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Results - Fault Tree Analysis

Results from analyses of the fault tree model of the 415V diesel-backed bus

Let's look at the results from the fault tree analysis of the 415V diesel-backed bus.
We have three questions:

1. How safe are my day-to-day operations?
2. Which are my systems weaknesses?
3. For how long periods of time will I be unable to deliver power and how often is my system likely to fail?

Let's look at what answers the fault tree analysis give us:

1. The probability (mean unavailability) of ending up in the unwanted event: "No power on the 415V bus" is: Q=1.71E-04. The MCS-list shows that the dominating combinations of events are failure of the external grid and failing connection switch or diesel generator.
2. The RDF, RIF-factor and the sensitivity measures, in the importance and sensitivity analysis, indicate that you would gain the most from improving the reliability and availability of the external grid. The event "Inadvertent disconnection of the isolator switch" (ISO-D) has a RDF-factor of 1.48 and RIF-factor of 590. Based on these results the recommendation is to install another isolator switch in parallel rather than trying to decrease the failure probability of the isolator switch. The event: "Diesel generator fail" (DIESEL) has a *RDF-factor of 2.01 and a **RIF-factors of 9.86. Recommendation: increase the diesel generators availability before installing a diesel generator in parallel.
3. The ***unconditional failure intensity indicate that the 415V bus, on an average, will fail once every 8.9 years of operations and during the same time period be ****unavailable 13 hour and 20 minutes (1.71E-04*8760*8.9= 13.33).

** RDF = Risk Decrease Factor:

Also known as risk reduction worth.

RDF for a basic event with the probability Qi is calculated using the following equation:

where QTOP is the top event probability and QTOP(Qi=0) is the top event result when the basic event Qi is perfectly reliable.

** RIF = Risk Increase Factor:

Also known as risk achievement worth.

RIF for a basic event with the probability Qi is calculated using the following equation:

where QTOP is the top event probability and QTOP(Qi=1) is the top event result when the basic event Qi is certainly failed.

*** The unconditional failure intensity:

The unconditional failure intensity is W= 1.28E-05/h. This means that the power system will fail approximately 0.11 times per year or once every 8.9 years of operation.

****Unavailability:

The MCS-analysis states that the mean unavailability for the 415V bus is Q= 1.71E-04. This means that during 8.9 years of operation the bus can be powerless approximately 13 hours and 20 minutes.

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