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AC-DC Interleaved Modular Multilevel Converter with Medium-Frequency Isolation Transformer for DC Micro-grids

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This article presents the features of a 3-phase AC-DC interleaved modular multilevel converter with medium-frequency transformer, that has been designed for the connection of low voltage DC micro-grids (380 V DC) to the medium voltage utility grid (13.8 kV). The converter meets the Brazilian grid code for the connection of micro-grids to the utility grid. A comparison analysis with the conventional 2-level voltage source converter is proposed, including the following points: overall operation, power transformer, semiconductor losses, efficiency and DC protection. The results of the comparison are useful to evaluate the suitability of this converter for the proposed application.
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AC-DC Interleaved Modular Multilevel
Converter with Medium-Frequency
Isolation Transformer for DC
Micro-grids
D. R. Joca1,2, B. Džonlaga2, L. H. S. C. Barreto1, D. S. Oliveira1, J. C. Vannier2, and L.
Quéval2
1Department of Electrical Engineering, Federal University of Ceará, Fortaleza, Brazil
2Group of electrical engineering - Paris (GeePs), CNRS UMR 8507, CentraleSupélec, UPSud,
UPMC, Gif-sur-Yvette, France
___________________________________________________________________________
Abstract
This article presents the features of a 3-phase AC-DC interleaved modular
multilevel converter with medium-frequency transformer, that has been designed
for the connection of low voltage DC micro-grids (380 VDC) to the medium
voltage utility grid (13.8 kV). The converter meets the Brazilian grid code for the
connection of micro-grids to the utility grid. A comparison analysis with the
conventional 2-level voltage source converter is proposed, including the
following points: overall operation, power transformer, semiconductor losses,
efficiency and DC protection. The results of the comparison are useful to
evaluate the suitability of this converter for the proposed application.
___________________________________________________________________________
I. Introduction
In Brazil, the connection of distributed generation to the utility grid is regulated by the
PRODIST norm from the National Agency of Electric Energy (ANEEL) [1]. This norm states
that for a generator with 75 kW or higher: the connection must be made to the three-phase AC
low voltage grid (380 V) or to the AC medium voltage grid (13.8 kV); a coupling transformer
must be used; in islanded mode operation both voltage and frequency must be controlled; and
the power factor at the connection point must be higher than 0.92. For the connection of DC
micro-grids to the utility grid, there is no norm yet, but it is expected that a similar regulation
will be applied.
In this context, we are developing an AC-DC interleaved modular multilevel converter
(IMMC) with a medium-frequency isolation transformer for the connection of low voltage DC
micro-grids (380 VDC) to the medium voltage utility grid (13.8 kV) [2].
In this article, we demonstrate that the proposed converter meets all of the requirements above.
This is done by simulation considering a 100 kW converter with four modules per arm and a
13.8 kV/380 V isolation transformer operating at 10 kHz. Additionally, we compare this
converter with a conventional 2-level voltage source converter (VSC) in series with a 13.8
kV/380 V transformer operating at 50 Hz. The following points are considered: overall
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operation for a given sequence, power transformer size and weight, semiconductor losses,
overall efficiency, and DC fault protection. The novelty of this article lies in the comparison of
the two converters, and the demonstration of the DC fault protection of the AC-DC IMMC.
II. AC-DC IMMC
We are developing a 3-phase bidirectional AC-DC interleaved modular multilevel converter
(IMMC) with a medium-frequency isolation transformer for the connection of low voltage DC
micro-grids (380 VDC) to the medium voltage utility grid (13.8 kV) [2].
The considered three-phase AC-DC IMMC is shown in Fig. 1 with the parameters of Table I.
The AC grid is connected to the converter input inductor Lin. The interleaving inductor Lx//Ly
divides the current through the two legs. This helps to decrease the semiconductors efforts, and
thus, their conduction losses [3]. The interphase inductors L1//L2 and L3//L4 share the current
among the arms of the modular multilevel converter. The medium-frequency transformer
(MFT) connects the interleaved modular multilevel converter at the high-voltage side to a
full-bridge converter at the low-voltage side. This transformer provides galvanic isolation and
voltage adjustment. Each converter leg has two arms. Each arm has 4 identical IGBT-based
half-bridge modules. This modular topology allows the selection of low-cost and low-rating
devices, improves the reliability and reduced maintenance costs.
Figure 1 - Three-phase AC-DC interleaved modular multilevel converter with
medium-frequency isolation (adapted from [2]).
The control scheme is described in details in [2] and can be summarized according to the
converter side:
The high-voltage controller controls the power factor and balances the energy stored in
the converter arms. The arms voltage modulation is implemented with a space vector
algorithm. It modulates the voltage across the converter input inductor Lin in order to
supply the transformer primary voltage, and it determines the number of modules that
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must be turned ON in each leg. A capacitor voltage balancing algorithm is applied to all
the converter modules, and helps to minimize the circulating currents [2][6-7].
The low-voltage controller (full-bridge converter) controls the DC side power flow with
a selected harmonic elimination sinusoidal pulse-width modulation, which reduces the
transformer harmonic distortion.
TABLE I - CONVERTERS SPECIFICATIONS!
Parameters
AC-DC IMMC
2-level VSC
Nominal power
Input voltage
13.8 kV
380 V
Output voltage
Transformer
10 kHz, 13.8 kV/380 V
50 Hz, 13.8 kV/380 V
Converter input impedance
Lin = 7.2 mH
Lin = 0.14 mH, Rin = 0.04 (a)
Number of modules per arm
N = 4
-
Switching frequency
Module capacitor
Cm = 5.5µF / 11.5 kV
-
Output capacitor
Co = 3.6 mF(b)
Co = 100 mF(c)
(a) Lin = 0.1 pu [4], (b) Co calculated according to [5] for the AC-DC IMMC, (c) Co calculated according to [4] for
the 2-level VSC.
III. 2-level VSC
In order to evaluate the suitability of the AC-DC IMMC for the connection of low voltage DC
micro-grids (380 VDC) to the medium voltage utility grid (13.8 kV), we propose to compare its
performances to a more conventional 3-phase 2-level voltage source converter (VSC) in series
with a 13.8 kV/380 V transformer operating at 50 Hz (Fig. 2).
The considered three-phase 2-level VSC is shown in Fig. 2 with the parameters of Table I. The
2-level VSC is connected to the AC grid through a 50 Hz transformer and a tie reactor (Rin, Lin).
The VSC is an IGBT-based PWM converter. The objectives of the converter control are to keep
the DC-link voltage constant, and to actively support the grid by regulating the reactive power.
The converter sizing, control design and PI tuning are described in details in [4].
Figure 2 - Three-phase 2-level VSC simulation scheme.
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IV. Simulation and comparison
For the following simulations, the AC grid is modeled as a “strong” grid with a short-circuit
level of 30 MVA and a X/R ratio equal to 10 [8]. The DC grid is modeled as a resistance Ro in
parallel with an ideal current source. The simulation is performed using Matlab/Simulink
SimPowerSystems toolbox [9]. The comparison focus on the following points: overall
operation, power transformer, semiconductor losses, efficiency, and DC fault protection.
1. Overall operation
In order to evaluate and compare the performances of both AC-DC IMMC and 2-level VSC, we
consider the following sequence:
Startup at 50% of the nominal power in rectifier mode (from t = 0.05 s to t = 0.2 s)
Load step to 100% of the nominal power in rectifier mode (from t = 0.2 s to t = 0.4 s)
Power flow inversion to 100% of the nominal power in inverter mode (from t = 0.4 s to
t = 0.6 s) and back to rectifier mode (from t = 0.6 s to t = 0.8 s).
Simulation results are shown in Figs. 3 and 4.
Figure 3 - Simulation results of the AC-DC IMMC.
a) Startup: Initially, the converters are not connected to the AC grid and the DC grid voltage is
zero. For the AC-DC IMMC, the module capacitors are pre-charged. The value of the load at
the DC side is the half of the nominal value. The connection to the grid occurs at t = 0.05 s. Both
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converters are able to regulate the DC grid voltages at 380 VDC with 0.998 power factor. The
DC capacitor being larger for the 2-level VSC, the charging time is longer. Unlike the AC-DC
IMMC, note that for the 2-level VSC, the DC grid charging is uncontrolled at first (because of
the IGBT antiparallel diodes), leading to a large inrush AC grid current. To address this issue,
pre-charge circuits or other soft-start methods should be added [10].
b) Step load: At t = 0.2 s, the load is increased from 50% to 100% in rectifier mode. The AC
currents increase while the DC voltages are stabilized at 380 VDC with 0.999 power factor. The
AC grid regulation is achieved in about 10 ms for the 2-level VSC and about 100 ms for the
AC-DC IMMC. The difference is linked to the complexity of the IMMC and to the time
constant of the module capacitor voltage control [2]. The DC grid regulation is achieved in
about 10 ms for both converters.
c) Power flow inversion: At t = 0.4 s, the power flow is inverted to 100% of the nominal power
in inverter mode. This means that the converter is supplying active power from the DC grid to
the AC grid. The power flow is reversed again at t = 0.6 s, bringing the converters to the
previous state. The regulation time is similar to the load step event. This represents a worst-case
scenario and thus demonstrates that the proposed AC-DC IMMC controller is stable.
Figure 4 - Simulation results of the 2-level VSC.
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2. Power transformer characteristics
The AC-DC IMMC uses three single-phase 10 kHz power transformers (Fig. 1). To evaluate
the advantage in terms of weight and volume, we performed a preliminary design of the
transformers using the core geometry approach [11]. The 2-level VSC uses a conventional
three-phase 50 Hz transformer [12]. To simplify the comparison, we assume that both
transformers are dry type transformers with Fe Si 3% cores. The results are summarized in
Table II. The transformer volume decreases when the operation frequency increases. As a
result, the AC-DC IMMC transformer is one order of magnitude lighter and smaller than the
2-level VSC transformer. Note that the use of a core material adapted for kHz-range frequency
operation, such as amorphous Fe [13], could allow one to decrease even more the AC-DC
IMMC transformer footprint.
TABLE II - TRANSFORMER COMPARISON
2-level VSC
AC-DC IMMC
Power
100 kVA
3× 33.3 kVA(a)
Phase
3-phase
3× 1-phase
Cooling
dry type
dry type
Transformer ratio
13.8 kV/380 V
13.8 kV/380 V
Frequency
50 Hz
10 kHz
Core material
Fe Si 3%
Fe Si 3%(b,c)
Losses
1492 W(d)
3× 260 W(d)
Weight
600 kg
3× 8 kg(e)
Volume
0.298 m3
3× 0.002 m3(e)
(a) Assuming no harmonics, (b) lamination thickness 0.25 µm, (c) at 10 kHz, the peak magnetic flux density is
reduced to 0.22 T [13], (d) full load losses, (e) active materials only.
3. Semiconductor losses
The semiconductor losses are the sum of the individual IGBT and diode losses. We used the
method described in [14] to estimate the conduction losses and switching losses, for both
converters, in steady-state operation. For the AC-DC IMMC modules, we selected 15 kV/40 A
SiC n-IGBT [15]. For the AC-DC IMMC full-bridge and for the 2-level VSC, we selected 600
V/600 A IGBTs [16].
(a) AC-DC IMMC (b) 2-level VSC
Figure 5 - Semiconductor losses. The IGBT and diode losses are added together.
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The estimated switching and conduction losses are shown in Fig. 5. For the AC-DC IMMC
arms, the conduction losses are lower than the switching losses. This is due to the interleaving
inductors which reduces the current efforts through the converter arms [3]. For the AC-DC
IMMC full-bridge and the 2-level VSC, the conduction losses are higher than switching losses
because of the high output current on the low voltage side. Overall, the total semiconductor
losses are about 8 times larger for the AC-DC IMMC than for the 2-level VSC. This is not
surprising considering the fact that the AC-DC IMMC has 18 times more switches than the
2-level VSC.
4. Converter efficiency
Considering the transformer losses (Table II) and the semiconductor losses (Fig. 5), the
efficiencies of the AC-DC IMMC and the 2-level VSC are estimated to 88.2 % and 97.1 %,
respectively. These figures are probably optimistic, and more work is required to decrease the
AC-DC IMMC losses in order to make it competitive.
5. DC fault protection
In a DC fault event, the converters connected to the utility grid must ensure safe operation, and
quick recovery after the fault. In Fig. 6, the simulation results for both converters are presented
where neither of them has a dedicated protection circuit. At t = 0.2 s, a DC short circuit of 50 ms
happens. The AC-DC IMMC (Fig. 6a) input/output current is limited within safe margin: the
converter is inherently able to block the DC fault current. For the 2-level VSC (Fig. 6b), the AC
side current increases up to 2.5 pu during the fault. This is due to the uncontrolled current
flowing through the antiparallel diodes when the voltages drop. To address this issue, a specific
protection strategy should be considered [17].
(a) AC-DC IMMC (b) 2-level VSC
Figure 6 - Simulation results during a DC fault event.
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V. Conclusions
In this paper, we have presented the structure, operation and control of an AC-DC IMMC
designed for the grid connection of low voltage DC micro-grids (380 VDC) to the medium
voltage utility grid (13.8 kV). By comparing it with a more conventional 2-level VSC, we can
reach the following conclusions:
The AC-DC IMMC doesn’t require pre-charge circuits or soft-start methods at startup,
while the 2-level VSC does.
The proposed control method is stable, even during worst case scenario events, but the
response time of the high-voltage side control needs to be improved in future works.
By using a medium-frequency transformer in the AC-DC IMMC, one can achieve a
significant reduction of weight and volume, in comparison to the conventional 50 Hz
transformer used with the 2-level VSC.
For the considered operating point, the efficiency of the high-frequency transformer is
twice better than its 50 Hz counterpart.
The total semiconductor losses of the AC-DC IMMC are 8 times larger than the ones of
the 2-level VSC.
The AC-DC IMMC has 108 switches, while the 2-level VSC has only 6. The losses per
switch is therefore lower for the AC-DC IMMC.
The estimated full load efficiency of the AC-DC IMMC is 88.2 %, to be compared to
the 97.1 % of the 2-level VSC. To make this converter more competitive, one would
need to decrease the losses and therefore the number of switches.
During a DC-link short circuit, the AC-DC IMMC provides a fault current blocking
capability: the AC grid is not feeding the fault current as the 2-level VSC does.
In conclusion, this study demonstrates the feasibility of the proposed AC-DC IMMC with
medium-frequency isolation transformer for the grid connection of DC micro-grids. Despite a
high number of switches, it shows interesting features in relation to reliability. More work is
being done in order to fully explore the capability of this converter. Besides, the models
presented here are now being used to design a small-scale prototype.
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... Dans ce contexte, nous développons actuellement un convertisseur AC-DC modulaire multiniveaux entrelacé triphasé (IMMC) intégrant un transformateur de fréquence moyenne. Il est dimensionné pour connecter un micro-réseau à courant continu basse tension (380 V DC) au réseau électrique à courant alternatif moyenne tension (13.8 kV) [2,3]. D'une puissance de 100 kW, il comprend 4 modules par bras et un transformateur de rapport 13.8 kV/380 V opérant à 10 kHz. ...
... Le côté basse tension du transformateur est connecté à un pont plein. L'article [2] montre le fonctionnement du convertisseur pendant plusieurs événements. Cette topologie permet de bloquer le courant de court-circuit côté DC et par rapport au VSC à 2 niveaux, elle a moins de pertes per interrupteur et la taille du transformateur réduite. ...
... Nous supposons que le convertisseur est connecté à un réseau AC fort [2,6]. Le réseau DC est modélisé par une résistance Ro en parallèle avec un condensateur Co et une source de courant. ...
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This article presents the overall operation of an AC-DC interleaved modular multilevel converter (IMMC) designed to connect a DC micro-grid of 380 V to the utility grid of 13.8 kV. Aiming at improving its control, we consider two different PI controller tuning methods: the factor K method and the symmetrical optimum method. We compare the behavior of the converter for the following events: start-up with two different references, operation as a rectifier and operation as an inverter.
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