Utility Design Method to Eliminate Voltage Collapse

1. Utility Design Method to Eliminate Voltage Collapse Presented by Jason Taylor

2. Motivation • Interest in utility system design and planning • Potential “Hot Spot” of activity • Utility systems were originally designed to be overly conservative. With the onset of deregulation, companies are looking for ways to optimize cost and still improve power quality • Current method for analyzing the potential voltage collapse situations is lengthy, shorting the options for corrections

3. Method Design • Developed using industry standard software • Easy to follow procedure • Prompt results that are conservative in nature • A non-customer specific application

4. Voltage Collapse • Voltage collapse is the uncontrollable drop in system voltage following a large disturbance • Two basic ways to incite voltage collapse • Sharp rise in system load • Major outage

5. Theory I

6. Increase in Load • As load increases the operating point moves further along the nose curve until it passes the nose • This results in voltage collapse

7. Change in System • Large disturbances in the system can change the properties of the system, leading to voltage collapse Pre-disturbance Post-disturbance

8. Analysis Method Model system parameters Run load flow simulation Is impedance within limits Run dynamic simulation Within safety margin No Yes Is voltage within limits Yes No Develop correction scheme

9. Utility System Model Model system parameters • Utility System can be represented by the Thevenin equivalent • Changes in the line impedance (Zline) will determine the potential for voltage collapse • Changes in Zline are be caused by: • Unintentional line outages • Utility line switching for system maintenance Zline=R + jX + + Service Point Vs - -

10. Customer SystemRepresentation Model system parameters Utility Service Generator 1 Generator 2 • IEEE standard one-line diagram for a refinery from IEEE color books • Actual configuration consists of • 38 busses • 16 transformers • 45 branches • 20 loads modeled as 50% motor load Bus 69 Bus 3 Bus 4 Bus 27 M M M M Bus 28 M M

11. Optimal Operation Run load flow simulation • Implemented load flow simulation using industry standard software (PSS/E) • Both generators in operation • No impedance nose point developed

12. Generator 1 Offline Run load flow simulation • Nose curve generated • Impedance nose point located at: 0.0750 + j0.600 pu

13. Generator 2 Offline Run load flow simulation • Changes is system result in different shaped nose curve • Impedance nose point located at: 0.0450 + j0.360 pu

14. Utility Service Only Run load flow simulation • Results in the smallest nose curve point • Impedance nose point located at: 0.0057 + j0.0456 pu

15. Impedance Nose Point Is impedance within limits • Decide which case is applicable with given model and the predicted customer operations • Conservative values decrease possibility of collapse but may result in an increase in the cost of counteractive measures • If predicted line impedances are not within determined limits then dynamic simulation may be necessary to determine voltage limits Worst Case ?

16. Dynamic Simulation Run dynamic simulation • Simulate using PSS/E and defined dynamic load characteristics • Dynamic simulation will provide the voltage response as a function of time for the system • Validation of method by dynamic simulation: • Are the method’s predicted impedance results conservative • The effects of different load models

17. Method Validation Run dynamic simulation • Impedance is increased in increments until voltage collapse occurs • In this case the load flow nose point does not result in voltage collapse • Voltage collapse occurs at the next increment of the impedance • In this case the load flow results are conservative Voltage at load flow impedance Voltage one Increment higher

18. Method Validation • The load flow values for a 90% motor load simulation will not be applicable • User must be aware of the properties of the modeled system to judge accuracy of load flow solution Load flow predicted Collapse point

19. Validated Analysis Method Model system parameters Run load flow simulation Is impedance within limits Run dynamic simulation Within safety margin No Yes Is voltage within limits Yes No Develop correction scheme

20. Voltage Limits • Are the determined voltage values within the determined dynamic limits? • Is there enough margin left for inaccuracy of the line impedance measurements? • If not then a correction scheme will need to be implemented Is voltage within limits

21. Correction Scheme • Corrections to improve the impedance or protection of the system vary in cost and effectiveness • Examples of utility corrections are: • Tree Trimming • Protection equipment • Redundant Lines • Customers can install other load configurations that correct voltage collapse potential Develop correction scheme

22. Cost Optimization Develop correction scheme • A potential customer wishes to locate a large industrial plant in utility service area using a 1000 hp motor • Using the designed method, the user can determine the potential for collapse • If system is within limits than no corrections needed • If outside of limits then dynamic simulation is necessary • Dynamic study results show a potential for a voltage collapse situation • Utility side solution: • Construct 10 mile redundant 115 kV line • Total cost: $1.5 Million • Customer side solution: • Install 2 500 hp motors in the place of the 1000 hp motor with an utility provided incentive • Total cost: $100,000

23. Method Benefits • Foresee the effects of utility system operations • Provide strong system at minimal cost when attracting and planning for future customers • Provide a screening method to remove cases from study that have no potential for collapse • Quick analysis can allow for corrections before a system is implemented

24. Future DesignPossibilities • Voltage collapse occurs at buses 4 and 34 • Plot the equipment time voltage curve to determine equipment dropout • This can be applied to method by accounting for changes in power as loads are removed

25. Conclusion • The designed method allows for conservative screening of non-potential voltage collapse situations • The user then can concentrate efforts in analyzing the situations that lead to collapse • By removing unnecessary analysis corrections can be made before the system is in place

26. References [1]H.O. Wang, E.H. Abed, R.A. Adomaitis, A.M.A. Hamdan “Control of Nonlinear Phenomena at the Inception of Voltage Collapse” Institute for Systems Research, University of Maryland, March 1993. [2]T.V. Cutsem, C. Vournas, Voltage Stability of Electric Power Systems, Kluwer Academic Publishers, Boston, 1998. [3]Y. Mansour, Voltage Stability of Power Systems: Concepts, Analytical Tools, and Industry Experience, IEEE Press, New York, 1990. [4]H.G. Kwatny, A.K. Parrija, L.Y. Bahar “ Static Bifurcation in Electrical Power Networks: Loss of Steady-State Stability and Voltage Collapse” IEEE Trans., Circuits Systems, 1986. [5]B.D. Hasssard, N.D. Kazarinoff, Y.H. Wan “Theory and Applications of Hopf Bifurcation” Cambridge University Press, Cambridge, 1981. [6] A Seidman, H.W. Beaty., H. Mahrous, Handbook of Electric Power Calculations, McGraw Hill , New York ,1997. [7]R.C. Dungan, M.F. McGranaghan, H.W. Beaty, Electric Power System Quality, McGraw Hill, New York, 1996. [8]J.D. Glover, M. Sarma, Power System Analysis and Design, PWS Publishing Co., Boston1994. [9]C.L. DeMarco, A.R. Bergen, “A security Measure for Random Load Disturbances in Nonlinear Power System Modals”, IEEE Transactions on Circuits and Systems, vol. CAS-34, no. 12, December 1987. [10] T.V. Cutsem, C.D. Vournas “Voltage Stability Analysis in Transient and Mid-term Time Scale”, IEEE Transactions on Circuits and Systems, vol. 11, no. 1, February 1996. [11] H.D. Chang, “Chaos in Simple Power System” IEEE Transactions on Circuits and Systems, vol. 8, no. 4, November 1993. [12] D.J. Hil, “Nonlinear Dynamic Load Models with Recovery for Voltage Stability Studies”, IEEE Transactions on Power Systems, vol. 8, no. 1, February 1993.