11-04-2011, 11:00 AM
PRESENTED BY:
JAYASREE.N
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LETS BEGIN WITH AN ANALOGY
Reactive power analogies
The “loft” in a baseball throw.
The “lift” in an airplane.
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.
As the power flow increases up to maximum, (determined by length of line, conductor size, spacing of conductors) power system will be stable.
Any further increase in line loading will cause the angle to increase into unstable region and an out of step condition of generators will follow.
The amount of power that can be sent over a transmission line is limited. The limits vary depending on the length of the line
For a short line, the heating of conductors due to line losses sets a "thermal" limit. If too much current is drawn, conductors may sag too close to the ground, or conductors and equipment may be damaged by overheating.
For intermediate-length lines on the order of 100 km (60 miles), the limit is set by the voltage drop in the line
Approximately, the power flowing over an AC line is proportional to the sine of the phase angle between the receiving and transmitting ends. Since this angle varies depending on system loading and generation, it is undesirable for the angle to approach 90 degrees.
Series capacitors or phase-shifting transformers are used on long lines to improve stability. HVDC lines are restricted only by thermal and voltage drop limits, since the phase angle is not material to their operations
Reactive Power System Sources & Sinks
Transmitting Reactive Power
Reactive Power Location
Behaviour of Transmission lines
Transmission lines have the properties of capacitance as well as inductance.
Therefore the lines behave as though comprising of number of distributed inductors and shunt capacitors
When power flow occurs on a line there will be always a shift between sending end and receiving end voltages and the shift increases with increase in the line loading.
Transformers, transmission lines, and motors require reactive power
Transformers and transmission lines introduce inductance as well as resistance
Both oppose the flow of current
Must raise the voltage higher to push the power through the inductance of the lines
Unless 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
The value of the SIL to a system operator helps in realizing that:
When a line is loaded above its SIL, it acts like a shunt reactor -absorbing MVAR from the system and
When a line is loaded below its SIL it acts like a shunt capacitor - supplying MVAR to the system.
Importance of Reactive Power
Permissible Voltage Limits
Maintain Proper Stability of the Power System