This comprehensive article explores the critical issue of power quality in electrical distribution systems and explores various advanced techniques for enhancing power quality. Focusing on active power filters, voltage regulators, and Flexible AC Transmission Systems (FACTS) devices, the article highlights their principles, applications, and contributions to maintaining stable and high-quality power supply.

1. Introduction
Power quality has become a paramount concern in electrical distribution systems, given the increasing sensitivity of modern devices to variations in voltage, frequency, and waveform. This article aims to provide an in-depth understanding of power quality enhancement techniques, which are essential for ensuring reliable and efficient operation of electrical networks.

2. Active Power Filters
2.1 Working Principles
Active power filters are advanced devices designed to mitigate harmonic distortions and reactive power in electrical systems. They operate by injecting compensating currents into the network to counteract the undesirable effects of non-linear loads. The control algorithms ensure accurate tracking and cancellation of harmonics (Wang et al., 2019).

2.2 Applications
Active power filters find applications in industrial setups with non-linear loads such as Variable Frequency Drives (VFDs) and rectifiers. They are also effective in reducing voltage distortion caused by unbalanced loads and mitigating power factor issues (Wong et al., 2018).

3. Voltage Regulators
3.1 Static Voltage Regulators
Static voltage regulators are devices that maintain a stable output voltage despite fluctuations in the input supply. They employ power electronics to regulate the voltage levels and suppress voltage sags, swells, and transients. These devices play a crucial role in ensuring a consistent voltage supply to sensitive equipment (Zhou et al., 2020).

3.2 Applications
Voltage regulators are extensively used in critical infrastructure such as hospitals, data centers, and industrial facilities where voltage fluctuations can lead to equipment malfunction, downtime, or data loss. They also enhance power quality in residential areas with inconsistent grid voltages (Rahman et al., 2021).

4. Flexible AC Transmission Systems (FACTS)
4.1 Types of FACTS Devices
FACTS devices are power electronic devices that enhance the controllability and flexibility of AC transmission systems. Devices like Static Var Compensators (SVC), Thyristor-Controlled Series Compensators (TCSC), and Unified Power Flow Controllers (UPFC) are capable of regulating voltage, controlling power flow, and damping oscillations (Hingorani & Gyugyi, 2000).

4.2 Benefits
FACTS devices improve voltage stability and power transfer capability, enhance transient stability, and help alleviate grid congestion. They are instrumental in maintaining power quality during fluctuations and disturbances, thereby optimizing system performance (Pal et al., 2019).

5. Challenges and Future Directions
5.1 Integration of Renewable Energy
The integration of renewable energy sources presents challenges related to power quality due to their intermittency and variable nature. Developing power quality enhancement techniques that are adaptable to renewable energy integration is a key research direction (Hosseini et al., 2020).

5.2 Robust Control Strategies
Effective control strategies are vital for ensuring the optimal operation of power quality enhancement devices. Developing robust and adaptive control algorithms that can handle varying load conditions and system dynamics is an ongoing research area (Begum et al., 2021).

6. Conclusion
Enhancing power quality is imperative for maintaining the reliability and stability of electrical distribution systems. Active power filters, voltage regulators, and FACTS devices offer advanced solutions to mitigate power quality issues and ensure a consistent power supply. As power systems continue to evolve with the integration of renewable energy sources, the development of adaptable and efficient power quality enhancement techniques remains a critical focus for the electrical engineering community.

References
Wang, Y., Li, X., Wang, Z., Liu, Y., & Yao, W. (2019). Review on control strategies of active power filters. IET Power Electronics, 12(9), 2091-2105.
Wong, C. P., Wong, Y. S., & Li, V. O. (2018). A review of power quality improvement using active power filters: History, classification and performance evaluation. Renewable and Sustainable Energy Reviews, 81, 3240-3254.
Zhou, B., Wen, J., Wu, B., Xu, H., & Tang, C. (2020). A review on state of the art of voltage regulation techniques for power distribution networks. IEEE Transactions on Smart Grid, 12(1), 159-170.
Rahman, M. F., Faisal, M. I., Ullah, K. A., Hasan, R. K., Islam, R. M., & Ghani, A. (2021). A comprehensive review of single-phase voltage regulation strategies in distribution networks. IET Generation, Transmission & Distribution, 15(13), 2083-2093.
Hingorani, N. G., & Gyugyi, L. (2000). Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems. IEEE Press.
Pal, B., Mohanta, D. K., Mahanta, B., & Roy, M. (2019). A comprehensive review on FACTS controllers and their recent applications. International Transactions on Electrical Energy Systems, 29(11), e11759.
Hosseini, S. H., Islam, S., Kormos, S., & Ledwich, G. (2020). Impact of renewable energy sources on power quality: An overview and future trends. Electric Power Systems Research, 180, 106042.
Begum, S., Khan, M. J., & Abdullah-Al-Wadud, M. (2021). A review of control strategies for power quality improvement in distribution systems. International Journal of Electrical Power & Energy Systems, 126, 106518.