Standards Development

Scope

The general objectives of this Technical Specification are to (1) describe hydraulic transient phenomena of hydro turbines, storage pumps and pump-turbines and the factors that affect them, (2) recommend modelling and measurement best practice guidelines and resulting limitations (3) define relevant equipment design criteria and (4) and identify potential mitigation solutions. Definitions of the relevant terms and quantities are provided along with descriptions of the system components that should be considered.

Various types of hydraulic transient phenomena are covered in the current specification, including waterhammer, surge tank mass oscillation, and speed variation of the hydraulic machine for operational modes such as start-up, normal shutdown, emergency shutdown, and load rejection and acceptance. Combinations of the individual transient modes are also considered. The links between these transient characteristics and operation across the relevant quadrants for each machine type are illustrated. Methods for mitigating extreme transient values of pressure, surge tank water levels, and unit rotational speed are suggested.

In this Technical Specification, abnormal transient phenomena are also defined and described, including component malfunction and catastrophic events like component rupture. The probability of the occurrence of these extreme events and how this might influ ence the necessary safety margin is described.

Different stages of hydroelectric development correspond to unique sets of available data with differing levels of detail and uncertainty. The various stages of project development are described in relation to transient analysis. The corresponding uncertainty margins associated with each stage and analysis technique are discussed.

This Technical Specification provides guidelines and commonly accepted practices to model and compute transient conditions. It provides a summary of the basic hypotheses and equations, together with relevant characteristic quantities and system time constants. Accepted methods of modelling hydraulic components, and related numerical simulation methods are identified. This specification details the required input data, including best practices for model testing of hydraulic machines, valves, gates, etc. in o rder to acquire reliable transient modelling.

Different calculation methods with different levels of uncertainty are described so that the most suitable approach for the available data or project stage can be selected. Limitations of one-dimensional modelling methods are described. Additional means of investigation, such as physical model tests or CFD computation, can be used to improve the simulation results.

For various configurations of equipment and operating regimes, typical scenarios for consideration are defined.

Procedures to determine uncertainty margin, with respect to the modelling and computation methods and available input data are recommended.

Finally, this Technical Specification describes methodologies for on -site measurements with respect to transient such as load rejection tests, runaway tests, etc. Recommendations are provided for quantities to be monitored during these tests, with related instrumentation, calibration and data acquisition systems. Procedures for comparing on-site measurements with numerical simulation results are proposed.

Note that the following aspects influence hydraulic transients and are therefore introduced; however, the associated modelling and related design guidelines will not be addressed in the Technical Specification:

• waterway design and optimization

• upstream and downstream free surface flows

• fluid structure interactions

• cavitation and water column separation

• unsteady friction and viscoelasticity

• two phase flows

• turbine governor design

• flow induced pressure fluctuations

Purpose

Proper treatment of hydraulic transients is important for utilities, equipment owners, producers as well as equipment suppliers. During transient operational modes, hydro power equipment can be exposed to large dynamic loads that, in extreme cases, can lea d to catastrophic events. Insufficient understanding of basic transient phenomena and related modelling can lead to inadequate design safety factors that can put both equipment and related plant personnel at risk.

The occurrence of transient phenomena during hydro power production is unavoidable. Simulation and testing of hydraulic transients provides critical values throughout the project development that should be used to establish and validate guarantees for maxi mum and minimum allowable pressures, rotational speeds, torques, etc. The primary goal of a hydraulic transient analysis is to support the safety of the hydroelectric equipment and any personnel directly related to the environment produced by a hydraulic t ransient event.