Download A DC Distribution Network with Alternative Sources

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A DC Distribution Network with Alternative
Sources
R. Magureanu, M. Albu, M. Priboianu and A.M. Dumitrescu
University Politehnica Bucharest/Department of Electrical Engineering, Bucharest, Romania
Abstract— A DC grid with different type of generators and
load is presented. It is considered an islanded and grid
connected operation. A low voltage laboratory model is
simulated and tested and is expected in the future the results
to be extended to high voltage applications. It is
demonstrated that the solution can have not only scientific
but also commercial viability.
I. INTRODUCTION
Romania has two main sources for renewable energy,
The first one is hydro, which is producing around 30 % of
total electrical energy and the second one is biomass,
produced by the forests which are covering over 40 % of
total country area. Less developed, but with a possible
positive results are wind and solar energy produced by
wind turbines and solar cells [1], [2], [3].
The disadvantage is the fact that generator’s current is
no longer sinusoidal but has a high content of harmonics
and the rectified voltage has relatively high ripple (Fig. 3).
Either the rectification is done by a unity power factor
converter, at a high cost and complicated control or it can
be used a six-phase rectifier with 12 valves (or a twelvepulse system), supplied by a synchronous generator with
two separate sets of three phase windings [4], resulting in
a six phase system (Fig. 5).
Fig. 2. Water turbine flow/speed/efficiency characteristic
ia2/10 (Amps)
800
600
400
200
Fig. 1 Map of Romania
II.
-200
va2 (V)
As all these types of energy are distributed and
sometimes they are used only for local consume and other
times are connected to the main grids, such a network has
to present also storage facilities as the production of
energy is variable from day to night and winter to
summer.
0
-400
-600
-800
0.97
0.98
0.985
Time (s)
0.99
0.995
1
a) Voltage and current for three phase generator
900
DC SOLUTIONS FOR SYNCHRONOUS GENERATION
OPERATION
750
vDC (V)
The generation of electrical energy is typically done by
synchronous or asynchronous generators for hydro and
wind turbines and by power electronics for sun and
electrochemical sources.
Typical for hydro and wind generators is the fact that
their efficiency is function of wind and water flow and
converter rotation speed. Due to the fact that synchronous
generator has to operate at constant speed; these
conditions are difficult to be fulfilled (Fig. 2).
Consequently, a possible solution is to rectify by diode
bridges generator outputs and to parallel them in DC.
0.975
600
450
300
150
0
0.98
0.982
0.984
0.986
0.988
0.99 0.992
Time (s)
0.994 0.996
0.998
b) Rectified output voltage
Fig. 3 Voltages and currents of the three-phase generator
1
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Other solution is to use the simple rectification plus a
booster which to maintain the output voltage at a constant
level. In Fig. 4 there is presented the voltage and the
current for a load that is increasing from 50% of nominal
power to 100 % of nominal power.
Fig. 5 Six-phase generator-rectifier system
a) Voltage and current with booster
ia1/10 (Amps)
400
300
200
100
0
va1 (V)
-100
-200
-300
-400
0.97
0.975
0.98
0.985
0.99
1
0.995
Time (s)
a)
b) The rectified voltage with booster
Fig. 4 The voltages and currents with booster
I. DC GRID STRUCTURE PROPOSED
In the case in which a three phase with neutral
distribution line (Fig. 7a) is available and is wanted to be
transformed in a DC network, [9] [10] the system can be
transformed in a two lines configuration. Using per unit
values, the AC network can transport a power equal with
3, but the DC one only 2.44, which means a transmission
efficiency of 0.81 (Fig. 7b).
vdc2 (V)
900
750
600
vdc (V)
vdc1 (V)
450
300
150
0
0.97
0.975
0.98
0.985
Time (s)
0.99
1
0.995
b) Rectified voltages
Fig. 6 Voltages and currents of the six-phase generator
R = 1Ω
Uˆ = 1, 41U Uˆ = 2, 44U
R = 1Ω
I = 1A
Uˆ = 1, 41U Uˆ = 2, 44 U
R = 1Ω
Uˆ = 1, 41U
a)
PCA = 3
The disadvantage is the fact that generator’s current is
no longer sinusoidal but has a high content of harmonics
and the rectified voltage has relatively high ripple. Either
the rectification is done by a unity power factor
converter, at a high cost and complicated control or it can
be used a six-phase rectifier with 12 valves (or a twelvepulse system), supplied by a synchronous generator with
two separate sets of three phase windings [4], resulting in
a six phase system (Fig. 5). In order to reduce the
harmonic level of the rectified output, we suggested [5] a
solution similar to that employed in the case of three
winding transformers: a configuration of the generator
two winding systems in a star/wye connection, thus
obtaining a six phase system [6], [7], [8]. In this way the
harmonics are considerably reduced, the sixth harmonic
of the rotating magnetic field in generator air gap is
practically eliminated, improving not only the efficiency
of the generator but also reducing the DC ripple of the
DC output voltage and current (Fig. 6).
Voltage and current for the first three phases
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R = 1, 73 Ω
−
Uˆ = 2 ⋅ 3 = 2, 44U
+
charged when there is an excess of energy in the system
and to discharge then when energy is necessary. That
means that we need a bidirectional DC/DC converter
which to be controlled in such a way that this process to
be optimally done (Fig. 8).
Uˆ = 2U
b)
+
RL
I1 = 1A
PCC = 2, 44
U CC = 2, 44
AC
−
PCC = 2, 44
U CC = 2, 44
AC
+
RL
PCA = 4,88
−
I1 = 1A
c)
Fig. 7 The three-phase line operating in AC (a), single DC (b) and
double DC (c)
Fortunately, we have available the extra phase and the
neutral which can be used also doubling the power of the
transport line, resulting 4.88 with a transport efficiency of
1.62 (Fig. 7c). As the current will be DC, the losses will
be smaller, compared with the AC ones, and the current
can be increased by at least 10% which is equivalent with
total transport efficiency about 1.8.
II.
INTEGRATION OF ALTERNATIVE SOURCES IN A DC
SYSTEM
Romania is a country with a medium solar energy
potential and although at present this is not a practical
solution, taking into consideration that in the future, and
the price of solar cells and of the energy produced will
decrease, in the future it might be an interesting
possibility, reason for each we introduced it in our
experimental model, [11] [12].
Beside hydro, solar and wind synchronous generators,
the system can use for electrical energy generation from
biomass a fuel cell converter supplied through a biogas
generator and a reformer. As the voltage produced by each
cell is low (a little bit over 1 V) and is variable with the
load, a unidirectional DC/DC converter is used in order
that the output voltage to be controlled in such a way that
this to be about equal with the DC line voltage [13].
There are different solutions for energy storage. We
considered here only two as in our case it is expected that
the energy available in the system from time to time is
over the energy which can be consumed or delivered to
the grid. The first solution is to produce hydrogen by the
mean of a hydrolyser and, when electrical energy is
necessary, that hydrogen to be used to produce energy. A
second solution is to use led acid batteries which to be
Fig. 8 Double DC distribution network
As we have two parallel lines, an equalizing system was
suggested, transferring the energy from one leg to the
other by the mean of a bidirectional DC/DC converter.
The connection with the grid is done by two
bidirectional front-end converters which allows not only
transforming the energy from DC in AC and DC in AC,
but can also improve the quality of energy on AC side. In
Fig. 9 is presented one of the two front-end converters of
60 kVA which can operate as presented (Fig. 10a, b).
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III.
CONCLUSIONS
The use of DC current in transport of energy improves
efficiency of operation and the quality of power. The
basic elements are the power electronic converters of
different types, able to convert the AC and DC currents in
a suitable way.
In this paper, new grid architecture for integrating
dispersed generation is proposed. Simulation results
concerning the power electronic converters and their
operation into the proposed system are presented.
Further work will emphasize laboratory testing of a DC
grid in kAmps range, as a first step towards a commercial
exploitation (20 MW, 10 kV DC).
ACKNOWLEDGMENT
The authors are much obliged to the Ministry of
Education and Research for financing the project trough
three grants in the period of 2006-2008.
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[1]
[2]
Fig.9 The 60 kVA IGBT inverter
400
[3]
300
[4]
Ua (V) Ia (Amps)
200
100
a)
0
[5]
-100
[6]
-200
-300
Ia (Amps)
-400
0.12
0.125
0.13
0.135
0.14
Time (s)
0.145
0.15
0.155
[7]
0.16
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[8]
300
[9]
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[10]
b)
0
[11]
Ua (V)
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[12]
-300
-400
0.12
0.125
0.13
0.135
0.14
Time (s)
0.145
0.15
0.155
0.16
Fig. 10 Voltages and currents of the three level inverter:
a) grid to source; b) source to grid
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