PhD defence by Unnur Stella Gudmundsdottir on Modelling of long High Voltage AC cables in Transmission Systems
- Thursday 10. June 2010 at 13:00 - 16:00
Modelling of long High Voltage AC cables in Transmission Systems
Unnur Stella Gudmundsdottir
Associate Professor, Michael Bech, Aalborg University
Professor Frede Blåbjerg, Aalborg University (chairman)
Professor Akihiro Ametani, Doshisha University, Kyotanabe
Dr. Hermann W. Dommel, University of British Columbia
The research documented in this thesis addresses Modelling of long High Voltage AC cables in Transmission Systems. The aim is to deliver a reliable cable model, which can be used as an instrument in planning and problem solving for long distance cables and a transmission system that is mostly or fully underground.
In recent years, the interest towards using underground cables in power transmission has increased considerably. In Denmark, the entire 150 kV and 132 kV transmission network shall be undergrounded during the next 20 years. Even 400 kV transmission lines will be undergrounded gradually as more experience is gathered. Precise modelling of long and many (meshed) underground cable lines is therefore essential and it is important that differences between simulations and measurements are identified, studied and eliminated. A study of the cable model accuracy for transmission line modelling is the topic of the research documented in this thesis.
The usual practice for validating a cable model has been to compare the simulation results to frequency domain calculations transformed to the time domain by use of Inverse Fast Fourier Transform (IFFT). This however, does not ensure the accuracy of the entry parameters of the modelling procedure, the parameter conversion and the modelling assumptions. Therefore, in order to analyse how cables behave field tests are performed. The purpose of the field measurements is to analyse the cable model, investigate the accuracy of the model, identify origin of disagreement between measurement and simulation results and validate the improved simulations when identified origin of disagreement has been eliminated by more accurate modelling.
The main conclusions from field measurements are:
- The existing cable model is precise and accurate for short cables or cables with no crossbonding points
- There is deviation between simulation and field measurement results on long cables. The existing cable model is not of acceptable accuracy for crossbonded cable lines
- Inaccurate modelling of the cable screen is the reason for deviation between simulation and field measurement results. This is because of intersheath mode reflecting from the crossbonding points.
The existing EMT-based models have the configuration for cables: conductor-insulation-conductor-insulation, whereas a transmission line single core XLPE cable will normally have the configuration: conductor-insulation-conductor-SC layer-conductor-insulation. Furthermore the existing cable models use analytical equations to calculate the series impedances and shunt admittances of the cable line. These analytical equations include skin effect, whereas they do not include proximity effect.
The cable model is firstly improved in such a way, that the correct physical layout of the screen is implemented in the model. At higher frequencies the proximity effect will force the current to be more constrained to smaller regions, resulting in a change in the impedance of the conductor. Therefore the cable model is secondly improved in such a way, that the impedance matrix is no longer calculated from the analytical equations but from a finite element method including the proximity effect.
The main conclusions from improving the cable model are:
- By improving the cable model with respect to correct physical layout of the screen, a correct damping will appear in the simulation results
- The correct physical layout of the cable screen does not eliminate high frequency oscillations that appear
- By including the proximity effect in the model, the impedance will change at high frequencies resulting in accurate damping of the high frequency oscillations
- By combining the proximity effect and the correct physical layout of the screen, the simulation results agree with field measurement results within the tolerance of the field measurements. This is the case for a non-crossbonded cable where the intersheath
- By combining the proximity effect and the correct physical layout of the screen, the simulation results agree with field measurement results within the tolerance of the field measurements. This is the case for a non-crossbonded cable where the intersheathmode is explicitly excited, for a 2.5 km cable with two crossbonding points and for a 55 km long cable line with 33 crossbonding points.
Some major contributions from the research presented are as follows:
- Field measurements for model validation
- New cable impedance equivalent for a layered cable screen
- Subdivision of conductors for a full 3 phase underground cable system
- Description of how the ground can be subdivided and practical simulations performed
- Validation of subdivision of conductors by use of high frequency field measurements.
All are welcome. The defence will be in English.
After the public defence there will be an informal reception in Pontoppidanstræde 101 room 25/27.
- Pontoppidanstræde 101, Room 23