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#1

# Overload of Generator and its response?

What happens if capacity of generator is 35MW and connected load to generator is 36MW.

Can it be able to supply?
How will frequency and voltage change?
If generator can supply and it can achieve stability. What will be frequencies and voltages of generator?

If someone can plot the situation using some software would be very helpful for me.
thankyou
11-25-2013 02:27 AM
Top #2
Sorry, can't do a simulation right now but if generator's Pmax is 35 MW and you load more than that the power angle delta will become >90deg and generator will drop out of synchronization. It means the frequency of rotor rotation frequency won't be 50/60 Hz anymore and P=0, so generator will stop producing energy.
11-25-2013 04:56 AM
Top #3
Most likely, no problems.
In the tally of the connected load (to 36 MW) it is quite often overlooked that not all loads are connected, and draws full power simultaneously, and if this should happen, it might be possible to install some load shedding mechanism for some of the less essential loads.
11-25-2013 07:14 AM
Top #4
Satish,

If this is a purely theoretical question then as the applied load exceeds the rating of the prime move the speed will drop by approximately 1-(36/35) x 100% = 2.9% and the output of the prime mover will match the input required of the load at the lower speed.

In real life things are rarely so clear. Most machines allow for variations up to +/- 5-10% of their ratings. Then there's the governor and protective relaying settings that may have sharper cut-offs, so you'll need to account for that as well.

With regard to the voltage, there are too many variables to answer this directly. If the machine is on manual (fixed) excitation then the voltage will drop as the speed does but this ignores the decrease in voltage drop as the current goes down. If the machine is on Automatic Voltage Regulator (AVR) then the voltage will probably remain fixed up to any limits in the AVR and protective relaying.

All of the above assumes an isolated system undergoing a smooth transition (incremental loading), if the load is applied in big steps then the dynamics of the system (transient stability criteria) will rule. You will learn much more about system behavior if you explore this on your own, so I leave that as an exercise to the readers.
11-25-2013 09:21 AM
Top #5
dear satish,
well, if your generator capacity is 35 MW and the load is 36 MW, there are two answer to it :
1. If your generator and load at the same terminal (there are no impedance between generaot and load), the generator will supply the load (assuming there are no relay in generator). But the generator will overheat and the generator shaft may be broken.
2. If there are impedance between your generator and load, i agree with George. But it will depends the impedance. as larger impedance will bring Pmax smaller (Pmax=V1*V2/X).

The response of voltage and frequency will depends on the exciter and governor control.
11-25-2013 12:18 PM
Top #6
Rizky,

"...the generator will overheat and the generator shaft may be broken..."

It is extraordinarily unlikely that a mere 3% overload of generator will cause a catastrophic failure such as a broken shaft when a safety factor of at least 2 to 1 (and more likely 4 to 1) is used when designing such a critical component.

Shafts can break under severely out of phase synchronization, inadvertent breaker closure on under or zero-speed machines, severe misalignment resulting in physical contact between rotor and stator, or after long periods of sub-synchronous oscillations. Nor is it likely that a generator will severely overheat to the point of failure unless it is run for long periods with a faulty cooling system.

With regard to overloading a machine, much has to do with the nature of the load, since many loads will naturally adjust to the changes that are taking place at the generator end as well as the mechanical load at the other end of the motor.

Think of a pump at the end of an induction motor, as the pumping load increases beyond the capability of the generator, the speed of the generator will drop, this will cause a decrease in the speed of the pump which in turn causes the amount of fluid being pumped to decrease which means the load has decreased.

The net result is a new operating/equilibrium point for the generator, pump, and load. Things do not suddenly "fall of a cliff" as suggested by George unless the changes are too sudden, too large, or certain combinations in between.

Alan
11-25-2013 02:53 PM
Top #7
In addition to the above comments, assuming the prime mover as the capability of supplying the additional power, the generator would have an increased temperature rise (i.e., it is going to run hotter).
11-25-2013 04:56 PM
Top #8
George and Rizky, As stated by Alan, the machine is not going to lose synchronism unless there is a fault on the system or some other major sudden disturbance.

Let's assume that there is no governor on the generator (or the governor is blocked or its maximum setting is 35 MW) and the mechanical input to the generator cannot exceed 35 MW. Let's also assume that the 36 MW load is constant MVA and has no frequency dependence, the generator has no losses so the in the steady state Pmech = Pelect, and the transmission connecting the generator to the load is lossless (i.e., Z = 0 + jX). In this case there is a mismatch between the generator output (35 MW) and the load (36 MW). The extra 1 MW to balance the system will come from the inertia of the generator, which will cause the system to slow down. Since we assumed no frequency dependence on the load (which is highly unlikely) the system will remain synchronized, but will decline in frequency until the generator stops. Long before this happens, the generator and/or the load will be tripped by underfrequency protection or volts/Hz relays. Also, during this frequency decline, the AVR of the generator will try to maintain the terminal voltage of the generator. As long as the AVR can maintain terminal voltage, the angle between the generator terminals and the load will remain constant (since the power transferred across the transmission line is a constant 36 MW and V1 and V2 are not changing).
11-25-2013 07:40 PM
Top #9
Very good explanation by Peter, except for two insignificant mistakes:
1: Since the frequency drops, but the reactance of the cable must physically remain the same, the X in his "Z = 0 + jX" equation must be proportional to the frequency, so for this thought experiment, the angle between generator and load must diminish with the reduced frequency.
2: The inertia of the load must be added to the generator inertia to determine the magnitude of the slow down.
11-25-2013 10:34 PM
Top #10
Ole, Thanks for catching my errors. Both of your comments are absolutely correct. Typically the inertia of the load is much smaller than that of the generator and turbine so the difference in results due to this effect will be small. The change in cable or overhead line impedance with frequency is much more significant. I am so used to dealing with systems where the frequency deviations are small (less than 2%) that it completely slipped my mind that X is a function of frequency.

Now that you mention it, I believe that the PSS/E software package allows you to specify line impedances as frequency dependent to capture this effect.
11-26-2013 01:25 AM
Top #11
Dear Alan,
I am sorry if my word is wrong, I know it is extraordinarily just 3% overload generator will cause broken shaft. I just say it is possible if there load changes too sudden or too large.