Is there any thumb rule or ratio that needs to be maintained.? In some cases we have the Generator Transformer combination directly connected with no circuit breaker in between. In other cases , we excite the Generator and the Transformer is closed on to the Generator using a Circuit Breaker.

The question is what is the Maximum transformer size(Ratio) that can be directly put on an excited Generator ? Is there any governing relationship here?

Is the transformer already connected to the load?

In a power plant other than Hydro, Wind and DG Set, Auxiliary power is required for making power available initially for running such Auxiliary Plant and Machinery that are prerequisite for start up of the generating unit.
If a Generator Transformer (GT) has been provided, it is but obvious that the function of the GT is to step up the Voltage for supplying generated power to High Voltage System. Therefore, when Generator is not running power is available only at High Voltage and for providing Station Power and power for Auxiliaries of start-up, a Step Down Transformer has to be provided. Such Transformer is termed "Station Service Transformer (SST)". However, a third transformer called "Unit Auxiliary Transformer (UAT)" is also provided. It is situated between Generator and GT and its Primary is at Generator Voltage and Secondary at Station Voltage, so that when the Generator is running all its essential Auxiliaries are catered by the UAT. The purpose is that in case of sudden OPENING of Generator Circuit Breaker, loss of power to essential Auxiliaries does not take place.
If the Station has more than One Generating Unit, it is possible to avoid investment in the "Station Service Transformer" by selecting kVA rating of the UAT such that it can safely supply Unit Auxiliary Power to two units. In such a case a Circuit Breaker is provided between the Generator and the UAT, GT combination, apart from the Generator Circuit Breaker on GT HV or HT Side. If by chance one of the UATs develops a fault and is unavailable, Unit Start-up Power for that Generating Unit can be then taken from the UAT of one of the other units.
If, however, there is only a single Generating Unit in the plant, then a SST is provided and in that case no Circuit Breaker is provided between the Generator and the UAT, GT combination. If the UAT becomes faulty, it is isolated for repairs and Generating Unit is restarted with the help of SST and thereafter Essential Auxiliaries of the Unit are kept running on the power obtained from SST. A DG Set of adequate Size is definitely provided in such cases so that extremely important Auxiliaries are not starved of power in case of a Grid Blackout.

Mohan,

Using "thumb rules" on something as expensive and critical as a GSU has no place, only solid knowledgeable engineering solutions should be used. That being said, what are your doubts? GSUs are generally sized to the turbine-generator set capability, it then becomes a question of whether the synchronizing breaker is on the high or low- side.

If you are concerned about the inrush currents when closing onto the low-side of a de-energized transformer that has no connected load, and the synchronizing breaker is on the high-side, the solution is simple. Set the excitation to manual with minimum (ideally zero) field current, spin the machine up and slowly increase the excitation as you approach synchronous speed and manually set the terminal voltage. Smooth and inrush free.

If you are trying to do this where you have a main transformer that is fed from many machines and this is the first machine in the group closing on the synchronizing bus, you can still use this method as long as there is no other load/connection of any kind on either side of the transformer or synchronizing bus. Of course your protective/control scheme has to be designed to do this without triggering all sorts of alarm and relay actions as a result.

Alan

Mr John : Assume that the load is diconnected.

Mr Anand / Mr Alan : I completely understand what you are saying. I have a reason to ask this question. Let me explain with a little more detail(probably I should have done so earlier).

To put it short.....The system comprised a Gas Turbine set (GsT) at 5.65KV and Steam turbine set (ST) at 6.3KV. These were stepped up to 33KV using Generator Transformers (GT). Further it was stepped up using a main transformer (MT) (25MVA) to 132KV for export of power. During the first start up of the plant (and my first experience) I had a chance to observe the MT being charged directly at 132KV winding from the grid(on no load). When the breaker(132KV SF6) was closed it made a huge sound which was quite loud and audible in the control room which was well insulated. This formed the basis of my question.

Now take the case of GT and ST in this system. These were directly coupled to transformers with no circuit breakers in between.This perfectly alllowed for a smooth voltage build up and brings both the generator and transformer up and running in unison.I guess this is what Alan was reffering to.

I also understand that these are high capital intensive machinery and design has to be on sound engineering principles.......The question here is , if there is any breaker between the transformer and a generator then what is the maximum size of a transformer that can be directly closed on a fully excited generator.I was looking for some ratio between the Transformer and Generator.Sorry , I had to use the word "thumb rule". Or is it that one has to avoid this situation by design of the system .(Like in this case the Generator and Transformer were closely coupled with no breaker in between allowing for no issues with In rush currents)

Mohan,

Not to beg the question, but if I were faced with charging a 100MVA transformer from the low-side with a 10MVA generator I would use the slow-field approach, but if it were a 50MVA transformer I would go ahead and try it directly. Quite frankly the difficulty lies with your protective relaying and keeping it from tripping on these short but sharp transients. Sometimes only a carefully thought out experiment is the best way to go.

Alan

Mr Alan/MrAnand

Thank you for the posts. I went through the URL posted by Mr Anand. It has clarified many of my doubts. I notice the advanced products available to counter some of the concerns.I also got a lead to look for certain kind of information and that should address my anticpated future concerns on this subject.

I guess this thread can be closed