Pyeongik Hwang School of Electrical Engineering Seoul National University Korea

1. A control method of distributed generators in smart distribution system considering system loss and voltage Pyeongik Hwang School of Electrical Engineering Seoul National University Korea Hwang – Korea – RIF Session 4a – 0324

2. Introduction • Increased installation of distributed generations(DGs) • The characteristics of the distribution system is changed • Voltage profile, system loss, power flow, etc. • Introduction of the smart distribution system • The status of the distribution system can be measured and calculated more accurately • The power output of DGs can be controlled using the communication infrastructures.  Chance to more effective operation using DGs Hwang – Korea – RIF Session 4a – 0324

3. DG control problem formulation • The objectives of the proposed method • Minimize the system loss • Maintain the system voltage within its limit - Minimize - Subject to Hwang – Korea – RIF Session 4a – 0324

4. Sequential Linear Programming • Relationship among loss, voltage, and output of DGs is highly non-linear • Formulated DG control problem is a non-linear optimization problem • Sequential Linear Programming(SLP) method is adopted • Optimal solution is calculated by solving series of linear programming (LP) problem linearized at the operation point • Operation point is determined at the previous iteration Hwang – Korea – RIF Session 4a – 0324

5. SLP application to DG control • Sub-functions of SLP • LP formulation • Step size adjustment • Convergence test • Decision variable for LP Hwang – Korea – RIF Session 4a – 0324

6. LP formulation Loss sensitivity matrix • Linearized Optimization problem -Minimize -Subject to Voltage sensitivity matrix Injection power sensitivity matrix Hwang – Korea – RIF Session 4a – 0324

7. LP formulation • Differences between distribution system and transmission system • Existence of mutual impedance in line parameter • Unbalanced connection of DGs • Bus admittance matrix with mutual line impedance • Used for calculation of loss and voltage sensitivity matrices A : bus incidence matrix, [y] : primitive admittance matrix. Hwang – Korea – RIF Session 4a – 0324

8. LP formulation • Differences between distribution system and transmission system • Existence of mutual impedance in line parameter • Unbalanced connection of DGs • Bus admittance matrix with mutual line impedance • Used for calculation of loss and voltage sensitivity matrices A : bus incidence matrix, [y] : primitive admittance matrix. Hwang – Korea – RIF Session 4a – 0324

9. LP formulation • Injection power sensitivity matrix calculation method Hwang – Korea – RIF Session 4a – 0324

10. SLP application to DG control • Step size adjustment • Prevent oscillation in SLP • Convergence test Hwang – Korea – RIF Session 4a – 0324

11. Proposed method • Flow chart of the proposed method Hwang – Korea – RIF Session 4a – 0324

12. Case Study • IEEE 37 node test feeder system with three DGs DG 1 A-B-C phase DG 2 A-B phase DG 3 B-C phase Hwang – Korea – RIF Session 4a – 0324

13. Initial voltage vs. voltage limit • Under voltage violation is occurred in case 2 and case 3 Under Voltage Hwang – Korea – RIF Session 4a – 0324

14. Performance of the proposed method • The proposed method is implemented as a Matlab code • Matlab provided function “linprog” is utilized as the LP solver • Comparing with results of the function “fmincon” • Maximum error is less than 0.1% • Proposed method is at least 90 times faster than fmincon Hwang – Korea – RIF Session 4a – 0324

15. Case 1 ( Vmin = 0.97 p.u., Vmax = 1.03 p.u. ) • The system loss is reduced about 19 %(97kW 78 kW)  Operation cost can be reduced by minimizing the loss Hwang – Korea – RIF Session 4a – 0324

16. Case 2 ( Vmin=0.98 p.u., Vmax=1.02 p.u.) • Without proposed method, tap position of OLTC must be changed to eliminate the voltage violation • Increasing operation cost • With proposed method, Under violation is eliminated without tap changing  System operation cost can be reduced by preventing the tap changing of OLTC  System stability can be improved by maintaining system voltage within its limit Hwang – Korea – RIF Session 4a – 0324

17. Case 2 ( Vmin=0.98 p.u., Vmax=1.02 p.u.) • System voltage Hwang – Korea – RIF Session 4a – 0324

18. Case 3 ( Vmin=0.985 p.u., Vmax=1.015 p.u.) • Tap changing to eliminate the under voltage violation  New over voltage violation is occurred Over voltage Hwang – Korea – RIF Session 4a – 0324

19. Case 3 ( Vmin=0.985 p.u., Vmax=1.015 p.u.) • System voltage can be maintained within its limit  Power quality can be enhanced by controlling the voltage more tightly Hwang – Korea – RIF Session 4a – 0324

20. Conclusions • DGs control problem was formulated as a non-linear optimization problem. • Sequential Linear Programming (SLP) based DGs control method was proposed • Effects of the proposed method were identified • Operation cost reduction • System stability improvement • Power quality enhancement Hwang – Korea – RIF Session 4a – 0324

21. Thank You ! (hpi@powerlab.snu.ac.kr) Hwang – Korea – RIF Session 4a – 0324