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?
There are LOTS of manufacturers that can meet most, if not all, of the technical requirements for a particular piece of equipment. And since excitation systems ("AVRs") are not all that complicated (unless you get in to some very large systems with the requirement for Power System Stabilizers and some region-specific technical and regulatory requirements) the decision should come down to a couple of simple things.
First, what is the reputation and experience of the company/supplier that will be configuring, installing and commissioning the exciter? What is their experience with systems of your size? The best way to judge this is to ask the supplier for references on jobs of similar size and configuration and then call or email those sites and ask for their thoughts on how the overall job went--particularly the installation and commissioning. The best piece of equipment, technically, can be thought of as the worst if it's not configure properly, installed with a minimum of trouble and on schedule, and commissioned properly. The last two can cause schedule over-runs which can impact production and revenue and availability, and both can affect reliability over time.
Second, how available is the supplier for answering questions and providing after-installation service and parts? Are they close, geographically? What is the knowledge and experience of their technical support staff?
Salespeople will make all sorts of claims, but by asking for references and contacting them you will be doing the most important due diligence--because, again, the best equipment (technically) if not configured, installed and commissioned properly can be perceived as the worst. And you could spend a lot of time (lost production and revenue included) trying to sort out problems.
It's not easy contacting other sites and asking for their experience and feedback--but it's the best gauge of how the overall job will likely progress at your site.
As far as a recommendation for a company's products to consider, Basler Electric (www.basler.com) has some very reasonably-priced and very capable equipment that has proven to be very reliable over time. It's easily programmed, it has lots of bells and whistles (not the least of which is it's ability to track and trend parameters when properly configured) and it is not overly complicated at all.
Also, you need to understand if you are connecting to a local electrical power distribution and transmission system (grid) if they have any regulations or requirements for new excitation systems.
Perhaps I didn't understand the original question(s). So let's give this another try.
When you contact potential suppliers for a new (upgrade; retrofit) excitation system they are going to ask you for a LOT of information. They will ask for the existing excitation systems's nameplate information, the make and model of the existing excitation system, and information about the generator itself (they will likely ask for generator nameplate data, as well as generator data sheet information: rotor inertia time constant, resistance, impedance and so on). Some of the information will be easy to obtain--just take pictures of nameplates. Other information will be more difficult to find or obtain (most of it will be in the manuals provided with the generator and prime mover).
The supplier(s) will then look at their offerings and will decide which of their products can meet the specifications and then make their recommendations. As I wrote previously, just about any excitation system can be programmed to do just about anything these days (for different ranges and types of excitation systems), so ceiling voltage limit and response ratio, for example, are mostly programmable parameters which can be set to match the existing system's parameters.
So, that being said, the hard work on your part is to gather all of the requested information, and then to ask for and contact the references provided by the one or two or three suppliers you are seriously considering to see how the overall project of upgrading/retrofitting their excitation system went. And, that's where the real differences will become apparent to you.
I can GUARANTEE you that there will be more than one person at your site or owner/operator that will be strictly looking at price and wanting to choose the lowest priced offering. BUT, price is not always the best indicator of value. I have a new saying these days: "Buy once; cry once." It means, you may "cry" over the cost of a more expensive product, but if you don't have to cry multiple times when the lower/lowest cost product fails or gives problems over time and has to be replaced, you have only cried once. The decision to purchase a particular offering should be based NOT only on price alone, but on past experiences of other sites who have done work with the supplier in the past. Your site may also have some experience with one or more suppliers that makes them seem more attractive even though their price is a little higher. But, if you make your decision on price alone, it's very likely you're going to "get what you pay for"--meaning a low-end product with poor installation and commissioning (the MOST important part of a product in my personal opinion--and the part that leaves the greatest impression after the project is over, too!) and potential long-term issues that may or may not be easily resolved.
So, let the supplier do the legwork of choosing and recommending the model that meets the requirements of the application. Sure, you have a responsibility to review their recommendations and parameters, but you are just reviewing and comparing recommendations from suppliers. AND, once you have narrowed down the field to one or two or three possible models/suppliers, then you need to check references. If you don't do this last thing, you may be very, very sorry. I know of many sites that didn't ask for and check references, and had an awful experience during installation and commissioning and afterwards, and then learned from other sites who had similar experiences they weren't the only ones. In other words, they found out after their project was over from other sites that other sites had had a less than desirable, or worse, experience--and that information would have saved them from the same fate.
Suppliers generally have what are called application engineers--it's their job to take the information from the site and choose and make preliminary parameter configurations and then provide that information to the salesperson. They have (or should have) the knowledge and experience to look at all the relevant details and pick the model that best suits the needs of your generator and system. Again, it's usually only your job to review and compare the recommendations provided by the application engineer and salespeople as a PART of the decision-making process. Once they provide you with recommendations/proposals, then you can review them and double-check them to see if they meet the system requirements. That's generally how things are done--you make a request for proposal; you provide a LOT of information and data (some of it you will have; some you may not). Their application engineer(s) then look over the information and data and choose an appropriate model and that information is passed on to the salespeople who prepare the quote. You review the quote(s) and compare them to each other and to the requirements of your system and generator as best as you can, and then you ask for and check references. (A lot of sites don't ask for or check references, and many wish they had later. Learn and benefit from the experience of others, or be doomed to make the same mistakes as others.)
Maybe this is the information you were looking for? It's not typical that the site researches several systems in depth and then chooses the model and asks for a quote for that specific model. Some suppliers have multiple models which meet the requirements for a specific generator but which have different characteristics which might make them better for a specific application. I have personal experience with several sites over more than three decades that have decided on a particular piece of equipment (for example, a very high horsepower AC motor, and an exciter, are two that come immediately to mind as I write this) and went to a seller and ordered the equipment and then installed and commissioned the equipment.
In the case of the large AC motor, it kept having multiple vibration-related problems with the rotor finally breaking apart and causing serious damage to the motor and compressor. It was then learned that there was another AC motor in the motor manufacturer's line of motors that was specifically designed for reciprocating compressor applications that would have NOT had the issues the first motor did--and had the site allowed the manufacturer's application engineer to review the application the proper motor would have been selected and installed with no problems (because the proper motor was eventually installed and has worked flawlessly for decades).
In the case of the exciter, the one specified by the site did not have the proper capability to reach the maximum reactive load of the generator and this significantly reduced the site's ability to enter the purchased power market. There was also another, slightly larger (and slightly more expensive) exciter that would have met the needs of the application, but because the site didn't choose that one they missed out on a large revenue stream.
Hopefully this will help you understand the general process for choosing equipment in a power plant (and many other plants and sites, as well). And, also some of the pitfalls of not following the general process--including asking for and checking references of past jobs. This is the hardest part of the process of selecting a supplier and piece of equipment--but it's the part with the most potential for pain and suffering on the long run, too.