# Analysis of Fault and Protection Strategy of a Converter Station in MMC-HVDC System

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## Abstract

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## 1. Introduction

- (a)
- This paper proposes to set a certain number of hot standby sub-modules and cold standby sub-modules in each bridge arm.
- (b)
- The causes of the AC system fault and internal fault of the converter in the converter station of the MMC-HVDC system are analyzed.
- (c)
- The sub-module fault is studied. For the fault, innovative sub-module redundancy protection is designed, and the correctness of the scheme is verified.
- (d)
- The bridge arm reactor fault in the converter station is studied. For the fault, innovative bridge arm overcurrent protection strategies are designed, and the correctness of the scheme is verified.

## 2. The Establishment of the MMC-HVDC Mathematical Mode

#### 2.1. MMC Topology

#### 2.2. How MMC Works

- (1)
- Charging mode: current flows in from A, passes through diode D1, flows through capacitor forward, and flows out from B. The capacitor is then plugged into the circuit and charged.
- (2)
- Bypass mode: current flows in from B, passes through diode D2, and flows out from A. The capacitor is not connected to the circuit and is in a bypass state.Input state: T1 open, T2 off
- (3)
- Discharge mode: current flows in from B, flows through the capacitor in reverse, passes through conduction T1, and flows out from A. The capacitor is then plugged into the circuit and discharged.
- (4)
- Resection status: T1 resection and T2 opening
- (5)
- Bypass state: when the current flows in from A, it flows out from B through conducting T2. The capacitor is not connected to the circuit and is in a bypass state.
- (6)
- Bypass state: when the current flows in form B, it passes through diode D2, and flows out from A. The capacitor is not connected to the circuit and is in a bypass state.

_{dc}/N. The output level is changed by changing the number of sub-modules input by the upper and lower bridge arms. The output level is equal to the number of sub-modules on each bridge arm plus one, that is, if there are N sub-modules in each phase, the output level is N/2 + 1.

_{g}is the AC voltage at the outlet of the converter, U

_{dc}is the DC side voltage of the converter, U

_{u}and U

_{p}are the voltage of the upper and lower bridge arms, I

_{u}, I

_{p}, I

_{g}, and I

_{c}are the current of the upper bridge arm, the current of the lower bridge arm, the phase current of the AC port and the internal circulation, L and R are the inductance and equivalent resistance of the bridge arm respectively. The bridge arm resistor can simulate the power loss inside each bridge arm of the MMC.

_{u}, lower bridge arm current I

_{p}, AC port phase current I

_{g}, and internal circulation I

_{cir}can be expressed as follows:

_{u}, the voltage of lower bridge arm U

_{p}, common-mode voltage of upper and lower bridge arm U

_{com}and differential mode voltage of upper and lower bridge arm U

_{diff}can be expressed as follows:

_{ref}, Q

_{ref}, U

_{acref}, U

_{dcref}, and other reference values of each active and reactive power class control, which are transmitted to the converter station level control layer. Then the station level control receives the setting values of system-level control to generate modulation ratio and phase shift angle signals, which are transmitted to the valve level control layer, finally achieving the control goal.

#### 2.3. MMC Modulation Method

_{u}and N

_{p}are respectively:

_{s}represents the voltage instantaneous value of the modulated wave, and U

_{c}represents the average value of capacitor voltage of all sub-modules on a phase bridge arm. Round(x) is the nearest rounded function, taking the nearest integer to x. For example, if U

_{s}/U

_{c}= 5.2, invest 5 sub-modules; if U

_{s}/U

_{c}= 5.8, invest 6 sub-modules.

#### 2.4. Internal Circulation Suppression Strategy

_{2fi}is the fundamental wave component of double frequency, I

_{2fi}is the interphase circulation, P is the differential operator, and L

_{arm}and R

_{am}are the equivalent reactance and resistance.

_{2fd}and I

_{2fq}are the D and Q axis components of the interphase circulation in the double frequency negative sequence rotation coordinate system respectively. According to this equation, a mathematical model of internal circulation can be obtained, as shown in the figure below.

_{2fd}and I

_{2fd}

_{ref}are the reference values of the internal circulation, and the reference values should be set to 0 to suppress the circulation.

## 3. Analysis of Internal Fault Characteristics of the Converter station

#### 3.1. Fault Analysis of MMC Sub-Module

#### 3.2. Fault Analysis of Bridge Arm Reactor

#### 3.3. Analysis of Converter Station Protection Strategy

- (1)
- When the sub-module running in one of the bridge arms fails, the system starts the protection strategy through signal detection, then bypasses the faulty sub-module and sends a locking signal to the faulty sub-module. At the same time, the sub-module in the hot standby state is put into use and sends a pulse signal to it, that is, to put it into a normal operation state;
- (2)
- If the number of fault modules is over the hot standby state of sub-modules, or hot standby module failure cannot be put into operation, the cold standby module will be started, and the first child module of the cold standby state bypass switch will be turned on, making its fault module, the module running in for participation in the stable operation of the whole system.

## 4. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

- Alam, M.; Abido, M.; Hussein, A.; El-Amin, I. Fault Ride through Capability Augmentation of a DFIG-Based Wind Integrated VSC-HVDC System with Non-Superconducting Fault Current Limiter. Sustainability
**2019**, 11, 1232. [Google Scholar] [CrossRef] [Green Version] - Xue, S.; Lu, J.; Sun, Y.; Wang, S.; Li, B. A reverse travelling wave differential protection scheme for DC lines in MMC–HVDC system with metallic return. Int. J. Electr. Power Energy Syst.
**2022**, 135, 107521. [Google Scholar] [CrossRef] - Yang, S.; Xiang, W.; Li, R.; Lu, X.; Wen, J. An Improved DC fault Protection Algorithm for MMC HVDC Grids based on Modal Domain Analysis. IEEE J. Emerg. Sel. Top. Power Electron.
**2019**, 8, 4086–4099. [Google Scholar] [CrossRef] [Green Version] - Li, Z.; Ping, W.; Zhu, H.; Chu, Z.; Li, Y. An Improved Pulse Width Modulation Method for Chopper-Cell-Based Modular Multilevel Converters. IEEE Trans. Power Electron.
**2012**, 27, 3472–3481. [Google Scholar] [CrossRef] - Alsokhiry, F.; Adam, G.P. Multi-Port DC-DC and DC-AC Converters for Large-Scale Integration of Renewable Power Generation. Sustainability
**2020**, 12, 8440. [Google Scholar] [CrossRef] - Xue, S.; Gu, C.; Liu, B.; Fan, B. Analysis and Protection Scheme of Station Internal AC Grounding Faults in a Bipolar MMC-HVDC System. IEEE Access
**2020**, 8, 26536–26548. [Google Scholar] [CrossRef] - Tan, X.; Ren, L.; Tang, Y.; Shi, J.; Li, Z. Analysis of R-SFCL with Shunt Resistor in MMC-HVDC System using novel R-Q method. IEEE Trans. Appl. Supercond.
**2020**, 30, 5601405. [Google Scholar] [CrossRef] - Zhou, J.; Wei, J.; Xie, G.; Ran, L.; Zhang, Y. Architecture Design of Digital Twin Platform for AC&DC Hybrid Transmission System with MMC-HVDC. J. Phys. Conf. Ser.
**2021**, 1754, 12041–12047. [Google Scholar] - Mohammadi, F.; Nazri, G.; Saif, M. A new topology of a fast proactive hybrid DC circuit breaker for MT-HVDC grids. Sustainability
**2019**, 11, 4493. [Google Scholar] [CrossRef] [Green Version] - Darbas, C.; Olivier, J.C.; Ginot, N.; Poitiers, F.; Batard, C. Cascaded Smart Gate Drivers for Modular Multilevel Converters Control: A Decentralized Voltage Balancing Algorithm. Energies
**2021**, 14, 3589. [Google Scholar] [CrossRef] - Qoria, T.; Guillaud, X. Control of power electronics-driven power sources. In Converter-Based Dynamics and Control of Modern Power Systems; Elsevier: Amsterdam, The Netherlands, 2021. [Google Scholar]
- Chen, Z.; Yan, J.; Lu, C. Coordinated Control between Prevention and Correction of AC/DC Hybrid Power System Based on Steady-state Security Region. IEEE Access
**2021**, 9, 47842–47855. [Google Scholar] [CrossRef] - Laha, P.; Chakraborty, B. Cost optimal combinations of storage technologies for maximizing renewable integration in Indian power system by 2040: Multi-region approach. Renew. Energy
**2021**, 179, 233–247. [Google Scholar] [CrossRef] - Vozikis, D.; Psaras, V.; Alsokhiry, F.; Adam, G.; Al-Turki, Y. Customized converter for cost-effective and DC-fault resilient HVDC Grids. Int. J. Electr. Power Energy Syst.
**2021**, 131, 107038. [Google Scholar] [CrossRef] - Stefani, A.; Yazidi, A.; Rossi, C.; Filippetti, F.; Casadei, D.; Capolino, G.A. Doubly Fed Induction Machines Diagnosis Based on Signature Analysis of Rotor Modulating Signals. IEEE Trans. Ind. Appl.
**2008**, 44, 1711–1721. [Google Scholar] [CrossRef] - Nandi, A.K.; Wang, Q.; Yu, Y.; Darwish, M.; Ahmed, H.O. Fault Detection and Classification in MMC-HVDC Systems Using Learning Methods. Sensors
**2020**, 20, 4438. [Google Scholar] - Skinner, B.; Mancarella, P.; Vrakopoulou, M.; Hiskens, I. Incorporating new power system security paradigms into low-carbon electricity markets. Electr. J.
**2020**, 33, 106837. [Google Scholar] [CrossRef] - Khorasaninejad, M.; Radmehr, M.; Firouzi, M.; Koochaki, A. Application of a resistive mutual-inductance fault current limiter in VSC-based HVDC system. Int. J. Electr. Power Energy Syst.
**2022**, 134, 107388. [Google Scholar] [CrossRef] - Ghadi, R.J.; Mehrasa, M.; Adabi, M.E.; Bacha, S. Lyapunov theory-based control strategy for multi-terminal MMC-HVDC systems. Int. J. Electr. Power Energy Syst.
**2021**, 129, 106778. [Google Scholar] [CrossRef] - Xia, X.; Xu, L.; Zhao, X.; Zeng, X.; Yi, H. Modular multilevel converter predictive control strategy based on energy balance. J. Power Electron.
**2021**, 21, 757–767. [Google Scholar] [CrossRef] - Rodriguez, J.; Franquelo, L.G.; Kouro, S.; Leon, J.I.; Portillo, R.C.; Prats, M.; Perez, M.A. Multilevel Converters: An Enabling Technology for High-Power Applications. Proc. IEEE
**2009**, 97, 1786–1817. [Google Scholar] [CrossRef] [Green Version] - Mukherjee, D.; Chakraborty, S.; Ghosh, S. Power system state forecasting using machine learning techniques. Electr. Eng.
**2021**, 104, 283–305. [Google Scholar] [CrossRef] - Gemmell, B. Prospects of multilevel VSC technologies for power transmission. In Proceedings of the 2008 IEEE/PES Transmission and Distribution Conference and Exposition, Chicago, IL, USA, 21–24 April 2008. [Google Scholar]
- Wu, J.Y.; Lan, S.; Xiao, S.J.; Yuan, Y.B. Single Pole-to-ground Fault Location System for MMC-HVDC Transmission Lines Based on Active Pulse and CEEMDAN. IEEE Access
**2021**, 9, 42226–42235. [Google Scholar] [CrossRef] - Zhou, G.; Han, M.; Filizadeh, S.; Cao, X.; Huang, W. Studies on the combination of RSFCLs and DCCBs in MMC-MTDC system protection. Int. J. Electr. Power Energy Syst.
**2021**, 125, 106532. [Google Scholar] [CrossRef] - Song, Q.; Liu, W.; Li, X.; Rao, H.; Xu, S.; Li, L. A Steady-State Analysis Method for a Modular Multilevel Converter. IEEE Trans. Power Electron.
**2013**, 28, 3702–3713. [Google Scholar] [CrossRef] - Li, X.; Song, Q.; Liu, W.; Rao, H.; Xu, S.; Li, L. Protection of Nonpermanent Faults on DC Overhead Lines in MMC-Based HVDC Systems. IEEE Trans. Power Deliv.
**2013**, 28, 483–490. [Google Scholar] [CrossRef] - Guan, M.; Xu, Z.; Li, H. Analysis of DC voltage ripples in modular multilevel converters. In Proceedings of the IEEE 2010 International Conference on Power System Technology, Hangzhou, China, 24–28 October 2010; pp. 1–6. [Google Scholar]
- Yuebin, Z.; Daozhuo, J.; Jie, G.; Pengfei, H.; Zhiyong, L. Control of Modular Multilevel Converter Based on Stationary Frame under Unbalanced AC System. In Proceedings of the IEEE 2012 Third International Conference on Digital Manufacturing & Automation, Guilin, China, 31 July–2 August 2012; pp. 293–296. [Google Scholar]
- Tu, Q.; Xu, Z.; Chang, Y.; Guan, L. Suppressing DC Voltage Ripples of MMC-HVDC under Unbalanced Grid Conditions. IEEE Trans. Power Deliv.
**2012**, 27, 1332–1338. [Google Scholar] [CrossRef] - Guan, M.; Xu, Z. Modeling and Control of a Modular Multilevel Converter-Based HVDC System Under Unbalanced Grid Conditions. IEEE Trans. Power Electron.
**2012**, 27, 4858–4867. [Google Scholar] [CrossRef] - Adam, G.P.; Anaya-Lara, O.; Burt, G.; Finney, S.J.; Williams, B.W. Comparison between Two VSC-HVDC Transmission Technologies: Modular and Neutral Point Clamped Multilevel Converter. In Proceedings of the IEEE 35th Annual Conference of the IEEE Industrial Electronics Society, Porto, Portugal, 3–5 November 2009; pp. 1–4. [Google Scholar]
- Li, R.; Fletcher, J. AC Voltage Control of DC/DC Converters Based on Modular Multilevel Converters in Multi-Terminal High-Voltage Direct Current Transmission Systems. Energies
**2016**, 9, 1064. [Google Scholar] [CrossRef] [Green Version] - Schmitt, D.; Wang, Y.; Weyh, T.; Marquardt, R. DC-side Fault Curret Management in Extended Multiterminal- HVDC-Grids. In Proceedings of the 9th International Multi-Conference on Systems, Signals and Devices, Chemnitz, Germany, 20–23 March 2012; pp. 1–5. [Google Scholar]
- Miyara, R.; Nakadomari, A.; Matayoshi, H.; Takahashi, H.; Hemeida, A.M.; Senjyu, T. A Resonant Hybrid DC Circuit Breaker for Multi-Terminal HVDC Systems. Sustainability
**2020**, 12, 7771. [Google Scholar] [CrossRef] - Fu, Z.; Sima, W.; Yang, M.; Sun, P.; Yuan, T.; Wang, X.; Long, Y. A Mutual-Inductance-Type Fault Current Limiter in MMC-HVDC Systems. IEEE Trans. Power Deliv.
**2020**, 35, 2403–2413. [Google Scholar] [CrossRef] - Tahir, M.; Hu, S.; Meng, Y. Unit Partition Resonance Analysis Strategy for Impedance Network in Modular Power Converters. Front. Energy Res.
**2022**, 10, 823938. [Google Scholar] [CrossRef] - Ahmed, H.; Nandi, A.K. Three-Stage Hybrid Fault Diagnosis for Rolling Bearings with Compressively Sampled Data and Subspace Learning Techniques. IEEE Trans. Ind. Electron.
**2019**, 66, 5516–5524. [Google Scholar] [CrossRef] [Green Version] - Zhang, Y.; Tang, F.; Qin, F.; Li, Y.; Gao, X.; Du, N. Research on Dynamic Reactive Power Compensation Scheme for Inhibiting Subsequent Commutation Failure of MIDC. Sustainability
**2021**, 13, 7829. [Google Scholar] [CrossRef] - Katyara, S.; Hashmani, A.; Chowdhary, B.S.; Musavi, H.A.; Aleem, A.; Chachar, F.A.; Shah, M.A. Wireless Networks for Volt age Stability Analysis and Anti-islanding Protection of Smart Grid System. Wirel. Pers. Commun.
**2021**, 116, 1361–1378. [Google Scholar] [CrossRef] - Zhang, Y.; Wang, S.; Liu, T.; Zhang, S.; Lu, Q. A traveling-wave-based protection scheme for the bipolar voltage source converter based high voltage direct current (VSC-HVDC) transmission lines in renewable energy integration. Energy
**2021**, 216, 119312. [Google Scholar] [CrossRef] - Hossain, M.I.; Abido, M.A. SCIG Based Wind Energy Integrated Multiterminal MMC-HVDC Transmission Network. Sustainability
**2020**, 12, 3622. [Google Scholar] [CrossRef]

**Figure 7.**Rectifier station system two waveforms after sub-module fault removal. (

**a**) is Three-phase AC; (

**b**) is A-phase circulation current.

**Figure 8.**System simulation results when the reactor fails. (

**a**) Three-phase AC current; (

**b**) DC current.

**Figure 10.**Sub-module Fault and Redundant Protection System Variable Waveforms. (

**a**) Capacitor voltage of sub-module of upper bridge arm in phase A; (

**b**) Current of sub-module of upper bridge arm in phase A; (

**c**) Dc voltage; (

**d**) Ac output current.

Working Status of the Sub-Module | Switch State | Current Path Circuit |
---|---|---|

Locked state | VT1 Shut off | |

VT2 Shut off | ||

Input | VT1 conduction | |

VT2 Shut off | ||

Excise | VT1 Shut off | |

VT2 conduction |

The voltage level of the main network on both the rectifier side and the inverter side | 220 kV |

The Ynd type is connected to the converter transformer with the longitude ratio | 220/230 kV |

The ac rated bus voltage | 230 kV |

The number of sub-modules of each bridge arm of MMC | N = 30 |

The inductance value of the reactor on each bridge arm | 70 mH |

The equivalent resistance of the system | 0.1 ω |

The rated DC voltage | ±230 kV |

The rated transmission active power | 1200 MW |

The reactive power | 480 MVA |

Working Status of the Sub-Module | Switch State | Current Path |
---|---|---|

Hot standby | K1 broken VT1 broken VT2 open | |

Cold standby | K1 closed VT1 Shut off VT2 Shut off |

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## Share and Cite

**MDPI and ACS Style**

Zhao, C.; Jiang, S.; Xie, Y.; Wang, L.; Zhang, D.; Ma, Y.; Zhang, Y.; Li, M.
Analysis of Fault and Protection Strategy of a Converter Station in MMC-HVDC System. *Sustainability* **2022**, *14*, 5446.
https://doi.org/10.3390/su14095446

**AMA Style**

Zhao C, Jiang S, Xie Y, Wang L, Zhang D, Ma Y, Zhang Y, Li M.
Analysis of Fault and Protection Strategy of a Converter Station in MMC-HVDC System. *Sustainability*. 2022; 14(9):5446.
https://doi.org/10.3390/su14095446

**Chicago/Turabian Style**

Zhao, Chong, Siyu Jiang, Yu Xie, Longze Wang, Delong Zhang, Yiyi Ma, Yan Zhang, and Meicheng Li.
2022. "Analysis of Fault and Protection Strategy of a Converter Station in MMC-HVDC System" *Sustainability* 14, no. 9: 5446.
https://doi.org/10.3390/su14095446