Hi, i am currently analysing a static exciter setup of which i included the following link to the schematic:It is a compound-source exciter, e.g. power to the generator field is delivered by a voltage source (Power Potential transformer) and a current source (Boost Current Transformer). This type of exciter is very similar of the General Electric SCT/PPT exciter, with the exception that the current transformer is not saturable.
A Basler whitepaper says the following: "The SCT-PPT excitation system uses both a generator stator voltage source (PPT) and a generator stator current source (SCT) as the power source for the main field. When the generator is offline and not supplying current to a load, the excitation field current is supplied by the 3-phase, power potential transformer. When the generator is on-line and supplying current to the system load, a portion of the excitation field current is supplied by the three saturable current transformers. Since there is both a current source and a voltage source used in this compound-type excitation system, linear reactors are utilized to prevent the PPT from being shorted when the SCTs are saturated. These three magnetic components (PPT, SCTs and Linear Reactors) constitute the power magnetics for this compound-type excitation system, all of which have been designed specifically for each generator to ensure the correct amount of excitation field current is available for all load conditions. This makes the SCT-PPT compound-type excitation system very desirable." Why does the SCT short the PPT?
Basler also claims the following: " Under normal steady-state conditions, the phasor summation between the PPT/linear reactors and the SCTs provides the correct compensated voltage and current to the generator field for all loads at any power factor." Can someone explain to me how this works?
The Boost Current Transformer is a pretty weird device. It essentially is a wound CT with 1 primary coil (3 windings) and 2 secondary coils (19 & 3 windings). They secondaries are series connected to eachother in a sort of 'delta way'. Isn't it illegal to connect two current sources in series. Does someone have a explanation of this arrangement?
There is a second 'current transformer' in the schematics, T1, which i do not know the function of. The output of this transformer is also essentially a current source. So you have a rectified current source and a rectified voltage source, wich are added and put in the field. Can someone explain how this thing works?
The field current is controlled by a thyristor setup which shunts current from the middle phase of U2, essentially shorting the Current Transformer for half a cycle. Is there some relation to coil L2?