Treeing electrical

Modern subsea trees, manifolds, (EH), etc., are commonly controlled via a complex Electro-Hydraulic System. Electricity is used to power the control system and to allow for communication or command signalling between surface and subsea. Signals sent back to surface will include, for example, subsea valve status and pressure/ temperature sensor outputs. Hydraulics are used to operate valves on the subsea facilities (e.g. subsea tree and manifold valves). The majority of the subsea valves are operated by hydraulically powered actuator units mounted on the valve bodies. 

Generally, trees occur under the relatively high voltages associated with power cables (11—13). Trees can be classified in three classes electrical, water, and electrochemical. 

Electrical trees consist of visible permanent hoUow channels, resulting from decomposition of the material, and show up clearly in polyethylene and other translucent soHd dielectrics when examined with an optical microscope. Eresh, unstained water trees appear diffuse and temporary. Water trees consist of very fine paths along which moisture has penetrated under the action of a voltage gradient. Considerable force is required to effect this... 

Electrochemical treeing is appHed in those cases of water treeing in which the water contains solute ions which move under the action of an electric field and are detected within the insulation layer, or at an electrode surface after having passed through the insulation. They are not encountered as often as the first two classes, for example, trees formed in a cable exposed to a hydrogen sulfide environment called sulfide trees. 

For current consoHdation, the basic circuits, used at each of the multiple power take-off points, are stacked into a Christmas tree topology to form a single power take-off terminal pair. Scale-up of these devices to commercial sizes is not expected to be a problem, as standard electrical components are available for all sizes considered. A different type of consoHdation scheme developed (117), uses dc to ac converters to connect the individual electrodes to the consoHdation point. The current from each electrode can be individually controUed by the converter, which can either absorb energy from or deHver energy to the path between the electrode and the consoHdation point. This scheme offers the potential capabiHty of controlling the current level of each electrode pair. 

Hence, it may be imagined that fault trees hang from each mode, as shown in Figure 3.4.5-3. Electric power... 

Function event trees include primarily the engineered safety features of the plant, but other systems provide necessary support functions. For example, electric power system failure amid reduce the effectiveness of the RCS heat-removal function after a transient or small UJ( A. Therefore, EP should be included among the systems that perform this safety function. Siipfiort systems such as component-cooling water and electric power do not perform safety functions directly. However, they significantly contribute to the unavailability of a system or group of systems that perform safety functions. It is necessary, therefore, to identify support systems for each frontline ssstcm and include them in the system analysis. 

The use of component logic models to build system fault logic has been discussed by several authors for chemical and electrical systems (Powers and Thompkins, 1974 Fussell, 197.S and Powers and Lapp, 1976). In addition, generic sabotage fault trees have been used for some time in the analysis of security concerns for nuclear power plants (NUREG /CR-0809, NUREG/CR. 121,... 

Direct Introduction - This involves locating, in the event tree, support systems (e.g., electric power) before the systems they affect. The event tree is constructed so that systems requiring the support systems are bypassed as failed. This is the technique in WASH-1400. 

Fig. 3.5.7-1 Fault Tree Diag Emergency Electric Powe.

Accident progression scenarios are developed and modeled as event trees for each of these accident classes. System fault trees are developed to the component level for each branch point, and the plant response to the failure is identified. Generic subtrees are linked to the system fault trees. An example is "loss of clcciric power" which is analyzed in a Markov model that considers the frequencies of lo,sing normal power, the probabilities of failure of emergency power, and the mean times to repair parts of the electric power supply. 

Fussell, J. B. 1975, Computer Aided Fault Tree Construction for Electrical System and Fault Tree Analysis, SIAM, Philadelphia, PA, p 37. 

Fault Tree diagram of the emergency electric power system. F... 

RED of a Simple Injection System. Fault Tree of Emergency Electric Power... 

The component failure rate data used as input to the fault tree model came from four basic sources plant records from Peach Bottom (a plant of similar design to Limerick), actual nuclear plant operating experience data as reported in LERs (to produce demand failure rates evaluated for pumps, diesels, and valves), General Electric BWR operating experience data on a wide variety of components (e.g., safety relief SRV valves, level sensors containment pressure sensors), and WASH-1400 assessed median values. 

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