I've heard rumours before that as few as 3-5 of the 105 nuclear powerplants in North America, can screw up enough to take down the powergrid. Is this another rumour for the scare mongers, or is this somewhat based on fact? The question is spouted after reading the news that Duke Nuclear has finished it's remediation process, and questioning, what about the rest of them? Thanks for any help on this...

-- Anonymous, June 18, 1999

Answers

Grid stability is more than just a function of supply and demand. The grid can fail with every plant operating. The grid could be unstable but viable with every N-plant down. There is no definite number. Right now there are 10 offline not counting Trojan which just shut down for good.

-- Anonymous, June 18, 1999

Sean,

Please understand that I do not profess to represent myself as an expert on nuclear power plant operations. My background is primarily control area operations in generation, transmission and distribution systems as seen from the fossil fuel side of the energy business. However, I do not think you will find any appreciable difference between electric generators powered by nuclear steam plants than those found in the fossil fuel world. And it is, after all, the loss of the electric generator that you are speaking of in terms of its affect on the grid. Having made this distinction, I would go on to clarify that I am not attempting to address the nuclear safety issue. Ill leave that to someone who knows what they are talking about.

Essentially, I agree with Slappy. There is no definite number. Interconnected as it is (with thousands of loops and radial ties), the grid represents a colossal and formidable form of inertia. The following, rather poor attempt at an analogy may help. It did for me many years ago.

For this imaginary one-way journey, lets imagine thousands of locomotives pulling millions of loaded freight cars of gold. Why gold? Lets just say its my ticket, ok. Youre welcome to come along but I must insist on your tolerance. Anyway, all currently operating locomotives are contributing a diverse level of energy to travel down an endless track at 60 miles per second (mps). (now I did say imagine, so bear with me) All non-operating locomotives are idle and disconnected from the common drive train (grid). Speed must be maintained at 60 mps because we have schedules to keep. Now four of the largest on-line locomotives shut down and automatically disconnect (trip) themselves from the drive train. Sean, you pick the reason for the failure Y2K or whatever you choose.

Load now exceeds capacity and speed begins to decay. During that period of time (less than 1 second) between initial failure and final disconnect, the speed of the train decreases to 59 mps.

All remaining locomotives begin to throttle up in proportion to their size and capability. Alas, it is not enough response and the speed continues to decay. Even though we are in first class, Sean, we are going to be late to our next junction.

Now, a protective relay in the control cab, is designed to disconnect a number of freight cars (dump load), either one by one or in groups, when the speed of the train begins to decay below 58.85 mps. This relay initiates a release command on a sliding scale depending on the degree of decay from 60 mps. The greater the decay, the more cars released.

Ok, now the speed has decayed to 58.0 and the relay continues to dump cars at an accelerating rate until the decay has been arrested and speed begins to return toward 60 mps. By this time, additional locomotives have been quick-started, up to speed and are on hot- standby. Speed decay has been arrested, has recovered and reaches 58.86 mps.

The relay has done its job, ceases execution and has dropped a flag to indicate it had operated. At this point, all on-line locomotives continue to operate at full throttle as the speed begins to reach toward 60 mps.

At 60 mps, additional locomotives on hot standby are tied to the drive train and begin to throttle up. Speed reaches 61 mps and all throttles on all locomotives begin to back off to maintain set point at 60 mps. At this point, load and capacity are relatively balanced, the train, speed and all locomotives are stable but there are thousands of freight cars full of gold that are no longer recoverable. Awww MAN!

However, we have resumed our journey at the correct speed but we will be late to the next junction. The lead locomotive operator calls all the other operators and instructs them to raise their throttle set points to 62 mps and hold that for 4 hours. After 4 hours at this speed all lost time is recovered and we will arrive on time. Sean and I head to the club car for a round of drinks and laughs.

Simple huh? Now obviously there are holes in this example that Im sure some bright engineer will ably point out. And thats fine with me. I would appreciate the help. Admittedly, the speeds given are arbitrary and used for simplicity and this example obviously does not assume other failures. But the biggest difference I see here between the example and electric systems is the complexity and that while the dumped freight cars are lost forever behind the train, in electric systems, those lost customers are recoverable. I hope this helps Sean.

-- Anonymous, June 19, 1999

Great analogy, JT.

-- Anonymous, June 20, 1999

Personally, I like CL's explanations better. Big Fat wires starting out from the Big Concrete structures. Then going to fenced-in control places, with smaller and smaller wires spreading out across the land, finally going into some sort of box in your yard and then to the plug in your wall. And people ready all along the way to manually throw switches and levers to keep it all flowing. Much easier to visualize.

-- Anonymous, June 22, 1999