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Introduction to PV

Photovoltaic (PV) Plant Basics

PV plants are comprised of several devices and components that work together to harvest energy from sunlight and transform it to AC electricity to be used on the grid.  This tech note will illustrate all the major components and the role of each.

PV modules – PV modules convert sunlight into DC electricity.  As photons of light strike the solar cell, a DC current is generated by a process of absorbing incident photons to create electron-hole pairs and then separating the electron-hole pairs at a p-n junction.  The external circuit completes the path allowing current flow as it connects across the p-n junction   A module is comprised of many solar cells connected together electrically.   The voltage and current of each module varies by design type.  PV modules degrade over time at a rate disclosed on the datasheet.  The basic things to remember about PV modules is that they each produce a DC voltage and DC current when exposed to sunlight.

PV strings – PV modules are manufactured with two DC wire leads DC(+) and DC (-).  These module leads have connectors installed on each such that the modules can be connected in series into a “string”.


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String voltages today range from 1000 volts DC to 1500 volts DC.  The modules are connected together in series strings to increase the DC voltage to a value suitable for conversion to AC electricity.


Description automatically generatedPV Combiners – Combiners, or combiner boxes, serve two purposes.  They provide a collection point such that the PV strings can be paralleled to sum the current from each string.  Combiners also provide a terminal point where the overcurrent protective devices (OCPD) and surge protective devices (SPD) are installed.     Each string circuit is fused in the combiner box.  Combiner boxes can also provide a disconnect point for the upstream device, the inverter.




Inverter – The inverter is the workhorse of a PV plant.  Its role is to converter the DC electricity to AC electricity so it can be fed into a grid connection, typically a substation for large projects, or a breaker panel for small (home) projects.  Transformers are used to match the inverter output AC voltage to the grid interconnection if needed.


*Small string-level inverters

 Large central inverter


The inverter creates an AC waveform by switching the DC voltage on and off very fast combined with a pulse width modulator.

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The image above represents the DC voltage (Vdc)being switched on and off in different durations which results in the inverter AC output.  This is the blue trace superimposed above.  The inverter has components to smooth the AC waveform into a near-perfect sine wave which are capacitors and inductors.  The inverter synchronizes this AC waveform to the grid connection.   After it is operating and all the checks and balances are complete, the inverter begins feeding AC current into the grid, thus completing the goal of converting sunlight into useable energy.

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