Voltage Positive Feedback Research Articles

Detecting Speed Improvement and System Stability Enhancement for DC Microgrids Islanding Detection Based on Impedance Characteristic Analysis

Islanding detection is the essential basis for the safety and protection of dc microgrids. Due to the advantages of rapid detection speed and small nondetection zone, the positive feedback islanding detection technique has attracted much interest in recent years. The implementation of a positive feedback loop facilitates the islanding detection for dc microgrids, but it may reduce the system stability margin at the grid-connected state. In this article, the dc output impedance model of voltage source converter and distributed generators are developed to reveal the dynamic interactions and instability mechanism accurately between the two subsystems considering the impact of islanding detection with voltage positive feedback of selected frequency (VPFOSF). Then, the stability region of the interconnected system based on the output admittance model is derived to guide the design method of the damping compensator. Moreover, a grid-current-based active-damping control scheme is further proposed to mitigate instability caused by the VPFOSF loop. Thus, the conflict among islanding detection rapidity, power quality, and grid-connected system stability is effectively addressed. In the end, both the simulations and experimental results are provided to validate the effectiveness of the proposed control method.

Islanding Detection With Positive Feedback of Selected Frequency for DC Microgrid Systems

The oscillation frequency of the point of common coupling (PCC) voltage is proposed as a new detecting freedom for dc-based islanding events in this article. In the presented method, the voltage positive feedback with a frequency selection is applied to a distributed generator to make the PCC voltage oscillate at the selected frequency when islanding events occur. With the frequency information, islanding events can be identified by the small-amplitude oscillation superimposed on the normal dc voltage without the necessity of voltage amplitude to be shifted out of the normal range. Thus, the islanding events can be effectively distinguished from other abnormal conditions that lead to large voltage fluctuations, and so, the accuracy of islanding detection is improved. Also, the power quality at islanding events is enhanced, which provides important benefits for seamless transfer from the unintentional islanding conditions to the controlled islanding conditions. Moreover, the frequency selection is designed to only transfer the voltage signal at the sensitive frequency band of the islanding condition. As a result, the deterioration of stability caused by the voltage positive feedback can be relieved under the grid-connected condition compared to the existing methods. Finally, the effectiveness of the proposed method has been verified by both time-domain nonlinear simulation and experimental results.

A Robust Controller for a Grid-Tied Inverter Connected Through an LCL Filter

This article presents a control strategy for three-phase inverters connected to grids through an LCL output filter. For this topology, it is known that when output current and capacitor current are fed back through certain optimal gains, the system is unfailingly unstable for a grid inductance value called “critical.” For the critical inductance value, the system's resonance frequency is equal to one-sixth of the sampling frequency. In this article, it is first checked that capacitor voltage positive feedback can stabilize this system. In addition, it is found that there exists a proportional relationship between the common coupling point voltage and the capacitor voltage. Based on this relationship, it is proposed to add grid voltage feedforward instead of capacitor voltage feedback for obtaining a stable system for every grid inductance value, including the critical value. In order to validate our assertion, this control law is tested via simulations and also, experiments on a prototype are included. Simulation and experimental results show the good performance of the proposed scheme.

Power Quality Assessment of Voltage Positive Feedback Based Islanding Detection Algorithm

Islanding refers to a condition where distributed generators (DGs) inject power solely to the local load after electrical separation from power grid. Several islanding detection methods (IDMs) categorized into remote, active, and passive groups have been reported to detect this undesirable state. In active techniques, a disturbance is injected into the DG's controller to drift a local yardstick out of the permissible range. Although this disturbance leads to more effective detections even in well-balanced island, it raises the total harmonic distortion (THD) of the output current under the normal operation conditions. This paper analyzes the power quality aspect of the modified sliding mode controller as a new active IDM for grid-connected photovoltaic system (GCPVS) with a string inverter. Its performance is compared with the voltage positive feedback (VPF) method, a well-known active IDM. This evaluation is carried out for a 1 kWp GCPVS in MATLAB/Simulink platform by measuring the output current harmonics and THD as well as the efficiency under various penetration and disturbance levels. The output results demonstrate that since the proposed disturbance changes the amplitude of the output current, it does not generate harmonics/subharmonics. Thereby, it has a negligible adverse effect on power quality. It is finally concluded that the performance of the sliding mode-based IDM is reliable from the standpoints of islanding detection and power quality.

Improved voltage shift islanding detection method for multi‐inverter grid‐connected photovoltaic systems

This study presents an improved voltage shift islanding detection method with the new control mode. The proposed method adopts the modulation index shift scheme based on the pulse-width modulation control principle, which can promote the detection performance in the multi-inverter grid-connected photovoltaic (PV) systems, compared with the conventional voltage positive feedback (VPF) method. The superposition algorithm of the proposed method can make the rapid reduction of voltage magnitude at the point of common coupling during the islanding condition, and minimise the non-detection zone. Through the self-varying positive feedback gain, the synchronisation of voltage shift rates from multiple PV inverters can be effectively improved under the varying active power conditions. According to the characteristic of maximum power point tracking control, the gain will be automatically changed along with the variable PV voltage. The advantage of the proposed method is analysed by the simulation results in comparison with the conventional VPF method. Moreover, the experimental results are provided to verify the effectiveness of the proposed method.

Voltage positive feedback based active method for islanding detection of photovoltaic system with string inverter using sliding mode controller

Islanding is an undesirable condition that is potentially dangerous to the maintenance or repair staff and may cause damages to sensitive equipments. Therefore, islanding detection is a mandatory feature for distributed generators (DGs), including grid–connected photovoltaic systems (GCPVSs). This article presents a new active islanding detection method for GCPVSs with string inverter based on sliding mode controller. In the proposed method, a feedback of point of common coupling (PCC) voltage has been inserted to the reference signal of sliding mode controller as a new maximum power point tracking (MPPT) technique. Through applying this voltage positive feedback (VPF), the inverter output power and consequently, the PCC voltage decreases in islanding condition. This procedure is repeated until the PCC voltage falls below the under voltage (UV) relay setting. The conventional VPF technique attempts to increase and decrease the output power in elevated and declined PCC voltage cases after grid disconnection, respectively. However, due to the input power limitation and MPPT in GCPVSs in the PCC voltage raised cases, the output active power cannot be elevated any further. In this study, the active power perturbation has been only applied in deceleration mode, considering the disturbance definition in the inverter voltage control loop. Therefore, unlike the conventional VPF scheme, it has the advantage of not violating the “conservation of energy law”. The performance of the proposed method is evaluated by simulating a GCPVS with string inverter supplying a local load, connected to the grid. Simulation results show troubleshooting the conventional VPF scheme drawbacks as well as successful operation of the proposed method in islanding and non–islanding conditions even in multi–inverter systems.

Anti-Islanding Detection for Single-Stage Photovoltaic System Based on Passivity

Based on the basic principles of anti-islanding detection, this paper puts forward an anti-islanding (AI) detection method which combined/q-axis voltage decoupling and positive feedback with Euler-Lagrange (EL) model .The proposed method is classified as an passive islanding detection method and it is based on the voltage change at the point of common coupling (PCC) that occurs in islanding .In the method, Add the d-axis decoupling voltage of PCC to the d-axis reference current of the EL model. A simulation study shows that the method is simple and feasible.

Islanding Detection Algorithm Based on Adaptive Voltage Positive Feedback

Islanding detection is an essential function of the grid-connected inverter of distributed power generation system. The deficiency of traditional islanding detection methods should be given due importance, especially the phenomena such as islanding detection failure when the output power of the inverter balances with the load power, and the use of active frequency and phase shifting method affecting the quality of power. In this regard, an improved islanding detection method based on adaptive voltage positive feedback was proposed. A mathematical model was established, and the hardware circuit was tested. The constraint conditions on the success of islanding detection and the pattern of voltage variation at the point of common coupling before and after incorporation to the grid were studied. The simulation result shows that this method has the advantages of high detection speed, small blind area and high reliability. The index requirements specified in IEEE Std.2000-929 are met. The reasonability of the proposed scheme is verified. DOI : http://dx.doi.org/10.11591/telkomnika.v12i6.5475 Full Text: PDF

New active islanding detection scheme for constant power and constant current controlled inverter‐based distributed generation

This study presents an active islanding detection scheme based on voltage positive feedback control strategy in synchronous d–q frame with a variable voltage gain. The proposed method employs the injection of a variable voltage gain at the DG end in order to monitor the responses of the proposed islanding detection technique. In this study, a complex frequency-dependent ZIP-exponential load model is included to assess the impact of load's active power frequency dependence on the performance of the proposed islanding detection scheme. The upper and lower limits of the voltage gain are designed for constant power and constant current controlled inverter with frequency-dependent ZIP-exponential load. The complete scheme is developed and tested in MATLAB/simulink environment and the results obtained after extensive testing indicate that the proposed scheme can be reliably detect islanding events in distributed generation.

Voltage Shift Acceleration Control for Anti-Islanding of Distributed Generation Inverters

This paper proposes voltage-shift acceleration control for islanding detection of distributed-generation (DG) inverters. The proposed method is conceptually based on a traditional <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -based voltage positive feedback control. A main improvement of the proposed control is that the anti-islanding performance and its impact on inverter output control can be consistent regardless of inverter operation points. An analytical approach based on a small-signal model of an inverter controller is used to prove the proposed control. Also, an analytical design method is proposed to obtain proper control gain that satisfies two conflicting issues: successful anti-islanding and limited impact on inverter control. Simulation and experimental tests were conducted to validate the proposed detection algorithm and its design method. In addition, simulation results on a two DGs system are shown to investigate the performance in multiple DGs operation.

Novel voltage stability constrained positive feedback anti-islanding algorithms for the inverter-based distributed generator systems - RETRACTED

A simple and effective control scheme for the anti-islanding of inverter-based distributed generators (DGs) is presented. The proposed control objective is achieved by a voltage positive feedback loop properly designed in the synchronous d-q frame. The test system configuration and parameters are prepared on relevant international standards. The system operating conditions and the overall power flow control scheme of the studied DG are firstly described. Then, the design methodology of an adaptive voltage shift acceleration gain based on small-signal stability and step input response is presented. The proposed control approach is intended to achieve a reliable detection with quasi zero non-detection zone, minimum effects on power quality and easy implementation without additional sensing devices or equipments. Performances on constant power and constant current control modes are investigated. The feasibility and effectiveness of the proposed anti-islanding approach and the gain design method are verified with comprehensive simulation and experimental results.

Influence of Voltage Positive Feedback Anti-Islanding Scheme on Inverter-Based Distributed Generator Stability

This letter investigates the influence of the voltage positive feedback anti-islanding scheme on the stability of inverter-based distributed generators (DGs). Modal analysis is used to determine the relationship between the positive feedback gain of the anti-islanding control and the stability limit. It was found that strong positive feedback gain will destabilize the distribution generator when the power transferred from the distribution system to the DG local load is small.

Islanding Detection Method for Multi-Inverter Distributed Generation

Islanding detection is an essential function for safety and reliability in grid-connected distributed generation (DG) systems. Several methods for islanding detection are proposed, but most of them may fail under multi-source configurations, or they may produce important power quality degradation which gets worse with increasing DG penetration. This paper presents an active islanding detection algorithm for Voltage Source Inverter (VSI) based multi-source DG systems. The proposed method is based on the Voltage Positive Feedback (VPF) theory to generate a limited active power perturbation. Theoretical analyses were performed and simulations by MATLAB /Simulink /SimPowerSystems were used to evaluate the algorithm’s performance and its advantages concerning the time response and the effects on power quality, which turned out to be negligible. The algorithm performance was tested under critical conditions: load with unity power factor, load with high quality factor, and load matching DER’s powers.