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PV System Performance

DC monitoring raises the bar for solar power plants

Fortnightly Magazine - July 2011

that exist heterogeneously in a PV array. Without these technologies, modules can and will be thrown off their most efficient operating point by modules performing sub-optimally due to shading, soiling, cloud transients, temperature gradients, module degradation related to increasing series resistance and insulation breakdown, and other real-world operating conditions, all of which reduces overall array performance and light energy conversion efficiency.

From an operations and maintenance (O&M) perspective, DC monitoring becomes especially vital as the number and size of PV projects increase. A commissioning test is only a snapshot in time that demonstrates system operability upon being placed in service; with time, power generation equipment will be exposed to operating stress and thermal cycling, resulting in the degradation of system performance. Without DC monitoring, this declining trend in performance might be identified, but can be hard to localize and remedy; imagine O&M crews combing hundreds of acres of PV modules and wiring searching for a blown fuse. DC monitoring systems enable troubleshooting teams to efficiently locate and fix broken or underperforming modules, ground faults, blown fuses, defective equipment and open disconnects and circuit breakers, thus reducing performance losses due to downtime.

Safety is the primary concern of anyone building or servicing PV systems. Several smart electronics packages available today are able to pull double duty as both DC monitoring systems and electrical safety systems. These devices withhold voltage and current contributions to the DC energy collection system, effectively turning off the strings or modules they protect unless a series of tight operating parameters are met. The net effect is that workers have less exposure to high voltage DC wiring when building and servicing PV systems. Building-integrated and rooftop PV arrays that use these systems have less high-voltage DC wiring running on, around and through buildings, and additional ground-fault and arc-fault protection features are included. Theft detection is also an added feature on many of these offerings.

The 2011 NEC article 690.11 makes the inclusion of DC arc fault protection equipment mandatory for certain building-integrated and rooftop PV systems. Like DC monitoring systems, arc-fault protection systems can generally be realized at the module, string or feeder (inverter) level. The inclusion of arc fault protection equipment provides a unique opportunity towards wider industry adoption of DC monitoring systems. Although no arc-fault protection devices have yet made it to market, several devices currently under development with arc-fault protection and other safety features also have the ability to provide DC monitoring and performance data.

Risks and Costs

All photovoltaic solar systems are custom-engineered projects; no two systems are alike in design or performance, which introduces risk in the form of uncertainty. The resulting uncertainty has contributed to the inclusion of performance guarantees (PEGU) in engineering, procurement and construction (EPC) contracts, where system builders and integrators provide an energy backstop to limit the end customer’s risk.

Reducing the risk in a project makes financing cheaper and easier. Lenders are more likely to finance a project if they are confident that the project will deliver a timely return on investment by producing the expected amount of energy while