Grounding

By ELASSAD Hakam

Microelettrica Scientifica

MS RESISTANCES

ELECTRICITY

The Electricity is produced & generated

outside cities at high voltage

On Nuclear plant

On diesel ,or steam generateur

On wind or photovoltaique

On hydro plant

NUCLEAR POWER PLANT

Diesel Generation plant

Steam power process

Steam generator

Photovoltaic plant

WIND POWER PLANT

Transmission & Distribution

Electricity should be transported & distributed

The transportation is made through Cables (

High voltage head over lines Transmission )

63 , 132,220 , 330 , 400 or 750 kV the voltage

vary from countries , distance , total power to be

transported Etc.

Head over lines

Transmission & distribution

process

Sub-Station

Types of Neutral Grounding in Power

Distribution

In industrial high voltage network , ground

or earth is the reference point from which

voltages are measured , a common return

path for electrical current or direct physical

connection to the earth

Method of grounding (Earthing)

To determine the Earthing mode a

compromise between three requirements

–Damp over voltages

–Limit damage & disturbances cause by

the earth fault

–Provide simple selective protection

devices

Criteria to Choose

Voltage level: The insulation level of material must

be in accordance with induced over voltage at the

time of Short circuit

Insulation coordination : Ground fault current will

induce locally over voltage which must be

compatible with the insulation of components

Limitation of Fault current : To reduce the

electrodynamics stress on material , limit the induced

voltage on telecommunication lines

Grounding Method

Insulated or Ungrounded Neutral

Ungrounded system is no longer recommended

No connection between Neutral & Ground

Solidly earthed or direct Earthling

The Neutral of Transformer or

Generator is directly connected

to ground by Bus Bar

No limitation of Fault current

Is usually used in low voltage

600 V and lower

Impedance fault current Limitation

There is three types

By neutral grounding

Resistor

By Neutral grounding

reactor

By tuned reactor (

Petersen coil or

ground fault

neutralizer )

Through a Neutral Earthing Reactor

The Neutral of Transformer is connected to earth through a Reactor

The ground fault current should be at least 60% of the three phase fault current

Reactor grounding is not considered as an alternative to the resistor grounding

This solution is rarely used in industry or bear cities due to induced magnetic field

Neutral grounding Reactor dry

type

PETERSEN COIL

An Earthing reactor connected between the

neutral of a system and earth and having

relatively high value of reactance

The reactive current to earth under fault

conditions balances the capacitance current

to earth so that the earth current at the fault

is limited to practically zero

Arc suppression coil

(Peterssen coil )

Through a Neutral Earthing Resistor

The Neutral of Transformer is connected to

earth through a Resistor

The fault current is limited to chosen value

I f = (System Voltage /3)/R

There is two types of resistors

LOW RESISTANCE & HIGH RESISTANCE

Neutral Grounding Resistor

Low Resistance Grounding Connected to earth ground through low resistance

Limits ground fault current to a 20 % of 3 phase fault

current (10-3000 A)

Resistor is connected between neutral of power source

High Resistance Grounded

Connected to earth ground through a high resistance

Limits ground fault current to a few amperes (1-10 A is common)

Commonly used on low voltage systems in industrial plants

Used by utilities for large generators

Where Do We Ground?

Single source feeding bus

Source connected

Derive neutral with Y- or zig-zag transformer

Connect grounding resistor to this derived neutral

Where Do We Ground?

Single transformer feeding bus

Transformer secondary Y connected

Connect grounding resistor to neutral of transformer

Where Do We Ground?

Single generator

feeding bus

Generator Y

connected

Connect grounding

resistor to neutral

of generator

Where Do We Ground?

Multiple sources feeding bus

Sources Y or connected

Derive neutral with Y- or zig-zag transformer

Connect grounding resistor to this derived neutral

Where Do We Ground?

Double-ended

substation with

normally open tie

Transformers Y

connected

Connect grounding resistor to each

transformer neutral

NEUTRAL GROUNDING

RESISTOR

Neutral Earthing Resistor

NGR is not a protection components

NGR is installed to reduce the fault current before clearing

the fault

Phase - Neutral Voltage = V = System voltage / 3

V=U/1.732

Rated fault Current = V/R ( A )

Rated time (sec)

Required Temperature Rise °K

NOTA : IEEE-32-1990 is the only existing standards to

design the resistor

Rated Voltage

NGR is inserted between the Neutral of

Transformer ( secondary ) & Earth ,

The rated voltage of NGR = Phase to

Neutral Voltage = System Voltage /3

Rated Fault Current

I fault= (U/3)/ R

Phase to Neutral Voltage / resistance value

In High Resistance , The fault current =1 to 10 A

In Low Resistance : The fault current = 10 to 3000A

In case of NO indication we limit the fault current to the rated current of transformer

Rated Time & temperature Rise Rated time = 10 sec , 30 sec or 1 minute /hour

The time rating indicate the time that the NGR can operate under fault conditions without damage & without exceeding the specified temperature rise

385 °C for continuous rating

760°C for less than 10 minutes

As per I EEE 32 Standards ,

Insulation

As per IEEE 32 Standards the

applied voltage should be = 2.25 times

the line to neutral voltage + 2 kV

EXTRACT FROM IEEE 32-1990

Resistances material

Different kind of resistance material

1: Cast iron ( not used any more)

2: Liquid resistor ( not used any more)

Stainless Steel ( AISI 304 , 430,310,32/20 or

inconel 601 ) NiCr alloy

Temperature Coefficient of resistance material

Temperature coefficient =

indicate the variation of resistivity of resistance material with temperature

Resistance material used for NGR should have a high variation of resistivity to limit the fault current & keep the voltage constant

The Stainless Steel AISI 430 is mostly used for NGR

Protection Degree

The NGR is a passive component , resistor

will absorb the energy developed by the

current during rated time and should

evacuate it

The Best Protection Degree for NGR = IP23

IP 55 type tests

IP55

Sub station in the desert Egypt

Components can be installed into Resistor

RECOMMANDED

Current & voltage transformer

On or off load disconnectors ( contactors)

Surge Arrestors

Space heater

Lamp

Doors

NOT RECOMMANDED

Electronic devices like relays or others monitors

( temperature into resistors will reach very high level & cause

damage to electronic

accessories

ROUTINE & Type Tests

Routine Tests Visual Inspection

Dimensional Check

Measurement of Insulation Resistance

Measurement of Resistance

High Potential Test (Dielectric test @ 50Hz – 1min)

Type Tests

As per IEEE-32 Impulse tests are not required for Resistor

Heat run tests = Temperature Rise tests , application of constant voltage during

rated time

measurement of current & resistance value at 0 sec up to end rated time

Measurement of temperature rise by application of tempearture sensor

Calculation of temperature rise tests as per following formula

R2=R1*(1+αΔθ)

α = Temperature coefficient of resistance material

Δθ = Temperature Rise

Temperature Rise tests & installation of temperature sensor

The End