Virtual Impedance Based Decentralized Control for a Microgrid
Virtual Impedance Based Decentralized Control for a Microgrid System Ramkrishna Mishan, MS Department of Electrical and Computer Engineering and Technology December 07, 2020
Outline Ø Introduction Ø Research Background Ø Research Objectives Ø Grid-Connected Microgrid Simulink Model Ø Islanded Microgrid Simulink Models Ø Islanded Microgrid Experiments and Results Ø Conclusions 2
Renewable Energy Technology and DG Climate change and coal, gas, oil etc. depletion cause electric power generation shifts from non-renewable to renewable sources. Renewable energy sources are continuously renewed by nature When renewable sources produce electric power denotes as a distributed generator. 3
Microgrid Operational Modes Microgrid Definition: • A microgrid facilitates the interconnection of different distributed generations • regulates the bidirectional power flow between consumer and generating stations Ø Grid-connected Mode-- When DGs connect to the traditional grid Ø Islanded Mode-- When DGs operate as standalone without connecting traditional grid 4
Research Background As more DGs integrate to power generations, the microgrid transits from grid-connected mode to islanded mode. In islanded mode, the grid inertia is very low without proper control system using power electronics converter and energy storage. As no longer the system inertia is provided by traditional grid, it is difficult to maintain the stable grid with proper power sharing under this mode. The intermittent nature of DG power generation and demand power makes it further deteriorate the grid stability in islanded mode. 5
Research Objectives To Maintain Proper Active-Reactive Power Sharing To Stabilize Grid Voltage and Frequency To Maintain Power Quality or THD 6
PQ Controlled DG • Resistive Transmission Line 7
VF Controlled DG • Calculated dynamic power references to get the stable grid and proper power-sharing in islanded mode. 8
Grid-Connected Simulink Model • Applied two PQ controlled DGs in converter 2 and 3, where two PI controller controls active and reactive power by changing the continuous reference voltage and phase angle. • Converter 1 is considered as a utility grid that will provide the additional inertia to the system to get the power sharing and grid stability. 9
Grid-Connected Output • Both active and reactive power sharing were obtained based on the power reference in PQ-controlled DGs. • In the case of impedance mismatch between DG 1 and DG 2, the same gain ratio has to be applied to maintain equal power-sharing. • Both Active and reactive power sharing is properly maintained as the primary grid can provide large inertia to the grid. 10
Islanded Mode of Microgrid Traditional Droop Control. Virtual Resistance Droop Control. Negative Virtual Impedance Droop Control. Decentralized Virtual Impedance Droop Control. 11
P-f/Q-V Droop Control for Inductive Transmission Line 12
P-V/Q-f Droop Control for Resistive Transmission Line 13
Negative Virtual Impedance Droop Control 14
Negative Virtual Impedance Grid Parameters • Reactive power droop coefficient depends on the transmission line reactance. • Virtual impedance dependency on transmission line makes this design impractical. 15
Model Output • Reactive power-sharing disrupts when the power factor of connected load is changed ; unless the reactive power coefficient changes. 16
Large Virtual Resistance Droop Control Simulink Model • • No need to measure transmission line impedances. Large virtual resistance for using P-V/Q-f droop control has been applied to maintain accurate power-sharing without distorting grid voltage. 17
Model Output • A stable grid frequency and voltage with proper active power-sharing has been found. • This model eliminates the tradeoff between grid voltage and power-sharing in traditional droop control. 18
Decentralized Design 19
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Explaining the Power Sharing in Intermittent Nature of DG and Demand Power PQ 2 Turn On PQ 1 Turn On • • at t=5 s, PQ 2 was completely shut down, VF 1 provide the power. at t=6 s, PQ 2 is turn on, VF 1 withdraw the power from the grid. at t=8 s, PQ 1 was completely shut down, VF 1 provide the power. at t=10 s, PQ 1 is turn on, VF 1 withdraw the power from the grid. 21
Stable Grid Frequency & Voltage • A stable grid voltage and frequency is observed even when one DG is completely shut down. • Frequency changes from the range of 59. 9 -60. 08. 22
Experiment Design DG 1 Load DG 2 e. Zdsp 23
Inverter Synchronization Transient Response • Used PLL to synchronize two inverter voltage, it only took 70 ms to properly aligned. 24
Power Sharing Using Conventional Droop Control • A phase shift between inverter current is observed in case of transmission line impedance mismatch between DG inverters • This phase shift depends on the line impedance difference 25
Power Sharing Between Two DG Using Conventional Droop Control • Instantaneous active power-sharing is achieved for resistive load • Two inverters' active power generation is about 25 W 26
Inverters Current and Voltage by using Virtual Droop Control • Minimizing the grid circulation current by synchronizing both inverter current and voltage in case of transmission line impedance mismatch 27
Power Sharing Between Two DG’ Inverters In Virtual Droop Control • Proper power sharing among DG inverter is maintained. 28
Transient Response of Microgrid • Transient response microgird, when inverter A is added into the microgrid • Microgrid maintain an smooth power sharing transition between these two inverters without disrupting both inverter current and voltage. 29
Power Sharing Under Transient Response • Microgrid almost instantaneously maintain smooth power sharing between two inverters. 30
Third Harmonic Distortion DG Inverter • Third harmonic distortion current is almost minimized for smooth power operation of microgrid. 31
Conclusions • In grid-connected mode, the microgrid inertia is provided by utility grid. • Virtual impedances model eliminates the tradeoff between grid voltage and power-sharing in traditional droop control. • Using virtual droop control, the circulation current is minimized. • To overcome the limitation of negative virtual impedance droop control in case of transmission line impedance mismatch, this paper propose a large virtual impedance method which is independent of transmission impedances. • Virtual impedance based decentralized control converts one PQ DG to VF DG to provide grid inertia and the rest of DGs operates as PQ controlled. • Using large virtual resistance, the experiment is capable of synchronizing current to make the system ready for reactive power sharing as well. 32
-Thank You- 33
- Slides: 33