Solar energy harvesting demands that systems achieve extremely high levels of efficiency. There are two key factors that have the biggest influence on conversion circuit efficiency. The first is topology, and the second is the type of components – including semiconductor switching devices, magnetic elements and capacitors, to name a few -- and their operational characteristics.
Among the various inverter topologies that have been implemented, two have achieved higher efficiencies for grid-connected centralized inverters than alternatives. The first topology is called the Highly Efficient and Reliable Inverter Concept (HERIC). The HERIC topology uses an extra switch and diode pairs at the output, which reduces losses by decoupling the output inductor from the input capacitor. The second, more complex, topology is the multilevel inverter (see Figure 2
). Multilevel inverters feature half the voltage stress in each switch as compared to a HERIC topology. The multilevel inverter approach uses much lower voltage, which yields higher efficiency and lower device costs. An additional benefit of multilevel inverters is that the size of the electromagnetic interference (EMI) level and output filter (for cleaning the harmonics) can be reduced, which reduces overall system cost.
Figure 2: Three-level central inverter
Once the topology is chosen, designers must select the components that will be used. In addition to an inverter, a PV system can also have a DC-to-DC conversion stage for maintaining a constant and controlled input voltage level at the inverter, and decoupling the control of voltage and power. A drawback of using a DC-to-DC conversion stage is that it can negatively impact system efficiency. To mitigate this potential hit on efficiency, designers can employ a number of techniques. One is to use silicon carbide (SiC) power transistors, which offer several advantages over traditional silicon or even gallium arsenide (GaAs) solutions, allowing for much greater power handling and higher switching rates. A number of projects are in the works to develop utility-grade devices with an eye toward creating solid-state power transformers and high-power inverters for wind and solar farms.
A second way to improve PV system efficiency is to use maximum power point tracking (MPPT) algorithms, which enables the PV system to better control the power inverter as it reacts to changes in operating conditions. A number of MPPT algorithms are available, each with advantages and disadvantages. These algorithms regulate the PV output for maximum power delivery, ensuring that the system maintains the optimum operating point. These algorithms also ensure that the inverter draws no more than the maximum PV array output power, thus preventing inverter collapse. There are two ways to deploy an MPPT algorithm – either in the main controller, or in the individual PV modules. The latter approach allows each to track independently, which is preferred in cases where the operating conditions for individual PV modules differ significantly. This finer control over power conversion can greatly enhance the efficiency of the conversion process in applications using a larger number of modules.
A third approach for improving efficiency is to employ pulse width modulation (PWM) technology to control power switching components in the inverter circuit during DC-to-AC conversion. A PWM algorithm is used to control the switching component's states. This ensures that the time-average value of the voltage command is met. PWM algorithms can reduce losses in the inverter while optimizing the voltage utilization of the DC bus. They also offer the benefit of being well understood, and they can easily be implemented in either hardware or software.
One last technique for improving efficiency is to use power factor correction (PFC). Capacitive and inductive loads cause a poor power factor, which is the ratio of real power to reactive power where real power is useful and reactive power is wasted (the result of current and voltage being out of phase). With a power factor of one, the voltage and current are in phase, which provides maximum power. By actively correcting the power factor, designers can, in effect, improve system efficiency.