外文文献翻译单相AC AC变换器补偿电压骤降和骤升.doc

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 180°phase-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 Vnom>VdDQ for a voltage sag and Vnom<VdDQ 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 90°phase 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 90°lag.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