Surge Protective Devices (SPD)

Surge Protective Devices prevent impulse overvoltages induced in the photovoltaic field, on the grounding system, conducted via the AC power network or signal lines, from damaging electronic equipment.

The Cabur range of surge protectors includes varistor cartridges and gas-filled cartridges, for protection in single-phase, three-phase, and DC lines up to 600 V or 1000 V.

Where and How SPDs Should Be Used

In the event of a transient overvoltage, the only way to protect equipment is to limit the potential difference between the various conductors entering or leaving the device. For this reason, in PV systems, surge protection must always be installed on both the DC and AC sides, ensuring equipotentiality between all system conductors—whether the overvoltage comes from the PV field, the AC network, or the ground.

For overvoltages on the PV field, DC-side SPDs create an instantaneous short circuit between the positive, negative, and ground conductors, equalizing them transiently. As a result, the three DC conductors of the inverter may rise to thousands of volts. However, because SPDs limit the potential difference between these three conductors to 4 kV, no damage occurs on the inverter’s DC side, whose impulse overvoltage withstand must exceed 4 kV.

This alone is insufficient to protect the inverter completely: if the three DC conductors rise, for example, to 10 kV, and there are no SPDs on the AC side capable of creating transient equipotentiality with the DC side, the DC side at 10 kV “sees” the 230–400 Vac output of the inverter as a lower potential and discharges through the inverter insulation or components, destroying them. The same occurs if the overvoltage originates from the AC side.

The concept of equipotentiality implies using SPDs on all conductors entering and leaving the inverter, because only by limiting potential differences between DC, AC, and ground within the insulation ratings and impulse withstand of the device can destructive discharges through insulation or components be avoided.

Safe Use of SPDs up to 1,000 Vdc

The varistor, which constitutes the active element of the SPD, can withstand a limited number of discharges. It may fail in short-circuit if subjected to a discharge exceeding its Isc max, or after many smaller discharges have degraded it to the end of life. In this condition, its resistance, normally tens of MΩ, drops to a few hundreds or tens of Ω. The varistor overheats due to the current between line and ground and may catch fire.

Class II SPD standards require a device that disconnects the SPD from the line at the end of its life. The device consists of a series contact on the line side, with terminals soldered in tin, one of which is spring-loaded. When the overheated varistor exceeds the tin melting temperature, the spring-loaded conductor detaches, opening the contact and disconnecting the varistor from the line, preventing damage.

In AC SPDs, this disconnect device can extinguish the arc during the AC zero crossing of the current passing through the failed varistor.

In PV systems, DC voltages from 500 to 1000 V and the absence of zero-crossings make DC arc interruption more difficult because air and surface clearances designed for AC are insufficient. This problem is solved by using three varistors in a “Y” configuration. In the Y configuration, the discharge is split across three varistors instead of the two used in the classic scheme, significantly reducing the likelihood of failure. Even if one varistor fails in a short between line and ground, after the overvoltage passes, the second intact varistor returns to MΩ resistance, cutting the current through the failed varistor contact.

Cabur does not recommend using gas discharge arresters connected to ground on the DC side, because although they provide insulation to ground, in the event of a short or partial short on a varistor, the gas arrester would remain triggered by the DC voltage, causing the string Isc to pass through the varistor, potentially leading to overheating or fire.