Wind, solar and biomass comprise the primary segments of the non-hydro renewable energy market. Biomass is increasing quite substantially because of the stimulus money coming from the government, said Jim Setterstein, ABB's product manager, wind turbine converters, at ABB Automation & Power World this week in Houston. He explained and compared the AC drive technologies used for wind and solar applications, looking at the pros and cons of the technologies.
Fundamentally, to create wind energy, the wind hits the blades, which supply a draft. This causes a mechanical rotation, and then some type of mechanical-to-electrical conversion takes place. From the converter, the energy moves to the transformer and then on to distribution.
The major difference with solar energy is that the panels provide a DC output, which has to be converted to AC. After the inversion, it follow the same path as the wind energy to the transformer and then to distribution.
Winds of Change
The nacelle on a wind turbine is the housing or enclosure at the base of the blades where the motors, drives and switches are located. Currently, three main topologies exist in the wind industry, explained Setterstein.
"The first topology consists of a dual-winding generator with a gear box and switch gears between the generator and the nacelle" he said. "The basic concept is the same as an industrial motor/generator: If you run below synchronous speed you're motoring. If you're above synchronous speed, you're generating."
The second type is the doubly fed concept. "This is an asynchronous generator with slip recovery," said Setterstein. "The speed range is fairly limited. When you're at synchronous speed, you have zero torque. The benefit is, with the converter system, you have a big efficiency gain. Below synchronous speed, you're consuming some of the power. One of the drawbacks with this design is the slip rings require maintenance fairly often."
Becoming increasingly popular is the full converter system, Setterstein added. "One hundred percent of the power is going through the converter to the grid, but some losses occur in the converter system," he said. "Any disturbances you see on the grid are causing problems in the gear boxes. It can be an induction machine or, more commonly, a permanent magnet synchronous machine. With direct drive, you can get rid of the gear box, and it has much better ride-through capabilities."
The majority of systems in place are the DFIG (doubly fed induction generator) type, but Setterstein predicts more of the full-converter type being used in the next five years. "You've got the isolation and other potential benefits with the full converter," he explained.
ABB converter solutions cover the wind market's three main categories: small wind (below 100 kW); community wind (100 kW-1 MW); and utility wind (above 1 MW).
"As you get into larger frames, we go into liquid cooled," said Setterstein. "ABB offers a variety of solutions up to 6 MW."
The single-converter configuration, which is connected to the generator and then to the local grid, is the most common, said Setterstein. In the double-converter configuration, two separate drives can take advantage of efficiency gains at the lower end and the higher end because, with lower speeds, efficiencies start to drop.
"We have liquid-cooled and air-cooled models available," said Setterstein. "The air- cooled is narrow. These are three-phase modules."
The liquid-cooled models allow completely enclosed systems in which 98% of the heat is transferred to the liquid and then dissipated in a liquid-to-liquid or liquid-to-air exchanger.
The IP54 totally enclosed cabinet means no ventilation openings, which is beneficial in harsh environments where exposure to the elements is an issue. "These meet demands for induction and permanent magnet generators," said Setterstein. "As more wind turbines come on-line, they'll continue making the grid code requirements tougher. The content of the grid code will depend on the region."
The solar side isn't as complicated as wind conversion. "There's no direct connection with a solar panel," explained Setterstein. "You have to have some type of inverter to go from DC to AC. The DC voltage has quite a wide range, so the inverter needs a wide range. You end up having different line-filtering requirements than you would on a wind turbine. But, similar to the wind, you have power tracking for maintaining the DC bus."
The DC bus is going to vary quite a bit, from 900 V down to 400 V, where you'd have to shut down, he explained. For these types of applications, ABB offers an air-cooled inverter solution.