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Active and reactive powerRenewables Summer Course17.7.2014Eetu Ahonen
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Contents• Power• Apparent power• Active power• Reactive power
• Creation of reactive power• Consumption of reactive power
• Power factor• Transmission line impedance• Reactive power & renewables• Summary
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Apparent power (näennäisteho)
• Power that is transferred by the conductors• (Joule’s law)• Measured in volt-
amperes• Transmission lines
”see” only apparent power
Relation of apparent power S, active power P and reactive power Q. Figure from wikipedia.
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Active power (pätöteho)
• Real part of apparent power• Transfers real energy, does work•Measured in watts (W)• In a resistive circuit:• • Current and voltage in phase• Energy is dissipated at power .
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Reactive power (loisteho)
• Deadweight, foam of the beer• Reactive power does not do work• Result of current transferring no energy•Measured in VArs (volt-ampere reactive)• Imaginary part of apparent power
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Creation of reactive power• In a capacitive circuit• Current leads the
voltage by 90 degrees
• Capacitive load creates reactive power
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Voltage and current in a capacitive circuit
Consumption of reactive power• In an inductive circuit• Current lags the
voltage by 90 degrees
• Capacitive load consume reactive power
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Voltage and current in an inductive circuit
Power factor
• Ratio of active power to apparent power
• Low power factor• More demands for conductors• Higher reactive and apparent power• More distribution losses
• Example: Power factor of 0.2 (really low), active power demand 1 kW• Needed apparent power 5 kVA• Can be compensated with capacitors
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Transmission line impedance• Transmission lines have
resistance and reactance• Resistance from the metal• Reactance from the
capacitive and inductive properties of the circuit Simplified model of a transmission line. stands for shunt
resistance
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Transmission line impedance• Transmission lines have
significant impedance• Restricts the amount of
power transferred• Voltage can drop or rise(!)
over the line• Less than 10% voltage drop
acceptableVoltage drop over a typical 200 km 100 kV transmission line as a function of active power demand for different values of
load power factor
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Reactive power & renewables• Renewable sources do not provide reactive power• High renewable production• Only few large power stations online• Reactive power transferred over great distances• Voltage collapse due to insufficient line capacity
• Large plants cannot be used for power control• Smaller modular generating units near the loads are needed• Other mitigating methods
• Capacitors• Synchronised generators
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Summary• Power• Apparent power seen by the conductors• Active power transfer energy• Reactive power
• Does not transfer energy• Created by capacitive loads• Consumed by inductive loads
• Power factor, measure of a load ”goodness”• Reactive power demands have to be taken into account in
network design• Especially in networks with high penetration of renewables
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