DETERMINACIÓN DE LA PENA

Converting the power from ac to dc efficiently has been historically an overwhelming challenge. Although ac/dc converters have been used in electronics and other low voltage applications, converting bulk power (higher than hundreds of MW) from ac to dc was not feasible until recent decades. There are currently two dominant technologies in use in these high-power converters: line-commutated converters (LCC) and voltage-source converters

(VSC).

2.3.3.1. Line-commutated converter

Most of HDVC converters in operation are LCC [32]. Modern LCC are based on controllable switching devices, employing thyristor valves to perform the commutation. In a LCC, the dc current does not change its direction; it flows through a large inductance and can be considered almost constant. On the ac side, the converter behaves approximately as a current source, injecting both grid-frequency and harmonic currents into the ac network. For this reason, LCC for HVDC is also known as a current-source converter. Because the direction of current cannot be varied, reversal of the direction of power flow (when required) is achieved by reversing the polarity of dc voltage at both stations [32].

LCCs rely on a strong ac system to function. In fact, an ac voltage is necessary to allow the commutation from one switching device to another. Consequently, LCCs depend upon a synchronous ac grid to ensure the electricity conversion. Moreover, a line-commutated converter in steady-state operation at 1 p.u. (rated) active power will consume 0.6 p.u. of reactive power [33], which has to be compensated with capacitor banks, thus implying a larger substation size. Nevertheless, LCCs have lower losses than their counterpart – VSC converters – does; LCC losses are indeed reported as 0.7% of the converted power [34].

Despite the lower losses, an HVDC LCC substation needs much more space. Compared to a VSC-HVDC station, a complete LCC ones needs double the space [35]. This is mainly due to the need of capacitor banks as mentioned, and filter banks to cope with the harmonics. LCC converters generate harmonics, which are to be filtered [36]; hence implying that there is need to accommodate filters in the substations. According to [33], an LCC substation in the hundred-MW range would need between 1600 and 5000 m2 only for the filter banks.

Even though LCC converters are an interesting option for bulk power transmission, they are not suitable for offshore applications⁷. Taking in consideration that the cost of an offshore platform is highly dependent on its size (the more the needed space, the bigger the offshore structure), installing an LCC converter would translate in a strong increase in the offshore

7 However, it has been stated that some of the LCC issues for offshore wind (such as the space needed for filter banks) could be resolved with alternative control strategies [65].

substation cost. Additionally, LCC need a strong ac grid to properly commutate the current between the thyristor valves and it is not capable to provide black start capability to the offshore wind farms [9]. Therefore, there is not a single offshore wind farm operating with an LCC converter. Consequently, LCC converters will not be considered as an option in the present work.

2.3.3.2. Voltage-source converter

Alternatively, VSCs overcome most of the LCC challenges regarding offshore transmission, and offer other significant advantages. It is an incipient technology; the first VSC converted being installed onshore in 1997. The use of those converters in offshore transmission is relatively new, as the first VSC converter was installed offshore in 2005. Back then they used two-level topologies, whereas nowadays, VSC employ modular multi-level converter (MMC) concepts.

VSC are based on IGBT transistors, which are fully controllable – they can be turned both on and off – on contrary to the thyristors used in LCC converters. Furthermore, in VSCs the active and the reactive power can be controlled independently, so there is no need for reactive power compensation [37]. Additionally, in VSC converters reversal of the power flow is done by reversal of the current – again on the contrary to LCC converters, where the voltage polarity is inverted – thus making them more suitable for dc grid applications [38]. Moreover, as their ac voltage has a low harmonic content, the need for filters is greatly reduced or even eliminated [39].

Another factor which makes VSC converters more interesting for offshore applications is their black start capability. In case of a blackout, VSC converters can lead the grid restoration (provided its capacitors are charged and can function as a voltage source) as they are not dependent on an ac grid to function, nor require an external power supply. According to [40], VSC converters make restoration of the system more reliable and less complicated, by providing a stable frequency and voltage during the recovery process.

On the other hand, one of the main drawback of VSC converters is their losses. Initially, losses on the first VSC converter were reported as 3% of the converted power for the first designs [41]. Nowadays, modern modular multi-level converters achieve less than 1% losses per converter [42].

In conclusion, MMC VSC converters have been proved to be more suitable for offshore wind power-conversion than LCC converters. Notably their reactive power control and lower need for space makes them the preferred option for offshore HVDC power transmission technology. Table 3 compares the main features of both LCC and VSC technologies.

Regarding the converter configuration, different topologies are possible (e.g. bipolar topology, or homopolar topology) with various configurations of converters and cabling systems [12].

Regarding the converters, to diminish the transmission system costs, this thesis will consider a configuration with only one single converter per converter station, i.e. one converter offshore and one converter onshore.

Table 3 Comparison between LCC and VSC converter technologies

LCC-HVDC VSC-HVDC

Maximum rated values per

converter 2000MW ± 800 kV (onshore) 1800MW ± 500 kV [43]

Transistors Partially controllable, thyristors Fully controllable, IGBT

Losses 0.7% 1%

Volume V0 0.5·V0

Black start capability No Yes

Power reversal Invert voltage polarity Invert current flow

Control of reactive power Uncontrolled. Approximatively rated as 60% of the active transferred power

Control of the reactive power produced