“Reactive Power Management and Voltage Stability”

Sharma Kolluri, IEEE FellowManager of Transmission Planning

Entergy Services Inc

Presentation at 2012 Southeast Symposium on Contemporary

Engineering Topics (SSCET)

October 26, 2012

New Orleans, Louisiana

Outline

• Background/Introduction• VAR Basics• Voltage Stability• FIDVR• Technology • Summary

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List of recent blackouts1. NE blackout - August 14, 2003

2. Greece – July 12, 2004

3. Florida – February 26, 2008

4. Southeast Power Outage/San Diego – September 8, 2011

5. India – July 31, 2012

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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

Reactive Power BasicsReactive Power Basics

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

P

Q

North American Electric Reliability Corporation

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

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.

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.

Reactive Power Management

• Effectively balancing capacitive and inductive components of a power system to provide sufficient voltage support.

• Essential for reliable power system operation.– Prevention of voltage collapse

Benefits• Improves efficiency of power delivery.

• Improves utilization of transmission assets.

• Reduces congestion and increases power transfer capability.

• Enhances grid security.

Reactive Power for Voltage Support

Reactive Loads

VARs flow from High voltage to Low voltage; import ofVARs indicate reactivepower deficit

Voltage Stability Voltage Stability

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”

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

Key Concerns

Minimize motor tripping

Limit UVLS activation

Voltage (pu)

P-V Curve

Q-V Curve

200

Q-V Curve with Detailed Load Model

Peak Load with Fixed Taps

-80

-60

-40

-20

0

20

40

60

80

100

120

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Voltage (p.u.)

Mv

ars

Base Case

Contingency

Voltages at Goslin 138kV Station

Time (seconds)

Vo

ltag

e (

volt

s)

Common Solutions for Voltage Instability

• Install/Operate Shunt Capacitor/Reactor Banks • Add dynamic Shunt Compensation in the form of

SVC/STATCOM/DVAR to mitigate transient voltage dips• Add Series Compensation on transmission lines in the

problem area• Construct transmission facilities• Coordinate Voltage Schedules/Reactive Power Flows • Implement UVLS Scheme

Fault Induced Delayed Voltage Recovery (FIDVR)

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

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.

Technology Technology

for Addressing Reactive for Addressing Reactive

Power/Voltage Stability Power/Voltage Stability

ProblemsProblems

Porter SVC

Porter Static Var Compensator (SVC)

Maintains system voltage by continuously varying VAR output to meet system demands. Controls capacitor banks on the transmission system to match reactive power output to the load requirements.

Series Capacitor – Dayton Bulk 230kV Station

The Capacitor offsets reactance in the line, making it appear to the system to be half of its actual length. Power flows are redirected over this larger line, unloading parallel lines and increasing transfer capability.

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

Transformer

DC-AC switching converter

IX

System bus

Cs

Vdc

V

E

Schematic diagram of STATCOM

Natchez DVARNatchez DVAR

s s s s

D-VAR

MT

Metro138 kV

34.5 kV FDR

34.5 kV25.2 MVAR

480 V

34.5 kV

Summary

• The increasing need to operate the transmission system at its maximum safe transfer limit has become a primary concern at most utilities

• Reactive power supply or VAR management is an important ingredient in maintaining healthy power system voltages and facilitating power transfers

• Inadequate reactive power supply was a major factor in most of the recent blackouts

Questions?