Solar power is one of the most convenient and readily available form of renewable energy. Scientists have devised the methodologies and technologists have created the stuff employed for conversion of heat and light of solar rays into electricity available at our home outlets. Some specialized genre of semi conductor materials that exhibits photovoltaic effect i.e. these material produce direct current in response to incident light, are used in the fabrication of solar plates combined in the form of arrays. Each single plate is a combination of a huge number of solar cells with each cell producing voltage output in the range of 0.6 to 0.8 volts DC depending upon various factors such as environmental conditions. Home appliances in most of the countries in the world need alternating current (AC) for operation so the output cultivated from solar plates must be converted to AC and the device which performs this conversion is known as solar power inverter.
The inverter operation is inherently very simple, it takes the input in the form of DC from a solar panel, the rated voltage varies from system to system but a typical standard value is 17 Volts, and inverts it to alternating current. The complexity of the system begins when it comes to the purity of sine wave, protection from short circuit and overvoltage and last but not the least meeting the load requirements. If the sine wave is not pure it induces harmonics which in turn cause efficiency degradation of the inverter. The solar inverters are segregated into various types but a major classification describes them as Grid connected inverter systems or stand alone. The Grid connected inverters are a part of huge solar panels bank and feed to the utility electric grid, which means it synchronizes itself with the grid in terms of voltage and frequency profiles. The stand alone types are used for small scale commercial or domestic applications.
There is another taxonomy of solar inverters comprising of micro inverters, string inverters and central inverters. In case of micro inverters each solar panel has its own inverter system connected to it and this makes the whole system very efficient and robust as implementation of maximum power point tracking becomes more feasible and in case of a single unit failure, the panel can be identified and replaced without affecting rest of the system and also makes it possible to use different makes of the solar modules in single system. The draw backs of micro inverters include high costs in terms of dollars per watt. The inherent design of micro inverters at a larger scale makes the system complex for both installation and maintenance.
For commercial and domestic requirements, the most abundantly found kind of the inverter is the string inverter and a large array of solar panels requires only one inverter unit to transform DC to AC. The advantages of string inverter over micro inverter is the low cost, flexibility of the design, robustness and moreover it also allows remote system monitoring because it is can be installed at a certain distance from the panels. The cons of the string inverters include the incapability of the panel level monitoring as present in micro inverters and also creates high voltage hazards. The MPPT is also a bit complex to manage as each solar array can exhibit different characteristics depending upon exposure to light, environmental conditions and variations caused during the manufacturing process. There is yet another type of inverters known as central inverters. This is derived from string inverters by increasing array size so design constraint and pros and cons of these both types are almost comparable.
A term MPPT( maximum power point tracking ) has been mentioned numerous times so it becomes necessary now to put some thoughts on this mechanism. Each solar power system, either grid connected or stand alone, employs a battery bank to store electrical energy and batteries have a nominal voltage and current rating which should not be violated under any circumstances as doing the opposite will deteriorate the batteries thus reducing its life time. For each PV module, there is an ideal voltage at which it can output the maximum power, and this voltage varies by sunlight intensity and temperature. The battery voltage also faces drifts depending upon state of charge and load factor, for a 12V battery voltage may swing between 11 to 14 volts approx. To charge battery, the voltage of PV module must be higher than that of the battery. The balance between these two voltages is maintained by a special DC-DC converter which is usually known as maximum power point tracker, the tracker intelligently finds that point and adjusts the voltages accordingly to get maximum power output.
The selection of an inverter system for any application varies from system to system, A basic rule of thumb is to design the inverter at a slightly higher power level in comparison to the system requirement. The sine wave purity is also part and parcel of the selection as power loss in the form of heating occurs otherwise. The inverter should also synchronized with the frequency and AC voltage levels of the grid it is feeding to because loss of synchronization may cause in power system islanding. In future, the capacitors used for voltage regulation will be removed from the larger solar power systems and capability of monitoring of each single module will be introduced making it possible to replace faulty arrays or modules without affecting the system integrity.