Outline

1. “Importance of Reactive Power Management, Voltage Stability and FACTS Applications in today’s Operating Environment” Sharma Kolluri Manager of Transmission Planning Entergy Services Inc Engineering Seminar Organized by IEEE Mississippi Section Jackson State University August 20, 2010

2. Outline • Introduction • VAR Basics • Voltage Stability • FACTS • Applications at Entergy • Summary .

3. Voltage Profile during Aug 14th Blackout • Voltages decay to almost 60% of normal voltage. This is probably the point that load started dropping off. • However, the recovery is too slow and generators are not able to maintain frequency during this condition. • Many generators trip, load shedding goes into effect, and then things just shut down due to a lack of generation.

4. A “Near” Fast Voltage Collapse in Phoenix in 1995 North American Electric Reliability Council, System Disturbances, Review of Selected 1995 Electric System Disturbances in North America, March 1996.

5. Recommendation#23 • Strengthen Reactive Power and Control Practices in all NERC Regions “Reactive power problem was a significant factor in the August 14 outage, and they were also important elements in the several of the earlier outages” -Quote form the outage report

6. Reactive Power

7. Laws of Reactive Physics • System load is comprised of resistive current (such as lights, space heaters) and reactive current (induction motor reactance, etc.). • Total current IT has two components. • IR resistive current • IQ reactive current • IT is the vector sum of IR & IQ • IT = IR + jIQ IT IQ IR North American Electric Reliability Corporation

8. Laws of Reactive Physics • Complex Power called Volt Amperes (“VA”) is comprised of resistive current IR and reactive current IQ times the voltage. • “VA” = VIT* = V (IR – jIQ) = P + jQ • Power Factor (“PF”) = Cosine of angle between P and “VA” • P = “VA” times “PF” • System Losses • Ploss = IT2 R (Watts) • Qloss = IT2 X (VARs) VA Q P North American Electric Reliability Corporation

9. Reactive Physics – VAR loss • Every component with reactance, X: VAR loss = IT2 X • Z is comprised of resistance R and reactance X • On 138kV lines, X = 2 to 5 times larger than R. • One 230kV lines, X = 5 to 10 times larger than R. • On 500kV lines, X = 25 times larger than R. • R decreases when conductor diameter increases. X increases as the required geometry of phase to phase spacing increases. • VAR loss • Increases in proportion to the square of the total current. • Is approximately 2 to 25 times larger than Watt loss. North American Electric Reliability Corporation

11. Reactive Power for Voltage Support Reactive Loads VARs flow from High voltage to Low voltage; import ofVARs indicate reactivepower deficit

12. Reactive Power Management/Compensation What is Reactive Power Compensation? • Effectively balancing of capacitive and inductive components of a power system to provide sufficient voltage support. • Static and dynamic reactive power • Essential for reliable operation of power system • prevention of voltage collapse/blackout Benefits of Reactive Power Compensation: • Improves efficiency of power delivery/reduction of losses. • Improves utilization of transmission assets/transmission capacity. • Reduces congestion and increases power transfer capability. • Enhances grid reliability/security.

13. Transmission Line Real and Reactive Power Losses vs. Line Loading Source: B. Kirby and E. Hirst 1997, Ancillary-Service Details: Voltage Control, ORNL/CON-453, Oak Ridge National Laboratory, Oak Ridge, Tenn., December 1997.

14. Static and Dynamic VAR Support • Static Reactive Power Devices • Cannot quickly change the reactive power level as long as the voltage level remains constant. • Reactive power production level drops when the voltage level drops. • Examples include capacitors and inductors. • Dynamic Reactive Power Devices • Can quickly change the MVAR level independent of the voltage level. • Reactive power production level increases when the voltage level drops. • Examples include static VAR compensators (SVC), synchronous condensers, and generators.

15. Voltage Stability

16. Common Definitions Voltage stability- ability of a power system to maintain steady voltages at all the buses in the system after disturbance. Voltage collapse - A condition of a blackout or abnormally low voltages in significant part of the power system. Short term voltage stability - involves the dynamics of fast acting load components such as induction motors, electronically controlled loads, and HVDC converters. Long term voltage stability - involves slower acting equipments such as tap-changing transformer, thermostatically controlled loads, and generator limiters.

17. What is Voltage Instability/Collapse? • A power system undergoes voltage collapse if post-disturbance voltages are below “acceptable limits” • voltage collapse may be due to voltage or angular instability • Main factor causing voltage instability is the inability of the power systems to “maintain a proper balance of reactive power and voltage control”

18. Voltage Instability/Collapse • The driving force for voltage instability is usually the load • The possible outcome of voltage instability: • loss of loads • loss of integrity of the power system • Voltage stability timeframe: • transient voltage instability: 0 to 10 secs • long-term voltage stability: 1 – 10 mins

19. Voltage stability causes and analysis Causes of voltage instability Increase in loading Generators, synchronous condensers, or SVCs reaching reactive power limits Tap-changing transformer action Load recovery dynamics Tripping of heavily loaded lines, generators Methods of voltage stability analysis Static analysis methods Algebraic equations, bulk system studies, power flow or continuation power flow methods Dynamic analysis methods Differential as well as algebraic equations, dynamic modeling of power system components required

20. Generator Capability Curve Over-excitation Limit Lagging (Over-excited) 0.8 pf line Stator Winding Heating Limit Normal Excitation (Q = 0, pF = 1) - Per unit MVAR (Q) + MW Turbine Limit Leading (Under-excited) Under-excitation Limit Stability Limit

21. P-V Curve

22. Q-V Curve with Detailed Load Model Peak Load with Fixed Taps 120 200 100 80 60 40 Base Case 20 Mvars Contingency 0 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 -20 -40 -60 -80 Voltage (p.u.) Q-V Curve

23. Limit UVLS activation Minimize motor tripping Key Concerns Voltage (pu)

24. Possible Solutions for Voltage Instability • Install/Operate Shunt Capacitor Banks • Add dynamic Shunt Compensation in the form of SVC/STATCOM to mitigate transient voltage dips • Add Series Compensation on transmission lines in the problem area • Implement UVLS Scheme • Construct transmission facilities

25. Voltage Collapse

26. Fault Induced Delayed Voltage Recovery (FIDVR) • FIDVR Definition • Load Models

27. Fault Induced Delayed Voltage Recovery (FIDVR) • What is it? • After a fault has cleared, the voltage stays at low levels (below 80%) for several seconds • Results in dropping load / generation or fast voltage collapse • 4 key factors drive FIDVR: • Fault Duration • Fault Location • High load level with high Induction motor load penetration • Unfavorable Generation Pattern

30. Load characteristics The accuracy of analytical results depends on modeling of power system components, devices, and controls. Power system components - Generators, excitation systems, over/under excitation limiters, static VAr systems, mechanically switched capacitors, under load tap changing transformers, and loads among others. Loads are most difficult to model. Complex in behavior varying with time and location Consist of a large number of continuous and discrete controls and protection systems Dynamics of loads, especially, induction motors at low voltage levels should be properly modeled.

31. Induction motor characteristics Impact of fault on transmission grid Depressed voltages at distribution feeders and motor terminals Reduction of electrical torque by the square of the voltage resulting in slow down of motors The slow down depends on the mechanical torque characteristics and motor inertias With fault clearing Torque -per unit Square-law load torque Electric torque Constant load torque 1.0 Speed – per unit Fig. 1 Induction motor characteristics • Partial voltage recovery • Slowed motors draw high reactive currents, depressing voltage magnitudes • Motor will reaccelerate to normal speed if, electrical torque>mechanical torque • else, the motors will rundown, stall, and trip • The problem is severe in the summer time with large proportion of air conditioner • motors

32. Air conditioner motor characteristics Characteristics Main portion (80-87%) consumed by compressor motor Electromagnetic contactor drop out between (43-56%) of the nominal voltage and reclose above drop out voltage Stalling at (50-73%) of the nominal voltage Thermal overload protection act if motors stall for 5-20 seconds The operation time of thermal over load (TOL) protection relay is inversely proportional to the applied voltage at the terminal Air conditioner should be modeled to analyze the short term voltage stability problem Quite important for utilities in the Western interconnection

33. Load modeling Old models – Loads are represented as lumped load at distribution feeder Does not consider the electrical distance between the transmission bus and the end load components The diversity in composition and dynamic behavior of various electrical loads is not modeled Modeling WECC interim model 20% of the load as generic induction motor load 80% constant current P and constant impedance Q Transmission Bus OLTC Distribution Bus Distribution Capacitor Lumped Load (ZIP load) Fig. 2 Traditional load model

34. Composite load modeling Representation of distribution equivalent Feeder reactance Substation transformer reactance Parameters of various load components Discharge lighting Electronic Loads Constant Impedance loads Motor loads Distribution Capacitor Transmission Bus Bus 1 OLTC Distribution Bus Bus 2 Substation Capacitor Feeder Equivalent Distribution Feeder Bus 3 Dynamic Loads (Small motor, Large motor, trip motor loads) Static Loads (Constant impedance, constant current, constant impedance loads) Distribution Capacitor Fig. 3 Composite load model structure

35. FACTS

36. What is FACTS? • Alternating Current Transmission Systems Incorporating Power Electronic Based and Other Static Controllers to Enhance Controllability and Increase Power Transfer Capability. • power semi-conductor based inverters • information and control technologies

37. Major FACTS Controllers • Static VAR Compensator (SVC) • Static Reactive Compensator (STATCOM) • Static Series Synchr. Compensator (SSSC) • Unified Power Flow Controller (UPFC) • Back-To-Back DC Link (BTB)

38. Inter-connected RTO System S/S BTB BTB UPFC Power Generation Voltage Control Power System Stability Power Flow Control System Reliability Inter-area Control Inter-tie Reliability Enhanced Import Capability S/S Load Load STATCOM STATCOM Improved Power Quality Increased Transmission Capacity Inter-connected Power System Load S/S SSSC FACTS Applications

39. Static VAr compensator (SVC) Variable reactive power source Can generate as well as absorb reactive power Maximum and minimum limits on reactive power output depends on limiting values of capacitive and inductive susceptances. Droop characteristic V I TCR Firing angle control XC XL Fig. 4 Schematic diagram of an SVC

40. Static compensator (STATCOM) Voltage source converter device Alternating voltage source behind a coupling reactance Can be operated at its full output current even at very low voltages Depending upon manufacturer's design, STATCOMs may have increased transient rating both in inductive as well as capacitive mode of operation System bus V Transformer I X E DC-AC switching converter Cs Vdc Fig. 5 Schematic diagram of STATCOM

41. Technology Applications at Entergy

42. Technology Applications at Entergy to Address Reactive Power Issues • Large Shunt Capacitor Banks • UVLS • Series Compensation • SVC • Coordinated Capacitor Bank Control • DVAR • AVR

43. Determining Reactive Power Requirements in the Southern Part of the Entergy System for Improving Voltage Security – A Case Study Sharma Kolluri Sujit Mandal Entergy Services Inc New Orleans, LA Panel on Optimal Allocation of Static and Dynamic VARS for Secure Voltage Control 2006 Power Systems Conference and Exposition Atlanta, Georgia October 31, 2006

44. Areas of Voltage Stability Concern North Arkansas Mississippi West of the Atchafalaya Basin (WOTAB) Southeast Louisiana Western Region Amite South/DSG

45. Study Objective • Identify Voltage Stability Problems in the DSG area • Determine the proper mix of reactive power support to address voltage stability problem • Determine size and location of static and dynamic devices.

46. Ninemile Units 1 - 50 MW 2 - 60 MW 3 - 128 MW 4 - 740 MW 5 - 750 MW Michoud Units 1 - 65 MW 2 - 240 MW 3 - 515 MW 115 kV 115 kV - 230 kV 230 kV Downstream of Gypsy Area - Critical Facilities Little Gypsy-South Norco 230kV line Waterford-Ninemile 230kV line

47. DSG Issues • Area load growth • 1.6% projected for 2003 - 2013 • Weather normalized to 100º F • Projected peak load – 3800 MW • Area power factor - Low • 94% at peak load • Worst double contingency • Loss of the Waterford to Ninemile 230 kV transmission line and one of the 230 kV generating units at Ninemile or Michoud Michoud Ninemile New Orleans area voltage profile on June 2, 2003 (with 2 major generators offline) • Area Problems • Thermal overloads of underlying 115 kV and 230 kV transmissionsystem • Depressed voltages throughout New Orleans metro area potentially leading to voltage collapse and load shedding

48. Various Steps Used for Determining Reactive Power Requirements • Step 1 – Problem identification • Step 2 – Determining total reactive power requirements • Step 3 – Sizing and locating dynamic devices • Step 4 – Sizing and locating static shunt devices • Step 5 – Verification of reactive power requirements

49. Tools & Techniques Used • Various tools and techniques used for analysis purposes • PV analysis using PowerWorld • Transient stability using PSS/E Dynamics • Mid-term stability using PSS/E Dynamics • PSS/E Optimal Power Flow • Detailed Models used • Motor models and appropriate ZIP model for dynamic analysis • Tap-changing distribution transformers, overexcitation limiters, self-restoring loads modeled in mid-term stability study

50. Improve post-fault voltage Minimize motor tripping Criteria/Requirements Voltage (pu)