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The debate on the merits and use of direct current (DC) versus alternating current (AC) systems has been around since the days of Thomas Edison and George Westinghouse, and continues today. Edison advocated for DC systems, while Westinghouse pushed for AC. The first electric generator was a DC unit, and the first electric "transmission line" was constructed with DC components. But with the advent of the induction motor, the workhorse in the industries, and the introduction of transformers, AC systems eventually prevailed.

Today's grid is basically an AC grid, with a few high-voltage direct-current (HVDC) systems in the mix. High-voltage alternating-current (HVAC) transmission will continue to constitute the backbone of power transmission grids. However, due to the nature and location of most renewable energy sources, HVDC may play a much larger role going forward.

TYPES OF HVDC SYSTEMS


There are two main categories of HVDC converter technologies: line-commutated converters (LCCs) and voltage source converters (VSCs). The two HVDC technologies are quite comparable, sharing a similar knowledge base and auxiliary subsystems, but the salient differences between the two technologies are as follows:

- LCC HVDC has very high power transmission capacity, while VSC provides more reactive power flexibility;
- LCC HVDC converters absorb a large amount of reactive power, which has to be supplied locally. The reactive power absorbed is at about 0.5 MVAr/MW under ideal conditions or higher and is a function of the active power being transmitted; and
- VSC HVDC can either produce or consume reactive power on demand. This creates a scenario where no separate reactive power is needed (other than the reactive power needed for filtering).

ADVANTAGES AND DISADVANTAGES OF HVDC

HVDC has been widely used to connect regions with different frequencies or when regions have the same frequency but are not synchronized. Some advantages of traditional HVDC use include the following:

Advantages For Overhead Transmission Lines


A long-distance, high-power HVDC transmission generally has lower capital costs, decreased losses and simpler line construction. Compared to HVAC, it wins on the fronts of charging current requirements, corona and radio interference. HVDC systems are more controllable than HVAC systems. They don’t require synchronous operation, nor do they contribute to short-circuit current levels. Magnetic fields from HVDC lines are negligible compared to corresponding magnetic fields for AC lines.

Advantages For Underground Systems


For HVAC cables, the conductor’s carrying capacity could be diminished in supplying the cable’s charging current, thus limiting its length and power-carrying capacity. HVDC cables have no such limitation. Although some DC leakage current continues to flow through the dielectric insulators, this is a very small concern compared to the cable rating - and the leakage is much less than with AC cables. Also, in AC systems, additional energy losses occur as a result of dielectric loss in the cable insulation.

Advantages For Asynchronous Connections


HVDC allows power transmission between unsynchronized AC systems; therefore, it contributes positively to system stability. Controllable AC output voltage and frequency of VSC HVDC links have the added benefit of increasing grid stability and operating margins. Consequently, HVDC can prevent cascading failures (such as those resulting from large loss of load due to major disturbances) from propagating to other parts of the grid. In addition, the full control of power flow enables HVDC to provide efficient power trading between regions and operators.

Disadvantages Of HVDC

HVDC is not a panacea for transmission systems for several reasons:

- Converters are expensive;
- LCC systems require reactive power sources;
- Converters generate harmonics and require filters; and
- Multi-terminal or network operation is not easy.

NEW LIFE FOR HVDC

In recent years, with deregulation and the addition of renewable energy sources, the number of built and planned HVDC interconnections has increased significantly (see Figure 1). Nonetheless, the amount of power being handled today by HVDC systems is still less than 2% of the installed global generation capacity.

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Offshore Wind Power

HVDC can offer opportunities in terms of renewable integration. One example is in the interconnection of offshore wind power plants. The benefits of such applications include:

- AC faults appearing in a wind park or main grid will not be propagated by the VSC transmission system - reducing the mechanical stress on wind turbine generators;
- The inherent VSC voltage and frequency control capability simplifies wind farms’ black starts; and
- With the VSC HVDC, the wind farm may behave like a power plant (albeit, with intermittent operation) for dispatching purposes.

HVDC And The Supergrids In Europe

Another example of the use of HVDC relates to the fact that the present European AC transmission grid has almost reached its transport capacity, and large electricity transport through the grid may lead to stability problems.

A Pan-European HVDC power grid would offer a valid solution with the following advantages:

- Strengthening the entire transmission grid, especially under demanding load conditions and during system disturbances. With HVDC, operators have an effective instrument to easily regulate the flow of energy in the region. It might also protect the separate AC networks from the uncontrolled spreading of disturbances and overvoltages;
- Removing local bottlenecks in the AC grid;
- Facilitating transport of electricity over very long distances. This allows connections of large remote wind resources (on land and off shore) into the continent, as well as connection of large solar farms in the south - even from the north of Africa;
- More effectively utilizing regional sources, regardless of the source (hydro, thermal or nuclear);
- Mitigating the effects of intermittency by averaging and smoothing the outputs of large numbers of geographically dispersed wind or solar farms;
- Lowering the necessary overall reserves, thus reducing the associated greenhouse gas emissions; and
- Facilitating a virtual power plant (VPP) concept across the different regions.

BARRIERS

Research is ongoing to enable wider use of HVDC systems, but mass deployments of HVDC can be thwarted if key barriers are not addressed. Some are technological in nature; others are related to renewable energy policy. 

- Converters remain expensive, and large capital-intensive transmission projects are often cost-prohibitive;
- Cable development needs improvements, such as the use of higher voltage level applications, cross-linked polyethylene (XPLE), and nanotechnology; and
- As regional DC grids grow into meshed grids, advanced control and protection schemes and breakers will be needed. Further development is needed to fully meet all future needs and regulatory demands, including the areas of protection and fault current interruption improvement. Monitoring and control systems for multi-terminal concepts require more research and development, as do communication aspects.

CONCLUSION

For over 100 years, HVAC has worked well as the grid’s backbone, but large interconnections pose some challenges to it. HVDC interconnections can strengthen those interconnections and can be critical in the development of future sustainable and robust, interconnected transmission grids.

HVDC (and the technology involved) plays a role in:

- Regional grids that interconnect with one another (LCC and VSC HVDC);
- Bulk power transmission from concentrated, large, remote renewable energy sources, such as large-scale hydropower plants and pumped hydro (LCC HVDC);
- Offshore wind farms (VSC HVDC);
- Use of embedded HVDC links for improving grid performance and facilitating the introduction of renewable energy into the grid (VSC HVDC); and
- Connectivity between countries and power market regions.

If certain barriers are overcome, HVDC’s inherent properties may make it a more convenient and efficient power transmission and remotely located renewable energy source.

Nicholas C. Abi-Samra is an IEEE Smart Grid subject matter expert. He is senior vice president of electricity transmission and distribution at DNV KEMA and is experienced in power systems, planning, operations and maintenance. Abi-Samra served as the general chair and technical program coordinator for the IEEE General Meeting of 2012. He is a professional engineer. He can be reached at nicholas.abi-samra@dnvkema.com.


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