Combiner boxes, which combine outputs from a number of solar strings into a single circuit and provide overcurrent protection, will continue to be a key element of the next generation of solar photovoltaic (PV) installations. Historically, these devices have been basic box and fuse holders that consolidate string output circuits into one circuit. An increased emphasis on safety and efficiency, however, has yielded many recent innovations in the design and functionality of combiner boxes.
Evolving to meet changing application requirements, today’s combiner boxes can provide cost-effective system monitoring and diagnostics, optimized safety and streamlined installation. These capabilities are allowing the industry to implement solutions that can improve overall system performance and capital use, while also helping with industry compliance.
Cost-Effective Monitoring
Central inverters maximize power output by tracking the maximum power point of the entire array. Though, because maximum power point tracking (MPPT) represents the average of all arrays, power loss occurs from all of the strings whose power curve is not in alignment. Ultimately, this can reduce revenue for integrators, power-purchase-agreement (PPA) providers and anyone with a financial interest in system performance.
A number of years ago string-level monitoring was added to combiner boxes to provide discrete data for troubleshooting. Recently, the move has been to add the monitoring at the recombiner level. While the data may not be as specific, it does provide a higher level of monitoring than at the inverter alone, at a cost that is lower than string level.
Optimizing Safety and Meeting Evolving Safety Standards
The National Electrical Code (NEC) 2011 code changes address safety concerns and improves design practices. Previously, combiner boxes were often located far from the inverter and without a disconnect switch nearby to interrupt current for servicing. A visible means of disconnect was required with the 2011 NEC 690.16(B) to be within sight of and no more than 50 feet from the fuses. Additionally, arc fault protection also became required, but the code does not specify where it should be located.
Once a small segment of the installations, combiner boxes with integral disconnects have become the norm relatively quickly, as they address a number of functions. Integral disconnects enhance operator safety during maintenance operations by opening the circuit and serving as a lock-out, tag-out (LOTO) device. Visible blade disconnects provide an additional level of safety by providing the operator with a visual check to ensure that the circuit has been opened. It is important to note that all integral disconnects should be tested to meet Underwriters Laboratories (UL) 98B standard. The testing required by this standard ensures that the switch has been tested for PV application.
Beyond the integral disconnect requirement, the 2011 NEC code requires arc-fault protection. Modern switched combiners equipped with contactors or shunt-trip switches allow for the integration of arc-fault protection. At the time of this writing, there are no listed PV AFCI boxes available. One of the key issues facing the integration of an AFCI component is that the device must react to arc events, while providing nearly 100% nuisance-trip characteristics. The “load” in PV systems is the inverter, and the noise associated with different manufacturers’ inverters must be recognized. Incorporating the AFCI within the combiner, can provide a cost effective solution that provides a high degree of fault troubleshooting
Streamlined Installation
Lower installation costs are driving product innovations in combiner boxes as well as a host of other balance of system technologies. Quick-connect boxes, whips and pre-punched enclosures are some of the items that have been developed to help reduce installation time and costs. These innovations can have a higher up-front cost, so installers should compare the added material costs to the lower labor costs before making purchasing decisions.
In summary, combiner boxes are a key element of solar balance of system solutions and will continue to evolve to meet changing application requirements. The next generation of combiner boxes can be applied to help enhance personnel safety, improve overall performance and provide robust system diagnostics. Further, large manufacturers are bringing economies of scale and research resources that will help advance the performance of solar PV installations that will lower the overall system cost and drive to grid parity.
By Brendon Foley, Product Line Manager, Eaton
Maaliq says
If I close the switch (Disconnect Switch) can an arc-flash happen