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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