The generator or the grid
in support of solar pumping

What a second source adds to a solar installation

An installation may have to hold a pressure at the sprinkler, the gun or the centre pivot, hold a flow throughout the watering window, or reach a programmed daily volume. When the solar resource does not supply all the power drawn, the second source tops up the supply. The study then states the solar share, the energy required from the backup source and the operating conditions of the chosen architecture.

This logic is that of constant pressure: as soon as the setpoint becomes an agronomic requirement, it stops being a seasonal average. The kiwi case study in South-West France gives a full-scale reading of it.

Three architectures for adding a backup source

Each range organises the backup source in its own way, and each manufacturer's booklet describes the entry point of the second source as well as the control arrangement that goes with it; LE LAB reproduces that description as it stands. The choice between the three follows the pump, and the pump follows the operating point: the flow at the total dynamic head of the site designates the range, and the backup source is fitted afterwards.

SQFlex: the automatic return to solar

The MSF 3 motor of the SQFlex carries its own control module and accepts both a direct supply of 30 to 300 VDC (100 to 300 VDC for the 2,500 W motor) and a single-phase alternating supply of 1 × 90 to 240 V, at 50 or 60 Hz. The pump can therefore receive the photovoltaic array and an alternating source without an intermediate converter.

The IO 101 box controls the supply of a SQFlex installation paired with a generator. Starting the generator puts the system into operation; stopping it triggers the automatic return to the solar supply. The manual puts it this way: “The generator must run steadily for the pump to start.”

SP and SPE on an RSI: the backup supply input

The RSI solar converter is compatible with direct current and with alternating current. Its technical booklet puts it in one sentence: the RSI can be connected to the grid or to a generator as a backup supply in case of interruptions of the solar array. This happens on three terminals, L1, L2 and L3, described as the three-phase AC power supply, alongside the direct bus that receives the array.

The alternating input voltage follows the version of the converter: 208 to 240 VAC in the low-voltage range, 380 to 480 VAC in the high-voltage range, that of the large machines. The input is documented as a backup supply, and the organisation of the control is defined at the cabinet, site by site, with the switchgear chosen by the installer. The same compatibility serves the workshop: the booklet notes that it makes it possible to connect the converter to the workshop three-phase supply to prepare it off site, which makes installation quick and simple on site.

This is the architecture of large volumes, that of the SPE and SP pumps on an RSI. Solar pumping on a grid connection finds here its most direct case: a connected farm keeps its supply contract in reserve and lets the array work the rest of the time.

PSk: the continuous combination of the two sources

The PSk3 controller has a built-in alternating input. It can switch between solar energy and the alternating source or combine the two to hold a flow or a pressure. In hybrid mode, the controller adapts the contribution of the backup source to the variations of the photovoltaic array in order to keep the setpoint foreseen by the sizing.

The two sources meet on the direct bus. To allow them to work simultaneously, the real Vmp of the array must stay above the rectified alternating voltage. On the PSk2 generation, this function is carried by the smartPSUk2 cabinet. The voltage thresholds, the ranges and the connection rules are detailed on the LORENTZ PSk hybrid page.

The control mode is set once and for all at commissioning. The LORENTZ application note “solar by default” describes the result: the operator switches on the generator and the system goes into hybrid mode; the operator switches it off and the system returns to solar mode, automatically. The smartStart adds control of the generator itself, through a volt-free contact that closes as soon as the backup source is called.

On the grid side, the PSk3 works from 380 to 480 V at plus or minus 10%, from 45 to 60 Hz: it monitors the voltage and connects when it sits within this range. The detail of PSk hybrid operation is set out on a separate page.

Granada: an example of the split between solar and the backup source

For a need of 60 m³ a day, six hours of sprinkling at 3.0 bar and a total dynamic head of 96 m, the study selects a LORENTZ PSk3-15 C-SJ17-18, an array of 8,755 Wp and a generator of 6.5 kW at continuous load. The energy split is established on sixteen years of real solar resource, hour by hour.

Choosing the generator on the operating point

The quantity that governs the choice is the continuous power of the generator, the power it delivers at continuous load, and the starting point is the power drawn at the operating point. LORENTZ publishes the rule and illustrates it: 6 kW drawn give a 9 kW generator in the hybrid sizing note, and 29.7 kW drawn give 44.55 kW, therefore at least 45 kW, in the Spanish irrigation case. The factor of 1.5 covers the converter efficiency, the power factor and the derating in continuous service. In Granada, the 4.26 kW drawn give 6.5 kW once rounded up to the next half kilowatt.

The controller works with generators whose continuous power is higher, equal to or lower than that of the motor: when the generator is smaller, a speed limit set at commissioning adjusts the demand to what it can deliver. This is the specific gain of the hybrid architecture, written as such in the smartSolution note: it allows a generator markedly more compact than the one a pumping system running on diesel alone would call for.

• Voltage. 380, 400, 415, 440, 460 or 480 V three-phase, according to the specifications supported by LORENTZ smartSolution.
• Frequency. 50 or 60 Hz.
• Remote start. Two-wire function as soon as the smartStart controls the generator; the smartStart supplies a volt-free contact, with a breaking capacity of 250 VAC or 30 VDC at 2 A (PSk2 manual, 2023 edition).
• Input current. The alternating input works as a rectifier and draws a non-sinusoidal current, which slightly raises the maximum value: all components are sized on it.
• Public grid. Where a harmonic distortion limit applies, filters are fitted at the input; they also reduce the rms current drawn from the source.
• Protections. The input receives its fuse and its manual disconnection, sized on the nominal current of the controller.

The backup source enters the study at step 6

At step 6, declaring another power source adds the backup source to the project. LE LAB then computes the split between solar and the second source over sixteen years of real solar resource, hour by hour, shows the typical day of the design month and gives the annual solar share. The installation diagram represents the connection point corresponding to the chosen range.

The professional report devotes a whole section to this split: the pressure setpoint and the position of the sensor, then the recommended generator at continuous load, the solar share, the backup energy, the running time and the estimated diesel, with its consumption assumption.

The documents behind this page

Every equipment behaviour described here comes from the official documentation of the manufacturer concerned. The figures of the worked example come from the LE LAB application, on the hourly PVGIS SARAH2 database.