1 Power System Protection Ground faults in isolated neutral systems 2 Briefly about the speaker • Professor at Norwegian Univ. Science and Technology – Dept. Electrical Engineering – Power system transients and protection – High Hi h voltage lt engineering, i i stress t calculations l l ti – Recent focus on Power Transformers • Developer of ATPDraw • Sabbatical at MTU – Room 628 628, phone 487 487-2910 2910 – hhoidale@mtu.edu 3 Solid vs. isolated neutral + No over-voltages in fault situations - High Hi h ffault lt currentt + Easy fault detection + G Ground ou d faults au s pe persist ss • Fast trip and reclosing - Poor power quality - Extra stress - Undefined voltages to ground. Sqrt(3) rise during ground faults faults. MOV dimensioning. + Low fault current - Difficult fault detection/location + Can continue to operate during ground fault. Increased power quality • Safety issues: down/broken conductors 4 Why is the fault current low in i l d neutrall systems? isolated ? • Fault current must return through line capacitances. High return impedance. Ground faults: • The most common fault • Often temporary – T Trees/branches, /b h snow/ice/wind, – Birds – Lightning • If 3 V f Z1 Z 2 Z 0 3R f Simultaneous faults…. 5 Usage of isolated neutral • In Norway Dy Dy – LV system 230 V IT – MV system 12-24 kV – Distribution level 66 66-132 132 kV • The 230 V IT system is gradually replaced by 400 V TN ((solidly yg grounded)) due to risk of undetected ground faults, double ground faults, and lower loss in a 400 V TN system. • In I MV; MV requirements i t to t detect d t t a 3k ground d ffault lt • Power quality is very important to the industry – Fast trip & reclosing not acceptable 6 Zero sequence measurements • Current I0: Sum Ia, Ib, Ic – Summing the current numerically – Residual connection 3I0 3I0 I0 Lower accuracy due to CT differences – Summation transformer (Toroidal/Rogowski coil) • Voltage U0: Sum Ua, Ub, Uc – Open delta 3U0 – Neutral point (isolated or resonance earth) U0 7 Isolated neutral network • Normal to only consider ground capacitance – – – – symmetrical y system, y ignore conductive line charging no voltage drop along line imbalance in loads canceled by y D-coupled p transformers • Fault current mainly returns in the two healthy phases • Fault resistance important VN h = e j120º hVpa h2Vpa Vpa Rf If Cg 8 Consequences • Fault current is independent on where the fault occurs • For calculation of feeder current and voltages g capacitances can be concentrated g to g ground will rise for the healthy y • The voltage phases • Easy to detect that there is a ground fault (by looking at V0), difficult to detect where (by looking at I0 in various feeders) 9 Phasors and equivalent • Equivalent circuit during fault (phase A): N + If Vpa 3Cg C A If Rf VN V0 V pa 1 3 j C g R f V pa 3 jC g 1 3 j C g R f I f C 3 j C g A B Before If Rf= Line-voltages unchanged B A Rf=0 UN After 10 Zero sequence in single radial • Same zero-sequence voltage in the entire system; selectivity is challenging • The fault is traditionally y located byy sectionalizing (often manual) • Zero sequence current varies along the line: I0 If U0 constant 3I0 pos. Rf 11 From fault to zero sequence current x l-x Vpa VN h2·Vpa h·Vpa C’g·x C’g·(l-x) • Currents: I a ( x) (VN V pa ) jC 'g (l x) I b ( x) (VN h 2 V pa ) jC 'g (l x) d Fault location x=d x l I c ( x) (VN h V pa ) jC 'g (l x) I f H (d x) VN j3C 'g x, x d 3I 0 ( x) ( I a I b I c ) VN j3C 'g (l x) I f H (d x) VN j3C 'g (l x), x d ignore voltage drop, no load-effect 12 Multiple radials • Three radials fed by the same transformer – Fault current in the faulty feeder is the negative sum of the current in the two healthy feeders I 01 VN jC g1 I 02 VN jC g 2 I 03 ( I 01 I 02 ) • With two radials the currents are equal in size so directional protection is required • With an increasing number of feeders simple over overcurrent protection becomes possible 13 Directional over over-current current relay • Often the preferred solution • Measure zero sequence voltage and zero sequence q current in each radial • Then for each radial: I01 I02 V0 I03 Forward direction Healthy feeder V0 I0n Tripping T i i zone Faulty feeder 14 Relaying logic • Is zero sequence voltage above threshold? – V0>Vlim (type 59G over-voltage relay) • Is zero sequence current above threshold and in correct quadrant (3rd)? – I0>Ilim I0 Ili (type (t 67N di directional ti l over-currentt relay) l ) + 59G (V0>Vlim) - 67N (I0>Ilim) 52 trip 15 Example- earth fault protection Example 22 kV 28 mi y 50 miles line in total Total capacitance is 50 miC’g [F/mi] 14 mi 8 mi 5 nF/mi assumed in this case. NB! Much higher in cable systems. • Fault current ((independent p on location)) – Depends on fault resistance I f ,min min 2.4454.7 A @ 3 k 22 kV / 3 If ' R f 1/ ( j3 50mi C g ) R f j 4244 I f ,max 3.090 A @ 0 U pa 16 Earth fault line 1 (28 mi) • P Partition titi off the th earth th fault f lt currentt measured d for f each radial • Healthy lines 2&3 feed 14 + 8 parts into line 1 22/50 14/50 28 mi 50/50 14 mi 8/50 8 mi 17 Earth fault line 2 (14 mi) • Partition of the earth fault current measured for each radial parts into line 2 • Healthyy lines 1&3 feed 28 + 8 p 28/50 28 mi 14 mi 36/50 8/50 50/50 8 mi 18 Earth fault line 3 (8 mi) • Partition of the earth fault current measured for each radial p into line 3 • Healthyy lines 1&2 feed 28 + 14 parts 28/50 28 mi 14/50 14 mi 8 mi 42/50 50/50