In hundreds MW plant we are using static excitation system, but it require more maintenance than brushless. Why not we are going for brushless system? What is the reason behind this?
As far as I know, the increased maintenance requirement is associated with the slip rings and brushes. It is not so easy to replace a static excitation system with a brushless system, as you'd have to change things in the machine rotor and shaft (remove the connections to the brushes and slip rings, add a diode bridge mounted in the machine rotor, add a new rotating machine connected to the shaft, etc.). Probably so expensive (once the unit is built) that it not worth considering it.
Besides, a static excitation system usually provides a much faster response, which improves stability. Also, a power system stabilizer is more effective in damping electromechanical oscillations than when applied to a brushless excitation system.
Note that in certain regions in North America the grid code essentially requires high-gain, fast-response excitation systems (high initial response excitation systems) due to stability considerations. These requirements, in practice, makes it almost impossible to apply a brushless (well, rotating) excitation system in these regions.
Brushless excitation systems have become far more common in recent years due to improvements in the response characteristics of brushless systems and the reliability of components. But, brushless is not appropriate for such a large generator. Look at 100 MW and 200 MW generators and you will find that most have a brushless (rotating diode) excitation system. By the time you reach 300 MW you will see that most are static. Each type has its benefits.
Brushless rotating exciters are quite common. And they can be applied to large units, but it all depends on regional requirements or grid codes. I would like to mention one very important issue related to brushless vs. static excitation systems: black-start capability.
Typically, static excitation systems are fed by an excitation transformer derived from the generator terminals or the auxiliary services bus of the plant. Then, the plant has to have an independent power supply to supply the excitation transformer (at least for the first unit in the plant) or the plant will not be able to black-start.
Black-start capability is a lot easier to achieve with a brushless excitation system, since a small PMG can be used as the independent source for the excitation.
For this reason, black start diesel generators almost always use a PMG with a brushless excitation system. But I haven't seen a PMG (permanent magnet generator) on a medium or large generator (say >50MW). They have auxiliary equipment which must run, such as lube oil pumps, so they are rarely used for black start. Small combustion turbines can work well in a black start string also. And you are right, if you are designating the unit as black-start, you would want at least the first machine to have a PMG. Otherwise you need to keep some battery power in reserve for excitation power.