HARMONIC ANALYSIS AND MITIGATION IN SELECTED NIGERIAN 33KV DISTRIBUTION NETWORKS

Abstract

ABSTRACT The non-linear characteristics of power electronic loads introduce harmonics, which may result in power distribution losses and reduction of the operational efficiency of equipment. Therefore, minimization of power harmonics is highly desirable in electrical power networks. Various techniques to mitigate against harmonics have been developed but do not meet the ultimate quality power supply and voltage level of 5% for total harmonic distortion (THD) and 3% for any individual harmonics, as specified by IEEE standard 519-1992. This study aimed at the development of an improved technique for minimisation of harmonic contents in distribution networks. Power flow analysis of a selected Nigerian 33 kV distribution network in Southwestern Nigeria was carried out to obtain its bus voltage and power characteristics (active and reactive power) using Newton-Raphson numerical technique. An algorithm was developed to induce power harmonics on the bus voltages resulting from the power flow analysis in order to obtain harmonic contents in each distribution line of network considered. Single and n-stage cascaded harmonic reduction networks were developed using standard procedure. The distribution network, with induced harmonics, was inserted into the developed harmonic reduction network. Effects of harmonics on distribution lines less than 1 km (short) and long lines of 7 km were observed. Data were analysed using Anova at p=0.05 Bus voltages at the 1st harmonic range from 32.553 to 32.997 kV, with percentage deviations varying from 0.01 to 1.35% of the nominal voltage of 33 kV. Reactive power (MVar) in the network was 61.97% of the active power supplied to the system at a given load. Unwanted harmonics generated in the network is in the frequency range of 100-1450Hz. Harmonic impedances for 2nd and 29th harmonics ranges from 101.962Ω to 102.118Ω and 101.556Ω to 102.007Ω respectively at different switching times. Mitigation, using single and 2nd stage cascaded harmonic reduction networks revealed that the peak harmonic impedance amplitude was reduced by 46.0% and 98.0% respectively. The number of harmonic components in the induced harmonics were reduced by single and 2nd stage reduction techniques, with 2nd stage reduction technique having much reduced harmonic amplitudes. The 2nd harmonic was completely removed in the single stage reduction technique, while the 2nd stage reduction technique eliminated the 2nd and 6th harmonics. The average impedance amplitudes of the remaining harmonics for single and 2nd stage reduction techniques were 102.1Ω and 102.0Ω, respectively. Harmonic impedance at the 2nd harmonic remained constant for short distribution line distance of less than 1 km. The 3rd harmonic and above were associated with distribution feeders of long distances of up to 7 km. Analysed data showed that the 2nd stage is significantly better than the single stage technique of harmonic reduction. The developed algorithm for harmonic analysis and its mitigation in power system resulted in improved supply voltage in short distribution feeders. Further mitigation in the 2nd stage technique enhanced power quality in the selected distribution network.