Module damage, degradation, dirt, and shadowing all can affect photovoltaic (PV) system output. These systems can contain thousands of individual PV strings and larger power bus cables, which create an obstacle for monitoring. As a solution, Phoenix Contact developed the string monitoring system SOLARCHECK-RSD, which provides diagnostics and performance validation for PV systems

Rapid shut down

Much of the research that went into SOLARCHECK-RSD centered on listening and watching the changing safety requirements put forth by the National Electrical Code (NEC), as well as listening to the various organizations and groups pushing for those changes to the NEC.

“As soon as Section 690.12 of the 2017 NEC was released, everyone knew it would evolve in the next code revision,” says Jeremy Valentine, interface product manager at Phoenix Contact, referring to the rapid shutdown requirement legislation. “We simply wanted to ensure we were aligned with whatever those changes would be. In turn, we ended up deciding we would design for individual PV panel level shutdown to zero volts. This way, no matter what the 2017 code ended up requiring, we would satisfy the requirement.”

Valentine says most of the challenges in developing SOLARCHECK-RSD, or rapid shut down, were in its design. Because this technology did not previously exist, Phoenix Contact had to start from scratch.

“It certainly pushed our design engineers, but they were able to meet those challenges. As a result, we ended up with several patents on the technology that resides within SOLARCHECK-RSD,” Valentine says.

SOLARCHECK-RSD continuously monitors the electrical health or integrity of a rooftop solar array. Two components work together to monitor and perform rapid shut down of the array; both are upstream from a string inverter. An isolator sits on the back of each PV panel and monitors it. In the event of an emergency or manual shutdown at the inverter, the isolators open the circuit connected to the PV panel and remove it from the string.

The second component is a starter that re-starts the PV array string when the system is safe.

In the event of a disruption to the array’s normal operating parameters, SOLARCHECK-RSD will automatically shut down each PV panel of the rooftop array to zero volts within seconds.

“The actual amount of time will depend upon the capacitance bleed of the inverter SOLARCHECK-RSD is connected to,” Valentine says. “There is also the ability to shut down the system manually. This means no voltage will be present at the panels. In a nutshell, this is the essence of the 2017 NEC code.”

Continuous monitoring

As soon as the system has been shut down, the starter unit tries to restart the system. Several checkpoints prevent restart until the system is ready; such as the presence of the inverter, an uninterrupted string line, and healthy PV modules.

“If all these bullet points in the checklist are OK, the starter unit initiates the system restart automatically. This ensures that the generator only comes back to life when it is technically OK, and no person working in the system will be exposed to any danger,” Valentine says. “So the downtime between shutdown and restart depends on the measures which have to be taken to repair the system. The system comes back as soon as technically safe conditions are given, and then it just takes the typical initiation time of the inverter to start up again.”

SOLARCHECK provides communication and measurement for up to eight strings on the same cable pair, minimizing the number of wires and power supplies required. The system’s Modbus interface can be combined with Phoenix Contact’s portfolio of controller and communication products to provide data access.

The system cannot be monitored remotely out of the box, but Valentine points out that since SOLARCHECK-RSD is constantly monitoring the presence of an inverter, a user could add a remote switch to the inverter for remote use.

SOLARCHECK-RSD will be available in the United States in the summer of 2016, and its intended market is commercial-scale rooftop PV installations.

“This product is pivotal to ensuring safety of maintenance workers and first responders that must deal with rooftop PV installations. Since PV panels are run in series (or cumulative), voltage from those panels can reach up to 1,000VDC by the time it gets back to the inverter,” Valentine says. “It’s all about ensuring preservation of life and property. By automatically shutting all PV panels down to zero volts, SOLARCHECK-RSD does just that. No cumulative voltage goes back to the inverter, meaning routine maintenance, updates, testing, and even emergency work can be done without risk of safety hazards.”