Connection of disturbing loads in micro-grids

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Connection of disturbing loads in micro-grids

• National Advanced Polytechnique Institute of
  Yaounde - Cameroon

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Objectifs du projet

• Many papers may be found in the literature
  studying the impact of dispersed generation on
  power quality, where the generators are
  considered as disturbing installations (sources
  of flicker, harmonics, or higher-frequency
  components). We report on a study with another
  point of view  what about the connection of
  "classical" disturbing loads in a micro-grid fed
  by renewable energy generators ?

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Description

• The system which was modeled is shown in Fig.1.
  It simply consists of a DC Generator connected to
  load by DC/AC converter.
•  
• Fig. 1 Electrical System to study

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The analysis of dynamic behavior of this system

• Knowing that load power can be explained as a
  function of the short circuit power and the
  relation between positive and negative sequence
  voltages, we can write that
• 
  (1)
• The determination of the load power can be done
  using
• 
  
• 
  (2)

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• After the calculation of the short circuit power
  we can get the short circuit impedance through
  the relation
• 
  
  
• 
  (3)
• The objective of this calculation is to compare
  the Zcc obtainedby simulation (DC Source-
  Converter- Unbalanced Load) with the Zcc of a
  similar system where the DC Source-Converter can
  be replaced by an AC Source.

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Results of simulations

• Fig. 2 Voltages obtained at the phases (a, b,
  c)
• of the unbalanced load

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• Fig. 3 Current at the DC side of the converter
  when
• AC connected to unbalanced load

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• What we were looking for, is the equivalent short
  circuit impedance (given by Scc  Sload x Ud/Ui).
  The value obtained is around 1.083 O.
• The short circuit impedance obtained from the
  result of the simulation (1.083 O) is not very
  greater than the calculated value from the filter
  (1.0101 O).
• Thus, with the simple model which was used for
  the PWM AC/DC converter (no active filtering
  function), the "short-circuit" impedance
  (Thevenin equivalent) of the generator is
  practically equal to the impedance of the filter.
  The question is then  how to choose the
  parameters L and C ? (the influence of C being
  low for 50 Hz phenomena, but important at
  harmonic frequencies).

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Choice of filter parameters

• It can be shown that the filter parameters L and
  C result from the choice of
• - zcc (pu) the series 50 Hz impedance of the
  generator, and
• - um Um/U (pu) the accepted relative
  magnitude of the modulation frequency component
•  
• 
  (H)
• 
  (F) 

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• Where
• - U phase-to-phase 50 Hz voltage of the
  generator (V).
• - ? 2?f 2?50 (rad/s).
• - Sn ?3UIn nominal power of the generator
  (VA).
• - n fm/f ratio between modulation frequency
  and power frequency.
• If we want to have zcc as low as possible, we
  have to accept if and um as high as possible.
• If we accept 0.5  for the magnitude of the HF
  voltage component and 10  for the magnitude of
  the no-load current in the filter then 

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• zcc 2.2  at 15 kHz
• zcc 5  at 10 kHz
• zcc 20  at 5 kHz.
• It appears that the achievable short-circuit
  power strongly depends on the modulation
  frequency. We know that a classical value for the
  impedance of a MV/LV transformer is 4 . Such a
  low impedance may be obtained with the PWM
  generator if the modulation frequency is higher
  than 10 kHz.