Gents,

We have a GE gas turbine generator with following specs:

Model: 217210 - 20MVA, 6.6kV, 50Hz, 4 pole, 1500rpm, 0.85 PF, 1750A. Field Amps at rated load is 800A.

We want to tweak the settings of the excitor limiters as follows:

Excitation Limiters	Existing/Original Settings	Proposed Settings
High Level	1560A	1800A
High Time	8sec	5sec
Medium Level	800A	1400A
Medium Time	1sec	20sec
Low Level	800A	810A

Why we want to change them?
Because the excitor was giving over excitation alarms every time a few large motors were being started at site. So we consulted excitation system provider i.e. Basler Electric, and they suggested the change.

Next step was to consult OEM, GE.
We contacted GE, but they asked us for 'money' to carry out 'dynamic simulation' after which they tell us if these settings, proposed by Excitation system provider, are correct or not.

Honestly speaking, the management does not want to spend money and they seem contended on Basler Electric's settings.

Since the excitation current directly impacts rotor, it is clear that the Limiters are based on Generator Capability Curve. The field amp at rated load is 800A (which is low level setting of excitor limiter and the same current, 800A for 1 sec, is Medium level of excitor setting as well). The high level excitor setting is kept at 1560A for 8sec.(I have no idea for the basis, but it was kept by GE).

With new settings, we will be increasing the low level limit from 800A to 810A and Medium from 800A for 1 sec to 1400A for 20secs and finally high level limiter from 1560A to 1800A for 5 secs.

My question is regarding the Medium and High settings. How can we ensure that these settings don't harm the rotor? I mean rotor is rated for 800A but Medium level will now be 1400A for 20secs, so if the field current rises from the low level excitor setting, the rotor will be exposed to much more current. Motor start ups usually last 15 to 17 secs.

What things we should look at if we don't want to have a dynamic simulation conducted? Excitation system manufacturer is confident these settings which he is proposing are correct.

Has anyone done anything similar at any site? What were the basis? and what were the checks done before the change?

Sincerely,
MRasool

 

We can offer advice and experience but ultimately, will your management accept the advice from an online forum? Your management needs to decide if the ability of the generator to ride thru a large motor start is worth the cost of the GE dynamic simulation. Otherwise, you are the fall guy when the bars jump out of the rotor.

 

We can offer advice and experience but ultimately, will your management accept the advice from an online forum? Your management needs to decide if the ability of the generator to ride thru a large motor start is worth the cost of the GE dynamic simulation. Otherwise, you are the fall guy when the bars jump out of the rotor.

I have already given my stance to the Management that OEM must be consulted and risk is involved if they go ahead.

This is for my own technical understanding.

 

I have already given my stance to the Management that OEM must be consulted and risk is involved if they go ahead.

This is for my own technical understanding.

Gotcha, I misunderstood the post question.
I have not changed settings like this but I have worked with Basler quite a bit. They should be able to offer some info on this if you have a contact.
My concern would be the torque on the rotor and what condition the rotor is in. 20 sec at almost the original High Level is a long time.
Has it been in service for 20yrs without an inspection or rewind or is it fairly new? Lots of variables to answer for your situation

 

Gotcha, I misunderstood the post question.
I have not changed settings like this but I have worked with Basler quite a bit. They should be able to offer some info on this if you have a contact.
My concern would be the torque on the rotor and what condition the rotor is in. 20 sec at almost the original High Level is a long time.
Has it been in service for 20yrs without an inspection or rewind or is it fairly new? Lots of variables to answer for your situation

I talked with Basler. They actually proposed the new settings and when I asked for reason or their basis for the values being recommended they said it's from experience. I am not convinced by this response.

Machine is quite old, more than 20 years for sure but inspection(Mechanical and Electrical) is done after every 5 years.

I was wondering about the basis of these settings. I have the Ia(Stator) vs. If(Rotor) curves of the machine from GE which are drawn till 800A(If). So how come GE has set 1560A for 8 secs setting as the High level excitation settings?

Also the GT is not alone, 4ea gas Turbines are working in parallel in droop mode in the system. So I guess when a large motor starts up on the Bus this particular GT is connected, it's excitor raises If to cater for the voltage drop but it touches the low level OEL limit and generates an alarm and the rest of gas turbines provide the net voltage balance to the system. The medium level setting is never reached. But if this is the case, there remains no point of a Medium and High level settings of OEL.