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 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

Figure 1 – The primary voltage is either stepped up or stepped down depending on the number of turns in the windings of both sides.
Figure 1 – The primary voltage is either stepped up or stepped down depending on the number of turns in the windings of both sides.

Variable Space

The change in the magnetic flux is the same on both sides. This yields the following equations.

Faraday

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

Designing Commercial Rooftop Solar Projects for Fire Code Compliance

This is a recent article in the August/September 2014 issue of SolarPro Magazine which features Blue Oak Energy’s take on Commercial Rooftop PV Arrays, Designing for Fire Codes.

Many thanks to the teamwork from our talented engineers Jayme Garcia, PE and Matt Stornetta for  writing this comprehensive article in their limited free time!

Check it out:

Commercial Rooftop PV Arrays, Designing for Fire Codes_Page_01

Designing Commercial Rooftop Solar Projects for Fire Code Compliance

Solar in the United States

We were updating the map on our webpage recently to show the locations where we are currently working and where we actively hold licenses in the the United States.  The result is this remarkable map below:

Solar Across the USA
Solar Across the US

The Past

It was only 5 years ago (in 2009) when we were actively licensed in about a dozen states and the lion’s share of our work was focused in California. Electricity is so expensive in California and our solar resource is so plentiful that solar energy makes great business sense when the effects of tax credits are entered into the equation. Plus, most of the solar projects we worked on in 2009 were rooftop 100kW to 500kW load-side connected projects.

The Present

As Jimmy Cliff once said, “The present is a gift, that’s why it’s called present.”

Today, we have found the 1MW to 20MW solar farm to be making up a large percentage of our engineering and design work. Solar carports have become increasingly common at schools and corporate campuses as rooftop space is often limited or obstructed. Because we’re paying well under $1.00/Watt for solar modules that will last thirty years or more, customers can afford the additional materials associated with fabricated steel carport structures and keep their investment returns under their hurdle rates.

It is also evident from the map that we have grown as an EPC (Engineering, Procurement and Construction) contractor to provide a single seamless point of responsibility for our customers.  The benefits to our customers has been the ability to deliver full multidisciplinary engineering and construction activities simultaneously. Thus, we are able to accelerate solar Commercial Operation Dates (CODs) and interconnection schedules far beyond our that of our competitors. Given the pace of this industry and the challenges from many directions, we needed to help our customers make projects happen at a pace greater than every thought possible. Since 2009, we have added more Project Engineers, plus Project Managers, Construction Managers, and we built a Civil Engineering team thanks to our dedicated staff. The rate of change in the renewable energy industry is nothing short of dizzying. But that’s what is required when you’re trying to move the needle from the solar energy industry’s current 0.57% makeup of nation’s energy production.

The Future

The US federal tax credit incentive is set to reduce from 30% to 10% for businesses on Jan 1, 2017. The reduction in this incentive is sure to change our map of active projects and licenses. However, we fully expect the solar energy market to continue growing and making up a larger dent in the global energy mix despite the many hurdles this industry faces. We need an energy mix, no doubt, but we also need a clear priority. That priority is clean energy.

 

Background on Steel Pile Corrosion for Ground Mounted Utility Scale Solar Projects

A major concern on ground mounting PV (photovoltaic) systems is the corrosion related to the solar module mounting system’s soil embedded steel piles.  Steel piles are typically driven into the soil and experience degradation over time due to either atmospheric or soil corrosion, or both.

Atmospheric corrosion occurs when the air invades the impurities of the steel pile.   The rate of corrosion will differ across geographies because of variations in water vapor and contaminants in the air.  For example, urban environments will see higher levels of atmospheric corrosion due to car emissions that contaminate the air quality.  The contaminants will increase the effectiveness of the conductive medium or electrolyte.  Even worse is the industrial environment where heavy concentrations of sulfur dioxide, phosphates, and nitrates aggravate the atmospheric corrosion even faster.   Marine environments also present a problem for steel piles.  Salt water spray in aquatic areas such as Hawaii can cause aggressive corrosion on steel piles, even galvanized or coated steel.  Salt water contains numerous ions due to the dissolved salts in the water that can increase corrosion.  The flow of these ions creates a stronger current, and therefore accelerates the corrosion process.  Humid environments like those found in Florida similarly affect the corrosion of steel piles.  The humidity coats the steel with a thin wet film of water and works in combination with the impurities in the steel to corrode the pile.

Soil corrosion occurs when two different metals with different properties form a flow of electrons due to a conductive liquid connecting them.  The electrons flow from the metal with the higher potential (anode) to the lower potential (cathode) through the conductive medium. The conductive medium can be comprised of air, water or a combination of air and water. The flow of electrons slowly strips material away from the anode over time.

 

 MetalsSusceptibility

Figure 1: Courtesy of Scott Canada. ‘Corrosion Impacts on Steel Piles’. Solar Pro Magazine. Issue 5.1, Dec/Jan ‘12

 

For an ideal ground mount solar farm, the steel piles will be driven into perfectly compacted soil, which will see little to no corrosion if the piles remain undisturbed and dry.  Unfortunately this is not the case for aerated or disturbed soils, which are more realistic in the real world.  When aerated soil mixes with water, the conductive liquid, the reaction between anodes and cathodes will cause metals lower on the galvanic scale to corrode.  Connecting metals found on opposite ends of the galvanic scale (figure 1) will cause the flow of electrons to accelerate faster than if similar metals on this scale were to be connected.  And that’s exactly why bare copper equipment grounding wires cannot touch bare steel piles in any location.  This is a common problem we see on solar farms and these dissimilar metals are very reactive over time.

A last concern for soil corrosion is the moisture content and pH level of the soil.  In general, soils with lower pH values will have a higher corrosion rate and the phenomenon of solubility plays a big role into why this is true.  Solubility is the property of a chemical substance called a solute to dissolve into a solvent.   In a 1998 study conducted by the Natural Resources Conservation Services, values of pH below 5.5 were highly soluble to aluminum, iron, and steel.  Aggregates in the steel become highly reactive with the chemicals found in soils with low pH values and ultimately result in corrosion.  Soils with pH values greater than 7.0 will be less corrosive due to the high presence of calcium and magnesium, which will prove to be less soluble to the alloys found in steel.

Finally, the amount of moisture in the soil has a direct relationship with the resistivity of the soil.  Soil resistivity is a measurement of how much the soil resists the flow of electricity and is given in the units (Ohm-cm) and has a direct relationship with soil water content. The higher the water content, the lower the resistivity.  As mentioned earlier, corrosion occurs as a result of a current being conducted between an anode and a cathode.  Increasing the resistance to this current with a soil high in resistivity will result in slowing the corrosion rate of the material, in this case steel.  Therefore, since water conducts electricity, adding it to the soil will increase the corrosion of the steel pile due to the drop in soil resistance from the water moisture.  An increase in water content in the soil results in an increase in the corrosivity of the soil. As a result, steel piles in areas with high soil resistivity, such as the desert, can require less concern for corrosion.

Effects of corrosion on steel piles is most commonly mitigated by:

  • Providing galvanization of the surface of the material
  • Increasing the steel cross sectional area or thickness
  • Providing a coating of epoxy or paint
  • Installing a sacrificial anode (typically a magnesium strip)
  • Cathodic protection system which injects a low voltage current on the pile surface

For more information and elaboration, please visit our Tech Talk 14 on Mitigation the Corrosion of Steel Piles at:

http://www.blueoakenergy.com/tech-talk

The Scoop on California Community Solar (SB-43)

California has passed the “Green Tariff Shared Renewables Program”, better known as SB-43.   This bill allows customers without a suitable location to source solar from a community plant via the utility.  According to PG&E, a customer can expect to pay 3-4 cents/kWh for this energy.   While this bill requires each of the large CA utilities to set up rate schedules, PG&E’s plan is currently most available.

Here’s what you can expect:

Dates:

  • March 1 2014: Utilities file their green tariff shared renewables program with CPUC.
  • July 1, 2014: CPUC issues decision.
  • Q2 2015: PG&E expects program to start.

Specifics:

  • For PG&E there are two options:
  1. Customers can pay for the output from a pool of small to mid-sized solar projects within PG&E’s service territory.
  2. Customers can pay for the output from a solar project near them of their choosing.
  • Utility procures newly developed sites – not customer.
  • Pay fixed price for solar energy less PG&E’s avoided generation cost. Now expected to be $0.03-$0.04/kWh but lessen as traditional generations costs increase.
  • Plants and users in same utility territory.

Caps and Limits:

  • 20MW per site cap.
  • 600MW total in state, 100MW to residential, 100MW to 1MW sites in CalEPA disadvantaged locations, 20MW City of Davis.
  • 272MW in PG&E; 136MW from residential in PG&E.
  • 2MW per customer max except governments or schools.
  • No “single entity” shall subscribe more than 20% of any single calendar year’s total cumulative rated generating capacity.

Sources and Additional Info: