A photovoltaic system needs surge protective devices (SPDs) on the DC side, at the inverter input and, depending on the distance, in the field, and on the AC side. DC SPDs are governed by IEC 61643-31 and applied according to IEC 61643-32. Local electrical codes usually require them to be coordinated, installed with short leads, and the lightning risk of rooftop or open-field installations to be assessed.
Why a solar system needs SPDs
A photovoltaic array covers a large area, outdoors and normally at the highest point of its surroundings. A lightning strike, direct or nearby, induces overvoltages in the conductors that run through the array and carries them to the inverter, whose power electronics are sensitive. Switching operations on the AC grid also produce transient overvoltages that arrive on the AC side.
A surge protective device (SPD) permanently connects the overvoltage to ground when it exceeds its protection level, and returns to a high-impedance state once the transient has passed. It protects the equipment downstream, not people or the structure in the event of a direct strike.
Two sides, two standards
DC side. SPDs connected to the DC side of photovoltaic installations are governed by IEC 61643-31:2018, which covers devices for circuits up to 1500 V DC. Its scope does not include systems with battery storage or two-port SPDs with series impedance. IEC 61643-32 gives the selection and application principles for those SPDs. The set of IEC standards for the system is in IEC standards for photovoltaic systems.
AC side. Low-voltage SPDs per IEC 61643-11 are used, installed at the inverter output panel. For coordination, IEC 61643-12 applies on AC and IEC 61643-32 on DC.
Downstream, on the medium-voltage side of the plant, overvoltage protection is provided by surge arresters (metal-oxide arresters) at the transformers and in the substation; the guide on surge arresters explains them.
Types and the data you read
PV SPDs are classified as Type 1, Type 2 or combined, according to the current waveform they are tested with. Type 1 withstands direct-strike impulse currents and is installed where there is external lightning protection. Type 2 withstands induced and switching overvoltages. Two data points on the device datasheet matter most:
- Ucpv: maximum continuous operating voltage of the SPD in the photovoltaic circuit. It must be equal to or higher than the maximum system voltage, that is, the string Voc corrected to the site’s minimum temperature.
- Protection level (Up): the residual voltage it lets through. It must be lower than the impulse voltage the protected equipment withstands.
The SPD also declares the discharge current it withstands, its thermal disconnection and the backup protection it requires. That protection is defined by the manufacturer; for the DC side, see gPV fuses and DC disconnectors.
Where they are installed
- PV array. The strike induces an overvoltage in the string conductors
- DC SPD at the combiner box or in the field. Limits the overvoltage to ground when the run to the inverter is long
- DC SPD at the inverter input. Protects the MPPT inputs of the inverter
- Inverter. Converts to AC with its internal protections
- AC SPD at the panel. Limits switching and grid overvoltages
The general rule is to protect each sensitive piece of equipment at its input and to place the SPD as close as possible. If the distance between the array and the inverter is large, the transient can add voltage along the run, which is why SPDs are installed at both ends. In the field, the DC SPD usually goes inside the string combiner box. The distance that calls for it and the SPD class come from the risk assessment, not from a fixed rule.
Local electrical codes add criteria that apply to both sides, and these are common in practice:
- The distance between the SPD terminals and those of the equipment must be as short as possible, so that inductance is minimal; in low voltage, the distance to the connection should not exceed 50 cm.
- If several stages are installed, a zoning methodology is applied and they are coordinated by energy and not only by current.
- SPDs built only with voltage-switching technology are not installed on power networks, unless coordination with limiting devices shows effective operation.
- The short-circuit rating of the SPD must be coordinated with the fault level of the node where it is installed; if that is not possible, it is justified and a series overcurrent protective device is added.
Risk assessment and shielding
Rooftop, ground-mounted and open-field photovoltaic installations should undergo a lightning risk assessment. The general method is defined by IEC 62305, with the PV-specific criteria of IEC TR 63227. The ground flash density of the site, the size of the structure and its consequences determine whether external protection is needed, of what level, and which SPDs accompany it. Shielding is covered in the guide on lightning shielding, and the effect of overvoltages on the plant equipment in the one on transient overvoltages in the plant.
SPDs discharge to ground: their effectiveness depends on a short equipotential connection and on proper grounding of the array, the structures and the inverter. All exposed metal parts, including module frames and structures, should be bonded to a common equipotential point.
Common mistakes
- AC SPD on DC, or with a Ucpv lower than the maximum string voltage.
- Long or looped connection leads, which cancel the protection level. The routing of string cables is covered in solar cable and MC4 connectors.
- Uncoordinated SPDs with the inverter, without backup protection or with a rating that does not match the node.
- No inspection after a storm: many SPDs have a status indicator or remote contact, and an exhausted SPD leaves the installation unprotected without anyone noticing.
What we do at Tiza Group
Specifying the DC SPDs in the array belongs to the array designer. Our primary engineering team designs, remotely and worldwide, the shielding, the grounding and the insulation coordination of the medium and high voltage part. In the field we commission solar plants, in Colombia or elsewhere on request, with the interconnection protection and the tests the utility requires.
Related guides
References
- IEC 61643-31:2018, Low-voltage surge protective devices, Part 31: Requirements and test methods for SPDs for photovoltaic installations.
- IEC 61643-32:2017, Surge protective devices connected to the d.c. side of photovoltaic installations: Selection and application principles.
- IEC 61643-11 and IEC 61643-12, AC low-voltage SPDs and selection and application principles.
- IEC 62305, Protection against lightning, and IEC TR 63227, lightning risk assessment of photovoltaic systems.
Frequently asked questions
Can an AC SPD be used on the DC side?
No. The DC voltage of a string does not extinguish itself like AC does, and an AC SPD is not tested for it. On the DC side, PV SPDs per IEC 61643-31 are used, with a maximum continuous operating voltage on DC (Ucpv) equal to or higher than the maximum system voltage.
How many SPDs does a solar installation need?
It depends on the distance between the array and the inverter, on the lightning risk of the site and on whether there is external lightning protection. In general the DC side is protected at the inverter and, if the run is long, also in the field or at the combiner box, and the AC side at the inverter output panel. The risk assessment defines how many and of what class.
Does the SPD replace the lightning rod or the shielding?
No. Shielding and external protection conduct a direct strike to ground; the SPD limits the overvoltages that arrive along the conductors. They are measures of one and the same system and are coordinated with each other, following IEC 62305 and the risk analysis of the site.