Renewable energies and power quality: new generation of converters
Regarding this topic of research, the team works on the development of hardware and software tools to estimate and improve the efficiency of the
energy conversion chain for household photovoltaic (PV) applications.
In particular, this section of the document gives an example of result that deals with the energy efficiency of a household grid connected PV system.
Figure 2 reminds the key elements of a grid connected PV system. In particular, it is possible to work on the PV module and the DC-AC converter
(which is generally composed of an MPPT ["Maximum Power Point Tracking"] system [DC-DC stage] and an inverter [DC-AC stage]) to increase the
energy efficiency of the whole system.
Figure 2 : key elements reminder of a grid connected photovoltaic system.
Our research team works on every link in the PV chain. An example of achievement in relation with inverters is described below.
Nowadays, H-Bridge inverters are commonly used in consumer applications because of their quite simple design and cost-effectiveness. However, the
use of this kind of topology can lead to two typical drawbacks.
The first problem deals with earth leakage currents generated by the stray capacitances, especially between the silicon cell and the aluminum structure
of the single glass PV modules (see Figure 3). The values of these capacitances could typically reach 450 nF for 3 kW PV plants. Many standards,
such as the photovoltaic system design manual as described in the UTE C 15-712 document, require protecting the electrical installation with a 30 mA
circuit breaker. Unfortunately, the leakage current values generated by the PV modules could increase well over 30 mA and lead to the installation shut
down. As a consequence, the limitation of earth leakage current levels is an important challenge.
Figure 3 : stray capacitances of a photovoltaic module.
The well-known NPC (Neutral Point Clamping) inverter could be the right one to eliminate earth leakage currents. This kind of topology uses a neutral
point which is connected to the DC voltage source and clamped by diodes. Therefore, no leakage current can be generated. In symmetrical
configuration, the output voltage is divided by 2 for three-level NPC compared to the H-Bridge topology. Moreover, it is necessary to double the DC
voltage source level of an H-Bridge converter to have the same output voltage value as the NPC inverter one. In such cases, the voltage stress across
each semiconductor switch could be important and limit the robustness of the inverter. One of the solutions to dealing with this issue consists in using
a five-level NPC topology. This kind of inverter improves the THD (Total Harmonic Distortion) of the output voltage and no earth leakage current can
flow through the structure. However, a five-level inverter is composed of 8 semiconductor switches and the number of clamping diodes (i.e. 6) is
increased in comparison with the H-Bridge topology. Furthermore, a five-level NPC inverter involves doubling the number of DC voltage sources and
decoupling capacitors. The reliability of these capacitors is often discussed in literature. It is therefore logical that this kind of structure could not be
commonly used because of its poor situation in terms of cost-effectiveness.
Taking into consideration all of these issues, the research team has recently developed a new mixed 5-level inverter to limit earth leakage currents,
while at the same time generating adequate robustness and cost-effectiveness of the system. This kind of topology (see Figure 4) is based on the
mixture between a 2-level H-Bridge converter and a 3-level NPC structure. A comparative simulation analysis between a 5-level NPC inverter and the
new mixed 5-level converter is introduced to get a better understanding of the differences between the two topologies. The results show that the new
Page 1
inverter proposed enables to reduce the voltage stress across the semiconductor switches and to improve the output signal quality. In particular, the
THD, which has approximately the same value as a 5-level NPC inverter, is divided by four compared to the well-known H-Bridge structure. The
number of power devices and input capacities are reduced compared to the NPC topology one (10 even 9 power switches and 2 input capacitances for
the new mixed 5-level inverter compared to 14 semiconductor devices and 4 input capacitances for the 5-level NPC topology). Thus, the robustness of
the converter could be improved. Many experimental measurements are helpful to validate the operation of the new mixed 5-level inverter. The first
results allow confirming the limitation of earth leakage currents and the increase of the robustness of this converter to improve the energy efficiency of
domestic photovoltaic systems.
Figure 4 : new mixed 5-level inverter for 3 kW household photovoltaic applications.
Page 2