BY M.V.Ramana Murty

Sch no: 072113101M.Tech(power

system)1

Purpose of Presentation: of Presentation:

Identify key challenges for all Identify key challenges for all transmission, distribution in transmission, distribution in maintaining voltage and reactive maintaining voltage and reactive power control and reliabilitypower control and reliability

Scope of Presentation:

Fundamentals of voltage and reactive power.

Relationship between voltage and overall reactive power and overall system reliability.

Voltage & Reactive power planning And Assessment Practices.

What is Reactive Power?While active power is the energy supplied to run a motor,

heat a home, or illuminate an electric light bulb, reactive power provides the important function of regulating voltage.

If voltage on the system is not high enough, active power cannot be supplied.

Reactive power is used to provide the voltage levels necessary for active power to do useful work.

Reactive power is essential to move active power through the transmission and distribution system to the customer.

How Are Voltages Controlled?Voltages are controlled by providing

sufficient reactive power control margin to “modulate” and supply needs through:Shunt capacitor and reactor compensationsDynamic compensationProper voltage schedule of generation.

Voltages are controlled by predicting and correcting reactive power demand from loads

Reactive Power Management and Control is Critical for Overall System Stability:

Voltage Must be Maintained Within Acceptable Levels:

• Under normal system conditions, both peak or off peak load conditions, the voltages need to be maintained between 95% and 105% of the nominal.

• Low voltage conditions could result in equipment malfunctions:1. Motor will stall, overheat or damage2. Reactive power output of capacitors will be reduced exponentially3. Generating units may trip.

• High voltage conditions may:1. Damage major equipment – insulation failure2. Automatically trip major transmission equipment.

Reactive Reactive Power System Sources & Sinks: System Sources & Sinks:

Transmission SystemSources Sinks

Synchronous Condensers& Generators

Capacitors

Reactive flow across

interconnections

Line Charging (Source)

Reactive Losses (Sink)

Synchronous Condensers& Generators

Reactors

Reactive flow across

interconnections

Reactive Load

Voltage and Reactive Power:

Voltage and reactive power must be properly managed and controlled to:

Provide adequate service quality

Maintain proper stability of the power system.

Maintain Maintain Proper Stability of the Power System Stability of the Power System::

Synchronous

CondensersGenerators Capacitors

Line

Charging Tie Lines

Tie Lines Reactors LoadsGenerators

Regulators Losses

Voltage Collapse Phenomenon:

Which results in:

Inadequate reactive support

Which results in:

Voltage drops

Reduced reactive power from capacitors and line charging

Less reactive power support

Greater voltage drops

Tripping of generating units due to low auxiliary voltage level or TCUL actions or generator field current limiters

Which results in:

Which results in:

Which results in:

VOLTAGE COLLAPSE

August 14, 2003 Blackout:

Several Causes:

• Sufficient voltage studies were not conducted.

• Inadequate practices without correction were used.

August 14, 2003 BlackoutConclusion:

• The August 2003 Blackout study team concluded that inadequate voltage and reactive power planning were contributing factors to the incident.

Remedy:

• voltage and reactive power planning and assessment practices to be implemented by electric utilities.

Voltage & Reactive Power Planning And Assessment Practices:

Reactive power cannot be transmitted over a long distance or through power transformers due to excessive reactive power losses.

Reactive power supply should be located in close proximity to its consumption.

Sufficient static and dynamic voltage support is needed to maintain voltage levels within an acceptable range.

Sufficient reactive power reserves must be available to regulate voltage at all times.

Key Principles:

Voltage & Reactive Power Planning And Assessment Practices:

Transmission and Distribution planners must determine in advance the required type and location of reactive correction.

Reactive power devices must be maintained and functioning properly to ensure the correct amount of reactive compensation.

Distribution reactive loads must be fully compensated before transmission reactive compensation is considered.

Key Implications:

Transmitting Reactive Power:

MVARs

High ImpedanceMountain

Reactive power cannot be effectively transmitted across long distances or through power transformers due to high I2X losses.

Reactive Power Location:

MVARs

High ImpedanceMountain

Reactive power should be located in close proximity to its consumption.

MVARs

Static vs. Dynamic Voltage Support:

The type of reactive compensation required is based on the time needed for voltage recovery.

Static Compensation is ideal for second and minute responses. (capacitors, reactors, tap changes).

Dynamic Compensation is ideal for instantaneous responses. (condensers, generators)

A proper balance of static and dynamic voltage support is needed to maintain voltage levels within an acceptable range.

Reactive Reserves During Varying Operating Conditions:

Ideally, the system capacitors, reactors, and condensers should be operated to supply the normal reactive load.

As the load increases or following a contingency, additional capacitors should be switched on or reactors removed to maintain acceptable system voltages.

The reactive capability of the generators should be largely reserved for contingencies on the EHV system or to support voltages during extreme system operating conditions.

Load shedding schemes must be implemented if a desired voltage is unattainable thru reactive power reserves.

Voltage Coordination:

The reactive sources must be coordinated to ensure that adequate voltages are maintained everywhere on the interconnected system during all possible system conditions.

. The consequences of uncoordinated operations would include: Increased reactive power losses A reduction in reactive margin available for

contingencies and extreme light load conditions Excessive switching of shunt capacitors or reactors Increased probability of voltage collapse conditions.

Voltage and Reactive Power Control:

Requires the coordination work of all Transmission and Distribution disciplines.

Transmission needs to: Forecast the reactive demand and required

reserve margin install the required type and location of reactive

correction Maintain reactive devices for proper

compensation Recommend the proper load shedding scheme if

necessary.

Voltage and Reactive Power Control:

Distribution needs to: Fully compensate distribution loads before

Transmission reactive compensation is considered

Maintain reactive devices for proper compensation

Install and test automatic under voltage load shedding schemes.

REFERENCES:

4. Q. H. Wu, et al., "Power System Optimal Reactive Power Dispatch ",IEEE Transaction on Power Systems Vol. 10, No. 3, pp.1243- 1249,August 1995

2 .F. C. Lu, Y. Y. Hsu, “Reactive Power/ Voltage Control in a Distribution Substation ”, IEE Proceedings, Generation Transmission Distribution, Vol. 142, No. 6, November 1995.

3. T. Van Cutsem, T. and C. Vournas, voltage Stability of Electric Power Systems, Kluwer Academic Publishers, 1998.

1. Voltage and Reactive Power for Planning and Operation by Harrison K Clark

5. H. Vu, et al., "An Improved Voltage Control on Large-Scale Power System“, IEEE Transaction on Power Systems, Vol. 11, No. 3, pp.1295-1303, August 1996.