# Green Solution for Insulation System of a Medium Frequency High Voltage Transformer for an Offshore Wind Farm

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## Abstract

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## 1. Introduction

_{2}emissions from electricity generation worldwide [1]. However, it remains of great importance to reduce the cost of these energy sources throughout their life cycle. Thus, the energy generated by the offshore windfarms is transferred to the shore by means of submarine high voltage cables. Since AC transmission becomes inefficient at distances longer than approximately 60 km [2], it is necessary to switch to High Voltage Direct Current (HVDC) transmission for sea-based wind farms. Today’s HVDC concept of offshore wind farms require a transformer and a converter station platform, which serves as the hub for the collection network and as a connection point of the HVDC cable to land. This solution is economically inefficient, and therefore, a reduction in costs of platforms is highly desirable.

## 2. Overview of the Design of the Mineral-Oil-Based HVDC Medium Frequency Transformer

#### 2.1. Design Method Development and Verification

_{P}is the rms of the DC voltage on the primary winding, A

_{c}is the effective cross section of the transformer core, φ

_{m}is the maximum flux in the core, B

_{m}is the maximum flux density of the core, N is the number of turns for the primary winding and f is the switching frequency [17].

#### 2.2. A Real Scale 10 MW HVDC MFT

_{1}and N

_{2}are 12 and 120, respectively. A ferrite magnetic core having 2240 mm

^{2}as its physical core area including 10 core stacks is selected. A Litz wire bearing 9000 strands, each with 0.2 mm in diameter and a physical dimension of 26.6 × 17.5 mm

^{2}is used. The primary winding is the layer type with 4 layers, each consisting of 3 turns. Each turn is made from 8 parallel Litz conductors. A 20 mm oil gap between the layers are considered for cooling purposes. The secondary winding is a disc type winding, each disc consisting of 6 turns. The HV winding contains twenty discs with a 5.5 mm oil gap between two adjacent discs.

#### 2.3. Insulation System Behaviour under DC Stress

## 3. Conductivity Measurement

## 4. FEM Simulation

#### 4.1. Initial Stress Distribution

#### 4.2. Final Stress Distribution

#### 4.3. Transient Stress Distribution

#### 4.4. Dielctirc Design Evaluation

#### 4.4.1. The Maximum Stress in OIP

#### 4.4.2. The Safety Factors in Oil Gaps

_{av}, is obtained as follows [5,25]:

_{bd}) [5,25]:

_{0}is 17.5 or 21.5 kVrms/mm for bare and covered electrodes, respectively, for mineral oil application, and d is distance in mm. The safety factor of an oil gap is considered as the minimum value of the related safety factor curve. It is worth mentioning that the partial discharge inception voltage and breakdown voltages of synthetic ester oil is slightly higher than mineral oil [37], and therefore, (5) is assumed to be valid for ester oil with a safer margin.

#### 4.4.3. The Safety Factors in Creepage Surfaces

_{av}stress curve shall be divided by 0.7, giving:

#### 4.4.4. The Safety Factors in the Combined Oil Gaps and Creepage Surfaces

_{av}curve of the combined path will be higher compared to that of the oil gap and creepage paths. Consequently, according to (4), this can lead to a lower minimum SF for combined path with respect to the SFs of its forming oil gap and creepage paths (Figure 14d).

#### 4.4.5. The Automatic Dielectric Evaluation

## 5. Discussion

- The measured conductivity of the oil/OIP insulation materials are temperature and electric stress dependent. This fact is considered in the FEM simulations adopted for finding the insulation withstand level of the transformer design.
- The conductivity values of the ester oil/OIP are generally higher than the mineral oil/OIP, which causes lower time constants for ester oil/OIP and consequently faster convergence to the steady state condition (shorter transient phase) by using ester oil.
- For both ester and mineral oils, the lower the operational temperature, the lower the conductivity values. As a result, the transient state is longer at low temperatures.
- The temperature dependency of ester oil/OIP conductivities are lower, which causes the transformer to be less sensitive to temperature variations during energization or variable loading conditions. As a result, the transformer filled with mineral oil behaves completely different at 90 °C compared to lower temperatures.
- For a successful insulation design, it is insufficient to check the stress distribution only during the initial and steady state conditions, but also during the transient state when instantaneous maximums in the field strength may occur.
- By using ester oil, the stress in the OIP at steady state condition is at the same level as for a similar transformer filled with mineral oil. Similarly, the minimum SFs at the oil gaps, creepage paths as well as the combined paths are almost at the same order for both ester and mineral oils applications.
- The introduced combined method for safety factor calculation can be used effectively in a transformer insulation design evaluation. The SF value of an insulation design discovered by this method is equal or lower than the minimum of the SF values found by conventional methods on the independent oil gaps and creepage paths. Therefore, the new proposed method can be considered as a conservative method for insulation design evaluation.

## 6. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Abbreviations

HVDC | High Voltage Direct Current |

MFT | Medium Frequency Transformer |

OIP | Oil Impregnated Pressboard |

DAB | Dual Active Bridge |

LCC | Life Cycle Cost |

LMW | Linear Maxwell–Wagner |

NLMW | Non-Linear Maxwell–Wagner |

FEA | Finite Element Analysis |

FEM | Finite Element Method |

CR | Conductivity Ratio |

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**Figure 1.**The series DC concept where the DC outputs of the wind turbines are directly delivering the high voltage to be transferred to land.

**Figure 2.**Manufactured prototype: (

**a**) Complete transformer; (

**b**) Active part; (

**c**) DC insulation test setup.

**Figure 3.**Dimensions of the real scale Ferrite 10 MW, 1.8/18 kV, 0.32 T, 5555/555 A, 12/120 Turns, 5 kHz MFT: (

**a**) Top view; (

**b**) Side view.

**Figure 7.**The initial permittivity-based stress distribution in kV/mm and the electric field streamlines in the dielectric structure of the MFT using: (

**a**) mineral oil; (

**b**) ester oil.

**Figure 8.**The final steady state stress distribution in kV/mm and the electric field streamlines in the dielectric structure of the MFT at different temperatures using: (

**left**) mineral oil; (

**right**) ester oil.

**Figure 9.**The position of some typical points in the dielectric structure of the MFT to investigate their stresses in time domain.

**Figure 10.**The time variation of stress curves of P1 to P8 using: (

**left**) mineral oil; (

**right**) ester oil.

**Figure 11.**The streamlines by using ester oil at 10 s and 30 °C: (

**a**) A sample streamline (thick pink line); (

**b**) The electric field curve along this streamline starting from the rightmost point.

**Figure 12.**The streamlines by using ester oil at 10 s and 30 °C: (

**a**) A sample creepage surface (thick pink line); (

**b**) The electric field curve along this creepage surface (accounted from the right bottom).

**Figure 14.**The streamlines by using ester oil at 10 s and 30 °C: (

**a**) A sample combined path in an oil gap and two creepage paths (thick pink line, creepage path 1 is on the insulation of the HV shield and creepage path 2 is on the oil duct barrier); (

**b**) The electric field curve along this combined path; (

**c**) the sorted electric field curves; (

**d**) the safety factor curves (the safety factor of the creepage path 1 is higher than 40 and that is why the green line is not shown).

**Figure 15.**The flowchart of automatic insulation design evaluation applied in COMSOL Live Link with MATLAB.

1 kV/mm | 3 kV/mm | 6 kV/mm | 12 kV/mm | ||
---|---|---|---|---|---|

Mineral Oil | 30 °C | 5.0 × 10^{−14} | 3.9 × 10^{−14} | 5.8 × 10^{−14} | - |

50 °C | 1.2 × 10^{−13} | 9.5 × 10^{−14} | 1.1 × 10^{−13} | - | |

90 °C | 5.2 × 10^{−13} | 4.6 × 10^{−13} | 3.9 × 10^{−13} | - | |

Mineral OIP | 30 °C | 1.7 × 10^{−16} | 2.2 × 10^{−16} | 2.7 × 10^{−16} | 4.0 × 10^{−16} |

50 °C | 2.6 × 10^{−15} | 3.3 × 10^{−15} | 4.0 × 10^{−15} | 5.8 × 10^{−15} | |

90 °C | 2.6 × 10^{−13} | 3.0 × 10^{−13} | 3.8 × 10^{−13} | 5.1 × 10^{−13} | |

CR | 30 °C | 294 | 177 | 215 | - |

50 °C | 46 | 29 | 27 | - | |

90 °C | 2 | 1.5 | 1 | - |

1 kV/mm | 3 kV/mm | 6 kV/mm | 12 kV/mm | ||
---|---|---|---|---|---|

Ester Oil | 30 °C | 1.4 × 10^{−11} | 1.0 × 10^{−11} | 0.8 × 10^{−11} | - |

50 °C | 5.0 × 10^{−11} | 3.6 × 10^{−11} | 3.2 × 10^{−11} | - | |

90 °C | 2.9 × 10^{−10} | 2.3 × 10^{−10} | 2.3 × 10^{−10} | - | |

Ester OIP | 30 °C | 6.1 × 10^{−13} | 6.9 × 10^{−13} | 4.9 × 10^{−13} | 3.4 × 10^{−13} |

50 °C | 2.0 × 10^{−12} | 2.2 × 10^{−12} | 1.6 × 10^{−12} | 1.2 × 10^{−12} | |

90 °C | 1.4 × 10^{−11} | 1.5 × 10^{−11} | 1.2 × 10^{−11} | 1.1 × 10^{−11} | |

CR | 30 °C | 23 | 15 | 16 | - |

50 °C | 25 | 16 | 20 | - | |

90 °C | 21 | 15 | 19 | - |

**Table 3.**The maximum stresses in OIPs, the minimum SFs in the oil gaps, on the interface creepage paths and combined paths and the related time of occurrences.

Temp. (°C) | Max. Stress in OIPs (kV/mm) | Time (s) | Min. SF in Oil Gaps | Time (s) | Min. SF at Creepage Surfaces | Time (s) | Min. SF at Combined Paths | Time (s) | |
---|---|---|---|---|---|---|---|---|---|

mineral oil | 30 | 17.5 | 4467 | 1.5 | 1 | 2.4 | 2239 | 1.5 | 1 |

50 | 16.2 | 1778 | 1.5 | 1 | 2.4 | 891 | 1.5 | 1 | |

90 | 8.3 | 398 | 1.5 | 1 | 2.5 | 158 | 1.5 | 1 | |

ester oil | 30 | 17.7 | 28 | 1.7 | 1 | 2.3 | 12.6 | 1.6 | 1 |

50 | 17.7 | 8 | 1.7 | 1 | 2.4 | 4 | 1.6 | 1 | |

90 | 17.3 | 2 | 1.8 | 1 | 2.5 | 1.4 | 1.7 | 1 |

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## Share and Cite

**MDPI and ACS Style**

Kharezy, M.; Mirzaei, H.R.; Thiringer, T.; Serdyuk, Y.V.
Green Solution for Insulation System of a Medium Frequency High Voltage Transformer for an Offshore Wind Farm. *Energies* **2022**, *15*, 1998.
https://doi.org/10.3390/en15061998

**AMA Style**

Kharezy M, Mirzaei HR, Thiringer T, Serdyuk YV.
Green Solution for Insulation System of a Medium Frequency High Voltage Transformer for an Offshore Wind Farm. *Energies*. 2022; 15(6):1998.
https://doi.org/10.3390/en15061998

**Chicago/Turabian Style**

Kharezy, Mohammad, Hassan Reza Mirzaei, Torbjörn Thiringer, and Yuriy V. Serdyuk.
2022. "Green Solution for Insulation System of a Medium Frequency High Voltage Transformer for an Offshore Wind Farm" *Energies* 15, no. 6: 1998.
https://doi.org/10.3390/en15061998