Benefits and Risks of Real Time Thermal Rating Systems

FBG sensors in real time thermal ratingA real-time thermal rating system has been developed initially for overhead transmission lines using actual meteorological data and real-time conductor temperatures and line loadings. Such a real-time thermal rating system provides much higher ampacity ratings than other conventional methods. A natural convective heat equation is developed for stranded conductors. The temperature of the conductor is solved directly without resorting to an iterative solution.

The temperature of an asset itself, such as a power cable, is a key for a real-time thermal rating system. This can be measured continuously if equipment utilizes a distributed temperature sensing (DTS) system. Distributed temperature sensing optical fibers are installed along with the fiber cable. The fiber cable can also be utilized for telecoms purposes as DTS systems typically utilize standard telecoms fiber optics.

Thermal headroom typically is determined using static ratings which are based upon probabilistic methods and are representative of worst-case scenarios. The “static” design calculation methods provide simple and conservative estimates of network capacity. In reality, networks can be complex and operational ratings can be influenced by multiple factors including weather conditions and loading. Soil condition, buried depth, burial configuration, cable size, and type must be considered for underground equipment.

The real-time thermal rating system works with such assets as:

 

  • Underground and subsea cables

 

Experience has shown that cable depth, soil type, and the shape of the load curve have a material impact on ratings. The real-time thermal rating system can determine actual thermal headroom indicating whether some unused network capacity can be released or locations where networks are constrained.

 

  • Overhead lines

 

  • Sag-based
  • Tension-based
  • Temperature-based
  • Current rating-based

 

  • Transformers

 

The real-time thermal rating systems for transformers utilize measurements including transformer load, ambient, and transformer temperatures based on the equations set out in IEC 6007. With the exception of emergency ratings, P15 recommends using an average ambient temperature and a weighted average that products the same aging if the temperature varies over a load cycle.

Optromix is a fast-growing vendor of fiber Bragg grating (FBG) products line: fiber Bragg grating sensors, FBG interrogators and multiplexers, distributed temperature sensing (DTS) systems. We create and supply a broad variety of top-notch fiber optic solutions for the monitoring of various facilities all over the world. Our main goal is to deliver the best quality fiber optic products to our clients. We produce a wide range of fiber optic devices, including our cutting-edge customized fiber optic Bragg grating product line and fiber Bragg grating sensor systems.

If you are interested in Optromix distributed temperature sensing systems, please contact us at info@optromix.com

Real Time Thermal Rating systems

RTTR, or Real-Time Thermal Rating, is a method of assessing real-time operational thermal rating of the equipment, or the amount of electrical current that a power line or an electrical facility can endure before suffering critical damage. Thermal rating devices can be used to measure the temperature of overhead power lines, transformers, underground and subsea cables.

Real-time thermal rating systems rely on real-time data from environmental conditions rather than theoretical assumptions and predictions. These systems are able to not only measure the thermal ratings in real-time but also to measure the stress levels of certain areas and determine their capacity. The calculation of the thermal ratings happen on the basis of: 1) weather conditions; 2) electrical current; 3) temperature of the equipment. However, other factors may need to be considered; this depends on the environment where real-time thermal rating needs to be performed. For example, soil condition, burial depth, and configuration must be considered for temperature measurement of underground cables, the mass of the transformer and type of the cooling mechanisms for temperature measurement of transformers, etc.

The key to the real-time thermal rating system, the temperature of the asset, should be continuously measured to avoid heating of the asset to dangerous levels. Distributed temperature sensing (DTS) systems should be used for this purpose, otherwise, only predictions about the temperature can be made.

The main advantage of real-time thermal rating systems is their ability to accurately measure the thermal behavior of assets, taking into consideration factors that static ratings do not. Static rating calculations are often overly conservative, therefore some power lines are often not used up to their full potential, while others are overloaded, which causes premature aging.

Thermal rating systems may be implemented into different types of power assets. Underground cable monitoring benefits greatly from introducing real-time thermal rating systems as they measure soil ambient temperature and soil thermal resistivity; these measurements help to determine actual thermal headroom to indicate unused network capacity.  

The thermal rating of the overhead power lines, depending on the actual system used, takes either the sag of the lines, the tension of the line conductor, the temperature of the line conductor, or environmental conditions into account during calculation.

Transformer load, ambient and transformer temperatures, oil temperature, and winding hot spot temperature are utilized by RTTR systems for transformers.

DTS optic sensing fibers are important for real-time thermal rating; they are installed along the length of the power cable and provide a continuous temperature profile.

If you would like to purchase DTS (Distributed Temperature System), please contact us: info@optromix.com or +1 617 558 98 58.

RTTR – Real Time Thermal Rating System

A real-time thermal rating is a monitoring system. It helps to effectively use current-carrying capacity. Basically, it allows avoiding making assumptions about the current load, and instead to ensure that it is used in the most efficient way and the probability to exceed the acceptable temperature is low.

Smart grid technology, a real-time thermal rating system, has been created to rate the electrical conductors affected by the local weather conditions. It provides accurate real-time temperature measurements and current reading along the entire high-temperature wire. The RTTR is embedded in the cable and calculates the capacity of the current under specific conditions. It is a perfect solution to monitor power cable performing under abnormal conditions such as different emergencies, energy outages, etc.

RTTR is often used with the DTS system of temperature sensors because it gives more accurate data and allows monitoring operations in the real-time mode. For cables that have temperature sensors (DTS) embedded or touching it, the temperature is monitored continuously and the rating can be indicated accordingly. The cables without DTS have their operational temperature are calculated based on the real-time installation condition and loading. There are two types of RTTR to monitor power cable: self-contained and environmentally based.

Self-contained real-time temperature collects the data along with the entire circuits; the embedded fiber optic cable measures the internal temperature, and the attached one measures the sheath temperature.

Environmentally based RTTR measures soil temperature and its direct effect on the cable. It also measures soil thermal resistivity, which affects the heat exchange rate between the cable and the external environment.

RTTR usually uses the following parameters for the calculations:

  • The ground type (soil, clay, sand, gravel, thermal backfill)
  • Burial Depth
  • Cable Type
  • Cable Structure
  • Other cables laid in close proximity

Rating calculations of the high-temperature wire are based on the data derived from monitoring the underground cable. Standard static ratings are usually conservative and understate the real feeder capacity; hence the feeders are not loaded fully most of the time. The real-time thermal rating allows determining the times when the cable is not loaded fully and when certain actions need to be taken.