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Has files: YesSubject: search.filters.subject.Electric power system stability

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    Voltage stability in distribution network
    (2020-09) Masikana Sboniso Brutus; Sharma, Gulshan; Akindeji, Timothy Kayode
    Voltage stability studies and to maintain the flat voltage profile is quite important in order to maintain the healthy operation of electric power network as well as to provide the quality and cheap electric energy to the modern power users. Further with the advancement of power electronics technologies and its application to design flexible alternating current transmission devices (FACTS) have made it easier to alleviate the voltage stability problem in a quicker and cheaper way in the modern DNs. Therefore, this research work shows an attempt to investigate and solve the problem of voltage instability in the distribution network (DN) with the help of FACTS. All buses and lines are calculated in terms of voltage stability index (VSI) and to identify the optimal location of FACTS. The bus or line with minimum voltage profile in terms of VSI are more sensitive to the voltage collapse and it may further lead to blackouts. Hence, the FACTS are permanently installed at the weakest point to enhance voltage profile and improve the voltage stability in the DN. The present study is tested on standard IEEE-15 bus DN and application results are shown to verify the feasibility of the present studies for DN. The beauty and future promise of UPFC in power quality improvement was authenticated on the IEEE-15 bus DN carried out using MATLAB software tool, five different scenarios were considered by increasing the load up to 40% at an interval of 10% from its nominal operating load. With the aim of determining the impact of UPFC on bus voltage and system losses, the load flow analysis was contributed on each scenario with and without UPFC placement in the DN. After UPFC placement there was a significant enhancement of voltages of all busses as well as weakest bus voltage jump from 0.5750 to 0.9750 p.u. and shifting that bus as well as system from voltage instability to stable zone. The active and reactive power loses were decrease by 9.83% and 27.27% that fulfil the beauty of the UPFC installation in the DNs as well as it promise to mitigate the voltage instability problem of the modern DNs
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    Transient fault analysis of a VSC-based multi-terminal HVDC scheme
    (2020-09) Malanda, Sindisiwe Cindy; Davidson, Innocent Ewaen; Adam, Grain Philip
    A multiterminal HVDC system includes the connection of different HVDC terminals to a common grid. Most of the MTDC networks are realized in voltage source converter (VSC) high voltage direct current (HVDC). Over long distances, HVDC transmission is preferred to high voltage direct current (HVAC). Furthermore, HVDC is subjected to minimal harmonics oscillation problems due to the absence of frequency. HVDC enables the interconnection of systems at different frequencies, and the system becomes free of angular stability problems. VSCs employ insulated gate bipolar transistors (IGBTs) switches, and High-frequency pulse width modulation is used to operate the IGBTs in order to achieve high-speed control of active and reactive power. The growth of MTDC networks may require a new type of VSCs topology, which is resilient and efficient to dc and ac network fault. This research investigation focuses on the transient dc-side fault analysis in a two-level Monopolar VSC- Based Multi-Terminal HVDC Scheme consisting of four asynchronous terminals sharing a rated 400kV DC-grid was carried out in PSCAD software. During dc-side fault analysis, a pole-to-ground fault was taken into consideration as it’s more likely to occur, although it is less severe compared to pole-to-pole. The converters are interconnected through 100 km dc cables placed 0.5 gm apart and at a depth of 1.5 m underground. It was observed that during the steady-state analysis, the dc voltage in the grid was maintained at the rated value 400 kV, the currents measured at the converters bus was 0.5 kA, and the current flowing through the cables was 0.25 kA. Under the fault condition, the dc voltage drop needs to be maintained to a closed range to avoid the grid to collapse. The voltage droop technique was incorporated in the dc voltage controller to keep the dc voltage at the narrow range. Depending on the value and nature of ground fault resistance, the fault current magnitude varies, and distance variation along the cable has a significant contribution in the fault current. It is observed that fault close to the converter (5 km’s measured 9 kA) results in high fault currents compared to fault away from the converter (50 km’s measured 7.8 kA). The protection design of the VSC needs to be able to detect whether its ground fault or short circuit since the location of the fault needs to be identified and repaired. Another observation made when the fault is inserted 50 kms away from the converter, meaning the fault is at the center of the two converters, the outcome results in high currents in both converters. The isolation of the fault should be fast and selective as the critical time is very short. The dc circuit breakers are mostly recommended to be used as primary protection; however, different protection techniques need to be incorporated with dc circuit breaker in order to quickly identify, select and reliable isolate the faulted line. Moreover, the protection should be able to isolate the line before the fault reaches the maximum fault current to avoid the damage in the converter components.
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    The impact of poor power quality and harmonics on the performance of an electrical power network
    (2019) Nkonyane, Mfanasibili Stanley; Ojo, Eshiemogie; Rigby, Bruce
    Poor power quality has a negative impact on electrical protection systems, rotating machines, transformers, control circuits, electronics, and power electronics equipment. The demand from industries to use power electronics equipment leads to more poor power quality issues – especially relating to harmonics. Determining the level of harmonics in an electrical network has become a necessity, as most electrical equipment are susceptible to harmonics. Previously, electrical networks on the customer side consisted mostly of direct current and induction motors, which resulted in simple networks that were easy to model using various types of simulation software. Today’s electrical network is considered complex due to power electronic equipment such as variable frequency drives (VFDs), uninterruptible power supplies (UPSs), switch mode power supplies (SMPSs) and other electronics equipment. Power electronic equipment are primary source of harmonics and are also susceptible to harmonics. To protect the electrical network infrastructure, it is very important to identify the level of harmonics content in an electrical network, so that solutions can be developed to minimise the harmonic level to acceptable limits – as determined by power quality or harmonics standards. This dissertation presents an analysis of the performance of an island electrical network for an offshore crude-oil drilling ship. A real-time digital simulator was used as a systematic analytical tool to study the level of harmonics in a reduced-scale electrical network model, used to represent the real drilling ship power network. The study objective was to evaluate the behaviour of the power network when direct on line (DOL) starters and variable frequency drives are used to run induction motors coupled with mechanical loads. The waveforms from a limited number of field measurements on the actual network are compared to those obtained from the reduced-scale real-time simulation model of the plant. The study reviews the theory and literature of power quality, generators, transformers, variable frequency drives, and induction machines, and focuses on poor power quality as contributed to by harmonics. The investigation was based on 12 pulse rectifiers for all variable speed drives, which are standard for drilling ships and other offshore installations – as they offer advantages in reducing the fifth and seventh harmonics. Both the field measurements and real-time simulation results in the dissertation indicate the presence of similar harmonic waveforms, and with comparable frequencies – but with different amplitudes. Unfortunately, the simulation results could not be closely matched to the field results, as most operating parameters that are needed for better representation of the plant in the simulation model, could not be obtained within the limited time available for field measurements. Nevertheless, the model developed could be used with a greater degree of accuracy to demonstrate the level of harmonics for an offshore drilling ship power network, provided all operating conditions’ parameters are available.