by Laura Carrabine, Senior Editor
Company uses reliability and predictability software to analyze a solar power system.
Enphase Energy designed a new microinverter unit to create a scalable, distributed solar power generation system. In traditional solar energy installations, a single central-ized inverter converts direct current (DC) energy to grid-compliant alternating current (AC) energy. One inverter may be linked to many solar panels, meaning that the failure of a traditional inverter results in a loss of the power generated by all of the panels connected to it. In contrast, Enphase Energy’s microinverters are smaller, more compact units directly attached to each solar panel in an array to convert power independently at the panel. The new system consists of multiple branches of microinverter units and AC interconnects that function in parallel, similar to a bank of batteries. This configuration introduces complex redundant and dependent relationships into the system that influence its reliability. Enphase Energy contracted Relex Consulting Services to assist in determining the reliability of this system.
The firm was contracted to analyze the avail-ability, capacity, expected number of power failures, and mean time between failures (MTBF) of systems employing various numbers of microinverter units. Using Relex Reliability Prediction and Relex OpSim software, the team analyzed the following:
• Performance metrics based on a model that is scalable to the number of microinverters in the system: to test its scalability, the team considered systems with 23, 58, 176, 678, and 5,719 units
• System performance after 20 years under two scenarios: system is repaired and system is not repaired.
• System performance considering three possible “failed” states: 99.5, 99, and 98% required capacity.
To model the anticipated performance of complex system configurations using dependency and redundancy, analysts used a method known as reliability block diagram (RBD) that supports capacity. Using the model, the team was able to calculate system performance measures including point availability, mean availability, capacity, state probabilities, and losses associated with performance degradation. This methodology may be summarized as follows:
Because failure data did not exist for the newly-designed microinverter unit, the Relex software was used in accordance with Telcordia SR332 Issue 2 to calculate a base failure rate given the operating parameters of the system.
An RBD was constructed to account for the configuration of components in the system, the mechanisms of failure, and the effects of failure on system capacity.
Both functions of the microinverter – AC power generation and AC interconnection – were considered separately because the effects of failure are different for each.
Capacity was represented as a function of the total number of units in a system, taking into consideration that the failure of the AC interconnection between units n and n+1 results in the loss of capacity supplied by microinverters 1 through n of the failed unit’s capacity.
Because of the loss of a connector is modeled differently from the loss of capacity, the failure rate may be modeled as a summation of the failure rates of the upstream and downstream connectors.
RBDs to the rescue
The software helped Enphase Energy realize several factors:
• Diagram linking enabled the microinverter unit to be modeled individually with the correct failure rate and capacity parameters.
• All figures in a branch diagram can be linked to the microinverter unit diagram to capture changes in the contributing capacity of one unit as it would be applied to all of the units in the system.
• Branch diagrams can be configured with any number of microinverters with the microinverter figure used multiple times so that the model can be scaled to accurately represent any size system.
With the system modeling in an accurate, scalable fashion, calculations can be performed to return availability, capacity, expected number of failures (ENF), and mean time between failures in hours.
PTC
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