while designing a control circuit, i have encountered in most of the cases that the output parameter of PI controller is named different variable than the input. for example: in HVDC VSC inverter control circuit, i found that measured DC voltage(Vm) and reference DC voltage(Vref) were compared to produce an error. this error when fed to a PI controller produces 'id'(as mentioned in block diagram).
why the o/p of PI is other than the input?
(refer to figure 2 of attached research paper)
The Vdc is compared with its reference value and the DC voltage error (e(t)) is processed by the PI controller. The PI controller for the DC-link voltage sets the amplitude of the active current of the VSI to regulate the DC-link voltage based on its reference value covering the inverter losses. Subtracting the measured load current, the reference value of the VSI current is obtained. The DC-link capacitor will be charged or discharged by the difference of the active load current and grid current, forcing the PI controller to change its output correspondingly, until both active currents are equal. VSI will compensate all non active components of the load current and regulate the DC-link voltage.
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