Synchronous generator and excitation system response to GIC

Jankee, Pitambar

Synchronous generator and excitation system response to GIC

Master Thesis

2021

Abstract

Solar cycle 25 has started catching the attention of space scientists, physicists, and power engineers. The resultant Geomagnetically Induced Currents (GIC) lead to periodic part wave saturation of transformers, harmonic pollution, increased non-active power demand and potential voltage collapse. While most power system studies have focussed on the transformer's response to GIC, there has been very little research investigating the response of synchronous generators and excitation systems to such disturbances. Moreover, conventional GIC modelling assumes a dc voltage induced in the neutral or along transmission lines, but this may not adequately represent the actual dynamic power system's response to GIC. Using a 4-bus and a multimachine power system modelled in MATLAB Simulink, this project shows that the power system's response to low frequency GIC (acGIC) is different to the conventional dc approximation (dcGIC). In both cases, harmonic analysis was carried out. It was observed that under unbalanced system conditions, harmonics do not follow the conventional phase sequence. This means that multiples of the third harmonic no longer have a zero sequence. Such harmonics may affect the generator despite delta winding connections of generator step-up transformers (GSUs). The increased var demand due to saturating transformers was compared using an IEEE1459 meter and a General Power Theory meter. The analysis showed that conventional power theories using the term “reactive power” underestimate the increased var demand. The differences arise in the definitions of conventional reactive power and non-active power defined by IEEE1459. Hence, the burden of var on generators and var compensators can be higher than expected. Additionally, the responses of different complexities of generator models were compared. The results showed that ac equivalent sources might not represent the true dynamic power system's response to GIC. The multi-machine simulation results revealed that voltage dip below the 10 % limit, due to GIC, can be reduced by up to 8.75 % using proper excitation system models and control parameters. Moreover, it was shown that while one generator might not be able to provide enough non-active power to meet the increased demand, a group of generators pushing out var within the generator's capability limits, can help to reduce this deficiency of non-active power. This is because the total var output from the generators is higher. The findings of this study aim to raise concern on the conventionally used dc model of GIC which are not representative of a realistic GIC profile. Excitation system control appears to play a significant role in voltage drop reduction. As such, contingencies such as Geomagnetic Disturbances (GMDs) can be used to determine and tune optimal parameters of the excitation system control.

Keywords

Electrical Engineering

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