System Reliability

System Reliability

DOI: 10.4018/978-1-5225-4941-3.ch002


Power system adequacy and security with reference to probabilistic and deterministic approaches are dealt in the second chapter. Increased investment leads to improved power availability and underinvestment leads to reduction in quality of power supply. The related optimal reliability with respect to cost is explained here. Extensive computerization of data is required for assessing the benefit of expansion and reinforcement/augmentation schemes for obtaining improved availability in the context of deregulated utility environment.
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Deterministic Vs. Probabilistic Approach

Essentially, reliability studies provide predictions. They predict the future behavior of a device or system, based on past information and experience. Since predictions cannot be made with certainty, they are inherently probabilistic.

This is equally true for most engineering design techniques which also involve predictions of future performance. However, very rarely are probabilistic approaches used in these methods - they are mostly based on deterministic techniques. This also applies to the older methods used in reliability studies. While more complex and difficult, probabilistic thinking is gradually establishing itself in many areas of engineering (Endrenyi, 2000).

The Traditional (Deterministic) Approach

In this approach, system design and operating policies are based on pre-selected tests: failure criteria are defined so that certain combinations of system and load conditions must not represent immediate system breakdown or even excessive component stress. To make sure that these criteria are met, “worst-case conditions” are analyzed and the calculated stresses and strengths for the case are set apart by a “safety factor”.

The Drawbacks of the Traditional Approach

  • Variability in input data is ignored (data provide spectra, not fixed numbers).

  • Selection of “worst-case” conditions is arbitrary: important conditions may be omitted, unlikely conditions included.

  • The assumption of no failure risk in designs satisfying traditional criteria is misleading; in fact, the approach provides no idea how safe the design actually is.

  • The effort to stay on the safe side often results in over design.

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