System Reliability Considerations

Abstract

The cost and reliability are two outstanding barriers in the promulgation of solar PV technologies on mass scale. In solar PV power system context reliability involves two factors: (i) the solar array need the use of an optimum load to deliver maximum power. This requirement varies with solar irradiance, temperature and battery bank characteristics and often difficult to be realized in practice. (ii) The reliability also means the ability of the system to continue functioning without failure for the period of time intended and under the given operating conditions; for example, during the array operation in field, the partial shadowing, soiling, cracking (provoked by hail impact) or opening of a string may occur due to solder melting or damage to encapsulate and in extreme cases it can lead to system failure. In both cases, the system reliability can only be guaranteed on a probabilistic basis. It essentially involves probability of success/failure, adequate performance, on/off time and operating conditions. Thus the probability provides the numerical input for the assessment of reliability and also the first index of system adequacy. For mission-oriented project this definition of reliability can be regarded as suitable measure. However, it becomes rather unsuitable measure for those continuously operated systems that can accommodate failure. The suitable measure used for these systems is availability. For example, in power system, availability is the percentage of time the system will deliver power to its load (Stember 1981). The required availability is different for different types of systems. Availability may be obtained by dividing “up-time” by “up-time plus down time” or it can be expressed aswhere A = availability, MTTR = mean time to repair and MTBF = mean time between failure.