Lightning and surge protection in photovoltaic (PV) systems has up until now not been regulated by testing Standards. This will change with the upcoming introduction of a Standard that will increase transparency and thus facilitate decision-making on the part of users. In addition, the testing procedures will be aligned significantly closer to the actual operating conditions of the protection components, writes Phoenix Contact.

Because of their exposure to the elements, PV systems are typically subjected to extreme weather conditions, such as the effects of lightning. In order to ensure that the systems can be operated safely and profitably over a long period of time, it is recommended to install lightning and surge protection. Previously, it was often not easy for system planners to find the proper protection components. However, a new industry standard will provide greater transparency and make it easier to compare commercially available components in the future.

prEN 50539-11 – Requirements and Testing for Surge Protection Devices

The testing of surge protection devices for PV installations has not been clearly regulated up to now. In the future, the prEN 50539-11 standard will outline testing procedures for these devices. Previously, testing procedures were typically taken from existing standards that did not take into account the special characteristics of PV systems. Under almost all operating conditions, the panels in a PV generator supply current at an approximately constant level, which is at the same time close to the system’s short-circuit current. The current drops abruptly only if the maximum voltage has been reached.

In contrast, there are the ‘normal’ direct-current sources with linear current-voltage characteristic curve behavior. By comparing these characteristic curves, it becomes clear that the interrupting rating of the surge protection in a PV system must be high enough to handle much higher power levels than in other DC applications (Fig. 2). The power level that must be interrupted in the event of a fault is significantly higher for the PV characteristic curve than for the DC source. In this case, the power is the product of the voltage and current, which corresponds to the area below the characteristic curve. The interrupting rating of the surge protection in a PV system must therefore be approximately twice as great as in other DC applications.

The Standard provides for two scenarios for the failure behavior of the surge protection in the event of a fault. In the first failure behavior option, the protection component interrupts the current path (OCM – open circuit mode). The second option provides for a short circuit (SCM – short-circuit mode). However, since some inverters cannot handle a short circuit at the inputs, the use of a protective component with OCM behavior is recommended here.

Selecting the technical data

Part 11 of the Standard, which was partially explained in the previous section, describes the requirements and tests to be performed on a surge protection device for PV applications. The application standard CLC/TS 50539-12 provides additional information. This Standard defines the technical parameters and the selection of the suitable protection components.

The parameters that are significant for the customer are the highest continuous operating voltage UCPV, the protection level UP, and the discharge capacity of the Type 1 or Type 2 arrester, as well as the failure behavior of the protection components in the event of a fault. The highest continuous operating voltage depends on the application in which the arrester is to be used. Here, the inverter manufacturers’ current values are provided: 1000V and 600V. As defined in the Standard, these values are used to determine the no-load voltage UOCSTC (OC – open circuit; STC – standard test conditions) from which the protection level follows.

The no-load voltage is 1.2 times lower than the highest continuous operating voltage. The reason for this is the fluctuation of the generator no-load voltage, which is caused by temperature fluctuations and changing solar radiation conditions. The factor of 1.2 thus provides a sufficient safety margin. To provide a safe level of protection, the protection level should be approximately 20% lower than the surge voltage capacity of the devices to be protected. If no other information is provided, this surge voltage capacity is assumed to be five times the no-load voltage. For example: UCPV = 1000V means that UOCSTC = 833V. This results in a protection level of UP = 5 * UOCSTC – 20% = 3.33kV.

Selecting the correct protective component

If a PV system is installed on a public building, it must be equipped with external lightning protection in accordance with VDE 0100-443 and VDE 0185-305 as well as the relevant regional building codes. In addition, the German Insurance Association requires the use of external lightning protection for systems with ratings above 10 kW, in accordance with VdS 2012 (VdS Schadensverhütung GmbH). Generally, however, both internal and external lightning protection should always be provided to ensure high system availability.

Selection begins as early as the design phase for the external lightning protection system, which protects the modules against direct strikes. The separation clearance s between the module frame and the lightning rods must also be maintained. For rooftop systems, this clearance is not usually maintained due to spatial constraints. In this case, the module frame must be connected to the external lightning protection using a 16mm² cable in order to prevent an electrical arc between the external lightning protection and the PV modules. However, this connection conversely involves a significantly increased risk potential for the PV system, since partial lightning currents can flow through the module frame (Fig. 3).

CLC/TS 50539-12 provides information about selecting the correct surge protection and the associated installation site for the various environmental conditions affecting a PV system:

Case 1: The building is not equipped with external lightning protection (Fig. 4).
Case 2: External lightning protection is installed and the separation clearance s is maintained.

In both cases, the use of Type 2 surge protection is sufficient, since lightning pulses on the PV cables do not have to be factored in.

Case 3 (Fig. 3): The building has external lightning protection, but the separation clearance s is not maintained. The conductor length L between the modules and the inverter is less than 10 m.

In this case as well, a Type 2 surge arrester is sufficient.

Case 4 (Fig. 3): Same as Case 3, except with a cable length L of more than 10m.

Here, Type 1 lightning protection is recommended, since the DC cables run parallel to the potential equalisation cable over long distances, increasing the likelihood of coupling.

In addition, in all cases, if the cable length L is greater than 10 m, protection components must be installed on both ends of the cable, at the building entry point and before the inverter, since the risk of coupling of a surge voltage rises significantly.

Summary

The new Standard for photovoltaic surge protection devices will give users more transparency and support when selecting and installing the lightning and surge protection required for their applications. In addition, the testing of protective components is defined such that it is significantly closer to practical requirements, since PV source characteristics are now taken into account.

Since Phoenix Contact is actively involved in creating the standard, all resulting ramifications can be included in the early stages of product design. This will ensure that the company is able to continue providing its customers with safe and standards-compliant lightning and surge protection solutions from the Trabtech family.