Browsing by Author "Oyedokun, David Temitope"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Open Access
    Geomagnetically induced currents (GIC) in large power systems including transformer time response
    (2015) Oyedokun, David Temitope; Gaunt, C Trevor; Folly, Komla A
    Geomagnetically induced currents (GIC) are the result of changing geomagnetic fields which are a consequence of a geomagnetic disturbance (GMD). The flow of GIC through transmission lines and transformers across the power network could have severe consequences, if the magnitudes of the GIC are high enough. Problems that could arise from the flow of GIC in transmission networks include an increase in the amount of reactive power demand by GIC-laden transformers, half-wave saturation, excessive heating in transformers, incorrect operation of transmission line protection schemes and voltage collapse in affected sections of the network. In the past, GIC were calculated without taking the transformer's response time into account. The limitation of this approach is that the size and core type of the transformer is neglected. This may affect the assessment of GIC in the power network as the flux pattern and winding inductance distribution are not uniform across all transformer core structures. This thesis postulates that these characteristics could have far-reaching effects on the GIC that flows through a transformer as a function of time. Based on this assumption, a novel way of calculating GIC is introduced in this thesis. This method combines the uniform plane wave model and the network Nodal Admittance Matrix (NAM) method and incorporated for the first time, the transformer time response, which does not appear to have been considered in previous calculation methods. A general formula, which describes the transformer's time response to GIC was derived, followed by the derivation of the electric field induced in each transmission line. A key input to the prospective GIC with transformer time response calculation, is a set of piecewise linear equations derived from a laboratory test and PSCAD simulations. These suitably characterise the response of three transformer core structures, namely: bank of single phase (3(1P-3L)), three-phase three-limb (3P-3L) and three-phase five-limb transformers (3P-5L). Each of these core types were considered as a Generator Step-up Unit (GSU) and a Transmission Transformer (TT). The results of the laboratory experiment and simulations in PSCAD led to the conclusion that the transformer time response to GIC is irregular across the transformer cores that were tested. The 300 VA transformer core structure with the shortest response time is the 3P-3L, followed by the 3P-5L and the 3(1P-3L). For the 500 MVA transformers, the order was: 3P-3L; 3(1P-3L); and 3P-5L. The 3P-3L transformers permit the flow of GIC through the windings of the transformer over a shorter length of time. Therefore based on the order in response time, during GMDs leading to higher GIC, the prospective GIC with or without transformer time response flowing through 3P-3L transformers will be similar. Furthermore, the response time to GIC in 3P-3L, 3P-5L and 3(1P-3L) transformer core types are load-dependant. The 3(1P-3L) and 3P-5L transformers operating as TT's (modelled as transformers at 40 % load) have the longest response time to GIC, while 3P-3L transformers operating as a GSU (modelled as transformers at full load) have the longest response time to DC. The shortest response time to DC was with a GSU at light load (modelled as transformers at 80 % load), which was consistent across the three transformer core types. This correlates well with the notion that power networks could stand a better chance of surviving a high GMD when all generating units and loads are online. Three different core structures were modelled with a variation of DC current levels and load conditions, both in PSCAD and in the laboratory. These results are unique to the transformer models used, but are representative of major types of core configurations used on power networks. These results provide an indication that it is incorrect to lump the responses of all transformers and transformer time response should be taken into consideration, especially when sampling at intervals as low as 2 seconds.
  • Thumbnail Image
    Item
    Open Access
    Power flow and rotor angle stability studies of HVAC-HVDC power system interconnections using DigSILENT
    (2010) Oyedokun, David Temitope; Folly, Komla A
    The backbone of all industrialised nations is the success of the power sector, which involves an efficient, reliable and secure means of power generation, transmission and distribution. Industrial growth and urbanisation have together endlessly stimulated electrical engineers to ensure that the electrical power needs of the society are met.
  • Thumbnail Image
    Item
    Open Access
    Real-time power system impedance estimation for DG applications: Using PV-inverter based harmonic injection method
    (2017) Knezevic, Aleksa; Oyedokun, David Temitope
    On-line power system (PS) Thévenin equivalent impedance (TEI) estimation involves the reduction of the PS's complex circuit into a simple form that provides valuable insight into its state and behaviour. It finds application in numerous areas such as voltage stability monitoring and islanding detection. In the context of distributed generation (DG), on-line TEI estimation can be easily implemented in existing hardware to add functionality and improve the operation of power converters – the key components of DG systems. Two distinct methods of on-line PS TEI estimation exist. The passive method involves only measurement of voltage and current, whereas the active method involves injection of current into the PS and measurement of the response. This work is focused on the active method. Through a review of the available literature, limitations of past work are highlighted. It is shown that the nature of current injection varies greatly in different works and that evaluation of implementation performance is generally not thorough. Little consideration has been made of the effect of injection current level and frequency on the performance of on-line TEI estimation. Furthermore, the behaviour of the grid and its impact has not been thoroughly investigated. In this work, the active method is implemented in a three-phase PV-inverter and thoroughly tested in terms of its TEI estimation accuracy. Dependence of said accuracy on parameters such as the level of injected current and its frequency is shown to be high through tests performed on the live PS at two locations. These parameters are optimised such that TEI accuracy is maximised and the performance of the device is shown to be good compared to calibration equipment. The accuracy of PS TEI tracking is evaluated and quantified. Considerations are also made of the device's hardware limitations and their effect. A process by which a device's TEI estimation accuracy can be thoroughly evaluated is developed through this work. The behaviour of the PS's TEI is also investigated over long periods and characterised. It is found that the TEI remains steady around an average level in both test locations, with a low standard deviation. Consistency in results is found to be high between the two tests.