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To tackle the complicated problem of filter planning for a residential feeder, primarily, it can be subdivided to the followings: 1. Basically, each filter is tuned to a harmonic order. The main aim is to provide a system-wide mitigation of that harmonic order. The main question is how many filters are needed and where they have to be placed? Figures below show how a single filter location in a very simple feeder affects the system-wide harmonic distortion indices: 2. Then, one has to verify that the planned filters are avoided to introduce any magnitude amplification of other harmonic orders in the system. The undesired harmonic increase may happen by a parallel resonance in the system. Methodologies have to be developed to address the issue. Substation A block of residential loads (e.g. a neighbourhood) Filter Normal Harmonic flow New harmonic flows after the filter is installed Harmonic Filters For Residntial Feeders Pooya Bagheri M.Sc. Department of Electrical and Computer Engineering, University of Alberta [email protected] Nowadays, the rapid growth of nonlinear electronic home appliances has resulted in significant voltage and current waveform distortions in residential distribution systems. Although each of the consumer electronic devices is not individually a big source of harmonics, the collective effect will be considerable and can become a serious concern of utility companies. Hence, there is an essential need of research for effective harmonic filter planning strategies in residential distribution systems. Introduction Modern vs. Traditional Harmonic Problems Power and Energy Innovation Forum ♦ University of Alberta ♦ November 2011 The Proposed Research Supply System Filter Industrial H-source loads The transformer impedance avoids attraction of system harmonics toward the filter The 240/25 kV Substation Downstream loads Upstream loads Filter 1 Telephone line sections Filter 2 (if necessary) Monitoring Location F 1 F 2 C A B C N 3C/a 2 1:a Trans. Series impedance Open Circuit Equivalent Zero-Seq Circuit Equivalent Pos/Neg-Seq Circuit Industrial Plants (the traditional harmonic problem): Residential Feeders (The modern harmonic problem): The following issues make the filter planning methods required for the residential feeders totally different (and more complicated) from the traditional methods usually applied to industrial harmonic problems: • Harmonic sources are not big loads concentrated in one location (like the industrial ones). There are several small sources dispersed through all the network. Therefore, the filter location to achieve a system-wide harmonic mitigation is not anymore obvious. • There is no transformer associated impedance (like the one in industrial plants). A filter can attract the upstream harmonics which may be even causing higher distortions in some segments of the system. •Harmonic resonance analysis will not be as simple as the methods used in an industrial plant filter design. Zero-Sequence Filter Design for a Real Telephone Interference Problem • Since a system-wide harmonic reduction is not essential for a telephone interference problem mitigation, the required filter placement is not too complicated. The only issue is about undesired attraction of upstream harmonics by the installed filter. For this project, this issue is investigated by some analytical and simulation studies and it was verified that for this specific feeder, it is not problematic and one filter was found to be sufficient. • The 9 th harmonic was identified as the main contributor to the interference problem. Hence, the filter was tuned to this harmonic order. • As the main privilege of zero-sequence filters, they appear as open-circuit for equivalent nonzero-sequence circuits and consequently do not have any effect on the pos/neg-sequence harmonics. Therefore, there is no concern of possible parallel resonance in the other harmonics caused by such filters installation! • During the project it was revealed that present analytical (such as k-factor) indices are not sufficient for such filters loading assessment. Hence, a new methodology was developed to evaluate the filter overloading. 3rd 5th 7th 9th 11th 13th 15th 0 2 4 6 IDD(%) Harmonic order No Filter One Filter installed at location F1 Two Filters installed at both locations (F1 & F2) 1st 3rd 5th 7th 9th 11th 13th 15th Total rms 0 50 100 150 Induced Voltage(V) Harmonic order No Filter One Filter installed at location F1 Two Filters installed at both locations (F1 & F2) The Zero-Sequence Filter Schematic 0 10 20 30 40 50 60 70 80 90 100 0 0.5 1 Filter location: x (% of feeder length) Scaled to Maximum Average Voltage THD Average Current THD Sub. F x
