Compare Excitation Systems 4

[oresakri]

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?

 

[waross]

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?)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oresakri]

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?

 

[oresakri]

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

 

[waross]

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.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

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

Muthu

http://www.edison.co.in

 

[oresakri]

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

 

[FreddyNurk]

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

 

[ScottyUK]

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.

 

[oresakri]

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

 

[redlinej]

"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.

 

[ScottyUK]

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.

 

[redlinej]

Thank you.

 

[davidbeach]

Try searching for negative field forcing.

 

[ScottyUK]

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.

 

[oresakri]

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

 

[MarcMarc22]

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.

 

[waross]

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.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rockman7892]

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?

 

[waross]

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).

Bill
--------------------
"Why not the best?"
Jimmy Carter