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Power Quality, distributed generation, and intermittent

sources of energy

Math BollenSTRI AB

Ludvika, Sweden

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What is Power Quality?

• Interaction between the power network and its customers.

• Voltage quality: network impacts customer equipment.

• Current quality: customer equipment impacts the network.

• Many other definitions exist.

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What’s the difference?

• Network and system operators have no control over the generation.

• Generated power is non-predictable and/or highly variable.

• Generator units do not contribute to ancillary services.

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Power Quality and DG

• 1. Voltage quality impacts the DG units.• 2. DG units impact the current quality and

through the network other customers.• 3. Tripping of DG units impacts system

security and reliability.

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1. Voltage Quality and DG

T&D network

DGvoltage

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Voltage Qualitya Matter of Design

• Normal operation (variations)• Units should tolerate the existing levels of voltage quality like all

other equipment.• Normal events

• Units should tolerate normal events (capacitor switching, transformer energizing).

• Abnormal events• The immunity against abnormal events is part of the economic

optimization of the DG units.• The approach is the same as for normal equipment

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2. Current Quality and DER

T&D network

DERother

customers current

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Current QualityA Multi-Dimensional Problem

Low-frequency harmonics

High-frequency harmonics

Voltage variations

Voltage flicker

UnbalanceTransformer energizing dips

Capacitor-energizing transients

Tap-changer operation

Voltage dips due to faults Frequency swingsHarmonic resonances

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Hosting Capacityaddressing one dimension at a time

Per

form

ance

index

Penetration levelHostingcapacity

Limit

more investment

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Hosting Capacity: Examplerm

s vo

ltage

%DG

95% 10-minute

99% 3-

seco

nd110%

106%

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3. Tripping of DG Units

voltageT&D network

DGothercustomers trip

Sudden loss of large amounts of DG will cause reliability, security, and quality problems in the power system.

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Potentially Dangerous Events

0 50 100 150 20049.2

49.4

49.6

49.8

50

50.2

50.4

50.6

Time [s]

Fre

quen

cy [

Hz]

0 10 20 30 40 50

-15

-10

-5

0

5

10

15

Time [Cycles]

Vol

tage

[kV

]Frequency swing due to loss of large power station

Voltage dip due to a fault not properly cleared by the protection

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Immunity Requirements

• Protection (maximum tripping time)• No protection mal-trip• No unintentional islanding• (Thermal) limits of DER components

• Immunity (minimum tripping time)• Limited risk of massive DER tripping• Reliability of grid connection

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Voltage Tolerance of DER100%

Duration

Res

idual

voltag

e Immunity requirementIm

mun

ity p

erfo

rman

ce

Protection requirement

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Further work

• Hosting-capacity approach• Stochastic models of load and DG.• Performance indices and objectives for a

wide range of disturbances.• Tripping of DG on voltage events

• Reliability methods to be applied.• What is an acceptable risk?