Technical Papers

In the field of renewable energy technologies feed-in converters or frequency converters can enhance efficiency and economic viability. If the control technology is suited to the application and operating conditions with pinpoint accuracy, the output power is increased significantly.

On the occasion of the EU Sustainable Energy Week (EUSEW) the statistical office of the European Union has published figures on the development of the share of renewable energies in the EU27 energy supply: From 1999 to 2009 the share almost doubled. Germany registers a growth from two to eight percent of the gross domestic energy demand and is therefore one of the fastest-growing EU member states in this field.

Part of this growth comes from small systems generating electricity close to the consumer from wind, water, solar, biomass or geothermal power The top priority of private operators, farmers or small business owners is best possible efficiency with regard to amortization. The output of small wind turbines, for example, can be further optimized by a system technology developed for the particular demands of the application.

A Proper Wind Collector

If one wishes to supply energy produced by a small wind turbine to a domestic grid and excess energy to the public low-voltage grid, he is in need of a grid-tie inverter. This device converts the three-phase current generated by the wind turbine to a grid-compatible current with suitable frequency and voltage to feed it into the 230 V grid. The feed-in converter aeocon by SIEB & MEYER (based in Lueneburg, Germany) is developed for this purpose. Since inverters used for these unit sizes are usually produced in small series and in most cases additional equipment is necessary to ensure safe operation, the investment costs are comparatively high. On this account aeocon provides all components required for the operation – rectifier, inverter incl. galvanic isolation and ballast circuit – structurally and functionally combined in one compact housing. Thus, the purchase process is tightened up and above all installation and setup of the device are much easier.

In order to achieve the best possible output and fast amortization the device development was focused on the special operating conditions in this application field with pinpoint accuracy: Compared to photovoltaic systems which tend to have slow status changes, the wind power systems must cope with very dynamic and irregular conditions due to different wind speeds, turbulences or gusts. Therefore aeocon is not only in charge of feeding electricity into the grid but also takes over parts of the operating control.

The system adjustment and the entered characteristic curve suitable for the connected generator play an important role for the efficiency of the wind turbine. aeocon features the possibility to configure the characteristic curve optionally as performance over speed curve P(f) or as performance over voltage curve P(U). The integrated ballast circuit with connected external ballast resistor ensures that the rotor is moved along the programmed characteristic curve all the time. The "brain" of these converters is the control which is exactly adapted to the requirements of small wind turbines, allowing fast control during different wind conditions under consideration of the characteristic curve of the turbine. Owing to the direct use of the rotor speed signal provided by the generator frequency and the eight interpolation points used for precise adjustment of the characteristic curve, excellent control performance is reached. The wide-range power input of 40 to 340 V (AC) allows for feeding in energy even during times of low wind speeds. Since small wind turbines are nearer to ground level and sometimes placed in built environment, the effective use of low wind speeds is of considerable interest for the operator. Thus, in the partial-load range (in which turbines often run) an efficiency above average is achieved.

aeocon is also convincing in regard to other renewable energy technologies, for example small hydroelectric power plants or mini cogeneration (CHP) units. The latter are more and more used in single- and multi-family houses as well as in companies and in housing developments. Principally aeocon is suitable for all applications featuring the conversion of generated current into grid-compatible voltage. Using the device is particularly beneficial when the generator supplies various performances, that means the unit is a speed variable system that requires corresponding application prerequisites – a permanent magnet synchronous generator (PMG), a power range of 3 to approx. 15 kW and a voltage range between 40 and 340 V (AC), ideally the rated voltage is 300 V (AC). Another advantage is the high dynamic range of aeocon.

Control Technology for Top Performance

Systems with ORC (Organic Rankine Cycle) also generate electric energy. Today the systems for residual and waste heat utilization are considered technically mature and become more and more popular. The underlying thermodynamic process allows cost-effective conversion of small thermal energy sources into electricity. The waste heat of CHP units, for example, is used to heat a low-boiling working fluid in an evaporator. This way pressure is generated, which is then released by means of an ORC turbine and converted into electric energy via a generator. "The ORC process allows generating electricity from residual heat that would be wasted otherwise", says Ralph Sawallisch, key account manager for feed-in technology at SIEB & MEYER. "The efficiency range is between nine and fourteen percent. That means a CHP unit with an electric efficiency of up to 40 percent can increase its overall efficiency to more than 50 percent at best. These results are very interesting for unit operators with respect to amortization and they provide a persuasive argument for manufacturers of ORC systems." The process is also suited to the utilization of industrial process heat. Since the process heat arises during production anyway, it can be used to generate electricity without additional primary energy demand.

SIEB & MEYER has developed a feed-in converter for operation with high speed compressors especially used in CHP units with higher performance ranges and downstream ORC systems. The device is based on the frequency converter SD2 for synchronous and asynchronous high speed electric motors. The converter is designed for speeds of 100.000 rpm, for example, and higher. In combination with other components the generated current is converted into grid-compatible current. Depending on the ORC unit the Lueneburg based company provides various forms of feed-in converters customized for the individual application.

This technology is comparatively new and requires solid know-how in regard to control systems for high speed compressors and the specific design of the feed-in converter. "This field of application also requires a clever control method with high dynamics. Devices based on solar power inverters can not provide a control suitable or optimized for these demands", says Sawallisch. If the compressor runs without control, a failure would occur. For this reason the controller must react quickly to changing operating states; in case of a pressure increase, for example, the control must readjust promptly. Otherwise the overspeed threshold would be exceeded quickly causing the compressor to operate in a range where its functionality is not ensured anymore.

In this field of technology the trend is away from asynchronous machines toward synchronous machines. The reason for this is the revision of the Low-Voltage Directive expected by July 2011. VDE-AR-N 4105:2010-07 Generators connected to the low-voltage distribution network –Technical requirements for the connection to and parallel operation with low-voltage distribution networks embodies new requirements with respect to phase shift and reactive power compensation that can not be fulfilled by many asynchronous devices available on the market at present. "This is an impasse without downstream electronics", the key account manager explains. Using a permanent magnet synchronous generator has the advantage that the system can be operated at variable speeds which increases the efficiency of the power control and the overall efficiency. Considering the savings in energy costs over the total running time of the system this investment also pays off.

Knowing how the wind blows

Besides the efficient control technology the user benefits from the long-time know-how of the SIEB & MEYER engineering, development platforms and a modern, flexible production. This combination provides the perfect environment for quick, customized solutions, which are suited exactly to the individual application requirements concerning the technical orientation and achieve the maximum output during operation. Many industrial automation solutions require individual device characteristics. A system that is especially adapted to the application offers decisive advantages in contrast to standard products. In addition many system operators break new ground when using comparatively new technologies. "Know-how is what counts at this point for proper consulting and finding solutions"; Ralph Sawallisch points out. This provides a solid argumentation for manufacturers of ORC units or small wind turbines regarding marketing and sales of their products.