KEYNOTE PCIM 2021: HVDC grid challenges and opportunities

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Seddik Bacha, Scientific Director and Scientific Committee Chair of the Supergrid Architecture & Systems research program at the SuperGrid Institute, delivered a keynote address on HVDC grid challenges and opportunities at PCIM Digital Days 2021.

Electrical power needs to be transmitted over long distances from generating stations to electrical substations from which it is sent to end-consumers. Though direct current (DC) transmission systems came first, they were quickly replaced by alternate current (AC) systems because DC systems were unable to transmit power for more than a few kilometers.

This is because as transmission distance increases, voltage drops considerably due to I2R losses. In a DC system, there is no simple way to change voltage like there is in an AC system with transformers. So, with the advent of AC, it became easier to transfer power over longer distances by stepping up voltage for transmission and stepping it down again for utilization in mains systems.

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Today, a vast majority of transmission systems use three-phase high-voltage AC (HVAC) power, however, developments in power electronics are having us consider a potential 180 and return to using DC power for transmission purposes.

HVAC vs HVDC

Power electronics will play a huge role in the electrical transmission systems of tomorrow. Due to the huge (and ever-increasing) quantity of renewable energy being integrated into the power grid, experts overwhelmingly agree that the current transmission system will need to be adapted in order to meet the demand for new energy sources.

How this can be achieved, however, has split opinions: While some believe that enhancing the existing HVAC grid will be enough, others think that an all-new HVDC (high-voltage direct current) grid will need to be built in parallel, explains Seddik Bacha of the SuperGrid Institute in his keynote address, HVDC Grid Challenges Locks and Opportunities, delivered at PCIM Digital Days 2021.

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According to Bacha, who argued for the latter during his keynote address, there are several new requirements and challenges that come with renewable energy. These include sending bulk power over long distances between countries, increasing transmission capacity, integrating offshore power generation, increasing renewable integration, and stability issues. There’s also an inherent need for more flexibility to address these challenges.

These, he says, makes existing HVAC grid systems unsuitable due to several key limitations including limited distances due to the inductive behavior of overhead lines and the capacitive behavior of cables. In contrast, a HVDC system is far more suitable due to fast control, power flow controls, and the potential coupling of AC grids. A HVDC system also suffers no impedance effect and unlocks the possibility of using cables for long distance power transmission that can’t be achieved in traditional HVAC grids.

Watch the whole keynote here:

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HVDC grid challenges locks and opportunities

Options for HVDC

Bacha’s keynote highlights two solutions that will leverage HVDC to solve these challenges.

The first solution is based on commissioning new lines associated with FACTS (flexible AC transmission system) devices. The second is based on the planning of a complete HVDC structure that will integrate with the existing HVAC grid. Realistically, Bacha says, the best solution will be a compromise between the two possibilities.

In both cases, power electronics will play a pivotal role on the actual operation and future grid development via HVDC applications, such as controlling the energy flows through optimal pathways, enabling interconnects to unsynchronized areas, and to transfer energy over longer and longer distances and make large subsea interconnections a realistic possibility.

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While HVDC has many benefits over HVAC so far as compatibility with renewables is concerned, there are some drawbacks. Not only do HVDC systems require more capital expenditure and result in more energy losses but they also require special protection systems (e.g., breakers and surge arresters), special AC filters, reactive power compensation, and special interoperability considerations when they are to be integrated on an existing system.

On balance, however, HVDC offers the potential to dramatically enhance power grid performance and make possible widespread utilization of renewable energy sources, something which is now a goal for virtually every nation on earth.

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