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Photovoltaic (PV) Module Level Remote Safety Disconnect

National Renewable Energy Laboratory

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Figure 1: System configuration of emergency module-level disconnect using module-level &lsquo;Isolation Detection Units&rsquo; (IDU).<br />
Figure 1: System configuration of emergency module-level disconnect using module-level ‘Isolation Detection Units’ (IDU).

Technology Marketing Summary

 

The ability to de-energize PV modules in the event of an emergency is of great interest for the firefighting community. A simple, low-cost method to do so involves integrating fault monitoring controls into the junction box of a PV module such that detection of a ground fault, either through local failure or intentional grounding for safety disconnect purposes will result in de-energization of the individual PV module.

Smart bypass diodes in a PV module are already designed to short out a section of a PV module under abnormal (partial shading) conditions. Typically, one of these devices will be placed in parallel with a series string of 20 to 24 PV cells, as would a passive bypass diode. An active or ‘smart’ bypass diode is simply a metal-oxide-semiconductor field-effect transistor (MOSFET) with low on-resistance that is automatically actuated when it sees its terminal voltage go negative, as would happen under partial shading conditions. When the partial shade condition is removed, the terminal voltage returns above zero and the MOSFET stops conducting.

Description

 

Engineers at the National Renewable Energy Laboratory (NREL) have invented a device that replaces conventional bypass diodes in a PV module junction box, while providing shade tolerance capabilities along with enhanced fault monitoring and emergency disconnect capability.  An advantage of this technology is that the control of the emergency disconnect is completely distributed throughout the PV system, so a system-level ‘watchdog’ signal to control the emergency disconnect is not required.  All that is required is a local detection of ground isolation between module power leads and equipment ground greater than a certain threshold value (e.g. 100Ω).  By intentionally shorting the negative conductor lead to ground at a main system disconnect point (e.g. at the inverter DC disconnect), individual modules will have their module-level emergency disconnect feature activated.

In addition to operating the bypass switch in the case of local reverse bias due to, e.g. partial shading, separate Isolation Detection Units (IDU’s) will continually detect the impedance between one or more DC conductors of a PV module and the module frame ground.  In case of loss of system ground isolation due to either local failure or intentional emergency system grounding, the IDU’s will engage the bypass smart diode, rendering the PV module to a safe (<1V) terminal voltage.

Benefits

 

·        Enhanced fault monitoring and emergency disconnect capability

·        Emergency disconnect is completely distributed throughout the PV system

Applications and Industries

 

·        Photovoltaic solar

·        Emergency response

Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL ROI 15-33ProposedAvailable10/30/201510/30/2015

Contact NREL About This Technology

To: Bill Hadley<bill.hadley@nrel.gov>