ground fault protection resonance neutral

7/29/2019 Ground Fault Protection Resonance Neutral

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1

Power System Protection

grounded systems


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Professor at Norwegian Univ. Science and

Technology Dept. Electrical Engineering Power system transients and protection

g vo age eng neer ng, s ress ca cu a ons

Recent focus on Power Transformers

Sabbatical at MTU

, - [email protected]


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Solid vs. resonance neutral

+ No over-voltages in faultsituations

- Undefined voltages to ground.High at resonance.

+ Ideall zero fault current. Arc

- g au curren

+ Easy fault detection

+ Ground faults ersist

self-extinguish.

- Difficult fault detection/location.Requires special competence

Fast trip and reclosing

- Poor power quality

and maintenance.

- In dynamic systems the coilmust adapt. Expensive

- Extra stress .+ Can continue to operate duringground fault. Increased power

ualit .

Safety issues: down/broken

conductors


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How can be reduce fault current further

from isolated neutral systems?

.

Resonance principles:

C L

I

+

V

-

L=-1

Cg

21 I V j C LC

j L j L

At resonance (2LC=1) the current is zero

g


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CurrentI0: SumIa,Ib,Ic

Summing the current numerically

I0Loweraccuracy

Residual connection

Summation transformer

3I0

differences3I0

(Toroidal/Rogowski coil)

a, ,

c

Open delta 3V0

Neutral point (isolated or resonance earth)V0


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Inductance added in the neutral point

Compensate for the capacitive current through Cg

; help

the arc self extinguish

changes (Petersen-coil (patent from around 1920)

Adjust air gap The coil changes it value incrementally and notice how the

neutral voltage changes. From a theoretical curve the resonance

point is estimated.

Over-compensation preferred to limit VN Avoid resonance also when parts of the system become

disconnected

The unit Ampere is used to quantify the coil;IL=Vp/L


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Variable inductance

h2V

hVpaN

TRENCH

Vpa

L

Increases with

resistance

VN Normal

operation

esonance

No-fault situation

L~


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First attempt to analyze the

neutral voltage

normal operation. Set U0 pick-up above this limit I V V C

V

h2Vpa

pa

2( )

( )

b N p a

c N p a

I V h V j C

I V h V j C

L C

03 ( ) 3

0? ?

Na b c N

N

I I I I V j Cj L

V3I0

At resonancej3C+1/jL=0 and the neutral

Need to add sophistication here


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Effect of asymmetry and

conductive line chargingN Increases with

Va

h2Vpa

paasymmetry and

resistance

VN Normal

C+CL GC

point of

operation

2

( ) ( )

( ) ( ( ) )

a N p a

b N p a

I V V G j C

I V h V G j C C

Resonance

No-fault situation

IL~1/LOver-comp.Under-comp.

20

2

3 ( ) 3 ( ) ( )

c N p a

Na b c N N p a

p a p p

VI I I I V G j C V h V j C

j L

h V j C V V C ,m ax

3 3 1 / 31

N N Ns dG j C j C j L Gj

u u


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Resonance grounded system

during fault (L=constant)

2(1 3 ) aV L CN A

Vpa

+If

3G2(1 3 )

f

f gj L R L CRf3Cg

L

0 2(1 3 )

pa

N

f g

V Vj L R L C

Vpa at resonance,

also for largeRf

elements (G||1/Rf) affect fault current and VN

to find the fault (identify faulty feeder)


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From ATP simulations

In reality the resonance peek is typically lower and measurements are required

25

30

35

V]rm

s

Stay away from this

resonance area

10

15

20

tralvoltage[

Fault0k

Fault3k

NofaultLCC

0

5

0 2 4 6 8 10 12 14 16 18 20

Ne

1pu

Normal point

of operation

100 k

U

V

SAT

Y

LCC

50. km

WRITE

max

50. km

LCC

50. km

LCC

50. kmM1

Coil"current"[A]0.1 nF 0-3k


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Directional over-current relay

resonance grounded systems Start when V0 goes above a fixed limit; pre-

measure the coil response max(VN,healty);

After a delay connect resistorR (|| toL)

nd

VNea y

0

Trip zone set based on natural conductive

current and asymmetry

IL

I01

I02

-I0

For each feeder

V0

I03LR

CG

V00CG I0L

I

Trip zone


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The resistive component ofI0 (in phase

with V0) is used

Trip zone

- 0

I0CGI

- 0

For each feeder

0

R

Healty feeder

0

I0R

R

Faulty feeder

I0CG r p zone

03 ( )

i a i b i c iI I I I 0

3 ( )

3 3 /

i a i b i c iI I I I

V G C V V R

0

13 ( ) 3 ( )

t i t iV G G j C C

j LAngle quite close to 90


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V0>Vlim (type 59G over-voltage relay)

and in correct quadrant (4th)? > m ype rec ona over-curren re ay

+

67N (I0>Ilim)

52 trip-


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Used in the 132 kV resonance earthed s stem

Measure the transient surge and use time ofarrival principles

Communication required

Trip the two relays that first identify the fault ens t ve to stur ances e g tn ng

ground fault protection resonance neutral

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