String inverters have a proven trajectory. Because of their established reliability, accessibility and high efficiency, they remain well-suited for most residential and small commercial systems under performance incentives. With added competition from micro-inverters and two-stage inverter systems, solar string inverter designers continue to make advancements in efficiency and power density using new and better components and topology innovation. In addition, they also face the demand to provide improved grid management and safety functions. Enhancing power control features, providing environmental data analytics, and addressing serial arc fault circuit interruption are challenges facing every solar string inverter designer.
Power Management Via Power Control Interface
Successful PV policies, technologies and business models have increased the penetration of PV systems using string inverters in many communities. The added uncertainties of distributed generation present challenges to utilities:
• Feeders and transformers may be overloaded by large production during periods of low consumption.
• The risk of overvoltage increases with production at the remote parts of the feeder.
• Power quality disturbances may increase.
• Protections may operate incorrectly. Failure to detect an uncontrolled island is a major concern.
In these scenarios, a power control interface enables PV plants to take part in the feed-in power management of the AC grid. The interface allows users and utilities to remotely set the active power limitation and the reactive power control of the string inverters in a PV plant.
Some proposed units would provide dry contact digital inputs for flexibility of connection to a control receiver, or similar device. The meter or Energy Service Interface (ESI) in the context of a smart grid may become the control receiver.
The set-point signals received can be translated into control commands for all solar string inverters connected on the inverter network. Any active or reactive power adjustment or power factor adjustment event is then logged with all records available via the plant communications system.
The communication between the power control interface and the inverters needs to be robust so industrial busses like CAN bus are ideal. Others, including wireless mesh networks, are also favored under some deployment circumstances—for example when inverters are in separate buildings but remain part of the same PV plant.
Environmental Performance And Analytics
The power control interface is ideal for added functions common in larger PV plants, but not yet in everyday use with smaller PV systems. The combination of environmental information with software applications for analytics can be a major part of the management of the AC grid.
With inputs available for irradiation, PV module temperature, ambient temperature and wind speed sensors, a full environmental picture can be obtained. In combination with power data from the inverters, the PV system performance can be checked and the performance ratio found, making it possible to compare PV systems anywhere in the world regardless of size or location. A comparison between actual and target performance can also be done so the user can see if the photovoltaic system provides the expected yield, or if maintenance is required to correct shading, dirt or a gradually declining array performance.
Series Arc Detection
In an effort to increase safety in PV installations, the NEC 2011 includes provisions for serial arc detection in the DC side for solar strings above 80VDC. It is expected that Europe will follow in this direction. The Arc Fault Circuit Interrupt (AFCI) applied to AC is familiar, however defining and identifying the signature of a serial DC arc in PV systems is complex. Furthermore, the code requires that after the AFCI is activated, the system can only be reset manually at the site. This provision makes the realistic cost of false alarms high.
AFCI devices must be immune to any external interference. This includes those inherent to the site, such as machinery or transmitters in low RF bands, as well as EMI generated by the inverter. The required performance of the serial arc protection is defined by UL1699B.
While AFCI is not a function inherent to the inverter, string inverters commonly incorporate other functions such as a DC disconnect and string combiner box. With current technology requiring one detector per string, the location of the AFCI at the inverter appears to be the most cost effective approach. However, pursing the purpose of eliminating fire hazards and protecting firefighters, parallel arc detection and PV module level shut down (PV Array Response to Emergency Shutdown) may be required in future revisions of the code.
High efficiency solar string inverters will continue to improve in their basic performance and reliability. Inverter designers continue to address and incorporate the challenging demands from users, communities and regulatory bodies. With the increased use of PV systems, string inverter designs have evolved with the full system in mind: grid management including smart grid models, system performance data to decrease performance uncertainty, and system safety.
By: Alberto de Leon, Sales Director for Renewable Energy in the Americas at Eltek