# Shunt Active Power Filter: A Review on Phase Synchronization Control Techniques

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## Abstract

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## 1. Introduction

## 2. Shunt Active Power Filter: Working Principle and Control Scheme

- (1)
- Harmonic Extraction Algorithm.The main function of this algorithm is to extract harmonic information from a harmonic-polluted power system and applied the extracted information to form a reference current signal ${i}_{ref}$. In this aspect, the distorted load current signal ${i}_{L}$ is processed in a way that allows its harmonic ${i}_{H}$ and fundamental ${i}_{1L}$ elements to be separated.After separating the elements, ${i}_{ref}$ can be derived by using either the harmonic or fundamental elements, each having its own derivation setting. Note that quality of the reference current signal will mainly determine how well the SAPF is going to work, thus it must be generated in a quick and accurate manner. This algorithm is also referred as reference current generation algorithm in some literature, such as in [18,19] due to the reason that reference current is being generated as the final output of this algorithm. Few examples of commonly applied techniques for this algorithm include synchronous reference frame (SRF) or $dq$ theory [20,21,22], instantaneous power $pq$ theory [23,24,25], fast Fourier transform (FFT) [26,27], discrete Fourier transform (DFT) [28,29] and artificial neural network (ANN) [30,31,32].
- (2)
- Synchronization Algorithm.The main function of this algorithm is to track angular position of source voltage signal ${v}_{S}$ and subsequently generate a phase synchronization angle $\theta $ to match the phase of the generated ${i}_{ref}$ with the phase of the operating power system. Many harmonic extraction algorithms do not possess a phase tracking feature and are fully dependent on an explicit synchronization algorithm to provide them with an effective synchronization angle. Nevertheless, there are some harmonic extraction algorithms that are inherited with phase tracking ability, for example, the algorithm designed based on instantaneous power $pq$ theory technique [25,33]. In this case, an explicit synchronization algorithm can be omitted. Further details on the available techniques for this algorithm are presented in the next section.
- (3)
- DC-link Capacitor Voltage Regulation Algorithm.The main function of this algorithm is to estimate the amount of dc-link charging current ${i}_{dc}$ needed by the SAPF to constantly maintain dc-link voltage ${V}_{dc}$ at a desired level. This algorithm continuously compares the measured ${V}_{dc}$ with a predetermined set-point value and minimizes the resulted error by using either proportional-integral (PI) [34,35,36] or fuzzy logic control (FLC) [37,38,39] techniques, via a voltage control loop. When the error between the measured ${V}_{dc}$ and the predetermined set-point value has been minimized, the effective magnitude ${I}_{dc}$ of ${i}_{dc}$ will be generated. Subsequently, by utilizing the phase synchronization angle $\theta $, ${i}_{dc}$ can be coordinated in-phase with the operating power system.
- (4)
- Current Control Algorithm.The main function of this algorithm is to convert ${i}_{ref}$ delivered by harmonic extraction algorithm and ${i}_{dc}$ delivered by dc-link capacitor voltage regulation algorithm into gate switching pulses $S$ via a pulse-width modulation process, while ensuring that the feedback signal ${i}_{inj}$ or ${i}_{S}$ is able to track ${i}_{ref}$ via a current control loop. This algorithm is also commonly referred as switching algorithm due to its function in generating gate switching pulses. Few examples of commonly applied techniques for this algorithm include hysteresis control [40,41,42,43], sinusoidal pulse-width modulation (SPWM) [44,45,46] and space vector PWM (SVPWM) [47,48,49,50].

## 3. Phase Synchronization Techniques

#### 3.1. Zero-Crossing Detection (ZCD) Technique

#### 3.2. Phase-Locked Loop (PLL) Technique

#### 3.3. Adaptive Linear Neuron (ADALINE) Technique

- $W=\left[\begin{array}{c}{W}_{11}\\ {W}_{21}\end{array}\right]$ is the weight factor,
- $Y=\left[\begin{array}{c}sin\left(k\omega \Delta t\right)\\ cos\left(k\omega \Delta t\right)\end{array}\right]$ is the fundamental sine and cosine components,
- $e\left(k\right)={v}_{S}\left(k\right)-{v}_{Fund\_est}\left(k\right)$ is the error between the measured and estimated voltage signal, and $\alpha $ is the learning rate.

#### 3.4. Fundamental Component Extraction Technique

- Extraction of fundamental (sinusoidal characteristic) source voltage ${v}_{Sfund}\left(k\right)$ from the measured source voltage ${v}_{S}\left(k\right)$. In the first stage, the extraction process is conducted in $\alpha \beta $-domain where a Clarke-transformation ($abc-\alpha \beta $ transformation) is applied according to Equation (4). In $\alpha \beta $-domain, source voltage which is subjected to distortion can be expressed as$$\left[\begin{array}{c}{v}_{\alpha}\left(k\right)\\ {v}_{\beta}\left(k\right)\end{array}\right]=\left[\begin{array}{c}{v}_{\alpha \left(dc\right)}\left(k\right)+{v}_{\alpha \left(ac\right)}\left(k\right)\\ {v}_{\beta \left(dc\right)}\left(k\right)+{v}_{\beta \left(ac\right)}\left(k\right)\end{array}\right]$$Here, a STF (as in Equation (6)) is applied to extract ${v}_{\alpha \left(dc\right)}\left(k\right)$ and ${v}_{\beta \left(dc\right)}\left(k\right)$ which is subsequently converted into a pure sinusoidal signal ${v}_{Sfund}\left(k\right)$ by means of inverse Clarke-transformation ($\alpha \beta -abc$ transformation), as in Equation (10). Note that, the application of STF is only needed when the source voltage is subjected to any distortion. In the case where the source voltage is ideal (balanced-sinusoidal), STF can actually be omitted to reduce structure complexity.$$\left[\begin{array}{c}{v}_{Sfund,a}\left(k\right)\\ {v}_{Sfund,b}\left(k\right)\\ {v}_{Sfund,c}\left(k\right)\end{array}\right]=\sqrt{\frac{2}{3}}\left[\begin{array}{cc}1& 0\\ -\frac{1}{2}& \frac{\sqrt{3}}{2}\\ -\frac{1}{2}& -\frac{\sqrt{3}}{2}\end{array}\right]\left[\begin{array}{c}{v}_{\alpha \left(dc\right)}\left(k\right)\\ {v}_{\beta \left(dc\right)}\left(k\right)\end{array}\right].$$
- Magnitude ${V}_{Fund\_mag}\left(k\right)$ calculation by using the extracted fundamental voltage component (${v}_{\alpha \left(dc\right)}\left(k\right)$ and ${v}_{\beta \left(dc\right)}\left(k\right)$). In this stage, the fundamental magnitude of source voltage is computed as follow$${V}_{Fund\_mag}\left(k\right)=\sqrt{{v}_{\alpha \left(dc\right)}{\left(k\right)}^{2}+{v}_{\beta \left(dc\right)}{\left(k\right)}^{2}.}$$
- Direct magnitude ${V}_{Fund\_mag}\left(k\right)$ division from ${v}_{Sfund}\left(k\right)$ to obtain its unity form. In the final stage, the desired synchronization signal $sin\left(k\omega \Delta t+\theta \right)$ is obtained as follow$$sin\left(k\omega \Delta t+\theta \right)=\frac{{v}_{Sfund}\left(k\right)}{{V}_{Fund\_mag}\left(k\right)}$$

#### 3.5. Unit Vector Generation Technique

## 4. Comparative Highlights on Phase Synchronization Techniques and Possible Future Works

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Typical power circuits connection of voltage source inverter (VSI)-based shunt active power filter (SAPF) and the associated control algorithms in its control system.

**Figure 2.**Theoretical voltage and current waveforms showing the condition of power system with a SAPF installed.

**Figure 3.**Overview of synchronization techniques applied to SAPF. ZCD: zero-crossing detection; PLL: phase-locked loop; ANN: artificial neural network; ADALINE: adaptive linear neuron.

**Figure 4.**Control structure of a basic phase-locked loop (PLL) technique [72]. VCO: voltage-controlled oscillator.

**Figure 7.**Block diagram of decoupling network in decoupled double SRF (DDSRF)-PLL [53].

**Figure 8.**Control structure of adaptive linear neuron (ADALINE)-based synchronization technique [58].

Synchronization Technique | Single Phase | Three Phase | Non-Ideal Grid | Strengths | Weaknesses |
---|---|---|---|---|---|

ZCD | Yes | Yes | No | Simplest control structure [9]. | Require hardware circuit [51]. Sensitive to noise [9]. Poor dynamic response [69]. Poor performance when grid is subjected to harmonics [67]. |

SRF-PLL | Yes | Yes | No | Easy to implement [10]. Accurate synchronization under ideal grid conditions [10,54]. Most widely applied techniques [54]. | PI controller needs to properly be tuned [35,84]. Unable to cope with distorted and unbalanced grid conditions [10,53,54]. |

STF-PLL | Yes | Yes | Yes | Suitable for highly distorted and unbalanced grid conditions [55]. | PI controller needs to properly be tuned [35,84]. Gain parameter of STF needs to carefully be selected [59,76]. Integration of STF increases control complexity [55]. |

DDSRF-PLL | No | Yes | Yes | Suitable for highly distorted and unbalanced grid conditions [53]. | PI controller needs to properly be tuned [35,84]. Additional SRF loops increases computational burden [53]. |

ADALINE | Yes | Yes | No | Low computational burden [58]. Can be applied for harmonic extraction purposes [58]. | Learning rate needs to be properly tuned [30,58,81]. Unable to cope with distorted and unbalanced grid conditions [58]. |

Fundamental Component Extraction | No | Yes | Yes | Easy to implement [59,60]. Effective for highly distorted and unbalanced grid conditions [59,60] Can be applied for harmonic extraction purposes [59,60]. | Gain parameter of STF needs to carefully be selected [59,76]. |

Unit Vector Generation | No | Yes | No | Simple control structure [61,62]. | LPF needs to properly be tuned [85]. LPF imposes additional delay [61,62]. Unable to cope with distortion and unbalanced grid conditions [61,62]. |

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**MDPI and ACS Style**

Hoon, Y.; Mohd Radzi, M.A.; Mohd Zainuri, M.A.A.; Zawawi, M.A.M.
Shunt Active Power Filter: A Review on Phase Synchronization Control Techniques. *Electronics* **2019**, *8*, 791.
https://doi.org/10.3390/electronics8070791

**AMA Style**

Hoon Y, Mohd Radzi MA, Mohd Zainuri MAA, Zawawi MAM.
Shunt Active Power Filter: A Review on Phase Synchronization Control Techniques. *Electronics*. 2019; 8(7):791.
https://doi.org/10.3390/electronics8070791

**Chicago/Turabian Style**

Hoon, Yap, Mohd Amran Mohd Radzi, Muhammad Ammirrul Atiqi Mohd Zainuri, and Mohamad Adzhar Md Zawawi.
2019. "Shunt Active Power Filter: A Review on Phase Synchronization Control Techniques" *Electronics* 8, no. 7: 791.
https://doi.org/10.3390/electronics8070791