Transformers are used in solar power plants to step up the voltage from the photovoltaic system to the distribution voltage of the electric grid. Transformers work by transforming voltages through electromagnetic induction between a primary and secondary winding. The ratio of turns between the windings determines whether the voltage is stepped up or down. For solar power plants, a common transformer size is 0.75-2.5 MVA and helps step up the 15kV output of the solar system to the 34.5kV distribution voltage.

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Transformer Basics for Solar Power Plants
All grid-tied photovoltaic systems include a main power transformer to
provide galvanic isolation, step up the voltage and transfer energy back to
the utility grid. A common transformer size for most medium voltage solar
facilities is the 0.75 to 2.5MVA, 15kV class step-up product range. Medium
sized PV systems are required to step up their output voltage up to
34.5kV in order to interconnect at common North American distribution
voltages.
Fundamentals of how a transformer works.
Transformers work by transforming voltages from the input to the output
due to the physics around electromagnetic fields. The electrical current
running through the primary (input) windings produces a magnetic field
through a metallic core of the transformer. This magnetic field has a certain
magnetic flux associated with it. The magnetic flux flows through the
surface area of the transformer core until it reaches the secondary (output)
winding. The magnetic flux induces an electromagnetic force in the
secondary windings, which produces a voltage. The number of turns on
the secondary winding is directly proportional to the secondary voltage. The
number of turns in the secondary winding relative to the primary winding
determines whether the voltage is stepped up or down.
Transformers usually step voltages up for transmission purposes. On utility
grid distribution and transmission systems, electrical energy will be
produced at lower voltages and then stepped up to higher voltages. Higher
voltages minimize the losses from the inductance or resistance in the wire
during the transmission process. Transformers are used mostly on AC
electrical systems to move electrical energy from the power plants to the
substations to individual businesses, houses or loads. This transmission
of energy would require a step down from the higher voltages in the power
plant to the lower voltage. This voltage step down is done through the use of

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a transformer and the ratio of how much the voltage is decreased depends
on the number of turns in the transformer.
Figure 1 - Transformer Diagram
Figure – The primary voltage is either stepped up or stepped down
depending on the number of turns in the windings of both sides.
The change in the magnetic flux is the same on both sides. This yields the
following equations.

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This Ideal transformer steps the primary voltage down by a factor of 0.3! The
number of windings affects how much the transformer will either step up or
step down the voltage. In the example, the primary voltage was reduced by
3/10. This happened because the number of turns on the secondary source
was less than the number of turns on the primary source. This decreases
the primary voltage by the ratio between the turns. This effect could also
be used to increase the voltage. By increasing the number of turns on the
secondary source, the primary voltage could also be stepped up. People
with solar systems (120V/240V) that produce more energy than their
household consumes have to step up to the medium voltage range (12kV) in
order to send energy back to the utility grid.
When choosing a transformer in a PV system, it is important to select the
correct power rating and voltage rating on both the primary side and the
secondary side of the transformer. Transformers are typically rated in kVA
(kilo* volt * amp). The kVA rating of the transformer shall match (or be
slightly larger than) the kVA rating of the generation source (in our case this
is the PV system inverter).
The low voltage side of the transformer must match the output voltage of the
inverter. The high voltage side of the transformer must match the grid
interconnection voltage supplied on the utility company transmission /
distribution system.

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The transformer should be able to handle the power requirements on both
the low voltage and high voltage sides. All this information is taken into
account when designing a PV system. There are other nuances also, such as
the transformer efficiency, the coolants types, the bushings, fuses, cabinet
features and other options to choose from.
On utility-scale solar energy facilities, voltages are transformed after
the inverter for delivery to the utility grid voltage. A transformer on a solar
power facility is primarily used to step-up the voltage to deliver the
renewable energy to the utility grid. However, the transformer has some
added benefits in that it provides galvanic isolation between the solar facility
and the utility grid.
A transformer is essentially and air gap between two conductor
windings. This galvanic isolation is used for safety and equipment
protection by preventing ground fault loops.
These are the basic operative points behind transformers, how they work
and why they are important as a primary component on solar electric power
plants. Transformers come in all shapes and sizes with many different
features. .