01-03-2013, 11:01 AM
VOLTAGE FED TRANS - Z SOURCE INVERTER
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INTRODUCTION
MOTIVATION
With the increasing energy consumption and increasing costs associated with it, tendency towards generating power with smaller generators that may be dispersed in a wide area and most of them are renewable, as they have greater advantages due to their environmentally friendly nature. This concept is commonly known as distributed generation. Most of these DG systems are connected to the grid through the power electronic Inverter. Inverter is an electrical system which converts direct current to alternating current. The alternating current is then applied directly to the commercial electrical grid or to the local off-grid electrical network. There exist two traditional inverters voltage-source and current-source inverters. Both of these inverters have a limited operating range, even though both are widely used in DG applications. This limitation overcomes by using a dc-dc converter at front end. So that it behaves like a two stage inverter. To obtain single stage conversion we go for a ZSI .The voltage-fed ZSI topology provides a modern approach to the boost voltage conversion techniques, but it has got more component count and control more complicated; these all barriers are avoided by using the proposed voltage–fed Trans –z source inverters.
This circuit is beneficial to applications, in which a high voltage gain is required. The low component count is an attractive figure of merit that makes the trans-Z-source inverters suitable for low to medium power applications. When the turns-ratio of the two windings is over 1, the voltage-fed trans-Z-source inverter can obtain a higher boost gain with the same shoot-through duty ratio and modulation index, compared with the original Z-source inverter
THESIS OBJECTIVES
The following objectives are hopefully to be achieved at the end of the project.
1) To study the different voltage fed inverter topologies and how the new trans-ZSI is more advantageous in comparison to the old ones.
2) To study the proposed voltage –fed Trans-ZSI and design the parameters of the proposed converter.
VOLTAGE SOURCE INVERTER
Single-Phase Voltage Source Inverter
The function of an inverter is to change a dc input voltage to a symmetrical ac outputs voltage of desired magnitude and frequency. Inverters can be broadly classified into two types:
1. Single-phase inverters
2. Three-phase inverters
These inverters generally use PWM controlled signals for producing an ac output voltage. The inverters are classified into three types according to the input source as follows:
1. Voltage source inverter if the input voltage remains constant
2. Current source inverter if the input currents are maintained constant.
3. Variable dc linked inverter if the input voltage is controllable.
Single-phase voltage source inverters (VSIs) can be found as half-bridge and full-bridge topologies. Although the power range they cover is the low one, they are widely used in power supplies, single-phase UPS, and currently to form elaborate high-power static power topologies, such as for instance, the multicell configurations.
Half-Bridge VSI
Fig.1.1 shows the power topology of a half-bridge VSI, where two large capacitors are required to provide a neutral point N, such that each capacitor maintains a constant voltage Vi/2. Because the current harmonics injected by the operation of the inverter are low-order harmonics, a set of large capacitors C+ and C- is required. It is clear that both switches S+ and S- cannot be ON simultaneously because a short circuit across the dc link voltage source Vi would be produced. There are two defined states 1 and 2 and one undefined state 3 switch states as shown in Table 1.1. In order to avoid the short circuit across the dc bus and the undefined ac output voltage condition, the modulating technique should always ensure that at any instant either the top or the bottom switch of the inverter leg is on.
Full-Bridge VSI
Fig.1.2 shows the power topology of a full-bridge VSI. This inverter is similar to the half-bridge inverter; however, a second leg provides the neutral point to the load. As expected, both switches S1+ and S1- or S2+ and S2- cannot be on simultaneously because a short circuit across the dc link voltage source Vi would be produced. There are four defined states 1, 2, 3, and 4 and one undefined state 5 switch states as shown in Table 1.2. The undefined condition should be avoided so as to be always capable of defining the ac output voltage. It can be observed that the ac output voltage can take values up to the dc link value Vi, which is twice that obtained with half-bridge VSI topologies. Several modulating techniques have been developed that are .applicable to full-bridge VSIs. Among them are the PWM bipolar and unipolar techniques.
Three Phase Voltage Source Inverters
Single-phase VSIs cover low-range power applications and three-phase VSIs cover the medium- to high-power applications. The main purpose of these topologies is to provide a three-phase voltage source, where the amplitude, phase, and frequency of the voltages should always be controllable. Although most of the applications require sinusoidal voltage waveforms e.g., ASDs, UPSs, FACTS, VAR compensators, arbitrary voltages are also required in some emerging applications e.g., active filters, voltage compensators
Fig.1.3 shows the traditional three-phase voltage-source converter structure. A dc voltage source supported by a relatively large capacitor feeds the main converter circuit, a three-phase bridge. The dc voltage source can be a battery, fuel-cell stack, diode rectifier, and/or capacitor. Six switches are used in the main circuit; each is traditionally composed of a power transistor and an antiparallel i.e., freewheeling diode to provide bidirectional current flow and unidirectional voltage blocking capability.
The standard three-phase VSI topology is shown in Fig. 1.3 and the eight valid switch states are given in Table 1.3. As in single-phase VSIs, the switches of any leg of the inverter S1 and S4, S3 and S6, or S5 and S2 cannot be switched on simultaneously because this would result in a short circuit across the dc link voltage supply. Similarly, in order to avoid undefined states in the VSI, and thus undefined ac output line voltages, the switches of any leg of the inverter cannot be switched off simultaneously as this will result in voltages that will depend upon the respective line current polarity. Of the eight valid states, two of them 7 and 8 in Table 1.3 produce zero ac line voltages. In this case, the ac line currents freewheel through either the upper or lower components. The remaining states 1 to 6 in Table1.3 produce non-zero ac output voltages. In order to generate a given voltage waveform, the inverter moves from one state to another. Thus the resulting ac output line voltages consist of discrete values of voltages that are Vi, 0, and -Vi for the topology shown in Fig.1.3.
Z– SOURCE INVERTER
The V-source converter is widely used, but it has the above mentioned conceptual and theoretical barriers and limitations. The voltage fed Z-source converter overcomes the above mentioned conceptual and theoretical barriers and limitations of the traditional V-source converter and provides a novel power conversion concept. So a Z-source inverter was proposed. The Z-source inverter intentionally utilizes the shoot-through zero states to boost dc voltage and produces an output voltage that is greater than the original dc voltage. At the same time, the Z-source structure greatly enhances the reliability of the inverter, because the momentary shoot-through states that might be caused by electromagnetic interference noise can no longer destroy the inverter. Fig. 2.1 shows the configuration of the Z-source inverter. The ZSI is including two capacitors and two inductors and an inverter bridge and provides unique buck boost characteristics. Moreover, unlike traditional inverter, it does not need dead time. Due to these unique features, the Z-source inverter has found applications in numerous industrial processes including DG system. The ZSI topology provides a modern approach to the boost voltage conversion techniques.
CONTROL METHOD
All the traditional pulse width-modulation (PWM) schemes can be used to control the Z-source inverter and their theoretical input–output relationships still hold. In every switching cycle, the two non shoot-through zero states are used along with two adjacent active states to synthesize the desired voltage. When the dc voltage is high enough to generate the desired ac voltage, the traditional PWM is used. While the dc voltage is not enough to directly generate a desired output voltage, a modified PWM with shoot-through zero states will be used as shown in Fig.2.4 to boost voltage. It should be noted that each phase leg still switches on and off once per switching cycle. Without change the total zero-state time interval, shoot-through zero states are evenly allocated into each phase. That is, the active states are unchanged. However, the equivalent dc-link voltage to the inverter is boosted because of the shoot-through states .It is noticeable here that the equivalent switching frequency viewed from the Z-source network is six times the switching frequency of the main inverter, which greatly reduces the required inductance of the Z-source network.
VOLTAGE–FED TRANS-Z-SOURCE INVERTER
Inorder to improve the properties of ZSI a Quasi ZSI is proposed. QZSI has been developed which feature several improvements and no disadvantages when compared to the ZSI. The QZSI when compared to the ZSI has below features:
1. lower dc voltage of capacitor C1
2. continuous input current
3. does not require input capacitance
4. common dc rail between the source and inverter
Additionally, the QZSI topology has no disadvantages when compared to the ZSI topology.The QZSI topology therefore can be used in any application in which the ZSI would customarily be used. The trans-Z-source inverters can be derived from the voltage-fed quasi Z-source inverters or the voltage-fed Z-source inverters. The trans-Z-source-inverters inherit their unique features, and they can be controlled using the PWM methods applicable to the Z-source inverters. This paper will begin with the derivation of two voltage-fed trans- Z-source inverters from one of the quasi Z-source inverters.
WORKING OF VOLTAGE–FED TRANS-ZSI
Another trans-Z-source inverter can be reconfigured as shown in Fig.4.6, if C1 is removed in Fig. 4.1 instead of C2.This trans-Z-source inverter has the same operation principle, voltage gain, and voltage stress as the previously developed voltage-fed trans-quasi-Z-source inverter, except for different capacitor voltage stress and different input current drawn from the dc source. Therefore, Figs. 4.1, 4.2 and 4.6 can be classified as a class of voltage-fed trans-Z-source inverters.