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.
- Digital Signal Processing (DSP)
- Dynamic voltage restorer (DVR)
- Energy storage integration
- Phase locked loop (PLL)
- Ultracapacitor (UCAP).
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.
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 phases will 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 UCAP based 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.
- H. Woodley, L. Morgan, and A. Sundaram, “Experience with an inverter-based dynamic voltage restorer,” IEEE Trans. Power Del., vol. 14, no. 3, pp. 1181–1186, Jul. 1999.
- S. Choi, B. H. Li, and D.M. Vilathgamuwa, “Dynamic voltage restoration with minimum energy injection,” IEEE Trans. Power Syst., vol. 15, no. 1, pp. 51–57, Feb. 2000.
- M. Vilathgamuwa, A. A. D. R. Perera, and S. S. Choi, “Voltage sag compensation with energy optimized dynamic voltage restorer,” IEEE Trans. Power Del., vol. 18, no. 3, pp. 928–936, Jul. 2003.
- W. Li, D. M. Vilathgamuwa, F. Blaabjerg, and P. C. Loh “A robust control scheme for medium-voltage-level DVR implementation,” IEEE Trans. Ind. Electron., vol. 54, no. 4, pp. 2249–2261, Aug. 2007.
- Ghosh and G. Ledwich, “Compensation of distribution system voltage using DVR,” IEEE Trans. Power Del., vol. 17, no. 4, pp. 1030–1036, Oct. 2002.