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How Do PV Inverters Differ From Power Conversion Systems?

The photovoltaic inverter (PV inverter) can only be used for grid-connected applications, and the Power Conversion System (PCS) can be used for on-grid/off-grid applications.

They are exactly the same in topology.

How Do Photovoltaic Inverters Differ From Power Conversion Systems?

Regarding the phase difference:

Three-phase inverter/converter adopts a three-level I-type/T-type topology, ANPC or NPC circuit. The single-phase inverter/converter adopts a topology of type H5/H6.

They are almost identical in hardware, except for some differences in the DC side wiring interface.

Why can photovoltaic inverters only be used in grid-connected applications?

The DC side of the photovoltaic inverter is connected to photovoltaic modules. Photovoltaic module panels are current sources.

Let’s use the product specification sheet of Trina Solar’s N-type i-TOPCon double-sided double-glass module model TSM-NEG21C.20 to understand photovoltaic power generation characteristics.

The picture below shows the I-V curve of the photovoltaic module.

I-V Curves of SPP375QHES at different irradiance

Under a certain environment, such as when the irradiance is 1000W/m2, the module’s current value is stable above 18A in the voltage range of 0⁓35V. As the voltage increases, the current shrinks.

It can be concluded from the I-V curve of the solar panel that the current is stable during power generation, so the photovoltaic panel has current source characteristics.

The voltage value changes due to factors such as irradiation intensity, temperature, air quality, surface cleanliness, etc., and the voltage value is also constantly changing.

Solar panel power generation (P) = voltage (U) x current (I).

Along the I-V curve, the area of the rectangle formed by the V value on the abscissa and the I value on the ordinate is the component power generation value.

There is a maximum area value in these rectangles, and this power value is the legendary MPPT (Maximum Power Tracking Point).

Solar modules’ MPPT is located on small hillsides. It can be concluded from the P-V curve that photovoltaic modules also generate power constantly.

Since photovoltaic panels cannot output stable voltage and power when generating electricity, the photovoltaic inverter cannot establish the AC side voltage and frequency when inverting power generation output.

It can only be used in grid-connected applications.

Run the phase-locked loop control strategy to inject electric energy following the power grid voltage and current sinusoidal waveforms.

Photovoltaic power sources are often called current sources, also known as P/Q sources.

Why does a Power Conversion System have a wide range of applications?

The DC side of the Power Conversion System is connected to an electrochemical/recyclable rechargeable battery.

Taking the lithium iron phosphate battery as a representative, learn about electrochemical batteries’ charge and discharge characteristics.

Charging and discharging SOC-V curves and tables for lithium iron phosphate batteries

The voltage of a lithium battery varies with the remaining power saturation. Its voltage value is stable during transient conditions and does not fluctuate in size.

Therefore, the lithium battery has voltage source characteristics.

Power Conversion System charges and discharges lithium batteries through rectifying or inversion.

Similarly, charge and discharge power (P) = voltage (U) x current (I). The voltage value is certain. You only need to control the current size and direction to control the power output.

When PCS is used for grid connection (grid-following) in charging and discharging/inverter rectification, it runs a phase-locked loop control strategy and injects or absorbs electric energy following the voltage and current sinusoidal waveforms of the grid.

When dealing with off-grid (network) operation, since the voltage and power of the DC side power supply are controllable, the Power Conversion System can build the voltage and frequency of the AC side, and the DSP chip controls the wave generation to establish the grid voltage/current sine wave curve and 50/60Hz frequency.

Therefore, energy storage power supplies used in off-grid applications are usually called voltage sources, also known as V/F sources.

Therefore, PCS can be widely used in energy storage systems, solar energy storage systems, wind power generation systems, etc. Provide stable power for enterprises, factories, airports, hotels, and other places where electricity is scarce or electricity bills are expensive.

PVMARS provide a full set BESS system with solar
Zoey Zhao is a content developer at PVMARS Solar. She has a BA in English and a MA in Journalism. Prior to joining PVMARS, Zoey worked as a writer for various brands in the automotive and construction industries. She is passionate about sustainability and is currently exploring all things renewable!

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