Reply to XCel Energy post by Frank Prager

Facing the realities of deep renewable penetration

I wrote this in response to Frank Prager's post on "Xcel Reveals Winds of Change:"

It was appropriate to give renewables a hall pass to avoid paying their fair share of the load-following and backup costs of the grid for a while, but now I agree it is time to face reality and force renewables to cover their costs. This implies quite a number of ancillary services ranging from frequency control (fast) to load shifting (slow). The slow energy storage options (pumped storage, massive batteries, compressed air energy storage) require large facilities that are much less expensive if implemented in rural areas. Both energy farms and load shifting energy storage will usually be sited quite remotely from cities for economic reasons. Thus transmission has a critical role in making deep penetration of renewables practical, even before considering the importance of locating wind and solar generators.

When one also considers the importance of networking together wind and solar generators in multiple weather systems, it becomes obvious that we need a mix of transmission (to access distant resources) and various forms of energy storage to balance loads from non-dispatchable renewable energy sources. One should also consider the advantages of a wider robust network to share the loads between cities, not just for emergency situations but for routine 2-way power flows. HVDC holds the potential to unlock the potential of a supergrid, to create a free market for electricity over a market area.

HVDC schemes have so far been hamstrung by the redundancy standard: that the grid must survive loss of any single resource, such as a powerline or generator without crashing the grid. It is precisely because HVDC schemes have been proposed one at a time that this limitation has been a problem: so far, the redundancy for every HVDC scheme has had to be via the AC grid, and because of this it has been impossible even to deliver the maximum capacity of current technology HVDC (about 7 GW, equal to the highest capacity Chinese HVDC line), because the AC grid cannot handle simultaneous loss of 7 GW. Although I have been working on the next generation of HVDC power lines, capable to about 30 GW, it is clear to me that before there can ever be a market for a 30 GW powerline, the redundancy question and circuit breakers for HVDC must be solved. On redundancy, this post:

points out that the smallest “unit cell” of the future supergrid comprises an HVDC loop. Such a loop, with circuit breakers in the loop between every set of next neighbor AC/DC converters, is self-redundant, because for any pair of AC/DC converters on the loop, they are connected by two independent lines, the clockwise connection and the counterclockwise connection.

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