# Control Strategies of Full-Voltage to Half-Voltage Operation for LCC and Hybrid LCC/MMC based UHVDC Systems

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## Abstract

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## 1. Introduction

## 2. Converter Station Configurations of UHVDC Systems

- (1)
**Normal operation:**both poles are in full-voltage operation; the whole system power capacity is 100%;- (2)
**Partial monopole off:**one-pole is in full-voltage operation while the other is partially off (25% of the rated capacity); the whole system power capacity is 75%;- (3)
**Monopole off**: one-pole is in full-voltage operation while the other is off; the whole system power capacity is 50%;- (4)
- Partial bipole off:
- Both poles are in (half-voltage operation) partial monopole off (each pole has 25% of the rated capacity); the whole system power capacity is 50%;
- One-pole is off while the other is (half-voltage operation) partial monopole off (25% of the rated capacity); the whole system power capacity is 25%;

- (5)
**Bipole off:**the whole system is shut down; the system power capacity of the whole system is 0.

## 3. Valve-Group Bypassing Strategy for LCC-UHVDC

- (1)
- The extinction angle γ of the HV valve-group in the inverter will be ramped up to 90˚ once a bypassing order is received from the system upper level control. At the same time, the bypassing order will be transmitted to the rectifier;
- (2)
- The firing angle α of the HV valve-group in the rectifier will be ramped up to 90˚ once the bypassing order is received;
- (3)
- When the voltage of the HV valve-groups decrease to 0.1 p.u., the BPS is closed. Then, the valve-groups are blocked. At the same time, the bypass isolator will be closed;
- (4)
- The BPS is opened once the bypass isolator is fully closed;
- (5)
- The valve-group isolators are opened once the BPS is fully opened;
- (6)
- The full-voltage to half-voltage process completes after the above process.

## 4. Valve-Group Bypassing Strategy for Hybrid LCC/VSC UHVDC

#### 4.1. The VSC in the Hybrid System is Half-Bridge Modular Multilevel Converter (HB-MMC)

_{ac}to V

_{dc}, where V

_{ac}is the converter valve-side line voltage and V

_{dc}is the DC side pole-to-ground voltage. Therefore, it cannot adjust the DC voltage from V

_{dc}to 0 through its DC voltage controller. In order to bypass the valve-groups, the DC current needs to be firstly reduced to a minimum value (0.1 p.u.). Then, both the HV and LV LCCs and the HV MMC need to be blocked. The LV MMC can keep operating as a static synchronous compensator (STATCOM) during the process.

_{1}and BPS

_{2}cannot be closed directly after blocking the converters because the LV valve-groups may be damaged by the DC line residual voltage. In order to speed up the voltage decaying process, a discharging resistor is employed to achieve a fast de-energization. Before inserting the discharging resistor R, the DC side switches S

_{1}and S

_{2}are opened to isolate the converters from the DC line. Then, the DC line will discharge through the resistor quickly. After opening the switches S

_{1}and S

_{2}, the BPS

_{1}and BPS

_{2}and isolators can operate to bypass the HV valve-groups of the LCC and MMC.

_{1}will be closed. The LV LCC will be deblocked to charge the DC line. The DC switch S

_{2}will be closed when the DC line voltage reaches to the DC voltage of the LV MMC. The power transmission will be interrupted for a short time during this full-voltage to half-voltage process.

- (1)
- Reduce the DC current to a minimum value (0.1 p.u.) once a bypassing order is received from the system upper level control;
- (2)
- Block the HV and LV LCCs and the HV MMC valve-groups once the DC current is reduced to the target value;
- (3)
- The DC terminal switches are tripped to isolate the converters from the DC line; Then, the discharging resistor is inserted; The HV valve-groups are bypassed during this process;
- (4)
- The discharging resistor is removed once the DC line is fully de-energized;
- (5)
- The LV LCC is deblocked to charge the DC line once its DC side switch is re-closed;
- (6)
- The LV MMC’s DC side switch is closed once the DC line voltage reaches to its DC terminal voltage;
- (7)
- The full-voltage to half-voltage process completes after the above process.

#### 4.2. The VSC in the Hybrid System is Full-Bridge Modular Multilevel Converter (FB-MMC)

_{dc}to −V

_{dc}[26]. Based on the modulation and control principle of MMC, the DC voltage of a FB-MMC can be given as

_{cap}is the capacitor voltage of a sub-module (SM), S

_{pi}and S

_{ni}are the switching function of the SMs in the upper arms and lower arms. The switching function has three states. For instance, the output voltage of the SM will be V

_{cap}when S

_{i}= 1; the output voltage is 0 when S

_{i}= 0 and the output voltage is −V

_{cap}when S

_{i}= −1. In order to achieve a stable online valve-group bypassing, the following modulation strategy is employed. The number of inserted SMs in the upper and lower arms is shown below:

_{up}and N

_{down}are the inserted SM number for the upper and lower arms, V

_{dcref}is the DC voltage reference which will be decreased during the full-voltage to half-voltage operation with a ramp-down rate, V

_{acref}is the MMC output ac voltage reference calculated from the control system, V

_{crated}is the rated SM capacitor voltage. Then the DC voltage of the MMC is

_{cap}remains stable. The DC voltage outer loop controller is shown in Figure 8. To achieve an online valve-group bypass, the target valve-group of the FB-MMC will start to decrease the DC voltage. At the same time, the target valve-group of the LCC will be controlled to follow the voltage drop. The power transmission will not be interrupted during this full-voltage to half-voltage process.

- (1)
- The DC voltage of the HV valve-group of the FB-MMC will be controlled to ramp down to a minimum value (0.1 p.u.) once a bypassing order is received from the system upper level control. At the same time, the bypassing order will be transmitted to the rectifier;
- (2)
- The firing angle α of the HV valve-group in the rectifier will be ramped up to follow the voltage decrease controlled by the FB-MMC;
- (3)
- When the voltage of the HV valve-groups decrease to 0.1 p.u., the BPS is closed. Then, the valve-groups are blocked. At the same time, the bypass isolator will be closed;
- (4)
- The BPS is opened once the bypass isolator is fully closed;
- (5)
- The valve-group isolators are opened once the BPS is fully opened;
- (6)
- The full-voltage to half-voltage process completes after the above process.

## 5. Power Rescheduling Strategy for Full-Voltage to Half-Voltage Operation

_{0}before the full-voltage to half-voltage process, therefore, the power transmitted in each pole is 2P

_{0}and the total power of the system is 4P

_{0}. The rated power of each valve-group is P

_{rated}. Therefore, P

_{0}≤ P

_{rated}. The detailed sequences of the power rescheduling are summarized below:

- (1)
- If P
_{0}≤ ½P_{rated}, the new power reference for the half-voltage operating valve-group will be set as 2P_{0}. The power reference for the full-voltage operating pole is not changed. In this case, there is no power loss. - (2)
- If ½P
_{rated}< P_{0}≤ ¾P_{rated}, the new power reference for the half-voltage operating valve-group will be set as P_{rated}. In this case, the increased power of the half-voltage valve-group is (P_{rated}‒P_{0}), the lost power is P_{0}‒ (P_{rated}‒ P_{0}) = (2P_{0}‒ P_{rated}) which can be undertaken by the full-voltage operating pole. It is known the original power in the full-voltage operating pole is 2P_{0}. As P_{0}≤ ¾P_{rated}, therefore (2P_{0}‒ P_{rated}) + 2P_{0}= (4P_{0}‒ P_{rated}) ≤ 2P_{rated}. Therefore, the new power reference for the full-voltage operating pole is set to (4P_{0}‒ P_{rated}). In this case, power loss of the system is 0. - (3)
- If ¾P
_{rated}< P_{0}≤ P_{rated}, the new power reference for the half-voltage operating valve-group will be set as P_{rated}. The power reference for the full-voltage operating valve-group will be set as 2P_{rated}. In this case, the power loss in the system is (4P_{0}‒ 3P_{rated}).

## 6. Case Studies

#### 6.1. LCC-UHVDC Point-to-Point Link

_{dcRec}is the DC terminal voltage of the rectifier. As shown in Figure 9a, V

_{dcRec}started to drop once the firing angle is controlled to increase to 90˚. The V

_{dcRecMid}is the voltage between the HV and LV valve-groups of the rectifier. It can be seen that the V

_{dcRecMid}and the DC current I

_{dc}were almost not affected during the full-voltage to half-voltage control process. It is because the voltage reduction of the HV valve-group does not affect the voltage of the LV valve-group which can keep controlling the DC current constantly. According to the proposed power reschedule strategy, the current (power) was then set to 1 p.u. when the BPS and isolators successfully operated to isolate HV valve-groups. The power transmission is not interrupted during the full-bridge to half-bridge process. Figure 9b,c show the firing angles and extinction angles of the rectifier and inverter, respectively. The firing angles and extinction angles of the HV valve-groups were controlled to ramp up once the bypassing signals were received. It should be mentioned that the HV valve-groups may have been blocked before their firing or extinction angle reaches to 90˚.

#### 6.2. Hybrid LCC/HB-MMC UHVDC Point-to-Point Link

#### 6.3. Hybrid LCC/FB-MMC UHVDC Point-to-Point Link

## 7. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Appendix A

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**Figure 4.**Possible operation modes of the positive pole of an LCC-UHVDC system. (

**a**) Normal operation; (

**b**) High-voltage (HV) valve-groups operation; (

**c**) Low-voltage (LV) valve-groups operation; (

**d**) Mixed valve-groups operation.

**Figure 5.**The sequence of bypassing the HV valve-group. (

**a**) Normal operation; (

**b**) BPS closed; (

**c**) Valve blocked, isolator closed; (

**d**) Half-voltage operation.

**Figure 9.**System dynamics during the full-voltage to half-voltage operation of the LCC-UHVDC link. (

**a**) DC terminal voltage and current of the sending end; (

**b**) Firing angles of the LV and HV valve-groups of the sending end; (

**c**) Extinction angles of the LV and HV valve-groups of the receiving end.

**Figure 10.**System dynamics of the negative pole. (

**a**) DC terminal voltage and current of the sending end; (

**b**) Firing angle of the sending end.

**Figure 11.**System dynamics during the full-voltage to half-voltage operation of the Hybrid LCC/HB-MMC UHVDC link. (

**a**) DC terminal voltage and current of the sending end; (

**b**) Firing angles of the LV and HV valve-groups of the sending end.

**Figure 12.**System dynamics during the full-voltage to half-voltage operation of the Hybrid LCC/FB-MMC UHVDC link. (

**a**) DC terminal voltage and current of the sending end; (

**b**) Firing angles of the LV and HV valve-groups of the sending end.

Initial Power of Each Valve-Group | Power Reference for Half-Voltage Pole | Power Reference for Full-Voltage Pole | System Power Loss |
---|---|---|---|

P_{0} ≤ ½P_{rated} | 2P_{0} | 2P_{0} | 0 |

½P_{rated} < P_{0} ≤ ¾P_{rated} | P_{rated} | 4P_{0} ‒ P_{rated} | 0 |

¾P_{rated} < P_{0} ≤ P_{rated} | P_{rated} | 2P_{rated} | 4P_{0} ‒ 3P_{rated} |

Parameters | Real Value |
---|---|

Capacity (MVA) | 5000 |

Rated DC voltage (kV) | ±800 |

AC grid frequency (Hz) | 50 |

Rated AC voltages (kV) | 525 (Rec) |

500 (Inv) | |

Transformer ratios (kV/kV) | 525/185 (Rec) |

500/175 (Inv) | |

Transformer capacity per valve (MVA) | 703.125 (Rec) |

625 (Inv) | |

Transformer leakage inductance (p.u.) | 0.14 |

Parameters | Real Value |
---|---|

Capacity (MVA) | 5000 |

Rated DC voltage (kV) | ± 800 |

AC grid frequency (Hz) | 50 |

Rated AC voltages (kV) | 500 |

Transformer ratios (kV/kV) | 500/260 |

Transformer capacity per valve (MVA) | 1250 |

Transformer leakage inductance (p.u.) | 0.14 |

Number of SM | 167 |

SM capacitor (mF) | 18 |

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Li, G.; Liu, W.; Joseph, T.; Liang, J.; An, T.; Lu, J.; Szechtman, M.; Andersen, B.; Zhuang, Q. Control Strategies of Full-Voltage to Half-Voltage Operation for LCC and Hybrid LCC/MMC based UHVDC Systems. *Energies* **2019**, *12*, 742.
https://doi.org/10.3390/en12040742

**AMA Style**

Li G, Liu W, Joseph T, Liang J, An T, Lu J, Szechtman M, Andersen B, Zhuang Q. Control Strategies of Full-Voltage to Half-Voltage Operation for LCC and Hybrid LCC/MMC based UHVDC Systems. *Energies*. 2019; 12(4):742.
https://doi.org/10.3390/en12040742

**Chicago/Turabian Style**

Li, Gen, Wei Liu, Tibin Joseph, Jun Liang, Ting An, Jingjing Lu, Marcio Szechtman, Bjarne Andersen, and Qikai Zhuang. 2019. "Control Strategies of Full-Voltage to Half-Voltage Operation for LCC and Hybrid LCC/MMC based UHVDC Systems" *Energies* 12, no. 4: 742.
https://doi.org/10.3390/en12040742