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2016China International Conferenceon Electricity Distribution (CICED 2016)
Xi’an, 10-13 Aug, 2016
Solid State Transformer application to Grid Connected
Photovoltaic Inverters
BAOLONG LIU, YABING ZHA, TAO ZHANG AND SHIMING CHEN
College of Information System and Management
National University of Defense Technology
Changsha, Hunan Province, China
The transformers have been widely used in
different fields, the primary functions of which are
voltage-transformation, isolation, voltage-stability and
energy-transfer.
Actually
the
conventional
copper-and-iron based transformers are highly reliable,
relatively inexpensive and rather efficient, but have
some disadvantages: (1) heavy weight and large sizes
for iron cores or copper windings; (2) voltage drop
under load; (3) sensitivity to harmonics; (4) low
performance under load unbalance or in presence of
dc-offset; (5) environmental concerns regarding
mineral oil[1-2]. The Solid State Transformer (SST) is
a new power conversion equipment, which is based on
power electronic converter and high frequency link.
SST has the advantage of
voltage regulation,
reactive power compensation, bi-directional power
flow control features etc[3-5]. SST has been regarded
as one of the ten most emerging technologies in 2011
by MIT technology review[6].
In the grid connected PV system, two-stage PV
converter with high gain dc-dc converter cascaded by
inverter is most popular topology[7-8]. In this
structure, the bulky electrolytic capacitors are likely to
have reliability of thirty times less than other types
under the same conditions[9-10]. Large electrolytic
capacitor can be replaced by small film capacitors to
extend the lifetime, but small dc-link capacitor may
suffer from large voltage swing[11]. The large twice
grid frequency ripple will not only affect the control
system but also degrade the MPPT performance.
which may not only affect the control system but also
CICED2016 Session 3
Paper No 1009
II. Proposed Converter
The scheme of the proposed converter is shown
in Fig.1. It consists of two decoupled power
processing stages. The DHB converter boosts the PV
voltage and directly performs the MPPT. A single
phase full bridge inverter will coordinate with the
DHB converter to achieve the MPPT and delivery the
maximum PV power to the grid.
SST Inverter
inverter stage
isolated stage
S1
C1
Lf
C3
S3
C4
S4
iL f
S2
C2
S5
S7
S6
S8
Grid
I. NTRODUCTION
result in low-frequency ripple energy propagating into
the PV side and degrade the MPPT performance.
Dual half bridge (DHB) could be used as the
isolated stage in the low power SST application[12] .
This paper proposes a novel single phase PV system
based on dual half bridge (DHB) with small DC-link
capacitor. In the proposed approach, the input PV
voltage is directly controlled by the shift phase
between the primary half bridge and secondary half
bridge. Due to the use of DHB, the PV arrays are
immune to large voltage ripple in the dc-link. The
effectiveness of the proposed approach is verified by
simulation results.
VLVDC
Abstract—In the paper, an architecture that includes a
Solid State Transformer (SST) which is different from
the conventional style is proposed. The photovoltaic
system with SST consist of two power stages. First a
dual half bridge (DHB) converter implement the
MPPT functions. Second a grid connected inverter is
used to stabilize the dc-link voltage and make the
output current sinusoidal. The control technique is
presented and validated by simulation implemented on
a photovoltaic system with SST. The simulation results
prove the effectiveness of the proposed photovoltaic
system and the proposed control technique.
Index Terms—Solid State Transformer (SST),
Photovoltaic Inverter, dual half bridge (DHB)
Fig. 1. Proposed inverter topology
A) energy buffer
Ideally, the voltage and current at the output of
the grid connected inverter are line frequency, e.g.[13],
50Hz. Therefore the output power of the grid
connected inverter consist mainly of average power
(which is in dc) and ripple power (which is the
second-order harmonic content at 100Hz).
(1
)
Where V represent grid RMS voltage, I represents
output RMS current,
, and
is
the grid frequency. On the other hand, in the PV
maximum power point (MPP), the PV current and
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2016China International Conferenceon Electricity Distribution (CICED 2016)
voltage are constant thus the power delivered by the
PV is constant. This means that the dc link bus must
absorb the 100Hz ripple power. Because 100Hz is a
relatively low frequency, large bus capacitance is
needed to reduce the bus voltage fluctuation and
achieve sufficiently high ripple current ratings.
B) PV
Fig. 2 shows the equivalent circuit of the ideal PV
cell. The basic equation that mathematically describes
the I-V characteristic of ideal PV cell[14] is
Xi’an, 10-13 Aug, 2016
(4)
where
is the phase shift ratio from the primary side
H-bridge to secondary side H-bridge.
Fig. 3 gives the average circuit model of the DHB
converter. The average current flowing out of the
secondary side H-bridge(see
in Fig.3) during
half of a switching cycle can be given by
(5)
(2)
id  avg 
vo
iLVDC
C
Where
is the current generated by the incident
light (it is directly proportional to the Sun irradiation),
is the reverse saturation or leakage current of
the diode,
is the electron charge
(1.60217646
C),
is the Boltzmann constant
(1.3806503
J/K),
(in Kelvin) is the
temperature of the
junction, and
is the diode
ideality constant.
Load
Fig. 3. average circuit model of the DHB converter
D) grid connected inverter
The dynamics of grid connected inverter for the
inner ac current control is described by
(6)
Fig. 2. Single-diode model of the theoretical PV cell and
and the dc link voltage operation
equavalent circuit of a practical PV device
Practical arrays are composed of several
connected photovoltaic cells and the observation of the
characteristics at the terminals of the photovoltaic
array[15] is described as
(3)
Where
and
are the photovoltaic and saturation
currents of the array and
is the thermal
voltage of the array with
cells connected in series.
C) DHB converter
DHB converter can realize zero-voltageswitching for all the switching devices without
auxiliary switch device, which enables the high
switching frequency operation with low switching
losses. DHB also enable seamless bidirectional power
flow. As a result, high power density and high
efficiency can be achieved. For DHB, the power
transferred is given by[16]
CICED2016 Session 3
Paper No 1009
(7)
where
and
is the grid current and voltage,
is the dc link voltage,
and
is the filter
inductance and dc link capacitor.
III. Control Scheme
The control system is represented in Fig .4, in
which there are three main control aims to be
performed. First, to achieve the MPPT under the
changing atmosphere, the proposed inverter system
will controlled to track the maximum power by the
MPPT unit which calculates the PV voltage reference.
The DHB regulates the PV voltage by controlling the
phase shift. Since the DHB converter is equivalent to a
controlled current source[17] and is simplified as a
first-order system, a PI controlled outer voltage is
selected to control the phase shift and consequently the
power drawn from PV module.
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i pv
v pv
iac
*
vdc

 

Predictive
current
control
id*
PI
vac
First order filter
and DQ transformation
FMA
MPPT
v*pv
DC/AC
Grid
DC/DC
vdc
v pv
i pv
2016China International Conferenceon Electricity Distribution (CICED 2016)
PI
idq
Xi’an, 10-13 Aug, 2016
Max power
1526W
Transformer turns ratio
11:16
Switching frequency
12.8KHz
Leakage inductance (referred to primary) 0.372mH
Dc link capacitor
vdq
Fig. 5 shows the voltage, current and power of
the PV arrays. As it was expected the PV power reach
the maximum power point.
iq*
Fig. 4. Control system block diagram
Second, the dc link voltage should be regulated
around its average value with desired bandwidth and
phase margin. Since the ripple at frequency twice the
grid is large, such a result will yield to grid current
distortion which is not desirable. A Fast moving
average (FMA) filter[18] that has a time window with
the length corresponding to the period of second
harmonic ripple is employed to filter the measured dc
link voltage. A standard proportional integral (PI)
controller is designed to control the filtered dc-link
voltage, which give the current reference of the inner
loop of the grid connected inverter.
Third, the output current of the grid connected
inverter is synchronized with the grid voltage for unit
power factor. Due to the time-varying nature of single
phase grid connected inverter, it is also difficult to
achieve good performance. It is much easier to
perform analysis and design controllers for single
phase rectifier in DQ frame because all time-varying
state variables of the single phase rectifier become DC
time-invariant[19]. The second orthogonal imaginary
circuit is created to satisfy the requirement of a
minimum of two independent phases in the
implementation of DQ controllers. a first-order
all-pass filter is used to create the second orthogonal
signal, which improves the response of the controller
by removing the
delay. The predictive current
control is selected in the current control loop because
of its faster dynamic response.
Fig. 5. PV arrays simulation waveforms
The DHB current is shown in Fig. 6. As can be
seen the current reaches the same point after half
switching period.
Fig. 6. primary side current of the DHB converter
The dc link voltage is shown in Fig. 7. As it was
expected the dc link voltage is stabilized around 400V.
The ripple twice the grid frequency is observed in the
result.
IV. Simulation Results
The proposed inverter system was simulated in
Matlab/Simulink. The simulated circuit parameters are
shown in table I. Main simulation results are shown in
Figs. 5-8.
Fig. 7. dc link voltage simulation waveforms
TABLE I.
SIMULATION PARAMETERS
Description
Value
PV Max Voltage
273.5V
Dc bus average voltage
400V
CICED2016 Session 3
Paper No 1009
The grid current and grid voltage is shown in Fig.
8. A unit power operation is achieved by the current
control.
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2016China International Conferenceon Electricity Distribution (CICED 2016)
Fig. 8. grid current and grid voltage simulation waveforms
V. CONCLUSION
In this paper, a new topology for interconnection
of PV modules to the grid is proposed. The topology
consists of two stage converter: a DHB converter, and
a grid connected inverter. The DHB converter
implement the MPPT functions. The grid connected
inverter injects sinusoidal current into the grid. A DQ
frame predictive current control is employed to
achieve the fastest dynamic response of the grid
current. The simulation result shown the effectiveness
of the proposed PV inverter system.
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Baolong Liu was born Changchun, China in 1988. He is currently
working towards the Ph.D. degree in College of Information System
and Management of National University of Defense Technology
(NUDT). His main interests are power management and control in
Energy Internet, control methods of power converters. E-mail:
[email protected]
Yabing ZHA was born in Jiangxi, China in 1968. He received his
Ph.D. degree in system simulation from National University of
Defense Technology (NUDT) in 1995. Now he is the dean of
College of Information System and Management, NUDT. His
research interests include conceptual modeling, system simulation
and integration, simulation Verification, Validation and
Accreditation (VV&A).
Tao Zhang was born in Anhui, China in 1976. He received his Ph.D.
degree in management science and engineering from NUDT in 2004.
Now he is a professor of NUDT. His research interests include
complex system modeling and optimization, battery management
technology, energy internet technology.
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