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1、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 continuo

2、us 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 advan

3、tages 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

4、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 di

5、sturbances 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 know

6、n 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 el

7、ectrical 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 sensiti

8、ve 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 ca

9、pacitors 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 c

10、ompensate 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 sinusoid

11、al （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 regu

12、lation 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 us

13、ing 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 percen

14、t 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

15、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 o

16、ccurs, 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 t

17、he 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 generat

18、e 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

19、 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-S4neve

20、r 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 o

21、f 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 switchi

22、ng 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 compe

23、nsation 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）

24、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 analy

25、sisIt 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

26、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 ampli

27、tude 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

28、 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 d

29、uty 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 wi

30、ll 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

31、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 independen

32、tly 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 vo