The third key aspect of hybrid grids: The mix of delivery
This is the fourth blog on hybrid grids. In the first blog, I talked about the definition of hybrid grids and introduced the four key aspects: mix of generation, mix of transmission and distribution, mix of delivery and mix of control. In today’s blog, I will elaborate on the key aspect mix of delivery, as well as some of its implications for the electric power system.
With “mix of delivery,” the technological form or characterization of the electric power during intermediate stages of delivery or ultimately at the point of use is set. It can be direct current (DC), alternating current (AC) at a relative low frequency (e.g. 50, 60 or 400 cycles/second), high frequency AC or even microwaves.
Note that the customers do not care if the electric power is delivered as alternating current, direct current or any other form. He/she wants energy functions like force, light, entertainment (music, TV, game) and heat …and most of all, that the delivered electric power is:
– Safe to use
– Affordable with high reliability
– Functional, suitable for the job
– Efficient (limited losses/conversion stages)
I will now restrict myself and elaborate on the choice between AC or DC, which has led and still leads to a lot of discussions over the years. The electric power transmission system has undergone significant development through the implementation of new HVDC systems. These systems have enhanced the bulk power transmission capability, enabled more effective transmission over long distances (including long submarine cables), enabled interconnection of regions with different grid frequencies, boosted the integration of large-scale renewable energy sources, improved the flexibility and controllability of AC systems, and in some cases minimized environmental impacts.
There is a natural (technical) choice for DC when bulk power needs to be transmitted over long distances with low losses to connect remote resources, such as hydropower, to distant load centers. This realm is dominated by ultra-high voltage DC, and can be found at the highest voltage levels, presently up to ±800kV.
At the other end of the spectrum we find DC applications — predominantly at the power distribution equipment level — at voltages well below 400V; think of household appliances, battery (charging) systems, LED lighting, electric vehicles, solar PV systems, and many others. The fast development of power electronics, the availability of efficient AC to DC and DC to DC converters and interoperability allow easy connectivity here.
The AC power system, as we know it, will not be converted to a DC system as some experts advocate. It is more likely that the power system will gradually evolve into a hybrid system combining best of both worlds (AC and DC), a kind of “sandwich structure” with DC technology dominant at top and bottom layers, and AC still governing the middle layer.
Source DNV GL
Grids are evolving into combinations of AC and highly controllable DC systems that can be more active in providing end-users with affordable, reliable, and sustainable energy.
The trend towards a hybrid transmission grid with embedded HVDC is observable in Europe and China, and also the US (to a lesser extent) due to the availability of efficient power electronic modules. The biggest impact from the introduction of HVDC becoming embedded into the transmission system is that it signals the end of the single frequency gird (50Hz in Europe and Asia and 60Hz in US) as parts of the grid will, and can, operate at (slightly) different frequencies. The hybrid transmission grid —in spite of its superior features— will become more complex to operate, and will exhibit different, and maybe unexpected, behavior.
A suite of new simulation, control, and laboratory tools are required to perform studies, verify the component and system integrity, and to operate and maintain the system within safe boundaries. If you want to learn more, you can download our position paper on hybrid grid – towards a hybrid AC/DC transmission grid, take a look at some of my blogs and stay tuned for the fifth and final blog that will address more in detail the in my opinion most complicated technology aspect “mix of control”.