Active Frequency Drift Method Research Articles

Adaptive Active Frequency Drift Islanding Detection for PV Inverters

The penetration level of renewable energy resources (RESs), such as photovoltaic (PV) plant and Wind plant (WP), in the power system is increasing exponentially, such plants connected with grid via inverter. This increasing rises from the attention about the undetected islanding operation. The islanding can be defined, according to IEEE std.1547, as a situation in which part of the power system becomes isolated from the rest of the system.
 Islanding detection methods (IDMs) are divided into passive and active IDM. Among active IDMs, active frequency drift (AFD) is the most IDM applied in the literatures. AFD bases on injecting a distortion waveform to the original waveform of inverter reference current, this to drift the inverter frequency during islanding event to be out of the nominal range. Due to the distortion waveform high harmonics will be injected to the system. Recently, to decrease this harmonic an improved active frequency drift (IAFD) was presented in the literature. IAFD uses a constant step change in in 2nd and 4th quarters of the inverter reference current. IAFD has lower total harmonic distortion (THD) injected to the system compared with conventional AFD.
 Since the non-detection zone (NDZ) has been considered as a performance measure for any IDM. The IAFD method did not introduce any improvement in NDZ over the AFD method. Thus, in this paper an adaptive step change is proposed to improve the performance of IAFD method, where a positive feedback of voltage frequency is used in this work to vary the distortion factor of IAFD. The adaption manner of step change enhances in increasing of injected perturbations during islanding event only. Which, it will reduce the NDZ during islanding event and decrease the injected THD in steady state operation. The proposed method has been theoretically analyzed and modeled using MATLAB/Simulink environment. As a result, the proposed method improves the performance of IAFD regard to the non-detection zone (NDZ).

Improved Active Islanding Detection Technique with Different Current Injection Waveform

The active frequency drift (AFD) method is an effective method to detect islanding in grid-connected photovoltaic systems. However, it has some inherent drawbacks, such as generating higher harmonics. In order to reduce the harmonics and non-detection zone (NDZ), various improved AFD methods have been proposed, but they still suffer from high harmonics and reduced detection speed. To overcome these limitations, this paper proposes an innovative islanding detection technique based on AFD. Analysis reveals that the proposed method reduces harmonics by 68% compared to conventional AFD and has a larger chopping factor. Therefore, this technique offers several distinct advantages, including accelerated detection speed, reduced NDZ and harm caused by disturbances, and improved power quality. Furthermore, to verify the harmonic impact of this proposed islanding detection method, simulations and analyses are conducted using simulation software of Matlab/Simulink. An experimental prototype is set up in Laboratory. The simulation and experimental results demonstrate the superiority of the proposed method.

Review, analysis, and performance evaluation of the most common four active methods for islanding detection in grid-connected photovoltaic systems

• The AFD, SFS, SMS, and SVS active islanding detection methods. • The studied methods provide good detection times within the new grid standards. • The methods have good detection times under different parameters and scenarios. • The active methods are studied and compared for the first time in a detailed study. This paper systematically analyzes the islanding performance under different case studies and scenarios of the most well-known active islanding detection methods (IDMs) for single-phase grid-connected photovoltaic (PV) systems. They are named as follows: Active Frequency Drift (AFD), Sandia Frequency Shift (SFS), Slip Mode frequency Shift (SMS), and Sandia Voltage Shift (SVS) anti-islanding detection strategies. The performance of these four active anti-islanding methods has been examined in detail using Matlab/Simulink. Moreover, the quality factor and non-detection zone (NDZ) influence on islanding detection is also analyzed. According to the new grid codes and standards, the studied active IDMs provide good detection times. Furthermore, the case studies illustrate that these active detection techniques can successfully detect the islanding operation mode under different quality factors, types of loads, solar irradiance changes, and fault-ride through (FRT) operation mode.

Implementation of the Improved Active Frequency Drift Anti-Islanding Method into the Three-Phase AC/DC Converter with the LCL Grid Filter

The article presents a modified standard Active Frequency Drift (AFD) method used to detect unintentional island operation in converters generating electricity from renewable energy sources to the power grid. The primary aim of each of the island operation detection methods is the possibility of shortening the energising of a separate part of the power grid. The proposed method eliminates fragments of the reference current signal when it reaches a constant value for a particular time. This part of the signal is replaced with the hyperbolic sine function. It allows reducing the value of Total Harmonic Distortions (THD) while maintaining the same effectiveness of island operation detection. The article contains a detailed description of the newly proposed type of disturbance generation. The proposed solution is verified by conducting simulation and laboratory tests. The possibility of shortening the island operation detection time is proven by increasing the maximum distortion introduced into the current without exceeding the permissible THD limit for converters connected to the power grid.

A modified active frequency islanding detection method based on load frequency and chopping fraction changes

To carry out the study and simulations of anti-islanding systems, it is necessary to take into account the generic system proposed by the IEEE 929-2000 and IEEE 1547 standards, where the network, the RLC load, and PV inverter are connected in parallel to the PCC. The IEEE 929-2000 standard defines the quality factor to have a standard test condition. The paper presents a modified Active Frequency Drift (AFD) algorithm to detect the islanding of grid-connected photovoltaic (PV) systems with low harmonics levels without degrading their power factor. Severe issues such as deterioration in power quality and electric shock can occur when a PV inverter operates in islanding mode. Various islanding detection techniques have been investigated in the literature in the last two decades among which, AFD is a typical method. However, a prominent drawback of AFD is its relatively large Non-Detection Zone (NDZ). The improved AFD method that this paper suggests allows the frequency of the inverter current to be controlled slightly lower (or higher) than the frequency of the terminal voltage. The islanding condition is detected by calculating the changing parameter that relates to the change of the chopping fraction and the line frequency periodically. The frequency deviation is then detected to determine the chopping and a changing parameter, which accumulates to the threshold value when the inverter is shut down. The proposed algorithm overcomes the conventional AFD issues by improving the accuracy of detection, eliminating the NDZ, and reducing the current harmonic distortion. The proposed method is validated with the help of simulated and experimental results of a 350 W PV inverter using IEEE Standards.

Improved Active Islanding Detection Technique for Multi-Inverter Power System

Due to the increased penetration of multi-inverter distributed generation (DG) systems, different DG technologies, inverter control methods, and other inverter functions are challenging the capabilities of islanding detection. In addition, multi-inverter networks connecting the distribution system point of common coupling (PCC) create islanding at paralleling inverters, which adds the vulnerability of islanding detection. Furthermore, available islanding detection must overcome more challenges from non-detection zones (NDZs) under reduced power mismatches. Therefore, in this study, a new method called parameter self-adapting active islanding detection was utilized to minimize the dilution effect and reduce NDZs in multi-inverter power systems. The method utilizes an active frequency drift (AFD) method and applies a positive feedback gain of adoption parameters, which significantly minimizes NDZs at parallel inverters. The simulation and experimental outcomes indicate that the proposed method can effectively weaken the dilution effect in multi-inverter networks connecting the distribution system PCC.

Study on a Correlation-Based Anti-Islanding Method under Wider Frequency Trip Settings for Distributed Generation

Islanding phenomenon of distributed generation (DG), such as photovoltaic (PV) generation, is undesirable because it causes safety issues for utility service personnel and power system equipment. Many anti-islanding methods have been studied since DG appeared in electric power systems (EPSs). Most anti-islanding methods focus on disconnecting DG from the grid using functionality to detect islanding under narrow frequency trip settings, because safety issues have a higher priority. However, as DG plays a key part of an EPS, a significant loss of DG due to a short disturbance could result in a reliability issue for the EPS. Corresponding to this matter, new international standards, such as IEEE standard 1547–2018, require more sophisticated and complex functionalities for grid-connected DGs by adopting ride-through technologies and wider voltage/frequency trip settings. Since most anti-islanding functions of inverter-based DG have been based on the frequency of the inverter voltage, it is more difficult to detect islanding under wider frequency trip settings. This paper presents a correlation-based anti-islanding method (AIM) without depending on the frequency trip of inverter-based DGs. Simulation results are provided to verify the performance of the correlation-based anti-islanding method. As a result, the proposed method detects islanding at 0.116 s under wider frequency trip setting by the IEEE Std. 1547–2018 test condition, while the popular active frequency drift method with positive feedback does not detect islanding using the same current disturbance.

An active frequency drift method for island problem of grid‐connected photovoltaic power generation system

Island problem in grid‐connected photovoltaic power generation system has a large influence on the safety and reliability of system, which needs to be timely detected. In this article, the photovoltaic grid‐connected generation and island problems were briefly introduced, and then active frequency drift (AFD) method for island detection was studied. It was found that AFD might affect the quality of output current. Therefore, an improved AFD method was proposed, which added a phase shift change to the original reference current. The simulation experiment suggested that the improved AFD method proposed in this article could detect the islanding problem in 0.1 s, and the total harmonic distortion of the output current was 1.62%, much less than that of the AFD method. The experimental results show that the improved AFD method has better performance in island detection, with high security and reliability, and is worth popularizing widely in photovoltaic grid‐connected power generation systems. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Improved Three-Phase AFD Islanding Detection Based on Digital Control and Non-Detection Zone Elimination

This paper proposes an improved active frequency drift (AFD) islanding detection method of a three-phase inverter. Different than the existing single-phase AFD method, in the three-phase system, the disturbance is added to the phase angle, which takes part in coordinate transformation calculations. Thus, the frequency drift of the three-phase reference current can be realized by the disturbance of the phase angle. It is unnecessary to add frequency drift to each phase respectively with the proposed method, which can simplify the control system. Furthermore, the non-detection zone can be eliminated by updating certain parameters; therefore, the detection method is sensitive. In this paper, the application of the proposed method in a digital control system is discussed in detail. The non-detection zone is analyzed, and an elimination method is proposed. Finally, the simulation and experimental results are given to verify the theoretical analysis.

A Novel Islanding Detection Method for Photovoltaic Grid Connected System

It is very important to detect islanding effect timely and accurately in the case of power grid failure. Therefore, a new periodic perturbation positive feedback active frequency drift method is proposed in this paper. When the power grid fails, the inverter output voltage frequency always has obvious change under the different load of inverter, and the islanding effect can be detected quickly and accurately by this control method. Experimental results show that the proposed method can detect grid faults quickly and accurately under different load conditions.

Research on Island Detection Based on Improved Positive Feedback Active Frequency Shift

Island detection is the essential function of photovoltaic grid connected generation system. The detection of the island effect must be accurate and fast, and the effect of the island detection method should be minimized on the quality of grid connected current. The positive feedback active frequency drift method is the most widely used method in current application. Aiming at the shortcomings of the detection blind area and the affect of quality of grid connect current of the algorithm. An improved positive feedback active frequency drift method is proposed. By detecting the magnitude the frequency offset of the initial time, the method is applied to determine the initial chopping factor and the direction of the disturbance by detecting the frequency deviation of the initial size, and the absolute value of the cumulative frequency deviation as positive feedback effect act on the common point. The improved island detection method not only improves the detection accuracy, but also reduces the current harmonic distortion rate. The simulation results shows that the improved AFDPF algorithm island detection time is faster than traditional AFDPF algorithm 0.04 s.

Two level islanding detection method for distributed generators in distribution networks

Use of distributed generators in distribution networks is proved highly reliable and economical. In utility practice at the present time, accidental islanding is an undesirable mode of operation as it can harm the personnel and other connected systems. Therefore, islanding must be detected and the islanded distributed generators must be disconnected from the rest of the system. This paper proposes a new islanding detection method which provides a reliable detection in two levels. At first level wavelet energy entropy is used as an indicator to suspect islanding and activate second level method. Second level method is an active frequency drift method with positive feedback and pulsating chopping fraction that vary the output voltage frequency of distributed generators to realize the anti islanding protections. The performance of the proposed method is simulated in MATLAB environment. The proposed algorithm provides fast islanding detection with least disturbance in inverter’s grid connected operation.

Active Frequency Drift Method for Islanding Detection Applied to Micro-inverter with Uncontrollable Reactive Power

This paper proposes active frequency drift (AFD) as an anti-islanding method applied to micro-inverters with uncontrollable reactive power. When using ordinary inverter topologies, such as full bridge inverters in photovoltaic systems, the islanding phenomenon can be detected with reactive power-based methods, such as reactive power variation. However, when the inverter topology cannot control the reactive power, conventional anti-islanding methods with reactive power cannot be utilized. In this work, the topology used in this paper cannot control the reactive power. Thus, an anti-islanding method that can be used in topologies that cannot control the reactive power is proposed. The conventional anti-islanding method of the topology that cannot control reactive power is introduced and analyzed. Unlike the conventional AFD method, the proposed method extends a zero current interval every predetermined cycle. The proposed method offers certain advantages over conventional AFD methods, such as total harmonic distortion. The proposed method is validated through simulation and experiment.

A New Islanding Detection Method Based on Feature Recognition Technology

Three-phase grid-connected inverters are widely applied in the fields of new energy power generation, electric vehicles and so on. Islanding detection is necessary to ensure the stability and safety of such systems. In this paper, feature recognition technology is applied and a novel islanding detection method is proposed. It can identify the features of inverter systems. The theoretical values of these features are defined as codebooks. The difference between the actual value of a feature and the codebook is defined as the quantizing distortion. When islanding happens, the sum of the quantizing distortions exceeds the threshold value. Thus, islanding can be detected. The non-detection zone can be avoided by choosing reasonable features. To accelerate the speed of detection and to avoid miscalculation, an active islanding detection method based on feature recognition technology is given. Compared to the active frequency or phase drift methods, the proposed active method can reduce the distortion of grid-current when the inverter works normally. The principles of the islanding detection method based on the feature recognition technology and the improved active method are both analyzed in detail. An 18 kVA DSP-based three-phase inverter with the SVPWM control strategy has been established and tested. Simulation and experimental results verify the theoretical analysis.

Impedance-Based Analysis of Active Frequency Drift Islanding Detection for Grid-Tied Inverter System

Islanding detection is critical to the safety of grid-tied inverters. This paper presents an impedance-based analysis of the active frequency drift (AFD) islanding detection method. To study this method, the output impedance of a grid-tied inverter is modeled. The $Z_{qq}$ of the inverter impedance features a negative incremental resistor, whose magnitude is equal to the dc impedance magnitude of the local load defined by the test standard. With a large value of feedforward gain $N$ in the AFD method, the phase of $Z_{qq}$ is shown to drop below 180°. Under an islanding condition, due to a lack of phase margin, the inverter system becomes unstable with a frequency drift away from its steady state. The islanding condition is then identified by a frequency detection unit. Experimental results verify the analysis.

Implementation of Active Anti-Islanding Methods Protection Devices for Grid Connected Photovoltaic Systems

Single-phase grid tied inverter is one among types of inverters widely used in photovoltaic generation systems due to the advantages they offer, in the absence of the grid, islanding situations occur. This paper describes model and simulation of such inverter in operation as distributed generation in electrical power system under islanding phenomena. In this configuration, it is imperative that the inverter automatically disconnects from the grid without remaining any residual voltage that can be dangerous for the plants and stakeholders. It is important to detect the islanding situation. For this purpose several techniques are used. We present in this article the implementation of the method called Slide-Mode frequency Shift (SMS) in the control of a single phase photovoltaic inverter, for which we provide a description of our SIMULINK implementations and their evaluations performed on the basis of comparison with Active Frequency Drift (AFD) method. Simulation results show that the developed algorithm detects the islanding phenomenon in the time approaching that required by the IEC and IEEE standards that secures the PV system connected to the grid.  Index Terms—PV systems, electrical grid, islanding detection methods, single phase PV inverter, distributed generation systems

계통연계형 태양광발전시스템의 AFS 기법을 이용한 단독운전 방지

계통연계형 인버터의 중요 성능 중 단독운전 검출기능 및 방지기능은 계통을 유지보수하기 위한 작업자의 안전을 위하여 고려되어야 할 필수 사항이다. 본 논문에서는 태양광을 이용한 계통연계형 태양광발전시스템에서의 AFS(Active Frequency Shake) 단독운전 검출기법을 제안하고자 한다. 먼저 AFS 단독운전 검출 알고리즘을 제안하기 위하여 기존의 능동 주파수 변이기법 알고리즘에 대해 기술한다. 그리고 단독운전 불검출영역을 줄이고, THD(Total Harmonic Distortion)를 개선하며, 기존 알고리즘 보다 검출 속도 또한 향상 시킬 수 있는 알고리즘을 제시하였다. 아울러 제안된 단독운전방지 기법을 3kW 계통연계형 태양광 인버터에 적용하고 시뮬레이션 및 실험을 통하여 제안하는 알고리즘의 타당성을 검증하였다. The islanding detection and prevention function in the important performance of the grid-connected inverter is the essential details which should be considered for worker's safety to maintain the system. This paper tries to suggest the active frequency shake islanding detection technique in the grid-connected PV generation system using sunlight. The existing active frequency drift method algorithm is described to suggest active frequency shake method algorithm. And, the algorithm which can reduce the non-detection zone, improve THD, and improve the detection speed in comparison with the existing algorithm is suggested. And we applied the proposed algorithm to the 3kW grid-connected photovoltaic inverter generation and verified the feasibility of the technique through the simulation and experiments.

Investigation and evaluation of active frequency drifting methods in multiple grid-connected inverters

As the development of distributed generation goes on, more grid-connected inverters are being inevitably connected to the same local electrical power system and the perturbations exerted by each inverter for islanding detection might interfere with each other. The islanding detection principle of commonly used active frequency drift (AFD), sandia frequency drift and slip-mode frequency shift are comprehensively investigated and improved algorithms are proposed as well for better detection performance. Moreover, the interaction of these strategies applied in multiple-inverter systems especially with frequency measurement errors is systematically analysed. The corresponding detection performance is summarised, which can also serve as an effective guideline for designing AFD methods in multi-inverter systems. Finally, simulation and experimental results are provided to verify the islanding detection performance of these methods under multiple-inverter operation.

Simulation of Islanding Detection for Photovoltaic Grid-Connected Inverters Based on Positive Feedback Intermittent Frequency Drift

Islanding detection is a absolutely necessary function for Photovoltaic Grid-Connected Inverters and it is very important to detect the islanding accurately and quickly with less adverse impact on power quality. This paper proposes a positive feedback intermittent active frequency drift method which can decrease the adverse impact of frequency disturbance effectively within the time restrict of islanding detection according to that of traditional islanding detection methods. The theoretical analysis and simulation results show that this algorithm which is simple, effective, fast and have less adverse impact on power quality and has certain application value.

개선된 AFD기법의 모델링 및 분석

In this paper, among the active islanding detection techniques, the modified active frequency drift method was analyzed, which is relatively easy to apply to the single-phase grid-connected PV PCS. The existing designs for turbulences in these applications were empirically conducted, and do not have sufficient reliability and performance. Therefore, three application forms of the modified active frequency drift technique were modeled, based on which the proper magnitude of turbulence, which is the frequency acceleration component, was calculated. Using the results, the magnitude of and injection method for turbulence for ensuring the islanding detection performance and improving the output power quality were proposed, and they were verified via simulations and experiment to prove that the reliable islanding detection technique can be developed merely by measuring the basic output power quality, without the need for expensive islanding simulation equipment.