M.V. LIMITING FUSE WITH THERMAL PROTECTION 1
INDEX 1.- Why Limiting Fuses? 2.- When does those situations happen? 3.- Market Solutions 4.- MERLIN GERIN Thermal Protection 5.- Technical, economical and safety advantages 6.- Note 7.- Real example 2
M.V. LIMITING FUSE WITH THERMAL STRIKER 1 WHY LIMITING FUSES Most of M.V. fuse that are used world wide to protect transformers are back-up type fuses. Frequently, those fuses are combined with Switch- disconnectors which are provided with automatic opening after the tripping of any signal, as a striker, coil, relay, etc. As a matter of fact, these type of limiting fuses and their holders are not able neither to break and actuate with currents lower than 3.5* In ( minimum breaking current of the fuse ) nor to support long electrical overcurrents. When these fuses are mounted in compact switchgears, these electric/thermic overcurrents increase the risk of material destruction putting in danger the security of the network and people. 2 WHEN DOES THOSE SITUATIONS HAPPEN? Normally M.V. limiting fuses are usually working in service conditions, well under their rated current and in consequence without any abnormal overheating. However, there are some situations which can give incorrect overheating situations originated by the characteristics of application (Long and/or frequent overcurrents), the characteristics of the compact switchgear, of human mistakes as wrong rating selection, bad installation, etc., 3 MARKET SOLUTION The market has some solutions which can avoid those dangerous situations: 1º Using fuses with Full Range protection ( Full Range class ). 2º Using a combination of fuses and protection relays. 3º Incorporating thermic mechanism which actuates at lower temperatures than the ones considered dangerous for the fuse. First two solutions require much higher cost and investment than a normal back up fuse. 3
4 - MERLIN GERIN THERMAL STRIKER. Schneider Electric has successfully developed the thermal striker, which has been incorporated at all our range of M.V. limiting Fuses. This device protects thermically the fuse and the fuse enclosure from any overheating risk which can produce dangerous situations in the installation. This new thermal characteristic has been implemented without any price increase on our prices. 5 - TECHNICAL, ECONOMICAL AND SECURITY ADVANTAGES. Lot of test have been made by MESA to check and verify the good reliability of the new device chosen for our fuses. This solution allows to limit the temperature of the components of our fuses in different type of installations and situations, as for instance: End cap temperature < 120 ºC. The thermal striker in our fuses provides the following additional advantages: Better protection to the fuse and their environment against non-acceptable overheating in installations which are combined with switch- disconnectors with possible automatic opening. Meet unexpected working conditions, with long or frequent overcurrents ( fig.1), as well as human mistakes in selection the correct rating or against installations with unventilated conditions. Reduce the operation costs originated by: - Destruction of the equipment in case of overcurrents below I3. - Quality service degradation. - Repairing times, staff, Increase security in the network. 6 - NOTE The development is available for all the range of fuses MERLIN GERIN, which are provided with tripping striker. 4
7º REAL EXAMPLE See enclosed the result of a test for a 800 kva transformer at 13,2 kv, protected by a fuse type FUSARC CF 24kV/63A in normal conditions installed in a fuse holder of a RMU. the result is as shown in figure 1. 90ºC 122ºC 76ºC 80ºC Figure 1 In case of a internal fault in the transformer, which provoque an over current of 1,7 times the rated current of the transformer and the installed fuse is not provided with thermal striker, the reached temperatures and the thermic situation will be as shown in figure 2. 213ºC 299ºC 170ºC 180ºC Figure 2 However, if the fuse FUSARC CF 24kV/63 A is equipped with thermal striker and combined with the switch-disconnector of the RMU, the maximum temperatures registered in overcurrents conditions (1,7 times the rated current of the transformer ). As shown in figure 3. 130ºC 190ºC 100ºC 110ºC Figure 3 5
Curve of actuation of the thermal striker Melting curve of the element fuse FIG 1 Time / current average curve for thermal striker and fuse element 6
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