Corona Tutorial Dave Phillips Submarine Operating Authority Engineering Space and Naval Warfare Systems Center San Diego, California IEEE Joint Technical

Corona Tutorial

Dave PhillipsSubmarine Operating Authority Engineering

Space and Naval Warfare Systems CenterSan Diego, California

IEEE Joint Technical Committee Meeting 2007 Orlando, Florida

7 January – 10 January

What Is Corona?What Is Corona?




Corona is a luminous discharge Corona is a luminous discharge due to ionization of the air due to ionization of the air surrounding an electrode, surrounding an electrode, caused by a voltage gradient caused by a voltage gradient exceeding a certain critical exceeding a certain critical value.value.

Example of Conductor Corona

What’s The Fuss?What’s The Fuss?


Corona from conductors and hardware Corona from conductors and hardware may cause audible noise and radio noisemay cause audible noise and radio noise


Corona from conductors and hardware may cause audible noise and radio noise

Audible noise from conductors may Audible noise from conductors may violate noise standardsviolate noise standards



Audible noise from conductors may violate noise standards

Radio noise from conductors may Radio noise from conductors may interfere with communications or interfere with communications or navigationnavigation




Radio noise from conductors may interfere with communications or navigation

Corona loss may be significant when Corona loss may be significant when compared with resistive loss of conductorscompared with resistive loss of conductors




Radio noise from conductors may interfere with communications or navigation

Corona loss may be significant when compared with resistive loss of conductors

Corona can cause possible damage to polymeric Corona can cause possible damage to polymeric insulatorsinsulators

Corona Discharge Currents are Impulsive

Ic(t)

t0.01 – 0.1 μsec

Physical Parameters of CoronaPhysical Parameters of Corona

Corona is caused by the ionization of the Corona is caused by the ionization of the media (air) surrounding the electrode media (air) surrounding the electrode (conductor)(conductor)


Corona is caused by the ionization of the media (air) surrounding the electrode (conductor)

Corona onset is a function of voltageCorona onset is a function of voltage



Corona onset is a function of voltage Corona onset is a function of relative air Corona onset is a function of relative air

densitydensity



Corona onset is a function of voltage Corona onset is a function of relative air

density Corona onset is a function of relative Corona onset is a function of relative

humidityhumidity

Corona and the Electric FieldCorona and the Electric Field

Corona is Corona is NOTNOT solely a function of the solely a function of the Electric FieldElectric Field


Corona is NOT solely a function of the Corona is NOT solely a function of the Electric FieldElectric Field

Corona is a function of the electric field on Corona is a function of the electric field on the surface of the electrode (conductor)the surface of the electrode (conductor)




Corona is also a function of the radius of Corona is also a function of the radius of curvature of the electrode (conductor)curvature of the electrode (conductor)




Corona is also a function of the radius of Corona is also a function of the radius of curvature of the electrode (conductor)curvature of the electrode (conductor)

Corona is also a function of the rate of decay Corona is also a function of the rate of decay of the electric field away from the electrode of the electric field away from the electrode (conductor)(conductor)


For the preceding reasons, selecting the For the preceding reasons, selecting the conductor with the smallest electric field at conductor with the smallest electric field at its surface is not correct.its surface is not correct.

One utility found out (the hard way) that One utility found out (the hard way) that simply choosing large diameter conductors simply choosing large diameter conductors did not work well because the electric field did not work well because the electric field decayed slowly away from the surfacedecayed slowly away from the surface

Corona and the Relative Air DensityCorona and the Relative Air Density


Corona has an inverse relationship with air Corona has an inverse relationship with air densitydensity


Corona has an inverse relationship with air Corona has an inverse relationship with air densitydensity

Standard line designs that perform well at Standard line designs that perform well at sea level, may have significant corona sea level, may have significant corona issues if used on lines that are installed over issues if used on lines that are installed over mountainous areasmountainous areas

Corona and the HumidityCorona and the Humidity


Corona has an inverse relationship with Corona has an inverse relationship with humidity at power frequencieshumidity at power frequencies


Corona has an inverse relationship with Corona has an inverse relationship with humidity at power frequencieshumidity at power frequencies

Fair weather corona is more prevalent in Fair weather corona is more prevalent in low humidity environmentslow humidity environments

Corona Is Dependent Surface Corona Is Dependent Surface Condition Of The ConductorsCondition Of The Conductors


Corona is enhanced by irregularities on the Corona is enhanced by irregularities on the conductor surfaceconductor surface



Irregularities include: dust, insects, burrs Irregularities include: dust, insects, burrs and scratches and water drops present on and scratches and water drops present on new conductorsnew conductors



Irregularities include: dust, insects, burrs Irregularities include: dust, insects, burrs and scratches and water drops present on and scratches and water drops present on new conductorsnew conductors

Corona will generally be greater on new Corona will generally be greater on new conductors and will decrease to a steady-conductors and will decrease to a steady-state value over a period of approximately state value over a period of approximately one year in-serviceone year in-service



Irregularities include: dust, insects, burrs and Irregularities include: dust, insects, burrs and scratches and water drops present on new scratches and water drops present on new conductorsconductors

Corona will generally be greater on new Corona will generally be greater on new conductors and will decrease to a steady-state conductors and will decrease to a steady-state value over a period of approximately one year in-value over a period of approximately one year in-serviceservice

Corona is significantly increased in foul weather.Corona is significantly increased in foul weather.

Corona Is Dependent On Local Corona Is Dependent On Local Electrode GeometryElectrode Geometry


N o n -C ritica l S u rfaces

N o n -C ritic a l S u rface

C ritic a l S u rfacesN u m b er o f E lem en ts = 2 1 0

C ritica l S ec tio n



r

S

G ro u n d P lan e V = 0

S p h ere V = V0


0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

500

1000

1500

2000

2500

3000Actual Electric Field vs. Distance

Distance (cm)

Ele

ctr

ic F

ield

(V

/cm

)

1 cm

2 cm3 cm

4 cm

5 cm

6 cm

7 cm8 cm

9 cm

Why is it a special problem for voltage upgrades?

If the voltage of a transmission line is increasedwithout changing the line design, the electric fieldat the surface of the line conductors (and hardware)will increase. This increase will cause additionalcorona

The same can be said for compact transmissionlines.

Practical Consequences

1. Larger conductors better – to a point

2. Use conductor bundles to reduce corona

3. Corona phenomena much worse in foul weather, high altitude

4. Compact lines more susceptible

5. New conductors can lead to poor corona performance for a while

Cautions Regarding Radio NoiseCautions Regarding Radio Noise


Radio noise from corona should not be Radio noise from corona should not be confused with radio noise generated by confused with radio noise generated by spark dischargespark discharge



High frequency complaints are High frequency complaints are almostalmost always due to sparksalways due to sparks




Causes of spark discharges can be located Causes of spark discharges can be located and repairedand repaired




Causes of spark discharges can be located Causes of spark discharges can be located and repairedand repaired

Corona is a DESIGN ISSUECorona is a DESIGN ISSUE


Radio noise caused by corona cannot be Radio noise caused by corona cannot be reduced by changing the line configuration reduced by changing the line configuration near where the noise problem is occuring. near where the noise problem is occuring. The source of the problem can be several The source of the problem can be several miles away miles away

Audible Noise

Can get “broadband noise” and “hum”

Load can affect audible noise For measurement see:

ANSI/IEEE Standard 656-1992 – “IEEE StandardFor Measurement of Audible Noise from OverheadTransmission Lines

Audible NoiseAudible Noise

May be present as “120 Hz Hum” or May be present as “120 Hz Hum” or broadband noisebroadband noise

Can be affected by loadCan be affected by load






Can be affected by loadCan be affected by load

Audible Noise in Context

50

55

60

65AudibleNoise dBA30 metersfromcenterline

Numerouscomplaints

Moderatecomplaints

Nocomplaints

Source: BPA study

Common design limit – 53 dBA

EPA RegulationsEffect Level Area

Outdooractivity

Indooractivity

Ldn < 55 dBA

Residential areasand farms where.. quiet is a basisfor use

Leq(24) < 55 dBA

Areas wherepeople spend limited time

Ldn < 45 dBA

Leq(24) < 45 dBA

Residential

Other

Ldn (has a -10 dB adder nightime) Leq(24) (24 hour average)

x

y

Tradeoff: Magnetic Field vs. Audible NoiseEffect of Changing Conductor Phasing

A

B

C

A or C

B

C or A

White – ABC – ABC Super BundleRed – ABC – CBA Low Reactance

500 kV Double Circuit Line

0

5

10

15

20

25

0 20 40 60 80 100 120

Distance from Center Line

Mag

neti

c F

ield

(mG

)

ABC – ABC Super Bundle

ABC – CBA Low Reactance

Magnetic Field is Reduced

52

54

56

58

60

62

64

0 20 40 60 80 100 120

Distance from Center Line (m)

Aud

ible

Noi

se (d

BA

)

ABC – CBA (Low Reactance)

ABC – ABC (Super Bundle)

Audible Noise is Increased

Corona Loss

What is it?

Whenever corona occurs on a transmission line, there is acorresponding loss of energy called “corona loss.” This lossis one of the reasons why the transmission of electric energy between two points is not perfect.

When can it be a problem?

Generally, corona loss is significantly less than resistive loss.However, at higher voltages, high altitudes and during foulweather, corona loss can actually exceed resistive loss.

A Word On Corona LossesA Word On Corona Losses


Whenever corona occurs on a transmission line, there is a corresponding loss of energy called “corona loss.” This loss is one of the reasons why the transmission of electric energy between two points is not perfect


Whenever corona occurs on a transmission line, there is a corresponding loss of energy called “corona loss.” This loss is one of the reasons why the transmission of electric energy between two points is not perfect

Generally, corona loss is significantly less than resistive loss. However, at higher voltages, high altitudes and during foul weather, corona loss can actually exceed resistive loss

Example Losses (sea level)

LineVoltage (kV)

Bundlen x 2a

LoadMVA

I2R LosskW/km

Corona Loss (kW/km)

Average Maximum

362

550

800

2 x 3.16

3 x 3.3

4 x 3.3

400

900

2000

41

52

93

2

4

8

26

78

208

Corona On Hardware

Transmission line hardware is usually tested in the laboratory prior to being selected to determine whether is corona free. This test is often done on a single phase system in order to conserve laboratory space.

It has been found that the traditional method of energizing the system to 110% of the rated line to ground voltage is not sufficient, especially for linesof more compact design. Apparently, the hardware is exposed to higher gradients in the field than those to which the hardware was tested.

Corona On HardwareCorona On Hardware


Transmission line hardware is usually tested in the laboratory prior to being selected to determine whether is corona free. This test is often done on a single phase system in order to conserve laboratory space


Transmission line hardware is usually tested in the laboratory prior to being selected to determine whether is corona free. This test is often done on a single phase system in order to conserve laboratory space

It has been found that the traditional method of energizing the system to 110% of the rated line to ground voltage is not sufficient, especially for lines of more compact design. Apparently, the hardware is exposed to higher gradients in the field than those to which the hardware was tested

Documents

Corona Tutorial Dave Phillips Submarine Operating Authority Engineering Space and Naval Warfare Systems Center San Diego, California IEEE Joint Technical