Technical performance and stability analysis of eskom power network using 600kv, 800kv, and 1000kv hvdc.

Master of Medical Science in Electrical Engineering. University of KwaZulu-Natal, Durban 2016.In designing electric power networks or implementing major expansions to existing networks, a number of the key issues regarding the technical performance of the network at both transmission and distribution level must be ascertained, namely: voltage regulation, voltage fluctuations, electrical losses, transmission/distribution plant loading and utilization, fault level, generation stability, harmonics, phase balancing, supply availability and system security. System studies and analysis conducted from time to time to ascertain the operating state of a network, taking into account, load growth projections for the future. Undue stresses on the system or anticipated problems are determined from power flow analysis or during operation and maintenance. Using a modified Eskom network (KwaZulu-Natal sub-grid) as a case study, the technical and stability analysis for different high voltage direct current (HVDC) transmission voltages: 600kV, 800kV and 1000kV were carried out using DIgSILENT PowerFactory engineering software tool, as an alternative for bulk power transfer using high voltage alternating current (HVAC) link along the major corridors. Static analysis using PV and QV curves; dynamic analysis using RMS time domain and electromagnetic EMT analysis were carried out. Dynamic analyses were performed to determine the system fault levels and critical fault clearing time. Results obtained from this investigation show that 600kV and 800kV HVDC transmission systems have greater power capacity than equivalent HVAC line. HVDC delivery systems were observed to have lower electrical losses, better voltage profile, increase fault clearing time, enabling robust protection schemes to be installed. Voltage distortion due to harmonic content and imperfect current waveform in Cahosa-Bassa LCC-HVDC link were also investigated, and re-engineering with the use of VSC-HVDC technology has been proposed. This option provides reduced harmonic content, excellent sinusoidal waveform and minimal vulnerability to commutation failure. A financial and economic analysis of a 500kV HVAC double circuit and ±600kV HVDC transmission network were compared. HVDC system was proposed the most suitable scheme for bulk transmission of electric power over long distances due to high efficiency and better economics

Stability of the grid incorporating multi terminal HVDC: case study of a south African network.

Doctoral Degree. University of KwaZulu-Natal, Durban.Transmission lines make one of the significant parts of power systems; faults or disturbances along any of the transmission medium often transcend to both the generating ends and the loads' end. Besides, the strength of any particular grid depends solely on the impedance of the tie-lines of that grid. Therefore, in this thesis, the line commutated converter (LCC) multiterminal high voltage direct current (MTDC) system is modelled and improved for the stability of an AC network. The converter control architecture and modelling are emphasized and explained. The effective short circuits ratio (ESCR) of the interconnecting AC lines is first described and analyzed as well. The existing CIGRE control techniques for a point-to-point LCC HVDC system have been enhanced and adapted for this study. The control and the filter parameters have also been calculated to generate a better and efficient result during a steady-state and dynamic analysis of the study. The work carried out in this study is divided into four sections, with each section focusing on each of the research objectives. In the first section, dynamic modelling and control of LCC MTDC systems were carried out with consideration to the ESCR of the inverter side of the AC substation. The impact of large-disturbance at the inverter is investigated. This analysis has been proposed to study the impact of AC short circuit fault on the three substations. The results from this study, which are shown on a subplot, show that the system experienced a large transient overcurrent and non-severe commutation failures. Also, a voltage dip at the faulted inverter station was recorded; however, the efficacy of the converter controller disallowed the transfer of such voltage dip to the other two converters. The second section of this study focuses on the application of MTDC system. We have carried out a comparative analysis of MTDC and AC transmission line on a single machine infinite bus (SMIB) network. The main focus of the investigation was on the transient and rotor angle stability of the SMIB network with or without MTDC link. The study also carried out a power-angle curve with the use of equal area criterion. The third section focuses on the interarea oscillation reduction in a power system. Kundur's two-area four-machine network was adapted to suit the scenarios of this study. Different fault analysis was carried out, and the response of the generator active power, frequencies, and DC-bus voltages are recorded. The results in this study show the better performance of the MTDC implemented in this study over the other well-known method of AC transmission medium. Also, the integration of the MTDC link is constrained by the variation of the current order of the overall power controller. The result is observed in the damping rate of the interarea oscillation of the network. The final section of this study carried out dynamic modelling of the South African grid, and detailed dynamic response to different stability studies was carried out. An auxiliary controller for the MTDC system capable of reducing the active power oscillation by generating a new current order is proposed. This secondary control for the MTDC system is based upon dynamic sensitivity analysis of the oscillations, and thereby generate a DC current compensation for the reduction of active power oscillations in the MTDC converters' station. Two network configurations were considered in this section. System disturbance during the first configuration shows a loss of synchronizing effect from both the AVR and PSS, which causes the generator to lose synchronism with subsequent oscillations. A negative damping torque for the rotor angle and negative synchronizing torque for the interarea oscillations was also observed. Meanwhile, the results during the second configuration recorded quick damping of the interarea oscillations with a significant improvement to the voltage profile. Among all of these benefits, the power carrying capacity at a reduced loss and cost stood out. The conclusion from this section is that the implementation of the MTDC link on the South African grid provided a better system performance. Therefore, the adoption of this research into South African transmission network will surely help enhance the stability margin of the grid. The proposed secondary controller also provided potential mitigation of excessive active power dip of the MTDC link during the system disturbance

Impact of LCC–HVDC multiterminal on generator rotor angle stability

Multiterminal High Voltage Direct Current (HVDC) transmission utilizing Line Commutated Converter (LCC-HVDC) technology is on the increase in interconnecting a remote generating station to any urban centre via long distance DC lines. This Multiterminal-HVDC (MTDC) system offers a reduced right of way benefits, reduction in transmission losses, as well as robust power controllability with enhanced stability margin. However, utilizing the MTDC system in an AC network bring about a new area of associated fault analysis as well as the effect on the entire AC system during a transient fault condition. This paper analyses the fault current contribution of an MTDC system during transient fault to the rotor angle of a synchronous generator. The results show a high rotor angle swing during a transient fault and the effectiveness of fast power system stabilizer connected to the generator automatic voltage regulator in damping the system oscillations. The MTDC link improved the system performance by providing an alternative path of power transfer and quick system recovery during transient fault thus increasing the rate at which the system oscillations were damped out. This shows great improvement compared to when power was being transmitted via AC lines

Prevalence of Anemia among Pregnant Women Registered at Antennal Clinic of Ondo Specialist Hospital, Ondo State, Nigeria

Anemia remains a major risk factor for unfavorable outcome of pregnancy both for the mother and the fetus. It is the world’s second leading cause of disability and one of the most serious global public health problems among children and pregnant women. Its diagnosis remains a challenge in poor and underfunded hospitals and primary health centers. This study is a hospital-based cross-sectional study conducted in Ondo Specialist Hospital, Ondo town to assess anemia among pregnant women attending antenatal care clinic from August to October 2015. One hundred and fifty pregnant women were enrolled in this study. Data were collected using pretested questionnaire, which contains socio-demographic characteristics of the pregnant women. Blood samples were collected to measure hemoglobin and Packed Cell Volume (PCV) levels. Data were entered and statistical analysis was performed using SPSS version 20.0 software. Association between variables was done using chi square, and statistical significance was considered at p<0.05. The mean age of pregnant women was 28.92±4.89 years and the prevalence of anemia obtained in this study using the Tallquist, Hemoglobin cyanide methods and PCV was 36%, 36.7% and 47.3% respectively, based on the World Health Organization criterion for the diagnosis of anemia in pregnancy (hemoglobin <11.0 g/dl; PCV <33%). Our study revealed a high prevalence of anemia in pregnant women and calls for more health intervention including health education about causes of anemia and its risk factors. Antenatal care follow up should also be improved on

A Review of LCC-HVDC and VSC-HVDC Technologies and Applications

High Voltage Direct Current (HVDC) systems has been an alternative method of transmitting electric power from one location to another with some inherent advantages over AC transmission systems. The efficiency and rated power carrying capacity of direct current transmission lines highly depends on the converter used in transforming the current from one form to another (AC to DC and vice versa). A well configured converter reduces harmonics, increases power transfer capabilities, and reliability in that it offers high tolerance to fault along the line. Different HVDC converter topologies have been proposed, built and utilised all over the world. The two dominant types are the line commutated converter LCC and the voltage source converter VSC. This review paper evaluates these two types of converters, their operational characteristics, power rating capability, control capability and losses. The balance of the paper addresses their applications, advantages, limitations and latest developments with these technologies

Dynamic Voltage Stability Studies using a Modified IEEE 30-Bus System

Power System stability is an essential study in the planning and operation of an efficient, economic, reliable and secure electric power system because it encompasses all the facet of power systems operations, from planning, to conceptual design stages of the project as well as during the systems operating life span. This paper presents different scenario of power system stability studies on a modified IEEE 30-bus system which is subjected to different faults conditions. A scenario whereby the longest high voltage alternating current (HVAC) line is replaced with a high voltage direct current (HVDC) line was implemented. The results obtained show that the HVDC line enhances system stability more compared to the contemporary HVAC line. Dynamic analysis using RMS simulation tool was used on DigSILENT PowerFactory

Advanced Distributed Cooperative Secondary Control of Islanded DC Microgrids

In an islanded DC microgrid with multiple distributed generators (DGs), the droop control is employed to realize proportional current sharing among the DGs in the microgrid. The action of the droop control causes a deviation in the DC bus voltage which is exacerbated by the line impedance between the DG and the DC bus. In this paper, an advanced distributed secondary control scheme is proposed to simultaneously achieve accurate voltage regulation and cooperative current sharing in the islanded DC microgrid system. The proposed distributed secondary controller is introduced in the cyber layer of the system, and each controller shares information with neighbouring controllers via a communication network. The distributed technique maintains the reliability of the overall system if some part of the communication link fails. The proposed controller uses the type-II fuzzy logic scheme to adaptively select the secondary control parameters for an improved response of the controller. The sufficient conditions to guarantee the stability of the proposed controller are derived using the Lyapunov method. Comprehensive tests under different operating scenarios are conducted to demonstrate the robustness of the proposed control scheme

Enhancing the Performance of Eskom’s Cahora Bassa HVDC Scheme and Harmonic Distortion Minimization of LCC-HVDC Scheme Using the VSC-HVDC Link

Cahora Bassa, a thyristor-based High Voltage Direct (HVDC) link, transmits 1920 MW of power from a hydro-power plant in Zambezi River, north of Mozambique, to Apollo Substation in Johannesburg, South Africa. The high degree of harmonics distortion that is transferred into the AC side of the transmission network and the continuous increase in the rate at which commutation failure occurs during systems disturbance are both flaws in the utilization of this HVDC converter technology. AC and DC filters with rugged controllers are often used to minimize this effect but are limited in scope. Modern converter technology, such as the Voltage Source Converter (VSC), was proposed in this study to reduce harmonics content level, increase power transfer capabilities, enhance network stability, and reduce the rate of commutation failure occurrence. This paper, therefore, evaluates the performance analysis of the Cahora Bassa HVDC link and its level of harmonic distortion in the line commutated converters. A proposed method of utilizing VSC HVDC is provided as a suitable solution using three modular-level voltage source converter technology. Current and voltage waveform characteristics during a three-phase short circuits fault were analyzed, and the latest developments in the area of VSC HVDC were discussed. The results show a lower total harmonics distortion with the usage of VSC HVDC converter technology at the inverter station. The continuous occurrence of commutation failure was minimized by implementing a new converter architecture. The network simulation and analysis were carried out using the DIgSILENT PowerFactory engineering software tool

Enhancing the Performance of Eskom&rsquo;s Cahora Bassa HVDC Scheme and Harmonic Distortion Minimization of LCC-HVDC Scheme Using the VSC-HVDC Link

Cahora Bassa, a thyristor-based High Voltage Direct (HVDC) link, transmits 1920 MW of power from a hydro-power plant in Zambezi River, north of Mozambique, to Apollo Substation in Johannesburg, South Africa. The high degree of harmonics distortion that is transferred into the AC side of the transmission network and the continuous increase in the rate at which commutation failure occurs during systems disturbance are both flaws in the utilization of this HVDC converter technology. AC and DC filters with rugged controllers are often used to minimize this effect but are limited in scope. Modern converter technology, such as the Voltage Source Converter (VSC), was proposed in this study to reduce harmonics content level, increase power transfer capabilities, enhance network stability, and reduce the rate of commutation failure occurrence. This paper, therefore, evaluates the performance analysis of the Cahora Bassa HVDC link and its level of harmonic distortion in the line commutated converters. A proposed method of utilizing VSC HVDC is provided as a suitable solution using three modular-level voltage source converter technology. Current and voltage waveform characteristics during a three-phase short circuits fault were analyzed, and the latest developments in the area of VSC HVDC were discussed. The results show a lower total harmonics distortion with the usage of VSC HVDC converter technology at the inverter station. The continuous occurrence of commutation failure was minimized by implementing a new converter architecture. The network simulation and analysis were carried out using the DIgSILENT PowerFactory engineering software tool