# Control Strategy for a Grid Connected Converter in Active Unbalanced Distribution Systems

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

- This paper presents an improved GCC control technique capable of maintaining adequate operation under asymmetrical voltages, showcasing the main benefits in comparison to the classical technique.
- The paper gives an extensive experimental verification of the improved technique behavior under different voltage sag types (according to the ABC classification), including operation under the most severe conditions. The developed GCC control technique is an improvement of the classical control technique with relatively low computational complexity and, in that regard, can be implemented in new GCC, as well as in the GCC already existing in the ADS.

## 2. Grid Connection Requirements in Active Unbalanced Distribution Networks

_{n}y

_{n}type, these transformers do not influence the voltage sag types at the PCC. In case of the Dd, Dz, and Yy transformers, the zero sequence voltage will be removed. The most important change comes from the Dy, Yd, and Yz transformers, since they change the line and phase voltages, resulting in an altered voltage sag type in the process. In the same context, the load connection (Y or D) will also significantly affect the voltage sag type at the PCC.

_{max}), the small radius (r

_{min}), and the inclination (φ

_{inc}). The values of the parameters can be calculated from the positive (superscript p) and negative (superscript n) sequence voltages as [28]:

## 3. Control Methodology for a Grid Connected Converter under Asymmetrical Voltages

_{d}, equals a quarter of the ratio between the sampling and the grid frequencies. Due to known issues with the discretization of the DSC technique, special attention should be given to the selection of the sampling frequency, since the mentioned parameter, n

_{d}, should be an integer number. If the discretization introduces a delay time of ΔT to the n

_{d}T

_{s}product, the expected error for the negative sequence separation will be:

_{g}, the integral gain parameter of the PI controller can be calculated as following:

## 4. Experimental Verification of the Improved GCC Control Strategy

#### 4.1. Experiment I—10% Voltage Sag in One Phase

#### 4.2. Experiment II—20% Voltage Sag in One Phase and a 5 Degree Phase Shift

#### 4.3. Experiment III and IV—Voltage Sags Type C and E

#### 4.4. Experiment V—10% Overvoltage in One Phase

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

- Kamh, M.Z.; Iravani, R. Unbalanced Model and Power-Flow Analysis of Microgrids and Active Distribution Systems. IEEE Trans. Power Deliv.
**2010**, 25, 2851–2858. [Google Scholar] [CrossRef] - Twidell, J.; Weir, T. Renewable Energy Resources, 2nd ed.; Routledge: London, UK, 2005; ISBN 978-0-419-25330-3. [Google Scholar]
- IRENA. Renewable Energy Prospects for the European Union: Preview for Policy Makers; IRENA: Abu Dhabi, UAE, 2018. [Google Scholar]
- Khushalani, S.; Schulz, N. Unbalanced Distribution Power Flow with Distributed Generation. In Proceedings of the 2005/2006 IEEE/PES Transmission and Distribution Conference and Exhibition, Dallas, TX, USA, 21–24 May 2006; pp. 301–306. [Google Scholar]
- Nasr-Azadani, E.; Canizares, C.A.; Olivares, D.E.; Bhattacharya, K. Stability Analysis of Unbalanced Distribution Systems with Synchronous Machine and DFIG Based Distributed Generators. IEEE Trans. Smart Grid
**2014**, 5, 2326–2338. [Google Scholar] [CrossRef] - Sharma, I.; Bozchalui, M.C.; Sharma, R. Smart operation of unbalanced distribution systems with PVs and Energy Storage. In Proceedings of the 2013 IEEE International Conference on Smart Energy Grid Engineering (SEGE), Oshawa, ON, Canada, 28–30 August 2013; pp. 1–6. [Google Scholar]
- Jia, J.; Yang, G.; Nielsen, A.H. A Review on Grid-connected Converter Control for Short Circuit Power Provision under Grid Unbalanced Faults. IEEE Trans. Power Deliv.
**2018**, 33, 649–661. [Google Scholar] [CrossRef] - Ma, K.; Liserre, M.; Blaabjerg, F. Power controllability of three-phase converter with unbalanced AC source. In Proceedings of the 2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition, Long Beach, CA, USA, 17–21 March 2013; pp. 342–350. [Google Scholar]
- Ji, Y.; Sun, P.; Wu, Y.; Du, X.; Tai, H.-M.; Gu, S. Power oscillation analysis and control of three-phase grid-connected voltage source converters under unbalanced grid faults. IET Power Electron.
**2016**, 9, 2162–2173. [Google Scholar] - Liu, Y.; Li, N.; Fu, Y.; Wang, J.; Ji, Y. Stationary-frame-based generalized control diagram for PWM AC-DC front-end converters with unbalanced grid voltage in renewable energy systems. In Proceedings of the 2015 IEEE Applied Power Electronics Conference and Exposition (APEC), Charlotte, NC, USA, 15–19 March 2015; pp. 678–683. [Google Scholar]
- Jiang, W.; Ma, W.; Wang, J.; Wang, L.; Gao, Y. Deadbeat Control Based on Current Predictive Calibration for Grid-Connected Converter Under Unbalanced Grid Voltage. IEEE Trans. Ind. Electron.
**2017**, 64, 5479–5491. [Google Scholar] [CrossRef] - Tran, T.; Yoon, S.-J.; Kim, K.-H. An LQR-Based Controller Design for an LCL-Filtered Grid-Connected Inverter in Discrete-Time State-Space under Distorted Grid Environment. Energies
**2018**, 11, 2062. [Google Scholar] [CrossRef] - Zhang, W.; Rocabert, J.; Candela, J.I.; Rodriguez, P. Synchronous Power Control of Grid-Connected Power Converters under Asymmetrical Grid Fault. Energies
**2017**, 10, 950. [Google Scholar] [CrossRef] - Ozsoy, E.; Padmanaban, S.; Mihet-Popa, L.; Fedák, V.; Ahmad, F.; Akhtar, R.; Sabanovic, A. Control Strategy for a Grid-Connected Inverter under Unbalanced Network Conditions—A Disturbance Observer-Based Decoupled Current Approach. Energies
**2017**, 10, 1067. [Google Scholar] [CrossRef] - Yazdani, A.; Iravani, R. A Unified Dynamic Model and Control for the Voltage-Sourced Converter Under Unbalanced Grid Conditions. IEEE Trans. Power Deliv.
**2006**, 21, 1620–1629. [Google Scholar] [CrossRef] - Revelo, S.; Silva, C.A. Current reference strategy with explicit negative sequence component for voltage equalization contribution during asymmetric fault ride through: Current Strategy with Negative Sequence Component. Int. Trans. Electr. Energy Syst.
**2015**, 25, 3449–3471. [Google Scholar] [CrossRef] - Lai, N.; Kim, K.-H. An Improved Current Control Strategy for a Grid-Connected Inverter under Distorted Grid Conditions. Energies
**2016**, 9, 190. [Google Scholar] [CrossRef] - Xiao, P.; Corzine, K.A.; Venayagamoorthy, G.K. Multiple Reference Frame-Based Control of Three-Phase PWM Boost Rectifiers under Unbalanced and Distorted Input Conditions. IEEE Trans. Power Electron.
**2008**, 23, 2006–2017. [Google Scholar] [CrossRef] - Galecki, A. Control system of the grid-connected converter based on a state current regulator with oscillatory terms. Przegled Elektrotechniczny
**2015**, 1, 67–71. [Google Scholar] [CrossRef] - Bobrowska-Rafal, M.; Rafal, K.; Jasinski, M.; Kazmierkowski, M. Grid synchronization and symmetrical components extraction with PLL algorithm for grid connected power electronic converters—A review. Bull. Pol. Acad. Sci. Tech. Sci.
**2011**, 59, 485–497. [Google Scholar] [CrossRef] - Pilo, F.; Jupe, S.; Silvestro, F.; Abbey, C.; Baitch, A.; Bak-Jensen, B.; Carter-Brown, C.; Celli, G.; El Bakari, K.; Fan, M.; et al. Planning and Optimization Methods for Active Distribution Systems; CIGRE: Paris, France, 2014. [Google Scholar]
- Liu, J.; Gao, H.; Ma, Z.; Li, Y. Review and prospect of active distribution system planning. J. Mod. Power Syst. Clean Energy
**2015**, 3, 457–467. [Google Scholar] [CrossRef][Green Version] - Xu, W.; Dommel, H.W.; Marti, J.R. A generalised three-phase power flow method for the initialisation of EMTP simulations. In Proceedings of the 1998 International Conference on Power System Technology, Beijing, China, 18–21 August 1998; Volume 2, pp. 875–879. [Google Scholar]
- Wang, D.T.-C.; Ochoa, L.F.; Harrison, G.P. Modified GA and Data Envelopment Analysis for Multistage Distribution Network Expansion Planning Under Uncertainty. IEEE Trans. Power Syst.
**2011**, 26, 897–904. [Google Scholar] [CrossRef][Green Version] - Dumnic, B.; Popadic, B.; Milicevic, D.; Vukajlovic, N.; Delimar, M. Grid Connected Converter Control Technique in Active Unbalanced Distribution Systems. In Proceedings of the 2018 Mediterranean Conference on Power Generation, Transmission, Distribution and Energy Conversion, Dubrovnik (Cavtat), Croatia, 12–15 November 2018; pp. 1–6. [Google Scholar]
- Overview of Power Quality and Power Quality Standards. In Understanding Power Quality Problems; IEEE: Piscataway, NI, USA, 2009; ISBN 978-0-470-54684-0.
- Ignatova, V.; Granjon, P.; Bacha, S.; Dumas, F. Classification and characterization of three phase voltage dips by space vector methodology. In Proceedings of the 2005 International Conference on Future Power Systems, Amsterdam, The Netherlands, 16–18 November 2005; IEEE: Amsterdam, The Netherlands, 2005. [Google Scholar]
- Bachschmid, N.; Pennacchi, P.; Vania, A. Diagnostic significance of orbit shape analysis and its application to improve machine fault detection. J. Braz. Soc. Mech. Sci. Eng.
**2004**, 26, 200–208. [Google Scholar] [CrossRef] - Popadic, B.; Katic, V.; Dumnic, B.; Milicevic, D.; Corba, Z. Synchronization method for grid integrated battery storage systems during asymmetrical grid faults. Serb. J. Electr. Eng.
**2017**, 14, 113–131. [Google Scholar] [CrossRef] - Popadic, B.; Dumnic, B.; Milicevic, D.; Corba, Z.; Vukajlovic, N. Behavior of the Grid Connected Converter in Unbalanced Smart Power Systems. In Proceedings of the 2018 International Symposium on Industrial Electronics (INDEL), Banja Luka, Republic of Srpska, Bosnia and Herzegovina, 1–3 November 2018; IEEE: Banja Luka, Republic of Srpska, Bosnia and Herzegovina, 2018; pp. 1–6. [Google Scholar]
- Popadic, B.; Dumnic, B.; Milicevic, D.; Katic, V.; Sljivac, D. Grid-connected converter control during unbalanced grid conditions based on delay signal cancellation. Int. Trans. Electr. Energy Syst.
**2018**, 12, e2636. [Google Scholar] [CrossRef] - Blaabjerg, F. (Ed.) Control of Power Electronic Converters and Systems; Academic Press: London, UK, 2018; ISBN 978-0-12-805245-7. [Google Scholar]
- Vujkov, B.; Dumnic, B.; Popadic, B.; Milicevic, D.; Vukajlovic, N.; Katic, V. Advanced research and development facility for digital control of power electronic based drives. In Proceedings of the 2018 International Symposium on Industrial Electronics (INDEL), Banja Luka, Republic of Srpska, Bosnia and Herzegovina, 1–3 November 2018; IEEE: Banja Luka, Republic of Srpska, Bosnia and Herzegovina, 2018; pp. 1–6. [Google Scholar]

**Figure 1.**The cumulative installed capacity of renewable energy sources in Europe [3].

**Figure 3.**Fault ride-through requirements by different distribution systems’ (DSs) and transmission systems’ (TSs) grid codes.

**Figure 9.**Converter (grid injected) current waveforms at the point of common coupling in one phase for the standard and improved technique—10% voltage sag in one phase.

**Figure 11.**Grid connected converter power (

**a**), positive sequence current (

**b**), and negative sequence current (

**c**) during the experiment—10% voltage sag in one phase.

**Figure 12.**Grid connected converter operation under one phase 20% voltage sag with a phase shift of 5 degree—voltage space vector trajectory (

**a**), current waveforms for 2 A reference (

**b**), power for 2 A reference (

**c**), positive sequence currents for 2 A reference (

**d**), negative sequence current for 2 A reference (

**e**), and current waveforms for 3 A reference (

**f**).

**Figure 13.**Improved control technique operation for a one phase 20% voltage sag with a 5 degree phase shift—current waveforms (

**a**), negative sequence currents (

**b**), and power (

**c**).

**Figure 14.**Improved control technique operation for the C type voltage sag—current waveforms (

**a**), negative sequence currents (

**b**), and power (

**c**).

**Figure 15.**Improved control technique operation for the E type voltage sag—current waveforms (

**a**), negative sequence currents (

**b**), and power (

**c**).

**Figure 16.**Converter (grid injected) current waveforms at the point of common coupling in one phase for the standard and improved technique—10% overvoltage in one phase.

**Figure 18.**Grid connected converter power (

**a**), positive sequence current (

**b**), and negative sequence current (

**c**) during the experiment—10% overvoltage in one phase.

Type | Voltage Value | Voltage Vector |
---|---|---|

A | ${\overline{v}}_{a}=(1-k)V$${\overline{v}}_{b}=-\frac{1}{2}(1-k)V-j\frac{\sqrt{3}}{2}(1-k)V$${\overline{v}}_{c}=-\frac{1}{2}(1-k)V+j\frac{\sqrt{3}}{2}(1-k)V$ | |

B | ${\overline{v}}_{a}=(1-k)V$${\overline{v}}_{b}=-\frac{1}{2}V-j\frac{\sqrt{3}}{2}V$${\overline{v}}_{c}=-\frac{1}{2}V+j\frac{\sqrt{3}}{2}V$ | |

C | ${\overline{v}}_{a}=V$${\overline{v}}_{b}=-\frac{1}{2}V-j\frac{\sqrt{3}}{2}(1-k)V$${\overline{v}}_{c}=-\frac{1}{2}V+j\frac{\sqrt{3}}{2}(1-k)V$ | |

D | ${\overline{v}}_{a}=(1-k)V$${\overline{v}}_{b}=-\frac{1}{2}(1-k)V-j\frac{\sqrt{3}}{2}V$${\overline{v}}_{c}=-\frac{1}{2}(1-k)V+j\frac{\sqrt{3}}{2}V$ | |

E | ${\overline{v}}_{a}=V$${\overline{v}}_{b}=-\frac{1}{2}(1-k)V-j\frac{\sqrt{3}}{2}(1-k)V$${\overline{v}}_{c}=-\frac{1}{2}(1-k)V+j\frac{\sqrt{3}}{2}(1-k)V$ | |

F | ${\overline{v}}_{a}=(1-k)V$${\overline{v}}_{b}=-\frac{1}{2}(1-k)V-j(\frac{\sqrt{3}}{6}(1-k)V+\frac{\sqrt{3}}{3}V)$${\overline{v}}_{c}=-\frac{1}{2}(1-k)V+j(\frac{\sqrt{3}}{6}(1-k)V+\frac{\sqrt{3}}{3}V)$ | |

G | ${\overline{v}}_{a}=\frac{2}{3}V+\frac{1}{3}(1-k)V$${\overline{v}}_{b}=-\frac{1}{3}V-\frac{1}{6}(1-k)V-j\frac{\sqrt{3}}{2}(1-k)V$${\overline{v}}_{c}=-\frac{1}{3}V-\frac{1}{6}(1-k)V+j\frac{\sqrt{3}}{2}(1-k)V$ |

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Dumnic, B.; Popadic, B.; Milicevic, D.; Vukajlovic, N.; Delimar, M.
Control Strategy for a Grid Connected Converter in Active Unbalanced Distribution Systems. *Energies* **2019**, *12*, 1362.
https://doi.org/10.3390/en12071362

**AMA Style**

Dumnic B, Popadic B, Milicevic D, Vukajlovic N, Delimar M.
Control Strategy for a Grid Connected Converter in Active Unbalanced Distribution Systems. *Energies*. 2019; 12(7):1362.
https://doi.org/10.3390/en12071362

**Chicago/Turabian Style**

Dumnic, Boris, Bane Popadic, Dragan Milicevic, Nikola Vukajlovic, and Marko Delimar.
2019. "Control Strategy for a Grid Connected Converter in Active Unbalanced Distribution Systems" *Energies* 12, no. 7: 1362.
https://doi.org/10.3390/en12071362