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Experimental Investigation on Humidity Effects on
the Variations of Positive DC Corona Discharge
Xu Zhang, Xingming Bian, Xiang Cui, Tiebing Lu, Haibing Li, Qinyuan Li, Yijia Zhu
State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources
North China Electric Power University
Beijing, P. R. China
[email protected]
Abstract—In order to study the effect of humidity on the
variations of positive DC corona discharge, an artificial climate
chamber was developed to perform corona experiments. Based
on the testing system, the photon counting rate, the total electric
field strength and the ion current density on the ground level
were measured by the UV imaging detector, the rotating DC
electric field mill and the Wilson plate respectively. The relatively
humidity during the experiments was kept within the range 40%
to 90%. Moreover, the relationship between the calibration
coefficient of the field mill and relatively humidity was also
investigated. Experimental results showed that the photon
counting rate and the total electric field increased with the raise
of relative humidity, while the ion current density and the
calibration coefficient decreased with the raise of relative
humidity.
Keywords—humidity; DC corona discharge; artificial climate
chamber; field mill; calibration coefficient
I. INTRODUCTION
Corona effects and its related electromagnetic environment
on the operated conductors are important problems for high
voltage transmission lines [1-2]. The climate environment is
quite complex in China, HVDC transmission lines may go
through many high relative humidity areas, consequently, the
corona performance of HVDC transmission lines will be
greatly affected [3-4].
The relationship between weather conditions and the
corona performance of transmission lines has in particular been
the subject of considerable work by both physicists and
engineers. Calva investigated the effect of humidity on both
positive and negative corona V-I characteristics at low air
pressure conditions, and noted that the effect of humidity
became greater as the pressure decreased [5]. Abdel-Salam
proposed a mathematical model to calculate the corona
inception voltage and corona current in humid air. It was found
that the photoelectrons distribution within the ionization layer
played an important role in deciding whether the inception
voltage increased or decreased with relative humidity [6].
Xingming Bian studied the variation of dc corona inception
voltages with air pressure and humidity, using a point-plane
electrode system. The mechanisms of the influence of the
photo-absorption coefficient and Townsend’s second
coefficient on the negative and positive corona were also
considered [7-8].
In the present work, an artificial climate chamber was
developed to study the effect of humidity on the variations of
positive DC corona discharge. The relationship between the
calibration coefficient of the field mill and relative humidity
was obtained. The effects on the photon counting rate, the total
electric field strength and the ion current density among
different relative humidity were measured and analyzed. The
charging properties of the suspended water particle in the
ionized field were also investigated to account for the
experimental phenomena.
II. EXPERIMENTAL PLATFORM AND MEASUREMENT
A. Experimental Platform
The experimental platform is shown in Fig. 1. The
experiments were made indoors in an artificial climate
chamber, which consisted of three portions The temperature in
the test was kept within the range from 4℃ to 7℃, and the air
pressure was 101.1kPa.
Fig. 1. General view of the artificial climate chamber used in the experiment.
The first portion was the wire-to-plane electrode. A 2.4 mlength stainless steel cylinder wire with the diameter of 5.0 mm
was used as the conductor, which was placed above a grounded
aluminum plate. Both of the ends of the conductors were
interlinked with shielding metal spheres in order to prevent
point discharge. The distance between the conductor and the
plate was 0.37 m. The conductor was connected with a direct
current high voltage power supply with the output voltage of U.
The HV power supply employed in the experiment was typed
as Matsusada AU-120 and the output voltage was stable to
within 0.1%.
The DayCor superb UV imaging detector made by Ofil
Corporation was utilized to measure the photon counting rate,
which gave images based on visible light together with
superimposed images based on the ultraviolet light from
corona [10].
The second portion was the relative humidity control
apparatus. The water vapor was generated by a humidifier,
which was diffused evenly in the chamber. The relative
humidity was measured by a precise hygrometer with an error
less than ±3%. The third portion was a cuboid Plexiglas cover
with the dimension 2.0 m × 1.0 m × 1.0 m.
The ion current plate, is also known as Wilson plate, which
is a thin copper-clad printed circuit board with the dimension
26.7 cm × 16.8 cm and a sampling resistor 1.0 MΩ, mounted
on the surface of the ground plate [9]. A 1.0cm-wide grounded
guard band is on the ion current plate to reduce the fringing
field effects. The voltage of the resistor is measured to obtain
the ion current density.
B. Calibration Experiment of The Field Mill Under Different
Relative Humidity
Rotating electric field meter is a shutter-type field mill,
which has a sensing electrode that is periodically exposed and
shielded from the external electric field by the grounded
rotating shutter [9]. The field strength can be determined by
measurement of the induced charge, current, or voltage across
an impedance that is located between the sensing electrode and
ground.
With the presence of high relative humidity, the
measurement accuracy of the field mill may be affected.
Therefore, the field mill should be calibrated under different
relative humidity before the corona discharge experiment. The
calibration platform of the field mill is shown in Fig. 2. The
uniform electrostatic field was generated by two parallel
circular plate electrodes. Between the two electrodes the
potential difference was U0 and the distance was d.
The field mill is used to measure the electric field strength
on the ground level, the measurement principle is discussed
detail above.
III. THE EXPERIMENT RESULTS AND ANALYSIS
A. The Calibration Coefficient of The Field Mill
According to the calibration principle of the field mill, the
calibration coefficient C is defined as:
C
Ereal
U
 0
Etest dEtest
(1)
where U0 denotes the applied voltage, d is the distance between
the two parallel electrode and Etest denotes the measured
electric field strength from the field mill.
The relationship between the calibration coefficient C and
relative humidity are shown in Fig. 3 and can be described by
the Eq. (2) with a maximum discrepancy of 1%.
C  15.73e RH /20.31  1.22
(2)
The original measurement value of the field mill was
affected by the relative humidity, so the calibration coefficient
varied with the relative humidity. The higher relative humidity
was, the lower calibration coefficient of the field mill would be.
Fig. 2. Schematic view of the calibration platform for the field mill.
C. Measurement of Positive DC Corona Discharge
In this paper, the DC voltages applied to the conductors
were raised from 0 kV to 100 kV until the corona discharge
was very fierce. The photon counting rate, the electric field
strength and the ion current density at the ground level were
measured by the UV imaging detector, the rotating DC electric
field mill and the Wilson plate respectively.
Fig. 3. The relationship between the calibration coefficient C and RH.
B. Variations of Positive DC Corona Discharge Under
Different Relative Humidity
The photon count is the number of photons per second
averaged over the past minute and represents the intensity of
the corona discharge [10].
humidity is higher than that under low relative humidity as
intensity of corona discharge turns to be stronger. Furthermore,
the corona inception voltages of the conductors decrease with
the increase of the relative humidity.
Fig. 4 shows the positive DC corona discharge images of
the conductors under different relative humidity at the same
applied voltage 90kV.
As can be seen, with the increase of the relative humidity
from 45% to 85%, the photon counting rate increases rapidly
from 10231 to 14273.
Fig. 5. Variation of the photon counting rate under different relative humidity.
Fig. 6 shows the effect of relative humidity on the total
electric field strength on the ground level. Three different
relative humidity were investigated in this experiment: 45%,
65% and 85%. Under a certain applied voltage, the total
electric field with the presence of the high relative humidity is
higher than that with low relative humidity. For instance, when
the applied voltage is 90 kV and the relative increases from
45% to 65% and to 85%, the total electric field strength on the
ground level increases from 160 kV/m to 184 kV/m (increasing
by 24%) and to 197 kV/m (increasing by 37%).
Fig. 6. Variation of the total electric field under different relative humidity.
Fig. 4. Corona discharge images of the conductors under different relative
humidity conditions at U=90kV.
The number of photons released from corona discharge on
the conductors under three different relative humidity is
compared in Fig. 5.
It is obvious that under the same applied voltage, the
photon counting rate of the conductors under high relative
Fig. 7 shows the effect of relative humidity on the ion
current density on the ground level. It is obvious that under the
same applied voltage, the ion current density decreases with
the increase of the relative humidity. For instance, when the
applied voltage is 90 kV and the relative increases from 45% to
65% and to 85%, the ion current density on the ground level
decreases from 4.17 μA/m2 to 3.60 μA/m2 (decreasing by 14%)
and to 3.01 μA/m2 (decreasing by 28%).
Then the mobility kw of the charged water particle can be
obtained, which is an important parameter to describe the
motion of the charged particles in the electric field [14].
kw 
Fig. 7. Variation of the ion current density under different relative humidity.
vt 2aE  0 r

E
 r  2
(6)
The mobility of the charged water particle is calculated
with different particle radius for the electric field equals 20
kV/m, 30 kV/m and 40 kV/m. As is shown in Fig. 8, the
mobility of the charged water particles increases linearly with
the diameter. The magnitude order of the charged water
particle’s mobility is about 10-8 to 10-7, which is 3 to 4 orders
smaller than the ionic mobility 10-4 [15]. Thus the effective
ions’ mobility is lowered, more space charges suspend in the
air and less space charges move to the ground electrode,
consequently, the total electric field increases, while the ion
current density decreases.
C. Discussion
When the electric field near the surface of the conductor
exceeds the corona inception value, corona discharge will
occur and space charges will be generated [11]. Under this case,
the suspended water particles will be charged. With the
diameter larger than 0.2μm, the water particles are mainly
dominated by field charging [12].
By assuming that the water particle is a homogeneous
dielectric sphere and is mainly charged by field charging.
Consider that the water particle obtains the saturation charge
amount q in the field as shown in Eq. (3) [13].
q =12πa 2 E
ε 0ε r
εr  2
(3)
The Coulomb force Fq acted on the water particle can be
written as Eq. (4).
εε
Fq  Eq =12πa E 0 r
εr  2
2
2
(4)
IV. CONCLUSION
where E is the external field, a is the water particle radius, ε0 is
the permittivity of free space and εr is the relative permittivity
of the water particle.
During the water’s motion in the electric field, the water
particle is subjected to Coulomb force Fq and Stokes drag
Fs=6πaηv. The gravitational force is ignored, which is much
smaller than Coulomb force under certain conditions [13].
Where η is the viscosity of the air and v is the water particle
velocity through the air.
When the Coulomb force equals the Stokes drag, the
particle reaches the dynamic equilibrium with a terminal
velocity vt written as Eq. (5).
vt 
2aE 2  0 r
 r  2
Fig. 8. Variation of the mobility of the charged water particle under different
particle radius.
Based on the experiments and calculations, the following
conclusions may be drawn.
1) The calibration coefficient of the field mill varies with
the relative humidity. The higher relative humidity is, the lower
calibration coefficient will be.
2) With the presence of high relative humidity, the positive
DC corona discharge turns intensely, the photon counting rate
and the total electric field increases.
3) The effective ions’ mobility decreases with the raise of
relative humidity, so the ion current density on the ground level
decreases.
ACKNOWLEDGMENT
(5)
This work was supported by National Natural Science
Foundation of China (51377096), Science and Technology
Project of SGCC (GYB17201400185), Fok Ying-Tong
Education Foundation, China (Grant No.151058) and the
Fundamental Research Funds for the Central Universities
2016YQ001.
[8]
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