Converter Transformer: The converter side voltage is controlled by the converter transformer to achieve the required active and reactive power. Also, the converter transformer provides coupling reactance between the VSC and the AC system and it is used to match the voltage between the AC system and the VSC. Also, it provides galvanic isolation and the path for zero sequence current to flow between the AC system and VSC.
AC Filter: The AC filters used in the VSC-HVDC have a lower rating than LCC-HVDC and are not required to provide reactive power support. AC filters are used to suppress the high-frequency harmonics and avoid interaction with the fundamental frequency components.
DC Capacitor: The DC capacitors are the energy storage element in the VSC-HVDC. It provides stiff DC voltage between switching instants and reduces the DC voltage harmonics.
DC Filter: Instead of increasing the size of DC capacitors, the DC filters may be used to filter out the targeted harmonics. It is connected in parallel with DC capacitors to decrease the total equivalent impedance of the DC circuit.
AC Reactor: AC reactor is added in series with converter side of transformer to increase the series reactance to achieve the large transformer leakage inductance where it is not possible. Also, it is used to reduce the DC fault currents and peak switch currents for AC faults.
DC Reactor: The reactor is connected on the DC side to reduce the rate of change of the DC fault current and harmonic current on the DC side. The typical value of a DC reactor is lower than 5 mH which is smaller than LCC-HVDC.
2.1. Advantages of VSC-HVDC Transmission Systems
VSC-HVDC has active and reactive power control independently. So, the VSC-HVDC can be operated with weak AC systems (short circuit ratio SCR<2).
VSC can generate leading and lagging reactive power independently and so it can provide voltage support while transmitting active power at any level.
If there is no power transmission in VSC-HVDC, both converter stations can be operated independently as STATCOM to provide reactive power/voltage support to the local AC networks.
With the use of PWM (with a switching frequency in kHz), the harmonic filters are at higher frequencies. Hence, it reduces the size, losses, and cost.
VSC maintains constant DC voltage and therefore Power flow reversal is possible by adjusting the PWM sequence. Also, it is more suitable for building the multi-terminal networks and HVDC grids.
The VSC has a good response to the AC faults and AC fault ride-through capability.
Black-start capability: VSC can start or restore the power to the AC network without power generation units.
The VSC uses IGBT and therefore it can block the short circuit fault current. However, the fault current can flow via an antiparallel diode in 2-level VSC. But this problem can be avoided in MMC-based HVDC.
2.2. Limitations of VSC-HVDC Transmission Systems
VSC-HVDC is vulnerable to DC side faults. The rate of rise of the DC fault current is higher because of the low reactor on the DC side. The 2-level VSC behaves like an uncontrollable diode bridge during DC side faults.
It gives higher losses than LCC-HVDC because the IGBTs in VSC-HVDC have high on-state losses (i.e., the losses in one VSC terminal are about 1.6 %).
High-frequency switching in VSC-HVDC can lead to high switching losses.
Lower power ratings because IGBTs in VSC-HVDC have lower power capability than thyristors in LCC-HVDC.
IGBTs in VSC-HVDC have lower current overload capability than thyristors in LCC-HVDC.
References
Dragan Jovcic, High voltage direct current transmission: converters, systems and DC grids. John Wiley & Sons, 2019.
Christian M Franck, "HVDC circuit breakers: A review identifying future research needs." IEEE transactions on power delivery, 2011.