DC Surge Protector

DC surge protectors (DC SPDs) limit transient overvoltages and safely redirect surge currents to ground. Under normal conditions, they remain at high impedance. During a surge, they switch to low impedance, diverting excess current and automatically resetting after the event to provide continuous DC surge protection.

A: The lifespan of a DC surge protector depends on the number and intensity of surges it encounters. Regular inspection for signs of wear or end-of-life indicators is recommended. Replacing SPDs proactively ensures continuous DC surge protection and prevents equipment damage.

A: Yes, DC SPDs are suitable for a wide range of applications, from residential solar systems to industrial DC power grids. Selecting the appropriate type and rating ensures effective DC surge protection tailored to each environment.

Surge Protective Devices (SPDs) provide protection against electrical surges and spikes, including those caused directly and indirectly by lightning.

At locations with frequent lightning, unprotected PV systems will suffer repeated and significant damages. This results in substantial repair and replacement costs, system downtime and loss of revenue.

Properly installed surge protective devices (SPDs) will minimize the potential impact of lightning events.

Sensitive electrical equipment of PV systems like AC/DC Inverter, monitoring devices, and PV array must be protected by surge protective devices (SPD).

A surge protective device (SPD) is designed to prevent higher energy voltage peaks from reaching sensitive equipment and thus potentially causing damage.

If properly designed, how does an SPD work in a DC system?

Excess voltage (beyond the rating of the equipment) is prevented from building up by controlled energy discharge between the affected DC or AC conductors.

If a ground connection is present on the SPD, the SPD also monitors voltage differential between the ground and the other conductors.

If necessary, energy is discharged to prevent excessive voltage differences such as in a surge event. For this to work properly, the path to the ground must be of low resistance.

SPDs cannot protect from prolonged over-voltage for multiple seconds or minutes. This must be prevented by correct system sizing.

1. Make sure your system and SPD has a good, low-resistance connection to the ground.

2. Match the surge protection device to the inputs of your power conversion equipment you want to protect by ensuring the “U_c” voltage in the surge protection device datasheet is at or just slightly (preferably 0 to 10 V) above the maximum continuous voltage on the conductors to be protected, or the maximum voltage rating of the power equipment connected.

If the SPD’s “U_c” rating is well above the maximum voltage rating of the power equipment connected, it can no longer effectively protect from voltage surges. The SPD will protect devices or equipment by activating well above the maximum continuous operating voltage “U_c” and will not interfere at voltages below “U_c”.

3. LSP recommends protecting at least the PV input of the charge controller or inverter/charger and if using a public electric grid, protect the AC input as well.

4. If used on the PV conductors, ensure the surge protective device is rated for DC voltages, if used on the AC input, ensure the SPD is rated for AC voltages.

Surge protective devices help reduce downtime occurring from surges. At PV plants, SPDs have to fulfill specific requirements to ensure continuous operation and energy generation.

When designing a PV plant, it is important to consider the installation of surge protection devices (SPDs). Surges and network disturbances can lead to downtime, reducing the performance of the plant.

Therefore, any conditions affecting energy generation and distribution should be taken into consideration when designing the electrical installation.

Solar panels are installed outside to convert solar energy into electricity. This outdoor location makes them directly exposed to harsh conditions like rain, wind and dust. Among the weather conditions lightning strikes require specific attention as they can severely affect the safety and performance of a PV plant.

They originate in a cumulonimbus cloud and terminate on the ground. When the lightning strike hits the ground, it discharges energy, affecting the electrical field on the ground. For the solar PV plant this poses two risks:

• A direct impact which can physically destroy the solar equipment on a rooftop
• Transitory overvoltages passing through cables by magnetic coupling, which can lead to the damage of sensitive components such as printed circuit boards (PCB).

As far as the direct impact is concerned, ‘External Lightning Protects’ (ELP) provides the required protection according to IEC 62305, which describes how to evaluate if your location needs such protection, and what should be the preferred option (meshed cages, air terminal, etc.).

The concept is simple: make sure the lightning will strike a metallic rod installed on the highest point of your plant and dispel the energy directly to the ground through a copper down conductor.

When it comes to transitory overvoltages, however, SPDs are required. They are installed in parallel into the circuit protection boards to divert the energy to the ground and limit the overvoltage up to such value acceptable to the end equipment.

As soon as ELP is installed at a PV plant, it is mandatory that an SPD be installed, too. If the PV plant is not equipped with an ELP, the installation of an SPD is highly recommended to limit network perturbations (transient overvoltages).

To guarantee the energy will flow to the ground first to limit overvoltages the most important component is the Metal Oxide Varistor (MOV).

This component has such propriety that in normal conditions (no overvoltages) the resistance is high enough to not make possible nominal currents passing through it.

Starting at a certain overvoltage level, the resistance will quickly drop, opening the path to the ground and coming back to a normal condition once the energy has been dissipated.

This process allows a limitation of the overvoltage level reaching all equipment connected downstream.

There are different types of SPDs available which vary in terms of resistance: Type 1, Type 2, and Type 1+2. A Type 1 SPD can cope with a direct strike which brings an energetic surge, whereas Type 2 limits overvoltages from various sources. Both characteristics can be combined into a “Type 1+2” for complete protection.

In PV plants the challenge is to choose the appropriate surge protection to withstand pure energy 10/350 µs waveform currents (almost 10 times more powerful than type 2 of 8/20 µs waveform) while at the same time taking space into consideration.

In an inverter or junction box space always is top priority. To maximize the available space, LSP’s SPDs use the depth of the enclosure for stronger components with an increased depth of the device.

With the new FLP-PV & SLP-PV series, both AC and DC circuit protection boards in solar installations can be protected against overvoltages due to lightning strikes or network disturbances.

Solar arrays, like all electronic devices, are prone to surges in voltage that can harm components and increase downtime. Surge protection devices can help keep systems running and profitable.

A surge protector helps prevent damage to electronics by diverting the extra electricity from the “hot” power line into a grounding wire.

In most common surge protectors, this is achieved through a metal oxide varistor (MOV), a piece of metal oxide joined to the power and grounding lines by two semiconductors.

Solar arrays are also electronic devices and so are subject to the same potential for damage from surges. Solar panels are especially prone to lightning strikes due to their large surface area and placement in exposed locations, such as on rooftops or ground-mounted in open spaces.

If the solar panels are struck directly, lightning can burn holes in the equipment or even cause explosions, and the entire system is destroyed.

But the effects of lighting and other overvoltages aren’t always so strikingly apparent. The secondary effects of these events can not only affect major components such as modules and inverters, but also monitoring systems, tracker controls and weather stations.

Loss of a PV module will only mean loss of a string, while loss of central inverter will mean loss of the power generation for a large section of the plant.

Because all electrical equipment is susceptible to surges, SPDs are available for all solar array components. The industrial versions of these devices also use metal oxide varistors (MOV) in combination with other sophisticated equipment to conduct surge overvoltages to grounding. Therefore, SPDs are generally installed after a stable grounding system is in place.

Think of an electrical single-line diagram of your installation and cascade SPDs from the utility service to the array equipment, locate robust protection on main entrances to protect against large surge transients and smaller units down critical paths to the equipment end-point.

An SPD network should be installed throughout the solar array’s AC and DC power distribution to protect critical circuits. SPDs should be installed on both the DC inputs and AC outputs of the system’s inverter(s) and be deployed with reference to ground on both the positive and negative DC lines. AC protection should be deployed on each power conductor to the ground. Combiner circuits should also be protected, as should all control circuits and even tracking and monitoring systems to prevent interference and data loss.

When it comes to commercial and utility-scale systems, LSP suggests using the 10m rule. For installations with DC cable lengths under 10 m), DC solar surge protection should be installed at a convenient point such as at inverters, combiner boxes or closer to the solar modules. For installations with DC cabling over 10 m, surge protection should be installed at both the inverter and module ends of the cables.

Residential solar systems with microinverters have very short DC cabling, but longer AC cables. An SPD installed at the combiner box can protect the home from array surges. An SPD on the main panel can protect the home from array surges as well, in addition to those from utility power and other internal equipment.

In any size system, SPDs should be installed by a licensed electrician in accordance with manufacturer recommendations and installation and electrical codes to maximize safety and effectiveness.

Additional steps, such as adding lightning air terminals, can be taken to further protect a solar array specifically from lightning. SPDs can’t prevent physical damage from direct lightning strikes.

Overvoltage may occur in electrical installations for various reasons. It may be caused by:

• The distribution network as a result of lightning or any work carried out.
• Lightning strikes (nearby or on buildings and PV installations, or on lightning conductors).
• Variations in the electrical field due to lightning.

Like all outdoor structures, PV installations are exposed to the risk of lightning which varies from region to region. Preventive and arrest systems and devices should be in place.

The first safeguard to put in place is a medium (conductor) that ensures equipotential bonding between all the conductive parts of a PV installation.

The aim is to bond all grounded conductors and metal parts and so create equal potential at all points in the installed system.

L is the sum of:

• the sum of distances between the inverter(s) and the junction box(es), taking into account that the lengths of cable located in the same conduit are counted only once, and
• the sum of distances between the junction box and the connection points of the photovoltaic modules forming the string, taking into account that the lengths of cable located in the same conduit are counted only once.

Ng is arc lightning density (number of strikes/km2/year).

The number and location of SPDs on the DC side depend on the length of the cables between the solar panels and inverter. The SPD should be installed in the vicinity of the inverter if the length is less than 10 metres. If it is greater than 10 metres, a second SPD is necessary and should be located in the box close to the solar panel, the first one is located in the inverter area.

To be efficient, SPD connection cables to the L+ / L- network and between the SPD’s earth terminal block and ground busbar must be as short as possible – less than 2.5 metres (d1+d2<50 cm).

Safe and reliable photovoltaic energy generation

Depending on the distance between the “generator” part and the “conversion” part, it may be necessary to install two surge arresters or more, to ensure protection of each of the two parts.

When a PV system is located on an industrial site, the business operations and equipment are also at jeopardy. Inverters are expensive, but for industrial applications, an even more expensive failure is the cost of downtime.

When lightning strikes a solar PV system, it causes an induced transient current and voltage within the solar PV system wire loops.

These transient currents and voltages will appear at the equipment terminals and likely cause insulation and dielectric failures within the solar PV electrical and electronics components such as the PV panels, the inverter, control and communications equipment, as well as devices in the building installation.

The array box, the inverter, and the MPPT (maximum power point tracker) device have the highest points of failure.

To prevent high energy from passing through electronics and causing high voltage damage to the PV system, voltage surges must have a path to ground.

To do this, all conductive surfaces should be directly grounded and all wiring that enters and exits the system (such as Ethernet cables and ac mains) be coupled to ground through an SPD.

A surge protection device is needed for each group of the strings within the array box, the combiner box, as well as the dc disconnect.

Height, pointed shapes, and isolation are the dominant characteristics that determine where lightning strikes. It is a myth that metal attracts lightning.

However, it is important to note that no matter where the PV farm is located, or the shape of any nearby objects, SPDs are essential for every PV system due to their inherent susceptibility to direct and indirect strikes.

Click edit button to change this text. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.

• the lightning round flash density;
• the system’s operating temperature;
• the system’s voltage;
• the system’s short circuit current rating;
• the level of waveform that is to be protected against (indirect or direct lightning);
• the nominal discharge current.

The SPD requirements for an installation that is protected by an external lightning protection system (LPS) depend on the selected class of the LPS and whether the separation distance between the LPS and the PV installation is isolated or non-isolated.

IEC 62305-3 details the separation distance requirements for an external LPS.

To have a protective effect, an SPD’s voltage protection level (U_p) should be 20 % lower than the dielectric strength of the system’s terminal equipment.

It is important to use an SPD with a short circuit withstand current greater than the short circuit current of the solar array string that the SPD is connected to.

The SPD that is provided on the dc output must have a dc MCOV equal to or greater than the maximum photovoltaic system voltage of the panel.

When lightning strikes at point A (see Figure 1), the solar PV panel and the inverter are likely to be damaged. Only the inverter will be damaged if the lightning strikes at point B.

However, the inverter is typically the most expensive component within a PV system, which is why it is essential to properly select and install the correct SPD on both the ac and dc lines. The closer the strike is to the inverter, the more damaged the inverter will be.

PV sources have very different current and voltage characteristics than traditional dc sources: they have a non-linear characteristic and cause long-term persistence of ignited arcs.

Therefore, PV current sources not only require larger PV switches and PV fuses, but also a disconnector for the surge protective device which is adapted to this unique nature and capable of coping with PV currents.

SPDs installed on the dc side must always be specifically designed for dc applications. The use of an SPD on the incorrect ac or dc side is hazardous under fault conditions.

When SPDs are used on the dc side, they must also be used on the ac side due to the potential differences.

Surge protection is just as important for the ac side as it is for the dc side. Ensure that the SPD is specifically designed for the ac side.

For optimal protection, the SPD should be sized specifically for the system. The proper selection will guarantee the best protection with the longest lifespan.

On the ac side, multiple inverters can be connected to the same SPD if they share the same grid connection.

SPDs should always be installed upstream of the devices they are going to protect. NFPA 780 12.4.2.1 says that surge protection shall be provided on the dc output of the solar panel from positive to ground and negative to ground, at the combiner and combiner box for multiple solar panels, and at the ac output of the inverter.

The proper installation of an SPD relies on three values, which are:

• Maximum continuous operating voltage: The voltage that the SPD will activate.
• Voltage protection level: The equipment’s overvoltage category must be higher than the SPD’s voltage protection level.
• Nominal discharge current: The peak value of waveform (8/20 μs for type 2 SPDs) that the SPD is capable of withstanding after repetitive surges.

Cables

The cables in PV systems are often extended across long distances so that they can reach the grid connection point. However, long cable lengths are never recommended, and PV systems are far from an exception.

This is because the effect of field-based and conducted electrical interference that is caused by lightning discharges increases in relation to increasing cable lengths and conductor loops. When a transient overvoltage occurs, any inductive voltage drop in the connecting cables can weaken the SPD’s protective effect. This is less likely to happen if the cables are routed to be as short as possible.

Surge voltage is a significant contributor to cable failure, and each impulse on a cable will contribute to the deterioration of the cable’s insulation strength.

If a surge is injected into a stand-alone PV system (a system that is far from the power grid), any equipment operations that are powered by solar electricity, such as medical equipment or water supply, may be disrupted.

The location and quantity of SPDs to install on the dc side depend on the length of cable between the solar panels and the inverter (see Table).

If the length is less than 10 meters, then only one SPD is necessary and the SPD should be installed within the same vicinity as the inverter. If the length of the cable is more than 10 meters, then install one SPD within the vicinity of the inverter as well as a second SPD in the box that is close to the solar panel.

Route cables in such a way that avoids large conductor loops. Ac and dc lines and data lines must be routed together with the equipotential bonding conductors along the entire route to ensure that conductor loops are not formed from being routed over several strings or when connecting the inverter to the grid connection.

Note:

The length of cable connecting an SPD to the load should always be as short as possible and never more than 10 meters long. If the cable length is longer than 10 meters, a second SPD is necessary. The greater the distance, the greater the reflection of the lightning wave.

How to Combine SPDs with Inverters

PV farms are comprised of very sensitive equipment that needs expansive protection. Because PV farms create direct current (dc) power, inverters (which are necessary to convert this power from dc to ac) are an essential component to their electrical production.

Unfortunately, inverters are not only highly susceptible to lightning strikes but they are incredibly expensive. NFPA 780 12.4.2.3 requires additional SPDs at the dc input of the inverter if the system inverter is more than 30 meters from the closest combiner or combiner box.

Install the SPD between the fuses and the inverter if there are string protectors (such as fuses, dc breakers or string diodes)

Conclusion

To operate photovoltaic equipment without proper surge protection is more than risky business – it is reckless.

For solar systems to be the future of a greener world, they must be protected.

The occurrence of lightning is unstoppable and thus, protection is essential.

Photovoltaic systems’ vulnerability to lightning strikes – both direct and indirect – means that they must be built with reliable and properly installed surge protection.