Parallel connection via
interphase transformers permits the implementation of rectifiers for high current applications. Series connection for high voltage is also possible, as shown in the full wave rectifier of figure shown. With this arrangement, it can be seen
that the three common cathode valves generate a positive voltage respect to the
neutral, and the three common anode valves produce a negative voltage. The
result is a dc voltage twice the value of the half wave rectifier. Each half of
the bridge is a three-pulse converter group. This bridge connection is a two-way
connection, and alternating currents flow in the valve-side transformer
windings during both half periods, avoiding dc components into the
windings, and saturation in the transformer magnetic core. These
characteristics made the also called Graetz Bridge the most widely used line
commutated thyristor rectifier. The configuration does not need any special
transformer, and works as a six-pulse rectifier. The series characteristic of
this rectifier produces a dc voltage twice the value of the half-wave
rectifier. The load average voltage is given by:
Where VMAX is the peak phase-to-neutral voltage at
the secondary transformer terminals, Vf-N
rms its rms value, and V f-f sec the
rms phase-to-phase
secondary voltage, at the valve terminals of the rectifier.
The figure shows the voltages of
each half wave bridge of this topology, VD
pos and VD neg, the total
instantaneous dc voltage VD, and the anode-to-cathode
voltage vAK
in
one of the bridge
thyristors.
The maximum value of VAK is Ö3·VMAX, which is the
same as of the half-wave converter, and the interphase
transformer rectifier. The double star rectifier presents a maximum anode-to-cathode
voltage of 2 times VMAX. The figure
shows the currents of the rectifier, which assumes that LD is large enough
to keep the dc current smooth. The example is for the same DY transformer
connection shown in the topology of figure. It can be noted that the secondary
currents do not carry any dc component, avoiding the overdesign of
windings, and
transformer
saturation. These two figures have been drawn for a firing angle a of approximately 30°. The perfect
symmetry of the currents in all windings and lines is one of the reasons why
this
rectifier
is the most popular in its type. The transformer rating in this case is:
As it can be noted, the
transformer only needs to be oversized 5%, and both, primary and secondary
windings have the same rating. Again, this value can be compared with the
previous rectifier transformers: 1.35PD for the half
wave rectifier, 1.55PD for the
six-pulse rectifier, and 1.26PD for the
interphase transformer rectifier. The Graetz Bridge makes an excellent use of
the power transformer.
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| Voltage Waveforms for the THREE-PHASE FULL-WAVE CONTROLLED RECTIFIER or, Graetz Bridge |
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| Current Wave forms for the THREE-PHASE FULL-WAVE CONTROLLED RECTIFIER or, Graetz Bridge |
DOUBLE STAR CONTROLLED RECTIFIER WITH INTERPHASE CONNECTION
This topology works as two half-wave
rectifiers in parallel, and is very useful when high dc current is required. An
optimal way to do that, reaching a good balance and at the same time harmonic
elimination, is through the connection shown in figure. The two rectifiers are shifted
by 180°, and their secondary neutrals have been connected through a middle-point
auto transformer, called “interphase transformer”. The interphase transformer is
connected between the two secondary neutrals, and the middle point at the load
return. In this way, both groups operate in parallel. Half the direct current
flows in each half of the interphase transformer, and then its iron core does
not become saturated. The potential of each neutral can oscillate
independently, generating an almost triangular voltage waveform (vT) in the
interphase transformer, as shown in figure. As this converter work like two
half-wave rectifiers connected in parallel, the load average voltage is the
same as shown in the equation,
where Vf-N rms is the
phase-to-neutral rms voltage at the valve side of the transformer
(secondary).
The figure also shows the two half-wave rectifier voltages, related to their respective neutrals. The voltage VD1 represents the potential between the common cathode connection and the neutral N1. The voltage VD2 is between the common cathode connection and N2. It can be seen that the two instantaneous voltages are shifted, giving as a result, a voltage VD smoother than VD1 and VD2.
The figure shows how VD, VD1, VD2, and VT change when the
firing angle changes from a=0° to a=180°.
The transformer rating in this case is:
and the average rating power will
be 1.26 PD, which is better
than the 1.35 for the half wave rectifier, and 1.55 for the six pulse rectifier.
Thus the transformer is well utilized.
SIX-PULSE OR DOUBLE STAR CONTROLLED RECTIFIER
The thyristor side windings of the transformer shown in figure form a six-phase system, resulting in a six-pulse star point (midpoint connection). Disregarding commutation overlap, each valve only conducts during 60° per period. The direct voltage is higher than that from the half wave rectifier, and its average value is given by,
The dc voltage ripple is also smaller than the one generated by the half wave rectifier, owing to the absence of the third harmonic with its inherently high amplitude. The smoothing reactor L D is also considerably smaller than the one needed for a three-pulse (half wave) rectifier.
The ac currents of the six-pulse rectifier are shown in figure. The currents in the secondary windings present a dc component, but the magnetic flux is compensated by the double star. As can be observed, only one valve is fired at a time, and then this connection in no way corresponds to a parallel connection. The currents inside the delta show a symmetrical waveform, with 60° conduction. Finally, due to the particular transformer connection shown in figure, the source currents also show a symmetrical waveform, but with 120° conduction.
THREE-PHASE HALF-WAVE CONTROLLED RECTIFIERS.
The three-phase half-wave rectifier topology is shown in the figure. To control the load voltage, the half wave rectifier uses three, common-cathode thyristor arrangement. In this figure, the power supply, and the transformer are assumed ideal. The thyristor will conduct (ON state), when the anode-to-cathode voltage VAK is positive, and a firing current pulse IG is applied to the gate terminal. Delaying the firing pulse by an angle α does the control of the load voltage. The firing angle α is measured from the crossing point between the phase supply voltages, as shown in figure. At that point, the anode-to-cathode thyristor voltage VAK begins to be positive. The figure shows that the possible range for gating delay is between α=0° and α=180°, but in real situations, because of commutation problems, the maximum firing angle is limited to around 160°. In figure, when the load is resistive, the current Id has the same waveform of the load voltage.
As the load becomes more and more inductive, the current flattens and finally becomes constant. The thyristor goes to the non-conducting condition (OFF state) when the following thyristor is switched ON, or the current, tries to reach a negative value.
With the help of figure shown, the load average voltage can be evaluated, and is given by:
where V MAX is the secondary phase-to-neutral peak voltage, V f-Nrms its rms value, and ω is the angular frequency of the mains power supply. It can be seen from equation that changing the firing angle α, the load average voltage VD is modified. When α is smaller than 90°, VD is positive, and when α becomes larger than 90°, the average dc voltage becomes negative. In such a case, the rectifier begins to work as an inverter, and the load needs to have the capability to generate power reversal by reversing its dc voltage.
The DC current waveforms under various type of load is illustrated here,
DC CURRENT WAVE FORM.
The ac currents of the half-wave rectifier are shown in figure. This drawing assumes that the dc current is constant (L D very large). Disregarding commutation overlap, each valve conducts during 120° per period. The secondary currents (thyristor currents also) present a dc component that is undesirable, and makes this rectifier not useful for high power applications. The primary currents show the same waveform, but with the dc component removed. This very distorted waveform requires an input filter to reduce harmonics contamination.
The currents waveforms shown in figure are useful for the design of the power transformer. Starting from:
VA prim and VA sec are the ratings of the transformer for the primary and secondary side respectively. P D is the power transferred to the dc side. The maximum power transfer is with α=0° (or α=180°). Then, to establish a relation between ac and dc voltages, equation for α=0° is required:
where a is the secondary to primary turn relation of the transformer. On the other hand, a relation
between the currents is also obtainable. With the help of figure,
between the currents is also obtainable. With the help of figure,
Combining equations, it yields:
DC CURRENT WAVE FORMS OF THREE PHASE HALF WAVE CONTROLLED RECTIFIER.
The DC current waveforms of three phase half wave controlled rectifier under various type of load is illustrated here,
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