All grid-tied photovoltaic systems include a main power transformer to provide galvanic isolation, step up the voltage and supply power 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 to around 12kV in order to interconnect at common North American distribution voltages. Let’s take a moment to consider the 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 in increments such as 128, 230, 345, 500 and 765 kV. 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 of 120 V typically found in U.S. residential homes. This voltage step down is done through the use of a transformer and the ratio of how much the voltage is decreased depends on the number of turns in the transformer. Let’s look at the theory behind the ideal transformer.
Figure 1 – Transformer Diagram
The change in the magnetic flux is the same on both sides. This yields the following equations.
Looking at figure 1, we can tell that: Vs=(3/10)Vp. 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 keep the correct power ratings on both the primary side and the secondary side of the transformer. Transformers are typically rated in kVA (kilo* volt * amp). The low side of the transformer should be rated to the output power rating of the inverter. The high side of the transformer should be rated to the grid at the interconnection voltage specified by the utility company. The transformer should be able to handle the power requirements on both the low voltage and high voltage sides. All this information Blue Oak Energy takes into account when designing a PV system.
On utility-scale solar energy facilities, voltages are stepped up from the DC-AC inverter 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 air gap provides a safety and separation between the grid and the power source which helps protect the grid from power surges, effects from lightning strikes and faults. 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. There are many types of cooling measures, coolants types, bushings, fuses, cabinet features and other options to choose from.
For more information like this, please visit our website and view Tech Talk #8 at http://www.blueoakenergy.com/tech-talk