Field Forcing effect on Generator Short Circuit Decrement

[ish1284]

Hey all I have some questions on generator field forcing.

I am looking at a generator decrement curve and trying to determine if the field forcing effect is included in the decrement curve or not. As you can see, the curve levels out at about 1s at about 2.5PU.

Would the current theoretically just go to 0 if there is no field forcing? I've seen other decrement curves where the fault current dips down, and then increases again before it levels off in which I'm certain that is a result from field forcing. But when it just levels off without increasing again, is there still field forcing taking place? The generator includes a main output breaker in which I would like to set it to trip in its short time pickup region when it senses a fault. Field forcing affects where the STPU needs to be set in order to catch it.

The particular generator I'm looking at is a 1MW, 480V MTU permanent magnet engine generator.

 

[RumRunner]

Hey all I have some questions on generator field forcing.

I am looking at a generator decrement curve and trying to determine if the field forcing effect is included in the decrement curve or not. As you can see, the curve levels out at about 1s at about 2.5PU.

Would the current theoretically just go to 0 if there is no field forcing? I've seen other decrement curves where the fault current dips down, and then increases again before it levels off in which I'm certain that is a result from field forcing. But when it just levels off without increasing again, is there still field forcing taking place? The generator includes a main output breaker in which I would like to set it to trip in its short time pickup region when it senses a fault. Field forcing affects where the STPU needs to be set in order to catch it.

The particular generator I'm looking at is a 1MW, 480V MTU permanent magnet engine generator.

View attachment 21972 View attachment 21973

Hmmm!

Over eighty views and no response.
I guess the generator gurus are still on their holiday. :roll:

Let me get this right.

Initially, you are saying that you are observing the behavior of the stator output through the differing decrements in the event of damped harmonic motion that result in dips and eventual levelling-off of current. Short circuit can also cause this nuance.

Your are also certain that the dips and rise are the result of field forcing.

Having said that, it is inferred that you have a wound rotor. . . this is where (purportedly) your observations are being conducted.

However, in your conclusion, you distinctly stated that this is a permanent magnet-equipped generator.

That is where the conflict arises.

When you have a permanent magnet generator, you would not have a wire-wound field rotor. So, field-forcing is totally irrelevant. You will have either one or the other. . . not both.

Since there is no wire-wound field coil in a permanent magnet generator. . . no field -forcing scenario would even be possible.

Correct me if I'm wrong.

I'll be here.

 

[ish1284]

Hmmm!

Over eighty views and no response.
I guess the generator gurus are still on their holiday. :roll:

Let me get this right.

Initially, you are saying that you are observing the behavior of the stator output through the differing decrements in the event of damped harmonic motion that result in dips and eventual levelling-off of current. Short circuit can also cause this nuance.

Your are also certain that the dips and rise are the result of field forcing.

Having said that, it is inferred that you have a wound rotor. . . this is where (purportedly) your observations are being conducted.

However, in your conclusion, you distinctly stated that this is a permanent magnet-equipped generator.

That is where the conflict arises.

When you have a permanent magnet generator, you would not have a wire-wound field rotor. So, field-forcing is totally irrelevant. You will have either one or the other. . . not both.

Since there is no wire-wound field coil in a permanent magnet generator. . . no field -forcing scenario would even be possible.

Correct me if I'm wrong.

I'll be here.

While I'm not disputing that you are wrong because your take is technically sound, can you explain the ramp up in current on the attached short circuit decrement curve? This is a PMG and as you can see, the output current decreases as one would expect and then at about 0.4s the output current ramps back up. It appears to be some sort of field forcing effect. Is there something else that can be built into the PMG's controls/voltage regulator that can simulate field forcing if there is no such thing as field forcing in a PMG?

 

[Russs57]

Keep in mind a PMG generator will respond slower. Something to consider if you are comparing curves from other units.

Personally I wouldn't worry about any forcing in setting up my breaker trip points. The is taking place because generator voltage has dropped due to increased load. You need enough forcing to keep current high enough to allow breaker to operate.

IMHO short circuit events are so quick that field can't be forced/changed that fast.

 

[topgone]

Keep in mind a PMG generator will respond slower. Something to consider if you are comparing curves from other units.

Personally I wouldn't worry about any forcing in setting up my breaker trip points. The is taking place because generator voltage has dropped due to increased load. You need enough forcing to keep current high enough to allow breaker to operate.

IMHO short circuit events are so quick that field can't be forced/changed that fast.

Agree with [MENTION=144899]Russs57[/MENTION]. Regulators need time to respond.

 

[GoldDigger]

Agree with [MENTION=144899]Russs57[/MENTION]. Regulators need time to respond.

That's why a Code cycle takes so long? :angel:

Sent from my XT1585 using Tapatalk

 

[RumRunner]

While I'm not disputing that you are wrong because your take is technically sound, can you explain the ramp up in current on the attached short circuit decrement curve? This is a PMG and as you can see, the output current decreases as one would expect and then at about 0.4s the output current ramps back up. It appears to be some sort of field forcing effect. Is there something else that can be built into the PMG's controls/voltage regulator that can simulate field forcing if there is no such thing as field forcing in a PMG?
View attachment 22006

Apologies for the slow response.

I have this bad habit of holding comment before other members had the chance to comment first.

I try very hard not to shoot myself in the foot.


There is a Market Research Report regarding your concept. . . I think that it is workable.

Several ideas came up in the following report that will cover this field.

For a small fee you can have an electronic copy of this report on the web.

https://www.giiresearch.com/report/mama603452-variable-speed-generator-market-by-type-permanent.html


The report covers the growing interest in variable speed generators.
This technology is basically aimed at all kinds of power generation. ( NICE)

Permanent Magnet, Doubly Fed Induction, Wound Rotor and Prime Mover (Wind, Hydro, IC Engine, Steam & Gas Turbine.)

In this (my) response I would address only your concern which is field forcing with PMG

Your goal is to take advantage of the Short Time Pick Up region (STPU). . . I would call it “window” because of the limited opportunity available for making corrections to bring the current up after the short circuit event.

On your chart, the longest time available to you is the three phase power source. . . about ten seconds as opposed to the 0.4 you have alluded to.
Keep in mind however that the above corrections are “likely” achievable in a PMG-- just for the sake of argument.
If one of the reports become a reality of course.

Adjusting the STPU of a “garden variety” circuit breaker is not something available from the shelves of Home Depot.
A lot of these CBs are still using the bi-metal tripping mechanism. . . although some electronic modern CBs are available.
I haven't seen the latter at HD yet.

This type tripping device is illustrated on your chart which indicate the thermal limit from each particular power source.

In order to achieve your goal and take of advantage of your limited opportunity, is to use super-fast- acting Circuit Breakers.
This would come in a totally different shape than the usual snap-ins that you see inside the sheet metal cabinet mounted on your garage wall.

You would need an arrangement of MOSFETs to achieve the benefit this super-fast- acting circuit breaker.
You are asking for a possible alternative to control the generator output in the absence of controllable wire-wound field rotor. . . and this is what comes to mind.

All the best.

 

[Bugman1400]

I would expect the transition from the higher current in the beginning to the lower current is just the transition from the sub-transient reactance to the transient reactance to the synchronous reactance.

 

[topgone]

I would expect the transition from the higher current in the beginning to the lower current is just the transition from the sub-transient reactance to the transient reactance to the synchronous reactance.

Nope. The sub-transient period is around up to 3 cycles (50ms). The transient period comes next between 3 cycles and about 12 cycles of the power frequency (200ms).

It is important to note that in this transient period is where your breakers are bound to respond and activate. Since the OP was talking about 1 second, then the idea that it is the transition period goes out of the window!

 

[RumRunner]

I would expect the transition from the higher current in the beginning to the lower current is just the transition from the sub-transient reactance to the transient reactance to the synchronous reactance.

Your expectations are sound and can be substantiated.

These transitions are the result of short circuit event. The sub-transient which is the first transition-- is the region where the instantaneous change from high current to lower current takes place. It's a very short event that happens between .4 ms to 50 ms.. . . depending on the frequency.

Your analysis is a textbook-based short circuit analysis which could pave the way for a paradigm on how to take a closer look on short circuit that affects current on the generator output.

The procession of these reactances-- from sub-transient to transient reactance and the eventual steady state (synchronous) reactance are irrefutable . It is just the way we were taught to accept. . . and are academically proven to be relied on--hence can be represented with a chart.

OP's focus relates to a “possible” way of utilizing this transitional event to control the output of the generator. . . and somehow come up with a device that can be put into use.

It's a tough “act to follow” because in a sustained short circuit (as illustrated on the chart) the current collapses to ZERO during and after STPU (short time pick up) transitional event.

The current will recover despite the time constraint--if the short circuit is momentary. Unfortunately, it is sustained-- according to the chart.

Until the short circuit is cleared-- I see no corrective measure that can be easily applied to bring the current back up to the high level before the short circuit event.

It is a challenge and the result is what OP is hoping to achieve.

The above is in response to your comment. . . and probably more


If I miss something. . . let me know.

 

[Sahib]

Your expectations are sound and can be substantiated.

These transitions are the result of short circuit event. The sub-transient which is the first transition-- is the region where the instantaneous change from high current to lower current takes place. It's a very short event that happens between .4 ms to 50 ms.. . . depending on the frequency.

Your analysis is a textbook-based short circuit analysis which could pave the way for a paradigm on how to take a closer look on short circuit that affects current on the generator output.

The procession of these reactances-- from sub-transient to transient reactance and the eventual steady state (synchronous) reactance are irrefutable . It is just the way we were taught to accept. . . and are academically proven to be relied on--hence can be represented with a chart.

OP's focus relates to a “possible” way of utilizing this transitional event to control the output of the generator. . . and somehow come up with a device that can be put into use.

It's a tough “act to follow” because in a sustained short circuit (as illustrated on the chart) the current collapses to ZERO during and after STPU (short time pick up) transitional event.

The current will recover despite the time constraint--if the short circuit is momentary. Unfortunately, it is sustained-- according to the chart.

Until the short circuit is cleared-- I see no corrective measure that can be easily applied to bring the current back up to the high level before the short circuit event.

It is a challenge and the result is what OP is hoping to achieve.

The above is in response to your comment. . . and probably more


If I miss something. . . let me know.

Unlike self-excited types, a PMG generator would have sustained short circuit current for the breaker to clear the fault.

 

[RumRunner]

Unlike self-excited types, a PMG generator would have sustained short circuit current for the breaker to clear the fault.

True, but that's not the gist of OP's post with his accompanying CURVE.

In his post, he expects to have the current level off and eventually rise to high level... while the short circuit is still present. (sustained)
And then I countered it with my statement –verbatim: “until the short circuit is cleared”.

His chart did not clearly indicate that there is rise in current. . . it just levelled off at ZERO.

With ZERO current-- how would the circuit breaker trip. . . and thus clear the fault?

 

[Sahib]

True, but that's not the gist of OP's post with his accompanying CURVE.

In his post, he expects to have the current level off and eventually rise to high level... while the short circuit is still present. (sustained)
And then I countered it with my statement –verbatim: “until the short circuit is cleared”.

His chart did not clearly indicate that there is rise in current. . . it just levelled off at ZERO.

With ZERO current-- how would the circuit breaker trip. . . and thus clear the fault?

In generators that depend on residual magnetism to build up terminal voltage, the terminal voltage collapses and no current to trip the breaker when a short is placed across the terminals. But in case of PMG generator, the short circuit current is sustained so that the breaker trips. That is one advantage of PMG generator and OP statement seems to deny it.

 

[topgone]

In generators that depend on residual magnetism to build up terminal voltage, the terminal voltage collapses and no current to trip the breaker when a short is placed across the terminals. But in case of PMG generator, the short circuit current is sustained so that the breaker trips. That is one advantage of PMG generator and OP statement seems to deny it.

A better understanding of how generators are allowed to be field-forced will help you there. The allowed field forcing is very short, about 30 seconds! Maximum field forcing is even limited to just about 3 seconds in most machines, before reducing to about 110% of normal excitation. Please refer to IEEE C50.13 for a better grasp of the subject.
Back to the OP, the presented decrement plot clearly shows the effect of field-forcing (shown on the plot that starts to bend up at around 300 ms), the OP's problem was to definitely point-out the value of the STPU setting of his main breaker to catch a "beyond normal" fault ( and trip the breaker) as opposed to allowing the breaker to ride thru a normal generator current with field forcing effect. If I were to decide, I would make sure my settings will not damage my unit by exceeding the thermal limits of the unit.

 

[SSNATHAN]

In generators that depend on residual magnetism to build up terminal voltage, the terminal voltage collapses and no current to trip the breaker when a short is placed across the terminals. But in case of PMG generator, the short circuit current is sustained so that the breaker trips. That is one advantage of PMG generator and OP statement seems to deny it.

What is OP sir?

 

[Dennis Alwon]

What is OP sir?

Original post

 

[xptpcrewx]

Hey all I have some questions on generator field forcing.

I am looking at a generator decrement curve and trying to determine if the field forcing effect is included in the decrement curve or not. As you can see, the curve levels out at about 1s at about 2.5PU.

Not enough information but field forcing is probably being shown.

Would the current theoretically just go to 0 if there is no field forcing?

No. Assuming the AVR does not adjust or trip-out, the excitation would be in whatever state the field was in prior to the short-circuit.

I've seen other decrement curves where the fault current dips down, and then increases again before it levels off in which I'm certain that is a result from field forcing. But when it just levels off without increasing again, is there still field forcing taking place?

Not sure do you have a decrement curve you want me to look at in particular?

The generator includes a main output breaker in which I would like to set it to trip in its short time pickup region when it senses a fault. Field forcing affects where the STPU needs to be set in order to catch it.

One suggestion is to model the generator decrement curve at no load excitation then set the breaker accordingly. If a fault occurs, you can be assured the breaker will trip with no load excitation (worst case) and also naturally when field forcing is in effect.

The particular generator I'm looking at is a 1MW, 480V MTU permanent magnet engine generator.

View attachment 2388532 View attachment 2388533

I understand this to be a PMG (Permanent Magnet Generator) supply to the regulator (so Brushless, rotating field generator).