Page 93 - Hybrid-Renewable Energy Systems in Microgrids

Multilevel inverters: an enabling technology 77

Right now there are two Transrapid systems functioning in the world: the 31.5 km
long test facilities in Emsland, Germany, and a commercial 30 km segment connect-
ing the Long Yang Road subway station with the Pudong International Airport in
Shanghai, China. The last one comprises of 12 converter units of 15.6 MVA each.

4.4 Utilization in grid connected systems

Renewable energy systems technology has underwent a significant development in the
past three decades. Photovoltaic (PV) system is one of the fastest upcoming renewable
energy sources and lots of research works are advancing in this area. The installed
capacity of energy from PV systems all over the global has been increasing exponen-
tially in the following decades and currently it has reached a level of 303 GW at the
end of 2016. Conventionally, the PV system topologies are grouped as four categories:
(1) central inverter, (2) module-integrated inverter, (3) string inverter, and (4) multi
string inverter.
Central inverters are generally used in large-scale PV installations, with a power
rating ranging from 20–400 kW, where PV arrays are linked in parallel strings, and
the DC–AC conversion is accomplished into one common inverter. Line commutated
inverters, using thyristors, was actually developed for the grid-connected applications,
but has been reinstated by self-commutated inverters employing insulated gate bipo-
lar transistors (IGBTs), or field-effect transistors (FETs) for less power. Pulse-width
modulation (PWM) and digital signal processing (DSP)-based circuit controllers have
improved the quality of the developed energy, with almost sinusoidal current output,
and this averts the usage of large reactive compensation units. Currently, inverters with
space vector modulation control have been actuated, and more effort has also been
centered on establishing new inverter topologies, to achieve high partial-loads effi-
ciency. Central inverter topology consists of several PV strings (PV panels connected
in series), which are connected in parallel along with one blocking diode per string to
structure a single DC-link. This topology is simple in structure, has a reliable control,
and is having less investment at the beginning stage. Anyhow, with a single central-
ized maximum power point tracking (MPPT) control, the energy generated can be
easily decremented by the reason of partial shading and panel mismatch. Sectioning
PV panels into minor groups with separate MPPT control can measure the problem.
An AC module is an integrated amalgamation of a single solar module and a single
inverter. Currently though, the design idea of multi-string inverters has come onto the
market, these are intended as an intermittent approach between string inverters and
AC modules. Module-integrated inverter topology comprises of one converter that
operates with only one or a few PV panels so that the power loss created in panel
mismatch can be reduced and the effect of partial shading can be subdued. Other than
that, the voltage amplification by either transformer or DC-DC boost stage is required.
The module-integrated inverter topology is proposed for small PV systems lower than
500 W [16].
The string inverter conceit was introduced onto the European market by summer
1995 when SMA did the launch of SWR 700 Sunny Boy inverter. String inverters are
positioned on a modular concept, in which PV string arrays are linked to inverters in