• December 2019
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  • November 2019
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  • October 2019
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  • September 2019
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  • August 2019
    Introducing myPV™ IQ Monitoring & Control Solution

  • July 2019
    How Having an “In-rush” Plan Can Save Your Solar Farm from the Start

  • June 2019
    Shining a Bright Light on the Dark Side of Developing Solar Farms

October 2019



By Brad Micallef, President and Managing Director & President

Since 2017, an increasing level of power quality requirements for Qualified Facilities (QFs) has become common place in the Carolinas, as Duke Energy has broadened the scope and honed the technical criteria for interconnecting new PV Solar projects. This post discusses two (2) related requirements that new PV Solar projects may face in the late stages of development, and how to address them.

Developing a PV Solar project often takes years of negotiations and financial modeling to secure Interconnections Agreements (IA), Power Purchase Agreements (PPA), rights to land, local permits, financing, procurement agreements, engineering, and construction contracts. Due to the long-term nature and complexity of these development efforts, the prospect of long lead times for key components, and looming reductions in the Investment Tax Credit (ITC), developers and EPCs often order long lead time equipment as early as possible.

This practice of early ordering, however, may cause unexpected consequences as additional interconnection power quality requirements are defined late in the development stage or the project.

Following a technical review called the “Advanced Study”, Duke Energy evaluates the proposed PV Solar project’s “Transformer magnetizing inrush & harmonics risk” impact at the point of interconnection.

There are three (3) general results to this study that developers may receive:

  • 1. The Advanced Study’s simulation has resulted in modeled current flows and voltage changes that are within acceptable tolerances, no system design changes are required.

  • 2. The Advanced Study’s simulation has resulted in modeled current flows and voltage changes that are not acceptable, the proposed PV Solar project’s transformers must be energized in a sequential manner to reduce the electrical disruptions to an acceptable level.

    Most commonly classified as a problem with "Transformer In-Rush Current".

  • 3. The Advanced Study’s simulation has resulted in modeled current flows and voltage changes that are not acceptable, not even a single transformer of the proposed PV Solar project’s may be energized.

    Most commonly classified as a problem with "Rapid Voltage Change".

If these power quality issues sound odd or new to you in relation to PV Solar projects, you are not alone. Many seasoned Developers and Engineers have not been asked to address these concerns in previous years, nor have they had to account for these power quality issues in previous project designs.

The following diagram illustrates both electrical phenomena. Both phenomena are directly related as current and voltage and inversely proportional.

The graph above illustrates the relatively large current in-rush when a PV solar project’s transformer are initially energized. Immediately following energization, the voltage drops sharply at the same time. As the harmonic disturbance of the current in-rush decays, the voltage also recovers to normal levels.

These abnormal electrical conditions are a result of the PV Solar project’s transformers magnetizing, electrically appearing to the grid a large and sudden load. This condition only occurs when the PV Solar project's grid connection is restored following a grid outage or when a facility is first interconnected for back-feed and during commissioning activities.

To further simplify, these events do not take place often and are not part of the normal daily operation of the PV Solar project.

For PV Solar projects that receive an Advanced Study response that requires mitigation of either (response 2) transformer in-rush current, or (response 3) rapid voltage change; the impact can be catastrophic to project construction timelines and budgets.

It is not simply a matter of knowing that a potential problem exists that was unknown prior to receiving the Advanced Study, it is the tightly coupled relationship between financial models that establish project margins. Margins based on: the federal ITC, which requires a project to be placed in service within the current tax calendar year, the long lead times associated with medium voltage (MV) equipment, and the unplanned material and labor costs associated with introducing new equipment and design changes late in the construction process.

In short, a potential poison pill.

The good news is, there are proven solutions to mitigate both transformer in-rush current, and rapid voltage change.

Mitigating Transformer In-Rush Current

Let's look at a hypothetical PV Solar project with three(3) transformers, only one (1) of which can be energized following a grid outage.

To reduce the total system in-rush current during energization, transformers 2 and 3 are energized in sequence ten seconds (10s) apart. The diagram above shows how automated time delay switching may be added to delay when transformer 2 and 3 are energized. The delay of 10s is purely a representative time delay, the actual time required to mitigate the inrush current is much less, measured in a few cycles.

While there are multiple methods to address the sequential delayed energization of multiple transformers to mitigate transformer in-rush current, all of them require advanced planning for both electrical design and procurement. The myPV XFMR-Sync Controller provides a turnkey automation solution for transformers that are equipped with internal motorized MV disconnects, and is also available as a pad mount switch solution for late stage procurement and project retrofits.

Mitigating Rapid Voltage Change

Let’s look at the same hypothetical PV Solar project with three (3) transformers, but in this situation none of the transformers can be energized following a grid outage.

To energize the PV Solar project’s MV loop feed a “pre-insertion” device will need to be installed between the utility interconnection and the first transformer. The pre-insertion device provides resistance to limit the current flow during initial energization, but also provide unrestricted current flow when the PV Solar project is generating.

Pre-insertion devices include resistors, high-impedance back-to-back transformer arrangements, and also cap-banks. All three (3) devices require careful consideration of costs and procurement lead-times. Additionally, the automated switching sequence to introduce and then remove the pre-insertion device from the MV loop feed requires advanced controls automation design and planning to ensure safe operation and proper fail-to-safe operation on power loss.

The myPV RVC Solution provides a turnkey automation and switching solution for rapid voltage change mitigation that simplifies installation to only a few electrical connections and is available with standard lead-times of 10-12 weeks.

In Closing

As renewable energy continues to expand and more deeply penetrate existing hub-and-spoke centralized generation models, new and evolving power quality standards should be expected to ensure grid reliability and overall system resiliency.

Transformer In-Rush Current and Rapid Voltage Change are two (2) examples of these new and evolving standards that can now be addressed and mitigated cost effectively and with proven market solutions.

For more information on products specifically developed to address the changing landscape PV Solar and Storage grid integration and operation, please visit

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