IEEE Power Electronics Magazine - September 2022 - 19

whereas some ceramic capacitors show significant
decreases in capacitance. More studies on cryogenic
operations of different types of capacitors and inductors
with new core and insulation materials are still
needed [37], [38], [39].
Partial Discharge
Currently, for MEAs, both ac and dc power distribution
have been implemented. For example, Boeing 787 has
over 1 MVA of electric power distribution, both 115 V 400
Hz ac distribution and ±270 V dc distribution are utilized.
One of the main reasons that the dc voltage is limited to
±270 V is partial discharge at high altitude. Partial discharge
(PD) is a localized discharge that only partially
bridges the insulation between conductors under high
voltage stress. Corona at the surface of conductor or insulation
material and cavity discharge inside insulation
material are two common PD phenomena.
A. High Altitude
The Paschen's curve, as shown in Figure 3,
indicates that the breakdown of the air can
happen just slightly above 300 V when the
air pressure is low. Figure 4(a) shows a
motorette sample under tests in an altitude
chamber, which emulates a section of the
stator of an electric machine, being tested
in an altitude chamber. Figure 4(b) shows
the partial discharge inception voltage
(PDIV) and partial discharge extinction
voltage (PDEV) of the motorette at different
pressures. It can be seen that at 50 torr,
which is roughly the air pressure at 62 000
ft (18897.6 m) above the sea level, PDIV of
the motorrete occurs at 270 V. At high altitude,
reduced PDIV can be observed for
100000
busbars, connectors, aviation wires, and cables which are
all essential for power converters.
B. High-Speed Switching
With WBG devices, partial discharge becomes a more
pressing issue because of the higher switching speed. Studies
show that PDIV of all types of insulation structures will
decrease with decreased rise and fall time of applied voltages
[28], [29]. Figure 5 shows a SiC-based 2 kV 1 MVA integrated
modular motor drive (IMMD) and the detailed view
of a power electronics submodule.
As indicated in the figure, PD is more prone to happen
at surfaces of stacked PCB boards, external connection
terminals, edges of busbars and the triple points inside
power modules. As shown in Figure 6, to study the PD
of triple points, test coupons were designed and tested.
Test results show that at short pulses, shorter rise time
Breakdown Voltage vs. Pressure
(Air - 0.1 inch Gap)
10000
V
1000
100
1.00E-02 1.00E-01 1.00E+00 1.00E+01
Pressure - Torr
FIG 3 Paschen curve for the air [27].
1.00E+02 1.00E+03
Test results within Phase A
550
500
450
400
350
300
250
200
0 100 200 300
(a)
PDIV-Phase A
PDEV-Phase A
400
500 600 700
Pressure (Torr)
(b)
FIG 4 (a) A motorette being tested in an altitude chamber. (b) PDIV and PDEV of the sample at different air pressure (altitude).
September 2022 z IEEE POWER ELECTRONICS MAGAZINE 19
Breakdown Voltage
Voltage (V)

IEEE Power Electronics Magazine - September 2022

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - September 2022