There are many variable factors involved in being able to accurately predict PV Array performance. These variations occur in real time and affect the final power output of the array. Initial estimates can be different from true performance in the field. Factors such as Radiance, Temperature & Load all affect the output performance of the PV panel. Lack of maintenance will also have an impact.
A realistic performance check can be done in the field under actual environments conditions where Radiance and Temperature are constantly changing. In this way real time data of the actual output watts of the solar array can be collected and used to determine performance acceptability based on predetermined criterion.
Because environmental conditions can change quickly, fast and synchronized sampling of V-I characteristics is essential for meaningful data collection. Described in this article are methods of how this can be achieved.
Also monitoring the array performance in real-time can be an effective way to maximize peak output power performance. The real time data can be used to calculate the maximum power point (MPP, from the I-V curves) at present conditions and used to affect panel loading for peak power performance by a smart grid inverter.
Considerations for system measurement hardware will depend on array size and data requirement definitions.
The Typical Installation
The Monitoring System can be attached to a typical solar installation through a Quick Disconnect Junction Box (fig-1) allowing Monitoring Equipment to remain mobile for testing at other installations. The junction box connections to the array need to include pairs of voltage sense lines for each panel and a pair of power conductors in series with each string to allow remote connection to the programmable electronic load. This is needed to allow the retrieval of unique I-V data for each panel. Voltage sense line connections provide Kelvin measuring points to obtain a more accurate performance measurement of the individual panels. Planning for these connections during array design would be desirable but retrofitting arrays is not a difficult task.
A sample array defined for illustrative purposes is shown in fig-2.
A pair of power conductors appropriately sized for the maximum string current interrupts the connection of the string to the grid inverter bus. These conductors are routed to the monitoring system via the Quick Disconnect junction box. High Voltage power relays in the monitoring system are used in conjunction with bypass relays to take each string off-line, one string at a time, for the purpose of establishing performance characteristics of the string panels. Voltage sense lines for each P.V. panel are also routed to the junction box allowing data collection by the monitoring systems signal multiplexer. A place holder in the array (see fig-2, string 8a) can be used for data collection of a reference panel or used as an input for an external irradiance standard.
Test Plan and Approach
The test and monitoring plan will involve taking each PV string off-line one at a time with a power-mux and connect the string in series with a programmable electronic load. The programmable load is then stepped through a series of current measurement points from zero amps (Voc) through the max short circuit current (Isc) of the string while simultaneously taking data for V-out of each panel. Increasing the number of current measurement points increases the accuracy of determining the peak power point of each panel. This can be especially important during low light conditions. The I-V data taken under real-time conditions (ambient radiance and temperature) forms the basis from which to derive the real-time P-V curves for each panel in the string. Once the data for one string is completed it will be switched back on-line and the next string will be switch off-line and connected to the programmable load. This process is repeated until each string of the array has been characterized.
Once the I-V data has been stored and time stamped the P-V results can be extracted and displayed in an easily readable graphical format (see fig-4).
Hardware selection will depend of array size and data desired. The monitoring system will need to measure, each PV panel voltage, each string current and the V & I from a reference panel or read the data from a solar radiance standard. (ref fig-1)
Basic Equipment List:
* -Computer (IPC)
* -Monitoring Software
* -Programmable DMMs and Functions Generators
* -Programmable Electronic Load
* -Programmable Power-Multiplexer and Signal-Multiplexer
* -Reference PV panel or Solar Radiance Standard
* -Sensor ports for Panel and Ambient Temperature
* -Modem or network interface, etc. (See fig-3)
To minimize equipment costs multiplexers for string current and panel voltage measurements are employed. Referring to the example array (fig-2) an eight channel power multiplexer is required to switch each string to the programmable load one at a time. Also a 32 channel differential multiplexer is required to measure the individual panel output voltages while under load.
Additional muxing channels would be required for temperature inputs and reference panels.
A Function generator, to drive the analog input of the load, for stair step generation could be used if the programmable load does not have built in stepping functions.
DMM’s connected through the signal muxes are used to measure panel output voltages. A precision shunt could be used for the string current if higher accuracies than the programmable load can provide are required.
Digital Outputs (Douts) control and timing is critical to the synchronized voltage and current data collection.
Consideration should be given for equipment selection to allow enough operating margin for worst case conditions. For instance normal operation voltages may be one value but open circuit voltage can be much higher especially at higher elevations, lower latitudes and in cooler climatic conditions. Consideration for conductor size is important as long routing paths will increase power losses due to current flow (I-R drops) and reduce overall array performance. Also power conductors and their return paths should be routed as parallel conductors thus keeping the array loop inductance at a minimum and improving system stability.
Configuration and Optimizing Data
Collection In optimizing hardware configuration is important to balance cost with accurate data collection and fast throughput.
It is important that I & V data of an individual panel be taken together (synchronously) since a change in radiance during data collection would give erroneous results. A scheme that achieves this is to take one string off line at a time and connect it to a programmable electronic load. Since serial communication tends to slow data collection down the Programmable Load will be used in the analog control mode (or internal programmable step mode) driven by a stair step function from zero load current through full load current while simultaneously taking panel Voltage data at each Load current step. The stair step current ramp is repeated until voltage data for each panel in the string has been collected. The program will step through each string in the array collecting a full compliment of I-V and Temperature data including the last string which may be connected to a standard panel or string used for data comparison.
The scheme described above should be expected to collect a complete set of data for each panel in the array in less than 4 seconds.
System Software can be a Versatile Tool
Software / Soft-panel Evaluation Features:
After collecting real time performance data the software can be used to analyze the data to check relative and absolute performance of the individual panels in the array strings by comparing:
* I. Array string performance to each other.
* II. String performance to a Standard Panel.
* III. String performance to Solar Radiance Detector.
* IV. Panel to Panel performance within a string or to standard.
A Measurement System of this type is key in analyzing true panel performance and accurate enough for comparative panel performance evaluation which may be of special interest when evaluating engineering prototype panels and new photovoltaic technologies. A Measurement System like this can be used at many different environmental sites due to its small size, flexibility and portability. In addition to panel performance, the information gathered with this system could be used to help predict and schedule maintenance and alert to required repairs so that the array can be kept at peak performance. (source www.chromausa.com)
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