【正文】 ated using singlephase dq theory, obtaining a fast response time and continue regulation, with a high efficiency. One of the advantages in this structure is that the taps of the coupling transformer are not required to change the polarity of the pensation voltage, and the converter drives only a percent of the load power. Design, construction and performance are detailed, and several simulations and experimental results obtained with a laboratory prototype are showed to validate the approach. ANALYSIS The structure of the proposed approach is shown in Fig. 1. Conceptual design of the proposed approach Its principal objective consists in supply a pensation voltage in order to keep always the nominal value of the AC mains. When voltage sag occurs, the converter supplies the necessary voltage to maintain regulation in the output voltage. In the same way, when voltage swell occurs, the converter reproduces the necessary voltage to cancel out the overvoltage. The topology of the singlephase ACAC converter is shown in Fig. 2. SinglePhase ACAC converter The converter has the following elements: Four current and voltage bidirectional switching devices connected to the AC mains [5],[6],[7]. Two lowpass filters to reduce the high frequency associated to switching in input current and output voltage[8]. The ACAC converter generates a PWM AC voltage to cancel the variations in the AC mains and to pensate the voltage sags and swells. S1,S2,S3 and S4 are used to generate the PWM voltage with the polarity required. The adequate operation of the switches allows producing an output voltage Vout on phase or 180176。 the voltage fast variation effects are function of the amplitude and the duration of the event. Some studies show that 92% of all disturbances in the electrical power distribution systems are produced by voltage sags [1]. It is important to eliminate the voltage fast variations because they are the most frequently cause of disrupted operations for many industrial processes, particularly those using modern electronic equipment, which are highly sensitive to short duration source variations [2]. Dynamic Voltage Restorer(DVR) and Uninterrupted Power Supply (UPS) systems had been researched and developed along the last decades and they are capable to pensate voltage sags and swells. Nevertheless, they depend on devices to store energy, like large capacitors or batteries bank. The nominal power operation is a function of size and capacity of those devices。2,177。 with this values it is possible to generate a table determining the maximum percent of pensation that can be obtained with the converter. Different values of a and n used to design the coupling transformer Table percent of pensation depending on the transformation relation. LOOP ANALYSIS A closed loop scheme is used in order to reduce the difference between the reference voltage and the voltage generated by the converter. In this case, the instantaneous amplitude of the AC mains is a time variant CD signal. Due to the above and to the necessity to have a good voltage regulation, a PI controller is proposal to improve the dynamic response of the singlephase ACAC converter. shows the closed loop block diagram of the singlephase ACAC converter. Closed loop block diagram of the singlephase ACAC converter The PI controller transfer function is: (12) RESULTS In order to verify the operation of the singlephase ACAC converter, several simulations of the converter in open and closed loop were performed. In these simulations it is used a load of 5kW with a nominal voltage of 127 Vrms, 60 Hz. The mutation frequency is equal to 20 KHz, the output low pass filter has a cutoff frequency of 918 Hz and the selected transformation relation is 3:1. The electric scheme used is shown in . shows open loop simulation results when a voltage swell occurs. Waveform (b) is the voltage of the AC mains. The converter generates Vout=0 while the voltage maintains its nominal value. When there is an amplitude disturbance in the time P,(corresponding to voltage swell)the converter reproduces the necessary ponent to pensate the load voltage. The fast system response can be noticed. Simulation results from the singlephase ACAC converter (change from 180 to 220 Vpeak).(a) Duty cycle D,(b) AC mains, (c)Compensation voltage generated by the converter,(d) Load voltage. In this case, the duration of the transient T caused by the disturbance P is minimum. shows open loop simulation results when swell and sag are present in the AC mains. Simulation results from the singlephase ACAC converter (change from 220 to 150 Vpeak).(a) Duty cycle D,(b) AC mains, (c)Compensation voltage generated by the converter,(d) Load voltage. Closed loop simulation results are shown below. The time response of the converter when a voltage swell occurs is