14-11-2012, 12:57 PM
UPQC-S: A Novel Concept of Simultaneous Voltage Sag/Swell and Load Reactive Power Compensations Utilizing Series Inverter of UPQC
UPQC-S.doc (Size: 4.69 MB / Downloads: 73)
Abstract
This paper introduces a new concept of optimal uti- lization of a unified power quality conditioner (UPQC). The series inverter of UPQC is controlled to perform simultaneous 1) volt- age sag/swell compensation and 2) load reactive power sharing with the shunt inverter. The active power control approach is used to compensate voltage sag/swell and is integrated with theory of power angle control (PAC) of UPQC to coordinate the load re- active power between the two inverters. Since the series inverter simultaneously delivers active and reactive powers, this concept is named as UPQC-S (S for complex power). A detailed mathemati- cal analysis, to extend the PAC approach for UPQC-S, is presented in this paper. MATLAB/SIMULINK-based simulation results are discussed to support the developed concept. Finally, the proposed concept is validated with a digital signal processor-based experi- mental study.
INTRODUCTION
THE MODERN power distribution system is becoming highly vulnerable to the different power quality problems [1], [2]. The extensive use of nonlinear loads is further contributing to increased current and voltage harmonics issues. Furthermore, the penetration level of small/large-scale renew- able energy systems based on wind energy, solar energy, fuel cell, etc., installed at distribution as well as transmission levels is increasing significantly. This integration of renewable energy sources in a power system is further imposing new challenges to the electrical power industry to accommodate these newly emerging distributed generation systems [3]. To maintain the controlled power quality regulations, some kind of compensation at all the power levels is becoming a common practice [5]–[9]. At the distribution level, UPQC is a most at- tractive solution to compensate several major power quality problems [7]–[9], [14]–[28].
VOLTAGE SAG/SWELL COMPENSATION UTILIZING UPQC-P AND UPQC-Q
The voltage sag on a system can be compensated through active power control [16]–[22] and reactive power control [23], [24] methods. Fig. 3 shows the phasor representations for volt- age sag compensation using active power control as in UPQC-P [see Fig. 3(a)] and reactive power control as in UPQC-Q [see Fig. 3(b)]. Fig. 3© and (d) shows the compensation capability of UPQC-P and UPQC-Q to compensate a swell on the system. For a voltage swell compensation using UPQC-Q [see Fig. 3(d)], the quadrature component injected by series inverter does not intersect with the rated voltage locus.
PAC APPROACH UNDER VOLTAGE SWELL CONDITION
The phasor representation for PAC of UPQC-S during a voltage swell on the system is shown in Fig. 8. Let us represent a vector VSr3 responsible to compensate the swell on the system using active power control approach. For simultaneous compensation, the series inverter should supply the V Sr1 component to support the load reactive power and V Sr3 to compensate the swell on the system. The resultant series injected voltage V would maintain the load voltage magnitude at a desired level while supporting the load reactive power.
UPQC-S CONTROLLER
The control block diagram for series inverter operation is shown in fig 10. The instantaneous power angle δ is determined using the procedure given in [15]. Based on the system rated specification, the value of the desired load voltage is set as reference load voltage k. The instantaneous value of factors kf and nO is computed by measuring the peak value of the supply voltage in real time. The error signal of actual and reference series voltage is utilized to perform the switching operation of series inverter of UPQC-S. The control diagram for the shunt inverter is as given in [15].
CONCLUSION
In this paper, a new concept of controlling complex power (simultaneous active and reactive powers) through series inverter of UPQC is introduced and named as UPQC-S. The proposed concept of the UPQC-S approach is mathematically formulated and analyzed for voltage sag and swell conditions. The developed comprehensive equations for UPQC-S can be utilized to estimate the required series injection voltage and the shunt compensating current profiles (magnitude and phase angle), and the overall VA loading both under voltage sag and swell conditions.
The simulation and experimental studies demonstrate the effectiveness of the proposed concept of simultaneous voltage sag/swell and load reactive power sharing feature of series part of UPQC-S. The significant advantages of UPQC-S over general UPQC applications are: 1) the multifunction ability of series inverter to compensate voltage variation (sag, swell, etc.) while supporting load reactive power; 2) better utilization of series inverter rating of UPQC; and 3) reduction in the shunt inverter rating due to the reactive power sharing by both the inverters.