Browsing by Author "Overett, Jonathan Stephen"

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    Assessment of the harmonic behaviour of a utility-scale photovoltaic plant
    (2017-12) Overett, Jonathan Stephen; Vermeulen, H. J.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: The rapidly increasing deployment of utility-scale solar photovoltaic (PV) plants worldwide presents new technical challenges including the risk of unacceptably high harmonic voltage distortion in the utility network. PV plants affect the harmonic voltage distortion at their point of common coupling primarily by two distinct mechanisms: generation of harmonic currents due to switching action of the inverters and introduction of series and parallel resonances caused by connection of the plant to the network. The objective of this study is to investigate the harmonic behaviour of utility-scale PV plants based on the case-study of a 75 MW PV plant in the Northern Cape province of South Africa. In this work, a harmonic simulation model of a PV plant is developed in DIgSILENT PowerFactory which represents the passive network components with sufficient accuracy to study the frequency response over the harmonic frequency band from 50 to 2500 Hz. 10-minute and 3-second aggregated harmonic measurement data and event-triggered waveform data were captured over a 12-day period using an IEC 61000-4-30 Class A power quality meter installed at the plant’s 132 kV point of connection (POC). A quality assessment of harmonic measurement data shows that the accuracy of harmonic voltage and current measurements are likely to be significantly affected by quantisation and transducer error for even and higher-order harmonics. Analysis of the 3-second aggregated harmonic quantities indicates that their fluctuation during each 10-minute period is sufficiently small that the 10-minute average values adequately approximate the short-term and long-term behaviour of the plant. The harmonic behaviour of the plant is investigated by considering the relationship between harmonic voltage and current emissions and active power, reactive power and time of day. Certain harmonic voltages increase with increasing plant active power output whilst others decrease but all harmonic currents remain constant or increase with increasing active power. It was not possible to quantify the relative impact of active and reactive power variation as the plant operates in voltage control mode. Brief periods of high harmonic current distortion were noted during start up and shut down of the plant. High harmonic currents at the 23rd harmonic order correspond with a series resonance identified in the model thus validating the POC frequency sweep simulation. A comparative analysis of different published methods for assessment of the harmonic emissions contribution of the PV plant is performed. All methods show similar trends in identifying dominant harmonics and general harmonic behaviour but the magnitudes of the calculated emissions differ significantly in some instances. Where accurate frequency sweep data of the utility network and phasor measurement data are both available, the plant model enables application of the harmonic vector method to discriminate between utility and plant contributions. Simulated voltage emissions based using the manufacturer’s quoted inverter current emissions do not closely match measured emissions due to the complexity of active source interactions within the real power system. Comparison of the mean, 75th percentile and 90th percentile emissions assessments demonstrate the effectiveness of percentile assessment in eliminating outlying high distortion periods from the assessed results.