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FIBER OPTIC SYSTEM FOR MONITORING OVERHEAD LINES

General description

Fiber-optic geotechnical monitoring system (FOGMS) includes 3 basic components: analyzer (or interrogator), distributed (uninterrupted) fiber-optic sensor and a dedicated software (Fig.1). The analyzer is a powerful diagnostic instrument for distributed measurement of strain and temperature over 65 km per channel, allowing measurement of thousands of locations by means of a single sensing cable. Use of internal optical switch featuring 2 measuring channels allows uninterrupted measurement of up to 135 km in two opposite directions from the analyzer. The number of measurement channels can be further extended by using an external multiple optical switch module and star topology. Once an external switch is connected to the analyzer, up to 21 sensors can be used for distributed sensing.

Distributed sensor is an uninterrupted fiber-optic cable that is often custom-designed for a specific application and environment. Each millimeter of sensor is used as a sensitive element offering a great alternative to numerous point sensors. Taking into account high spatial resolution of the analyzer each 50 centimeters of sensor can be considered as an individual point sensor, therefore 50 centimeter sensor section is equivalent to 100 000 point sensors. Fiber-optic distributed sensors are fully passive and do not require connection to power supply. One sensor is usually connected to the analyzer from one or two ends and is adjacent to the entire length of a monitoring facility. Optical fiber integrated in sensor design is sensitive to various external parameters (temperature, elongation/compression, acoustic pressure, etc.) by changing its optical properties. Thus, fiber-optic geotechnical monitoring system offers a wide range of performances and suitability for different applications.

The system automatically controls an extended facility in every section with an installed sensor providing accurate measurement capabilities in real time. The superior sensing technique and operation flexibility makes FOGMS a unique and unrivalled solution to most demanding applications. For the details of system’s operation principles and advantages of fiber-optic sensors, please refer to Section “Resources”.


A)  

B)

 

C)

Fig. 1. FOSGTM includes a set of components which consist of:

A) Analyzer unit and supporting equipment mounted in a 19” rack cabinet; B) Fiber-optic sensor; C) Dedicated software to be installed in a server room.

 

Fire prediction in HVcollector cabling systems

Although HV overhead lines are still the dominant transmission construction around the world, in the latest decades largest cities of the advanced countries are showing tendency of replacement HV overhead lines with cable lines. Though such replacement tends to minimize operation costs of a transmission line, the fire safety issue at collector cabling systems is still of a great importance. Distributed fiber optic temperature sensor can address the fire safety challenges induced to the power industry.


 
Fig. 2. Examples of modern HV collector cabling systems.

 

 

Fig.3. Consequences of inflammation in cable collector. Moscow, 2000.

 

Distributed fiber optic temperature sensor module with optical fibers can be integrated to the design of HV cable at the manufacturer’s plant. The fiber optic temperature sensor can be either integrated to the HV cable design during manufacture to enhance speed of response or installed directly on the HV cable or between HV cable lines.


Distributed fiber optic sensor is fully dielectric, totally immune to EMI and features long operation lifetime.

Current load optimization in HV cables

Efficient transmission of energy generated by power grids to the end users requires controlling heat conditions of HV cables. This is achieved by increasing current load (output power) while heat limits determined for a specific power cable are maintained. To do so modern HV lines are based on integrated fiber optic modules (as illustrated in Fig.4), that can be used for both data and control signals transmission and control of temperature distributions along the cable with use of Raman or Brillouin analyzer.

 
Fig. 4. Examples of modern HV cables with integrated optical fiber.

 

 

Real-time data obtained by monitoring system is transferred to control system for analysis and design of HV transmission lines. The analysis systems are designed to store the modern varieties of HV cable designs. By using heat models and boundary conditons such system converts temperature of an optical fiber located in different parts of the cable (depending on the specific design, as illustrated in Fig.4) to the temperature of conductor core. Thus, transient heat processes occuring in the cable can be forecasted and controlled. A variety of meausurands can be acquired to provide diverse information, including:


Induced current in the cable in terms of time-temperature;
Temperature analysis in terms of time-current load (capacity);
Time calculation to achieve a required temperature and current load (capacity);
Transmission capacity calculation in terms of time;
Simultaneous analysis of multiple cables in a bundle;
Simultaneous and consequential current measurements for further analysis of the cable loads.

An alarm module that can be optionally included to the system is used to transmit information about temperature thresholds that had been pre-determined in the monitoring system. When the cable temperature exceeds the threshold defined in the system, a transmitted alarm enables to prevent emergiencies. Alarm signal can be associated to the following values:

Absolute or relative temperatures of the core and/or cladding;
Velocity of temperature change;
Allocation of various measurands associated to different segments.

Fig. 5. Temperature analysis of HV cable

 

 

The monitoring system provides better performance of the existing transmission lines with improved power quality (larger output capacity) and operational safety. Fiber optic systems are largely recognized by power industry, whereas such applications as overheadlines and on-shore subsea power cables are the most appropriate to it.

Deicing at OPGW cables (ground wires)

OPGW cables are most frequently subjected to icing that presents a quite challenging engineering issue (Fig. 6). On the one hand, icing process leads to significant increase of the cable mass and overload. On the other hand, the formation of ice results to increased cable diameter and stronger wind loads. Both factors have serious negative effects leading to greater tensile strength, especially at the connection point between cable and pylons, which is then causes a cable rupture.

 
     
 
Fig. 6. Examples of icing and consequential damage to HV cables.

 

 

When the ice sheet is growing significantly deicing is the most common procedure made by the operators to prevent possible ruptures of the power supply. High alternating or direct current is used for heating a critical section of the ground wire for a certain time period. But it shall be taken into account that a continuous ice fusion can cause a cable overheat, so knowing the temperature evolution of the HV cable is important. Ice fusion systems that are commonly used in the world (Russia, Canada, Alaska and Norway as being most exposed to icing) are based on point temperature sensors that can be installed only in most critical sections. The main disadvantage of such systems is that they do not allow inspection of early icing signs, or may on the contrary contribute to overheating. Only by monitoring temperature along the whole distance of the ground wire a secure energy supply can be ensured. Raman or Brillouin analyzer together with an optical fiber inside of the ground wire offers the most effective industry solution.

 

 
Fig. 7. Examples of modern ground wires integrated with an optical fiber (OPGW).

 

Reliability assessment of transmission overheadlines. Control of tension in overhead wires.

Overhead wires with long spans between the poles are exposed to abnormally high tensile stresses. Additionally, high loads are typical for transmission overhead lines located in the zones with high humidity (in the vicinity of rivers, swamps, etc.) that consequently induce water condensation. Water may infiltrate into the cable and during winter time when it is exposed to freezing temperatures ice will form on the line. Another threat is wire sagging (as illustrated in Fig.8). During the summer, the generally warmer temperature heats the wires and they expand in length and width. So if the temperature exceeds the wire’s thermal rating, the steel loses its tensile strength and the wire sags. Therefore, for safe operation of the transmission overheadlines it is necessary to control cables tension. Such control is either not maintained at the existing overhead wires, or limited to the most critical areas by using local strain sensors that are placed at certain poles. An alternative method based on the use of Fiber Optic Geotechnical Monitoring System (FOGMS) provides strain monitoring along the whole length of the fiber. To achieve sensing on such large distance a strain sensor module shall be integrated to a power cable. This configuration would help to evaluate strength and safety margin of the monitored overhead wire. Currently CJSC “Laser Solutions” together with All-Russian Scientific Research and Development Cable Institute (VNIIKP), major electric power cable manufacturer in Russia are working on the design of power cable with built-in optical fibers. Followed by test and validation results such power cables with strain sensing module can be applied both at new and redesigned overhead lines.

 
Рис. 8. Examples of phase wires sagging and consequential damage to the transmission overheadline.

 

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