# Enhanced Quasi Type-1 PLL-Based Multi-Functional Control of Single-Phase Dynamic Voltage Restorer
###### Abstract  
This paper considers the reference signal generation problem for the multi-functional operation of single-phase dynamic voltage restorers. For this purpose, a single-phase quasi type-1 phase-locked loop (QT1-PLL) is proposed. The pre-loop filter part of this PLL is composed of a frequency-fixed delayed signal cancellation method and a two-stage all-pass filter. Thanks to the frequency-fixed nature, the pre-loop filter is easy to implement and can provide rejection of any measurement offset. Moreover, this PLL benefits from the excellent harmonic robustness property of the conventional QT1-PLL. Small-signal modeling and gain tuning procedures are detailed in this paper. In order to track the reference voltage signals generated by the proposed PLL, a super-twisting sliding mode controller is also presented, which helps to achieve fast dynamic responses. Laboratory-scale prototype-based experimental studies were conducted to validate the developed reference generator and the controller. Experimental results show that the proposed method is fast in detecting and compensating any grid voltage anomalies to maintain constant load voltage despite voltage sag, swell, and harmonic distortions. grid-synchronization; dynamic voltage restorer; converter control system; sliding mode control +
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the desired voltage. This results in maintaining desired load voltage despite various power quality disturbances at the grid. The detection of voltage sag, swell, and harmonics plays a vital role in ensuring the effective operation of DVR. Voltage sag and swells are typically short-lived incidents with a time range of few milliseconds to a minute [5]. As such, fast and accurate detection of sag and swell is essential for fast responsive DVR operation. In the ideal case, voltage sag/swell can be detected very quickly by comparing the voltage magnitude with respect to the ideal magnitude. However, calculating the magnitude can be tricky in the presence of nonlinearities such as harmonics. The first step in compensation voltage calculation is to generate the reference voltage, which should be in-phase with the measured grid voltage. If the grid voltage is ideal, i.e., has a frequency of 50 Hz (or 60 Hz) and contains no harmonics, then generating the reference voltage is straightforward. However, this is not the case in practice. According to the European standard EN 50160 [6], grid voltage can vary between \(-3\) Hz and \(+2\) Hz of the nominal frequency. However, the grid frequency has to be within 1% of the nominal value, i.e., \(\pm 0.5\) Hz for 99.5% of the time. Moreover, harmonics are almost always present in the grid due to the ever increasing penetration of nonlinear loads and converter-interfaced distributed energy sources. All these factors complicate the reference voltage calculation. In order to address the non-ideal characteristics of the measured grid voltage, researchers often rely on phase-locked loop (PLL) [7; 8; 9; 10; 11; 12; 13; 14; 15] or similar techniques to generate the reference voltage for DVR. Using PLL, first, the instantaneous phase of the grid voltage fundamental component is estimated. This can then be used as the unit template for the reference voltage. By multiplying the unit template with the desired amplitude, the actual reference grid voltage can be calculated. In this study, we are considering a single-phase DVR. However, traditional PLLs in the form of synchronous reference frame (SRF) PLLs work only for three-phase systems. For single-phase systems, an additional orthogonal signal generator (OSG) is required to implement SRF-PLL. In the single-phase DVR literature, various OSGs have already been used. In [16], a second-order generalized integrator (SOGI) is used as the OSG. The effect of DC offset is not considered in [16]. Moreover, despite having band-pass filtering property, traditional SOGI-PLL cannot completely reject the dominant harmonics. Thus, multiple SOGIs need to be placed in parallel in order to reduce the effect of dominant harmonic components.