Earth fault hazard voltages and their limitation

Transcription

1 Earth fault hazard voltages and their limitation

2 The effect of electricity on human body - combustion - chemical changes in blood - heart fibrillation Currents corresonding to 10% and 1% robabilities of heart fibrillation, when the current coincides with the dangerous quarter of uming cycle. Duration t/s I 10% / ma I 1% / ma > , , , ,

3 Voltages corresonding to the 10 % heart fibrillation robability as a function of stress duration. The resistance of a human body

4 Earth voltages versus touch voltages - touch voltage - ste voltage - only art of earth voltage is hazard voltage Earth surface otential caused by an earth fault (V) and the corresonding touch voltage (U t ), and ste voltage (U s ).

5 Maximum allowed touch voltages according to CENELEC THE GOUNDING IS ACCEPTABLE, IF: 1) Global earth: city areas where grounding electrodes are connected 2) Earth voltage < 2 x maximum touch voltage 3) Earth voltage < 4 x maximum touch voltage & otential grading electrode 4) It is shown by measurements or calculations that the touch voltage limits are met

6 How to reduce touch voltages Earth voltage: U e I ef e I ef earth fault current e grounding resistance 1) larger grounding electrodes 2) reduction of the fault current - divide networ into arts, comensation 3) reducing fault time - relay rotection, automation

7 Comutation of the grounding resistance - soil secific resistivity r - tyical value in Finland r 2300 Wm Some earthing electrodes Some earthing electrodes

8 Examle: Earthing resistance of a 50 m rod - straight conductor 50 m - in 0,7 m deth - soil resistivity r 2300 Wm - 16 coer wire Þ d» 4,5 mm r 2L 2 L ln 1,85hd W 2 50 ln 1,85 0,7 0,0045 W Examle: Earth fault current 20 A earth voltage U m I ef m V 1900 V base case: touch voltage 1/2 U m 950 V Û not allowed! with otential grading: touch voltage 1/4 U m -U T 1/4 U m 475 V Û max. duration 0,2 s If the CB oeration taes 100 ms, the relay must tri in 0.1 s

9 Examle - MV substation with 300 m of 20 V overhead line -c 0»6 nf / m - r 2300 W/m - rotection set to tri in 0,4 s - grounding electrode 16 mm 2 Cu in 0,7 m deth How long electrode must be? Networ earth caacitance C 0 c 0 l 1,8 mf Earth fault current in unearthed system case: I ef 3 Uw C 0, when f 0 W I ef , ,6 A t 0,4 s Û max allowed touch voltage U T» 280 V base case: max allowed earthing voltage: U m 2 U T 560 V

10 Examle continued... U m Um 560 V m Ief Þ 27 I 20,6 A ef W The resistance of the grounding electrode is comuted as: m Þ r 2L ì r 2300 Wm 2 L ï ln í h 0,7 m 1,85hdï 2 î d 0,0045 m (16 mm ) L/m m /W , , ,05 Þ 215 m In the case of otential grading electrodes: : Þ Þ max U m m U I ef m 4 U 54 W 1120 V 97 m

11 MEANS TO LIMIT THE TOUCH POTENTIALS - reduction of grounding resistance - reduction of earth fault current - dividing the networ into arts - earth fault current comensation Comensation : SI EC U 0 L I c I c I f - coil L is usually tuned so that its current is 95% of the caacitive current - resistive leaage current is tyically 5-8 % of the caacitive current I L

12 Examle Fault current of the revious examle after comensation: I I x r I c 8 % ; (1-0,95) 20,6 0,05 1,03A Total fault current: I I 2 r I r I 0,08 20,6 1,65 A 2 1,94 A Base case: U m 560 V Þ m 288 W Þ L 13 m! (electrode length) Sustained oeration of the networ with an earth fault CENELEC : allowed, if U T 75 V Þ U m 2 U T 150 V (base case) In comensated neutral case : I ef» 1.94 A U m 150 V Þ 77.3 W I 1.94 A ef

13 Earthings System earthing - a art of the ower system is connected to earth - e.g. the earthing of low voltage networ neutral - the aim is to limit the hase to earth voltages Equiment (or rotective) earthing - a normally dead metallic art is connected to earth - e.g. metallic body of a iece of electrical equiment - the aim is to revent hazardous touch voltages Temorary earthing - rotective earthing used when woring on ower system comonents

14 Connection of the MV equiment earth and the LV system earth Searate earthings : - a MV fault causes high stress between tan and LV-winding - a fault between LV winding and tan causes a hazardous voltage (> 110 V) Connected earthings: - the voltage of MV faults sreads in LV system over the 0-conductor The earthings must be connected if it is difficult to mae sure that they will remain isolated from each other (urban areas).

15 Earthings in tower footings - the aim is to reduce the lightning voltages in the tower and shielding wires Þ number of bac flashovers reduced - the decrease in earthing resistances increases earth fault currents and imroves the sensitivity of earth fault rotection Comare distance relay: X 0,3-0,4 W/m - an imortant goal also is to reduce the touch otentials of towers Examle: a 110 V line - 60 W tower footing earthing ; U m» 50 V - with shield wires & additional earth electrodes Þ m < 1 W Þ U m in acceted limits - Note: the screening effect of earth wires: the earth fault current in soil is reduced by about %

16 Tower basic and additional earthing a) basic, b) radial earthing, c) continuous counterouse Examles of a 110 V transmission line earthings z u shield wires r no additional earth. 1) additional earthing Wm W/m /W e /W /W e /W 500 3,3 60 3,1 15 1,6 2 x 35 St , ,7 32 2, , ,5 50 2, , ,3 15 0,7 2 x Imatra , ,5 32 1, , ,4 50 1,3 u imedance of the shielding wire footing earthings solely e the effect of shielding wires included

17 Tower basic and additional earthing The chain of tower footings and shielding wires u (1) the resistance of tower footing W u the imedance of shielding wire (2-3 W/m) (the length between two towers) In the middle of the line e 0,5 (2 directions) Examle: 60 W & u 0,6 W Þ 6 W e 3 W

18 u u u 2 u u 2» Þ Þ Tower basic and additional earthing The derivation of equation (1): u u u u 1 2? u An infinite chain : 1 2 u u 2 2 u 1 ) ( ) ( ) ( Þ Þ Þ ( >> ) u Þ

19 Electrical injuries The number of deaths due to electrical accidents in Nordic countries in average 90 s 0,6-0,9 / mill.eole, yr 80 s 1,0-1,5 / s 1,6-2,1 / s 2,3-3,3 / - - The number of deaths due to electricity Year Number Deaths The of because of use of deaths faulty electricity equiment (ersons) (ersons) (GWh) (0) (1) (1) (1) sum 13 2(3) The number of deaths in electrical injuries in Finland High voltage Low voltage Electrical technician Trainee or student Common eole Equiment not allowed by security code Faulty equiment or installation The cause of victim 1) Other cause 2) SUM ) uncarefulness, lay, ublic disobeyance 2) inadequate guidance, third arty, unnown

20 Number of deaths in electrical injuries in Professionals and commoners

21 Electrical injuries in Finland Year Electric shoc - Professionals - Commoners Electric arc - Professionals - Commoners Injuries Deaths

22 Fires caused by electrical henomena Causes : - misuse of electrical equiment - short circuit and earth fault - loose connections - overload - static electricity - lightning About 40 % of fires are caused by electricity! Fires caused by electricity in Sweden in years CAUSE number cs % flash over, short circuit ,2 Faulty comonent ,1 misuse of heating equiment 219 7,6 mechanical, chemical or thermal fault 205 7,1 covering the heating equiment 180 6,2 Loose contact 117 4,0 erroneous oeration or use 59 2,0 sar igniting gas 55 1,9 sie in cable 49 1,7 overload, oversized fuses 39 1,3 other causes 265 9,3 not nown ,8 sum ,0

23 Electrical fires and deaths in Finland Year Electric fires Deaths in Electric fires All deaths in fires

24 Electrical fires and causes in Finland Year KITCHEN OVEN LAMP SAUNA STOVE ELECTIC CENTE WASHING MACHINE ELECTIC WIES MICOWAVE OVEN OTHE ELECTIC NTWK POCESS EQUIPMENT COLD EQUIPMENT