The Inverter Provides A.C. Load Voltage from a D.C. Voltage Source. the Semi- Conductor

INVERTERS

The inverter provides a.c. load voltage from a d.c. voltage source. The semi- conductor switches can be BJTs, thyristors, Mosfets, IGBTs etc. The choice of power switch will depend on rating requirements and ease with which the device can be turned on and off.

A single-phase inverter will contain two or four power switches arranged in half-bridge or full-bridge topologies. Half-bridges have the maximum a.c. voltage limited to half the value of the full d.c. source voltage and may need a centre tapped source. Full-bridges have the full d.c. source voltage as the maximum a.c. voltage. Where the d.c. source voltage is low, e.g. 12V or 24V, the voltage drop across the conducting power switches is significant and should be taken into account both in calculation and in selection of the switch. The a.c. load voltage of the inverter is essentially a square wave, but pulse- width-modulation methods can be used to reduce the harmonics and produce a quasi-sine wave. If higher a.c. voltages than the d.c. source voltage are required, then the inverter will require a step-up transformer. The output frequency of the inverter is controlled by the rate at which the

switches are turned on and off, in other words by the pulse repetition frequency of the base, or gate, driver circuit. Thyristors would only be used in very high power inverters, since on the source side there is no voltage zero, and a forced commutation circuit would be required to turn the thyristor off. Some typical single-phase inverters are considered in the following sections. The switching device shown is a BJT, but could be any switch, the choice being determined by availability of required rating and ease of turn-on and turn-off.

Care must be taken not to have two switches 'on' together, shorting out the d.c. source. There must be either a dead-time between switches or an inhibit circuit to ensure this does not happen.

The main objective of static power converters is to produce anac output waveform from a dc power supply. These are thetypes of waveforms required in adjustable speed drives(ASDs), uninterruptible power supplies (UPS), static var compensators, active filters, flexible ac transmission systems(FACTS), and voltage compensators, which are only a fewapplications. For sinusoidal ac outputs, the magnitude,frequency, and phase should be controllable. According tothe type of ac output waveform, these topologies can beconsidered as voltage source inverters (VSIs), where theindependently controlled ac output is a voltage waveform.

These structures are the most widely used because theynaturally behave as voltage sources as required by manyindustrial applications, such as adjustable speed drives(ASDs), which are the most popular application of inverters;see Fig. . Similarly, these topologies can be found ascurrent source inverters (CSIs), where the independentlycontrolled ac output is a current waveform. These structuresare still widely used in medium-voltage industrial applications,where high-quality voltage waveforms are required.

Single-Phase Voltage Source

Inverters

Single-phase voltage source inverters (VSIs) can be found ashalf-bridge and full-bridge topologies. Although the powerrange they cover is the low one, they are widely used in powersupplies, single-phase UPSs, and currently to form elaboratehigh-power static power topologies,Themain features of both approaches are reviewed and presentedin the following.

Half-Bridge VSI

Figure shows the power topology of a half-bridge VSI,where two large capacitors are required to provide a neutralpoint N, such that each capacitor maintains a constant voltagevi/2. Because the current harmonics injected by the operationof the inverter are low-order harmonics, a set of largecapacitors (C+ and C-) is required. It is clear that bothswitches S+ and S- cannot be on simultaneously because ashort circuit across the dc link voltage source viwould beproduced. There are two defined (states 1 and 2) and oneundefined (state 3) switch state as shown in Table 14.1. Inorder to avoid the short circuit across the dc bus and theundefined ac output voltage condition, the modulating tech-nique should always enure that at any instant either the top or

the bottom switch of the inverter leg is on.

Single-phase half-bridge VSI.

TABLE Switch states for a half-bridge single-phase VSI

Figure shows the ideal waveforms associated with thehalf-bridge inverter shown in Fig. . The states for theswitches S+ and S- are defined by the modulating technique,which in this case is a carrier-based PWM.

Full-Bridge VSI

Figure shows the power topology of a full-bridge VSI.This inverter is similar to the half-bridge inverter; however, asecond leg provides the neutral point to the load. As expected,both switches S1+ and S1- (or S2+ and S2-) cannot be onsimultaneously because a short circuit across the dc link

voltage source viwould be produced. There are four defined(states 1, 2, 3, and 4) and one undefined (state 5) switch statesas shown in Table .The undefined condition should be avoided so as to bealways capable of defining the ac output voltage. In order toavoid the short circuit across the dc bus and the undefined acoutput voltage condition, the modulating technique shouldensure that either the top or the bottom switch of each leg ison at any instant. It can be observed that the ac output voltagecan take values up to the dc link value vi, which is twice thatobtained with half-bridge VSI topologies.

Several modulating techniques have been developed that areapplicable to full-bridge VSIs. Among them are the PWM(bipolar and unipolar) techniques.

Single-phase full-bridge VSI.

TABLE Switch states for a full-bridge single-phase VSI

The full-bridge VSI. Ideal waveforms for the unipolar SPWM

Three-Phase Voltage Source

Inverters

Single-phase VSIs cover low-range power applications andthree-phase VSIs cover the medium- to high-power applica-tions. The main purpose of these topologies is to provide athree-phase voltage source, where the amplitude, phase, andfrequency of the voltages should always be controllable.

Although most of the applications require sinusoidal voltagewaveforms (e.g., ASDs, UPSs, FACTS, var compensators),arbitrary voltages are also required in some emerging applications (e.g., active filters, voltage compensators).

The standard three-phase VSI topology is shown in Fig. and the eight valid switch states are given in Table .

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 onsimultaneously because this would result in a short circuitacross the dc link voltage supply. Similarly, in order to avoidundefined states in the VSI, and thus undefined ac output linevoltages, the switches of any leg of the inverter cannot beswitched off simultaneously as this will result in voltages thatwill depend upon the respective line current polarity.

TABLE Valild switch states for a three-phase VSI

Current Source Inverters

The main objective of these static power converters is toproduce ac output current waveforms from a dc currentpower supply. For sinusoidal ac outputs, its magnitude,frequency, and phase should be controllable. Due to the factthat the ac line currents ioa, iob, and ioc (Fig. ) feature highdi/dt , a capacitive filter should be connected at the acterminals in inductive load applications (such as ASDs).

Thus, nearly sinusoidal load voltages are generated thatjustifies the use of these topologies in medium-voltage industrial applications, where high-quality voltage waveforms arerequired. Although single-phase CSIs can in the same way asthree-phase CSIs topologies be developed under similar principles, only three-phase applications are of practical use and are analyzed in the following.In order to properly gate the power switches of a three-phase CSI, two main constraints must always be met: (a) theac side is mainly capacitive, thus, it must not be short-circuited; this implies that, at most one top switch (1, 3, or

5and one bottom switch (4, 6, or 2 )

Three-phase CSI topology.

should be closed at any time; and (b) the dc bus is of thecurrent-source type and thus it cannot be opened; therefore,there must be at least one top switch (1, 3, or 5) and onebottom switch (4, 6, or 2) closed at all times. Note that bothconstraints can be summarized by stating that at any time,only one top switch and one bottom switch must be closed.

There are nine valid states in three-phase CSIs. The states 7,8, and 9 (Table ) produce zero ac line currents. In thiscase, the dc link current freewheels through either the switchesS1 and S4, switches S3 and S6, or switches S5 and S2. Theremaining states (1 to 6 in Table ) produce nonzero acoutput line currents. In order to generate a given set of ac linecurrent waveforms, the inverter must move from one state toanother. Thus, the resulting line currents consist of discretevalues of current, which are

. The selection of the states in order to generate the given waveforms is done by themodulating technique that should ensure the use of only thevalid states.