Comparison of Three Topologies and Controls of a Hybrid Energy Storage
System for Microgrids
A. Etxeberriaa,b , I. Vechiua , H. Camblonga,c , J.-M. Vinassab
a ESTIA-Recherche
b IMS
laboratory, Ecole Supérieure des Technologies Industrielles Avancées, 64210 Bidart, France
laboratory, CNRS UMR 5218-ENSEIRB, Université Bordeaux 1, 33405 Talence-Cedex, France
c University of Basque Country (E.U.P.-D), 20018 Donostia-San Sebastian, Spain
Abstract
A high penetration rate of the Renewable Energy Sources (RES) can create stability, reliability and power
quality problems in the main electrical grid. The microgrid should be a feasible alternative to solve these
issues. As it is a weak electrical grid, the microgrid is very sensitive to any variation and consequently an
Energy Storage System (ESS) is necessary to reduce their effect. A microgrid needs an ESS with high energy
and power densities, but none of the currently available storage technologies satisfies both requirements at
the same time. Consequently, it is necessary to associate more than one storage technology creating a Hybrid
Energy Storage System (HESS).
The objective of this work is to compare by means of simulations three different topologies that are used
to control a HESS formed by a SuperCapacitor (SC) bank and a Vanadium Redox Battery (VRB). The
compared topologies are the parallel active topology, the floating topology and the Three-Level Neutral
Point Clamped (3LNPC) converter topology. The analysed microgrid is formed by the mentioned HESS
and different renewable energy sources. This microgrid is interconnected to a weak grid based on a Diesel
Generator (DG). Two case studies have been analysed: the effect of the RES while the microgrid is connected
to the weak grid and the effect of the disconnection of the microgrid from the weak grid. The different
topologies are compared analysing their capability of facing the mentioned variations, their power losses and
their Total Harmonic Distortion (THD). The Matlab/Simulink software and the SimPowerSystems toolbox
have been used to carry out the simulations.
Keywords: energy management, energy storage, microgrid, power converters, renewable energy
1. Introduction
sequently difficult to control. Due to this drawback,
a high penetration of the RES can create stability,
reliability and power quality problems in the main
The use of the Renewable Energy Sources (RES)
electrical grid. Thus, an optimum way of integrating
is continuously increasing due to the global climate
the energy obtained from the RES must be designed.
change, the limitation of the fossil fuels and the soThe microgrid is being analysed as a solution to
cial awareness concerning this subject. The energy
this
problem. A microgrid is a system that has at
obtained from the RES is clean and creates no polluleast
one distributed energy resource (renewable or
tion, but on the other hand it is stochastic and connot), energy storage systems, power conversion systems, control systems and loads [1](see Figure 1). Its
Email address: a.etxeberria@estia.fr (A. Etxeberria)
main characteristic is its ability to work not only conPreprint submitted to Energy Conversion and Management
May 27, 2013
nected, but also disconnected from the main electrical reaction rate [8].
grid.
The use of a supercapacitor in parallel with the
battery allows reducing the power rating of the VRB
and thus also the cost. The supercapacitor stores the
energy in electrical form, without converting it into
any other kind of energy in order to save it. The most
Wind Turbine
important advantages of a supercapacitor are its very
high efficiency (95%), very high power density (up to
Phot ovolt aic Panel
10 000W/kg), its tolerance to have deep discharges,
and its very long life-cycle (500 000 cycles at 100%
Diesel
depth-of-discharge) [9]. However, its energy density
Generat or
is very low and it has a high self-discharge current
Supervisory syst em
Hybrid Energy
(5% per day [3]). Thus, its use is not oriented for
DC bus
St orage Syst em
Power conversion syst em
long-term applications.
St at ic swit ch
The association of a SC and a VRB permits to
Load
take advantage of the characteristics of both ESSs
obtaining a high energy density, high power density,
Figure 1: Schematic representation of a microgrid.
high life-cycle and high efficiency HESS.
This paper compares three different power elecAn Energy Storage System (ESS) is usually nec- tronics topologies and the associated controls that
essary in a microgrid to maintain the power and en- can be used to manage the HESS: the parallel conergy balance as well as to improve the power quality. nection of the ESSs using a DC/DC converter to conNowadays there are many different types of storage trol each storage device [10], a topology where the
technologies [2, 3], but unfortunately none of them VRB is directly connected to the DC bus [6] and a
satisfies the requirements of the microgrid applica- topology where a Three-Level Neutral Point Clamped
tion: an ESS must have a high power density in order (3LNPC) converter is used to interface both ESSs
to face fast power variations, and at the same time [11]. The objective of this comparison is to analyse
it must have a high energy density to give autonomy the flexibility, the power losses and the THD of the
to the microgrid. For that reason, it is necessary to different topologies in two case studies. The operaassociate more than one storage technology creating tion of a microgrid formed by the HESS and different
a Hybrid Energy Storage System (HESS) [4].
RES has been analysed when it is (dis)connected to a
In this work a HESS based on the association of a weak electric grid based on a Diesel Generator (DG)
Vanadium Redox Battery (VRB), as long-term stor- as shown in Figure 1. The first case study analyses
age device, and a SuperCapacitor (SC), as a short- the operation of the microgrid when it is connected to
term storage device, is investigated.
the main weak grid and the second case study analyThe VRB is a flow battery which has independent ses the disconnection of the microgrid from the weak
energy and power densities. It has a long life-cycle, grid. The simulations have been carried out using the
up to 10 000 charge/discharge cycles [5]. The VRB Matlab/Simulink software and the SimPowerSystems
has theoretically no depth-of-discharge limitation and toolbox.
a good efficiency, between 75-85% [2, 6]. Although
The article is divided as follows: in Section 2 the
the electrochemical response time of the VRB is un- modelling of the two storage devices is shown. Secder 0.5ms [7], the response time of the VRB is limited tion 3 explains the topologies that have been comby the flow of the electrolyte, which is controlled by pared, with the advantages and disadvantages of each
pumps. As the electrochemical reaction occurs, it is one of them. Section 4 shows the simulation results.
necessary to introduce new active species in the stack, Finally, Section 5 compares the obtained results and
and therefore the flow rate must be adapted to each summarises the main conclusions of this work.
2
2. Modelling
dynamic model only the electrode capacitance is represented [13]. The electrical equivalent circuit of the
VRB that has been used is shown in Figure 3.
This section shows the dynamic models of the two
ESSs that have been used during simulations. The
complexity of the dynamic models has been chosen
according with the investigated phenomena.
Celectrodes
2.1. Supercapacitor
The dynamic model that has been used to represent the SC is shown in Figure 2.
Vvrb
Rfixed
Vstack
I pump
Rreactor
Rresistive
Rsc
Csc
Figure 3: Dynamic model of the VRB [13].
Figure 2: Dynamic model of the supercapacitor.
The SC is represented using a constant capacitor and a constant resistance, which represents the
electronic serial resistance in the conductors and the
ionic resistance in the electrolyte. Both capacitance
and the serial resistance of the SC are dependent on
frequency, temperature and voltage [12]. However,
taking into account the simulation time of this work
(some seconds), the effect of the temperature and
voltage variations can be neglected because they are
almost constant. Furthermore, the frequency variations of the SC current are low enough to assume
that the capacitor value is also constant. The selfdischarge of the SC is also neglected.
3. Compared Topologies
This section presents the details of the compared
topologies and the applied control algorithms. As
the objective of this work was to compare the operation of each topology and not to design new control
strategies, classical controllers have been used. The
control methods are based on the use of a low pass
filter and PI controllers to regulate the different currents and voltages. The main objective of the control
algorithm is the same in all topologies: to divide the
power variations of the system into two parts depending on the frequency. The low frequency part is absorbed/supplied by the VRB and the high frequency
2.2. Vanadium Redox Battery
part by the SC. Similar controllers have been used in
The dynamic model of the VRB that has been used
all topologies in order to obtain comparable results.
is presented in detail in [6, 13–15]. The model was
first presented to the knowledge of the authors in [13]
as a valid dynamic model to simulate the effect of a 3.1. Parallel DC/DC converters topology
This topology is based on the use of parallel bidiVRB for wind power smoothing application. The parameters of the model are determined assuming that rectional DC/DC converters to control the power flow
at the operation point where the stack current is max- of each storage device. The schematic representation
imum and the state-of-charge (SOC) 20%, the power of this topology is shown in Figure 4.
Each storage device can be independently conlosses of the VRB are of 21%. Therefore, at this operation point the overall efficiency is supposed to be trolled, which gives flexibility to the control algorithm. A typical two-level inverter is used to generate
79%.
The transient behaviour of the VRB is related to the AC side 230Vrms /50Hz voltage. An inductancethe electrode capacitance as well as to the concen- based first-order low pass filter is used to filter the
tration depletion close to the electrodes. In the used high frequency harmonics introduced by the inverter
3
Lsc
I sc
Vsc
I o1
I bus
Vbus
Pulse Width Modulation (PWM) is used to generate
the switching signals.
Lfilt er
3
4
SC
Cbus
3.1.2. Control algorithm of the two-level inverter
As the microgrid can work connected or disconnected from the main weak grid, the control algorithm is adapted to each operation mode. When
the microgrid is operating in grid-disconnected mode,
the inverter fixes the AC side voltage/frequency and
when it is connected to the DG the active/reactive
power injected to the weak grid are controlled.
In grid-disconnected mode the control of the twolevel inverter is done using a dq0 rotating frame and
with two control loops, an inner one for the current
and an outer one for the voltage. The Third Harmonic Injection PWM (THIPWM) is used to generate the switching signals.
On the other hand, when the microgrid works connected to the DG the Phase-Locked Loop (PLL) locks
the phase of the DG and this reference is used by the
inverter to inject/take power to/from the grid. In
this operation mode the inner current controllers are
the same of the other mode, and the current references are directly generated from the active and reactive power references. The reactive power reference
is 0VAr while the active power reference is generated
according to the authors’ choice.
The transition between both control modes is
smoothed restarting the PIs of the grid-disconnected
mode with the last value of the grid-connected mode
as the Initial Condition (IC) value.
The control structure of the two-level inverter is
shown in Figure 6.
The controllers of all the power converters have
been designed using the small-signal transfer functions obtained from the linearised average equations
of the converters [16, 17]. All loops are controlled
through PI controllers.
5
Vinv
I inv
2 level invert er
I vrb
Vvrb
1
Lvrb
VRB
2
DC/ DC convert ers
Figure 4: Parallel DC/DC converters topology.
switches. The capacitor Cbus is used to smooth the
DC bus voltage.
3.1.1. Control algorithm of DC-DC converters
The control algorithm applied to both DC/DC converters is shown in Figure 5.
I bus
Vbus
0
I sc
Vsc
I*
vrb
LP filter
LP filter
PI
Vvrb
V*bus
d vrb
I*
sc
PWM
Switching
signals
I vrb
PI
Vbus
PI
PI
d sc
PWM
I sc
Figure 5: Control algorithm of the parallel topology.
The SC side DC/DC converter control is made using two control loops: the outer voltage loop maintains constant the DC bus voltage and the inner loop
controls the SC current. The control of the VRB
has a unique loop to control the current of the battery. The reference signal of the current of the VRB
is generated mainly by the low-frequency part of the
power required by the load of the inverter: the power
through the inverter is filtered and the low frequency
part is used as a part of the reference signal. The
other part is generated by a PI controller in order to
maintain at 0A the average current of the SC. The
3.2. Floating topology
In this topology the DC/DC converter of the VRB
battery is eliminated and the battery is directly connected to the DC bus (refer to Figure 7). For this
reason, and in order to maintain the same DC bus
voltage, the voltage rating of the VRB is double as
much as in the other two topologies.
4
3.3. Three-Level Neutral Point Clamped converter
topology
The 3LNPC converters are typically used in high
and medium power applications due to their ability
to reduce the voltage ratings of the semiconductors
(compared to the ratings that are necessary with conventional two-level (2L) inverters) and their improved
output power quality [18, 19].
This converter can be useful from the point of view
of the association of two ESSs in a microgrid context.
The storage devices can be used as the voltage sources
of the 3LNPC converter, whereas the capacitors are
maintained in order to filter the effect of the switches
(see Figure 9).
Figure 6: Two-level inverter control structure
1
I sc Lsc
Vsc
I bus
Vbus
Lfilt er
1
3
4
VRB
SC
Cbus
Ib
Vsc
5
Cin
SC
Vinv
I inv
DC/ DC convert er
Lfilt er
2 level invert er
3
4
2
I vrb I n
Vvrb
Figure 7: Floating topology.
VRB
LP filter
0
I sc
Vsc
I*sc
PI
d sc
Vinv
I inv
Cin
2
3.2.1. Control algorithm
The block diagram of the control algorithm used
in this topology is shown in Figure 8. The SC is used
to absorb/supply the fast power variations of the DC
bus, to prevent the battery from peak currents. At
the same time, the average power of the SC is controlled in order to maintain it at 0A; this is achieved
using a low pass filter applied to the power of the SC
and then a PI controller.
The design of the control of the two power converters used in this topology is the same of the parallel
DC/DC topology.
I vrb
Vvrb
5
I nput filt er
3 level invert er
Figure 9: 3LNPC converter topology.
One of the most important problems of the 3LNPC
inverter is the neutral point voltage balance, i.e. to
maintain an equal distribution of the DC bus voltage between the two capacitors. Several methods for
maintaining this equality can be found in the literature, like the use of redundant vectors in the Space
Vector Modulation (SVM) [20] and the zero-sequence
component injection in the carrier based PWM modulations [21–23]. These methods can be designed
with a different objective and be used to control the
currents of the storage devices.
PWM
PI
LP filter
3.3.1. Modulation strategy
The possibility of using the small redundant vectors depends directly on the modulation rate. A
Figure 8: Control algorithm of the floating topology.
5
trade-off between the DC bus voltage value and the
use of the small redundant vectors have been carried
out to determine the reference DC bus voltage value.
Based on this analysis, a value of 900V has been selected instead of the typical 700V one.
The multicarrier Third Harmonic Injection PWM
(THIPWM) is used, as it allows a modulation index increase of 15% in the linear modulation range,
compared to the Sinusoidal PWM, when an one-sixth
third harmonic is injected [24]. This way, the same
modulation index increase of the Space Vector Modulation is obtained and the possibility of using the
small redundant vectors is increased. The addition of
the third harmonic does not affect the output waveform in the case of the three phase-three wire systems
[25].
On the other hand, the zero-sequence component
injection has been used to control the use of the
higher or lower small redundant vectors, which determines the current taken from the input voltage
sources. A controlled offset is added to the modulating signal in order to control the current of the VRB
(see Figure 10).
As the use of the storage devices will be different,
voltage differences may appear which would create
undesirable distortions at the output of the inverter
[11, 18]. In order to avoid that and to tend to equalise
both voltages, the amplitudes of the carrier signals
are modified as explained in [11].
final modulating signal is then compared with the
carrier signals to generate the switching orders.
Vsc
Vvrb
Carrier
signals
THIPWM
d abc
dq0
to
abc
Vsc
Vvrb
Ib
Vbus
LP filter
In
Switching
signals
I *vrb
PI
Offset
limits offset
I vrb
I sc
Figure 10: Control algorithm of the 3LNPC converter.
4. Simulation Results
The operation of the three topologies has been
compared in the simulation scenario shown in Figure 11. The power generated by different RES is
simulated as a variable DC current and it is injected
directly in the DC bus. The HESS is used to smooth
the variability of the incoming power. The microgrid
can operate connected or disconnected from the main
weak grid, which is based on a DG. There are two
resistive loads: the main load is directly connected
to the microgrid and a second load is connected to
the DG. Mainly resistive grid-lines have been used to
simulate the distance between the elements.
Two case studies have been simulated. In the first
one, the microgrid operates connected to the main
weak grid, while a variable current generated by RES
is injected in the DC bus. In the second case study
the disconnection of the microgrid from the main
weak grid has been analysed.
In both cases the ability of each topology to divide
the power between the SC and the VRB, as well as
the power losses and the THD are analysed.
The parameters of the supercapacitor bank used
in the simulations have been extrapolated from the
commercial unit BMOD0083 P048 of Maxwell Technologies manufacturer. The parameters that have
3.3.2. Control algorithm
The control algorithm of the 3LNPC converter is
similar to the control applied to the two-level inverter, but an offset is added to the modulating signal
in order to control the use of the redundant vectors.
This additional part is shown in Figure 10. The statespace averaging method has been used to obtain the
equations of the inverter and then the dq0 transformation is carried out to control the inverter in this
reference system [26].
The current reference of the VRB is generated
adding the measured input inverter power in the DC
bus level with the measured power of the SC and subtracting the power signal of the neutral point power.
Then, the resulting value of the addition is filtered
and used as the reference of the VRB current. The
6
Ideal semiconductor elements have been used during simulations, and their current (ic ) has been measured in order to calculate the power losses. Only the
conduction and switching losses have been taken into
account as these are the most important losses in a
power converter. Commercial semiconductors have
been used to calculate the power losses, using the information provided in the data-sheets and applying
the equations (1) and (2) [27].
Pconduction = Vce ic + Rce i2c
Figure 11: Simulation scenario
Pswitching = (Eon + Eoff )fs
been used are equivalent to a SC bank that has 250
cells connected in series. The total capacity of the
used SC is 6F, the serial resistance is 143.1mΩ, the
rated voltage is 450V and the maximum continuous
charge/discharge current is 61A.
The VRB used in this work has a rated stack voltage of 450V (at 50% of SOC and open circuit), a
maximum charge/discharge current of 50A and an
autonomy of 0.75 hours. The parameters of this storage device are shown in Table 1. Notice that as in
the floating topology the VRB is directly connected
to the common DC bus and the voltage in this point
is 900V (as explained in Section 3), the parameters
of the VRB in this topology will be different from
the ones shown in Table 1. These parameters can be
easily obtained using a rated voltage of 900V.
VDC
Vbase
(1)
(2)
4.1. First case study
A current profile generated by different RES has
been introduced to the DC bus in order to analyse
the behaviour of each topology. The resistive loads
Rload and Rdg are maintained constant.
The first 0.7seconds of the simulations of both case
studies are used to carry out the connection process
of the microgrid to the main weak grid.
The power losses of each topology that are obtained
in the first case study are shown in Table 3.
Table 3: Power losses of the first case study
Topology
Power converter
Conduction
Switching
DC/DC SC
12.58W
139.14W
Parallel
DC/DC VRB
23.79W
141.58W
2L inverter
27.22W
413.18W
DC/DC SC
11.88W
140.68W
Floating
2L inverter
27.16W
412.72W
3LNPC
3LNPC inverter
37.1W
320.45W
Table 1: VRB model parameters
Parameter
Value
N
322
Vequilibrium
1.4V
k
0.05138
Kpump
0.9877
Rresistive
0.54Ω
Rreaction
0.81Ω
Rf ixed
295.245Ω
Celectrodes
0.0186F
The average THDs that are obtained in this
first case study are 3.24%/3.96% with the parallel
topology, 3.22%/3.95% with the floating one and
2.03%/3.49% with the 3LNPC converter topology for
voltage/current respectively.
The values of the grid-lines, different elements of 4.1.1. Parallel DC-DC topology
the power converters, the Diesel Generator and other
Figure 12 shows the VRB current, the SC current,
parameters that are used in the simulations are shown the current coming from the RES and the DC bus
in Table 2.
voltage during the first case study. As it can be seen,
7
Grid-line
Param.
Value
Rline
7.6mΩ
Lline
66.53µH
Table 2: Simulation parameters
Power converters
Diesel Generator
Param.
Value
Param.
Value
Lf ilter
3mH
Prated
33kW
Lsc
3mH
Rdg
5.81Ω
Lvrb
4.9mH
J
3 kg · m2
T (pu)
Cin
10mF
Kd
0.6 ω(pu)
fs
15kHz
p
2
the desired power division between the SC and VRB
is achieved.
Figure 12(b) shows that the DC bus voltage is well
maintained at its reference value. The inverter is
supplying a constant active power and no reactive
power. During the first 0.7seconds the inverter works
in grid-disconnected mode and it fixes the frequency
and the voltage of the microgrid. During this period,
the power required by the load is saved, and this value
is used as the active power reference value when the
inverter commutes to work in grid-connected mode.
25
20
ISC 15
0
IRES 10
−20
5
−40
0
0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7
time [s]
(a)
900.5
VDC [V]
20
fast variations of the DC bus are absorbed/supplied
by the SC. The VRB supplies the current difference
between the average power coming from the RES and
the power required by the load. As the VRB is directly connected to the DC bus and its voltage is dependent on its SOC, the voltage amplitude varies and
is slightly different from 900V (refer to Figure 13(b)).
IVRB
IRES [A]
IVRB,ISC [A]
40
Others
Param.
Value
Cbus
4700µF
Rload
15Ω
900
Figure 13: (a) Currents of the ESSs and RES current profile,
(b) DC bus voltage.
899.5
0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7
time [s]
(b)
Figure 12: (a) Currents of the ESSs and RES current profile,
(b) DC bus voltage.
4.1.3. 3LNPC topology
Figure 14 shows the results obtained by the 3LNPC
topology. From Figure 14(a) it can be seen that the
4.1.2. Floating topology
fast power variations are mostly absorbed/supplied
The results obtained using the floating topology by the SC. Taking into account that the state-ofare shown in Figure 13. As shown in Figure 13, the charge of the VRB is 50%, once the connection to
floating topology is also able to divide the power be- the DG is done the current reference of the VRB is
tween the SC and VRB. The current of the VRB can- selected to be 0A. This way, the average power gennot be maintained completely constant as only the erated by the RES will be transferred to the AC side.
8
If the SC gets discharged, the reference of the VRB
could be increased to charge it.
Figure 14(b) shows the evolution of the DC bus
voltage. As the DC bus voltage in this case is formed
by the voltages of the SC and the VRB, it depends
on the state-of-charges of both storage devices. However, the variations are not higher than the ones obtained in the floating topology.
VDC [V]
25
20
IVRB 15
0
IRES 10
−20
5
−40
0
0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7
time [s]
(a)
920
20
study
Switching
141.25W
168.11W
405.47W
145.03W
404.56W
331.67W
4.2.1. Parallel DC-DC topology
Figure 15(a) shows the evolution of the currents of
the ESSs during the disconnection of the microgrid
from the weak grid. When the disconnection occurs,
the current trough the inverter increases and thus a
decrease of the DC bus voltage occurs, as shown in
Figure 15(b). However, the controller of the DC/DC
converter of the SC reacts and minimises the DC bus
voltage variation by injecting a high peak current in
the DC bus. Therefore, the fast variation is supplied
completely by the SC. Once the disturbance is eliminated and the output of the low pass filter of the SC
power increases, the VRB increases its current in order to supply entirely the new load power. As it can
be seen in Figure 15(c), the effect of the disconnection
in the AC side voltages is negligible.
ISC
IRES [A]
IVRB,ISC [A]
40
Table 4: Power losses of the second case
Topology
Power converter
Conduction
DC/DC SC
5.98W
Parallel
DC/DC VRB
56.97W
2L inverter
24.69W
DC/DC SC
10.16W
Floating
2L inverter
24.62W
3LNPC
3LNPC inverter
33.75W
900
880
860
0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7
time [s]
(b)
Figure 14: (a) Currents of ESSs and RES current profile, (b)
DC bus voltage.
4.2. Second case study
In the second case study the disconnection of the
microgrid from the main grid has been analysed. As
in the first case study, during the first 0.7seconds the
connection process of the microgrid to the weak grid
is carried out and the power required by Rload is
saved. Then, the grid-connection is done and the
power reference of the inverter is reduced slowly until reaching the half of the initial value at the instant
1.2seconds (refer to Figure 15(d)). This is done in
order to create a power difference that the inverter
must face when the disconnection occurs.
The power losses of this case study are shown in
Table 4.
The average THDs of the second case study are
3.78%/4.98% for the parallel topology, 3.68%/4.96%
for the floating topology and 2.39%/3.57% for the
3LNPC topology.
Figure 15: (a) Currents of the ESSs, (b) DC bus voltage, (c)
AC voltages, (d) Power of the inverter.
4.2.2. Floating topology
Figure 16 shows the behaviour of the floating topology when the disconnection of the microgrid occurs.
9
The fast power variation is supplied by the SC, but
a small fast current variation can be noticed in the
VRB current. As the battery is directly connected to
the DC bus, when the disconnection occurs the current taken by the inverter increases rapidly and the
DC/DC converter of the SC needs a certain time to
increase its output current. Although the fast current variation of the VRB is perceptible its value is
small in amplitude, as well as its ratio in relation to
the current step of the SC. The effect of the disconnection on the AC side is negligible (refer to Figure
16c).
Figure 16: (a) Currents of the ESSs, (b) DC bus voltage, (c)
AC voltages, (d) Power of the inverter.
4.2.3. 3LNPC topology
Figure 17 shows the results obtained with the
3LNPC topology. Figure 17(a) shows the current division between the two ESSs. The sudden disconnection of the microgrid obliges the inverter to supply
the total power of the load (Rload ). This fast power
variation is supplied completely by the SC, as it is
shown in Figure 17(a).
From Figure 17(c) it can be seen that the AC side
voltage is properly maintained at the reference values. Only a small transitory voltage decrease can be
noticed when the disconnection occurs.
5. Conclusions
The presented research work has proved the feasibility of the parallel topology, the floating topology
Figure 17: (a) Currents of the ESSs, (b) DC bus voltage, (c)
AC voltages, (d) Power of the inverter.
and the three-level neutral point clamped converter
topology to control a HESS in a microgrid context.
The results of the first case study show that the
three topologies are able to smooth the power coming from RES dividing it between the SC and the
VRB. The DC bus voltage is only controllable in the
case of the parallel topology. However, in the case of
the other two topologies, the selected DC bus voltage
gives enough flexibility to reduce the voltage (according to the state-of-charge of the storage devices) while
a proper operation of the inverters is assured. The results, concerning the power division, are similar in the
three cases, but the 3LNPC topology obtains these
results with a reduction of 52.8% and 39.65% of the
power losses comparing to the results of the parallel
and floating topologies respectively.
On the other hand, the results of the second case
study show that the three analysed topologies are
able to face the disconnection of the DG and to continue supplying power to the loads with a correct AC
side voltage. The sudden power increase is well divided between the SC and the VRB in the case of
the parallel and 3LNPC topologies; in the case of the
floating topology there is a small fast variation in the
current of the VRB due to the reaction time needed
by the DC-DC converter of the SC. The effect of the
disconnection on the AC side voltages is negligible
in the floating and parallel topologies; in the case of
the 3LNPC topologies there is a small voltage reduction during a short period of time. However, this
10
transitory part is acceptable. Concerning the power
losses, the 3LNPC obtains a reduction of 54.46% and
37.47% comparing to the losses of the parallel and
floating topologies respectively. These power losses
reduction could be reduced even more using a modulation strategy designed for that objective, as for
example in [28, 29].
In both case studies the 3LNPC converter topology
obtains the best THDs.
Although the 3LNPC improves the power losses
and the THD, it has some disadvantages: namely
the loss of flexibility of the controller, the necessity
of maintaining equal the voltages of both ESSs and
having a variable DC bus voltage. Comparing to the
flexibility of the parallel and floating topologies, the
flexibility is more limited as the control of the DC and
AC side variables is interdependent. The controller
tries to manage the current supplied/absorbed by the
ESSs while this current is used to generate the AC
side signals. Consequently, a complex control algorithm is necessary to manage properly the 3LNPC
topology.
Analysing the advantages and disadvantages of the
3LNPC converter topology, the authors conclude that
even if this topology has some flexibility limitations,
the 3LNPC topology can be the most interesting
choice to manage a HESS and to improve the power
quality in a microgrid. The fact of controlling the
HESS using just one power converter with reduced
power losses and improved THD is highly interesting
from the distributed generation application point of
view, where the storage system would be installed in
many different locations.
During this first stage of the presented research
work, the investigated system has been tested
by means of simulations using the software Matlab/Simulink. The investigated microgrid and the
main grid based on the DG are actually installed for
the laboratory tests in order to prove the advantages
of the 3LNPC topology compared to the floating and
parallel topologies.
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