a 500 kv 1 b 220%kV 母线节点 500%kV 母线节点 500%kV 母线 3-4 入中性点地中性点开关入地电流 I i 电 %kV 母线中性点开关公共绕阻电组 T ci 变电站接地电阻串接绕组电阻 T si + 变电站 7-8

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1 ELECTRI C MACHINES AND CONTROL Vol. 14 No. 7 July TM TM 622 A X Evaluation and mitigation for DC bias in Hydropower plant ZHANG Lu 1 PAN Zhuo-hong 1 LEI Ming 2 TAN Bo 1 ZHANG Zeng 1 WEN Xi-Shan 1 1. School of electrical engineering Wuhan University Wuhan China 2. Luoyang power supply company Luoyang China Abstract The evaluation model for transformer s DC bias in hydropower plant under HVDC ground return operating model was presented. The developed software based on this model was established. Comparision between mesured value and calculated result proved the effectiveness of the model. Current injection method was analyzed and was stressed on the ways of surpress DC bias. Take JingPing hydropower plant for example suggestion on construction and operation of the hydropower plant were presented. With the practical condition of hydropower plant different DC bias mitigation methods were analyzed. Monitoring data verified the effectiveness of the current injection method and the impact of soil resistivity and compensation distance on compensation degree was quantitatively analyzed. Simulation and application of the method show that current injection actually functions as compensation of potential difference to the bus of substation. Increase in soil resistivity demand or decrease in compensation distance lead to the increase in injection current. But it leads to lower compensation degree and the compensation degree usually bellows The most advantage of current injection method is to keep neutral unchanged and save space of station. Only ones set of the equiment can safely supress the transformer DC bias in hydroplant. Key words HVDC transmission transformer DC bias current injection method CB

2 a 500 kv 1 b 220%kV 母线节点 500%kV 母线节点 500%kV 母线 3-4 入中性点地中性点开关入地电流 I i 电 %kV 母线中性点开关公共绕阻电组 T ci 变电站接地电阻串接绕组电阻 T si + 变电站 7-8 地电位 - Pi (a) (b) kv 11 Fig kv substation using autotransformer 接地网变压器 GV = SP 20 G 7-8 P = YI + MI DC 流 1 V P 10 S 9-11 Y I M I DC I = XV 3 X Visual C ~ 3 G - SYX V = SMI DC 4 II I = X G - SYX - 1 SMI DC

7 31 7-11 2014 92 112 193 2 2009a Visual C ++ 2005 3 3 3 Fig. 3 3 The diagram of Sichuan power grid 500 kv 2 57 km Fig.

3 a Visual C Fig. 3 3 The diagram of Sichuan power grid 500 kv 2 57 km Fig. 2 Local connection diagram of AC grid 320 km mm MW Table Horizontal 4 layer soil structure parameter ρ i / Ω m h i /m MW A A Table 2 Comparison between computational result and measuring data /A /A 5. 2 A 6 A / Ω 0. 7 Ω 1. 0 Ω 1. 5 Ω 2. 0 Ω ~ Ω m A

4 ρ 1 = 50 Ω m ρ 2 = Ω m 21 5 M 3 中性点直流电流 /A 中性点直流电流 /A R=0.4% 赘 R=0.7% 赘 R=1.5% 赘 R=2.0% 赘 80 p1=50 赘 m 70 p2=100 赘 m p3=500 赘 m 60 p4=1 赘 m %%%%%0.6%%%%%0.8%%%%%%%1%%%%%%1.2%%%%%1.4%%%%%1.6%%%%%%1.8%%%%%%%2 接地电阻 / 赘 %%%%%2%%%%%%4%%%%%6%%%%%%8%%%%10%%%%12%%%14%%%%16%%%%18%%%%20 地壤电阻率 /k 赘 m 4 Fig. 4 uniform soil s result KC i R 10 i C 6 K 2 5 V mi = - C i R mi Fig. 5 layers horizontal soil s result R mi i V mi 3 R mi < R i K 1 19

5 7 33 C i R i - R mi 19 K R i C 4 - R mi /R i 3 ρ d Table 3 ρ and d s impact on current injection method ρ /Ω m d /km C i /A η 4 1 GV = SP + C 6 C 2 P = YI + MI DC - ZC 7 Z I C η < 1 d ρ C η ρ d C η ρ d ρ < 6100 Ω m d 0. 3 ~ 1 km η ~ d > 1 km η ρ > 500 Ω m d 1 km η 0. 4 ρ > Ω m d 2 km η > 0. 4 母线 Ω m 1 km 3 4 电流注入装置 ( 置于主变洞外 ) 控制测量变压器 4 Table 4 Test data of current injection method 保护双向可控电流源 I 主变洞 C i /A I 1 /A I 2 /A η 站外补偿接地极 C 水电站接地网 C i I 1 I 2 6 Fig. 6 Injection current method in hydropower station 4 4 ρ = Ω m d < 500 kv 3 km η < kv Ω d 500 kv 220 kv η A - 5 A ρ d ρ d R C η

6 Ω 19 7 J 2005 ZHU Yiying 8 J CAO Zhaojun 1 km 8 J ZHANG Bo ZENG R HE J et al. Numerical analysis of potential distribution between ground electrodes of HVDC system con- sidering the effect of deep earth layers J 11 J 0. 6 J. High Voltage Engineering J JIANG Wei WU Guangning XIAO Hua et al. Effects of transformer DC bias and the relationship between UHVDC grounding current and grounding resistances J. High Voltage Engineering 1. M E. M J GIC in power systems J. IEEE Transactions on Power Delivery LI Xiaoping WEN Xishan CHENG Cixuan. Simulating analysis of exciting current of single phase transformer on DC bias J. High Voltage Engineering SHANG Chun. Measure to decrease the neutral current of AC J LI Xiaoping WEN Xishan LAN Lei et al. Test and simulation for single-phase transformer under DC bias J. Proceedings of the CSEE HUANG Fucheng RUAN Jiangjun ZHANG Yu et al. DC J magnetic bias induced current effects on transformer and restricting methods J. High Voltage Engineering ZHONG Lianhong LU Peijun QIU Zhicheng et al. The influence of current of DC earthing electrode on directly grounded trans former J. High Voltage Engineering J ZENG Liansheng. Impact of HVDC ground electrode current on the adjacent power transformer J. High Voltage Engineering JIANG Weiping ZHENG Zhaohua et al. Studying on measures of restraining DC current through transformer neutrals J. Proceedings of the CSEE HE Junjia YE Huisheng et al. Method for the calculation of DC current distribution in AC system when HVDC operating in ground - return mode J. High Voltage Engineering ZHANG Bo ZHAO Jie ZENG Rong et al. Estimation of DC current distribution in AC power system caused by HVDC transmission system in ground return status J. Proceedings of the CSEE. Generation Transmission & Distribution IET ± 800kV LIU Lianguang CUI Mingde SUN Zhongming et al. Influence scope of AC net work by DC grounding electrode rated ± 800 kv KAPPENMAN G NORR S R SWEEZY G A et al. GIC mitigation a neutral blocking /bypass device to prevent the flow of 14. HVDC J transformer in HVDC ground-return system J. High Voltage Engineering J

7 J J ZHAO Jie LI Xiaolin LU Jinzhuang et al. Applying series resistor to restrain power transformer DC biasing J. Automation of Electric Power Systems HVDC Power Systems J ZENG Rong ZHAO Jie SHANG Chun. Measures to Restrain the neutral current of the AC transformer in HVDC ground return system J. High Voltage Engineering Guangdong Electric Power J DU Zhongdong DONG Xiaohui WANG Jianwu et al. Test and analysis on restraining transformer DC bias by changing electric potential of grounding grid J. High Voltage Engineering J KUAI Dizheng WAN Da ZOU Yun. Analysis and handling of the impact of geomagnetically induced current upon electric network equipment in DC transmission J. Automation of Electric 20. J MA Zhiqiang. A potential compensation method to eliminate transformer neutral direct current produced by HVDC J. 21. M 檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪 PWM control J ZHAO Rende HE Yikang. Simulation study on the virtual line flux oriented vector control of the PWM rectifier J. Proceedings of the CSU - EPSA PWM doubly-fed induction machine drive based on computer simulation J ZHAO Rende HE Yikang. Virtual line-flux-linkage oriented vector control of three-phase voltage source PWM rectifier without line voltage sensors J. Proceedings of the CSEE PWM J WU Fengjiang WANG Zhiwen SUN Li. Improved virtual flux o- riented vector control of PWM rectifier J. Electric Machines and 8 MARIUSZ Malinowski MARIAN P Kazmierkowski STEFFAN Hansen et al. Virtual-flux-based direct power control of threephase PWM rectifiers J. IEEE Transactions on Industry Applications WANG Song DING Yunshi. Stability analysis of field oriented J. Electric Power Components and Systems MALINOWSKI M SZCZYGIEL W KAZMIERKOWSKI M P et al. Sensorless operation of active damping methods for threephase PWM converters J. IEEE ISIE

34 22 f t = f 0 w t + f r t f w θ t = F cos p - ω 0 t - φ 1 2 f r θ t = F cos p - ω 0 t - φ 2 3 p ω 0 F F φ 1 φ 2 t A B s Fig. 1

34 22 f t = f 0 w t + f r t f w θ t = F cos p - ω 0 t - φ 1 2 f r θ t = F cos p - ω 0 t - φ 2 3 p ω 0 F F φ 1 φ 2 t A B s Fig. 1 22 2 2018 2 Electri c Machines and Control Vol. 22 No. 2 Feb. 2018 1 2 3 3 1. 214082 2. 214082 3. 150001 DOI 10. 15938 /j. emc. 2018. 02. 005 TM 301. 4 A 1007-449X 2018 02-0033- 08 Research of permanent

a 500 kv 1 b 220%kV 母线节点 500%kV 母线节点 500%kV 母线 3-4 入中性点地中性点开关入地电流 I i 电 %kV 母线 中性点开关 公共绕阻电组 T ci 变电站接地电阻 串接绕组电阻 T si + 变电站 7-8

a 500 kv 1 b 220%kV 母线节点 500%kV 母线节点 500%kV 母线 3-4 入中性点地中性点开关入地电流 I i 电 %kV 母线中性点开关公共绕阻电组 T ci 变电站接地电阻串接绕组电阻 T si + 变电站 7-8