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Electrical riddle no.3 – Earthquake in nuclear power plant

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      After one sever earthquake happened in the hypothetical nuclear power plant area, following observations were reported by special investigation committee that were authorized to estimate earthquake demolition in power plant.

      The mentioned Power plant has two units. Each unit has one control room. The control rooms have many quite similar electrical circuits integrated in protection and control panels. The panels are fixed on the floor of the rooms. According to the committee technical reports, one of the generator protection panels that were installed in unit 1 control room, broke from bottom of its base plate and fell down due to the strong vibration. On the other hand protection panels located in other control rooms (unit 2) withstood against mechanical stress. After more verification they found that the protective relays installed in inversed panel had operated correctly and related generator circuit breaker had received correct trip commands. But the protective relays located in other protection panels (unit 2) couldn’t have suitable reflection against earthquake.

      Consequently, one of the panels damaged due to falling but it could protect relevant generator, vice versa other panel that withstood against vibration didn't do its assignments (generator protection).
      Regardless similar electrical circuits, the method of devices installation in dedicated panel were different from the other panel. In the selfless panel, heavy equipments (e.g. matching current and voltage transformers) were installed on the top of the panel and main generator protection relays were located in the bottom side but in the other panels, heavy equipments were installed down and protective relay were fixed on the top of the panels.

      How can you explain different behaviors of protection panels against earthquake waves?

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        Re: Earthquake in nuclear power plant06/30/2007 5:47 PM The S waves of a quake may like light or waves in water through a slit get refracted or reflected.The one panel may have been at a spot where the combined waves were superimposed,while the other in a spot were it was almost canceled. It is a scary thought that nuclear plants may be vulnerable to earthquakes as well. __________________ Old technology at work (play).


          Guest Re: Earthquake in nuclear power plant 07/02/2007 2:30 PM I think the vulnerability to earthquake always needs to be considered in design. Such plant are always vulnerable, just a question of how much damage will be caused. There are nuclear plants in California, and all are relatively close to active faults (zone 4). The designs take the seismic risk in to consideration and over build for it. On the other hand, some designs do not adequately take any risk consideration or redundancy into the design, chernobyl or any Russian reactor is a good example. In essence, with nuclear power plant, liek everything else, you get what you pay for.


            Hi, Interesting situation. I have some experience with this situation in NZ – mainly in HV generation and distribution stations. It is suggested that the outcome and the problems experienced could be related to: 1. Whether aseismic blocking units had been fitted – these are often used to prevent mal operation of Buchholz protection on transformers. 2. The centre of gravity and positioning of items in panels – as you have indicated. Mounting of heavy auxiliary CTs etc has a direct effect. 3. Seismic restraint of the panels. Control and relay panels are often supported at the top in addition to suitable mounting of the base of the panels to reduce the likelihood of swaying or tipping during earthquakes. Regards, PC


              The specific seismic class shall be considered for important electro technical equipment, particularly for nuclear power stations, where the acceleration to which the equipment is subjected will be specified in the response spectrum. As you know, the vibratory stresses to which the functional devices will be subjected must be evaluated, and there must be assurance that they would function correctly when put under these stresses. Therefore the nature of the transfer of ground seismic solicitation to functional cells (e.g. protective relays), must be taken into account at the design stage. The fragility threshold of functional devices (level of acceleration at which the equipment no longer performs its function) has to be determined be forehand. When a structure (generator protection panels) undergoes floor acceleration “a (t)” (i.e. an earthquake) everything happens, supposing that the floor is fixed, as if each element of mass m of the structure was subject to an inertia load Ft = -m . a (t) The equation of motion for damped forced vibrations is Ft =mx”+cx’+kx In the first mode of vibration, the acceleration of mass increased by its displacement. Therefore, the acceleration of mass in top of protection panel is more than its bottom portion. As a result, when generator protective relay installed in top of panel, it is possible to exceed acceleration of relay mass from its vibration withstand.

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