Interline Power Flow Controller

32.4.2.6 Interline Power Flow Controller

32.5 Voltage Source Converter

The interline power flow controller (IPFC) [43] is a UPFC-

derived device with the objective of controlling power flow

(VSC)-Based HVDC Transmission

between lines instead of one line as in the UPFC. Figure 32.41 shows a basic block diagram of an IPFC with two converters to The VSC-based HVDC technology is a suitable solution for control the power flow in lines 1 and 2. The minimum number

dc transmission systems through underground or undersea dc of converters connected back-to-back is two, but there may be cables. The basic circuit configuration for a VSC-based HVDC more. Each converter should be connected in series with a dif- system is shown in Fig. 32.42. It comprises two VSCs connected ferent transmission line and should control power flow in this back-to-back through dc cables. The dc capacitors are used at line with the following conditions:

the dc side of both converters for smoothing of the dc voltage [46]. One of the main advantages of the VSC-HVDC system is

• The reactive power control can be totally independent in that it allows independent active and reactive power control in each converter; each terminal, and thus it can be connected to weak power sys- • The active power flowing into or out of each converter has tems and to passive networks [47, 48]. Beyond that, the VSCs to be coordinated in such a way that the dc link voltage is have a faster dynamic response when compared with the con- kept controlled. ventional current source converter (CSC)–based HVDC, and

The dc link voltage control can be achieved in a similar thus it can increase the system stability, improve the power flow way as in the case of the UPFC. In this case, one of the control, and isolate the transients from one side to the other. series converters can control the compensation voltage freely These features provide more flexibility to the systems where and it may produce active power flowing into or from the dc the VSC-HVDC is connected, and hence the VSC-HVDC is link, which would charge or discharge the dc link capacitor. addressed in this Chapter. Therefore, the other converter has to be controlled to regulate

The VSC-HVDC system can also be seen as an extension this dc voltage. If there are n converters with n greater than of the IPFC concept, keeping in mind that the main idea is two, (n − 1) converters can absorb or generate active power, to transfer active power from one bus to another through a

874 E. H. Watanabe et al.

Series controller 1

q SE1 *

V d C d Main

Series controller 2

+ v C2 − FIGURE 32.41 Block diagram of IPFC with two converters.

System A

Converter 1

Converter 2

System B

FIGURE 32.42 Voltage source converter HVDC system.

dc link. The main differences are that the IPFC is connected the main power flows through the converters. Another appli- in series with the line and both converters are directly con- cation that the back-to-back VSC-HVDC can be applied is the nected in a back-to-back configuration, i.e., without the dc (full converter) connection of wind turbine units to the ac grid cables, whereas the VSC-HVDC system is shunt connected to [47, 49]. the bus and the converters are connected through dc cables.

The VSC-based dc transmission has an exactly dual configu- The VSC-HVDC system can also be connected in back-to-back ration of the conventional HVDC transmission system, which configuration, and one application can be the control of the is based on the thyristor-controlled CSC. The duality can be power flow in a transmission line. In this case, the VSC-HVDC explained by the fact that the thyristor-controlled HVDC sys- can be seen as an extension of the UPFC. The difference is tem controls the dc link current, whereas the system based on that the transmission line is segmented by the VSC-HVDC sys- VSC controls the dc link voltage. This concept can be used for tem and whole power flows through the converters, whereas the connection of asynchronous systems, systems with different the UPFC does not segment the line and only a fraction of frequencies or located in places where cable transmission is

32 Flexible AC Transmission Systems 875

Submodule SM

FIGURE 32.43 Modular multilevel converter topology.

more applicable than conventional transmission lines (as in For very high-power applications, a MMC is also an alternative, congested urban areas or underwater transmission). The num- and potentially preferable, to a multilevel converter [53]. The ber of VSCs can be two for point-to-point transmission or MMC is being presented as the preferred potential candidate more for multipoint transmission.

for VSC-HVDC system applications [54]. Features like mod- Although the CSC-HVDC transmission can be synthesized ularity and capability of a multimodule VSC to handle high for higher power as compared with the VSC-HVDC system, it power or high voltage indicate that a MMC may also be an can only control active power and may need large quantity of option to high-power STATCOM, SSSC, UPFC, or IPFC. reactive power. This means that reactive power compensation

Figure 32.43 shows the basic topology of a three-phase equipment is normally necessary, which does not happen to the MMC. Each leg is comprised of n submodules, which are com- VSC-HVDC system.

posed of two self-commutated switches and a storage capacitor. There are different VSC topologies suitable for the dc power Depending on the application, a full-bridge submodule might transmission. The most often used topologies are the two-level

be necessary instead of the half-bridge shown in this figure. three-phase converter followed by the three-level three-phase The submodule can be seen as a voltage source with two converter (neutral point clamped [NPC]), both are similar to possible stages 0 or +V C , where V C is the voltage over the the converters shown in Fig. 32.21. New topologies for VSC- capacitor. Thus, each arm can be understood as a controllable HVDC application are emerging, as in the case of the modular voltage with n possible voltage steps, and the higher the num- multilevel converter (MMC) [50]. The MMC is suitable for ber of submodules the more-approximated sine waveform can high-voltage or high-power application and thus it is suitable

be achieved at the ac side. The main advantages of the MMC for either HVDC transmission or FACTS applications [51, 52]. are low switching losses, low dv/dt over individual switch,

876 E. H. Watanabe et al. modularity, reconfigurable design, and increased energy stor-

17. D. G. Holmes and T. A. Lipo, Pulse Width Modulation for Power age [53]. Beyond that, the MMC can also be directly connected

Converters: Principles and Practice, Wiley-IEEE Press, 2003. to the power system, i.e., without any power transformer [55],

18. S. Mori, K. Matsuno, M. Takeda, and M. Seto, “Development of a which can reduce the overall substation footprint and cost,

large static var generator using self-commutated inverters for improv- resulting in a compact-area HVDC substation.

ing power system stability,” IEEE Trans. Power Delivery, vol. 8, no. 1, pp. 371–377, February 1993.