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GENERATOR NEUTRAL SYSTEM

This paper is submitted to fulfill one of the requirements of Ship Electrical Installation subjects

Group Member:

Mochamad Iqbal 4211101007Muhammad Arifin 4211101008Mohammad Hafidh R 4211101015

Johannes Michael 4211101020

FACULTY OF MARINE TECHNOLOGY

DEPARTMENT OF MARINE ENGINEERING

INSTITUT TEKKNOLOGI SEPULUH NOPEMBER

2 14

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PREFACE

Praise to God who has been giving the blessing and mercy to all of us and the writerto complete the paper entitled "Generator Neutral System". This paper is submitted to fulfillone of the requirements of Ship Electrical Installation subjects.

The Generator Neutral System are similar to fuses in that they do nothing untilsomething in the system goes wrong. Then, like fuses, they protect personnel andequipment from damage. Damage comes from two factors, how long the fault lasts andhow large the fault current is. Ground relays trip breakers and limit how long a fault lastsand Neutral grounding resistors limit how large the fault current is.

In finishing this paper, the writer really gives his regards and thanks for people whohas given guidance and help. Finally, the writer realizes there are unintended errors in writingthis paper. He really allows all the readers to give their suggestion to improve its content inorder to be made as one of the good examples for the next paper.

Surabaya, June 7, 2014

Authors

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CONTENTS

PAGESPREFACE ............................................................................................................... ii CONTENTS............................................................................................................ iii

CHAPTER 1 INTRODUCTION1.1 Background ...................................................................................................... 11.2 Objective ............................................................................................................ 1

CHAPTER 2 BASIC THEORY2.1 Importance of Neutral Grounding ................................................................. 22.3 System and Equipment Grounding ................................................................. 22.3 Methods of Neutral Grounding ....................................................................... 3

1) Ungrounded Neutral System .......................................................................... 32) Solid Neutral Grounded System ..................................................................... 53) Resistance Neutral Grounding System ........................................................... 6

o Low Resistance Grounding ...................................................................... 8o High Resistance Grounding .................................................................... 10

4) Resonant Neutral Grounding System ........................................................... 12

5) Grounding Transformer ............................................................................... 14

CHAPTER 3 DISCUSSION3.1 Power Distribution .......................................................................................... 163.2 Grounding System in Ship Electrical Network ............................................ 163.3 Electrical Fault ................................................................................................ 17

3.3.1 Earth Fault ................................................................................................ 183.3.2 Open Circuit Fault.................................................................................... 183.3.3 Significance of Earth Fault ...................................................................... 18

3.4 Low-voltage Power Distribution System Grounding Analysis ................... 19

CHAPTER 4 CLOSING4.1 Conclusion ....................................................................................................... 23

REFERENCES ...................................................................................................... 24

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CHAPTER I

INTRODUCTION

1.1 BackgroundPower Systems Grounding is probably the most misunderstood element of any PowerSystems design. Therefore, this paper will review the characteristics of different PowerSystems Grounding techniques as currently applied -- and misapplied within industrytoday. In many cases, misunderstood concepts and perceptions of the purpose and type ofPower Systems Grounding to be selected dates back to the 1940's and earlier. Since thattime much research, coupled with experience, has taken place that is now available toindustry. This paper will review the many different system grounding practices and

present information on different grounding methods. Safety, National Electric Code

requirements, and operational considerations, such as continuity of service, will beinvestigated. Finally, examples of proper applications within various industries will begiven.

1.2 ObjectiveThis paper is aimed at a better understanding of how to get an electrical system that canimproved a better system that can be more reliable and safety for a user or operator byusing grounding system for electrical

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CHAPTER 2

BASIC THEORY

2.1 Importance of Neutral Grounding Neutral grounding systems are similar to fuses in that they do nothing until something inthe system goes wrong. Then, like fuses, they protect personnel and equipment fromdamage. Damage comes from two factors, how long the fault lasts and how large the faultcurrent is. Ground relays trip breakers and limit how long a fault lasts and Neutralgrounding resistors limit how large the fault current is.

There are many neutral grounding options available for both Low and Medium voltage power systems. The neutral points of transformers, generators and rotating machinery tothe ground ground network provides a reference point of zero volts.

This protective measure offers many advantages over an ungrounded system, like: Reduced magnitude of transient over voltages 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.

2.2 System and Equipment Grounding

Careful consideration of the grounding arrangements of AC generators used in emergencyand standby power systems is essential for optimum continuity of power for critical loadsand for the safety of personnel. Specific considerations for emergency and standby

systems include selection of a system grounding method for the generator, when to usefour pole transfer switch equipment, requirements for indication only of a ground fault onthe generator, and the methods used in transfer equipment for switching the neutral pole.

The term grounding describes and encompasses both systems grounding and equipmentgrounding. The basic difference between system and equipment grounding is that systemgrounding involves grounding circuit conductors that are current carrying under normaloperation, where equipment grounding involves grounding of all non-current carryingmetallic parts that enclose the circuit conductors. A grounding electrode or severalgrounding electrodes tied together as a system provides the reference ground and the

means for connection to earth.

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System grounding refers to the intentional connection between a conductor of an AC power system and ground. The source of normal power for the system is typically a utilitysupplied transformer and the source of emergency or standby power is typically anowner-supplied on-site generator set. The power system conductor connected to ground

becomes the grounded conductor, which is typically the neutral circuit conductor on a 3- phase 4-wire system. System grounding, in other words, describes the practice ofgrounding one conductor of an AC power system.

Equipment grounding refers to the bonding and grounding of all non-current carrying(during normal operation) metal conduit, equipment enclosures, supports, frames, etc. forcurrent carrying circuit conductors and equipment. Equipment grounding contributes to

personnel safety by limiting the voltage to ground on these metallic parts and reduces thehazard of electric shock. All of those metallic parts are bonded together to make an equal

potential conductor. This provides a sufficiently low impedance path for ground fault

current to flow back to the system power source, through a bonding jumper to thegrounded circuit conductor (neutral) located at the service equipment.

2.3 Methods of Neutral Grounding

There are five methods for Neutral grounding:

1) Ungrounded Neutral System2) Solid Neutral Grounded System3) Resistance Neutral Grounding System

o Low Resistance Groundingo High Resistance Grounding

4) Resonant Neutral Grounding System5) Grounding Transformers

1) Ungrounded Neutral SystemIn ungrounded system there is no internal connection between the conductors andground. However, as system, a capacitive coupling exists between the systemconductors and the adjacent grounded surfaces. Consequently, the ungrounded

system is, in reality, a capacitive grounded system by virtue of the distributedcapacitance.

Under normal operating conditions, this distributed capacitance causes no problems. In fact, it is beneficial because it establishes, in effect, a neutral pointfor the system; As a result, the phase conductors are stressed at only line-to-neutral voltage above ground.

But problems can rise in ground fault conditions. A ground fault on one lineresults in full line-to-line voltage appearing throughout the system. Thus, avoltage 1.73 times the normal voltage is present on all insulation in the system.

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This situation can often cause failures in older motors and transformers, due toinsulation breakdown.

Figure 2.2.1. Ungrounded Neutral Systems Source: http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-

distribution-part-1

AdvantagesAfter the first ground fault, assuming it remains as a single fault, the circuit maycontinue in operation, permitting continued production until a convenient shutdown for maintenance can be scheduled.

Disadvantages The interaction between the faulted system and its distributed capacitance

may cause transient over-voltages (several times normal) to appear fromline to ground during normal switching of a circuit having a line-to groundfault (short). These over voltages may cause insulation failures at pointsother than the original fault.

A second fault on another phase may occur before the first fault can becleared. This can result in very high line-to-line fault currents, equipmentdamage and disruption of both circuits.

The cost of equipment damage. Complicate for locating fault(s), involving a tedious process of trial and

error: first isolating the correct feeder, then the branch, and finally, theequipment at fault. The result is unnecessarily lengthy and expensive downdowntime.

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2) Solid Neutral Grounded System Solidly grounded systems are usually used in low voltage applications at 600 voltsor less. In solidly grounded system, the neutral point is connected to ground.

Solidly Neutral Grounding slightly reduces the problem of transient over voltagesfound on the ungrounded system and provided path for the ground fault current isin the range of 25 to 100% of the system three phase fault current..

However, if the reactance of the generator or transformer is too great, the problemof transient over voltages will not be solved.

While solidly grounded systems are an improvement over ungrounded systems,and speed up the location of faults, they lack the current limiting ability ofresistance grounding and the extra protection this provides.

To maintain systems health and safe, Transformer neutral is grounded andgrounding conductor must be extend from the source to the furthest point of thesystem within the same raceway or conduit. Its purpose is to maintain very lowimpedance to ground faults so that a relatively high fault current will flow thusinsuring that circuit breakers or fuses will clear the fault quickly and thereforeminimize damage.

Figure 2.2.2 Solid Neutral Grounded Systems

Source: http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-1

If the system is not solidly grounded, the neutral point of the system would floatwith respect to ground as a function of load subjecting the line-to-neutral loads tovoltage unbalances and instability. The single-phase ground fault current in asolidly grounded system may exceed the three phase fault current. The magnitudeof the current depends on the fault location and the fault resistance.

One way to reduce the ground fault current is to leave some of the transformerneutrals ungrounded.

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AdvantagesThe main advantage of solidly grounded systems is low over voltages, whichmakes the grounding design common at high voltage levels (HV).

Disadvantages This system involves all the drawbacks and hazards of high ground faultcurrent: maximum damage and disturbances.

There is no service continuity on the faulty feeder. The danger for personnel is high during the fault since the touch voltages

created are high.

Applications Distributed neutral conductor 3-phase + neutral distribution Use of the neutral conductor as a protective conductor with systematic

grounding at each transmission pole Used when the short-circuit power of the source is low

3) Resistance Neutral Grounded System Resistance grounding has been used in three-phase industrial applications formany years and it resolves many of the problems associated with solidly grounded

and ungrounded systems. Resistance Grounding Systems limits the phase-to-ground fault currents.

The main reasons for limiting the phase to ground fault current by resistancegrounding are:

To reduce burning and melting effects in faulted electrical equipment likeswitchgear, transformers, cables, and rotating machines.

To reduce mechanical stresses in circuits/Equipment carrying faultcurrents.

To reduce electrical-shock hazards to personnel caused by stray groundfault.

To reduce the arc blast or flash hazard. To reduce the momentary line-voltage dip. To secure control of the transient over-voltages while at the same time. To improve the detection of the ground fault in a power system.

Grounding Resistors are generally connected between ground and neutral oftransformers, generators and grounding transformers to limit maximum faultcurrent as per Ohms Law to a value which will not damage the equipment in the

power system and allow sufficient flow of fault current to detect and operateGround protective relays to clear the fault. Although it is possible to limit fault

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currents with high resistance Neutral grounding Resistors, ground short circuitcurrents can be extremely reduced.

As a result of this fact, protection devices may not sense the fault.

Therefore, it is the most common application to limit single phase fault currentswith low resistance Neutral Grounding Resistors to approximately rated current oftransformer and / or generator.

In addition, limiting fault currents to predetermined maximum values permits thedesigner to selectively coordinate the operation of protective devices, whichminimizes system disruption and allows for quick location of the fault.

There are two categories of resistance grounding:

Low resistance Grounding

High resistance GroundingGround fault current flowing through either type of resistor when a single phasefaults to ground will increase the phase-to-ground voltage of the remaining two

phases. As a result, conductor insulation and surge arrestor ratings must be basedon line-to-line voltage. This temporary increase in phase-to-ground voltage shouldalso be considered when selecting two and three pole breakers installed onresistance grounded low voltage systems.

The increase in phase-to-ground voltage associated with ground fault currents also precludes the connection of line-to-neutral loads directly to the system. If line-toneutral loads (such as 277V lighting) are present, they must be served by a solidlygrounded system. This can be achieved with an isolation transformer that has athree-phase delta primary and a three-phase, four-wire, wye secondary.

Figure 2.2.3. Resistance Neutral Grounded Systems


Neither of these grounding systems (low or high resistance) reduces arc-flashhazards associated with phase-to-phase faults, but both systems significantlyreduce or essentially eliminate the arc-flash hazards associated with phase-to-ground faults. Both types of grounding systems limit mechanical stresses and

reduce thermal damage to electrical equipment, circuits, and apparatus carryingfaulted current.

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The difference between Low Resistance Grounding and High ResistanceGrounding is a matter of perception and, therefore, is not well defined. Generallyspeaking high-resistance grounding refers to a system in which the NGR let-through current is less than 50 to 100 A. Low resistance grounding indicates that

NGR current would be above 100 A.A better distinction between the two levels might be alarm only and tripping. Analarm-only system continues to operate with a single ground fault on the systemfor an unspecified amount of time. In a tripping system a ground fault isautomatically removed by protective relaying and circuit interrupting devices.Alarm-only systems usually limit NGR current to 10 A or less.

Rating of The Neutral grounding resistor:

Voltage: Line-to-neutral voltage of the system to which it is connected.

Initial Current: The initial current which will flow through the resistor withrated voltage applied.

Time : The on time for which the resistor can operate without exceedingthe allowable temperature rise.

A. Low Resistance Grounded

Low Resistance Grounding is used for large electrical systems where there is ahigh investment in capital equipment or prolonged loss of service of equipment

has a significant economic impact and it is not commonly used in low voltagesystems because the limited ground fault current is too low to reliably operate

breaker trip units or fuses. This makes system selectivity hard to achieve.Moreover, low resistance grounded systems are not suitable for 4-wire loads andhence have not been used in commercial market applications.

A resistor is connected from the system neutral point to ground and generallysized to permit only 200A to 1200 amps of ground fault current to flow. Enoughcurrent must flow such that protective devices can detect the faulted circuit andtrip it off-line but not so much current as to create major damage at the fault point.

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Figure 2.2.4. Low Resistance Neutral Grounded Systems


Since the grounding impedance is in the form of resistance, any transient overvoltages are quickly damped out and the whole transient overvoltage phenomenais no longer applicable. Although theoretically possible to be applied in lowvoltage systems (e.g. 480V),significant amount of the system voltage droppedacross the grounding resistor, there is not enough voltage across the arc forcingcurrent to flow, for the fault to be reliably detected.

For this reason low resistance grounding is not used for low voltage systems(under 1000 volts line to-line).

Advantages

Limits phase-to-ground currents to 200-400A. Reduces arcing current and, to some extent, limits arc-flash hazards

associated with phase-to-ground arcing current conditions only. May limit the mechanical damage and thermal damage to shorted

transformer and rotating machinery windings.

Disadvantages:

Does not prevent operation of over current devices. Does not require a ground fault detection system. May be utilized on medium or high voltage systems. Conductor insulation and surge arrestors must be rated based on the line

to-line voltage. Phase-to-neutral loads must be served through an isolationtransformer.

Used: Up to 400 amps for 10 sec are commonly found on medium voltagesystems.

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B. High Resistance Grounded

High resistance grounding is almost identical to low resistance grounding exceptthat the ground fault current magnitude is typically limited to 10 amperes or less.High resistance grounding accomplishes two things.

The first is that the ground fault current magnitude is sufficiently low enough suchthat no appreciable damage is done at the fault point. This means that the faultedcircuit need not be tripped off-line when the fault first occurs. Means that once afault does occur, we do not know where the fault is located. In this respect, it

performs just like an ungrounded system.

The second point is it can control the transient overvoltage phenomenon presenton ungrounded systems if engineered properly.

Under-ground fault conditions, the resistance must dominate over the systemcharging capacitance but not to the point of permitting excessive current to flowand thereby excluding continuous operation.

Figure 2.2.5. High Resistance Neutral Grounded Systems


High Resistance Grounding (HRG) systems limit the fault current when one phaseof the system shorts or arcs to ground, but at lower levels than low resistancesystems. In the event that a ground fault condition exists, the HRG typically limitsthe current to 5-10A.

HRG s are continuous current rated, so the des cription of a particular unit does not

include a time rating. Unlike NGR s, ground fault current flowing through a HRGis usually not of significant magnitude to result in the operation of an over currentdevice. Since the ground fault current is not interrupted, a ground fault detectionsystem must be installed.

These systems include a bypass contactor tapped across a portion of the resistorthat pulses (periodically opens and closes). When the contactor is open, groundfault current flows through the entire resistor. When the contactor is closed a

portion of the resistor is bypassed resulting in slightly lower resistance andslightly higher ground fault current.

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Requires a ground fault detection system to notify the facility engineer thata ground fault condition has occurred.

4) Resonant Neutral Grounding System

Adding inductive reactance from the system neutral point to ground is an easymethod of limiting the available ground fault from something near the maximum 3

phase short circuit capacity (thousands of amperes) to a relatively low value (200to 800 amperes).

To limit the reactive part of the ground fault current in a power system a neutral point reactor can be connected between the transformer neutral and the stationgrounding system. A system in which at least one of the neutrals is connected toground through:

Inductive reactance. Petersen coil / Arc Suppression Coil / Ground Fault Neutralizer.

The current generated by the reactance during an ground fault approximatelycompensates the capacitive component of the single phase ground fault current, iscalled a resonant grounded system.

The system is hardly ever exactly tuned, i.e. the reactive current does not exactlyequal the capacitive ground fault current of the system. A system in which theinductive current is slightly larger than the capacitive ground fault current is overcompensated. A system in which the induced ground fault current is slightlysmaller than the capacitive ground fault current is under compensated.

Figure 2.2.6. Resonant Neutral Grounding Systems

Source: http://electrical-engineering-portal.com/types-of-neutral-earthing-in- power-distribution-part-2

However, experience indicated that this inductive reactance to ground resonateswith the system shunt capacitance to ground under arcing ground fault conditionsand creates very high transient over voltages on the system. To control the

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transient over voltages, the design must permit at least 60% of the 3 phase shortcircuit current to flow underground fault conditions.

Example A 6000 amp grounding reactor for a system having 10,000 amps 3 phase short circuit capacity available. Due to the high magnitude of ground faultcurrent required to control transient over voltages, inductance grounding is rarelyused within industry.

Petersen Coils

A Petersen Coil is connected between the neutral point of the system and ground,and is rated so that the capacitive current in the ground fault is compensated by aninductive current passed by the Petersen Coil. A small residual current willremain, but this is so small that any arc between the faulted phase and ground will

not be maintained and the fault will extinguish. Minor ground faults such as a broken pin insulator, could be held on the system without the supply beinginterrupted. Transient faults would not result in supply interruptions.

Although the standard Peterson coil does not compensate the entire ground faultcurrent in a network due to the presence of resistive losses in the lines and coil, itis now possible to apply residual current compensation by injecting an additional180 out of phase current into the neutral via the Peterson coil. The fault current isthereby reduced to practically zero. Such systems are known as Resonantground ing with residual compensation , and can be considered as a spe cial case ofreactive grounding.

Resonant grounding can reduce EPR to a safe level. This is because the Petersencoil can often effectively act as a high impedance NER, which will substantiallyreduce any ground fault currents, and hence also any corresponding EPR hazards(e.g. touch voltages, step voltages and transferred voltages, including any EPRhazards impressed onto nearby telecommunication networks).

Advantages

Small reactive ground fault current independent of the phase to groundcapacitance of the system.

Enables high impedance fault detection.

Disadvantages

Risk of extensive active ground fault losses. High costs associated.

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5) Grounding Transformers

For cases where there is no neutral point available for Neutral Grounding (e.g. fora delta winding), an grounding transformer may be used to provide a return pathfor single phase fault currents.

Figure 2.2.7. Grounding Transformers

Source: http://electrical-engineering-portal.com/types-of-neutral-earthing-in- power-distribution-part-2

In such cases the impedance of the grounding transformer may be sufficient to actas effective grounding impedance. Additional impedance can be added in series ifrequired. A special zig -zag transformer is sometimes used for grounding delta

windings to provide a low zero-sequence impedance and high positive andnegative sequence impedance to fault currents.

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Comparison of Neutral Grounding System

Condition Ungrounded Solid

Grounded Low Resistance

Grounded High Resistance

Grounded ReactanceGrounding

Immunity toTransient Over

voltagesWorse Good Good Best Best

73% Increase inVoltage StressUnder Line-to-Ground Fault

Condition

Poor Best Good Poor

Equipment

ProtectedWorse Poor Better Best Best

Safety toPersonnel

Worse Better Good Best Best

Service Reliability Worse Good Better Best Best

Maintenance Cost Worse Good Better Best Best

Ease of LocatingFirst Ground Fault

Worse Good Better Best Best

Permits Designerto

CoordinateProtective Devices

Not Possible Good Better Best Best

Reduction inFrequency of

FaultsWorse Better Good Best Best

Lighting ArrestorUngroundedneutral type

Grounded-neutral type

Ungroundedneutral type



Current for phase-to ground fault in percent ofthree-

phase fault current

Less than 1%Varies, may be 100% or

greater5 to 20% Less than 1% 5 to 25%

Table 2.2.1. Comparison of Neutral Grounding System

Source: http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution- part-2

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CHAPTER 3

DISCUSSION

3.1. Power Distribution The function of a ship s electrical distribution system is to safely convey electrical

power to every item of equipment connected to it. The most obvious element in thesystem is the main switchboard. The main board supplies bulk power to motor startergroups (often part of the main board), section boards and distribution boards.Transformers interconnect the HV and LV distribution sections of the system. Circuit

breakers and fuses strategically placed troughout the system automatically disconnects afaulty circuit within the network. The main switchboard is placed in the engine

controlroom and from there engineroom staf monitor and control the generation anddistribution of electrical power. It is very important that every engineer has a profoundknowledge of the electrical distribution of the ship s power. The only way to aquire thisknowledge is to study the ship s power diagrams. Almost all oceangoing ships have anA.C. distribution system in preference to a direct current D.C. system. Usally a ship selectrical distribution scheme follows shore pratice. This allows normal industrialequipment to be used after being adapted and certified where and if necessary, so it canwithstand the conditions on board of a ship (e.g. vibration, freezing and tropicaltemperatures, humidity, the salty atmosphere, etc. encountered in various parts of theship). Most ships have a 3-phase A.C., 3-wire, 440V insulated-neutral system. Thismeans that the neutral point of star connected- generators is not earthed to the ship shull. Ship s with very large electrical loads have generators operating at high voltages(HV) of 3.3KV, 6.6KV, and even 11KV. By using these high voltages we can reducethe size of cables and equipment. High voltage systems are becoming more common asship size and complexity increase. The frequency of an A.C. power system can be 50Hz or 60Hz. The most common power frequency adopted for use on board ships is60Hz. This higher frequency means that generators and motors run at higher speedswith a consequent reduction in size for a given power rating. Lighting and low powersingle-phase supplies usually operate at 220 V. This voltage is derived from a step

down transformer connected to the 440 V system.

3.2. Grounding systems in shipboard electrical networks In electrical engineeering, the ground means reference in electrical circuits from whichother voltages are measured. The earth point means a solid connection to the earth,which due to its massive section and mass has almost no resistance for electricalcurrent. If the reference for your voltage measurements is the earth the earth becomesyour ground. By absense of the nearth on board of a ship, the ship s hull can be used asa substitute for the earth. Depending on the construction of the electrical networks they

may ar may not be connected to earth potential. In general we can have solidlygrounded, reactance grounded, resistance grounded and isolated networks. In isolated

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networks there is the challenge to detect earth faults. Ships distribution systems aretypically isolated in low voltage systems (1000V AC) and high resistance grounded inhigh voltage systems. High resistance grounding ensures the trip action in case of anearth fault and prevents short circuit faults in the network. High resistance grounding

can therefore not guarantee continuity of service.

Figure 3.2.1. Insulated and Earthed Neutral System

Source: http://magelhaes.hzs.be/willem/assorted/marine_electrical_knowledge.pdf

Table 3.2.1. Characteristic Grounding System in Ship


3.3. Electrical Fault There are three different kind of electrical faults.

Figure 3.2.2. Circuit Fault


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the safety of the ship. An insulated distribution system therefore requires twoearth faults on two different lines to cause an earth fault current to flow.Incontrast, an earthed distribution system requires only one earth fault to causean earth fault current to flow. An insulated system is, therefore more effective

than an earthed system in maintenance continuity of supply to essentialservices. Hence its adoption for most marine electrical systems. Note: Double-pole switches with fuses in both lines are necessary in aninsulated single-phase circuit.

3.4. Low-voltage Power Distribution System Grounding Analysis The basic construction of electricity power supply system to be called (380V) three-

phase three-wire and (380/220V) three-phase four-wire system, but these are not verystrict terminology content. International Electrotechnical Commission (IEC) madeunified regulations, known as TT systems, TN systems, IT systems. TN system isdivided into TN-C, TN-S, TN-C-S system. Here a variety of power supply systemreferred to above to make a brief analysis.

According to IEC provides a variety of protection, the term concept, low-voltagedistribution system divides into 3 ways by grounding systems different, which namelyTT, TN and IT systems, as described below.

1. TT System

TT mode power supply system is the way that the metal casing of electricalequipment will be grounded directly to the protection system, known as protectiveground system, also known as the TT system. The first symbol T means that the

power system directly grounded neutral point; second symbol T said metal shell andthe normal load equipment's metal parts are not charged directly connected with theearth, and has nothing to do with how the system ground.

The characteristics are as below: When the charged metal casing of electrical equipment (line touch shell or

equipment damage insulation leakage), due to a ground fault protection, cangreatly reduce the risk of electric shock. However, the low-voltage circuit

breaker (auto switch) does not necessarily trip, resulting in leakage ofequipment enclosures to the ground voltage above the safe voltage, is adangerous voltage.

When the leakage current is relatively hour, even if not necessarily blownfuse, so the leakage protection needed for protection, should not be trappedin the TT system, 380/220V Power Supply System.

TT system earthing consumption of steel and more, and difficult recovery,costs of working hours, fee material.

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2. TN SystemTN supply system means that power supply system is the metal casing of electricalequipment and metal parts not charged the normal work of the zero line with the

phase of the protection system, called then zero protection system, with the TN said.Its characteristics are as follows. Once the device appears live shell, then zero leakage current protection

system can rise to (220V) short-circuit current, the current large, is a TTsystem, many times, in fact, a single relatively short-circuit fault, the fuse ofthe fuse Will fuse, low voltage circuit breaker action immediately release thetrip, the faulty equipment power, more safety.

TN system can save material, it works in the country and is widely appliedin many other countries, and we can see more advantages than the TTsystem. According to the protection of the neutral line whether separated,TN mode power supply systems is classified as TN-C and TN-S.

a) TN-C systemTN-C mode power supply system is connected with the work of the zero linedouble as zero-line of protection, protection can be called the neutral line,can be expressed NPE.TN-C power supply system following features.o TN-C system lines when using leakage protection, leakage protection

behind the ground must be removed all duplicate, or GFCI not close;

Moreover, the zero line of work are not under any circumstances break.Therefore, the practical works in the zero line only allow leakage

protection on the side of a repeat ground.o TN-C mode power supply system is suitable for basic load balancing

phase (220V no load) condition.

b) TN-S systemTN-S power supply system is the way to the zero line of work for the

protection of line N and PE strictly separate the power supply system, calledTN-S power supply system, shown in Figure 1-4, TN-S power supplysystem following features. The system normal operation, for the protection of online no current, but

the zero line of work are unbalanced current. PE line on the ground thereis no voltage, so the metal casing of electrical equipment, then protectionis then zero line of protection in the PE on a dedicated, safe and reliable.

The neutral line of work is only used for single-phase lighting loadcircuit.

For the protection of PE not allowed to break lines, but also not allowedto enter a work GFCI zero line.

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The use of Route leakage protection, leakage protection may not be arepeat ground, while the PE grounding lines are repeated, but withoutleakage protection, so the TN-S system can also be installed on thesupply mains leakage protection.

TN-S method safe and reliable power supply system, for industrial andcivil buildings such as low-voltage power supply system. Before theconstruction of "leveling"(electricity, water supplies and roads and thehorizon - must be TN-S mode power supply system.

c) TN-C-S systemTN-C-S system in the way of construction of temporary power supply, if thefirst part is (no 220V load) TN-C mode power supply, and constructionspecification construction site must be TN-S mode power supply system,The system can be part of the scene after the separation of the total PE cabledistribution box,. This system known as TN-C-S power supply system. TN-C-S system is characterized as follows.

The neutral line of work for the protection of line N and PE phaseUnicom, after the main switch box circuit current imbalanceislarge,protected electrical equipment connected by the zero line of zero

potential. PE line behind the main switch boxes not current,t hat is, theseaction of the wire is no voltage drop, therefore, TN-CS system canreduce the electrical equipment enclosureto th eground voltage, however,can not completely eliminate this voltage, which depends on Wire

unbalanced load current size and N lines in the total length of the line before the switch box. Unbalanced load current increases, N and verylong lines, equipment enclosureson the ground the greater the voltageoffset. Unbalanced load current is not so much required, and the lineshould be repeatedin the PE ground.

PE line scan not, underany circumstancesinto the leakage protection,because the end of theleakage protection circuit will preventleakage protection before the class trip causing wide spread poweroutages,specification: There are zero then no cascaded protection of anyof the zero line Switches and fuses.

The addition of PE in the total box office lines and N lines connecting toother than the sub-boxes at the N line and were not linked PE cable, PEonline not allowed to install the switch and fuse, and the connection must

besecure.

Through the above analysis, TN-C-S power supply system isTN-Csystem inthe temporary work around. When the three-phase power transformergrounding work well, when the three-phase load is balanced, TN-CS systemresults in the construction of electricity is feasible in practice

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3. IT SystemThe first letter I means the power supply system ground side does not work, or

through the high impedance grounding. The second letter T, said load side groundfault protection of electrical equipment.

IT way power supply system in power is not very long distance, high reliability power supply, and good security. Place to allow the general power outage, or powercontinuously demanding areas such as continuous production equipment, largehospital operating rooms, underground mines and other places. Conditions inunderground mines are relatively poor power supply, cables, easy to dry. IT meansthe use of power supply system, even when the power is not neutral ground, oncethe equipment leakage, leakage current is still a single relatively small, will notdisrupt the balance of supply voltage, so the neutral point than the power supplysystem is also secure.

However, if the distance is long in the power supply, power supply line capacitanceof the distribution of the earth can not be ignored. Figure 1-6 shows the load short-circuit fault or leakage to the equipment enclosure charged, the leakage currentthrough the earth form a loop, protective equipment is not necessarily action, whichis dangerous. The power supply only keeps safe when the distance is not too long.This power supply is rare in the construction site of our company 35KV, 10KV,

6KV IT system using this method. IT is obvious shortcomings of the way, single phase line, the other two to the line voltage relative to ground voltage, over-voltageon the electrical equipment required is very high. Last year, three-wire overheadline construction is the construction unit severing the second phase to ground station6KV system B, A, C both increased relative to the voltage 1.732 times the voltagetransformer caused the second catalytic 6KV burned by over-voltage, the main fantrip, affecting normal device Production.

Summary of power lines symbol1) The International Electrotechnical Commission (IEC) power supply under

the symbol, the first letter means that the power (power supply) system inrelationships. Such as the T that is directly grounded neutral point; I meanthat all live parts of the insulation (not ground).

2) The second letter indicated that the exposed metal parts of electricalequipment on the ground relationship. T indicates that the device, such aschassis ground, which with any other system not directly related to accesslocations; N said load by then zero protection.

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CHAPTER 4

CLOSING

4.1 ConclusionThere is no best method for neutral earthing in marine power system. Each

application requires careful assessement of all safety, operational and commercialaspects.

The level of insulation required in marine system, particularly for rotating electricalmachines, does not depend on the method of neutral earthing adopted. We propose thatearth fault current is the most suitable criterion on which to base a choice of earthingmethod. For simplicity, three earth fault current range with associated earthing

impedances have been defined: high, intemediate, and lowHigh impedance neutral earthing is appropriate for hazardous areas and for

situations where immediate continuity of supply to unduplicated essential loads isnecessary.

Intermediate neutral earthing impedances enable automatic earth fault locationwithout excessive fault damage

Low impedance neutral earthing offers automatic earth fault location at minimumcost and with minimum overvoltages on healthy phases and is therefore appropriate fordomestic and hotel loads.

Proper System Grounding of electrical power systems can significantly improvereliability and safety. Retrofits of existing systems can be achieved utilizing groundingtransformers can be shown at the picture of diagram below. New systems can bedesigned using wye connected generators and delta-wye transformers.

The characteristics of different grounding techniques set forth in this paper should provide an intelligent basis for proper selection consistent with the needs of the powersystem in question.

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REFERENCES

http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-1

http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-2

http://www.fatech-surge-protection.com/Low-voltage-power-distribution-system-grounding-analysis-7.html

Marine Electrical Knowledge

http://magelhaes.hzs.be/willem/assorted/marine_electrical_knowledge.pdf
http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-1http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-1http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-1http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-2http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-2http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-2http://www.fatech-surge-protection.com/Low-voltage-power-distribution-system-grounding-analysis-7.htmlhttp://www.fatech-surge-protection.com/Low-voltage-power-distribution-system-grounding-analysis-7.htmlhttp://www.fatech-surge-protection.com/Low-voltage-power-distribution-system-grounding-analysis-7.htmlhttp://magelhaes.hzs.be/willem/assorted/marine_electrical_knowledge.pdfhttp://magelhaes.hzs.be/willem/assorted/marine_electrical_knowledge.pdfhttp://magelhaes.hzs.be/willem/assorted/marine_electrical_knowledge.pdfhttp://www.fatech-surge-protection.com/Low-voltage-power-distribution-system-grounding-analysis-7.htmlhttp://www.fatech-surge-protection.com/Low-voltage-power-distribution-system-grounding-analysis-7.htmlhttp://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-2http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-2http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-1http://electrical-engineering-portal.com/types-of-neutral-earthing-in-power-distribution-part-1

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