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Compare Excitation Systems

Compare Excitation Systems

Compare Excitation Systems

Hello Guys,

I study a generator (5 MVA) with an old Static Excitation System. I plan to change the Excitation System with a new model. Which parameters should be compared between new Excitation models (ceiling voltage, ceiling current, Response Ratio), such that i can arrive at a decision concerning which is the optimum solution? In other words, which parameters are the most important in deciding about what model to choose and, consequently? how can I conclude with certainty that i choose the correct model?

RE: Compare Excitation Systems

If this is an old set, does it use slip rings or is it a brushless excitation system?
Does it use a PMG (Permanent Magnet Generator).
You want to know the field resistance and the maximum field voltage.
Do you need provision for cross current compensation? (Will the set operate in parallel with other sets?)

"Why not the best?"
Jimmy Carter

RE: Compare Excitation Systems

Yes the generator use slip rings but not PMG. It will operate in parallel with other sets. The field resistance and the maximum field voltage depend on the existing Generator or the new AVR?

RE: Compare Excitation Systems

Note i would like to highlight that i plan to change only the AVR and not all the Excitation System!

RE: Compare Excitation Systems

An AVR for a brushless set will have a listed minimum field resistance and a maximum voltage.
These AVRs excite the brushless exciter, not the main field.
With static exciter you still must be concerned with the maximum current and voltage that the AVR must deliver.
I saw very few static exitation systems on the sets that I worked on. All had either rotating exciters, brushless exciters or rotating exciters converted to brushless exciters.
Let's wait and see what ScottyUK has to say. I defer to Scotty on larger sets.

"Why not the best?"
Jimmy Carter

RE: Compare Excitation Systems

Better to involve reputed AVR makers like Basler, Woodward etc. Static excitation systems are more complex than brushless systems.


RE: Compare Excitation Systems

Irrespective of vendors, how can I conclude with certainty that i choose the correct model? Which parameters (overshoot,rise time) should be compared?

RE: Compare Excitation Systems

oresakri, in cases like this, its feasible to ask vendors for their consideration on their most appropriate solution for replacement of a static excitation system. There are likely factors other than technical parameters that would guide selection of a vendor (and their AVR), including local support, and commissioning capabilities for their equipment.

You'll likely need to be aware (or be prepared to find out) what the relevant parameters are for your unit, which I'd presume to be rotor impedance among other things (like waross, I've only ever dealt with brushless systems). ScottyUK has more experience on larger units.

EDMS Australia

RE: Compare Excitation Systems

On a small set like this I don't think you will need to worry too much about the AVR capability other than ceiling voltage and maximum current. Most AVR's can drive the field with more current than the rotor can continuously withstand, and the rotor is protected by an I²t inverse characteristic.

Check if the power stage is a half-controlled or fully-controlled bridge. A fully-controlled type can actively drive the field current down as well as up, whereas a half-controlled type can only boost the field and allow it to decay according to the L/R time constant. This has implications if you have significant load shifts and wish to avoid avoid over-voltage excursions during load reduction.

There are numerous monitors, protections, etc available but typically found on much larger sets. 5 MW is pretty small for a slipring machine.

RE: Compare Excitation Systems

ScottyUK, thank you very much . As to bridge is fully-controlled .

RE: Compare Excitation Systems

"Check if the power stage is a half-controlled or fully-controlled bridge. A fully-controlled type can actively drive the field current down as well as up, whereas a half-controlled type can only boost the field and allow it to decay according to the L/R time constant" ScottyUK- I never knew that an AVR can drive the voltage down,I only thought it decay after reducing the excitation current. I Try google for more but nothing in detail.Do u have any literature explaining the operation in detail..Thank you.

RE: Compare Excitation Systems

The fully controlled bridge can operate in two quadrants: positive voltage with positive current, or negative voltage with positive current. The half-controlled bridge can only operate in the first quadrant with positive voltage, positive current, so once current is established in the R-L circuit a freewheel path must be provided to allow current to continue to circulate when the converter firing delay angle is increased as the field setpoint is reduced. Any decent power electronics text will cover full-converters and half-converters far better than I can manage here in a text-only forum: the waveforms are the key to understanding operation. If you're looking for a specific recommendation, I've had this book for over 25 years and it's still the one I reach for first, but there are plenty others to choose from and loads of online stuff in Google.

The AVR / field circuit is just a controlled bridge with a highly inductive load, i.e an energy store from which regeneration can take place if the converter design allows it. In the case of the fully-controlled bridge, the converter can regenerate into the source and actively push energy from the field back into the source, rather than waiting for the stored energy to dissipate in the resistive element of the R-L circuit, which gives a much faster response when trying to reduce the field current.

RE: Compare Excitation Systems

Thank you.

RE: Compare Excitation Systems

Try searching for negative field forcing.

RE: Compare Excitation Systems

It's worth noting that a 2-quadrant converter requires direct connection to the field winding. An on-shaft rectifier will obviate the potential benefit of a 2-quadrant field converter, so this type of converter is only seen with slipring machines. The exception to the rule is for on-shaft thyristor rectifiers, but I'm not aware of any in operational service today although these were trialed by the CEGB and the tame manufacturers many years ago. If anyone knows of any please shout up.

RE: Compare Excitation Systems

redlinej i think you should read the "Applying Static Excitation Systems" by R.C. Schaefer. In this paper is described, how 6 SCR system causes a negative voltage to be applied into the field

RE: Compare Excitation Systems

Hello oresakri,

First of all, you need to know what is the unit rated field current (at rated MW, Rated PF). Then, a good rule of thumbs is to add a margin on that value. Then with that value, that would determine the bridge size you need (vendors have different bridge size to reduce the cost since the most expensive component in an excitation system is the bridge).
In a full static excitation system, the main components are: The excitation transformer (not any transformer can do that job, it needs to be usually oversized compared to the rating), the power stage ( the thyristor bridge (or Diode + IGBT in smaller systems)) and the rotor windings.

When you say you only want to replace the AVR, what you actually mean is that you want to keep the bridge but just to replace the controller? Or you mean replacing the full excitation system? AVR stands for Automatic Voltage Regulator and relates to the controller itself typically. Then if you only replace the excitation system without touching the excitation transformer, your ceiling factor is pretty much fixed. The excitation transformer will most likely determine the ceiling factor (That and the rotor impedance). Typically we are shooting for 1.6pu for 10s for the ceiling current. The ceiling voltage depends on the transformer secondary voltage and the minimum firing angle (default value is 10degree and 150 degrees, where 90 degrees = and average around 0V, 10degrees = Maximum Positive and 150 = Maximum negative.). Keep in mind that higher ceiling is what we are looking for. High ceiling means faster response and higher support in case of a fault. Even though these numbers might exceed the rotor capability, all new exciter have what we call OEL (over-excitation limiter) that will limit the field current so your excitation system can be oversized. It usually looks like a staircase (goes to 1.6 for 10s, then reduce to a lower value something like 1.05 for indefinite period of time)
As mentioned by others, having a full bridge (6 thyristors) allows you to do negative field forcing by firing the bridge in inverter (firing above 90 degrees). Keep in mind though that this is only possible since the load is inductive and that once the load is discharged you can't fire in inverter anymore (that comes from the fact that as the current decay in an inductive load, the voltage inverse (V = Ldi/dt)). Inverter is one of the reason why people go full static (the other is to reduce the time constant Te by removing the rotating exciter / rotating diode)

If many units are sharing a single step-up transformer you'll need either a Qstatic compensation (also called a droop) value to ensure that both units are not going to fight ( think of a car driven by 2 drivers that don't talk with each other) As another member mentioned, the evolution of a droop is the CCC or Cross-current compensation. 2 drivers but know they talk one with the other so they know what is happening.

Not all vendors can do CCC, all vendors can do droop though. Droop reduce your MVars support to the grid and therefore between the two you should shoot for the CCC if available.

RE: Compare Excitation Systems

You can use a CT and a resistor to generate a quadrature voltage that may be used to bias the sense voltage to achieve CCC with any AVR.

"Why not the best?"
Jimmy Carter

RE: Compare Excitation Systems

Can someone briefly describe the difference between rotating excitation and brushless excitation. I always thought they were one in the same but it appears that there may be inherent differences.

I understand that a rotating exciter uses an external excitation source (AC or DC) to provide field current to an "exciter" mounted on the generator shaft with the exciter then providing the main field current to the main field.

How does a brushless excitation system differ from that? From what I've searched it appears that a brushless system may use a PMG in place of an external excitation source to provide the field current to the exciter?

Is there one type of excitation system (static, rotating, or brushless) that is a preferred method given the technologies that are available today?

RE: Compare Excitation Systems

The old original exciters were a DC generator rotating on the shaft. The output of the generator was controlled by varying the strength of the stationary exciter field. The output of the generator was fed back into the field via slip rings.
Then brushless exciters were developed.
A brushless exciter is a three phase alternator with a stationary field.
The output of the alternator is controlled by varying the strength of the stationary exciter field.
The output of the alternator is fed to a shaft mounted diode plate and then directly to the main alternator field.
Many of the original DC generator exciters have been converted to brushless exciters.
A Permanent Magnet Generator (PMG) is mounted on the shaft behind the brushless exciter to supply power to the Automatic Voltage Regulator (AVR).

"Why not the best?"
Jimmy Carter

RE: Compare Excitation Systems

In the days before high power electronics became readily available it was common to find a both pilot exciter and main exciter on the generator shaft, with the pilot being the externally-controlled machine and the main exciter being the 'power amplifier' which boosted the current to the level required by the field. Each stage adds a lag into the control response, so these machines were fairly sluggish to respond when compared a modern static AVR.

A static exciter with sliprings will give the best outright performance under dynamic conditions.

RE: Compare Excitation Systems

Hello rockman7892,

As you mentioned there are 2 great categories of excitation system :

1) Full Static :
This is the most common. Typically an excitation transformer will be shunt fed by the generator output, then that transformer will feed the excitation bridges that will convert the AC into DC.

The main advantage of a full static is the response time. A full static doesn't have a time constant Te like the rotating machine does and therefore the only delay is the one of the electronic to calculate a new firing angle and apply it to the thyristor. This is the most common type of excitation system and now also the prefered one. A lot of places they converted from rotating to Static (by removing the rotating machine on the shaft). You will require an external field flash supply source unless you system is small enough to be aux fed.

2)Rotating exciter :
2.1: AC : This is the most common among the rotating exciter. You have a static part of the excitation system that will excite the stator of a rotating machine. Then the rotor will produce AC then you have many options. 1) If the diodes are on the shaft and spinning with it, this is called rotating diode and it will be brushless (since at no point you have to transfer power to a rotating part). Some places they will use stationary diodes then you will have 2 sets of brushes. One to feed the diodes and one from the diodes to the rotor winding.

2.2 : DC rotating machine. Way simpler, this tends to disappear. No rotating diodes are required (you are already in DC) but the static part works about the same.

Using a PMG or not doesn't have any incidence of being brushless or not. PGM is used as a supply for the static excitation system. For instance, you will have a static excitation system that will be PMG fed and that excitation system will feed the stator of a rotating exciter. Since you have an amplification stage (the rotating machine) then your PMG doesn't need to be big and therefore it can be big enough to supply the needs of the excitation system. Most if not all nuclear units are done like that. A lot of really big coal machine as well. It allows having a way smaller excitation system (smaller bridge means way cheaper). Also, being PMG fed allow you to have a certain black start capability since if you can make the rotor spin then you have supplied to the excitation system. Also, no field flash supply required for the same reason

For instance a 1000MVA unit that uses a rotating exciter, can be feed by a really small bridge (supplying 400-500 amps depending on the design it can be more or less than that but this is a rough idea) while if you go full static you will need multiple bridges in parallel to supply 4000-5000A some places up to 10 000A which require a really big excitation transformer, 5-6 super big bridges and all the electronics that comes with it.

Some nuclear units are converting to static because of grid stability problems. If you have a really long Te which is common in nuclear units, then you'll need super high gains to try to reduce the Te incidence and this created problems. Being full static allows getting a faster response with way smaller gains.

I heard some rumours about instead of using Diodes on the shaft, they will use thyristors that are fired remotely (using Bluetooth or a similar technology) and then that remove the needs for that stationary part. I never saw one myself and I have my doubts since we can't even read the field current and voltage on a brushless unit (which would be way easier to do than firing thyristors) so anyway maybe in a near future.

Finally, the reason why people go brushless (over another type of rotating exciter) is to reduce maintenance. When you have a machine spinning at 3600RPM you'll have to replace the brushes quite often and that means downtime. Well if you want to do it safely.

RE: Compare Excitation Systems

Great information for large machines.
Not applicable to a few million diesel sets.
Almost universal now for sets from about 10 KVA to a few Mega Watts is the brushless exciter.
A PMG may be added. The PMG powers the AVR.
The AVR controls the exciter which is powered by the prime mover.
An issue with alternators is voltage collapse under fault conditions. As a result of voltage collapse there may not be enough current to reliably operate the protection devices.
One way to mitigate voltage collapse is with a PMG.
An older method of mitigating voltage collapse is with a current boost circuit.
In the current boost scheme the output of the current boost CTs is used to develop voltages across resistors. That voltage is used to support the control of the brushless exciter.
I don't know what size of machines that Rockman uses, but I doubt that they are nuclear class.

"Why not the best?"
Jimmy Carter

RE: Compare Excitation Systems

Hello Waross,

It`s still applicable to small units, however, as you said you will see way more rotating exciter on a small machine because it's way cheaper.
As for the PMG you stand very correctly, having the PMG give you some fault ride through capability. Since the machine has inertia (typically bigger than 1.8MW-s/MVA in North America, it will prevent the unit from accelerating too fast in the event of a fault (your electrical power is gone so nothing balance the mechanical power out, so you'll accelerate, if you get to a certain point, you'll trip)). Therefore, if you still have your voltage supply this will allow you to support the grid and PMG produces a voltage as long as they spin so this is the perfect power source to allow that.

As you mentioned this is the problem with shunt fed excitation transformer when you need the unit to support, your voltage is low so you lose a part of your capability. This is part of the reason why all units must have a ceiling way higher than the operating voltage. This is also why some units are using compounding. They basically install CT on the neutral side (it sees the same current) and then when you have a fault, current increase but voltage collapse, so you'll use that current to produce a voltage that will feed the rotor windings and boost your support. However, not sure why most of the customer get rid of it during upgrades.

Thanks for sharing.

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