compensationofvoltagesagsandswellsusingasingle-外文文献(编辑修改稿)

compensationofvoltagesagsandswellsusingasingle-外文文献(编辑修改稿)内容摘要：

of the harmonics and therefore, to propose the cutoff frequency of the lowpass filters. In this case, it is just necessary to multiply the magnitude of (4) by the amplitude of the AC mains. where: 1 ~ mo,x 2 1612 Ah = A= v, = T= x= m= Y= Harmonics magnitude. AC mains voltage amplitude. Commutation pattern amplitude. Pulse width. Commutation pattern period. 0, +I, *2, t3, ..... Switching frequency. Once calculated the amplitude and frequency of the harmonics, the cutoff frequency of the lowpass filters is selected. It is noted that the output voltage in the ACAC converter depends on the average duty cycle D: In the same form, D is related with the pensation and regulation of the load voltage: (7) The duty cycle is affected by the relation of transfonnation n ofthe coupling transformer selected. In this case, it is chosen a buck transformer, such that current of the ACAC converter will be lower than current flowing through the AC mains. The equation that determines D value in open loop is as follows: Where V, is the peak of reference voltage and I39。 dDo is the peak voltage related to the singlephase dq transfonnation of v!,. Equation (8) shows that V, V~Q for a voltage sag and Ynom Vap for a voltage swell. This allows that D stays within 1 y 1. B. Four sfep switching technique The four step switching technique offers a safe transition of inductive load current fiom one bidirectional switch to another, and ensures a safe PWM operation. This technique controls independently each switching device within a bidirectional switch element that depends on the input voltage and load current polarity. In the case of the ACAC converter, operation state of Si, SI, S3 and S4 wilt depend on 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 Fig. 4. To pensate a swell it is just necessary to change SI by Sj and S4 by S,. The switching pattern as much for sags as for swells turns on two switching devices in altemated form to offer a safe transition for inductive load current, that is, it is used S, and S, during the first pulse control and SI and S, during the second one. The above guarantee that switch mutation and conduction looses are equilibrated. .@ State machine representation for voltagesags. Fig. 5 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 pattem. 1v. REFERENCE 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 dg 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 90 phase shift with respect to real ponents [9]. In this paper, it is employed the proposal in [lo] which is based on the concept that the imaginary orthogonal circuit has a 90 lag. Fig. 6 shows the real and orthogonal imaginary variable used to determine the dq transformation from the AC mains. I Real and imaginary variables. 1613 The matrix transformation from real and imaginary circuit to the dq rotating frame is expressed by: 5:l (9) % I % where: Vd = Voltage of the real circuit. Vq = Voltage of the imaginary. A. 1 The dq transformation provides information about the active ponent to pensate. As an example, the Yd and V, ponents for a sinusoidal signal Ygin(wt) (without harmonic content) are P39。 and 0 respectively. % I % V. COUPLING TRANSFORMER DESIGN From Fig. 1, the pensated voltage measured in the load is: Using electrical system equations。