Electronics News
October, 2007 Page: 36
Today's advanced wind turbines are three-bladed, variable speed turbines. The rotor blades are adjusted and controlled via three independent electromechanical propulsion units for the pitch systems. On a pitch-controlled wind turbine the turbine's electronic controller checks the power output of the turbine several times per second. When the power output becomes too high, it sends an order to the pitch mechanism, which immediately turns the rotor blades slightly out of the wind. Conversely, the blades are turned back into the wind whenever the wind drops again. Thus aerodynamic efficiency and reduced loads on the drive train are maintained, providing reduced maintenance and longer turbine life. To enhance the level of safety, newer wind turbine technology uses the wind not only to produce wind energy but also for its own safety.
The converters feature aerodynamic braking by individual pitch control. The rotor attains the full braking effect with a 90ยบ off position of all three blades. Even if a blade pitch unit fails, the other two rotor blades finish off the braking process safely. To enhance the level of safety, each of the autonomous pitch systems is equipped with an emergency power supply to immediately ensure the reliable functioning of the fast blade pitch system if there is a total power failure or for maintenance.
Currently, batteries are the most widely used component for emergency power supply. They are sized to satisfy the peak power demands to adjust the rotor blades, even if those demands occur for only a few seconds. If high power is needed, the deficiencies of battery storage systems are varied and they create many design challenges for pitch system engineers. Batteries have a known low-temperature performance in addition to a very limited lifetime under extreme conditions.
Batteries require repeated replacement throughout the life of the wind energy plant and they are not designed to satisfy the most important requirements of pitch system power source, which is to provide bursts of power in the seconds range for rotor blade adjustments over many hundreds of thousands of cycles.
With no moving parts, ultracapacitors provide a simple, solid-state, highly reliable solution to buffer short-term mismatches between the power available and the power required. When appropriately designed with a systems approach, they offer good performance, wide operating temperature range, long life, flexible management, reduced system size, and are cost effective as well as reliable.
Maxwell Technologies has available the Boostcap ultracapacitors in several sizes, ranging from prismatic 5, 10 and 100 farad cells to cylindrical 2600 farad large cells. To facilitate adoption of ultracapacitors for applications which require integrated modules consisting of multiple ultracapacitor cells, the company provides fully integrated power packs that satisfy the energy storage and power delivery requirements of fast blade pitch systems. Large cell ultracapacitors have been designed into the pitch systems of many wind turbine manufacturers and pitch system designers.
Each autonomous pitch system is equipped with an ultracapacitor emergency power pack to ensure the functioning of the fast blade pitch system with high reliability. The following is a layout example of a pitch system. Pitch systems are in the rotating rotor hub of the wind turbine. The power supply and control signals for the pitch systems are transferred by a slip ring from the non-rotating part of the nacelle. The slip ring is connected to a unit that includes clamps for distributing power and control signals for the three individual blade drive units. Each of them consists of a switched mode power supply, a fieldbus, the motor converter, an emergency system and the ultracapacitor bank. When the power supply is switched on, the ultracapacitor module is charged to its nominal voltage. Typical charging time is about a minute.
The capacitor module has a high enough energy content to run the system for more than 30s with nominal power. The ultracapacitor module is directly connected to the DC link of the motor converter. The converter then drives a three-phase, four-pole asynchronous motor, mounted directly to the gearbox of the blade drive. The motor is designed to give maximum torque at very low rpm. Each blade has sensors that control the blade position.
Manufacturers continue to reach for the stars as installations grow ever larger. Megawatt class turbines dominate much of the actual world market, pushing the average installed capacity per turbine above the 1 MW mark. Several wind energy plant manufacturers are developing multi-megawatt turbines, as the offshore market may demand such installations. The largest turbines can produce power up to 5 MW with rotor diameters of up to 110m.
To ensure the functioning of the fast blade pitch system even for such large installations, bigger emergency power packs have to be integrated. The company's integrated packs assembled with large ultracapacitors are suitable for these megawatt class turbines. To obtain the standard nominal voltage of 300 DC used for such wind turbines, four 75 V sub-modules are connected in series.
Due to their high reliability, efficiency, and operating lifetime, ultracapacitors are especially suitable for offshore and remote wind energy applications. Ultracapacitors are fundamentally viewed as maintenance-free devices that do not require costly test runs and expensive management systems versus batteries, which require ongoing evaluation of their state of health and state of charge.
Reliable and maintenance-free operation is a 'must' for offshore applications because the power plants are several kilometres away from the coast. In winter or during stormy weather conditions, the inspection cycles can even extend to several months. Currently, there are more than 60,000 wind turbines operating worldwide, which represent 32 GW of installed capacity. Of these, offshore installations account for 3% of the world market.
It is expected that soon, offshore wind energy generation will account for 14% of the world's new wind capacity. Though the wind energy contributes less than 0.5% of the total world electricity supply today, it is estimated, that by 2012, wind's growing contribution will reach 2%. Therefore, another 145 GW of new capacity is expected to be installed. The implication of these estimates is that a staggering number of new turbines will be added to networks by then, representing a high potential for advanced three-bladed variable speed turbines that feature aerodynamic braking by individual pitch control. Without question these 'future' turbines require ultracapacitors as they provide a simple, solid-state, cost-effective, long-life solution that ensures the functioning of the fast blade pitch system with the highest reliability.
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