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4 comments
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Join Date: May 2012
Posts: 9

Frequency Insensitive System Impact on Generator

05/06/2012 1:54 PM

Our power plant has Oil Fired Boilers & Steam Turbines (4 x 300 MW = 1200MW). Our national grid has large variations in grid frequency since we have extensive difference in between demand and supply of power.


We have 50 Hz system. When grid gets over loaded, frequency starts declining & mostly it gets around even 48.5Hz. Consequently, our generators also respond to this change according to droop setting (droop setting=5% on all 4 units). BUT sometimes our generator MW output hits +320MW or even more.

I just want to inquire, if we make our generators output (MW) FREQUENCY INSENSITIVE for >= 300MW i.e. it won't respond to frequency change when it's already operating at 300MW then IN SUCH CASE, WHAT WOULD BE ITS HARMFUL IMPACT ON OUR GENERATOR/TURBINE ?

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#1

Re: Frequency Insensitive System Impact on Generator

05/07/2012 11:41 AM

The short answer is that you have proposed an abnormal operating condition that can't be met. Assume that the grid frequency is 50Hz and your identical generators are operating at 300MW each on normal governor load control. You now switch from synchronous (frequency following) mode to isochronous (fixed frequency) mode. Now assume that the grid frequency starts drifting down. Unfortunately your generators will now try and maintain the frequency in the only way they can, which is to start pushing out more power to try and keep the frequency fixed at 50Hz; i.e., it is picking up all the load that the grid cannot support (that's why the grid frequency is falling). It is unlikely that your protective relaying will operate fast enough (within a half cycle) to trip your units off-line before your units become extremely overloaded, unstable and start slipping poles.

The grid will now suffer the loss of your generation at a time that it cannot keep up with the load, leading to a further and more rapid decline infrequency, possibly leading to cascading failures and/or a total collapse of its system.

In order to tell you how disastrous this will be requires that you provide information on the capacity of the national grid and at least the impedance between your generators and the source where your utility is located, but at the very least you will be shutdown until a complete inspection of all the rotating elements has been completed. The mechanical shocks are enormous and turbine-generators are not meant to be subjected to them. The boilers, if properly maintained and tested, should be able to withstand the full-load steam rejection that follows the trip.

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#2
In reply to #1

Re: Frequency Insensitive System Impact on Generator

05/07/2012 3:35 PM

Many thanks for your detailed reply.

I just want to further clarify the point you mentioned , If we operate our generator (connected to National Grid) in Isochronous (fixed frequency) mode & grid frequency starts drifting down then does this drifting down in frequency will enforce our prime mover speed to slow down in Isochronous mode?

If it slows down our prime mover speed then would it allow to the governors to inject extra energy in Isochronous mode to compensate this drifting down in speed which could lead to potential threat of Tripping?

Looking forward for your expert opinion.

Thanks

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#3
In reply to #2

Re: Frequency Insensitive System Impact on Generator

05/07/2012 8:42 PM

Isochronous mode is normally used when the generator is the only source in the network, also known as black-start or isolated mode. In isochronous mode the governor will only compare the speed of the turbine-generator against an internal reference frequency and adjust the steam valves to maintain that speed. As the load increases incrementally the governor sees the speed drop and admits more steam to maintain the speed, it knows nothing about load and will keep opening the valves until the stops are reached, so it is up to the operator to carefully manage the additional loading.

So if you insist on improperly operating in isochronous mode while attached to the grid your governor will try its best to counteract the speed drop without any regard for the load that is being imposed on it. The threat (guarantee) of tripping is the fact that unless your generators represent a significant portion of the grid loading (80% or more) then there will quickly be a speed difference between your generators and the rest of the grid, and it is this speed difference which will cause the generators to lose synchronism with the grid.

It is no different than when you are driving your manual transmission car 50km/h on a level surface and you encounter a hill. You (the governor) want to maintain the 50 km/h speed up the hill, so you increase the fuel supply but the engine speed never changes. If the hill is too steep or the load in the car is too great then the car slows down once the pedal reaches the floor, you have exceeded the rating of the motor. If your motor can provide the power but the clutch slips then it is equivalent to slipping poles; i.e., there is not enough magnetic field to couple the turbine power from the rotor to the generator stator. Be careful as you reach the top of the hill because if you keep the pedal buried you will surely overspeed the engine as you reach the crest and gravity aids you instead of impeding you.

Hope this helps, and thanks for the GA.

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#4

Re: Frequency Insensitive System Impact on Generator

05/13/2012 7:12 AM

hello evil, the inpact on it is will burn your AVR and you rectifier diod but incase of that happen this is the diod supplyer http://www.powersupply-parts.com/sdp/1198946/4/pd-5469021/9773278-2125666/Stamford_rectifier_diode_RSK2001.html and also you can prevent that for installing a step up transformer on you generator if you supplying large facilities i required to you this you need two TR @ 380 ac volt input output 33kv second 33kv input @ 380 /415 acvolts this will connect in series but you need to check the kva of your genset, genset kva are always 2x higher than the first transtformer, i share this to you because i done this before when our genset shutdown and burn the AVR and rectifier diod, we are also supplying village here, i forgot to tell you we use 1.5megawatts genset with this set up this is the genset that i repair with the broken diod,

hope this will help you my friend

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