【文章內(nèi)容簡介】 the input voltage polarity, the pensation to realize(a sag or swell)and the control signal of the switching devices. The diagram of the operation sequence for voltage sags is shown in . To pensate a swell it is just necessary to change S1 by S3 and S4 by S2. The switching pattern as much for sags as for swells turns on two switching devices in alternated form to offer a safe transition for inductive load current, that is, it is used S3 and S4 during the first pulse control and S1 and S2 during the second one. The above guarantee that switch mutation and conduction looses are equilibrated. State machine representation for voltage sags shows the operation sequence of singlephase ACAC converter: Operation sequence of the ACAC converter A Field Programmable Gate Arrays (FPGA) is used to program the state machine and to generate a dead time between the turn on and the turn off for the bidirectional switches. The FPGA permits to reduce the processing time of the DSP. In this case, The DSP only generates the voltage reference and the switching pattern. GENERATION The singlephase dq theory is used to realize the pensation process and to select the duty cycle. The dq theory transforms fundamental frequency signals into DC ponents, allowing a fast transient response to pensate voltage sags and swells. To achieve the singlephase dq transformation, an imaginary orthogonal system concept is introduced. The main idea is that the imaginary orthogonal variable keeps exactly the same system ponents and parameters, keeping always 90176。phase shift with respect to real ponents [9].In this paper, it is employed the proposal in [10] which is based on the concept that the imaginary orthogonal circuit has a 90176。lag. shows the real and orthogonal imaginary variable used to determine the dq transformation from the AC mains. Real and imaginary variables The matrix transformation from real and imaginary circuit to the dq rotating frame is expressed by: (9) where: Vd = Voltage of the real circuit. Vq = Voltage of the imaginary. The dq transformation provides information about the active ponent to pensate. As an example, the Vd and Vq ponents for a sinusoidal signal Vpsin(wt)(without harmonic content) are “Vp”and “0”respectively. TRANSFORMER DESIGN From , the pensated voltage measured in the load is: (10) Using electrical system equations and considering a factor a that represents an amplitude percent of increment or decrement of the AC mains when the voltage sag or swell occur: (11) shows different values of a and n obtained from the equation (11).It is noted that there are two values of a for each n。 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)Com