Quasi Type-1 PLL With Tunable Phase Detector for Unbalanced and Distorted Three-Phase Grid

International audienc

Novel PLL for power converters under unbalanced and distorted grid conditions

A novel phase-locked loop (PLL) with simple structure is proposed in the present work for three-phase power converters under adverse grid conditions. Based on a synchronous rotating frame PLL (SRF-PLL), multi-resonant harmonic compensators with the ability of accommodating frequency deviations are employed in the feedback path of a pre-filter. As a result, the negative-sequence component and harmonic distortions of grid voltage can be attenuated. Different from existing methods, only classical regulators are used, avoiding complicated networks for the decoupling of unbalance and harmonics and thus greatly simplifying the control algorithm. The proposed method is analysed and designed in both the continuous s-domain and discrete z-domain, whereby stable, fast, accurate, and robust responses are achieved. Simulation results have been obtained to show the improved performance of the proposed PLL compared with two widely used methods

Robust Estimation of Fundamental Frequency Positive-Sequence Component for Grid-Integration Applications in Energy Systems

This article studies the problem of fundamental frequency positive-sequence component separation in unbalanced three-phase systems for grid-integration applications. We propose a-simple-to-implement approach involving a modified delayed signal cancellation method and moving average filtering. A delay-based linear-regression framework is considered to make the sequence component separation frequency-adaptive, providing fast and accurate frequency estimation. Comparative experimental results demonstrate the suitability of the proposed method over conventional approaches

Fast voltage detection method for grid-tied renewable energy generation systems under distorted grid voltage conditions

© The Institution of Engineering and Technology. A fast voltage detection method to assist with the low-voltage ride-through operation of grid-tied renewable energy generation systems is proposed in this study. It is designed to detect every phase voltage, so that it can be applied in both three-phase and single-phase applications. The whole voltage detection approach consists of two stages, the pre-filtering stage and the voltage detection stage. In the pre-filtering stage, a cascaded delayed signal cancellation (CDSC) module and a low-pass filter are connected in series to filter low-order harmonics and high-frequency noises. For eliminating the low-order harmonics of interest, different types of CDSC methods are studied in detail. Subsequently, a new orthogonal signal generator is built to calculate the voltage amplitude in the voltage detection stage. Finally, the proposed voltage detection method is verified by experimental results

Adaptive Vectorial Filter for Grid Synchronization of Power Converters Under Unbalanced and/or Distorted Grid Conditions

This paper presents a new synchronization scheme for detecting multiple positive-/negative-sequence frequency harmonics in three-phase systems for grid-connected power converters. The proposed technique is called MAVF-FLL because it is based on the use of multiple adaptive vectorial filters (AVFs) working together inside a harmonic decoupling network, resting on a frequency-locked loop (FLL) which makes the system frequency adaptive. The method uses the vectorial properties of the three-phase input signal in the αβ reference frame in order to obtain the different harmonic components. The MAVF-FLL is fully designed and analyzed, addressing the tuning procedure in order to obtain the desired and predefined performance. The proposed algorithm is evaluated by both simulation and experimental results, demonstrating its ability to perform as required for detecting different harmonic components under a highly unbalanced and distorted input grid voltage

Mitigation of current harmonic distortion in three-phase LCL-type filter interfaced inverters

It is a common practice to use an LCL-filter to attenuate the high-frequency harmonic distortions in grid-connected converters to stay within the limit set by different harmonic limit standards. However, to deal with low-order harmonics, optimal ac current-control techniques are preferred as they are cost-effective solutions. Several previous studies showed that whether LCL-filter is damped actively or passively causes more losses. Moreover, it increases the cost and design complexity of the system since LCL-filter introduces new state variables in the dynamical model compared to simple an L-type filter. Thus, harmonic mitigation under distorted and unbalanced grid conditions in LCL filter interfaced inverters is still an open research topic. In this thesis, two current-control techniques: proportional-integral (PI) and proportional-resonant (PR) controllers for current-fed passively damped LCL-filter interfaced grid-connected inverter are examined. Inverter-side current-control is adopted in LCL-filter interfaced inverters to have a safe operation. However, the presence of the harmonic distortion in the grid-side cannot be adequately compensated under sever grid conditions. Thus, it is necessary to adopt proportional grid-voltage feedforward in dq-domain and capacitor-current feedforward in αβ-domain to inject pure sinusoidal currents to the grid. Moreover, the conventional synchronous reference frame PLL (SRF-PLL) that generates the angle and frequency for the current controllers lacks the capability to suppress high disturbance under unbalanced and distorted grid voltage conditions. Hence, the scarcity of the SRF-PLL to attenuate disturbances is reduced by introducing a prefiltering stage that utilizes delayed signal cancellation (DSC) techniques. Based on the techniques mentioned above, the harmonic content of the grid-side current was kept below what standards recommend. The feedforward terms in both domains showed an excellent performance in suppressing the low-order harmonics from the grid-current in distorted grid voltage case. There were also some crucial differences in the performances of the current- control techniques. Enhancements on the quality of the grid current were also made by utilizing αβCDSC-PLL and dqADSC-PLL in distorted and unbalanced grid voltage conditions, respectively. The operation and performance of the two current-control methods were verified by experimental results from the laboratory test bench at Tampere University Hervanta campus