PSI - Issue 28

Evgeniia Georgievskaia et al. / Procedia Structural Integrity 28 (2020) 836–842 Evgeniia Georgievskaia/ Structural Integrity Procedia 00 (2020) 000–000

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1. Introduction In the past two decades, one of the world's trends, as noted by Trudel (2017), Favrel et al. (2019) and many other researchers, this is the widespread and rapid spread of new renewable energy sources such as wind, solar, tidal, wave, which characterized very nonstable levels of power output. To support the stability and reliability of energy systems new requirements for significantly extending the operational modes of hydraulic turbines (HT) appear because HT is the most maneuverability unit among traditional power generating ones. As a result, HT used in off-design operation modes more and more frequently, longer and longer, sometimes in the rough zones. The technical term “off-design” modes/conditions should be understood as non-stationary or transient modes such as startups and shutdowns, low and medium partial load operations, synchronized compensation, and speed-no-load (SNL) that not provided at the design stage for long-term operation. At transient and off-design conditions, the dynamic loads can increase several times to compare the stable operating at the best efficient point (BEP). This phenomenon is widely investigated in the last time, for example, Favrel et al. (2019, 2020), Duparchy et al (2017), and Sakamoto et al. (2019). Long-term operation in these modes and frequently start-stops cause accelerated progress of defects, premature crack growth, significant reduction of a lifetime, and ultimately, can leads to a grave accident with large losses.

Nomenclature BEP

the best efficient point hydraulic power plant hydraulic turbines

HPP

HT

SNL PAS

speed-no-load

predictive analytics system

2. Main problems of the equipment owner Increasing the reliability, efficiency, machine production, and minimization of the maintenance costs of the HT is directly related to the potential failures that can occur on the turbine’s components. There are three main problems which most relevant today. The first problem concerns the age of the equipment. Most of the hydraulic turbines had designed for 25, 30, or rarely 40 years and today their design lifetime is over but they continue to rotate and generate power without anything serious reconstruction, modernization, changing main elements, or limiting operating conditions. The risk of failure increases with every year of operation and especially dangerous for large units. Another problem is changing operating conditions provided at the design stage. Today, long-term operation at off design modes and sufficient increasing the number and duration of transient processes as a comparison to the base operation near the BEP are typical. The significant higher stress level of the unit’s component under these regimes explains the necessity of more frequent repair, especially for exhausted design lifetime units. So, the reliability and power output of such turbines are decreased in contrast with the cost of unit maintenance that increases from year to year. So, and the owner needs more and more money to ensure the trouble-free operation of the unit at HPP. The third problem is the individuality of hydraulic units that connected to original machine design and construction, deviations during the manufacture and installation, features of the welding technology, machine refurbishment and modifications, environmental factors, maintenance history, and specific operating conditions. Every year brings new differences from the original turbine design. Unlike mass-produced products, hydraulic turbines are unique technical objects. Thus, it is not possible to use statistical data on the growth of defects, failures, and damages in their elements. Only an individual lifetime assessment can guarantee the reliability and safety of HT. Also, for large and expensive machines such as HT, the crash-test is not possible even for a separate machine component not to mention the whole unit.