MPPT charge controllers (Maximum Power Point Tracking) have been used in off-grid battery charging systems for several years. The benefit of tracking the maximum power point is that the solar PV system can run on the Vmp voltage to convert the full power of the array during bulk charging. This results in an increase in performance compared to bulk charging a PWM switching regulator (pulse width modulation) that uses a "nominal voltage" array and operates with battery voltage below Vmp.
Most MPPT controllers are rated for a maximum Voc voltage of 150V. This makes a lot of sense as installers can use the popular circuit breakers that are rated for 150V. PV voltages above 150 V may be subject to additional code requirements, such as: B. US NEC 690.7 (C), according to which the cabling of the PV source must only be accessible to qualified persons. In addition, buck conversion circuits used in most MPPT controllers do not work effectively with higher voltages and higher power. This applies to all MPPT controllers with a buck converter. Most MPPT controller efficiency ratings show that efficiency drops dramatically at higher input voltage and power levels.
While a maximum input voltage of 150V is often a good choice for many applications, there are many cases where a higher voltage is desired. Some advantages associated with a higher input voltage MPPT controller include:
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Higher voltages allow lower currents and thus a lower voltage drop.
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Smaller wire diameters can be used from the array to the controller to provide the same amount of power to power the batteries.
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Longer wire runs from the array to the controller can be achieved with much lower wire costs.
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Faster, easier installation and the ability to make smaller radial wire bends around obstacles and tight spaces that would otherwise not be possible with larger diameter wire.
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Longer string dimensioning of PV modules with higher voltage means that often no combiners and even no string fuses are required.
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The economic benefits of a 600V MPPT controller can be achieved on cable runs longer than 30 meters where the cost of wire, combiner, breaker, and other savings are higher than the higher cost of the 600V MPPT controller .
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The single-string solution allows modules to be added step-by-step without any problems with string size.
In addition to the off-grid grid, some high voltage regulators (HVCs) can be used in new and retrofitted tie-in systems with battery backup using the same string size as a typical 600V grid interactive string inverter. This represents an alternative to AC coupling and offers advantages in terms of flexibility, scalability, stability and battery charging efficiency.
In addition to using MPPT controllers greater than 150V, customers often want more charging current over 60 amps. The most reliable solution is to connect multiple 60 amp controllers in parallel. However, some consumers unknowingly purchase MPPT controllers rated more than 60 amps, which often require multiple fans to prevent overheating. Many consumers believe that they are getting more for their money, but what they often gain from higher input voltages and maximum charging current, they lose reliability. Because these MPPT controllers are designed to exceed the limits of a 150V / 60A MPPT controller, the electronic components are less efficient, become overheated, and are more likely to fail over time.
Morningstar's TriStar MPPT 600V controller is an example of a controller that works with very high efficiencies of 95 to 98% with operating input voltages of 250 to 425 V. This high efficiency means that the electricity charges the batteries instead of heating the electronics. Hence, no fans are needed, which makes it as reliable as the TriStar 150V MPPT controller, which also has no fans. The developers of this controller took a new approach in its design and the result is that it can be operated with an efficiency of> 97% with an input power of 1-3 kW at 300 V for charging 48 V batteries.
Morningstar engineers considered how to meet the US National Electric Code and other international electrical codes. For controllers used with arrays in a home, this high voltage controller requires a ground fault protection device (GFPD). The existing GFPD solutions have several disadvantages, especially for a 600 V MPPT controller:
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The GFPDs available on the market interrupt the grounding so that the battery and loads are not grounded and float during a ground fault. This can potentially expose users to high array voltages between ground and battery under load.
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Most GFPDs use fuses that must be replaced after a ground fault is detected. This can be extremely problematic at many off-grid locations with difficult access and without replacement backup on site.
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Activation times for GFPDs that use fuses or breakers are typically around 1 amp, which is around three times higher than recommended.
Morningstar was able to develop 150V and 600V GFPD solutions that overcame the shortcomings of these existing solutions. These GFPDs break both positive and negative conductors from the PV array. This approach better isolates the ground fault and keeps the battery and loads grounded so that people are not exposed to high array voltages during a ground fault. Instead of using fuses, a high quality sensor is used to detect a ground fault and trip at 300 milliamps. A 2-pole breaker switch disconnects the conductors of the array so that it can be reset and therefore no fuses need to be replaced.
Train customers
Even after solving these challenges, manufacturers face the challenge of educating customers about the benefits of using a high voltage MPPT controller. As these units continue to be used successfully and new applications for their use become increasingly popular, the rest of the market will see the eagerness with which early adopters embraced developments in high voltage charge controllers and next generation earth fault protection.
The next step is to configure the 600V MPPT controllers in parallel inputs with up to 12 to 15 kWp input from a solar, wind, or hydro system. Then there will be a 600 V DC transfer switch for a new solution for on-grid battery backup for retrofits and new installations.
Written by Mark McHenry, Marketing Manager, and Doug Grubbs, Applications Sales Engineer at Morningstar Corporation in Newtown, Pennsylvania (USA).