Fig. 11 shows the entire active-reactive power area where the VSC can be operated with the 1.0 p.u. value being the MVA rating of each converter.
The use of VSC as opposed to a line commutated CSC
offers the following advantages:
• Avoidance of commutation failures due to disturbances
in the AC network.
dc
4
_
+
Vdc
4
_
+
Vdc
4
_
Vdc
4
_
• Independent control of the reactive and active power
consumed or generated by the converter.
• Possibility to connect the VSC-HVDC system to a
“weak” AC network or even to one where no
generation source is available and naturally the short-circuit level is very low.
Fig. 13: Five-level flying capacitor VSC phase leg topology.
1
Fundamental
0
−1
• Faster dynamic response due to higher (PWM) than the 0
Time 2π
fundamental switching frequency (phase-controlled)
operation, which further results in reduced need for
Fig. 14: Three-level PWM line-to-neutral voltage waveform.
filtering and hence smaller filter size. 2
• No need of transformers for the conversion process. 1
IV. MULTILEVEL VSC TOPOLOGIES FOR HVDC 0
In this Section, different selected VSC topologies suitable -1 for the implementation of a VSC-HVDC system are discussed. Multilevel converters extend the well-known -2
advantages of low and medium power PWM converter
Fundamental
2 π
Time
technology into the high power applications suitable for high-voltage high-power adjustable speed drives and large converters for power systems through FACTS and VSC-based HVDC power transmission [55]-[62].
There are numerous multilevel solid-state converter
topologies reported in the technical literature [57]. However, there are two distinct topologies, namely, the