LCC-HVDC vs VSC-HVDC Transmission Systems

HVDC power transmission systems can be classified into 2 categories based on converter technologies that are given below:

• Line commutated converter (LCC) based HVDC transmission system

• Voltage source converter (VSC) based HVDC transmission system

1.  Line Commutated Converter (LCC) Based HVDC Transmission Systems

The Line commutated converters (LCC)-based HVDC transmission system is developed based on thyristors. A thyristor is a current-controlled device and therefore it is called a current source converter (CSC) based HVDC transmission system. HVDC converters have one or more six-pulse thyristor bridges. Each bridge consists of six thyristor valves which contain hundreds of individual thyristors. For large systems, the bridges are connected in series in 12-pulse or 24-pulse configurations. The 12-pulse monopole LCC-HVDC is shown in Fig. 1.

In LCC-HVDC transmission system, the active power flow is controlled by adjusting the firing angle (turn-on) and extinction angle. The LCC-HVDC transmission system applies to high voltage and power ratings. It is strong on the DC side due to the large inductance connected to the DC side. The typical smoothing inductance value on the DC side for a large HVDC system is in the range of 0.1 H to 0.5 H. In LCC-HVDC, the converter requires reactive power of around 50 - 60 % of the converter power rating and these are supplied by the filters and capacitor banks connected on the AC side.

1.1.  Advantages of LCC-HVDC Transmission Systems

• High voltage and power rating (for example, power levels up to 6400 MW).

• Low losses because thyristor has low on-state losses and negligible switching losses. The losses in one terminal are about 0.7 % of the rated current.

• LCC-HVDC has a low rate of rise of DC fault current because of large inductance on the DC side.

1.2. Limitations of LCC-HVDC Transmission Systems

• The power flow is unidirectional. The reversal of active power flow requires a change in the DC link voltage polarity of the system. So, building multi-terminal HVDC grid is difficult.

• It cannot be operated with weak AC systems (Low short circuit ratio SCR<2).

• Risk of commutation failure during AC side faults & fault-ride through capability.

• Independent control of active and reactive power is not possible.

• The thyristor has only a turn-on capability, and it does not have a turn-off capability that will not interrupt the fault current.

• LCC converter consumes a large amount of reactive power and so it requires large filters that will increase the capital cost.

• Converter transformers are expensive with high insulation requirements to withstand harmonics and voltage stresses during power reversal.

• DC cable must withstand voltage polarity reversal during power reversal. The XLPE cable cannot be used if voltage reversal is needed.

2.  Voltage Source Converter (VSC) Based HVDC Transmission Systems

The VSC-based HVDC transmission system is developed based on self-commutating devices such as Insulated Gate Bipolar Transistor (IGBT). The IGBT is a voltage-controlled device and therefore it is called a voltage source converter (VSC) based HVDC system. The self-commutating devices allow the application of high-frequency (over 1 kHz) pulse-width modulation (PWM) techniques that allow the precise control of active and reactive powers by controlling the full AC system voltage. The VSC has multiple topologies such as 2-level VSC, 3-level VSC and Modular Multi-level Converter (MMC). The symmetric monopole VSC-HVDC is shown in Fig. 2.

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.