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PV protection from electrical surges

PV systems can come at a price. Electrical surges and lightening strikes can cause catastrophic damage. But implementing a low-cost surge protection system can help avoid such problems, while expanding system life expectancy and doubling your payback

PV protection systems prevent damage caused from electrical surges
PV protection systems prevent damage caused from electrical surges

Despite the tough economic conditions, solar PV is forecast for massive growth worldwide. Ensuring safe integration with grid networks is paramount and critical to this is protection against electrical surges.

Electroctechnical company DEHN, which specialises in protection and safety products, says: “PV systems are especially threatened by lightning discharges during thunderstorms. Causes for surges in PV systems are inductive or capacitive voltages deriving from lightning discharges, as well as lightning surges and switching operations in the upstream power supply system.” Left unchecked, a voltage surge can “have serious consequences for the operation of the system,” says DEHN. “High repair costs, for example, those of the inverter, have a negative effect and, secondly, the system failure can result in considerable profit cuts for the operator of the plant.”

UK firm Surge Protection Devices (SPD) agrees. The company says: “Damages due to overvoltages can be catastrophic and the payback period of replacing damaged equipment can double.” Many manufacturers will guarantee a system life expectancy of greater than 20 years, while the inverter is generally guaranteed for only five to ten years.

But firm Citel says many PV systems don’t reach maturity. This is due to the exposed nature of these applications and interconnection back to the AC utility grid. The French firm, which specialises in transient voltage surge suppression products and components, says: “Solar PV arrays, with their metallic frame and mounted in the open, or on roof tops, act as a very good lightning rod. For this reason, it is prudent to invest in surge protection to eliminate these potential threats and maximise the system’s life expectancy.

The company says the cost for a comprehensive surge protection system is less than 1 per cent of the total system expenditure. Citel says comprehensive risk assessments prior to proceeding with a PV project are vital. These should cover the following four areas: “geographic risk” because areas with severe lightning and unstable utility power are more vulnerable; “application surface area risk” because the greater the surface area of the solar PV array, the more exposure to direct and/or induced lightning surges; “power interconnection risk” because the AC utility grid is a likely source of switching transients and/or induced lightning surges; and “operational downtime risk” because consequences of system downtime are not only limited to equipment replacement.

Additional losses can result from lost orders, idle workers, overtime, customer or management dissatisfaction, expedited freight charges and expedited shipping costs. Citel says all available power and communication networks should be addressed with surge protection to eliminate PV system vulnerabilities.

This includes the primary AC utility power supply, inverter AC output, inverter DC input, PV string combiner and other related data or signal lines, such as RS-485, 4-20mA current loop, PT-100, RTD and telephone modems. Implementing an earthing system from the outset is essential best practice too.

“Surge protectors shunt transients to the earth grounding system,” the company says. “A low impedance ground path, at the same potential, is critical for the surge protectors to function properly. All power systems, communication lines, grounded and ungrounded metallic objects need to be equipotentially bonded for the protection scheme to work efficiently.”

Firm MHHsolartechnik, which has a range of dedicated surge protectors designed to provide lightning and surge protection on both the DC and AC current sides of a system, agrees. The company says one of the more important safety considerations when installing a photovoltaic system is effective external and internal lightning protection. A lightning protection system for photovoltaic arrays should include components that protect against direct lightning strikes (external protection), as well as mechanisms to protect system components from electrical surges (internal protection).

MHHsolartechnik says: “External lightning protection involves intercepting direct lightning strikes using a lightning rod and diverting the energy to the ground, without damaging the building or the photovoltaic components. Internal surge protection prevents dangerous electrical surges from travelling through the wires from the photovoltaic system. During the planning phase it is also important to determine what kinds of lightning and surge protection your system insurer requires.” Citel adds that the connection between the external Solar PV Array and the internal power control equipment should be underground or electrically shielded to limit the risk of direct lightning strikes and/or coupling.

Surge arrestors A key defence for PV systems against the impacts of overvoltages are surge arrestors. UK solar installation firm Geosolar, says: “Good system design, shielding, earthing and bonding may not always provide adequate protection. Surge arrestors can provide expensive or sensitive electronic equipment with a last defence to such damage.”

Surge arrestors generally act as voltage limiting devices, switching to a low internal resistance once a threshold voltage is exceeded. “Connected between a conductor and earth under normal conditions, no current will flow,” Geosolar says. “However, surges above a clamping voltage are shorted to earth. Clamping voltages will vary for any arrestor depending upon the current being conducted.” Various types of arrestor are available: silicon oxide varistors (SOV); metal oxide varistors (MOV); and silicon avalanche diodes (SAD). Geosolar says MOVs have been commonly used to provide protection.

However, they degrade over time, consume small amounts of power and can catch fire unless fuse protected. DC rated SOVs don’t have these problems and are available in single simple to mount packages ready to connect between DC positive, DC negative and earth. Similar units are also available for an AC supply. A large number of systems have surge suppression built into the PV systems on both the DC array side and the AC inverter side. “The risk of induced voltages and transients along the incoming service cable is greater, especially in rural areas where supply is by long overhead lines,” Geosolar says. “As such, surge suppression devices are best fitted at the main incoming point of AC supply, if the risk of surges and transients merits it.

This makes surge protection more effective in being located at the main likely point of entry and access to the incoming conductors and main earthing terminal more convenient.” Arrestors can be fitted at the inverter end of the DC cabling, or at both ends for systems with a long DC cable run. They can also be fitted to the AC cabling either at the inverter or at the supply point. Geosolar says, where used to protect equipment, maximum protection is achieved by fitting the arrestors as close as is practical to the device. These devices however must be electrically safe, and pose no electrical fire hazard. Backed up and fuseless Firm Carlo Gavazzi extended its solar energy portfolio earlier this year, with the introduction of surge protection devices that provide backup without internal fuses.

The company says: “The DSF surge arrestors embody a special internal backup that ensures they remain safe at the end of varistor life, thus ensuring the safest transient overvoltage protection for combiner boxes in photovoltaic systems.” Carlo Gavazzi’s DSF surge arrestors comply with new and emerging standards for DC-side surge protection in PV systems, in particular the latest French UTE C 61-740-51 and EN 50539-11. The standards specify that surge protection devices must include backup protection and prohibit the use of fuses for this purpose. The firm says three of its DSF models take the maximum nominal voltage up to 1200V DC.

They are available in two or three-pole configurations and provide both differential and common-mode protection. Built-in technologies cater for both long duration temporary overvoltage and instantaneous surges lasting microseconds. Carlo Gavazzi says the former is addressed by a path combining a current limiter with varistor, while faster events are routed to a path with varistor and gas tube surge arrester. Internal varistors are protected from the leakage current that could damage and age them when the nominal DC voltage of a plant is temporary exceeded. During mornings when solar panels are cold, for example. The company says when the varistor nears the end of its life, DSF technology ensures the arrester is intrinsically safe with regard to short-circuit behaviour.

DSF arrestors are based on removable cartridges, which can be replaced without leaving the installation unprotected. Exhausted cartridges are indicated by a red optical window on the front panel and a voltage-free contact is provided for remote function monitoring. Carlo Gavazzi says internal wiring and labeled terminals contribute further to quicker, simpler installation and maintenance.