International Journal of Research in Computer and ISSN (Online) 2278- 5841
Communication Technology, Vol 4, Issue 1, January- 2015 ISSN (Print) 2320- 5156
To Resolve The Voltage Unbalance Of The Dc Links In Different
H-Bridges Based Solid State Transformer (Sst)
Peravali Jyotsna,P.Suguna Ratna Mala
M.Tech(Research Scholar),Associate Professor in Dept. of EEE,
Godavari Institute Of Engineering & Technology,AP,
Email Id: jyotsnaperavali@yahoo.com,suguna.236@gmail.com
Abstract:
As the expansion of the dc distribution system and
the augment of the penetration of distributed
generations an intelligent transformer with the
capability to actively supervise the power and
allowing for the easy connection of the distribution
resources is becoming essential. The solid state
transformer (SST) has the features of immediate
voltage regulation, voltage sag compensation, fault
isolation, power factor correction, harmonic
isolation and dc output. Acting very much like an
energy router, each SST has bidirectional energy
flow control potential allowing it to control active
and reactive power flow and to handle the fault
currents on both low- and high-voltage sides. Its
large control bandwidth offers the plug-and-play
feature for distributed resources to speedily
recognize and respond to changes in the system.
This paper proposes a 20-kVA cascaded H-Bridge
multilevel converter-based SST to directly interface
with 7.2-kV single-phase distribution voltage level.
The SST consists of a cascaded multilevel ac/dc
rectifier, dual active bridge (DAB) converters with
high-frequency transformers. The DAB converter
regulates the 400-V-low-voltage dc bus and added
dc/ac inverters can be added to present a 60 Hz
120/240-V ac residential voltage.
Keywords: Cascaded H-Bridge converter, dq
vector control, solid-state transformer (SST),
voltage and power balance.
Introduction:
The paper recommends a new voltage and power
balance control for the cascaded H-Bridge
converter-based SST. Based on the single-phase dq
model a novel voltage and the power control
strategy is proposed to stability the rectifier
capacitor voltages and the real power through
parallel DAB modules. In addition the intrinsic
power constraints of the cascaded H-Bridge voltage
balance control are derived and analyzed. With the
proposed control methods the dc-link voltage and
the real power through each module can be
balanced. The SST switching model simulation and
the prototype experiments are presented to validate
the performance of the proposed voltage and power
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balance
controller.
Low-frequency
PWM
techniques for STATCOM are presented. The dclink voltage is balanced by using different
switching patterns to charge and discharge each HBridge capacitor but the reactive power is not
controlled. Barrena et al. present an individual
voltage-balancing scheme to balance the dc-link
voltage with PWM. The method preserves
delivered reactive power equally distributed among
all H-Bridges. However the method is based on the
STATCOM application and no power unbalance
constraints are mentioned in this paper.
Related Work:
In the proposed electric configuration of the smart
grid system low voltage (120 V), residential class
distributed renewable energy resource (DRER),
distributed energy storage device (DESD) and
loads are connected to the 400-V dc distribution
bus and then to distribution bus (12-kV three phase
or 7.2-kV single phase) through a SST. The SST is
used to facilitate active management of DRER,
DESD, and loads, rather than a 60-Hz conventional
transformer. The SST has a 400-V dc port that will
make possible more professional connection of
certain classes of DRERs and DESDs. The
regulated 400-V dc bus is distributed for easier
connection of battery and other distributed
resources. The rectifier stage is a seven-level
cascaded H-Bridge converter which controls the
input power factor and regulates the 3.8-kV highvoltage dc link.
Existing Method:
Iman-Eini et al. present a method that makes sure
the dc-link voltages congregate to the reference
value when load power is different. The method
results in different switching frequencies for the HBridges
and
a
complicated
controller
implementation. Leon et al. and Barrena et al.
presents different PWM methods to balance the dclink voltages. However the balance range and
power control are not included.
Disadvantages:
One of the main disadvantages of the cascaded Hbridge rectifier is the voltage unbalance of the dclink voltages at different H-Bridges. Relevant
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International Journal of Research in Computer and ISSN (Online) 2278- 5841
Communication Technology, Vol 4, Issue 1, January- 2015 ISSN (Print) 2320- 5156
research mainly focuses on the unbalance issues in
static synchronous compensator (STATCOM) or
drive applications and no power control method. It
necessitates a high frequency modulation and both
real and reactive power control. Besides due to the
intrinsic constraint of the cascaded rectifier
topology the voltage balance control is limited and
the power balance control is therefore required.
Proposed Method:
The modelling of the SST, including ac/dc rectifier,
DAB converter are developed. A single-phased dq
vector controller is designed for the rectifier stage;
therefore the real power and reactive power can be
controlled separately. Based on the single-phase dq
control, a voltage balance control system is
proposed to resolve the voltage unbalance that
could appear on the dc links of different H-bridges.
The projected voltage and power control are
verified by the switching model simulation. Finally,
a SST scale-down prototype is implemented by
using 600-V insulated gate bipolar transistor
(IGBT) devices.
Advantages:
It has balanced dc-bus voltage, balanced real
power, evenly distributed reactive power between
H-bridges and use simple phase-shift pulse width
modulation (PWM).
System Architecture:
-
frequency transformers to provide a regulated 400V dc distribution, and an optional dc/ac stage that
can be connected to the 400-V dc bus to provide
residential 120/240 Vac. The SST is rated as
single-phase input voltage 60 Hz, 7.2 kV, output
voltage 400-V dc. The first stage of the SST is a
high-voltage cascaded H-Bridge ac/dc rectifier that
converts 60 Hz, 7.2-kV ac to three cascaded 3.8-kV
dc links. The second stage consists of three highvoltage high-frequency dc/dc converters that
convert 3.8 kV to 400-V dc bus and then additional
inverter can be connected to 400-V dc bus to invert
400-V dc to 60-Hz, 240/120 V. The switching
devices in high-voltage H-bridges and low-voltage
H-bridges are 6.5-kV IGBT and 600-V IGBT,
respectively. The switching frequency of the 6.5kV IGBT devices is 1 kHz, because the device
current is very low resulting in low switching
losses. The 20-kVA SST unit is envisioned as a
building block for construction of a larger rated
SST. The controller of each stage in the SST is
independent from each other.
Rectifier Controller:
The function of the rectifier controller is to control
the reactive power (or power factor) and regulate
the dc-link voltages. With the chosen phase-locked
loop, the voltage vector is aligned with the
direction of the d-axis during steady state. The
grid-voltage component in the d-direction is equal
to its peak value and the q-component of the grid
voltage is equal to zero. Thus, the d-component of
the current vector (in steady state parallel to the
grid-voltage vector) becomes the active current
component (d-current) and the q-component of the
current vector becomes the reactive current
component (q-current) [15]. The decoupled dq
vector controller for each H-bridge is shown in Fig.
3. The three sinusoidal pulse width modulation
(SPWM) carriers for the cascaded H-bridge are
phase shifted so that the rectifier has seven voltage
levels to reduce the voltage stress and harmonics.
Dual Active Bridge (Dab):
The DAB topology offers zero voltage switching,
relatively low-voltage stress for the switches, low
passive component ratings and complete symmetry
of configuration that allows seamless control for
bidirectional power flow. Real power flows from
the bridge with leading phase angle to the bridge
with lagging phase angle, the amount of transferred
power is controlled by the phase angle difference
and the magnitudes of the dc voltages at the two
ends.
The solid-state transformer (SST) is an interface
device between ac distribution grids and dc
distribution systems. The SST consists of a
cascaded multilevel ac/dc rectifier stage, a dual
active bridge (DAB) converter stage with high-
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Page 74
International Journal of Research in Computer and ISSN (Online) 2278- 5841
Communication Technology, Vol 4, Issue 1, January- 2015 ISSN (Print) 2320- 5156
where, VdcH is input side high dc-link voltage, fH
is switching frequency, L is leakage inductance, V
dcL is output side low dc bus voltage referred to
input side, and ddc is the ratio of time delay
between the two bridges to one-half of switching
period. For the DAB converter, the phase-shift
control is applied to regulate the low-voltage dc
voltage to the reference 400 V under different load
conditions. First the difference between the low dc
voltage Vdc and the reference voltage is compared.
Then, the
phase-shift angle is adjusted by the proportionalintegral (PI) controller to regulate VdcL according
to this voltage error.
Voltage And Power Balance Control:
The three H-Bridge dc-link voltages become
unbalanced after the power change. The H-bridge
that transfers more power has the highest dc-link
Voltage. Fig. 8 shows the three dc-link voltages
with voltage balance control. With the voltage
balance controller, the three dc-link voltages are
equally regulated in the steady state. The different
resistors simulate a 20% power difference, which is
caused by no power balance control in the DAB
modules. So when the power balancing control is
implemented in DAB, the power difference is
dominated by converter power losses, the power
unbalance is much smaller than 20%. The
switching model simulation is implemented to
verify the proposed power balance control. The
DAB module with smaller leakage inductance has
large current and transfers more power. Figs. 13
and 15 illustrate the DAB primary currents and
power with power balance control. The transformer
current and the power transferring through each
DAB module is balanced. So the power balance
control guarantees the power unbalance of the
parallel DAB modules meets the (21), so that the
dc-link voltages can always be balanced with the
proposed voltage balance control.
Voltage Balance Constraints:
Due to the intrinsic constraints of the cascaded HBridge structure, in order to maintain the dc-link
balanced, the real power unbalance range of the HBridges is limited. This limitation is determined by
the input ac voltage, dc-link voltage reference,
input inductance, and the number of H-bridges.
where, Vn is the Nth H-Bridge voltage vector, Vline
is the single phase input ac voltage, Iline is the
input ac current, dn and qn are the d-axis and q-axis
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duty cycle generated by the controller, and E is the
dc-link reference voltage of each H-Bridge.
Experimental Results:
The dc-link voltage is regulated to the reference
400 V total (133 V each H-Bridge) and the input
current is in phase with the input voltage, which
indicates a unity power factor. The SST rectifier
stage not only converts the input ac to regulated dc
voltages, but also has reactive power compensation
capabilities. Depending on the reactive power
reference in the SST controller, the SST can
generate or absorb reactive power to the power
grid.
Conclusion:
A new voltage balance control method is proposed
to resolve the voltage unbalance of the dc links in
different H-bridges. The power intrinsic unbalance
constraints of the voltage balance control for the
cascaded H-Bridge rectifier is derived and verified
by simulations and experiments. Meanwhile, a
power balance control method is proposed to
regulate the real power transferring through the
parallel modules. Finally, the switching model
simulation and SST scale-down prototype are
implemented with the proposed controller. The
single-phase dq vector modeling and control of the
SST, including ac/dc rectifier, DAB converter is
developed.
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