Choosing between resistor and reactor

Choosing Between

Resistor and Reactor

for Neutral Ground

Impedance

Faults involving ground produce high fault

current magnitude especially when the

transformer(s) neutral is solidly grounded.

The neutral ground circuit in the transformer

provides the return path for the fault

currents. To limit this current, impedance

either in the form of a reactor or a resistor

is installed in the neutral circuit. See the

figure below.

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02-04-2015http://peguru.com/2013/01/choosing-between-resistor-and-reactor-for-neutral-ground...

Ground Fault Current Path

The table below lists the differences between

utilizing a resistor or reactor as the

impedance in the transformer neutral circuit.

Resistor

Image

courtesy:Postglover

Reactor

Image

courtesy:Trench

Pros

1. It uses actual

resistance of the

design material

1. It uses inductive

reactance of the

coil to create an

impedance path

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to limit the fault

current.

2. It is universally

applied without

any limitation.

for the fault

current.

2. It is well suited

for applications

which require

several thousand

amps to flow

through it for a

short duration.

Cons

1. It is expensive to

build since it

must have

enough mass to

absorb the fault

current energy.

The price

increases with

the increase in

fault current it

can handle and

time rating.

1. It is typically

applied in

systems where

there is a desire

to limit the

ground fault

current to a

magnitude that is

25% to 60% of the

three phase fault

current. If the

ground fault

current is limited

to less than 25%

of the three-

phase fault

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current then the

neutral winding

can be subjected

to transient over-

voltages

(explained

below).Therefore,

a surge arrester

should be

installed in with

the reactor to

achieve the

reduction in fault

current and

prevent transient

over-voltages

from damaging

the neutral

winding of the

transformer.

More on transient over-voltages:

Transient over-voltages are produced by arcing faults,

not surges. The over-voltage occurs when the arc

strikes due to a line-to-ground fault and charges the

system capacitive reactance. When the arc momentarily

extinguishes, the charge needs to dissipate. When the

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neutral ground resistor is used as the impedance,

its resistance is usually less than the capacitive

reactance, thereby allowing the voltage to discharge.

However, when the reactor is used and when its

reactive impedance is high (to limit ground fault

current to less than 25% of the three phase current), the

voltage cannot discharge. As the arc re-fires, the

charge can continually build, thus creating the over-

voltage.

Summary:

If it is desired to limit the fault current to a

really low magnitude using the actual resistance

then a resistor is recommended. On the other

hand, if several thousand amps of fault current

is permissible in the system then the reactor is

recommended. In the either case, the reactor can

be an economical solution. Keep in mind, we

are talking about shunt air core reactor in the

transformer neutral for current limiting purpose.

Series reactors, however, are expensive. The

price of any neutral ground impedance device

increases with the increase in the continuous

current rating (for reactors), impedance rating,

and time rating.



* * * * *

grounding neutral grounding reactor

resistor transient overvoltage

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