Integration of UPQC for Power Quality improvement in distributed generation network - a review <s> III. UPQC IN DOUBLE FEEDER DISTRIBUTION SYSTEM <s> Multilevel voltage source converters are emerging as a new breed of power converter options for high-power applications. The multilevel voltage source converters typically synthesize the staircase voltage wave from several levels of DC capacitor voltages. One of the major limitations of the multilevel converters is the voltage unbalance between different levels. The techniques to balance the voltage between different levels normally involve voltage clamping or capacitor charge control. There are several ways of implementing voltage balance in multilevel converters. Without considering the traditional magnetic coupled converters, this paper presents three recently developed multilevel voltage source converters: (1) diode-clamp, (2) flying-capacitors, and (3) cascaded-inverters with separate DC sources. The operating principle, features, constraints, and potential applications of these converters are discussed. <s> BIB001 </s> Integration of UPQC for Power Quality improvement in distributed generation network - a review <s> III. UPQC IN DOUBLE FEEDER DISTRIBUTION SYSTEM <s> A power line conditioner (PLC) using a cascade multilevel inverter is presented for voltage regulation, reactive power (VAr) compensation and harmonic filtering in this paper. The cascade M-level inverter consists of (M-1)/2 H-bridges in which each bridge has its own separate DC source. This new inverter: (1) can generate almost an sinusoidal waveform voltage with only one time switching per line cycle; (2) can eliminate transformers of multipulse inverters used in the conventional static VAr compensators; and (3) makes possible direct connection to the 13.8 kV power distribution system in parallel and series without any transformer. In other words, the power line conditioner is much more efficient and more suitable to VAr compensation and harmonic filtering of distribution systems than traditional multipulse and pulse width modulation (PWM) inverters. It has been shown that the new inverter is specially suited for VAr compensation. This paper focuses on feasibility and control schemes of the cascade inverter for voltage regulation and harmonic filtering in distribution systems. Analytical, simulated and experimental results show the superiority of the new power line conditioner. <s> BIB002 </s> Integration of UPQC for Power Quality improvement in distributed generation network - a review <s> III. UPQC IN DOUBLE FEEDER DISTRIBUTION SYSTEM <s> This paper proposes a novel configuration of a unified power quality conditioner (UPQC) which can be connected to the distribution system without series injection transformers. The operation of the proposed system was analyzed through simulations with PSCAD/EMTDC and experimental works with a scaled hardware model, assuming that the UPQC is connected with the 22.9-kV distribution line. The proposed UPQC has the ultimate capability of improving power quality at the point of installation on power distribution systems. It has flexibility in expanding the operation voltage by increasing the number of H-bridge modules. <s> BIB003 </s> Integration of UPQC for Power Quality improvement in distributed generation network - a review <s> III. UPQC IN DOUBLE FEEDER DISTRIBUTION SYSTEM <s> This paper presents a new unified power-quality conditioning system (MC-UPQC), capable of simultaneous compensation for voltage and current in multibus/multifeeder systems. In this configuration, one shunt voltage-source converter (shunt VSC) and two or more series VSCs exist. The system can be applied to adjacent feeders to compensate for supply-voltage and load current imperfections on the main feeder and full compensation of supply-voltage imperfections on the other feeders. In the proposed configuration, all converters are connected back to back on the DC side and share a common DC-link capacitor. Therefore, power can be transferred from one feeder to adjacent feeders to compensate for sag/swell and interruption. The performance of the MC-UPQC as well as the adopted control algorithm is illustrated by simulation. The results obtained in PSCAD/EMTDC on a two-bus/two-feeder system show the effectiveness of the proposed configuration. <s> BIB004
A new connection proposal for UPQC, as shown in Figure 6 , to improve the power quality of two feeders in a distribution system has been made in  which is termed Interline UPQC (IUPQC). The purpose of the IUPQC is to hold the feeder voltages constant against voltage sag/swell and temporary interruption in either of the two feeders. It has been demonstrated that the IUPQC can absorb power from one feeder to hold the other feeder voltage constant in case of a voltage sag in the other source voltage. This UPQC can also be implemented in a Custom Power Park or Microgrid system. This can further be improved as a multi-converter based UPQC BIB004 where multiple VSI are connected back to back on the dc side. IV. CAPACITY ENHANCEMENT OF UPQC In high power applications, the filtering task cannot be performed for the whole spectrum of harmonics by using a single converter due to the limitations on switching frequency and power rating of the semiconductor devices. Therefore, compensating the reactive harmonic components to improve the power quality of the DG integrated system as well as to avoid the large capacity centralised APF, parallel operation of multiple low power APF units are increasing. Like APF, UPQC can also be placed at the PCC or at a high voltage distribution line as a part of DG integrated network or in microgrid system to work both in interconnected or islanded mode. At this place, capacity enhancement is achieved by using Multi-level topologies to reach the higher power levels. These options are as follows: i. Multi-level converter based UPQC ii. Multi-module converter based UPQC iii. Multi-module (power cell) unit based UPQC A multi-level converter is proposed to increase the converter operation voltage, avoiding the series connection of switching elements. However, the multilevel converter is complex to form the output voltage and requires an excessive number of back-connection diodes or flying capacitors BIB001 or cascade converters BIB002 . A basic form of multi-level UPQC is shown in Figure 7 . A multi-module H-bridge UPQC can also be connected to the distribution system without series injection transformers. It has the flexibility in expanding the operation voltage by increasing the number of H-bridge modules BIB003 , as shown in Figure 8 . Here each phase consists of several pairs of H-bridge modules isolated through a single-phase multiwinding transformer. These Multi-module techniques [1318] allow the symmetrical distribution of the load power among the components of the topology, but the classical design procedure must be modified or refined to ensure the power cell components should be within its maximum ratings. Therefore, a new design procedure of UPQC with a feature of extending capacity based on a modular approach is presented in   , shown in Figure 9 , where H-bridge power cells are added in each single phase arrangement depending on the required compensating power. Some advantages and disadvantages are also outlined in Table3. • It is found that research in recent years has placed more emphasis on CPDs, especially on UPQC, and its application in DG or microgrid system. • Single or modular type UPQC has been proposed to deal with power quality issues, with an addition to voltage interruption compensation, active power transfer, related to DG with integrated or microgrid mode. • Capacity enhancement has been achieved using multi-level or multi-module and central control mode, however, the flexibility of UPQC to increase its capacity in future and to cope up with the increase load demand in low voltage distribution level has not been achieved. • The economics for the capacity enhancement of UPQC should also be analysed.