Wavelet packet-based passive islanding detection method for grid connected photovoltaic inverters

This study presents a new method for detecting an islanding event in a microgrid with embedded converter‐based distributed generation (DG). Unlike other schemes, the proposed scheme injects negligible perturbations into the microgrid after the generation of an alert signal. The proposed scheme then uses another index called superimposed impedance, ΔZ. The ΔZ is characterised by a low steady‐state magnitude during the grid‐connected mode and a high magnitude during the islanded mode. Furthermore, for a fault at the point of common coupling (PCC), the magnitudes of ΔZ and PCC voltage are both very low – except during the initial transient period, where |ΔZ | momentarily crosses the threshold. Therefore, an islanding event can be detected if the magnitude of ΔZ is high for some specified time. Moreover, a fault event will not be misdirected as an islanding event because the steady‐state magnitudes of both the ΔZ and PCC voltage are very low in the case of a fault. The robustness of the proposed detection scheme is evaluated against different islanding conditions and also, for a fault at the PCC, first by using MATLAB‐based simulations and later by using a laboratory‐based experimental setup. A comparison with a recently published detection scheme shows the superiority of the proposed schemes.

Islanding is a major problem that is related to a grid connected distribution generator. During islanding the distribution generator gets disconnected from the grid while still being energized. It should be detected as fast as possible in order to prevent the distribution generator from getting damaged or to ensure the safety of the personnel working. The islanding detection makes use of system parameter to detect islanding at the time of occurrence. In this paper a passive islanding detection method based on ripple content of voltage at the point of common coupling is being used to differentiate between islanding and non-islanding conditions. A threshold level is set upon the ripple content of voltage at the point of common coupling and islanding is detected whenever the threshold level is exceeded for some certain period of time. This method effectively distinguishes the islanding condition from other non-islanding condition and detects the islanding condition efficiently. The working of this method is being ensured by simulating a variety of islanding and non-islanding condition. No non-detection zone was shown by this method and islanding conditions were detected within 150ms of the allowed delay time. Theoretical analysis and simulation results are presented to validate the proposed method.

In this paper, the performance of a passive anti-islanding method is experimentally tested for three phase $(3\phi)$ 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 $3\phi$ apparent powers $(s_{d} \mbox{and} s_{q})$ , when evaluated at the point of common coupling (PCC). This passive anti-islanding method is founded based on the nature of instantaneous $3\phi$ 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 $s_{d}$ and $s_{q}$ . 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 $3\phi$ cogeneration system under various loading and power delivery conditions. Performance results reveal accurate, fast, and reliable detection and response to the islanding condition.

This paper proposes a point of common coupling (PCC) voltage compensation method for islanding microgrids using an improved power sharing control scheme among distributed generators (DGs) without communication. The PCC voltage compensation algorithm is implemented in the droop control scheme to reduce the PCC voltage deviation produced by the droop controller itself and the voltage drop on the line impedance. The control scheme of each individual DG unit is designed to use only locally measured feedback variables and an obtained line impedance to calculate the PCC voltage. Therefore, traditional voltage measurement devices installed at the PCC as well as communication between the PCC and the DGs are not required. The proposed control scheme can maintain the PCC voltage amplitude within an allowed range even to some extent assuming inaccurate line impedance parameters. In addition, it can achieve proper power sharing in islanding microgrids. Experimental results obtained under accurate and inaccurate line impedances are presented to show the performance of the proposed control scheme in islanding microgrids.

In future low voltage grids, with multiple inverter interfaced sources connected, voltage regulation may become a necessary task. The potential exists for inverter interfaced sources to be deployed to regulate the voltage at the point of common coupling (PCC) of each inverter interfaced sources. The PCC voltage regulation is attainable with inverter interfaced sources by dynamically controlling the amount of reactive power injected to the power distribution grid by individual systems. In the current research, a closed-loop controller is proposed to regulate the PCC voltage of a solar photovoltaic (PV) system that is connected to a single-phase power distribution feeder (with R to X ratio greater than 1). The plant model of the PCC voltage controller of the PV system is derived considering both reactance and resistance of the network to which the PV system is connected. Three different compensators are evaluated to identify a suitable compensator for the closed-loop PCC voltage controller to regulate the PCC voltage at a given reference voltage. Simulation studies and experimental verification confirm that the theoretical approach taken to derive the control plant model of the PCC voltage controller is accurate and the procedure that is followed to design the controller is robust. The control design procedures illustrated in the current research leads to a PCC voltage control system with acceptable dynamic and steady state performance.

Interconnecting microgrids in LV power system presents appealing features such as self-healing or power quality. When networked microgrids are not connected to a strong utility grid, their Point of Common Coupling (PCC) voltage and their power reserves vary with the operating point. An external droop control architecture is proposed that allows active and reactive power sharing among the different microgrids, thereby stabilizing the system frequency and PCC voltage, and the maximum achievable droop gains are supplied. Next, the design of appropriate primary controllers for the Distributed Energy Resources inside each microgrid allows to achieve a specified aggregated external droop controller at the connection point with little communication requirements. This methodology is applied to a modified CIGRE benchmark and shows good results while keeping a standard decentralized control architecture.

The quick detection of islanding in active distribution networks is the main challenge for power engineers. The late detection of islanding may have hazardous impacts on the power systems. Due to the advancement in photovoltaic (PV) technology and their eco-friendly nature, these systems are widely in demand. The islanding methods in the power system are classified into active, passive, remote, and hybrid. The passive methods depend on the measurement of electrical parameters such as voltage, frequency, and phase angle at the point of common coupling (PCC) during islanding condition. The purpose of this paper is to analyze the performance of different passive methods in 100 kW grid-connected PV array. This research paper compares over/under frequency, over/under voltage and rate of change of frequency (ROCOF)-based islanding detection methods (IDMs). The performance of these methods has been analyzed at different conditions. Computer simulations are performed using MATLAB/Simulink.

In this paper, a new approach in the passive-islanding detection method has been studied for the grid-connected photovoltaic (PV) inverter system using the wavelet packet transform (WPT) and back propagation neural network (BPNN). This proposed method is based on the point of common coupling voltage measurement and processing of this signal with a WPT to find the normalized logarithmic energy entropy. The BPNN is used for decision-making mechanism to avoid threshold. With this technique, the islanding operation is able to detect within 40 ms. In addition, it is effective even when the power supplied by distributed generation and the power consumed by the load is matched. In order to verify the capability of the proposed method, the simulation and experimental results of a 1.2-kW PV inverter by IEC Std. 62116 are provided. The proposed technique can be applied to both single- and multi-PV systems.

Brushless doubly-fed induction generators have higher reliability, making them an attractive choice for not only offshore applications but also for remote locations. These generators are composed of two back-to-back voltage source converters, a grid side converter and a rotor side converter. Existing techniques use the rotor side converter for reactive current control; however, it is more suitable for stabilizing steady state behavior. In order to stabilize the voltage fluctuations at the point of common coupling (PCC) due to sudden inductive load introduction, the grid side converter may be a better choice due to faster response and higher control bandwidth. Therefore, this paper proposes a control scheme for the grid side converter to suppress the PCC voltage fluctuations when a large inductive load is suddenly connected. The proposed technique is based on an analytical model of the transient behavior of the voltage drop at the PCC. The analysis shows that reactive current control using the grid side converter introduces a double fundamental frequency component to the PCC voltage. To block this harmonic, we designed a notch filter. The simulation results in Matlab/Simulink show that the proposed technique can not only significantly reduce the voltage drop but also results in an 82% reduction in voltage distortion at the PCC.

This paper proposes a point of common coupling (PCC) voltage compensation algorithm using a current limitation strategy for use in distributed generation (DG). The proposed strategy maintains the PCC voltage by prioritizing currents when an output current reference is larger than the current capacity of the power condition system (PCS) of the DG. With this strategy, the DG outputs the active current, reactive current, and the negative sequence current. The DG uses the reactive current for maintaining the PCC voltage within a normal range; the negative sequence current is used for reducing the PCC voltage unbalance. The proposed method was verified using PSIM simulation and experimental results.

To suppress the background harmonics in the point of common coupling (PCC) voltage on the injected current, feedforward schemes of PCC voltage have been widely used for LCL-type grid-connected inverters. Especially, the unit feedforward scheme is attractive due to its simplicity and utility. However, when the unit feedforward scheme is applied to single-loop current control, unstable open-loop poles may appear due to the variation of grid impedance, which will degrade the robustness of the system or even lead to system instability. Therefore, the robust region of LCL resonance frequency against grid impedance variation is limited to be lower than f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> /3 (fs is the sampling frequency). Nevertheless, in terms of the cost and volume of the filter, it is preferable to design the LCL resonance frequency higher. Aim at the trouble in the application of the unit feedforward scheme under LCL resonance frequency beyond f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> /3, a weighted average feedforward scheme is proposed in this paper, in which PCC voltage and the filter capacitor voltage are fed forward complementarily. The proposed method can guarantee the robustness of the inverter without the constraint of the LCL resonance frequency. Simulations and experimental results validate the effectiveness of the proposed feedforward scheme.

The determination of the true current harmonic distortion produced by a nonlinear load is problematic due to the fact that the voltage waveform provided by the power supply at the Point of Common Coupling (PCC) rarely is free of harmonic distortion. This paper proposes a method based on Artificial Neural Networks (ANN) in order to realize the estimation of the harmonic content presented in the nonlinear load current waveform supposing a pure sinusoidal power supply. However, this estimation is made when the nonlinear load is in fact connected to a distorted power supply. For this, a dynamic ANN with time delays is utilized for modeling the nonlinear load admittance in a pre-established condition of PCC voltage with little or none harmonic distortion. Once achieved a satisfactory training, now for a PCC voltage with any level of harmonic distortion, the ANN is utilized for predicting the nonlinear load current waveform by a power supply estimation that preserves similarities with that applied during its training. One of the advantages of the method is that only the instantaneous values of voltage and current are necessary for modeling the loads. It does not need the knowledge of any load or power system parameter.

As the non‐linear loads increase along the low‐voltage distribution network (LVDN), voltage harmonic components will appear at the point of common coupling (PCC). To improve the voltage power quality of PCC, an active PCC voltage power quality restoring strategy based on droop controlled grid‐connecting microgrid (DCGCM) is proposed. The load current of LVDN and the grid‐connecting current of DCGCM are sampled and are calculated through an additional PCC voltage restoring controller adopted in the secondary level of system. Then, the generated voltage harmonic reference offset is sent to the primary level and tracked by voltage controlled inverter. In this way, the required harmonic current is injected into the LVDN to supply the nonlinear loads at the cost of slight voltage distortion of DCGCM's output voltage. Therefore, the voltage power quality of PCC can be restored. At last, the simulation results from SIMULINK/MATLAB have been presented to verify the validity of the proposed control strategy.

In this article, a photovoltaic (PV) grid-connected inverter (GCI) is employed for multifunctional control [i.e., real power flow control from PV panels, mitigation of current, and voltage-related power quality (PQ) problems], and the GCI input reference dc voltage is formulated as a function of GCI output currents for loss reduction during low currents through inverter switches. In the latest literature, the PV GCI is used for mitigating current-related PQ problems, and it is also used for mitigating minor voltage-related PQ problems (voltage sag, voltage swell, and voltage imbalance). During voltage sag conditions, GCI output current is high enough to maintain rated voltage at the point of common coupling, and in this case, the reference dc voltage requirement is high. However, during rated grid voltage condition, low power generation from PV, and minimum load condition, keeping maximum dc voltage at the input of the GCI leads to heavy burden on GCI switches. In the proposed method, in order to reduce losses in the GCI, the input dc voltage of the GCI is reduced during low GCI output current. The proposed method is validated with a MATLAB simulation and a hardware experiment by comparing data existing in literature with the proposed method.

In this paper, an improved load sharing strategy is proposed for distributed generation units (DGs) connected in a microgrid. Conventional frequency and voltage droop control result in unacceptable active and reactive power sharing. The proposed method formulates a suitable algorithm for load sharing in the islanded microgrid. The feeder power loss and the line impedance voltage drops are minimized so as to regulate the voltage at the point of common coupling (PCC) at its nominal value. The desired DG output voltages are calculated and a linear relationship is obtained between the shared active and reactive powers and the DG output voltages. A master DG controller sets the frequency which is followed by other DG units. The reference powers for the DG units are adjusted so as to maintain the rated PCC voltage. The proposed strategy is verified taking into account the DG ratings, unequal line impedance drops, feeder losses, change in system impedance and effect of DG local loads and formulates an improved power sharing strategy that also facilitates PCC voltage regulation under variable loading conditions. Simulation and experimental results are presented to verify the effectiveness of the proposed method.