Transient analysis

What's a transient and why does it matter?

KGAL's transient analysis can help prevent catastophic hydraulic shocks in pipelines

By Nick Crosby on 28th Apr 2017

Every generating set changes load to grid control instruction and responds to changes in grid frequency automatically as determined by the governor settings.

Massive losses of load or trips in response to emergency conditions require that the huge quantity of energy flowing into the prime mover be switched off to prevent dangerous overspeed on the generating set. On a steam turbine, the steam valves close in a few hundred milli-seconds and the boiler safety valves lift to disperse the energy still being produced by the combusting fuel. Diesel engines and gas turbines cut the fuel supply rapidly and as close to the point of use as possible to reduce the amount of fuel in the supply pipes that might cause an overspeed.

And what of a hydro generating set? How does it respond when the load is lost or equipment trips?

With no combustion involved, it might seem a simpler operation to just turn the tap off that controls the water supply. But with a long supply pipe there may be many thousands or even hundreds of thousands of tonnes of water on the move. Stopping such a mass of moving water is a complex problem, one that is known as water hammer.

 When a hydro turbine responds to a load rejection, its governor begins to turn off the “tap” of water flow. Pressure waves are created that travel back through the water column at the speed of sound in water, waves that reflect off bends, transitions, bifurcations and leave pressure drops in their wake. To minimise the pressure waves, the closing of the water flow tap needs to be controlled with time in the order of seconds. The energy flow into the turbine continues for the whole of this period and the energy flow can actually increase as the pressure waves reflect, increasing the driving force of the water at the turbine. The resulting overspeed is typically in the range 40% to 50% with a hydro generating set compared to 5% to 10% with thermal generating plant.

To optimise the ratio between the pressure-rise and the speed-rise, it is necessary to analyse the pipeline, the turbine and the generator. Each hydropower station is unique; the type of turbine, the generator inertia, the pipeline layout - each station must be carefully analysed on its own merits.

The penalties for getting it wrong are major. At the extreme end, the consequences can include the destruction of the generating set or even the whole facility. Fortunately, such events are rare.

KGAL has a sophisticated transient analysis package that covers all aspects of hydraulic, mechanical and electrical transients in hydropower. And most importantly, KGAL has the knowledge, experience and data base to use the software on any hydropower station with any type of turbine based on only the most basic of data about the installation – vital experience when considering refurbishment of existing power stations where data and characteristics might have been lost in the mists of time.

The same package can be used to analyse any pipeline not just in a hydropower plant. Pipelines through dams with control valves, pumping stations and gravity fed water supply pipes can make use of the same analysis package.

So if you want to control water in a pipeline you need KGAL’s transient analysis expertise. 

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