IMPLEMENTATION OF THREE PHASE UPS SYSTEM USING SPWM AND FUZZY LOGIC CONTROLLERS FOR LINEAR AND NON-LINEAR LOADS

This paper presents the design of a highperformance sinusoidal pulse width modulation (SPWM) controller for three phase uninterruptible power supply (UPS) systems that are operating under highly nonlinear loads. The classical SPWM method is quite effective in controlling the RMS magnitude of the UPS output voltages. However, it is not good enough in compensating the harmonics and the distortion caused specifically by the nonlinear currents drawn by the rectifier loads. The distortion becomes more severe at high power where the switching frequency has to be reduced due to the efficiency concerns. This study proposes a new design strategy that overcomes the limitations of the classical RMS control. It adds inner loops to the closed-loop control system effectively that enables successful reduction of harmonics and compensation of distortion at the outputs, the experimental results demonstrate that the controller successfully achieves the steady-state RMS voltage regulation specifications as well as the total harmonic distortion and the dynamic response requirements of major UPS standards. The classical SPWM method and space vector modulation are quite effective in controlling the RMS magnitude of the UPS output voltages but couldn't effectively compensate the harmonics and distortions caused specifically by the nonlinear loads. The distortion becomes more severe at high power where the switching frequency has to be reduced due to the efficiency concerns. It adds inner loops to the closed loop feedback control system effectively that enables successful reduction of harmonics and compensation of distortion at the outputs. The fuzzy based multi-loop controller eliminates the periodic errors on the output voltages due to inverter voltage nonlinearity and load disturbances. The steady-state RMS voltage regulation, total harmonic distortion and dynamic performance of the three-phase UPS is investigated in detail. The theory of the control strategy is analyzed by means of simulink using the state-space model of inverter. 1.INTRODUCTION The increased use of rectifiers in critical loads employed by the information technologies, and medical and military equipment mandate the design of uninterruptible power supplies (UPS) with high-quality outputs [1]–[3]. The highly nonlinear currents drawn especially by high-power single-phase rectifier loads greatly distort the UPS outputs. The distorted UPS voltages cause generation of low dc voltage at the output of the rectifier loads, which causes high current flow, increased power losses, and possibly the malfunction of the critical load or the UPS. The distortion is resulted mainly by the voltage drop across the inductive element of the LC filter due to the non-sinusoidal current at the output of the inverter [4]–[6]. In a UPS system, the inverter is responsible for synthesizing sinusoidal voltages from a dc source through the pulse width modulation (PWM) of the dc voltage The inductive element here is needed to remove the switching frequency harmonics from the current waveform that are generated by the PWM operation of the inverter. So, for the selected switching frequency and the power level, an optimum filter with a smallest inductance can be designed, but the distortion cannot be completely avoided, and the regulations and the customer specifications may not be satisfied. The solution to this problem is to design a proper inverter controller such that it generater a control signal with multiple functionalities. This signal must carry information to produce sinusoidal voltages with small steady-state RMS error, to pro-vide fast transient response and corrective actions to reduce distortion even under highly nonlinear loads. Therefore, a major research has been conducted to design such controllers International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 03 Issue: 12 | Dec -2016 www.irjet. net p-ISSN: 2395-0072 © 2016, IRJET | Impact Factor value: 4.45 | ISO 9001:2008 Certified Journal | Page 447 for the high-performance UPS systems [2]–[5], [7]– [36]. The high-performance controllers in general employed multi loop state feedback control strategies to achieve the regulation specifications [9]–[12]. Moreover, the dead-beat control method [13] and the predictive and repetitive control methods [14]–[21] have been widely investigated and proposed among researchers. In addition, the iterative [22] and adaptive learning control methods [23], the H-infinity control method [24], [25], the feedback linearization method [26], and recently the multi sampled control approach to improve the control performance [27] have been studied and evaluated. High-quality output voltages with substantially low total harmonic distortion (THD) and fast dynamic response have been demonstrated with these methods. However, the disadvantages such as implementation complexity and the problems caused by highly unbalanced loading may limit some of the benefits of these methods. The inductive element of the LC filter due to the non-sinusoidal current at the output of the inverter [4]–[6]. In a UPS system, the inverter is responsible for synthesizing sinusoidal voltages from a dc source through the pulse width modulation (PWM) of the dc voltage. 2. ANALYSIS OF THE UPS INVERTER POWER STAGE This section obtains the state-space model of the inverter stage of a three-phase UPS in order to design the controller for the inverter. The developed model is also used to study the controller performance for the lowest THD of the output voltage while maintaining the stability and a good dynamic response under all load conditions. The model is developed based on the circuit schematic given in Fig. 2. As shown in Fig. 2, an insulated gate bipolar transistor (IGBT)-based threephase inverter is used to produce pulse-width modulated voltages across the terminals labeled as 1, 2, and 3. Moreover, L is the external filter inductor used to reduce ripple at the line current, Llk1 is the primary side leakage, and Lμ is the magnetizing inductance of the trans-former; then Llk2 is the secondary side leakage inductance, C_ is the filter capacitor, and finally R_ is the load resistance (the prime symbol represents the parameters referred to the -side of the transformer). Writing the voltage equations at the -side of the transformer yields the following sets of equations for the line-to-line voltages across the inverter terminals, (1) as shown at the bottom of the page. Fig . 1 UPS inverter stage including the Δ-zigzag transformer equivalent circuit and the resistive load. Similarly, writing the current equations yields the following sets of equations for the derivative of the transformer secondary currents: International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 03 Issue: 12 | Dec -2016 www.irjet. net p-ISSN: 2395-0072 © 2016, IRJET | Impact Factor value: 4.45 | ISO 9001:2008 Certified Journal | Page 448 Likewise, the following sets of equations are obtained for the derivative of the referred output voltages based on the output parameters: In addition, the inverter terminal voltages given in (1) can also be written in terms of the moving average of the switching functions, the modulation ratio ma, and the dc bus voltage Vdc. The moving average of the switching functions is actually equal to the control signals that are applied to the comparators to generate the PWM signals for the inverter switches. Consequently, the following sets of equations give the averaged model of the PWM generator and the threephase inverter operation. where ucont abc represents the control signals produced by the controller for each inverterleg.The modulation ratio ma is equal to the peak of the control signal Ucont divided by the peak of the triangular carrier waveform V tri. Although the parameter V tri is always constant, Vdc may be changing depending on the grid conditions. It is relatively constant when grid is okay due tothecontrolled rectifier,but it is varying when gridis goneand the inverter works from the battery pack. These conditions are studied during the performance testing of the controller. Deriving the derivative of the delta winding currents from(1) first, then using them in (2), and also using (4) for the inverter terminal voltages,the following sets of the state-space equations