PV inverters are only suitable for grid-connected applications, while pcs can be used for both on-grid and off-grid applications. PV inverters and pcs share the same topology. Three-phase inverters/converters use a three-level I-type/T-type topology, ANPC or NPC circuits. Single-phase inverters/converters use an H5/H6 topology. The hardware of PV inverters and pcs is nearly identical, with only slight differences in the DC-side wiring interfaces.
The DC side of a photovoltaic inverter is connected to photovoltaic modules. The figure below shows the I-V curve of a photovoltaic module. Under certain conditions, such as an irradiance of 1000W/m², the module's current remains stable at over 18A within the voltage range of 0-35V. As the voltage increases, the current decreases. The I-V curve shows that the photovoltaic cell module maintains a stable current when generating electricity, thus exhibiting the characteristics of a current source. However, its voltage varies continuously, influenced by factors such as irradiance intensity, temperature, air quality, and surface cleanliness.
The power generated by a photovoltaic module (P) = voltage (U) x current (I). Along the I-V curve, the area of the rectangle formed by the V value on the horizontal axis and the I value on the vertical axis represents the module's power generation value. Within these rectangles, the maximum area is the power value at which the MPPT (Maximum Power Point Tracking) is located. See the P-V curve for the module below.
The MPPT of a module is located at the top of a hill, similar to the peak of a small hill. The P-V curve shows that the power generated by a photovoltaic module is constantly changing.
Because photovoltaic batteries cannot generate stable voltage and power during power generation, photovoltaic inverters cannot establish AC voltage and frequency during power generation. They can only be used for grid-connected applications, running a phase-locked loop (PLL) control strategy to inject power following the grid's voltage and current sinusoidal waveforms. Therefore, photovoltaic power sources are often referred to as current sources, also known as P/Q sources.
The DC side of the pcs is connected to an electrochemical/rechargeable battery. A typical example is LFP battery. The following figure shows a charge and discharge SOC-V curve and table for LFP battery. The voltage of a lithium battery changes only with the SOC. During transient conditions, its voltage is stable and does not experience sudden increases or decreases. Therefore, a lithium battery has the characteristics of a voltage source.
pcs charge and discharge lithium batteries through rectification or inversion. Similarly, charge and discharge power (P) = voltage (U) x current (I). Given a fixed voltage, power output can be controlled simply by controlling the magnitude and direction of the current. When performing charging/discharging/inversion rectification for grid-connected (following) operation, the pcs employs a phase-locked loop (PLL) control strategy, injecting or absorbing energy in accordance with the grid's voltage and current sinusoidal waveforms. During off-grid (connecting to the grid) operation, since the voltage and power of the DC power supply are controllable, the pcs can establish the AC voltage and frequency. A DSP chip controls the generation of the grid voltage/current sinusoidal curves and 50/60Hz frequencies. Therefore,energy storage power supplies used in off-grid applications are often referred to as voltage sources, also known as V/F sources.