1. By: Irvan A Estrella, Emil Andes & Ranier de Asis
2. Short circuits (S.C) are generally caused by insulation
failure, flashovers, short circuits, broken conductors, physical
damage or human error. Short circuits involving all three phases
simultaneously are of symmetrical nature, whilst those involving
only one or two phases are asymmetrical faults. The balanced three
phase faults are normally analysed using equivalent single phase
circuits. Use of symmetrical components helps to resolve the
asymmetrical system faults.
3. Short-circuit calculation should be calculated at every bus
and points where relays or any short-circuit protective device are
installed in order to:
4. 1. Determine the duty rating protective devices and busses.
2. Determine the proper size of the cables 3. Determine the setting
of the relays 4. Properly coordinate the protective device 5.
Determine whether the short-circuit MVA is sufficient to start
large motors w/o excessive voltage dip
7. 1. DRAW THE SYSTEM SINGLE LINE DIAGRAM INDICATING VOLTAGE IN
ALL LEVELS, EQUIPMENT RATINGS AND THEIR PARAMETERS AND LENGTH AND
TYPE OF CABLES.
8. 2. MARK POINTS WHERE FAULT CURRENT IS DESIRED TO BE KNOWN 3.
ASSUME A COMMON POWER BASE 4. CONVERT ALL PARAMETERS FROM ORIGINAL
POWER BASE INTO THE NEW COMMON POWER BASE. 5. DRAW THE REACTANCE
DIAGRAM INDICATING PARAMETERS OF EQUIPMENT DEVICES & CKT
ELEMENTS AS COMPUTED IN STEP 4.
9. 6. PERFORM THE OPERATION OF CKT REDUCTION UNTIL SINGLE
IMPEDANCE IS LEFT FROM THE SOURCES OF FAULT CURRENT UP TO THE POINT
WHERE FAULT CURRENT IS DESIRED TO BE KNOWN. (IN THE PROCESS OF THE
CKT REDUCTION WHERE RESISTANCE IS NEGLIGIBLE RELATIVE TO THE
INDUCTANCE, IMPEDANCE OF SERIES CKT CAN BE RAGARDED AS INDUCTIVE
RACTANCE W/O APPLICABLE ERRORS.)
10. The voltage used in a short circuit calculation is the line
to line value. The cable parameter to be used in a three phase
short ckt calculation is the line to neutral while in a single
phase must be line to line value The power base must be used
throughout the calculation at all voltage levels and is a
three-phase value.
11. Utility power sc duty is given in three phase value. For
single phase sc application, the computed per unit value based on a
common power base from three phase supply shall be multiplied twice
to get the per unit value. Impedance info. available from three
phase xformers is the per unit value on the base determined by its
rating. Impedance in per unit for the 3 phase unit is the same as
that for each individual xformer.
12. In a 3-phase system; and %Z voltage is POWERBASEZIIVP LLL 2
3 33 1.% eq V ZI Z N L
13. Multiplying eq.1 by VN/ VN gives, 2 3 3 3*% 22 L LL L L L N
LN N N N L V ZIV V ZI V V ZIV V V V ZI Z 2.% 2 eq V ZPOWERBASE Z
L
14. Multilying eq.1 by IL/IL gives, Where IL=IF3 3 3 3*% 2 LL L
L POWERBASE NN L N N N L IV POWERBASE I VIV ZI I I V ZI Z 3. 3%
POWERBASE IF3 eq ZVL
15. Given a cable connected from 480V, 3phase, 15MVA system. L=
200ft, R/100ft=0.026ohms, XL/100ft=0.0048ohms solve for %Z based on
15MVA
16. 385.3 480 052.01015 % 2 6 2 x V RPOWERBASE R L 625.0 480
0096.01015 % 2 6 2 x V XPOWERBASE X L 46.1044.3625.0385.3%%%
jXjRZ
17. Determine the 3phase short ckt current at the load end of
the cable in example 1.
18. A ZVL 4805 34807548.3 15x10 3% POWERBASE I 6 F3
19. The short-circuit MVA of an equipment is equal to its rated
MVA divided by its %Z or %X. Example: 3phase induction motor,
0.5MVA, 2.3kV, %X=0.25. MVAsc=0.5MVA/0.25=2 In sc calculation; KVA
base= HP rating (for induction motor and 80%pf sync motor) KVA
base=0.8HP rating (for unity pf sync. Motor)
