Undergrounding power lines- the great challenge

Transcription

1 Undergrounding power lines- the great challenge 1 Calculation of 1. short-circuit currents. To be revised! 2. temporary overvoltages. 3. electromagnetic transients 1

2 Undergrounding power lines and the capacitive current switching ~ Equivalent Voltage Source Step-down transformer n underground cables Distri ibution sy ystem A B C 2 Single Capacitor Bank

3 Types of the capacitive switching Type Switching lines/cables Inrush current peak Very low Inrush current rate of rise high Duty for CBs Light Switching low low light single-step capacitor bank Back-to-back k highh highh Very heavy capacitor bank 3

4 Traditional approach to switching isolated capacitor bank I peak ~ V max cos(t) L S V0 di V0, L / dt L S C S Ipeak = 1.6 ka, di/dt = 2.7 ka/ms Bus-bar Inrush current of isolated capacitor bank C 4 Field tests of isolated 24 kv capacitor bank: I peak = 7 ka, di/dt = 3300 ka/ms

5 Suggestion: Inrush current is a sum of current surges in the cables [kv,pe ak] Vlt Voltage change at tthe MV busbars Voltage [kv, V,peak] Distance from busbars b [km] Feeder Time [sec] MV busbars Vo oltage [kv,pe eak] cable Incident voltage wave (V in ) Distance from busbars [km] Feeder Incident voltage wave (V in ) cable in 5

6 Cur rrent [ka,pe eak] Suggestion: Inrush current is a sum of current surges in the cables Time[microsec] Distance from busbars [km] Feeder cable Incident I(t)= I I in 0.5 in =V in /Z W current I(t) 2 Capacitor inrush current MV busbars eak] Cur rrent [ka,p Curre ent [ka,peak k] 1 wave (I in ) Distance from busbars [km] Feeder cable Incident 0.5 I in =V in /Z W 1 current wave (I in )

7 Since measured inrush currents significantly exceed the "inrush current of isolated capacitor bank", their detailed research is necessary for selection of the capacitor bank switchgear. Main goals of the research: 1. Analytical study of the phenomenon 2. Case study of switching 24 kv capacitor banks 7

8 Analytical study cable L S R S Z CW ~ V m cost L Cable C termination ti Z LW impedance Single phase circuit to analyze the capacitor bank inrush current 8

9 Analytical study L S R S Z CW L L Z LW -V 0 C C -V 0 Inrush current through the system impedance Inrush current due to traveling waves in cables 9

10 Analytical study: inrush current of isolated capacitor bank L S R S L -V 0 C 10 where I RS 2L S i () t I e sint tr peak peak V0 1, L / C L C S t S

11 Analytical study: inrush current due to the traveling waves General case: underground cables have unequal lengths L Z CW Inrush current due to the initial current surges C -V 0 i( p) VC 0 2 LCp Z CW Cp 1 Inrush current in the Laplace domain 11

12 Z Analytical study: closed form-expressions of current waveforms Damping reactor is not applied Z CW Damping reactor is applied 2ZW Z 2Z t it () 2Ie sint t it () 2Ie sinht where W 12 I L C V 0 CW,, 2 2 ZCW 4Z L W Characteristic Z Z C impedance of the CW n circuit it formed by inductance L and capacitance C CW W Z Z 4Z 2 2L 2 2 CW W Equivalent surge impedance of n underground cables

13 I peak Analytical study: inrush current peak 2I I is reached when t ln 2 Limiting case of non-oscillatory inrush current : 13 L 0 Limiting case of oscillatory inrush current : L C Z C V I n 0 peak I C Z Peak V 0 L ZC n

14 Analytical study: maximum value of di/dt di dt V 0 L The maximum rate of rise of the inrush current is limited only by the equivalent inductance L. If L includes only stray inductance (damping reactor is not applied) the maximum value of di/dt can be extremely high. 14

15 Analytical study: inrush current due to the traveling waves Particular case: n underground cables have equal length Z CW L C -V 0 Z LW Inrush current due to superposition of the initial current surges and reflected current surges i S ( p) VC 0 2 LCp ZCWCp 1 Inrush current in the Laplace domain k k 1 k a [( 1)( 2)] L p p p p k [( p p1)( p p2)] k 1

16 Analytical study: closed form-expression of current waveforms when the initial waves and the first 3 reflected waves are considered 16 t it () 2 Ie sinh t i S () t i() t i ( t ) i ( t ) i ( t ) i t Ie a t a t t a t t t1 ( ) 2 [ sinh( ) sinh( ) cosh( )] t i ( t ) 2 Ie 2 [ a sinh( t ) a t sinh( t ) a t cosh( t ) a23 t2 sinh( t2) a24 t2 cosh( t2)] i ( t ) 2 Ie [ a sinh( t ) a t sinh( t ) a t cosh( t ) t a33 t3 sinh( t3) a34 t3 cosh( t3) a35 t3 sinh( t3) t3 cosh( t3 )] a t t

17 Case study: the circuit description Transformer of 161/24 kv, n feeders S= 45 MVA, e k =18% Infinite busbar Z C l C Z L l L L Q 24 kv Types of capacitors: busbar 1. Isolated 9MVAR capacitor bank Single-core, coaxial cables, XLPE copper 300/25 crosssection Overhead line ACSR 150/25 2. Multi-step t 35 MVAR capacitor bank (switching the 1 st step) 17

18 Case study: results of the field tests versus EMTP simulation Switching 9 MVAR, 24 kv capacitor bank Measured peak inrush currents Test Number Peak inrush currents [ka, peak] Result of EMTP simulation 6.7 ka Maximum di/dt = 3600 ka/ms I R I S I T Maximum measured di/dt = 3300 ka/ms

19 Case study: Parametric study of the inrush current peak Dependence on the number of underground cables 19 current peak [ka,peak] Inrush Ipeak versus number of cables 9 MVAR 5 MVAR 9MVAR L= 5 µh 5 MVAR 9 MVAR 5 MVAR Number of underground cables L= 100 µh L= 1000 µh I peak V n Z 0 C (proportional to n) C IPeak V0 L (practically independent of n)

20 Case study: Parametric study of the inrush current peak current pea ak [ka, peak k] Inrush Dependence on the equivalent inductance, n =8 Ipeak versus inductance 9 MVAR 5MVAR Increase of the equivalent inductance L leads to reduction of the inrush current peak Ipeak 20 Total inductance[microhenry]

21 Parametric study of Ipeak Concept of the minimal length of cable Minimal length of cable lminl as the shortest cable length among n cables that makes it possible for the inrush current to reach its peak value Ipeak before the reflected wave arrives at the substation busbar For the banks without t damping reactor For the banks with damping reactor lmin versus L 21 minimal le ength of cable [km] MVAR 5 MVAR Stray inductance [microhenry] length of cable [km] minimal lmin versus L 9 MVAR 5 MVAR Equivalent inductance [microhenry]

22 Inrus sh current [ka,peak] Case study: n underground cables have equal length Verification of the derived analytical expressions: lc= 1km, Q=9 MVAR, L = 5H, n =8 Inrush current waveform Numerical waveform Analytical waveform Very good agreement between the analytical waveforms and the EMTP waveforms enables to accept the hypothesis that the inrush currents occur due to the traveling waves in the alongside underground cables! 22 Time [microsecond]

23 Inrus sh current [k ka, peak] Switching Capacitor bank Back-to-Back to Underground Cables Case study: n underground cables have equal length Verification of the derived analytical expressions: lc= 5 km, Q=9 MVAR, L = 5H, n =8 Inrush current waveform 6.4 ka EMTP waveform 70kA Analytical 6 waveform For n identical long cables the maximum value of the inrush current is always achieved at the first peak of the current waveform. The following crest values of the current waveform are smaller than the 1st peak because of the attenuation of the traveling waves. 23 Time [microsecond]

24 Case study: Parametric study of the inrush current peak Dependence on the cable length: Q=9 Q 9 MVAR, L = 5H, n =8 eak [ka,peak k] Maximu um current p Maximum crest value versus lc Imax = 6.4 ka Variation of the cable length from 0 to lmin results in increase of the maximum current peak from zero to the inrush current peak Ipeak. The following growth of the cable length does not cause increase of the maximum lmin = 0.7km current peak because of the attenuation of the traveling waves in the cables. 24 Cable length [km]

25 Case study: Undergrounding the distribution power lines and the inrush current Total inrush current Inrush current through the transformer Total inrush current Inrush current through the transformer Capacitor bank switching back-to back to 8 underground cables Capacitor bank switching back-to back to 8 overhead feeders 25

26 Switching 170 kv, 200 MVAR capacitor bank back-to-back to underground cables (damping reactor is not applied) Calculation method inrush current of isolated capacitor bank Switching back-to-back to 6 underground cables Ipeak [ka, peak] di/dt [ka/ms]

27 Operational experience: Analysis of failures of 24 kv, minimumoil circuit breakers used for switching isolated capacitor banks Handling capacitor inrush currents (Ipeak = 4-6 ka, di/dt = ka/ms) Quick oil deterioration; problems with CB mechanical system Restrikes during opening CB, Inrush currents having Ipeak = 8-12 ka 27 Explosions of CB breaking unit accompanied by a short circuit on the substation busbar

28 Conclusions 1. Since the inrush current during the capacitor bank switching back-to-back to the underground cables significantly exceeds the "inrush current of isolated capacitor bank" it should taken into account in the selection of the capacitor bank switchgear. 2. It seems that switching capacitor bank back-to-back to the underground cables should be addressed in the standards concerning the capacitive current switching 28