ABSTRACT
Cost of various energy storage technologies is decreasing
rapidly and the integration of these technologies into the power grid is
becoming a reality with the advent of smart grid. Dynamic voltage restorer
(DVR) is one product that can provide improved voltage sag and swell compensation
with energy storage integration. Ultra-capacitors (UCAP) have low-energy
density and high-power density ideal characteristics for compensation of
voltage sags and voltage swells, which are both events that require high power
for short spans of time. The novel contribution of this paper lies in the
integration of rechargeable UCAP-based energy storage into the DVR topology.
With this integration, the UCAP-DVR system will have active power capability
and will be able to independently compensate temporary voltage sags and swells
without relying on the grid to compensate for faults on the grid like in the past.
UCAP is integrated into dc-link of the DVR through a bidirectional dc–dc
converter, which helps in providing a stiff dc-link voltage, and the integrated
UCAP-DVR system helps in compensating temporary voltage sags and voltage
swells, which last from 3 s to 1 min. Complexities involved in the design and
control of both the dc–ac inverter and the dc–dc converter are discussed. The simulation
model of the overall system is developed.
KEYWORDS
1.
Digital Signal
Processing (DSP)
2.
Dynamic voltage
restorer (DVR)
3.
Energy storage
integration
4.
Phase locked loop
(PLL)
5.
Ultracapacitor
(UCAP).
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM
Fig.
1. One-line diagram of DVR with UCAP energy storage.
Fig.
2. Model of three-phase series inverter (DVR) and its controller with
integrated
higher order controller.
EXPECTED SIMULATION
RESULTS
Fig.
4. (a) Source and load RMS voltages Vsrms and VLrms during
sag.(b) Source voltages Vsab (blue), Vsbc (red), and Vsca (green)
during sag. (c) Load voltages VLab (blue), VLbc (red), and VLca
(green) during sag. (d) Injected voltages Vinj2a (blue), Vinj2b
(red), and Vinj2c (green) during sag. (e) Vinj2a (green)
and Vsab (blue) waveforms during sag.
Fig.
5. (a) Currents and voltages of dc–dc converter. (b) Active power of grid, load,
and inverter during voltage sag.
Fig.
6. (a) Source and load rms voltages Vsrms and VLrms during swell.
(b) Source voltages Vsab (blue), Vsbc (red), and Vsca (green)
during swell. (c) Load voltages VLab (blue), VLbc (red), and VLca
(green) during swell. (d) Injected voltages Vinj2a (blue), Vinj2b
(red), Vinj2c (green) during swell. (e) Vinj2a (green)
and Vsab (blue) waveforms during swell.
Fig.
7. (a) Currents and voltages of dc–dc converter during swell. (b) Active and
reactive power of grid, load, and inverter during a voltage swell.
Fig.
8. (a) UCAP and bidirectional dc–dc converter simulation waveforms Ecap
(CH1), Vfdc (CH2), Idclnk (CH3) and Iucav (CH4) during
voltage sag. (b) Inverter simulation waveforms Vsab (CH1), VLab (CH2)
and Vinj2a (CH3) and ILa (CH4) during the voltage sag.
Fig.
9. (a) UCAP and dc–dc converter simulation waveformsEcap (CH1), Vfdc
(CH2), Idclnk (CH3), and Iucav (CH4) during voltage swell. (b)
Inverter simulation waveforms Vsab (CH1), VLab (CH2) and Vinj2a
(CH3) and ILa (CH4) during the voltage swell.
Fig.
10. (a) Inverter experimental waveforms VLab (CH1), Vsa (CH2), Vsb
(CH3), and ILa (CH4) for during an unbalanced sag in phases a and
b. (b) Bidirectional dc–dc converter waveforms Ecap (CH1),
Vfdc (CH2), Idclnk (CH3), and Iucav (CH4) showing
transient response during an unbalanced sag in phases a and b.
CONCLUSION
In
this paper, the concept of integrating UCAP-based rechargeable energy storage to
the DVR system to improve its voltage restoration capabilities is explored.
With this integration, the DVR will be able to independently compensate voltage
sags and swells without relying on the grid to compensate for faults on the
grid. The UCAP integration through a bidirectional dc–dc converter at the
dc-link of the DVR is proposed. The power stage and control strategy of the
series inverter, which acts as the DVR, are discussed. The control strategy is
simple and is based on injecting voltages in-phase with the system voltage and
is easier to implement when the DVR system has the ability to provide active
power. A higher level integrated controller, which takes decisions based on the
system parameters, provides inputs to the inverter and dc–dc converter
controllers to carry out their control actions. Designs of major components in
the power stage of the bidirectional dc–dc converter are discussed. Average
current mode control is used to regulate the output voltage of the dc–dc
converter due to its inherently stable characteristic.
The simulation of the UCAP-DVR system, which
consists of the UCAP, dc–dc converter, and the grid-tied inverter, is carried
out using PSCAD. Hardware experimental setup of the integrated system is presented
and the ability to provide temporary voltage sag and swell compensation in all
three phases to the distribution grid dynamically is tested. Results for
transient response during voltage sags/swells in two phaseswill be included in
the full-version of this paper. Results from simulation and experiment agree
well with each other thereby verifying the concepts introduced in this paper.
Similar UCAPbased energy storages can be deployed in the future on the distribution
grid to respond to dynamic changes in the voltage profiles of the grid and
prevent sensitive loads from voltage disturbances.
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