IEEE Power Electronics Magazine - June 2022 - 56

coil terminal forms the output node, which is connected to
the critical load.
From varistor current, i var
Figure 10(b).
vv vv v
in=+ =+
ary turns, turns ratio, nn /nsp
If the transformer has np
=
varloadps
primary turns and ns
, then vnvsp
load voltage can be expressed as:
vv vnvv nv1
load=+ -= -- p
varvarpp
^h
(9)
second=
. Hence the
(10)
Because the design should lower the voltage seen by the
load to a voltage less than the varistor clamping voltage.
vv nv10
load12( -^hvar
p
Because v
p 02 for a positive transient,
nn
- 22( 1
10
(11)
(12)
To provide a complete analysis with design calculations
leading to a testable prototype of the SCASA is beyond the
scope of this paper. These details are available in [1, 12-16].
A summary useful for a designer is given below, based on
the preliminary prototypes build and the first commercial
product developed which will be discussed later.
i) The coupled inductor's secondary should work as a stepup
winding to lower the transient related voltage appearing
at the load.
1,200
1,000
800
600
400
200
and load current branches in
ii) Technique provides the unique advantage of transient
related load voltage will be less than the varistor's clamping
voltage.
iii) There is a limit to increasing the turns ratio, to prevent
transient load voltage reaching a negative value
iv) Coupled inductor's windings should not create any
excessive series inductance into the load loop at the
50 Hz line frequency.
v) Supercapacitor sub-circuit shown in Figure 10(c) assists
the dissipation of the surge energy by forming a closed
loop due to SC path, without any unwanted ringing
waveforms related to the transient surge energy.
For readers to gain more insight into this technique in
terms of theoretical concepts, Ref [10] is suggested.
Selection of Supercapacitor and the Magnetic Core
To achieve a practically useful SPD based on the SCASA
technique, correctly selecting the supercapacitor and the
core for the coupled inductor are the most important key
design decisions. At the early development stage of the technique,
research team considered using a film capacitor in
the order of tens of nano-Farads, instead of a SC. This was
to justify the value of using a small supercapacitor, to assist
the absorption and dissipation of the transient surge energy,
so that SPD's ability to survive without major degradation of
the MOVs, when subjected to repeated surges under UL
1449- 3rd Edition tests.
Figure 11 depicts the case where the SC is replaced by
Vvar
Vload
a similar can size 47 nF capacitor. Figure 11(a) shows the
experimental results and Figure 11(b) depicts the simulation
results. In both cases, it can be observed that the voltage
across the load creates a ringing effect with an approximate
frequency around 120 to 125 kHz.
Figure 12 shows the transient performance of the actual
24.3 µs
0102030
Time (µs)
(a)
3.5
2.5
1.5
0.5
-0.5
-1.5
-2.5
-3.5
40 50
SCASA circuit based on a 1 F SC and a resistor subjected
to C62.41 standard based surges, where peak voltages at
1 kV and 6.6 kV were supplied from a lightning surge simulator.
In both experimental and simulated results, it can
be observed that the transient related load voltage does
not create any ringing effects, as in the case of using film
capacitors in the sub-circuit.
This discussion indicates clearly that the supercapacitor
Vload
Vvar
25 µs
05 10 15 20 30 35 40 45 50
Time (µs)
(b)
25
FIG 11 Effect of sub-circuit capacitor on load voltage during
the surge (a) experimental results for 47 nF capacitor (b) simulation
results for 47 nF capacitor.
56 IEEE POWER ELECTRONICS MAGAZINE z June 2022
assisted new topology helps in the surge absorption, based
on a coupled inductor approach at the input end. In developing
this new topology, the coupled inductor acts as a loosely
coupled transformer, when the MOV enters into conducting
stage. Design of this coupled inductor was based on a permeance
(inductance per square turns) based approach, as
detailed in [10], [16]-[19]. Without a detailed discussion, at
the first stage of research and development, the research
team had a concluding result that powdered alloys work
effectively in the SCASA design while ferrite cores were
not effective. Due to high permeability/permeance of ferrites
(Figure 13), they cannot be used in their original form
for SCASA design. But, in the gapped form (single-gapped
or double-gapped), ferrites perform satisfactorily. More
Voltage (kV)
Voltage (V)
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