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Voltage Stability and Reactive Voltage Stability and Reactive

Power PlanningPower Planning

Entergy Transmission Planning Summit Entergy Transmission Planning Summit

New Orleans, LA New Orleans, LA

July 8, 2004July 8, 2004

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• Voltages decay to almost 60% and initiates loss of load• Slow recovery leads to generators tripping• 60% voltage level also observed in 1995 Phoenix-area blackout

Voltage Profile During Aug 14th Blackout

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East Lake 5 Exciter Operation

Exciter trips to manual

Exciter trips completely

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Power System Outage Task Force

• 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 from the outage report

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#7. Evaluate reactive power and voltage control practices. Evaluate the effectiveness of existing reactive power and voltage control standards

and how they are being implemented in the NERC regions

Recommend revisions to standards or process improvements to ensure voltage control and stability issues are adequately addressed

#14. Improve system modeling data and data exchange practices. Establish and begin implementing criteria and procedures for validating data used in

power flow/stability models by benchmarking model data with actual system performance

Validated data shall be exchanged on an inter-regional basis

NERC Recommendations Pertinent to Reactive Power Planning

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What is Voltage Instability/Collapse?

• A power system undergoes voltage collapse if post-disturbance voltages are below acceptable limits:– Less than 0.8 PU

– 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

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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:– Short term/transient voltage instability: 0 - 30 seconds

• Motor dynamics/stalling• OEL’s

– long-term voltage stability: 1 – 60 minutes• Tap changers/Voltage regulators• OEL’s

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Power-Voltage (P-V) Curve

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Power – Voltage (P-V) Curve

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Possible Solutions forVoltage Issues

• 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 under-voltage load shed (UVLS) program

• Construct transmission/generation facilities

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MW

Stator Winding Heating Limit

- P

er u

nit

MV

AR

(Q

) +

0.8 pf line

Under-excitation Limit

Lag

gin

g

(Ove

r-ex

cite

d)

Lea

din

g

(Un

der

-exc

ited

)

Normal Excitation (Q = 0, pF = 1)

Over-excitation Limit

Stability Limit

Generator Capability Curve

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Reactive Power Requirementsfor Generators

• The facility should generate reactive power in accordance with the voltage schedule prescribed by the system dispatcher.

• Above 230 kV: 1.02 PU• 230 kV: 1.01 PU• 69 kV – 161 kV: 1.00 PU

• The facility shall have a reactive power capability to maintain a power factor between 0.95 lagging and 0.97 leading.

• Units must be operated with the voltage regulator in auto mode.

• Generator may be required to operate at its maximum reactive capability to meet required voltage schedules.

• Ensure that adequate reactive reserves are available so that the system can be restored satisfactorily.

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Areas of Voltage Stability Concern

West of the Atchafalaya Basin(WOTAB)

North Arkansas

Southeast Louisiana

Western Region

Amite South/DSG

Mississippi

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Reactive Power Projects Implemented in Entergy

Down Stream of Gypsy (DSG) Area: 300 MVAR Shunt Capacitor Banks - 2004 Automatic Under Voltage Load Shedding Program - 2004 Static exciter at Ninemile 4 Unit - 2004

200 MVAR Shunt Capacitor Banks – 2005 300 MVAR SVC at Ninemile 230 kV – 2005

Western Region: Automatic Under Voltage Load Shedding Program – 1998/modified

2004 Series Compensation on China-Jacinto 230 KV line – 2001

300 MVAR SVC at Porter 230 kV – 2005 Series Compensation on China – Porter 230 kV line - 2005

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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.

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QUESTIONS ???