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PowerViews
The Right Device for a High Yield
David Bassie, Product Manager, Solar Division, Mastervolt
Page 2
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Maximum power point tracking
The optimum working voltage, or maximum power point (MPP) of solar modules depends on temperature and insolation levels. Inverters are equipped with MPP trackers which regulate the voltage in the module array for maximum energy production under any conditions. Under uniform conditions, modern inverters can reach an MPPT efficiency of over 99.8%.
Avoiding dynamic tracking losses
More than half of the annual yields in Europe are generated under variable insolation conditions (figure 3). Any delay or inaccuracy in adjusting the maximum power point results in a brief energy loss. Over the course of a year, such dynamic tracking losses can add up to as much as 2%. In order to reduce these losses, inverters must respond correctly and accurately to changing conditions.
Figure 3. Erratic insolation fluctuations on March 2, 2012 (Source: Austrian institute of Technology, Vienna)
Minimize losses due to shading
Since shading significantly reduces a PV installations’ electricity production it should be avoided by careful planning. However, there is always the risk of the surroundings changing after a solar installation has been built, which may result in new potential sources of shading.
A shaded PV system has multiple maximum power points which an inverter cannot distinguish unless it has been optimized for a shaded module array. This requires a special MPPT algorithm which is capable of identifying the appropriate MPP quickly. Various inverter manufacturers, including Mastervolt, have introduced an MPPT that has been optimized accordingly.
Maximum operating time
In the course of a typical day, most inverters go into idle mode several times, and some may even shut down altogether. Such a startup and shutdown strategy reduces the total operating time, leads to the (unnecessary) tripping of protections due to grid disturbances and, last but not least, impairs the reliability of the inverter design.
Startup strategy
Inverters use a special strategy in order to avoid frequent startups and shutdowns. In its simplest form, an inverter is activated when a relatively high voltage threshold is reached at which sufficient power is available.
This means that inverters in a PV system with voltages much lower or higher then this threshold are activated later than it would be desirable. One way of getting around this problem would be to manually adjust of the startup voltage, but this still does not compensate for changes in temperature over the course of the year.
To tackle this problem, Mastervolt has developed a procedure that determines the solar modules’ current MPP even before the inverter is activated. As a result, the device is always activated at the earliest possible moment. The same principle applies to the late hours of the evening when the inverter shuts down.
The combination of an intelligent startup and shutdown strategy and low energy consumption increases the annual operating time of Mastervolt inverters by up to 200 hours, leading to additional yields of up to 4%.
Response to grid disturbances
When the grid voltage is high, modern inverters can compensate for some of the increase in voltage by feeding reactive power into the grid. This prevents frequent shutdowns and the subsequent loss of energy production.
This only solves just one part of the problem, though. According to the applicable grid guideline, inverters must shut down within fractions of a second in response to minor grid disturbances such as voltage drops. One to five shutdowns a day are not unusual. The grid guideline stipulates a subsequent interval of one to five minutes before the inverter may resume feed-in.
Nevertheless, it is possible to significantly reduce the number of shutdowns. Devices with a “Low Voltage Ride Through” function continue to feed in, even in the event of a brief disturbance, while still complying with the grid guidelines. Mastervolt inverters make use of the full scope of the grid guideline, which enables them to avoid up to 175 hours of operation interruptions per year and boosting yields by up to 4%.
Powerful cooling
High inverter temperatures lead to reduced output power as the electronics components have to be protected from overheating. However, the ambient temperatures are highest when insolation, and therefore potential energy production, reaches a maximum. The inverter’s cooling system must therefore be capable of cooling the inverter electronics adequately in ambient temperatures of up to 40 to 50°C.
The adverse effects of high temperatures on the efficiency of the electronics components are less well-known. Where inverters do not have active cooling, efficiency losses of over 1% have been measured in sunny conditions at high outdoor temperatures.
Conclusion
By systematically minimizing the causes of energy loss and maximizing operating hours, PV systems can generate up to 10% more energy. Inverters can reduce the energy loss in a PV system by increasing the system voltage, through optimized weighted efficiency, optimized dynamic tracking and by staying cool under hot conditions. Inverters can maximize PV system operating hours with an improved startup strategy and by optimizing the response to grid events.
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About the Author
David Bassie is Product Manager at the Solar Division of Mastervolt, and responsible for the complete range of Mastervolt solar inverters globally. He has more than 10 years of experience within the Solar department of Mastervolt.
We welcome the opportunity to publish your opinions. Please email us at editorial@darnell.com.
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