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The VSC is becoming increasingly popular for HVDC applications. The rst HVDC VSCs were introduced in 1997 by ABB and the rst converters were installed in Sweden in a demonstrator project [55]. The demonstrator was a 3 MW, ± 10 kV converter with 10 km of overhead DC lines. Today the largest VSC commissioned is the INELFE bipole link connecting the French and Spanish transmission systems, which enables the transfer of 2000 MW at ± 320 kV between the two systems [56]. The semiconductor device used for HVDC VSCs is the Insulated Gate Bipolar Transistor (IGBT). The IGBTs can be controlled to turn o as well as on which means the VSC can provide exible and independent control of real and reactive power [26]. This ability allows the VSC to connect to small or weak AC systems, making it the preferred technology for connecting intermittent remote generation such as wind generation [25]. The IGBT is limited in current and voltage ratings and more devices are required in the circuit when compared with the thyristor based CSC devices. However ratings are starting to become comparable to thyristors with the latest generation of IGBTs rated for 4.5 kV and 2 kA [57]. The rst generation of VSC, the two-level converter is shown in Figure 1.7a.

Ia Ib Ic Vdc La Lb Lc Idc

(a) Circuit diagram (b) Sample AC waveform

Figure 1.7.: Two-level VSC

The converter works by holding a constant DC voltage and the power ow is controlled by changing the value of the DC current. The power ow is reversed by changing the polarity of the current. A DC bus capacitor is required on the DC terminals of the converter to stabilise the DC voltage and to absorb the harmonic switching content [58]. In the two-level VSC the AC waveform is generated by switching between the positive and negative DC voltage. Pulse Width Modulation (PWM) is used to improve the shape of the AC waveform, as shown in Figure 1.7b. PWM uses high switching frequencies which increases the switching losses of the converter and ltering is needed to remove the switching content from the AC waveforms [59]. Filtering increases the losses and the volume of the converter, however the ltering requirement is lower than the CSC as VSCs do not require reactive power compensation [60]. The power loss gure for the rst generation of two-level VSCs was approximately 3 % [25].

In order to reach the high voltages required for HVDC the two-level converter can have hundreds of IGBTs connected in series to block the DC voltage. It is a challenge to switch all of the series connected devices simultaneously. If one devices turns o prior to the other devices it will not be able to block the full DC bus voltage

and will fail [58]. The precautions to prevent this become increasingly burdensome and leads to a restriction in the DC voltage that can be achieved with this design and subsequently limits the power transfer capability of the converter.

The three-level converter switched between the positive and negative DC bus voltage and zero to produce the AC waveforms. As the number of levels increases, each level requires an additional two semiconductor switches per phase [61]. ABB's second generation of HVDC Light used three-level converters with power losses in the region of 2 %, which was still still more than twice the losses of the CSC [25]. The use of Selective Harmonic Elimination (SHE) PWM for two-levels VSCs improved their loss gure to 1.5 % ABB [25]. The lower loss gure is a result of the SHE PWM lowering the switching frequency and thus reducing the switching losses of the converter [62].

Increasing the number of DC voltage output levels available from the converter can reduce the Total Harmonic Distortion (THD) in the AC waveforms and thus reduce the ltering requirement for the converter, and reduce the switching frequency required [21]. A new topology of VSC was introduced in the early 2000s, the Modular Multilevel Converter (MMC) [63]. The MMC creates an staircase waveforms by using hundreds of DC voltage levels available from the converter. The topology is described in Chapter 2.

VSC for MTDC Grids

When considering MTDC networks, the VSC is better suited than the CSC for several reasons. The primary one being the ease of reversing power ow by changing the current polarity. DC switching arrangements are not required as the voltage polarity does not change [19]. Another advantage of the VSC is that it allows the use of cross-linked polyethylene (XLPE) cables which cannot be used with CSC technology as the breakdown strength of the cable is aected negatively by the

reversal of voltage polarity [64]. These cables are also lighter, cheaper and more environmentally friendly than oil-lled cables [28, 64]. As XLPE is lighter this means that longer cables can be made and more power can be transported per kilogram of cable [65].

Newer modular VSCs have a smaller footprint than the CSC, as they do not have the same ltering requirement [50]. This is advantageous in oshore applications where space is a premium. Additionally the VSC enables the connection of weak or islanded AC systems, such as oshore wind farms, and can thus be placed anywhere within an AC system [26].

At time of writing there were two MTDC VSC grids operating in the world, both in China. The rst was the Nanao project with three terminals, it operates at ± 160 kV and the three terminals are rated for 200MW, 100 MW, and 50 MW [66]. The second operating scheme is the Zhoushan project, which is a ve terminal network [67]. This project operates at ± 200 kV and the rst three terminals are rated for 400 MW, 300 MW, and three terminals at 100 MW [68].