Medium Voltage Earthing Systems – Arrangements Medium Voltage Earthing Systems – Arrangements and Comparison and Comparison Neutral point connection method Neutral point connection method First, let’s define the different medium voltage First, let’s define the different medium voltage earthing systems earthing systems and then compare the and then compare the advantages and disadvantages of each one. Earthing systems in medium voltage can be advantages and disadvantages of each one. Earthing systems in medium voltage can be differentiated according to the differentiated according to the neutral point connection method neutral point connection method. The various earthing systems in medium voltage systems are different in the way they The various earthing systems in medium voltage systems are different in the way they operate and each has its advantagesand disadvantages, which we shall now consider. operate and each has its advantagesand disadvantages, which we shall now consider. Directly earthed neutral (Direct earthing) Directly earthed neutral (Direct earthing) 1. Unearthed neutral Unearthed neutral 2. Resistance earthing Resistance earthing 3. Reactance earthing Reactance earthing 4. Petersen coil earthing Petersen coil earthing 5. Comparison of Medium Voltage Earthing Systems | EEP http://electrical-engineering-portal.com/medium-voltage-earthing-systems... 1 of 7 8/4/2015 1:38 PM
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Medium Voltage Earthing Systems – ArrangementsMedium Voltage Earthing Systems – Arrangementsand Comparisonand Comparison
Neutral point connection methodNeutral point connection method
First, let’s define the different medium voltage First, let’s define the different medium voltage earthing systemsearthing systems and then compare the and then compare the
advantages and disadvantages of each one. Earthing systems in medium voltage can beadvantages and disadvantages of each one. Earthing systems in medium voltage can be
differentiated according to the differentiated according to the neutral point connection methodneutral point connection method..
The various earthing systems in medium voltage systems are different in the way theyThe various earthing systems in medium voltage systems are different in the way they
operate and each has its advantagesand disadvantages, which we shall now consider.operate and each has its advantagesand disadvantages, which we shall now consider.
Description:Description: An electrical connection is made between the neutral point and earth. An electrical connection is made between the neutral point and earth.
Operating technique:Operating technique: Compulsory switching on occurrence of the first insulation fault. Compulsory switching on occurrence of the first insulation fault.
Advantages:Advantages:
Reduces the risk of overvoltages occurring.Reduces the risk of overvoltages occurring.
Authorizes the use of equipment with a normal phase to earth insulating level.Authorizes the use of equipment with a normal phase to earth insulating level.
Disadvantages:Disadvantages:
Compulsory tripping upon occurrence of the first fault.Compulsory tripping upon occurrence of the first fault.
Very high fault currents leading to maximum damage and disturbance (creation ofVery high fault currents leading to maximum damage and disturbance (creation of
induced currents in telecommunication networks and auxiliary circuits).induced currents in telecommunication networks and auxiliary circuits).
The risk for personnel is high while the fault lasts; the touch voltages which developThe risk for personnel is high while the fault lasts; the touch voltages which develop
being high.being high.
Requires the use of differential protection devices so that the fault clearance time isRequires the use of differential protection devices so that the fault clearance time is
not long. These systems are costly.not long. These systems are costly.
Go back to Index ↑Go back to Index ↑
2. Unearthed neutral2. Unearthed neutral
Description:Description:
There is no electrical connection There is no electrical connection between the neutral point and earthbetween the neutral point and earth, except for, except for
measuring or protective devices. A measuring or protective devices. A high impedancehigh impedance is inserted between the neutral point is inserted between the neutral point
and earth.and earth.
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Operating technique:Operating technique:
No switching on occurrence of the first insulation faultNo switching on occurrence of the first insulation fault
– – it is thus compulsoryit is thus compulsory::
To carry out permanent insulation monitoring;To carry out permanent insulation monitoring;
To indicate the first insulation fault;To indicate the first insulation fault;
To locate and clear the first insulation fault;To locate and clear the first insulation fault;
To switch upon occurrence of the second insulationTo switch upon occurrence of the second insulation
fault (double fault).fault (double fault).
Advantages:Advantages:
Provides continuity of service by only tripping upon occurrence of the second fault,Provides continuity of service by only tripping upon occurrence of the second fault,
subject to the network capacity not leading to a high earth fault current that wouldsubject to the network capacity not leading to a high earth fault current that would
be dangerous for personnel and loads on occurrence of the first fault.be dangerous for personnel and loads on occurrence of the first fault.
Disadvantages:Disadvantages:
The unearthed neutral involves:The unearthed neutral involves:
The use of equipment whose phase-to-earth insulation level is at least equal to thatThe use of equipment whose phase-to-earth insulation level is at least equal to that
of the phase-to-phase level. Indeed, when a permanent phase-earth faultof the phase-to-phase level. Indeed, when a permanent phase-earth fault
occurs, the voltage of both unaffected phases in relation to earth takes on the valueoccurs, the voltage of both unaffected phases in relation to earth takes on the value
of the phase-to-phase voltage if tripping is not triggered on occurrence of the firstof the phase-to-phase voltage if tripping is not triggered on occurrence of the first
fault. Cables, rotating machines, transformers and loads must therefore be chosenfault. Cables, rotating machines, transformers and loads must therefore be chosen
with this in mind;with this in mind;
The risk of high internal overvoltages making it advisable to reinforce the equipmentThe risk of high internal overvoltages making it advisable to reinforce the equipment
insulation;insulation;
The compulsory insulation monitoring, with visual and audible indication of the firstThe compulsory insulation monitoring, with visual and audible indication of the first
fault if tripping is not triggered until the second fault occurs;fault if tripping is not triggered until the second fault occurs;
The presence of maintenance personnel to monitor and locate the first fault duringThe presence of maintenance personnel to monitor and locate the first fault during
use;use;
Some difficulties implementing selective protection devices upon occurrence of theSome difficulties implementing selective protection devices upon occurrence of the
first fault;first fault;
The risk of ferroresonance.The risk of ferroresonance.
Description:Description: A reactor is inserted between the neutral point and earth. A reactor is inserted between the neutral point and earth.
Operating technique:Operating technique: Switching upon occurrence of the first insulation fault. Switching upon occurrence of the first insulation fault.
Advantages:Advantages:
Limits the Limits the fault currentsfault currents (reduced damage and disturbance). (reduced damage and disturbance).
Allows the implementation of simple selective protection devices if Allows the implementation of simple selective protection devices if IILL >> I >> ICC..
The coil, being of low resistance, does not have to dissipate a high heat load.The coil, being of low resistance, does not have to dissipate a high heat load.
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Disadvantages:Disadvantages:
May cause high overvoltages during earth fault clearance.May cause high overvoltages during earth fault clearance.
Compulsory tripping upon occurrence of the first fault.Compulsory tripping upon occurrence of the first fault.
A reactor L tuned to the network capacities is inserted between the neutral point andA reactor L tuned to the network capacities is inserted between the neutral point and
earth so that if an earth fault occurs, earth so that if an earth fault occurs, the fault current is zerothe fault current is zero ..
IIff = I = ILL + I + ICC
Where:Where:
IIff – fault current – fault current
IILL – current in the neutral earthing reactor – current in the neutral earthing reactor
IICC – current in the phase-earth capacitances – current in the phase-earth capacitances
Operating technique:Operating technique: No switching upon occurrence of the first fault. No switching upon occurrence of the first fault.
Advantages:Advantages:
If the reactance is such that If the reactance is such that 3 L3 L00 C C00 ω ω00 = 1 = 1 is respected, the phase-earth fault current is is respected, the phase-earth fault current is
zero:zero:
Spontaneous clearance of non-permanent earth faults;Spontaneous clearance of non-permanent earth faults;
The installation continues to operate in spite of there being a permanent fault, withThe installation continues to operate in spite of there being a permanent fault, with
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Share with engineers //Share with engineers //
tripping necessarily occurring on the second fault;tripping necessarily occurring on the second fault;
The first fault is indicated by the detection of the current flowing through the coil.The first fault is indicated by the detection of the current flowing through the coil.
The coil is dimensioned so that permanent operation is possible.The coil is dimensioned so that permanent operation is possible.
Disadvantages:Disadvantages:
Difficulties establishingDifficulties establishing the condition 3 L the condition 3 L00 C C00 ω ω22 = 1 = 1 due to uncertain knowledge due to uncertain knowledge
of the network’s capacity: the result is that throughout the duration of the fault, aof the network’s capacity: the result is that throughout the duration of the fault, a
residual current circulates in the fault. Care must be taken to make sure this currentresidual current circulates in the fault. Care must be taken to make sure this current
is not dangerousfor personnel and equipment.is not dangerousfor personnel and equipment.
The risk of overvoltages occurring is high.The risk of overvoltages occurring is high.
Requires the presence of monitoring personnel.Requires the presence of monitoring personnel.
Impossible to provide selective protection upon occurrence of the first fault if the coilImpossible to provide selective protection upon occurrence of the first fault if the coil
has been tuned to: has been tuned to: 3 L3 L00 C C00 ω ω22 = 1 = 1
If it is systematically out of tune If it is systematically out of tune (3 L(3 L00 C C00 ω ω22 ≠ 1) ≠ 1) selective protection upon selective protection upon
occurrence of the first fault is complex and costly.occurrence of the first fault is complex and costly.
Risk of ferro-resonance.Risk of ferro-resonance.
Go back to Index ↑Go back to Index ↑
Resource:Resource: Protection of Electrical Networks – Christophe Prévé ( Protection of Electrical Networks – Christophe Prévé (get this book fromget this book from
AmazonAmazon))
Recommended EE articles //Recommended EE articles //
About Author //About Author //
Edvard CsanyiEdvard Csanyi
EdvardEdvard - - Electrical engineer, programmer and founder of Electrical engineer, programmer and founder of EEPEEP..
Highly specialized for design of LV high power busbar trunkingHighly specialized for design of LV high power busbar trunking
(<6300A) in power substations, buildings and industry fascilities.(<6300A) in power substations, buildings and industry fascilities.
Designing of LV/MV switchgears.Professional in AutoCADDesigning of LV/MV switchgears.Professional in AutoCAD
programming and web-design.Present on programming and web-design.Present on Google+Google+
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