The frequency converter for a solar-powered pumping installation is an essential component in order to achieve a reliable and performing system, with reduced installation and maintenance costs. There are a number of factors that should be considered when selecting the most suitable drive. For the performance of the system, it is important to know the control strategy and to determine the reliability of the system you have to look at things like where the drive is installed, condensation, the temperature range, and the backup supply.
Control strategy
Many solar-powered drives for pumping applications in the market operate as under-voltage regulators, reducing the pump speed (and so the absorbed power) when the DC voltage reaches a minimum voltage set-point. This simple control mode usually impacts the PV modules performance, since they operate at a non-optimal voltage.
A more refined approach is followed in other drives operating at a constant DC voltage, selected depending on the electrical characteristics of the installed PV strings and seeking to maximize the hydraulic efficiency of the pump. This constant voltage set-point can be corrected in some models depending on the ambient temperature through an external temperature sensor. Other features such as dry well detection and “go to sleep” and “waking up” routines can also be incorporated.
Some solar-powered drives for pump applications feature MPPT (Maximum Power Point Tracking) control to ensure an optimal operation voltage for the PV modules. A good MPPT controller should be able to follow instantaneous irradiance or temperature variations and skip local maximums, which are generated when there exist partial shading conditions or degraded modules in the PV array. Otherwise, the operation point could be a local maximum providing a bad performance.
A good MPPT control can provide more running hours (earlier start and later stop) and higher efficiency. Consequently, more water is pumped per Wp of installed PV modules, leading to reduced investments and shorter payback time.
Cabinet or outdoor installation
IP 20/21 drives require installation inside a cabinet suitable for outdoor installation. Other elements as fuses, power switch or surge protections are also placed in the cabinet. This approach is not recommended because the cabinet needs to be ventilated in order to avoid overheating of the enclosed elements (this overheating would produce derating and reduced the lifetime of the drive). This airflow needs to be filtered in order to avoid dust inside the cabinet and these filters need to be periodically replaced /cleaned, increasing maintenance costs.
For outdoor installation without a cabinet, drives should be at least IP 54 (recommended IP 66/NEMA 4X). Drives can be mounted in the same structure of the PV modules, taking advantage of their shading to avoid overheating due to direct sun on the drive. The rest of the components (fuses, surge protection …) would be installed in the same structure in a smaller sealed cabinet. High IP rate drives offer much better reliability for the entire system and reduce maintenance costs. Consider that these installations are normally located in remote areas and service is really an issue.
Condensation
Drives operate in many applications continuously and this allows internal temperature to be always higher than the ambient one and so the risk of internal condensation is highly minimized. Nevertheless, in solar-powered pumping installations, the intermittent operation (only during the day) together with the ΔT between night and day internal temperatures promotes cyclical condensation-evaporation processes inside of drive.
IP 20/21 drives inside a cabinet are quite sensitive to internal condensation and so the cabinet should provide anti-condensation measures. The typical solution of temperature controlled heaters can be an issue in a solar-powered pumping system because AC grid is not normally available on site and an auxiliary power source should be considered for this purpose. Anti-condensation solutions not requiring energy supply should be considered instead in this case.
IP 66/NEMA 4X drives installed outdoor are quite well protected against water and dust ingress but, even inside a complete tight enclosure, condensation can occur (absolute humidity is going to be maintained from the last time the enclosure was opened for service purposes, for example). A drive having gore vents to remove internal moisture and avoid condensation is recommended in this case.
Temperature range
Solar-powered pumping systems are normally installed in Sun Belt locations where extremely cold temperatures are not an issue, but hot temperatures can be. Maximum ambient temperature on site should be considered when selecting the appropriate drive model. This maximum temperature should be contained in the operating temperature range for the drive, preferably in the range without derating, in order to avoid power losses and extend the lifetime of the drive. Take into account that the temperature inside a cabinet where an IP 20/21 drive is installed is always higher than the ambient one. Consider also the overheating effect due to direct sunlight, if the cabinet is not mounted in the shade.
Back-up supply
One of the most interesting fields for solar-powered pumping systems is as a complement for irrigation systems powered by diesel generators (to provide relevant fuel cost savings) or very weak grids (to increase pumping system availability). Drives supporting commutation from PV to grid/diesel generator supply and vice-versa are required for this kind of applications. Also harmonics mitigation measures (integrated DC-chokes in the drive, for example) should be considered in order to reduce the impact of the harmonics produced by the drive in the grid/diesel generator.
Article by Abraham González Ponce, Application Knowledge Manager at Danfoss Drives