System Grounding

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2Introduction

The objective of a grounding system are:

1.

To provide safety to personnel during normal and fault conditions by limiting step and touch potential.

2.

To assure correct operation of electrical/electronic devices.3.

To prevent damage to electrical/electronic apparatus.

4.

To dissipate lightning strokes.5.

To stabilize voltage during transient conditions and to minimize the probability of flashover during transients.

6.

To divert stray RF energy from sensitive audio, video, control, and computer equipment.

3Introduction

A safe grounding design has two objectives:

1.

To provide means to carry electric currents into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service.

2.

To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock.

The PRIMARY goal of the grounding system throughout any facilities is

SAFETY.

Why ground at all?

PERSONNEL SAFETY FIRST

EQUIPMENT PROTECTION SECOND

4Introduction

What are the three main types of grounding?

1. EQUIPMENT GROUNDING (SAFETY)2. SYSTEM GROUNDING3. LIGHTNING/SURGE GROUNDING

Types of Faults

Phase Faults (limited only by positive sequence impedance of system)

High Fault Currents.

Only limited by inherent impedance of Power System.

Earth Faults

Solid Earthing means high earth fault currents

Only limited by inherent zero sequence impedance of Power system.

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Consequences

Heavy currents damage equipment extensively.

Danger of fire hazard.

This leads to long outage times.

Lost production and lost revenue.

Heavy currents in earth bonding gives rise to high touch potentials -

dangerous to human life.

Large Fault currents are more hazardous in igniting gases.

Explosion Hazard.

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Solutions

Phase Segregation (separating phases far apart)Eliminates phase-to-phase faults.

Resistance Earthing

Means lower earth fault currents

Value can be chosen during design stage to limit current to desired value -

say 400Amps

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Benefits (1)

Fault damage now minimalReduces fire hazard (especially in mines)

Lower outage timesLess lost production, less lost revenue.

Touch potentials kept within safe limits.Protects human life.

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Benefits (2)

Low Fault Currents reduce possibility of igniting gases.Minimizes explosion hazard.

Lower Magnetic or thermal stresses imposed on plant during fault.

Transient overvoltages limitedPrevents stressing of insulation, breaker restrikes.

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Neutral GroundingPower System Grounding

System grounding

means the connection of ground to the neutral

points of current carrying conductors such as the

neutral point of a circuit, a transformer, rotating machinery, or a system, either solidly or with a current limiting device.

Ungrounded system.

Solid grounding

Impedance grounding (R and X)

Resonant grounding

Earthing Methods 1

Ungrounded SystemNeutral connection on Generator/ Transformer is not

connected to earth at all

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Ungrounded Systems

An ungrounded system is one in which there is no intentional connection between the conductors and earth ground.

The ungrounded system

is, in reality, a capacitive grounded neural system

by virtue of the distributed capacitance. The capacitance being

the

conductor capacitance to ground.

The ungrounded neutral system is a capacitive grounded neutral system,

In normal operation the capacitive current of all three lines is

leading the respective line to neutral voltage by 90o

, and the vector sum of all three

currents is zero.

Early Electrical systems are almost universally operated ungrounded. On small systems an insulation failure on one phase did not cause an outage.

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Ungrounded Systems

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Ungrounded Systems

In addition to the cost of equipment damage, ungrounded systems present fault locating problems.

This involves a tedious process of trial and error; first isolating the correct feeder, then the branch, and finally the equipment at fault.

The result is unnecessarily lengthy and expensive downtime.

Despite the drawbacks of an ungrounded system, it does have one main advantage.

The circuit may continue in operation after the first ground fault, assuming it remains as a single fault.

This permits continued production, until a convenient shutdown can be scheduled for maintenance.

The interaction between the faulted system and its distributed capacitance may cause transient over-voltages (several times normal) to appear from line to ground during normal switching of a circuit having a line to ground fault (short). These over-voltages may cause insulation failures at points other than the original fault.

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Grounded Systems

All power systems of today operate with grounded neutrals. It is

important because:

1. The earth fault protection is based on the method of neutral grounding.

2. The system voltage during earth fault depends on neutral grounding. 3. Neutral grounding has its associated switchgear.4. Neutral grounding gives protection against arcing ground, unbalanced

voltage with respect to earth, and protection from lightning.

The intentional connection of the neutral points of transformers, generators and rotating machinery to the earth ground network provides a

reference

point of zero volts.

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Grounded Systems

This protective measure offers many advantages over an ungrounded system, including:

Reduced magnitude of transient overvoltages

Simplified ground fault location

Improved system and equipment fault protection

Reduced maintenance time and expense

Greater safety for personnel

Improved lightning protection

Reduction in frequency of faults.

Earthing Methods 2

Solid earthingNeutral connection on Generator / Transformer is connected to earth by a solid Conductor Cost Reductions due to avoidance of sensitive relays and earthing device, Grading of insulation towards neutral end.But Circulation of third harmonic currents between neutrals

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Solidly Grounded Systems

A solidly grounded system is one in which the neutral points have been intentionally connected to ground with a conductor having no intentional impedance.

It is a simple and effective method of grounding and inexpensive.

The neutrals of any star connected transformers, generators are connected to ground.

It minimizes the magnitude of the overvoltage that will appear on the unfaulted

phases during a ground fault, resulting in a reduction in the stress on

insulation.

This partially reduces the problem of transient overvoltages found on the ungrounded system, provided the ground fault current is in the range of 25 to 100% of the system three phase fault current.

While solidly grounded systems are an improvement over ungrounded systems, and speed the location of faults,

they lack the current limiting ability

of resistance grounding.

Earthing Methods 3

Resistance earthingNeutral connection on

Generator / Transformer is connected to earth (0V) through a fixed resistance to limit the earth fault current

Mainly used below 33 KVFull line to line insulation

required towards neutral

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Resistive Grounded Systems

Resistance grounding is by far the most effective and preferred method.

It solves the problem of transient overvoltages, thereby reducing equipment damage.

It accomplishes this by allowing the magnitude of the fault current to be predetermined by a simple ohms law calculation

Thus the fault current can be limited, in order to prevent equipment damage.

I =V/Rwhere: I = Limit of Fault Current.

V = Line-to-neutral Voltage of SystemR = Ohmic Value of Neutral grounding Resistor

Limiting fault currents to predetermined maximum values permits the designer to selectively co-ordinate the operation of protective devices, which minimizes system disruption and allows for quick location of the fault.

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Resistive Grounded Systems

There are two broad categories of resistance grounding: low resistance high resistance.

In both types of grounding, the resistor is connected between the neutral of the transformer secondary and the earth ground.

Low Resistance Grounding

Low resistance grounding of the neutral limits the ground fault current to a high level (typically 50 amps or more) in order to operate

protective fault clearing relays and current transformers.

These devices are then able to quickly clear the fault, usually within a few seconds.

Low resistance grounding resistors are commonly found on medium and high voltage systems.

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Resistive Grounded SystemsHigh Resistance Grounding

High resistance grounding of the neutral limits the ground fault

current

to a very low level (typically under 25 amps).

It is used on low voltage systems of 600 volts or less, under 3000 amps.

By limiting the ground fault current, the fault can be tolerated

on the system until it can be located, and then isolated or removed at a convenient time.

High resistance neutral grounding can be added to existing ungrounded systems without the expense of adding fault clearing relays and breakers.

This provides an economical method of upgrading older; ungrounded systems.

High resistance neutral grounding combined with sensitive ground fault relays and isolating devices, can quickly detect and shut down the

faulted circuit.

Earthing Methods 4

Reactance earthingNeutral connection on

Generator / transformer is connected to earth (0V) through a fixed reactance to limit the earth fault current

Can be cheaper compared to resistance

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Earthing Methods 5- Petersen Coil (arc suppression)

Peterson coil is a tunable iron cored reactor connected between the neutral and ground.

Neutral connection on transformer is connected to earth (0V) through a variable reactance to neutralize the capacitive earth fault current. Results in arc extinction.

Elimination of the fault current that could cause the arcing ground condition.

Normally it does not carry current

During fault: reactive component of current I = capacitive component of current I

No current at the fault, preventing restrikes and eliminates the cause of voltage buildup

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Earthing Methods 6

In HV delta systems no earth connection is available. A 3 phase neutral earthing compensator is connected to allow earth fault currents to flow -

allowing detection

of these faults.

Due to its composition, a zigzag transformer is more effective for grounding purposes because it has less internal winding impedance going to the ground than when

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NEC earthing (with and without resistance)

Earthing Methods 6

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When a ground fault occurs downstream of the Zigzag transformer, ground fault current flows through the fault, back through ground and the NGR to the Zigzag where the current is divided equally in each leg of the Zigzag. Since these three currents are all equal and in time phase with each other (zero sequence), and because of the special Zigzag winding connections, they see a very low impedance. This allows the ground fault current to flow back into the system. It can be seen that the ground fault current is only limited by the resistance of the ground fault, the NGR, and the small reactance of the Zigzag.

Slide Number 1Slide Number 2Slide Number 3Slide Number 4Types of Faults ConsequencesSolutionsBenefits (1)Benefits (2)Slide Number 10Earthing Methods 1Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Earthing Methods 2Slide Number 18Earthing Methods 3Slide Number 20Slide Number 21Slide Number 22Earthing Methods 4Earthing Methods 5- Petersen Coil (arc suppression)Earthing Methods 6Earthing Methods 6