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Improving the protective effect of surge arresters by optimizing the electrical property of ZnO varistors
Electric Power Systems Research 178 (2020) 106041
Contents lists available at ScienceDirect
Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Improving the protective effect of surge arresters by optimizing the electrical property of ZnO varistors
T
Pengfei Menga, Chao Yuana, Heng Xub, Shuai Wanb, Qingyun Xiec, Jimou Hec, Hongfeng Zhaod, ⁎ Jun Hua, , Jinliang Hea a State Key Laboratory of Control and Simulation of Power System and Generation Equipment, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China b Wuhan NARI Limited Liability Company, State Grid Electric Power Research Institute, Wuhan 430074, China c Xidian Surge Arrester Co., Ltd., Xi’an, Shaanxi 710200, China d College of Electrical Engineering, Xinjiang University, Urumqi, Xinjiang 830046, China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO varistors Multiple dopants Sintering process Electrical properties Surge arrester Overvoltage
In the present research, we studied ZnO varistors having multiple dopants and optimized their microstructure and electrical properties by tailoring the sintering process. At certain concentrations of the rare-earth elements yttrium and indium, the voltage gradient decreased from 683.7 to 293.0 V/mm as the sintering temperature was increased from 1160 to 1320 °C. In addition, the nonlinear coefficient first increased and then decreased as sintering temperature was increased. When sintered at 1240 °C, the multiple doped ZnO varistors samples showed excellent performance, exhibiting a voltage gradient of 520 V/mm, a residual voltage ratio of 1.55, and a density of 63.7 A/cm2 under a current waveform of 8/20 μs, and a nonlinear coefficient of 74.5. These optimized properties will be of great utility in the manufacture of metal oxide arresters. When the high-performance varistors were used in surge arresters of ultra-high voltage grids, the overvoltage level along the transmission line and in the substation were reduced by 31.14% and 33.53%, respectively. This combination of characteristics will dramatically improve the protective effect of surge arresters for deeply suppressing over-voltages in power systems, significantly reduce the insulation requirement and cost of power apparatus as well as greatly increase the safety of power systems.
1. Introduction Several 1000-kV ultra-high voltage (UHV) AC power transmission projects are operating in China, and more UHV projects will be constructed in the near future [1,2]. Hence, it has become important to reduce the construction costs and improve the efficiency of such UHV projects [3,4]. The insulation coordination of UHV AC systems is mainly controlled by the switching overvoltage level [4,5]. Among various methods used to control switching of overvoltages, improving the protection performance of surge arresters is the best way [5]. Currently, installing surge arresters at transmission line terminals is a complementary method to control the switching overvoltage along the transmission line [6]. To protect the substation, usually, three sets of surge arresters are installed in a UHV substation, one set of surge arresters is installed at the entrance of the transmission line to substation, the second set is installed on busbars, and the third set is installed to protect the power transformers [7].
⁎
ZnO varistors are polycrystalline n-type semi-conductive ceramics that can be prepared by mixing and sintering ZnO with other minor oxide additives [8,9]. Owing to their excellent nonlinear characteristics and surge energy absorption capabilities, these components have been widely applied in electrical systems as surge protection devices [10,11]. The insulation and protective level in high-voltage power systems depend mainly on the residual voltage ratio of the metal oxide arresters. Thus, decreasing the residual voltage ratio of ZnO varistors can effectively reduce the insulation requirements and the costs of power systems. It has been reported that reducing the sintering temperature and sintering time improves the voltage gradient [12]. In this study, we extend our previous research [13,14] to investigate the electrical properties of novel ZnO varistors co-doped with the rare-earth element yttrium and indium additives. We also examined the protective effect of these varistors when implements in surge arresters.
Corresponding author. E-mail address: [email protected] (J. Hu).
https://doi.org/10.1016/j.epsr.2019.106041 Received 15 March 2019; Received in revised form 31 August 2019; Accepted 19 September 2019 Available online 03 October 2019 0378-7796/ © 2019 Elsevier B.V. All rights reserved.
Electric Power Systems Research 178 (2020) 106041
P. Meng, et al.
coefficients α were calculated by the equation: α = 1/(logE2 − logE1), where E2 and E1 represent the electric fields at current densities of 1.0 and 0.1 mA/cm2, respectively. The capacitance–voltage (C–V) characteristics at 1 kHz were tested with a broadband dielectric device (Novocontrol Concept 80, Germany). The applied voltage dependence of the capacitance was determined as follows: (1/C − 1/C0)2 = 2(ϕb + V)/(qεsNd) [15], where C is the capacitance per unit area of a grain boundary, V is the applied voltage per grain boundary, C0 is the value of C when V = 0 V, q is the electronic charge, and εs is the permittivity of the ZnO grains. The donor density of the ZnO grains (Nd) and the barrier height of the double Schottky barrier at the grain boundaries (ϕb) of each sample can be determined from the slope and intercept of the line of (1/C − 1/C0)2 versus V. The power loss over the frequency range from 0.1 to 106 Hz under a 1.0-V applied AC voltage at room temperature of the samples treated at different sintering temperatures were measured with a Novolcontrol Alpha-A high-performance frequency analyzer (GmbH, Winston-Salem, Germany). The electromagnetic transient software EMTDC was used to model and simulate the power system. The simulation calculation model and design method were based on our previous research [14]. As the calculation model was the 1000 kV double-circuit UHV transmission system in Huainan–Wannan with a reference voltage of 1000 kV, and a base overvoltage of 898 kV, and Fig. 2 shows the circuit model diagram simulated by the EMTDC of the power system.
Fig. 1. The sintering processes of different ZnO varistor samples.
2. Experiments and simulation The multiple dopant-based ZnO varistor samples were prepared with the following compositions: 94.075 mol% ZnO, 1.0 mol% Bi2O3, 0.75 mol% MnO2, 1.0 mol% Co2O3, 0.5 mol% Cr2O3, 1 mol% Sb2O3, 1.2 mol% SiO2, 0.45 mol% Y2O3, and 0.025 mol% In(NO)3·9H2O. The analytical-grade raw materials were mixed in the determined ratios with deionized water in a planetary ball mill for 10 h. The resulting mixture was dried at 90 °C for 12 h and then pressed at 400 kg/cm2 into discs 30 mm in diameter and 2.0 mm thick. The discs were divided into five groups, all the samples were sintered in a furnace (Nabertherm LH60/14, Germany) under air at a heating rate of 3 °C/min to 700 °C, and then heated at a rate of 2 °C/min to 1100, 1150, 1200, 1250, and 1300 °C, and sintered for 2 h. All the samples were cooled at a cooling rate of 2 °C/min, as shown in Fig. 1. Then, a disc grinding machine was used to grind the surfaces of zinc oxide varistor samples and use the ultrasonic cleaner to remove the oil and dust on the surfaces of the samples. The samples were then placed in a drying chamber at 120 °C for 24 h to remove the moisture adsorbed in the samples. Finally, the both ends of the samples were covered with silver paste to act as electrodes. The surfaces of the samples were examined with a scanning electron microscope (SEM, Hitachi 8010 instrument, Japan). We used a source meter (Keithley 2410, USA) to measure the electric field-current density (E–J) characteristics of the samples in the pre-breakdown region. The voltage gradient (E1mA) was measured at a current density of 1.0 mA/ cm2, whereas the leakage current density JL was measured at 0.75 E1mA. An impulse current with a waveform of 8/20 μs was generated by an impulse generator (Keytek EMC Pro, USA). The residual voltage ratio K was calculated by the following formula: K = Un/U1mA, where Un is the voltage under a current density of 63.7 A/cm2. The nonlinear
3. Results and discussion The SEM images of the multiple dopant-based ZnO varistor samples prepared by different sintering processes are shown in Fig. 3. It can be observed from the scanning electron microscope image that the main compositions are spinel, ZnO grains and pores. The ZnO grains had a spinel phase and an intergranular phase. The average grain sizes (d) of the samples measured by the lineal intercept method [16] are summarized in Table 1. As observed, d increased from 4.5 to 8.6 μm as the sintering temperature increased from 1160 to 1320 °C. On the one hand, the growth rate of zinc oxide grains is faster at higher temperature, on the other hand, the effective growth time of grains is longer at higher sintering temperature. Therefore, the grain size of zinc oxide increases with the increase of sintering temperature. Fig. 4 shows the E–J characteristics from the pre-breakdown region to the upturn region of the samples for various sintering processes. Detailed electrical parameters, such as the voltage gradient (E1mA), leakage current (JL), and nonlinearity coefficient (α) of the samples deduced from the E–J characteristics are summarized in Table 1. As observed, the voltage gradient decreased sharply from 683.7 to 293.0 V/mm as the sintering temperature increased from 1160 to 1320 °C. Furthermore, the residual voltage ratio decreased from 1.69 to 1.55 as the sintering temperature increased from 1160 to 1240 °C. Nevertheless, as the sintering temperature increased to 1320 °C, the residual voltage ratio showed a slight increase to 1.61. In addition, the
Fig. 2. The circuit model diagram simulated by the EMTDC of the power system. 2
Electric Power Systems Research 178 (2020) 106041
P. Meng, et al.
Fig. 3. The SEM images of the samples prepared with various sintering processes.
nonlinear coefficient α showed the opposite trend; when the sintering temperature increased to 1240 °C, the nonlinearity coefficient increased from 31.7 to 74.5. When the sintering temperature increased to 1320 °C the parameter decreased to 52.0. Furthermore, the lowest leakage current of 0.58 μA/cm2 was obtained at a sintering temperature of 1240 °C. Fig. 5 shows the C–V characteristics: (1/C − 1/C0)2 as a function of Ugb for the multiple dopant ZnO varistors prepared by various sintering processes. The Nd of the grains and the ϕb of the double Schottky barrier at the grain boundaries were determined from the slope and intercept of the line (1/C − 1/C0)2 versus Ugb. The value of Ni can be calculated from the formula: ϕb = e Ni2/2 Ndε0ε. Parameters such as Nd, Ni, and ϕb are summarized in Table 1. The values of Nd and Ni of the samples increased as the sintering temperature increased from 1160 to 1320 °C. As a result, the highest barrier height ϕb of 1.97 eV was obtained at a sintering temperature of 1240 °C. The dielectric loss tan δ measured over the frequency range from 0.1 to 106 Hz is shown in Fig. 6. The lower dielectric loss tan δ at power frequency was beneficial for the aging performance and the stability of the power system assembled with surge arrestors. As observed, at low frequencies, the dielectric loss decreased with increasing sintering temperature, owing to the decrease of the double Schottky barrier height [17], which was consistent with experiment results of the electrical field-current density characteristics (E–J) as well as the C–V characteristics. The opposite trend was observed at high frequencies with increasing sintering temperature compared with the performance at low frequencies.
Fig. 4. The E–J characteristics from pre-breakdown region to upturn region of the samples with various sintering processes.
the two terminals of the electric transmission line. And Method B is installing closing resistor cooperate with arresters at the two terminals of the electric transmission line. Besides, Method C1 is arranging one set of surge arrester in the middle of the transmission line on the basis of Method A, and Method C3 is arranging three sets of surge arresters along the transmission line on the basis of Method A, respectively. We also compare three different arresters to limit the effects of operating overvoltage. The ordinary arrester (OA) is the arresters being used in the 1000 kV system, which operates at a rated voltage of 1.3 pu. The low residual arrester (LRA) is an arrester assembly with the obtained optimal performance ZnO varistors sintered at 1240 °C, and the rated
4. Analysis on application effects Four different ways to limit the effects of the operating overvoltage, namely Method A, Method B, Method C1, and Method C3 as described in our previous study [14]. Method A is installing different arresters at
Table 1 Microstructure and electrical parameters of the samples prepared with different sintering processes. Sintering temperature (°C)
E1mA (V/mm)
JL (μA/cm2)
α
d (μm)
K
Nd (1023 m−3)
Ni (1016 m−2)
ϕb (eV)
1160 1200 1240 1280 1320
683.7 617.1 520.0 404.2 293.0
1.19 1.23 0.58 0.59 2.42
37.1 51.3 74.5 71.3 52.0
4.5 5.4 6.5 7.3 8.6
1.69 1.63 1.55 1.60 1.61
7.3 7.5 8.1 8.3 9.3
3.5 3.6 3.9 3.9 4.0
1.79 1.84 1.95 1.92 1.82
3
Electric Power Systems Research 178 (2020) 106041
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shown in Fig. 7. According to the simulation results of the electromagnetic transient software EMTDC, we summarized various switching overvoltage and restriction effects of various restriction strategies that might appear in UHV systems. The overvoltage level along the transmission line is shown in Table 2. As the closing resistance cannot limit a single-phase ground overvoltage and fault clearing overvoltage, the limiting effect of Method A is the same as that of Method B for the switching overvoltage. The limited effect of the ordinary arrester cooperating with the closing resistor (Method B-OA) defines the present overvoltage level of the UHV transmission line. Furthermore, we observed that various switching overvoltage levels have a great degree of reduction when installed with low residual arresters or high electric load rate low residual arresters. For Method C1, when the LRA and the HLRA are used to replace the OA, the various switching overvoltages can be reduced by 8%–20% and 10%–20%, respectively. Additionally, for Method C3, the various switching overvoltages can be reduced by 18%–29% and 20%–31%, with the use of LRA and the HLRA to replace the OA, respectively. The overvoltage level in the substation is shown in Table 3. As observed, LRA or HLRA have advantages for limiting the switching overvoltage in the substation compared with the transmission line. The switching overvoltage at the substation can be reduced to levels of approximately 1.22 and 1.17 pu when using LRA and HLRA, respectively, which is beneficial for substation equipment. Similar to the overvoltage level along the transmission line, the methods C1 or C3 can achieve the best protective effect. Thus, installing three sets of LRA with a nominal voltage of 1.3 pu along the electric transmission line reduces all types of switching overvoltage by 6%–33%, which is a considerable improvement compared with the original overvoltage level of the UHV system. This result can markedly suppress the switching overvoltage, especially in the case of closed resistors unable to limit the overvoltage of a single-phase ground and fault clearing.
Fig. 5. The C–V characteristics of the samples with various sintering processes.
5. Conclusions In the present research, we studied the effects of sintering processes on the micro-characteristics and electrical properties of rare-earth element yttrium and indium dopant-based ZnO varistor ceramics. The Nd and Ni increased as the sintering temperature increased, and the highest barrier height ϕb of 1.97 eV was obtained at a sintering temperature of 1240 °C. The residual voltage ratio decreased from 1.69 to 1.55 as the sintering temperature increased from 1160 to 1240 °C. The varistor samples sintered at 1240 °C showed excellent all-round performance, exhibiting a nonlinear coefficient of 74.5, a leakage current of 0.58 μA/ cm2, a grain size of 6.5 μm, a 1-mA residual voltage of 520.0 V/mm, and a residual voltage ratio of 1.55 with a density of 63.7 A/cm2 under a current waveform of 8/20 μs. Furthermore, the use of the arresters assembled with the obtained optimal performing ZnO varistors to limit the switching overvoltage has considerable advantages compared with ordinary arresters. When an optimized restriction strategy is used, the overvoltage level along the transmission line and in the substation can be reduced by 31.14% and 33.53%, respectively. This optimal performance shows great potential for improving the protective effects of surge arresters assembled with ZnO varistors and the stability of power systems as well as realizing the target of suppressing lightning and switching overvoltages. As a result, the insulation requirement as well as the cost of power apparatus will be reduced, and the safety of power systems will be improved significantly with immeasurable economic benefits.
Fig. 6. The dielectric loss tan δ of the samples with different sintering processes with the frequency range from 0.1 to 106 Hz.
Fig. 7. The volt-ampere characteristic curves of different surge arresters.
Declaration of Competing Interest
voltage was 1.3 pu. The high electric load rate low residual arrester (HLRA) is a low residual arrester with a reduced rated voltage of 1.2 pu. The volt-ampere characteristic curves of different surge arresters are
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 4
Electric Power Systems Research 178 (2020) 106041
P. Meng, et al.
Table 2 The limited effect of arresters for the switching overvoltages along the transmission line. Method
A
B
C1
C3
Overvoltage type
Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing
OA [14]
LRA
HLRA
Overvoltage level (p.u.)
Overvoltage level (p.u.)
Percentage change (%)
Overvoltage level (p.u.)
Percentage change (%)
1.83 1.81 1.55 1.78 1.47 1.09 1.55 1.78 1.74 1.72 1.50 1.68 1.67 1.60 1.50 1.57
1.54 1.65 1.55 1.68 1.30 1.09 1.55 1.66 1.39 1.44 1.35 1.53 1.18 1.23 1.23 1.22
−14.85 −8.84 0 −5.62 −11.56 0 0 −6.74 −20.11 −16.28 −10.00 −8.93 −29.34 −23.13 −18.00 −22.29
1.43 1.59 1.55 1.61 1.23 1.09 1.55 1.61 1.38 1.39 1.27 1.51 1.15 1.17 1.19 1.15
−21.86 −12.15 0 −9.55 −16.33 0 0 −9.55 −20.69 −19.19 −15.33 −10.12 −31.14 −26.88 −20.67 −26.75
Table 3 The limited effect of arresters for the switching overvoltage in the substation. Method
A
B
C1
C3
Overvoltage type
Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing
OA [14]
LRA
HLRA
Overvoltage level (p.u.)
Overvoltage level (p.u.)
Percentage change (%)
Overvoltage level (p.u.)
Percentage change (%)
1.77 1.48 1.23 1.47 1.47 1.07 1.23 1.47 1.69 1.48 1.23 1.45 1.67 1.47 1.22 1.45
1.20 1.22 1.17 1.21 1.21 1.07 1.17 1.21 1.19 1.21 1.16 1.21 1.18 1.21 1.16 1.19
−32.20 −17.57 −4.88 −17.69 −17.69 0 −4.88 −17.69 −29.59 −18.24 −5.69 −16.55 −29.34 −17.69 −4.92 −17.93
1.13 1.16 1.16 1.15 1.17 1.06 1.16 1.16 1.13 1.15 1.15 1.15 1.11 1.14 1.14 1.13
−36.16 −21.62 −5.69 −21.77 −20.41 −0.94 −5.69 −21.09 −33.14 −22.30 −6.50 −20.69 −33.53 −22.45 −6.56 −22.07
Acknowledgements
(1982) 2694–2702. [7] J.L. He, Y. Zhou, Z.Q. Li, J. Yuan, Feasibility of using one-column-varistor arresters in 1000-kV UHV substations, IEEE Trans. Power Deliv. 31 (4) (2016) 1533–1541. [8] D.R. Clarke, Varistor ceramics, J. Am. Ceram. Soc. 82 (1999) 485–502. [9] M. Matsuoka, Nonohmic properties of zinc oxide ceramics, Jpn. J. Appl. Phys. 10 (6) (1971) 736–746. [10] T.K. Gupta, Application of zinc oxide varistors, J. Am. Ceram. Soc. 73 (7) (1990) 1817–1840. [11] P.F. Meng, S.Q. Gu, J. Wang, J. Hu, J.L. He, Improving electrical properties of multiple dopant ZnO varistor by doping with indium and gallium, Ceram. Int. 44 (2018) 1168–1171. [12] Q.H. Chen, J.L. He, K.X. Tan, S.M. Chen, M.Y. Yan, Influence of grain size on distribution of temperature and thermal stress in ZnO varistor ceramics, J. Sci. China Ser E 45 (4) (2002) 337–347. [13] P.F. Meng, S.L. Lyu, J. Hu, J.L. He, Tailoring low leakage current and high nonlinear coefficient of a Y-doped ZnO varistor by indium doping, Mater. Lett. 188 (2017) 77–79. [14] P.F. Meng, Y. Zhou, J.B. Wu, J. Hu, J.L. He, Novel ZnO varistors for dramatically improving protective effect of surge arresters, International Conference on Lightning Protection, September, Rzeszow, Poland, 2018, pp. 2–7. [15] K. Mukae, K. Tsuda, I. Nagasawa, Capacitance-vs-voltage characteristics of ZnO varistors, J. Appl. Phys. 50 (6) (1979) 4475–4476. [16] J.C. Wurst, J.A. Nelson, Lineal intercept technique for measuring grain size in twophase polycrystalline ceramics, J. Am. Ceram. Soc. 55 (1972) 109–111. [17] P.F. Meng, X. Yang, J.B. Wu, Q.Y. Xie, J.M. He, J. Hu, J.L. He, Stable electrical properties of ZnO varistor ceramics with multiple additives against the AC accelerated aging process, Ceram. Int., in press, Accepted manuscript, Available online 1 February 2019.
This work was supported by the Science and Technology Project of SGCC (Research and Application of Key Technologies of Lightning Protection for ± 1100 kV UHVDC Transmission Lines) and the National Natural Science Foundation of China under Grant No. 51762038. References [1] D.C. Huang, Y.B. Shu, J.J. Ruan, Y. Hu, Ultra high voltage transmission in China: developments, current status and future prospects, Proc. IEEE 97 (3) (2009) 555–583. [2] D.X. Gu, P.H. Zhou, M.H. Xiu, S. Wang, M. Dai, Y. Lou, Study on overvoltage and insulation coordination for 1000kV AC transmission system, High Voltage Eng. 32 (12) (2006) 1–6. [3] J.L. He, C. Li, J. Hu, R. Zeng, J. Yuan, Elimination of closing resistors for breakers in 1000-kV UHV system by surge arresters, IEEE Trans. Power Deliv. 27 (4) (2012) 2168–2175. [4] C. Li, J.L. He, J. Hu, R. Zeng, Switching transient of 1000-kV UHV system considering detailed substation structure, IEEE Trans. Power Deliv. 27 (1) (2012) 112–122. [5] J.L. He, C. Li, J. Hu, R. Zeng, Deep suppression of switching overvoltages in AC UHV systems using low residual arresters, IEEE Trans. Power Deliv. 26 (4) (2011) 2718–2725. [6] IEEE Working Group on Switching Surges, Switching surges: part IV – control and reduction on AC transmission lines, IEEE Trans. Power App. Syst. PAS-101 (8)
5
Contents lists available at ScienceDirect
Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Improving the protective effect of surge arresters by optimizing the electrical property of ZnO varistors
T
Pengfei Menga, Chao Yuana, Heng Xub, Shuai Wanb, Qingyun Xiec, Jimou Hec, Hongfeng Zhaod, ⁎ Jun Hua, , Jinliang Hea a State Key Laboratory of Control and Simulation of Power System and Generation Equipment, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China b Wuhan NARI Limited Liability Company, State Grid Electric Power Research Institute, Wuhan 430074, China c Xidian Surge Arrester Co., Ltd., Xi’an, Shaanxi 710200, China d College of Electrical Engineering, Xinjiang University, Urumqi, Xinjiang 830046, China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO varistors Multiple dopants Sintering process Electrical properties Surge arrester Overvoltage
In the present research, we studied ZnO varistors having multiple dopants and optimized their microstructure and electrical properties by tailoring the sintering process. At certain concentrations of the rare-earth elements yttrium and indium, the voltage gradient decreased from 683.7 to 293.0 V/mm as the sintering temperature was increased from 1160 to 1320 °C. In addition, the nonlinear coefficient first increased and then decreased as sintering temperature was increased. When sintered at 1240 °C, the multiple doped ZnO varistors samples showed excellent performance, exhibiting a voltage gradient of 520 V/mm, a residual voltage ratio of 1.55, and a density of 63.7 A/cm2 under a current waveform of 8/20 μs, and a nonlinear coefficient of 74.5. These optimized properties will be of great utility in the manufacture of metal oxide arresters. When the high-performance varistors were used in surge arresters of ultra-high voltage grids, the overvoltage level along the transmission line and in the substation were reduced by 31.14% and 33.53%, respectively. This combination of characteristics will dramatically improve the protective effect of surge arresters for deeply suppressing over-voltages in power systems, significantly reduce the insulation requirement and cost of power apparatus as well as greatly increase the safety of power systems.
1. Introduction Several 1000-kV ultra-high voltage (UHV) AC power transmission projects are operating in China, and more UHV projects will be constructed in the near future [1,2]. Hence, it has become important to reduce the construction costs and improve the efficiency of such UHV projects [3,4]. The insulation coordination of UHV AC systems is mainly controlled by the switching overvoltage level [4,5]. Among various methods used to control switching of overvoltages, improving the protection performance of surge arresters is the best way [5]. Currently, installing surge arresters at transmission line terminals is a complementary method to control the switching overvoltage along the transmission line [6]. To protect the substation, usually, three sets of surge arresters are installed in a UHV substation, one set of surge arresters is installed at the entrance of the transmission line to substation, the second set is installed on busbars, and the third set is installed to protect the power transformers [7].
⁎
ZnO varistors are polycrystalline n-type semi-conductive ceramics that can be prepared by mixing and sintering ZnO with other minor oxide additives [8,9]. Owing to their excellent nonlinear characteristics and surge energy absorption capabilities, these components have been widely applied in electrical systems as surge protection devices [10,11]. The insulation and protective level in high-voltage power systems depend mainly on the residual voltage ratio of the metal oxide arresters. Thus, decreasing the residual voltage ratio of ZnO varistors can effectively reduce the insulation requirements and the costs of power systems. It has been reported that reducing the sintering temperature and sintering time improves the voltage gradient [12]. In this study, we extend our previous research [13,14] to investigate the electrical properties of novel ZnO varistors co-doped with the rare-earth element yttrium and indium additives. We also examined the protective effect of these varistors when implements in surge arresters.
Corresponding author. E-mail address: [email protected] (J. Hu).
https://doi.org/10.1016/j.epsr.2019.106041 Received 15 March 2019; Received in revised form 31 August 2019; Accepted 19 September 2019 Available online 03 October 2019 0378-7796/ © 2019 Elsevier B.V. All rights reserved.
Electric Power Systems Research 178 (2020) 106041
P. Meng, et al.
coefficients α were calculated by the equation: α = 1/(logE2 − logE1), where E2 and E1 represent the electric fields at current densities of 1.0 and 0.1 mA/cm2, respectively. The capacitance–voltage (C–V) characteristics at 1 kHz were tested with a broadband dielectric device (Novocontrol Concept 80, Germany). The applied voltage dependence of the capacitance was determined as follows: (1/C − 1/C0)2 = 2(ϕb + V)/(qεsNd) [15], where C is the capacitance per unit area of a grain boundary, V is the applied voltage per grain boundary, C0 is the value of C when V = 0 V, q is the electronic charge, and εs is the permittivity of the ZnO grains. The donor density of the ZnO grains (Nd) and the barrier height of the double Schottky barrier at the grain boundaries (ϕb) of each sample can be determined from the slope and intercept of the line of (1/C − 1/C0)2 versus V. The power loss over the frequency range from 0.1 to 106 Hz under a 1.0-V applied AC voltage at room temperature of the samples treated at different sintering temperatures were measured with a Novolcontrol Alpha-A high-performance frequency analyzer (GmbH, Winston-Salem, Germany). The electromagnetic transient software EMTDC was used to model and simulate the power system. The simulation calculation model and design method were based on our previous research [14]. As the calculation model was the 1000 kV double-circuit UHV transmission system in Huainan–Wannan with a reference voltage of 1000 kV, and a base overvoltage of 898 kV, and Fig. 2 shows the circuit model diagram simulated by the EMTDC of the power system.
Fig. 1. The sintering processes of different ZnO varistor samples.
2. Experiments and simulation The multiple dopant-based ZnO varistor samples were prepared with the following compositions: 94.075 mol% ZnO, 1.0 mol% Bi2O3, 0.75 mol% MnO2, 1.0 mol% Co2O3, 0.5 mol% Cr2O3, 1 mol% Sb2O3, 1.2 mol% SiO2, 0.45 mol% Y2O3, and 0.025 mol% In(NO)3·9H2O. The analytical-grade raw materials were mixed in the determined ratios with deionized water in a planetary ball mill for 10 h. The resulting mixture was dried at 90 °C for 12 h and then pressed at 400 kg/cm2 into discs 30 mm in diameter and 2.0 mm thick. The discs were divided into five groups, all the samples were sintered in a furnace (Nabertherm LH60/14, Germany) under air at a heating rate of 3 °C/min to 700 °C, and then heated at a rate of 2 °C/min to 1100, 1150, 1200, 1250, and 1300 °C, and sintered for 2 h. All the samples were cooled at a cooling rate of 2 °C/min, as shown in Fig. 1. Then, a disc grinding machine was used to grind the surfaces of zinc oxide varistor samples and use the ultrasonic cleaner to remove the oil and dust on the surfaces of the samples. The samples were then placed in a drying chamber at 120 °C for 24 h to remove the moisture adsorbed in the samples. Finally, the both ends of the samples were covered with silver paste to act as electrodes. The surfaces of the samples were examined with a scanning electron microscope (SEM, Hitachi 8010 instrument, Japan). We used a source meter (Keithley 2410, USA) to measure the electric field-current density (E–J) characteristics of the samples in the pre-breakdown region. The voltage gradient (E1mA) was measured at a current density of 1.0 mA/ cm2, whereas the leakage current density JL was measured at 0.75 E1mA. An impulse current with a waveform of 8/20 μs was generated by an impulse generator (Keytek EMC Pro, USA). The residual voltage ratio K was calculated by the following formula: K = Un/U1mA, where Un is the voltage under a current density of 63.7 A/cm2. The nonlinear
3. Results and discussion The SEM images of the multiple dopant-based ZnO varistor samples prepared by different sintering processes are shown in Fig. 3. It can be observed from the scanning electron microscope image that the main compositions are spinel, ZnO grains and pores. The ZnO grains had a spinel phase and an intergranular phase. The average grain sizes (d) of the samples measured by the lineal intercept method [16] are summarized in Table 1. As observed, d increased from 4.5 to 8.6 μm as the sintering temperature increased from 1160 to 1320 °C. On the one hand, the growth rate of zinc oxide grains is faster at higher temperature, on the other hand, the effective growth time of grains is longer at higher sintering temperature. Therefore, the grain size of zinc oxide increases with the increase of sintering temperature. Fig. 4 shows the E–J characteristics from the pre-breakdown region to the upturn region of the samples for various sintering processes. Detailed electrical parameters, such as the voltage gradient (E1mA), leakage current (JL), and nonlinearity coefficient (α) of the samples deduced from the E–J characteristics are summarized in Table 1. As observed, the voltage gradient decreased sharply from 683.7 to 293.0 V/mm as the sintering temperature increased from 1160 to 1320 °C. Furthermore, the residual voltage ratio decreased from 1.69 to 1.55 as the sintering temperature increased from 1160 to 1240 °C. Nevertheless, as the sintering temperature increased to 1320 °C, the residual voltage ratio showed a slight increase to 1.61. In addition, the
Fig. 2. The circuit model diagram simulated by the EMTDC of the power system. 2
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Fig. 3. The SEM images of the samples prepared with various sintering processes.
nonlinear coefficient α showed the opposite trend; when the sintering temperature increased to 1240 °C, the nonlinearity coefficient increased from 31.7 to 74.5. When the sintering temperature increased to 1320 °C the parameter decreased to 52.0. Furthermore, the lowest leakage current of 0.58 μA/cm2 was obtained at a sintering temperature of 1240 °C. Fig. 5 shows the C–V characteristics: (1/C − 1/C0)2 as a function of Ugb for the multiple dopant ZnO varistors prepared by various sintering processes. The Nd of the grains and the ϕb of the double Schottky barrier at the grain boundaries were determined from the slope and intercept of the line (1/C − 1/C0)2 versus Ugb. The value of Ni can be calculated from the formula: ϕb = e Ni2/2 Ndε0ε. Parameters such as Nd, Ni, and ϕb are summarized in Table 1. The values of Nd and Ni of the samples increased as the sintering temperature increased from 1160 to 1320 °C. As a result, the highest barrier height ϕb of 1.97 eV was obtained at a sintering temperature of 1240 °C. The dielectric loss tan δ measured over the frequency range from 0.1 to 106 Hz is shown in Fig. 6. The lower dielectric loss tan δ at power frequency was beneficial for the aging performance and the stability of the power system assembled with surge arrestors. As observed, at low frequencies, the dielectric loss decreased with increasing sintering temperature, owing to the decrease of the double Schottky barrier height [17], which was consistent with experiment results of the electrical field-current density characteristics (E–J) as well as the C–V characteristics. The opposite trend was observed at high frequencies with increasing sintering temperature compared with the performance at low frequencies.
Fig. 4. The E–J characteristics from pre-breakdown region to upturn region of the samples with various sintering processes.
the two terminals of the electric transmission line. And Method B is installing closing resistor cooperate with arresters at the two terminals of the electric transmission line. Besides, Method C1 is arranging one set of surge arrester in the middle of the transmission line on the basis of Method A, and Method C3 is arranging three sets of surge arresters along the transmission line on the basis of Method A, respectively. We also compare three different arresters to limit the effects of operating overvoltage. The ordinary arrester (OA) is the arresters being used in the 1000 kV system, which operates at a rated voltage of 1.3 pu. The low residual arrester (LRA) is an arrester assembly with the obtained optimal performance ZnO varistors sintered at 1240 °C, and the rated
4. Analysis on application effects Four different ways to limit the effects of the operating overvoltage, namely Method A, Method B, Method C1, and Method C3 as described in our previous study [14]. Method A is installing different arresters at
Table 1 Microstructure and electrical parameters of the samples prepared with different sintering processes. Sintering temperature (°C)
E1mA (V/mm)
JL (μA/cm2)
α
d (μm)
K
Nd (1023 m−3)
Ni (1016 m−2)
ϕb (eV)
1160 1200 1240 1280 1320
683.7 617.1 520.0 404.2 293.0
1.19 1.23 0.58 0.59 2.42
37.1 51.3 74.5 71.3 52.0
4.5 5.4 6.5 7.3 8.6
1.69 1.63 1.55 1.60 1.61
7.3 7.5 8.1 8.3 9.3
3.5 3.6 3.9 3.9 4.0
1.79 1.84 1.95 1.92 1.82
3
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shown in Fig. 7. According to the simulation results of the electromagnetic transient software EMTDC, we summarized various switching overvoltage and restriction effects of various restriction strategies that might appear in UHV systems. The overvoltage level along the transmission line is shown in Table 2. As the closing resistance cannot limit a single-phase ground overvoltage and fault clearing overvoltage, the limiting effect of Method A is the same as that of Method B for the switching overvoltage. The limited effect of the ordinary arrester cooperating with the closing resistor (Method B-OA) defines the present overvoltage level of the UHV transmission line. Furthermore, we observed that various switching overvoltage levels have a great degree of reduction when installed with low residual arresters or high electric load rate low residual arresters. For Method C1, when the LRA and the HLRA are used to replace the OA, the various switching overvoltages can be reduced by 8%–20% and 10%–20%, respectively. Additionally, for Method C3, the various switching overvoltages can be reduced by 18%–29% and 20%–31%, with the use of LRA and the HLRA to replace the OA, respectively. The overvoltage level in the substation is shown in Table 3. As observed, LRA or HLRA have advantages for limiting the switching overvoltage in the substation compared with the transmission line. The switching overvoltage at the substation can be reduced to levels of approximately 1.22 and 1.17 pu when using LRA and HLRA, respectively, which is beneficial for substation equipment. Similar to the overvoltage level along the transmission line, the methods C1 or C3 can achieve the best protective effect. Thus, installing three sets of LRA with a nominal voltage of 1.3 pu along the electric transmission line reduces all types of switching overvoltage by 6%–33%, which is a considerable improvement compared with the original overvoltage level of the UHV system. This result can markedly suppress the switching overvoltage, especially in the case of closed resistors unable to limit the overvoltage of a single-phase ground and fault clearing.
Fig. 5. The C–V characteristics of the samples with various sintering processes.
5. Conclusions In the present research, we studied the effects of sintering processes on the micro-characteristics and electrical properties of rare-earth element yttrium and indium dopant-based ZnO varistor ceramics. The Nd and Ni increased as the sintering temperature increased, and the highest barrier height ϕb of 1.97 eV was obtained at a sintering temperature of 1240 °C. The residual voltage ratio decreased from 1.69 to 1.55 as the sintering temperature increased from 1160 to 1240 °C. The varistor samples sintered at 1240 °C showed excellent all-round performance, exhibiting a nonlinear coefficient of 74.5, a leakage current of 0.58 μA/ cm2, a grain size of 6.5 μm, a 1-mA residual voltage of 520.0 V/mm, and a residual voltage ratio of 1.55 with a density of 63.7 A/cm2 under a current waveform of 8/20 μs. Furthermore, the use of the arresters assembled with the obtained optimal performing ZnO varistors to limit the switching overvoltage has considerable advantages compared with ordinary arresters. When an optimized restriction strategy is used, the overvoltage level along the transmission line and in the substation can be reduced by 31.14% and 33.53%, respectively. This optimal performance shows great potential for improving the protective effects of surge arresters assembled with ZnO varistors and the stability of power systems as well as realizing the target of suppressing lightning and switching overvoltages. As a result, the insulation requirement as well as the cost of power apparatus will be reduced, and the safety of power systems will be improved significantly with immeasurable economic benefits.
Fig. 6. The dielectric loss tan δ of the samples with different sintering processes with the frequency range from 0.1 to 106 Hz.
Fig. 7. The volt-ampere characteristic curves of different surge arresters.
Declaration of Competing Interest
voltage was 1.3 pu. The high electric load rate low residual arrester (HLRA) is a low residual arrester with a reduced rated voltage of 1.2 pu. The volt-ampere characteristic curves of different surge arresters are
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 4
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Table 2 The limited effect of arresters for the switching overvoltages along the transmission line. Method
A
B
C1
C3
Overvoltage type
Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing
OA [14]
LRA
HLRA
Overvoltage level (p.u.)
Overvoltage level (p.u.)
Percentage change (%)
Overvoltage level (p.u.)
Percentage change (%)
1.83 1.81 1.55 1.78 1.47 1.09 1.55 1.78 1.74 1.72 1.50 1.68 1.67 1.60 1.50 1.57
1.54 1.65 1.55 1.68 1.30 1.09 1.55 1.66 1.39 1.44 1.35 1.53 1.18 1.23 1.23 1.22
−14.85 −8.84 0 −5.62 −11.56 0 0 −6.74 −20.11 −16.28 −10.00 −8.93 −29.34 −23.13 −18.00 −22.29
1.43 1.59 1.55 1.61 1.23 1.09 1.55 1.61 1.38 1.39 1.27 1.51 1.15 1.17 1.19 1.15
−21.86 −12.15 0 −9.55 −16.33 0 0 −9.55 −20.69 −19.19 −15.33 −10.12 −31.14 −26.88 −20.67 −26.75
Table 3 The limited effect of arresters for the switching overvoltage in the substation. Method
A
B
C1
C3
Overvoltage type
Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing Line energizing Single-phase reclosure Single-phase ground Fault clearing
OA [14]
LRA
HLRA
Overvoltage level (p.u.)
Overvoltage level (p.u.)
Percentage change (%)
Overvoltage level (p.u.)
Percentage change (%)
1.77 1.48 1.23 1.47 1.47 1.07 1.23 1.47 1.69 1.48 1.23 1.45 1.67 1.47 1.22 1.45
1.20 1.22 1.17 1.21 1.21 1.07 1.17 1.21 1.19 1.21 1.16 1.21 1.18 1.21 1.16 1.19
−32.20 −17.57 −4.88 −17.69 −17.69 0 −4.88 −17.69 −29.59 −18.24 −5.69 −16.55 −29.34 −17.69 −4.92 −17.93
1.13 1.16 1.16 1.15 1.17 1.06 1.16 1.16 1.13 1.15 1.15 1.15 1.11 1.14 1.14 1.13
−36.16 −21.62 −5.69 −21.77 −20.41 −0.94 −5.69 −21.09 −33.14 −22.30 −6.50 −20.69 −33.53 −22.45 −6.56 −22.07
Acknowledgements
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