Chapter 3 Step-up/down Voltage Stabiliser for Voltage Optimisation
3.4 Implementation & Experiments Results
A prototype based on the proposed topology has been implemented and tested with closed
loop control. The parameter of main components is shown as Table 3-1. The cut-off
frequency of the applied LC filter is about 1/10 of the switching frequency to ensure the
unwanted high frequency harmonics being filtered out properly. The bidirectional switches
are made up of two HGTG5N120BND insulated gate bipolar transistors (IGBTs) from
Fairchild. Each pair of two IGBTs (21A, 1200V) is connected in emitter to emitter
configuration. The dead time of commutation is set as 1µs.
The input voltage, output voltage and output current are measured and logged at 100 kHz for
a resistive load step change scenario experiments in step down mode by using National
Instruments (NI) data acquisition system. Meanwhile, the chopped voltage on the primary
winding of the transformer and switching sequence are observed by using oscilloscope at 100
0 0.5 1 1.5 2 200 220 240 Time [s] V o lt a g e [ V ] 0 0.5 1 1.5 20 2 4 6 8 10 C u rr e n t [A ] Vin Vout Iout
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MHz in order to investigate the switching process of the proposed voltage stabiliser. The
implemented hardware along with the measurement connection is shown as Figure 3.23:
Figure 3.23 Picture of implemented hardware with measurement connection.
Figure 3.24 illustrates the chopped voltage on the primary winding. It can be observed that
this voltage is equal to either the input voltage in active state or tiny voltage level in
freewheeling state, which is in exact agreements with working principle.
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In addition, the turn-on resistance of IGBTs leads to some voltage drop on the switches
therefore the voltage on primary winding is not exactly zero in freewheeling state as it can be
also noticed in Figure 3.24.
The instantaneous input voltage, output voltage and output current are illustrated in Figure
3.25. The output voltage demonstrates a good sinusoidal shape and it is in phase with the
input voltage with slight reduced amplitude. As a resistive load is connected in this scenario,
no phase delay can be observed between the current waveform and voltage waveform in
Figure 3.25. When the load increases from 2.5A to 6A, the output voltage also keeps an
excellent sinusoidal shape and in phase with the input voltage.
Figure 3.25 Experiments results of input voltage, output voltage and current RMS values.
Figure 3.26 shows the RMS values of input voltage, output voltage and current in long time
period. It can be observed that the input voltage fluctuates slightly over time while the output
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the beginning, the output voltage is equal to the input voltage before the controller engaged.
When the controller starts to operate, the output voltage keep reducing for a while and then
reach 220V with a duty ratio 0.52 as shown in zoomed area of Figure 3.25. After the step
change of the load occurs, the output drops instantly and then the controller plays its role
again to raise the voltage up to 220V. As it shown in Figure 3.25, the duty ratio reduces to
0.47 to maintain the output voltage as the reference after the load increases.
26.0 on T s 0.52 D 23.6 on T s 0.47 D
Figure 3.26 Experiments results of input voltage, output voltage and current RMS values.
3.5 Summary
This chapter describes a topology of step up/down voltage stabiliser which combines a
conventional PWM AC converter with a multi-windings transformer to optimise voltage.
Compared with the conventional topology, this topology is capable of not only reducing the
output voltage level but also increasing the output whenever required. A switch S0 is
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mode. Furthermore, the connection of the transformer splits the total current into two
branches, namely main current loop with the load and the inner current loop with primary
winding of the transformer. Hence, only a fraction of total current flows through the IGBTs.
To achieve the same power rating as the conventional topology, the proposed voltage
stabiliser requires IGBTs with less current rating, which results in large reduction on
manufacturing cost and the size.
Meanwhile, the open loop mathematical model of the proposed voltage stabiliser is
established and then a control strategy based on PI-controller is designed for regulating the
output voltage level. The first step of controlling the proposed voltage stabiliser is to decide
the operating mode of it. Therefore, a sign determiner is applied into the control strategy.
With the operating mode decision mechanism, a PI controller is then introduced to eliminate
the error between the output voltage and the reference voltage. This close loop mathematical
model with the designed controller is validated with the circuit model through simulation in
MATLAB. The results demonstrate that the mathematical model is in excellent agreement
with the circuit model. At last, the close loop system is also simulated with different typed of
loads, which includes resistive, inductive and non-linear loads. The simulation results show
that the proposed voltage stabiliser along with the controller works properly with all types of
loads from 0A to around 10A
Finally, a prototype with 20A current rating and 220V voltage rating is implemented and then
tested with a step changed resistive load in step-down mode. Although the controller is tuned
slower than the simulation work in this real implementation, the results are still very
promising and it can be concluded that the proposed step up/down voltage stabiliser works
perfect both theoretically and experimentally and is an excellent variant for voltage regulation
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