1.
2.
3.
4.
being DC to three phase [150]. In the case where more than one string is connected in
parallel and significant differences in the irradiance from string to string are expected,
blocking diodes are required to prevent current circulation inside strings.
Many different inverter topologies are utilized as three-phase inverters. Sometimes
they are organized as a single DC to three-phase unit but sometimes as three separate DC
to AC single phase units working with a phase displacement of 120 °C each. Having all
the PV modules connected in a single array in such a centralized configuration offers the
lowest specific cost (cost per kW p of installed power). Since central inverters only use a
few components, they are very reliable which makes them the preferred option in largescale PV power plants.
In spite of their simplicity and low specific cost, central inverter systems suffer from
the following disadvantages:
Due to the layout of the system, a large amount of power is carried over
considerable distances using DC wiring. This can cause safety issues because
fault DC currents are difficult to interrupt. Special precautions must to be taken
such as thicker insulation on the DC cabling and special circuit breakers, which
can increase costs.
All strings operate at the same maximum power point, which leads to mismatch
losses in the modules. This is a significant disadvantage. Mismatch losses
increase even more with ageing and with partial shading of sections of the array.
Mismatches between the different strings may significantly reduce the overall
system output.
Low flexibility and expandability of the system. Due to the high ratings, a system
is normally designed as one unit and hence is difficult to extend. In other words
the system design is not very flexible.
Power losses in the string diodes (if any), which are put in series with each string
to prevent current circulation inside the strings.
Micro inverters
A very different architecture is that of the micro inverters, as shown in Figure 19.6 (b).
These inverters operate directly at one or several PV modules and have power ratings of
several hundreds of watts. Because of the low voltage rating of the PV module, these
inverters often require a two-stage power conversion. In the first stage, the DC voltage is
boosted to the required value while it is inverted to AC in the second stage. Often, a high
frequency transformer is incorporated providing full galvanic isolation, which enhances
the system flexibility even further. These inverters are usually placed close to the PV
2.
3.
4.
being DC to three phase [150]. In the case where more than one string is connected in
parallel and significant differences in the irradiance from string to string are expected,
blocking diodes are required to prevent current circulation inside strings.
Many different inverter topologies are utilized as three-phase inverters. Sometimes
they are organized as a single DC to three-phase unit but sometimes as three separate DC
to AC single phase units working with a phase displacement of 120 °C each. Having all
the PV modules connected in a single array in such a centralized configuration offers the
lowest specific cost (cost per kW p of installed power). Since central inverters only use a
few components, they are very reliable which makes them the preferred option in largescale PV power plants.
In spite of their simplicity and low specific cost, central inverter systems suffer from
the following disadvantages:
Due to the layout of the system, a large amount of power is carried over
considerable distances using DC wiring. This can cause safety issues because
fault DC currents are difficult to interrupt. Special precautions must to be taken
such as thicker insulation on the DC cabling and special circuit breakers, which
can increase costs.
All strings operate at the same maximum power point, which leads to mismatch
losses in the modules. This is a significant disadvantage. Mismatch losses
increase even more with ageing and with partial shading of sections of the array.
Mismatches between the different strings may significantly reduce the overall
system output.
Low flexibility and expandability of the system. Due to the high ratings, a system
is normally designed as one unit and hence is difficult to extend. In other words
the system design is not very flexible.
Power losses in the string diodes (if any), which are put in series with each string
to prevent current circulation inside the strings.
Micro inverters
A very different architecture is that of the micro inverters, as shown in Figure 19.6 (b).
These inverters operate directly at one or several PV modules and have power ratings of
several hundreds of watts. Because of the low voltage rating of the PV module, these
inverters often require a two-stage power conversion. In the first stage, the DC voltage is
boosted to the required value while it is inverted to AC in the second stage. Often, a high
frequency transformer is incorporated providing full galvanic isolation, which enhances
the system flexibility even further. These inverters are usually placed close to the PV