Recent developments in FACTS research have led to a new device: the Interline Power Flow Controller (IPFC). This element consists of two (or more) series voltage source converter-based devices (SSSCs) installed in two (or more) lines and connected at their DC terminals. Thus, in addition, to serially compensating the reactive power, each SSSC can provide real power to the common DC link from its own line.

The IPFC gives them the possibility to solve the problem of controlling different transmission lines at a determined substation. In fact, the under-utilized lines make available a surplus power that can be used by other lines for real power control. This capability makes it possible to equalize both real and reactive power flow between the lines, to transfer power demand from overloaded to underloaded lines, to compensate against resistive line voltage drops and the corresponding reactive line power, and to increase the effectiveness of a compensating system for dynamic disturbances (transient stability and power oscillation damping). Therefore, the IPFC provides a highly effective scheme for power transmission at a multi-line substation. The IPFC is a multi-line FACTS device.

Figure 1: Schematic diagram of IPFC

An Interline Power Flow Controller (IPFC) consists of a set of converters that are connected in series with different transmission lines. The schematic diagram of IPFC is illustrated in Figure.1. In addition to these series converters, it may also include a shunt converter which is connected between a transmission line and the ground. The converters are connected through a common DC link to exchange active power. Each series converter can provide independent reactive compensation of its own transmission line. If a shunt converter is involved in the system, the series converters can also provide independent active compensation; otherwise not all the series converters can provide independent active compensation for their own line.

Compared to the Unified Power Flow Controller (UPFC), the IPFC provides a relatively economical solution for multiple transmission line power flow control, since only one shunt converter is involved. The IPFC also gains more control capability than the Static Synchronous Series Compensator (SSSC), which is like the IPFC but without a common DC link, because of the active compensation. From a probabilistic point of view, the performance of the IPFC will be better when more series converter involves in the IPFC system.

However, because the converters are connected through the common DC link, the converters should be physically close to each other. The common DC link will become a location constraint for the IPFC and limits its commercial application in the future network. Therefore, a method that can eradicate the IPFC common DC link and provide the active power exchange between converters will be interesting.

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