DC Overvoltage Suppression Strategy for MMC-MTDC Based on Bridge Arm Modulated Wave Adjustment

doi:10.11930/j.issn.1004-9649.202311116

Abstract

Abstract:

Modular multilevel converter based multi-terminal DC transmission (MMC-MTDC) system can realize multi-source power supplies and multi-terminal power-receiving. Flexible in operation modes, it is thus an effective technical means to solve the problem of grid-access and accommodation of clean energy. However, when AC fault occurs at the AC side of the receiving-end system, the created surplus power of the system will lead to serious DC overvoltage. In view of this, this paper proposes a DC overvoltage suppression strategy for multi-terminal flexible HVDC system based on bridge arm modulated wave dynamic adjustment. The proposed strategy introduces DC voltage deviation control into the modulation wave of the converter valve bridge arm, dynamically adjusting the DC voltage reference value of the bridge arm modulation wave during transient periods, thereby reducing the number of sub modules invested in the bridge arm, and ultimately achieving the goal of suppressing DC overvoltage. In order to verify the effectiveness of the proposed control strategy, a four-terminal flexible HVDC transmission system for photovoltaic and hydropower transmission is built in PSCAD/EMTDC, and the parameter design method of the proposed control strategy is given. Finally, by setting different operating conditions for the receiving-end AC system, a comparative study is made on the DC overvoltage characteristics of the system after implementation of the proposed control strategy. The results show that the proposed control strategy can effectively suppress the DC overvoltage of the MMC-HVDC transmission system caused by the receiving-end AC-side fault.

Key words: multi-terminal flexible HVDC system, surplus power, DC overvoltage, bridge arm modulated wave dynamic adjustment, voltage deviation control

Maolan PENG, Lei FENG, Yu WANG, Liqing XU, Wei ZHAO, Chunyi GUO. DC Overvoltage Suppression Strategy for MMC-MTDC Based on Bridge Arm Modulated Wave Adjustment[J]. Electric Power, 2024, 57(4): 171-181.

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URL: https://www.electricpower.com.cn/EN/10.11930/j.issn.1004-9649.202311116

https://www.electricpower.com.cn/EN/Y2024/V57/I4/171

Figures/Tables 15

Fig.1 Four-terminal flexible HVDC system

Table 1 System Parameters

系统参数		数值
额定直流电压/kV		±800
4个换流站额定直流功率/MW		10000
光伏场站额定有功功率/MW		7500
水电站额定有功功率/MW		2500
直流线路I长度/km		2000
直流线路Ⅱ长度/km		400
直流线路电阻R₀/(Ω·km^–1)		0.0042
直流线路电感L₀/(H·km^–1)		0.0008
直流线路电容C₀/(μF·km^–1)		0.0065
受端交流系统短路比		5
单个换流阀额定直流电压/kV		400
单个换流阀额定直流功率/kW		1250
单个换流阀的桥臂子模块个数		220
子模块额定电容电压/kV		2.1

Fig.2 The bridge arm modulated wave generation and sub-module modulation process

Fig.3 Modulated wave adjustment method with additional voltage deviation control

Fig.4 The relation curve between Udci_max2 and Udci_max1 with K=0.8

Fig.5 The DC voltage waveform of converter stations under AC fault of inverter station 1

Fig.6 The number of sub-modules put into one-phase bridge arm under AC fault of inverter station 1

Fig.7 The capacitor voltage waveform of sub-module under AC fault of inverter station 1

Fig.8 The DC power waveform of converter station under AC fault of inverter station 1

Table 2 Comparison between theoretical calculation and simulation values of DC overvoltage of all converters under AC fault of inverter station 1

换流站	直流过电压理论计算值/kV	直流过电压仿真值/kV	相对误差/%
整流站	894	892	0.2
逆变站1	845	838	0.8
逆变站2	824	844	2.4

Fig.9 The DC voltage of converter station under AC fault of inverter station 2

Fig.10 The number of sub-modules put into one-phase bridge arm under AC fault of inverter station 2

Fig.11 The capacitor voltage waveform of sub-module under AC fault of inverter station 2

Fig.12 The DC power waveform of converter station under AC fault of inverter station 2

Table 3 Comparison between theoretical calculation and simulation values of DC overvoltage of all converters under AC faults of inverter station 2

换流站	直流过电压理论计算值/kV	直流过电压仿真值/kV	相对误差/%
整流站	932	937	0.5
逆变站1	855	882	3.1
逆变站2	861	878	1.9

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