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In some applications such as battery chargers and a class of dc and ac motor drives it is necessary for the dc voltage t be controllable. The ac to controlled-dc conversion is accomplished in line0frequency phase-controlled converters by means of thyistor. In the past, these converters were used in a large number of applications for controlling the flow of the electric power. Owing to the increasing availability of better controllable switches in high voltage and current ratings, new use of these thyistor converters nowadays is primarily in three-phase, high-power applications. This is particularly true in applications, most of them at high power levels, where it is necessary or desirable to be able to control the power flow in both directions between the ac and the dc sides. Example of such applications are converters in high-voltage dc power transmission and some dc motor and ac motor and ac drives with regenerative capabilities.
As the name of there converters implies, the line-frequency voltages are present on their ac side. In these converters, the instant at which a thyistor begins or ceases to conduct depends on the line-frequency ac voltage waveforms and the control inputs. Moreover, the transfer or commutation of current from one device to the next occurs naturally because of the presence of these ac voltages.
A fully controlled converter is shown in Fig 2-1a in block diagram form. For given ac line voltages, the average dc-side voltage can be controlled from a positive maximum to a negative minimum value in a continuous manner. The converter dc current can not change direction. Therefore, a converter of this type can operate in only two quadrants (of the plane), as shown in Fig 2-1b. Here, the positive values of imply rectification where the power flow is from the ac to the dc side. In a inverter mode, becomes negative ( and the power is transferred from the dc to the ac side. The inverter mode of operation on a sustained basis is possible only if a source of power such batteries, is present on the dc side.
Fig2-1 line frequency controlled converter.
2-2 Thyistor circuits and their control
For given ac input voltages, the magnitude of the average output voltage in thyistor converters can be controlled by delaying the instants at which the thyistor are allowed to start conduction. This is illustrated by the simple circuits of Fig2-2.
2-2-1 basic thyistor circuits
In Fig2-2a, a thyistor connects the line-frequency source to a load resistance. In the positive half-cycle of, the current is zero until, at which time the thyistor is supplied a positive gate pulse of a short duration. With the thyistor conducting, . for the rest of the positive half-cycle, the current waveform follows the ac voltage waveform and becomes zero at . Then the thyistor blocks the current from flowing during the negative half-cycle of . the current stays zero until , at which time another short-duration gate pulse is applied and the next cycle of the waveform begins. By adjusting , the average value of the load voltage can be controlled.
In Fig2-2b, the load consists of both Rand L. initially, the current is zero. The thyistor conduction is delayed until. Once the thyistor is fired or gated on at during the positive half-cycle of when the voltage across the thyistor is positive, the current begins to flow and . the voltage across the inductor can be written as
where . In Fig 2-2b, is plotted and is shown as the difference between. During is positive and the current increases, since
where is a variable of integration. Beyond , becomes negative and the current begins to decline. The instant at which the current becomes zero and stays zer due to the thyistor is dictated by Eq2-2.graphaically in Fig 2-2b, is the instant at which are equals area and the current becomes zero. These areas represent the time integral of , which must be zero over one cycle of repetition in steady state. It should be note that the current continues to flow for a while even after has become negative. The reason for this has to do with the stored energy in the inductor, a part of with is supplied to R and the other part is absorbed by when it becomes negative.
In Fig 2-2c, the load consists of an inductor and a dc voltage . Here, with the current initially zero, the thyistor is reverse biased until, as shown in Fig2-2c. Therefore, in can not conduct until. The thyistor conduction is further delayed until , when a positive gate pulse is applied. With the current flowing
In terms of ,
where is an arbitrary variable of integration. The current peaks at where . the current goes to zero at, at which instant area equals area , and the time integral of the inductor voltage over one time period of repetition becomes zero.
Fig2-2 basic thyistor converter
2-2-2 Thyistor Gate triggering
By controlling the instant at which the thyistor is gated on, he average current in the circuits of Fig3-3 can be controlled in a continuous manner from zero to a maximum value. The same is true for the power supplied by the ac source.
Versatile integrated circuits, such as the TCA780, are available to provide delayed gate trigger signals to the thyistor. A simplified block diagram of a gate trigger control circuit is shown in Fig3-3. Here, a saw tooth waveform is compared with the control signal , and the delay angle with respect to the positive zero crossing of the ac line voltage is obtained in terms of and the peak of the saw tooth waveform :
another gate trigger signal can easily be obtained, delayed with respect to the negative zero crossing of the ac line voltage.
Fig 2-3 Gate trigger control circuit
2-2-3 Practical Thyistor Converters
Full-bridge converters for single and three-phase utility inputs are shown in Fig2-4. The dc-side inductance may be a part of the load, for example, in dc motor drives, prior to analysis of the full-bridge converter in Fig 2-4; it will be helpful to analyze some simple and possibly hypothetical circuits. This simplification is achieved by assuming ac-side inductance to be zero and the dc side current to be purely dc. Next, the effect of on the converter waveforms will be analyzed. Finally, the effect of the ripple in will be included. These converters will also be analyzed for their inverter mode of operation.
Fig2-4 Practical thyistor converter
2-2-4-1 Single-phase thyistor converter and inverter
The practical circuit of Fig 2-4a, with the assumption of and a purely dc current , is shown in Fig2-5a. it can be redraw as in Fig 2-5b. the current flows through one thyistor of the top group (thyistor 1 and 3) and one thyistor of the bottom group (2 and 4). If the gate currents to the thyritor were continuously applied, the thyistors in Fig2-5 would behave as diodes and their operation would be similar. The voltage and current waveforms under these conditions are shown in Fig2-6a.
Fig2-5 single-phase thyistor converter with and constant dc current
The instant of natural conduction for a thyistor refers to the instant at which the thyistor would begin to conduct of its gate current were continuously applied. Therefore, in Fig 2-6b, the instant of natural conduction is for thyistors 1 and 2 and for thyistor 3 and 4.
Fig 2-6 Waveform in the converter of Fig 2-5
Next consider the effect of applying gate current pulses that delayed by an angle (called the delay angel or firing angle) with respect to the instant of natural conduction. Now prior to , the current is flowing through thyistor 3 and4, and . as shown in Fig2-6b, the voltage across thyistor 1 becomes forward biased beyond ,but it can not conduct until when a gate current pulse is applied. The situation is identical for thyistor2. As a consequence of this finite delay angle , becomes negative during the interval from 0 to.
At , the commutation of current from thyistor 3 and 4 to thyistor 1 and 2 is instantaneous due to the assumption of . When thyistor 1 and 2 are conducing ,. Thyistor 1 and 2 conduct until when thyistor 3 and 4 are triggered, delayed by the angle with respect to their instant of natural conduction (. A similar commutation of current takes place from thyistor 1 and 2 to thyistor 3 and 4.
Comparing the effect of the delay angel on the waveform in Fig 2-6b with that in Fig2-6a shows that the average value can be obtained as
Let be the average value of the dc voltage in Fig2-6a with and =0, where
Then, the drop in the average value due to is
This "lossless" voltage drop in is equal to the volt-radian area shown in Fig 2-6b divided by .
The variation of as a function of is shown in Fig2-7, which shows that the average dc voltage becomes negative beyond . This region is called the inverter mode of operation and is discussed later. The average power through the converter can be calculated as
With a constant dc current (,
Fig 2-7 Normalized as a function of
2-2-4-2 inverter mode of operation of single-phase thyistor converter
It was mentioned in chapter2-1 that the thyistor converters can also operate in an inverter mode, where has a negative value, as shown in Fig 2-1b, and hence the power flows from the dc side to the ac side. The easiest way to understand the inverter mode of the operation is to assume that the dc side of the converter can be replaced by a current source of constant amplitude, as shown in Fig2-5a. For a delay angle greater than 90Ëš but less than 180Ëš, the voltage and current wave forms are shown in Fig2-8b. The average value of is negative, given by
Where 90Ëš. Therefore, the average power is negative, that is, it flows from the dc to the ac side. One the ac side, is also negative because .
Fig 2-8 (a) Inverter assuming a constant dc current (b) waveforms
There are several points worth noting here. This inverter mode of operation is possible since there is a source of energy on the dc side. On the ac side, the ac voltage source facilitates the commutation of current from one pair of thyistor to another. The power flows into this ac source.
Generally, the dc current source is not a realistic dc-side representation of systems where such a mode of operation may be encountered, Fig 3-6a shows a voltage source on the dc side that may represent a battery, a photovoltaic source, or a dc voltage produced by a wind-electric system. It may also be encountered in a four-quadrant dc motor supplied by a back-to-back connected thyistor converter.
An assumption of a very large value of allows us to assume to be a constant dc, and hence the waveforms of Fig 2-8b also apply to the circuit of Fig 2-9a. Since the average voltage across is zero,
The equations is exact if the current is constant at , otherwise, a value of at , should be used in this Eq instead of . Fig 2-9b shows that for a given value of , for example, , the intersection of the dc current and hence the power flow .
During the inverter mode, the voltage waveform across one of the thyistor is shown in Fig 2-10. An extinction angle is defined to be
during which the voltage across the thyistor is negative and beyond which it becomes positive. As the extinction time interval should be greater than the thyistor turn-off time. Otherwise, the thyistor will prematurely begin to conduct, resulting in the failure of current to commutate from one thyistor pair to the other, an abnormal operation that can result in large destructive currents.
Fig2-9 (a) thyistor inverter with a dc voltage source (b) versus
Fig 2-10 Voltage across a thyistor in the inverter mode
2-2-5 Three-phase Converters and Inverters
The practical circuit of Fig 2-4b with the assumption of and a purely dc current is shown in Fig 2-11a. It can be redrawn as in Fig 2-11b. The current flows
Though one of the thyistor of the top group (thistor1, 3, 5) and one of the bottom group(2. 4. 6). If the gate currents were continuously applied, the thyistor in Fig2-11 woud behave as diode and their operation would be similar to that described in the previous chapter. Under these conditions (), the voltages and the current in phase are shown in Fig 2-12a. the average dc voltage is as in Eq:
Using the same definition as in 2-2-4-1, instants of natural conduction for the various thyistor are shown in Fig 2-12a by 1, 2â€¦ The effect of the firing or delay angle on the converter wave forms are shown in Figs 2-12b to d. Focusing on the commutation of current from thyistor 5 to 1, we see that thyistor 5 keeps on conducting until , at which instant the current commutates instantaneously to thyistor 1 due to zero . The current in phase is shown in Fig 2-12c. Similar delay by a angle takes place in the conduction of the other thyistor. The line to line ac voltages and the dc output voltage are shown in Fig2-12d.
The expression for the average dc voltage can be obtained from the waveforms in Figs 2-12b and d. the volt-second area (every 60) results in the reduction in the average dc voltage with a delay angle compared to in Fig 2-12a. Therefore,
From Fig 2-12b, the volt-radian area is the integral of . This can be confirmed by Fig 2-12d, where is the integral of . With the time origin chosen in Fig 2-12,
Substituting in Eq.2-4 and using Eq.2-3 for yield
The above procedure to obtain is straightforward when . For we get the same result but an alternate derivation may be easier
Equation 2-5 shown that is independent of the current magnitude so long as flows continuously (and =0). The control of as a function of is similar to the single-phase case shown by Fig 2-10. The dc voltage waveform various values of is shown in Fig 2-13. The average power is
Fig 2-12 Waveform in the converter of Fig 2-11
Fig2-13 the dc-side voltage waveform as a function of where
2-2-5-1 Inverter mode of three-phase thyistor converter
To understand the inverter mode of operation, we will assume that the dc side of the converter can be represented by a current source of constant amplitude, as shown in Fig 2-14. For a delay angle greater than 90Ëš but less than 180Ëš, the voltage and current waveforms are shown in Fig 2-15a. The average value of is negative according the Eq. . On the ac side, the negative power implies that the phase angle between and is greater than 90Ëš, as shown in Fig 2-15b.