Transcript Présentation PowerPoint

Connection of disturbing
loads in micro-grids
National Advanced Polytechnique Institute of
Yaounde - Cameroon
Tchuidjan Roger, PhD Electrical
Engineer
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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 higherfrequency 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 ?
Tchuidjan Roger, PhD Electrical
Engineer
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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
Tchuidjan Roger, PhD Electrical
Engineer
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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
Ra  
and
S
load

U
R
b

2
bc
R
Tchuidjan Roger, PhD Electrical
Engineer
c
(2)
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
After the calculation of the short circuit power we can
get the short circuit impedance through the relation
Z
cc

U
S
2
(3)
cc

The objective of this calculation is to compare the Zcc
obtainedby simulation (DC Source- ConverterUnbalanced Load) with the Zcc of a similar system
where the DC Source-Converter can be replaced by an
AC Source.
Tchuidjan Roger, PhD Electrical
Engineer
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Results of simulations
200
Votlages a, b , c [ V ]
150
100
50
0
-50
-100
-150
-200
2.9
2.92
2.94
2.96
2.98
3
T imes [ ms ]
Fig. 2 : Voltages obtained at the phases (a, b, c)
of the unbalanced load
Tchuidjan Roger, PhD Electrical
Engineer
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10
C urrent [ A ]
8
6
4
2
0
-2
2 .9
2 .9 2
2 .9 4
2 .9 6
2 .9 8
3
T im e s [ m s ]
Fig. 3: Current at the DC side of the converter when
AC connected to unbalanced load
Tchuidjan Roger, PhD Electrical
Engineer
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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 Ω.
 The short circuit impedance obtained from the result of
the simulation (1.083 Ω) is not very greater than the
calculated value from the filter (1.0101 Ω).

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).
Tchuidjan Roger, PhD Electrical
Engineer
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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




U2
L  z cc
S n
C
(H)
1
umn  L
2
2
Tchuidjan Roger, PhD Electrical
Engineer
(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:
Tchuidjan Roger, PhD Electrical
Engineer
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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.
Tchuidjan Roger, PhD Electrical
Engineer
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