Converters From Direct Current To Alternating Current
In the early days of photovoltaic solar energy, the electrification facilities used electricity for consumption at the same voltage and with the same form that they received from photovoltaic solar panels and accumulators, that is, 12, 24, 48 volts in direct current. This made a big difference with the users that had electricity distribution network or generator sets at 220 volts AC.
The appliance market has adapted to the majority of users and we can find any appliance with 220 volts of alternating current. Therefore, getting reliable, quality, and reasonably priced appliances that work at low voltage and direct current is more difficult.
Therefore, there is a need for equipment that converts direct currents with low voltage values into alternating currents of 220 volt voltage values. These are the inverters (also known as inverters or converters). Today, there are available thanks to advances in technology with GTO transistors.
Converters direct current / alternating current (inverters, inverters) can convert the 12, 24, 48 volt direct current produced by the solar panels and stored in the battery, in alternating current of 125 or 220 V (currently, 230 V) , like the one normally used in places where the conventional electricity grid is.
Advantages and disadvantages of converters
- This is the type of current that is used throughout the world and, therefore, gives a point of normality.
- Facilitates the purchase of appliances to access those that are more efficient.
- It allows to maintain stable values of voltage and waveform, despite the variability of the state of charge of the batteries.
- The fact of working with higher voltages (220 V is 18 times 12 V) allows to work with low electric currents and, therefore, thinner conductors can be used, usual electrical protections and losses are minimized.
Not all are advantages, they also have some drawback:
- The installation consists of one more element, the converter. Therefore, the reliability of the system decreases.
- The inverter has electrical losses to compensate by generating more electricity to the modules (5%).
- In small installations, the converter can represent an important part of the budget; for example, for an installation of about 100 Wp of module power, a 250 W converter can represent 20% of the total cost.
Characteristics of a current converter
Main characteristics that define a converter
- Input voltage (Vcc): this value must be equal to the value of the accumulator (12, 24, 48 V).
- Output voltage (Vca): this value must be normalized (230 VAC).
- Stability of the output / input voltage: variations of up to 10% are allowed for square wave converters and 5% for sinusoidal wave converters. They are values that the norms admit for the voltage of the conventional electrical networks, independently of the power demanded by the consumption. On the other hand, in installations with accumulators, the input voltage may not be higher than 125% nor lower than 85% of the nominal input voltage of the converter.
- Wave type: currently, inverters must present a normalized AC type format with a pure sine wave.
- Overload capacity (peak powers) and thermal protection: very useful in installations with motors, since at the moment of starting the power needed for nominal operation can be doubled, even if only for a few seconds. It must be borne in mind that any motor, at the time of starting up, can consume a current up to five times the rated current and that, as a rule, approximately three times.
- Energy efficiency or converter efficiency is the ratio between the energy that the converter facilitates to the consumptions in alternating current and the energy that this input (battery) converter needs. If the converter designed for a given power works at a fraction of this power, the performance will go down. A sinusoidal converter must be required to have a performance of 70% working at 20% of the rated power and 85% when working at a power greater than 40% of the rated power.
- Automatic start and standby state: allows the power parts of the same converter to be disconnected in the absence of consumption and reconnected when they detect an energy demand above a previously fixed threshold.
- Protection against reversal of polarity and short-circuits: basic options, given the possibilities of error or malfunction of the consumption circuits that are high during the life of the converter.
- Low harmonic distortion: parameter related to the quality of the generated wave. Harmonics are normally eliminated by means of filters, although this leads to losses. The variation of the frequency of the output voltage will be less than 3% of the nominal.
- Possibility of being combined in parallel: it will allow a possible growth of the installation and of the power consumption.
- Good behavior with temperature variation: operating margin between -5ºC and 40ºC.
- Technical documentation suffices. At least:
- Input and output working voltage.
- Nominal power.
- Nominal frequency and distortion factor.
- Shape of the output wave.
- Supported working temperature range.
- Performance depending on the power demanded.
- Overload that resists.
- Resistance to short circuit.
- Power factor.
Last review: August 29, 2018