Topic 8 Fault Calculation Methods
-
Upload
raja-nurulhani -
Category
Documents
-
view
248 -
download
1
Transcript of Topic 8 Fault Calculation Methods
-
7/31/2019 Topic 8 Fault Calculation Methods
1/13
Topic 8: Fault Calculation
Methods
Problems in electrical circuits
Open circuits Supply is disconnected to the load
Short circuits Supply bypasses the load
Very dangerous Lead to high fault currents
Disastrous effects on equipment
Thermal heating
Electromechanical effects
Fires
Protection must detect abnormal fault currents Isolate in a time consistent with short circuit fault
current
Therefore fault current must be accurately predictedfor a fault in any location
-
7/31/2019 Topic 8 Fault Calculation Methods
2/13
-
7/31/2019 Topic 8 Fault Calculation Methods
3/13
Types of short-circuits and their currents
Balanced three-
phase short-circuit
Line-to-line short-
circuit
Types of short-circuits and their currents
Line-to-line short-circuit
with earth connection
Line-to-earth short-circuit
-
7/31/2019 Topic 8 Fault Calculation Methods
4/13
Comparison of magnitude of short-
circuit currents
Fault type Magnitude
3-phase (most severe) (E/Z) x multiplier
Line-to-line 0.87 x 3-phase fault
Line-to-ground(usually least severe)
Depends on systemgrounding
Per Unit System
Fault calculations must be done using per-unit system
Vpu = V/VB VB is the voltage base
Ipu = I/IB IB is the current base
Spu = S/SB SB is the kVA base
Zpu = Z/ZB ZB is the impedance base
Usually base values VB and SB are specified
IB and and ZB are determined from VB and SBIB = SB/VBZB = VB
2/SB = VB/IB VB is taken as the rated system voltage
SB is arbitrarily specified (100/10/1 MVA)
Rating of transformer is normally used as the base
-
7/31/2019 Topic 8 Fault Calculation Methods
5/13
Use of pu system
For balanced symmetrical 3-phase faults
Fault calculation must be done on single phase
basis
Use the pu phase impedance of one-line
diagram
Care must be taken to ensure the proper phase kVA
and voltage levels are used in the calculation
IB = (SB/3)/(VB/3) = SB/ (3 VB)
ZB = [(VB/3)2]/(SB/3) = VB2/SB
Where VB is the line voltage
And SB is the three phase kVA value
Change of base
Fault calculations must include all significant components ofimpedance
Must be expressed in pu terms using the appropriate base value
Sometimes may need to be changed if they are expressed usingdifferent bases, e.g transformer impedances
Zpu = Z/ZB = Z(SB/VB2)
For change of kVA base (SB): Zpu(new) = Zpu(old)[SB(new)/SB(old)]
Change of voltage base (VB): Zpu(new) = Zpu(old)[VB2
(old)/VB2
(new)]
Change of both kVA and voltage bases at the same time:
Zpu(new) = Zpu(old)[SB(new)/SB(old)][VB2
(old)/VB2
(new)]
Impedances of transformers, motors, etc will be given in puterms based on their rated voltage and power levels
Impedances of cables, overhead lines, etc will be given in ohmsand must be converted to pu with the appropriate base
-
7/31/2019 Topic 8 Fault Calculation Methods
6/13
Fault Calculation Effects and Requirements
Fault levels in a power system must be calculated atthe design stage to determine:
Overcurrent protection
Peak electromagnetic forces
Thermal heating effects
Maximum and minimum fault current
Time discrimination requirements of protection
Touch voltages on earth objects
Sources of fault currents
Electrical utility supply system
In-house generation systems operating at time of fault
Motors operating within the system at time of fault
Any electrical storage elements in the system, e.g.capacitors
Fault current contribution
Static equipment are not sources of fault
current
Inverters, converters, transformers, induction
heaters
Capacitors and battery operated UPS are low-
level sources of fault current
Supply utility and in-house generation providesconstant fault current (stiff source)
Motors will provide decaying fault current as
their magnetic excitation fields collapse
-
7/31/2019 Topic 8 Fault Calculation Methods
7/13
Fault current contribution from motors
Time-varying impedance
Sub-transient reactance (Xd) effective impedance in
first few cycles
Transient reactance (Xd) - effective impedance in 2 20
cycles
Synchronous reactance (Xs) impedance in steady state
Synchronous reactance is generally not used since
protection will operate before it comes into effect
For synchronous motors, only the sub-transient and
transient reactance are normally used
For induction motors, only the sub-transient reactance is
used
DC Offset
Must be included in fault calculations,
especially LV systems
DC offset can increase initial current levels
substantially
Magnitude of DC offset depends on X/R ratio
of fault circuit
-
7/31/2019 Topic 8 Fault Calculation Methods
8/13
Short-circuit current waveforms
(far from generator short-circuit)
Short-circuit current waveforms
(near to generator short-circuit)
-
7/31/2019 Topic 8 Fault Calculation Methods
9/13
Short-circuit current waveforms from
different sources
Utility
Generator
Synchronous
motor
Induction
motor
Symmetrical short-circuit current from 3 sources(utility, motors and generator combined into a total)
-
7/31/2019 Topic 8 Fault Calculation Methods
10/13
Fault calculation methodsTo simplify calculations, the following assumptions are made:
Fault is balanced 3-phase symmetrical
Pu impedances are pure reactances for MV
systems, any resistance is neglected
For LV systems where resistance is important,
impedance is [Z] = (R2 + X2)1/2
All significant component impedances are
included
The fault itself has zero impedance (i.e. boltedshort circuit)
Fault calculation methods
Earth circuit impedance is neglected
balanced 3-phase nature of fault eliminates earth
impedance
Rated voltage is used as the voltage base
X/R for all equipment
used to calculate level of DC offset multiplier
after symmetrical fault current has been calculated
Convert all impedances to pu values Use these to draw single line diagram of fault circuit
All possible sources modeled as ideal voltage source
With appropriate source impedance value connected
-
7/31/2019 Topic 8 Fault Calculation Methods
11/13
Fault calculation methods
Circuit simplification
Impedance diagram reduced to single pu
impedance Zf
Connected to true earth and to ideal voltage
source
Fault current (pu):
If (pu) = Vpu/Zf(pu)
= 1/Zf(pu) (since Vpu = 1) If= If(pu) . IB Amps
Parameters for fault calculation
-
7/31/2019 Topic 8 Fault Calculation Methods
12/13
-
7/31/2019 Topic 8 Fault Calculation Methods
13/13