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VOLTAGE DIPS

In the case of voltage dips, surveys are not useful for transferring their data to particular sites, for the following reasons:
? The data on the occurrence of voltage dips is not fully reliable as indices are not freely or easily available and network or user system reconfigurations may change it very
significantly.
? A proportion of voltage dips are unlikely to be designated interruptions and may well be generated locally by user heavy loads, especially during start-up. This further complicates information relating to the occurrence of voltage dip and depth.
? Device/system immunity plays a much bigger role than in the case of interruptions; voltage dip immunization is more case specific and more sophisticated than the equivalent for interruptions.
When analyzing PQ costs there is always the question of whether this analysis is deterministic or stochastic – put simply, the balance between certainty, risk and uncertainty.
With interruptions, especially long interruptions, the level of certainty is reasonablyhigh, whereas with voltage dips and even more so with harmonics and other voltage distortions/variations, the term probability is highly relevant. Consequently, to the equation for cost per event times number of events per time unit (usually one year), one should add the additional factor of probability as this answers the question of whether the event under consideration will result in cost and wastage.
The first thing to be determined is, as in the case of interruptions, the frequency of occurrence. Network operators may have some data, e.g. that derived from continuous
measurement of voltage (at each phase), at different points and different voltage levels of the network.
To gain a better understanding of voltage dips that occur in the European MV supply networks, the former UNIPEDE group of experts DISDIP [12] carried out a coordinated
measurement campaign over a period of three years in nine countries with different climatic conditions and network configurations.
The survey was carried out at 126 sites with standardized measurement and evaluation criteria, with the maximum duration of a dip set at 60 s in order to include less frequently
occurring longer-duration dips. The measurements were taken at theLVbusbars of distribution transformers and at various locations with the aim of ensuring that the results could be seen as representative of public LV networks in general. The results are presented in Table 5.1.
Further, for each site a so-called vulnerability study was performed. Devices like switching apparatus – namely, relays or contactors, electronic controls – have different immunities described by immunity curves. This study was trying to determine at which dip duration and depth the process will stop or will be influenced. Determining the level of
vulnerability of each single component to influence the tripping of the entire process would accomplish the task. Methodologies exist on how to carry out this part of the analysis, but it should be emphasized that such work requires knowledge, skill and experience.
Table 5.1 Frequency of occurrence of dips per annum with a 95% confidence interval (Reproduced from Voltage Dips and Short Interruptions in Electricity Supply Systems. UNIPEDE Report 91)

VOLTAGE DIPS

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