AC Surge Protection Considerations
*Image: 3Phase & 1Phase AC SPDs
Lightning flashes from clouds to the lightning protection system, the PV structure or a nearby ground cause a local ground-potential rise with regard to distant ground references. Conductors spanning these distances expose equipment to significant voltages. The effects of ground-potential rises are primarily experienced at the point of connection between a grid-tied PV system and the utility at the service entrance—the point where the local ground is electrically connected to a distant referenced ground.
Surge protection should be placed at the service entrance to protect the utility side of the inverter from damaging transients. The transients seen at this location are of a high magnitude and duration and therefore must be managed by surge protection with appropriately high-discharge current ratings. Controlled spark gaps used in coordination with MOVs are ideal for this purpose. Spark gap technology can discharge high lightning currents by providing an equipotential bonding function during the lightning transient. The coordinated MOV has the ability to clamp the residual voltage to an acceptable level.
In addition to the effects of ground-potential rise, the ac side of the inverter may be affected by lightning-induced and utility-switching transients that also appear at the service entrance. To minimize potential equipment damage, appropriately rated ac surge protection should be applied as close to the ac terminals of the inverter as possible, with the shortest and straightest route for conductors of sufficient cross-sectional area. Not implementing this design criterion results in higher-than-necessary voltage drop in the SPD circuit during discharge and exposes the protected equipment to higher transient voltages than necessary.
DC Surge Protection Considerations
*Image: Type2 1000v DC SPD
Direct strikes to nearby grounded structures (including the lightning protection system), and inter- and intra-cloud flashes that may be of magnitudes of 100 kA can cause associated magnetic fields that induce transient currents into PV system dc cabling. These transient voltages appear at equipment terminals and cause insulation and dielectric failures of key components.
Placing SPDs at specified locations mitigates the effect of these induced and partial lightning currents. The SPD is placed in parallel between the energized conductors and ground. It changes state from a high-impedance device to a low-impedance device when the overvoltage occurs. In this configuration, the SPD discharges the associated transient current, minimizing the overvoltage that would otherwise be present at the equipment terminals. This parallel device does not carry any load current. The selected SPD must be specifically designed, rated and approved for application on dc PV voltages. The integral SPD disconnect must be able to interrupt the more severe dc arc, which is not found in ac applications.
Connecting MOV modules in a Y configuration is a commonly used SPD configuration on large commercial and utility-scale PV systems operating at a maximum open-circuit voltage of 600 or 1,000 Vdc. Each leg of the Y contains an MOV module connected to each pole and to ground. In an ungrounded system, there are two modules between each pole, and between both pole and ground. In this configuration, each module is rated for half the system voltage, so even if a pole-to-ground fault occurs, the MOV modules do not exceed their rated value.
In spite of the high lightning risk that many PV installations are exposed to, they can be protected by the application of SPDs and a properly engineered lightning protection system. Effective SPD implementation should include the following considerations:
- Correct placement in the system
- Termination requirements
- Proper grounding and bonding of the equipment-ground system
- Discharge rating
- Voltage protection level
- Suitability for the system in question, including dc versus ac applications
- Failure mode
- Local and remote status indication
- Easily replaceable modules
- Normal system function should be unaffected, specifically on non-power systems