An on-site calibration system for high frequency harmonic voltage measurements in high voltage power systems

The power system landscape is undergoing a transformative shift due to the penetration of renewable resources, advancements in transmission and distribution infrastructure, and the use of non-linear device based consumer appliances. As a result of these developments, a significant increase in the use of power electronic converters have been observed recently. Modern power electronics employ force commutated semiconductor devices switching at relatively high frequencies, contributing to an increase in harmonic emissions beyond 2 kHz. The measurement and assessment of high-frequency harmonics ranging from 2 kHz to 150 kHz have been overlooked due to their negligible magnitudes and perceived minimal impact. However, anticipating a future growth in emissions, technical bodies are working to regulate harmonic emissions and to develop measurement techniques covering this range.

Associated with the above, there is a growing interest in the accurate measurement of high frequency harmonics. State-of-the-art power quality analysers provide accurate measurements at low voltage (LV) levels. For medium voltage (MV) and high voltage (HV) levels, an instrument transducer (IT) is required to reduce the primary voltages into measurable quantities. The majority of ITs are inductive and capacitive voltage transformers, designed for the accurate voltage transformation at rated frequency. These transformers have non-linearities at higher frequencies, affecting their frequency response. Hence, an accurate calibration is required for the correction of measurement errors. Conventionally, the IT calibration is performed using an LV sinusoidal frequency sweep. However, recent work shows that ITs could demonstrate comparatively different voltage ratio and phase displacement errors when determined under distorted high voltage waveforms. However, implementation of such calibration systems for frequencies above 2 kHz has been challenging. The underlying reasons being the lack of voltage sources that can generate distorted HV waveforms with superimposed high frequency harmonics, and also the lack of reference transducers of which the frequency response have been experimentally proven not to be affected by the distorted HV waveforms. Hence, there exists a need to develop such HV sources and reference transducers along with acceptable experimental test procedures to prove their performance. Research work and outcomes presented in this thesis addresses these requirements.

This thesis presents a novel IT calibration system consisting of a composite voltage source to generate HV distorted waveforms comprising of a fundamental component up to 132/√3 kV, with superimposed spectrum of harmonics up to 10 kHz. A salient feature of the developed source is its capability to superimpose multiple high frequency harmonics with sufficient amplitudes that are essential for the accurate calibration of ITs with large voltage transformation ratios. Compared to existing HV sources, whose harmonic generation capability diminishes with increasing frequencies, the developed source can generate sufficient harmonic voltage magnitudes up to 10 kHz. This enables the investigation of IT accuracy at high frequencies under different conditions and would contribute to the expansion of knowledge on both conventional and novel instrument transducer technologies.

This research also addresses the limitations of reference transducers by developing a precision capacitive voltage divider of which the performance in terms of its voltage ratio and phase error was demonstrated to have a negligible impact due to distorted high voltage waveforms. Rigorous experimental procedures have been developed, which are expected to have a profound impact on the ongoing work by the various technical bodies with regard to harmonic measurement in high voltage networks, where there are existing deficiencies in transducer calibration.

Employing the above calibration system and based on the case study results obtained through characterization of two inductive voltage transformers under high voltage distorted conditions, it was identified that the transformer response around resonance frequencies can exhibit significant differences in voltage ratio and phase displacement errors under high voltage distorted waveforms compared to frequency response results obtained with low voltage frequency sweep. Results also illustrate that, with lower order harmonic frequencies below 2 kHz, the transformer responses do not show a significant difference in their accuracy when subjected to distorted high voltage waveforms in comparison to those obtained using frequency sweep. This could be attributed to the fact that the tested transformers do not show resonance effects at frequencies below 2 kHz. However, the results support the fact that high frequency characterization of inductive transformer should be undertaken with distorted waveforms in place of frequency sweep.

The work presented in this thesis contributes towards the linear and non-linear modelling of inductive transformers under LV sinusoidal and HV distorted waveforms. The frequency response of the investigated transformer under LV sinusoidal conditions can be successfully represented by higher order transfer functions. However, linear models fail to represent the high frequency behaviour of inductive transformers where resonance effects are observed. In contrast, the non-linear models, which have been previously used to model non-linearities of instrument transformers for harmonics below 25th order, can be successfully employed to represent the behaviour transformers at high frequencies. The development of such measurement based non-linear models at high frequencies has not been previously possible due to the limitations in the voltage sources required to generate distorted high voltage waveforms consisting of high frequency harmonics. Hence, the contributions made in this thesis are novel and significant not only in terms of investigating the accuracy performance of high frequency harmonic measurements in HV networks, but also in the expansion of knowledge with respect to high frequency harmonic measurement.