Earthing systems, Harmonics Currents and GFP

Contents Basic Principle 3 Earthing System Technique and Suitable Devices 6 o IEC 60364 o NEC and GFP GFP Technique 19 Harmonics and Neutral conductor 27 o network studies o Multi-source and Multi-grounding o conclusion NEC

Basic Principle Definition n The 3 phases n The Neutral conductor N n The Protection conductor PE Role n Active conductors: 3 phases + Neutral Normal current in these conductors n Protection conductor PE Insulation Fault current Measuring this fault current is the principle of Earthing System E56885 L1 L2 L3 N PE

Basic Principles PE conductor The A point (star point of LV transformer) can be: o solidly grounded o isolated o linked with an impedance to the PE Fault current in A, the vectorial sum of the current is equal to 0 SIph + IN+ I PE = 0 o 2 ways to measure If ( =IPE) o don’t forget the Neutral current E56886 L1 L2 L3 N A PE

Basic Principles Main Ideas Measuring the current in the PE conductor is the principle of the Earthing System Techniques Any other Normal current mustn’t flow in the PE (especially the Neutral current) in order to avoid the tripping nuisances

Earthing System Techniques The target n Protection of people n Protection against fire risks The economical constraints n Continuity of the power supply n Avoiding tripping nuisance due to the EM Effects IEC 60364 and NEC standards have got the same targets and the same constraints E56887

Earthing system IEC 60364 TT system Definition n The Neutral point of LV transformer is directly connected to an earth electrode n The exposed conductive parts of the installation are connected to an electrically separate earth electrode E56888 L1 L2 L3 N PE Rn Ru

Earthing system IEC 60364 TT system Earth fault study The fault current generates a dangerous touch voltage Uc > UL The SCPD is usually not suitable for eliminating this type of fault (If = a few A eg from 1 to 10 A) Earth Leakage Relays (ELR) must be used (Setting at 300 mA to 10 A) E56889 Uo = 230 V L1 L2 L3 N 400V / 230V If = 11,5A Exposed conductive metallic part Uf = 115 V Rn 10 W Ru 10 W

Earthing system IEC 60364 TN system Definition The star point of the LV transformer is directly connected to an earth electrode (source earth) The exposed conductive parts of the installation are connected by the PE to the same earth electrode E56890 L1 L2 L3 N PE

Earthing system IEC 60364 TN-S system Definition The PE and Neutral conductor are separate E56890 L1 L2 L3 N PE

Earthing system IEC 60364 TN-C system Definition The PE and Neutral conductor are common = the PEN E56892 L1 L2 L3 PEN

Earthing system IEC 60364 TN system Earth fault study Fault current is equal to a Phase/Neutral short-circuit The fault current generates dangerous touch voltage Tripping of the circuit breaker by Short Time Protection or Magnetic E56893 L1 L2 L3 N PE

Earthing system IEC 60364 IT system Definition The star point of the LV transformer is not connected to an earth electrode The exposed conductive parts of the loads are connected by the PE conductor to a common earth electrode E56894 L1 L2 L3 N PE

Earthing system IEC 60364 IT system Earth Fault Study There isn’t a fault current n The fault is not dangerous but it must be signaled by PIM Principle of PIM Injection of current, n Measurement of IR (*). (*) Insulation Resistance E56895 L1 L2 L3 N PE

Earthing system IEC 60364 IT system Principle of the FTD (*) Detection of fault current Type of FTD (*): Movable tracking n Fixed tracking (*) Fault Tracking Device E56896 L1 L2 L3 N PE

NEC / US Installation Standard Defines IT - or IG (Insulated Grounding) - System and TN-S - or SG (Solidly Grounded) - System TN-S details o Neutral conductor is not broken the protective conductor PE is achieved by bonding the metallic parts of equipment o protection against fire is achieved by GFP

NEC / US Installation Standard Example of an installation complying with the NEC Protection against fire o the NEC imposes a maximum protection sensivity of: 1200 A E56897

Earthing System conclusions IEC 60364 NEC Earthing TN-C TN-S TT IT idem System TN-S I fault High High Low Very low High Protection SCPD SCPD RCD No problem SCPD of people Fire Forbidden Not Recommended Recommended GFP Protection recommended with RCD with RCD TT and IT Systems naturally manage the Fire Risk by limiting the fault current

Ground Fault Protection GFP GFP types External CT installation o measurement of all Currents (don’t forget the Neutral) GFP co-ordination o with the CB Magnetic or ST tripping o with the other GFP

GFP Types Residual Sensing: vectorial sum of the currents in the phases and the Neutral Source Ground Return: single sensor placed on the Neutral earth link Zero Sequence: - for low current detection - single sensor placed all live conductors E56898 L1 L2 L3 N L1 L2 L3 N E56899 E56900 L1 L2 L3 N

External CT installation 3 poles CB on 3 phases + N: on T option, the External CT must be compulsory both installed on primary and o connected on secondary according to this diagram E56901

GFP coordination Upstream GFP must be coordinated with any downstream Short Circuit Protection Device - SCPD - or GFP Discrimination types o current and Time discrimination with the respect of - asymptote at 1200A - 3000 A / 1s point o logic (ZSI) E56902 GFP upstream GFP downs- tream

Upstream GFP / downstream SCPD Discrimination rules Fuses o GFP sensibility setting up > 10 to 15 times down. fuse rating o GFP I²t function Circuit breakers o GFP sensibility setting up > 1,5 times down. CB short time setting up o GFP short delay setting up > down. CB breaking time setting T E56903 Down- stream short delay Down- stream fuse 2 Upstream GFP 1 step 2 step 1 D I 30 % I down- stream I up- stream I

Upstream GFP / downstream GFP Discrimination rules Ampermetric o up. GFP sensibility setting up > 30% down. GFP senstivity setting up Chronometric o up. GFP short delay setting up > down. SCPD breaking time setting o Dt1 (GFP1) > D t2 (GFP2) + dt2 (SCPD2) E56904 T Downstream GFP 2 Upstream GFP 1 30 % 3000 A 1s step 2 step 1 I down- stream I up- stream 1200 A 3000 A I

Fluorescent lighting Electrical diagram capacitive impedance for harmonics current Is current THDI = 53% Harmonics breakdown E56914 E56915 Ph Is FL T N E56916

Computer power supply Electrical diagram single phase loads high level of harmonics Is current THDI = 115% Harmonics breakdown I3=85%, I5=65%, I7=40%, I9=20% E56891 E56913 Is C T E56917

Harmonics current Their effects On the equipment: o capacitor, transformer, motor, alternator, … o overload and overheating, loss of the available power, added vibration On the installation E M disturbance On the Neutral conductor cumulating of harmonic 3 and their multiples E56905

Harmonics current conclusion IEC 60364 NEC Earthing TN-C TN-S TT IT idem System TN-S EM Forbidden Good Very good Very good Good disturbances PE and But be careful No problems No problems But be Neutral are avoiding PE even if Neutral even if Neutral careful together and Neutral to and PE are in and PE are in avoiding (PEN) be in contacts contacts contacts PE and Neutral to be in contacts There isn't any fault current in TT and IT System even if the PE and Neutral are in contact

Application Study of network one source / two sources o with harmonics TN- S with Neutral conductor not broken (according to NEC standard) o with GFP E56912

Harmonics current / Earth fault (one source) Harmonics current without earth fault no fault current measured by GFP Harmonics current and earth fault the right fault current is measured by GFP E56906 E56907 PE N GFP PE N GFP Ground bus Ground bus Service ground Service ground

Harmonics current / Earth fault (two sources or more) Both the main circuit breakers are closed, the bus coupler is open E56907 GFP GFP Neutral PE Ground bus Loads Service ground Service ground

Harmonics current / Earth fault (two sources or more) Harmonic current without earth fault a "false" earth fault current is measured, harmonic current are present throughout the metallic frames E56909 GFP GFP Neutral PE Ground bus Loads Service ground Service ground

Harmonics current / Earth fault (two sources or more) Earth fault with / without harmonic current the "right" earth fault current is not measured by the "right" GFP, a "false" earth fault current is measured by the other one this current may be present throughout the metallic frames E56910 GFP GFP Neutral PE Ground bus Loads Service ground Service ground

How to improve Network dependability ? Dealing with the problem by using particular GFP diagrams (Modified GFP, for instance) Avoiding the problem by using a 4 poles switch on the bus coupler (to break the Neutral) E56911 GFP GFP Neutral PE Ground bus Loads Service ground Service ground

How to improve network dependability ? Breaking the Neutral conductor Is the more efficient solution Is the sole solution to avoid EM effects E56911 GFP GFP Neutral PE Ground bus Loads Service ground Service ground