A. S. Aljankawey

Influences of Power Electronic Converters on Voltage–Current Behaviors During Faults in DGUs—Part I: Wind Energy Conversion Systems

The growing interest in clean and sustainable electric energy pushes toward increasing the interconnection of wind energy conversion systems (WECSs) to utility grids. The designs of the majority of WECSs employ power electronic converters (PECs), which generally have nonlinear and switched characteristics. The characteristics, operation, and control of PECs in WECSs can result in nonconventional voltage-current behaviors during faults. This paper investigates the voltage-current behaviors during faults that occur in grid-connected WECSs. Two types of WECSs are considered in this paper, which are the doubly fed induction generators and permanent magnet generator based WECSs. The voltage-current behaviors are investigated in experimentations for different faults occurring in different parts of the test grid-connected WECSs.

Apparent Power-Based Anti-Islanding Protection for Distributed Cogeneration Systems

In this paper, the performance of a passive anti-islanding method is experimentally tested for three phase (3φ) cogeneration systems. The tested method is based on determining the wavelet packet transform (WPT) high-frequency subbands present in the d-q-axis components of instantaneous 3q apparent powers (sd and sq), when evaluated at the point of common coupling (PCC). This passive anti-islanding method is founded based on the nature of instantaneous 3φ apparent powers that have components continuously exchanged between both sides of the PCC. An islanding condition can be considered as a transient disturbance that creates nonperiodic and nonstationary high frequency components in sd and sq. These frequency components can be parameterized by WPT high-frequency subbands, which can provide accurate detection of the islanding condition. The d-q WPT-based anti-islanding method is tested for a 3φ cogeneration system under various loading and power delivery conditions. Performance results reveal accurate, fast, and reliable detection and response to the islanding condition.

Influences of Power Electronic Converters on Voltage–Current Behaviors During Faults in DGUs—Part II: Photovoltaic Systems

Photovoltaic (PV) systems have become a popular type of distributed generation units (DGUs) for small and medium levels of renewable energy generation. When grid connected, these DGUs are designed and operated to deliver electric power to their host grid through power electronic converters. The nonlinear and switched natures, along with the control actions, of these PECs can influence the voltage-current behaviors during faults in grid-connected PV systems. This paper experimentally investigates the voltage-current behaviors during faults occurring in grid-connected PV systems. Results of these investigation show that voltages and currents at the point-of-common-coupling (PCC) experience changes in their magnitudes and harmonic distortions due to faults occurring on both sides of PCC. These results can be incorporated in the improvement of the accuracy and reliability of protective devices used for grid-connected PV systems.

Passive Islanding Detection Approach Based on Tracking the Frequency-Dependent Impedance Change

This paper presents a passive islanding detection approach based on measurements of the frequency-dependent impedance at the point of common coupling. The approach exploits the presence of harmonics injected by the electric power system (EPS) and harmonics injected by the distributed generator (DG). Models of impedance are developed to characterize the interconnection topology among the DG, the EPS, and the local load. The interconnection topology changes when islanding occurs resulting in a frequency-dependent change of the impedance as seen by the DG. Islanding is detected when certain changes in the impedance magnitude are detected at certain harmonics. The approach is new: 1) an impedance-based analytic model is derived from the interconnection topology and this may be used as a basis for feature extraction and threshold selection; 2) a passive algorithm is proposed as an enhancement to the under/overfrequency (UF/OF) method; 3) a grid impedance parameter space is derived to characterize the detection zone; and 4) a local load parameter space is used to compare the nondetection zone (NDZ) of the proposed method with that of the UF/OF method without the enhancement. The proposed approach is shown to reduce the NDZ when combined with the UF/OF method.

A new impedance-based approach for passive islanding detection scheme

This paper presents a new approach for islanding detection that is based on monitoring measures of impedance at the point of common coupling (PCC). A frequency dependent model that characterizes the change in circuit topology when an island occurs is developed as a first step in systematically selecting impedance metrics and thresholds for islanding detection. To implement passive islanding detection, the monitoring operation exploits the presence of harmonic distortion in the current and voltage at the PCC as a basis for computing measures of impedance at frequencies where there is sufficient harmonic content. Unlike signals based schemes that rely on heuristics, the proposed approach is based on an analytic model that reflects the interconnection topology. The main contribution of the work is the development of an analytic model that reflects the interconnection topology of distributed generators (DGs) with the electric power system (EPS) for DGs that include power converters and DGs that do not include power converters.

Performance evaluation for grid impedance based islanding detection method

This paper develops an approach for islanding detection based on measurements of the frequency dependent impedance at the point of common coupling (PCC) that exploits the presence of harmonics introduced by the electric power system (EPS) and harmonics introduced by the distributed generator (DG). The approach is new: i) an analytic model for the frequency dependent impedance is derived from the interconnection topology and this may be used as a basis for selecting features of the impedance that change when islanding occurs, and ii) the variation of a frequency dependent feature is used to characterize a non detection zone (NDZ) and iii) the load parameter space is used to compare the NDZ of the grid impedance based scheme with under/over frequency schemes. Although the focus is passive islanding detection, it also encompasses active schemes where certain harmonics are intentionally injected rather than inherent.

A passive islanding detection index based on variation of signal energy

A new index for passive islanding detection is proposed. The approach exploits the inherent harmonic distortion produced by grid-connected power converters as well as the frequency dependent changes in the grid impedance as seen by the distributed generator before and following islanding. The new index is computed from the variation of signal energy over a certain band of frequencies contained within a virtual power signal. The virtual power signal is obtained from measurements of voltage and current at the point of common coupling. The approach is validated via simulation and is demonstrated on a 7kW distributed generator system.

Developing and testing phaselet frames-based digital protection for distributed generation units

This paper presents the development and performance testing of a new digital protection for interconnected distributed generation units (DGUs). The developed digital protection is structured to detect and respond to transient disturbances (fault and non-fault conditions) based on the magnitudes and phases of the high frequency sub-band contents extracted from the d-q axis components of the currents flowing through the point-of-common-coupling (PCC). These magnitudes and phases are extracted by employing a set of 6 phaselet frames. The employed phaselet frames are realized by a modulated filter bank that is composed of 6 digital high pass filters (HPFs). The coefficients of digital HPFs are determined by bi-orthogonal phaselet basis functions. Extracted magnitudes and phases of the high frequency sub-band contents of the PCC d-q axis current components provide signature information for accurate detection and identification of faults. The performance of phaselet frames-based digital protection is experimentally tested for two wind energy conversion systems and a photovoltaic system under different fault and non-fault conditions. Test results of the proposed digital protection demonstrate reliable and timely responses, along with negligible sensitivity to the type and control of DGUs, type and location of faults, and loading levels.

On the Experimental Performance of a Coordinated Antiislanding Protection for Systems With Multiple DGUs

This paper presents the implementation and performance evaluation of a coordinated antiislanding protection for systems with multiple distributed generation units (DGUs). The proposed coordinated antiislanding protection is structured to process the d - q-axis components of the instantaneous three phase apparent powers (sd and sq) determined at the point-of-common coupling for each DGU. The processing of sd and sq, for each DGU, is carried out by the wavelet packet transform (WPT) in order to extract their low-and high-frequency subband contents. The contents of WPT low-and high-frequency subbands offer signature information that can facilitate detecting the islanding condition and identifying the islanded DGU(s). The coordinated antiislanding protection is implemented in real time for experimental testing on a laboratory collector system that has three different DGUs. Experimental results reveal fast and accurate responses to islanding events, accurate identification of islanded DGUs, and negligible sensitivity to the type or ratings of protected DGUs. In addition, test results show that the d - q WPT-based coordinated antiislanding protection can accurately distinguish between islanding and nonislanding events, including faults, step changes in power delivery to the grid, unintentional loss of grid connection, low-voltage ride through, and sudden harmonic distortion on the grid side.

Impacts of grounding configurations on responses of ground protective relays for DFIG-based WECSs

One of the requirements for safe, stable, sustainable, and profitable operation of doubly-fed induction generators (DFIGs)-based wind energy conversion systems (WECSs) is the accurate and reliable protection against electrical faults, in particular ground faults. The performance of protective devices employed to achieve this requirement is highly dependent on the grounding configuration of the DFIG-based WECS. This paper investigates impacts of the grounding configuration on the performance of protective devices used to protect DFIGs-based WECSs from electrical ground faults. Investigated grounding configurations include solid-grounding, low-resistance grounding, high-resistance grounding, and no-grounding. The impacts of the grounding configurations on protective devices are observed through their ability to identify faults, as well as their speed to respond to identified faults. Simulation and experimental results reveal that adequately designed low-resistance grounding offers the minimum impacts on protective devices used for ground protection of DFIG-based WECSs.