Standard values for d (overshoot voltage) and ts on avr

[uhpo]

I know there is some standard to define the overshot voltage and ts time (time to reach a steady state), i would like to know about the calculation values, cause if you see on table there is terrible high values, 80% overshoot i think it´s too big..
I ask for the standard that define this, and also if you think this values of the table a aplied to hydro units with salient poles, and what do you think about ts of 10 seconds, do you know the usual values for example hydro unit about 300 rpm and 5 to 10 MW.
thanks in advance

 

[uhpo]

this is the curve i refered also

 

[rasevskii]

This is very old stuff you are looking at. With a modern static exciter the overshoot is only a couple percent, with a brushless system (not applicable to slow speed hydro units usually, but it has been done as retrofits) Even with the usual speed rise of 30% on 100% load rejection of a hydro unit, a modern AVR and static exciter system should hold the voltage down to a couple of percent overshoot. This is from site experience, however.

With old mechanical regulators, another story. Replace them with modern equipment.

rasevskii

 

[ratz1]

The attached figures are related to step change in reference. Let say with the generator at no-load step change the reference value from 95 to 105% of the rated one.
In this case also the 80% is not huge; it means step from 95 to 105% => 10% step and 80% overhoot =>8% ; final result is the voltage jumping from 955 to 113% and then settling to 105%.

Other things are the voltage overshoots generated after the sudden load removal (extreme case = opening the generator CB at rated load).


factors that determine overshoots:

1. Speed increase ( how much and will depend on the shaft inertia and how fast the valve could be closed); for hydro units overspeeds of 150-200% are nothing strange


2. Voltage increase (overshoot)

The overshoot magnitude depends of:

a) generator transient reactance X'd ( higher X'd = higher overshoot); this portion could not be improved with the AVR

b) lood level before breaker opening, in particular mode the reactive power => excitation current


c) Excitation type ( static exciter or brushless exciter) and AVR type (with or without negative forcing capability);
The forcing capability of the exciter will define how fast the rotor will be demagnetized. Fast static systems ( brush type generators) with negative forcing capability are the best choice to quickly restore the voltage at reasonable levels; brushless generators do not have possibility to force the negative voltage on field circuit, so the voltage decays with the natural generator time constant (T'do) which could be pretty long (5-10 seconds).

The voltage increase up to 130-160% and restoring in 5-10 seconds for brushless machines is quite normal. With the brush machine and static excitation the values are much lower, 120 -130% of V restoring in 2-3 seconds.

 

[rasevskii]

for ratz1:

Are you quite sure?

"130-160% and restoring in 5-10 seconts (etc)" for a brushless would seem quite high in my experience. The stator would saturate long before 160% was ever reached (in my opinion) and at least the OV protection should trip the unit and the overfluxing protection should trip if the unit were connected to its own transformer. Are you speaking from site experience???

Many years ago I was involved in performance tests for a 30MW Kaplan unit with a top-class manufacturer(World Class OEM), at full load rejection and static excitation optimized, the stator voltage trace on the UV recorder hardly showed a blip at all when the GCB was opened at 100% load rejection. But that was an OEM of fame.

The same for "120-130% of V restoring (etc)" (for static systems): a good static system should hold it down to less than 5% with negative field forcing voltage, if the regulator is set up properly and a high ceiling to no-load voltage ratio is available (3 times or more). The speed rise is irrelevant if the excitation system is properly designed.

Also a speed rise of 30% (130% speed) is normal for a hydro unit, (Francis and Kaplan) but 60% can be eypected for tube turbines that have direct connected generators with low inertia (space constraint). For Pelton turbines only a few percent rise due the fast opeation of the deflectors. You said a speed rise of 200% (100% overspeed)- really? Where was that and how fast did people get out of the station...

Again: are you speaking from site experience?? I am.

Of course with today`s quality of equipment and el-cheapo OEMs anything is possible. Forgive the criticism.

Comments anyone? Wolf39?

rasevskii

 

[uhpo]

I wonder how much overvoltaGE is permited and how much time to reach the steady state, cause i have some discusion with manufacturer.... and if somebody know some standard of IEEE or something similar please let me know
thanks all

 

[uhpo]

and another point, here is no negative forcing...just DECS 200

 

[rasevskii]

For uhpo:

Is this a hydro as you mentioned earlier of 300RPM and 5 to 10MW? Brushless or Static system? A brushless at that slow speed would be unwieldy, but it has been done as a retrofit to replace the old rotary DC exciter at at least one plant in Sweden.

The negative forcing voltage (if a static system) serves to demagnetize the main field quickly (as ratz1 said)to reduce the V quickly. If a brushless only the exciter field is demagnetized quickly, but the main field will decay more slowly. That coupled with the speed rise will result is some OV, I would not hazard a guess, but more than 10% would likely not be acceptable. It depends on what was specified if it was a retrofit or a new unit. What is the actual equipment and what was specified?

rasevskii

 

[rasevskii]

For uhpo:

Also, if the DECS 200 system cannot produce negative forcing voltage (only zero voltage)upon load rejection and if this is a hydro with 30% speed rise, then possibly the system cannot be accepted. I am assuming this is a retrofit or upgrade situation. but assuming anything is dangerous...

rasevskii