Improved Real/Reactive Power Management and Controls for Converter-Based DERs in Microgrids

Transcription

1 Improved Real/Reactive Power Management and Controls for Converter-Based DERs in Microgrids Masoud Karimi and Thaer Qunais Mississippi State University

2 1. Introduction: Electric Power System Generation Transmission PCC PV Fuel Cell Power Electronic Converter Power Electronic Converter Storage Power Electronic Converter Main Grid Distribution Interface Switch Consumption Conventional: - Central generation - Uni-directional power flow Power Electronic Converter/ Ind. or Syn. Generator Load Gen Power Electronic Converter Load Wind A distribution microgrid with various DERs

3 2. Problem Statement Principles of Power Management and Control in Power System The real power is drooped with frequency. The reactive power is drooped with voltage magnitude. These are compatible with principles of operation of synchronous generators. f V fnl VNL Droop Principles P But most DERs are non synchronous! Q What is the best drooping strategy?

4 2. Problem Statement (continued) Reverse Droop: Has shown improved stability for microgrids containing DERs. Has shown improved efficiency (lower transmission and distribution losses). What if the MG operates parallel to the grid? (Incoherence!) Flexible Droop: sin 90 : 0 : normal droop reverse droop cos 0 90 What is the best value for?

5 3. Approach: study system and performance System Performance: Stability Stability robustness Transmission loss Power sharing Voltage profile Seamless Transition Study System

6 3. Approach: universal controller Structure of modified universal controller MUC3

7 3. Approach: Mathematical Model 1 Inverter and Controller: ) ) 2 = = sin cos cos sin Output L filter: Local Load: 1 1 Line: Common Load:

8 3. Approach: Mathematical Model 2 Linearized Model:

9 3. Approach: Performance Indices 1 1. Stability Indices Dominant Poles High Frequency Dominant Poles Pole with Low Frequency Dominant Poles Pole with Pole with High Frequency Critical Dominant Poles (HFCD Poles) Damping Improvement Index (DII) Pole with Low Frequency Critical Dominant Poles (LFCD Poles) Stability Indices Distance Stability Improvement Index (DSII)

10 3. Approach: Performance Indices 2 2. Voltage Regulation Error Index (VREI): : MG nominal voltage : the number of critical buses Power Loss Improvement Index : total transmission loss 4. Power Sharing Indices Active and reactive power sharing error indices (PSEI and QSEI) : number of inverters. : total transmitted apparent power and : output powers of each inverter and : ideal power shares of each inverter. 5. Sensitivity Index (SI): 100 : number of dominant poles that move to the right when the physical parameter is changed and : real parts of those poles at the initial and final values of that parameter.

11 4. Results System Parameter Value Inverter Rating 2 kva L filter Inductance 5 mh DC Bus Voltage 500 V Local Loads 400 W 300 Var Common Load 2.2 kva 0.82 PF lagging RMS Grid Voltage 208 V Grid Frequency 60 Hz Switching Frequency 10 khz Line Impedance j0.083 Ω/km Grid Impedance ( ) Ω 5.7 mh Grid Rating 20 kva Grid Load 12 kw 9 kvar Example 1: double DER microgrid

12 4. Results: indices for standalone mode Stability indices for HFCD poles Standalone Mode voltage regulation error index Stability indices for LFCD poles Standalone Mode P/Q sharing error indices. Power loss improvement index Sensitivity index when changes from 3 to 7mH

13 4. Results: simulations for standalone mode Voltage amplitudes (V): Load increase at t=2 s. More stability Less power loss More regulated voltages As decreases from 90 towards 30 degrees! Output real power of DG 2 (W). At 90 (top). At Load increase at t=2 s. 0 (bottom)

14 4. Results: indices for standalone mode at low R/X lines (R/X=0.85) DII and DSII for HFCD poles PLII More or less same results as high R/X ratio: Good for both distribution and transmission levels. DII and DSII for LFCD poles

15 4. Results: indices and responses for gridconnected mode Stability indices for HFCD poles Voltage regulation error index Power loss improvement index Stability indices for LFCD poles d axis current of grid (left). Zoomed in view (right). 0 (bottom). Load increase at t=10 s 90 (top) and

16 t1 4. Results: indices for grid-connected mode at low R/X (0.85) Unstable region DII and DSII for HFCD poles PLII Stability is (significantly) compromised as decreases! DII and DSII for LFCD poles

17 Slide 16 t1 tq39 3/22/2018

18 4. Results: transition mode Output Real power of the grid for transition from standalone to grid connected at t= 5 s. Seamless transition is compromised as decreases. Global conclusion is that a modest selection of cross coupling terms e.g will establish a desirable trade off between the system stability (in both grid connected and islanded modes) and efficiency while improves the voltage regulation.

19 5. Results: IEEE 13-Bus Case Study System Parameter Value DG2 and DG3 Ratings 4 kva DG1 and DG4 Ratings 8 kva Grid Rating 40 kva RMS Grid Voltage 208 V Grid Frequency 60 Hz Line Impedance j0.083 Ω/km Modified IEEE 13 bus distribution system

20 5. Results: IEEE 13-Bus (indices for standalone mode) (a) VREI % (b) PLII % (b) PSEI and QSEI % More stability Less power loss More regulated voltages As decreases from 90 towards 30 degrees!

21 5. Results: IEEE 13-Bus (responses for standalone mode) Improved stability & Improved Voltage Regulation are observed as decreases. P_DG2 when a load is connected at bus 692 at t=3 s Voltage amplitude of selected buses (V)

22 5. Results: IEEE 13-Bus (indices for gridconnected mode) (a) VREI % (b) PLII % (b) PSEI and QSEI % Less power loss As decreases from 90 towards 60 degrees!

23 5. Results: IEEE 13-Bus (responses for grid-connected mode) Stability is compromised as decreases. (Left) d axis current of grid. (Right) Apparent power of grid. Load connected at bus 634 increases at t= 5 s

24 5. Results: IEEE 13-Bus (responses for transition mode) Output Real power of the grid for transition from standalone to grid connected at t= 3 s. Seamless transition is compromised as decreases.

25 8. Conclusion Grid Connected Mode 1. Conventional droop controller will guarantee the highest level of stability. 2. Small amount of cross coupling can improve the system efficiency without compromising stability. Islanded Mode 1. A large amount of cross coupling up to the extreme point ( system stability and efficiency. 0 ) will both improve Global Conclusion 80 will establish a desirable 1. Modest selection of cross coupling terms e.g. 70 trade off between the system stability (in both modes) and efficiency while improving voltage regulation.

26 Thank You