Rapid Shutdown of Photovoltaic Module Series Strings

Rapid Shutdown of Photovoltaic Module Series Strings

With the increasing number of Photovoltaic (PV) power systems being installed on buildings and the concern of emergency first responders (firemen) that these high voltages on buildings represent a hazard to first responders, the National Electrical Code (NEC) has added requirements intended to address this hazard. Many PV power systems use a design with many series connected PV modules (termed ‘strings’) to simplify wiring and increase efficiency. Presently these systems can use strings with cold open circuit voltages of up to 600 volts DC on residences, up to 1000 volts on non-residential buildings, and up to 1500 volts in utility scale installations. The problem is that even if the wires from the PV array are disconnected, high voltages remain and emergency responders could be exposed to high voltages even if all electrical power is shut off for the building.

To address this hazard, the NEC added Section 690.12 Rapid Shutdown of PV Systems on Buildings in the 2014 edition. The requirements were refined in the 2017 edition. Methods and designs for achieving the proper rapid shutdown are not addressed by the NEC but instead are addressed in the product standards (such as UL Standards) for this type of equipment. The figure below from the September/October issue of Solar Pro magazine (see solarprofessional.com) illustrates the requirements.

RapidShutdown2014 vs 2017

There are many ways that the PV industry is addressing these requirements:

Module Level Electronics (MLE), the inclusion of electronic devices within the PV modules:

  • Micro Inverters, one per PV module (or sometimes for 2 modules), to produce AC power from the PV array area. Due to inverter requirements, these inverters must shut down any time the AC power is disconnected or fails (utility outage).
  • Power conditioners for each PV module that are controlled by the central inverter and are designed to shut down whenever the inverter is shutdown.
  • Other shutdown devices such as small remote switching devices, controlled by the inverter or a separate rapid shutdown switch..

Array rapid shutdown devices that are placed within the array boundary and disconnect the PV array whenever the inverter is shutdown.

Inverter location – simply placing the string inverter close to the PV array, within the array boundary. Inverters are being designed to allow such low profile mounting.

In Arizona there is no state wide building code, cities and counties determine the applicable codes and versions of codes.  For instance, Phoenix is now on the 2017 version of the NEC and the 1' boundary applies, while Scottsdale is on the 2014 version of the NEC and the 10' boundry applies. The installer will have to obtain building permits and follow the applicable code version.


Activating rapid shutdown:  PV systems installed following either the 2014 or 2017 versions of the NEC will have a reflective label (left image) near the electrical service showing the switch that will put the PV system in shutdown mode.  This label may be either on the AC disconnect (center image- for systems that shutdown upon loss of the utility connection) or on a separate switch (right image).  PV systems using micro-inverter and installed following earlier NEC versions may not be labeled.


Snow can damage PV arrays

The recent snow storms in Arizona (February 2019) have resulted in snow accumilations that can damage solar mounting structures.  A good illustration is from Germany in 2013.

Snow beforeIn 2012, motor home manufacturer ’Adria’ built this fancy new 1MW solar plant over its parking lot.

It collapsed under about 3-4ft of snow Feb 2013.

Snow fail

Are there any Arizona Examples?

APS announces 'Solar after Sunset" battery storage inititative

In utility-scale electricity generation over the course of a day there is a timing imbalance between peak demand and renewable energy production. Especially in Arizona, the peak demand often occurs after sunset, when solar power is no longer available. A graph of power production with various future estimated solar production levels tends to develope into a clasic duck shape and has been named the duck curve. (the image below is from NREL and not specific to APS)

Duck Curve1

Arizona electric utilities have been dealing with this mismatch of load and generation by ramping up/down their various generation sources. Another solution to the problem is storing some of the solar generated energy for later use to reduce the peak generation required of the conventional sources.

APS has announced their plan for testing a large scale battery system to help solve this problem. AES will develop a 100MW battery for APS. In addition to being able to shift energy, the installation will be used to provide "critical peaking capacity" and “increased” grid reliability. AES will provide a system with a four-hour duration based on Fluence’s Advancion platform. It will be leased under a 20-year contract with AES the owner and operator of the project. The cost per kWh shifted has not been disclosed.

APS states "APS will add battery storage to its existing fleet of solar power plants, build new solar plants with storage, and use storage to deliver cleaner energy to customers at times of peak energy usage. As a result, APS customers will be able to use solar energy even after the sun goes down. Family dinners, prime-time television and bedtime reading lights will all be powered by a cleaner energy mix."

For further detail, see the APS news release.

Good Intentions, but----

tep 2019 logoBack in January 2015 Tucson Electric Power Company (TEP) and UniSource Energy Services (UES) set up a Voluntary Solar Contribution Program (VSCP) that directed contributions to a fund managed by the Arizona Community Foundation (ACF). The program was to be used to install solar photovoltaic systems on newly built residential single-family homes constructed by the Arizona chapters of Habitat for Humanity International ("Habitat") on Habitat homes for TEP customers. This was part of the approved TEP 2014 Renewable Energy Standard Implementation Plan and related Distributed Energy Administrative Plan.

Good intentions, but it never worked out.  In spite of a lot of publicity at the time (search "VOLUNTARY SOLAR CONTRIBUTION PROGRAM"), and an initial donation of $25,000 by TEP, Additionally, as shown by the most recent VSCP Report filed on September 28, 2018, no contributions have been made by another company or any member of the general public. The fund balance initially contributed by TEP and UNSE, now $24,684.91, is from its shareholders, not ratepayers. The total annual administrative costs were $500 in 2017 to maintain an online donation page on ACF's website and will be $750 in 2018. The fees are deducted monthly from the fund balance.

According to a docket filed with the Arizona Corporation Commission remaining accumulated funds should be donated to Habitat.  No solar photovoltaic systems were installed.