Remote Trip

The expansion of the Ferndale project, from 1.8 MW to 5.1 MW will require the installation of a remote trip mechanism. The Hydro One substation will send a signal to wind farm to tell it to shut down when a reclosure occurs.

The electricity distribution network in Ontario, like most of North America, is equipped with automatic reclosures. Europe has few of these, and so the experience with installing wind turbines in Europe, where there are many more deployed, is not transferrable. When an event on the feeder is detected, such as induction from lightning, or a branch grounding one of the wires, the reclosure will open the circuit, and then reclose in about 500 msec (1/2 second). Customers will experience a momentary dip in the voltage, which will cause the lights to flicker, and sometimes reset clocks, computers etc. This occurs on my feeder about 6 times per year, according to Hydro One.

Utility engineers are deathly afraid of “islanding”. An island would occur when a feeder is isolated from the main electricity grid, but the generation on that feeder was sufficient to supply the load on the feeder. The concern is that when the switch closes, the two systems – the island, and the rest of the grid – would be out of phase. When the main system is reconnected to the island, the systems would likely be out of phase, and a “collision” would occur. This would cause substantial torque in the generator, and could damage the turbine. So their solution is to ensure that generation is shut off before reclosing the switch. This is accomplished with remote trip, also called transfer trip.

Transfer trip is simple in concept, but expensive to implement. Basically, the reclosure station is equipped to send a signal to the wind farm to tell it to go off line, and the wind farm is equipped to receive and act on that signal. The wind farm must shut down within 1/2 sec. The signal can be sent by a line of sight radio signal, FreeWave, or by telephone. Because of the length of time required to send the signal, and act on it, there is no time for a modem to dial and place a call. So instead of using a dial up line, a constantly monitored leased line is required. These are expensive. In addition, because utility engineers are so afraid of islanding, they can sometimes require full redundancy, so two leased lines, or two FreeWave transmitters and receivers are required.

One of the concerns with remote trip schemes is how the logic is implemented. The logic can be implemented in a way such that if the communications fails, the wind farm shuts down. This would provide 100% protection from islanding. Or it could be implemented so that if communications fails, that the wind farm can continue to operate, but at the risk of the wind farm owner, if islanding occured, damaging the generator upon closure of the switch. Communications will fail. And losing production during a communications failure is naturally a concern to a wind farm owner. And so is equipment damage.

On the Ferndale project, the cost of a single leased line is $865/month. The turbine is in an area served by Amtelecom, and Hydro One is served by Bell. And it is about 60 km. This no doubt increases the cost. In addition, Hydro One’s only approved product to do this is an ABB NSD570 – a very expensive peice of gear – about $25,000 each end, without battery, rack, or installation. The original quote from Hydro One on my project when I submitted a bid on RFP 1 was almost $200,000 in capital cost, including installation, commissioning etc. At the time, two leased lines were required.

FreeWave signals will go about 20-25 km. So I also would need a repeater station. And I would need to find a line of sight tower to locate the repeater. Hydro One staff and I took a tour of the Peninsula today, looking for suitable repeater locations. We think we found one on the tower on Highway 6 north of Wiarton. Now I need to find out who owns it, and see if they are agreeable to locating antennae and repeaters there, and confirm that it will work.

With the original single turbine installation, Hydro One was satisfied that the turbine would detect a reclosure event on its own, and shut down. This is because the maximum production from the turbine is 25 amps, and the minimum load on the feeder is 43 amps. So when the switch opens, the voltage rapidly drops, which the turbine detects and shuts down. With 3 turbines, islanding is now theoretically possible, and therefore remote trip is required. The rule of thumb used by Hydro One in determining if remote trip is required is you must install remote trip if maximum production exceeds 50% of the minimum load on the feeder.

The questions that still need to be asked are: What are the odds of islanding occuring? And if islanding occurs, what would happen (what damage, if any, would be caused)?

The odds of islanding occuring are very small. Wind turbines are monitoring voltage, amperage, frequency, on each phase, and if something is outside of very tight paremeters, the turbine will shut down. For example, if the frequency drops to 58 Hz, or exceeds 62 Hz, the turbine shuts down. And if the voltage varies by more than 5% of nominal, or 10% with low voltage ride through, the turbine shuts down. I have heard some utility engineers estimate the odds of islanding at 1 in 1 trillion. Others, think it is much less. But the bottom line is that it is very low. The load and the generation have to match precisely, and for some period of time. Since wind varies, so does production, as gusts increase or decrease output dramatically. And load does the same thing, as appliances and machines come on and off. If the odds are truly 1 in 1 trillion, then is the investment in transfer trip gear necessary?

And what would happen if islanding did occur, and the switch closed, causing this collision of two systems? Vestas believes that nothing would happen to their equipment. The generator on the V80 in particular, is designed to accept a 10% slip, so the generator would simply absorb the difference in phases. But because it never occurs, they of course can’t provide proof of this. Hydro One is rightly concerned about what may happen to others on the feeder in the event of collision, as motors attached to the grid, that were operating in a slightly different phase would also experience a rapid phase shift when the main grid reconnects.

Wind turbines are usually induction generators. That means the magnets they use are fed by the grid. As such, they are different than most of the other synchronous generators on the grid, such as in nuclear and fossil plants, and hydraulic plants. So utilities have very little experience with them. Some believe that induction generators, because they depend on the grid to be energized, simply cannot island. Again, because islanding never happens, absolute proof of this is hard to find.

Are utilities too conservative? Or are they right to try to protect customer equipment? Is 100% guaranteed protection appropriate? Or should risks be taken with one in 1 trillion occurences?

My brother Lyle descibes such debates as “technical butt sniffing.” When two dogs meet, they need a good sniff to know what they are dealing with. Engineers, and especially software people are similar. They need to test each other before they decide to respect the other’s point of view. In this case both points of view have merit. And I, as a non technical person, have no way to assess which point of view is correct. Is it truly 1 in 1 trillion? Will nothing happen if islanding occurs? I don’t know. And it seems nobody does. So what I do, is point out that laying high costs onto wind farms, makes them less competitive with conventional generation, and ensures that incumbent forms of fossil and nuclear generation have a competitive edge. If remote trip is truly needed, then fine. If not, then rather than smell rear ends, I smell smog.

One of the challenges with our current market rules is that Hydro One and other distributors, until recently, paid nothing for the installation of systems like transfer trip. So why would they take any risk over possible customer lawsuits? Zero chance was all that was acceptable. The Ontario Energy Board has ruled that now the electricity distributor, such as Hydro One, must pay for the cost of transfer trip systems at their end. They must pay for the gear at the Owen Sound station. Of course, they are allowed to recover this from the rate base, but the change may encourage a shift in thinking. Some of the Hydro One engineers agree that we may not need transfer trip, or that we should find lower cost solutions to implement it. I was travelling with two such engineers today. The successful widescale implementation of wind depends on individuals like this, who think outside the box, and slowly will implement lower cost or better rules. Co-operation, and innovation, will move us forward.

I am hopeful that the Ferndale wind farm will be able to include remote trip with a low cost FreeWave solution. It will meet the needs of the conservative utility engineers and their organization, for whom no risk is tolerable, at a more affordable cost. But I will continue to ask questions, to ensure that even an affordable cost is indeed necessary.

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