arc flash hazards in ac and dc systems - walter scott,...
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Arc Flash Hazards in AC and DC Systems
End of Semester Report, fall 2012
David Smith
Prepared to Partially Fulfill the Requirements of ECE 401
Department of Electrical and Computer Engineering
Colorado State University
Fort Collins, Colorado 80523
Project Advisor: Dr. George Collins
Industry Sponsor: Dohn Simms, Praxis Corporation
Approved by: Dr George Collins
Abstract
For my senior project, I chose to focus on the subject of arc flash hazards in electrical
systems, specifically in utility‐scale wind farms and photovoltaic arrays. As an electrician, this
subject combines my interests in electrical worker safety and the continued rapid pace of
the development of alternative methods of generating electricity. This subject is of great
practical interest to Facility and Project Managers who are responsible for the safety of their
workers but must also balance this concern with the need to maintain a reliable system with
minimal downtime.
An arc flash is a sustained arcing current that propagates through the conductive plasma
created by the breakdown of a gaseous dielectric medium, typically air. Given the right
conditions, the current will continue to flow unabated until interrupted by an upstream
over‐current protective device. Such arcs release enormous energy, resulting in high
temperatures, sound levels, pressures and the ejection of high‐speed molten debris. Arc
flashes and blasts in electrical transmission, distribution and utilization systems are a leading
cause of injuries and fatalities to electrical workers and are estimated to cost hundreds of
millions of dollars each year in downtime and damaged equipment.
As a specific hazard to electrical workers that needed to be actively managed, arc flashes
and blasts were largely ignored until the 1985 publication of Ralph Lee’s seminal paper “Arc
Flash: The Other Electrical Hazard,” which presented the first theoretical model for
predicting incident energies workers could be exposed to as a function of arcing time,
available short circuit current and distance from the arc.
This model and much of the ensuing research into predicting arc flash energies is specific to
alternating current, however. The empirically derived equations in the Institute of Electrical
and Electronics Engineer’s ‘Guide to Performing Arc‐Flash Hazard Calculations’ (IEEE
Standard 1584) are specific to arcs in AC systems. Currently, no consensus standard exists for
calculating arc energies in DC systems. However, the 2012 edition of the National Fire
Protection Association’s ‘Standard for Electrical Safety in the Workplace’ (NFPA 70E)
references two papers that offer theoretical and semi‐empirical methods for estimating DC
arc energy.
The primary goal of my project will be twofold: first, to perform a comparative evaluation of
the consensus standards for predicting arc energy on AC systems as conducted on a wind
farm, and two, to perform a comparative evaluation of the two methods given in the 2012
NFPA 70E for predicting DC arc energy as conducted on a photovoltaic array.
ContentsAbstract ......................................................................................................................................................... 2
List of Figures and Tables .............................................................................................................................. 4
Introduction .................................................................................................................................................. 5
Arcs in AC Systems ........................................................................................................................................ 6
Short Circuit Current Analysis ................................................................................................................... 7
Utility ................................................................................................................................................... 10
Transformers ....................................................................................................................................... 11
Turbines .............................................................................................................................................. 11
Conductors .......................................................................................................................................... 11
Arc Energy ................................................................................................................................................... 15
Lee’s Method .......................................................................................................................................... 15
IEEE 1584 Standard ................................................................................................................................. 16
Conclusions ................................................................................................................................................. 23
Plans for spring, 2013 Semester ................................................................................................................. 24
Arcs in DC Systems .................................................................................................................................. 24
References .................................................................................................................................................. 25
Appendix A: Abbreviations ......................................................................................................................... 26
Appendix B: Expected Project Costs ........................................................................................................... 26
Appendix C: Project Outlines ...................................................................................................................... 27
Acknowledgements ..................................................................................................................................... 29
ListofFiguresandTables
Symmetrical Short Circuit Current…………………………………………………............7
Asymmetrical Short Circuit Current…………………………………………………………..7
Short Circuit Current DC Component………………………………………………………..8
System Diagram………………………………………………………………………………………..9
Short Circuit Current Diagram…………………………………………………………………..9
Per‐Unit Impedance Network………………………………………………………………….13
Vector Diagrams for Lee’s Method………………………………………………………….15
Arcing Current vs. Bolted Fault Current < 1000V……………………………………..17
Arcing Current vs. Bolted Fault Current >1000V………………………………………17
Incident Energy vs. Bolted Fault Current < 1000V……………………………………18
Incident Energy vs. Bolted Fault Current > 1000V……………………………………18
Incident Energy Diagram…………………………………………………………………………20
Time‐Current Curves for XFMR fuse and Collector Circuit Relay……………..21
Time‐Current Curves of XFMR Fuse and Turbine Circuit Breaker……………..22
IntroductionThe system of electrical transmission, distribution and utilization equipment that we
informally refer to as the “grid” constitutes an enormous and enormously complex network
that is subject to many types of failures and hence must have robust and well‐designed
protection systems. These systems must fulfill the dual function of keeping the network up
and running as consistently and reliably as possible as well as protecting the people that
maintain and repair it. Finding the balance between reliability and safety can be difficult, and
too often these goals are considered to be contradictory.
Because the costs of downtime for consumers of electricity can be extremely high, there is
enormous pressure on electrical workers to perform their duties on energized systems, work
that is extremely hazardous even for well‐trained and qualified personnel. Clearly, a major
hazard is the potential for injury due to electric shock, but there is also the potential for
serious injury from electrical arc flashes and blasts that can occur when an unintentional
low‐impedance path between phase conductors or between a phase conductor and ground
is created. If a low‐impedance fault escalates into a sustained electric arc, tremendous
energies can be released in the form of intense heat and concussive blasts. Many of these
types of faults are triggered by electricians and linemen working on energized equipment,
work that necessarily involves being in close proximity to the arc and which often results in
devastating burns, impacts from explosive blasts, falls and a host of other equally serious
injuries.
Until relatively recently, injuries from electric shock were considered to be most common
and serious hazard faced by electrical workers, but it is now recognized that many of the
burns suffered by victims of electrical accidents that were once ascribed to being part of the
conduction path of electric current are actually caused by exposure to arc flashes and blasts.
While the statistics on how many injuries and fatalities are caused by shock vs. arc flashes
are a matter of some debate, it is generally accepted that arc flash events are not
uncommon and constitute a significant portion of the 320 deaths and 4000 major injuries
that are caused by electrical accidents on average each year in the US[1]. While the costs of
individual accidents vary widely and are difficult to calculate, electrical accidents often have
higher costs than other types of accidents, and one study by the Electric Power Research
Institute in Palo Alto estimates the average total direct and indirect costs to be as high as 15
million dollars per case[1].
Further, reducing the intensity of an arc flash protects equipment as well as workers. A high
energy, long duration arcing fault inside the wiring compartment of an oil‐filled transformer,
for example, will typically destroy the wiring lugs or paddles, the feeder conductors, and can
even rupture the tank, leading to the complete loss of a very expensive piece of equipment,
as well as costs due to system downtime and labor for repairs. Less energetic arcs will
generally cause less damage, and systems with low calculated incident energies are easier to
properly maintain with less downtime, since the personal protective equipment that must
be worn is proportionately less burdensome.
While shock hazards must mostly be mitigated through work practices—training and
permitting, personal protective equipment requirements, and the like—arc flash hazards can
often be significantly reduced by making judicious changes to the electrical system itself. To
correctly identify where and how improvements can be made, one must be able to estimate
with some degree of accuracy the potential arc flash intensity at all points in the electrical
system where electrical maintenance or repair is likely to occur.
ArcsinACSystemsFree‐burning arcs in open air are chaotic, complex and unpredictable in nature, difficult to
accurately model and are typically evaluated from a “black box” perspective. The most
commonly used theoretical model for predicting arc energy is Lee’s method, which relies on
basic circuit theory and the maximum power transfer theorem. Developed by Ralph Lee in
his 1982 paper “The Other Electrical hazard: Electric Arc Blast Burns,” Lee’s method, along
with various but relatively minor refinements, was the accepted methodology until the
publication of the IEEE 1584 Standard in 2002, which presented empirically derived
equations based on extensive laboratory test data. It should be noted that Lee’s method is
still the consensus standard when the system to be evaluated falls outside of the scope of
the 1584 standard.
The difference between the two methods will be explored in detail later, but since both
methods rely on accurately calculating the maximum short circuit current (SCC) available in
the electrical system, a discussion of short circuit study process will be helpful here. It is
generally accepted that the vast majority of line‐to‐line or single‐line‐to‐ground faults in a
three phase AC system will quickly escalate into three phase faults due to the propagation of arcing current through the conductive plasma and vaporized conductive material created by
the initial fault. The energy radiated by the arc is a function of the current through the arc,
which in turn depends on the on the SCC available at the fault point.
ShortCircuitCurrentAnalysisThe available SCC is defined as the maximum current that will flow from all sources in the
system to a bolted (zero impedance) fault, which occurs during the first half cycle after fault
inception. It is important to understand that the available SCC at a particular node in the
system is independent of the steady‐state current drawn by the downstream load, and is a
function of the sources of SCC—the electric utility, generators and motors—and the
impedance of those sources as well as the impedance of the conductors and transformers in
the system.
Because of this load independence, we find that the equivalent impedance looking back into
a typical system from the fault point is predominantly inductive, so the fault current
waveform will lag the voltage by close to 90 degrees. This means that the symmetry of the
current waveform about the zero axis will depend on the point in the voltage cycle at which
the fault occurs. If the voltage is at its peak value when the fault occurs, the current will start
at zero magnitude and follow the voltage down through the zero‐crossing and be
symmetrical about the zero axis. If, however, the voltage is at zero when the fault occurs,
the current will again start at zero, but since it must follow the voltage by 90 degrees, the
current will rise above the zero axis and vary as a sinusoid about a DC offset value:
Figure 1: Symmetrical SCC[2] Figure 2: Asymmetrical SCC[2]
It is common to analyze the asymmetrical fault current as the sum of a symmetrical sinusoid
and a DC component that decays as a function of the circuit’s X/R ratio:
Figure 3: SCC as Sinusoid and DC Component[2]
The maximum asymmetrical SCC can be found theoretically by analyzing the transient
response of an equivalent series RL circuit just after the time of fault inception. This,
however, is a relatively complicated calculation in a real system and simplified methods have
been developed where the steady‐state symmetrical SCC is adjusted by a table of multiplying
factors based on the relative X/R ratio at the fault point and the test X/R ratio of the
interrupting device. These methods are described in several IEC and ANSI standards for
different interrupting devices, most notably ANSI C37.04 and ANSI C37.010 for high voltage
circuit breakers rated on a symmetrical basis.
However, these standards are intended to guide the proper selection of an interrupting
device capable of opening under worst‐case fault conditions, for which the maximum
asymmetrical SCC must be known. For the purposes of estimating arc energy, the
symmetrical RMS SCC is the value we want, since the power in an arc is found as the product
of the effective (RMS) voltage across the arc and the effective current through the arc. The
current through the arc will be lower than the available symmetrical SCC due to arc
impedance, but we begin by finding the symmetrical SCC and adjusting it accordingly.
This is accomplished by reducing the system to a per‐unit impedance network at a common
base MVA and a common driving voltage, after which the available symmetrical SCC at each
node can be calculated simply as I=V/Z, where V is the system driving voltage and Z is the
equivalent impedance of the network as seen by the fault point. As an illustrative example,
we will use a small system consisting of two 2MW wind turbines feeding a 115KV substation,
all protected by a variety of over‐current devices. The following diagrams depict the system
schematic (Figure 4) and the system with the available symmetrical RMS short circuit current
shown as calculated by ESA’s EasyPower software, based on actual device data (Figure 5):
Figure 4: System Diagram
Figure 5: Short Circuit Current Diagram
To verify the above values—and see how they are obtained—we will choose a base of
10MVA and a per‐unit driving voltage of 115KV/115KV=1.
Next we need to find the base impedance at each voltage level in the system. From the
relationships V=IZ and S=IV, we can obtain and use Z=V2/S to find the base impedances:
Z115kv = (115,000)2/10MVA=1322.5
Z34.5KV = (34,500)2/10MVA=119.03
Z0.6kv = (600)2/10MVA=0.036
Next, each element in the system must be converted to per‐unit impedance using the
relationship Zpu=Zactual/Zbase:
UtilitySCC contributions from the electric utility that feeds a system must be obtained in order
accurately calculate SCC values in the system. A common mistake in short circuit analysis is
to assume that the utility is an infinite bus—an ideal voltage source with zero internal
resistance capable of supplying unlimited SCC—under the rationale that this is a worst‐case
approach. The problem is that overestimating the short circuit current at a particular node in
a system can lead to believing that a given over current protective device may detect and
clear a fault faster than it would in reality. Many over current protective devices have
inverse time‐current characteristics, meaning that the higher the current detected is, the
faster the device will open. Overestimating the available short circuit current can lead to
significantly underestimating the device opening time and hence, the incident energy.
The SCC a utility can provide at a particular point is easily obtained from the utility itself, and
is typically given as amperes at particular voltage and X/R ratio. In our example, the utility
contribution is 3003 amperes at 115KV with an X/R=6.08. To use the relationship above, we
need to find the actual impedance looking back into the utility:
∗√
= 22.1 .
.0.0167
Then we need to separate the impedance into its real and imaginary parts:
→ tan 6.08 80.66°
cos 80.66° 0.00271
sin 80.66° 0.0165
Hence the per‐unit utility impedance is: Zpu=0.00271 + j0.0165
TransformersPractical power transformers are designed such that the reactance is much larger than the
resistance of wire that makes up the transformer coils. Transformer impedances are given by
the manufacturer in per‐unit values (%Z), often without an accompanying X/R ratio or power
factor, so this impedance can be assumed to be purely reactive for the purposes calculating
SCC. The %Z is valid for the rated (nameplate) VA and must be scaled to the arbitrarily
chosen base MVA:
TX‐1
%100
9.04100
1048
.
XFMR‐1 and XFMR‐2
%100
5.66100
102
.
TurbinesThe impedance of the wind turbine—like any generator—will be given as three values: X”d
(subtransient reactance), X’d (transient reactance) and Xd (synchronous reactance). Since we
are interested in maximum symmetrical SCC, we want to use X”d, the value that will give the
SCC in the first half‐cycle. In practice, the generator will be much like a transformer and have
an X >> R, even though the nameplate may provide an X/R or power factor. In our example,
the turbine nameplate gives an X/R=24.97, so we will neglect the real part of the impedance.
The per‐unit impedance must again be scaled by the base impedance:
%100
16.7100
102
.
ConductorsThe impedance of the conductors in the system can significantly reduce SCC at the fault
point and must be considered. This is particularly true in an example like ours, a wind farm,
where the collector cables between turbines can be thousands of feet long. To find the per‐
unit conductor impedances, we need to first find the actual impedance, which we can obtain
from the manufacturer or from a reference table. We’ll use table 9 from the National
Electrical Code:
1000kcmil: 0.019+j0.037 (ohms/1000’)
500kcmil: 0.029+j0.039 (ohms/1000’)
4/0 awg: 0.091+j0.041 (ohms/1000’)
For the 4/0 and 1000kcmil lines, we only have 1 conductor per run, with each run 1000’ long,
so we can simply divide the above impedances by the base impedance at 34.5KV:
1000kcmil: . .
.. .
4/0: . .
.. .
For the 500kcmil lines, we have four parallel conductors per run at a total of 350’ long, so
the impedances of each conductor will add inversely:
1 40.35
10.029 0.039
→1
0.00254 0.00341
Then we divide the actual impedance by the base impedance at 600 volts to find the per‐unit
impedance:
0.00254 0.003410.036
. .
Finally, we can draw the per‐unit impedance network:
Figure 6: per unit impedance network
To find the SCC at node ‘A,’ we find the equivalent impedance at that point as the parallel
combination of the three branches:
/ 0.0708 1.213
0.0700 1.213
/ 0.00363 0.03598
1 1 1 1→ . .
Then, we can find the per‐unit SCC:
10.00335 0.03397
2.875 29.15 29.30 .
To find the actual SCC at 34.5KV, we need to calculate the base current:
√3 ∗
10
√3 ∗ 34.5167.35
And finally, the actual SCC at node A is the product of the base current and the per‐unit
current:
∗ . .
We see from the SCC diagram above that the value calculated by the EasyPower software at
that node is 4917A with an angle of ‐83.2, differences that are probably due to rounding
errors in the hand calculations. The SCC at any other node in the system can be found
similarly, though it is clearly easier to use the software! However, it is critical to be able to
assess the SCC values given by the software to verify the accuracy of the equipment data. If,
for example, the impedance of TX‐1 were inadvertently entered as 3.04% rather than 9.04%,
the SCC at node A would be 7,353A.
ArcEnergyThe primary difficulty in predicting the energy radiated by an electric arc is accurately
determining arc impedance. Since the arc power or energy per time is the product of the
driving voltage and the arcing current, the arc impedance would need to be known to
calculate the difference between the bolted fault current and the arcing current. In
applications where arcs are deliberately formed, such as welders and high‐intensity
discharge lighting, the arc gap, pressure and chemical composition of the dielectric is
precisely known to achieve the desired effect. In the inadvertent arcs that we are interested
in, struck by a carelessly dropped tool or by operating a malfunctioning switch, these
parameters may vary greatly, and accurately predicting the arc impedance is nearly
impossible.
Lee’sMethodIn order to work around this difficulty, Ralph Lee began with the assertion that the sources
of SCC in a practical electrical system are primarily inductive and that the impedance of an
arc is primarily resistive, such that the source voltage and arcing current would be shifted by
90 degrees, with the voltage across the arc necessarily being phase with the arcing current.
From this, the voltage drops across the source inductance and the arc resistance can be
drawn in quadrature:
Figure 7: Vector Diagrams for Lee’s Method [3]
From this, we can see that the maximum power will be transferred to the arc under
condition 2, when Es2=Ea1 or Xsource=Rarc, according to the maximum power transfer theorem.
From the vectors for condition 2, we can solve for maximum power:
cos 45° √ →√
0.707 , and by the same logic I2=0.707Isc
Hence Pmax=V2*I2=0.5*V0*Ib f= 0.5*VAbf
Then, treating the arc as a spherical radiant heat source, the fraction of energy absorbed by
a spherical surface per‐unit area will be inversely proportional to the square of the distance
between the arc and that surface:
2.142 ∗
Where:
E is incident energy in J/cm2
MVA is the bolted fault SCC
t is arcing time in seconds
D is distance from the arc in mm
This method has several limitations, the most significant of which is that at the maximum
power point, we will always have that the voltage across the arc and the current through it are
equal to 70.7% of the system voltage and the bolted fault current, with the result that predicted
arc energy will increase linearly with the system voltage and the available bolted fault current.
This is problematic given the non‐linear nature of arc VI characteristics, where past a certain
point, the voltage across the arc is primarily a function of arc length, and current through the
arc may increase almost independently of arc voltage. A further issue is that the maximum
power method does not provide us with a way to accurately predict the arcing current, which
may be significantly different than the 70.7% of bolted fault current present at maximum power
point.
IEEE1584StandardIn 2002, the IEEE 1584 working group published empirical equations for arc energy derived
from extensive laboratory test data using curve‐fitting techniques [4]. The group created arcs at
several common system voltages (208, 400, 450, 480, 600, 2300, 4160 and 13800 volts) over a
range of bolted fault currents up to 106,000 amperes while varying electrode gap and
composition, enclosure size and distance from the arc. Importantly, the group measured
current delivered by the system under a variety of configuration and provided equations for
predicting arcing current as a function of system voltage, arc gap and bolted fault current
(equations for arcing current and incident energy are reproduced in the appendix). The test
results demonstrated that arcing current as a percentage of bolted fault current increased
quickly as the system voltage was increased. Below is a plot of arcing current vs. bolted fault
current using the 1584 equation at the five voltages under 1000V that the group tested at, with
all other variables held constant:
Figure 8: Arc Current vs. Bolted Fault Current for <1000V
At 208V, we see that the arcing current is roughly 20% of bolted fault current, while at 600V,
arcing current is closer to 50% bolted fault current.
The group also tested at 2.3KV, 4.16KV and 13.8KV, finding that at these voltages, arcing
current was almost entirely a function of bolted fault SCC. The equation provided for arcing
current is the same for all three system voltages and we see that the arcing current is almost
equal to the bolted fault current:
Figure 9: Arc Current vs. Bolted Fault Current >1000V
What this means is that when we compare Incident Energy computed by the two methods over
a range of bolted fault current values, we see that Lee’s method will be consistently—but not
excessively—conservative at the lower system voltages:
Figure 10: Incident Energy vs. Bolted Fault Current < 1000V For 1584 Curves: Arc Gap = 32mm, Arc Time = 0.1 sec, Calc Factor=1.5, Dist. Exp.=1.473, Working Dist. = 18”
For Lee’s Curves: Arc Time = 0.1 sec, Working Dist. = 18”
However, above 1000 volts, Lee’s method provides very conservative results. In the plot
below, we see that the 1584 equations return the same results for all three of the test
voltages, while the values predicted by Lee’s method will continue to increase as a direct
function of increasing system voltage:
Figure 11: Incident Energy vs. Bolted Fault Current > 1000V
For 1584 Curves: Arc Gap = 153mm, Arc Time = 0.1 sec, Calc Factor=1, Dist. Exp. = 0.973, Working Dist. = 38”
For Lee’s Curves: Arc Time = 0.1 sec, Working Dist. = 38”
This is problematic because the 1584 group only tested up to 13.8KV, and hence any system
with voltages above this level must be evaluated using Lee’s method. What this means is
that electrical workers performing energized work on systems above 13.8KV are likely
donning Personnel Protective Equipment (PPE) rated far higher than what might actually be
necessary to prevent an incurable burn. PPE rated for 40 cal/cm2 is heavy, uncomfortable,
restricts motion and visibility and hence lowers worker productivity, and could conceivably
increase the likelihood of an accident occurring.
It is also sometimes the case that facility or project managers choose to evaluate incident
energy using the maximum power method, reasoning that this will always give a worst‐case
value. In a system where the over‐current interrupting devices are instantaneous circuit
breakers or definite‐time relays—where the current at which the device will trip is
independent of time—this is probably a good notion; the above plots provide a pretty good
indication of the differences one would expect to see between methods. Given the relatively
minor margin by which Lee’s method overestimates incident energy on low voltage systems,
one could certainly use this method with a high degree of confidence that electrical workers
wearing PPE rated to withstand the estimated energy would be adequately protected
without that PPE being unnecessarily burdensome.
However, as previously mentioned, Lee’s method does not provide a way to predict arcing
current. In systems where over‐current devices have inverse time‐current characteristics—
where the time it takes for the device to open decreases as the current through the device
increases—an accurate estimate of the device clearing time is essential to properly predict
incident energy. Since Lee’s method calculates the maximum power in an arc using the
bolted fault SCC/square root of 2, this is the amount of current we must logically assume is
present in the arc, and hence is the amount of current sensed by the clearing device. As the
1584 test results show, the arcing current is a strong function of system voltage under
1000V, and on these systems, Lee’s method may vastly overestimate the arcing current that
is present. This would lead to underestimating the arc clearing time and hence the arc
energy.
Returning to our previous example of the small wind farm and calculating the incident
energy using both Lee’s method and the 1584 equations, we get values shown in the
following diagram:
Figure 12: Incident Energy; all values for open air
Since the portions of the system from the utility up to the primary side of XFMR‐1 and
XFMR‐2 are at 115KV and 34.5KV, and hence are outside the range of the 1584 model, the
only values shown are those given by Lee’s Method (the fault clearing time is given by the
pick‐up time of the differential relay—not shown—plus the opening time of the 115KV main
breaker. Given what we have seen at the tested medium voltages (2.3KV, 4.16KV, 13.8KV), it
is likely that these values are quite conservative. It is, however, difficult to quantify just how
conservative, and a prudent adherence to best work practices dictates that electrical
workers don PPE rated for the levels shown.
Between the 600V side of the XFMRs and the MCCB breakers, we see that Lee’s Method
predicts incident energy will be approximately 25% lower than what is given by the 1584
equations. Incident energy at these nodes is determined by the trip characteristics of the FS‐
1 and FS‐2 fuses. As seen in figure 13 below, the 1584 equations predict that the arcing
current for faults at these nodes will be approximately 18KA, while 70.7% of the bolted fault
SCC available on the XFMR secondary is approximately 21.4KA. Due to the inverse time‐
current characteristics of the fuse, this relatively small difference results in Lee’s Method
predicting that the fuse will clear 8 cycles faster than the 1584 prediction, resulting in the
lower incident energy values:
Figure 13: Time‐Current Curves for XFMR fuse and Collector Circuit Relay
At the terminals of the turbine, the situation is potentially much worse. Here, the
Instantaneous pickup of the MCCB1 breaker—the lower right portion of the purple curve‐‐is
set to 14KA, meaning that the device should open immediately if any current is sensed
above that level. However, as is always the case, there is some uncertainty here: the
manufacturer gives a range of currents at which the device has been observed to operate
during testing. This range is given graphically by the width of the trip curve in each region of
operation. Further, this range is only valid for devices in the condition they were in the day
they left the factory. Old and poorly maintained devices are considerably more
unpredictable.
As seen in Figure 14 below, the calculated arcing current is approximately 14.5KA, while
70.7% of the bolted fault SCC seen by MCCB‐1 is 16.9KA. Given the uncertainty described
above, EasyPower correctly assumes that the arcing current may not be sufficient to trip the
breaker, so the total arc clearing time is based on the Short Term setting of 0.3 seconds plus
the breaker opening time of 0.04 seconds. Using Lee’s Method—estimating the arcing
current to be 70.7% of the bolted fault SCC—we would think that there is sufficient arcing
current to operate the breaker instantaneously in the event of a fault, resulting in an
incident energy value fully 10 times lower than that predicted when using the 1584
methodology.
Figure 14: Time‐Current Curves of XFMR Fuse and Turbine Circuit Breaker
Ideally, facility managers and engineers would set the Instantaneous pickups on their devices
to the lowest possible setting that still permits normal, reliable operation of their system,
and the small the small difference in estimated arcing current shown in this example would
not be right on the transition zone between trip regions, but be well above the
Instantaneous setting. We would see a much smaller effect if we assume that both current
values are sufficient to operate the device in the Instantaneous region. With a total clearing
time of 0.06 seconds, the estimated incident energy using the 1584 equations is 2.6 cal/cm2
compared to the 3.1 cal/cm2 using Lee.
In practice, however, it is often the case that the inrush currents necessary to start
transformers and motors are quite large and difficult to calculate precisely, and the margins
can indeed be as narrow as shown in this example. The key takeaway here is that we cannot
assume that Lee’s method will always provide conservative results, and further, that as
much room as possible needs to be given between relay settings and arcing current
regardless of what method is used, since there is considerable uncertainty in both the
clearing device threshold and the estimation of the arcing current.
ConclusionsMuch of the foregoing analysis is well known in the industry and does not present new
methods for conducting an Arc Flash Hazard Analysis outside those given in the two
consensus standards I have evaluated. It does, however, amply demonstrate the care that
must be taken in deciding what method to apply after careful consideration of site‐specific
conditions. Since Lee’s model provides a theoretical maximum arc energy but no method for
finding arcing current, the empirical equations for both arc energy and arcing current are
likely to give values that most closely approximate reality, but it is critical to realize that the
values obtained are indeed approximations.
Further, there is considerable value in taking a close look at the disparity between the
maximum power method and empirical data in AC case as an entry point to investigating
how we might predict arc energies in DC systems. Existing research has shown that the V‐I
characteristics of arcs in AC and DC systems are non‐linear, behavior that Lee’s maximum
power method does not account for, so it is reasonable to predict that the situation may be
much the same in DC systems, and that more accurate results will be returned by an
empirical model.
Plansforspring,2013Semester
ArcsinDCSystemsAs of this writing, there is no consensus standard for calculating arc flash hazards on a Direct
Current system. However, the 2012 edition of the National Fire Protection Association’s
Standard for Electrical Safety in the Workplace’ (NFPA 70E) [5] references two papers that
offer theoretical and semi‐empirical methods for estimating DC arc energy. The theoretical
method is based on a paper by Dan Doan [6] that presents a maximum power approach to
determining arc energy very similar to that given by Lee. The semi‐empirical method,
presented in a paper by Dr. Ravel Ammerman [7] and several colleagues, is based on a survey
of extant historical data going back over 100 years.
My plan for the coming semester is to perform a comparative evaluation of these two
methods in much the same fashion as for Lee’s method and the IEEE 1584 model, and apply
both methods in attempting to predict the arc energies that may exist on the DC bus of a
photovoltaic (PV) array. There are, however, two complicating factors specific to applying
either model to the DC bus of a PV system. First, the resistance of a PV cell is non‐linear in
nature, hence the current the cells can provide to a given fault point may change
significantly and rapidly as the load resistance goes from that of the inverter to the
resistance at the fault point. In other words, the source resistance—that of the PV cells—is
not fixed to the same extent as the resistance of a string of batteries is.
Second, over‐current protection on PV array is fundamentally different than in a typical AC
system. The short circuit current a PV cell can supply is only slightly higher than the current it
will produce at standard test conditions with a load matched to the characteristic resistance
of the array. This means that the fuses protecting individual strings in the array are intended
primarily for overload protection as distinct from fault protection, and may not serve to
interrupt faults at all. Identifying the fault clearing devices and their associated clearing
times and accurately predicting the arcing fault current will be the primary challenges in
completing the study.
References
[1] Chicago Electrical Trauma Research Institute website [Online]. Available at:
http://www.cetri.org/statistics.html
[2] General Electric Application Information, Short Circuit Current Calculations [online]. Available at:
http://www.geindustrial.com/publibrary/checkout/GET‐3550F?TNR=White%20Papers|GET‐
3550F|generic
[3] The other Electrical Hazard: Electric Arc Blast Burns, Ralph H. Lee, 1982
[4] Guide for Performing Arc‐Flash Hazard Calculations, IEEE 1584‐2002, 2002.
[5] Standard for Electrical Safety in the Workplace, NFPA 70E, 2012
[6] Arc Flash Calculations for Exposures to DC Systems, Daniel R. Doan, 2007
[7] DC Arc Models and Incident Energy Calculations, Ammerman, Gammon, Sen, Nelson, 2009
AppendixA:Abbreviations
IEEE: Institute of Electrical and Electronics Engineers
NFPA: National Fire Protections Association
SCC: Short Circuit Current
IE: Incident Energy
RMS: Root Mean Square
TCC: Time‐Current Curve
XFMR: Power Transformer
AppendixB:ExpectedProjectCosts
Travel:
– 2 trips to Alamosa for data collection. 255 miles each way
• At $0.555/mi (gsa.gov)
• 255*4*$0.555=$566.10
Research paper costs:
– $10 each from IEEE explore
• Estimate 6 total for $60
E‐days poster:
– $20
Total estimated expenses: $646.10
AppendixC:ProjectOutlines
Project Outline, Revision #1, September 9, 2012
1) Research standard methods for calculating arc energies in AC systems. Identify and summarize significant differences using a small
wind farm as a real‐world example. Evaluate why the standard methodologies don’t apply to DC systems:
a. Lee’s method
b. IEEE 1584
c. NFPA 70E table method
Deliverables:
1) Short circuit current and potential arc energy values for small wind farm using above methods
Estimate 60 hoursOctober 15, 2012
2) Report evaluating differences between methods, strengths and shortcomings of each and an explanation of why they
cannot be applied to DC systems
Estimate 20 hoursOctober 29, 2012
2) Seek out and evaluate existing research into methods for calculating arc energies on DC systems:
a. IEEE working group
b. ESA
c. Dan Doan paper
Deliverables
1) Report summarizing and evaluating assumptions and scope of existing research
Estimate 40 hoursNovember 30, 2012
3) Develop the above into an algorithm that can be applied specifically to worker accessible system nodes on a utility‐scale photovoltaic
array
Deliverables
1) Equations for calculating arc energy as function of system voltage, available short circuit current, arc impedance and arc
gap
Estimate 60 hours February 28, 2012
2) Report explaining the process of developing the equations and justifying the assumptions made
Estimate 20 hoursMarch 15, 2012
4) Visit Iberdrola Renewables San Luis solar farm (30MW):
a. Collect Data (OCPD, conductors, PV panels, inverters, power transformers,etc.)
Deliverables
1) One‐line diagram of farm
Estimate 20 hoursMarch 29, 2012
5) Implement equations using a combination of Excel or Matlab and ESA’s EasyPower software and evaluate the San Luis Solar Farm:
Deliverables
1) Available short circuit current values at all significant nodes in the system
Estimate 10 hoursApril 8, 2012
2) Equipment Duty Study
Estimate 10 hoursApril 15, 2012
3) Time‐Current Curves of OCPD found on farm
Estimate 10 hoursApril 22, 2012
4) Potential incident energy values over a range of likely working distances
Estimate 10 hoursApril 29, 2012
6) If time allows, evaluate above results and offer the following:
1) Recommendations for system changes or work practices to lower potential incident energy
2) Recommendations for system changes or work practices to reduce shock exposures
Project Outline, Revision #2, December 7, 2012
1) Complete algorithm for finding iterative solution to equation for arc V‐I characteristic based on the Stokes and
Oppenlander data given in Dr. Ammerman’s paper
Deliverables
a. Matlab code and report explaining methodology and fit comparison to empirical curves
January 30, 2012
2) Understand resistance of the PV cell and how it changes as a function of load resistance. This is critical for applying the
equations from part 1 above. Ideally, I would like to use the Max Power Point Tracking in the inverter to rapidly vary
the load resistance seen by the PV array and record the results, but it is unlikely I would be allowed to do so. I will
probably have to use the characteristic resistance as defined by Vmax power/Imax power. This should give worst case value
Deliverables
a. Report justifying the value used for the equivalent resistance of the PV array looking in from the fault point
February 28, 2012
3) Visit Iberdrola Renewables San Luis solar farm (30MW):
b. Collect Data (OCPD, conductors, PV panels, inverters, power transformers,etc.)
Deliverables
2) One‐line diagram of farm
March 29, 2012
4) Implement equations using a combination of Excel or Matlab and ESA’s EasyPower software and evaluate the San Luis
Solar Farm:
Deliverables
5) Available short circuit current values at all significant nodes in the system
April 8, 2012
6) Equipment Duty Study
April 15, 2012
7) Time‐Current Curves of OCPD found on farm
April 22, 2012
8) Potential incident energy values over a range of likely working distances
April 29, 2012
Acknowledgements
I would like to thank Mr. Simms and Dr. Collins for their guidance throughout the semester
and for their patience as I slowly learned the background material necessary for
understanding this subject. I would also like to thank John Kolak for his constant optimism
and tireless encouragement when it was most needed.