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C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177
1Department of Electrical and Computer Engineering, Faculty of Engineering, Naresuan University, Phitsanulok, Thailand.
2School of Renewable Energy and Smart Grid Technology, Phitsanulok, Thailand, 65000, Thailand.
*Corresponding author: P. Pachanapan; E-mail: piyadanip@nu.ac.th.
Implementation of an Off-grid Single-phase Hybrid PV – HV
Battery Inverter with Interleaved Bidirectional DC-DC
Converter for Power Balancing Control in an Isolated
Electrical System
Chayakarn Saeseiw1, Piyadanai Pachanapan1,*, Tanakorn Kaewchum2, and
Sakda Somkun2
A R T I C L E I N F O
Article history:
Received: 12 October 2022
Revised: 20 January 2023
Accepted: 15 March 2023
Keywords:
High voltage battery
Hybrid inverter
Hybrid PWM
Interleaved DC-DC converter
Off-grid operation
A B S T R A C T
Two challenges for operating an isolated electricity system, such as an island or a
mountain, are 1) maintaining the satisfied system frequency and voltage when the energy
sources and load are rapidly changed and 2) handling the increasing harmonic distortion
level from non-linear loads. This paper presents an off-grid single-phase hybrid
photovoltaic (PV) and high-voltage (HV) battery inverter which can perform the fast
power balancing mechanism under linear and non-linear load conditions. This hybrid
inverter is comprised of a DC-AC inverter, a boost DC-DC conversion on the PV side,
and a bidirectional DC-DC converter on the HV battery side. The power balance is
controlled by changing and discharging the HV battery using the interleaved bidirectional
DC-DC converter. In addition, the two-stage interleaved topology also benefits in
reducing DC bus voltage and battery current ripples and increasing power conversion
efficiency. Furthermore, the LCL filter is installed to reduce harmonic components from
the DC-AC inverter outputs. Finally, the control performance of the hybrid inverter
prototype is investigated in various off-grid operation scenarios. The experimental
conclusions indicated that the proposed hybrid inverter efficiently managed fluctuations
in PV power and load requirements as well as low and high levels of harmonic distortion.
1. INTRODUCTION
Hybrid photovoltaic (PV)-battery systems are becoming
popular renewable-based energy sources for residential
loads in many remote areas, such as villages, islands and
hilly areas, where access to utility power is difficult [1]. As
solar panel and battery costs have decreased, the hybrid PV-
battery solution has sparked global interest in replacing the
use of traditional diesel generator for isolated electricity
systems [2]. With the assistance of battery energy storage
(BES), The hybrid PV-battery system has the potential to
increase energy management abilities and deliver a
continuous power supply while the system is operating
independently. Furthermore, the BES also supports a power
balancing mechanism among PV system and local loads, as
well as reducing the power fluctuation caused by PV
generation uncertainly [3], [4]. Hence, the system frequency
and voltage in an isolated system are improved
For small-scale isolated systems with capacities less-
than 5 kW, a single-phase hybrid PV – battery system is
regularly used for economic reasons. In Fig. 1, it is shown
that the DC-coupled hybrid inverter is combined PV and
BES with the load. which is simple and cost-effective.
Besides, the hybrid inverter performs similar functions to an
off-grid solar inverter but also includes an integrated battery
controller inside a common unit. With the various battery
technologies available today, hybrid inverters are classified
by battery voltage level divided into low-voltage (LV) and
high-voltage (HV) batteries. Additionally, the LV batteries
basically operate at 48 V, while the HV batteries operate in
between 200-500 V [5].
Fig. 1. Diagram of a DC-coupled hybrid PV-battery system.
Off-grid Hybrid
Inverter
HV – Battery
PV Panels
Domestic loads
Isolated Electrical System
168 C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177
-
+CDC
vDC
PCC Load
Line
LCL Filter
Full-bridge DC/AC inverter
is
vAB vS
L1
Cfvcf
ic
PV iLB LBT
-
+CBT
vPV
Boost DC/DC converter
Bidirectional DC/DC converter
-
+
L1
L2
L2
CBatt
ƩiLBn LB1
LB2
HV Battery
iBatt
vBatt
-
+
SBT
S1
S2
S3
S4
DBT T1
T2
T3
T4
iPV
Single-phase Hybrid Inverter
Fig. 2. The proposed single-phase hybrid inverter implemented in this work.
In the past, conventional LV batteries like lead acid were
often employed as a source of energy storage due to their
cost-effectiveness and simple installation. However, their
weight and size are major drawbacks, necessitating a large
installation space [6]. The use of LV batteries in high power
applications is also limited due to their large battery current
and notable loss in the voltage conversion process.
Alternatively, the new generation HV batteries, such as
lithium-ion, are becoming a preferred energy storage
solution since their smaller size and weight, which can store
more energy and provide faster charging and discharging
rates than the lower one. Furthermore, as the voltage range
of the PV array, which is 300 to 600 V, is extremely close to
that of the voltage batteries. Temperature and cable losses of
hybrid inverter is reduced, increasing the power conversion
efficiency [7].
The off-grid hybrid inverter with DC coupling can
convert DC power from either PV or BES to AC power via
the voltage-controlled voltage source inverter (VSI). The
amplitude and frequency of AC output voltage can be
accomplished using pulse width modulation (PWM) control.
Unipolar PWM and Bipolar PWM are two modulation
techniques commonly used for open-loop voltage control of
power inverters [8]. The parasitic capacitance of PV panels
is the source of leakage current, which is why the hybrid
PWM method has been devised to reduce it [9]. This
undesirable leakage current should be mitigated to ensure
safety and electromagnetic compatibility. In addition, an
LCL filter, which has smaller filter volume and a better
switching ripple reduction characteristic comparing to the
simplest L filter, is installed to eliminate high frequency
signals from the switching process [10], [11].
At the DC side, to manage the power flow for charging
and discharging the HV battery system, the bidirectional
DC-DC converter is employed. There are several topologies
of non-isolated DC-DC converter reviewed in [12], [13]. It
was found that the most promising bidirectional DC-DC
converter in terms of high efficiency, current ripple
cancellation, better cooling performance and high-power
density is an interleaved topology. The lower current ripple
requires the smaller size of filter capacitance when
compared to the conventional topology, resulting in a faster
control response. Also, the two-phase half-bridge
interleaved topology is the most common choice for
exchanging power between energy storage and DC bus [14],
[15]. More than that, the interleaved bidirectional DC-DC
converter is also widely applied for HV battery in electric
vehicle applications [16].
In cases of residential-scale PV systems (3-5 kW),
a higher voltage (i.e., 300 V – 600 V) boost DC-DC
converter with the maximum power point tracking (MPPT)
algorithm is typically used to extract the greatest amount of
power from the PV panels. Several boost DC-DC converter
topologies for PV systems were review in [17], [18].
Additionally, the popular MPPT techniques such as
perturbation and observation and incremental conductance
methods [19] can be applied to operate fixed PV panels at
the maximum power point under various solar irradiance
conditions. Moreover, the power limiting control may be
included to curtail the active power produced through a PV
system so as to improve system stability and voltage
regulation [20].
C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177 169
vDC
vAB
L
vS
iS
T1
T2
T3
T4
vA0 vB0
vPE
iCM
CPV
(a) Full-bridge DC-AC inverter
(c) PWM and output waveforms
(b) Hybrid PWM
Fig. 3. Full-bridge DC-AC inverter with hybrid PWM control strategy.
Contribution: This paper implements the hardware
prototype of an off-grid single-phase DC-coupled hybrid PV
- HV battery inverter that can provide a fast power balancing
mechanism in an isolated electrical system under highly
non-linear load conditions. Furthermore, the HV battery
controller, which relies on a bidirectional interleaved DC-
DC converter, is an important component in the power
balancing operation by absorbing excess power from the PV
system or injecting it into the load. The DC-AC inverter with
an LCL filter and an open-loop voltage control based on
hybrid PWM are used to generate an AC voltage output with
lower harmonic components and leakage current from
parasitic capacitance. Finally, the control performance of the
proposed hybrid PV – HV battery inverter is investigated in
various scenarios, including sudden load change, ramp-up,
and ramp-down PV generation.
2. OFF-GRID SINGLE-PHASE HYBRID INVERTER
Figure 2 shows the implementation of the DC-coupled
hybrid inverter used in this research, which consists of two
DC-DC converters connected by the DC-link connection to
a full-bridge DC-AC inverter. The interleaved bidirectional
DC-DC converter is linked to an HV battery for energy
storage management, while the typical boost DC-DC
converter with MPPT algorithm is utilized to obtain as much
power as possible from the PV arrays. Moreover, the LCL
filter is chosen to mitigate the harmonic distortions from the
high-frequency switching process.
During daylight hours, the dc-coupled hybrid system can
continuously send power to the loads from either PV array
or BES. If the quantity of electricity created through the PV
system is higher than the amount of power needed by the
load, the surplus power will be used to charge the battery. In
contrast, when the energy produced by the PV is insufficient,
the battery operates to supply additional power to the load.
When the battery runs out, the DC-AC inverter shuts down,
leaving the connected loads unsupplied and resulting in an
interruption. Then, the battery voltage proceeds to increase
when power directly from the PV system is sent straight to
the BES. Once fully charged, the DC-AC inverter can turn
back on and supply power to the local loads again.
Full-bridge DC-AC inverter
The single-phase VSI with an H-bridge structure is typically
used to convert DC power to AC power, as shown in Fig. 3
(a). This type of inverter consists of four switches which
mostly made of insulated-gate bipolar transistors (IGBTs).
The PWM method is widely used for controlling frequency
and amplitude of an AC signal's output voltage. In addition,
the high switching frequency (kHz range) and the passive
filter, L or LCL, can make the waveform of output voltage
nearly sinusoidal. If the off-grid hybrid inverter is used as a
stand-alone system without any other grid-forming energy
sources, the open-loop voltage control, which is simple and
robust, could be effective enough to allow the full-bridge
DC-AC inverter to supply the power at the domestic level (<
5 kW).
When PV panels are installed, they always exhibit
capacitance between PV panels and the ground towards their
environment, known as “parasitic” capacitance, CPV. If the
inverter is a non-isolated type, the leakage current or
common-mode current, iCM, is introduced which can become
a shock hazard. This leakage current may trip the residual-
current device, causing the inverter to disconnect from the
load temporarily. For a 5-kW system, the value of CPV is
330-500 nF in the case of monocrystalline and
polycrystalline modules [21].
To mitigate the iCM, the hybrid PWM is selected instead
of conventional bipolar and unipolar PWM methods. Fig. 3
m(t)
-m(t)
+
-
+
-
T3
T4
T1
T2
Vcarr(t)
Comparator
Comparator
170 C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177
(b) and (c) shows the hybrid PWM method and output
waveforms of the DC-AC inverter. It was discovered that
the switching frequency of the hybrid PWM is roughly half
that of the unipolar PWM, making the switching loss is
decreased. Also, the unfiltered output voltage, vAB, is similar
to that of unipolar PWM method, whereas the common
mode voltage, vPE, is improved, resulting in less leakage
current. However, the output current ripple generated by the
hybrid PMW method is greater than that created by the
unipolar PWM method. As a result, the larger size of filter
is required.
-
+
vDC
PV iLB LBT
-
+CBT
vPV
DBT
iPV
MPPT PI PI PWM
SBT
SBT
vPV
iLB vPVref
vPV
iLBref
iLB
Zinv
CDC
D
DBTp
Fig. 4. The boost DC-DC converter control construction
MPPT boost DC-DC converter
The PV-side DC-DC converter operates in boost mode,
focusing on obtaining the maximal power as practicable
from solar irradiation. The PWM controlled boost DC-DC
converter with the MPPT algorithm is shown in Fig. 4. As
in the steady state condition, the inductance current, iLBT is
equal the PV output current, iPV. Therefore, the maximum
power point is tracked by measuring PV output voltage, vPV,
and iLBT. The algorithm for the MPPT defines the voltage
and current references, vPVref and iLBref that can extract the
highest power. The vPV and iLBT are regulated using PI
controllers, resulting in a change in duty ratio, D, of PMW
signal.
The MPPT controller is only activated when the vPVref is
less than the reference DC bus voltage, vDCref. If the vPVref
exceeds the vDCref, the IGBT switch is turned off and the iPV
flows directly to the DC-AC inverter, via the bypass diode,
DBTp. In the case of a two-string PV system, two exactly
alike boost DC-DC converters with independent MPPT
controllers are employed.
Interleaved bidirectional DC-DC converter
The interleaved bidirectional DC-DC converter with a two-
phase topology is implemented for charging and discharging
the HV battery. From Fig. 2, the two-phase interleaved DC-
DC converter consists of four switches made of IGBTs, and
filter capacitor, CB. Inductors LB1 and LB2 are used to store
and release the energy during the switching operation. If PV
power exceeds the load demand, the energy from PV arrays
can be used to charge the HV battery through this DC-DC
converter which works in the buck mode. Whereas, it runs
in boost mode to discharge power from the HV battery to the
load if the PV power is relatively low. The switching signals
are generated using PWM method.
S´2 , S´4
iBatt
iLBn
CB
ƩiLBn
vBatt
LBn
vDC CDC
iDC S1 , S3n = 1, 2
(a) Charging mode
vDC CDC
iBatt
iLBn
iDC
CB
S´1 , S´3
S2 , S4vBatt
LBn
n = 1, 2
ƩiLBn
(b) Discharging mode
Fig. 5. Charging mode and discharging mode of interleaved bidirectional DC-DC converter.
iLB1
iLB2
iBatt
S1,S'2
DTs
DTs
Δ iLB1
Δ iLB2
Δ ibatt
Ts/2
0Ts3Ts/2 t
Ts/2
S3,S'4
(+)
(+)
(+)
(a) Charging mode
iLB1
iLB2
Ts/2
DTs
DTs
iBatt
Ts/2
0Ts3Ts/2 t
Δ iLB1
Δ iLB2
Δ ibatt
S'1,S2
S'3,S4
(-)
(-)
(-)
(b) Discharging mode
Fig. 6. Current waveforms of interleaved bidirectional DC-DC converter.
C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177 171
In charging mode, switches S1 and S3 are controlled while
switches S2 and S4 are switched off. The flow of currents in
the charging mode operation is shown in Fig. 5(a). Since the
DC bus voltage is relatively close to the battery voltage, the
switching operation is carried out with duty ratio, D, set in
the range of 0.5 < D ≤1.0 and working in the continuous
conduction mode. The switching signals for S1 and S3 are
identical but shifted by 180o in a two-phase interleaved
design. Fig. 6 (a) depicts the battery current, iBatt, which is a
combination of the two inductor currents ILB1 and ILB2. It can
be seen that the two-phase interleaved topology can lower
the battery current ripple.
PI
PI PWM
S1
vDCref
vDC
iBatt iLB1
D1
*S2
PI PWM
S3
iLB2
D2
S4
iLB1
*
iLB2
*
1
2
Fig. 7. The interleaved bidirectional DC-DC converter control
block diagram for off-grid operation.
1
sL1
PWM
m
Vcarr
vAB 1
sCf
1
sL2
vcf
icis
vs
Fig. 8. The single-phase VSI with LCL filter control block
diagram.
In discharging mode, on the other hand, the switching
signals are sent to trigger switches S2 and S4, whereas
switches S1 and S3 are not activated. The duty ratio of
switching signals is between 0 ≤ D ≤ 0.5. The flow of
currents in the discharging mode, as well as the current
waveforms are demonstrated in Fig. 5 (b) and Fig 6 (b),
respectively. It is found that the flow of iBatt is in the opposite
direction when compared to the charging mode.
The control of interleaved bidirectional DC-DC
converter for off-grid operation is shown in Fig. 7. The
closed-loop control structure is composed of the following
two loops: The average inductor current controller (loop of
inside control) and the DC voltage controller (the outer
control loop). The voltage loop regulator differentiates the
reference voltage, vDCref, with the vDC. The voltage error is
cancelled out by the PI controller and produces the battery
current reference, i*Batt, for the current control loop.
In the case of two-phase interleaved circuit, i*Batt is
divided in half (i*Batt/2) and transmitted to the current
control loop. These divided currents i*LB1 and i*LB2 are the
reference inductor currents in parallel circuits, regulating
iLB1 and iLB2. PI controllers are used to eliminating the current
errors. The output of the current control loop is the duty ratio
of PWM signals sent to control switches S1 and S3 or
switches S2 and S4.
LCL filter
The LCL filter is used to minimize output current switching
ripple, which is the downside of implementing hybrid PWM.
Compared to the L filter, the LCL filter requires lower
reactive power and has better harmonic attenuation. From
Fig. 2, The LCL filter has three elements: an inductor on the
converter side, L1, filter capacity, Cf and an inductor on the
grid side, L2. The LCL filter state equations are as follows:
-
c
AB cf
di t
L v t v t
dt
1
(1)
cf
f c s
dv t
C i t i t
dt
(2)
s
cf s
di t
L v t v t
dt
2
(3)
From (1) to (3), the block diagram of the single-phase
VSI with the LCL filter can be drawn as Fig. 8. Moreover,
the current ripple attenuation, A, is derive by.
Interleaved
Bidirectional DC-
DC converter
CBatt
Single-phase
Full-bridge DC-
AC Inverter
(a) Hardware prototype
(b) Experimental setup
Fig. 9. Hardware prototype and experimental setup.
Line
Impedance
PV
sim Grid
sim
Battery
sim RLC
load
Non-linear
load
DSPs
DC/AC
Inverter
Interleaved
Bidirectional DC
-DC converter
172 C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177
3
1 2 1 2
1
s
AB f
is
v s s L L C s L L
(4)
2
2
3
1 2 1 2
1
f
c
AB f
s L C
is
v s s L L C s L L
(5)
2
2
1
1
s
cf
is
Ai s s L C
(6)
From (4) to (6), it is found that the LCL filter can provide
the ripple attenuation of -60 dB/decade while the L filter has
the attenuation of only -20 dB/decade [11]. To avoid the
resonance issue, which can cause the system to become
unreliable, the LCL filter components are chosen using the
design process as explained in [10], [11]. The L1 is
calculated by using the ripple factor of the switching
frequency component to the rated current. The Cf is limited
by the reactive power absorption. The L2 is determined by
considering the attenuation ratio.
(a) Current loop controller
PI
vDCref Current
loops
*
iBatt ƩiLBn
vBatt
PBatt
PPV
PLoad
1
s(vDCref CDC)vDC
Sensor & Filter
(b) Voltage loop controller
Fig. 10. The Interleave bidirectional DC-DC converter with
current-voltage loop controllers.
Fig. 11. Bode diagram of interleave bidirectional DC-DC
converter control system.
3. EXPERIMENTAL SETUP AND CASE STUDY
The hardware prototype of the off-grid single-phase hybrid
inverter is implemented, as illustrated in Fig. 9. Table 1,
summarizes the key data of the proposed hybrid inverter.
Table 2, displays the designed parameters of a 5-kW full-
bridge DC-AC inverter with an MPPT boost DC-DC
converter and an LCL filter. Besides, a 3-kW interleaved
bidirectional DC-DC converter is employed coupled with a
300 V battery. Two real-time control units are developed on
TMS320F28379D 32-bit microcontrollers which one for
controlling DC-AC inverter and boost DC-DC converter,
and the other for controlling the interleaved bidirectional
DC-DC converter.
Table 1. Off-grid Hybrid Inverter Technical Data
Parameters
Value
AC nominal voltage (rms)
220 Vac
AC nominal frequency
50 Hz
AC maximum current (rms)
23 A
AC maximum power
5 kW
Nominal DC link, vDC
400 Vdc
PV input voltage, vPV
130 – 600 Vdc
Battery voltage, vbatt
300 Vdc
PWM technique
Hybrid
Switching frequency for
- Full-bridge DC-AC inverter
20 kHz
- PV Boost DC-DC converter
20 kHz
- Bidirectional DC-DC converter
20 kHz
Table 2. Parameters of Hybrid Inverter’s Power Stages
Parameters
Value
Boost inductor, LBT
2 mH
Boost filter capacitor, CBT
75 μF
Inverter side inductor, L1
0.8 mH
Grid side inductor, L2
0.4 mH
DC bus capacitor, CDC
1,200 μF
Bidirectional inductor, LB1 and LB2
1 mH
Bidirectional filter capacitor, CBatt
195 μF
IGBTs
650 V, 80 A
Diodes
600 V, 60 A
The PI controllers for voltage and current controllers of
interleaved bidirectional DC-DC converter are considered
by applying an extended symmetrical optimization method,
as proposed in [22],[23]. This method recommends that, to
have an acceptable overshoot, the PI controller’s phase
margin should be in the range of 30o - 65o to assure the
stability of the control system. From the closed loop control
diagrams in Fig. 10, the parameters of PI controllers are
1
2
*
iBatt iLBn
*
PI PWM vDC
vBatt
1
sLBn
iLBn
Sensor & Filter
D
C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177 173
designed based on switching frequency of 20 kHz and the
bandwidth is 30 Hz. Table 3, shows the parameters used in
voltage and current control loops. Fig. 11 depicts the Bode
diagram of the interleaved bidirectional DC-DC converter
control system. It is discovered that the phase margin is 45o
which can achieve the recommended value. Additionally,
Section 4 presents of the reactions in the time domain.
Table 3. Parameters of Close-loop Controllers
Parameters
Value
Current loop: Proportional gain, Ki
0.2667
Current loop: Integral time, Ti
0.65 ms
Voltage loop: Proportional gain, Kv
0.7685
Voltage loop: Integral time,, Tv
5.1 ms
PWM delay time, Td
40 μs
Current loop: Equivalent delay time, Tdi
0.85 ms
Sensor & Filter delay time, Tf
0.2 ms
Table 4. List of Experimental Test
Time
Detail
0-20 s
PV = 0 kW, Load = 1 kW
20-35 s
PV ramps up to 3 kW, Load = 1 kW
35-65 s
PV = 3 kW, Load steps up to 2 kW
65-80 s
PV ramps down to 0 kW, Load = 2 kW
80-100 s
PV = 0 kW, Load = 2 kW
The test system is similar to Fig. 1 which consist of
single-phase hybrid PV-battery system connects with AC
linear and non-linear loads via the line impedance of 0.24 +
j0.15 Ohm. As well, the experimental setup consists of the
PV simulator, the Battery simulator, and the RLC simulator,
which represent PV systems, energy storage systems (the
HV lithium-ion 300V), and a linear load, respectively. The
diode-bridge rectifier with resistance loads is chosen to
represent the non-linear load.
The off-grid operation is explored using fluctuations in
PV power generation and load demand for power to
investigate the control capabilities of the proposed hybrid
inverter. The variations of PV power are created by
adjusting the values of iLBref of boost DC-DC converter in
Fig. 4. At initial, the PV is unavailable and the load demand
is 1 kW at power factor of 0.95. After that, the PV ramps the
power up to 3 kW and the load is stepped up to 2 kW with a
0.95 power factor. The detail of experimental examination
is described in Table 4.
vS
iS
vBatt
vDC
PLoad
PPV
300 V
400 V
Prms = 1 kW
3 kW
Charging mode Discharging modeDischarging mode
2 kW 1 kW
- 2 kW
Zoom A
0 S 10 S 20 S 30 S 40 S 50 S 60 S 70 S 80 S 90 S 100 S
- 1 kW
Vrms = 221 VVrms = 216 V
irms = 4.79 A irms = 9.87 A
iBatt
PBatt
Prms = 2 kW
(a) The main results of the hybrid inverter
3.33 A
3 kW
6.67 A
BES 2 kW BES 1 kW
400 V
irms = 4.79 Airms = 9.87 A
Vrms = 221 VVrms = 216 V
Prms = 1 kW Prms = 2 kW
300 V
Zoom A
Δibatt = 4.20A
Δibatt = 2.10 A
ΔVDC = 7 VΔVDC = 14.4 V
vS
iS
vBatt
vDC
PLoad
PPV
iBatt
PBatt
(b) The changes between 45 s and 55 s (Zoom A)
(c) P and Q (rms values) and harmonic distortions
Fig. 12. Experimental results in Case 1) Non-harmonic load
The growth of domestic non-linear loads, such as smart
devices, inverter-based electric appliances, and LED
lightings, can raise the level of harmonic distortions in the
isolated electrical system. The proposed hybrid inverter
should perform well even in environments with very high
174 C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177
harmonic pollution. Therefore, this hybrid inverter is tested
under two different load types, as follows:
Case 1) Non-harmonic load.
Case 2) Harmonic load which is a mix of linear and non-
linear loads.
The goal of this experiment is to verify the off-grid hybrid
inverter operation in a range of situations that can occur in
an actual isolated system. In a hybrid PV-battery system, the
power balancing mechanism should be provided for
controlling the DC link voltage via the interleaved
bidirectional DC-DC converter. Rapid charging and
discharging are essential for the effective management of the
isolated system. Moreover, the full-bridge DC-AC inverter
can function properly even when a highly non-linear load is
connected.
4. EXPERIMENTAL RESULTS
The waveform outputs are measured by high bandwidth
oscilloscope with 8 channels (YOKOGAWA model
DLM4000). The harmonic distortions, RMS values of active
and reactive powers are recorded by the digital power meter
(YOKOGAWA model WT333E) with the sampling time of
1 second.
RL RL
R R
0.5 kW 0.5 kW
0.5 kW 0.25 kW
Line
Linear load
Diode rectifier load
Harmonic load
vs
is
Fig. 13. Harmonic load and its waveforms
Vrms = 221 V
Charging mode Discharging modeDischarging mode
3 kW
400 V
300 V
0 S 10 S 20 S 30 S 40 S 50 S 60 S 70 S 80 S 90 S 100 S
Zoom B
Vrms = 216 V
irms = 5.7 Airms = 9.3 A
Prms = 1 kW Prms = 1.75 kW
- 1.75 kW
1.25 kW
2 kW
- 1 kW
vS
iS
vBatt
vDC
PLoad
PPV
iBatt
PBatt
(a) The main results of the hybrid inverter
3 kW
PAC,rms = 1 kW PAC,rms = 1.75 kW
BES 2 kW
400 V
300 V
Charging mode
BES 1.25 kW
6.67 A 4.17 A
Vrms = 221 V
irms = 5.7 A
Vrms = 216 V
irms = 9.3 A
Zoom B
ΔiBatt = 3.25 A ΔiBatt = 5 A
ΔVDC = 11.20 V ΔVDC = 16.20 V
vS
iS
vBatt
vDC
PLoad
PPV
iBatt
PBatt
(b) The changes between 48 s and 60 s (Zoom B)
(c) P and Q (rms values) and harmonic distortions
Fig. 14. Experimental results in Case 2) Harmonic load.
Case 1) Non-harmonic load
Fig. 12 (a) shows the main test results which include: AC
output voltage and current (vS and iS), battery current,
voltage and power (vBatt, iBatt and PBatt), DC bus voltage
(vDC), PV power (PPV) and load demand (PLoad). The
experimental results demonstrate that the proposed off-grid
hybrid inverter can handle rapid variations in PV generation
C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177 175
and load consumption while maintaining low switching and
battery current ripples. The control performance in each
section is explained below.
During the time interval 0-20s: There is no PV generation
at first. As a result, the battery is the only energy source
capable of supporting the AC load at 1 kW and 0.33 kVar
(power factor of 0.95). To discharge power from the HV
battery, the bidirectional DC-DC converter works in the
boost mode.
During the time interval 20-35s: There is a load
requirement of 1 kW, but the PV power grows from 0 to 3
kW. The HV battery is charged when the PV power is in
excess of the load requirement, and the bidirectional DC-DC
converter switches to buck mode, absorbing the excess
power (typically approximately 2 kW).
During the time interval 35-65s: The power demand from
the load unexpectedly stepped from 1 kW to 2 kW, while the
PV power remained exactly the same at 3 kW. Additionally,
the reactive power is also changed to 0.82 kVar to maintain
the power factor of 0.95 (see Fig. 12 (c)). However, the
charging power is gradually reduced from 2 kW to 1 kW
while the bidirectional DC-DC converter continues to
operate in the buck mode. As shown in Fig. 12 (b), it is found
that the battery current is changed from 6.67 A to 3.33 A
within 3 ms while the voltage at the DC link momentarily
falls before before recovering to the reference value of 400
V. During the time interval 65-80s: The PV power steadily
decreases from 3 kW to 0 kW, while the load demand
remains constant at 2 kW. When the PV energy drops below
2 kW, the bidirectional DC-DC converter changes to the
boost mode to discharge extra power in conjunction with the
PV power to supply the AC load.
During the time interval 80-100s: The load requirement
is 2 kW, but the PV power is 0 kW. Hence, the HV battery
is the only available supply capable of satisfying the load
requirements. The bidirectional DC-DC converter remains
in boost mode to supply 2 kW to the AC load.
It is found that using hybrid PWM with the LCL filter
allows the DC-AC inverter to generate AC output signals
with very low switching ripples throughout the test. The AC
output current and voltage waveforms, as illustrated in Fig.
12 (b), are nearly pure sinusoidal. Fig. 12 (c) shows that the
total harmonic distortion of output current, THDi, is less
than 1 %, and the total harmonic distortion of output voltage,
THDv, is less than 1.2 %.
At the DC side, because of the interleaved topology, the
DC link voltage ripple (peak to peak, ΔvDC) ranges from
7 V to 14 V, and the battery current ripple (peak to peak,
ΔiBatt) ranges from 2.1 A to 4.2 A.
Case 2) Harmonic load
At initial, the 1 kW harmonic load is made up of a 0.5 kW
RL load and a bridge diode rectifier with 0.5 kW resistive
load. The voltage and current waveform of this 1 kW
harmonic load is shown in Fig. 13. The harmonic distortions
of output voltage and current are demonstrated in Table. 5
which mostly are 3rd, 5th and 7th harmonic orders. The THDi
and THDv are 56.8 % and 3.8 %, respectively. The load
stepping up is achieved by increasing linear load to 1 kW
and non-linear load to 0.75 kW. Besides, the reactive power
is increased from 0.74 kVar to 1 kVar, which causes the
power factor to increase from 0.81 to 0.87.
Table 5. Harmonic Distortions of Non-Linear Load
Order
%HDv
%HDi
1 kW
1.75 kW
1 kW
1.75 kW
3
1.30
1.29
41.04
41.04
5
2.32
2.32
31.47
31.49
7
2.33
2.33
20.03
20.06
9
1.16
1.16
9.54
9.56
11
0.60
0.60
3.14
3.15
13
0.14
0.14
0.56
0.56
15
0.19
0.18
0.63
0.63
Similar to the case 1, Fig. 14 (a) illustrates that the
proposed hybrid inverter can perform the power balancing
among fast variations of PV power and load demand, even
though the level of harmonic distortion in the isolated
system is dramatically increased. Furthermore, the ripples of
DC voltage and battery current are significantly higher than
in the non-harmonic load case, as can be seen in Fig. 14
(b). It is found that the ΔvDC ranges from 11.2 V to 16.2 V,
and the battery current ripple (peak to peak, ΔiBatt) ranges
from 3.25 A to 5 A.
As shown in Fig. 14 (c), the addition of the non-linear
load causes an obvious increase in THDi and THDv of AC
output voltage and current over the duration of the test,
which THDi is in between 46 % and 57 % and THDv is in
the range of 3.8 % to 4.6 %. Then, the concern of power loss
causing by the raise of ripples and harmonic distortions
becomes more serious, and a larger size of filter capacitor
and passive filter may be considered.
5. CONCLUSION
The experimental testing verifies that the proposed off-grid
single-phase hybrid PV – HV battery inverter with a two-
stage interleaved bidirectional DC-DC converter can deal
with rapid changes in PV output and load demand in an
isolated system, both low and high harmonic distortion
conditions. The interleaved bidirectional DC-DC converter
can handle the power balancing mechanism by controlling
the HV battery charging/discharging current with a fast
response. The two-phase interleaved topology also provides
a low battery current ripple, improving power conversion
efficiency. However, increasing the harmonic load intends
176 C. Saeseiw et al. / GMSARN International Journal 18 (2024) 167-177
to exacerbate the ripples of DC bus voltage and battery
current, which can reduce the battery’s service life.
Moreover, the DC-AC inverter with LCL filter and Hybrid
PWM control technique facilitates output ripples and
leakage current, making the AC output waveform close to
the pure sinusoidal.
ACKNOWLEDGMENT
This work is in “Interleaved Bi-directional Converter for
Hybrid inverter with PV and battery system” project
(number R2566E006) supported by Centre of Excellence on
Energy Technology and Environment (CeTe) and Faculty of
Engineering, Naresuan University, Thailand.
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