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Cause of Reverse Power
Cause of reverse power.

Can anyone kindly tell me what are the causes of reverse power in synchronization? Thanks... It'll be a great help.

1 out of 2 members thought this post was helpful...

Reverse power in synchronization?

I don't really understand the question involving reverse power and synchronization because synchronization is usually used to describe the procedure for connecting a generator to a grid with other generators and under such conditions there should not generally be reverse power flowing.

Reverse power generally describes a condition where the prime mover of a generator is not supplying sufficient torque to keep the generator rotor spinning at the same frequency as the grid to which the generator is connected. In other words, the generator has actually become a motor and is drawing current from the grid to which is connected and is supplying torque to the prime mover which is supposed to be supplying torque to the generator.

During synchronization, it might be possible to have the synchroscope rotating in the Slow direction (anti-clockwise) and then close the breaker. Under this condition, the generator would then be drawing a small amount of current from the grid instead of supplying a small amount of current to the grid (which occurs when the breaker is closed with the synchroscope rotates in the Fast (clockwise) direction).

I have seen some prime mover control systems (and exciter regulator control systems and synchronizers) that try to increase load slightly as soon as the generator breaker is closed. On a couple of sites, after a maintenance outage, I have seen the output of the control mis-connected to the control valve and cause the load to actually decrease instead of increase and cause the generator to trip on reverse power. One case was a paper mill that had a DC motor on the steam control valve actuator and the motor leads were reversed.

blackstoneboy,
it seems that you are trying to synchronise a generator, and as soon as its circuit breaker closes, the generator operates in reverse power. If this is the case, then there could be two causes of the problem:

1. If you are doing a manual synchronisation, ensure that you set the generator to run slightly faster than the grid frequency (shown by a synchroscope rotating clockwise). If you are doing this and still ending up in reverse power, check the connections of your synchroscope. You might have the generator and grid connections reversed)

2. If you are doing an automatic synchronisation, through a synchronising relay, again check that the generator and grid voltage connections are not inverted. In addition ensure that that the relay is set to close the circuit breaker only when the speed of the generator is equal to or higher than that of the grid (most probably indicated by positive slip in the relay setting). If the relay does not use slip for synchronisation, but synchronises only when the generator speed is the same as that of the grid (only a deviation in phase angle is accepted), then devise something such that as soon as the circuit breaker is closed, a raise command is given to the generator prime mover governor.

By mehmet bozkurt on 10 June, 2011 - 5:48 am

Hi..!

Can you help me with a problem which a have in a hydroelectric power plant.

the problem is: reverse power

We start the generator and all values are in a normal range. Then we synchronize the machine to the national grid of turkey, but after that we see the generator protection relay trips and gives W< reverse power fault. We tried that to increase the excitation before the synchronization and then we push the button of synh. and the generator works in normal operation with synch. the relay is Microelettrica Scientifica.

Thanks,

Increasing the excitation will only affect the MVars generated and have no effect on MWs. Water flow and head determine the megawatts generated.

What type of turbine is this? Pelton or Francis/Kaplan? Provided water is flowing and the gates are open it is actually quite difficult to reverse power a hydro turbine. I've come across some original installations which had no reverse power trip protection. Time delayed minimum position limit switches on the wicket gates/needle valves would provide the protection.

Increasing water flow will increase the megawatts generated. What's happening to your wicket gate position immediately after breaker closure? It should increase a couple percent to increase power, and for a Pelton turbine the deflector should open out of the stream.

Are your reverse power relays settings realistic for a hydro turbine with a suitable delay? For the larger units it is normal practise to synchronize at a lower speed than the grid. This means immediately after breaker closure reverse power will be present until the wicket gates have slightly opened. Has the CT and PT wiring been checked?
Can this unit run as a synchronous condenser?

Please tell us more about your unit.

1 out of 2 members thought this post was helpful...

Hello!

When did this problem start? After some outage? Is this a new unit?

What have you done to try to troubleshoot the problem? And what were the results of your efforts?

Reverse power is generally sensed by the "direction" of current flowing at or near the generator terminals. The CTs (current transformers) used for the sensing input to the reverse power relay are polarity-sensitive, and so if they were not verified during installation correctly, or if the wiring may have been disturbed during some outage and not verified prior to re-start, then the signal being applied to the reverse power relay may be "reversed".

Your answer is very clear and correct the same problem was happened in my site also thank you for your clear answer.

I agree. The wrong ct installation happened on my new gt fr6 power plant. The polarity of the ct was connected reversely. Soon after the generator Synchronized to the grid, we experienced reverse power and set the breaker to trip (only breaker not turbine-generator). No matter we thrust the turbine up by increasing fuel after sync, the reverse power always cut out.

Sooner we identified the misconnection of the ct, because this is the new power plant and the things like this happen in this situation.

But if reverse power occurs suddenly in well running generator, it is likely as the result of the failure of control system to establish proper amount of fuel after sync. You can start from inspecting the related device.

Good luck

By Namatimangan08 on 10 June, 2011 - 1:55 pm

A hydro turbine especially the one that has a long penstock has one noticeable characteristic. When you attempt to increase the load by increasing the opening of flow control mechanism, the actual load will go down first probably for around 3-5 seconds. When you try to reduce the load by reducing the opening of flow control mechanism the actual load will go up first for about 3-5 seconds before it eventually goes down.

I will tell you why these are true.

What load you set prior to synchronizing?

There are other possibilities I wish to share later. First thing first. Please provide the answer to my question above.

"When you try to reduce the load by reducing the opening of flow control mechanism the actual load will go up first for about 3-5 seconds before it eventually goes down.

I will tell you why these are true. "

We too waiting for your reply.

Take care
G.Rajesh

By Namatimangan08 on 24 June, 2013 - 11:31 am

When you open the wicket gate, the water water passes across it will increase almost instantaneously due to the present of very high pressure difference over short distance. The same may not true for the flow from the wicket gate to the intake. It takes quite sometime for this massive water to accelerate owing to huge inertia it has.

Due to this the area just before the wicket gate has to go through flow "starvation" i.e. water leaving that section is higher that replenishment rate. As a result, high pressure drop will occur.

Since power is as a function of mass flow rate and pressure, temporary pressure drop will reduce power output if such drop in pressure cannot be compensated by additional mass flow. In this case, since flow starvation just occurs before the wicket gate, there is no way pressure drop can be compensated by additional mass flow. Therefore the output will go down for a while..

After a few seconds new steady state will achieve. Pressure rises back and mass flow rate reaches new steady state. Only then the output will increase and remain at the new steady state condition.

Reverse power in synchronization?

i just want to ask how this reverse power trip be tested. our generators circuit breaker is mitsubishi AE1000-SW

thank you for your soonest reply!

1 out of 2 members thought this post was helpful...

Hi,

Reverse power occurs when the energy flow-rate into the prime mover is not sufficient to maintain rated speed while the generator breaker remains closed. In this case the generator actually becomes a motor, drawing amperes from the grid in order to maintain rate speed. In effect this results in the prime mover being rotated by the motorized generator instead of the prime mover rotating the generator.

To test the reverse power relay (this isn't a test of the breaker) just reduce the energy flow-rate into the prime mover until the reverse power relay operated--opening the generator breaker.

Hope this helps!

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.

1 out of 2 members thought this post was helpful...

Jaypot... testing requires knowledge of the relay used.

Is it analog, solid-state or microprrocessor based. Also, a single-phase unit is normally used, but sometimes it can be three-phase.

Can you provide detail as to the manufacturer?

Regards,
Phil Corso

0 out of 1 members thought this post was helpful...

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?

2 out of 3 members thought this post was helpful...

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 from the grid 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:

  +"reverse power"
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 CSA,

I have been reading a few of your posts and i have learned quite a lot in the past 2 hours. Thank you so much for sharing your knowledge.

I am trying to find out what else could cause a reverse power condition. Could a bad exciter cause a reverse power trip?

1 out of 2 members thought this post was helpful...

SPG,

I suppose under a very limited set of circumstances a bad exciter could contribute to a reverse power trip--but only if the real power (watts) was at or slightly less than zero to begin with.

Reverse power is related to the inability of the generator prime mover to keep the generator spinning at synchronous speed so that the generator draws power (watts) from the grid to keep it--and the prime mover running at synchronous speed.

Hi experts,

Out of curiosity, can Reverse Power occur in a:

- 2 or more GTGs running in Island Mode?

- Single GTG trying to de-couple from Grid to Island Mode?

0 out of 1 members thought this post was helpful...

dinster,

Long time no write.

>Out of curiosity, can Reverse Power occur in a:
>
>- 2 or more GTGs running in Island Mode?

Yes.

>- Single GTG trying to de-couple from Grid to Island Mode?

Not likely, where would the reverse power come from?

Thanks CSA,

just need someone to validate my hypothesis.. :-)

By Pedro Vega on 25 August, 2016 - 9:00 am

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

1 out of 2 members thought this post was helpful...

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