I aplogize for not being clear, The Game and Fish Commision requested SNL to shock Walley. The main point is the Genertor ACB was opened and the unit ran untied to the grid for 8 hour. Normally when a unit is at speed the Gen ACB is closed except during testing or when the units being brought online and being sync'd.
An operator on Friday went from full load to SNL and opened the Gen Breaker, Wicket gates were at 10%. This continued for 8 hours. Note: This unit was not condensing.
I was under the impression when the unit is tied electrically to the grid it is in a much more stable condition since its being controlled by frequency via the Governor System - versus untied and being controlled by just the governor system.
Also with the Gen Breaker open is all relay protection still in place?
I've been operating for 34 years and have never seen this done. I dont understand the logic here - simply why would some one think it is ok to do that? This is a new guy.
At low power levels, some generators experience increased cavitation damage to the turbine blades. I am not sure if this can happen at SNL, but it sounds unusual to have the generator at SNL for a whole shift. Cavitation damage occurs over a lengthy period of time, rather than an abrupt failure.
When I visited a hydro plant with synchronous condenser capability, the operator explained that the condenser units are dewatered using compressed air after synchronization so that they spin freely.
It does not make much sense to run the unit at SNL, unsynchronized for extended periods. Why not run it on line at a small loading like a few percent of rated, safely out of the cavitation zone. With 10% gate opening to get SNL. this sounds like a large slow speed Francis or Kaplan unit. If synchronized, then a fast load pickup is always possible if required by the dispatcher. Furthermore, the unit can contribute to VAR sharing with the other unis that are on line.
Thanks for all the input, my big concern was the Generator breaker being opened, therefore the unit was unprotected since the unit CT's read accross the closed contacts of the breaker. If the unit had failed electrically the CT's would not have seen a differential accross the breaker.
The Walley are fine, G&F shock the fish every spring to bleed their eggs for the fish hatchery. I tried throwing a net back there once - they didn't care for it much.
The differential would have seen current from the neutral end and nothing from the terminal end, easy trip, but what to trip? Wicket gates, but then what?
"Shock" Walley?? Is this a pressure shock when Walley passes through the turbine?
or
Do you throw electrodes in the water and shock Walley electrically?
Bill
--------------------
"Why not the best?"
Jimmy Carter
On the 65 MW hydraulic units I ran, any trip from protection would open the unit and field breakers, close and pinch the wicket gates, start the jacking oil pump, and apply brakes once the unit speed had dropped below ~ 50%. A headgate drop would only occur due to overspeed or loss of relay oil operating pressure needed to drive the wicket gates.
Since the above were equipped with turbine-type runners and cavitation became a definite problem at any wicket gate opening below about 70%, operation at SNL was avoided as much as possible, and minimized when unavoidable.
As bacon4life alluded to, units whose runners are sited below tailrace elevation can employ compressed-air 'tailrace suppression' to enable units to stay on line as synchronous condensers, but this is an expensive option and would have to have a significant favourable cost-benefit ratio to even be considered.
Question: was there a new requirement to provide a minimum flow over downstream riffle beds where fish might rest and recoup before attempting to jump some falls or rapids?
I don't know this unit, but I would assume it is impedance grounded, and that most likely a fault would be a phase to ground fault which would pickup the neutral protection. Not much current would be needed, just voltage.
Some of us use the hydro units more as a control for water, not just power. So it might be assumed they were just letting water through, because they did not need the energy.
Sometimes they run them at SNL for reserve power, especially since its hydro (some call it spinning reserve but normally spinning reserve is connected to the grid already).
Even though the generator breaker is open, protection is typically still in service (don't typically monitor breaker status to activate protective relays). Relays typically monitor the breaker status and are smart enough to know what to activate etc. A big factor here could be the switchyard configuration.
![[wink]](/data/assets/smilies/wink.gif "[wink] [wink]")