[Note: "Tables" and various equations (denoted by "e.") are
not yet avail., but coming soon.]
1. Introduction
Mechanical forces and thermal effects produced by high fault currents can
damage or destroy substation equipment. This damage is a serious safety issue
for a number of reasons. All substation equipment can suffer damage from mechanical
forces caused by short circuits.
Examples include bending of bus work, sudden expansion of transformer coils,
and breaking of insulators and bushings. In addition, even faults with rather
moderate magnitude may cause longterm effects such as accelerated aging of
dielectric insulation as a result of repetitive mechanical stresses. Solving
the problem of increased fault currents means repeating portions of the original
design process. Because substation design has become an automated procedure,
the uprating should be in the nature of a design review. This section is adapted
from EPRI (2006).
2. Definitions
Fixed bus bar end. The end of a rigid bus bar that is not free to rotate.
Pinned bus bar end. The end of a rigid bus bar that is free to rotate.
SCADA Systems. Supervisory controls and data acquisition systems.
Strain bus. A bus made of flexible conductors, which hangs from suspension
insulators, such that
eqn.1
…where…
= distance between supports, in m
= length of bus conductor, in m
= length of one insulator chain, in m.
Slack bus. A bus made of flexible conductors, which hangs from post insulators,
such that 13.2
FIG. 1 Threephase sidebyside bus bar configuration.
3. ShortCircuit Mechanical Forces on Rigid Bus Bars
3.1 ShortCircuit Mechanical Forces on Rigid Bus BarsCircular Cross
Section
The shortcircuit forces on three, rigid, sidebyside busbars are shown in
FIG. 1 and are calculated according to IEEE Standard 605 (IEEE, 2008):
eq.3
...where ...
= fault current force in N/m
= symmetrical rms fault current in A
= conductor centertocenter spacing in m
= a constant whose value is 1.00 for phasephase faults on either conductor,
0.866 for threephase faults on conductor B,
0.808 for threephase faults on conductor A or C.
...is the decrement factor.
Where fault durations are less than 1 s or the
ratio is greater than 5, the asymmetry of fault current waveforms produces
additional heating, which must be taken into account:
eqn.4
...where ...
= fault duration in s
= time constant in s
= mounting structure flexibility factor.
This is 1.0 unless the heights of the mounting structures are greater than
3m (IEEE, 1998, Figure 4).
The shortcircuit forces on a conductor are added to the other forces to produce
a total force, which must be less than the minimum yield stress of the conductor
material (Table 1) (IEEE, 2008, Section 11). The magnitude of the total
force is 13.5.
The angle of the force below the horizontal is 13.6 where
= total vector force on the bus, in N/m
= wind force in N/m
= total bus unit weight, including ice loading and connectors, in N/m
= 1 if the bus conductors are vertical, otherwise 0
= 1 if the bus conductors are horizontal, otherwise 0
Table 1 Allowable Stress for Common Conductor Materials
The maximum allowable length between spans is limited by the maximum of the
vertical deflection or the span length for fiber stress.
The vertical deflection is primarily an aesthetic concern, which is not affected
by shortcircuit forces. The maximum allowable length based on fiber stress
is calculated from the maximum allowable stress in Table 1:
eqn.7 where
= maximum length of the bus in cm
= constant based on the number of spans and end types (Table 2)
= maximum yield stress in kPa2
= section modulus in cm^{3}.
When increased forces are anticipated owing to increased shortcircuit levels,
supporting them with additional insulators can protect substation bus bars.
Table 2 Conductor Span Constant KS
3.2 ShortCircuit Mechanical ForcesRectangular Cross Section
In the case of substations with rectangular crosssection bus bars, a proximity
factor is used:
...eqn.8 where is taken from FIG. 2. is equal to 1.0 for a round conductor
and is almost 1.0 for a square conductor. Conductor shape is most significant
for thin, strip conductors.
FIG. 2 Dwight curves for proximity factor (Dwight, 1945).
4. Dynamic Effects of Short Circuits
When excited by a displacement force, a rigid conductor will vibrate at its
natural frequency, subject to damping forces (IEEE, 2008). The stimulus of
a short circuit will provide a twice power frequency periodic force which may
be amplified if the natural frequency of the bus bar is greater than or equal
to the power frequency.
If additional supports are being added to stiffen the bus owing to increased
fault currents, the dynamic effects should be checked as well. The natural
frequency of the bus bar is 13.9 where
= natural frequency of the bus, in Hz
= pinning factor:
1.00 if both ends are pinned, 1.22 if one end is fixed and one end is pinned,
1.51 if both ends are fixed.
= modulus of elasticity (Table 3), in kPa
= moment of inertia of the crosssection, in
= mass per unit length, in kg/m.
If the resonant frequency calculation leads to the suspicion of a possible
resonance problem, a dynamic or static finite element analysis should be performed.
Table 3 Modulus of Elasticity for Common Conductor Materials Material Modulus
of Elasticity (kPa) Al alloy 6061T6 Al alloy 6063T6 6.895 × 10^{7} Al alloy
6101T6 Cu 11.03× 10^{7} IEEE (2008).
In the case of hollow bus bars, internal weights or stiffeners may be added
to dampen vibration modes.
5. ShortCircuit Thermal Effects
Heating of bus bars can cause annealing, thermal expansion or damage to attached
equipment. The limit for thermal expansion adopted during the design of the
substation is generally used. Annealing can occur at temperatures of 100°C
or more.
The amount of current required to heat a conductor from the ambient to a given
final temperature during the duration of a fault can be calculated:
eqn.10 where
= maximum allowable rms symmetrical fault current in A
= constant: 92.9 for aluminum conductors, 142 for copper conductors
= fault duration in s
= final conductor temperature in °C
= initial conductor temperature in °C
= constant: 15150 for aluminum, 24500 for copper
= International Annealed Copper Standard (IACS) conductivity as a percentage.
When the ends of a bus bar are not fixed, heating will result in thermal expansion,
which may cause damage to attached equipment, such as switches, insulators,
and other devices. The amount of expansion is 13.11
where
= initial bus bar length in m
= change in bus bar length in m
= coefficient of thermal expansion in 1/°C.
If both ends of the bus bar are fixed, then the resulting a force results
may damage attached equipment:
eqn.12
…where…
= crosssectional area in cm^{2} Where thermal expansion is anticipated owing
to increased fault currents, expansion fittings can be added to long bus structures.
6. Flexible Conductor Buses
Flexible conductor buses may be constructed as strain buses, suspended from
insulator strings (FIG. 3). This type of construction is usually used
for substation main buses at high voltages . The slack bus construction (FIG.
4) with post insulators is normally used for connections between equipment
within a substation. When high currentcarrying capability is needed, conductors
are often bundled (FIG. 5) separated from 8 to 60 cm with spacers at regular
intervals of 2 to 30 m.
FIG. 3 Strain bus from suspension insulators (EPRI, 2006).
FIG. 4 Slack bus from post insulators (EPRI, 2006).
FIG. 5 Details of flexible conductor bundle with spacers.
The following are the effects of high fault currents on flexible conductor
buses:
1. Increased tension on the conductors
2. Increased tension on insulators
3. Unwanted forces on support structures
4. Increased thermal stress on the conductors
5. Possibility of arcing due to decreased minimum clearance between conductors
during swing
6. Possibility of damage due to increased drop force
7. Pincheffect damage to conductors due to clashing, to spacers due to compression,
and to suspension insulators and supports due to impulse tension.
Flexible conductor substation buses are discussed in detail in IEC Standard
60865 (IEC, 1994; IEC, 2011a) with further explanations in (CIGRE, 1996). The
design standard is intended for horizontal buses up to 60 m long in a temperature
range of 20 to +60°C and maximum sag of 8%. Automatic reclosing does not increase
the effect of short circuits on flexible conductors. The simplified calculation
procedure used in the standards has been verified by tests and detailed finite
element method (FEM) simulations (Stein, Miri, and Meyer, 2000; Miri and Stein,
2002; Herrmann, Stein, and Keißling 1989).
6.1 Conductor Motion During a Fault
The shortcircuit force on flexible conductors for a phasetophase or threephase
fault is approximately...
eqn.13
The forces will cause the conductors to separate (swing out), gravity will
then bring them together (drop force), and they will oscillate with a characteristic
period. The forces will also cause an elastic expansion of the conductor material,
while the high currents will cause thermal expansion. If the bus is constructed
from bundled conductors instead of single conductors, then the short circuit
will force them together through the pinch effect, which produces tension on
the conductor.
The IEC standard (IEC, 2011a) defines the ratio of electromagnetic force from
the short circuit to the weight of the conductor:
eqn.14
...where ...
= number of subconductors
= mass of the subconductors in kg/m
= gravitational constant in m/s^2 then the direction of the resultant force
is eqn.15
Without current flow, the static conductor sag is eqn.16 where
= is the static force on the conductors in N.
If the conductor oscillates at small swingout angles, again, with no current
flow, the period of oscillation is eqn.17
With a short circuit, the period is eqn.18
The maximum angle which the conductor can swing out for a given shortcircuit
current magnitude and fault duration can now be calculated. The swingout angle
at the end of the short circuit is eqn.19
FIG. 6 Curves for determining the factor
? from IEC Standard 60865, Figure 7 (EPRI, 2006).
FIG. 7 Horizontal displacement and distance between midpoints of a slack
bus (EPRI, 2006).
Once the conductors have reached their maximum height, they will fall, experiencing
the drop force:
eqn.37
6.2 Pinch Forces on Bundled Conductors
In bundled conductor configurations (FIG. 5), shortcircuit forces cause
the subconductors to come together rapidly. This discussion applies to subconductors
arranged [...]
FIG. 8 Curves for determining the factors v1 and v2 from IEC Standard
60865, Figure 9 (EPRI, 2006).
FIG. 9 Curves for determining the factor v3 from IEC Standard 60865,
Figure 10 (EPRI, 2006).
FIG. 10 Curves for determining the factor ? as a function of j and from
IEC Standard 60865, Figure 11 (EPRI, 2006).
FIG. 11 Curves for determining the factor
? from IEC Standard 60865, when , Figure 12a (EPRI, 2006).
FIG. 12 Curves for determining the factor ? from IEC Standard 60865,
when , Figure 12b (EPRI, 2006).
FIG. 13 Additional curves for determining the factor ? from IEC Standard
60865, when , Figure 11 (EPRI, 2006).
7. Force Safety Devices
When shortcircuit forces increase, force safety devices (FSDs) can mitigate
pinchforce effects.
A FSD (Miroshnik, 2003) is a deformable mechanical link which can be placed
in series with a flexible substation bus to limit damage due to shortcircuit
forces. It is similar to a fuse, in that it is nonrecoverable, and must be
replaced after a short circuit. The principle of operation (FIG. 14) is
that of a metallic cramp, having two weakened crosssectional areas which are
calibrated for an actuation force of . The FSD is connected between the support
structure and the suspension insulator (FIG. 15). When the total force
exceeds , the FSD will be deformed, limiting the force. A graph of force limitation
is plotted in FIG. 16.
FIG. 14 Operation of force safety device (EPRI, 2006).
FIG. 15 Connection of FSD to flexible substation bus structure (EPRI,
2006).
FIG. 16 Limitation of bus tension by FSD (EPRI, 2006).
8. Substation Cable and Conductor Systems
There are many types of cables and conductors used in substations (IEEE, 2007c).
These include the following:
1. Highvoltage power cables, defined as >1000 V. These may connect to
other substations, to substation equipment, or to customer loads.
2. Lowvoltage power cables, defined as >1000 V. These supply auxiliary
power to substation equipment.
3. Control cables. These include instrument transformer secondary cables.
4. Instrumentation cables. These are used primarily for SCADA systems.
5. Overhead secondary conductors. In distribution substations, these medium
voltage openwire lines are the termination points of distribution feeders.
8.1 Cable Thermal Limits
Cables are subject to thermal damage from prolonged exposure to shortcircuit
currents.
Protective relay operating times and circuitbreaker clearing times must be
fast enough to prevent prolonged overheating (IEEE, 1993, Section 5.6.2). Although
the protection requirements of the National Electrical Code (NEC) (NFPA, 2014)
do not apply in most substations, they should be considered when evaluating
cable protection systems.
In addition to the NEC requirements, it is recommended that cable protection
adhere to the limits of the cable damage curve for the insulation type as published
by the cable manufacturers.
eqn.52 where ...
= symmetrical shortcircuit current in A
= conductor crosssection in circular mils
= initial conductor temperature in °C, the maximum continuous conductor temperature
for the insulation system is used, typically 75 or 90°C for lowvoltage cables.
= final conductor temperature in °C, the shortcircuit temperature limit of
the insulation system is used, typically 250°C for lowvoltage cables.
= heating temperature constant for the conductor material, 234°C for Cu and
228°C for Al.
Similar limits are available for the sheaths of mediumvoltage cables. They
should be used for ground fault currents. In the case of increased fault current
levels, protective relay settings should be changed, as necessary, to protect
the cables. If this is not possible, resizing of the cable may be necessary.
8.2 Cable Mechanical Limits
When a short duration fault bends a cable, the mechanical effect is more significant
than the thermal effect. Permanent deformation may occur in plasticinsulated
singlecore cables. When cleats confine a cable such that shortcircuit forces
create outward bows with small bending radii, the cable may be damaged. Friction
between the cleats and the cable may damage the outer sheath. Softening of
the insulation by simultaneous heating further increases the damage caused
by bending of the conductors. Proper support of cables can prevent this type
of damage from occurring.
9. Distribution Line Conductor Motion
When overhead distribution lines enter substations, the opportunity exists
for the substation to be exposed to damage from distribution faults (Ward,
2003). A fault on the distribution line causes the overhead conductors to swing
sidetoside closer to the substation. As a result, the conductors may move
close enough to arc (0.1 m) or even touch.
This causes a second fault, which may cause increased stress on the substation
transformer and cause backup protective devices to operate.
Ward prepared a computer program that calculates critical clearingtime curves
for overhead distribution conductors, based on conductor motion. Critical clearing
time increases as spans decrease. The following are possible solutions to the
problem of damage caused by distribution conductor motion:
Using faster recloser time curves. This is the preferred solution, if it is
possible.
Installing fiberglass spacers at mid span to shorten the effective span. This
is fairly inexpensive.
Adding intermediate poles to shorten spans.
This is expensive.
Increasing the phase spacing. This requires replacing cross arms, and is expensive.
Removing slack in the lines to reduce conductor motion. This is time consuming
and expensive.
The first two options, faster reclosing times and fiberglass spacers at mid
span, are the best alternatives if increased levels of fault current result
in added stress to the substation due to overhead distribution line faults.
10. Effects of High Fault Currents on Substation Insulators
Brittle fracture of nonceramic insulators (NCIs) have mostly occurred in
polymer suspension insulators, however, the same problems
could occur in posttype insulators. In terms of shortcircuit stresses, exceeding
the mechanical loading limits could result in cracks or splits in the rod or
in damaged seals. Water intrusion leads to brittle fracture, through the leaching
of acids in combination with tensile stress.
10.1 Station Post Insulators for Rigid Bus Bars
High fault current forces on rigid bus bars are transmitted to supporting
insulators, which will be subject to forces that may exceed their design limits.
The effects on the insulators could be cracks, fractures, or breakage. These,
in turn, will weaken the support structure of the bus, resulting in greater
damage should a second fault occur before the damage is repaired. The action
of reclosers is of particular concern here.
The shortcircuit force on a bus bar is transmitted to the insulator (IEEE,
2008, Section 12) through the bussupport fitting (Figures 17 and 18):
eqn.53
...where ...
= bus shortcircuit force transmitted to the bus support fitting in N
= effective length of the bus span in m.
FIG. 17 Insulator configuration for vertical bus (EPRI, 2006).
FIG. 18 Insulator configuration for horizontal bus (EPRI, 2006).
Similarly, the gravitational forces are
...where...
= gravitational force transmitted to the bus support fitting in N
= weight of the bus in N.
The cantilever force on the insulator is then 13.54 where
= overload factor for wind forces, typically 2.5
= overload factor for fault current forces;
this should also be 2.5, unless certain resonance criteria are met.
= overload factor for gravitational forces, typically 2.5
= wind force on the insulator in N
= height of the insulator in cm
= height of the bus centerline above the insulator in cm.
If the cantilever force is exceeded as prospective fault currents increase,
two possible solutions are (i) to increase the number of insulators, thus decreasing
or (ii) to replace the insulators with units having greater cantilever strength.
If insulator spacing is changed, the mechanical resonant frequencies will have
to be recalculated, and a dynamic study may need to be performed. Experimental
results (Barrett et al., 2003) show that the IEEE method is conservative, and
it is unlikely that increased fault currents will damage an IEEEdesigned insulator
structure.
10.2 Suspension Insulators for Flexible Conductor Buses
In accordance with (IEC, 2011a), the forces calculated shall be used with
multipliers according to the type of insulator, as shown in Table 4.
Table 4 Insulator Force Multipliers
Force Post Insulator
Chain Insulator
Tension 1.5 1.0
Drop 1.0 1.0
Pinch 1.0 1.0
11. Effects of High Fault Currents on GasInsulated Substations
(GIS)
Gasinsulated substations (GIS) are designed and tested in accordance to (IEEE,
2011), and have shortcircuit ratings as listed in Table 5.
These are for external faults, where the GIS is tested in the same manner
as circuit breakers.
The internal arcing faultwithstand capability of GIS is based on the thickness
of the metal walls and the gas pressure (IEEE, 2011), and is thus not easy
to upgrade. The withstand times are listed in Table 6. IEC standards are
similar in regard to shortcircuit ratings as well as burnthrough times. The
time to puncture an aluminum plate is approximately (Boeck and Krüger, 1992)
13.55 where
= time in ms
= constant
= thickness of the aluminum in mm
= current in kA.
Table 5 GIS ShortCircuit Ratings ShortTime CurrentCarrying Capability
(kA, rms) for a Specified Time of 1 or 3 s
Table 6 GIS PhasetoGround BurnThrough Times
FIG. 19 GIS enclosure punctured by a rotating arc (EPRI, 2006).
A rotating arc can puncture a GIS wall in two different ways (Boeck, 2003).
If there is an oblique arc, which rotates, the burst will be similar to that
shown in FIG. 19. If an insulating barrier stops the moving arc, the vertical
arc will puncture a hole in a much shorter time. The internal design of GIS
is intended to keep the arcs moving and to prevent them from sweeping over
the same locations more than once.
A statistical analysis (Trinh, 1992) shows that: "An increase in the
mean fault current from 20 to 30 kA raises the risk of burnthrough from 0.34
to 0.78, which illustrates the importance of designing the GIS in terms of
the distribution of the local fault current." This
is expressed as a probability formula:
eqn.56 where ...
= risk of burnthrough of a GIS having an envelope thickness associated with
a fault at a certain location on the transmission system
= probability density of the local fault current ic
= probability that the burnthrough time will not exceed tc
= probability density of the faultclearing times.
When a GIS unit is inspected and maintained, or replaced after a fault, very
specific safety procedures should be followed (IEEE, 2011).
Sulfur hexafluoride is nontoxic, but produces numerous toxic byproducts during
arcing and burning.
