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LIOS offers Real Time Thermal Rating / Dynamic Cable Rating (RTTR/ DCR) solutions for ampacity predictions of the power cable and overhead transmission line installation transparently integrated with its field proven DTS systems.
Temperature monitoring is a key factor for the optimisation of underground power transmission and distribution installations. The conductor temperature depends on the load, but other factors such as the thermal soil resistivity, the power line arrangement, adjacent cables and other sources dissipating heat into the surrounding area have an important impact on the performance of the installation.
The availability of Distributed Temperature Sensing (DTS) systems that measure in real time temperatures all along the cable is a first step in monitoring the transmission system capacity. Dynamic or Real Time Thermal Rating (RTTR) provides the ability to the Operator to predict the behaviour of the transmission and distribution system upon major changes made to its initial operating conditions.
Why adding RTTR to Distributed Temperature Sensing?
Real Time Thermal Rating (RTTR) or Dynamic Cable Rating removes all uncertainty left by the DTS. The DTS measures the real time temperature at the sheath or jacket of a cable. The sheath temperature gives a good idea of the temperature of conductor, but unless an accurate model for the conductor is provided there will be some uncertainly left. The uncertainty is small during steady state operation, but it could be (very) large during an emergency situation. The following figure illustrates the temperature of the jacket and conductor during an emergency situation.
One can appreciate that while the temperature difference between the jacket and the conductor can be small in steady state. However, moments after the onset of an emergency situation the temperature difference could be very large. The reason is that cable insulation has a large inertia and therefore the heating of the conductor can only be detected at the jacket several minutes (to hours) later. Additionally, the temperature difference changes with the loading level. The temperature difference is larger for larger loading levels.
Typically the actual maximum temperature reading of each configured cable section and the actual electrical current reading are computed to build the dynamic cable rating of the installation, based on the IEC 60287 and IEC 60853 standards.
Key capabilities of the integrated RTTR solution:
- Distributed temperature measurement along the power cable
- Calculation of the power cable conductor temperature at the core of the conductor directly derived out of the online measurements (Steady State and Transient Operation)
- Prediction of emergency ratings, transient calculations for Time, Current, Temperature (Transient Operation)
The RTTR engine computes the current-carrying capacity (or ampacity) under given conditions of the underground cable installation for the steady state and transient. Cable Operators will greatly appreciate the transient simulation that will allow them to estimate the current that can be safely transferred from another circuit to the monitored installation, due to unusual operating conditions in situations such as emergencies, maintenance, outages, faults, etc.). The RTTR engine can be used for emergency ratings from 10 minutes and up to 2000 hours. This covers the entire emergency rating spectrum since typically emergencies last a few hours or days.
Modelling Capabilities
Virtually every cable construction available in the market can be modelled: one-core, three-core, sheathed cables, concentric neutrals, armoured cables, screens, shields, beddings, servings, jackets, combined sheath, etc. Most of the installation types can be modelled: duct banks, backfills, directly buried, buried ducts, buried pipes, cables in air (including groups of cables and riser poles) and cables in tunnels. The installation may include adjacent heat sources/sinks such as steam or water pipes. Unique to LIOS RTTR is its ability to model several materials with different thermal resistivities, for example: stratified soil layers, multiple duct banks and multiple backfills.
Transient Calculations
The cable operating temperature very much depends on the load shape applied to the cable. In other words, the temperature of a cable depends on the intensity of the current and its time variations. Therefore, cables have different ratings, i.e. steady state, cyclic, emergency and short circuit. Since cables installations have thermal inertia, it takes time to heat up the cable and its surroundings.
The emergency rating provides the following information useful for the cable operator under newly given operating conditions:
- Based on a higher load applied for so many hours: What the cable temperature will be at the end of the emergency case?
- Given the operating temperature and the applied (over) load, the RTTR software predicts the temperature of the cable in the future.
- Based on a higher load for a given period of time When will the installation reach its design emergency temperature?
- Given the operating temperature and the applied (over) load, the RTTR gives the time that it will take to the cable to reach a specified emergency temperature.
- Based on initial conditions and a maximum operating temperature What is the maximum current that can be pushed in the system?
- Given the operating temperature and a time frame for an over load, the RTTR computes the maximum current that the circuit can carry to reach certain emergency temperature.
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