Surge Protection for Agrivoltaics and Solar Water Pumping: DC SPD Selection for Open-Field PV

Contents

1. Why open-field PV differs from a rooftop
2. Protection points in a pumping system
3. PV-specific DC SPD versus a generic DC SPD
4. Ucpv, polarity and short-circuit current
5. Long DC cable runs and both-end protection
6. DC selection checklist
7. Earthing reality in open-field sites
8. FAQ

Solar water pumping and agrivoltaic installations combine two conditions that make surge protection harder than it looks: they sit in open fields with no surrounding structure to shield them, and they run on long DC cables with a grounding network that is often thinner than anything found in a building. Getting the DC side right is the difference between a system that survives a storm season and one that loses a controller or inverter in the first thunderstorm.

This article explains how to approach agrivoltaic surge protection and solar water pump protection, with the focus on DC SPD selection: the Ucpv rating, polarity and short-circuit behaviour, and where the device is installed. It references the two IEC standards that govern this area and describes the selection logic, not a specific product line.

Why open-field PV differs from a rooftop

On a rooftop, the building structure often carries the lightning current through its own protection system, and the array sits close to a well-developed earthing arrangement. In an open field, the array frame and the module cables are themselves the most exposed conductive objects, and the earthing is typically a few rods in dry or sandy soil spread across a wide area.

Three consequences follow. First, the DC cable run between the array and the pump controller is frequently the longest conductor on site, so induced transients build along its length. Second, the earthing reference between the array end and the equipment end can differ during a surge. Third, the pump controller and inverter concentrate the value of the whole installation, so a failure there stops production entirely.

There is also an operational angle. Agrivoltaic sites and remote pumping stations are typically visited rarely, so a protection device that fails unnoticed becomes a future, more expensive failure. Status indication and the ability to see that indication at the controller are worth more here than in a building where someone walks past the board every day.

This combination — high exposure, long runs, weak earthing and rare attendance — is what justifies treating open-field PV as its own selection case rather than importing rooftop practice. The same reasoning extends to the wider agrivoltaic sector, where arrays share land with crops or livestock and the electrical infrastructure is spread even thinner across the site.

For anyone specifying these systems, the discipline is the same whether the load is a pump, a borehole controller or a battery-assisted hybrid: record the array, the cable and the earthing first, then choose the DC protection. The agrivoltaic surge protection decision follows from those three facts rather than from a catalogue habit.

Protection points in a pumping system

A solar pumping system can be read as a short chain: the PV array, the DC combiner box, the DC cable, the pump controller or inverter, and the motor. Surge protection is considered at the points where an exposed conductor meets a sensitive interface — at the combiner box on the source side, and at the controller or inverter input on the load side.

Where the pump is driven through a variable-speed drive or a soft starter, the motor side of that drive is a further interface, but the DC side is the focus of this article because that is where the PV-specific requirements live. Our DC PV SPD range covers the DC side of PV generators and inverter inputs; the DC power SPD range covers other DC distribution duties, and the two are not assumed to be interchangeable.

The chain is also a useful way to explain the scope of this article to a supplier: when a proposal mentions "surge protection for the pump", the first clarification is which side — DC, AC, or the signal and control loops. The DC side is where the PV-specific reasoning applies, and it is the part most often left under-specified.

PV-specific DC SPD versus a generic DC SPD

The first selection question is whether the device is actually a PV-specific SPD. A photovoltaic array is not a generic DC source: it behaves as a current source, its open-circuit voltage rises as the module temperature falls, and its short-circuit current is only modestly above the operating current. Those characteristics drive a dedicated test basis, set out in IEC 61643-31 for requirements and test methods on the DC side of PV installations up to 1500 V DC.

The practical effect is that a DC SPD for PV pumping is specified from the array data, not from the pump motor size. The pump tells you about the AC or motor side; the array and the DC cable tell you about the DC side, and the two are selected separately even though they sit in the same enclosure group.

A generic DC SPD — tested for a rectifier supply or a battery system — may look similar on a datasheet, but its behaviour under PV conditions has not been established by its test basis. That does not mean it fails; it means the fit has to be proven case by case, which is a different level of assurance from a device that declares its PV ratings directly.

Selection and application on the DC side follow IEC 61643-32, which also covers the AC side of the PV system up to the connection point. One scope boundary worth noting: SPDs for PV systems with energy storage, such as batteries or capacitor banks, are not covered by the PV test basis, so a pumping site that adds battery storage is a case where the selection is re-examined rather than extended.

Ucpv, polarity and short-circuit current

Three fields decide most DC SPD selections. The first is Ucpv, the maximum continuous operating voltage on the DC side. It is checked against the string open-circuit voltage at the lowest expected site temperature, because that is when the voltage is highest — selecting against the nominal operating voltage underrates the device.

The second is polarity and topology. The SPD is matched to how the strings and the inverter are connected, including whether the DC side is earthed and how. The third is short-circuit and follow-current behaviour, which matters because a PV source sustains its short-circuit current in a way a battery or rectifier does not, and the device's end-of-life behaviour has to be consistent with that source.

Long DC cable runs and both-end protection

Because the DC cable is long, protection is usually considered at both ends rather than at a single point. A device only at the combiner box protects the array side; the controller still sees the residual that arrives along the cable. Two devices, coordinated so that they share the duty rather than simply duplicating it, are the common arrangement in open-field pumping.

The installation detail matters as much as the rating. Short connection leads to the earth point, correct sealing for an outdoor enclosure, and terminal torque are what keep the installed protection close to the datasheet value. This is the same discipline as any power installation, but the consequences of cutting corners are higher in a field enclosure that is visited rarely.

Climate adds its own requirements. Dust, moisture and wide temperature swings all reach the enclosure before they reach the device, so the enclosure rating and the terminal treatment are part of the selection, not an afterthought. A device that performs correctly on a clean indoor test bench can still fail early in a field cabinet if the sealing does not match the location.

Maintenance planning closes the loop. Remote sites argue for a device whose status can be read at the controller — a spare module held on site and a planned visit beats a return trip after a fault. The maintenance approach is part of the selection conversation, because a protection device nobody can see is only half specified.

DC selection checklist

The checklist summarises selection logic for the DC side. Ratings are confirmed against the applicable datasheets and the site electrical documentation; it is not a substitute for the IEC 61643-32 selection process for a specific project.

Two of the fields deserve emphasis because they are easy to get subtly wrong. The temperature correction on Voc is often forgotten: a string specified at its nominal voltage can exceed a marginal Ucpv on a cold morning. And the installation position is not interchangeable — a device sized for the combiner box and one sized for the inverter input are selected against different points in the circuit, even if the two numbers look similar.

Earthing reality in open-field sites

The earthing discussion deserves its own heading because it is where open-field installations most often fall short. A solar water pump in a remote location may have a single rod at the pump and a long DC cable to an array that is earthed separately, or not at all. The SPD does not create an earth; it works against the reference the installation provides.

Where the earthing is weak, the selection and the bonding design are reviewed together. Sometimes the right answer is a modest improvement to the earthing before the SPD can perform as rated — a point worth raising early, because it affects both cost and lead time. Our solar PV solutions page describes how the DC SPD, the earthing and the system layout are treated as one design rather than three separate purchases.

Selecting a solar water pump SPD on the DC side therefore comes down to a short chain of decisions: confirm the array voltage and configuration, confirm the earthing reference, then place the device where an exposed conductor meets a sensitive input. The device rating is the last of these steps, not the first, which is a useful habit for any remote-site project.

Engineering review note. This article describes DC SPD selection logic for agrivoltaic and solar water pumping installations, referencing IEC 61643-31 and IEC 61643-32. It does not constitute a project-specific selection. Ucpv, polarity, short-circuit behaviour, discharge ratings and installation details are confirmed against the module datasheets, the site electrical documentation and the SPD datasheets before any device is specified.

FAQ

What does Ucpv mean and why does it matter?

Ucpv is the maximum continuous operating voltage for the DC side of a PV installation. It is checked against the string open-circuit voltage at the lowest site temperature, which is when the voltage is highest.

Can I use any DC SPD on a solar pump system?

Functionally a DC SPD may work, but a PV-specific device carries a test basis (IEC 61643-31) that covers PV source behaviour. A generic device's fit has to be proven case by case, so it is not a like-for-like substitution.

Do I need protection at both ends of the DC cable?

Where the cable run is long, protection is usually considered at both the combiner box and the controller/inverter input, coordinated so the two devices share the duty. A single device protects only one end.

What if the site has no proper earthing?

The SPD works against the reference the installation provides, so a weak earth limits what the device can achieve. The bonding design and device selection are reviewed together, and improving the earthing is often part of the answer.

Which standards apply to the DC side of PV?

IEC 61643-31 sets requirements and test methods for DC-side PV SPDs, and IEC 61643-32 covers selection and application principles. Both are referenced when specifying the DC side of a pumping or agrivoltaic system.

What information do you need for a DC SPD check?

The PV array voltage (Voc and maximum system voltage), the DC cable length and routing, and the inverter or controller input details. Send them through our contact page and we will confirm the selection.