Real-time monitoring is critical for sustaining solar PV energy output

SCADA technology monitors, controls, and delivers system performance data down to the string level.

By Marlee Rosen August 15, 2013

Producing clean energy is the primary function of any renewable energy system. However, regardless of its source of generation, an industrial energy system must consistently produce enough power to support the operations of the facility in which it’s located.     

When contemplating the use of a solar electric photovoltaic (PV) system, it is first important to assess how much energy the system can produce according to location, orientation, and plant conversion efficiency. That assessment ultimately will depend on the kind of technology and quality of system components. 

Making sure an industrial energy system is always producing sufficient power requires monitoring its performance around the clock. That can only be done through use of a real-time performance monitoring system. And it’s becoming increasingly clear that industrial automation technology—particularly the class of systems known as SCADA (supervisory control and data acquisition)—provides an ideal platform for both controlling and monitoring the performance of solar electric PV systems—in real time.   

From sunlight to dc current

PV systems use cells to convert, with determined efficiency, sunlight into direct current (dc) electricity. Usually made of silicon, cells come wired together in a panel or a group of panels mounted together on a frame, which is called a PV array. PV systems also include several pieces of equipment in addition to the PV array. The balance of system (BOS) comprises components that typically include racks and other mounting equipment for the solar panels, combiners, inverters, wiring, transformers, and (if desired) a form of electricity storage (typically batteries).

We are all familiar with our residential electric meter used by the utility company to record and bill us monthly for the kilowatt-hours consumed. Over the course of a year, these bills can be compared to determine monthly consumption. While this scenario illustrates usage consumption, it is different for monitoring production with PV systems. A meter is also used to measure the energy produced but, instead of a monthly basis, we are interested in the amount of energy produced during short time intervals—perhaps every hour or every 5 minutes. The recording frequency requires more sophisticated meters than the residential ones called data loggers. Data loggers feed data into a memory system that can be archived for use at a later time. They also have communication interfaces (Ethernet or serial ports), which allow a computer to connect to it and retrieve the data.

Distributed generation standards

Most electric utilities in the United States have adopted standard criteria and guidelines for interconnection of distributed generation (DG) to their electric distribution systems. Photovoltaic system installations effectively reduce the customer load and, during minimum loading conditions, may export energy back to the utility in a transaction known as “net energy metering” (NEM). A set of guidelines (IEEE P1547.6) were recommended by the Institute of Electrical and Electronic Engineers (IEEE) to be incorporated within the design of PV systems and operate in parallel with utility systems.

One such system designer is Staer Sistemi, which tackled the design of the first PV automated management system in late 2009 and has since revised it over the last 4 years. The initial design comprised the use of a simple DAS (data acquisition system), but Staer Sistemi quickly identified that due to the volatility of solar radiation at ground level—mainly due to atmospheric turbulence—a pretty fast sampling pace (5 seconds or less) would be required. Due to this requirement, Staer Sistemi decided the best approach would be to develop the final application in an industrial established SCADA environment. This would allow designers to manage data streams in the range of several thousand measures per second.

After conducting numerous system tests as a proof of concept, Staer Sistemi configured a low-weight, industrial SCADA software, PcVue from Arc Informatique, to meet its PV application requirements. This SCADA provided flexibility in monitoring and controlling the various plant component and operations, including trackers, inverters, substations, and meters. PcVue was used in all company flagship installations with some PV plants exceeding 5 MWp (megawatt peak) individually and in some larger multi-tenant, multi-site systems too.

Designed for monitoring performance, the system logs any problem and triggers alarms so that the engineering staff can fix or change components or fine-tune the process of plant operation.

Performance management methodology

The system monitors the performance by means of a sophisticated mathematical model initialized once at installation time with some plant design data: PV panels’ peak power, inverters, manufacturer-provided electric parameters, number of strings, strings length, etc. The model is then continuously fed with local weather data and calculates in real time what would be the correct energy production at 100% of plant capacity.

The automatic comparison between the calculated and the real production figures (supplied by the already mentioned data logger) will give a precise indication of the plant performance or plant health every minute or less.

Today monitoring and performance analysis of solar PV plants has become extremely critical due to the increasing cost of operation and maintenance as well as reducing yield due to performance degradation during the lifecycle of the plant equipment. This means that the use of a monitoring system can become essential to ensure high performance, low downtime, and fault detection of a solar PV power plant during the entire lifecycle.

From a technical point of view, it is interesting to understand how the overall data acquisition is performed starting from the dc level. Here, string combiner boxes designed for PV installations have built-in string probe units that measure the values of dc current voltage and power made available through a serial RS485 port (different methods or wireless can be used) for communication to the SCADA via ModBus. Some RTUs (remote terminal units) are installed at the field location that connects to multiple string junction boxes on the already mentioned RS485 multi-drop loops.

At the ac level, inverters expose RS485 ports to allow an easy connection. The native communication drivers from PcVue collect data from control boxes and RTUs with a time stamp for real-time processing, storage, alarming, reporting, and displaying. This is in order for both dc and ac side parameters, status, and diagnostics to be continuously acquired. The SCADA capabilities are further used in monitoring of grid protection relays, energy meters, weather monitoring station/sensors, LT (low tension) and HT (high tension that starts from 3 kV) control panels, dc switches, transformers, and in general any devices capable of affecting—directly or indirectly—plant production.

Additionally, to make PV applications as efficient, sustainable, and scalable as possible, it’s important to take into consideration other aspects of the SCADA application. These include dynamic configuration, stand-alone and client-server configurations, redundancy for data protection, historical and real-time trends analysis, as well as advanced alarm management. Looking further at compliance, the support of such protocols as IEC 61850 and DNP3 is considered an asset if you have to communicate with various electric substation devices, for example. 

To access all data points, a user-friendly graphical interface with 2D and 3D displays, report generator, scheduler, and an event-driven engine all make the process much smoother. Finally, Web access capabilities provide all kinds of mobility and access to remote devices the application may need.

Marlee Rosen is an energy market research analyst with Rosen Associates. Edited by Sidney Hill, Jr., a CFE Media contributing content specialist, This article is part of the Industrial Energy Management supplement for CFE Media publications. 

Online extra

See additional advice on solar PV installations at the bottom of the article.