ground fault protection isolated neutral

7/28/2019 Ground Fault Protection Isolated Neutral

1/18

1

Power System Protection

systems


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Professor at Norwegian Univ. Science and

Technology Dept. Electrical Engineering Power system transients and protection

g vo age engneerng, s ress cacua ons

Recent focus on Power Transformers

Sabbatical at MTU

, - [email protected]


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3

Solid vs. isolated neutral

+ No over-voltages in faultsituations

- Undefined voltages toground. Sqrt(3) rise

- g au curren

+ Easy fault detection

+ Ground faults ersist

.MOV dimensioning.

+ Low fault current

Fast trip and reclosing

- Poor power quality

- Difficult faultdetection/location

+ Can continue to o erate- Extra stress

during ground fault.Increased power quality

down/broken conductors


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Why is the fault current low insoate neutra systems

capacitances. High return impedance.

The most common fault

Often temporary rees ranc es,

snow/ice/wind,

Birds

Simultaneous faults.

3 V

1 2 03

f

fZ Z Z R


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In Norway DyDy

LV system 230 V IT MV system 12-24 kV

-

The 230 V IT system is gradually replaced by400 V TN solidl rounded due to risk ofundetected ground faults, double ground faults,and lower loss in a 400 V TN system.

n ; requremen s o e ec a groun au Power quality is very important to the industry


6/18

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CurrentI0: SumIa,Ib,Ic

Summing the current numerically

I0Loweraccuracy

Residual connection

Summation transformer

3I0

differences

3I0

(Toroidal/Rogowski coil)

a, ,

c

Open delta 3U0

Neutral point (isolated or resonance earth)U0


7/18

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Normal to only consider ground capacitance

symmetrical system,

ignore conductive line charging no voltage drop along line

imbalance in loads canceled b D-cou led transformers

Fault current mainly returns in the two healthy phases

Fault resistance important

hVpa h = ej120VN

Vpa

h2Vpa

gff


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occurs For calculation of feeder current and volta es

capacitances can be concentrated

The volta e to round will rise for the healthphases

Easy to detect that there is a ground fault (by

looking atV0), difficult to detect where (bylooking at I0 in various feeders)


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N A

V

+If

3pa gV j CI

fg 1 3 g fj C R

V I C0

1 3 3N

g f g

V Vj C R j C

f

A

UN

Rf=Rf=0

BBefore

Line-voltages

unchanged

B After


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Same zero-sequence voltage in the entire

system; selectivity is challenging The fault is traditionally located by

sectionalizing (often manual)

Z r n rr ntv ri l n th lin :I0

If

3I0U0 constant

Rf

pos.


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From fault to zero sequence current

VpaVN

x l-x

h2Vpa

hVpa

CgxCg(l-x)

2

( ) ( ) ' ( )a N pa gI x V V j C l x

Fault locationx=dx

( ) ( ) ' ( ) ( )

3 '

b N pa g

c N pa g f

x xI x V h V j C l x I H d x

V C x x d

03 ( ) ( ) 3 ' ( ) ( )

3 ' ( ),a b c N g f

N g

I x I I I V j C l x I H d xV j C l x x d

ignore voltage drop, no load-effect


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Three radials fed by the same transformer

Fault current in the faulty feeder is the negative sumof the current in the two healthy feeders

01 1N gI V j C

02 2N gI V j C

03 01 02( )I I I

With two radials the currents are equal in size sodirectional protection is required

-

current protection becomes possible


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- Often the preferred solution

Measure zero sequence voltage and zerose uence current in each radial

Then for each radial:

01

I02

Healthyfeeder

V0 03 V0

I0nForward

rppng zoneFaulty feeder

direction


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

and in correct quadrant (3rd)? > m ype rec ona ove -curren re ay

+

67N (I0>Ilim)

52 trip-


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-22 kV

y28 mi

50 miles line in totalTotal capacitance is50 miCg [F/mi]

14 mi

8 mi

5 nF/mi assumed in

this case.

Fault current inde endent on location

cable systems.

Depends on fault resistance

2.44 54.7 A 3 kIU ,'

,max 3.0 90 A @ 042441/ ( 3 50 )f

fff g

IIR jR j mi C


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ar on o e ear au curren measure oreach radial

28 mi

22/5050/50

14 mi

8 mi8/50


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Partition of the earth fault current measured for

each radial Health lines 1&3 feed 28 + 8 arts into line 2

28/5028 mi

8 mi

36/50

8/50

50/50


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Partition of the earth fault current measured for

each radial Health lines 1&2 feed 28 + 14 arts into line 3

28/5028 mi

14 mi

14/50

8 mi

42/50

ground fault protection isolated neutral

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