The physical constraint
The strongest wind resource is not always near the highest electricity demand. Generation can exceed the power that a transmission boundary safely carries at a particular time. A system operator must maintain network limits and balance generation with demand; it may accept bids or offers from generators to adjust output.
When wind output is curtailed, other generators, demand response or storage elsewhere may be needed to balance the system. The cost can include both the payment associated with reducing output and the cost of replacing it. A headline payment to one wind farm is not identical to the net total balancing cost.
What matters when judging costs
Three different measures often get confused: megawatt-hours of curtailed renewable generation, cash paid or received under balancing actions, and the net cost of actions needed to keep the system secure. They answer different questions.
A complete assessment would compare the lifetime cost of expanding a transmission link with the cost of continuing to curtail power, allowing for uncertainty in future wind build-out, prices, storage and demand. The most attractive answer may vary by location and timing.
Can storage solve the problem?
Storage can absorb surplus electricity and return it later, but its usefulness depends on the power rating, stored energy, location, duration, efficiency and prices. A battery close to a congested wind farm may not remove a distant network bottleneck. Multi-day low-wind periods raise different questions from a few hours of local congestion.
Other options include building transmission, locating flexible demand near generation, changing network operation and adjusting the pace or location of new generation. No one measure removes every constraint.
The policy test
Ask whether each proposed reinforcement has been tested against lower or higher renewable penetration and changing demand. Publishing the assumptions and a range of projected constraint costs would help readers distinguish a necessary investment from an overoptimistic forecast.
Sources and further reading
Reader appreciation — not a vote on the evidence.