The sampling theorem based methodology for harmonics analysis
- Bokoro, P., Pretorius, J., Case, M.
- Authors: Bokoro, P. , Pretorius, J. , Case, M.
- Date: 2012
- Subjects: Harmonics analysis
- Type: Article
- Identifier: uj:4831 , http://hdl.handle.net/10210/12424
- Description: This paper investigates the validity of the sampling theorem in the analysis of supply side line current harmonics as applied to 6-pulse AC-DC thyristor converters. The common pattern of the percentage harmonic factor, for the converter characteristic harmonic current, per incremental change of the thyristor firing angle is used as the basis of the study. The RMS values of the peak amplitudes of the characteristic harmonic currents induced in the input-phase of the converter supplying a DC motor are described in terms of the sampling theorem equations and further evaluated in terms of their respective individual harmonic contributions as the thyristor firing angle is changed. The MatLab/Simulink package is also employed to simulate the forward drive speed control operation of a DC motor using the 6-pulse AC-DC thyristor converter. However the percentage harmonic factor trends observed in the sampling theorem method appear to be generally increasing which is in contradiction to the MatLab/Simulink produced trends.
- Full Text:
- Authors: Bokoro, P. , Pretorius, J. , Case, M.
- Date: 2012
- Subjects: Harmonics analysis
- Type: Article
- Identifier: uj:4831 , http://hdl.handle.net/10210/12424
- Description: This paper investigates the validity of the sampling theorem in the analysis of supply side line current harmonics as applied to 6-pulse AC-DC thyristor converters. The common pattern of the percentage harmonic factor, for the converter characteristic harmonic current, per incremental change of the thyristor firing angle is used as the basis of the study. The RMS values of the peak amplitudes of the characteristic harmonic currents induced in the input-phase of the converter supplying a DC motor are described in terms of the sampling theorem equations and further evaluated in terms of their respective individual harmonic contributions as the thyristor firing angle is changed. The MatLab/Simulink package is also employed to simulate the forward drive speed control operation of a DC motor using the 6-pulse AC-DC thyristor converter. However the percentage harmonic factor trends observed in the sampling theorem method appear to be generally increasing which is in contradiction to the MatLab/Simulink produced trends.
- Full Text:
The sampling theorem based methodology for harmonics analysis
- Bokoro, P., Pretorius, J., Case, M.
- Authors: Bokoro, P. , Pretorius, J. , Case, M.
- Date: 2012
- Subjects: Sampling theorem , Line current harmonics , Thyristor converters , Harmonic analysis
- Type: Article
- Identifier: uj:6271 , http://hdl.handle.net/10210/8897
- Description: This paper investigates the validity of the sampling theorem in the analysis of supply side line current harmonics as applied to 6-pulse AC-DC thyristor converters. The common pattern of the percentage harmonic factor, for the converter characteristic harmonic current, per incremental change of the thyristor firing angle is used as the basis of the study. The RMS values of the peak amplitudes of the characteristic harmonic currents induced in the input-phase of the converter supplying a DC motor are described in terms of the sampling theorem equations and further evaluated in terms of their respective individual harmonic contributions as the thyristor firing angle is changed. The MatLab/Simulink package is also employed to simulate the forward drive speed control operation of a DC motor using the 6-pulse AC-DC thyristor converter. However the percentage harmonic factor trends observed in the sampling theorem method appear to be generally increasing which is in contradiction to the MatLab/Simulink produced trends.
- Full Text:
- Authors: Bokoro, P. , Pretorius, J. , Case, M.
- Date: 2012
- Subjects: Sampling theorem , Line current harmonics , Thyristor converters , Harmonic analysis
- Type: Article
- Identifier: uj:6271 , http://hdl.handle.net/10210/8897
- Description: This paper investigates the validity of the sampling theorem in the analysis of supply side line current harmonics as applied to 6-pulse AC-DC thyristor converters. The common pattern of the percentage harmonic factor, for the converter characteristic harmonic current, per incremental change of the thyristor firing angle is used as the basis of the study. The RMS values of the peak amplitudes of the characteristic harmonic currents induced in the input-phase of the converter supplying a DC motor are described in terms of the sampling theorem equations and further evaluated in terms of their respective individual harmonic contributions as the thyristor firing angle is changed. The MatLab/Simulink package is also employed to simulate the forward drive speed control operation of a DC motor using the 6-pulse AC-DC thyristor converter. However the percentage harmonic factor trends observed in the sampling theorem method appear to be generally increasing which is in contradiction to the MatLab/Simulink produced trends.
- Full Text:
A frequency-domain analysis of varistor current under distorted supply voltage
- Bokoro, P., Jandrell, I., Hove, M.
- Authors: Bokoro, P. , Jandrell, I. , Hove, M.
- Date: 2014
- Subjects: Varistor arresters , Third harmonic current , Hormaonics injection , Time-domain waveform , Frequency-domain waveform
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/20427 , uj:16097 , Citation: Bokoro, P., Jandrell, I. & Hove, M. 2014. A frequency-domain analysis of varistor current under distorted supply voltage. Proceedings of the 22nd Southern African Universities Power Engineering Conference (SAUPEC 2014), 30-31 January 2014, University of KwaZulu-Natal and Durban University of Technology, Durban, South Africa. p. 69-72. ISBN: 978-1-86840-619-7
- Description: Abstract: In this paper, the impact of supply voltage harmonics on the third harmonic current-based condition assessment of varistor is experimentally verified. Therefore, time-domain current and voltage waveforms, measured from ten identical varistor samples, are decomposed in frequency-domain. The flattop window of the FFT technique is used to determine the rms values and subsequently the third harmonic amplitude of the varistor current, before and after injection of harmonics. The harmonic-generating load consists of a triac-based ac voltage controller driving a resistive load unit at fixed firing angle of 10 degrees. All varistor devices used in this work were subjected to rated ac operating voltage. However, the results obtained indicated that the operation of a harmonic source connected across the varistor arrestor has the effect of increasing the magnitude of the third harmonic component of the varistor current.
- Full Text:
- Authors: Bokoro, P. , Jandrell, I. , Hove, M.
- Date: 2014
- Subjects: Varistor arresters , Third harmonic current , Hormaonics injection , Time-domain waveform , Frequency-domain waveform
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/20427 , uj:16097 , Citation: Bokoro, P., Jandrell, I. & Hove, M. 2014. A frequency-domain analysis of varistor current under distorted supply voltage. Proceedings of the 22nd Southern African Universities Power Engineering Conference (SAUPEC 2014), 30-31 January 2014, University of KwaZulu-Natal and Durban University of Technology, Durban, South Africa. p. 69-72. ISBN: 978-1-86840-619-7
- Description: Abstract: In this paper, the impact of supply voltage harmonics on the third harmonic current-based condition assessment of varistor is experimentally verified. Therefore, time-domain current and voltage waveforms, measured from ten identical varistor samples, are decomposed in frequency-domain. The flattop window of the FFT technique is used to determine the rms values and subsequently the third harmonic amplitude of the varistor current, before and after injection of harmonics. The harmonic-generating load consists of a triac-based ac voltage controller driving a resistive load unit at fixed firing angle of 10 degrees. All varistor devices used in this work were subjected to rated ac operating voltage. However, the results obtained indicated that the operation of a harmonic source connected across the varistor arrestor has the effect of increasing the magnitude of the third harmonic component of the varistor current.
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EMTP-based analysis of pre-insertion resistor and point on wave switching methodology
- Zondi, S., Bokoro, P., Paul, B.
- Authors: Zondi, S. , Bokoro, P. , Paul, B.
- Date: 2015
- Subjects: Capacitor banks , Transient overvoltage and inrush current , Single and back to back switching
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/20405 , uj:16092 , Citation: Zondi, S., Bokoro, P. & Paul, B. 2015. EMTP-based analysis of pre-insertion resistor and point on wave switching methodology. Proceedings of the IEEE AFRICON, September 2015, Addis Ababa, Ethiopia. p. 962-966. ISBN: 978-1-4799-7497-9
- Description: Abstract: The switching of high-voltage capacitor banks for reactive power and voltage control usually produces significant transients. These high amplitude disturbances have a tendency to damage or reduce the lifespan of capacitor units, thus leading to complete failure of capacitor banks. Pre-insertion resistors or inductors as well as synchronous switching (Point on Wave) methods can be used to mitigate transient phenomenon. In this work, EMTP simulations of transient voltage for single and back to back capacitor banks indicate that pre-insertion resistors methodology can significantly reduce transients, and could therefore be combined with synchronous switching to effect transient mitigation.
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- Authors: Zondi, S. , Bokoro, P. , Paul, B.
- Date: 2015
- Subjects: Capacitor banks , Transient overvoltage and inrush current , Single and back to back switching
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/20405 , uj:16092 , Citation: Zondi, S., Bokoro, P. & Paul, B. 2015. EMTP-based analysis of pre-insertion resistor and point on wave switching methodology. Proceedings of the IEEE AFRICON, September 2015, Addis Ababa, Ethiopia. p. 962-966. ISBN: 978-1-4799-7497-9
- Description: Abstract: The switching of high-voltage capacitor banks for reactive power and voltage control usually produces significant transients. These high amplitude disturbances have a tendency to damage or reduce the lifespan of capacitor units, thus leading to complete failure of capacitor banks. Pre-insertion resistors or inductors as well as synchronous switching (Point on Wave) methods can be used to mitigate transient phenomenon. In this work, EMTP simulations of transient voltage for single and back to back capacitor banks indicate that pre-insertion resistors methodology can significantly reduce transients, and could therefore be combined with synchronous switching to effect transient mitigation.
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A case study of induced current unbalance as a result of capacitor failure
- Zondi, S., Bokoro, P., Paul, B.
- Authors: Zondi, S. , Bokoro, P. , Paul, B.
- Date: 2015-01-28
- Subjects: Point on wave switching , Transient voltage , Capacitor banks , Current unbalance
- Type: Article
- Identifier: uj:5143 , ISBN 978-0-86970-786-9 , http://hdl.handle.net/10210/14124
- Description: Capacitor banks rated 132 kV/ 72 MVAr are installed at the Durban North Substation of Ethekwini municipality to compensate for inductive power losses. The POW or synchronous switching technique is used for on-line switching of these banks, in a bid to minimise high magnitude induced transient voltage and current. in this study, the voltage waveform records and the percentage current unbalance, measured on and six months post commissioning of the capacitor banks are analysed in order to test the effectiveness of POW switching method, when implemented with mechanically linked 3-phase, 3-pole SF6 circuit breakers. The results obtained indicate that within 6 months of commissioning, capacitive impedance of the banks failed, as result of high magnitude induced transient current and voltage, and the percentage unbalance level observed in the neutral current grew from 2.3% to 5.7%.
- Full Text:
- Authors: Zondi, S. , Bokoro, P. , Paul, B.
- Date: 2015-01-28
- Subjects: Point on wave switching , Transient voltage , Capacitor banks , Current unbalance
- Type: Article
- Identifier: uj:5143 , ISBN 978-0-86970-786-9 , http://hdl.handle.net/10210/14124
- Description: Capacitor banks rated 132 kV/ 72 MVAr are installed at the Durban North Substation of Ethekwini municipality to compensate for inductive power losses. The POW or synchronous switching technique is used for on-line switching of these banks, in a bid to minimise high magnitude induced transient voltage and current. in this study, the voltage waveform records and the percentage current unbalance, measured on and six months post commissioning of the capacitor banks are analysed in order to test the effectiveness of POW switching method, when implemented with mechanically linked 3-phase, 3-pole SF6 circuit breakers. The results obtained indicate that within 6 months of commissioning, capacitive impedance of the banks failed, as result of high magnitude induced transient current and voltage, and the percentage unbalance level observed in the neutral current grew from 2.3% to 5.7%.
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Impact of system expansion on specification of personal protective equipment : arc flash analysis
- Authors: Edwards, D. , Bokoro, P.
- Date: 2015-01-28
- Subjects: Arc flash incident energy , System expansion , Short circuit capacity , Fault clearing time , Personal protective equipment
- Type: Article
- Identifier: uj:5140 , ISBN 978-0-86970-786-9 , http://hdl.handle.net/10210/14107
- Description: Electricity network expansion require the installation of new substations and switchgear equipment. This, in many cases, is intended to cater for electrical energy demand much needed for industrial operations. Such an increase in power available for use by consumers translates in high short circuit capacity or fault level of the network, which in turn causes the arc flash incident energy to be high in case of a fault. In this paper, the impact of network expansion on the arc flash incident energy is assessed using the digsilent software version 15.1. The influence of protective relays time setting on the reduction of arc flash energy, available during three-phase fault conditions, is also analysed for the purpose of PPE specification. The results obtained show that an increase in the short circuit capacity or fault level as well as in the fault clearing time, causes the arc flash incident energy to increase to higher level than the corresponding PPE prescribed.
- Full Text:
- Authors: Edwards, D. , Bokoro, P.
- Date: 2015-01-28
- Subjects: Arc flash incident energy , System expansion , Short circuit capacity , Fault clearing time , Personal protective equipment
- Type: Article
- Identifier: uj:5140 , ISBN 978-0-86970-786-9 , http://hdl.handle.net/10210/14107
- Description: Electricity network expansion require the installation of new substations and switchgear equipment. This, in many cases, is intended to cater for electrical energy demand much needed for industrial operations. Such an increase in power available for use by consumers translates in high short circuit capacity or fault level of the network, which in turn causes the arc flash incident energy to be high in case of a fault. In this paper, the impact of network expansion on the arc flash incident energy is assessed using the digsilent software version 15.1. The influence of protective relays time setting on the reduction of arc flash energy, available during three-phase fault conditions, is also analysed for the purpose of PPE specification. The results obtained show that an increase in the short circuit capacity or fault level as well as in the fault clearing time, causes the arc flash incident energy to increase to higher level than the corresponding PPE prescribed.
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Failure analysis of metal oxide arresters under harmonic distortion
- Authors: Bokoro, P. , Jandrell, I.
- Date: 2016
- Subjects: Time to failure , Metal oxide arrester , Voltage harmonics
- Language: English
- Type: Article
- Identifier: http://hdl.handle.net/10210/123304 , uj:20774 , Citation: Bokoro, P & Jandrell, I. 2016. Failure analysis of metal oxide arresters under harmonic distortion.
- Description: Abstract: The probability of accelerated degradation or reduced time to failure of metal oxide arresters (MOA), when continuously exposed to distorted ac conduction, is analysed in this study. Metal oxide-based surge arresters of similar size and electrical characteristics are tested using accelerated degradation at elevated temperature and distorted ac voltage stress. The three-parameter Weibull probability method is applied to analyse the obtained time to failure distribution. The Fourier series’ expansion is also relied upon in order to evaluate the content of the harmonic resistive components of the measured leakage current. The results obtained indicate that for 6.24% and 5.58% content of the 3rd and the 5th harmonic component, respectively embedded in the applied voltage stress, the probability of reduced time to failure or accelerated degradation is found to be 58.93 %, and the mean life reduction obtained is just above 40%. These results correlate with the pronounced shift of the V I curve as well as the increase in the magnitude of the respective harmonic resistive current components of the arrester samples.
- Full Text:
- Authors: Bokoro, P. , Jandrell, I.
- Date: 2016
- Subjects: Time to failure , Metal oxide arrester , Voltage harmonics
- Language: English
- Type: Article
- Identifier: http://hdl.handle.net/10210/123304 , uj:20774 , Citation: Bokoro, P & Jandrell, I. 2016. Failure analysis of metal oxide arresters under harmonic distortion.
- Description: Abstract: The probability of accelerated degradation or reduced time to failure of metal oxide arresters (MOA), when continuously exposed to distorted ac conduction, is analysed in this study. Metal oxide-based surge arresters of similar size and electrical characteristics are tested using accelerated degradation at elevated temperature and distorted ac voltage stress. The three-parameter Weibull probability method is applied to analyse the obtained time to failure distribution. The Fourier series’ expansion is also relied upon in order to evaluate the content of the harmonic resistive components of the measured leakage current. The results obtained indicate that for 6.24% and 5.58% content of the 3rd and the 5th harmonic component, respectively embedded in the applied voltage stress, the probability of reduced time to failure or accelerated degradation is found to be 58.93 %, and the mean life reduction obtained is just above 40%. These results correlate with the pronounced shift of the V I curve as well as the increase in the magnitude of the respective harmonic resistive current components of the arrester samples.
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Mean life estimation of metal oxide surge arresters under harmonic distortion
- Authors: Bokoro, P. , Jandrell, I.
- Date: 2016
- Subjects: Metal oxide surge arrester , Mean life, accelerated ageing , Harmonic-distortion
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/123288 , uj:20772 , Citation: Bokoro, P & Jandrell, I. 2016. Mean life estimation of metal oxide surge arresters under harmonic distortion.
- Description: Abstract: In this paper, similar metal oxide-based arresters are tested using accelerated ageing tests with low and high total harmonic distortion content in the AC voltage stress. The extrapolated ageing times obtained are modelled using the threeparameter Weibull statistical distribution. The hypothesis testing based on the likelihood ratio test is invoked in order to ensure no possibility of equal probability density functions, which are therefore applied to estimate or predict the mean life of MOSA under distorted ac voltage stress. The results obtained indicate that the likelihood ratio test is found to be -1.2855 and the mean life obtained for MOSA samples aged with a total harmonic distortion content of 8.5% in the voltage stress is reduced by 30.13%.
- Full Text:
- Authors: Bokoro, P. , Jandrell, I.
- Date: 2016
- Subjects: Metal oxide surge arrester , Mean life, accelerated ageing , Harmonic-distortion
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/123288 , uj:20772 , Citation: Bokoro, P & Jandrell, I. 2016. Mean life estimation of metal oxide surge arresters under harmonic distortion.
- Description: Abstract: In this paper, similar metal oxide-based arresters are tested using accelerated ageing tests with low and high total harmonic distortion content in the AC voltage stress. The extrapolated ageing times obtained are modelled using the threeparameter Weibull statistical distribution. The hypothesis testing based on the likelihood ratio test is invoked in order to ensure no possibility of equal probability density functions, which are therefore applied to estimate or predict the mean life of MOSA under distorted ac voltage stress. The results obtained indicate that the likelihood ratio test is found to be -1.2855 and the mean life obtained for MOSA samples aged with a total harmonic distortion content of 8.5% in the voltage stress is reduced by 30.13%.
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Statistical analysis of operating times of high voltage SF6 circuit breakers
- Mabunda, N., Bokoro, P., Nicolae, Dan-Valentin
- Authors: Mabunda, N. , Bokoro, P. , Nicolae, Dan-Valentin
- Date: 2016
- Subjects: Circuit breakers , Hypothesis testing
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/123312 , uj:20775 , Citation: Mabunda, N., Bokoro, P & Nicolae, D. 2016. Statistical analysis of operating times of high voltage SF6 circuit breakers.
- Description: Abstract: In this paper the operating times of high voltage SF6 live tank circuit breakers are tested using standard timing tests. The time measurements obtained have proven to be adequate to the normal distribution. The hypothesis testing methodology is applied in order to determine the mean difference value between the tested phase operating times. The results obtained showed that the time difference between the operating times of the tested circuit breakers is located between - 0.661 and 0.536 ms, which indicates that the operating mechanism of the breakers is still reliable and in good condition.
- Full Text:
- Authors: Mabunda, N. , Bokoro, P. , Nicolae, Dan-Valentin
- Date: 2016
- Subjects: Circuit breakers , Hypothesis testing
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/123312 , uj:20775 , Citation: Mabunda, N., Bokoro, P & Nicolae, D. 2016. Statistical analysis of operating times of high voltage SF6 circuit breakers.
- Description: Abstract: In this paper the operating times of high voltage SF6 live tank circuit breakers are tested using standard timing tests. The time measurements obtained have proven to be adequate to the normal distribution. The hypothesis testing methodology is applied in order to determine the mean difference value between the tested phase operating times. The results obtained showed that the time difference between the operating times of the tested circuit breakers is located between - 0.661 and 0.536 ms, which indicates that the operating mechanism of the breakers is still reliable and in good condition.
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Condition assessment of SF6 circuit breakers using analysis of variance (ANOVA)
- Authors: Bokoro, P. , Malandala, K.
- Date: 2017
- Subjects: SF6 circuit breakers , Analysis of variance , Tripping time
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/241876 , uj:24934 , Citation: Bokoro, P. & Malandala, K. 2017. Condition assessment of SF6 circuit breakers using analysis of variance (ANOVA).
- Description: Abstract: In this study, the condition of high-voltage three-phase SF6 circuit breakers is assessed using analysis of variancebased statistical test. 40 sulphur hexafluoride (SF6) circuit breakers, with similar technical characteristics and operating conditions, were tested on the field using the SA10 switch analyser. The chi-square test for normality was performed on the measured coil tripping times, before analysis of variance was applied. Results obtained indicate the difference between and within the coil tripping times is not significant.
- Full Text:
- Authors: Bokoro, P. , Malandala, K.
- Date: 2017
- Subjects: SF6 circuit breakers , Analysis of variance , Tripping time
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/241876 , uj:24934 , Citation: Bokoro, P. & Malandala, K. 2017. Condition assessment of SF6 circuit breakers using analysis of variance (ANOVA).
- Description: Abstract: In this study, the condition of high-voltage three-phase SF6 circuit breakers is assessed using analysis of variancebased statistical test. 40 sulphur hexafluoride (SF6) circuit breakers, with similar technical characteristics and operating conditions, were tested on the field using the SA10 switch analyser. The chi-square test for normality was performed on the measured coil tripping times, before analysis of variance was applied. Results obtained indicate the difference between and within the coil tripping times is not significant.
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Design of a DC overcurrent fault detection and protection scheme for DC collectors of a photovoltaic plant
- Holland, I., Bokoro, P., Paul, B., Ndlovu, E.
- Authors: Holland, I. , Bokoro, P. , Paul, B. , Ndlovu, E.
- Date: 2017
- Subjects: Photovoltaic power plants - Protection
- Language: English
- Type: Conference proceedings
- Identifier: http://ujcontent.uj.ac.za8080/10210/370432 , http://hdl.handle.net/10210/221791 , uj:22242 , Citation: Holland, I. et al. 2017. Design of a DC overcurrent fault detection and protection scheme for DC collectors of a photovoltaic plant.
- Description: Abstract: Faults in the DC collector circuits of a photovoltaic plant are a cause for major concern due to the damage they may cause to equipment. Fault protection is required for ground faults, line-to-line faults and arc faults. Typically, the magnitude of short-circuit currents of PV modules are similar to that of load currents which makes fault detection and protection difficult to carry out on the DC system. The DC collectors of a PV plant consist of combiner boxes where PV arrays are connected and inverter DC buses where combiner boxes are connected. This paper investigates the fault contribution from the PV modules on the combiner boxes and inverter DC buses in relation to typical PV module data on an isolated section of the DC network of a plant using the ETAP simulation tool. A fault detection and protection scheme for the main faults in a DC PV system is proposed based on previous research, for complete protection of the DC collector circuits.
- Full Text:
- Authors: Holland, I. , Bokoro, P. , Paul, B. , Ndlovu, E.
- Date: 2017
- Subjects: Photovoltaic power plants - Protection
- Language: English
- Type: Conference proceedings
- Identifier: http://ujcontent.uj.ac.za8080/10210/370432 , http://hdl.handle.net/10210/221791 , uj:22242 , Citation: Holland, I. et al. 2017. Design of a DC overcurrent fault detection and protection scheme for DC collectors of a photovoltaic plant.
- Description: Abstract: Faults in the DC collector circuits of a photovoltaic plant are a cause for major concern due to the damage they may cause to equipment. Fault protection is required for ground faults, line-to-line faults and arc faults. Typically, the magnitude of short-circuit currents of PV modules are similar to that of load currents which makes fault detection and protection difficult to carry out on the DC system. The DC collectors of a PV plant consist of combiner boxes where PV arrays are connected and inverter DC buses where combiner boxes are connected. This paper investigates the fault contribution from the PV modules on the combiner boxes and inverter DC buses in relation to typical PV module data on an isolated section of the DC network of a plant using the ETAP simulation tool. A fault detection and protection scheme for the main faults in a DC PV system is proposed based on previous research, for complete protection of the DC collector circuits.
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Effect of electro-thermally induced degradation on the capacitance of metal oxide varistors
- Authors: Muremi, L. , Bokoro, P.
- Date: 2017
- Subjects: Metal Oxide varistors , Thermal stresses , Degradation
- Language: English
- Type: Conference proceedings
- Identifier: http://ujcontent.uj.ac.za8080/10210/384404 , http://hdl.handle.net/10210/254468 , uj:26637 , Citation: Muremi, L. & Bokoro, P. 2017. Effect of electro-thermally induced degradation on the capacitance of metal oxide varistors.
- Description: Abstract: In this study, changes in the capacitance of MOV-based arresters resulting from thermal stresses are analysed. Low-voltage MOV samples are artificially degraded at different thermal stresses, while the simultaneously applied ac field was kept constant. Prior to ANOVA being applied, the change in capacitance measured at room temperature, before and after degradation process, is tested for compliance with normality using Anderson-Darling test. Results show significant reduction in capacitance after electro-thermal degradation.
- Full Text:
- Authors: Muremi, L. , Bokoro, P.
- Date: 2017
- Subjects: Metal Oxide varistors , Thermal stresses , Degradation
- Language: English
- Type: Conference proceedings
- Identifier: http://ujcontent.uj.ac.za8080/10210/384404 , http://hdl.handle.net/10210/254468 , uj:26637 , Citation: Muremi, L. & Bokoro, P. 2017. Effect of electro-thermally induced degradation on the capacitance of metal oxide varistors.
- Description: Abstract: In this study, changes in the capacitance of MOV-based arresters resulting from thermal stresses are analysed. Low-voltage MOV samples are artificially degraded at different thermal stresses, while the simultaneously applied ac field was kept constant. Prior to ANOVA being applied, the change in capacitance measured at room temperature, before and after degradation process, is tested for compliance with normality using Anderson-Darling test. Results show significant reduction in capacitance after electro-thermal degradation.
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On-line monitoring of metal-oxide surge arresters using improved equivalent model with evolutionary optimisation algorithm
- Authors: Doorsamy, W. , Bokoro, P.
- Date: 2017
- Subjects: Surge arresters , Resistive current extraction , Evolutionary optimisation
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/237719 , uj:24360 , Citation: Doorsamy, W. & Bokoro, P. 2017. On-line monitoring of metal-oxide surge arresters using improved equivalent model with evolutionary optimisation algorithm.
- Description: Abstract: A resistive-current extraction algorithm to assist with on-line monitoring of surge arresters is proposed in this paper. The algorithm is based on the improved equivalent model for surge arresters and combines evolutionary optimisation with the base-frequency approximating method. A genetic algorithm is used to obtain the optimal capacitance such that the phase shift between the base-components of the branch voltage and current is minimised thereby yielding a good approximation of the resistive current. In this paper, the algorithm is implemented and tested using Matlab/Simulink. Results indicate that the proposed algorithm is able to efficiently and accurately obtain the resistive component of the leakage current, using the improved equivalent model, under both ideal and distorted supply conditions.
- Full Text:
- Authors: Doorsamy, W. , Bokoro, P.
- Date: 2017
- Subjects: Surge arresters , Resistive current extraction , Evolutionary optimisation
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/237719 , uj:24360 , Citation: Doorsamy, W. & Bokoro, P. 2017. On-line monitoring of metal-oxide surge arresters using improved equivalent model with evolutionary optimisation algorithm.
- Description: Abstract: A resistive-current extraction algorithm to assist with on-line monitoring of surge arresters is proposed in this paper. The algorithm is based on the improved equivalent model for surge arresters and combines evolutionary optimisation with the base-frequency approximating method. A genetic algorithm is used to obtain the optimal capacitance such that the phase shift between the base-components of the branch voltage and current is minimised thereby yielding a good approximation of the resistive current. In this paper, the algorithm is implemented and tested using Matlab/Simulink. Results indicate that the proposed algorithm is able to efficiently and accurately obtain the resistive component of the leakage current, using the improved equivalent model, under both ideal and distorted supply conditions.
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Selection and coordination of surge arresters for switching transient mitigation in photovoltaic power plants
- Oliveira, R., Bokoro, P., Paul, B., Ndlovu, E.
- Authors: Oliveira, R. , Bokoro, P. , Paul, B. , Ndlovu, E.
- Date: 2017
- Subjects: Renewable energy sources , Photovoltaic power plants , Surge arresters
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/221807 , uj:22243 , Citation: Oliveira, R. et al. 2017. Selection and coordination of surge arresters for switching transient mitigation in photovoltaic power plants.
- Description: Abstract: Photovoltaic energy generation has become a popular renewable alternative to conventional energy generation that utilise fossil fuels. However, given the diversity and complexity of these PV plants, it is imperative that such plant equipment be protected against the greatest contributor to equipment failure; surges. Software simulation using EMTP-RV version 3.3, this paper implements a proposed methodology for the insulation coordination study of a PV plant. The overvoltages associated with the opening of vacuum circuit breakers, at various test points along the network are considered in order to recommend possible selection criteria of surge arresters as well as location thereof. The study finds that for a reduction of surge magnitudes from 8 p.u to within 1.2 p.u would require surge arrester energy capabilities to be greater than 2.8 kJ/kV for the medium voltage (MV) arresters , and capabilities exceeding 259kJ/kV for the low voltage (LV) arrester. For the high voltage (HV) section of the plant, no surge propagation was identified thus exempting it from the insulation coordination. The above mentioned, along with surge current and overvoltage levels comprise the findings of the study providing parameter guidelines for arrester selection.
- Full Text:
- Authors: Oliveira, R. , Bokoro, P. , Paul, B. , Ndlovu, E.
- Date: 2017
- Subjects: Renewable energy sources , Photovoltaic power plants , Surge arresters
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/221807 , uj:22243 , Citation: Oliveira, R. et al. 2017. Selection and coordination of surge arresters for switching transient mitigation in photovoltaic power plants.
- Description: Abstract: Photovoltaic energy generation has become a popular renewable alternative to conventional energy generation that utilise fossil fuels. However, given the diversity and complexity of these PV plants, it is imperative that such plant equipment be protected against the greatest contributor to equipment failure; surges. Software simulation using EMTP-RV version 3.3, this paper implements a proposed methodology for the insulation coordination study of a PV plant. The overvoltages associated with the opening of vacuum circuit breakers, at various test points along the network are considered in order to recommend possible selection criteria of surge arresters as well as location thereof. The study finds that for a reduction of surge magnitudes from 8 p.u to within 1.2 p.u would require surge arrester energy capabilities to be greater than 2.8 kJ/kV for the medium voltage (MV) arresters , and capabilities exceeding 259kJ/kV for the low voltage (LV) arrester. For the high voltage (HV) section of the plant, no surge propagation was identified thus exempting it from the insulation coordination. The above mentioned, along with surge current and overvoltage levels comprise the findings of the study providing parameter guidelines for arrester selection.
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Soil resistivity : a limiting determinant to zero-sequence currents for grounded conductors in South African low voltage networks
- Authors: Louw, Q. , Bokoro, P.
- Date: 2017
- Subjects: Soil resistivity , Zero-sequence current , Grounded conductor
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/237995 , uj:24393 , Citation: Louw, Q. & Bokoro, P. Soil resistivity : a limiting determinant to zero-sequence currents for grounded conductors in South African low voltage networks.
- Description: Abstract: In this paper, the dependence of soil resistivity on the geological structure and meteorological conditions are discussed. The South African context of changing resistivity and its inevitable consequences on the behaviour of prospective zero-sequence currents in low-voltage distribution systems is highlighted. Therefore, field measurements of soil resistivity using the Wenner Array technique is conducted on expansive clay-based soil. The resulting zero-sequence current inherent to the soil conditions is also assessed. The results obtained indicated that the soil resistivity value of 488.74 Ω. m, which corresponds to the soil conditions at the time and measurement season, represents an increase of 62.91% of the standard limit value for expansive clays, and causes 37.14% decrease in the zero-sequence current likely to flow at the limit value of expansive clay’s resistivity.
- Full Text:
- Authors: Louw, Q. , Bokoro, P.
- Date: 2017
- Subjects: Soil resistivity , Zero-sequence current , Grounded conductor
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/237995 , uj:24393 , Citation: Louw, Q. & Bokoro, P. Soil resistivity : a limiting determinant to zero-sequence currents for grounded conductors in South African low voltage networks.
- Description: Abstract: In this paper, the dependence of soil resistivity on the geological structure and meteorological conditions are discussed. The South African context of changing resistivity and its inevitable consequences on the behaviour of prospective zero-sequence currents in low-voltage distribution systems is highlighted. Therefore, field measurements of soil resistivity using the Wenner Array technique is conducted on expansive clay-based soil. The resulting zero-sequence current inherent to the soil conditions is also assessed. The results obtained indicated that the soil resistivity value of 488.74 Ω. m, which corresponds to the soil conditions at the time and measurement season, represents an increase of 62.91% of the standard limit value for expansive clays, and causes 37.14% decrease in the zero-sequence current likely to flow at the limit value of expansive clay’s resistivity.
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Condition monitoring of metal-oxide surge arresters using leakage current signature analysis
- Authors: Doorsamy, W. , Bokoro, P.
- Date: 2018
- Subjects: Metal-oxide surge arrestors , Online condition monitoring , Leakage current signature
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/278893 , uj:29937 , Citation: Doorsamy, W. & Bokoro, P. 2018. Condition monitoring of metal-oxide surge arresters using leakage current signature analysis.
- Description: Abstract: Online condition assessment of surge arresters is necessary for early detection of problems and subsequent replacement of the device to avoid unplanned downtime and/or damage to the equipment being protected. The most common technique of monitoring and assessing the condition of the arrester is extraction of the resistive component of the leakage current. There are many drawbacks to this method arising from inaccuracies in the model-based approach and deficiencies in the understanding of the leakage current signal. This paper presents leakage current signature analysis (LCSA) as a method of assessing the condition of the surge arrester where the harmonics components of the current signal, extracted using Bayesian spectrum estimation, are used as indicators of degradation. An experimental methodology comprising measurement of the leakage current of arresters undergoing constant-stress accelerated degradation testing is employed. Results indicate that a consistent increase in the DC component of the leakage current signature in the case of degrading samples is the most likely indicator of condition.
- Full Text:
- Authors: Doorsamy, W. , Bokoro, P.
- Date: 2018
- Subjects: Metal-oxide surge arrestors , Online condition monitoring , Leakage current signature
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/278893 , uj:29937 , Citation: Doorsamy, W. & Bokoro, P. 2018. Condition monitoring of metal-oxide surge arresters using leakage current signature analysis.
- Description: Abstract: Online condition assessment of surge arresters is necessary for early detection of problems and subsequent replacement of the device to avoid unplanned downtime and/or damage to the equipment being protected. The most common technique of monitoring and assessing the condition of the arrester is extraction of the resistive component of the leakage current. There are many drawbacks to this method arising from inaccuracies in the model-based approach and deficiencies in the understanding of the leakage current signal. This paper presents leakage current signature analysis (LCSA) as a method of assessing the condition of the surge arrester where the harmonics components of the current signal, extracted using Bayesian spectrum estimation, are used as indicators of degradation. An experimental methodology comprising measurement of the leakage current of arresters undergoing constant-stress accelerated degradation testing is employed. Results indicate that a consistent increase in the DC component of the leakage current signature in the case of degrading samples is the most likely indicator of condition.
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Investigation of lightning surge effects on a grid-connected PV plant
- Authors: Bokoro, P. , Doorsamy, W.
- Date: 2018
- Subjects: Photovoltaic plant , Induced surges , Lightning protection
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/278891 , uj:29936 , Citation: Bokoro, P. & Doorsamy, W. 2018. Investigation of lightning surge effects on a grid-connected PV plant.
- Description: Abstract: This paper investigates the indirect effects of lightning on a medium-scale photovoltaic plant. The purpose of the investigation is to analyse the surge effects on the photovoltaic plant and interconnected systems in order to develop an understanding of the associated risk of damage to the plant and ascertain the requirements for lightning protection. The investigated plant consists of four photovoltaic arrays, each comprising 2500 modules connected in series-parallel configurations, with inverters and connection to the high voltage transmission grid via a system of parallel buses, insulated power cables and power transformers. The presented study makes use of the electro-magnetic transient program-restructured version (EMTP-RV) software platform to investigate induced effects of lightning on grid-connected PV systems and provide system designers with a means of analysing and mitigating lightning risk. Results obtained show significant risk associated to voltage rise of 7.4% on the DC side and 1.2 p.u on the AC side.
- Full Text:
- Authors: Bokoro, P. , Doorsamy, W.
- Date: 2018
- Subjects: Photovoltaic plant , Induced surges , Lightning protection
- Language: English
- Type: Conference proceedings
- Identifier: http://hdl.handle.net/10210/278891 , uj:29936 , Citation: Bokoro, P. & Doorsamy, W. 2018. Investigation of lightning surge effects on a grid-connected PV plant.
- Description: Abstract: This paper investigates the indirect effects of lightning on a medium-scale photovoltaic plant. The purpose of the investigation is to analyse the surge effects on the photovoltaic plant and interconnected systems in order to develop an understanding of the associated risk of damage to the plant and ascertain the requirements for lightning protection. The investigated plant consists of four photovoltaic arrays, each comprising 2500 modules connected in series-parallel configurations, with inverters and connection to the high voltage transmission grid via a system of parallel buses, insulated power cables and power transformers. The presented study makes use of the electro-magnetic transient program-restructured version (EMTP-RV) software platform to investigate induced effects of lightning on grid-connected PV systems and provide system designers with a means of analysing and mitigating lightning risk. Results obtained show significant risk associated to voltage rise of 7.4% on the DC side and 1.2 p.u on the AC side.
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Investigation of very fast-front transient overvoltages for selection and placement of surge arresters
- Oliveira, R., Bokoro, P., Doorsamy, W.
- Authors: Oliveira, R. , Bokoro, P. , Doorsamy, W.
- Date: 2018
- Subjects: Very fast-front transient , Voltage magnification , Surge arrester
- Language: English
- Type: Conference proceedings
- Identifier: http://ujcontent.uj.ac.za8080/10210/384178 , http://hdl.handle.net/10210/274101 , uj:29226 , Citation: Oliveira, R., Bokoro, P. & Doorsamy, W. 2018. Investigation of very fast-front transient overvoltages for selection and placement of surge arresters.
- Description: Abstract: An analysis of very fast front transient overvoltages, generated as a result of induced effects of lightning, across a photovoltaic plant is investigated. The EMTP-RV version 3.3 computer software platform is used to simulate surge voltage magnification. Selection and placement of surge arresters for effective protection of power transformers are conducted. Results show that selected arresters placed on bus 4 and coordinated with those placed on bus 5 and 3 ensure surge amplitude reduction of 33% on bus 4.
- Full Text:
- Authors: Oliveira, R. , Bokoro, P. , Doorsamy, W.
- Date: 2018
- Subjects: Very fast-front transient , Voltage magnification , Surge arrester
- Language: English
- Type: Conference proceedings
- Identifier: http://ujcontent.uj.ac.za8080/10210/384178 , http://hdl.handle.net/10210/274101 , uj:29226 , Citation: Oliveira, R., Bokoro, P. & Doorsamy, W. 2018. Investigation of very fast-front transient overvoltages for selection and placement of surge arresters.
- Description: Abstract: An analysis of very fast front transient overvoltages, generated as a result of induced effects of lightning, across a photovoltaic plant is investigated. The EMTP-RV version 3.3 computer software platform is used to simulate surge voltage magnification. Selection and placement of surge arresters for effective protection of power transformers are conducted. Results show that selected arresters placed on bus 4 and coordinated with those placed on bus 5 and 3 ensure surge amplitude reduction of 33% on bus 4.
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Validation of peak point-on-wave switching regime for the generation of switching transients in low voltage AC applications
- Authors: Oliveira, R. , Bokoro, P.
- Date: 2018
- Subjects: POW , Switching transients , Overvoltage
- Language: English
- Type: Conference proceeding
- Identifier: http://hdl.handle.net/10210/290439 , uj:31530 , Citation: Oliveira, R. & Bokoro, P. 2018. Validation of peak point-on-wave switching regime for the generation of switching transients in low voltage AC applications.
- Description: Abstract: Point-On-Wave switching is often associated with zero crossover switching in which a contact is opened or closed at the zero crossover to reduce the presence of switching transient phenomena. In this paper presents a Point-On-wave switching circuit designed to switch a contact on at the peak point on wave for the purpose of switching transient overvoltage studies. The overvoltage generated is analysed based on the wave shape and its respective parameters and is validated against the defined parameters stipulated by international standards.
- Full Text:
- Authors: Oliveira, R. , Bokoro, P.
- Date: 2018
- Subjects: POW , Switching transients , Overvoltage
- Language: English
- Type: Conference proceeding
- Identifier: http://hdl.handle.net/10210/290439 , uj:31530 , Citation: Oliveira, R. & Bokoro, P. 2018. Validation of peak point-on-wave switching regime for the generation of switching transients in low voltage AC applications.
- Description: Abstract: Point-On-Wave switching is often associated with zero crossover switching in which a contact is opened or closed at the zero crossover to reduce the presence of switching transient phenomena. In this paper presents a Point-On-wave switching circuit designed to switch a contact on at the peak point on wave for the purpose of switching transient overvoltage studies. The overvoltage generated is analysed based on the wave shape and its respective parameters and is validated against the defined parameters stipulated by international standards.
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Improving the diagnosis of incipient faults in power transformers using dissolved gas analysis and multi layer perceptron
- Authors: Nkosi, S. , Bokoro, P.
- Date: 2020
- Subjects: Component , Formatting , Style
- Language: English
- Type: Conference Proceedings
- Identifier: http://hdl.handle.net/10210/465274 , uj:41601 , Citation: Nkosi, S. & Bokoro, P. 2020. Improving the diagnosis of incipient faults in power transformers using dissolved gas analysis and multi layer perceptron.
- Description: Abstract: Please refer to full text to view abstract.
- Full Text:
- Authors: Nkosi, S. , Bokoro, P.
- Date: 2020
- Subjects: Component , Formatting , Style
- Language: English
- Type: Conference Proceedings
- Identifier: http://hdl.handle.net/10210/465274 , uj:41601 , Citation: Nkosi, S. & Bokoro, P. 2020. Improving the diagnosis of incipient faults in power transformers using dissolved gas analysis and multi layer perceptron.
- Description: Abstract: Please refer to full text to view abstract.
- Full Text: