Solar farms have the capacity to provide an additional service to the grid, beyond supplying power. It’s the inverters, needed to convert PV’s natural DC output to AC, that create the opportunity, Colin Schauder, a Fellow at Satcon Technology Corporation, explained in a phone interview. Generators with inverter banks have properties that make them compatible with the grid in a way that ordinary generators are not, he said. In the process of converting DC to AC they act as power managers that can control current from one moment to the next.
The question arose from the number of microFIT projects that, in recent news reports, have been told they won’t be allowed to connect. Hydro One, the distributor to which most of these are trying to connect, is worried that many of its transformer stations are already at or near the limit of the power they can properly connect, without risking safety as well as Hydro One and customer equipment. One issue is the maximum fault level that can be handled at Hydro One’s transformer stations. It’s the circuit breakers on the low voltage side of those transformers: in some cases the current on those lines is right at the limit of what those switches can interrupt, Hydro One is saying. What might then happen, in the case of a fault on one of the feeders, is a small amount of current leaking over from an adjacent distribution feeder line with one or more of these small generators connected to it, and even though small, that would be more than the breakers could interrupt. This might destroy the breaker while allowing some current through to threaten line workers, among other things.
The maximum allowable fault current is also mandated by the OEB’s Transmission System Code.
In response, in stakeholder meetings, inverter suppliers have been arguing that the electronics included with their equipment allows very rapid sensing of line faults, so that the inverter, the gateway between the solar generation equipment and the local grid, can simply turn supply off – preventing the above situation before the mechanical breaker has the problem. To this, Hydro One has responded that an inverter can only shut off if it detects a fault. A fault in an adjacent feeder cannot and should not be detected by an inverter and it will not shut off, so it is still contributing to the current the breaker must interrupt.
In addition, scientists at the University of Western Ontario are developing technology that is intended to employ solar installations to assist with voltage regulation on the grid. This can alleviate transmission constraints at times and reduce the cost of voltage regulation equipment required at nearby intermittent generators such as wind farms. Overall, it has the potential to move more power through existing transmission lines, at much lower cost than building additional transmission capacity.
Rajiv Varma, an associate professor at the University of Western Ontario has patents pending on this equipment and two distributors in Ontario have plans to use the technology at select solar sites. See “How to get more value out of a transmission line,” IPPSO FACTO, April 2010.
Discussions back and forth, on this and other matters, some of extreme technical complexity, are continuing.
Satcon has previously supplied inverters for large PV projects in Ontario, and is currently competing for several proposed new projects.
Related stories in this feature:
What’s behind Ontario’s PV boom?
Enbridge, First Solar announce more projects
First Solar and the Sarnia solar farm
Ontario’s solar sector evolves to meet the challenge of large scale application
The evolution of PV technology
Competition in the PV industry evolving quickly
How much is solar actually costing us?