Environmental Engineering Reference
In-Depth Information
The output current should not have any direct current bias, since it pre-
magnetises transformers within the grid and may also impair the function of
earth leakage protection switches.
Feed-in current and grid voltage should not show any phase shift (cos φ = 1), to
prevent reactive power from oscillating between the grid and the inverter that
may cause additional losses. Future inverter generations should allow for active
reactive power compensation to enhance supply quality and reduce transmis-
sion losses. The operation of such a plant thus creates an additional value for
the network operator.
In case of abnormal operating conditions (such as missing or excessive mains
voltage, strong deviations from the target frequency, short-circuits or isolation
errors) the inverter must automatically disconnect from the grid. To monitor
parameters characterising the grid, such as voltage and frequency, the monitor-
ing of all three phases has requested also for single-phase inverters in the past.
Since the launch of the ENS (device for grid monitoring with allocated series-
connected switching) photovoltaic applications up to 5 kW have been consid-
erably simplified. By measuring grid impedance, dynamic grid impedance
changes as well as grid voltage and frequency, this system detects grid failure
and disconnects the inverter from the mains using two independent switching
devices. For safety purposes monitoring devices must be of redundant design.
According to the plant concept such an ENS may either be provided individu-
ally for every inverter or one central ENS for several inverters.
Further safety components such as isolation or earth leakage protection
switches suitable for AC and DC currents must be provided in accordance with
the inverter concept.
Ripple control signals integrated into the grid voltage by the power supply
companies must not be distorted by the inverter nor disturb its operation.
The entry side should be well adapted to the solar generator, e.g. by Maximum
Power Point Tracking (MPPT). The commonly applied MPPT algorithms de-
termine the maximum power point of the photovoltaic generator by performing
search functions at regular intervals, e.g. every few seconds or minutes. For
this purpose the working voltage of the solar generator is modified by a small
quantity; if, following this operation, the output power of the inverter is in-
creased, the search direction is maintained during the next search function, oth-
erwise it is inverted. The optimum voltage value determined by this procedure
will be maintained until the next search. Due to this methodological approach
the working voltage is subject to fluctuations of a certain range around the ac-
tual maximum power point (MPP). Other MPPT processes pass within regular
intervals through a certain section of the characteristic curve of the photo-
voltaic generator to determine the maximum power point, which is also main-
tained until the next search function.
Input voltage fluctuations (voltage ripple) should be low (< 3 %) for single-
phase inverters that feed energy into the grid at 50 Hz to enable inverter opera-
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