Quantifying and compensating for the influence of instrument transformers on harmonic measurements for grid code compliance

Abstract

Accurate harmonic current measurement is critical for grid code compliance in modern power systems. This research addresses a key challenge in the modeling of medium- and high-voltage hairpin-type current transformers (CTs) by developing a simulation model that incorporates stray capacitance into the CT equivalent circuit - an element traditionally neglected in harmonic analysis. A comprehensive literature review identified this modeling gap, leading to the development of a theoretical framework and simulation approach. The model was validated through emulated field testing on outdoor CTs under various burden scenarios, with a particular focus on inductive-bearing burden. Experimental results demonstrated strong correlation with simulation outputs, with harmonic ratio errors reduced to within 1-2% up to 3 kHz using the proposed novel engineering method. These findings not only improve the accuracy of harmonic measurements but also support more reliable grid code compliance assessments, particularly within the South African context. Key findings include: (1) Stray capacitance, owing to the dielectric design of a CT, significantly affects harmonic measurement accuracy and must be included in CT models, (2) proposed simulation model accurately predicts CT behaviour up to 3 kHz and (3) a correction method can reduce harmonic ratio errors to within 1-2%, improving measurement reliability for grid code compliance. Novel contributions of this research include: (1) The integration of stray capacitance into CT modelling for harmonic analysis, (2) quantification of burden effects on harmonic current measurement accuracy, and (3) a engineering method to correct measurement errors for grid code compliance in South Africa. Limitations of this research include the focus on a single CT geometry and regional design constraints. Future research opportunities exist to extend the model to other CT types and improve accuracy at lowerorder harmonics - considering magnetic core impacts, as proposed in various literature on this subject. This thesis advances the field of electrical engineering by improving harmonic current measurement accuracy and supporting the development of more robust frameworks for grid code compliance.