# An Overview of HVDC Technology

^{1}

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## Abstract

**:**

## 1. Introduction

## 2. HVDC Technology Overview/Background

#### 2.1. Advantages of HVDC

- Asynchronous connection (can connect two AC systems of different frequency).
- Overcoming technical limitations. HVDC can supply via a long cable system when an AC system cannot. This is due to the high cable capacitance causing a large capacitive current (reactive power flow) that leaves reduced current capacity for the transmission of real power when AC transmission is used.
- No increase in short-circuit capacity (there is no need to upgrade protection equipment due to the link).
- Controllable real power transfer (independent of Z, V & f). The ability supply power to any pre-specified criteria (the controller can be set to a variety of functions).
- Higher power transfer for a given conductor.
- No stability distance limitation.
- The lack of reactive voltage drop means better voltage regulation for both heavy loading and light loading (no Ferranti effect).
- Narrower right-of-way (better land use).
- Higher power transfer for a given conductor.
- Advanced control features can improve the stability of the AC systems and act as a fast generation reserve.

#### 2.2. Power Electronic Switches

#### 2.3. Current Source Converters

_{dc}

_{0}) that results from the converter conversion process. To match a level DC voltage (V

_{dc}) a DC smoothing reactor is necessary [17]. This large DC reactor makes the converter current stiff, hence the term Current Source Converter (CSC).

#### 2.4. Voltage Source Converters

#### 2.5. Topologies

## 3. Comparison of Current Source and Voltage Source Converters

#### 3.1. Introduction

#### 3.2. Current Source Converters

_{ac}) is displayed in Figure 12.

_{1}and T

_{2}in Figure 13). The extinction angle is the angle between when a thyristor stops conducting and the next zero-crossing when commutation can no longer occur. Hence $\gamma =180-\left(\alpha +\mu \right)$. The maximum firing angle α is limited by the need to allow for the thyristor to re-establish its blocking ability and a margin to allow for voltage and current perturbations that naturally occur in the system. If the commutation from thyristor 1 to 3 has not been completed fully by T

_{2}then Thyristor 1 will pick up the current again and hence commutation failure has occurred. The term Line-Commutated Converter (LCC) is often used for CSC as it indicates that the conversion process relies on the line voltage of the AC system to which the converter is connected in order to facilitate commutation from one thyristor to the next.

#### 3.3. Voltage Source Converters

_{dc}/2 or −V

_{dc}/2. To improve the output voltage waveform the periods of V

_{dc}/2 or −V

_{dc}/2 would be modulated to approximate a sinewave. In sinusoidal PWM a triangular or carrier waveform is compared for a sinusoidal control/reference signal and the devices are switched when they cross as illustrated in Figure 17. Note there are two control variables, amplitude (by changing the modulation index, M

_{a}) and phase angle (Φ) (by changing the control sinusoidal phase angle), and this allows independent control of the real and reactive power transferred to/from the AC system. In theory, the operational area in a P/Q diagram is circular; however, constraints on voltage and current modify this shape as illustrated in Figure 18, which shows the four quadrant operation of typical VSC in a PQ diagram [50]. In Figure 19 the single-phase PWM of Figure 17 is expanded to show three-phase PWM and the resulting phase-to-phase voltage waveform. PWM can also be used in multilevel converters, with a lower switching frequency required with more levels.

- Neutral-point clamped circuit (also called a diode-clamped circuit)
- Flying Capacitor
- Cascaded H-bridge
- Modular Multi-level Converter (MMC)

- Circulating currents are inherent in the MMC topology.
- These currents cause variations in the capacitor voltages and increase converter losses.
- Capacitor voltages variations increase with increase in load current and circulating currents.

## 4. Historical Development

## 5. Innovations in HVDC

#### 5.1. Capacitor Commutated Converter

- Reduced reactive power demand hence reducing the amount of shunt compensation
- Reduced area requirements
- Simplified AC switchyard
- Increased immunity to commutation failures
- Increased stability at low SCR
- Smaller overvoltages at load rejection
- No AC side zero sequence currents
- Improved control properties
- Reduces shunt bank switching and transformer OLTC operations (reduces operation and maintenance costs)

#### 5.2. Continuously Tuned AC Filter

#### 5.3. Active DC Side Filters

#### 5.4. Reinjection Concept

#### 5.5. DC Faults

## 6. Discussion

#### 6.1. VSC HVDC Systems

#### 6.2. CSC-Based HVDC Systems

- Load rejection overvoltages
- Temporary overvoltage after recovering from an AC system fault
- Voltage change on reactive switching
- AC network frequency and stabilisation/modulation control
- Possible subsynchronous torsional interactions with nearby turbine-generators.

#### 6.3. Hybrid HVDC Systems

## 7. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Correction Statement

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**Figure 3.**Basic Solid-state switches (

**a**) Thyristor or Silicon Controlled Rectifier (

**b**) Insulated Gate Bipolar Transistor.

**Figure 7.**Two 6-pulse bridges forming a 12-pulse CSC using: (

**a**) Three-winding converter transformer (

**b**) Two two-winding converter transformers.

**Figure 8.**Arrangements of High Voltage Direct Current (HVDC) poles: (

**a**) Monopole with metallic return path; (

**b**) Monopole with earth return path; (

**c**) Bipole with metallic return path; (

**d**) Bipole with earth return path.

**Figure 9.**Operating states of a bipole HVDC link: (

**a**) Normal operation; (

**b**) Monopole due to a fault on one pole and using earth return; (

**c**) Monopole operation and using metallic return path.

**Figure 10.**HVDC schemes for remote generation: (

**a**) Unit connected HVDC scheme; (

**b**) Group connected HVDC scheme.

**Figure 12.**AC Current Waveforms of two 6-pulse bridges and the 12-pulse converter formed from the two: (

**a**) AC current from Star/Star connected bridge; (

**b**) AC current from Star/Delta connected bridge; (

**c**) AC Current from the combination.

**Figure 13.**The commutation process in a 6-pulse bridge (

**a**) Equivalent circuit for commutation between thyristor 1 and 3 (

**b**) Voltage waveforms (rectification) (

**c**) the current waveforms.

**Figure 15.**Voltage waveforms for the basic 6-pulse VSC using fundamental frequency switching. Waveforms (

**a**–

**c**): Phase-to-DC-midpoint voltages; waveforms (

**d**–

**f**): Phase-to-phase voltages; waveform (

**g**) Neutral voltage with respect to DC-midpoint; waveform (

**h**) Phase-to-neutral voltage of phase a.

**Figure 20.**PWM with 3rd and 9th Harmonic: (

**a**) Control Waveforms; (

**b**) Synthesized Voltage waveform (V

_{sw}) and its fundamental component (V

_{1}).

**Figure 23.**Submodule Operation: (

**a**) Circuit and schematic for the capacitor in-line; (

**b**) Circuit and schematic for the capacitor by-passed.

Characteristic | LCC HVDC | VSC HVDC |
---|---|---|

Store energy in | inductance | capacitance |

Semiconductor | withstands voltage in either polarity | Combination can pass current in either direction |

Semiconductor switch | turned ON by control action | turned ON & OFF by control action |

DC Voltage | changes polarity, reserves the power flow direction | Direction does not change |

DC Current | direction does not change | direction changes to reverse the power flow |

Turn-OFF | commutation relies on the external circuit | independent of external circuit |

P & Q | P & Q dependent | independent P & Q control |

Quadrants | 2 quadrant operation | 4-quadrant operation |

Real Power capability | Very High | Lower than LCC |

System Strength | Requires minimum SCR to commutate thyristors | Operates into weaker AC systems (or passive) |

Overload capability | Good | Weak |

DC line faults | Copes well. Control action can extinguish arc | More challenging as diodes provide path. |

Harmonic generation | Significant, AC & DC harmonic filters required | Small, minimal filtering required. |

Reactive power | Needed | Fine reactive power control in both directions |

“Black” start | requires additional equipment | capable |

**Table 2.**Selected HVDC Schemes using Line-Commutated Converters [69].

Name | Year | Technology | Length | DC Voltage | Power Rating |
---|---|---|---|---|---|

Cable/OHL | (kV) | P (MW) | |||

Gotland 1 | 1954 | Mercury-arc | 98/0 | 200 | 20 |

Cross-Channel | 1961 | Mercury-arc | 64/0 | ±100 | 160 |

NZ Inter-Island 1 | 1965 | Mercury-arc | 40/571 | ±250 | 600 |

SACOI ^{1} | 1965 | Mercury-arc | 365/118 | ±200 | 200 |

Konti-Skan 1 | 1965 | Mercury-arc | 87/89 | ±250 | 250 |

Zhoushan | 1987 | Mercury-arc | 54 | −100 | 50 |

Vancouver Isl. 1 | 1968 | Mercury | 42/33 | 260 | 312 |

Pacific DC Intertie | 1970 | Thyristor | 0/1362 | ±500 | 3100 |

Nelson River Bipole 1 ^{2} | 1977 | Mercury-arc | 0/895 | ±450 | 1620 |

Skagerrak 1 | 1977 | Thyristor | 130/100 | ±250 | 500 |

Cahora Bassa ^{3} | 1979 | Thyristor | 0/1420 | ±533 | 1920 |

Hokkaido—Honshu | 1979 | Thyristor | 44/149 | ±250 | 300 |

Zhou Shan ^{4} | 1982 | Thyristor | 44/149 | +100 | 50 |

Itaipu 1 | 1984 | Thyristor | 0/785 | ±600 | 3150 |

Nelson River Bipole 2 | 1985 | Thyristor | 0/940 | ±500 | 1800 |

Itaipu 2 | 1987 | Thyristor | 0/805 | ±600 | 3150 |

Fenno-Skan | 1989 | Thyristor | 200/33 | ±400 | 500 |

Rihand-Delhi | 1990 | Thyristor | 0/814 | ±500 | 1500 |

Quebec—New England | 1991 | Thyristor | 5/1100 | ±450 | 2250 |

NZ Inter-Island 2 | 1992 | Merc. & Thyr | 40/571 | +270/−350 | 1240 |

Baltic Cable | 1994 | Thyristor | 250/12 | 450 | 600 |

Garabi HVDC | 2002 | Merc. | 0/0 | ±70 | 2200 |

Three Gorges—Changzhou | 2003 | Thyristor | 0/890 | ±500 | 3000 |

Three Gorges—Guangdong 1 | 2004 | Thyristor | 0/980 | ±500 | 3000 |

Three Gorges—Guangdong | 2004 | Thyristor | 0/940 | ±500 | 3000 |

BassLink | 2006 | Thyristor | 298/72 | ±400 | 500 |

NorNed | 2008 | Thyristor | 580/0 | ±450 | 700 |

Yunnan–Guangdong | 2010 | Thyristor | 0/1418 | ±800 | 5000 |

XIangjiaba-Shanghai | 2010 | Thyristor | 0/1907 | ±800 | 6400 |

NZ Inter-Island 3 | 2013 | Thyristor | 40/571 | ±350 | 1200 |

Estlink 2 | 2014 | Thyristor | 157/14 | ±450 | 650 |

North-East Agra | 2017 | Thyristor | 0/1728 | ±800 | 6000 |

Nelson River Bipole 3 | 2018 | Thyristor | 0/1324 | ±500 | 2000 |

^{1}Later changed to be the first multiterminal link.

^{2}Largest mercury-arc valves ever made. The mercury-arc valves since replaced by thyristors.

^{3}First HVDC scheme ordered with thyristors, although operation was delayed. First to use a DC voltage greater than 500 kV. First HVDC link in Africa.

^{4}First HVDC Link in China.

**Table 3.**Selected HVDC Schemes using Voltage Source Converters [69].

Name | Year | Topology | Length (km) | Switching Frequency | DC Voltage | Power Rating | |
---|---|---|---|---|---|---|---|

Cable ^{1}/OHL | (Hz) | (kV) | P (MW) | Q(MVAr) | |||

Gotland VSC | 1999 | 2-level | 70/0 | 1950 | ±80 | 50 | −55 to 50 |

Tjäreborg | 2000 | 2-level | 4.3/0 | 1950 | ±9 | 7.2 | −3 to 4 |

Directlink | 2000 | 2-level | 59/0 | 1950 | ±80 | 180 | −165 to 90 |

Eagle Pass | 2000 | 3-level ^{2} | 0/0 | 1500 | ±15.9 | 36 | ±36 |

MurrayLink | 2002 | 3-level ^{3} | 176/0 | 1350 | ±150 | 220 | −150 to 140 |

CrossSound | 2002 | 3-level ^{3} | 40/0 | 1260 | ±150 | 330 | ±150 |

Troll A | 2005 | 2-level | 70/0 | 2000 | ±60 | 84 | −20 to 24 |

Estlink1 | 2006 | 2-level ^{4} | 105/0 | 1150 | ±150 | 350 | ±125 |

BorWin1 | 2009 | 2-level | 200/0 | ±150 | 400 | ||

Trans Bay Cable | 2010 | MMC | 85/0 | <150 | ±200 | 400 | ±170 |

Nanao Island ^{5} | 2013 | MMC ^{6} | 10/32 | ±160 | 200/100/500 | ||

Zhoushan Isl. ^{7} | 2014 | MMC | 134/0 | ±200 | 400 | ||

INELFE | 2015 | MMC | 64.5/0 | ±320 | 2 × 1000 | ||

BorWin2 | 2015 | MMC | 200/0 | ±300 | 800 | ||

HelWin1, | 2015 | MMC | 130/0 | ±250 | 576 | ||

HelWin2 | 2015 | MMC | 130/0 | ±320 | 690 | ||

Dolwin1 | 2015 | Casc. 2-L ^{8} | 165/0 | ±320 | 800 | ||

Dolwin2 | 2015 | MMC | 135/0 | ±320 | 900 | ||

Dolwin3 | 2018 | MMC | 162/0 | ±320 | 900 | ||

SylWin1 | 2015 | MMC | 205/0 | ±300 | 864 | ||

BorWin3 | 2019 | MMC | 160/0 | ±320 | 900 | ||

Zhangbei HVDC ^{9} | 2019 ^{10} | MMC | 170/648 ^{11} | ±500 | 1500/4500 |

^{1}Cable length maybe a combination of undersea and land based cable.

^{2}Back-to-Back scheme using Diode Clamped/ Neutral-Point Clamped converter.

^{3}Active Neutral-Point Clamped.

^{4}Optimal Pulse Width Modulation (PWM).

^{5}3-terminal HVDC system in parallel to and AC interconnection. Switching devices: Injection-Enhanced Gate Transistor (IEGT) and Insulated Gate Bipolar Transistors (IGBT).

^{6}Multiterminal DC (MTDC).

^{7}5-terminal HVDC system. Provides voltage support to the existing ±50 kV 60 MW LCC-HVDC system on Sijiao Island to prevent commutation failure.

^{8}Cascaded 2-Level converters.

^{9}4-terminal HVDC system.

^{10}Stage 1.

^{11}Total length in the DC grid.

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**MDPI and ACS Style**

Watson, N.R.; Watson, J.D.
An Overview of HVDC Technology. *Energies* **2020**, *13*, 4342.
https://doi.org/10.3390/en13174342

**AMA Style**

Watson NR, Watson JD.
An Overview of HVDC Technology. *Energies*. 2020; 13(17):4342.
https://doi.org/10.3390/en13174342

**Chicago/Turabian Style**

Watson, Neville R., and Jeremy D. Watson.
2020. "An Overview of HVDC Technology" *Energies* 13, no. 17: 4342.
https://doi.org/10.3390/en13174342