DC Surge Protection Devices for PV Installations
Because DC Surge Protection Devices for PV Installations must be designed to provide full exposure to the sunlight, they are highly vulnerable to the effects of lightning. The capacity of a PV array is directly related to its exposed surface area, so the potential impact of lightning events increases with system size. Where lighting occurrences are frequent, unprotected PV systems can suffer repeated and significant damage to key components. This results in substantial repair and replacement costs, system downtime and the loss of revenue. Properly designed, specified and installed surge protection devices (SPDs) minimize the potential impact of lightning events when used in conjunction with engineered lightning protection systems.
A lightning protection system that incorporates basic elements such as air terminals, proper down conductors, equipotential bonding for all current-carrying components and proper grounding principles provides a canopy of protection against direct strikes. If there is any concern of lightning risk at your PV site, I highly recommend hiring a professional electrical engineer with expertise in this field to provide a risk assessment study and a protection system design if necessary.
It is important to understand the difference between lightning protection systems and SPDs. A lightning protection system’s purpose is to channel a direct lightning strike through substantial current-carrying conductors to earth, thus saving structures and equipment from being in the path of that discharge or being directly struck. SPDs are applied to electrical systems to provide a discharge path to earth to save those systems’ components from being exposed to the high-voltage transients caused by the direct or indirect effects of lightning or power system anomalies. Even with an external lightning protection system in place, without SPDs, the effects of lightning can still cause major damage to components.
For the purposes of this article, I assume that some form of lightning protection is in place and examine the types, function, and benefits of the additional use of appropriate SPDs. In conjunction with a properly engineered lightning protection system, the use of SPDs at key system locations protects major components such as inverters, modules, equipment in combiner boxes, and measurement, control, and communications systems.
The Importance of SPDs
Aside from the consequences of direct lightning strikes to the arrays, interconnecting power cabling is very susceptible to electromagnetically induced transients. Transients directly or indirectly caused by lightning, as well as transients generated by utility-switching functions, expose electrical and electronic equipment to very high overvoltages of very short duration (tens to hundreds of microseconds). Exposure to these transient voltages may cause a catastrophic component failure that may be noticeable by mechanical damage and carbon tracking or be unnoticeable but still cause an equipment or system failure.
Long-term exposure to lower-magnitude transients deteriorates dielectric and insulation material in PV system equipment until there is a final breakdown. In addition, voltage transients may appear on measurement, control and communication circuits. These transients may appear to be erroneous signals or information, causing equipment to malfunction or shut down. The strategic placement of SPDs mitigates these issues because they function as shorting or clamping devices.
Technical Characteristics of SPDs
The most common SPD technology used in PV applications is the metal oxide varistor (MOV), which functions as a voltage-clamping device. Other SPD technologies include the silicon avalanche diode, controlled spark gaps, and gas discharge tubes. The latter two are switching devices that appear as short circuits or crowbars. Each technology has its own characteristics, making it more or less suitable for a specific application. Combinations of these devices can also be coordinated to provide more optimal characteristics than they offer individually. Table 1 lists the major SPD types used in PV systems and details their general operating characteristics.
An SPD must be able to change states quickly enough for the brief time a transient is present and to discharge the magnitude of the transient current without failing. The device must also minimize the voltage drop across the SPD circuit to protect the equipment it is connected to. Finally, SPD function should not interfere with the normal function of that circuit.
SPD operating characteristics are defined by several parameters that whoever is making the selection for the SPDs must understand. This subject requires more details that can be covered here, but the following are some parameters that should be considered: maximum continuous operating voltage, ac or dc application, nominal discharge current (defined by a magnitude and waveform), voltage-protection level (the terminal voltage that is present when the SPD is discharging a specific current) and temporary overvoltage (a continuous overvoltage that can be applied for a specific time without damaging the SPD).
SPDs using different component technologies can be placed in the same circuits. However, they must be selected with care to ensure energy coordination between them. The component technology with the higher discharge rating must discharge the greatest magnitude of the available transient current while the other component technology reduces the residual transient voltage to a lower magnitude as it discharges a lesser current.
The SPD must have an integral self-protecting device that disconnects it from the circuit should the device fail. To make this disconnection apparent, many SPDs display a flag that indicates its disconnect status. Indicating the SPD’s status via an integral auxiliary set of contacts is an enhanced feature that can provide a signal to a remote location. Another important product characteristic to consider is whether the SPD utilizes a finger-safe, removable module that allows a failed module to be easily replaced without tools or the need to de-energize the circuit.
AC Surge Protection Devices for PV Installations Considerations
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 Devices for PV Installations Considerations
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 on 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 a 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.
Nonpower System Surge Protection Considerations
Just as power system equipment and components are susceptible to the effects of lightning, so is the equipment found in the measurement, control, instrumentation, SCADA and communication systems associated with these installations. In these cases, the basic concept of surge protection is the same as it is on power circuits. However, because this equipment is usually less tolerant of overvoltage impulses and more susceptible to erroneous signals and to being adversely affected by the addition of series or parallel components to the circuits, greater care must be given to the characteristics of each SPD added. Specific SPDs is called for according to whether these components are communicating through twisted pair, CAT 6 Ethernet or coaxial RF. In addition, SPDs selected for nonpower circuits must be able to discharge the transient currents without failure, to provide an adequate voltage protection level and refrain from interfering with the system’s function—including series impedance, line-to-line and ground capacitance and frequency bandwidth.
Common Misapplications of SPDs
SPDs have been applied to power circuits for many years. Most contemporary power circuits are alternating current systems. As such, most surge protection equipment has been designed for use in ac systems. The relatively recent introduction of large commercial and utility-scale PV systems and the increasing number of systems deployed has, unfortunately, led to the misapplication to the dc side of SPDs designed for ac systems. In these cases, the SPDs perform improperly, especially during their failure mode, due to the characteristics of dc PV systems.
MOVs provide excellent characteristics for serving as SPDs. If they are rated properly and applied correctly, they perform in a quality manner for that function. However, like all electrical products, they may fail. Failure can be caused by ambient heating, discharging currents that are greater than the device is designed to handle, discharging too many times or being exposed to continuous over-voltage conditions.
Therefore, SPDs are designed with a thermally operated disconnecting switch that separates them from the parallel connection to the energized dc circuit should that become necessary. Since some current flows through as the SPD enters failure mode, a slight arc appears as the thermal disconnect switch operates. When applied on an ac circuit, the first zero crossing of the generator-supplied current extinguishes that arc, and the SPD is safely removed from the circuit. If that same ac SPD is applied to the dc side of a PV system, especially high voltages, there is no zero crossing of the current in a dc waveform. The normal thermally operated switch cannot extinguish the arc current, and the device fails.
Placing a parallel fused bypass circuit around the MOV is one method to overcome the extinguishing of the dc fault arc. Should the thermal disconnect operate, an arc still appears across its opening contacts; but that arc current is redirected to a parallel path containing a fuse where the arc is extinguished, and the fuse interrupts the fault current.
Upstream fusing ahead of the SPD, as may be applied on ac systems, is not appropriate on dc systems. The short-circuit available current to operate the fuse (as in an overcurrent protection device) may not be sufficient when the generator is at reduced power output. As a consequence, some SPD manufacturers have taken this into consideration in their design. UL has modified its earlier standard by its supplement to the latest surge protection standard—UL 1449. This third edition is specifically applicable to PV systems.
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