The SPWM current control has a constant switching frequency and also provides fast dynamic responses. After being compensated by a PI-regulator, the error signal is compared with a triangular waveform to generate SPWM signal for switching control. The current through the inductor or the capacitor is sensed and compared with the reference signal. In SPWM control technique, the output voltage feedback is compared with a sine reference signal and the error voltage is compensated by a PI-regulator to produce the current reference. Typical current and voltage control loops for UPS inverter. This hysteresis control has fast transient response but, the switching frequency varies widely. The output voltage tracks the reference signal within the upper and lower boundary levels. The duration between two successive levels is determined by the slope of the reference signal. The sign and predetermined amplitude of the error determine the output of the modulator. In a typical hysteresis regulator, the reference signal is compared with the feedback signal. The basic current regulators employed as minor current loop are: hysteresis regulators, sinusoidal PWM regulator, and predictive regulators. The minor current loop ensures fast dynamic responses enabling good performance with non-linear or step-changing loads. This error is also used as a reference signal for the inner current regulator loop, which uses the inductor or the capacitor output filter current as the feedback signal. The error is compensated by a Pi-integrator to achieve stable output voltage under steady-state operation. The outer control loop uses the output voltage as a feedback signal, which is compared with a reference signal. 24.14, there are two control loops: an outer and an inner. In an instantaneous voltage feedback SPWM control, the output voltage is continuously compared with the reference signal improving the dynamic performance of the UPS inverter.īetter performance even with non-linear and step-changing loads can be achieved by multiple control loop strategies. These approaches control only the amplitude of the output voltage and are good only at high frequencies. The average approach is basically the same but, the sensed voltage is converted to an average value and after that, is compared with a reference signal. In natural sampling type, the peak value of the output voltage is detected and compared with a reference voltage in order to obtain the error, which is used to control the reference to the modulator. The sinusoidal PWM (SPWM) can be of natural sampling type, average type, or instantaneous type. Analog techniques are used in continuous approach. The feedback control can be continuous or discontinuous. This strategy uses a single feedback loop to provide well-regulated output voltage with low THD. The advantage of this method is low output voltage harmonic and robustness. This method can be utilized for both single-phase and three-phase systems. The most common switching technique is Sinusoidal PWM. Other factors considered for a good control technique are nearly zero steady-state inverter output voltage error, good voltage regulation, robustness, fast transient response, and protection of the inverter against overload under linear/non-linear loads.
In addition, it should provide the proper mechanism to recharge the battery set and maintain high input power factor and low total input current harmonic distortion. The main task of the control system in a UPS unit is to minimize the output voltage total harmonic distortion in different loading profiles. Adel Nasiri Ph.D., in Power Electronics Handbook (Third Edition), 2011 24.5 Control Techniques