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2016 China International Conference on Electricity Distribution (CICED 2016)
Xi’an, 10-13 Aug, 2016
Impacts of the Frequency Effects on Partial Discharge
Characteristics of the High Frequency Power Transformer
Insulation
Han Shuai1, Wang Feng1, Bi Jiangang1, Zhang Bowen1, Li Qingmin2
1.China Electric Power Research Institute
2.North China Electric Power University
Abstract—The High-frequency power transformer
(HFPT) is a new type of intelligent distribution
equipment. It is easy to fail under high frequency
voltage, the main cause of which is partial discharge
(PD). A comprehensive test system is established to
study the impacts of the frequency on PD
characteristics. The PD inception voltage (PDIV), PD
times, and PD mean amplitude of polyimide film are
measured under the frequency 1~50 kHz. The adaptive
algorithm is established to decompose and reduce the
noise so as to acquire the pure PD signal and achieve
the effective characteristics. Also the insulation
lifetime is recorded to illustrate the relationship
between PD and aging state. Furthermore, the
frequency-dependent characteristics of several typical
PD parameters are elucidated in details using the
method of equivalent microcircuit which can explain
the PD activities under high frequency voltage. The
experimental results show that with the increase of
frequency, PD mean amplitude and times of polyimide
film see downward trends after first increase while the
PDIV keeps roughly constant with frequency variation.
Through the analysis, the frequency-induced space
charge accumulation effect plays a dominant role in
the above physical changes under high frequency,
whereas, the frequency stress changes the insulation
permeability as well as the tangential E-field vector,
which executes a prominent impact on the PD
development under low frequency. It is worth noting
that the lifetime is greatly shortened as the frequency
increase which is different form the trend of PD
characters, showing that there is no direct relationship
between PD and the insulation lifetime.
Index Terms— high-frequency power transformer,
partial discharge, insulation system, space charge,
frequency effect
I. INTRODUCTION
Intelligent high voltage electrical equipment in
substation is an important physical basis for the
realization of smart grid which not only relies on the
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intelligent monitoring of running state of electrical
equipment but also the intelligent operation and
control of electrical equipment itself [1]. With the
development of semiconductor devices and power
electronic technology, a new type of intelligent
substation equipment is coming into the
world—High-frequency Transformer (HFT) or Power
electronic transformer (PET), resolving most of the
problems and drawbacks that traditional transformer
has (e.g. large volume and high loss factor). Moreover,
PET has realized the flexible control over energy to
meet the various needs of power grid and proved to be
the most intelligent transformer fundamentally [2].
The HFT usually works under sinusoidal voltage with
the frequency 1-50 kHz. Despite the advantages
illustrated above, the insulation system of HFT which
is composed of polymer and air is fragile under high
frequency voltage [3]. Partial discharge (PD) is
considered to be the dominant factor to the insulation
failure under strong electrical press in all power
transformers. PD properties under power frequency
and square-wave voltage are widely studied. The PD
pulses are more likely to emerge on the rising edge of
the voltage whereas few on the falling edge in positive
or negative respectively [4]. The PD times and
amplitude remain substantially constant with
frequency under square wave voltage [5]. However,
the unique characteristics of HF sinusoidal voltage is
obvious which have sharper rising edge than the
normal power frequency voltage, no platform area
compared with square wave voltage, and much higher
frequency than those waveforms. Therefore, the
impact as well as the mechanism of the high frequency
sinusoidal voltage should be further explored.
In this paper, frequency based experiment ranging
from 1 kHz to 50 kHz is carried out. PD times, and
mean PD amplitude of polyimide film are measured.
Several frequency-dependent characteristics of PD are
elucidated in details using the method of equivalent
microcircuit in order to find out their intrinsic
relationship from the physics point of view.
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2016 China International Conference on Electricity Distribution (CICED 2016)
II. Experimental Setup
The schematic diagram of PD mesure system is shown
in Fig. 1. The adopted HF voltage source can generate
the sinusoidal voltage (0-15kV) with frequency 1 k- 50
kHz. Stick-plate electode according to IEC 60343 is
used to model the field disitrabution of the real
insulation system. The photomultiplier (PMT) is
employed to detect the emitted light so as to
standardize the PD inception voltage (PDIV).
Considering the unique characteristics of high
frequency PD, ETS-93686 high frequency current
transducer (HFCT) is adopted to detect PD pulses.
Polyimide films (PI) of 75 μm are chosen as the
insulation samples, and they are all cleaned by ethanol
and dried in 60℃ for 10 hours before the test. The test
is carried out under the standard atmospheric condition
(20 ℃, 0.1 MPa ).
In this test, the voltage is increased rapidly until the PD
happens and then it will be set to zero. Such step is
repeated 5 times to obtain the PDIV. The PD test
voltage is chosen to be 2kV which is the 1.5 times of
the PDIV, and the voltage is applied until the
breakdown happens.
Xi’an, 10-13 Aug, 2016
The process of PD signal extraction is shown in
Figure 2. Figure 2(a) displays the original signal in
which many interfering signals exist including the
environmental noise. Firstly, the adaptive threshold
method is used to remove the noise signal in the
original, and the waveform can be obtained by
reconstructing the remaining signal, as is shown in
Figure 2(b). To prove the effectiveness of the
de-noising method, the partial discharge signal
energy is calculated. The energy ratio between the
de-noised and the original signal is 93.56%, which
means that fine filtering effect can be achieved
without distorting the partial discharge signal. Then
according to the principle that the bipolar is
discharge and the unipolar is interference, the real
discharge signal can be acquired by excluding the
unipolar pulse interference from the reconstructed
signal, and the waveform of the result is shown in
Fig. 2(c). Finally, for the reason that a complete
discharge is an oscillation, the phase and amplitude
of partial discharge signal cannot be directly
obtained from the signal. As a result, the
open-window method by separating the different
discharge signals and selecting the maximum in
every oscillation is used to process the real discharge
signal. The processed result is shown in Figure 2(d)
and the final patterns of PD pulses can be seen in Fig.
3.
III. Experimental Result
Fig. 1. Schematic diagram of test system
Fig. 4 shows that when the frequency is below 10 kHz,
the PD times will see an increasing trend at first.
During the period 10~50 kHz, the PD times decrease
slightly and it end up with 1400 which is close to the 6
kHz point. The similar situation also takes place to the
curve of PD mean amplitude which is shown in Fig. 5,
however, it is not a monotone decrease until the
frequency reaches the 30 kHz (0.19V) and then it rises
slightly. Both these two curves reach their maximum
values at about 8~ 10 kHz and that is also the critical
point between HF and LF in the traditional sense. As
can be seen in Fig. 6, the PDIV keeps roughly constant
with frequency variation which is around 1.3kV.
The trends of the phases of the pulses in the test are
exhibited in Fig.7. It can be seen that the phases of both
the positive and negative PD pulses decline with the
frequency rises and their tendencies are basically the
same. It is worth to notice that the minimum value of
both the positive and negative PD phase is below
0°when the frequency is above 30kHz, and such
phenomena cannot be seen under power frequency
atmosphere.
Fig. 2. The extraction of PD Signals
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2016 China International Conference on Electricity Distribution (CICED 2016)
0.5
0
-0.5
-1
0
100
200
300
Phase /（ deg）
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
-2.5
0
(a) 1 kHz
250
Possitive pulse
Negative pulse
200
Phase of the PD pulse / °
PD Amplitude /（ V）
PD Amplitude /（ V）
1
Xi’an, 10-13 Aug, 2016
100
200
300
Phase /（ deg）
150
100
50
0
(b) 10 kHz
-50
Fig. 3. PD patterns at different frenquencies
0
10
20
30
Frequency / kHz
40
50
Fig. 7. Trend of the PD pulse Phase
1700
1600
IV. Analysis and Discussions
PD Times
1500
1400
1300
1200
1100
0
10
20
30
Frequency / kHz
40
50
Fig. 4. PD times under different frenquencies
The whole insulation-electrode system can be modeled
as an equivalent circuit (Figure 8). Cd and Cg are the
dielectric and air gap equivalent capacitor respectively.
Before the breakdown occurs, the two capacitors are in
series. When the air gap breakdown, the switch will be
closed and Cg will be in parallel with the plasma
resistance which varies with time. Obviously, the
voltage in the air gap Ua(t) can be illustrated by
equation (1).
0.21
(1)
PD mean amplitude
0.2
0.19
0.18
0.17
0.16
0.15
Cd
Ud
0
10
20
30
Frequency / kHz
40
50
Id
Ua
Gap Breakdown
Fig. 5. PD mean amplitude under different frenquencies
Ug
Rs
Cg
Fig.8 The equivalent microcircuit for partial discharge of the
gas-solid insulation
Fig. 6. Behavior of PDIV value with frequency
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Wherein, Ua(t) and Ud(t) are the applied voltage and
the voltage applied on the insulation dielectric. As the
field taken by air gap is much higher than that of
dielectric, the gap will breakdown first when the
voltage reach the certain value Ui. After the
breakdown event, Ug(t) will decrease dramatically
below Ui until the PD current extinguishes. Because of
the existing plasma resistance Rs, the charges in
capacitor will continue to decline, finally leaving a
residual voltage Ures which will hinder the PD when
the voltage polarity gets reversed [6]. Meanwhile, the
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2016 China International Conference on Electricity Distribution (CICED 2016)
current Id increases rapidly, charging the insulation
dielectric capacitor, and increases the charges
accumulating the Cd. During the discharge process, the
air gap voltage Ug(t) can be expressed by the equation
(2):
(2)
Owing to the space charge effect, once the polarity
gets reversed, the space charge is still captured in deep
traps of the PI film, making the
stay at a higher
value. When the applied voltage
decreases to a
value low enough, air gap breakdown will occur by the
voltage
. The higher the frequency is, the
smaller charge dissipation time will be, and also the
larger
will become because of the unchanged
constant . That is why the phases change with the
frequency and fall below 0° in Fig. 7.
When the frequency is below 10 kHz, the residual
space charge in the air gap will reduce to a low value
as the cycle is long enough for the dissipation of
charge. The frequency changes the insulation
permeability as well as the tangential E-field vector [7],
so that the PD can easily take place in larger area. The
compression of energy density of the applied voltage
together with the enlarged discharge area will make
the PD times and PD mean amplitude rise in lower
frequency. However, when the frequency becomes
higher, charge dissipation time t will be shortened in
half cycle and Ua(t)+Ud(t)-Ug(t) will become smaller,
leading to the reduce of PD parameters. Limited by the
applied voltage and charge density that the electrons
emit, the enlarged PD area will no longer be in the
dominant place. Moreover, due to the constriction of
cycle caused by frequency increasing, the chance that
Ug attain Ui has been largely reduced, thus the PD
times decrease.
In order to better study the properties of PD, statistics
of the average lifetime of the samples are obtained, see
TABLE I. It is obvious that samples under lower
frequency have longer lifetime which is different from
the trend of PD parameters. As a result, the main cause
of aging will not be PD, but the diametrical damage
caused by the high frequency effect.
In order to study the potential relationship between PD
characteristic and frequency, 1-50 kHz high frequency
voltage and a series of signal extraction methods are
chosen for the experimental arrangements.
The results indicate that with the increase of frequency
PD both the amplitude and times of PI film see
downward trends after first increase in the frequency
range of 1~50 kHz while the PDIV keeps roughly
1.3kV. Through the analysis, the frequency-induced
space charge accumulation effect plays a dominant
role in the above physical changes, in the meanwhile,
the frequency stress changes the insulation
permeability as well as the tangential E-field vector,
which executes a prominent impact on the PD
development. It is worth noting that the lifetime is
greatly shortened as the frequency increase which is
different from the trend of PD characters, showing that
PD is not the main cause of aging, but the diametrical
damage caused by the frequency effect. The research
above can provide theoretical and technological basis
for the development of the high-frequency power
transformer to be equipped in smart grid, which
presents academic significance and application
prospects.
ACKNOWLEDGMENT
The authors would here convey their appreciation for
the support by the technology project of the State Grid
Corporation of China (GY71-15-045).
REFERENCES
[1]
[2]
[3]
[4]
TABLE I. RELATIONSHIP BETWEEN LIFETIME AND
FREQUENCY
Frequency
Lifetime
1 kHz
16121s
5 kHz
5090s
10
30
50
kHz
kHz
kHz
2805s
1632s
1214s
[5]
[6]
[7]
V.
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Paper No CP0146
Xi’an, 10-13 Aug, 2016
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