Victor M. Moreno

Wavelet-Based Islanding Detection in Grid-Connected PV Systems

Distributed power generation systems (DPGSs) based on inverters require reliable islanding detection algorithms (passive or active) in order to determine the electrical grid status and operate the grid-connected inverter properly. These methods are based on the analysis of the DPGS voltage, current, and power in time or frequency domain. This paper proposes a time-frequency detection algorithm based on monitoring the DPGS output power considering the influence of the pulsewidth modulation, the output LCL filter, and the employed current controller. Wavelet analysis is applied to obtain time localization of the islanding condition. Simulation and experimental results show the performance of the proposed detection algorithm in comparison with existing methods.

An Anti-Islanding Method for Single-Phase Inverters Based on a Grid Voltage Sensorless Control

The detection of the islanding condition of a distributed generation (DG) system is crucial for safety reasons, as discussed in the IEEE standards and specifically required by some national codes. Several anti-islanding methods that are resident in the inverter have been investigated and classified as passive (measurement of the natural effects of islanding) or active (based on the measurement of the effects due to transients or harmonics deliberately introduced in the system). In case the power drained by the load matches the power generated by the DG inverter, the effect of islanding is small, and the passive methods fail. However, the active methods, which have been developed to overcome these limits, create disturbances that can interact with those generated by other DG systems. In this paper, a new anti-islanding method is proposed. It exploits the natural sensitivity of a grid-voltage sensorless control to disturbances to highlight the islanding condition. The adopted grid-voltage sensorless control is adapted to a single-phase system with the use of resonant controllers based on the internal model control law: resonant-controller-based observer results. Then, a Kalman-filter-based algorithm is used to detect the islanding condition based on the energy mismatch between the estimated third and fifth harmonics and the real ones. Experimental results support the analysis

A Comparative Analysis of Real-Time Algorithms for Power Signal Decomposition in Multiple Synchronous Reference Frames

The monitoring and rejection of voltage and current harmonics in power electronics applications such as power quality conditioners or distributed generation systems require correct estimation algorithms especially if the harmonic amplitudes are time varying. Power signal decomposition in multiple synchronous rotating reference frames (MSRFs) is considered one of the best solutions. The most commonly employed implementations of this signal transformation are based on phase-locked loops (PLLs), recursive discrete Fourier transforms (RDFT), or discrete Kalman filtering (DKF). In this paper, a rigorous analysis of the performance of these implementations has been carried out. Complete tests have been performed to evaluate the computational burden, the frequency domain response, and the tolerance to low frequency amplitude variations. The results make it possible to select the proper method depending on the requirements of each application.

Performance Evaluation of Active Islanding-Detection Algorithms in Distributed-Generation Photovoltaic Systems: Two Inverters Case

Grid-connected photovoltaic (PV) inverters employ an islanding-detection functionality in order to determine the status of the electrical grid. In fact, the inverter must be stopped once the islanding operating mode is detected according to standards and grid-code limits. Diverse islanding-detection algorithms have been proposed in literature to cope with this safety requirement. Among them, active methods based on the deliberate perturbation of the inverter behavior can minimize the so-called nondetection zone, which is a range of conditions in which the inverter does not recognize that it is operating in an undesired island. In most cases, the performances of these methods have been analyzed considering a highly dispersed generation scheme, where only one distributed-generation power system is connected to the local electrical power system (EPS). However, in some studies, it has been highlighted that if two or more PV inverters are connected to the same local EPS, their anti-islanding algorithms do not behave ideally and can fail in detecting the islanding condition. However, there is no systematic study that has investigated the overall capability of different anti-islanding methods employed on several inverters connected to the same EPS to detect islanding condition. This paper is a first attempt to carry out a systematic study of the performances of the most common active detection methods in a case of two inverters connected to the same EPS. In order to evaluate the global capability of the two systems to detect islanding condition, a new performance index is introduced and applied also to the case when the two inverters employ different anti-islanding algorithms.

Reference current estimation under distorted line voltage for control of shunt active power filters

Shunt active power filters (APF) are used in power systems for the compensation of harmonic currents generated for non linear loads. A new digital reference current estimation method for control of APF using a Kalman digital algorithm is presented. Its capability of prediction avoids the effects of computational lags derived from the digital signal processing. The characteristics of the proposed technique are: the harmonic current compensation in a global or a selective way, the fast dynamical response and its independence from disturbances in the line voltage waveform. Simulation and experimental results under distorted supply voltages demonstrate the usefulness of the presented technique to improve the filtering performance.

A Recursive Park Transformation to Improve the Performance of Synchronous Reference Frame Controllers in Shunt Active Power Filters

Load harmonic currents and load unbalances reduce power quality (PQ) supplied by electrical networks. Shunt active power filters (SAPFs) are a well-known solution that can be employed to enhance electrical PQ by injecting a compensation current at the point of common coupling (PCC) of the SAPF, the load, and the electrical grid. Hence, SAPF controllers must determine the instantaneous values of the compensation reference current, including nondesirable components of the load current. A family of SAPF controllers, which evaluates the compensation reference current using synchronous rotating frames (SRFs), employs a structure based on Park transformations: direct transform, low- pass filtering (LPF), and inverse transform. The cutoff frequency and the filter order of the LPF stage must be designed properly in order to obtain an accurate reference current and a fast dynamic response of these SAPF controllers. This paper proposes a recursive implementation of the direct Park transformation that avoids the filtering stage and allows accurate SRF controllers to be designed. Moreover, the proposed implementation is not dependent on PCC conditions. The proposed implementation is evaluated using a three-phase, three-wire SAPF and compared with LPF-based controllers by simulation and experiment.

Wavelet-Based Islanding Detection Algorithm for Single-Phase Photovoltaic (PV) Distributed Generation Systems

Distributed Generation (DG) systems based on inverters connected to low-voltage electrical grids, such as low power PV systems in buildings, require reliable islanding detection algorithms in order to determine the electrical grid status and operate the inverter properly. Passive and active islanding detection methods are local detection techniques which have been presented and analyzed in literature. The first approach is based on current and voltage measurements at the inverter side of the point of common coupling (PCC) between the electrical grid, the local load and the DG system. Active methods introduce a disturbance at the PCC in order to reveal the islanding condition. This paper proposes a new hybrid detection algorithm based on monitoring of high-frequency components of the DG system output power due to PWM, the output LCL filter and the employed current controller. Wavelet analysis is applied to obtain time localization of the islanding operation mode. Simulation results show the performance of the proposed detection algorithm.

Multilevel Phase-Shifting Carrier PWM Technique in case of Non-Equal dc-Link Voltages

This paper analyzes the phase-shifting carrier PWM techniques in case of multilevel cascaded H-bridges with non-equal DC-link voltages. In the past it has been demonstrated that these techniques succeed in cancelling the sideband harmonics in the produced multilevel voltage only if the DC-link voltages are equal. Two different approaches can be used to modulate the reference signal in multilevel converters when the DC-link voltages are different and in a non-integer ratio: the theory of symmetric polynomials and resultants, and the generalized hybrid modulation (GHM) technique, which extends the hybrid modulation technique to non-integer DC-link ratio. This paper extends the original PSC-PWM technique proposing a method to obtain the proper shifting angles in view of reducing the overall output voltage WTHD under every possible de-link voltages. The proposed new approach is compared, in simulation with the original phase-shifted cascaded multilevel technique

Modified FBD Method in Active Power Filters to Minimize the Line Current Harmonics

Harmonic line currents due to nonlinear loads cause nonsinusoidal grid voltages in electrical power systems. Shunt active power filters (SAPFs) can be employed to improve the system efficiency but their design must consider nonsinusoidal conditions. The Fryze, Buchholz, and Depenbrock method has been applied to controllers in SAPFs, allowing the compensation of the load nonactive power. A drawback of this method is a resultant line current with harmonic components which contributes to maintain the grid voltage distortion. This letter proposes a new definition of conductance for SAPF controllers in case of nonlinear loads and nonsinusoidal grid voltages which minimize the line current distortion

Accurate and Computationally Efficient Implementation of the IEEE 1459-2000 Standard in Three-Phase Three-Wire Power Systems

The definitions for the measurement of electrical power quantities under sinusoidal, nonsinusoidal, balanced, or unbalanced conditions are established in IEEE Standard 1459-2000. Measured magnitudes, such as apparent powers or effective quantities, are evaluated considering the frequency spectrum of voltage and current signals (fundamental, nonfundamental, and combined magnitudes). Previous papers implement these definitions in the frequency domain (DFT) and in the time domain (Clarke-Park Transformations). In the time domain, low-pass filters (LPFs) must be applied to obtain the fundamental magnitudes but, in this case, the quality of the measurements depends on the LPFs orders and their cutoff frequencies. This paper proposes a time-domain implementation of the standard definitions without LPFs and employs a recursive averaging algorithm which improves the measurements precision and reduces the computational burden. Simulation and experimental tests showing the proposed method performance are included