Limits on Standard Offer Connections

I attended a meeting with Hydro One this week, to determine if a project that I am involved with can be connected.  The information in this entry would be useful to any Standard Offer proponent.

The application to Hydro One is for two 10 MW projects.  They will be connected on the same distribution feeder line.  The project is outside of the restricted Orange Zone, and so is permitted to be connected.  The proposed projects feed into a substation that has two 25 MVA transformers, but of course since both applications are on the same feeder line, both projects will feed into just one of the transformers.

Last week, we received a letter indicating that we could not connect both projects.  This was after earlier feedback had indicated that we would be able to connect both projects.  How could this be?

Hydro One is learning.  And the criteria they are establishing to connect projects are changing, or being established.  So while the old criteria would have allowed connection of both projects, the new criteria will not.

Here is the math.  First, you need to convert MVA into MW.  To do this, multiply by .9.  So the maximum you can put into a 25 MVA transformer is 22.5 MW.  Then Hydro One multiplies by 60%, to make 13.5.  The 60% criteria is recently established.  It is not clear to me why 60% is the right number.  Why not 50%?  Why not 80%.  Apparently there is an active debate among the engineers at Hydro One, whose role after all is to ensure the reliability of the network, about what number is right.  Some think 60% is too high.  Hydro One has committed to learning what level is appropriate over time, and may have selected a low threshold.  So 60% of 22.5 is 13.5 MW.  To this, you can then add the minimum load on the feeder.  This is the minimum amount of electricity consumed on that feeder at any time in the year.  The minimum load on the substation, which has two transformers, and two sets of feeders, is 5.4 MW, which is roughly balanced between each transformer.  So to the 13.5 MW, you add 2.7 MW.  So the maximum generation that the feeder can accomodate, according to this criteria, is 16.2 MW of generation.

There are definitely some technical challenges faced in attaching generation to distribution feeders.  For example, the line has an automatic tap changer, which allows the voltage on the line to be adjusted up or down.  With 15 MW attached to the feeder, there will be times of high winds, and low demand, when flow through the transformer will be reversed.  When this occurs, according to Hydro One, the tap change adjustments must be disabled, or it could cause problems with the voltage on the line, leading to problems for customers, including the wind farm.  So there are issues to consider.

There are two transformers in the substation.  Apparently this design is fairly common in the Hydro One network.  There is a 115 KV line into the substation, and 27.6 KV feeders.  Part of the reason for having two transformers is redundancy.  If one tranformer failed, all of the feeders in that substation would be transferred to a single tranformer.  In times of high demand, it may be overloaded, but some overloading is allowed, so long as it is limited in time and amount.  In come cases, if the remaining transformer is not overloaded, it may take 3-4 months to replace the failed transformer.  So Hydro One has a dilemma.  They know that they can keep the lights on for customers in the area if a transformer fails.  And they know they can keep 16.2 MW of wind farm connected.  But clearly, according to their criteria, they could not support any more than 13.5 MW (maximum MW per transformer X 60%) plus 5.4 MW(minimum load) = 18.9 MW of wind connected in the event of a transformer failure.  So, can Hydro One connect another 16.2 MW wind farm to the other transformer?  And if they did, which wind farm would not be allowed to operate, if a transformer failed?  These are not easy questions.
The answer, of course, is for Hydro One to stock a spare transformer (or identify substations that they can “borrow” a transformer from), so that they can reduce their repair time to a week instead of 3-4 months.  A wind farm can afford a one week outage.  But this is a bigger policy question.  Hydro One cannot voluntarily increase spares – it is for the OEB or Ministry of Energy to tell them to do so.

There is a natural limit on the amount of generation that can be connected to distribution feeders, without significant upgrades.  Hydro One’s 60% criteria, and concerns about using the redundant transformer to service generation customers may make that limit far lower than needed.  At the same time, Hydro One has to proceed in a diligent way, to ensure the integrity of their network.  We need sound policy decisions by the Ontario Energy Board to support the Standard Offer, while not penalizing Hydro One financially.  We need sound technical decisions by Hydro One, to ensure network reliability, but also to maximize renewable energy connections.  And these technical criteria need to be based on facts and research, and established practices elsewhere.   This is not the place for excess conservatism, until it is proven that conservatism is justified.

There are a total of about 11 proposals to connect into the substation in question, presumably all for 10 MW.  Even with no conservatism, the maximum number of MW’s that can be connected is 50 MW (22.5 X 2 + 5.4).  There will be some projects unable to connect, without substantial and expensive upgrades to the substation, including replacing transformers, at over $1 million each.

Life is not simple in the electricity business.  Reforming the electricity network to accept the maxiumum number of renewable energy generators possible, as we surely must, will take time.  But we will get there.

Leave a Reply