Cause of Reverse Power

Hi

I know for Synchronizing voltage, phase and frequency should match. But what happens when synchronous generator voltage slightly lower than grid. Is there a possibility to flow reverse power?
 
WATT,

During synchronization it's customary to make the frequency of the incoming generator be just slightly higher than the frequency of the grid the incoming generator is being synchronized to. This is so that when the generator breaker is closed positive power will be produced by the generator and generator stator amperes and watts will flow "out" of the generator and on to the grid adding to the ability of the grid to power loads connected to the grid..

If a generator breaker is closed when the frequency of the incoming generator is less than the frequency of the grid the incoming generator is being synchronized to then reverse power will flow into the generator--amps and watts will be supplied to the generator <i>from the grid</i> and the generator becomes "motorized" and increases the speed (frequency) of both the generator rotor and the prime mover driving the generator. Reverse power draws amperes and watts from the grid, reducing the ability of the grid to power loads connected to the grid. (To answer the next question: Yes, it is possible to close a generator breaker when there is "negative" slip--that is, when the frequency of the incoming generator is less than the grid being synchronized to.)

During synchronization it is also customary to make the incoming generator terminal voltage slightly higher than the voltage of the grid the incoming generator is being synchronized to. This is so that when the generator breaker is closed positive reactive current will flow in the generator stator windings--lagging VArs; a lagging power factor. The real amperes won't change--they are a function of the amount of power being admitted to the generator's prime mover, and are related to the difference in frequency between the incoming generator and the grid frequency at the time the generator breaker is closed.

If a generator breaker is closed when the incoming generator voltage is less than the voltage of the grid the generator is being synchronized to, leading reactive current will flow in the generator stator windings--leading VArs; a leading power factor. Again, the real amperes won't change as they are a result of the difference in frequency between the incoming generator and the grid frequency at the time the generator breaker is closed.

Speed and frequency are related to real current (amperes); generator terminal voltage and reactive current are related. If one increases the amount of energy flowing through the generator's prime mover while the generator breaker is closed the amount of real power (watts) will increase. If one decreases the amount of energy flowing through the generator's prime mover while the generator breaker is closed the amount of real power (watts) will decrease. Decrease the energy flow-rate through the generator's prime mover so much that if the generator and prime mover were disconnected from the grid the generator speed would be less than that required to make the generator's frequency equal to grid frequency then reverse power (real amperes) will flow into the generator's stator windings.

If the amount of excitation applied to the generator rotor is exactly equal to what would be required to make the generator terminal voltage equal to grid voltage during synchronization and the generator breaker is closed then the reactive current flowing in the generator's stator windings would be zero--0 VArs; 1.0 power factor. If the excitation is then increased about the level required to make the generator terminal voltage equal to grid voltage lagging reactive current (lagging VArs) will begin to flow in the generator's stator windings--in addition the real amperes flowing in the generator stator windings at that time. The generator power would decrease from 1.0 in the lagging (sometimes referred to as the 'positive' direction).

Or, if the generator excitation were to be decreased below the level that would make the generator terminal voltage equal to grid voltage at the time of synchronization then leading reactive current (leading VArs) would begin to flow in the generator stator windings at that time--in addition to the real amperes flowing in the generator stator windings. The generator power factor would decrease from 1.0 in the leading (sometimes referred to as the 'negative' direction).

Real power and reactive power are both "relative" things when the generator breaker is closed and the machine is producing "power"--real and reactive power. The amount of energy flowing in the generator's prime mover affects the amount of real amperes flowing in the generator stator windings--the amount real power produced by the generator. The amount of excitation applied to the generator affects the reactive current ("power") flowing in the generator stator windings--the number of VArs, and therefore the power factor of the load of the generator.

Real power--positive or negative (reverse power)--is affected by the amount of energy flowing into/through the generator's prime mover. Reactive "power"--positive or negative (lagging or leading, respectively)--is affected by the excitation applied to the generator rotor.

Hope this helps! If you're new to control.com, there's a 'Search' field at the far right of the Menu bar of every control.com webpage. All of the previous posts on control.com are archived and can be accessed using the 'Search' function. For example, if you wanted to look at previous posts regarding reverse power, you would type the following search term and syntax into the 'Search' field:<pre> +"reverse power"</pre>and then press the ENTER key. And, voila! Tens, if not hundreds of questions and responses will appear on the computer screen you are looking at.
 
Hello,

I am from Panama on Central America and I have a problem with a generator of 200KW.

This generator works single, but when I put load immediately send me signal of power reverse. If apply more load more increase, the power reverse signal and then the engine shut down by power reverse.

Do you help me with this problem?

Pedro
 
Pedro,

When did this problem start? Is this a new problem on an older generator-set, or a problem with a new generator-set? Did the problem start after some work on the prime mover, or on the electrical system (power/metering wiring)?

Your post is not clear. When you say the generator works fine "single," what do you mean? That if it's powering a small load independently of the grid it works okay? But that when you synchronize it to a grid it trips on reverse power?

What mode is the governor operating in? Droop speed control mode, or Isochronous speed control mode? Because, if the unit is powering a small load (less than 200kW) independently of a grid and it's able to maintain frequency then it's probably operating in Isochronous speed control mode. And, when it is synchronized to a grid with other generators and their prime movers it needs to be operating in Droop speed control mode.
 
Hello,

we have a new generator prime 1250 KVA, connected to a load: heating pump 400KW, 10 Lifts 250 KW, fans and AHU 200 KW and 20 KW LED & fluorescent lighting. during the testing process on the existed load we notice the following on the generator control panel:

- power factor 0.96 leading

- connected load from 32% to 65%

- AVQ: 406 V L-L

after 55 minutes an alarm appeared (reverse reactive power); after 5 minutes the genset shut down. after 2 days we repeated the test and we get the same results.

please advise
 
Dear

I had similar problem with GE FRAME 5. AFTER synchronizing it trips immediately or after some more time? we checked everything in electric protection there was no problem.

After thorough checks we found the cause from the fuel system as it was unstable and could not supply the require fuel as per load the frequency drops and trip the machine on reverse power.

this information may helps you
 
B

briancacayuran

Sir what if your voltage is decreasing? Is the reverse power will occur???

>Or to test, also just invert the angle 180 degrees into the
>relay configuration, and instead reducing the power you can
>"simulate" the reverse power actually going forward. i.e.:
>reverse power at -5MW => inverse the angle in the relay and
>do a +5MW (the 52G will trip per reverse power).
>
>don't forget to put it back again to normal operation.
 
Top