Reactive Power Issues

• While active power is the energy supplied to run a motor, heat a home, or illuminate an electric light bulb, reactive powerprovides 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.

What is Reactive Power?

Reactive power analogies

• The “loft” in a baseball throw.

• The “lift” in an airplane.

The “loft” analogy

The AIRCRAFT “lift” analogy

Reactive Power is a Byproduct ofAlternating Current (AC) Systems

• Transformers, transmission lines, and motors require reactive power

• Transformers and transmission lines introduce inductance as well as resistanceBoth oppose the flow of current

• Must raise the voltage higher to push the power through the inductance of the linesUnless capacitance is introduced to offset inductance

• The farther the transmission of power, the higher the voltage needs to be raised

• Electric motors need reactive power to produce magnetic fields for their operation

• 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.

• 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.

What is Reactive Power?

• Voltages are controlled by providing sufficient reactive power control margin to “modulate” and supply needs through:1. Shunt capacitor and reactor compensations2. Dynamic compensation3. Proper voltage schedule of generation.

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

How Are Voltages Controlled?

Reactive Power Management and Control is Critical for Overall System Stability

• 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.

Voltage Must be Maintained Within Acceptable Levels

Reactive Power System Sources & Sinks

Sources Sinks

Line Charging (Source)

Reactive Losses (Sink)

Synchronous Condensers& Generators

Capacitors Reactive LoadReactors

Reactiveflow across

interconnections Reactiveflow across

interconnections

Transmission System

Synchronous Condensers& Generators

Voltage and Reactive Power

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

1. Provide adequate service quality

2. Maintain proper stability of the power system.

Maintain Proper Stability of the Power System

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.

• Operational voltage criteria that did not reflect actual voltage stability conditions and needs were used in voltage analyses.

• Inadequate practices without correction were used.

August 14, 2003 Blackout

Conclusion:

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

Recommendation:

• National Electric Reliability Council specifically recommended stricter voltage and reactive 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

• Metering must be in place and maintained to capture actual reactive consumption at various points.

• 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

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

Reactive Power Location

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

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.

• Maintaining acceptable system voltages involves the coordination of sources and sinks which include:1. Plant voltage schedules2. Transformer tap settings3. Reactive device settings4. Load shedding schemes.

Voltage Coordination

• The consequences of uncoordinated operations would include:1. Increased reactive power losses2. A reduction in reactive margin available for

contingencies and extreme light load conditions

3. Excessive switching of shunt capacitors or reactors

4. Increased probability of voltage collapse conditions.

Voltage Schedule

• Each power plant is requested to maintain a particular voltage on the system bus to which the plant is connected.

• The assigned schedule will permit the generating unit to typically operate:1. In the middle of its reactive capability range

during normal conditions2. At the high end of its reactive capability

range during contingencies3. “Under excited” or absorb reactive power

under extreme light load conditions.

Transformer Tap Settings

• Transformer taps must be coordinated with each other and with nearby generating station voltage schedules.

• The transformer taps should be selected so that secondary voltages remain below equipment limits during light load conditions.

Reactive Device Settings

• Capacitors on the low voltage networks should be set to switch “on” to maintain voltages during peak and contingency conditions.

And

• “Off” when no longer required to support voltage levels.

Load Shedding Schemes

• Load shedding schemes must be implemented as a “last resort” to maintain acceptable voltages.

Voltage and Reactive Power Control

• Requires the coordination work of all Transmission and Distribution disciplines.

1. Transmission needs to: • Forecast the reactive demand and required reserve

margin• Plan, engineer, and install the required type and

location of reactive correction• Maintain reactive devices for proper

compensation• Maintain meters to ensure accurate data• Recommend the proper load shedding scheme if

necessary.

Voltage and Reactive Power Control

2. Distribution needs to: • Fully compensate distribution loads before

Transmission reactive compensation is considered

• Maintain reactive devices for proper compensation

• Maintain meters to ensure accurate data• Install and test automatic under voltage load

shedding schemes.