Photovoltaic inverters can be classified in several ways. Based on their application, they can be divided into stand-alone power inverters and stand-alone grid-connected inverters (based on the presence or absence of a transformer, stand-alone grid-connected inverters can be further divided into transformer-type inverters and transformerless inverters). Based on their waveform modulation method, they can be divided into square wave inverters, stepped inverters, sine wave inverters, and combined three-phase inverters.
1. Square wave inverter
This inverter outputs a square wave voltage waveform. The inverter circuitry is simple, inexpensive, and relatively easy to implement. The disadvantage is that the square wave voltage contains a large number of high-order harmonic components, which will generate additional losses in the load and cause significant interference to communication equipment, requiring an external filter. This type of inverter is commonly found in earlier, small-capacity inverters with a design power of no more than a few hundred watts.
2. Stepped wave inverter
A stepped-wave inverter outputs a stepped voltage waveform. The advantage of a stepped-wave inverter is that the output waveform is close to a sine wave, a significant improvement over a square wave, and with reduced high-order harmonic content. When the stepped wave has more than 16 steps (f), the output waveform becomes a quasi-sine wave, resulting in high overall efficiency. However, this type of inverter often requires multiple DC power supplies and a large number of power switching transistors, which can cause inconvenience for photovoltaic array grouping and battery grouping.
3. Sine wave PWM inverter
The advantages of a sinusoidal inverter are that its output waveform is essentially a sine wave, resulting in minimal harmonic losses in the load, low interference with communication equipment, and high overall efficiency. The disadvantages are its complexity and high price. With advancements in power electronics technology and the widespread adoption of pulse width modulation (PWM) technology, large-capacity PWM-type sinusoidal inverters have gradually become the mainstream inverter product. Taking a typical single-phase full-bridge inverter as an example, the four diagonally opposite power transistors, grouped into pairs, are sequentially turned on and off, generating alternating positive and negative voltages at the load terminals, forming an AC output. When the frequency of this alternating conduction matches the AC frequency required by the load, the output voltage is a square wave. When the transistors switch at a frequency much higher than the inverter's AC output voltage, and the pulse width of each switch is modulated according to the amplitude of a sine wave, it becomes a sinusoidal pulse width modulated output inverter. After adding a filter, its output voltage waveform becomes a sinusoidal output inverter.
PWM inverters widely use power MOSFETs and insulated-gate bipolar transistors.
IGBTs and GTOs are used as switching transistors, while the control section uses dedicated PWM switching integrated circuits and DSP and microcontroller chips with PWM output. A practical inverter requires a main power circuit, a control circuit, and auxiliary circuits (such as protection, measurement, and monitoring). The inversion process is as follows: DC power from the photovoltaic array or battery enters the inverter's DC bus, and is converted into sinusoidal AC pulse waves with forward and reverse outputs by a switching circuit (such as a bridge circuit). This pulse-width modulated AC voltage is then filtered to become a sinusoidal AC voltage output. If a step-up transformer is needed, the AC power is then transmitted to the load via a transmission line. The frequency of the PWM modulation output signal is called the inverter's modulation frequency or switching frequency, which is generally tens to hundreds of times higher than the inverter's fundamental AC output frequency. A typical inverter has an AC output frequency of 50Hz, and the inverter's switching frequency can range from hundreds to tens of kilohertz. The higher the switching frequency of PWM modulation, the lower the harmonics of the inverter output waveform, but the greater the power loss caused by the switching process. A balance must be struck when selecting the switching frequency of the PWM modulation of the switching transistor.
The filter connected to the inverter output is typically a low-pass filter, consisting of an inductor and a capacitor forming a T-type low-pass filter. The filter design must consider both its filtering capability and potential electromagnetic resonance. Inverters are further classified into voltage-source inverters and current-source inverters based on their output type.
4. Frequency converter
A frequency converter consists of a three-phase rectifier, a passive inverter with a voltage source, and a controller. Due to the DC nature of power generated by photovoltaic (PV) systems, PV frequency converters do not require a three-phase rectifier; instead, the DC bus of the frequency converter is directly connected to the DC bus of the PV system. Given that PV power is significantly affected by the natural environment and sunlight, a battery is typically added to the DC bus to stabilize the frequency converter's operation. A low-voltage control signal is applied to the frequency converter's control terminals to continuously adjust the set frequency and change the output power, achieving maximum power point tracking (MPPT) with the PV array. As an adjustable load, the frequency converter must be used in conjunction with the PV array's MPPT for control. In PV systems, frequency converters should be used in conjunction with motor-type power loads to reduce the impact of motor starting current and allow for flexible adjustment of motor loads.
