Hi Muthulakshmi,
I am assuming that you are referring to the ST5C and AC1C excitation system models available in Simscape Electrical library.
The output of the ST5C block is a Simulink signal. This must be converted to a physical signal to connect to the "Synchronous Machine Field Circuit" block available in Simscape Electrical library. You can use the "Simulink-PS Converter" block to connect these two blocks. You can refer to the following documentation to know more about the "Simulink-PS Converter" block:
You can run the following code to open the example model that uses the AC1C block with the Simulink-PS converter block:
openExample('simscapeelectrical/SMControlExample')
The blocks are present within the "AVR and Exciter" subsystem in the example model. The output of the AC1C block is given to the converter block as an input and the output of the converter block is given to the Synchronous Machine Field Circuit block.
I hope this answers your query.
