# Causes of Reverse Power in Generators

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#### mohsin

What are the causes or reverse power in generators?

In my case sometime reverse power alarm came as I am using caterpillar c18 generators. One of my generator some time give this alarm. Kindly explain what are the causes and their solutions.

#### CSA

mohsin,

- Motors convert amperes into torque.

- Motors get their amperes from generators.

- Generators convert torque into amperes.

- Generators get their torque from the prime mover coupled to them (in your case, a reciprocating engine).

- Prime movers produce torque from the energy flowing into them (diesel fuel; natural gas; steam; etc.).

Synchronous generators--and their prime movers--<b><i>synchronized</b></i> together all run at the same speed (remember you have to start the prime mover and get the generator up to rated speed in order to be able to synchronize to the other generators and their prime movers--and once the generator breaker is closed the prime mover and generator can only run at a speed that is proportional to the frequency of the grid).

Most people think that when they increase the load of a generator and its prime mover that the speed of the two will increase--but it doesn't! Once synchronized to a grid with other generators (even a small grid!), the speeds of all the generators are fixed by the frequency of the grid. And, the torque being supplied by the prime movers to each of the generators they are coupled to must be sufficient to keep the generators spinning at rated speed at a minimum. If the torque being provided is more than required to maintain rated speed, then the generator will sense the extra torque and convert it to amperes. If the torque being provided is exactly equal to that required to maintain rated speed there will be ZERO amperes produced by the generator. If the torque being provided by the prime mover to the generator is less than that required to maintain rated speed then the other generators and their prime movers will provide amperes to the generator to keep it (and the prime mover) spinning at rated speed--because they all have to spin at the same speed. (You can't have one generator running at 49.7 Hz, and another one at 52.4 Hz, and another one running at 50.1 Hz, and have 50.0 Hz coming out of the receptacle on the wall. It doesn't work that way; there's no smoothing thing or averaging thing for frequency.)

When the torque being supplied to the generator is not sufficient to keep it running at rated speed the other generators and their prime movers will supply amperes to the generator to keep it spinning at the same speed. This is called "reverse power"--since the amperes will be flowing into the generator which isn't getting sufficient torque from its prime mover instead of flowing out of the generator.

In this case, the generator actually becomes a motor and drives the prime mover to keep them both spinning at synchronous speed (the same speed as all the other generators synchronized together as a function of the grid frequency). This is also called "motorizing" the generator. There is really very little difference, mechanincally, between motors and generators--just the directions of torque and amperes.

It's NOT good to use the generator (as a motor) to drive the reciprocating engine (or just about any other prime mover). So, there are protective relays to monitor the direction of current flow and open the generator breaker to protect the prime mover (again, the electrical machine coupled to the prime mover doesn't really care if it's a motor or a generator (it's designed to act as a generator), but the prime mover doesn't like being driven by a motor (or a generator which has become a motor)).

Since the torque produced by the prime mover is a function of the energy flowing into the prime mover, if the torque falls below that required to keep the generator spinning at its rated speed that means the prime mover control system (the governor) isn't doing it's job properly. Or, there is something which is restricting the flow of energy into the prime mover (in the case of a reciprocating engine, dirty fuel filters; low fuel supply pressure; a non-working fuel pump; etc.).

So, when a generator--and its prime mover--are tripped by reverse power that means the prime mover is not providing or producing sufficient torque to keep its generator spinning at its rated speed and the other generators synchronized to the grid will provide amperes to the generator, causing it to act like a motor, and spin the prime mover.

You need to understand why the prime mover isn't producing sufficient torque to keep the generator spinning at its rated speed.

There's one more possibility--but it gets very complicated. There are two basic governor modes: Droop and Isochronous (speed control modes). IN GENERAL, most governors are always in Droop speed control mode when multiple generators are synchronized together on a grid. If one governor is operated in Isochronous speed control mode when it should not be, then that could cause a problem.

<b>OR,</b> if there is a over-all control system for a group of generators (sometimes called a PMS, or Power Management System) that <i>attempts</i> to control the loads of multiple generators synchronized together there can be issues if the PMS is not properly configured OR operators think they need to over-ride the PMS in some instances....

That's about it. Without understanding a LOT more about the configuration of the generators and their prime movers at your site it's really not possible to say much more. BUT, again: The most common cause of reverse power tripping is insufficient energy flowing into the prime mover--regardless of it's governor mode, or whether there is a PMS or not. Insufficient energy flowing into the prime mover equals reverse power (amperes flowing INTO the generator instead of out). Amperes flowing into a generator causes the generator to become a motor which is keeping the generator--and its prime mover--spinning at a particular speed. And, most prime movers (especially reciprocating engines and steam turbines) DO NOT like to be spun by their generators (which have become motors). It's as simple as that.

Hope this helps!

#### amit06212

Hi Brother. Did you get ur problem solved. I am facing Similer problem in my DG Set( Reverse power tripping) . Can u provide me some solution.

#### CSA

amit0612,

While a sizable portion of original posters to this site provide feedback on their issues, a larger number do not. Many are just hoping for an exact answer to their problem so they can go back to whatever they do when they are getting paid to troubleshoot problems or operate power generation equipment. I subscribe to the theory of, "If you give a man a fish you feed him for a day; if you teach him to fish you feed him for life." In other words, I'm trying to use some of these questions as a teaching tool to teach people how power generation works so that they become better at their jobs since so many people don't have or don't receive much, if any, training in power generation fundamentals or principles.

Good on you, mate, for using the 'Search' feature of Control.com (I'm presuming you used the 'Search' feature; I hope you used the 'Search' feature!). And, I hope you're looking for some basic operational and fundamental information as you try to solve your problem; it comes with the response--free of charge!

What part of:

"So, when a generator--and its prime mover--are tripped by reverse power that means the prime mover is not providing or producing sufficient torque to keep its generator spinning at its rated speed and the other generators it is synchronized to the grid will provide amperes to the generator, causing it to act like a motor, and spin the prime mover.

You need to understand why the prime mover isn't producing sufficient torque to keep the generator spinning at its rated speed."

didn't you understand?

Are you operating or troubleshooting a diesel gen-set? If not, what is the prime mover--a steam turbine? A gasoline engine? A hydro turbine? A combustion turbine?

If you have a diesel gen-set, then it's not getting enough fuel for some reason. If the load is dropping before the reverse power trip then the problem is likely dirty fuel filters or low fuel level or some kind of mechanical problem with the fuel rack slipping.

If the diesel control gets signals from another, external control system, the external control system might be not working properly. Or the wiring between the gen-set and the external control system might be bad.

When you want help with a problem, you need to provide more details than, "Help; it keeps tripping on reverse power." Here are a few ideas if you really want help:

What is going on when it trips?

Is there a large load change in the facility the gen-set is powering? (It's presumed the gen-set is powering some facility/load which is not being powered by a large grid/utility at the time of the reverse power tripping; if that's not correct, please tell us.)

Is the frequency of the system going high before the reverse power trip? (Or is the frequency going low?)

What mode is the diesel controller operating in: Droop or Isochronous speed control?

How many other gen-sets is the diesel synchronized with? (It's presumed the gen-set with the reverse power tripping problem is NOT synchronized to a large, "infinite" grid; if that's not correct, please tell us if it is.)

Are the other gen-sets significantly larger than the one that's having problems (higher rated power)?

What is the setting of the reverse power relay on the gen-set that keeps tripping?

#### amit06212

Hi. I am new to control and automation. Got it here through google search prime mover is not providing or producing sufficient torque to keep its generator spinning at its rated speed:- Diesel Generator (500KVA) is working fine with all its parameter at rated load.

If the load is dropping before the reverse power trip : No

The DG set is working in parallel with similar rating DG set with same droop setting. The DG set is not connected to the grid but to an isolated load and its governor is in droop mode.

I think problem lies in CT wiring connected to Reverse power relay. I will update once I identify the problem.

Thanks.

#### PhilCorso

Amit06212...
Thank you for bringing this topic back to the forum. Following is my Readers-Digest-type version, using an Energy Approach:
1) The Gas Turbine part of GTG converts chemical-energy in fuel-burned to mechanical-energy.
2) The Generator part of GTG is a Synchronous-Machine that can operate as either a Generator or Motor. It converts mechanical-energy into electrical-energy.
3) Normal Energy-flow is typically unidirectional, meaning from the Synchronous-Machine operating as a Generator, to a "Load-Bus".
4) Reverse Energy-flow is just the opposite, meaning it's from the "Load-Bus" to the Synchronous-Machine causing it to operate as a Motor.
5) The cause of this disturbance is the inability of the GT to provide sufficient energy to maintain the Generator's energy requirement.
6) If excitation to the Synchronous-Machine is maintained, it remains synchronized to the line, but its operation becomes that of a Motor.
7) Protection against this "disturbance" has little to do with the Synchronous-Machine, but instead, is primarily for protection of the prime-mover, especially if it's a GT.
8) The title "Causes" should be Singular and not Plural. Except for the very, very rare event when the relay itself has mis-operated, there is no significant data pointing to other causes of Reverse Power !

Let us know if you would like additional detail of how protection works, or why the risk is more severe for the GT than all other types of prime-movers !

Regards, Phil Corso

#### Curious_One

CSA and PhilCorso thank you for your contributions: which always contain great information.

I just want to remind everyone that most modern generator breakers have 2.... yes two trip coils.

If the trip coil from the control system does not function and the trip coil from the protective relays does function.

Reverse power relay trip coil will alarm and open the generator breaker because the control system was unable to open the generator breaker.

It is not an actual reverse power.

At NO time has the poster described an engine/gt continuing to run without fuel powered by the generator.

Never the less, Thanks to CSA and PhilCorso for making this forum a place of learning.

#### CSA

Curious_One,

You said the second coil will trip the generator breaker if the first one doesn't.

amit0612 should be seeing an alarm SOMEWHERE if the something is trying to trip the generator breaker via the primary trip coil. This is just so typical of many plants and operators today: People don't look at the alarms, or if they do, they don't know how to detect the cause of a trip. And, so, these kinds of questions come up all the time.

It doesn't help that automation (the control systems) are annunciating more and more alarms--with cryptic text messages.

In my limited experience with these two breaker trip coil schemes, both coils get energized at the same time, so that if the primary doesn't trip the breaker the secondary--hopefully--will with as little time delay as possible.

And, I'm unclear how the secondary coil will trip/annunciate a reverse power condition UNLESS the prime mover has tripped without tripping the generator breaker. Unless I'm misunderstanding something--which is very possible. If the secondary coil will only energize on reverse power if the primary coil didn't open the breaker that seems to say the prime mover was tripped and the generator was drawing reverse power (hopefully with the excitation system still working).

In GE-design heavy duty gas turbine- and steam turbine applications when the prime mover gets tripped it the generator breaker is opened on reverse power--in order to prevent an overspeed condition. If the generator breaker were opened at the same time (or a split second after the prime mover is tripped) BUT the energy flow-rate into the prime mover wasn't halted then it's highly likely the unit will increase speed above rated, if not reaching the overspeed trip speed.

#### Curious_One

CSA,

I currently do not have any generator protection drawings with me. But if my memory is correct, I believe on our 7EA fleet the first trip coil is landed on P core. The generator breaker is opened at about 2 MW during shutdown.

The other trip coil is armed by the protective relay system including the EX2000 exciter.

I remember long ago we were getting a reverse power alarm and gen breaker would open at about -3 MW. Although the Mark V was asking the breaker to open but it would not. We found a faulty terminal strip in the GAC (generator cabinet). Once repaired everything was back to normal.

The bad part is the Mark V did NOT alarm when it was unable to open the breaker.

I remember a similar problem on a Mark V controlled large steam turbine. The operators were not allowing the steam turbine to properly speed match before issuing the sync command. The sync command was issued at around 3620 rpm. The turbine would slow to 3600 and sync. But before the momentum of the turbine could be changed to the forward direction i.e. positive MW the reverse relay would become active and open the gen breaker.

This is why I brought up the trip coil issue when trying to resolve reverse power alarms. Sometimes it is a simple as poor operation or loose connection. If you wish, I could get some drawings and constant setting to share.

#### PhilCorso

CuriousOne...
Are there no intentional delays in Reverse-Power tripping logic?
Regards, Phil

#### Curious_One

PhilCorso,

The reverse power relays I have had experience with utilize cycles as a measurement . If I recall correctly, it amounts to about 3 minutes. They also have some algorithms that speed up the process as the deviation continues to rise. Don't quote me on that. I very seldom look at protective relay settings unless asked to.

Protective relay is more of an art than a science. https://www.gegridsolutions.com/multilin/notes/artsci/artsci.pdf

Regards,
Curious_One

#### PhilCorso

Curious_0ne...
I couldn't agree more. My teacher and mentor was J. Lewis Blackburn. The course text was the first printing of "Silent Sentinels" in 1957 !
Phil

#### CSA

Most of the reverse power protective relays I have worked with had inverse time delay characteristics--meaning the faster the rate of increase in reverse power the quicker the relay would act to open the generator breaker. Beginning with Mark* V turbine control systems, GE incorporated a reverse power detection function (using the load transducer) to open the generator breaker on reverse power (negative MW), and sometimes (often) there was a short time delay (a second or two) to prevent nuisance tripping (on frequency excursions, mostly).

But, we're all speculating and postulating--because the person who posted the most recent question isn't responding or clarifying....

#### PhilCorso

CSA...
Since I posted the last response to Curious_One, it was to agree with him, i.e., Protective Relying is an Art! And, I've been involved since 1957 !
Regards, Phil

#### Syed Ahmed

mohsin,

- Motors convert amperes into torque.

- Motors get their amperes from generators.

- Generators convert torque into amperes.

- Generators get their torque from the prime mover coupled to them (in your case, a reciprocating engine).

- Prime movers produce torque from the energy flowing into them (diesel fuel; natural gas; steam; etc.).

Synchronous generators--and their prime movers--<b><i>synchronized</b></i> together all run at the same speed (remember you have to start the prime mover and get the generator up to rated speed in order to be able to synchronize to the other generators and their prime movers--and once the generator breaker is closed the prime mover and generator can only run at a speed that is proportional to the frequency of the grid).

Most people think that when they increase the load of a generator and its prime mover that the speed of the two will increase--but it doesn't! Once synchronized to a grid with other generators (even a small grid!), the speeds of all the generators are fixed by the frequency of the grid. And, the torque being supplied by the prime movers to each of the generators they are coupled to must be sufficient to keep the generators spinning at rated speed at a minimum. If the torque being provided is more than required to maintain rated speed, then the generator will sense the extra torque and convert it to amperes. If the torque being provided is exactly equal to that required to maintain rated speed there will be ZERO amperes produced by the generator. If the torque being provided by the prime mover to the generator is less than that required to maintain rated speed then the other generators and their prime movers will provide amperes to the generator to keep it (and the prime mover) spinning at rated speed--because they all have to spin at the same speed. (You can't have one generator running at 49.7 Hz, and another one at 52.4 Hz, and another one running at 50.1 Hz, and have 50.0 Hz coming out of the receptacle on the wall. It doesn't work that way; there's no smoothing thing or averaging thing for frequency.)

When the torque being supplied to the generator is not sufficient to keep it running at rated speed the other generators and their prime movers will supply amperes to the generator to keep it spinning at the same speed. This is called "reverse power"--since the amperes will be flowing into the generator which isn't getting sufficient torque from its prime mover instead of flowing out of the generator.

In this case, the generator actually becomes a motor and drives the prime mover to keep them both spinning at synchronous speed (the same speed as all the other generators synchronized together as a function of the grid frequency). This is also called "motorizing" the generator. There is really very little difference, mechanincally, between motors and generators--just the directions of torque and amperes.

It's NOT good to use the generator (as a motor) to drive the reciprocating engine (or just about any other prime mover). So, there are protective relays to monitor the direction of current flow and open the generator breaker to protect the prime mover (again, the electrical machine coupled to the prime mover doesn't really care if it's a motor or a generator (it's designed to act as a generator), but the prime mover doesn't like being driven by a motor (or a generator which has become a motor)).

Since the torque produced by the prime mover is a function of the energy flowing into the prime mover, if the torque falls below that required to keep the generator spinning at its rated speed that means the prime mover control system (the governor) isn't doing it's job properly. Or, there is something which is restricting the flow of energy into the prime mover (in the case of a reciprocating engine, dirty fuel filters; low fuel supply pressure; a non-working fuel pump; etc.).

So, when a generator--and its prime mover--are tripped by reverse power that means the prime mover is not providing or producing sufficient torque to keep its generator spinning at its rated speed and the other generators synchronized to the grid will provide amperes to the generator, causing it to act like a motor, and spin the prime mover.

You need to understand why the prime mover isn't producing sufficient torque to keep the generator spinning at its rated speed.

There's one more possibility--but it gets very complicated. There are two basic governor modes: Droop and Isochronous (speed control modes). IN GENERAL, most governors are always in Droop speed control mode when multiple generators are synchronized together on a grid. If one governor is operated in Isochronous speed control mode when it should not be, then that could cause a problem.

<b>OR,</b> if there is a over-all control system for a group of generators (sometimes called a PMS, or Power Management System) that <i>attempts</i> to control the loads of multiple generators synchronized together there can be issues if the PMS is not properly configured OR operators think they need to over-ride the PMS in some instances....

That's about it. Without understanding a LOT more about the configuration of the generators and their prime movers at your site it's really not possible to say much more. BUT, again: The most common cause of reverse power tripping is insufficient energy flowing into the prime mover--regardless of it's governor mode, or whether there is a PMS or not. Insufficient energy flowing into the prime mover equals reverse power (amperes flowing INTO the generator instead of out). Amperes flowing into a generator causes the generator to become a motor which is keeping the generator--and its prime mover--spinning at a particular speed. And, most prime movers (especially reciprocating engines and steam turbines) DO NOT like to be spun by their generators (which have become motors). It's as simple as that.

Hope this helps!
Dear CSA,
What would happen if the governor fails to shut of fuel supply during the increase of frequency. Like if there is 4 percent droop setting then with every 1 percent increment the load must decreases by 25 percent but what if the governor valve don't responds?

#### CSA

Syed Ahmed,

I'm not quite sure I completely understand the question, but if the frequency had increased or was increasing and the governor of a particular machine did not reduce it's fuel/load in response then the frequency would tend to stay high and not decrease--the machine would not be doing its part to support grid stability, in this case by not reducing its fuel/load to try to help return the grid to rated frequency.

When the grid frequency goes high that means there is an excess of generation for the amount of load on the grid--meaning there is more energy flowing into the prime movers than is required to support the load on the grid AND maintain rated frequency. For example, for some reason a large block of load is separated from the grid (a residential substation breaker opens, or a large user like a cement plant or a paper mill or a refinery utility tie breaker opens--and suddenly the amount of energy flowing into the prime movers is too much for the amount of load causing the frequency to increase. Those units operating in Droop Speed Control and operating at part load (not at Base Load and not close to zero load) should respond by reducing their energy flow-rates to reduce their fuel (and their load) to keep the grid frequency from continuing to increase, uncontrolled.

The opposite happens when the grid frequency drops below rated. The amount of generation on the grid is not sufficient to support the load on the grid AND maintain rated frequency. So, if a large generator, or a group of generators, is suddenly separated from the grid then the remaining generators and their prime movers do not have enough energy flowing into their prime movers to maintain the load AND also maintain rated frequency. So, those units operating in Droop Speed Control and not operating at Base Load should increase their energy flow-rates to help keep the grid frequency from continuing to decline, uncontrolled.

AC power generation is about supplying load WHILE MAINTAINING RATED FREQUENCY. To get a generator up to rated speed (frequency) and maintain rated frequency (such as during synchronizing) requires a certain amount of power just do maintain rated speed (frequency). When the generator breaker closes the prime mover has to continue to supply at least that much power (the power required to maintain rated speed (frequency))--or reverse power will flow from the grid into the generator to keep it spinning at rated speed (frequency). Just because the generator breaker closes doesn't mean the prime mover can stop supplying power to keep the generator spinning at rated speed (frequency). If the power provided to the generator by the prime mover is only equal to what's required to keep the generator spinning at rated speed (frequency), the power out of the generator will be zero (watts; kW; MW).

When the load was at 10 MW and the generator/system frequency was at rated (50.0 Hz), with a Droop of 4% the governor the governor speed reference was at 102%. That is the amount of energy required to BOTH supply the load on the system (10 MW) AND keep the generator running at rated frequency. When the group of pumps was started, the governor speed reference DID NOT CHANGE! Only the actual speed of the generator (system frequency) changed! This means that some of the energy that was being used to maintain rated speed/frequency was being used to help supply the additional load of the group of pumps. So, the system frequency decreased.

When the operator is satisfied with his brew and returns to the control board he sees the load on the system has increased and the system frequency has decreased. He then starts increasing the governor speed reference to add more energy to the prime mover to bring the system frequency back up to rated. When the operator is finished returning the frequency to rated, the governor speed reference will be at 103%. Because the total load on the system is 15 MW, which is equal to 3/4 of the rated load of the prime mover, and 3/4 of rated load converts to 3/4 of the amount of Droop--which is 3% of the total of the 4% Droop rating.

The opposite would happen if someone shut off that 5 MW group of water pumps without changing the governor speed reference (decreasing it from 102% to 101%). And, if the governor valve admitting energy into the prime mover didn't decrease the flow of energy into the prime mover then the speed (frequency) is going to increase, by an amount proportional to the extra energy--and this is not going to help support the grid stability and its ability to return to normal (frequency).

It's a real balancing act--load and frequency. A lot of people think Droop Speed Control will return frequency to normal by itself, but it doesn't. What it does is prevent the downward spiral (or upward spiral, as the case may be) and gives an operator a chance to correct the situation.

Hope this helps!

#### CSA

By the way,

A 1% speed difference (between reference and actual) for a 50 Hz machine with 4% Droop would correspond to a 0.5 Hz frequency change.

Just in case enquiring minds wanted to know.

#### Curious_One

By the way,

A 1% speed difference (between reference and actual) for a 50 Hz machine with 4% Droop would correspond to a 0.5 Hz frequency change.

Just in case enquiring minds wanted to know.
Dear CSA,
What would happen if the governor fails to shut of fuel supply during the increase of frequency. Like if there is 4 percent droop setting then with every 1 percent increment the load must decreases by 25 percent but what if the governor valve don't responds?

CSA is doing his best to explain droop, but I feel the question is deeper.
If turbine/generator controls are unable to handle droop, protective relays should.
Protective relays are installed to open generator breakers when the control system is unable respond in a timely matter.

CSA

#### CSA

If turbine/generator controls are unable to handle droop, protective relays should.
Protective relays are installed to open generator breakers when the control system is unable respond in a timely matter.
Thanks, Curious_One. This type of question, or one asked in this way, is difficult for me to respond to. Mostly, the questioner is likely looking for confirmation of a theory or reason they have for something which seemed to have occurred somewhere. And we get very little actionable data (recorded values over time), but bits and pieces of anecdotal data--mostly perceptions and assumptions. Very difficult to deal with.

The thing I should have asked is: Was this question for a single machine synchronized to a small number of other machines, or to a larger, "infinite" grid? When machines are synchronized together they are actually as one unit in that the frequency of ALL units is the same (they are synchronized together). No single machine's frequency can be greater or lesser than any other machine's frequency.

If one machine is not responding as it could or should then that will affect the frequency--and possibly the stability--of the entire group of machines synchronized together. That really depends on the size of the machine having the issue versus the entire group of synchronized units.

AND, if the one machine having the problem is large enough, relative to the entire group of synchronized units then it's possible, as you say Curious-One, that the frequency will reach a value that causes one or more protective relays to operate, possibly causing one or more units to be tripped--thus adding to the stability problem of the group of remaining synchronized unit(s).

Based on the information provided, it really was nearly impossible to respond appropriately in a concise (brief) manner. If I asked a lot of questions, I probably wouldn't have gotten much, if anything in response.

My apologies if my response wasn't very good to this question. And thanks for trying to nail me out, Curious-One.

#### CSA

Curious_One,

>>>CORRECTION<<<

"...And, thanks for trying to bail me out, ..." Not nail me out. Spell-checkers. Stoopud spelcjecqurs.