Joachim Holboell

Wideband Modeling of Power Transformers Using Commercial sFRA Equipment

This paper presents a new procedure to characterize an admittance matrix over a wide frequency band, for the purpose of black-box modeling of power transformers. Unlike previous approaches, our setup is based on a commercial sweep frequency-response analysis measurement instrument which is lightweight and considerably less expensive than dedicated high-precision laboratory equipment. The use of 50- Ω termination resistors allows measuring the terminal admittance matrix without the need for correcting for the measurement cables. The measured admittance matrix is next subjected to rational modeling using vector fitting and passivity enforcement. When applied to a 300-kVA distribution transformer, the resulting model accurately reproduces measured time-domain waveforms.

Multiple Lightning Discharges in Wind Turbines Associated With Nearby Cloud-to-Ground Lightning

This paper presents the analysis of five events where simultaneous lightning currents were registered in different wind turbines of a wind farm with lightning monitoring equipment installed. Measurements from current monitoring devices installed at the wind turbines and observations from auto-triggering video cameras were correlated with data from the U.S. National Lighting Detection Network. In all five events, the correlation showed that a cloud-to-ground (CG) lightning stroke with high peak current struck the ground within 10 km of the affected turbines at the time of the currents in the wind turbines. The polarity of the CG discharges was negative in two events and positive in the three remaining events, which were also accompanied by small intracloud pulses. According to the analysis of the data for each event, the simultaneous discharges at the wind turbines are most likely caused by the nearby CG strokes, involving mechanisms that vary depending on the polarity of the associated CG stroke. The analysis also suggests that the event of upward lightning from wind turbines triggered by nearby lightning activity occurs very often and therefore it should be considered carefully in the processes of assessing the lightning exposure of the wind turbines, designing the lightning protection, and defining validation tests.

Laboratory results from dielectric spectroscopy of field aged XLPE cables with respect to water trees

This paper deals with the application of frequency domain dielectric spectroscopy for the detection of water trees in XLPE cables. During the research, dielectric spectroscopy measurements have been carried out on cable samples with and without water trees while it was attempted to preserve many of the uncertainties present in the field. The experiments support the applicability of a high voltage test developed primarily at KTH in Sweden, which utilizes the nonlinearity of the dissipation factor. Subsequently it was investigated whether this high voltage method also seems reliable at low voltages. It turned out that the fundamentals of the high voltage method were not directly applicable at low voltages as the nonlinearity of the permittivity is not noticeable at low voltages, at least compared to the deviations. Instead a novel method for interpreting the low frequency measurements is proposed though it needs further verification. Finally a theoretically possible modification of the measurement system is proposed, which deals with the problem of a limited current capability of the amplifier

Lightning attachment to wind turbine surfaces affected by internal blade conditions

The efficiency of the blade lightning protection system depends to a great extend on the effectiveness of the receptor to intercept the lightning discharge. When the blade is exposed to a high electric field, streamers are mainly initiated from the receptor surface, but lightning attachment on the blade surface instead of the receptor is also possible, with the risk of damages in the composite structure as a consequence. The present paper focuses on electrical fields and streamer activity in connection to conductive components inside a wind turbine blade when a downward leader is approaching the wind turbine. High voltage tests have been carried out to study the development of streamers in conductive components inside a fiberglass structure, and the results have been correlated with the simulations of the electric field performed using Finite Element Method. Finally, potential applications of the results of this study are discussed.

Breakdown and tracking properties of rubber materials for wind turbine blades

The use of rubber materials in wind turbine blades, for example in controllable trailing edge flaps, requires research on their behavior under heavy exposure to electric fields and electrical discharges. Since the complex construction of blades usually involves several and often inhomogeneous materials, the testing methods selected should reflect the realistic conditions. In this paper the applicability of rubber materials to thunderstorm environments is studied by performing electric breakdown tests and tracking resistance tests on selected samples, and the findings are related to the possible future application of rubber materials to wind turbine blades.

Temperature and load correlated on-line PD measurements on a Danish 6,4MVA Power Generator

Monitoring of the PD level of a 10kV, 6.4MVA generator in a Danish Power Plant over a couple of years has caused increasing concern about the generators VPI insulation condition. This paper presents an analysis of the PD activity in the generator with an advanced PD monitoring system. The measurements were performed as on-line measurements during normal operation as well as during the start up and shut down sequences. Winding temperature and the generator load were recorded simultaneously with the PD measurements. A correlation has been established between the temporal development of PD intensity and factors like winding temperature and load level. Analyses were performed on the PD development with respect to pulse magnitude and frequency, including advanced pulse shape analyses, in order to determine PD-origin and characteristics. A comparable high PD level was recorded which, combined with experiences from early types of generators with global VPI insulation, led to the decision that the generator should be refurbished. The results described and analyzed in this paper are based on the service period with the original insulation until refurbishment.

Switching operation simulations in a large offshore wind farm with use of parametric variation and Frequency Domain Severity Factor

Transient voltages resulting from switching operations depend on an interaction between the breaker, the transformer, cables and a neighbourhood grid and imply a risk for the transformer and other components. In this paper the Frequency Domain Severity Factor (FDSF) is used to assess the severity of electrical stress imposed on wind turbine transformers by voltage waveforms produced during switching operations. The method is implemented in Matlab together with automatic and systematic variation of parameters. Simulations of a radial energization are performed on a 90MVA offshore wind farm model implemented in ATP-EMTP using standard component models and further validated against measurements. The results show the range of overvoltages on the transformer terminals as well as the corresponding FSDF for all turbines under a number of varying parameters. The maximum FDSF of 1.235 and frequency of 445 kHz has been found on the low voltage side of the transformer at the last wind turbine in the neighbouring radial to the one being energized.

Faults in the collection grid of offshore wind farms

This paper deals with transient conditions in the collection grid of offshore wind farms under different faults. A model of a standard wind farm was established in two versions, with a floating and a grounded collection grid respectively. Line to ground faults and three-phase to ground faults were applied at critical points at worst-case phase angle and the results compared. The simulations show that it is better with a grounded collection grid, since lack of a ground reference will prevent the system from recovering after a line to ground fault.