Improved Hybrid Techniques for Grid Fault Detection and Fault Ride-Through of Power Converters

Abstract

In recent years, electric power generation using renewable energy sources has experienced an exponential growth in the world energy market. Their unprecedented large scale penetration foresees a 100% renewable based power generation in near future. These sources require power electronic converters at various levels for power conversion and grid integration. The converters are fast acting devices and use advance control techniques in a hierarchical manner. With the retirement of the conventional synchronous generators, the modern power electronics based power system lacks the inertia property. Hence, they are more vulnerable to several grid transient events; for instance grid faults. Control functionalities classify these converters as grid-forming and grid-following. Both these types can act as grid supporting devices during grid fault. In contrast to grid-forming type, grid-following type converters are mostly used to support the grid. The act of supporting the grid with reactive power instead of tripping during a fault for a pre-defined duration is known as fault ride-through. These converters rely on a separate synchronization unit to inject grid current during both normal and fault condition. Recent grid fault events across the globe have revealed the inefficiency of such synchronization units. This is attributed to the delayed grid parameter estimation that eventually leads to the tripping of the converters rather ride-through. It indicates that the performance robustness of the synchronizing unit while considering the fault ride-through of converter needs to be further investigated thoroughly. In lieu of the above, accurate and fast grid voltage parameter estimation is essential for grid-connected converters. To achieve this objective, the contributions of this thesis are classified into two parts.

The first part of the thesis deals with the fault detection for converters during a grid fault using digital signal processing (DSP) techniques. Faster fault detection is vital to safeguard the power converter as they have limited fault current carrying capacity. Hence a hybrid fault detection technique is proposed. The technique combines the features of two DSP techniques, Hilbert-Huang Transform (HHT) and Teager Energy Operator (TEO). This is called Teager-Huang in this thesis. With this proposed technique, several grid faults, balanced and unbalanced, in both the grid voltage magnitude and phase-angle jumps are detected. Further, comparisons of the fault energies are presented, which provides a benchmark for the severity of the grid faults.

In the second part of the thesis, the synchronization aspect of the converter is investigated. For the purpose of analysis, the synchronization using the classical synchronous reference frame phase-locked loop (SRFPLL) is considered. Initially, the synchronization inefficiency of SRFPLL during the grid fault is explained in regards to the loss of synchronization (LOS) instability. It is shown that the cause for LOS during a fault may be initiated as results of very low grid voltage magnitude, high grid impedance or high current injection. The analysis emphasises on the occurrence of phase-angle jump (PAJ) during a fault. The thesis indicates that the conventional SRFPLL design parameters result in synchronization delay and insufficient damping to ride-through such PAJs.

The decrease in the SRFPLL synchronization robustness highly affects the grid-connected converters. To enhance the grid synchronization performance during a grid fault with PAJ, a hybrid grid synchronization concept is proposed. It consists of both hybrid phase-angle estimators and hybrid frequency estimators. The hybrid frequency estimators contain several improved adaptive and PLL independent frequency estimation techniques. The proposed technique is designed to be compatible with both the three-phase and single-phase grid synchronization. To avoid voltage transients during the transition between the estimators, a transition scheme is presented. This is controlled based on the instantaneous phase-angle error measured by the estimators.

The three-phase grid-connected converter is modelled using the proposed hybrid grid synchronization technique. The current controller of the converter is designed both in stationary and synchronous reference frame. Further, the fault ride-through (FRT) strategy is embedded in the converter controller. With the developed model, the FRT of the converter is tested during a fault. Both symmetrical and asymmetrical grid faults with PAJs are considered. The efficacy of the proposed technique is evaluated using both simulation and experimental validations.

The last part of the thesis explores the FRT of single-phase power converters employing the proposed hybrid grid synchronization transition. The synchronization performance along with the current controller robustness during FRT is investigated.