ECN publication
Estimation of the potential to pollute the electricity network with harmonics due to the use of small micro generators with inverters; Measurements of the Complex Conductance, an additional test method for inverters
Heskes, P.J.M.; Rooij, P.M.; Cobben, J.F.G.; Oldenkamp, H.E.
Published by: Publication date:
ECN Energy in the Built Environment 1-8-2004
ECN report number: Document type:
ECN-C--04-087 ECN publication
Number of pages: Full text:
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Currently the total power of PV-generators in the built environmentis small compared to the power of the distribution transformer. With the steady growth of PV installations, the penetration of PV-generators in the built environment will grow as well. This will lead to new phenomena, which may eventually result in grid instabilities [3]. In order to be able to predict whether these phenomena will occur, a simple though accurate harmonics model for the grid, the cables the loads and the inverters is presented, as well as a measuring method to determine the necessary frequency dependent parameters of inverters and loads in general. In a previous project [4] a preliminary harmonic interaction test method for inverters was proposed, this result was an important input for this project. This project focuses on harmonic pollution of the grid voltage at large-scale implementation of small grid connected micro generators with inverters, like photovoltaics. The goal of this project is to lay down a test method to qualify inverters on this subject. The upper lying target is to contribute to a new standard on inverter testing, NTA 8494 [1] (Netherlands Technical Agreement), a guideline for inverter testing. In the first part of this report the 'Complex Conductance Measurement Method' has been critically reviewed both theoretically and practically. In the second part of this report the 'Complex Conductance Measurement Method' has been adapted to situations in practice. With the latter method a few measurement examples are presented and discussed. The measuring method described in this report can be used to determine the frequency dependent parameters of inverters and loads. The results depend strongly on the inverter topology. Negative values for the normalized conductance appear to be possible for some types of inverters. As this may result in grid instabilities, inverters with negative conductance should only be used in projects with large-scale implementation of PV when sufficient compensation is available. Results of the mentioned Measurement Method can easily be used in a more complex simulation to predict the level of harmonic distortion and the possibility of resonance-effects. From the experiments can be remarked that a further research on the behaviour of different kinds of loads in relation to harmonics is needed to get familiar with test results.

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