Neutral Grounding Resistors

&

Temperature coefficient By Hakam ELASSAD

MS RESISTANCES

When designing an industrial High Voltage (HV) network, sub- station ,a suitable Neutral Ground arrangement must be selected: the neutral can either be insulated, or it can be connected to earth

Once the neutral ground solution has been adopted to determine the Grounding mode, a compromise has been to be made between three contradictory requirements:

To damp over voltages To limit damage & disturbances caused by an earth fault To provide simple, selective protective devices

GROUNDING METHODS

Direct or Solidly Grounding It is the most efficient grounding method to limit the over-voltage. However, In the event of a ground fault, the current is not limited; damage Interferences occur Flash hazard are important during the fault. This method is not used for High voltage distribution. Impedance Fault Current Limitation or impedance grounding Impedance grounding Limits point fault damage. Eliminates transient over-voltages. Reduces flash hazard and provides adequate tripping levels for selective ground fault detection and coordination. Impedance grounding consists of connecting a neutral grounding resistor or reactor in series between the transformer neutral & earth as shown below. In the event of phase to earth fault, the current will flow through resistor or reactor and be limited.

Grounding through Reactor Tuned Reactor This solution is sometimes used for public HV networks. Protective relays sensitive to the active component of the residual current must be used to obtain selectivity. Current limiting reactor This solution can result in over voltages; it can be used only where there are low limiting impedances.

Grounding through Resistors

Only the IEEE treats the Grounding

&

IEEE-142 the green book

We are in contact with the committee of IEC to build a new standards for Neutral Grounding Resistors

As Per IEEE-32

ln a resistance-grounded system, the neutral of the transformer or generator is connected to ground through a resistor. The resistance has a considerably higher ohmic magnitude than the system reactance at the resistor location. Consequently, the line-to- ground fault current is primarily limited by the resistor itself

The reasons for limiting the current by resistance grounding include the following: 1: To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables. and rotating machines. 2:T0 reduce mechanical stresses in circuits and apparatus carrying fault currents 3: To reduce electric-shock hazards to personnel caused by stray ground-fault current in the ground-return path. 4:To reduce the arc blast or flash hazard to personnel who may have accidentally caused or happen to be in close proximity to the ground fault. 5:To reduce the momentary line-voltage clip occasioned by the occurrence and clearing of a ground fault. 6:To secure control of transient over-voltages while at the same time avoiding the shutdown of a faulted circuit on the occurrence of the first ground fault (high resistance grounding).

Designing of Neutral Grounding Resistor

Resistor comprises: Active parts, Insulators, accessories all are housed and assembled into cubicle 5 steps to be followed during the designing to be in line with the related standards

1: Technology of active parts

2: Resistance material

3: Insulation

4: Protection degree of housing

5: Finishing

Technology of active parts

Different Resistance elements are used to build resistor, elements are connected together in bank, connection is made in serial or in parallel to obtain the electrical resistance value . The most known elements are 1 : GRID TYPE or flat obtained by punching , expanding or cutting 2 : EDGEWOUND COIL TYPE : obtained by wounding or wire . 3 : Mats type : obtained by woven the metallic wire and glass wire 4 : Liquide type :(this technology is not used anymore for NGR , it is only used for some application as soft starter for starting of slip ring asynchronous motors);

The neutral Grounding resistor work in adiabatic zone An “adiabatic” process is a thermodynamic process in which there is no heat transfer

Tem

pera

ture

Time

θ2

θ1

Adiabatic No Adiabatic

0 T1 T2

Δθ = θ2-θ2 ( °K ) : Temperature Rise

θ2 = temperature of resistor after rated time (°C ) θ1= ambient temperature (°C)

From 0 to T1, the resistance elements absorb & store the energy . This is adiabatic heating, the energy absorbed

is 2 1

Cp is the specific heat of the body or material used °

Where E is the energy dissipated by the body (in Joules),

m =is the mass of the body (in Kg), and

2 1 is the change in temperature (in Kelvins).

The energy from a current flowing through a resistance is based on the definition for electrical energy:

Where is the current (in Amps), is the resistance of the body which the current is flowing through (in Ω)

and is the duration of the current flow (in seconds).

From T1 to T2, the resistance elements exchange the heat with surrounding atmosphere, NO adiabatic heating is anymore applicable. The power exchanged is P=K.S.Δθ .

P = exchanged power

S = surface of active part of resistor elements

K = Thermal Exchange Coefficient of the used resistor Elements Technology

CONSTRUCTION OF RESISTORS

Different kind of construction: 1 : Air cool Resistor 1.1 Natural convection Most common arrangement, 90% of resistor are built to be cooled by natural air circulation The size of resistor is depending of the total energy to be dissipated by the resistance elements (W) Maintenance of resistor is easy the maintenance frequency is depending on the pollution in the area where resistor is installed. 1.2 : Forced air cooling . Is used when the energy to be dissipated by resistance elements is important and the available space reserved for resistor is reduced. This kind of arrangement requires air blower and ventilator. Is generally used for Railway transit resistor (Dynamic braking resistor) or load bank

This kind of arrangement requires good maintenance. 2: Oil Cool Resistors. Is used when the energy to be dissipated by resistance elements are important and the available space reserved for resistor is reduced , it is generally used for high voltage resistor ;Oil tank and oil cooling circuit are required 3: Water cool Resistor.

Resistance material

The Neutral Grounding Resistor is used to keep the voltage constant during Fault

Since the active material used in resistors has an appreciable temperature coefficient, the resistance is materially charged during the time of operation causing the voltage ta increase or the current to decrease, the resistor shall be rated for constant voltage and the rated voltage shall be taken equal to the line-to-neutral voltage.

The conductor element resistance changes to some extent with temperature. The change may be calculated from the temperature coefficient of resistivity

. α = θ θ

R2=R1(1+ α(θ2-θ1))

RI and R2 are resistances in ohms. at temperatures θ1 and θ2 in degrees Celsius, respectively, and α is the temperature coefficient of resistance.

As per Ohm law U=I x R

U = Voltage to be kept constant

The Fault current (I) must be limited and decreased

The Resistance (R) will increase due to Temperature Rise

AS per definition the resistance

R= ρ

ρ = resistivity of resistance material

l = length of resistance element

S= section of resistance element

ρ2 = resistivity at θ2 , ρ1 = resistivity at θ1

Considering the l & s will not change or slightly change

ρ2 = ρ1 x(1+α(θ2-θ1) → ρ2 = ρ1 x(1+α(θ2-θ1) →

The temperature coefficient α = θ θ

We know that the resistivity increases with temperature.

If the value of temperature coefficient is indicated in the specification , the temperature interval should be indicated also Example: from ambient temperature to 500°C

Despite the requirement of the new IEEE-C57-32 and as per our experience, the stainless steel AISI 430 or AISI 304 , are the best resistance material for Neutral grounding resistors , , because the variation of resistivity is high with temperature , which allow the fault current to decrease during fault and keep the voltage constant

The argument given in the New IEEE-C57-32 to use stainless steel with very low temperature coefficient does not hold, because a current transformer is always coupled with a protection relay connected to the neutral circuit. Simply choosing a relay with an adequate detection range is enough to avoid detection fails due to a strong drop of current caused by a strong increase in ohmic value.

The New IEEE-C57-32 is partially valid for High Resistance Grounding (HRG) communally used in the USA

LIMITATION OF TEMPERATURE RISE

As per IEEE-32 the temperature rise should be limited to see table

0200400600800

10001200

0 2 4 6 8 10 12

Time (sec) O\Ref: AISI 430

M.S. RESISTANCES

0100200300400500600700

0 2 4 6 8 10 12

Time (sec) O\Ref: AISI 430

M.S. RESISTANCES

0

5

10

15

20

25

30

0 2 4 6 8 10 12

Time (sec) O\Ref: AISI 430

M.S. RESISTANCES

0200400600800

10001200

0 2 4 6 8 10 12

Time (sec) O\Ref: AlClFe

M.S. RESISTANCES

12,5

13

13,5

14

14,5

0 2 4 6 8 10 12

Time (sec) O\Ref: AlClFe

M.S. RESISTANCES

0

200

400

600

800

1000

0 2 4 6 8 10 12

Time (sec) O\Ref: AlClFe

M.S. RESISTANCES

Insulation

The air temperature into the cubicle can reach a high value up to 500 °C after fault duration

The Insulators must be able to withstand high temperature.

The use of Epoxy insulators or bushing is strongly NOT RECOMMANDED)

The use of Porcelain insulators or Bushing is highly & strongly RECOMMANDED

The insulation is given by IEEE-32

The voltage applied from terminals of each assembly to its own frame shall be twice the rated voltage of the section of which the frame is part plus 1000V when rated voltage =600 V or les

Or 2,25 times the rated voltage+2000V when rated voltage is over 600V

TO simplify we will insulate the resistors using insulators level value higher than the line to neutral voltage in kV

As per IEEE-32 , No impulse tests required for Neutral Grounding Resistor if voltage is equal or less than

34.5 kV

Example : insulation of 11 kV resistor

Line to line voltage = 11 kV

Line to Neutral voltage = 11/√3 = 6.35 kV

Minimum of Applied voltage = 6.35*2.25+2=16.28 kV one minute 50 Hz

Size of insulators =7.2 kV as per IEC

Protection Degree There are different kind of protection degree for resistance : As per IEC standard 528, there is no air circulation into cubicle which is an IP 54 protection degree. In such case, the air into cubicle is subject to be heated and pressure increasing, an exploding risk occurs if no safety protection such valves or others.

Protection degree

Protection

Comments

IP00

No protection Suitable for indoor installation

IP23 Protected against solid objects greater than 12 mm and against spraying water.

- Suitable and recommended for indoor or outdoor installation - The cooling time is approximately 10 to 30 the heating time.

IP43 Protected against solid objects greater than 1 mm and against spaying water.

- Suitable for outdoor installation. The cooling time is approximately 20 to 30 minutes.

IP54 Dust protection and water splashing

- No recommended - The cooling time is approximately 40 to 50 minutes - Exploding risk if no safety protection against pressure rise - The hot spot must be reduced to very low value. - A space heater must be installed which required an AC (380, 220 or 115) or DC supply.

IP 55 or IP65 Dust protection or dust-tight and water jets

Are not recommended

For better cooling , the recommended protection Degree for Neutral Grounding Resistor = IP23

33 kV IP 55 400 kV IP23

HOUSING FINISHING:

- The hot dip galvanizing finishing of housing is the best protection against corrosion or aggressive environment such acid pollution. - For installation near the sea, the housing can be made from Nickel chromium stainless steel sheets AISI 316. The Nickel Chromium stainless steel must be AISI 316 - We do not recommend painting housing.

FINISHING

USE & PROTECTION

Hot dip Galvanizing - Best corrosion resistance,- best against acid , basic , saline or chemical pollution - Recommended for indoor or outdoor,- recommended for installation near the sea.

Nickel Chromium stainless steel AISI 304

- Not recommended for installation near the sea (salt and humidity).

Nickel Chromium stainless steel AISI 316

- Recommended for installation near the sea.

Mill Galvanizing - Not recommended near the sea due to corrosion risk. Painted - Not recommended (painting destruction, due to the elevation of temperature of

housing and corrosion risk )

Mill or Hot dip Galvanizing & Painting

- Better than only painting.

Tests into sulfur dioxide environment 500 hours

Stainless Steel AISI304 Stainless Steel AISI 441Stainless Steel AISI 430 Hot Dip Galvanized

Test into saline environment 500 hours

Stainless Steel AISI304 Stainless Steel AISI 441 Stainless Steel AISI 430 Hot Dip Galvanized

Routine & Type TESTS