Overview

In this blog article, we will explain the magneto-optic effect’s principle for monitoring surge currents in lightning strikes. We will use the example of a lightning monitoring system in wind power plants, monuments, and power transformation substations.

Lightning monitoring on exposed objects

Lightning strikes can cause severe damage to buildings and systems, especially in residential or commercial buildings. One can quickly evaluate the extent of damage and immediately repair it, preventing consequential damage. However, employees cannot continuously monitor exposed objects or large surfaces like wind power plants or railway systems. That’s why the use of Intelligent monitoring systems is becoming more common to permanently monitor system function states and send results to a central control unit. This allows for immediate response in case of malfunctions and prevents consequential damage and long downtimes. Until now, no measuring system could reliably detect and evaluate lightning strikes.

New monitoring technology with the Faraday effect

A new monitoring system uses the Faraday effect and magneto-optic effect to analyze surge currents in lightning arresters. Moreover, a fiber optic cable transmits signals, preventing lightning currents from influencing the light signal. This ensures reliable and EMC-protected signals for the evaluation unit electronics, ensuring efficient and reliable monitoring and measurement.

Structure of the measuring section

The measuring section is a transparent dielectric with polarizers at either end, positioned 90 degrees to the current flow direction in the down conductor. Consequently, Ensuring that the propagation direction of a light wave is parallel to the surge current’s magnetic field.

Dielectric

A dielectric is a nonconductive, nonmagnetic substance in gaseous, liquid, or solid states. Its charge carriers are typically subjected to electric or electromagnetic fields, preventing free movement.

Polarizers

Polarizers filter electromagnetic waves, producing circular, elliptical, or linear polarized light. To utilize the Faraday effect, Linear polarization is used in lightning measuring systems.

Magnetic effect on the plane of polarization

A light wave causes electron oscillation in a dielectric, affecting the electron’s movement and influencing the light’s polarization plane, which can rotate in any direction depending on the magnetic field direction.

Magneto-optic effect in the monitoring system

A light beam of a specified intensity is guided onto a measuring section via fiber optics. A linear polarizing filter at the input polarizes the directed light, causing electrons to oscillate and travel through the medium in the polarized plane. Moreover, the magnetic field of a surge current rotates the plane of polarization around the longitudinal axis, with the direction of rotation influenced by the direction of the magnetic field lines and current flow. The output polarizing filter, positioned at an angle of 45 degrees, only passes 50% of an uninfluenced light wave, resulting in a light signal that we can measure and evaluate.

Measuring result and evaluation

Positive lightning causes a clockwise rotation of the polarized light signal. With the amount passing through the second polarizing filter increasing to over 50% of the output value. A positive lightning strike occurs when the angle of rotation reaches 45 degrees, while a negative lightning strike occurs when the angle reaches -45 degrees. The progression of light over time determines the monitored lightning surge current, including maximum current strength, lightning current rate of rise, charge, and specific energy.

Electric field vector E

The electric field vector describes the progression and the position of the affected or rotated light wave. The arrow here shows the light wave’s direction

Calculation of the angle of rotation β

We can calculate the angle of rotation β, about which the plane of polarization rotates, by: β = V x d x B

Verdet constant V

The Verdet constant is an optical constant for dielectrics, describing the Faraday effect strength and rotation per unit of magnetic flux density, influenced by the medium and electromagnetic wavelength.

Installation of the sensor on a down conductor

The depth of the sensor’s immersion in the current carrying down conductor’s magnetic field determines the effective field strength in a circular magnetic field. Thus, A smaller radius results in greater field strength. Therefore, we recommend mounting the sensor tightly to maximize effective field strength.

Radius r

The radius of a sensor is used to measure the depth of immersion in a magnetic field and the effective magnetic field strength H. Hence, It is calculated from the sensor housing’s outer edge to the conductor’s center line, ensuring consistent measurement conditions across different system conditions.

System interfaces and signal transmission

The evaluation unit integrates into standard networks via RJ45 Ethernet, and an internal web server is used for data access and system configuration, accessible using IP addressing via a PC’s internet browser.

Remote monitoring

The LM-S lightning monitoring system offers an integrated web interface for remote access to measuring data on systems like offshore wind parks. Meanwhile, enabling users to monitor system load situations at any time using a smartphone.

Preventive maintenance

The evaluated data allows for precise system load estimation, enabling preventive maintenance and rapid measures to prevent damage. In addition, it also saves time and money by indicating minimum, uncritical system loads, reducing unnecessary maintenance work or servicing.

Remote contact

The evaluation unit features a switching relay with a remote contact for evaluating lightning strikes. Moreover, this N/C contact produces a short pulse for each event, allowing for simple or additional evaluations. Furthermore, the relay contact only returns to normal position after system start-up, and drops out in case of malfunction, allowing the querying of the system readiness via the remote contact.

LM-S applications

Lightning monitoring systems are commonly used in industrial plants, large buildings, antennas, energy distribution systems, and transport monitoring systems, ensuring safety and protection against lightning strikes.

Wind power plant

The system in a wind power plant consists of individual components, including sensors mounted on lightning arresters and an evaluation unit in a control cabinet. In addition, signals are transmitted via fiber optics, and an Ethernet connection is established via slip rings between the gondola and observation deck. The evaluation unit operates with 24 V DC and can be connected remotely for event evaluation.

Cultural monument

The Hermann monument in Detmold, Germany, uses a lightning monitoring system to detect potential damage. The statue, over 53 meters tall, is connected to three grounding cables, diverting lightning surge currents to the ground. Sensors are mounted on these down conductors, and an evaluation unit is installed inside the base. The main objective is not early monitoring of potential damage. It is to statistically evaluate lightning strikes and their strength due to the structure’s exposed location and height.

Power transformation substation

Lightning strikes on high-voltage cables cause transformer loading in power transformation substations. Surge protection elements, typically connected upstream of transformers, direct surge currents from coupled overvoltages to ground. Moreover, varistors have become the preferred solution as protective elements instead of spark gaps. Furthermore, LM-S allows for early monitoring of load limits and replacement of affected protective elements. Our Sensors are installed on down conductors, transmitted via fiber optics to an evaluation unit in a remote control cabinet.

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