DC bus voltage at the interconnection of solar, wind and battery is converted to AC by use of three phase inverter. The power electronic switches used in this project is IGBT based inverter. This inverter is fed by a PWM signal to control the switches in the three arms of the bridge. The PWM signal is a series of six signals (two for each set of IGBTs), which change widths depending upon the modulation waveform.
To get the desired load voltage, inverter is controlled by closed loop. Figure shows the closed method of inverter control. The control system will detect actual output voltage and converted to per – unit base. In this method, output voltage of the inverter is converted to rotating reference frame dqo i.e. output voltage in fixed sinusoidal abc form is converted to direct – quadrature dqo frame. For simplicity zero phase sequence component is ignored. The phase-locked loop (PLL) is used to detect phase angle and frequency of grid voltage and is used to get the output wave in phase with the input sine wave and to maintain the system frequency. By using a selector, only dq component of voltage is extracted. Comparator compares the actual output voltage and the reference voltage to produce the error. The error between the actual and reference q-axis voltage is minimized by a proportional integral (PI) controller .This error is processed by proportional integral controller to correct the error. The phase angle is generated by integrating the output of PI controller and constant angular frequency .Then again voltage in dqo is transformed to sinusoidal adc form and given to PWM generator to produce the pulses for IGBT inverter accordingly.
Here Sinusoidal pulse width modulation (SPWM) based inverter is used for producing gating pulses for inverter. Figure shows SPWM waveform. In this technique, sine wave also known as reference wave is compared with high frequency carrier wave (i.e. triangle wave).
Output will be high (or a logical 1), when reference wave is greater than carrier wave. On the other hand output will be low (or a logical 0) when the reference wave is lesser than carrier.
Figure 2.7 Three arm bridge inverter
The controlled output of the inverter will then feed into a harmonic filtering system. This filter has to be tuned properly. This is then fed into the three phase load.
Figure 2.8 SPWM Technique
Figure 2.9 Block diagram of Voltage Regulator