## 1 Introduction
For instance, one recent work is reported to have applied an innovative technique that utilizes the strength of the STF and SRF concept (named as STF-dq0) for controlling three-phase four-wire SAPF [22]. The STF-dq0 technique is revealed to perform effectively under non-ideal source voltage and unbalanced loads scenarios. Hence, it would be interesting to find out how other available techniques especially the ADALINE technique can be modified to suit three-phase four-wire operation. It will serve as an important alternative other than just depending on SRF and PQ theory techniques. Therefore, in this work, the typical ADALINE-based harmonics extraction algorithm which has been applied in three-phase three-wire system is modified to suit operation in three-phase four-wire system and eventually enhanced serving as a better alternative in generating reference current signal. The proposed algorithm is named as enhanced-ADALINE, and it is developed by integrating together ADALINE concept, HSF, and averaging function. An exhaustive simulation study inclusive of circuit connection of the three-phase four-wire system, design of control algorithms, and test and analysis of findings is performed in MATLAB-Simulink environment (R2012a, The MathWorks, Inc., Natick, MA, USA). To prove design concept of the proposed algorithm, performance demonstrated by SAPF while using the proposed algorithm is examined under various non-ideal source voltage and unbalanced loads scenarios. In addition, the recent STF-dq0 algorithm is also implemented where it serves as a benchmark to gauge the ability of the proposed enhanced-ADALINE algorithm. Nevertheless, the details on STF-dq0 algorithm will not be presented in this paper as they have clearly been described in the existing literature [22]. There are five main sections in this paper. Section 2 demonstrates operation of SAPF in a three-phase four-wire system, and subsequently clarifies the control algorithms which are considered in this work. Next, in Section 3, the design concept and operation of the proposed algorithm is thoroughly described, and the important modules of the proposed algorithm are highlighted. Section 4 provides all the simulation findings of this work. The simulation findings are presented in a comparative manner which providesa better view on achievements of the proposed algorithm. A concluding section is presented as Section 5 to summarize important findings and contributions of this work. Circuit Connection of Shunt Active Power Filter in a Three-Phase Four-Wire System and Associated Control Algorithms  
Conceptual functionality of SAPF in a three-phase four-wire system is presented in this section. To perform a mitigation operation in a three-phase four-wire system, two types of topologies can be considered for SAPF: three-phase three-leg with two dc-link capacitors split at neutral-point [23; 25] and three-phase four-leg topologies [26]. In this work, a standard three-phase three-leg topology is adopted, and the circuit connection is illustrated in Figure 1. The three-phase output of the SAPF is connected to the operating power system at point of common coupling (PCC) and its neutral-point is connected to the neutral-wire \(N\) of the power system. Typically, there will be an output filter interfacing between the SAPF and power system. The output filter which is commonly an inductor can potentially reduce switching ripples produced by the SAPF, allowing the desired injection current \(i_{inj}\) to be accurately injected into any harmonic-contaminated power system for mitigation purposes. Additionally, in a three-phase four-wire system, due to unbalanced loads, there is a high possibility that excessive neutral current \(i_{N}\) may be retained in the returning neutral-wire \(N\). This problem can be rectified via the additional wire which links the middle-point of the two split capacitors to the neutral-wire \(N\) of the power system.