TYPES OF CONVERTERS AC to DC Converters (RECTIFIER) Introduction One of the first and most widely used application of power electronic devices have been in rectification. Rectification refers to the process of converting an ac voltage or current source to dc voltage and current. Rectifiers specially refer to power electronic converters where the electrical power flows from the ac side to the dc side. In many situations the same converter circuit may carry electrical power from the dc side to the ac side where upon they are referred to as inverters. In this lesson and subsequent ones the working principle and analysis of several commonly used rectifier circuits supplying different types of loads (resistive, inductive, capacitive, back emf type) will be presented. Points of interest in the analysis will be. • Waveforms and characteristic values (average, RMS etc) of the rectified voltage and current. • Influence of the load type on the rectified voltage and current. • Harmonic content in the output. • Voltage and current ratings of the power electronic devices used in the rectifier circuit. • Reaction of the rectifier circuit upon the ac network, reactive power requirement, power factor, harmonics etc. • Rectifier control aspects (for controlled rectifiers only) In the analysis, following simplifying assumptions will be made. • The internal impedance of the ac source is zero. • Power electronic devices used in the rectifier are ideal switches. The first assumption will be relaxed in a latter module. However, unless specified otherwise, the second assumption will remain in force. Rectifiers are used in a large variety of configurations and a method of classifying them into certain categories (based on common characteristics) will certainly help one to gain significant
insight into their operation. Unfortunately, no consensus exists among experts regarding the criteria to be used for such classification. For the purpose of this lesson (and subsequent lessons) the classification shown in Fig 9.1 will be followed.
DC – DC CONVERTER There are three basic types of dc-dc converter circuits, termed as buck, boost and buck-boost. In all of these circuits, a power device is used as a switch. This device earlier used was a thyristor, which is turned on by a pulse fed at its gate. In all these circuits, the thyristor is connected in series with load to a dc supply, or a positive (forward) voltage is applied between anode and cathode terminals. The thyristor turns off, when the current decreases below the holding current, or a reverse (negative) voltage is applied between anode and cathode terminals. So, a thyristor is to be force-commutated, for which additional circuit is to be used, where another thyristor is often used. Later, GTO’s came into the market, which can also be turned off by a negative current fed at its gate, unlike thyristors, requiring proper control circuit. The turn-on and turn-off times of GTOs are lower than those of thyristors. So, the frequency used in GTO-based choppers can be increased, thus reducing the size of filters. Earlier, dc-dc converters were called ‘choppers’, where thyristors or GTOs are used. It may be noted here that buck converter (dc-dc) is called as ‘step-down chopper’, whereas boost converter (dcdc) is a ‘step-up chopper’. In the case of chopper, no buck-boost type was used.
With the advent of bipolar junction transistor (BJT), which is termed as self-commutated device, it is used as a switch, instead of thyristor, in dc-dc converters. This device (NPN transistor) is switched on by a positive current through the base and emitter, and then switched off by withdrawing the above signal. The collector is connected to a positive voltage. Now-a-days, MOSFETs are used as a switching device in low voltage and high current applications. It may be noted that, as the turnon and turn-off time of MOSFETs are lower as compared to other switching devices, the frequency used for the dc-dc converters using it (MOSFET) is high, thus, reducing the size of filters as stated earlier. These converters are now being used for applications, one of the most important being Switched Mode Power Supply (SMPS). Similarly, when application requires high voltage, Insulated Gate Bi-polar Transistors (IGBT) are preferred over BJTs, as the turn-on and turn-off times of IGBTs are lower than those of power transistors (BJT), thus the frequency can be increased in the converters using them. So, mostly self-commutated devices of transistor family as described are being increasingly used in dc-dc converters
TYPES OF DC –DC CONVERTERS Buck Converters (dc-dc) Boost Converters (dc-dc) Buck-Boost Converters (dc-dc) AC –AC CONVERTER An AC/AC converter converts an AC waveform such as the mains supply, to another AC waveform, where the output voltage and frequency can be set arbitrarily. AC/AC converters can be categorized into •
Converters with a DC-link.
•
Hybrid Matrix Converters.
•
Matrix Converters.
As shown in Fig 1. For such AC-AC conversion today typically converter systems with a voltage (Fig. 2) or current (Fig. 3) DC-link are employed. For the voltage DC-link, the mains coupling
could be implemented by a diode bridge. To accomplish braking operation of a motor, a braking resistor must be placed in the DC-link. Alternatively, an anti-parallel thyristor bridge must be provided on the mains side for feeding back energy into the mains. The disadvantages of this solution are the relatively high mains distortion and high reactive power requirements (especially during inverter operation).
An AC/AC converter with approximately sinusoidal input currents and bidirectional power flow can be realized by coupling a PWM rectifier and a PWM inverter to the DC-link. The DC-link quantity is then impressed by an energy storage element that is common to both stages, which is a capacitor C for the voltage DC-link or an inductor L for the current DC-link. The PWM rectifier is controlled in a way that a sinusoidal mains current is drawn, which is in phase or antiphase (for energy feedback) with the corresponding mains phase voltage.
Due to the DC-link storage element, there is the advantage that both converter stages are to a large extent decoupled for control purposes. Furthermore, a constant, mains independent input quantity exists for the PWM inverter stage, which results in high utilization of the converter’s power capability. On the other hand, the DC-link energy storage element has a relatively large physical volume, and when electrolytic capacitors are used, in the case of a voltage DC-link, there is potentially a reduced system lifetime. In order to achieve higher power density and reliability, it is makes sense to consider Matrix Converters that achieve three-phase AC/AC conversion without any intermediate energy storage element.
An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. An inverter is essentially the opposite of a rectifier. Static inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current
applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries. The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC. Basic designs In one simple inverter circuit, DC power is connected to a transformer through the centre 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 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 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. This type of electromechanical inverter switch, called a vibrator or buzzer, was once used in vacuum tube automobile radios. A similar mechanism has been used in door bells, buzzers and tattoo guns. As they became available with adequate power ratings, transistors and various other types of semiconductor switches have been incorporated into inverter circuit designs
Introduction Single phase uncontrolled rectifiers are extensively used in a number of power electronic based converters. In most cases they are used to provide an intermediate unregulated dc voltage source which is further processed to obtain a regulated dc or ac output. They have, in general, been proved to be efficient and robust power stages. However, they suffer from a few disadvantages. The main among them is their inability to control the output dc voltage / current magnitude when the input ac voltage and load parameters remain fixed. They are also unidirectional in the sense that they allow electrical power to flow from the ac side to the dc side only. These two disadvantages are the direct consequences of using power diodes in these converters which can block voltage only in one direction. As will be shown in this module, these two disadvantages are overcome if the diodes are replaced by thyristors, the resulting converters are called fully controlled converters. Thyristors are semi controlled devices which can be turned ON by applying a current pulse at its gate terminal at a desired instance. However, they cannot be turned off from the gate terminals. Therefore, the fully controlled converter continues to exhibit load dependent output voltage / current waveforms as in the case of their uncontrolled counterpart. However, since the thyristor can block forward voltage, the output voltage / current magnitude can be controlled by controlling the turn on instants of the thyristors. Working principle of thyristors based single phase fully controlled converters will be explained first in the case of a single thyristor halfwave rectifier circuit supplying an R or R-L load. However, such converters are rarely used in practice. Full bridge is the most popular configuration used with single phase fully controlled rectifiers. Analysis and performance of this rectifier supplying an R-L-E load (which may represent a dc motor) will be studied in detail in this lesson.
Single phase fully controlled halfwave rectifier
Fig shows the circuit diagram of a single phase fully controlled halfwave rectifier supplying a purely resistive load. At ωt = 0 when the input supply voltage becomes positive the thyristor T becomes forward biased. However, unlike a diode, it does not turn ON till a gate pulse is applied at ωt = α. During the period 0 < ωt ≤ α, the thyristor blocks the supply voltage and the load voltage remains zero as shown in fig 10.1(b). Consequently, no load current flows during this interval. As soon as a gate pulse is applied to the thyristor at ωt = α it turns ON. The voltage across the thyristor collapses to almost zero and the full supply voltage appears across the load. From this point onwards the load voltage follows the supply voltage. The load being purely resistive the load current io is proportional to the load voltage. At ωt = π as the supply voltage passes through the negative going zero crossing the load voltage and hence the load current becomes zero and tries to reverse direction. In the process the thyristor undergoes reverse recovery and starts blocking the negative supply voltage. Therefore, the load voltage and the load current remains clamped at zero till the thyristor is fired again at ωt = 2π + α. The same process repeats there after
for
There fore
Or
Three phase fully controlled bridge converter Introduction The three phase fully controlled bridge converter has been probably the most widely used power electronic converter in the medium to high power applications. Three phase circuits are preferable when large power is involved. The controlled rectifier can provide controllable out put dc voltage in a single unit instead of a three phase autotransformer and a diode bridge rectifier. The controlled rectifier is obtained by replacing the diodes of the uncontrolled rectifier with thyristors. Control over the output dc voltage is obtained by controlling the conduction interval of each thyristor. This method is known as phase control and converters are also called “phase controlled converters”. Since thyristors can block voltage in both directions it is possible to reverse the polarity of the output dc voltage and hence feed power back to the ac supply from the dc side. Under such condition the converter is said to be operating in the “inverting mode”. The thyristors in the converter circuit are commutated with the help of the supply voltage in the rectifying mode of operation and are known as “Line commutated converter”. The same circuit while operating in the inverter mode requires load side counter emf. for commutation and are referred to as the “Load commutated inverter”. In phase controlled rectifiers though the output voltage can be varied continuously the load harmonic voltage increases considerably as the average value goes down. Of course the magnitude of harmonic voltage is lower in three phase converter compared to the single phase circuit. Since the frequency of the harmonic voltage is higher smaller load inductance leads to continuous conduction. Input current wave shape become rectangular and contain 5th and higher
order odd harmonics. The displacement angle of the input current increases with firing angle. The frequency of the harmonic voltage and current can be increased by increasing the pulse number of the converter which can be achieved by series and parallel connection of basic 6 pulse converters. The control circuit become considerably complicated and the use of coupling transformer and / or interphase reactors become mandatory. With the introduction of high power IGBTs the three phase bridge converter has all but been replaced by dc link voltage source converters in the medium to moderately high power range. However in very high power application (such as HV dc transmission system, cycloconverter drives, load commutated inverter synchronous motor drives, static scherbius drives etc.) the basic B phase bridge converter block is still used. In this lesson the operating principle and characteristic of this very important converter topology will be discussed in source depth.
Operating principle of 3 phase fully controlled bridge converter A three phase fully controlled converter is obtained by replacing all the six diodes of an uncontrolled converter by six thyristors as shown in Fig. 13.1 (a)
For any current to flow in the load at least one device from the top group (T1, T3, T5) and one from the bottom group (T2, T4, T6) must conduct. It can be argued as in the case of an uncontrolled converter only one device from these two groups will conduct. Then from symmetry consideration it can be argued that each thyristor conducts for 120° of the input cycle. Now the thyristors are fired in the sequence T1 → T2 → T3 → T4 → T5 → T6 → T1 with 60° interval between each firing. Therefore thyristors on the same phase leg are fired at an interval of 180° and hence can not conduct simultaneously. This leaves only six possible conduction mode for the converter in the continuous conduction mode of operation. These are T1T2, T2T3, T3T4, T4T5, T5T6, T6T1. Each conduction mode is of 60° duration and appears in the sequence mentioned. The conduction table of Fig. 13.1 (b) shows voltage across different devices and the dc output voltage for each conduction interval. The phasor diagram of the line voltages appear in Fig. 13.1 (c). Each of these line voltages can be associated with the firing of a thyristor with the help of the conduction table-1. For example the thyristor T1 is fired at the end of T5T6 conduction interval. During this period the voltage across T 1 was vac. Therefore T1 is fired α angle after the positive going zero crossing of vac. Similar observation can be made about other thyristors. The phasor diagram of Fig. 13.1 (c) also confirms that all the thyristors are fired in the correct sequence with 60° interval between each firing. Fig. 13.2 shows the waveforms of different variables (shown in Fig. 13.1 (a)). To arrive at the waveforms it is necessary to draw the conduction diagram which shows the interval of conduction for each thyristor and can be drawn with the help of the phasor diagram of fig. 13.1 (c). If the converter firing angle is α each thyristor is fired “α” angle after the positive going zero crossing of the line voltage with which it’s firing is associated. Once the conduction diagram is
drawn all other voltage waveforms can be drawn from the line voltage waveforms and from the conduction table of fig. 13.1 (b). Similarly line currents can be drawn from the output current and the conduction diagram. It is clear from the waveforms that output voltage and current waveforms are periodic over one sixth of the input cycle. Therefore this converter is also called the “six pulse” converter. The input current on the other hand contains only odds harmonics of the input frequency other than the triplex (3rd, 9th etc.) harmonics. The next section will analyze the operation of this converter in more details.
Additional inductance: The addition of AC input inductance to the single phase drive improves the current waveform and spectrum from those shown in Figures 2 and 3 to those shown in Figures 7 and 8. It is particularly beneficial for the higher order harmonics, but the fifth and seventh is reduced by a useful degree. Only the third harmonic is little improved.
Since the three-phase rectifier has no third harmonic current, the AC inductor is even more beneficial, as shown in figure
Introduction:
In most practical situations, most of ac dc converters are supplied from transformers. The series impedance of the transformer can not always be neglected. Even if no transformer is used, the impedance of the feeder line comes in series with the source. In most cases this impedance is predominantly inductive with negligible resistive component. The presence of source inductance does have significant effect on the performance of the converter. With source inductance present the output voltage of a converter does not remain constant for a given firing angle. Instead it drops gradually with load current. The converter output voltage and input current waveforms also change significantly. In this lesson a quantitative analysis of these effects will be taken up in some detail. DC inductance Drives rated at 4kW or more usually have three-phase input and include inductance in the DC link. This gives the improved waveform and spectrum shown in Figures 11 and 12, which are for a hypothetical 1.5kW drive for ease of comparison with the previous illustrations.
Single phase fully controlled converter with source inductance Fig. 1.1(a) shows a single phase fully controlled converter with source inductance. For simplicity it has been assumed that the converter operates in the continuous conduction mode. Further, it has been assumed that the load current ripple is negligible and the load can be replaced by a dc current source the magnitude of which equals the average load current. Fig. 1.1(b) shows the corresponding waveforms. It is assumed that the thyristors T 3 and T4 were conducting at t = 0. T1 and T2 are fired at ωt = α. If there were no source inductance T 3 and T4 would have commutated as soon as T1 and T2 are turned ON. The input current polarity would have changed instantaneously. However, if a source inductance is present the commutation and change of input current polarity can not be instantaneous. Therefore, when T1 and T2 are turned ON T3 T4 does not commutate immediately. Instead, for some interval all four thyristors continue to conduct as shown in Fig. 15.1(b). This interval is called “overlap” interval.
During this period the load current freewheels through the thyristors and the output voltage is clamped to zero. On the other hand, the input current starts changing polarity as the current through T1 and T2 increases and T3 T4 current decreases. At the end of the overlap interval the
current through T3 and T4 becomes zero and they commutate, T1 and T2 starts conducting the full load current. The same process repeats during commutation from T1 T2 to T3T4 at ωt = π + α. From Fig. 15.1(b) it is clear that, commutation overlap not only reduces average output dc voltage but also reduces the extinction angle γ which may cause commutation failure in the inverting mode of operation if α is very close to 180º. In the following analysis an expression of the overlap angle “μ” will be determined.
. From the equivalent circuit of the converter during overlap period
for
α ≤ ωt ≤α + μ
can be represented by the following equivalent circuit
The simple equivalent circuit of Fig. 15.3 represents the single phase fully controlled converter with source inductance as a practical dc source as far as its average behavior is concerned. The open circuit voltage of this practical source equals the average dc output voltage of an ideal
converter (without source inductance) operating at a firing angle of α. The voltage drop across the internal resistance “RC” represents the voltage lost due to overlap shown in Fig. 15.1(b) by the hatched portion of the v0 waveform. Therefore, this is called the “Commutation resistance”. Although this resistance accounts for the voltage drop correctly there is no power loss associated with this resistance since the physical process of overlap does not involve any power loss. Therefore this resistance should be used carefully where power calculation is involved.
Three phase fully controlled converter with source inductance When the source inductance is taken into account, the qualitative effects on the performance of the converter is similar to that in the case of a single phase converter. Fig. 15.4(a) shows such a converter. As in the case of a single phase converter the load is assumed to be highly inductive such that the load can be replaced by a current source.
As in the case of a single phase converter, commutations are not instantaneous due to the presence of source inductances. It takes place over an overlap period of “μ 1” instead. During the overlap period three thyristors instead of two conducts. Current in the outgoing thyristor gradually decreases to zero while the incoming thyristor current increases and equals the total load current at the end of the overlap period. If the duration of the overlap period is greater than 60º four thyristors may also conduct clamping the output voltage to zero for sometime. However, this situation is not very common and will not be discussed any further in this lesson. Due to the conduction of two devices during commutation either from the top group or the bottom group the instantaneous output voltage during the overlap period drops (shown by the hatched portion of Fig. 15.4 (b)) resulting in reduced average voltage. The exact amount of this reduction can be calculated as follows. In the time interval α < ωt ≤ α + μ, T6 and T2 from the bottom group and T1 from the top group conducts. The equivalent circuit of the converter during this period is given by the circuit diagram of Fig. 15.5.
Therefore, in the interval α < ωt ≤ α + μ
at ωt = α + μ, ib = 0
Equation 15.20 holds for μ ≤ 60º. It can be shown that for this condition to be satisfied
To calculate the dc voltage For α ≤ ωt ≤ α + μ
Substituting Equation 15.20 into 15.24
Equation 15.25 suggests the same dc equivalent circuit for the three phase converter with source inductance as shown in Fig. 15.3 with
and commutation resistance It should be noted that RC is a “loss less” resistance, since the overlap process does not involve any active power loss.
