The basic and general function of DVR controller in microgrid system is to detect voltage sag/swell issues in the system thereby correcting voltage, generation of triggering pulses to the sinusoidal PWM inverter, and termination of the trigger pulses when the sag/swell issues has passed. DVR controller block diagram is similar to figure 2.9.
There are different type of control strategy in DVR. In this paper Proportional – Integral (PI) controller has been used for mitigating sag and swell in microgrid.
PI controller is a closed loop feedback controller which causes the steady state error to be zero for a step input. PI input is an actuating or error signal which is the difference between the reference voltage ( V_ref) and load voltage (V_L) output of the controller which is shown in Fig.
Output of comparator = V_ref – V_L ……………………………………………………. (3.5.1)
Actual load voltage is transformed to stationary reference frame dqo and in which only dq are selected. These values are compared with reference value and if there is a difference then an error signal is produced and passed through the PI controller. The controller has two gain constant named: the proportion gain K_p and the integral gain K_I. The proportional term K_p produces an output value which is proportional to the error value of current. Integral term K_I is proportional to both the duration and magnitude of the error.
Under normal operating conditions there is no error and hence no pulses are generated to the IGBT switch .When there is voltage sag or swell then an error occurs and based on the error value PWM generator generates pulses to the IGBT switch accordingly, the load voltage is maintained constant. The voltage error is passed through a PI controller and the output from PI controller is transformed to abc values using transformation block. The modulated signal V_(abc_inv) is compared against a triangular signal in order to generate gate pulses for the inverter using SPWM technique