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EARTH FAULT PROTECTION IN MV DISTRIBUTION NETWORKS:

SENSITIVITY VS. SECURITY

ENDESA SEMINAR, SEVILLA

Prof. José Pinto de Sá, IST, Lisbon

2012, November 27

1 EARTH FAULT PROTECTION: SENSITIVITY VS SECURITY

The problem:

In any protection system:

• Security: to not operate for non faults!

• Sensitivity: to operate even for very small faults…

The root of the problem in earth faults:

EDF practices in the 50´s and 60´s!...

EARTH FAULT PROTECTION: SENSITIVITY VS SECURITY

2

EDF old practices:


3

The problem is the residual currents:


4

ZaaZabZab

ZabZaaZab

ZabZabZaa

zm

zp

zm

[Zabc] = ,

+ Lines are not perfectly symmetrical

Ed

Yh- jw2/3CA 2/3 jwCA

Yd,h

Ih Uh

A model for the inter-action between zero and “usual” voltage

Typical MV residual currents are 0,2 – 0,6 A, in Portuguese lines! Still more in the neutral grounding ( 1 A and +)...!

Therefore, EDF requested sensitivity to 0,5 A earth faults causes too many unnecessary CB trips!

What are we afraid of? (1)


5

,

AlcatrãoSolo normal

Downed conductors:

... and indirect contacts:

ICC

IH

IT

Rposte

RT Rtoque

ICC

IT IHVtoque

What are we afraid of? (2)


6

,

... And other direct hazards:

ICC

ICC

Baínha

Iso lam ento

Condutor


Most cells are short long cells: nervous and skeletal muscles!

The cell membrane is very isolating (250 kV/cm disrupting field)!

Direct contacts with High V:


ELECTROPORATION:

Unlike the Joule effect, electroporation requires a very short time: 3-4 miliseconds only!

2 kV/cm are enough to cause electroporation in most cells!

For electroporation lasting t>10-20 ms, cell damage is extensive and recovery is very difficult!


Electricity burns ...by Joule effect? (1)

Burning requires time (energy). That´s why people can walk on hot coals or touch a stove to know whether it is hot...

10 A through an arm (hand to shoulder) increase the arm temperature by 8ºC /s!

A lot of heat is required to increase the body temperature, because of its heat capacity (similar to water: to heat up a 10 kg arm for 10ºC, 418 K J are required)!


Electricity burns ...by Joule effect? (2)

Considering the time required to burn live tissues and the body electrical resistance:

… therefore, electricity burning through Joule effect is unlikely!

For t<0,20 s, not even 20 kV can burn any part of an arm!


Electrical burns, not thermal burns!

65-75% of the survivors of a high voltage shock have to suffer some amputation!

Thermal versus electric field thresholds to burn live tissue.

Very often the most damaged cells are invisible muscles and nerves. Skin seems ok and can mislead doctors, until the member swells and necrosis. (Knowledge on electroporation since 1987 thanks to Raphael Lee,

engineer & surgeon)


Direct contacts: another effect

Large currents (if allowed by the earthing philosophy...): ARC FLASH!

ARC FLASH:


,

, in cal/cm2, for U 15 kV 3

25.12 10 cc

td

E x UI

Reduce Isc, reduce t (protection role), enlarge d, shield people against:


Lightning, the most deadly natural hazard!

According to data from the NOAA, in the years from 1959-1994, lightning was responsible for more than 3,000 deaths and nearly 10,000 casualties. The actual number of lightning casualties may be higher, because up to 50% may go unreported. Lightning kills more people each year in the United States than hurricanes, volcanoes, tornadoes, and earthquakes combined.

Few individuals experience the full energy of a lightning strike because only about 3-5% of injuries are from a direct strike. Most of the energy is mediated by other factors including the ground, the tree, or other object that, once hit, transmits the energy to the person. In fact, less than one third of affected persons have signs of burns. When burns do occur, they are usually superficial. Internal burns are rare. Myoglobinuria is rarely encountered in lightning injuries, whereas cardiac and respiratory arrest, vascular spasm, neurologic damage, and autonomic instability play a greater role. Blunt force injuries from falling, being thrown by muscle contractions, or barotrauma from the explosive force of a nearby lightning strike may occur. Lightning strikes are primarily a neurologic injury that affects all 3 components of the nervous system: central, autonomic, and peripheral.

Indirect contacts: the case for downed conductors


15

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Subestação CargaIdef

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a

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b

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c

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Mostly dependent on the earthing system

The case for downed conductors - when a problem really exists:


16

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Mostly dependent on the load!

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Downed conductors with high resistance to earth, another big problem:


17

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Subestação Carga

I def

Solo muito Resistivo

0defI 1

2

p pd

d dI I1

2

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So, for protective relays working with zero sequence currents:

1) Sending end fault with “usual” fault resistance: usually no problem!

2) Receiving end fault: only loads above some threshold can provide sensitivity (eg. 150 kVA for 2 A minimum sensitivity in 15 kV systems).

3) Sending end fault with high resistance : no solution with zero currents!

Can´t we detect those broken conductors through negative currents?


18

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0defI 1

2

p pd

d dI I1

2

p pd

i dI I

The answer is: not really (in MV Distribution networks)!

Because of:

1. Load unbalance (it does´nt exist for zero currents, but it exists for negative currents)

2. Voltage unbalance in the source (even if small);

3. Line asymmetries (larger for negative than for zero parameters)

There allways are negative currents:


19

,

Positive and negative currents in a real unfaulted system:

0 5 10 15 20 25 30 35 40 4550

100

150

200

250

300

350

Id (

A)

Amostra

0 5 10 15 20 25 30 35 40 450

1

2

3

4

5

6

Ii (

A)

Amostra

The ratio of Negative to Positive currents can almost reach 3%. A relay has to be insensitive to that!

So, do we really need such a sensitivity (eg. 0,5 A)?

May be, if we think of such a current

straight to the heart – but that´s highly unlikely – or not?


20

,


We have chosed a risk-based approach (because everything is random)

The random variables: The probability of dying from a given heart current… The body resistance (voltage to current ratio)… The shoes, their electrical resistance and withstanding voltage… The step length and the arm length (for poles)… The soil resistivity… The length of the conductor touching the soil (for downed conductors)…

ISC

II

Vpotential

Vtouch

Earth Rod

IH

0,5 3H shoe

SC H

R RI I

k V l

2step H foot shoe H

V R R R I


Ventricular fibrillation – how (1)

Threshold for muscle contraction: 15 mA (men) – 11mA (women). => Risk of fall! But... 0,13 A for 1 second kills half the people! How?

Here the heart can´t be excited. No danger...

...only in this 10% of the cycle is when the heart can be excited by an external current!


Ventricular fibrillation – how (2) Fibrillation starts!

...but even 5% of the current required to fibrillate is enough to cause a premature systole if it happens here, leading to a reduction of the fibrillation current with time.

The heart cycle lasts at least 0,6 to 0,9 s, therefore the susceptibility grows with time, for this current (Bilgermeier<1980)


Ventricular fibrillation – IEC 60479-1

Minimum threshold (probability =0)

0,1 A for 1,1s through the heart kills half the people – but for just 0,2s, 1 A is required...will it be?

Fibrillation probability =5%

Fibrillation probability =50%

Bilgermeier work (untill 1980) based on animal experiments


Ventricular fibrillation – yet A log-normal probability distribution fits well the data

density probability

For a given high current through the heart (eg, >2 A), fibrillation probability is a function of the current duration. Theart_cycle > 0,6s

Fast rise because of the heart cycle


Ventricular fibrillation – the current path

IF= I/F

So, if for 0,2s 1A is required to kill half the people (left hand to feet), for step contact 25A are required…


Human body electrical resistance

For dry skin and no shoes. For wet conditions the resistance is lower.

Therefore, in Low Voltage and dry skin, 95% of the people can withstand a naked hand to foot contact provided it lasts for less than 0,5 s!...


Human body electrical resistance + shoes

Shoes provide a high additional resistance – if dry…!


Applying Monte Carlo methods (Matlab):

Fault currents versus probability of fall when stepping a downed conductor. - With random shoes, - barefooted, X- with only a type of shoes (dry new black rubber).

For stepping a downed conductor the direct risk of death is surprisingly low – but the risk of loosing the legs control and falling on the conductor, and then having a chess to earth path for the current is much higher…


Applying Monte Carlo methods (2):

Time to heart fibrillation versus fault current for different probabilities (touching a faulted pole) – but again, the “let-go” threshold is much demanding!

1 2 3 4 6 810 20 3040 60 100 200300 500 1000 20000.1

0.2

0.3

0.4

0.6

0.81

2

3

4

6

810

I (A)

t (s

)

0.5% risk

2.5% risk

5% risk

25% risk


Summary:

Therefore, EDP has increased the tripping setting in zero current feeder relays from 0,7 to 3 A - but where people is mostly barefooted and there is a high risk of faults, the old practice may be justified. In addition, 30 A should be cleared fast (<0,2s).

It is also worth mentioning: 1. Resonant earthing in Central and northern Europe usually allows a fault

current of 3 A untill a better chance to repair the line; 2. Standard EN 50341 for "Overhead electrical lines exceeding AC 45kV“is also quite

tolerant regarding thouch voltage, but it considers the time to trip and how people attend the place.

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