instrumentation of the säntis tower in switzerland for ... · for tests in lausanne using a 2 ka...

I. INTRODUCTION

The knowledge of the lightning channel-base current is of primary importance because most of the studies dealing with the electromagnetic effects of lightning are based on it. Indeed, it is only through the current at the channel base that comprehensive statistical data are available since it can be measured directly. As a consequence, most of the models describing the electromagnetic field radiation by a lightning channel as well as those predicting the coupling of the radiated field to nearby transmission lines use as 'input data' the lightning current at the channel base. Lightning channel-base currents are obtained either by direct measurements using instrumented towers (e.g. [1-4]) or from artificially-initiated lightning by small rockets (e.g. [5-7]). Estimates of various lightning current parameters can also be obtained from the measurements of lightning electromagnetic fields assuming one or more empirical [8, 9] or theoretical [10] relations between the lightning current and its associated electromagnetic fields. The most complete description of lightning return stroke currents is due to Berger et al. [1] who used resistive shunts to measure natural lightning currents at the top of two 55-m tall towers located on Monte San Salvatore, in southern Switzerland. More recently, lightning current and current derivative measurements have been obtained at the top of tall telecommunication towers in Austria [4], Germany [3], Switzerland [11], and Canada [2]. This paper reports on the progress in a new project to instrument the Säntis telecommunications tower in the Saint Gallen region of Switzerland. The 125-m tower sits on top of the 2505-m mount Säntis (Fig. 1). An analysis of the lightning location

system data over the past 10 years has revealed that this tower is struck by lightning more often than any other tower in Switzerland. Indeed, according to the data from lightning location systems, this tower is struck by lightning more than 100 times per year. Fig. 2 shows lightning impacts in the region where telecommunication towers susceptible of being used in this study are located. The data refers to the time period between January 1st

Instrumentation of the Säntis Tower in Switzerland for Lightning Current Measurements

C. Romero1, A. Rubinstein2, M. Paolone3, F. Rachidi1, M. Rubinstein2, P. Zweiacker1, and B. Daout4

1Swiss Federal Institute of Technology (EPFL), Switzerland 2University of Applied Sciences, Switzerland

3University of Bologna, Italy 4Montena EMC, Switzerland

Abstract—This paper reports on the progress in a new project to instrument the Säntis telecommunications tower in the Saint Gallen region of Switzerland. The paper includes information on the Rogowski coils and magnetic loop sensors that will be used to measure the lightning current. In addition, information is given on the design of the control and monitoring system in the context of the particularly rough weather conditions of the site.

Keywords—Lighting current, Rogowski coil, automatic measurement system

Corresponding author: Abraham Rubinstein e-mail address: [email protected] Presented at the Fourth International Workshop on Electromagnetic Radiation from Lightning to Tall Structures, in July 2009

Fig. 1. Säntis Telecommunication Tower near St. Gallen in northeastern Switzerland.

86 International Journal of Plasma Environmental Science & Technology, Vol.4, No.1, MARCH 2010

1998 and August 31st 2001. The recorded number of lightning strikes to ground in a 5 km square area around the Säntis tower was more than 4300. The Säntis tower is therefore a unique structure to collect experimental data related to the lightning discharge.

II. MEASUREMENT SYSTEM

A. Overall system The lightning current will be measured at two

different heights, 20 m and 67 m, using, at each height, two sensors: (1) a Rogowski coil, whose output will be processed by an analog integrator to obtain a replica of

the current waveform, and (2) a loop antenna. A schematic diagram of the current measurement system is shown in Fig. 3. The magnetic loops will measure either the lightning current derivative, when an open-circuit configuration is used, or the lightning current directly, when a short-circuit configuration is adopted. Close-range, wide-band electric and magnetic field sensors as well as a field mill will also be installed in the vicinity of the tower.

An over-the-Internet remote maintenance, monitoring and control system has been designed and built using National Instruments CompactRIO modules linked via 100Base-FX fiber optics Ethernet to a control room in the vicinity of the tower that is connected to the Internet over a standard ADSL link on the Säntis.

Fig. 2. Lightning impacts on Säntis tower between 1.01.1998 -31.08.2001 (Data Courtesy of Siemens AG). +: positive lightning strikes to ground; -: negative lightning strikes to ground

Fig. 3. The schematic of the current measurement system on the Säntis Telecommunication Tower.

Romero et al. 87

B. Current measurement system

Two sets of Rogowski coils with analog integrators will be installed on the Säntis tower at different heights, to provide not only a redundant measurement, but also means to evaluate the transient phenomena along the tower. The main characteristics of the two Rogowski coils are presented in Table I.

The two Rogowski coils were calibrated, first in the premises of the Montena EMC laboratories and then at the high voltage laboratory of the Swiss Federal Institute

of Technology in Lausanne. In the first case, an impulse generator providing a standard 8/20 microseconds current waveform was used. Figs. 4 and 5 show the comparison of each coil’s output with a reference current waveform, measured by means of a Pearson type 110 current probe, for tests in Lausanne using a 2 kA signal with a risetime of 1 s. C. EMC Box Design

Sensitive equipment will be located near each measurement point on the tower inside a specially designed metallic box. The boxes were designed to withstand the requirements of the tower conditions, in terms of humidity, temperature, electromagnetic compatibility and space constraints.

The boxes were modeled in CAD along with the inner components (Fig. 6) in order to assure the mechanical positioning, securing and thermal constraints. Each box includes: A CompactRIO microcontroller system that

monitors and controls the heating and ventilation system, and also controls the DC inner power sources. This system is linked to the recording and control room via fiber optic (FO) for remote monitoring and calibration of the subsystems.

Fiber optic converters to relay the output of the Rogowski coils to the high-speed digitizer.

Rogowski coils integrators. Power supply units. Heating system.

TABLE I CHARACTERISTICS OF THE TWO ROGOSWKI COILS

First Coil (ROCOIL) Second Coil (PEM)LF (-3 dB) 0.1 Hz 0.01 Hz HF (-3 dB) 5 MHz 3 MHz dI/dt peak 100 kA/S 150 kA/S

Noise level 20 mVp-p 2 mVp-p Calibration Factor Output 1: 1 kA/V

Output 2: 10 kA/V Output: 20 kA/V

Fig. 4. Measurement of the impulse response of the ROCOIL

Rogowski coil. Current transformer output (red), Rogowski coil current (blue).

Fig. 5. Measurement of the impulse response of the PEM Rogowski coil. Current transformer output (red), Rogowski coil current (blue).

Fig. 6. CAD Model of the box and its components.


C.1 Thermal Insulation Design Due to the harsh conditions inside the tower, where the temperature ranges from -15°C to +35°C, a system comprising a heater, a ventilator, moisture exhaust holes, and a thermal insulation material was designed to keep the temperature within acceptable limits for the components and to avoid water condensation inside the PCB cards. The thermal component design was based on a finite element 2D simulation using the COMSOL Multiphysics FEM-simulation environment. The details are shown in Fig. 7, in which a heater, insulation material, a ventilation hole and an external heat source are represented to simulate the heat exchange between the inside and the outside of the box. The air mass movement due to the fan was disregarded and, in turn, a steady state was assumed. Fig. 8 shows the resulting temperature distribution. It was found that foam material with a thickness of 3 cm is sufficient to maintain the temperature inside the box within the operating ranges provided by the manufacturers of the equipment installed inside the boxes. D. Digitizers The Rogowski coil output is coupled to the high-speed digitizer via a fiber optic analog-to-digital / digital-to-analog link. The selected links, manufactured by Terahertz, exceed the output bandwidth of the Rogowski coils by a factor greater than 5 (25 MHz -3 dB cutoff and DC response with a sampling rate of 100 MS/s) and have a 12-bit resolution at ±5 V, giving a SNRmax = 74 dB. These are connected to an 850 nm monomode industrial fiber.

A high-speed digitizer (a NI PCI-5122 configured as shown in Table II) is connected to the fiber optic digital-to-analog converter in the control room. E. Electromagnetic field measurement system Electric and magnetic field sensors will be installed inside a multi-storey dielectric enclosure located about 50 m away from the tower (see Fig. 9) to record the electromagnetic field signature in the immediate vicinity of a tower struck by lightning. F. Remote maintenance, monitoring and control system The installation site of the measurement system requires the development of specific functionalities to allow the remote monitoring, control and maintenance of the different components of the system. These functionalities are characterized by the hierarchical structure shown in Fig. 10. The three levels that characterize the remote control system correspond to the units that compose the system itself: Level #1 is the server that provides the backup

storage of the measured lightning current waveforms. This server operates as a front-end, providing remote access to the data over the Internet.

Fig. 7. Subdomains definition for the thermal simulation.

Fig. 8. Thermal behavior of the box (scale on °K).

TABLE II CONFIGURATION OF THE DIGITIZER

Number of channels 4 Number of digitizers 2 Sample Rate (Each channel) 100 MS/s Sampling time window @ 100 MS/s 1.2 S Pretrigger 40% Resolution 14 bits Input impedance 50 Ω, 1 MΩ

(selectable) Input voltage range ±10 V

Romero et al. 89

Level #2 is the measurement station composed of an industrial PC that hosts the two digitizers and the GPS card. It provides both measurement and storage functionalities and the dialog with the Remote Terminal Units (RTU) that contain the current sensors.

Level #3 are the RTUs composed of the current sensors, their power supply, the relevant optical fiber links (measurement and communication) and controlled by the National Instruments CompactRIO microcontroller.

In what follows, details about the functionalities of each level of the remote control system are illustrated. In particular, we start with the description of Level #3 that has been designed to operate autonomously, even without the availability of the other levels. Level #3: Each RTU is controlled by means of a real-time microcontroller (National Instruments CompactRIO) equipped with an FPGA (Field Programmable Gate Array) bus. The microcontroller features a 400 MHz real-time processor with 2 GB nonvolatile storage, 128 MB DRAM memory linked with

a 1 Mgate FPGA working at 40 MHz. The functionalities implemented in each microcontroller are: a. Voltage measurement and control of the power

supply units connected to the Rogowski coil integrators and to the optical fiber link.

b. Temperature measurement and control of the shielded chamber that contains the RTU equipment.

c. Communication with the measurement station to allow the remote control of the RTU.

Functionalities a and b are executed by the microcontroller autonomously and, if necessary, can be remotely configured by accessing Level #2. Level #2 is composed of an industrial PC equipped with an Intel Dual Core CPU at 2.6 GHz, 4 GB RAM and a 500 GB SATA HD. This computer hosts the two digitizers and implements the following functionalities: a. Triggering, measurement and storage of the

current waveforms. b. Acquisition of the UTC-GPS time stamp. c. Backup of the measured current waveforms into

the Level #1 server.

Fig. 9. Dielectric enclosure located in the vicinity of the Säntis tower in which electric and magnetic field sensors will be installed.

Level #1Server (storage, web

access)

Level #2Measurement station

Level #3RTU-1

Level #3RTU-2

Fig. 10. Hierarchical structure of the remote control functionalities implemented into the measurement system.


d. Remote control of the remote terminal units RTU-1 and RTU-2.

Functionality a is the most complex considering that the digitizers (National Instruments PCI 5124) are sampling at 100 MHz within a time window of 1.2 s and, therefore, each waveform contains 120 MSamples per channel. To solve the problem of the transfer of the measured waveform from the digitizer’s memory to the PC, the digitizer memory is fetched in 10 MSamples segments and sequentially saved onto the PC to compose a single measurement file. Concerning the trigger, the two digitizers are synchronized by means of a PLL. Therefore, the trigger event, programmed in one of the two digitizers, is transmitted to the other within a time window that has been experimentally determined to be within the range of 0 - 20 ns. Functionality b is performed by the Meinberg GPS 170 PCI card, characterized by an overall time uncertainty of 100 ns. The trigger relevant to the measurement of the GPS time stamp is exported by one of the two digitizers by means of a digital signal. Functionalities c and d are directly performed by the software implemented into the industrial PC of the measurement station. Level #1 is composed of a standard PC featuring an Intel Core 2 Duo CPU operating at 2.8 GHz and 4 GB of fast RAM. Because the events recorded by the digitizing equipment are expected to produce very large files, the temporary backup storage provided by this server is ensured by a level 1 RAID (Redundant Array of Independent Disks) system with two 1.5 TB SATA hard drives. This kind of configuration provides simple but efficient data protection, duplicating the data across the two drives in real time, including the operating system and installed programs. Because the data are mirrored, the capacity of this array is actually 1.5 TB and not 3 TB, as one might expect. On the other hand, the full redundancy provided by this configuration allows the system to continue operating even if one of the drives fails. In the event of a drive failure, the data recorded on the healthy drive will be automatically mirrored to a new replacement drive as soon as this is installed to replace the damaged one. The data replication takes place while the system continues to operate normally, thanks to the fact that the RAID controller works as an independent unit. As already mentioned, a standard ADSL link available on-site will provide a connection to the Internet. A web interface will allow some basic interaction with Levels #2 and #3. Of particular interest is the possibility to plot data while it still remains locally stored on the backup server, without actually having to transfer the large data files. Nevertheless, we expect to systematically transfer the recorded data files over the Internet link on a regular basis. These files will ultimately be stored on servers belonging to the Swiss Federal Institute of Technology and the University of Applied Sciences of Western Switzerland. The use of a VPN (Virtual Private Network) will provide full access to the front-end server. This will

allow for full remote control of the front end and the industrial PC ensuring easy remote reconfiguration and reprogramming of the control and monitoring system.

III. CONCLUSION

During the past several years, the Säntis tower has

been struck by lightning over a 100 times per year, more often than any other tower in Switzerland, making it a unique structure to collect experimental lightning data.

The tower is being instrumented to measure the lightning current at two different heights, 20 m and 67 m, using, at each height, two sensors, a Rogowski coil set up to measure the current itself and a loop antenna to measure, depending on the selected configuration, either the current or the current derivative. Wideband electric and magnetic field sensors will be installed inside a dielectric enclosure located about 50 m away from the tower and a field mill will also be installed near the tower. Special boxes have been designed to house the electronics near the sensors. Criteria for the design include thermal and EMC constraints to deal with the electromagnetic and climatic environment in the tower. Maintenance, monitoring and control tasks will be carried out remotely using a remote control system over the Internet using a standard ADSL link on the Säntis.

ACKNOWLEDGMENT

This work has been carried out within the framework of the European COST Action P18. Financial support from the Swiss Office for Education and Research SER (Project No. C07.0037) and the Swiss National Science Foundation (Project No. 200021-122457) are acknowledged.

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