You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Integration of UPQC for Power Quality improvement in distributed generation network - a review 
Chapter Title:  I. INTRODUCTION 
1. Abstract of Cited Paper (BIB001):  This paper describes analysis results of a combined operation of the unified power quality conditioner with the distributed generation. The proposed system consists of a series inverter, a shunt inverter, and a distributed generator connected in the dc link through a rectifier. The proposed system can compensate voltage sag and swell, voltage interruption, harmonics, and reactive power in both interconnected mode and islanding mode. The performance of proposed system was analyzed using simulations with power system computer aided design/electromagnetic transients dc analysis program, and experimental results with the hardware prototype. The proposed system can improve the power quality at the point of installation on power distribution systems or industrial power systems.

Literature Review:
The increasing demand of Distributed Generation (DG) in recent years, to minimize the gap between the supply and load demand, is introducing some voltage and current disturbance and harmonics due to the generator types and the interfacing power electronics converters. Therefore, quality of power supply has become an important issue with the increasing demand of DG systems either connected to the grid through grid-tie inverters or work in isolated (microgrid) mode. The need for monitoring of power quality in low voltage distribution levels and ways to mitigate the problems are also increasing due to better customer service, reasonably priced meters, telecommunication development, network planning, operation and regulation requirements  . Implementation of Custom Power Devices (CPD) like UPQC in DG or microgrid systems to improve the power quality is gaining greater importance BIB002 BIB001    . UPQC is the integration of series and shunt active filters, connected back-to-back on the dc side, sharing a common DC capacitor BIB001 as shown in Figure 1 . level; balanced and distortion free. The shunt component is responsible for mitigating the current quality problems caused by the consumer: poor power factor, load harmonic currents, load unbalance etc. It injects currents in the ac system such that the source currents become balanced sinusoids and in phase with the source voltages. This paper deals with the review of research work that has been completed so far on this issue. Emphasis has been given on incorporation techniques of UPQC in DG or microgrid system along with their advantages and disadvantages. More DGs such as Photovoltaic or Wind Energy Systems are now penetrating into the grid or microgrid. Again, numbers of nonlinear loads are also increasing. Therefore, current research on capacity enhancement techniques of UPQC to cope up with the expanding DG or microgrid system is also reviewed.