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Solar Transformers: Sizing, Inverters, and E-Shields

Learn all about transformer sizing and design requirements for solar applications—inverters, harmonics, DC bias, overload, bi-directionality, and more.

Written by:
Ben Gulick & Nathan Stenzel

May 29, 2024

A Maddox padmount transformer infront of a solar array

Unique demands of solar applications

Let’s start by reviewing the unique demands that solar applications face. Solar generation relies on a discontinuous power source — the sun. Day and night cycles paired with environmental factors like precipitation and cloud cover influence its reliability. Power generation from this type of renewable source is cyclical rather than continuous.

This means your transformer will not run at 100% load for 24 hours. Depending on the time of year, it may only be at full load for 6 of those hours. This brings up a couple questions…

  • Can I reduce the size of my transformer since it’s only loaded part-time?
  • Can I overload the transformer during the day since it’s underloaded at night?

The short answer to both these questions is, no

In fact, these scenarios may increase the demand put on the transformer. To open up this concept, let’s look at the generation side of things starting with inverters.


Inverters are the part of the solar array that connects to the step-up transformer. Inverters convert DC generated solar power into AC. They handle the wide swings in power supplied from the solar array. They also steady the voltage supplied to the step-up transformer. The inverters do all this with special switching that regulates their power output. This switching often creates power quality problems in the system. These power quality issues result in additional heating at the transformer.

There are three considerations you need to take when working with inverters:

  1. Harmonics
  2. DC bias
  3. Output power 
inverters attached to solar panels


The main concern with harmonics at the transformer is overheating. Most inverters have filters to reduce harmonic distortion. With filters, inverters can keep their harmonic output below 5%. This does not account for any interaction with the transformer or other parts of the system. The total harmonic distortion across the system could be higher. You cannot always assume the inverter’s harmonic rating across the whole system.

DC Bias

Some inverters use an additional DC supply to regulate their AC output voltage. This DC component is superimposed on the AC output signal. The DC voltage cannot pass through the transformer to the grid. But, it does end up in the transformer low voltage winding. This can easily create overheating in the transformer core and insulation stress. Learn about transformer insulation and temperature rise.

This often shows up as high hydrogen gassing on a transformer DGA test.

Output Power

Some inverters output above their nameplate power rating. This means a transformer may be overloaded during the inverter’s peak output period. In such cases, size the transformer kVA to handle the maximum output of the inverter (not its nameplate rating). 

Other sources of increased inverter output stem from environmental factors. Solar panel output correlates with ambient temperature. Some seasons will produce more output than others when temperatures change.

A sunny day with patches of clouds can produce output power spikes. When certain types of clouds are high in the atmosphere, they amplify the brightness of the sun’s light. This phenomenon is called cloud lensing. On the positive side, this increases the output power of solar panels. But, the mixture of cloud shadows and cloud lensing at the panels creates voltage spikes. In longer durations, this causes transformer overloading.

Inverters and a green padmount transformer powering a solar array

Sizing for Overload

Correct transformer sizing allows for possible overload situations. The kVA should match with the inverter’s output characteristics. Wherever possible, consult both transformer and inverter manufacturers for their input. An in-depth power quality analysis of the solar system can reveal what kVA is best. When an in-depth PQ analysis is not in the cards, we recommend sizing for the worst case scenario. A dual rated temperature rise (55/65) works well in such cases. It provides an extra 12% of kVA above the base rating.

Unique features of solar transformers

Electrostatic Shielding

All Maddox solar transformer designs include an electrostatic shield (E-Shield) by default unless the customer specifies otherwise. This shield serves two purposes.

  1. Protection for the high voltage winding & utility grid
  2. Protection for the inverter
A transformer electrostatic shield (E-Shield) drawing

Protecting the Transformer & Grid

Harmonic disruptions from inverters can pass to the utility grid. These power disruptions cause voltage spikes and impulse-like effects in the high voltage winding. Such power disruptions can wreak havoc at the transformer and downwind on the grid. An electrostatic shield between the high voltage and low voltage transformer windings eliminates this problem. The shield metal is thin to reduce added eddy currents. It is connected to ground at one single point (internally) in the transformer. 

Protecting the Inverter

Transient overvoltage spikes on the utility side can also pass to the inverter. These overvoltage events can damage an inverter’s sensitive components. 

In this way, the E-shield offers bi-directional protection. Due to its important function, the electrostatic shield should always be engaged

Step-Up & Bi-directional Design

Renewable generation sources (like solar) interact with transformers in a unique way. At startup, power is fed from the utility to the solar inverter. Once the inverter receives a balanced voltage input, the solar side feeds back into the grid. The transformer plays the role of a step up and step down unit. This is why the term bi-directional often appears on solar equipment. 

A graphic showing how solar panels feed into inverters and batteries connected to a transformer and then go to transmission of an end user.

All transformers are by nature bi-directional as far as power flow goes. Current may be fed from either winding. By itself, this term does nothing more than define a normal transformer. It means more in the context of certain applications like solar. As mentioned already, energization happens on the utility side winding. The low side winding is excited after mutual induction is present between the coils. Unless the transformer is de-energized and re-energized repeatedly, inrush current is not a big issue. So, the word bi-directional has more to do with how the transformer gets the grid and inverter to, metaphorically, shake hands. This is done with the configuration of the primary and secondary windings.

Winding Configuration

The utility’s distribution feed determines the HV winding configuration. Utility systems are usually grounded. This means they require a wye connection with a grounded neutral point. Likewise, the inverter’s requirements determine the configuration on the LV winding. Most inverters prefer a connection to a wye service with a solidly grounded neutral point. If a neutral is connected to the inverter, it is usually for voltage sensing only. This is the reason most solar transformers are configured as wye wye. The most important thing is to match the configuration required by the inverter and grid. A wye wye connection is not always required, but it is the most common.


When connecting to the grid, the inverter needs to sense any voltage imbalance from the utility. If one phase of the utility feed is lost from a fault, the inverter needs to recognize this. If the inverter does not see the lost phase, it could backfeed power to the utility causing catastrophic damage or harm. This is known as islanding. When a fault occurs on the grid, the inverter must trip and shut down. The vector grouping of the transformer plays a key role in this. A lost phase with a wye connection will yield a voltage imbalance on the other two phases. This is why most inverters favor a wye connected service. For delta connected services, additional ground fault detection outside the inverter may be needed. Some inverters will support a wye or delta connection though. You must consult the inverter manufacturer to determine what winding configurations will allow the inverter to trip during a grid side fault.

It is important to realize that not all inverters and grid connections are alike. A truly bi-directional transformer design will always harmonize the needs of the grid and inverter.

A solar field with a Maddox padmount transformer, inverters, a 600v PV switchboard and a 15Kv interconnection meter gear

Solar Voltages

Renewable transformers also have different voltages than the standard industrial voltages you might have seen.

Solar array voltages: 800V, 630V, 600V, 480V, 208V

800, 630, and 600 are all common voltages used with solar arrays. 800V is more common with European inverter manufacturers; 630V is usually found in larger solar arrays; and 600V is the most common voltage for solar inverters. 

Monitoring and Gauge Alarm Contacts

Due to the remote nature of many renewable projects, solar transformers are often outfitted with alarm contacts on the gauges. These alarm contacts communicate remotely with the end user allowing them to monitor the transformers from miles away.

Because of the unique loading profile of solar transformers, temperature and pressure monitoring is essential. Early detection of overloading and overheating is the best way to prevent equipment failure and unwanted downtime. 

Learn more about remote monitoring on transformer gauges.

A graphic showing how gauges with contacts allow for transformer remote monitoring


Integrating renewable energy sources like solar introduces unique challenges for transformers. The cyclical nature of the source can lead to overheating, power quality issues, and overloading. This means it’s critical to size your transformer appropriately for your solar system.

If you are planning your next solar farm, and have questions or are looking for a transformer quote, fill out the form below. We have worked with renewable projects across the country, and have the transformers and the expertise to get your project up and running.

Maddox padmount transformer loaded on truck

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