Overhead line monitoring

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Overhead line monitoring (abbreviation FLM , also overhead line temperature monitoring or weather-dependent overhead line operation ) is a control method in power grids with which the transmission capacity of overhead lines is better utilized.

Definition and benefits

The transmission capacity of overhead lines is limited by the maximum operating temperature of the conductor cable . The main influencing factors for this operating temperature are the current flow in the conductor and the climatic environmental conditions. In the case of an overhead line operated without an FLM, the flow of electricity is limited using a conservatively defined standard climate. Instead of assuming a standard climate, overhead line monitoring either measures the operating temperature directly or the cooling effect of the weather is modeled along the route using real climate data.

Overhead line monitoring is used in particular on existing lines of the high voltage network and can increase their usable transmission capacity by up to 50% of the nominal output . Because of the expansion of renewable energies, especially with wind turbines , it is necessary to increase the capacity of the transmission networks. Overhead line monitoring is the main means alongside the use of high-temperature conductors. Under certain circumstances it can help to avoid the construction of new routes.

background

The transmission capacity of overhead lines, also referred to as the current carrying capacity in relation to the conductor cross-section , is the maximum current flow that can be permanently achieved in compliance with safety regulations. With a given material and cable diameter of the conductor, the transmission capacity is limited by the conductor temperature. An operating temperature greater than 80 ° C must be avoided in accordance with EN 50182 , as the cable sag above this becomes too great due to thermal expansion , and there is a risk of the cable coming into contact with the ground or vegetation or rollover . The rope material can also lose its strength due to greatly increased temperatures .

In addition to the heat input through the resistance that the electrical current experiences in the conductor, the cooling (and more rarely also heating) through the environment is the other essential factor for the conductor temperature. The essential factors for this are ambient temperature, wind speed, solar radiation and precipitation. The air humidity does not play an essential role during the warm seasons. A layer of ice on the cables, as can occur in cold seasons, has a significant influence on the thermal balance of the conductor. However, in terms of the maximum conductor temperature, winter is not a design-relevant operating state.

The European standard EN 50182 ("Conductors for overhead lines - conductors made of concentrically stranded round wires") specifies the continuous current carrying capacity, which under the worst-case assumption of a hot summer day without clouds and practical calm leads to compliance with the maximum permissible conductor temperature. This means that the minimum distances between the conductor and the floor or other objects are maintained. These environmental conditions are specified in the standard at 35 ° C outside temperature, full global radiation with 900 W / m² and 0.6 m / s wind flow at right angles to the conductor in accordance with EN 50341 ("Overhead lines over AC 45 kV"). Such climatic conditions rarely occur in Central and Northern Europe. As a result, overhead lines are usually operated there with considerable transmission reserves.

Procedure

With overhead line monitoring, the static worst-case assumptions in accordance with EN 50182 are dispensed with and instead, climate data along the route and, if necessary, the operating temperature of the conductor cables are monitored. In favorable weather conditions, for example strong winds or low outside temperatures, the ladder can be subjected to greater loads than would be the case under normal climatic conditions. The problem of overloaded transmission networks has arisen in Central Europe especially since the beginning of the strong expansion of renewable energies with wind energy peaks . The particularly powerful wind turbines are located in surplus areas with strong winds and few consumers. Especially at times when there is a high transmission requirement due to strong wind energy feed-in, there is naturally a lot of cooling wind. FLM can increase the current carrying capacity of overhead lines near the coast by up to 50%.

With overhead line monitoring, the conductor cable temperature is measured directly or indirectly. The measurement is done point by point with sensors or integratively over the entire length of a monitored section. The measurement signals are transmitted from the transducers to the control room ; the signal is usually filtered. The measurement signals are processed in the control room. Special software is used for this purpose, which, in addition to the measurement signals, also monitors the status of the system itself (e.g. battery range). The displays of this software show what percentage of the load a line is running. Since thermodynamic systems are inert, some systems also have a forecast function, which is fed by weather forecasts and the electricity schedule. This information can be used to control the transmission load in the control room. With newly built transmission lines, FLM can be integrated into the control center and its software from the start.

Measurement method

There are various methods for determining the conductor temperature and the cable sag. These differ according to:

  • Measured variable: temperature measurement vs. Measurement of another physical quantity from which the temperature can be derived.
  • Distance to the measurement object: direct temperature measurement with contact to the conductor vs. Distance measurement
  • Measuring method: Which physical measuring method is used?
  • Reference length: measurement at one measuring point vs. Measurement across an anchoring section or the entire line
  • Measurement resolution: integrative over the reference length vs. resolved at discrete points

The following measuring methods exist:

Procedure Measurand Distance to the measuring object Measurement method Reference length Measurement resolution
Cable measurement through load cells Tensile force in the conductor Directly, the load cells are connected to the rope with fork straps Piezoelectric One rope field each between two pylons Integrating
Temperature measurement by thermocouples Temperature in the conductor Direct, the thermocouples are in or on the rope Thermoelectricity Point measurement per sensor Near temperature field around the sensor ( cm range), in practice the resolution is determined by the number of sensors installed
Temperature measurement with DTS Temperature in the conductor Direct, the glass fibers are woven into the core of the conductor cable Raman effect , fiber Bragg grating , Brillouin scattering Over several kilometers Accurate to about one meter
Temperature measurement by surface wave sensors that can be queried by radio Surface temperature of the conductor Direct, the sensors are located on the conductor and are queried remotely AOW sensors Point measurement per sensor Several sensors can be queried with one antenna, otherwise point resolution by sensor
Temperature measurement with thermovision cameras Surface temperature of the conductor Indirect, the cameras are on the ground or on the mast Thermography Section of rope captured by the angle of view of the lens of the thermovision camera Due to the low resolution of the thermal image sensor, at least a few meters
Temperature determination using a thermostatic rope model Current and power loss Indirectly, the temperature results from the measured power loss
Sag calculation from current level and weather data from distributed measuring stations Electricity and weather data (see next line) Indirectly, the temperature results from the measured current and weather data
Measurement of weather data on the route Ambient temperature, effective wind force, thermal radiation Indirectly, the weather data is incorporated into a model. The weather stations are at a distance from the conductor cable and are installed at least at the height of the cable to record the microclimate . Common sensor-based measurement methods for temperature. Wind speed must be measured perpendicular to the rope. Measurement of thermal radiation can be combined with solar-powered energy supply. A constant microclimate around the weather station is assumed. Restriction to critical route sections is possible. One weather station per two rope fields is common there. One to two kilometers, depending on the distribution of the weather stations
Ground distance measurement using laser or ultrasonic methods indirect, distance between conductor and ground indirect, distance measuring point to sensor up to one kilometer trigonometric method, measurement with handheld devices or airborne laser scanning Maximum sag in a rope field or distance to critical trees or similar. critical distance per rope span, no continuous measurement.

Some of these methods are also combined. The cable pull method (e.g. CAT-1) and the use of weather stations in combination with power loss measurement are the two most common methods.

Transmission of the signals

The measurement signal must be transmitted reliably and without interference from the sensor to the control room. For this purpose, signal cables are common on the mast, from there to the control center GPRS is usually used. The signals are converted into SCADA formats at the control center. The sensors and communication elements need an energy supply that should be as self-sufficient as possible. The battery-buffered solar supply is common ; alternatively, the energy can also be drawn inductively from the conductor.

implementation

Attempts at overhead line monitoring began in the United States in the 1960s. The first practical use took place there in 1991 at Virginia Power . With the CAT-1 system, the rope temperature is not measured directly, but rather the rope sag is determined with load cells . By breaking down the measured tensile force into a horizontal and vertical component, the angle and thus the sag of the rope is determined. In addition to measuring the rope force, the local temperature is recorded on the recording devices. The system has to be calibrated frequently; the result is falsified in the event of wind peaks. Local temperature peaks, which can lead to material fatigue, cannot be recorded either. As of 2011, CAT-1 was being used 400 times worldwide by around 100 transmission system operators , 16 also in Europe.

In 1995 Edmund Handschin, Professor of Energy Systems at TU Dortmund University , published the load-bearing capacity of conductor cables as a function of wind speed and air temperature. At a wind speed of 10 m / s, i.e. 36 km / h and therefore moderate wind , the permissible continuous current load doubles compared to the values ​​in the standard EN 50182.

At the beginning of 2003, Alpiq (then Atel ) carried out a six-year FLM field trial on the Lukmanier line together with Swissgrid . The aim was to compare different FLM methods. The approximately 100 km long Lukmanier line leads through three different climate zones, each with a measuring station. The Erstfeld station is in a temperate climate, the Rueras station in the alpine and the Cugnasco station in the Mediterranean climate. Systems of the type PowerDonut (local flow and temperature measurement), CAT-1 and Line Thermal Monitoring (LTM) were used. With the latter method, the line resistance is determined from precise measurements of the active power loss between two stations. From this the global average of the conductor temperature can be calculated. Important findings of the experiment were the high dependence of the cooling effect of the wind on the direction of flow. The CAT-1 system did not prove to be reliable at high wind strengths, since the rope force measurement cannot distinguish between the horizontal component of the force, which can be changed by rope elongation, and the additionally applied wind load. With a high current flow and precise calibration, the LTM system turned out to be useful for measuring the global cable conductor temperature trend; a particular advantage is that it can do without additional hardware installations.

From 2006 E.ON Netz carried out an FLM field test in Schleswig-Holstein on the 110 kV Niebüll - Flensburg line . As a result, the transmission capacity of this overhead line could be temporarily increased by up to 50%, depending on the weather. This was followed by further field tests in northern Germany with 110, 220 and 380 kV lines. The climate data was collected along the route or obtained from meteorological service providers. The conductor temperature resulting from the resistance heat input and climatic cooling was then calculated in real time using a model. This data was made available to the network control center, where the transmission can be increased accordingly. As of 2011, TenneT had converted more than 900 km of extra-high voltage lines and 20 substations from Hamburg to Gießen to weather-controlled FLM using investments of 55 million euros.

With the weather-controlled method chosen by TenneT, certain safety assumptions had to be made in the model. The transmission capacity could be increased even further by measuring the conductor temperature directly. These can fiber optic temperature measurement (DTS) are used, which is in the US for FLM in use. If the fiber optic temperature measurement is combined with the measurement of the deformation ("strain"), the procedure is called DTSS ("Distributed Temperature and Strain Sensing"). The fiber optic cable required for measuring along the route is embedded in the conductor cable. In 2005, measurements were carried out in a pilot test using unimodal DTS / DTSS on an overhead line stretch of 16.7 km, with DTS achieving a temperature resolution of 0.7 ° C after 15 minutes, which increased to 0.3 ° C after two hours decreased. Measurement results were available in a 1 m grid with a spatial resolution of two meters. At the same time, the elongation of the conductor was measured with DTSS, with a measurement accuracy of 20 µm / m with a spatial resolution of 1.5 m.

The network study II published by the semi-public DENA 2010 assumes in its network forecast the use of two technical possibilities to increase the current carrying capacity of overhead lines: Overhead line monitoring (FLM) and high temperature cables (TAL). However, the transmission capacity through FLM in the strong wind scenario only increases by 50% in the north; Due to the decreasing weather correlation, weaker winds and the shielding effect of vegetation, 30% is expected in central Germany and only 15% in the south. The conclusions of DENA and the four large transmission system operators from the network study II were criticized because they limited the optimization potential in existing lines to FLM with weather management and TAL high-temperature cables. With direct conductor temperature measurement and ACCC or ACCR high-temperature cables, more could be achieved, and the need for expensive new construction lines, which are controversial among the population, would decrease accordingly.

When VDE , the project group developed weather-dependent transmission line operation of the Forum Network Technology / Network Operation (FNN) a VDE application, the 2011 and 4210-5 weather-dependent transmission line operation VDE-AR-N was released.

literature

Web links

Individual evidence

  1. ^ A b Klaus Heuck, Klaus-Dieter Dettmann, Detlef Schulz: Electrical energy supply: generation, transmission and distribution of electrical energy for study and practice , 8th edition. Vieweg + Teubner, Wiesbaden 2010, ISBN 978-3-8348-0736-6 , pp. 356-357.
  2. ↑ Ralf Puffer: Network optimization through weather-dependent overhead line operation and high-temperature conductors . (PDF; 1.9 MB) Lecture on May 6, 2010 at the Institute for High Voltage Technology at RWTH Aachen University , pp. 5–6.
  3. ↑ DIN EN 50182: 2001-12 : Conductors for overhead lines - Conductors made of concentrically stranded round wires; German version EN 50182: 2001 . Beuth, Berlin 2001.
  4. ↑ a b Renata Teminova, et al .: Use of passive surface wave sensors that can be queried by radio for temperature monitoring of conductor cables . In: Josef Kindersberger (ed.): Diagnostics of electrical equipment . VDE-Verlag, Berlin 2004, ISBN 3-8007-2817-6 , pp. 353-358.
  5. ↑ Georg Küffner: The country needs new lines . In: FAZ of September 23, 2011.
  6. ↑ a b c Lorenz Jarass, Gustav M. Obermair, Wilfried Voigt: Wind energy: reliable integration into the energy supply . 2nd Edition. Springer, Berlin 2009, ISBN 978-3-540-85252-0 , pp. 68-72. ( Subchapter "Overhead line monitoring" )
  7. ↑ Walter Sattinger et al .: Conductor temperature measurement on the Lukmanier . (PDF)  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. In: Electrosuisse Bulletin SEV / VSE , No. 5/2010, pp. 45–49.@1@ 2Template: Dead Link / www.electrosuisse.ch  
  8. ↑ New concept for wind power integration: E.ON Netz presents overhead line monitoring . E.ON Netz press release of September 18, 2006.
  9. ↑ Overhead line monitoring . ( Memento of the original from June 17, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 306 kB) E.ON Netz GmbH, brochure from 09/07. @1@ 2Template: Webachiv / IABot / apps.eon.com
  10. ↑ Overhead line monitoring - optimal capacity utilization of overhead lines (PDF; 666 kB) at TenneT TSO (accessed on January 31, 2012.)
  11. ↑ Case Studies: Overhead Powerline - Strain & Temperature Monitoring  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. . On Sensornet, Hertfordshire, UK. (Accessed January 31, 2012.)@1@ 2Template: Dead Link / www.sensornet.co.uk  
  12. ↑ dena grid study II - Integration of renewable energies into the German power supply in the period 2015-2020 with an outlook for 2025 . DENA, November 2010.
  13. ^ Jörg-Rainer Zimmermann: Bottleneck network . (PDF)  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. In: neue energie , no. 01/2011, pp. 29–39.@1@ 2Template: Dead Link / www.neueenergie.net  
  14. ↑ VDE-AR-N 4210-5 Weather-dependent overhead line operation ( Memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (Application rule of the VDE) @1@ 2Template: Webachiv / IABot / www.dke.de