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1、Open loop PWM control techniquesSinusoidal PWM (SPWM)SPWM technique is one of the most popular modulation techniques among the others applied in power switching inverters. In SPWM, a sinusoidal reference voltage waveform is compared with a triangular carrier waveform to generate gate signals for the
2、 switches of inverter. Power dissipation is one of the most important issues in high power applications. The fundamental frequency SPWM control method was proposed to minimize the switching losses. The multi-carrier SPWM control methods also have been implemented to increase the performance of multi
3、level inverters and have been classified according to vertical or horizontal arrangements of carrier signal. The vertical carrier distribution techniques are defined as Phase Dissipation (PD), Phase Opposition Dissipation (POD), and Alternative Phase Opposition Dissipation (APOD), while horizontal a
4、rrangement is known as phase shifted (PS) control technique. In fact PS-PWM is only useful for cascaded H-bridges and flying capacitors, while PD-PWM is more useful for NPCOEach of the mentioned multi-carrier SPWM control techniques have been illustrated in Fig, :1, respectively. The sinusoidal SPWM
5、 is the most widely used PWM control method due to many advantages including easy implementation, lower harmonic outputs according to other techniques, and low switching losses. In SPWM control, a high frequency triangular carrier signal is compared with a low frequency sinusoidal modulating signal
6、in an analog or logic comparator devices. The frequency of modulating sinusoidal signal defines the desired line voltage frequency at the inverter output .Fig. 1. Multi-carrier SPWM control strategies:(a) PD, (b) POD, (c) APOD, (d) PS.Fig. 2. An SPWM controlled inverter : (a) complete system, (b) mo
7、dulator block of invertermaSwitching freq. (Hz)Line voltages (V)Line currents (A)Switching angleTHD ratios (% THD)Line currentLine voltage10.61000202.816.19309.34107.2520.6200021216.19304.4692.7230.65000202.816.2730.41.7125.2140.610,000206.316.2829.890.885.3650.81000277.121.58308.3081.7260.82000295.
8、722.9429.865.4164.9170.85000293.823.0129.962.9733.6380.810,000303.924.0429.555.0117.28911000351.627.3130.148.2861.211012000355.727.6829.784.1651.78111500037729.2329.673.7832.6612110,000409.132.126.624.8214.23131.21000385.129.9830.057.9552.68141.22000387.730.2229.924.6048.42151.25000389.832.4722.3410
9、.5932.10161.210,000425.933.1526.47.5722.01171.41000399.831.1230.37.4349.37181.42000403.931.5829.934.1446.43191.45000424.432.4728.234.8327.75201.410,000441.333.3623.537.1315.31Table 1. Current and voltage THD analysis of full bridge inverter.Switching m(lfreq. (Hz)Linevoltages(V)Linecurrents(A)Switch
10、*ngTHD ratios (% THD)angleLinecurrentLinevoltageA three-phase full bridge inverter and the SPWM modulator that has been designed to generate switching signals for three-phase full bridge inverter is depicted in Fig. 2a and Fig. 2b . In the designed three-level inverter, modulated SPWM signals have b
11、een used to control MOSFET switches of the inverter, and THK Total Harmonic Distortion ) analysis of output voltage and current have been performed as seen in Table 1.The design parameters were as follows;DC voltage (VDC) = 300 VModulation index (m) = 0.6 m. 1.4Switching frequency (fsw) = 1 kHz fsw
12、10 kHzLoad resistance (RL) = 5Load impedance (L) = 5 mHRated power = 10 kV AIn the harmonic analyses of SPWM controlled inverter designed using Simulink; the lowest THD for current has been measured as 0.88% while modulation index is 0.6 and switching frequency at 10 kHz. The THD for line voltage ha
13、s been measured as 5.36% for the same operating conditions The line voltage has increased almost proportional to Eq. (1) in over-modulation range, while proportional to Eq. (2) in linear modulation range.In SPWM control technique, the output voltage is obtained in linear modulation range,(1)一:,一皿一、-
14、叽汗叱1(2)m 命r r r f 2-。叫土 1Another application has been implemented using SPWM technique to control a five-level CHB-MLI as seen in Fig 3. The designed VSI is based on the topology of Fig. 4 which includes dual H-bridge cells as shown in Fig. 5 per phase to generate a five-level output voltage. The ma
15、in topological calculations which are performed initially indicate the requirements of modulator and inverter blocks, and allow obtaining a well prepared modulation algorithm toincrease the performance of inverter.Fig.3.An SPWM|Three*Phase Cascade Inverter with SPWM ModuMorlcontrolled five-level CHB
16、-MLI controlled inverter:(a) complete system, (b) CHB-MLI block.Fig. 4. Three-phase five-level topology of cascaded H-bridge multilevel inverterFig. 5: three-level CHB-MLI.The proposed inverter includes six H-bridges and four SPWM switching signals to control each bridge respectively. The modulation
17、 algorithm has been performed in SPWM modulator block to generate 24 separate SPWM pulses for H-bridges. The SPWM modulator has aswitching bandwidth between 0 and 40 kHz to control H-bridges. Fig. 6 shows the values which are obtained at 5 kHz switching conditions, while m. = 1. Fig. 6a and b repres
18、ent the FFT analysis of the current THD (THDi) and voltage THD (THDv) ratios in Simulink.Fundamental (50Hz) = 24.32 , THD= 1.34%FFT window: 3 of 50 cycles of selected signal0204060Harmonic order801002 1 8 6 4 2 1O.O.O.O. (_Jgua)EwunLl.os)00204060Harmonic order80100Fundamental (50Hz) = 194.8 , THD= 2
19、3.59%5 0 5 0 52 2 11 (-JSuQJUJqDunLLEE 沼巨(a)(b)Fig. 6. THD analysis of inverter while fsw = 5 kHz and mi = 1:(a) THD for current is 1.34%, (b) THD for phase voltage is 23.59%.The switching frequency of SPWM modulator has been limited to 1-0 kHz, and modulation indexes are selected in0.6m.i1.4 ranges
20、 to analyze the effect offsw and m. on THD of inverter.It has been observed by the performed tests that reducing the THD of current and voltage is depended on increasing the switching frequency in linear modulation range as shown in Fig.7a and b respectively.100.0080.0060.0040.0020.000.00(a)%THD120.
21、00THD for voltage in % (fsw=l KHz) THD for voltage in % (fsw=5 KHz)THD for voltage in % (fsw=2 KHz) THD for voltage in % (fsw=10 KHz)110.78,00.4794.5091.8989.8167.8068.5329.6916.282.270.660.8235916.47I 78.58 32.3018321.01.4(b)Fig. 7. THD analysis of inverter at various switching frequencies and modu
22、lationindexes:(a) THD for current in %, (b) THD for phase voltages in %The output current and voltage values have been increased in over-modulation range since m. is over 1, but the THD rates have been changed nonlinearly. The lowest THD for current of the designed MLI has been measured as 0.1% duri
23、ng 10 kHz switching frequency and m. = 0.8 conditions. The THD analyses of two different inverter topologies controlled with SPWM havebeen depicted in Fig. 8 applying 5 kHz switching frequency while modulation index was 1.FFT window: 3 of 50 cycles of selected signalFFT window: 3 of 15 cycles of sel
24、ected signalFundamental (50Hz) =29.23 , THD = 3.78%21.50.5LilLI, Lill.l山nlll L ,204060801004060Harmonic order80(a)(b)Fig. 8. Current THD comparison of topologies, while mi = 1 fsw = 5 kHz:(a) five-level CHB-MLI inverter output, (b) three-level full bridge inverter output.The current THD of five-leve
25、l CHB-MLI has been found 1.34%, while current THD of three-level full bridge multilevel inverter was 3.78%, The least THD of CHB-MLI has been determined as 0.1% for 10 kHz switching frequency and m. = 1 conditions. The lower switching frequency in linear modulation range has caused to higher THD for
26、 current and voltage at the output of inverter.开环PWM控制技术正弦脉宽调制(SPWM)在功率开关应用中，SPWM技术是其中最流行的一种调制技术之一。在SPWM中，一 个正弦参考电压波形与一个三角形的载波信号波形相比较，产生门信号使开关逆变器工作。 应用大功率时，功率消耗是其中一个最重要的问题。SPWM控制基频算法可以最大限度地 减少开关损耗。基于SPWM控制的方法也已开始实施来提高多级逆变器的性能，按水平或者 垂直排列，对载波信号进行分类。垂直载体分布技术被定义相位耗散(PD)，相位相对耗散 (POD),选择性相位相对耗散(APOD)，而耗散水
27、平排列被称为相移(PS)控制技术。事实上 PS-PWM仅仅是有用的串级H-bridges和浮动电容器是比较有用的，而PD-PWM多用在 NPC。以上提到的各种基于SPWM的控制技术的分析如图1。由于许多优点，如容易实施， 低谐波输出借助其他的技术，以及低的开关损耗，正弦脉宽调制型是目前应用最广泛的PWM 控制方法。在SPWM控制中，高频三角载波信号与一种低频正弦调制信号在一个模拟或逻辑 比较器装置进行比较。正弦信号的调制频率定义所需的线电压，在逆变器输出。图 1： SPWM 控制技术分析：(a) PD, (b) POD, (c) APOD, (d) PS (图见英文)三相全桥逆变器和SPWM调
28、制器已经设计用来产生开关信号，三相全桥逆变器如图2。 在设计的三电平逆变器中，调制SPWM信号控制逆变器的MOSFET开关、谐波畸变率分 析输出电压和电流已完成，如表1。图2： SPWM控制的逆变器：(a)完整的体系(b)逆变器的调制器表1：全桥逆变器的电压、电流谐波畸变率分析设计参数如下；直流电压WDC)= 300V调制指数(m)= 0.6 叫 1.4开关频率(质=1KHz fsw 10KHz负载端电阻(RL)二 5负载阻抗(L)= 5 mH额定功率=10KVA在对SPWM控制的逆变器进行谐波分析中设计采用Simulink仿真;当前最低的谐波畸 变率为0.88%，而调制指数是0.6，开关频率
29、是10KHz。同样的操作条件下，线电压的谐波 畸变率为5.36%。非线性调制范围，线电压变化规律见式1,在线性调制范围变化如式2。(1)KrJ - Fr.r 一血j。 鸣1(2)在SPWM控制技术，输出电压是在线性调制范围内得到的。另一个使用SPWM技术的应用程序已经实施，它可以控制一个五级别的CHB-MLI，如 图3。逆变器的设计是基于拓扑图4,包括双H-bridge,如图5每阶段生成一种五级输出电压。 最初所做的主要拓扑计算能力表明提高逆变器的性能需要调制器和逆变器，和一个合适的调 制算法。图3： SPWM控制的五个级别的CHB-MLI逆变器:(a)的完整体系,(b)CHB-MLI图4：
30、H-bridge三相逆变器拓扑结构图5:三级CHB-MLI该逆变器中还包括六H-bridges和四个SPWM开关信号，分别控制每座桥。调制算法在 SPWM调制器中执行，给Hbridges产生24个独立的SPWM脉冲。SPWM调制器具有 切换的介于0和40千赫带宽来控制H-bridges。图6显示的值是在5KHz的实验条件下得 到的，mi = 1。图6a和b显示了 FFT分析电流的THD值和电压THD值比值的仿真。图6：逆变器的谐波畸变率分析(fsw = 5 kHz , mi = 1 ): (a)电流THD： 1.34%(b)相电 压 THD： 23.59%SPWM调制器的开关频率限制在1-10
31、KHz，调制指数选择在0-6m,1.4范围内，分析fsw和mt对逆变器THD 值得影响。测试表明，要想降低电流和电压的THD值，必须在线性调制范围提高开关频率，如图7。图7：不同开关频率和调制指数逆变器的THD分析：(a)电流THD (b)相电压THD从mi大于1,输出电流和电压值在非线性范围内增加，但总谐波失真率非线性改变。 在10KHz的开关频率和mi = 0.8的条件，所设计的MLI的电流最低总谐波失真是0.1%。 SPWM控制的两种不同的拓扑结构的逆变器总谐波失真分析已描绘图8 (5 kHz开关频率， 同时调制指数为1 )。图8：不同拓扑结构电流总谐波失真的比较，(mi = 1 , fsw = 5 kHz )(一)五级CHB- MLI的逆变器输出，(b)三级的全桥逆变器输出五级的CHB-MLI的电流总谐波失真为1.34%，而三级全桥多电平逆变的为3.78%CHB- MLI的总谐波失真至少已确定为0.1%（10KHz的开关频率，mi=1）。在逆变器的输出中，线 性调制频率范围内，较低的开关频率造成了高的电压和电流总谐波失真。
