ENGINEERING TRIPOS PART IIA
INGLIS ELECTRICAL LABORATORY
EXPERIMENT 3B 3
THYRISTOR CONVERTERS
OBJECTIVES
To gain an understanding of the basic features of controlled rectificationcircuits including
their operation in the inversion mode.
To accurately calculate the behaviour of such circuits, taking account of strayinductances
associated with the ac supply.
Note: This experiment may be adopted for your Full Technical writeup. Suggested extensions are
available from P.R. Palmer.
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Thyristor converters are used to control bi-directional power flow between ac and dc systems. They
find applications in motor drives and in power conversion between power systems with different
frequencies.
Power flow from ac to dc is known as rectification and can be achieved using uncontrolled diodes.
The voltage on the dc-side is then set by the amplitude of the voltage on the ac-side and the form of
the rectifier circuit (3-phase or single-phase, star or bridge etc.). Controlled dc-side voltage is possible
using thyristors in place of the diodes. The voltage is then controlled by synchronising the timing of
the firing signals applied to the gates of the thyristors and delaying their phase with respect to the
ac-side waveform. The ac power supply waveform enables the thyristors to switch off without any
auxiliary circuits in a manner known as line-commutation or natural commutation. Controlled
rectification is widely used for industrial and transportation motor drives and industrial heating.
Under some conditions, which are explored in this experiment, the dc-side voltage can be reversed and
the power made to flow from the dc to the ac-side. This condition is known as inversion. However,
current in the thyristors and dc-side can only flow in one direction, so if bi-directional current output is
needed, as in some motor drives, two converters must be used (or a reversing switch)
Power system interconnection requires two converters, one operating as a rectifier and the other as an
inverter, connected via a dc-link. Since the power is transmitted as dc, the two power systems need
not be synchronised. A cross-channel dc-link was built next to the Channel Tunnel. Voltages and
currents are high and achieved using series and parallel connection of the thyristors.
In the course of this lab, you will see the influence of strayinductances, and the use of filter circuits.
"!
2.1 The Thyristor Converter
The waveforms in thyristor circuits supplied at 50 Hz can be understood considering the four states of
thyristor operation:
1.
2.
3.
4.
Open circuit blocking mode;
Short circuit on-state (forward current only);
Conducting to blocking transition (if forward current falls to zero);
Blocking to conducting transition (if trigger pulse is applied and forward voltage is positive).
The waveforms of Fig. 1 apply to a three-phase bridge rectifier supplying a load in which the current is
continuous. The firing angle α = 30° is measured from a voltage crossover. Load voltage is indicated
by the difference between shaded lines in supply voltage waveforms. The load current Id is assumed
constant but its path through the thyristor changes depending on which one is in the conducting state.
Waveforms for other thyristor configurations and loads can be drawn given the supply voltage
waveforms and firing angles.
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2.2 AC. supply reactance
In Fig. 1, at each commutation instant, only two thyristors are involved. However, the current in the
supply lines cannot rise and fall in zero time due to inductance in the line, for example the leakage
reactance of a supply transformer. Thus commutation from one thyristor to the next takes a finite time.
As only two thyristors are involved each time, commutation is most easily illustrated using the
two-phase circuit as in Fig. 2(a). Fig. 2(b) shows the output voltage for constant load current and the
thyristor current waveforms; the angle of overlap (µ) is seen to represent the period during which both
thyristors are conducting. (Note that the current in a constant impedance load will not be constant, but
the current ripple can be reduced by using a series inductance and resistance whose time constant L/R
is large compared with the fundamental ripple period of the currentwaveform).
When T1 is fired e1 is positive with respect to e2, current i1 builds up and i2 decreases. The voltage e1-e2
forces current to build up in T1 and decrease in T2. The rates of change of i1 and –i2 are equal, so that
during overlap, the instantaneous output voltage e is the mean value of e1 and e2. (e = 0 during overlap
in the two-phase circuit with equal inductance in each line.)
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2.3 Calculations
For the 3-phase Bridge converter, the mean voltage across load is given by the integral of V from α to
α + π/3, less the voltage ’lost’ during overlap:
V dc =
3 2V
cos( ) −
3I dc l
(1)
Where V is the rms line voltage, ω the ac supply frequency and l the series stray inductance in one
phase of the supply.
The first term of Equation (1) shows that the dc voltage may be controlled by electronically setting α.
Equation (1) also shows that the effect of overlap is to introduce a term proportional to load current I d,
i.e. an output resistance term, into the equivalent circuit of the controlled rectifier (see Fig. 3).
Fig. 3
The value of l may be estimated using the angle of overlap µ measured at α = 30O, and the simple
equation:
V = l di
dt
(2)
Substituting known values,
I
V = e A − e B = 2l DCt
(3)
Where eA and eB are the instantaneous values of the two supply voltages at the moment of
commutation.
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2.4 Inversion
If load current remains constant, equation 1 shows that the equivalent source voltage of a two-phase
rectifier is negative for α > 90o. This implies power flow from the dc to ac side, since the current does
not (and cannot) reverse, and a generator is necessary on the dc side to maintain Id against the now
negative dc equivalent voltage at the converter output.
Fig. 4(b) shows the voltage and supply current waveforms for the circuit of Fig. 4(a) for α > 90o
(neglecting supply inductance). It shows that the fundamental component of supply has a phase angle
> 90o with respect to supply voltage, so that power is indeed being supplied to the ac side.
Similar arguments can be used for three (and higher) phase supplies.
3. Experiment
The Universal S.C.R. converter used in the experiment should be set for circuit 7 with only one
transformer secondary in circuit.
The connection to be investigated is that for a three-phase full-wave (bridge) rectifier, as shown in Fig.
1(a). Check the correct links are in place and the selectors set before proceeding.
N.B. To avoid connecting two points at different potential via the oscilloscope common or earth,
use only one voltage probe and the isolated current probe.
Following this advice allows you complete freedom to observe signals around the converter, as
isolation is provided by the converter transformers.
3.1 Rectification
The convertor should be connected to the smoothing reactor and load resistor as shown in Fig. 5. The
supply variac should be set to 100%.
Fig. 5
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Observe the converter output voltage and current on the oscilloscope. Set the delay angle to zero and
the rheostat to maximum and close the breaker.
It may be necessary initially to decrease the resistance to start the rectifier. With zero angle of delay
and approximately 4A output current, sketch the waveforms of
(1)
(2)
(3)
(4)
converter output voltage;
output current;
thyristor voltage
thyristor current.
Estimate the angle of overlap, and note the output voltage value. Set the angle of delay at 30o and
ajust the output current to 4A using the load resistor. Sketch the above waveforms, record the output
voltage and estimate the angle of overlap. Measure the transformer secondary line voltage V at your
variac setting on load using a DVM.
Compare the thyristor voltage waveshape and the output voltage magnitude with their theoretical
counterparts.
Calculate the output voltage for zero delay and 30 degrees delay, at 4A load current and compare with
the experimental measurements.
Questions
1.
2.
3.2
Why is the bridge convertor most suitable for h.v. dc application?
What prevents the rectifier from starting on inductive loads?
Inversion
Add the 75V, 4A power supply in series with the inductor and resistor, and make a note of the new
circuit. Keeping the variac at the same setting as before, set the angle of delay to 90o and switch on
the convertor. Switch on the dc supply and ascertain that the rheostat is fully in (50 Ω). Set the
power supply output to 75V and set the current limit to maximum. Close the breaker. Observe the
magnitude of the convertor output voltage as the delay angle approaches 180o. Return the delay angle
to 90o and cut out the rheostat (the current limit on the PSU will reduce the PSU voltage as
necessary).
Increase the delay to 150o and sketch the waveforms of thyristor voltage and transformer secondary
voltage and current. Observe the phase relationship between the latter (using zero crossing points).
Estimate the angle of overlap and record the converter output voltage and delay angle.
Compare the thyristor voltage waveshape with the theoretical waveshape.
Compare the actual and theoretical values of the converter output voltage.
Sketches should be of a reasonable size on the squared paper in your lab book. Some care with angles
and phase relationships at this stage helps when writing up. Also note the sense of the connections
used when measuring voltage. The sense of measured current waveforms is uncertain.
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4. Report
The report should contain your recorded results and the required comparisons with theoretical
predictions. Theoretical waveshapes and relationships for 3-phase converters can be found in the
reference textbooks. You may like to use the sinewave printout attached
Also prepare answers to the questions.
5. References
See for example:
N. Mohan, T.M. Undeland and W.P. Robbins; ’Power electronics, converters, applicationas and
design’,Wiley 1989. NT
Or
R.M. Davis, ’Power Diode and Thyristor Circuits’,Peter Peregrinus.
(pages 64-65 for 3-phase bridge circuit waveforms; pages 25-28 for angle of overlap in 2-phase
circuits; pages 122-123 for a description of inversion.)
P R Palmer, October 2002
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