Black Start, BlackOut and Diesels: Some Clarity

Vermont Yankee and Vernon Dam

This picture of Vermont Yankee shows the plant and Vernon Dam.  Vernon Dam has always been one of Vermont Yankee's potential sources of back-up power, along with the plant's own diesels, batteries and so forth.

Recently, the grid operator decided that, starting next year, Vernon Dam will no longer be "maintained as a black start unit." Due to that change, Vermont Yankee will have to ask the Public Service Board for permission (Certificate of Public Good) to buy a new diesel generator and pour a concrete slab to hold it.

Reading this material, it's hard to understand what is going on.  Does the plant need a new diesel generator for safety in blackout conditions? Is "blackout" the same as "black start"?

Intevenors Ready for Action

Whatever it means, the usual-suspect intervenors are getting set to intervene.  According to this article by Bob Audette in the Brattleboro Reformer, Ray Shadis of NEC has already offered his opinion, which includes the idea that "the state even today regulates certain aspects of existing emergency generators -- such as exhaust emissions, petroleum leaks and noise," he said.  Other groups are also considering intervening in the diesel placement.

Three Shades of Black

Blackout and blackstart are different. Grid blackout refers to the grid going down, and black start is about bringing the grid up again.  Confusingly, "station blackout" (not the same as grid  blackout) means that the grid is down AND the station's emergency diesels don't operate.  In other words, there are three shades of black: grid blackout, station blackout, and black start.

To avoid station blackout, all nuclear plants have other backup sources of power, all of which have to approved by the NRC.  At the end of the year, apparently, Vernon Dam will no longer be a black start plant, so the NRC will no longer approve it as a backup power source against Vermont Yankee station blackout.

Blackout 

All three "black" conditions start the same way: the grid goes down.  Power isn't flowing. The nuclear plant has a loss of offsite power. At that point, however the three terms diverge: blackout, station blackout, and black start  are different.

Nuclear plants have lost offsite power many times.  This is grid blackout. When this happens, the reactors scram automatically, because where are they going to send their energy?  No grid, no place to receive the  power.  At that point, the emergency diesels begin operating and cool the reactor.  This has happened many times all over the country.  That is grid blackout, and the plant takes care of itself.

It is important to note that the emergency diesels are not running the plant, and they are not sized to run the plant.  Operating the plant at full capacity takes a lot of power for pumps and so forth, and the plant uses grid or its own power for that work.  The emergency diesels are sized to keep the plant's core cool while the plant is off-line and generating only residual heat.  Vermont Yankee has two qualified emergency diesels, and it  also has a hard-wired connection to Vernon Dam, which has acted as a back-up source of emergency power.

If, for some reason, the plant diesels didn't start, the current next step would be Vernon Dam providing power.  Vernon Dam would prevent "station blackout."

However, the grid operator decided that Vernon Dam will lose its qualification as a Black Start station in the future. Vernon Dam will not be one of the "Black Start" stations that re-energizes the grid. Based on this grid operator decision on the future of Vernon Dam, the NRC decided Vernon Dam could not be a "station blackout" resource for Vermont Yankee in the future. (Note, Vernon Dam is still a qualified Black Start station right now..this is all about planning for the future.)

Black Start

Black Start refers to the grid.  When the grid is down, plants have to be brought on line to restart it.  There's an entire sequence of events to energize a grid, and I am beyond my depth to describe it all.  However, I do know that hydro plants have generally been  the "black start" plants.  It takes very little power to start a hydro plant (opening gates rather than starting big pumps) and then the hydro plant supplies power to the grid.  Next, other plants use the hydro power to start their pumps.  Eventually, the whole grid is re-energized.  The workers at Comerford Hydro plant first explained this to me when we visited the hydro plants on Energy Safari last year.  Wikipedia also has a good article on black start.

One of the reasons I like the picture of Vermont Yankee and Vernon Dam at the top of this post is to show the differences in power output. Vernon Dam, like all renewable sources, tends to be land-hungry.  The dam floods a large territory, and makes only 34 MW  of power.  Vermont Yankee makes 620 MW.

Small is Not Beautiful

The small size of Vernon Dam has become a problem to the ISO grid operator, Recently ISO-NE managers have decided that big plants, not small hydro plants, will become the black start plants.   As the Brattleboro Reformer article states: The current (ISO-NE) plan calls for smaller generators, such as the Vernon Hydro Dam, to go online first. The new plan calls for the biggest generators to startup first.

"Vermont Yankee currently credits the Vernon Hydro Dam as its NRC-required Station Blackout AC power source because the dam is a Blackstart facility," said Neil Sheehan, spokesman for the Nuclear Regulatory Commission. "After the end of the year, the Vernon Hydro Dam will no longer be maintained as a Blackstart facility."

Apparently, when the grid operator decided that Vernon Dam will lose its Black Start qualification, the NRC decided that Vernon Dam was not going to be maintained well enough to be a source of backup power for Vermont Yankee  With the loss of Vernon Dam as a qualifying back-up, the plant will have to buy another diesel generator.  Vermont Yankee will keep its connection to Vernon Dam.  However, that connection will not "count" in the NRC assessment of plant safety.

Why The Change

Combined Cycle Gas Turbine
Schematic from InfoPix

According to the Wikipedia article and the Brattleboro Reformer article, ISO-NE has been revising its black start procedures and incentives.   I didn't quite understand why ISO has made this change: hydro plants are traditional black start plants, and Vernon Dam hydro was recently refurbished and upgraded.  I called ISO-NE and Lacey Ryan was kind enough to answer my questions and send me a link  to ISO-NE's position paper on the "Black Start" change.

Basically, with small hydro plants as "Black Start", ISO predicts it will take many hours for the grid to be re-energized.  However, with more large natural gas plants on the grid, using these plants as Black Start plants will bring the grid up sooner.  Natural gas plants can come up to power very quickly, with limited auxiliary equipment.  This is why they are back-ups for wind turbines, and why ISO is designating them as Black Start plants on the New England grid. Much of the information about this choice is on pages 6 to 9 of the ISO document. Starting the grid will be "top down" in the future, with big plants coming on-line first, rather than bottom-up, with small plants first.

Note that plants that raise steam (like nuclear plants) cannot be "black start" plants. It takes time to raise steam.  Combined cycle gas turbines must run open cycle when they operate in black start mode: they can't raise steam either. (Information from ISO document.)

What Does It Mean?

Plant opponents will somehow claim that this shows Vermont Yankee wasn't safe and shouldn't be operated until it gets a new diesel.  That is not the case.  The Vernon Dam plant will be re-classified by ISO-NE, but nothing else has changed. Due to the re-classification, the NRC will also reclassify it and require a new diesel for Vermont Yankee in the future. The NRC's rule is a typical forward-looking safety requirement for the nuclear industry. Things change, and the NRC stays on top of the changes.

However, any time new equipment is required or a new hearing must be held, plant opponents become energized and active.  That's pretty much the only thing wrong with this story.

Acknowledgements

A huge thank you to Howard Shaffer for reading and correcting an earlier version of this draft, and to Lacey Ryan of ISO-NE for her help.  Grid operation is not my strongest knowledge base, and I welcome further comments and corrections from my readers.