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Electrical Power and Energy Systems 115 (2020) 105484
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The interface charge characteristics of oil-pressboard composite insulation and its impact on surface flashover under combined AC/DC voltages
T
⁎
Chunjia Gao, Bo Qi , Chengrong Li State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, 102206 Beijing, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oil-pressboard insulation Combined AC/DC voltage Interface charge characteristics Surface flashover Impacting mechanism
The interface charges of oil-pressboard insulation could distort the local electric field and even weaken the insulating strength of converter transformer. This paper employs the electrostatic capacitive probe to capture the accumulation characteristics of interface charges in oil-pressboard insulation with needle-plate electrode under AC, DC and combined AC/DC voltages. The research results indicate that, under AC voltage, the polarity of interface charge is the same to that of the instantaneous value of applied voltage, and density of interface charge remains constant with the AC voltage application prolonging. In the context of DC voltage, the density of negative interface charge is about 1.2–1.5 times of that of the positive one. Under AC/DC combined voltage, the time-dependent dynamic accumulating process of interface charge behaves similarly to the waveform of AC voltage, and the larger the amplitude of DC component in combined voltage, the weaker the fluctuation. The tests for the surface flashover of oil-pressboard insulation were also conducted, aiming at discovering the impacting mechanism of interface charge accumulation on flashover voltage. The surface flashover voltage of test model under negative DC superimposed AC voltage is 1.3 times higher than that of positive DC combined with AC voltage. The built interface polarization model made an explanation for the interface charge accumulation, and the impacting mechanism of interface charge on flashover voltage, namely the positive effect of homopolar interface charges, is also proposed.
1. Introduction As the high voltage direct current (HVDC) transmission technology develops rapidly, the converter transformer has been applied more and more widely. Oil-pressboard composite insulation, attributing to its electrical, mechanical and chemical merits, is applied as the main insulating structure of converter transformer, thus whose insulating strength is key to the safe and reliable operation of power system [1,2]. When it comes to the insulating properties of composite insulation structures under various voltage stresses, the characteristic of space/ interface charges, inevitably, have always been regarding as the hot research topics. From 1970s, there had appeared a variety of non-destructive measuring approaches for space/interface charge characteristic of insulation structure. As for the space charge characteristics of dielectric medium, the Pressure Wave Propagation method (PWP) and Pulsed Electroacoustic method (PEA) could play the main roles in measurement [3,4]. While in the concerned of surface/interface charge measurement techniques, the means of dust map, Pockels electro-optic effect and electrostatic capacitive probe are commonly utilized [5]. Dust map method can contribute to the qualitative observation and
⁎
analysis for field/charge distribution characteristics, but it failed to capture the quantitative results [6]. The Pockels electro-optic effect has the advantages of better accuracy and spatial resolution for charge measurement, but the existence of Pockels optical medium could exert a significant negative effect on the measurement results [7]. In comparison, the electrostatic capacitive probe method, which is based on the capacitance voltage dividing principles, could be adopted for the quantitative measurement of surface/interface charge without any interference to the surrounding dielectric medium [8,9]. Taking the advantage of electrostatic capacitive probe method in charge measurement, our former researches had been conducted to capture the characteristics of surface charge on some composite insulations under various voltage stresses, which indicated that the accumulation of surface charges could not only distort the electric field but also could even facilitate insulation failure [10–13]. Surface flashover is one of most serious insulating failures for the oil-pressboard insulation structures of converter transformer, which could create risks for the safe and stable operation of HVDC system. According to the complex structures of insulations in converter transformer, the tangential electric field strength along the insulation surface
Corresponding author. E-mail address: [email protected] (B. Qi).
https://doi.org/10.1016/j.ijepes.2019.105484 Received 21 April 2019; Received in revised form 4 July 2019; Accepted 12 August 2019 0142-0615/ © 2019 Published by Elsevier Ltd.
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could be larger than that normal to the surface, which could provide chances for charge accumulation and thus create risks for surface flashover. Certain studies have reported the relationship between the charge accumulation and surface flashover in some insulation structures. P. Leblanc, et al placed the research focus on the behaviour of interface charge accumulation at the pressboard/oil under DC voltage, and his achievement pointed out the potential effect of accumulated charges on the electrostatic hazard in power transformer [14]. Du, et al investigated the surface charge and flashover behaviour of oil-paper insulation under DC voltage at different operating temperatures, indicating there could accumulate more charges at higher temperature to participate in the procedure of flashover [15]. K. Kato’s studied revealed that the locations of charge accumulation could exert different effect on the impulse flashover characteristics of alumina dielectrics in vacuum [16]. The existing researches were mainly untaken under the DC or AC uniform electric field, neglecting one of the most common operating conditions of converter transformer, namely the combined AC/DC voltages. In addition, there still remains a gap in the studies for the accumulation mechanism of interface charge in oil-pressboard insulation and its impacting mechanism for surface flashover. In this paper, the accumulation characteristic of interface charges in oil-pressboard insulation under non-uniform electric field induced by AC voltage, DC voltage and combined AC/DC voltages were captured, and the impacting mechanism of which on surface flashover voltage was also proposed, aiming at filling in the research gap and providing meaningful references for the insulation design of oil-pressboard/paper insulating apparatus. This paper is organized as follows. Section 2 presents the details of experimental platform including the interface charge measurement platform, surface flashover test platform, test model and test cavity. Section 3 reports the accumulation characteristics of interface charge in oil-pressboard insulation under DC, AC and combined AC/DC voltages, and the influence of voltage amplitude, AC voltage phase and AC/DC ratio in combined voltage on interface charge accumulation was also carried out. Section 4 presents the surface flashover characteristics of test model under AC/DC combined voltages. Section 5 discusses the mechanism of interface charge accumulation and proposes the impacting mechanism of interface charge on surface flashover voltage.
Fig. 2. The schematic diagram of interface charge measurement unit.
interface charge accumulation points; Q represents the quantity of charges on the insulator surface underneath the probe, and σ stands for the interface charge density. From Fig. 1, the following equations could be obtained:
C1 C2 ⎞ Q = σA = U2 ⎛C3 + C1 + C2 ⎠ ⎝ ⎜
U1 =
⎟
C2 U2 C1 + C2
(1)
(2)
in which, A is the equivalent charge detection area of probe. The schematic diagram of interface charge measurement unit designed is this paper is illustrated in Fig. 2, and as it indicates that, there exits two layers of dielectric medium between the probe and interface charge, namely the oil and air. In consequence, the C2 is formed by connecting the equivalent capacitance Coil of the transformer oil layer and the equivalent capacitance Cair of the air layer, which could be obtained by the following Eq. (3).
C2 =
Coil Cair ε0 εoil εair A = Coil + Cair d2 εoil + d1 εair
(3)
in which, ε0, εoil and εair represent the permittivity of vacuum, oil and air respectively. Given that the C1 ≫ C2 ≫ C3, and combining the Eqs. (1) and (2), the following Eq. (4) could be obtained:
2. Experimental platform 2.1. Principle of interface charge measurement Fig. 1 illustrates the principle and equivalent circuit of electrostatic capacitance probe for interface charge measurement. In Fig. 1, C1 is the sum of capacitance of electrostatic capacitance probe and test circuit; C2 stands for the equivalent capacitance between the inductive surface of probe and interface charge; C3 represents the equivalent capacitance of interface charge relative to ground; U1 marks the output voltage of probe; U2 marks the equivalent voltage at the
σ≈
C1 U1 = MU1 A
(4)
in which, M marks the calibration coefficient, namely the coefficient correlation between the interface charge density and output voltage of probe. In order to clarify the reliability of the interface charge measurement unit and capture the coefficient M, the calibration method described in our former study was applied [11]. An Aluminum foil was adhered to the pressboard and applied with DC voltage with different magnitudes to simulate interface charge accumulation. The probe was vertically positioned 2.0 mm away from the foil to carry out the calibration result, as shown in Fig. 3. As the fitting line in Fig. 3 presents, the relationship between U1 and U2 could be expressed in the following Eq. (5):
U1 = 2.448 × 10−5 × U2 − 0.0042
(5)
The Schering Bridge method was applied to work out C1 = 129.000 pF. According to Eqs. (2) and (5), C2 could be worked out, namely C2 = 3.158 fF, and then the Eq. (3) makes its contribution to work out A = 1.001 mm2. Finally, the M in Eq. (4) could be obtained as 0.129 pC/(mm2·mV), and the relationship between interface charge density σ and probe output voltage U1 could be presented as follows.
Fig. 1. The principle and equivalent circuit of electrostatic capacitive probe. 2
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Fig. 3. Calibration curve. Fig. 5. Schematic diagram of the surface flashover test platform.
σ = 0.129 × U1 (pC/mm2)
(6) driving signal, judged by the input power frequency reference signal, can drive the high-voltage relay to cut off voltage at the selected instantaneous phase, which contributes to the research about the influence of the phase change of the AC voltage component on the characteristics of oil-pressboard interface charge. With a maximum allowable input voltage of 70 kV and an action delay of 15 ms of highvoltage relay, the cutting-off phase angle could be precisely managed within an error range of less than ± 5°. The apparent partial discharge magnitude of experimental platform could reach less than 5.0 pC when the test model was applied by the maximum test voltage.
As indicated from the calibration results, the charge resolution sensitivity and spatial resolution of designed interface charge measurement unit is 0.129 pC/(mm2·mV) and 1.0 mm2. The data acquisition device applied in this paper is Keithley2182A nano-voltmeter with an input impedance of 1015 Ω and a time constant of 1.07 × 105 s, which is much greater than the dissipation time constant of the interface charges. 2.2. Interface charge measurement platform Fig. 4 shows the structure diagram of developed interface charge measurement platform. The combined AC/DC voltage was applied to the test cavity in parallel. The DC voltage generator could supply steady DC voltage with the amplitude ranging from 0 to 200 kV, the ripple factor of less than 0.5% and the output voltage accuracy of more than 99.5%. A 400 MΩ/40 kΩ resistance voltage divider, which has a voltage ratio of 10,000:1 and a withstanding voltage of 200 kV DC, was applied for the DC voltage measurement. The AC voltage could be continuously adjusted from 0 to 120 kV, and it was connected in parallel with the DC voltage through the DC blocking capacitor C′3. The AC voltage component was measured by the DPO4034 oscilloscope via a capacitance voltage divider with a voltage ratio of 1000:1 and a withstanding voltage of 120 kV AC. The phase control system contains of oscilloscope, AC voltage phase control/driving circuit and high-voltage relay. The
2.3. Surface flashover test platform The developed surface flashover test platform is illustrated in Fig. 5. The flashover signals were collected through the photomultiplier tube (PMT), while the voltage signals at the moment of flashover were also recorded in synchronism. The PMT light intensity signals were recorded by the DPO4034 oscilloscope, and which were then communicated to the computer for data storage and subsequent analysis. 2.4. Test model and test cavity Fig. 6 shows the diagram of test model and test cavity, in which the needle-plate electrode contributes to generate the non-uniform electric field. The tungsten needle electrode has a length of 10 mm and the end curvature radius of which is 50 μm, and it is tightly stick to the T4 laminated pressboard with the size of 80 mm × 70 mm and the thickness of 5 mm. The brass plate electrode, with a chamfered edge, has a
Fig. 4. The structure diagram of developed oil-pressboard interface charge measurement platform.
Fig. 6. The test model and test cavity. 3
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height of 50 mm, and the distance between the needle electrode and plate electrode is 15 mm. The T4 high-density pressboard applied in this paper comes from the insulation manufacturer of the converter transformer, and the test oil is the KI50X transformer oil. In alignment with IEC 60641-2:2004 [17], the pressboard was firstly dried in vacuum for 48 h under the temperature of 105 °C, and then immersed in oil for 24 h at the temperature of 85 °C and in vacuum. The water content in pressboard is controlled less than 1.0%. Before test, the oil in the test cavity was filtered to remove the impurities. After the standard processes, the water content of oil is around 7.0–10.0 μL/L with little difference among different models, which aligns to the specifications stated in the IEC 60422:2013 [18]. 3. Oil-pressboard interface charge characteristic 3.1. Under DC voltage
Fig. 8. The interface charge density under AC voltages.
In the interface charge measurement tests under DC voltage, the applied voltages are set as ± 4 kV, ± 6 kV, ± 8 kV and ± 10 kV for 30 min. The charge measuring point is located at 7.5 mm to the GND electrode and 2 mm to the interface. After the voltage cutting off, the probe was immediately moved to measuring point to obtain the interface charge density, and the same measuring procedure was conducted for five times, of which the average value was recorded as results, as shown in Fig. 7. It is observed from the results that the polarity of interface charge is the same to that of applied voltage, while the density of which is proportional to the magnitude of applied voltage. With the voltage application time prolonging, the density of charge tends to increase gradually. Multiple test results also indicate that, under the same test condition, the charge density on the interface oil-pressboard under negative DC voltage is 1.2–1.5 times of that under positive voltage. Ieda et al. [19] provided explanations for such a phenomenon. The insulation pressboard applied in transformers contains cellulose and lignin, both of which are contained with much hydroxyl (eOH). In addition, the lignin also includes aldehyde (eCHO) and carboxyl groups (eCOOH). The oxygen atoms within the hydroxyl groups have the second-largest electronegativity of any atom. For this reason, the electrons in hydrogen atoms are attracted to the oxygen atoms, polarizing the oxygen atoms to be negative and the hydrogen atoms to be positive. The electropositive hydrogen atoms on the pressboard surface then absorbs the negative charges in oil, which results in the more negative interface charge accumulation.
(Root Mean Square) values of AC voltages are selected as 4 kVrms (5.66 kVpeak) and 8 kVrms (11.31 kVpeak) at the frequency of 50 Hz, and the voltage application time is set as 5 s, 10 s, 30 s, 60 s, 600 s, and 1800 s respectively. The voltages were cut off at the wave-peak phase of 85–95° and the wave-trough phase of 265–275° respectively, and then the oil-pressboard interface charge density was captured. The average values of five repetitive tests under AC voltages were demonstrated in Fig. 8. As indicated from Fig. 8 that there could still witness the charge accumulation on the oil-pressboard interface under AC voltages. The polarity of the charges is the same to the instantaneous polarity of cutoff AC voltage and the density of which is in relation with the instantaneous values of the AC charges but it remains constant with voltage application duration prolonging. Fig. 8 also tells that the positive charge density at the voltage peak is slightly larger than that of negative ones at the voltage trough, but such phenomenon is different from that under DC voltages which witnesses larger density of negative charges than positive ones. This is due to the fact that, in multilayer insulation structure, the distribution of AC electric field is related to dielectric constant whilst the conductivity of dielectric donates in the distribution features of DC electric field. The dielectric constant of the transformer oil is about half of that of the pressboards whereas whose conductivity is about 10–1000 times of the pressboard [20], in consequence, AC voltage is mainly endured in the transformer oil. Besides, in the oil-pressboard insulation, the cellulose pressboard tends to accumulate negative charges and the transformer oil are more likely to absorb positive charges [21].
3.2. Under AC voltage
3.3. Under AC/DC combined voltage
For the interface charge measurement under AC voltages, the RMS
3.3.1. Accumulation characteristic During the test under AC/DC combined voltages, the voltage ratios of AC and DC voltage components are marked as 1:1 and 1:5 respectively, the voltage amplitudes are set as ± 9.65 kV (4 kVrms AC/ ± 4 kV DC) and ± 8.55 kV (1.33 kVrms AC/ ± 6.67 kV DC) respectively, and the voltage application time is recorded as 30 min. The charge measuring point is located at 7.5 mm to the GND electrode and 2 mm to the interface. The accumulation characteristics of interface charges when the applied voltages are cut off at wave-peak and wave trough are presented in Figs. 9 and 10. Figs. 9 and 10 tell that, there exists great difference between the interface charge density at the wave-peak and wave-trough of combined voltages, which could be attributed to the strengthening effect of AC voltage peak and weakening effect of its valley on the combined electric field. Comparing the interface charge density curves under the same polarity voltages in Figs. 9 and 10, it can be found that the instantaneous value of interface charge density at voltage trough and
Fig. 7. The oil-pressboard interface charge density under different DC voltages. 4
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Fig. 9. The interface charge density under 1:1 AC/DC combined voltage. Fig. 11. The interface charge density at different voltage phase of ± 4 kVrms AC/DC combined voltage.
Fig. 10. The interface charge density under 1:5 AC/DC combined voltage.
peak time of combined voltage with the AC/DC ratio of 1:1 differs by about 12 pC/mm2, while when the ratio comes to 1:5, the difference is only about 4 pC/mm2.
Fig. 12. The interface charge density at different voltage phase of ± 8 kVrms AC/DC combined voltage.
3.3.2. Dependence of AC voltage phase on interface charge accumulation characteristics In order to carry out the relationship between AC voltage phase and interface charge accumulation characteristics, the test models were applied with positive and negative combined voltages with the AC/DC ratios of 1:1, 1:3 and 1:5, the amplitudes of voltages combing DC value with AC RMS value in different ratios are set as ± 4 kV and ± 8 kV, the details of experiment voltage conditions are listed in Table 1. After the polarization time of 2 min, the voltages were cut off at the phases of 0°, 30°, 90°, 150°, 180°, 210°, 270°, 330° and 360° to capture the instantaneous value of the charge density at the oil-pressboard interface. The charge measuring point is located at 7.5 mm to the GND
electrode and 2 mm to the interface. Each test was conducted for five times, of which the mean value was obtained. The results are shown in Figs. 11 and 12. From Figs. 11 and 12, it is observed that the dynamic process of interface charge appears as a fluctuation which behaves similarly to the waveform of power frequency AC voltage, and the larger the DC proportion in the combined voltage, the smaller the fluctuation. Comparing the experimental results in Figs. 7, 8 and 12, it could be found that, there could witness −0.61 pC/mm2 interface charge accumulation under −8 kVrms AC/DC combined voltage (1:1) when cutting off voltage at wave peak. In Fig. 7, the charge density under −4 kV DC voltage is −2.51 pC/mm2, while in Fig. 8, that under 4 kVrms AC voltage (cutting off at wave-peak) is 6.36 pC/mm2, and the sum of them is about 3.85 pC/mm2, which is different from the result (−0.61 pC/ mm2) in Fig. 12. Due to our former study, the negative interface charges are more likely to accumulate but more difficult to dissipate than the positive ones [11,22], in consequence, under the positive cycles of AC voltage, there is no enough time for the accumulated negative charges to dissipate. In addition, under DC voltage, the density of negative interface charge is bigger than that of positive one. Ultimately, there exists three kinds of interface charges under the AC/DC combined voltages: (1) negative interface charges under DC voltage; (2)
Table 1 Experiment conditions. AC/DC combined voltage ratio
DC component amplitude/kV
AC component RMS value/kV
AC component peak value/kV
1:1 1:1 1:3 1:3 1:5 1:5
±2 ±4 ±3 ±6 ± 3.33 ± 6.67
2 4 1 2 0.67 1.33
2.83 5.66 1.41 2.83 0.95 1.88
( ± 4 kV) ( ± 8 kV) ( ± 4 kV) ( ± 8 kV) ( ± 4 kV) ( ± 8 kV)
5
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Table 2 Surface flashover experiment conditions. Pre-stressing DC voltage/kV
RMS value of AC voltage at flashover/kV
Peak value of combined voltage/kV
−30 −15 +30 +15 0
43.8 46.1 32.3 31.5 28.0
91.9 80.2 75.6 59.5 39.6
manner, as illustrated in Fig. 13. Regardless of the positive or negative AC/DC combined voltage, the interface charge accumulated under enhanced combined voltage dissipates faster than that under weakened one. The dissipating time constant of enhanced interface charge is about 15 min, while for the weakened interface charge it is about 30 min. Fig. 13. Interface charge dissipation characteristics at AC voltage wave-trough.
4. Surface flashover characteristic undissipated negative interface charges in last negative cycles of AC voltage; (3) newly accumulated positive charges in positive cycles of AC voltage. All these combined three kinds of charges made the interface charge to be negative but has a smaller density. Likewise, there could witness more negative interface charges under −8 kVrms (1:1) AC/DC combined voltage (cut off at wave trough), compared to the sum of interface charge density under −4 kV DC voltage and that under 4 kVrms AC (cut off at wave trough) respectively
In the surface flashover experiments, the needle electrode was prestressed by DC voltage whilst the plate electrode was applied with AC voltage. Both negative and positive DC voltages were firstly applied for 20 min and then superimposed with AC voltage to generate the AC/DC combined electric field. In accordance with IEC 60243-1, the rapid-rise test approach is conducted at the speed of 5 kV/s to capture the surface flashover voltage. The experimental conditions and results are shown in Table 2 and Fig. 15. The surface flashover experimental results in Fig. 15 indicate that, the surface flashover peak values of test model under −15 kV and −30 kV pre-stressing DC voltage is greater than that under positive voltage conditions, with a maximum 31% growth. Taking the case under ± 15 kV pre-stressing DC voltages for examples, the interface charge densities along the 15 mm electrodes gap were obtained in Fig. 16. The interface charge measurement was undertaken along the 15 mm gap between the two electrodes at the measuring interval of 1.0 mm, and the measuring direction is perpendicular to the GND electrode. It could be indicated that, the density of interface charge tends to decrease as the distance between the measuring point and needle electrode grows. Under −15 kV pre-stressing DC voltage, the density of negative charge around the needle point is −23.45 pC/mm2 while under 15 kV it reads 19.7 pC/mm2. The analysis for the impacting mechanism of interface charge on surface flashover would be proposed in next chapter.
3.3.3. Dissipation characteristics In the experiment for interface charge dissipation characteristics, the applied voltage is set as ± 4 kVrms, ± 6 kVrms, ± 8 kVrms and ± 10 kVrms AC/DC combined voltage with the ratio of 1:1 and the voltage application time is 5 min. The charge measuring point is located at 7.5 mm to the GND electrode and 2 mm to the interface. Figs. 13 and 14 present the dissipation characteristics of the interface charges when cutting off voltage at the voltage wave-peak and wave-trough. As displayed from the results that the polarity of interface charge is the same as the polarity of DC voltage component in combined AC/DC voltage, but the density of interface charge is closely related to the cutting-off phase of AC voltage. In the case of cutting off the voltage at AC voltage wave-peak, the positive AC voltage makes a positive joint operation with positive DC voltage to increase the interface charge density, but there could witness a negative impact on charge accumulation under negative DC voltage. As for the case of cutting off at AC voltage wave-trough, the maximum negative AC voltage would take similar actions, and the charge density curve also behaves in similar
5. Discussions 5.1. Modelling analysis for interface charge accumulation In order to analyze the oil-pressboard interface charge accumulation
Fig. 15. The dependence of pre-stressing DC voltage on flashover voltage.
Fig. 14. Interface charge dissipation characteristics at AC voltage wave-peak. 6
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transformed to the following Eq. (11):
K L K L ⎛ o P 0P1 ⎞ Eno + ⎛⎜ p P1P 2 ⎞⎟ Enp = u (t ) ⎝ cos a ⎠ ⎝ cos β ⎠
(11)
Assuming that, A = KoLP0P1/cosα and B = KpLP1P2/cosβ, and the A and B are two constants. Combing Eqs. (8) and (11), the interface charge density in the following Eq. (12) could be obtained:
εp
∂Enp ∂t
∂ εo ∂t
(
(
A
)
+ γp + γo B Enp−
u (t ) − AEnp B
εp Enp − εo
(
)−γ
u (t ) B
u (t ) o B
−
A E B np
=0
) = σ (t )
(12)
Above theoretical analysis is only based on the surface polarity features of the oil-pressboard composite dielectric media and does not take into consideration the impacts of Schottky charge injection under strong tangential component of needle-plate electrode. In the case of tests under negative DC voltage, the strong tangential component of electric field could promote the Schottky barrier injection of electron, and the interface charges generated by the surface polarization of normal component are also negative ones. When it comes to test conditions under positive DC voltage, the needle electrode absorbs electron and the left charges around which are positive ones, which could be equivalent to the injection of positive charges. Consequently, the homopolar charges which come from Schottky injection are not able to change the polarity of the interface charges but only increases the density of that.
Fig. 16. The distribution of oil-pressboard interface charge density along the needle-plate axis under ± 15 kV pre-stressing DC voltage.
5.2. The impacting mechanism of interface charge on surface flashover As indicated in Table 2 and Fig. 15, there is a great difference in the flashover voltages of needle-plate model under positive and negative combined voltages, which could be explained by the different charges accumulation conditions at the oil-pressboard interface. Under the negative pre-stressing DC voltage, the surface polarization could generate the negative charge accumulation, and the Schottky injection could also produce electron. With the action of electric field force, the free electrons could migrate to the oil-pressboard interface and then were captured by the traps on interface, forming the negative interface charge accumulation. Fig. 18(a) indicates that, the electric field induced by negative interface charge, namely Eδ−, has the opposite direction to the applied negative electric field EDC, making the composite electric field EDC-Eδ− to be smaller than applied electric field. Similarly, in the scenario of pre-stressing positive DC voltage, the accumulated positive interface charge could also decrease the applied electric field, as shown in Fig. 18(b). The presence of homopolar charges can weaken the field strength nearby the needle electrode, and hence, the surface flashover voltage under AC/DC composite voltage could be largely enhanced compared with that under pure AC voltage. In the case of positive and negative pre-stressing DC voltage, the difference in flashover voltage is due to the large difference in interface charge density under positive and negative DC voltages. Under the negative DC voltage, the negative interface charge density is larger than the positive one during the positive DC voltage application. Therefore, the more negative charges will have a stronger weakening effect on the electric field near the needle electrode. When it comes to the condition of pure AC voltage, there witnesses rare weakened effect exerted by rare interface charge on applied electric field, resulting in a lower flashover voltage than that under AC/ DC combined voltages.
Fig. 17. The analysis model for the interface charge accumulation.
characteristics under electric field, an interface polarization model was built in Fig. 17. Referring to the dielectric interface conditions, the interface charge density of oil-pressboard satisfy the following Eq. (7).
εp Enp − εo Eno = σ (t ) (γp Enp − γo Eno) +
∂σ (t ) ∂t
=0
. (7)
in which, Enp and Eno are the normal electric field components on the two side of P1 point, namely the pressboard and oil; εp and εo stand for the relative permittivity of the pressboard and oil; γp and γo represent the conductivity of pressboard and oil; σ(t) marks the interface charge density. The following Eq. (8) could be obtained from the Eq. (7):
(γp Enp − γo Eno) +
∂ (εp Enp − εo Eno) = 0 ∂t
(8)
Given that, the dotted line with arrows from HV electrode towards GND electrode in Fig. 17 is any one of the electric force line, which has an intersection point with the interface, marked as P1. The applied voltage u(t) could be expressed by the following Eq. (9):
∫L
P 0P 2
E (L) dL =
∫L
E (L) dL +
P 0P1
∫L
P1P 2
E (L) dL = u (t )
(9)
According to the mean value theorem of integrals, in the case of E (L) ≠ ∞, the equation (9) could be simplified to the following equation (10):
LP 0P1 E (ζo) + LP1P 2 E (ζ p) = u (t )
(10)
in which, ζo and ζp stand for the certain point in the curved section P0P1 and P1-P2, respectively; LP0P1 and LP1P2 mark the length of curved section P0-P1 and P1-P2 respectively. Given that Ko = E(ζo)/Eo, Kp = E (ζp)/Ep, Eno = Eocosα and Enp = Epcosβ, and then Eq. (10) could be
6. Conclusion Adopting the electrostatic capacitive probe method, the characteristics of interface charge in oil-pressboard insulation with needle-plate 7
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homopolar interface charge could weaken the applied field strength near the needle electrode, and thus enhanced the flashover voltage. Beyond that, the higher density of negative interface charge than that of positive ones also makes the complementary explanation. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgement The research projects received financial support from the Natural Science Foundation of Beijing Municipality (Grant 3172033) and National Key R&D Program of China (2017YFB0902704).
(a) Under negative voltage
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ijepes.2019.105484. References [1] Li Y, Zhang Q, Ding Y, Zhao Y. Effect of cellulose impurities on partial discharges in oilpressboard insulation under DC voltage. IEEE Trans Dielectr Electr Insul 2017;24(4):2271–3. [2] Qi B, Wei Z, Li C. Creepage discharge of oil-pressboard Insulation in AC-DC composite field: phenomenon and characteristics. IEEE Trans Dielectr Electr Insul 2016;23(1):237–45. [3] Liu R, Wahlstrom G. Measurements of the DC electric field in liquid impregnated\pressboard using the pressure wave propagation technique. Proc. IEEE Int Symp Electr Insul, Pittsburgh, USA. 1994. p. 103–6. [4] Morshuis P, Jeroense M. Space charge measurements on impregnated paper: A review of the PEA method and a discussion of results. IEEE Electr Insul Mag 1997;13(3):26–35. [5] Faircloth D, Allen N. High resolution measurements of surface charge densities on insulator surfaces. IEEE Trans Dielectr Electr Insul 2003;10(2):285–90. [6] Murooka Y, Takada T, Hiddaka K. Nanosecond surface discharge and charge density evaluation, Part I: review and experiments. IEEE Electr Insul Mag 2001;17(2):6–16. [7] Kawasaki T, Arai Y. Two-dimensional measurement of electrical surface charge distribution on insulating material by lectro-optic pockels effect. Jpn J Appl Phys Part 1(Regul Pap Short Notes) 1991;30(6):1262–5. [8] Pedersen A. On the electrostatics of probe measurements of surface charge densities. Gaseous dielectrics V. New York (USA): Pergamon Press; 1987. p. 235–41. [9] Kumada A, Okabe S, Hidaka K. Resolution and signal processing technique of surface charge density measurement with electrostatic probe. IEEE Trans Dielectr Electr Insul 2004;11(1):122–9. [10] Qi B, Gao C, Li C, Xiong J. The influence of surface charge accumulation on flashover voltage of GIS/GIL basin insulator under various voltage stresses. Int J Electr Power Energy Syst 2019;105:514–20. [11] Qi B, Dai Q, Li C, Lv Y, Huang M, Zhang S. The influence of polarity reversal time on interface charges in oil-impregnated pressboard insulation under non-uniform electric field. IEEE Trans Dielectr Electr Insul 2018;25(5):1598–604. [12] Qi B, Zhao X, Li C, Wu H. Transient electric field characteristics in oil-pressboard composite insulation under voltage polarity reversal. IEEE Trans Dielectr Electr Insul 2015;22(4):2148–55. [13] Qi B, Chen Y, Li C, Gao C, Zhao L, Sun X. Oil-pressboard interface charges under combined AC/DC electric field: their characteristics and influence on surface flashover voltage. Proc Electr Insul Conf, Seattle, USA. 2015. p. 65–8. [14] Leblanc P, Paillat T, Morin G, Perrier C. Behavior of the charge accumulation at the pressboard/oil interface under DC external electric field stress. IEEE Trans Dielectr Electr Insul 2015;22(5):2537–42. [15] Du B, Chang R, Li J, Zhu W, Jiang J, Liang H, et al. Temperature-dependent surface charge and flashover characteristics of double-layer oil-paper insulation under DC voltage. Proc IEEE Int Conf Dielectr, Budapest, Hungary. 2018. p. 1–4. [16] Kato K, Kato H, Ishida T, Okubo H, Tsuchiya K. Influence of surface charges on impulse flashover characteristics of alumina dielectrics in vacuum. IEEE Trans Dielectr Electr Insul 2009;16(6):1710–6. [17] IEC Standard: IEC 60641-2:2004. Pressboard and presspaper for electrical purposes - Part 2: methods of tests; 2004-06-23. [18] IEC Standard: IEC 60422:2013. Mineral insulating oils in electrical equipment - supervision and maintenance guidance; 2013-01-10. [19] Ieda M, Goto K, Okugo H, Miyamoto T, Tsukioka H, Kohno Y. Suppression of static electrification of insulating oil for large power transformers. IEEE Trans Electr Insul 1988;23(1):153–7. [20] Kurita A, Takahashi E, Ozawa J, Watanabe M, Okuyama K. DC flashover voltage characteristics and their calculation method for oil-immersed insulation systems in HVDC transformers. IEEE Trans Power Deliv 1986;1(3):184–90. [21] Okubo H, Wakamatsu M, Inoue N, Kato K, Koide H. Charge behavior in flowing oil in oil/ pressboard insulation system by electro-optic field measurement. IEEE Trans Electr Insul 2003;10(6):956–62. [22] Qi B, Gao C, Zhao X, Li C, Wu H. Interface charge polarity effect based analysis model for electric field in oil-pressboard insulation under DC voltage. IEEE Trans Electr Insul 2016;23(5):2704–11.
(b) Under positive voltage Fig. 18. The interaction between interface charge electric field and external applied electric field.
electrode under AC voltage, DC voltage and AC/DC combined voltage were obtained, and based on the model analysis, the impacting mechanism of interface charge on surface flashover was proposed. Major achievements are concluded as follows. (1) Aiming at finding out the interface charge characteristics of oilpressboard insulation and its impact on surface flashover under AC/ DC combined voltages, an interface charge measurement platform with a charge resolution of 0.129 pC/(mm2·mV) and a space resolution of 1.0 mm2 and a surface flashover voltage test platform were developed in this paper. (2) In the context of AC voltage, the polarity of interface charge is same to that of instantaneous voltage value, and the density of interface depends positively linearly on voltage amplitude, but stays constant with voltage application duration prolonging. Under DC voltage, the polarity of charge is also consistent with that of applied voltage, while the density of negative interface charge is about 1.2–1.5 times of positive charges. (3) Under the combined voltages with AC/DC ratios of 1:1, 1:3 and 1:5, the polarity of interface charge is the same as that of the DC component, and the larger the DC voltage component, the bigger the interface charge density. As the duration of voltage prolongs, the dynamic process of interface charge appears as a fluctuation which behaves similarly to the waveform of power frequency voltage, and the larger the amplitude of DC component in combined voltage, the weaker the fluctuation. (4) The surface flashover voltage of test model under AC voltage combined with negative DC voltage is maximally 1.3 times of that under combined electric field with positive DC voltage. An interface polarization model was built to present the interface charge accumulation mechanism, and based on which the impacting mechanism of interface charge on flashover voltage was also proposed. In the oil-pressboard insulation structure with needle-plate electrode under AC/DC combined voltages, the electric field induced by 8
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The interface charge characteristics of oil-pressboard composite insulation and its impact on surface flashover under combined AC/DC voltages
T
⁎
Chunjia Gao, Bo Qi , Chengrong Li State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, 102206 Beijing, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oil-pressboard insulation Combined AC/DC voltage Interface charge characteristics Surface flashover Impacting mechanism
The interface charges of oil-pressboard insulation could distort the local electric field and even weaken the insulating strength of converter transformer. This paper employs the electrostatic capacitive probe to capture the accumulation characteristics of interface charges in oil-pressboard insulation with needle-plate electrode under AC, DC and combined AC/DC voltages. The research results indicate that, under AC voltage, the polarity of interface charge is the same to that of the instantaneous value of applied voltage, and density of interface charge remains constant with the AC voltage application prolonging. In the context of DC voltage, the density of negative interface charge is about 1.2–1.5 times of that of the positive one. Under AC/DC combined voltage, the time-dependent dynamic accumulating process of interface charge behaves similarly to the waveform of AC voltage, and the larger the amplitude of DC component in combined voltage, the weaker the fluctuation. The tests for the surface flashover of oil-pressboard insulation were also conducted, aiming at discovering the impacting mechanism of interface charge accumulation on flashover voltage. The surface flashover voltage of test model under negative DC superimposed AC voltage is 1.3 times higher than that of positive DC combined with AC voltage. The built interface polarization model made an explanation for the interface charge accumulation, and the impacting mechanism of interface charge on flashover voltage, namely the positive effect of homopolar interface charges, is also proposed.
1. Introduction As the high voltage direct current (HVDC) transmission technology develops rapidly, the converter transformer has been applied more and more widely. Oil-pressboard composite insulation, attributing to its electrical, mechanical and chemical merits, is applied as the main insulating structure of converter transformer, thus whose insulating strength is key to the safe and reliable operation of power system [1,2]. When it comes to the insulating properties of composite insulation structures under various voltage stresses, the characteristic of space/ interface charges, inevitably, have always been regarding as the hot research topics. From 1970s, there had appeared a variety of non-destructive measuring approaches for space/interface charge characteristic of insulation structure. As for the space charge characteristics of dielectric medium, the Pressure Wave Propagation method (PWP) and Pulsed Electroacoustic method (PEA) could play the main roles in measurement [3,4]. While in the concerned of surface/interface charge measurement techniques, the means of dust map, Pockels electro-optic effect and electrostatic capacitive probe are commonly utilized [5]. Dust map method can contribute to the qualitative observation and
⁎
analysis for field/charge distribution characteristics, but it failed to capture the quantitative results [6]. The Pockels electro-optic effect has the advantages of better accuracy and spatial resolution for charge measurement, but the existence of Pockels optical medium could exert a significant negative effect on the measurement results [7]. In comparison, the electrostatic capacitive probe method, which is based on the capacitance voltage dividing principles, could be adopted for the quantitative measurement of surface/interface charge without any interference to the surrounding dielectric medium [8,9]. Taking the advantage of electrostatic capacitive probe method in charge measurement, our former researches had been conducted to capture the characteristics of surface charge on some composite insulations under various voltage stresses, which indicated that the accumulation of surface charges could not only distort the electric field but also could even facilitate insulation failure [10–13]. Surface flashover is one of most serious insulating failures for the oil-pressboard insulation structures of converter transformer, which could create risks for the safe and stable operation of HVDC system. According to the complex structures of insulations in converter transformer, the tangential electric field strength along the insulation surface
Corresponding author. E-mail address: [email protected] (B. Qi).
https://doi.org/10.1016/j.ijepes.2019.105484 Received 21 April 2019; Received in revised form 4 July 2019; Accepted 12 August 2019 0142-0615/ © 2019 Published by Elsevier Ltd.
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could be larger than that normal to the surface, which could provide chances for charge accumulation and thus create risks for surface flashover. Certain studies have reported the relationship between the charge accumulation and surface flashover in some insulation structures. P. Leblanc, et al placed the research focus on the behaviour of interface charge accumulation at the pressboard/oil under DC voltage, and his achievement pointed out the potential effect of accumulated charges on the electrostatic hazard in power transformer [14]. Du, et al investigated the surface charge and flashover behaviour of oil-paper insulation under DC voltage at different operating temperatures, indicating there could accumulate more charges at higher temperature to participate in the procedure of flashover [15]. K. Kato’s studied revealed that the locations of charge accumulation could exert different effect on the impulse flashover characteristics of alumina dielectrics in vacuum [16]. The existing researches were mainly untaken under the DC or AC uniform electric field, neglecting one of the most common operating conditions of converter transformer, namely the combined AC/DC voltages. In addition, there still remains a gap in the studies for the accumulation mechanism of interface charge in oil-pressboard insulation and its impacting mechanism for surface flashover. In this paper, the accumulation characteristic of interface charges in oil-pressboard insulation under non-uniform electric field induced by AC voltage, DC voltage and combined AC/DC voltages were captured, and the impacting mechanism of which on surface flashover voltage was also proposed, aiming at filling in the research gap and providing meaningful references for the insulation design of oil-pressboard/paper insulating apparatus. This paper is organized as follows. Section 2 presents the details of experimental platform including the interface charge measurement platform, surface flashover test platform, test model and test cavity. Section 3 reports the accumulation characteristics of interface charge in oil-pressboard insulation under DC, AC and combined AC/DC voltages, and the influence of voltage amplitude, AC voltage phase and AC/DC ratio in combined voltage on interface charge accumulation was also carried out. Section 4 presents the surface flashover characteristics of test model under AC/DC combined voltages. Section 5 discusses the mechanism of interface charge accumulation and proposes the impacting mechanism of interface charge on surface flashover voltage.
Fig. 2. The schematic diagram of interface charge measurement unit.
interface charge accumulation points; Q represents the quantity of charges on the insulator surface underneath the probe, and σ stands for the interface charge density. From Fig. 1, the following equations could be obtained:
C1 C2 ⎞ Q = σA = U2 ⎛C3 + C1 + C2 ⎠ ⎝ ⎜
U1 =
⎟
C2 U2 C1 + C2
(1)
(2)
in which, A is the equivalent charge detection area of probe. The schematic diagram of interface charge measurement unit designed is this paper is illustrated in Fig. 2, and as it indicates that, there exits two layers of dielectric medium between the probe and interface charge, namely the oil and air. In consequence, the C2 is formed by connecting the equivalent capacitance Coil of the transformer oil layer and the equivalent capacitance Cair of the air layer, which could be obtained by the following Eq. (3).
C2 =
Coil Cair ε0 εoil εair A = Coil + Cair d2 εoil + d1 εair
(3)
in which, ε0, εoil and εair represent the permittivity of vacuum, oil and air respectively. Given that the C1 ≫ C2 ≫ C3, and combining the Eqs. (1) and (2), the following Eq. (4) could be obtained:
2. Experimental platform 2.1. Principle of interface charge measurement Fig. 1 illustrates the principle and equivalent circuit of electrostatic capacitance probe for interface charge measurement. In Fig. 1, C1 is the sum of capacitance of electrostatic capacitance probe and test circuit; C2 stands for the equivalent capacitance between the inductive surface of probe and interface charge; C3 represents the equivalent capacitance of interface charge relative to ground; U1 marks the output voltage of probe; U2 marks the equivalent voltage at the
σ≈
C1 U1 = MU1 A
(4)
in which, M marks the calibration coefficient, namely the coefficient correlation between the interface charge density and output voltage of probe. In order to clarify the reliability of the interface charge measurement unit and capture the coefficient M, the calibration method described in our former study was applied [11]. An Aluminum foil was adhered to the pressboard and applied with DC voltage with different magnitudes to simulate interface charge accumulation. The probe was vertically positioned 2.0 mm away from the foil to carry out the calibration result, as shown in Fig. 3. As the fitting line in Fig. 3 presents, the relationship between U1 and U2 could be expressed in the following Eq. (5):
U1 = 2.448 × 10−5 × U2 − 0.0042
(5)
The Schering Bridge method was applied to work out C1 = 129.000 pF. According to Eqs. (2) and (5), C2 could be worked out, namely C2 = 3.158 fF, and then the Eq. (3) makes its contribution to work out A = 1.001 mm2. Finally, the M in Eq. (4) could be obtained as 0.129 pC/(mm2·mV), and the relationship between interface charge density σ and probe output voltage U1 could be presented as follows.
Fig. 1. The principle and equivalent circuit of electrostatic capacitive probe. 2
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Fig. 3. Calibration curve. Fig. 5. Schematic diagram of the surface flashover test platform.
σ = 0.129 × U1 (pC/mm2)
(6) driving signal, judged by the input power frequency reference signal, can drive the high-voltage relay to cut off voltage at the selected instantaneous phase, which contributes to the research about the influence of the phase change of the AC voltage component on the characteristics of oil-pressboard interface charge. With a maximum allowable input voltage of 70 kV and an action delay of 15 ms of highvoltage relay, the cutting-off phase angle could be precisely managed within an error range of less than ± 5°. The apparent partial discharge magnitude of experimental platform could reach less than 5.0 pC when the test model was applied by the maximum test voltage.
As indicated from the calibration results, the charge resolution sensitivity and spatial resolution of designed interface charge measurement unit is 0.129 pC/(mm2·mV) and 1.0 mm2. The data acquisition device applied in this paper is Keithley2182A nano-voltmeter with an input impedance of 1015 Ω and a time constant of 1.07 × 105 s, which is much greater than the dissipation time constant of the interface charges. 2.2. Interface charge measurement platform Fig. 4 shows the structure diagram of developed interface charge measurement platform. The combined AC/DC voltage was applied to the test cavity in parallel. The DC voltage generator could supply steady DC voltage with the amplitude ranging from 0 to 200 kV, the ripple factor of less than 0.5% and the output voltage accuracy of more than 99.5%. A 400 MΩ/40 kΩ resistance voltage divider, which has a voltage ratio of 10,000:1 and a withstanding voltage of 200 kV DC, was applied for the DC voltage measurement. The AC voltage could be continuously adjusted from 0 to 120 kV, and it was connected in parallel with the DC voltage through the DC blocking capacitor C′3. The AC voltage component was measured by the DPO4034 oscilloscope via a capacitance voltage divider with a voltage ratio of 1000:1 and a withstanding voltage of 120 kV AC. The phase control system contains of oscilloscope, AC voltage phase control/driving circuit and high-voltage relay. The
2.3. Surface flashover test platform The developed surface flashover test platform is illustrated in Fig. 5. The flashover signals were collected through the photomultiplier tube (PMT), while the voltage signals at the moment of flashover were also recorded in synchronism. The PMT light intensity signals were recorded by the DPO4034 oscilloscope, and which were then communicated to the computer for data storage and subsequent analysis. 2.4. Test model and test cavity Fig. 6 shows the diagram of test model and test cavity, in which the needle-plate electrode contributes to generate the non-uniform electric field. The tungsten needle electrode has a length of 10 mm and the end curvature radius of which is 50 μm, and it is tightly stick to the T4 laminated pressboard with the size of 80 mm × 70 mm and the thickness of 5 mm. The brass plate electrode, with a chamfered edge, has a
Fig. 4. The structure diagram of developed oil-pressboard interface charge measurement platform.
Fig. 6. The test model and test cavity. 3
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height of 50 mm, and the distance between the needle electrode and plate electrode is 15 mm. The T4 high-density pressboard applied in this paper comes from the insulation manufacturer of the converter transformer, and the test oil is the KI50X transformer oil. In alignment with IEC 60641-2:2004 [17], the pressboard was firstly dried in vacuum for 48 h under the temperature of 105 °C, and then immersed in oil for 24 h at the temperature of 85 °C and in vacuum. The water content in pressboard is controlled less than 1.0%. Before test, the oil in the test cavity was filtered to remove the impurities. After the standard processes, the water content of oil is around 7.0–10.0 μL/L with little difference among different models, which aligns to the specifications stated in the IEC 60422:2013 [18]. 3. Oil-pressboard interface charge characteristic 3.1. Under DC voltage
Fig. 8. The interface charge density under AC voltages.
In the interface charge measurement tests under DC voltage, the applied voltages are set as ± 4 kV, ± 6 kV, ± 8 kV and ± 10 kV for 30 min. The charge measuring point is located at 7.5 mm to the GND electrode and 2 mm to the interface. After the voltage cutting off, the probe was immediately moved to measuring point to obtain the interface charge density, and the same measuring procedure was conducted for five times, of which the average value was recorded as results, as shown in Fig. 7. It is observed from the results that the polarity of interface charge is the same to that of applied voltage, while the density of which is proportional to the magnitude of applied voltage. With the voltage application time prolonging, the density of charge tends to increase gradually. Multiple test results also indicate that, under the same test condition, the charge density on the interface oil-pressboard under negative DC voltage is 1.2–1.5 times of that under positive voltage. Ieda et al. [19] provided explanations for such a phenomenon. The insulation pressboard applied in transformers contains cellulose and lignin, both of which are contained with much hydroxyl (eOH). In addition, the lignin also includes aldehyde (eCHO) and carboxyl groups (eCOOH). The oxygen atoms within the hydroxyl groups have the second-largest electronegativity of any atom. For this reason, the electrons in hydrogen atoms are attracted to the oxygen atoms, polarizing the oxygen atoms to be negative and the hydrogen atoms to be positive. The electropositive hydrogen atoms on the pressboard surface then absorbs the negative charges in oil, which results in the more negative interface charge accumulation.
(Root Mean Square) values of AC voltages are selected as 4 kVrms (5.66 kVpeak) and 8 kVrms (11.31 kVpeak) at the frequency of 50 Hz, and the voltage application time is set as 5 s, 10 s, 30 s, 60 s, 600 s, and 1800 s respectively. The voltages were cut off at the wave-peak phase of 85–95° and the wave-trough phase of 265–275° respectively, and then the oil-pressboard interface charge density was captured. The average values of five repetitive tests under AC voltages were demonstrated in Fig. 8. As indicated from Fig. 8 that there could still witness the charge accumulation on the oil-pressboard interface under AC voltages. The polarity of the charges is the same to the instantaneous polarity of cutoff AC voltage and the density of which is in relation with the instantaneous values of the AC charges but it remains constant with voltage application duration prolonging. Fig. 8 also tells that the positive charge density at the voltage peak is slightly larger than that of negative ones at the voltage trough, but such phenomenon is different from that under DC voltages which witnesses larger density of negative charges than positive ones. This is due to the fact that, in multilayer insulation structure, the distribution of AC electric field is related to dielectric constant whilst the conductivity of dielectric donates in the distribution features of DC electric field. The dielectric constant of the transformer oil is about half of that of the pressboards whereas whose conductivity is about 10–1000 times of the pressboard [20], in consequence, AC voltage is mainly endured in the transformer oil. Besides, in the oil-pressboard insulation, the cellulose pressboard tends to accumulate negative charges and the transformer oil are more likely to absorb positive charges [21].
3.2. Under AC voltage
3.3. Under AC/DC combined voltage
For the interface charge measurement under AC voltages, the RMS
3.3.1. Accumulation characteristic During the test under AC/DC combined voltages, the voltage ratios of AC and DC voltage components are marked as 1:1 and 1:5 respectively, the voltage amplitudes are set as ± 9.65 kV (4 kVrms AC/ ± 4 kV DC) and ± 8.55 kV (1.33 kVrms AC/ ± 6.67 kV DC) respectively, and the voltage application time is recorded as 30 min. The charge measuring point is located at 7.5 mm to the GND electrode and 2 mm to the interface. The accumulation characteristics of interface charges when the applied voltages are cut off at wave-peak and wave trough are presented in Figs. 9 and 10. Figs. 9 and 10 tell that, there exists great difference between the interface charge density at the wave-peak and wave-trough of combined voltages, which could be attributed to the strengthening effect of AC voltage peak and weakening effect of its valley on the combined electric field. Comparing the interface charge density curves under the same polarity voltages in Figs. 9 and 10, it can be found that the instantaneous value of interface charge density at voltage trough and
Fig. 7. The oil-pressboard interface charge density under different DC voltages. 4
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Fig. 9. The interface charge density under 1:1 AC/DC combined voltage. Fig. 11. The interface charge density at different voltage phase of ± 4 kVrms AC/DC combined voltage.
Fig. 10. The interface charge density under 1:5 AC/DC combined voltage.
peak time of combined voltage with the AC/DC ratio of 1:1 differs by about 12 pC/mm2, while when the ratio comes to 1:5, the difference is only about 4 pC/mm2.
Fig. 12. The interface charge density at different voltage phase of ± 8 kVrms AC/DC combined voltage.
3.3.2. Dependence of AC voltage phase on interface charge accumulation characteristics In order to carry out the relationship between AC voltage phase and interface charge accumulation characteristics, the test models were applied with positive and negative combined voltages with the AC/DC ratios of 1:1, 1:3 and 1:5, the amplitudes of voltages combing DC value with AC RMS value in different ratios are set as ± 4 kV and ± 8 kV, the details of experiment voltage conditions are listed in Table 1. After the polarization time of 2 min, the voltages were cut off at the phases of 0°, 30°, 90°, 150°, 180°, 210°, 270°, 330° and 360° to capture the instantaneous value of the charge density at the oil-pressboard interface. The charge measuring point is located at 7.5 mm to the GND
electrode and 2 mm to the interface. Each test was conducted for five times, of which the mean value was obtained. The results are shown in Figs. 11 and 12. From Figs. 11 and 12, it is observed that the dynamic process of interface charge appears as a fluctuation which behaves similarly to the waveform of power frequency AC voltage, and the larger the DC proportion in the combined voltage, the smaller the fluctuation. Comparing the experimental results in Figs. 7, 8 and 12, it could be found that, there could witness −0.61 pC/mm2 interface charge accumulation under −8 kVrms AC/DC combined voltage (1:1) when cutting off voltage at wave peak. In Fig. 7, the charge density under −4 kV DC voltage is −2.51 pC/mm2, while in Fig. 8, that under 4 kVrms AC voltage (cutting off at wave-peak) is 6.36 pC/mm2, and the sum of them is about 3.85 pC/mm2, which is different from the result (−0.61 pC/ mm2) in Fig. 12. Due to our former study, the negative interface charges are more likely to accumulate but more difficult to dissipate than the positive ones [11,22], in consequence, under the positive cycles of AC voltage, there is no enough time for the accumulated negative charges to dissipate. In addition, under DC voltage, the density of negative interface charge is bigger than that of positive one. Ultimately, there exists three kinds of interface charges under the AC/DC combined voltages: (1) negative interface charges under DC voltage; (2)
Table 1 Experiment conditions. AC/DC combined voltage ratio
DC component amplitude/kV
AC component RMS value/kV
AC component peak value/kV
1:1 1:1 1:3 1:3 1:5 1:5
±2 ±4 ±3 ±6 ± 3.33 ± 6.67
2 4 1 2 0.67 1.33
2.83 5.66 1.41 2.83 0.95 1.88
( ± 4 kV) ( ± 8 kV) ( ± 4 kV) ( ± 8 kV) ( ± 4 kV) ( ± 8 kV)
5
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Table 2 Surface flashover experiment conditions. Pre-stressing DC voltage/kV
RMS value of AC voltage at flashover/kV
Peak value of combined voltage/kV
−30 −15 +30 +15 0
43.8 46.1 32.3 31.5 28.0
91.9 80.2 75.6 59.5 39.6
manner, as illustrated in Fig. 13. Regardless of the positive or negative AC/DC combined voltage, the interface charge accumulated under enhanced combined voltage dissipates faster than that under weakened one. The dissipating time constant of enhanced interface charge is about 15 min, while for the weakened interface charge it is about 30 min. Fig. 13. Interface charge dissipation characteristics at AC voltage wave-trough.
4. Surface flashover characteristic undissipated negative interface charges in last negative cycles of AC voltage; (3) newly accumulated positive charges in positive cycles of AC voltage. All these combined three kinds of charges made the interface charge to be negative but has a smaller density. Likewise, there could witness more negative interface charges under −8 kVrms (1:1) AC/DC combined voltage (cut off at wave trough), compared to the sum of interface charge density under −4 kV DC voltage and that under 4 kVrms AC (cut off at wave trough) respectively
In the surface flashover experiments, the needle electrode was prestressed by DC voltage whilst the plate electrode was applied with AC voltage. Both negative and positive DC voltages were firstly applied for 20 min and then superimposed with AC voltage to generate the AC/DC combined electric field. In accordance with IEC 60243-1, the rapid-rise test approach is conducted at the speed of 5 kV/s to capture the surface flashover voltage. The experimental conditions and results are shown in Table 2 and Fig. 15. The surface flashover experimental results in Fig. 15 indicate that, the surface flashover peak values of test model under −15 kV and −30 kV pre-stressing DC voltage is greater than that under positive voltage conditions, with a maximum 31% growth. Taking the case under ± 15 kV pre-stressing DC voltages for examples, the interface charge densities along the 15 mm electrodes gap were obtained in Fig. 16. The interface charge measurement was undertaken along the 15 mm gap between the two electrodes at the measuring interval of 1.0 mm, and the measuring direction is perpendicular to the GND electrode. It could be indicated that, the density of interface charge tends to decrease as the distance between the measuring point and needle electrode grows. Under −15 kV pre-stressing DC voltage, the density of negative charge around the needle point is −23.45 pC/mm2 while under 15 kV it reads 19.7 pC/mm2. The analysis for the impacting mechanism of interface charge on surface flashover would be proposed in next chapter.
3.3.3. Dissipation characteristics In the experiment for interface charge dissipation characteristics, the applied voltage is set as ± 4 kVrms, ± 6 kVrms, ± 8 kVrms and ± 10 kVrms AC/DC combined voltage with the ratio of 1:1 and the voltage application time is 5 min. The charge measuring point is located at 7.5 mm to the GND electrode and 2 mm to the interface. Figs. 13 and 14 present the dissipation characteristics of the interface charges when cutting off voltage at the voltage wave-peak and wave-trough. As displayed from the results that the polarity of interface charge is the same as the polarity of DC voltage component in combined AC/DC voltage, but the density of interface charge is closely related to the cutting-off phase of AC voltage. In the case of cutting off the voltage at AC voltage wave-peak, the positive AC voltage makes a positive joint operation with positive DC voltage to increase the interface charge density, but there could witness a negative impact on charge accumulation under negative DC voltage. As for the case of cutting off at AC voltage wave-trough, the maximum negative AC voltage would take similar actions, and the charge density curve also behaves in similar
5. Discussions 5.1. Modelling analysis for interface charge accumulation In order to analyze the oil-pressboard interface charge accumulation
Fig. 15. The dependence of pre-stressing DC voltage on flashover voltage.
Fig. 14. Interface charge dissipation characteristics at AC voltage wave-peak. 6
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transformed to the following Eq. (11):
K L K L ⎛ o P 0P1 ⎞ Eno + ⎛⎜ p P1P 2 ⎞⎟ Enp = u (t ) ⎝ cos a ⎠ ⎝ cos β ⎠
(11)
Assuming that, A = KoLP0P1/cosα and B = KpLP1P2/cosβ, and the A and B are two constants. Combing Eqs. (8) and (11), the interface charge density in the following Eq. (12) could be obtained:
εp
∂Enp ∂t
∂ εo ∂t
(
(
A
)
+ γp + γo B Enp−
u (t ) − AEnp B
εp Enp − εo
(
)−γ
u (t ) B
u (t ) o B
−
A E B np
=0
) = σ (t )
(12)
Above theoretical analysis is only based on the surface polarity features of the oil-pressboard composite dielectric media and does not take into consideration the impacts of Schottky charge injection under strong tangential component of needle-plate electrode. In the case of tests under negative DC voltage, the strong tangential component of electric field could promote the Schottky barrier injection of electron, and the interface charges generated by the surface polarization of normal component are also negative ones. When it comes to test conditions under positive DC voltage, the needle electrode absorbs electron and the left charges around which are positive ones, which could be equivalent to the injection of positive charges. Consequently, the homopolar charges which come from Schottky injection are not able to change the polarity of the interface charges but only increases the density of that.
Fig. 16. The distribution of oil-pressboard interface charge density along the needle-plate axis under ± 15 kV pre-stressing DC voltage.
5.2. The impacting mechanism of interface charge on surface flashover As indicated in Table 2 and Fig. 15, there is a great difference in the flashover voltages of needle-plate model under positive and negative combined voltages, which could be explained by the different charges accumulation conditions at the oil-pressboard interface. Under the negative pre-stressing DC voltage, the surface polarization could generate the negative charge accumulation, and the Schottky injection could also produce electron. With the action of electric field force, the free electrons could migrate to the oil-pressboard interface and then were captured by the traps on interface, forming the negative interface charge accumulation. Fig. 18(a) indicates that, the electric field induced by negative interface charge, namely Eδ−, has the opposite direction to the applied negative electric field EDC, making the composite electric field EDC-Eδ− to be smaller than applied electric field. Similarly, in the scenario of pre-stressing positive DC voltage, the accumulated positive interface charge could also decrease the applied electric field, as shown in Fig. 18(b). The presence of homopolar charges can weaken the field strength nearby the needle electrode, and hence, the surface flashover voltage under AC/DC composite voltage could be largely enhanced compared with that under pure AC voltage. In the case of positive and negative pre-stressing DC voltage, the difference in flashover voltage is due to the large difference in interface charge density under positive and negative DC voltages. Under the negative DC voltage, the negative interface charge density is larger than the positive one during the positive DC voltage application. Therefore, the more negative charges will have a stronger weakening effect on the electric field near the needle electrode. When it comes to the condition of pure AC voltage, there witnesses rare weakened effect exerted by rare interface charge on applied electric field, resulting in a lower flashover voltage than that under AC/ DC combined voltages.
Fig. 17. The analysis model for the interface charge accumulation.
characteristics under electric field, an interface polarization model was built in Fig. 17. Referring to the dielectric interface conditions, the interface charge density of oil-pressboard satisfy the following Eq. (7).
εp Enp − εo Eno = σ (t ) (γp Enp − γo Eno) +
∂σ (t ) ∂t
=0
. (7)
in which, Enp and Eno are the normal electric field components on the two side of P1 point, namely the pressboard and oil; εp and εo stand for the relative permittivity of the pressboard and oil; γp and γo represent the conductivity of pressboard and oil; σ(t) marks the interface charge density. The following Eq. (8) could be obtained from the Eq. (7):
(γp Enp − γo Eno) +
∂ (εp Enp − εo Eno) = 0 ∂t
(8)
Given that, the dotted line with arrows from HV electrode towards GND electrode in Fig. 17 is any one of the electric force line, which has an intersection point with the interface, marked as P1. The applied voltage u(t) could be expressed by the following Eq. (9):
∫L
P 0P 2
E (L) dL =
∫L
E (L) dL +
P 0P1
∫L
P1P 2
E (L) dL = u (t )
(9)
According to the mean value theorem of integrals, in the case of E (L) ≠ ∞, the equation (9) could be simplified to the following equation (10):
LP 0P1 E (ζo) + LP1P 2 E (ζ p) = u (t )
(10)
in which, ζo and ζp stand for the certain point in the curved section P0P1 and P1-P2, respectively; LP0P1 and LP1P2 mark the length of curved section P0-P1 and P1-P2 respectively. Given that Ko = E(ζo)/Eo, Kp = E (ζp)/Ep, Eno = Eocosα and Enp = Epcosβ, and then Eq. (10) could be
6. Conclusion Adopting the electrostatic capacitive probe method, the characteristics of interface charge in oil-pressboard insulation with needle-plate 7
Electrical Power and Energy Systems 115 (2020) 105484
C. Gao, et al.
homopolar interface charge could weaken the applied field strength near the needle electrode, and thus enhanced the flashover voltage. Beyond that, the higher density of negative interface charge than that of positive ones also makes the complementary explanation. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgement The research projects received financial support from the Natural Science Foundation of Beijing Municipality (Grant 3172033) and National Key R&D Program of China (2017YFB0902704).
(a) Under negative voltage
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ijepes.2019.105484. References [1] Li Y, Zhang Q, Ding Y, Zhao Y. Effect of cellulose impurities on partial discharges in oilpressboard insulation under DC voltage. IEEE Trans Dielectr Electr Insul 2017;24(4):2271–3. [2] Qi B, Wei Z, Li C. Creepage discharge of oil-pressboard Insulation in AC-DC composite field: phenomenon and characteristics. IEEE Trans Dielectr Electr Insul 2016;23(1):237–45. [3] Liu R, Wahlstrom G. Measurements of the DC electric field in liquid impregnated\pressboard using the pressure wave propagation technique. Proc. IEEE Int Symp Electr Insul, Pittsburgh, USA. 1994. p. 103–6. [4] Morshuis P, Jeroense M. Space charge measurements on impregnated paper: A review of the PEA method and a discussion of results. IEEE Electr Insul Mag 1997;13(3):26–35. [5] Faircloth D, Allen N. High resolution measurements of surface charge densities on insulator surfaces. IEEE Trans Dielectr Electr Insul 2003;10(2):285–90. [6] Murooka Y, Takada T, Hiddaka K. Nanosecond surface discharge and charge density evaluation, Part I: review and experiments. IEEE Electr Insul Mag 2001;17(2):6–16. [7] Kawasaki T, Arai Y. Two-dimensional measurement of electrical surface charge distribution on insulating material by lectro-optic pockels effect. Jpn J Appl Phys Part 1(Regul Pap Short Notes) 1991;30(6):1262–5. [8] Pedersen A. On the electrostatics of probe measurements of surface charge densities. Gaseous dielectrics V. New York (USA): Pergamon Press; 1987. p. 235–41. [9] Kumada A, Okabe S, Hidaka K. Resolution and signal processing technique of surface charge density measurement with electrostatic probe. IEEE Trans Dielectr Electr Insul 2004;11(1):122–9. [10] Qi B, Gao C, Li C, Xiong J. The influence of surface charge accumulation on flashover voltage of GIS/GIL basin insulator under various voltage stresses. Int J Electr Power Energy Syst 2019;105:514–20. [11] Qi B, Dai Q, Li C, Lv Y, Huang M, Zhang S. The influence of polarity reversal time on interface charges in oil-impregnated pressboard insulation under non-uniform electric field. IEEE Trans Dielectr Electr Insul 2018;25(5):1598–604. [12] Qi B, Zhao X, Li C, Wu H. Transient electric field characteristics in oil-pressboard composite insulation under voltage polarity reversal. IEEE Trans Dielectr Electr Insul 2015;22(4):2148–55. [13] Qi B, Chen Y, Li C, Gao C, Zhao L, Sun X. Oil-pressboard interface charges under combined AC/DC electric field: their characteristics and influence on surface flashover voltage. Proc Electr Insul Conf, Seattle, USA. 2015. p. 65–8. [14] Leblanc P, Paillat T, Morin G, Perrier C. Behavior of the charge accumulation at the pressboard/oil interface under DC external electric field stress. IEEE Trans Dielectr Electr Insul 2015;22(5):2537–42. [15] Du B, Chang R, Li J, Zhu W, Jiang J, Liang H, et al. Temperature-dependent surface charge and flashover characteristics of double-layer oil-paper insulation under DC voltage. Proc IEEE Int Conf Dielectr, Budapest, Hungary. 2018. p. 1–4. [16] Kato K, Kato H, Ishida T, Okubo H, Tsuchiya K. Influence of surface charges on impulse flashover characteristics of alumina dielectrics in vacuum. IEEE Trans Dielectr Electr Insul 2009;16(6):1710–6. [17] IEC Standard: IEC 60641-2:2004. Pressboard and presspaper for electrical purposes - Part 2: methods of tests; 2004-06-23. [18] IEC Standard: IEC 60422:2013. Mineral insulating oils in electrical equipment - supervision and maintenance guidance; 2013-01-10. [19] Ieda M, Goto K, Okugo H, Miyamoto T, Tsukioka H, Kohno Y. Suppression of static electrification of insulating oil for large power transformers. IEEE Trans Electr Insul 1988;23(1):153–7. [20] Kurita A, Takahashi E, Ozawa J, Watanabe M, Okuyama K. DC flashover voltage characteristics and their calculation method for oil-immersed insulation systems in HVDC transformers. IEEE Trans Power Deliv 1986;1(3):184–90. [21] Okubo H, Wakamatsu M, Inoue N, Kato K, Koide H. Charge behavior in flowing oil in oil/ pressboard insulation system by electro-optic field measurement. IEEE Trans Electr Insul 2003;10(6):956–62. [22] Qi B, Gao C, Zhao X, Li C, Wu H. Interface charge polarity effect based analysis model for electric field in oil-pressboard insulation under DC voltage. IEEE Trans Electr Insul 2016;23(5):2704–11.
(b) Under positive voltage Fig. 18. The interaction between interface charge electric field and external applied electric field.
electrode under AC voltage, DC voltage and AC/DC combined voltage were obtained, and based on the model analysis, the impacting mechanism of interface charge on surface flashover was proposed. Major achievements are concluded as follows. (1) Aiming at finding out the interface charge characteristics of oilpressboard insulation and its impact on surface flashover under AC/ DC combined voltages, an interface charge measurement platform with a charge resolution of 0.129 pC/(mm2·mV) and a space resolution of 1.0 mm2 and a surface flashover voltage test platform were developed in this paper. (2) In the context of AC voltage, the polarity of interface charge is same to that of instantaneous voltage value, and the density of interface depends positively linearly on voltage amplitude, but stays constant with voltage application duration prolonging. Under DC voltage, the polarity of charge is also consistent with that of applied voltage, while the density of negative interface charge is about 1.2–1.5 times of positive charges. (3) Under the combined voltages with AC/DC ratios of 1:1, 1:3 and 1:5, the polarity of interface charge is the same as that of the DC component, and the larger the DC voltage component, the bigger the interface charge density. As the duration of voltage prolongs, the dynamic process of interface charge appears as a fluctuation which behaves similarly to the waveform of power frequency voltage, and the larger the amplitude of DC component in combined voltage, the weaker the fluctuation. (4) The surface flashover voltage of test model under AC voltage combined with negative DC voltage is maximally 1.3 times of that under combined electric field with positive DC voltage. An interface polarization model was built to present the interface charge accumulation mechanism, and based on which the impacting mechanism of interface charge on flashover voltage was also proposed. In the oil-pressboard insulation structure with needle-plate electrode under AC/DC combined voltages, the electric field induced by 8