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?

 

[ScottyUK]

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.

 

[MarcMarc22]

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.

​

 

[waross]

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.

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

 

[MarcMarc22]

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.