Replacing Rotary Exciters With Static Exciters

Well, the best people to talk to are those who design, install and maintain the equipment, or their successors.

Why does the want exist?

The armature of the old exciters are giving problems.Heavy groves in the commutator surface of the armature.

Why does that feature justify design substitution rather than repair- or replacement-based remedial maintenance?

What discussions have taken place already with the equipment manufacturers or their successors, and what was the outcome of those discussions?

What grows there, oranges, lemons, or nuts?

groves grooves

I have to agree with PWS, why do you want to do this on a 36 year old machine?

Do an economic analysis comparing repair and operating costs of the old system vs. the time and expense involved in removing/replacing the old excitation system, rewiring and commissioning the new one. There's some number of years in the future where you'll benefit from the swap, if the generator will still be in service at that point then go for it.

One more thing, someone has to determine if your old generator and its auxiliaries can take the much higher ceiling voltage/current that a new modern excitation system can deliver, otherwise it'll be like putting a turbocharger on a Cavalier engine; all show, no go.

Replace the units or call in an expert.

You will only cause much damage, not knowing what you are doing.

How many of these questions can you answer yes to?

http://www.basler.com/downloads/ex-rep.pdf

We recently started rehabilitation of our generators.Stator winding replaced from Class B to Class F.The exciters we want to replace to improve efficiency,fast recovery.Remember in 1977 system was limited which has expanded and we need quick response.As far as answer to the number of yes to questions.well yes are 9 out of 16

Jan63, Static Excitation will be far superior when compared with Rotary DC Excitation Machine.

Only drawback is during initial start you will require an independent power supply as Static Excitation cannot excite the machine under black start conditions. I mean you will require 3 phase 415V Auxiliary Power Supply either from Grid or another Power Pant or a Emergency Diesel Generator Set.

If yours is a Steam Power Plant, then not to worry as you cannot fire the boiler without Auxiliary Power (For ID FD Fuel Pumps, BFP etc.) and there must be provision of sufficient Start Up power (4 to 5MVA).

Another Problem can be during Electrical Faults close to Power Plant - we can find that fault currents suddenly dips to a value that it may not be sensed by protection relay.

In case of Rotary Exciter Machine - as long as Turbine is running the exciter keeps on supplying power for Excitation and maintains excitation during fault which is picked up by the protection relay, thus clearing the fault.

But in case of Static Excitation, since the Power is drawn from system, Dip in bus voltage due to 3 phase electrical fault close to Power Station, the power supply voltage to Static Excitation Device will dip. This will reduce excitation of Turbo-Generator, resulting in reduction of fault currents to low value. This may sometime pose problem because Protection Relays at Generating Stations are set at Relatively higher Time Multiplier compared to downstream relays for proper Protection Relay Co-ordination.

Therefore you will require to revise your relay scheme, for example, instead of standard Over current use Voltage Restrained Over current relay so that it turns in to an instantaneous protection scheme for faults where voltage dip is drastic and trips within 20 m secs. (It takes 2 to 3 cycles for voltage to dip)

Further you can augment your protection scheme by making Very Very Under Voltage to instantaneous setting and also if Carrier Tripping is available, inter trip via remote station on feeders leaving your Power Station.

By changing to static excitation, you will save lot of headache of maintenance of Rotary Exciters, such as, daily check for sparking at brushes, clean brushes, check brush holder spring tension, check pigtails of brush to holders for sound connections, besides annually undercutting of mica and chamfering of commutator grooves. In my youth I have worked on a AEG - KWU (now part of Siemens) sets in 72 t 82 and I know headache of DC machines. These sets were production of 1954.

Since you are using the machine for 36 years, its helical grooves on slip ring also must have vanished by now (on our machine we had helical grooves to dress contact surface of Carbon brushes and for cooling) In 1980 I had designed a jig to cut these grooves on slip rings by running TG on Turning gear (Barring Gear). I used the principal of Lathe Machine which I had played with during workshop practice for 1 semester in my Electrical Engineering studies. In case you are facing similar situation we can discuss it.

Now I have my own business and recently I have developed and supplied Static Excitation System rated at 300V, 300A for testing of Traction Alternators to Indian Railways.

I am not sure if Basler have Static Excitation available for 175MW Turbo-Generator, but you can find from ABB UNITROL range or you have to catch one of small manufactures, like me.

We are going ahead with replacement of the exciters.Our is a vertical hydel unit with Francis turbine.My initial question still remains.What will be effect on the rotor when the weight of the rotary exciters is removed.

Since yours is a hydropower plant please ensure that existing emergency DG Set has enough capacity to supply input power to static exciter of at least one hydro power generator.

Batter shall be to leave the rotor in position and just disconnect and remove leads form exciter end (or just keep all brush holders out). Reasons being:

1. By keeping armature in position, you are not reducing the moment of Inertia of the complete rotor assembly thus no risk of run away speed going above the designed.

2. If the complete assembly was dynamically balanced, removing exciter armature will require to dynamically balancing again.

Only advantage I see, reducing weigh on thrust pads, which should not be considered in light of disadvantages as the thrust pads were initially designed to handle the weight of complete assembly.

Best of Luck, Ramesh, Power Solutions.

Generator Rotor and exciter rotor are balanced separately. So removing excite should not have balancing problem.

We had removed exciter rotor of 12 MW Machine when shifting to Static exciter without any problem.

@jan63; I do not know if the following was done: A Total System evaluation with the manufacturer's experts and "your experts". If this was not done, it should be. Also every System has a life expectancy and some essential engineering parameters that should be adhered to and should guide your decision: (i) The Reliabillity, (ii) Maintainability (cost effectiveness)and Safety of Personnel and Plant. After 36 years it might be more prudent to replace the entire system, starting with the weakness "module". Call in the Consultants and review the entire plan. I do not think that the replacement strategy is a sound idea.

Dear Mr.jan63,

I have experience of working with STATIC EXCITER for an LT Alternator, for 14 years. Absolutely NO PROBLEM in all the 14 years, from Excitation aspect and it did not require any external supply either AC or DC.

The Static Exciter was getting a supply of 127 Volts from and with in the 3 phase windings of the alternator, and converted as DC and through Slip-Ring /Carbon Brush the Excitation was done with an AVR.

BHEL has designed and supplied the system.

DHAYANANDHAN.S

you do require power supply during initial start up as the residual magnetism in field is not enough to build the terminal voltage.Its field flashin

Dear Mr.jan63,

The installation what I referred is still working - which is a Sugar Factory - which is a Seasonal Industry, which works for 8 months and shut down for 4 months.

At the time of starting, after 4 months shut down, the Voltage builds up and we did not face any problem, during my 14 years of my service there.

DHAYANANDHAN.S

Please do not fight over power supply required or not required.

In your case the value of slope of excitation curve (No load characteristic) is more that resistance of Field winding and therefore it will self excite.

In case of other gentleman the slope of excitation curve is less than Resistance of field winding therefore he requires shock excitation every time he starts the machine.

By Slope I mean Divide V(occ) (open circuit voltage of machine which is feeding AVR) by the field current I(f), (slope = Perpendicular/ base)