Restriction in flow in Penstock

Well the gravity is doing the work, and as you use the gravity you are effectively lowering the head pressure, so less head pressure less flow....

https://www.omnicalculator.com/ecology/hydroelectric-power

Why restrict the flow.If we operate all the turbines and there are four relief valves also the flow of 84000 cfs can be achieved.But as soon as the flow increases above 60000 cfs one unit will trip.Flow restricted to 60000 against original capacity.Is the velocity in penstock a restriction

On what condition does <...one unit...trip...>? Overspeed? Overvoltage? Something else?

Please supply the forum with sufficient information to be able to give a useful answer.

"Head determines the maximum shaft power a turbine can produce. The power produced when the generator is synchronized is controlled by adjusting the amount of water flowing through the turbine. Power is RPM multiplied by torque and since a synchronous machine runs at constant speed, you get more or less power by producing more or less torque.

The way the flow is adjusted depends on the turbine type; for a Francis turbine, a series of what are called wicket gates are opened/closed to throttle the amount of water.
The offline generator is brought up to slightly above synchronous speed by adjusting the water flow before a sync check and breaker close."

https://control.com/forums/threads/controlling-the-speed-of-hydro-turbine.44345/

The generator, to match the frequency of the grid(50 or 60 hz), must run at the proper rpm, which is dictated by the water flow velocity, which is controlled by a throttling valve or gate...as the electrical load increases or decreases, you need a feedback mechanism that controls the amount of water flow to match the load requirement...If too much water is allowed to flow the generator goes overspeed and a mismatch of the frequency is created and trips the generator...

Is there an obstruction in the pipe: big lump of tree? Broken rowing boat? Dead animal?

One shouldn't expect the flow to be the same if <...a power house...> has been added, because some of the flow will be used to drive the thing that is producing the <...power...>, which has its own inherent pressure/flow characteristic.

The installation cannot be seen from here.

The unit trips by getting a signal from flowmeter that flow is above 60000 cfs

Then it seems nonsense to attempt operate the unit at <...84000 cfs..>!

• Where is the original commissioning documentation and what does it say on the topic?
• Where are the original installation design calculations and what do they say on the topic?
• Why is the trip set at <...60000 cfs...>?
• Has there been a change in temperature, and therefore density and viscosity, since the installation was commissioned?

Agreed but still there is capacity to release more than 60000 cfs but still flow is restricted

Something the size of a dead horse in the pipework is a possibility that needs to be eliminated.

Please eliminate the use of the word <...but...>. This word is only used to join two things that are in conflict and the above sentence contains two of them, making it difficult to follow.

At the moment, all possibilities need to be on the table, for investigation and elimination one by one.

<...Tried to get answer.but did not...>

Try asking more questions that are more probing and ask more widely. One will not succeed when one gives up.

I was about to comment when I saw your own comment.
I agree.
I would guess that the generators (or the available generators) are sized such that increasing flow beyond 60000 would cause damage to them.

regards,

Vulcan

You never should design or operate operate anything to the max capacity continuously.Some overhead capacity is needed to adjust for demand,likewise when demand is low,throttling is necessary.The design allowed for future expansion and demand,and is not currently requiring full capacity,therefore some units are shut down to prevent overloading the grid connection.As the future demand increases,the turbines will have the excess capacity to handle it.Electricity is addictive;there is never enough.