# Numerical Analysis of a Spiral Tube Damping Busbar to Suppress VFTO in 1000 kV GIS

^{*}

## Abstract

**:**

_{6}discharge during switching operations in a GIS may threaten the insulation of electrical equipment. In this paper, a novel VFTO suppression method with great prospects in engineering, called the spiral tube damping busbar, is proposed. The suppressing mechanism of the new method is analyzed. The structure and the design characteristics of the damping busbar are introduced as well. Moreover, a calculation method for the self-inductance of the damping busbar at high frequency is presented. According to the structural characteristics of the damping busbar, the inductance effect on suppressing VFTO is analyzed. A further improvement in damping VFTO is investigated by designing a spiral litz coil connected in series with the busbar, which increases the damping effect. The simulation results show that the improved damping busbar has a significant suppressing effect on the amplitude and the frequency of VFTO.

## 1. Introduction

_{6}discharge during switching operations in GISs. VFTO could cause damage to power devices. A reduction of the insulating capability of the dielectric gas in GISs is caused mainly by the peak magnitude and high-frequency oscillations of VFTO. The internal VFTO causes stress on the main insulation in the GIS, while the external VFTO poses a threat mainly to the main transformer and the secondary equipment within the substation [8]. These transients have extremely short rise time, in the nanoseconds range.

_{6}gas [21,22]. It could be well observed from the literature that an accurate design using the specific parameters of the GIS DS (Disconnector) contact system is required for VFTO mitigation by controlling the voltage conditions preceding voltage breakdowns in the disconnector contact system. As a consequence, additional costs will increase for new combinations.

## 2. The Suppression Mechanism of the New Method

#### 2.1. The Structure of the Damping Busbar

#### 2.2. The Equivalent Circuit of the Damping Busbar

_{i}(i = 0,1,2,…) represents the inductance of each unit coil of the metal spiral tube conductor. R

_{i}and L

_{Ri}(i = 0,1,2,…) represent the noninductive resistance and its residual inductance of the parallel connection of each turn, respectively, and the value of the resistance can be adjusted to achieve the best damping effect. g

_{i}(i = 0,1,2,…) is the hollowing gap of the damping busbar and r

_{i}(i = 0,1,2,…) is the arcing resistance of the gap, which represents the losses formed by the discharge channel.

#### Inductance Calculation of the Damping Busbar at High Frequency-Damping Busbar Parameters

_{6}gas discharge. Due to VFTC being assigned as current excitation for the simulation procedures, the VFTC equation was obtained for Fourier 8, with goodness of fit parameters R-square = 0.9768 and adjusted R-square = 0.9759.

## 3. Simulation Results

#### 3.1. Modelling of a 1000 kV GIS

_{0}= 8.85418782 × 10

^{−12}F/m, ε

_{r}= 1.0024 and μ

_{0}= 4π × 10

^{−7}H/m. According to the dimensions of the GIS busbar, the calculations showed that the propagation velocity is $295\text{}\mathrm{m}/\mathsf{\mu}\mathrm{s}$ and the surge impedance is about $93.382\text{}\mathrm{ohm}/\mathrm{m}$.

#### 3.2. Discussion

_{6}gas discharges, the disconnector gap restores the insulation state; then, the current disappears, the high-frequency transient components gradually decay, and, finally, the voltage wave continues as a sinusoidal wave with the supply voltage.

_{6}gas breakdown, the step magnitude is the breakdown voltage, and the narrow pulse at the ladder edge is the highest frequency of the VFTO, as shown in Figure 7c.

_{6}gas is equal to the transient recovery voltage of the gap, the SF

_{6}gas collapses and the gap transfers from the insulation state into a conductive state; then, the circuit produces a high-frequency transient current, as shown in Figure 7e. Additionally, when the critical breakdown voltage of SF

_{6}gas exceeds the transient recovery voltage of the gap, the current disappears during discharge of the SF

_{6}gas.

## 4. Inductance Effect on Suppressing VFTO

_{i}increases the round-trip time of the travelling wave, which in turn leads to a reduction in the high-frequency components of the VFTO. The travelling wave leads to a higher voltage on the inductance of the small circuit, which causes the parallel resistance ${R}_{i}$ to absorb the travelling wave energy, increase the consumption of active power, and reduce the amplitude of the VFTO.

## 5. The Improved Design of the Damping Busbar

_{s}is the cross-sectional area of a strand. Consequently, by inserting the operating frequency, the AWG (American wire gauge) can be chosen (i.e., when frequency range is from 1.4 MHz to 2.8 MHz, then the best choice is AWG = 48), which means that about 70% loss reduction is achieved. Eddy power losses were calculated, and the final design was determined according to the critical factor (diameter of litz wire no more than the width of the spiral slot on the damping busbar surface).

- Determine A
_{c}of the core, which was an aluminum cylinder (busbar body) in our study. - Determine the number of turns which was commensurate with the design.
- Determine the inductance value which achieves an effective design to avoid saturation.
- Calculate the air gap length as a first step to calculate the magnetic flux density in order to calculate the power loss in the litz wire for the optimal design of the coil. Thus, the length of the air gap was calculated by using Ampere’s law, as shown below:$${l}_{g}=\frac{{N}^{2}{A}_{c}{\mu}_{0}}{L}.$$

## 6. Summary

## 7. Conclusions

- We proposed a new design which has a significantly better damping effect when compared with other existing suppression methods. The proposed damping busbar can be easily implemented in any existing GIS without any change in its structure and is therefore very cost-effective.
- We investigated a calculation method for the self-inductance at high frequency.
- We improved the performance of the damping busbar by adding a spiral litz coil connected in series with the busbar in order to enhance the suppression effect, and we developed an algorithm based on air gap calculation to design the litz coil.
- We compared the damping effect with and without the damping busbar and the improved design.

## Author Contributions

## Conflicts of Interest

## References

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**Figure 1.**The new damping busbar installed in a gas insulated substations (GIS): (

**a**) general view of the damping busbar with its components; (

**b**) the placement of the damping busbar.

**Figure 7.**Very fast transient overvoltage (VFTO) and VFTC waveforms: VFTO at the source side (

**a**) before installing the damping busbar and (

**b**) after installing it; VFTO at the load side (

**c**) before installing the damping busbar and (

**d**) after installing it; VFTC (

**e**) before installing the damping busbar and (

**f**) after installing it.

**Figure 8.**Influence of the inductance value on VFTO suppression at the load side of a 1000 kV GIS: at inductance values (

**a**) L = 0 mH, (

**b**) L = 0.3 mH, (

**c**) L = 3 mH.

**Figure 10.**VFTO waveform at the load side: (

**a**) before installing the damping busbar, (

**b**) after installing the improved design of the damping busbar; VFTO at the load side for one breakdown at 0.013 s: (

**c**) before installing the damping busbar, (

**d**) after installing the improved design of the damping busbar.

Values | |||||||||
---|---|---|---|---|---|---|---|---|---|

i | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |

a_{i} | 1025 | 403.3 | −7.241 | 809.5 | −28.8 | 46.59 | 23.15 | −1149 | 1881 |

b_{i} | 0 | 642.6 | 2540 | −3334 | −808.4 | 161.9 | −859.5 | 1063 | −1489 |

ω | ${1.372\times 10}^{7}$ |

Parameters | ||||
---|---|---|---|---|

Number of Turns | Units | Resistance per Unit [Ω] | Inductance [mH] | |

Value | 28 | 14 | 6 Resistors in parallel Req./unit = 20 Ω | 0.33656 |

**Table 3.**The transient characteristics of VFTO under an opening operation at the load side before and after installing the damping busbar.

VFTO Characteristic Parameters | Simulation Results | |
---|---|---|

Before Installing the Damping Busbar | After Installing the Damping Busbar | |

Rise time [ns] | 10 | 30 |

Average breakdown steepness [kV/ns] | 71.53 | 48.725 |

Maximum amplitude of VFTO [p.u.] | 1.554 | 1.13 |

Average voltage overshoot coefficient | 0.487 | 0.244 |

Delay time [μs] | 24.4 | 47.2 |

VFTO Characteristics Parameters | Simulation Results | ||
---|---|---|---|

L = 0 mH | L = 0.3 mH | L = 3 mH | |

Rise time [ns] | 10 | 30 | 35 |

Maximum amplitude of VFTO [p.u.] | 1.554 | 1.13 | 1.0 |

Inductance Value [mH] | Simulation Results for Litz Coil Design | |||
---|---|---|---|---|

Number of Bundles | Number of Strands | Strand Diameter [mm] | Construction Type | |

0.1 | 5 | 140 | 0.899 | Stranded litz wire, type 2 |

0.3 | 5 | 140 | 0.838 | |

3.0 | 5 | 140 | 0.702 |

**Table 6.**The transient characteristics of VFTO under an opening operation at load side with and without installing the spiral litz coil (0.1 mH).

VFTO Characteristics Parameters | Simulation Results | ||
---|---|---|---|

Without Damping Busbar | Damping Busbar Only | Spiral Litz Coil in Series with the Damping Busbar | |

Rise time [ns] | 10 | 29 | 31 |

Maximum amplitude of VFTO [p.u.] | 1.554 | 1.13 | 1.081 |

Method | Mitigation Effect |
---|---|

Damping resistor | Up to 25% |

Slow-acting disconnector (low trapped charge voltage) | (15–25)% |

High-frequency resonator | Up to 20% |

Ferrite rings | Up to (10–30)% |

Damping busbar | (22.22–52.94)% |

© 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/).

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**MDPI and ACS Style**

A. Almenweer, R.; Su, Y.-X.; Xixiu, W.
Numerical Analysis of a Spiral Tube Damping Busbar to Suppress VFTO in 1000 kV GIS. *Appl. Sci.* **2019**, *9*, 5076.
https://doi.org/10.3390/app9235076

**AMA Style**

A. Almenweer R, Su Y-X, Xixiu W.
Numerical Analysis of a Spiral Tube Damping Busbar to Suppress VFTO in 1000 kV GIS. *Applied Sciences*. 2019; 9(23):5076.
https://doi.org/10.3390/app9235076

**Chicago/Turabian Style**

A. Almenweer, Reem, Yi-Xin Su, and Wu Xixiu.
2019. "Numerical Analysis of a Spiral Tube Damping Busbar to Suppress VFTO in 1000 kV GIS" *Applied Sciences* 9, no. 23: 5076.
https://doi.org/10.3390/app9235076