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PV System Performance

DC monitoring raises the bar for solar power plants

Fortnightly Magazine - July 2011

Our electric grid is changing as solar photovoltaic (PV) systems increase both in size and market penetration. As solar power plants achieve larger scale, the operating parameters of direct current (DC) systems become more critical. Stakeholders at all levels, from financiers to engineers, designers, developers, salespeople and end customers, stand to benefit from improved DC monitoring technology.

An incomplete understanding of the benefits that these systems present, coupled with their added costs, can prevent adoption of DC monitoring technology. But as the latest design methodologies and PV technologies evolve, DC monitoring systems will help PV systems produce a return on investment for their owners—and will allow the industry to rely increasingly on clean solar energy for decades to come.

String vs. Module Monitoring

DC monitoring can be implemented in many different forms and flavors, and all topologies have potential benefits and drawbacks. These systems monitor DC performance by measuring and recording DC voltages and currents at specified intervals, which can then be analyzed mathematically to correct for environmental operating conditions and calculate other parameters such as DC energy generated.

In general, DC monitoring can be implemented at the module, string and feeder level: it should be noted that many makes and models of inverters automatically report the aggregate DC input current, though that information doesn’t provide any insight into what is happening inside the PV array. As such, a DC monitoring system isn’t required to determine DC performance on a system-wide basis, and many PV builders and integrators have traditionally shunned DC monitoring systems due to increased cost, complexity and low energy-handling efficiency. The prevailing assumption was that the extra information provided by DC-monitoring systems, while nice to have, wasn’t worth the additional costs of including such a system, especially for smaller projects, 1 MW or less. It’s worth revisiting the efficacy of DC monitoring, especially as large-scale utility power plant systems come to the fore of the solar industry.

Module-integrated electronics packages and inverters provide the most granular, module-level voltages and currents and promise to eliminate DC mismatch error while minimizing the effect of inverter downtime and maximum power point tracking (MPPT) losses. This equipment usually is ordered in large quantities, as each unit is typically located on the back side of each PV module in a system, where it’s exposed to thermal cycling and other extreme operating conditions. While such systems have historically had lower power conversion efficiencies, the introduction of new and increasingly sophisticated technologies warrants a second look at these products. However, if implemented in a large scale PV power plant, module-level monitoring would produce a tsunami of performance information. It’s unclear whether this additional information will be needed in a PV power plant scenario, and practical standardized methods to manage this information at large scale haven’t yet been demonstrated.

String-level DC monitoring generally comes in two different forms: energy handling equipment and string monitoring boxes. Energy handling equipment includes string inverters and

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