Power electronics 2.docx

1、Power electronics 2 Converters Table of contentsIntroduction p.31. Rectifier p.4 a. simple rectifier p.4 b. simple cotrolled rectifier p.5 c. Full wave rectification p.6 d. Controlled full wave rectification p.82. Inverter p.10 a. circuit description p.11 b. basic designs p.12 c. advanced designConv

2、erters-Introduction Power electronics is a branch of electrical engineering that relates to devices for changing the form of electrical energy (converters). We can distinguish four different types of converters: AC to DC (rectifier) DC to AC (inverter) DC to DC (DC-to-DC converter) AC to AC (AC-to-A

3、C converter)Power electronics is a branch of electrical engineering that relates to devices for changing the form of electrical energy (converters). Power electronics includes the design, implementation and maintenance: -electronic components used in high power; -structures of converters; -the order

4、 of these converters; -industrial applications of these converters.1. Rectifiera. simple rectifierThis type of rectifier is achieved by replacing the diode rectifier who is not controlled by a thyristor. This is more than an educational assembly actually used example.V(t) Vs(t) because Th is conside

5、red like a diod. is the root-mean-square value of output voltageis the maximum value of v(t) is the average output voltage b. Simple controlled rectifierV(t)=sin(t) and Thyristor begins with a firing delay between 0 and . Once initiated, it behaves as a diode.c. Full wave rectificationA diode bridge

6、 or Graetz bridge is an assembly of four diodes bridge, single-phase regime, rectifies the alternating current into direct current, that is to say not flowing in one direction.For low-power devices, the simplest filter is made using one or more capacitors connected in parallel to the output. The cap

7、acitor stores energy when the current through the diodes and then returns.-For a given load, when the capacitance of the capacitor is higher, the filtering is more efficient. -For a given capacitor, the lower the load current calls (R is big), the more efficient the filter is-Generally, when the rec

8、tifier supplies a resistive load resistance R , the product RC is higher, the greater the filtering will be effective.This shows that the higher the capacitance of the capacitor is, the more we limit the corrugations. However, increasing the capacity corresponds to an increase in cost and space and

9、on the other hand, too large a capacity leads to very narrow peak current, which causes harmonic pollution on the distribution network. Finally, the transient current load current capacity to power the rectifier can cause destruction of the latter by exceeding the maximum allowable intensity.Importa

10、nt: For loads which require a high current, the voltage of the filter capacitor is not the most efficient. It is then preferred to place an inductor in series with the load.d. controlled full wave rectificationWhen trigger pulses are available: -Among the thyristors Th1 and Th3, who has the potentia

11、l of its most positive anode leads-Among the thyristor Th2 and Th4, who has the potential of its most negative cathode leads-When the gate pulse no longer exist, thyristors who drove before stopping pulses continue driving until the resumption of pulses.The average value of the output voltage can be

12、 calculated by the equation:The new period of output Vs(t) isT/2.N.B: In the diagram, we have a current source output to simulate a large inductanceAs we can see, the inductance doesnt change the output voltage. 2. Inverter An inverter is a power electronic device for delivering voltages and alterna

13、ting current from a power source providing a direct current. This is the inverse function of a rectifier. The inverter is a DC/AC converter. An inverter is used to produce an un-interrupted 220V AC or 110V AC (depending on the line voltage of the particular country) supply to the device connected as

14、 the load at the output socket. The inverter gives constant AC voltage at its output socket when the AC mains power supply is not available. Principal applications of inverter are portable device who have battery who provides a continuous current, solar generating systems who provides continuous cur

15、rent and some others. a. Circuit descriptionThe inverters are based on a H-bridge structure consisting mostly of electronic switches such as IGBTs, power transistors or thyristors. For a set of switches controlled appropriately (generally a pulse with modulation), the source is modulated to obtain a

16、 frequency AC signal that we want. There are two types of inverters: voltage inverters and power inverters. We also distinguish self and non autonomes UPS inverters: Stand-alone Inverters An autonomous inverter outputs a voltage with a frequency either fixed or adjustable by the user. It does not al

17、ways need to run electrical network, for example a travel converter that connects to the cigarette lighter socket of a car uses 12 VDC vehicle to generate 120 or 230 V AC 50 or 60 Hz; The inverters are especially used for TV reception in mobile mode (satellite receiver in a motorhome for example) wi

18、thout input low voltage power supply ( 12 V).Non-autonomous inverterA non-autonomous inverter is a rectifier circuit all thyristors (Graetz bridge) which, assisted natural switching network, which it is connected, enables inverter operation (eg energy recovery during braking periods in electric moto

19、r). At the base of the development of static variable speed drives for DC motors and AC cyclo, power inverters for synchronous and asynchronous machines, up to powers of several MW, this type of arrangement is gradually supplanted in favor of GTO or IGBT converters. b. Basic designsIn one simple inv

20、erter circuit, DC power is connected to a transformer through the center tap of the primary winding. A switch is rapidly switched back and forth to allow current to flow back to the DC source following two alternate paths through one end of the primary winding and then the other. The alternation of

21、the direction of current in the primary winding of the transformer produces alternating current (AC) in the secondary circuit.The electromechanical version of the switching device includes two stationary contacts and a spring supported moving contact. The spring holds the movable contact against one

22、 of the stationary contacts and an electromagnet pulls the movable contact to the opposite stationary contact. The current in the electromagnet is interrupted by the action of the switch so that the switch continually switches rapidly back and forth.The switch in the simple inverter described above,

23、 when not coupled to an output transformer, produces a square voltage waveform due to its simple off and on nature as opposed to the sinusoidal waveform that is the usual waveform of an AC power supply. Using Fourier analysis, periodic waveforms are represented as the sum of an infinite series of si

24、ne waves. The sine wave that has the same frequency as the original waveform is called the fundamental component. The other sine waves, called harmonics, that are included in the series have frequencies that are integral multiples of the fundamental frequency.Fourier analysis can be used to calculat

25、e the total harmonic distortion (THD). The total harmonic distortion (THD) is the square root of the sum of the squares of the harmonic voltages divided by the fundamental voltage:c. Advanced designsThere are many different power circuit topologies and control strategies used in inverter designs. Di

26、fferent design approaches address various issues that may be more or less important depending on the way that the inverter is intended to be used.The issue of waveform quality can be addressed in many ways. Capacitors and inductors can be used to filter the waveform. If the design includes a transfo

27、rmer, filtering can be applied to the primary or the secondary side of the transformer or to both sides. Low-pass filters are applied to allow the fundamental component of the waveform to pass to the output while limiting the passage of the harmonic components. If the inverter is designed to provide

28、 power at a fixed frequency, a resonant filter can be used. For an adjustable frequency inverter, the filter must be tuned to a frequency that is above the maximum fundamental frequency.Since most loads contain inductance, feedback rectifiers or antiparallel diodes are often connected across each se

29、miconductor switch to provide a path for the peak inductive load current when the switch is turned off. The antiparallel diodes are somewhat similar to the freewheeling diodes used in AC/DC converter circuits.H bridge inverter circuit with transistor switches and antiparallel diodes Fourier analysis

30、 reveals that a waveform, like a square wave, that is anti-symmetrical about the 180 degree point contains only odd harmonics, the 3rd, 5th, 7th, etc. Waveforms that have steps of certain widths and heights can attenuate certain lower harmonics at the expense of amplifying higher harmonics. For exam

31、ple, by inserting a zero-voltage step between the positive and negative sections of the square-wave, all of the harmonics that are divisible by three (3rd and 9th, etc.) can be eliminated. That leaves only the 5th, 7th, 11th, 13th etc. The required width of the steps is one third of the period for e

32、ach of the positive and negative steps and one sixth of the period for each of the zero-voltage steps.Changing the square wave as described above is an example of pulse-width modulation (PWM). Modulating, or regulating the width of a square-wave pulse is often used as a method of regulating or adjusting an inverters output voltage. Whe