it results in a high transient overcurrent that is harmful to the system. This article presents a new, simple, and effective resynchronization mechanism for the universal droop control (UDC) DG inverters to achieve a seamless transfer from the islanded mode to the grid-connected mode. Simulation and experimental results are provided to verify the effectiveness of the proposed method. The result shows that the high transient overcurrent can be avoided and a seamless connection/reconnection to the main grid can be achieved.

A general coordinated voltage regulation method in distribution networks with soft open points

Due to the high flexibility and fast response capability, power electronic devices are versatile, and increasingly being considered to mitigate voltage violations and improve system efficiency in distribution networks with high penetration of distributed generation (DG). This paper proposes a novel voltage regulation framework in distribution systems through the general types of Soft Open Points (SOPs) and reactive powers of DG inverters. By using the linearized DistFlow equations, we establish a biconvex problem that minimizes voltage deviation via a well-coordinated scheme for SOPs and DGs. The coordination of different types of power electronic devices can be simultaneously modelled into this problem which can be efficiently solved by alternative convex search algorithm. The proposed approach is validated by numerical simulations on modified IEEE 15-bus system, IEEE 33-bus system, and IEEE 69-bus system with different types of power electronic compensators.

Adaptive Slope Voltage Control for Distributed Generation Inverters With Improved Transient Performance

Reactive power injection in distributed generation inverters is an useful ancillary service for grid supporting purposes. For grid-feeding converters, the slope control method is the most common voltage regulation strategy used in local (communication-less) applications. Despite its simplicity, this method offers limited dynamic properties in scenarios with changing operation conditions. In this sense, this paper presents an adaptive slope voltage control that provides an improved transient performance against operating variations. To derive the control configuration, a control-oriented mathematical model is developed. The accuracy of the modeling and the performance of the proposed control are validated by selected experimental results.

Small Signal Stability Analysis of Distribution Networks With Electric Springs

This paper presents small signal stability analysis of distribution networks with electric springs (ESs) installed at the customer supply points. The focus is on ESs with reactive compensation only. Vector control of ES with reactive compensation is reported for the first time to ensure compatibility with the standard stability models of other components such as the interface inverter of distributed generators (DGs). A linearized state-space model of the distribution network with multiple ESs is developed which is extendible to include inverter-interfaced DGs, energy storage, active loads, etc. The impact of distance of an ES from the substation, proximity between adjacent ESs and the R/X ratio of the network on the small signal stability of the system is analyzed and compared against the case with equivalent DG inverters. The collective operation of ESs is validated through simulation study on a standard distribution network.

Integrated control of voltage regulators and distributed generation inverters

High penetration of distributed generation (DG) can cause fast voltage changes in distribution systems. Inverter-based DG can be a significant source of reactive power, which brings new opportunities to improve the performance of the distribution system. An integrated control strategy is formulated for the coordinated control of both distribution system equipment and inverter-based DG. The control strategy combines the use of inverter reactive power capability with the operation of voltage regulators in order to improve the expected value of a desired figure of merit (e.g., system losses) while maintaining appropriate system voltage magnitudes, by formulating chance constraints on the voltage magnitudes. The effectiveness of the proposed method is validated using the IEEE 123-node radial distribution system.

LQR Based Control Method for Grid Connected and Islanded DG System

AbstractThis paper presents an output and state feedback robust LQR (OSRLQR ) optimal controller for the control of active (P) and reactive power (Q) in grid-connected mode, and voltage (V) and active power in islanded mode of photovoltaic (PV) distributed generation (DG) system. The OSRLQR scheme is comprising of an output and state feedback controllers, which are designed based on LMI-LQR optimization approach. The DG inverter along with proposed controller is configured to work as a current source in grid-connected mode and a voltage source in island mode using P-Q and P-V control scheme respectively. A seamless transition between the grid connected and island modes under the presence of the proposed controller is achieved using a passive island detection and synchronization algorithm along with load shedding. The entire control scheme of the DG system is modelled and analyzed using MATLAB/SIMULINK/Robust Tool-Box, and the practical feasibility of the proposed control scheme is verified using dSPACE. A superior trade-off robustness among the stability and performance is the final outcome of the DG system with proposed control scheme. Performance of the proposed control scheme is compared with conventional PI controller and the comparative results indicate the superior performance of proposed control scheme over the conventional control scheme.

Restoration of Low-Voltage Distribution Systems With Inverter-Interfaced DG Units

The increasing share of distributed generation (DG) offers new chances in grid restoration of low-voltage distribution grids. Instead of relying on the transmission or high- and medium-voltage levels, establishing islanding operation in low-voltage grids might be a good option after a wide-area voltage collapse. This paper proposes a restoration strategy from zero-voltage conditions for inverter-interfaced DG under islanded conditions. In the approach, a flexible and scalable master DG inverter concept is introduced for DGs, where no communication is needed and an outage of the master can be balanced by other DG inverters. The control strategy ensures the tracking of nominal values of the system voltage and frequency without zero steady-state error. The influences of noncontrollable DG are also taken into account in the strategy with an effective countermeasure developed. Experimental results verify the effectiveness of the proposed approach.

Dynamic Stability of an Autonomous Microgrid Considering Active Load Impact With a New Dedicated Synchronization Scheme

Similar to distributed generation inverters, active load has to use a dedicated synchronization unit such as a phase-locked loop (PLL) to get the reference voltage of the microgrid. To avoid PLL drawbacks, a new method is proposed to track the microgrid reference voltage for active load synchronization. An autonomous microgrid is modeled and designed. Additionally, the impact of active loads on the dynamic stability of the autonomous microgrid is investigated. An optimal design is performed to obtain the optimal controller parameters. Based on the time-domain simulation, the control design problem is optimally solved by minimizing a weighted objective function to limit the error in the dc voltage and the measured active power. Different step change disturbances are applied to examine and verify the effectiveness of the new synchronization method and the proposed controllers. Finally, a comparison between the proposed synchronization scheme and the traditional PLL is provided to prove the superiority of the proposed technique. It is concluded that the proposed synchronization method has a potential to enhance the microgrid stability efficiently. With the proposed method, PLL disadvantages such as design complexity, bad accuracy, nonlinearity, tuning difficulties, slow response, and negative impact on the control performance can be avoided.

A centralized-based method to determine the local voltage control strategies of distributed generator operation in active distribution networks

The increasing penetration of distributed generators (DGs) exacerbates the risk of voltage violations in active distribution networks (ADNs). The var capacity provided by DG inverters is a potential solution for voltage regulation. The conventional centralized control strategies limited by computation and communication burdens are difficult to meet the requirement of rapid voltage control. However, the local control strategies based on real-time measurements have a fast response to the frequent fluctuations of DG outputs. The performance of these local controllers depends on the tuning of the control parameters. This paper proposes a centralized-based method to determine the local voltage control strategies for DGs. A centralized parameter tuning model of control curves is built, in which Q-V and Pcurt-V curves of DG inverters are mathematically formulated based on piecewise linearization. The original mixed-integer nonlinear programming (MINLP) model is converted into an effectively solved mixed-integer second-order cone programming (MISOCP) model using convex relaxation. The potential benefits of DG inverters are explored to regulate both reactive and curtailed active power, based on local voltage measurements. Case studies on a modified PG&E 69-node distribution system are carried out to verify the effectiveness of the proposed method. Results show that power losses of ADNs are significantly reduced and voltage profiles are also improved.

Decentralized Circulating Currents Suppression for Paralleled Inverters in Microgrids Using Adaptive Virtual Inductances

Droop-controlled inverters, which are the main interfaces between distributed generators (DGs) and grid AC bus, are widely adopted in today’s microgrids. Because the DGs are usually dispersed along with the microgrids, which may spread out in a wide area, the impedances of transmission lines that connect different DGs and the point of common coupling (PCC) may be different. Large circulating currents among paralleled inverters would be induced eventually, which not only results in decreased system efficiency, but also makes some DGs impossible to operate at their rated power. In this paper, the relationship between circulating current and line impedances among paralleled inverters with conventional droop control is analyzed in detail. To mitigate the circulating current, an adaptive virtual output inductance control method for the DG inverter is proposed. With the help of the added adaptive virtual inductances, huge reduction on the circulating currents can be obtained regardless of the differences in line impedances. The implementation details of the proposed method and the optimum design rules for system parameters are elaborated, which are swiftly followed by the operational principle and stability analyses. Because neither communications nor global signals are needed in the proposed method, it is a total discrete approach, which evidently indicates high flexibility and scalability. The validity of the proposed method is finally verified by simulations and experiments.

Distributed hierarchical control structure for voltage harmonic compensation and harmonic current sharing in isolated MicroGrids

Abstract In this paper, a distributed hierarchical control structure is proposed for compensating voltage harmonics of Sensitive Load Bus (SLB) and sharing current harmonics of load current among Distributed Generation (DG) units in isolated MicroGrids (MGs). In this structure, each DG has primary and secondary controller implemented locally. In the primary control level, a method is proposed for voltage harmonic compensation at SLB and sharing the compensation workload, which is based on non-fundamental apparent power, and directly takes into account the rated capacity limitation of DG inverter. Furthermore, in this level, another method is proposed for accurate harmonic current sharing among DGs which is based on circulating currents. The proposed method is capable of removing harmonic current sharing error without using any knowledge about MG topology and also impedances of distribution lines and loading conditions. Moreover, the effect of the applying proposed method on the amplitude and quality of DG output voltage waveforms will be insignificant. The secondary control level transmits the reference control signals to the primary control level in order to decrease the voltage harmonics at SLB and share harmonic currents among DGs. Details of designing control system parameters are presented and simulation results are given to show the efficiency of the proposed structure.

Improved P-f/Q-V and P-V/Q-f droop controllers for parallel distributed generation inverters in AC microgrid

Distributed generation inverters are generally operated in parallel with P-f/Q-V and P-V/Q-f droop control strategies. Due to mismatched resistive and inductive line impedance, power sharing and output voltage of the parallel DG inverters deviate from the reference value. This leads to instability in the microgrid system. Adding virtual resistors and virtual inductors in the control loop of droop controllers improve the power sharing and stability of operation. But, this leads to voltage drop. Therefore, an improved P-f/Q-V and P-V/Q-f droop control is proposed. Simulation results demonstrate that the proposed control and the selection of parameters enhance the output voltage of inverters.

Positive and Negative Sequence Control Strategies to Maximize the Voltage Support in Resistive–Inductive Grids During Grid Faults

Grid faults are one of the most severe perturbations in power systems. During these extreme disturbances, the reliability of the grid is compromised and the risk of a power outage is increased. To prevent this issue, distributed generation inverters can help the grid by supporting the grid voltages. Voltage support mainly depends on two constraints: the amount of injected current and the grid impedance. This paper proposes a voltage support control scheme that joins these two features. Hence, the control strategy injects the maximum rated current of the inverter. Thus, the inverter takes advantage of the distributed capacities and operates safely during voltage sags. Also, the controller selects the appropriate power references depending on the resistive–inductive grid impedance. Therefore, the grid can be better supported since the voltage at the point of common coupling is improved. Several voltage objectives, which cannot be achieved together, are developed and discussed in detail. These objectives are threefold: to maximize the positive sequence voltage; to minimize the negative sequence voltage; and to maximize the difference between positive and negative sequence voltages. A mathematical optimal solution is obtained for each objective function. This solution is characterized by a safe peak current injection, and by the optimization of the voltage profile in any type of grid connection. Therefore, the proposed control scheme includes advanced features for voltage support during voltage sags, which are applicable to different power facilities in different types of networks. Due to system limitations, a suboptimal solution is also considered, analyzed, and discussed for each of the optimization problems. Experimental results are presented to validate the theoretical solutions.

Reactive power and voltage control strategy based on dynamic and adaptive segment for DG inverter

The inverter of distributed generation (DG) can support reactive power to help solve the problem of out-of-limit voltage in active distribution network (ADN). Therefore, a reactive voltage control strategy based on dynamic and adaptive segment for DG inverter is put forward to actively control voltage in this paper. The proposed strategy adjusts the segmented voltage threshold of Q(U) droop curve dynamically and adaptively according to the voltage of grid-connected point and the power direction of adjacent downstream line. And then the reactive power reference of DG inverter can be got through modified Q(U) control strategy. The reactive power of inverter is controlled to trace the reference value. The proposed control strategy can not only control the local voltage of grid-connected point but also help to maintain voltage within qualified range considering the terminal voltage of distribution feeder and the reactive support for adjacent downstream DG. The scheme using the proposed strategy is compared with the scheme without the reactive support of DG inverter and the scheme using the Q(U) control strategy with constant segmented voltage threshold. The simulation results suggest that the proposed method has a significant improvement on solving the problem of out-of-limit voltage, restraining voltage variation and improving voltage quality.

Adaptive Power Management Strategy for Effective Volt–Ampere Utilization of a Photovoltaic Generation Unit in Standalone Microgrids

This paper proposes an adaptive power management strategy for photovoltaic (PV) generation units in a standalone microgrid. In existing microgrid configurations, the PV units are controlled using droop control technique, which ensures load sharing among the distributed generation (DG) inverters. However, the intermittency in power generation of PV-based DG poses many challenges to the standalone operation of a microgrid, the ineffective volt-ampere (VA) utilization being one among them. This paper proposes an adaptive droop control, which enables the maximum utilization of the VA rating of the interfacing inverter in a PV/battery hybrid system. With the proposed technique, as the generation goes down, the DG inverter is controlled to support the reactive power demand of the load, thereby utilizing the rating of the inverter switches effectively. The applicability of the proposed adaptive control method is analyzed using detailed simulation and real-time hardware-in-loop studies.

Smart Interface Devices for Distributed Generation in Smart Grids: The Case of Islanding

With the increasing presence of distributed generators (DGs), the implementation of a smart sensors and actuators network in distribution grids has become crucial for different monitoring, management, and protection functionalities. For DGs monitoring and control, sensors and actuators are usually embedded in so-called interface devices (IDs) or interface protection systems. Existing IDs lack in communication interfaces both to the electrical energy distribution system operators (DSOs) and to the DGs inverter. In this framework, the authors have developed a new ID solution and a proper communication architecture, which allow implementing new smart functionalities based on the interaction with the DSO, such as remote control of DG power production or islanding detection. Regarding this last aspect, a new islanding algorithm is proposed, based on both local measurements and communications with DSO. The effectiveness of the proposed solution is shown on the real case study of Ustica Island’s distribution network.

Distributed Generation Inverter as APF in Dual APF DG interfacing Scheme

<span>Power quality issues are the most concerned area for electrical engineers in these modern days due to indusrtrialization and advancements in electronics. Harmonics are one of the power quality issues that deteriorated the quality of source components delivered to load. Active power filters (APF) are the compensating devices used to mitigate harmonics. Distributed generation (DG) became a trendline due to economical, technical and environmental reasons. This paper presents the harmonic compensation using dual APF sharing compensating signals while active power is fed from distributed generation to grid. DG inverter acts as an interface between DG system and grid to feed active power from DG to grid. In this context, the DG inverter acts both as inverter and APF, used as inverter to invert supply from DG and feeds grid for specified time and also as APF for harmonic compensation thereafter along with dual APF. The proposed system is developed and results are obtained using MATLAB/SIMULINK software. Results are shown for DG inverter feeding active power to grid and also acting as APF. Dual APF characteristics are also shown. </span>

A Critical Evaluation of Power Quality Features using Dual APF under Grid Interfaced DG Scheme

This paper presents the concept of power quality enhancement reducing the harmonic distortion with power distribution system consisting of balanced and unbalanced non-linear type of loads. This paper presents power quality enhancement using dual active power filters (APF’s) under grid connected distributed generation scheme. Acive power filters effectively generate compensating signals for harmonic abolition and this paper presents APF for compensating currents to be induced in to distribution grid for harmonic elimination under balanced and unbalanced non-linear load conditions. Distributed generation (DG) feeds the grid and the scheme of grid interfaced DG was explained. APF is controlled using instantaneous P-Q theory and DG inverter is controlled using simple control strategy. Proposed system was developed and the results are obtained using MATLAB/SIMULINK software.