Short-circuits in ES Short-circuit: cross fault, quick emergency change in ES the most often fault in ES transient events occur during short-circuits

Transcription

1 Short-circuits in ES Short-circuit: cross fault, quick emergency change in ES the most often fault in ES transient eents occur during short-circuits Short-circuit formation: fault connection between phases or between phase(s) and the ground in the system with the grounded neutral point Main causes: insulation defect caused by oeroltage direct lightning strike insulation aging direct damage of oerhead lines or cables

2 Short-circuit impacts: total impedance of the network affected part decreases currents are increasing => so called short-circuit currents k the oltage decreases near the short-circuit k impacts causes deice heating and power strain problems with k disconnecting, electrical arc and oeroltage occurred during the short-circuit synchronism disruption of ES working in parallel communication line disturbing => induced oltages Note: n short-circuit places transient resistances arise. transient resistance is a sum of electrical arc resistance and resistance of other k way parts (determination of exact resistances is difficult) current and electrical arc length is changing during short-circuit => resistance of electrical arc is also changing

3 transient resistances are neglected for k calculation (dimensioning of electrical deices) perfect short-circuits Short-circuits types Symmetrical short-circuits: Three-phase short-circuit => all 3 phases are affected by short-circuit little occurrence in the case of oerhead lines the most occurrences in the case of cable lines => other kinds of faults change to 3ph short-circuit due to electrical-arc impact nbalanced (asymmetrical) short-circuits: phase-to-phase short-circuit double-phase-to-ground short-circuit single-phase-to-ground short-circuit: in MV a different kind of fault => so called ground fault in case of ground fault in MV (insulated or indirectly grounded neutral point) => no change in LV (grounded neutral point)

4 Shortcircuit type Diagram Occurrence probability (%) MV kv kv 3ph 5,6,9 ph 4,8,6 ph to ground 3,8 5,4 ph * 9 93,

5 WL dw P dt Li L Short-circuit current time behaiour transient eent Time behaiour: open-circuit, resistances neglected reactance, current of inductie character, higher k alues mpact of R on k attributes: finite R alues decrease short-circuit impacts R neglecting results in time constants prolongation = L/R

6 = max in the short-circuit moment k starts from zero (min. alue) Short-circuit components (f = 5 Hz): sub-transient exponential enelope, T k transient exponential enelope, T k steady-state constant magnitude t is caused by synchronous machine behaiour during short-circuit more significant during short-circuits near the machine.

7 Values symmetrical short-circuit current ks - steady-state, transient and sub-transient component sum, RMS alue sub-transient short-circuit current k - ks RMS alue in the period of sub-transient component t 3T k initial sub-transient short-circuit current k - k alue in the moment of short-circuit origin t = transient short-circuit current k - ks RMS alue in the period from the sub-transient component end to the transient component end t 3T k 3T k initial transient short-circuit current k - RMS alue of the steadystate and transient component for t = steady-state short-circuit current ku - ks after transient components end t 3T k

8 = in the short-circuit moment k starts from max. alue

9 Values DC component ka - disappears exponentially, T ka initial DC component ka - ka in the moment t =, forced by the current continuous behaiour unbalanced short-circuit current kns - steady-state, transient, subtransient and DC component sum, RMS alue peak short-circuit current km - the first half-period magnitude during the maximal DC component Short-circuit current power factor k arctg X R tot tot

10 lines oerhead cable (kv) X : R / / 5/ / 5/ 7/ /4 / /,7 Z : X,4,,,,5, 4,,4, k ( ) ,7 85, 86, 87,

11 Short-circuits in 3ph system Conersion between phase alues and symmetrical components C B BC T BC C B T 3 mpedance matrix in symmetrical components (for series sym. segment) Z

12 3ph system during short-circuit internal generator oltage E (or i ) E Z BC BC BC BC Symmetrical system (independent systems,, ) E Z

13 Generator symmetrical oltage only positie sequence component Reference phase : 3 E T E BC Negatie and zero sequence are caused by oltage unbalance in the faulted place.

14 n the fault point 6 quantities (, ) 3 equations necessary to be added by other 3 equations according to the short-circuit type (local unbalance description).

15 Three-phase (to-ground) short-circuit 3 char. equations C B Hence 6 equations for 6 unknowns

16 Components 3 T BC ; ; Phases BC T C B ; ; Only the positie-sequence component included.

17 Single-phase-to-ground short-circuit 3 char. equations ; C B Hence 6 equations for 6 unknowns

18 Components BC 3 3 T ll three components are in series.

19 Phases 3 T BC ; ; 3 C B BC T

20 Current phasor diagram

21 Voltage phasor diagram

22 Phase-to-phase short-circuit 3 char. equations ; ; C B C B Hence 6 equations for 6 unknowns

23 Components 3 j 3 j 3 3 B B B B ; ; Positie and negatie components in parallel.

24 Phases BC 3 j 3 j T C B 3 j ; 3 j ; BC T

25 Current phasor diagram

26 Voltage phasor diagram

27 Double-phase-to-ground short-circuit 3 char. equations ; C B Hence 6 equations for 6 unknowns

28 Components 3 3 ;

29 ll three components are in parallel. Phases BC T B ) ( ) ( C ) ( ) ( 3 T BC

30 Phasor diagrams

31 Components during short-circuit: 3ph positie ph positie, negatie ph ground positie, negatie, zero ph positie, negatie, zero

32 Short-circuits calculation by means of relatie alues Relatie alues related to a defined base. base power (3ph) S (V) base oltage (phase-to-phase) (V) base current () base impedance Z (Ω) S Z 3 f Relatie impedance Z Z z Z f Z f 3 3 f f Z S 3 f Z S

33 nitial sub-transient short-circuit current (3ph short-circuit) f k S Z z k z 3 S z S z 3 nitial sub-transient short-circuit power k k S z z 3 3 S

34 Similarly for ph short-circuit () 3 k z z ph short-circuit () 3 k z z z Note: Sometimes it is respected generator loading, more precisely higher internal generator oltage than nominal one. k k z k >