Both asynchronous as well as synchronous generators are used in CHP units. The advantages of asynchronous motors are that they have a simpler design, are easier to maintain and thus more economical than synchronous generators. The disadvantage of asynchronous generators is that they require a separate excitation to start, which is normally supplied from the power grid. The motor is synchronised automatically, but reactive power compensation is needed depending on the specifications of the energy provider, because the motor is drawing reactive power from the power grid. The installation of capacitor batteries may produce relief in this regard. Therefore, asynchronous motors in CHP units are only used up to a capacity of approx. 100 kW.
Synchronous generators are used if greater capacities are required or if the CHP unit is to be operated in isolation, that is, without being connected to the public grid. The technically more elaborate design of the synchronous generator requires the use of so-called synchronisation equipment used for the adaptation to the frequency of the grid to be connected. Thanks to the synchronous generator’s ability to draw as well as supply reactive power from and to the grid, the use of such a generator in the cogeneration unit can contribute to the reduction of the reactive power costs.
As an alternative to off-grid operation alone, the CHP unit may also be operated in parallel with the grid. This means that the cogeneration unit is still connected to the public power grid and that
one hundred percent of the generated electrical power is fed into the grid or
one hundred percent is used to cover own requirements and the additionally required power is covered by electrical power from the public grid, or
it is partly used to cover own requirements or fed into the public grid, depending on the development of the demand for electrical power.
The considered operating mode is dependent on the economic conditions, which must be contrasted with the grid supply and the costs for the procurement from the public grid. These points should be examined carefully during the pre-planning phase.