SGS Solar System Description

SGS Solar System Description

Technical details of the Solar Array

Display of Current Solar Production

January 1, 2006 Update

The Solar System located at the Springerville Generating Station (SGSSS) was designed to be built in phases with a fully completed continuous generating capacity of 8,000 AC kW. The system was designed around installation of 64 building block systems with a nominal size of 125 AC kW per system. Blocks were configured in two different sizes to accommodate PV modules of different efficiencies. While the fully completed system will use 88 acres of ground, the initial leveling was 38 acres, for 24 unit blocks. A second phase of ground preparation in 2003 developed an additional 20 block locations on 20 acres. Leveling was performed in a manner to barely disturb the ground to leave as much native vegetation in place as possible. A water permeable dust/erosion control agent was placed on the soil after leveling to promote growth of the native grasses. Growth of weeds is being controlled by manual removal until such time as the native grasses reestablish themselves as the dominant species. The native grasses generally reach a full growth height of about 8 inches and the lower edge of the PV modules are placed 8 inches above the ground. The modules are fixed, south facing with a tilt of 34 degrees from the horizontal. While the area is fenced to keep out cattle, lizards and small mammals have moved back into the area and use the PV modules for shade.

PV array blocks located on the east side of the field are built on a footprint of 300 feet north to south by 140 feet east to west, and will accommodate PV modules with efficiency of 10% or more. Today, twenty-six (26) of these blocks have been completed, all of them using crystal silicon PV modules of type 300 DG/50 from RWE Schott. Each unit uses 450 of the modules for an STC DC power rating of 135 kW.

Blocks located on the west side of the field are built on a footprint of 300 feet north to south and 250 feet east to west, and will accommodate PV modules with efficiency of 5.5% or more. Four (4) of these blocks have been completed each with 2,688+ Cad-Tel First Solar FS-50 PV modules for an STC DC rating of 134.4+ kW. Four (4) others units are completed each with 3000 a-Si BP Solarex MST-43 PV modules. These units have an STC DC rating of 129 kW.

All units use a Xantrex PV-150 inverter to convert the variable 300 to 600 volt DC power to 208 volt 3 phase AC power. The inverters have a maximum rating of 157 kVA, at which point they will limit output or come off line, followed by an automatic restart. Each unit has a DC disconnect, 208 to 480 volt step-up/isolation high efficiency transformer, revenue meter and AC disconnect. Because of the high conversion efficiency and fast response time of the inverters, in April 2005 the maximum inverter power level allowed was limited in software to 92% of rated capacity to prevent overloading the system AC grid components during cloud enhanced solar insolation conditions.

Groups of four (4) units are connected in parallel to each of eleven 500 kVA 480 volt to 34,500 volt high efficiency step up transformers. Each transformer has a continuous rating of 500 kVA and can accommodate up to 650 kVA for brief intervals. The high voltage sides of the transformers are connected in parallel in a daisy chain configuration to a 34.5 kV underground distribution line which connects to the overhead 34.5 kV distribution line which feeds the well field pumps of the Springerville Generating Station. The pumps operate continuously with an average load of about 10,000 kW. The SGS Solar System is a customer based distributed generation system providing power to the pumps. As the distribution line is connected to the main switchyard at SGS, any excess energy produced will flow through the switchyard to the grid.

The system produces the most power capacity during the cooler months of the year when the sun is near latitude angle. This typically occurs in March and April. Solar insolation at the 6650 foot elevation site has been measured at over 1,500 watts per sq-meter in these months for short periods and at over 1,300 for one minute averages. These events occur during cloud passage which creates strong sunsplash conditions. The actual AC output of the units during severe sunsplash have been measured at 157 kW for 10 second averages. Consequently, the short term power output rating of the system with 34 units installed is an actual 5,113 kW measured for a one minute average on December 6th, 2004. The revenue meters measure 15 minute power output. Actual measured 15 minute average unit AC energy outputs in 2004 total 4,644 kW for the full system. Thus we are rating the total 15 minute AC system output at 4,644 AC kW. However, while these system output conditions must be taken into account during system design, they are not an indication of long term energy output.

The annual net AC energy production in 2005 was 7,532,420 kWh. Based on experience with the installations to date, we expect the annual net AC energy production from the 34 units to be just over 7,650,000 kWh in 2006. Energy production reliability of the inverters was 99.92% and reliability of the entire PV system was 99.81% in 2005.

Continued expansion plans are currently on hold while module prices awaiting a reduction in module prices to below pre 2004 levels. Future plans are to install 125 AC kW units on ten (10) prepared pads in 2007 and to prepare land and power distribution infrastructure in 2007 for the final 20 units. The type of PV modules to be installed in that final phase will be in large part determined by actual energy production of existing systems as well as installed cost data bid for PV equipment.

The system operates as an unmanned site and is monitored continuously via an Internet based communications channel. Nearly all operational functions can be performed by remote control, including inverter reconfiguration. Alarms are logged. Data is taken from each of the 34 inverters and the revenue meters on 10 second scan cycles and averaged for one minute archiving. Performance information is developed from the raw data and reviewed in Excel spreadsheet and graphical chart form daily. Spare parts are maintained on site and local based service personnel dispatched to site to perform repairs in response to alarms. Near real time performance is available on the Internet at www.GreenWatts.com/pages/solaroutput.asp

If the need exists, there is sufficient room to place another 8,000 kW block south of the systems currently being installed. The distribution line is capable of supporting 36 MW of distributed generation.