Introduction to Transformerless InvertersThe recently announced AE 3TL three-phase string inverters are transformerless

inverters listed for use with ungrounded arrays. These inverters are more efficient

and lighter than the previous generation of PV Powered string inverters. This

paper discusses what a transformerless inverter is and what impact this has on

system design and installation.

ContentsIntroduction 1

Ungrounded Arrays 2

System Design & Installation 2Disconnecting Means 2

Color Codes 2

Equipment Grounding 3

Wiring Methods 3

Labeling 3

Module Compatibility 3

Safety 3

Conclusion 3

Transformerless InvertersJohn Foster, PEApplications EngineerNABCEP Certif ied PV Installer

Ungrounded ArraysPractically all electrical systems in the United States are grounded electrical systems. This means that one of the current carrying conductors is intentionally bonded to ground. When photovoltaic systems were first introduced to the US, it naturally followed that the photovoltaic arrays would be grounded. Unfortunately, this creates a complication for inverter designers. If there is a connection to ground on both the DC input of the inverter (because the array is grounded) and the AC output of the inverter (because our AC distribution system is grounded) galvanic isolation must be provided to prevent a mis-operation as the inverter converts DC to AC. This galvanic isolation is provided by an internal transformer. There are several drawbacks to having this transformer. Those drawbacks include electrical losses which decrease the efficiency of the inverter and the additional weight of the transformer.

Ungrounded arrays are common in Europe, but relatively new to the United States. They have actually been allowed by the National Electric Code since 2005, provided a number of requirements are met. But acceptance has been slow due to a lack of listed equipment and a lack of familiarity among system designers, installers, and code enforcement officials. Removing the requirement for grounding of the array opened the door for transformerless inverters. There are many benefits to moving to a transformerless design. Benefits of the transformerless AE 3TL three-phase string inverter include higher conversion efficiency for greater energy yield; lighter weight for lower shipping costs and faster installation; and finally safer operation due to the ability to detect ground fault currents at lower levels.

System Design and Installation Considerations With the change to transformerless inverters and ungrounded arrays, there are some new system design and installation requirements that must be taken into account. Many of these requirements are discussed in section 690.35 of the National Electrical Code.

Disconnecting Means With a grounded array, one of the conductors, typically the array negative, is bonded to ground. Since it is a grounded conductor, fuses and disconnect switches cannot be installed in it. With a transformerless inverter and an ungrounded array, none of the conductors are bonded to ground. All of the array conductors are “hot” conductors. Since both the positive and negative conductors are hot, both must be fused and switched. Note that the AE 3TL string inverters include a disconnect and fused combiner which fuses and disconnects both positive and negative conductors.

Color Codes Similarly, since there is no grounded conductor in an ungrounded array none of the conductors should be colored white or grey. Although no color code is specified for the ‘hot’ conductors, the use of red and black for the positive and negative conductors is permissible.

White Paper

2

Ungrounded Array/Transformerless InverterCombiner

Box

Array

InterconnectionCircuit Breaker

Inverter

DC Disconnect

Grounded Array/Transformer Based Inverter

CombinerBox

Array

GFDIFuse

InterconnectionCircuit Breaker

IsolationTransformer

Inverter

DC Disconnect

White Paper

3

Equipment GroundingEven with an ungrounded array an equipment grounding conductor, sometimes called a “safety ground” or “mechanical ground” is still required. “Ungrounded” means one of the current carrying conductors is not tied to ground. It is still necessary to ground all non-current-carrying metal parts, such as module frames and enclosures. The equipment grounding conductor should still be colored green or bare. A grounding electrode conductor may also be required.

Wiring MethodsTraditionally, type USE-2 wire has been used for array source circuit wiring. This is not allowed for ungrounded arrays. Ungrounded arrays must be wired with PV Wire, conductors installed in raceway, or nonmetallic jacketed multiconductor cables. This was one of the issues which slowed the initial acceptance of ungrounded arrays, since PV Wire was not available and modules were not design to be used with raceway. Currently, almost all module manufacturers use PV Wire for their module leads. Also, PV Wire is widely available from electrical and solar specialty distributors.

LabelingIn addition to the other labeling required by the National Electrical Code, the following warning label must be applied at each junction box, combiner box, disconnect, or other device where energized, ungrounded circuits may be exposed during service:

WARNING: ELECTRIC SHOCK HAZARD. THE DC CONDUCTORS

OF THIS PHOTOVOLTAIC SYSTEM ARE UNGROUNDED AND

MAY BE ENERGIZED.

Module CompatibilityIt is important to consider the module type when using a transformerless inverter. Some photovoltaic modules cannot be ungrounded. Some crystalline modules are required to be positively grounded. Some thin film modules are required to be negatively grounded. Modules that require a particular polarity with respect to ground cannot be used with a transformerless inverter which uses an ungrounded array.

SafetyAlthough they are unfamiliar, ungrounded arrays enjoy a safety advantage over grounded arrays. Transformerless inverters can detect a lower level of ground fault current. This potentially allows for earlier detection of compromised conductors. Also, with grounded arrays, it is very difficult for the inverter to detect a ground fault in the grounded conductor. There have been incidents where a fault in the grounded conductor goes undetected until a second fault develops in a hot conductor. At this point, the inverter is unable to interrupt the fault current and a catastrophic event may occur. With an ungrounded array, this blind spot does not exist – the first fault will be detected. The transformerless inverter will then disconnect all the conductors and give an indication that the fault has occurred. There is one additional advantage to the ground fault protection in a transformerless inverter. Transformer based inverters perform ground fault detection with a fuse that blows if a ground fault occurs. Once the ground fault is found and fixed, the fuse needs to be replaced. A transformerless inverter does not use a fuse to detect a ground fault, so there is nothing to replace after the ground fault is corrected.

ConclusionThe new AE 3TL Three-Phase String inverters offer higher efficiency, lower weight, and lower installed cost than their predecessors. These advances are due to a shift to a transformerless topology listed for use with ungrounded arrays. Small but important changes in installation and design practices will allow AE Solar Energy customers to benefit from these new inverters.

Advanced Energy® is a trademarks of Advanced Energy Industries, Inc.

For more information on AE inverters, visit www.advanced-energy.com/solarenergy.

Specifications are subject to change without notice.

AE Solar Energy • 115 Nicholson Lane • San Jose, CA 95134 USAwww.advanced-energy.com/solarenergy877.312.3832 • sales.support@aei.com • invertersupport@aei.com Please see www.advanced-energy.com for worldwide contact information.

© Advanced Energy Industries, Inc. 2013

All rights reserved. Printed in U.S.A.

ENG-TransformerlessInverters-270-02 0M 5/13