Transformerless Inverters for Solar PV

Transformerless inverters are increasing popularity in USA after European and Australian markets. This article presents an overview of the concept and advantages of transformerless inverters in solar applications

What is a transformerless (TL) inverter?

Transformers

The differences between standard or conventional inverters and transformerless inverters are:

  1. Conventional inverters are built with an internal transformer that synchronizes the DC voltage with the AC output.
  2. Transformerless (TL) inverters use a computerized multi-step process and electronic components to convert DC to high frequency AC, back to DC, and ultimately to standard-frequency AC.

Transformerless inverters are increasing in popularity in European and Australian markets. In 2010, SMA Solar Technology AG earned the first UL certification for their transformerless inverters, thereby increasing product availability for residential consumers in the United States.

Transformerless (TL) Inverter Appeal

Transformerless inverters are light, compact, and relatively inexpensive. Since transformerless inverters use electronic switching rather than mechanical switching the amount of heat and humidity produced by standard inverters is greatly reduced.
TL inverters maintain the unique ability to utilize two power point trackers that allow installations to be treated as separate Solar PV Systems. In other words with TL inverters, Solar PV Panels can be installed in two different directions (i.e. north and west) on the same rooftop and generate DC output at separate peak hours with optimal effects. Traditional inverters work through only one power point, which means panels that are performing at lower frequencies will lower DC output for the entire system.

Transformerless (TL) Inverter Considerations

Transformerless inverters do not have electrical isolation between DC and AC circuits. This may raise some grounding and / or lightning protection concerns. In order for transformerless inverters to comply with NEC specifications specially designed and more expensive PV Wire must be used.

Transformerless inverters have been developed for use with Grid-Tie Solar PV Systems, so Off-Grid systems users will not necessarily achieve the same benefit yet.

Inverter Efficiency

Inverter efficiency

Inverter efficiency is determined by the percentage measurement of energy convergence (i.e. the closer to 100% of DC to AC convergence for the longest amount of time the more refined inverter efficiency). When calculating efficiency it is important to include peak and off-peak performance percentages in addition to how often your inverter is operating at rated capacity.

In the above chart (Fig. 1) red represents standard rate of inverter efficiency, the green arrow indicates Solar PV panel power output (DC), and the gray area represents the operational window for inverter efficiency (85-95%).

inverter efficiency grapIn (Fig. 2) where orange (1) represents transformerless, unipolar switching SiC, purple (2) represents transformerless, unipolar switching Si, and turquoise (3) represents transformerless, bipolar switching, Si there exists a minimum of 97% TL inverter efficiency. That’s 1-3% higher than conventional inverters.

Studies show that even a small percentage increase in inverter efficiency means the power supply increase can be quite significant if factored throughout the life span of the inverter.

Installation Considerations for TL Inverters:

  • The positive and negative PV source circuits must BOTH be switched and over-current protected with TL Inverters.
  • The PV array equipment must still be grounded, but not the PV source.
  • The modules and the source circuits must use wire rated PV WIRE or PV CABLE.
  • The negative conductor of the PV array is not grounded, and therefore shall no longer be colored white when terminating at the inverter or disconnect. Refer to NEC 690.35 for some relavent TL inverter information
  • PV source circuits shall be labeled with the following warning at each junction box, combiner box, disconnect, and device where the ungrounded circuits may be exposed during service:

WARNING
ELECTRIC SHOCK HAZARD
THE DC CONDUCTORS OF THIS
PHOTOVOLTAIC SYSTEM ARE
UNGROUNDED AND MAY BE ENERGIZED
.

Possible Benefits of using a Transformerless Inverter:

  • Usually much lighter in weight than inverters with transformers.
  • Have higher efficiency ratings
  • Capable of dual MPPT inputs, depending on manufacturer

Comments

Question.  Is SMA the only company making a transformerless inverter or are there other products like this on the market?  Thanks.


In the US, you can buy SMA, Kaco and PowerOne transformerless inverters at this time.

I was looking through the SMA HF inverter manuals and couldn't find any reference to your comment ...

In order for transformerless inverters to comply with NEC specifications specially designed and more expensive PV Wire must be used.

Can you be specific as to exactly what wire can be used? I am currently in the middle of my first HF install and we are using #10 THHN/THWN as we almost always do. Will this suffice? Any insight would be appreciated.

-Chris

-Chris DiBernardo
President, 2K Solar, Inc.
Chris@2KSolar.com
www.2KSolar.com
P:845-673-5463
C:845-781-8876

Chris,

The PV source is ungrounded - so PV wire must be used in place of USE-2 wire. Once you are in the combiner box (and protected by conduit) you can use the THWN/THHN rated wires. PV wire has thicker insulation to (theoretically) reduce the chance of an exposed conductor energizing a metal structure. So, wherever you would have had USE-2 - from the module J-box to the DC combiner - you need PV wire.