T&D World Magazine

BPA Advances Transmission Technique for Wind Energy

Connecting wind energy to a decades-old electric grid requires new techniques. One technique Bonneville Power Administration engineers are developing is the increased use of dynamic transfer – a specialized use of the transmission system to accommodate the continual increase or decrease of power generation. On a pilot basis, BPA is allowing an increased use of dynamic transfer to help balance variations in wind energy.

The agency recently awarded five customers the ability to use this transmission technique starting Oct. 1, 2011, during the second phase of a Dynamic Transfer Capability pilot. BPA previously awarded DTC to two customers for the first phase in 2010.

"We saw more interest in the pilot this year, and we were able to offer DTC to more customers," says Brian Tuck, Dynamic Transfer Limits Study manager. "The additional awards will help us get a better sense of how the pilot operates and ultimately help us determine how to expand the pilot."

Dynamic transfer is not a new concept. BPA has historically allowed limited use of dynamic transfers across its transmission grid to help other utilities balance generation and load. But the use of dynamic transfers to correct wind schedules is new, and interest in it has risen with the growth of wind projects on BPA’s grid.

The technique allows a utility to remotely control and manage a power plant in another utility’s portion of the grid, its balancing authority area. This lets the utility balance unscheduled variations in wind generation moment to moment.

Dynamic transfer can also electronically move a wind generator – and all the variation that comes with it – from one balancing authority area to another. The next phase of the pilot could facilitate the transfer of 200 megawatts of wind generation out of the BPA balancing authority area, which would save the agency from maintaining the reserves to balance that wind energy.

The basic need is balance
Today, the federal hydroelectric system balances most of the unscheduled ups and downs of wind generation in BPA’s grid. BPA holds back water in reservoirs when wind blows more power than scheduled, or sends water through hydro turbines to produce power when there isn’t enough wind to meet schedules. The ability of the federal hydro system to provide this service is limited.

"Dynamic transfers allow our customers to rely on other sources to balance wind energy," says Tuck. "For BPA, it means reducing the amount of reserves we maintain to balance wind energy, and increasing the flexibility of the hydro system to generate power or store water for other needs."

Many resources that could be used for balancing wind energy are remote from the wind projects they could balance. Dynamic transfer allows utilities to access these remote resources.

While this new, increased use of dynamic transfer can be beneficial, the capability is limited because the continual ramp of resources can degrade the reliability of the transmission system. The pilot explores how to safely expand the amount of dynamic transfer BPA can provide.

How it works
To understand the difference between historic use of dynamic transfers and today’s needs, think of an airplane flight. In the past, dynamic transfer balanced differences between traditional power sources and load, which are relatively small. You can compare it to gentle increases and decreases in an airplane’s elevation throughout a flight.

Balancing wind requires much more dynamic transfer capability, because ramps are large and frequent. It would be like dramatically and continually changing an airplane’s elevation. A flight like this would be uncomfortable for passengers and put a lot of stress on the airplane and pilot. Large and frequent power ramps across the transmission system put a similar burden on system operators. The pilot is helping BPA develop the tools, training and equipment to accommodate this use of the transmission system reliably.

As wind generation increases and decreases throughout the hour, its varying energy is sent across the system. Signals from the wind plant tell the balancing resource when to ramp its balancing energy up or down – opposite the wind. If 100 megawatts of wind generation are scheduled, 100 megawatts will be produced, but they may come from a combination of the wind and the remote balancing generator.

One pilot enables another The 2010 Dynamic Transfer Capability pilot enabled a second BPA pilot project in which Iberdrola – the largest wind project owner in the Columbia Basin – provides its own balancing reserves. This Customer Supplied Generation Imbalance pilot helps BPA reduce the amount of balancing reserves it holds by about 300 megawatts. BPA and Iberdrola will use phase 2 of the DTC pilot to continue the CSGI pilot through 2013.

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