# Reducing Operational Costs of Offshore HVDC Energy Export Systems Through Optimized Maintenance

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## Abstract

**:**

## 1. Introduction

## 2. Reference HVDC Energy Export System

#### 2.1. Converter (VSC Modular Multilevel Converter)

#### 2.2. Converter Reliability

## 3. Maintenance

#### 3.1. MMC Maintenance

#### 3.2. Maintenance Date and Time Point

#### 3.3. Maintenance Assumtion and Model Simplifications

## 4. Model Description

## 5. Offshore Wind Farm

## 6. Case Study Discussion

#### 6.1. Maintenance Period

#### 6.2. Maintenace Staff

#### 6.3. Maintenance Start Day

#### 6.4. Maintenance Costs Calculation

## 7. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Nomenclature

Parameter | Description |

${n}_{min}$ | Minimum number of SMs per converter-leg |

${n}_{red}$ | Redundant number of SMs per converter-leg |

${n}_{Con}$ | Total number of SMs in the converter |

${U}_{SM}$ | SM operation voltage |

${U}_{d}$ | DC voltage |

${\widehat{U}}_{AC}$ | AC peak voltage |

${R}_{SM}$ | Reliability for one submodule |

${R}_{CL}$ | Reliability for one converter-leg |

${R}_{pos\_CL}$ | Reliability for positive converter-leg |

${R}_{neg\_CL}$ | Reliability for negative converter-leg |

${R}_{CP}$ | Reliability for one converter phase |

${R}_{C}$ | Reliability for the converter |

$t$ | Time in hours |

${\lambda}_{SM}$ | SM hazard rate |

$FI{T}_{SM}$ | SM failures in time |

$F\left(k,t\right)$ | Reliability function |

$n$ | Number of all component in a system |

$k$ | Number of components that is needed for operating a system |

${E}_{SM}$ | Expected number of defective SMs |

$i$ | Time in days |

$\beta \left(\mathrm{i}\right)$ | Maintenance matrix |

${P}_{Transf.}$ | Transformer power rating |

${P}_{HVDCSystem}$ | Power rating |

${E}_{yield}\left(\mathrm{i}\right)$ | Energy yield from wind farm |

${E}_{totallosses}$ | Maintenance-related losses |

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**Figure 1.**Schematic configuration of the offshore wind farm and the high-voltage direct current (HVDC) energy export system.

**Figure 2.**Single line diagram (SLD) of the HVDC energy export system and maintenance areas for the onshore and offshore station.

**Figure 3.**Schematic configuration of the three phase modular multilevel converter (MMC) and structure of the half bridge submodule (SM).

**Figure 4.**Converter reliability and probability of SM failure over the number of redundant SM after one and two year maintenance period.

**Figure 5.**Converter reliability and probability of SM failure over the number of redundant SMs after the three and four year maintenance period.

**Figure 6.**Maintenance time (

**a**) for the different maintenance areas of the HVDC system over different maintenance periods and (

**b**) for a two-year maintenance period over different number of maintenance staff in the converter yard.

**Figure 9.**Monthly energy yields of the offshore wind farm as average value over the years 1980 to 2010.

**Figure 10.**Maintenance-related losses for the total operation time of the HVDC system over different maintenance periods.

**Figure 11.**Maintenance-related losses for the total operation time of the HVDC system over different number of maintenance staff in the converter yard.

**Figure 12.**Maintenance-related losses for the total operation time of the HVDC system over possible maintenance shift for different number of maintenance staff in the converter-yard.

**Figure 13.**Maintenance staff costs (

**a**) and maintenance-related costs change (

**b**) for the total operation time of the HVDC system over possible maintenance shift for different number of maintenance staff in the converter-yard.

Parameter | Value |
---|---|

Power rating (${P}_{HVDCSystem}$) | 1000 MW |

DC voltage (${U}_{d}$) | ± 320 kV |

AC peak voltage (${\widehat{U}}_{AC}$) | 320 kV |

SM operation voltage (${U}_{SM}$) | 2.7 kV |

Submodule FIT ($FI{T}_{SM}$) | 1700 |

Minimum number of SM per converter-leg (${n}_{min}$) | 238 |

Number of transformers | 2 |

Transformer power rating (${P}_{Transf.}$) | 600 MW |

Parameter | Maintenance Periods [a] | |||
---|---|---|---|---|

1 | 2 | 3 | 4 | |

Minimum SM | 1428 | 1428 | 1428 | 1428 |

Redundant SM | 60 | 96 | 132 | 162 |

Increase of redundant SM | - | 60% | 37.5% | 22.73% |

All SM | 1488 | 1524 | 1560 | 1590 |

Increase of all SM | - | 2.42% | 2.36% | 1.92% |

Maintenance Area | Transmission Capacity |
---|---|

AC yard 1 | 0 MW |

Transformer yard | 600 MW |

AC yard 2 | 0 MW |

Converter yard | 0 MW |

DC yard | 0 MW |

Parameter | Maintenance Area | |||
---|---|---|---|---|

AC Yard 1 | Transf. Yard | AC Yard 2 | DC Yard | |

Interval [a] | 25 | 1 | 25 | 5 |

Maintenance work | check insulating parts, cleaning electrical contacts | oil test, cooling system check, isolation test | check insulating parts, cleaning electrical contacts | check insulating parts, cleaning electrical contacts |

Man-hour maintenance duration [h] | 144 | 48 | 16 | 7 |

Number of maintenance staff | 2 | 2 | 2 | 1 |

Sources | [35,36,37] | [38,39,40] | [35,36,37] | [41,42,43] |

Parameter | Value |
---|---|

Preparation time | 4 h |

Follow-up time | 4 h |

Replacement time for the SM | 3 h |

Persons per team | 2 |

**Table 6.**Expected number of defective SMs using the probability function for different maintenance periods.

Parameter | Maintenance Period [a] | |||
---|---|---|---|---|

1 | 2 | 3 | 4 | |

Expected defective SM | 22 | 45 | 69 | 92 |

Parameter | Value |
---|---|

Rated power | 8 MW |

Rotor diameter | 154 m |

Hub height | 103 m |

Cut in wind speed | 4 m/s |

Cut-out wind speed | 25 m/s |

Parameter | Value |
---|---|

Fixed Parameters | |

Operating time | 30 a |

Commissioning date | 01.06.1980 dd.mm.yy |

Maintenance date | every 01.06 dd.mm |

Working time per day | 12 h/d |

Number of maintenance staff (AC yard 1) | 2 |

Number of maintenance staff (AC yard 2) | 2 |

Number of maintenance staff (DC yard) | 1 |

Number of maintenance staff (transformer yard) | 2 |

Modified Parameters | |

Maintenance period | From 1 a to 4 a |

Number of maintenance staff in den converter yard | From 2 to 10 |

Possible maintenance shift | From 0 d to 10 d |

Cost Parameters | |

Remuneration (based on [53]) | 100 €/MWh |

Offshore staff costs per person (based on [54]) | 1500 €/d |

Onshore staff costs per person (based on [54]) | 960 €/d |

© 2020 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**

Unnewehr, J.F.; Waldl, H.-P.; Pahlke, T.; Herráez, I.; Weidlich, A. Reducing Operational Costs of Offshore HVDC Energy Export Systems Through Optimized Maintenance. *Energies* **2020**, *13*, 1146.
https://doi.org/10.3390/en13051146

**AMA Style**

Unnewehr JF, Waldl H-P, Pahlke T, Herráez I, Weidlich A. Reducing Operational Costs of Offshore HVDC Energy Export Systems Through Optimized Maintenance. *Energies*. 2020; 13(5):1146.
https://doi.org/10.3390/en13051146

**Chicago/Turabian Style**

Unnewehr, Jan Frederick, Hans-Peter Waldl, Thomas Pahlke, Iván Herráez, and Anke Weidlich. 2020. "Reducing Operational Costs of Offshore HVDC Energy Export Systems Through Optimized Maintenance" *Energies* 13, no. 5: 1146.
https://doi.org/10.3390/en13051146