Delays were so bad that developers bid for a place in the queue in the very early stages of planning a project. Grid problems led to curtailment and wasted wind power. In Texas, the most severely affected state out of six studied by the Lawrence Berkeley National Laboratory, 17.1% (3,872GWh) of wind was curtailed by the Electric Reliability Council of Texas (Ercot), which operates 75% of the state's grid.
But since then, enough lines have been added - or are planned - to enable US grid to cope. By early 2013, the interconnection backlog had fallen to 136GW, says AWEA's annual report, published in April.
Similarly, Ercot's wind curtailment is expected to have dwindled to almost nothing by the end of 2013, says Michael Goggin, AWEA's transmission expert. And by 2020, planned onshore transmission projects could carry an additional 70GW of wind, AWEA estimates.
"I would say that we're doing fairly well," Michael Skelly, president of transmission-line developer Clean Line Energy Partners, says about the state of US wind-friendly transmission. "We've been proving out the thesis that the US can actually build transmission projects around renewables." Clean Line is developing four long-distance high-voltage direct-current (HVDC) lines, each of which will "uncork" transmission for 3.5GW of wind power by around 2018.
What is not yet clear is how easily transmission policy will allow the next generation of wind expansion. "Until we deal with the base problem - the incumbent utilities using transmission to protect the equity in their own high-cost generation - it's going to be tough," says Ron Lehr, former chair and commissioner of the Colorado Public Utilities Commission. Lehr works with the Western Grid Group, which is trying to secure transmission for wind power in the US West.
The US grid comprises around 322,000 kilometres of high-voltage transmission lines and was designed for utility-owned fossil-fuel plants, which are not as location-constrained and dependent on interstate transmission as wind. Typically, the grid was constructed by local utilities, although independent transmission companies have proliferated more recently. Most of the grid, however, is more than 25 years old and predates America's surge in wind capacity.
Still, as the transmission market has become more competitive, the rate of building new capacity covering greater distance has increased. In the past five years, this rate has accelerated by 230% says the North American Electric Reliability Corporation (NERC), which oversees grid reliability. Current plans suggest the rate will increase by another 64% over the next five years. NERC further estimates that almost a quarter of new transmission is specifically linked to the integration of renewables generation.
Also important to the surge in transmission are the policies of three key regional transmission organisations - Texas' Ercot, the Midwest Independent Transmission System Operator (Miso), and the Southwest Power Pool (SPP). Each allows for regional planning and wider sharing of costs.
Ercot has introduced a system of competitive renewable energy zones, which streamline major wind-friendly transmission lines and allocate costs across Texas. Some $7 billion of projects to be completed by year-end will be able to transmit 18.5GW of wind from the windy parts of the state in the west and the Panhandle to its cities and industrial centres.
Miso's multi-value projects (MVPs) were likewise established to ease transmission's planning, financing and building. The region includes most of the Upper Midwest, a hotbed of wind development. By 2020, MVPs could carry 14GW of wind, says AWEA. Within Miso, the CapX2020 transmission programme - a smaller MVP-style project - could connect an additional 5GW of wind by the end of 2013.
SPP's region includes another wind development hotspot: Kansas and Oklahoma. SPP's priority transmission projects could unlock 3.2GW of wind projects by 2017.
A second broad improvement, which has helped reduce interconnection times, is a reform of the queuing systems, says AWEA's Goggin. In the areas covered by Miso and SPP, as well as in California, a project can now keep its place in the queue for connection even if it reduces its size.
However, transmission bottlenecks still exist, and they can occasionally lead to negative pricing, when power generators sell power for less than $0. The two worst bottlenecks for wind power are currently for transmitting eastwards from the western part of Miso's range, and eastwards from west Texas.
Another area known for congestion is New York state, which has significant wind curtailment due to constraints moving power from the north to the heavily populated south.
In the longer-term, it will still be policy, rather than technical or economic barriers, that will hamper the building of more wind-friendly transmission. More specifically, policies for siting, planning and cost allocation should be improved, say experts. AWEA argues that the siting issue would be eased if the Federal Energy Regulatory Commission (Ferc) had the sort of exclusive siting authority it has over interstate natural gas pipelines, which allows Ferc to intervene if one state is dithering over approving a gasline, says AWEA.
Another fundamental issue is how to split the cost of transmission lines. "The next generation of transmission lines will not happen if we do not have good cost-allocation policies," says AWEA's Goggins.
Cost allocation and planning are addressed in Ferc's ground-breaking Order No. 1000, which could lead to the biggest transformation of the grid in decades. The order was issued in July 2011 and proposes requiring transmission operators to plan regionally, usually at the multi-state level, and to allocate project costs to the beneficiaries regardless of their location.
Strict enforcement of Order 1000's requirement for cost allocation would alleviate the "chicken and egg" dilemma of whether project or grid investment comes first, and also the problem of a pioneering power plant having to shoulder all of the needed grid upgrades, which can cost millions of dollars and then benefit the pioneer's competitors.
Last year, transmission operators had to state how they were planning to comply with the order. But two of the transmission operators with the most planned wind projects could scupper the aim of Order 1000. Miso and PJM Interconnection - which covers Ohio, Virginia, Maryland and New Jersey - suggested that individual states should have more say in allocating cost.
Transmission lines will not be built if any one state has the power to veto, AWEA argues. But Ferc accepted Miso and PJM's proposals in March. While AWEA's Goggins says that adoption of any part of Order 1000 is a step forward, he adds: "It's just not as much as we would have liked."
Ferc is expected to rule on other compliance filings over the next few months. Then it will become clearer how easy it will be to build the grid needed for the government's goal of 20% of theUS's electricity to come from wind by 2030.