## 1 Introduction
Nevertheless, for both single-phase and three-phase applications, to ensure effectiveness of the ADALINE-based harmonics extraction algorithm, additional implementation of synchronizer such as zero-crossing detector (ZCD) [11] and phase-locked loop (PLL) [12] is required. The purpose of the synchronizer is to match the phase of the generated reference current signal with the phase of the operating power system. Specifically, the ADALINE technique alone is for estimating either fundamental or harmonic currents and at the same time the synchronizer will coordinate the phase of the estimated currents according to the phase of the operating power system. In other words, the estimated fundamental/harmonic currents signal with the correct phase is the reference current signal needed. Indeed, the additional ZCD and PLL circuitries will complicate the structure of the ADALINE-based harmonics extraction algorithm and thus the overall control structure. Alternatively, an ADALINE-based fundamental voltage extraction algorithm [16; 17] can be applied to replace the traditional ZCD and PLL. As the name indicates, this algorithm is developed by using ADALINE concept where it has a similar control structure with the ADALINE technique that is originally used for extracting fundamental/harmonic currents. This innovative algorithm is named as unified ADALINE. It contains two similar ADALINE structures: the first one for extracting fundamental voltage signal (for phase synchronization) and the second one for extracting fundamental current signal [16]. Since both ADALINE structures are similar, hence no additional design effort is needed in developing a unified ADALINE technique. However, from a practical point of view, the source voltage is most probably non-ideal (distorted and/or unbalanced), and thus worsens the performance of SAPF which is initially developed to operate with ideal source voltage (sinusoidal and balanced) scenarios. Owing to source voltage distortion and unbalances, the traditional ZCD and PLL may not perform as desired because their tracking ability can be degraded according to the severity of distortion and/or unbalances suffered by source voltages [18]. Meanwhile, according to the ADALINE-based fundamental voltage extraction algorithm, the synchronization phase is literally a unity representation of the source voltage, obtained by dividing the sinusoidal source voltage with its magnitude (extracted by the ADALINE algorithm) [16]. In this case, if the source voltage is distorted and/or unbalanced, the unified ADALINE (as a whole) will probably fail to work as desired [19]. Fortunately, techniques to deal with distorted and/or unbalanced source voltage scenarios have been developed such as by using adaptive notch filters (ANFs) [20] and highly selective filters (HSF) or self-tuning filters (STFs) [18; 19; 21]. For the purpose of generating reference current, STF has been integrated with ADALINE technique (named as STF-based ADALINE) and this innovative technique has been reported to work effectively under various distorted and/or unbalanced source voltage scenarios [19]. Hence, ADALINE-based approaches for the purpose of harmonics extraction have improved in a progressive manner, i.e., from single-phase two-wire to three-phase three-wire applications, from steady state to dynamic state operations, and from ideal to non-ideal source voltage scenarios. However, relevant studies to examine the possibility to adapt the ADALINE-based algorithm in a three-phase four-wire system which invariably carries unbalanced loads are rather limited. Nevertheless, for three-phase four-wire applications, two common techniques, i.e., SRF [22; 23] and PQ theory [7; 24] techniques are reported to be effective. Indeed, both of these techniques are actually modified from their respective counterparts designated for three-phase three-wire applications. Up to date, these two techniques are still most preferred for controlling the operation of a three-phase four-wire SAPF.