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Closed-Loop Black Start-Up of Dual-Active-Bridge Converter With Boosted Dynamics and Soft-Switching Operation


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This article proposes a closed-loop black start-up control for the dual-active bridge (DAB) converter. With a simple combination of the extended phase-shift and triple phase-shift modulation schemes, the DAB converter can start up with the maximum allowable pre-charge current while realizing soft-switching operation in the trapezoidal current mode simultaneously. Compared to the conventional start-up procedure that usually requires multiple steps of operation and parameter fine-tuning, the proposed start-up method can be implemented simply into the existing closed-loop controller and can shorten the start-up transients of the DAB converter significantly. The presented methods are validated experimentally on a downscaled DAB converter prototype.
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Closed-Loop Black Start-up of Dual-Active Bridge Converter
with Boosted Dynamics and Soft-Switching Operation
Jingxin Hu, Member, IEEE, Shenghui Cui, Member, IEEE, Rik W. De Doncker, Fellow, IEEE
Abstract—This article proposes a closed-loop black start-up
control for the dual-active bridge (DAB) converter. With a simple
combination of the extended phase-shift and triple phase-shift
modulation schemes, the DAB converter can start up with the
maximum allowable pre-charge current while realizing soft-
switching operation in the trapezoidal current mode simulta-
neously. Compared to the conventional start-up procedure that
usually requires multiple steps of operation and parameter fine-
tuning, the proposed start-up method can be implemented simply
into the existing closed-loop controller and can shorten the start-
up transients of the DAB converter significantly. The presented
methods are validated experimentally on a downscaled DAB
converter prototype.
Keywords—Dual-active bridge, black start-up, closed-loop con-
trol, soft switching, inrush current.
The dual-active bridge (DAB) is a popular dc-dc converter
topology to transfer a bidirectional power flow over a wide
voltage range with galvanic isolation and soft-switching op-
eration [1]. Since the DAB converter is widely used as a dc
transformer in grid and industrial applications, the black start-
up is a crucial function to restore the system after faults or
maintenance and enhance the system stability and availability
In the black start-up procedure, the fully discharged output
dc capacitor of the DAB converter needs to be pre-charged
from the input side. However, when directly applying the
steady-state switching pattern, e.g. the single phase-shift (SPS)
modulation [1], a high inrush current occurs in the ac link
which results in excessive thermal stress and potential failures
in the power semiconductor devices [3]. To address this
issue, soft start-up procedures are proposed in [4]–[6] which
first operate the output bridge only as a diode rectifier and
increase the inner phase-shift of the input bridge gradually
in an open-loop manner. When the output dc capacitor is
naturally charged to the maximum rectification voltage, the
output bridge starts switching, and the closed-loop control is
enabled to regulate the output voltage to the final reference
value gradually. To ensure a suppressed inrush current in the
whole start-up procedure, the aforementioned method requires
a careful manual fine-tuning of the parameters under different
load conditions [5], [6], i.e. the ramp slope-rate of both the
inner phase-shift and the reference voltage. This makes such
methods less generic and difficult to optimize. Moreover, as
This work is supported by European Union’s Horizon 2020 research
and innovation programme under grant agreement No. 957788, project HY-
PERRIDE, and the Federal Ministry of Education and Research (BMBF,
FKZ03SF0490), Flexible Electrical Networks (FEN) Research Campus. (Cor-
responding author: Shenghui Cui.)
J. Hu, S. Cui and R. W. De Doncker are with the Institute for Power
Generation and Storage Systems at E.ON ERC and the FEN Research Cam-
pus, RWTH Aachen University, Aachen, Germany. (e-mail: jhu@eonerc.rwth-;; post
Fig. 1: Circuit diagram of single-phase DAB converter.
Vs - case 1 Vs - case 2
ip - case 1
ip - case 2
open-loop ramp closed-loop ramp
Fig. 2: Key waveforms of the state-of-the-art soft start-up procedure (case 1:
high slope ramp-rate; case 2: low slope ramp-rate).
the maximum peak value of the inrush current is only limited
but not fully utilized in the whole procedure, those multi-step
approaches have an inherent limitation in the start-up speed.
In [3], the dual phase-shift (DPS) modulation is proposed to
reduce the inrush current during start-up, but further analysis
and evaluation are still needed. In [7], the extended phase-
shift (EPS) modulation is also considered to increase the pre-
charge current at the start-up in an open-loop manner, but the
mode selection and parameter calculation are rather complex.
Besides, multiple steps of operation are also required in the
start-up procedure.
In this article, a dedicated EPS mode is proposed which is
particularly suitable for a low output voltage operation with
a high output current. Combined with the triple phase-shift
(TPS) modulation [8], the DAB converter can realize a closed-
loop start-up control with a fully regulated inrush current, a
boosted start-up speed and soft-switching operation.
The single-phase DAB consists of two H-bridges with ac
voltages denoted by vAB,vCD and a high-frequency trans-
former with a leakage inductance Lσand a turns ratio Ntr as
depicted in Fig. 1. As illustrated in Fig. 2, the state-of-the-art
soft start-up procedure is generally divided into two steps.
The first step is known as the open-loop ramp phase, where
only the gate signals of S1S4in the input bridge are
activated. The inner phase-shift ratio of the input H-bridge
Dpincreases from 0 to 0.5 linearly with a slope ramp-rate
0Ts/2 Ts
0Ts/2 Ts
(a) (b)
0Ts/2 Ts
Q1, Q4Q2, Q3
Fig. 3: Switching patterns and waveforms of the selected modulation modes.
(a) TPS-TCM (d<1). (b) TPS-TZM. (c) Dedicated EPS-TZM.
dt. The typical ac voltages and current waveforms are also
shown in Fig. 2. The inner phase-shift determines the duty
ratio of vAB, which limits the peak value of the transformer
current when the output voltage is still low. Meanwhile, the
resulting triangular transformer current flows into the output
diode rectifier and charges the output capacitor until the output
voltage Vsreaches the input one Vp(a turns ratio Ntr =1is
assumed hereinafter without loss of generality). The dDp
dtis the
key design parameter in this stage. As shown in Fig. 2, a lower
dtin case 2 results in a lower peak inrush current but a longer
ramp time of the output dc voltage. However, considering the
non-linear slope of Vsespecially when a load is connected, a
generic analytical design of dDp
dtfor different load conditions
is challenging. This usually leads to a conservative choice of
dtand a long start-up duration.
The second step, known as the closed-loop ramp phase,
starts when Vsis equal or sufficiently close to Vp. The gate
signals of Q1Q4in the output bridge are also activated, and
the closed-loop voltage control is enabled to regulate Vs. If the
final reference value V
sis different from Vp, a ramp slope-rate
dtis also required to limit the peak current as depicted in
Fig. 2, which is also difficult to calculate analytically.
Therefore, the state-of-the-art soft start-up method requires
a careful fine-tuning of both dDp
dtand dV
dtin simulations or
even experiments to precisely limit the inrush current. It lacks
generality for different load conditions and additional degree
of freedom to optimize the start-up time.
The DAB converter has normally three control variables,
which are the inner phase-shift ratios Dp,Dsfor the input
bridge and the output bridge respectively and the outer phase-
shift ratio Dϕbetween two bridges as depicted in Fig. 3. To
minimize the start-up time, a operation mode is desired to
deliver the maximum output current with a limited transformer
peak current under the low-output-voltage condition.
(Ip,lim = 1.3 pu)
(Ip,lim = 1.3 pu)
(d < 1)
(d > 1)
Start-up trajectory
with Ip,lim = 1.3 pu
Fig. 4: Output current capability of different modulation schemes.
As already mentioned, the SPS modulation is not suitable
for the soft start-up operation due to the large inrush current,
although it can deliver the highest possible output current,
i.e. Vp
8f Lσ
=1 p.u.[1], as depicted in Fig. 4. On the
other hand, the triple phase-shift with the triangular-current
modulation (TPS-TCM) as depicted in Fig. 3(a) can limit the
inrush current, which is similar to the open-loop ramp phase
in the conventional soft start-up method. But the maximum
output current of the TPS-TCM mode is limited when the
voltage ratio d=NtrVs
Vpis very low as shown in Fig. 4. As
an extension of the TPS-TCM mode [8], the triple phase-
shift with the trapezoidal-current modulation (TPS-TZM) as
depicted in Fig. 3(b) can further increase the maximum output
current, but the difference is only remarkable when dis above
0.4 as depicted in Fig. 4. The analytical expression of the
output current for the aforementioned modulation modes can
be found in Table I [8].
To fill in the gap in the range of a very low d, the extended
phase-shift with the trapezoidal current modulation (EPS-
TZM) is proposed for d<1as shown in Fig. 3(c). As another
form of extension from the TPS-TCM mode, the EPS-TZM
fixes Ds=0.5to maximize the output current, and adjusts Dϕ
and Dpto limit the peak current while realizing zero-current
switching (ZCS) for the output bridge. The switches in the
input bridge can realize zero-voltage switching (ZVS). The
transformer current is calculated as
for 0 tt1,
ip(t1) − dVp(tt1)
for t1tt2,
ip(t2) − (1+d)Vp(tt2)
for t2tTs
where t1=DpTsDϕTsand t2=0.5TsDϕTs.Ts=1
the switching period, where fis the switching frequency.
Assigning ip(0)=ip(T s
The transformer peak current is expressed as
ippk =ip(t1)=
2f Lσ
TABLE I: Expressions of the output current for related modulation modes.
Mode DpDsIs,out Is,out,max Is,out,min
TPS-TCM (d<1)2d Dϕ
f Lσ(1d)
4f Lσ0
TPS-TCM (d>1)2d Dϕ
f Lσ(d1)
4f Lσd20
TPS-TZM d(12Dϕ)
4f Lσd(1+d)2
4f Lσ(1+d+d2)
4f Lσfor d<1or (d1)Vp
4f Lσd2for d>1
SPS 0.5 0.5 VpDϕ(12Dϕ)
f Lσ
8f Lσ0
EPS-TZM 2Dϕ+0.5d0.5 Vp[−8D2
4f Lσ
8f Lσ
4f Lσ
Mode selection &
Calculate control
Fig. 5: Generic closed-loop control block diagram of the DAB converter.
The output current of the EPS-TZM mode is expressed as
is,out =
4f Lσ
From (4), the maximum and minimum values of the output
current are derived
is,out,max =
8f Lσ
at Dϕ=1d
is,out,min =
4f Lσ
at Dϕ=0.
According to (5), is,out,max of the EPS-TZM mode decreases
quadratically with an increasing d. As depicted in Fig.4,
the EPS-TZM mode can deliver a significantly higher output
current than the TPS-TZM mode when d<0.6, which fills in
the gap under the condition of a very low output-voltage.
Thereafter, the maximum allowable pre-charge current with
a limited current stress can be conveniently determined in the
following steps. 1) Identify the nominal current stress ˆ
at the nominal voltage and power rating. 2) Pre-define the
maximum allowable current stress ˆ
Ip,lim ˆ
Ip,nom for the start-
up procedure depending on the current rating, safe operating
area (SOA) and the cooling capacity of the devices. The
potential maximum transformer peak current of the EPS-TZM
mode can be calculated by (3) with Dϕ=1d
4. When ippk
needs to be limited to a lower value ˆ
Ip,lim, steps 3) and 4) are
conducted. 3) Substitute ˆ
Ip,lim into (3) to obtain the maximum
allowable Dϕ,max of the EPS-TZM mode. 4) Substitute Dϕ,max
into (4) to obtain the maximum allowable pre-charge current
of the EPS-TZM mode. Similar procedure can be applied to
determine the maximum allowable pre-charge/output current
of the TPS-TZM mode.
As an example, the maximum allowable is,out of the EPS-
TZM and TPS-TZM modes for ˆ
Ip,lim =1.3 p.u.are depicted in
Fig. 4 (marked with *). Following the upper boundary of these
two modes constrained by Ip,lim, the DAB converter can start
up from zero to the nominal output voltage with the maximum
allowable output current. This can not only limit the inrush
current but also minimize the start-up time. Moreover, due to
the ZCS nature of the trapezoidal current, a smooth transition
of the transformer current is realized from the EPS-TZM to
the TPS-TZM mode with soft-switching operation in the whole
start-up procedure.
More importantly, the proposed EPS-TZM mode can be
generically implemented in the closed-loop controller together
with other modulation modes such as TPS-TZM and TPS-
TCM as depicted in Fig. 5. A standard voltage PI regulator
is adopted to produce the reference output current. With
the measured voltage ratio, the maximum allowable output
currents of the EPS-TZM, TPS-TCM and TPS-TZM modes
with a limited peak current ˆ
Ip,lim are then calculated and
compared with the reference output current to select the proper
mode as referred to Fig. 4. An anti-windup is required to
limit the output reference current and avoid a saturation of
the integrator. Due to a large voltage error in the beginning of
the black start-up, the reference output current will be instan-
taneously set to the maximum allowable output current of the
EPS-TZM mode. With an increasing d, the modulation mode
is automatically transited to TPS-TZM or TPS-TCM until
the reference output voltage is reached. Benefited from the
closed-loop control implementation, the proposed black start-
up method is inherently adaptive to different load conditions
which avoids complex process of parameter tuning as in the
conventional method.
Experiments are preformed on a down-scaled DAB con-
verter prototype (as shown in Fig. 6) to compare the black
start-up performance of the conventional and the proposed
methods under different load conditions. Detailed parameters
of the DAB converter are given in Table II. It is worth
mentioning that the potential maximum transformer peak
current during the black start-up procedure is Vp
4f Lσ
=34.5 A
at Vs=0 V with Dp=Ds=0and Dϕ=0.25, which is too
large for the designed prototype. Notice that the peak current
under the nominal voltage and power condition is only about
12 A. Therefore, in the experiments, the maximum transformer
peak current is always limited to 15 A, which is approximately
Fig. 6: DAB converter prototype.
Input dc voltage 80 V
Output dc voltage reference 90 V
Total leakage inductance 29 µH
Transformer turns ratio 1:1
Switching frequency 20 kHz
Output capacitance 2 mF
Proportional gain 1.244
Integral gain 39.081
TABLE II: Experimental parameters.
tstart = 21.2ms
ip-pk = 15.0A
vAB 60V/div
vCD 60V/div
ip 10A/div
Vs 20V/div
Fig. 7: Measured black start-up waveforms under the no-load condition with
a maximum transformer peak current of 15 A. (a) Conventional method. (b)
Proposed method.
tstart = 41.5ms
ip-pk = 15.0A
vAB 60V/div
vCD 60V/div
ip 10A/div
Vs 20V/div
tstart = 93.4ms
ip-pk = 15.2A
vAB 60V/div
vCD 60V/div
ip 10A/div
Vs 20V/div
Open-loop ramp Closed-loop ramp
Fig. 8: Measured black start-up waveforms with a load resistor Rload =13.5
(600 W at Vs=90 V) and a maximum transformer peak current of 15 A. (a)
Conventional method. (b) Proposed method.
25 % higher than the steady-state peak current for a nominal
power of 600 W at Vs=90 V.
Fig. 7 depicts the measured black start-up waveforms un-
der the no-load condition. For the conventional soft start-up
method, the slope ramp-rates of Dpand V
sare carefully tuned
to dDp
dt=0.022/ms and dV
dt=5 V/ms respectively to limit
the maximum peak current to 15 A. This results in a start-
up period of tstart =37.6 ms and a maximum peak current of
15.3 A as shown in Fig. 7(a). When the proposed method is
applied, Vsramps to the nominal voltage of 90 V within only
21.2 ms as shown in Fig. 7(b), which is significantly reduced
by 43.6 % compared to the conventional method. Moreover,
the transformer peak current in the proposed method is not
only limited to 15 A but also maintained close to the maximum
allowable peak current during the whole start-up procedure,
vAB 60V/div
vCD 60V/div
ip 10A/div
Vs 20V/div
ip-pk = 14.9A EPS-TZM
ip-pk = 15.0A
S3 ZVS-on
S1 ZVS-on
Q2,Q3 ZCS-off
Q1,Q4 ZCS-off
(a) (b)
ip-pk = 14.3A
(c) (d)
S3 ZVS-on
S1 ZVS-on
Q1 ZVS-on
Q4 ZCS-off
ip-pk = 12.1A
Fig. 9: Zoomed-in waveforms of the proposed black start-up control with
Rload =13.5in Fig. 8(b). (a) Initial EPS-TZM waveforms at Vs=0 V.
(b) EPS-TZM mode at Vs=35 V. (c) Mode transition from EPS-TZM to
TPS-TZM at Vs=52 V. (d) TPS-TZM mode at Vs=90 V.
Fig. 10: Measured no-load start-up trajectories (is,out against Vs) of the DAB
converter with the conventional- and the proposed method.
which is distinguished from the conventional method.
Fig. 8 depicts the measured black start-up waveforms when
a load resistor Rload =13.5(Pnom =600 W) is connected.
For the conventional method, the slope ramp-rates are re-tuned
to dDp
dt=0.0125/ms and dV
dt=0.8 V/ms for the same
maximum peak current, which result in a start-up period of
tstart =93.4 ms as shown in Fig. 8(a). It can be noticed that the
slope-rate of Vsbecomes very low in the end of the open-loop
ramp stage due to a relatively large load current. Therefore,
to minimize the start-up time, the closed-loop ramp stage has
already started when Vs64 V, i.e. 0.8Vp. On the other hand,
when the proposed method is applied, the start-up period is
41.5 ms with even a 55.6 % reduction as shown in Fig. 8(b).
Furthermore, Fig. 9 shows zoomed-in waveforms of the
proposed method with Rload =13.5. In Fig. 9(a), the DAB
converter starts up in the EPS-TZM mode with ippk =14.9 A
at Vs=0 V. In Fig. 9(b), the DAB converter continues to
operate in the EPS-TZM mode with the maximum allowable
peak current. The ZVS and ZCS are realized for the input-
and output bridge, respectively. In Fig. 9(c), when Vsincreases
to 52 V, a mode transition occurs from the EPS-TZM to the
TPS-TZM mode with a smooth transformer current. The DAB
converter continues to operate in the TPS-TZM mode until
reaching the steady state at Vs=90 V as shown in Fig. 9(d).
Enabled by the combination of the EPS-TZM and TPS-TZM
modes, the DAB converter realizes ZVS and ZCS operation
during the whole start-up procedure.
Fig. 10 shows a comparison of the measured start-up trajec-
tory, i.e. is,out against Vs, under the no-load condition between
the conventional and the proposed methods. It is validated that
a higher output current is delivered by the proposed method
at almost every value of the output dc voltage.
This article introduces a novel black start-up control of the
DAB converter, which can be generically implemented into
the closed-loop controller without complex tuning process.
The proposed control is enabled by the EPS-TZM mode
which is able to deliver a high output current with a limited
peak value at a very low voltage ratio. Combining the EPS-
TZM and TPS-TZM mode, the DAB converter can start up
with the maximum allowable output current while maintaining
soft-switching operation during the whole start-up procedure.
Experiments validate the effectiveness of the proposed method,
and demonstrate a significant reduction of the start-up time up
to 55.6 % compared to the state-of-the-art method.
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... In [8], [9], an open-loop start-up method using the inner phaseshift ratios is proposed to limit the inrush current. In [10], [11], a novel closed-loop start-up method, which uses the combination of the extended phase-shift (EPS) and triple phase-shift (TPS) modulation schemes, was proposed to speed up the start-up process with the maximum allowable current. That method employs EPS-TZM and TPS-TZM modulations which associate phase-shift ratios and reduce the complexity of calculation. ...
... 1) Assuming 0 ≤ D 1 ≤ D 2 ≤ 1, the combination of D 1 and D 2 can be calculated by (10). If the assumption is not satisfied, step 2) is conducted. ...
... In addition, the snubber circuit should be adopted to reduce the switching-off loss due to the hard turn-off behavior, and the soft switching-on performance enables the energy in the snubber capacitor to be fed back to the dc bus [16]. However, the serious straight through of bridge arms and direct discharge of snubber capacitors will occur once the soft turn-on behavior of the IGCT-HDCT is lost [17] Thus, it is crucial to ensure the soft switching-on operation of the IGCT-HDCT over the full power range [18]. ...
Full-text available
Medium-voltage large-capacity dc transformer is the key energy conversion link for building intelligent distribution grid. Increasing the voltage and capacity of single module in dc transformer can effectively improve the power density and reduce the system cost. Aiming at the IGCT-based high-capacity dc transformer (IGCT-HDCT), the asymmetric topology design and quasi-zero-loss switching composite modulation are proposed to achieve the construction of high efficiency and power density dc transformer. Through designing the snubber circuit and phase shift ratio trajectory ingeniously, the zero-voltage switching-on and zero-current switching-off operation of the IGCT-HDCT can be maintained, which greatly reduces the switching loss and gives full play to the extremely low on-state loss characteristics of IGCT. Meanwhile, the full-range soft-switching operation (from no-load to full-load) can be achieved in the composite modulation without adding any auxiliary component. And the design of the snubber circuit is greatly simplified, which can not only effectively limit the dv/dt and di/dt of power semiconductors, but also improves the power density and operating efficiency of IGCT-HDCT. Finally, the theoretical analysis of the proposed system solution is verified by building a 1.25MW IGCT-HDCT engineering prototype, and the maximum operating efficiency reaches 98.6%∼99.1%.
... Five-level control is one of the most advantageous control schemes to improve the performance (e.g., efficiency) of the NPC-based DAB converters [4], [5]. However, the capacitor voltage imbalance is commonly seen with this control scheme when the gate-driving signals fail to synchronize due to asymmetry in the pulsewidth-modulator in the microcontroller [6]. ...
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Capacitor voltage balancing is of importance in the neutral-point-clamped (NPC)-based dual-active-bridge (DAB) converters. Most of traditional voltage balancing methods adopt the transformer current models in the balancing process, as the direction of the neutral-point current is affected by the transformer current polarity. However, this approach requires heavy and repetitive offline pre-calculation for the transformer current polarity under various operating modes, and thus, leading to complicated implementation. To overcome this, a model-free voltage balancing scheme based on a fixed-switching-state (FSS) method is proposed in this letter. Two switching states are employed during the voltage balancing process in the proposed method, where the direction of the neutral-point current is independent on the transformer current polarity. Hence, the implementation can be simplified without the pre-calculation of transformer current polarity. Furthermore, the model-free feature of the proposed method is more robust against parameter variations or operating mode changes. Experimental tests are performed, which verify that the proposed method can achieve voltage balancing in terms of easy implementation and fast dynamics.
High power dual active bridge (DAB) converters are widely adopted in DC and hybrid AC/DC micro-grids, for which fast start-up dynamics of DAB converters is a key requirement. Conventional start-up methods usually require complex operation stages, and a time-consuming parameter-tuning process, resulting in slow dynamics. In this work, we propose a new simple and optimal start-up method for DAB converters. Major contributions lie in three-folds . (i) We theoretically analyze the transmission power and the peak inductor current of extended phase-shift modulation. (ii) A new Lagrange Multiplier method is proposed to solve the optimal phase-shift ratios of the underlying control problem, resulting in very fast control dynamics at maximum power transmission, fully respecting the system permissible current limit. (iii) An effective and simple strategy by solely manipulating the gate signals in the first switching period is newly proposed to eliminate the magnetic bias. The proposed method avoids complex parameter-tuning procedures. Experimental results demonstrate the effectiveness of the proposed method. Results confirm that with the proposed solution a much faster start-up process with a controllable inductor current without DC bias is achieved, in comparison with the recently reported start-up methods.
The dual active bridge (DAB) converter is widely used in renewable energy power generation systems with wide input voltage characteristics. An inappropriate duty cycle will lead to a larger inductor RMS current and low efficiency. In this paper, an efficiency-oriented optimized triple-phase-shift (OTPS) scheme is proposed for the dual active bridge (DAB) converter that can reduce the inductor root-mean-square (RMS) current for a wide input voltage and realize zero voltage switching (ZVS). The ZVS condition contains the direction and amplitude of the inductor current to make the ZVS area more accurate. In addition, the OTPS scheme also has the capability of voltage balancing without additional voltage equalizers under unbalanced load. The influence of deadtime on voltage balance is analyzed and a voltage balancing scheme with compensation of the duty cycle is proposed. To reduce resource occupation and operation time, an on-line implementation scheme of variable parameter control (VPC) based on field programable gate array (FPGA) is proposed. Without a look-up table, the sum of the operation time of the control module and modulation module is only 0.66 $\mu$ s, and the required memory bits are only 459k. Both operation time and memory bits are reduced by more than 90% compared to existing literature. Finally, the whole proposed process is verified by the experimental results.
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Power electronics-based three-stage smart transformers (ST) can be seriously damaged by inrush currents and overvoltages during the start-up phase if the control of the stages is not correctly coordinated. Hence, it is crucial to design properly the start-up procedure, especially in case of modular architectures with distributed DC-links. The design of the start-up procedure depends on the ST power stages topologies, their control systems and the operation modes. This work proposes a soft-shift start modulation technique that allows to limit the inrush current in the DC/DC isolation stage during the DC-link capacitors pre-charging. A fast voltage balancing control, performed by the DC/DC isolation stage, is introduced to avoid overvoltages and unbalanced voltage conditions among the different power cells. Under the proposed method, fast control dynamics is guaranteed thanks to the high frequency bandwidth of the DC/DC isolation stage converters. Theoretical analysis, based on a detailed small signal model of the ST, and simulations are used to demonstrate the principle of the operation. Experimental results, carried out in a ST prototype, confirm the performances of proposed solution in realizing a smooth start-up without voltage/current overshoots.
This letter is focused on developing a start-up scheme for a three-stage solid-state transformer that includes a high-frequency transformer. The proposed scheme is aiming at minimizing the high-frequency transformer current during the start-up transient. As a result, the input-inrush current is also eliminated. The scheme is implemented with no extra cost by synchronizing the start-up of rectifier stage and dc-dc converter stage. The scheme is compared with other start-up methods. The theoretical analysis and experimental results are provided to verify the proposed start-up scheme.
The derivation of an accurate small-signal model for a galvanically isolated, bidirectional dc-dc converter and the implementation of a corresponding controller on a DSP as well as key methods and functions required for the digital implementation are detailed in this paper. The investigated dc-dc converter, an automotive dual active bridge (DAB) system, enables power transfer between a low-voltage port (ranging from 11 to 16 V) and an HV port (240 to 450 V). The nominal power rating is 2 kW. The developed small-signal model yields highly accurate results for the DAB system, but the proposed modeling procedure could also be applied to arbitrary resonant power converters with unidirectional or bidirectional power transfer.
This paper proposes a novel dual-phase-shift (DPS) control strategy for a dual-active-bridge isolated bidirectional DC-DC converter. The proposed DPS control consists of a phase shift between the primary and secondary voltages of the isolation transformer, and a phase shift between the gate signals of the diagonal switches of each H-bridge. Simulation on a 600-V/5-kW prototype shows that the DPS control has excellent dynamic and static performance compared to the traditional phase-shift control (single phase shift). In this paper, the concept of ldquoreactive powerrdquo is defined, and the corresponding equations are derived for isolated bidirectional DC-DC converters. It is shown that the reactive power in traditional phase-shift control is inherent, and is the main factor contributing to large peak current and large system loss. The DPS control can eliminate reactive power in isolated bidirectional DC-DC converters. In addition, the DPS control can decrease the peak inrush current and steady-state current, improve system efficiency, increase system power capability (by 33%), and minimize the output capacitance as compared to the traditional phase-shift control. The soft-switching range and the influence of short-time-scale factors, such as deadband and system-level safe operation area, are also discussed in detail. Under certain operation conditions, deadband compensation can be implemented easily in the DPS control without a current sensor.
Three DC/DC converter topologies suitable for high-power-density high-power applications are presented. All three circuits operate in a soft-switched manner, making possible a reduction in device switching losses and an increase in switching frequency. The three-phase dual-bridge converter proposed is shown to have the most favorable characteristics. This converter consists of two three-phase inverter stages operating in a high-frequency six-step mode. In contrast to existing single-phase AC-link DC/DC converters, lower turn-off peak currents in the power devices and lower RMS current ratings for both the input and output filter capacitors are obtained. This is in addition to smaller filter element values due to the higher-frequency content of the input and output waveforms. Furthermore, the use of a three-phase symmetrical transformer instead of single-phase transformers and a better utilization of the available apparent power of the transformer (as a consequence of the controlled output inverter) significantly increase the power density attainable
Modulation and dynamic control of intelligent dual-active-bridge converter based substations for flexible dc grids
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J. Hu, "Modulation and dynamic control of intelligent dual-active-bridge converter based substations for flexible dc grids," Ph.D. dissertation, E.ON Energy Research Center, RWTH Aachen University, Aachen, 2019.