Impact of intergrating teebus hydro power on the unbalanced distribution MV network

Small hydro power sources have been identified as one of the renewable energy technologies that the South African government is focusing on in order to generate more electricity from renewable/independent resources. Due to the low carbon output of most renewable energy technologies and the carbon intensive power generation technologies that are currently being used in South Africa e.g. Hydro, coal, gas, and etc. further pressure is increasing to incorporate cleaner forms of generation. In 2002 a study focusing on the hydropower potential was compiled providing an assessment according to conventional and unconventional possibilities for all the provinces. Nowadays, the power electricity demand is growing fast and one of the main tasks for power engineers is to generate electricity from renewable energy sources to overcome this increase in the energy consumption and at the same time reduce environmental impact of power generation. Eskom Distribution Eastern Cape Operating Unit (ECOU) was requested to investigate the feasibility of connecting a small hydro power scheme located in the Teebus area in the Eastern Cape. The Eastern Cape in particular, was identified as potentially the most productive area for small hydroelectric development in South Africa for both the grid connected and off grid applications. These network conditions are in contrast to the South African electricity network where long radial feeders with low X/R ratios and high resistance, spanning large geographic areas, give rise to low voltages on the network. Practical simulation networks have been used to test the conditions set out in the South African Grid Code/NERSA standard and to test the impact of connecting small hydro generation onto the unbalanced distribution network. These networks are representative of various real case scenarios of the South African distribution network. Most of the findings from the simulations were consistent with what was expected when comparing with other literatures. From the simulation results it was seen that the performance of the variable speed generators were superior to that of the fixed speed generators during transient conditions. It was also seen that the weakness of the network had a negative effect on the stability of the system. It is also noted that the stability studies are a necessity when connecting the generators to a network and that each case should be reviewed individually. The fundamental cause of voltage instability is identified as incapability of combined distribution and generation system to meet excessive load demand in either real power or reactive power form

Study of the Thermal Behavior of a Three-phase Induction Motor under Fault Conditions

Electric motors play an important role in the industry, because nowadays almost everything in the industry works with the auxiliary of them, either for low or high power ratings. It is possible to divide the electric motors in induction motors and synchronous motors, however the most used in the industry are the induction motors. So, it is very important to monitor its behavior throughout the time. Due to their working conditions, which sometimes can be very adverse, the motor losses can increase the inner machine temperature causing degradation of the materials which will lead to serious faults. Most severe faults may lead to a machine breakdown and interruption of the industrial production inflicting severe financial loss. The main goal of this dissertation is to create a computational model of a three-phase squirrel cage induction motor to study and analyze its thermal behavior under healthy and faulty conditions. This will be made through the finite elements method (FEM), where Flux2D 12.1 (Cedrat) software will be used. Initially the computational modeling will focus on the electromagnetic study, in order to calculate the motor losses. After that, those values will be inserted in the thermal simulation to better understand the thermal behavior of the motor. The experimental tests will be carried out with the aid of five temperature sensors (PT100), where the acquisition of the experimental data will be done through a software developed in LabView programming language. As well as that, the results obtained experimentally will be compared with those obtained computationally. However, only the results of two sensors can be compared, since two of them are placed throughout the three-dimensional perspective of the motor and one is placed inner of the motor frame, which will not be defined in the simulation.Os motores elétricos desempenham um papel bastante importante na indústria, pois hoje em dia quase tudo na indústria funciona com o auxílio destes, seja em reduzidas ou elevadas potências. É possível dividir os motores elétricos em motores de indução e motores síncronos, no entanto os mais utilizados na indústria são os de indução, sendo então bastante importante monitorizar o seu comportamento ao longo do tempo. Devido às suas condições de funcionamento, que por vezes são bastante adversas, as perdas do motor podem aumentar causando a degradação dos materiais e levando a falhas graves, que podem prejudicar toda a produção de uma indústria, infligindo graves perdas financeiras O objetivo principal desta dissertação é criar um modelo computacional de um motor de indução trifásico de rotor em gaiola de esquilo para estudar e analisar o seu comportamento térmico, tanto sob condições normais como de avaria. Este trabalho será desenvolvido através do método de elementos finitos (FEM), sendo assim utilizado o software Flux2D 12.1 (Cedrat). Inicialmente a modelação computacional focar-se-á no estudo eletromagnético, de forma a calcularem-se as perdas do motor. Posteriormente, esses valores serão inseridos na simulação térmica, de forma a compreender-se melhor o comportamento térmico do motor. Os ensaios experimentais terão o auxílio de cinco sensores de temperatura (PT100) onde a aquisição dos dados experimentais é efetuada através de um software desenvolvido na linguagem de programação LabView. Posteriormente, os resultados obtidos experimentalmente serão comparados com os resultados obtidos computacionalmente. Porém, apenas os resultados de dois dos sensores podem ser comparados, pois existem dois sensores ao longo da perspetiva tridimensional do motor e um que está situado na periferia interna da carcaça, a qual não será definida na simulação

Advances in power quality analysis techniques for electrical machines and drives: a review

The electric machines are the elements most used at an industry level, and they represent the major power consumption of the productive processes. Particularly speaking, among all electric machines, the motors and their drives play a key role since they literally allow the motion interchange in the industrial processes; it could be said that they are the medullar column for moving the rest of the mechanical parts. Hence, their proper operation must be guaranteed in order to raise, as much as possible, their efficiency, and, as consequence, bring out the economic benefits. This review presents a general overview of the reported works that address the efficiency topic in motors and drives and in the power quality of the electric grid. This study speaks about the relationship existing between the motors and drives that induces electric disturbances into the grid, affecting its power quality, and also how these power disturbances present in the electrical network adversely affect, in turn, the motors and drives. In addition, the reported techniques that tackle the detection, classification, and mitigations of power quality disturbances are discussed. Additionally, several works are reviewed in order to present the panorama that show the evolution and advances in the techniques and tendencies in both senses: motors and drives affecting the power source quality and the power quality disturbances affecting the efficiency of motors and drives. A discussion of trends in techniques and future work about power quality analysis from the motors and drives efficiency viewpoint is provided. Finally, some prompts are made about alternative methods that could help in overcome the gaps until now detected in the reported approaches referring to the detection, classification and mitigation of power disturbances with views toward the improvement of the efficiency of motors and drives.Peer ReviewedPostprint (published version

Vibration Simulation of Electric Machines

This chapter deals with the basics of vibration calculations in electrical machines. It includes a brief introduction to the sources of vibration in electrical machines. In addition, the construction of electric machines is briefly summarized. It also describes the influence of individual parts of electric machines on vibration generation. The chapter also deals with the important steps that need to be taken when calculating vibration signal waveform using finite element method (Ansys). The individual sections summarize the most important requirements for setting the vibration calculation and it also deals with minimizing the calculation errors

The impact of voltage unbalance and regulation on the life expectancy of LV induction machines

The induction machine is the most widely used electrical machine in the world, they are used for industrial, commercial and industrial applications. When manufactured they have nameplate ratings that stipulate the voltages and currents at which they may be operated. The quality of supply from power utilities can lead to them being operated under unbalanced voltage conditions. Power utilities experience voltage unbalances and voltage dips when they provide services to end users. In South Africa ESKOM is the primary power utility, it has a license agreement with NERSA stipulating the allowable voltage levels. In most cases the operating levels are within the agreed limits however many consumers are exposed to voltage levels at the fringe of these specified limits. This can be detrimental to electrical equipment if operated under these conditions for considerable lengths of time. NRS048 part 2 provides limits for voltage regulation and voltage unbalance which can exist on the power network at various voltage levels. It is incumbent on each utility to ensure that the quality of power supplied to end users complies with the minimum standards specified in the NRS048. Most customers connected to rural 11/22kV networks are farmers, where a large portion of the load is pumps driven by low voltage AC induction motors. NRS048 dictates the voltage limits as ±10% of the nominal supply voltage of 400V. The voltage unbalance on three phase systems is limited to 2%, and 3% for predominantly single-phase systems. Utilities such as Eskom have standards and operating procedures in place to run the networks optimally within these limits. Variations in the voltage levels on rural networks can be significant during load changes. Voltage unbalance levels can also be high due to the use of single-phase loads on these networks. Normally, utilities have operating procedures in place to allow operation of the supply network within the limits specified in the NRS048. Although these operating levels are within specified limits, a large portion of the customer’s plant can however be exposed to voltage levels well above or below the rated value of the equipment. Operating under these conditions can eventually lead to failure of the customer equipment. The impact of operating AC motors with voltages levels above or below the rated levels has not been fully determined on rural networks in South Africa. Research is therefore required to assess the impact of these operating conditions on customer equipment in rural areas and whether utility operating procedures need to be revised to take into consideration customer equipment. This has consequences for Eskom Distribution that relates to: a) The way in which the network voltages are managed, and b) Claims lodged against the Distribution business resulting from damage to customer’s motors The purpose of this research project is to investigate and quantify the impact of voltage regulation (over and under) and unbalance conditions on a typical rural feeder on the lifespan of induction machines. There are various definitions of voltage unbalance by NEMA, the IEC and the IEEE. The IEC definition is used in this research report, it is known as the true definition and incorporates both magnitude and phase information. To estimate the loss of life in induction machines operating under unbalanced conditions, the positive and negative sequence per phase equivalent circuits must be determined and the thermal model needs to be obtained and quantified as well. The losses obtained from the per phase sequence circuits are inputs to thermal model which in turn is used to predict the induction machine stator windings temperatures. Factors that are considered when analysing the impact of voltage unbalance and regulation on the life expectancy of machines include, the induction machine manufacturer, the size of the induction machine, the voltage rating of the induction machine and the efficiency class of the induction machine. The research presented in this report is primarily focused on the impact of voltage unbalance and regulation on the life expectancy of low voltage induction machines. The operating conditions considered in this report are prevalent on a typical rural feeder. Most customers connected to rural 11/22kV networks are farmers and a large portion of load pumps are driven by low voltage induction machines. This report presents the effect of these operating conditions on the life expectancy of the machines. The thermal model presented is suitable for continuously operated machines (S1). Since the operating conditions considered are primarily prevalent on typical rural feeders, the machines considered were also the machines primarily used in those regions. This has consequence that relates to the way in which the network voltages are managed, and claims lodged against power utilities resulting from damage to end user’s induction machines. The thermal model presented in this report can be incorporated as an algorithm and be implemented in microprocessor devices which enhance the level of accuracy and flexibility. As a practical application of the thermal model real time data can be processed according to the firmware thermal algorithm program and results are compared with the expected values and stored in memory. If a machine protection device is used, it computes an analog value which is then compared with the output of the thermal model algorithm. In practice the MPD usually triggers the digital outputs if the compared analog values exceed the set thermal threshold

Detection of inter-turn faults in multi-phase ferrite-PM assisted synchronous reluctance machine

Inter-turn winding faults in five-phase ferrite-permanent magnet-assisted synchronous reluctance motors (fPMa-SynRMs) can lead to catastrophic consequences if not detected in a timely manner, since they can quickly progress into more severe short-circuit faults, such as coil-to-coil, phase-to-ground or phase-to-phase faults. This paper analyzes the feasibility of detecting such harmful faults in their early stage, with only one short-circuited turn, since there is a lack of works related to this topic in multi-phase fPMa-SynRMs. Two methods are tested for this purpose, the analysis of the spectral content of the zero-sequence voltage component (ZSVC) and the analysis of the stator current spectra, also known as motor current signature analysis (MCSA), which is a well-known fault diagnosis method. This paper compares the performance and sensitivity of both methods under different operating conditions. It is proven that inter-turn faults can be detected in the early stage, with the ZSVC providing more sensitivity than the MCSA method. It is also proven that the working conditions have little effect on the sensitivity of both methods. To conclude, this paper proposes two inter-turn fault indicators and the threshold values to detect such faults in the early stage, which are calculated from the spectral information of the ZSVC and the line currentsPeer ReviewedPostprint (published version

Reclosing transients in standard and premium efficiency induction machines in the presence of voltage unbalance

The impact of restarting an induction machine, while coasting, was investigated in this study. When restarting an induction machine, high peaking current and torque transients appear. As a result, possible damage to the rotor shaft could occur together with a destabilising of the power system which could lead to fluctuating supply voltage levels. This dissertation investigated the restarting of two different efficiency class induction machines and analyses how a shift in the phase between the supply voltage and the residual voltage affects the restarting transients. Previous studies have been carried out on standard efficiency induction machines, however this study aimed to compare the impact of restarting on the standard and premium plus efficiency induction machines. The design differences between standard and energy efficient induction machines play a significant role in how these transients behave. The energy efficient machines were found to be made of higher grade lamination steel and larger sized conductors in order to reduce the stator and rotor copper losses. Using material with a higher permeability, as is the case with energy efficient machines, ensures that the machines are able to retain less residual flux at the point of disconnect than standard motors. It is for this reason that the energy efficient machines have a larger residual voltage at the point of disconnect. However, the speed of the decay of the residual flux of the standard efficiency machine is higher than the energy efficient machine and, as a result, the energy efficient machine resulted with a higher residual voltage at the 180° phase shift. This leads to a larger potential difference for the energy efficient machines than the standard efficiency machines. However, the stator impedance of the energy efficient machine was established as a dominating factor in the outcome of the tests results. Energy efficient machines have a significantly lower impedance than standard efficiency machines and due to this, they are more susceptible to the detrimental effects of out-of-phase restarting. It was important to establish a critical point when the current and torque transients are at a maximum as the motors speed is winding down. It is necessary to minimise the effects of restarting at this critical point as the resulting developed torque can be mechanically detrimental to the shaft. The residual voltage needs to be minimised dramatically to a point where the out-of-phase reclosing will not give rise to significantly high torque transients. Additional to this study was the observation of the current and torque transients in the presence of voltage unbalance. Voltage unbalance presents a power quality issue which can potentially have a negative effect on the efficiency of an induction machine. Both machines were tested for 0%, 1% and 2% voltage unbalance with a focus on start-up and 180° restart. In addition, voltage unbalance was introduced at the rated voltage of 400 V with 10% variations above and below this rating for over and undervoltage respectively. It was seen that the energy efficient machines were more vulnerable to unbalance effects due to a larger negative sequence air gap torque being developed under transient conditions

Enhancing transient performance of microgeneration-dense low voltage distribution networks

In addition to other measures such as energy saving, the adoption of microgeneration driven by renewable and low carbon energy resources is expected to have the potential to reduce losses associated with producing and delivering electricity, combat climate change and fuel poverty, and improve the overall system performance. However, incorporating a substantial volume of microgeneration within a system that is not designed for such a paradigm could lead to conflicts in the operating strategies of the new and existing centralised generation technologies. So it becomes vital for such substantial amount of microgeneration among other decentralised resources to be controlled in the way that local constraints are mitigated and their aggregated response supports the wider system. In addition, the characteristic behaviour of connected microgeneration requires to be understood under different system conditions to ascertain measures of risk and resilience, and to ensure the benefits of microgeneration to be delivered. Therefore, this thesis provides three main valuable contributions of future attainment of sustainable power systems. Firstly, a new conceptual control structure for a system incorporating a high penetration of microgeneration and dynamic load is developed. Secondly, the resilience level of the host distribution network as well as the resilience levels of microgeneration during large transient disturbances is evaluated and quantified. Thirdly, a technical solution that can support enhanced transient stability of a large penetration of LV connected microgeneration is introduced and demonstrated. A control system structure concept based on “a cell concept” is introduced to manage the spread of heavy volumes of distributed energy resources (DERs) including microgeneration such that the useful features of DER units in support of the wider system can be exploited, and the threats to system performance presented by significant connection of passive and unpredictable DERs can be mitigated. The structure also provides simpler and better coordinated communication with DERs by allowing the inputs from DERs and groups of cells to be transferred as collective actions when it moves from a local to a wider system level. The anticipated transient performance problems surrounding the integration of microgeneration on a large basis within a typical urban distribution network are addressed. Three areas of studies are tackled; the increased fault level due to the present of microgeneration, the collective impact of LV connected microgeneration on traditional LV protection performance, and the system fault ride through capabilities of LV connected microgeneration interfaced by different technologies. The possible local impacts of unnecessary disconnection of large amount of microgeneration on the performance of the host distribution network are also quantified. The thesis proposes a network solution based on using resistive-type superconducting fault current limiters (RSFCLs) to prevent the impact of local transient disturbances from expanding and enhance the fault ride through capabilities of a high penetration of microgeneration connected to low voltage distribution networks. A new mathematical approach is developed within the thesis to identify at which condition RSFCL can be used as a significant device to maintain the transient stability of large numbers of LV connected microgeneration. The approach is based on equation solution to determine the minimum required value of the resistive element of RSFCL to maintain microgeneration transient stability, and at the same time additional headroom against switchgear short-circuit ratings is provided. Remote disturbances or a failure to clear remote faults quickly are shown to no longer result in complete unnecessary disconnection of substantial amount of microgeneration

An Assessment on the Non-Invasive Methods for Condition Monitoring of Induction Motors

The ability to forecast motor mechanical faults at incipient stages is vital to reducing maintenance costs, operation downtime and safety hazards. This paper synthesized the progress in the research and development in condition monitoring and fault diagnosis of induction motors. The motor condition monitoring techniques are mainly classified into two categories that are invasive and non-invasive techniques. The invasive techniques are very basic, but they have some implementation difficulties and high cost. The non-invasive methods, namely MCSA, PVA and IPA, overcome the disadvantages associated to invasive methods. This book chapter reviews the various non-invasive condition monitoring methods for diagnosis of mechanical faults in induction motor and concludes that the instantaneous power analysis (IPA) and Park vector analysis (PVA) methods are best suitable for the diagnosis of small fault signatures associated to mechanical faults. Recommendations for the future research in these areas are also presented