外文文献翻译单相AC AC变换器补偿电压骤降和骤升

外文文献翻译专业电气工程及其自动化学生姓名陈嘉俐班级BD电气071学号0720601103指导教师胡国文电气工程学院Compensation of Voltage Sags and Swells usinga Single-Phase AC-AC ConverterAbstract-In this paper, a topology to compensate voltage sags and swells simultaneously in critical loads is proposed. It consists in a single-phase AC-AC converter in a matrix arrangement, which keeps a continuous regulation in the output voltage. The proposed scheme has the capability to compensate up to 25% voltage sags and 50% voltage swells.Energy storage devices are not required by the AC-AC converter and it is connected between the AC mains and the load by using a series transformer. One of the advantages of this topology is that taps for the coupling transformer are no necessary to change the polarity of the compensation voltage. A four step switching technique is used to drive the AC-AC converter switches, executing snubber-less operation. The reference signal is generated using single-phase d-q theory, obtaining a fast response time and high regulation.Simulation and experimental results of a 5kW capacity, 127V, 60Hz equipment are presented.I.INTRODUCTIONThe quality of the AC mains has been affected by the use of new semiconductor-devices technologies. Nowadays, it is common to find disturbances in the amplitude or waveform shape of current and voltage in the electric systems. These conditions could produce fails in the equipments, raising the possibility of an energy interruption. The voltage fast variations that appear in the AC mains during 10 seconds or less are commonly known as voltage sags and swells. These variations are produced by normal operation of high power loads as well as theirs connection and disconnection; 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 compensate 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; if the power is increased, the size of the devices will increase. In spite of the above, the UPS systems are capable to support energy interruptions.Other option developed, which is able to compensate voltage sags is based on PWM AC-AC converter3,4.This solution uses an autotransformer composed by one primary side and two secondary windings presenting a good performance. The system compensates until 50% voltage sags and swells and can continuously shape the output voltage to be sinusoidal (with low THD). Nevertheless, the autotransformer drives all the load power due to it is connected between the load and the AC mains.In this paper a PWM AC-AC converter is presented, in order to compensate voltage sags and swells simultaneously in critical loads, and to maintain a continuous regulation in the output voltage. The system consists in a single-phase AC-AC converter in a matrix arrangement, and energy storage devices are not required. A four step switching technique is used to drive AC-AC converter switches, executing snubber-less operations. The reference signal is generated using single-phase d-q 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 compensation 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. II.CONVERTER ANALYSISThe structure of the proposed approach is shown in Fig. 1.Fig.l Conceptual design of the proposed approach Its principal objective consists in supply a compensation 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 theovervoltage.The topology of the single-phase AC-AC converter is shown in Fig. 2.Fig.2 Single-Phase AC-AC converter The converter has the following elements: Four current and voltage bi-directional switching devices connected to the AC mains 5,6,7. Two low-pass filters to reduce the high frequency associated to switching in input current and output voltage8. The AC-AC converter generates a PWM AC voltage to cancel the variations in the AC mains and to compensate the voltage sags and swells.S1,S2,S3 and S4 are used to generate the PWM voltage with the polarityrequired. The adequate operation of the switches allows producing anoutput voltage Vout on phase or 180phase-shifted with respect to Vin. When the utility voltage is at normal level, the switches S3 andS4 are closed (or S1 and S2) and the output voltage is equal to zero. Whenthere is a voltage sag, the switch S4 is closed, and S1 and S3 are operatedwith a duty cycle D, generating a Vout, for compensation. When there isa voltage swell, the switch S3 is closed, and S2 and S3 generate a Vout,with a polarity inverted for compensation. Switches S1-S3 and S2-S4never should be closed at the same time in order to avoid a short-circuitin the AC mains side. The switches are driven using a signal pattern which incorporate afour-step switching strategy, reducing the switching losses andeliminating the use of snubbers circuits. III.MODULATION TECHNIQUEThe function of the single-phase AC-AC converter is to reproduce avoltage with a peak amplitude lower or equal than the AC mains value. To achieve this, the voltage of the AC mains is modulated by using a switching pattern. The amplitude of the fundamental voltage will depend on themodulation index of the switching pattern. Fig.3 shows the scheme used to obtain a pulsed pattern. In thiscase, it is used a Digital Signal Processor(DSP) to generate the duty cycle D and to control the PWM of the switching devices. The operation mode of the scheme consists in to obtain a referencesignal that represents the compensation voltage Vc. In this case, it isused the d-q theory to transform the AC mains voltage in a DC signal. Thed component is compared with the nominal peak voltage of the AC mainsVnom, to obtain the Vc. (The d-q theory is explained in section IV).Fig.3 Scheme to generate the control pulsesThe C(s)controller calculates the duty cycle D from Vc and the Control logic determines which of the AC-AC converter switches will be turned on and which be turned off(S1,S2,S3 or S4).The switching pattern is obtained when D and a saw-tooth signal generated by the DSP are compared.A. Switching pattern analysisIt is possible to determine the harmonic content of the converteroutput voltage Vpwm from the analysis of the switching pattern. The sampling process theory is used to know the amplitude and frequency of each harmonic generated in the converter output. The representation of the switching pattern in Fourier series is given by (1): (1)Expressing (1)in complex form: (2)The Fourier complex coefficients of the switching pattern are calculated using equation (3). (3)Considering that the switching pattern has an amplitude Vx and that the pulse width is x: (4) Equation (5) permits to know the amplitude and frequency of the harmonics and therefore, to propose the cut-off frequency of the low-pass filters. In this case, it is just necessary to multiply the magnitude of (4) by the amplitude of the AC mains. (5)where:Ah = Harmonics magnitude.A = AC mains voltage amplitude.Vx = Commutation pattern amplitude.x = Pulse width.T = Commutation pattern period.m = 0,1,2,3,.ws = Switching frequency.Once calculated the amplitude and frequency of the harmonics, the cut-off frequency of the low-pass filters is selected. It is noted that the output voltage in the AC-AC converter depends on the average duty cycle D: (6) In the same form, D is related with the compensation and regulation of the load voltage: (7)The duty cycle is affected by the relation of transformation n of the coupling transformer selected. In this case, it is chosen a buck transformer, such that current of the AC-AC converter will be lower than current flowing through the AC mains. The equation that determines D valuein open loop is as follows: (8)Where Vnom is the peak of reference voltage and VdDQ is the peak voltage related to the single-phase d-q transformation of Vin. Equation(8) shows that VnomVdDQ for a voltage sag and VnomVdDQ for a voltage swell. This allows that D stays within 1 y-1.B. Four step switching techniqueThe four step switching technique offers a safe transition of inductive load current from one bi-directional switch to another, and ensures a safe PWM operation. This technique controls independently each switching device within a bi-directional switch element that depends on the input voltage and load current polarity.In the case of the AC-AC converter, operation state of S1,S2,S3 and S4 will depend on the input voltage polarity, the compensation 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 compensate 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 commutation and conduction looses are equilibrated. Fig.4 State machine representation for voltage sagsFig.5 shows the operation sequence of single-phase AC-AC converter:Fig.5 Operation sequence of the AC-AC converterA 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 bi-directional 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. IV.REFERENCE GENERATIONThe single-phase d-q theory is used to realize the compensation process and to select the duty cycle. The d-q theory transforms fundamental frequency signals into DC components, allowing a fast transient response to compensate voltage sags and swells.To achieve the single-phase d-q transformation, an imaginary orthogonal system concept is introduced. The main idea is that the imaginary orthogonal variable keeps exactly the same system components and parameters, keeping always 90phase shift with respect to real components 9.In this paper, it is employed the proposal in 10 which is based on the concept that the imaginary orthogonal circuit has a 90lag.Fig.6 shows the real and orthogonal imaginary variable used to determine the d-q transformation from the AC mains.Fig.6 Real and imaginary variablesThe matrix transformation from real and imaginary circuit to the d-q rotating frame is expressed by: (9)where:Vd = Voltage of the real circuit.Vq = Voltage of the imaginary.The d-q transformation provides information about the active component to compensate. As an example, the Vd and Vq components for a sinusoidal signal Vpsin(wt)(without harmonic content) are “Vp”and “0”respectively.V.COUPLING TRANSFORMER DESIGNFrom Fig.1, the compensated 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)Fig.7 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 compensation that can be obtained with the converter.Fig.7 Different values of a and n used to design the coupling transformerTable 1.-Maximum percent of compensation depending on thetransformation relation.VI.CLOSED LOOP ANALYSISA 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 single-phase AC-AC converter.Fig.8 shows the closed loop block diagram of the single-phase AC-AC converter.Fig.8 Closed loop block diagram of the single-phase AC-AC converter The PI controller transfer function is: (12)VII.SIMULATION RESULTSIn order to verify the operation of the single-phase AC-AC 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 commutation frequency is equal to 20 KHz, the output low pass filter has a cut-off frequency of 918 Hz and the selected transformation relation is 3:1. The electric scheme used is shown in Fig.1.Fig.9 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 component to compensate the load voltage. The fast system response can be noticed.Fig.9 Simulation results from the single-phase AC-AC 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. Fig.10 shows open loop simulation results when swell and sag are present in the AC mains.Fig.10 Simulation results from the single-phase AC-AC 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 shown in Fig.11. It can be seen that the converter compensates the disturbance in two cycles approximately.Fig.11 Closed loop simulation results(AC mains changes from 180 to 220 Vpeak).(a) AC mains,(b) Compensation voltage generated by the converter,(c) Load voltage.A closed loop simulation taking account the presence of harmoniccontent in the AC mains is shown in Fig.12.Fig.12 Closed loop simulation results(AC mains changes from 220 to 150 Vpeak).(a)AC mains,(b)Compensation voltage generated by the converter,(c)Load voltage.The AC-AC converter is capable to reproduce the necessary voltage to compensate the load voltage even when the AC mains present a distortion due to the harmonics.VIII.EXPERIMENTAL RESULTSA laboratory prototype to compensate voltage sags and swells was implemented in order to verify the analysis performed. The prototype is a single-phase equipment with 5kW capacity, 127V, 60Hz. It is able to compensate up to 25% voltage sags and 50% voltage swells.Fig.13 shows the PWM voltage in the AC-AC converter. In this case, the Vrms in the AC mains is equal to 100V and requires to be compensated.Fig.13 Experimental results of the AC-AC converter (a)AC mains,(b) AC-AC PWM voltage.It is noted that the single-phase AC-AC converter reproduces a pulsed voltage VPWM and that the four step modulation technique offers a safe transition for the input and output current.The reference voltage, the filtered output voltage of the converter and current of the AC mains in open loop are shown in Fig.14. A resistive load of 1kVA is connected in the output of the single-phase AC-AC converter.Fig.14 Experimental results of the AC-AC converter(a)AC mains, (b) Filtered output voltage of the converter,(c) Current in the AC mains.Fig.15 shows steady state compensation. A 15% of voltage sag is present in the AC mains. Waveform (a) is the voltage of the AC mains, (b) is the compensation voltage generated by the converter and (c) is the output voltage.During the compensation, the single phase AC-AC converter reproducesthe necessary voltage to maintain regulated the voltage in the load.Fig.15 Steady state compensation results(a)AC mains, (b) Compensation voltage generated by the converter,(c) Output voltage. Fig.16 Control signals generated by the DSP(a) Duty cycle, (b) AC mains polarity, (c) S1A switching device,(d)S1B switching device.IX.CONCLUSIONSIn this paper a topology to compensate voltage sags and swells simultaneously in critical loads as well as to maintain a continuous regulation in the output voltage has been proposed. The scheme uses a single-phase AC-AC converter in matrix arrangement and it has the advantage that energy storage devices are not required. A four step switching technique is used to drive the AC-AC converter switches toexecute snubber-less operations.Simulation and experimental results confirm the good performance of the single-phase AC-AC converter and shows a fast response to compensation.单相AC- AC变换器补偿电压骤降和骤升摘要在本文中，在一个拓扑补偿电压骤降和骤升的同时，提出了临界载荷。这是一个由单相矩阵中的AC - AC变换器安排，保持在一个连续调节输出电压。所提出的方案有能力赔偿高达25的电压骤降，50的电压骤升。储能器件都要求采用AC- AC的转换器和它之间的交流电源和连接通过使用负载系列的变压器。这种拓扑结构的其中一个优势是，在没有耦合变压器抽头时要改变补偿电压的极性。四步交换技术是用于驱动交流转换开关，执行缓冲无操作。该参考信号是使用单相d-q理论，获得快速响应时间和效率。5KW，127V，60Hz设备的仿真与实验结果介绍。 一导言交流电源一直使用新型半导体器件技术。如今，它经常会出现在振幅扰动或波形电流的电力系统。在这些条件下可能产生故障的设备，提高能源中断的可能性。电压快速变化，在交流电源出现在10秒内，通常被称为电压骤降和骤升。这些变化是由高功率负荷以及他们的连接和断开的正常运行所引起的；快速的电压变化影响幅度和持续时间的功能。一些研究表明，在配电系统的所有故障的92%由电压骤降产生1。消除电压快速变化是重要的，因为他们是最常见的中断原因，对于许多工业生产过程操作，特别是那些采用现代电子设备，这是高度敏感的短期源变化2。动态电压恢复器（DVR）和不间断电源（UPS）系统已经研制，沿着过去的几十年发展，它们有能力补偿电压骤降和骤升。然而，他们依赖于设备的储存能量，像大电容或电池银行。标称功率运行是这些设备规模和容量的能力；如果功率增加，该设备的大小会增加。鉴于上述情况，UPS系统有能力支持能源中断。其他选项发达，基于PWM的AC - AC变换器基础上的34能够补偿电压骤降。此解决方案是使用一个组成的自耦变压器原边和两个次级绕组呈现出良好的性能。该系统补偿50的电压骤降和骤升，可连续输出塑造电压是正弦（低总谐波失真）。然而，所有的负载功率，它是自耦变压器驱动器负载和交流电源之间的连接。本文提出了一种PWM交流-交流变换器，在关键的负荷能够同时补偿电压骤降和骤升并保持连续的调控输出电压。该系统由一个单相交交变频组成，能源存储设备不是必需的。四步切换技术用来驱动交流 交流转换器开关，执行无缓冲操作。该参考信号是使用单相d-q理论，获得了快速响应时间和继续调控，具有高效率。这种结构的优点之一，就是自耦变压器不需要改变极性补偿电压，转换器硬盘只有百分之几的负载功率。设计、建造和性能的详细信息，一些仿真和实验结果是由实验室样机显示出来的。 二变换器分析该方法的结构图所示1 图1 概念设计方法 其主要目的在于提供一个补偿，为了始终保持电压的交流面值水管。当电压骤降发生时，转换器供应需要的电压维持输出调节电压。以同样的方式，当电压膨胀发生时，再现了所需的电压转换器取消了过电压。单相交流-交流转换器拓扑如图所示2图2 单相AC- AC变换器该转换器具有下列内容： 四个电流和电压双向转换连接