Page 81 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

68 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
®

of new added capacity, overtaking wind energy installation that was below 12 GW. During 2014,
the PV technology lost some momentum, with only 7 GW of added capacity being the second in
the line, as reported by SolarPower Europe (formerly European Photovoltaic Industry Association
[EPIA]) [2].
PV has been one of the fastest growing energy technologies during the last years, thanks to the
booming of the European market until its saturation in 2012. During 2014, markets outside Europe
increased, especially in China and the Asia-Pacific region, as reported by EPIA. As a consequence,
at the end of 2015, there were over 222 GW of PV systems installed worldwide [2].
For 2016, it is reported that the global installed capacity has increased by 50 GW, and by 2019,
the worldwide capacity might reach 396 GW in the case of low installation scenario, while the opti-
mistic forecast is 540 GW [2].
Ground-mounted PV plants have reached power levels of several hundreds of MW, and during
the last years, these systems no longer worked in an “install and forget” mode, meaning that grid-
connected PV systems injected all the available energy into the electrical network. Presently, extra
services for supporting the grid with reactive power is a requirement even for systems in the kW
range, making PV inverters a key component in the energy transformation chain of solar power.
PV inverters convert the DC power supplied by the PV panels into grid-synchronized AC power
that can be injected into the electrical network. In the past, PV inverters were only passive elements
in the network, working as negative loads. The injected power was proportional with the available
sunshine, supplying only active power to the electrical grid. This passive behavior led to grid sta-
bility issues, mostly in low-voltage (LV) residential or rural areas, where PV penetration was very
high, thereby overwhelming the LV distribution lines and transformers and resulting in an increase
of the voltage in the distribution system level. This led to several outcomes and corresponding key
requirements for the PV inverters [3–5]:

• The PV inverter was required to disconnect from the grid due to the fact that the grid
voltage (V ) has risen above 110% of the nominal value (V ). Standards and grid codes
gn
g
refer to the voltage range for normal operation of converters connected to the grid, stat-
ing that converters can only stay connected to the grid in case the grid voltage is within
0.85 * V < V < 1.1 * V .
gn
g
gn
• The PV inverter was required to reduce the injection of active power with a rate of 40% of
the actual power per 1 Hz difference from the nominal grid frequency and the output power
can only be increased again in case the grid frequency reaches 50.05 Hz in the case of a
nominal frequency of 50 Hz.
• The PV inverter was required to support the grid with reactive power. The amount of reac-
tive power can be calculated from the actual active power or based on the actual grid volt-
age measurement.

Future PV systems should take advantage of the intelligence that can be built into the power elec-
tronic converters, thereby making renewable energy systems influential players on the energy market.
A general control scheme of the three-phase grid-connected PV inverter is shown in Figure 4.1.
In case the nominal PV string voltage does not reach the peak line-to-line grid voltage, a DC–DC
boost converter is added, forming a dual-stage PV inverter. For larger systems, (with sufficiently
high string voltages) a single-stage DC–AC inverter is controlling the power flow from the PV pan-
els to the grid by sensing the most important voltage and current signals.
In the next section, different converter structures and topologies are presented. This is then fol-
lowed by explaining the pulse width modulation (PWM) techniques applied for the listed topolo-
gies. Synchronization is indispensable in case of grid-connected applications, and this is discussed
in Section 4.4.2. The reference voltage for the PWM is supplied from a current control block, which
is discussed in Section 4.4.5, followed by a presentation of different maximum power point tracking
(MPPT) techniques.