arc flash analysis - Colorado State University

advertisement
ARC FLASH ANALYSIS
Final Report
Fall Semester 2010
Todd J Vedder
Prepared to Partially Fulfill the Requirements for ECE402
Department of Electrical and Computer Engineering
Colorado State University
Fort Collins, Colorado 80523
Project Advisor:
Dr. George Collins
.
1
ABSTRACT
I chose to do my senior design project on the topic of Arc Flash Analysis. The
last few years have developed a great increase in the awareness of arc flash hazards. The
analysis is applied to the overall electrical system of utility companies. The purpose of
the study is to develop the best way to limit the amount of an arc flash a lineman can be
exposed to while doing maintenance in the field.
The National Electric Safety Code
(NESC)-2007 Rule 410 states "employers shall ensure that an assessment is performed to
determine the potential exposure to an electric arc for employees who work on or near
energized parts or equipment. If the assessment determines a potential employee
exposure, greater than 2 cal/cm2 exists, the employer shall require employees to wear
clothing or clothing systems that have an effective arc rating not less than the anticipated
level of arc energy." Electricity has proposed a serious hazard since its discovery by
Benjamin Franklin. Why though, after over a hundred years, is the awareness of this
hazard drawing so much attention?
Arc flash analysis can propose a serious problem to utility companies. Most
injuries that occur out in the field with lineman who work for power utility companies
don’t result from electrocution, but rather from an arc flash. Over the years, these
companies install protective devices all throughout a system. However, much effort, time
and cost is involved to ensure these protective devices coordinate with other devices and
are used to their best efficiency. These protective devices cannot be changed
instantaneously to help arc flashes. One job of a consulting engineer is to calculate the
arc flash incident energy at the point of contact. It then can be determined if the up line
protective device is adequate, or if a recommendation to their protective devices needs to
2
be applied. When the first arc flash assessment was created there were a few key design
questions that needed to be thought about:
1. What are the requirements?
2. How long do we have to design?
3. How will it be tested?
4. What recommended improvements can be made to client?
This study has shown that a procedure can be implemented for utility companies.
Steps and procedures were developed to perform an accurate arc flash analysis.
However, is this the best method that can be used? How accurate are these calculations?
Although it has been learned that there isn’t a 100% right solution, an accurate and
reliable analysis can be created. Future work for this study will be to come up with the
most accurate and reliable method.
3
Table
of
Contents
Title
Page.................................................................................................................................... 1
Abstract....................................................................................................................................... 2
Table
of
Contents..................................................................................................................... 4
List
of
Figures............................................................................................................................ 5
Introduction .............................................................................................................................. 6
Arc
Flash
Characteristics ...................................................................................................... 8
Why
perform
an
arc
flash
assessment? ................................................................................................. 8
How
might
an
arc
flash
occur? .................................................................................................................. 8
What
are
the
results
of
an
arc
flash
and
why
is
it
such
a
concern? ........................................... 9
Dependents
of
survivability
from
arc
flash.......................................................................................... 9
Units
of
measurement................................................................................................................................... 9
Degree
of
burns..............................................................................................................................................10
Steps
Necessary
to
Prepare
for
Arc
Flash
Analysis ....................................................10
Checklist
Completed
by
Utility
Company ......................................................................11
Checklist
for
arc
flash
studies
for
electric
distribution
systems...............................................12
How
Does
an
Arc
Flash
Operate?......................................................................................15
Jacob’s
Ladder
Test ......................................................................................................................................15
Incident
Energy
Calculations
IEEE
1584........................................................................16
Arc
Flash
Analysis
Calculations ........................................................................................17
Improvements/Recommendations .......................................................................................................18
Official
Calculation
Spreadsheet ......................................................................................19
Arc
Flash
Boundary ..............................................................................................................21
Arc
flash
boundary
formula......................................................................................................................22
Economic
Tradeoff ................................................................................................................22
Future
Changes.......................................................................................................................24
Conclusion................................................................................................................................25
Appendix ..................................................................................................................................26
Official
calculation
spreadsheet..............................................................................................................27
Budget
analysis ..............................................................................................................................................28
Definitions ........................................................................................................................................................29
Recommendation
for
project
continuation .......................................................................................32
References........................................................................................................................................................33
Acknowledgements ......................................................................................................................................34
4
List of Figures & Tables
Fig 1: Power System Flow Layout ................................................................. 7
Fig 2: Jacob’s Ladder Test............................................................................ 15
Fig 3: Existing System Model ...................................................................... 17
Fig 4: Hazard Category as a Function of Incident Energy Table ................. 18
Fig 5: Recommended System Model............................................................ 19
Fig 6: Variable Factors For Different Types of Equipment ......................... 20
Fig 7: Fuse vs. Recloser Economic Cost ...................................................... 22
Fig 8: Fuse vs. Recloser Payoff Chart .......................................................... 23
Fig 9: Blank Calculation Spreadsheet........................................................... 27
Fig 10: Time Current Curve ......................................................................... 30
5
Introduction:
With the development of technology, society has seen an increase in benefits.
Things have become easier, more precise, and satisfying. Technology has brought a
significant increase to the world of power and electricity. Arc Flash injuries have been a
concern since the discovery of electricity. So why is it that these hazards are only
drawing high attention over the past few years? A primary reason is the high increase in
magnitude and volume to electricity being generated over the world. Many companies
are requiring more power and higher voltages. Probably the most important reason is the
increased need to perform work on energized equipment. If a line has to be de-energized
in order to perform maintenance, then buildings and homes must go without power. This
provokes not only a loss in revenue for the Power Company, but can upset many
customers. The exponential increase in technology has made it difficult for companies to
keep up with training standards to keep employees aware of the risk of arc flash hazard.
The last important reason arc flash hazards have become a concern is the increase in
liability from lawsuits. Companies are adopting procedures and methods relating to arc
flash hazards not only to provide safety to their employees, but the company as well.
The focus of this study will be on the distribution side of the power utility
system. This is the final stage of delivery of power to end consumers. A distribution
system carries power from the transmission system and delivers it to its consumers. The
diagram below shows how power is developed starting at a power plant, traveling to
transmission lines where it feeds into a power substation. The power substation
downsizes the voltage (typically from 69k volts to 12.47k volts). Here power is traveled
6
throughout the town until it reaches the transformer drum and downsized (typically from
12.47k volts to 480 volts). Here power is supplied to houses and commercial industries.
Figure 1: Power System Flow Layout
According to the NESC an arc flash is the result of a rapid release of energy due
to a short circuit between two conductors, or conductor and ground. The arc flash is not
the direct short between two conductors but rather it is the arc created in the air after a
short circuit occurs. During an arc the air acts as a conductor. This develops problems
for utility companies who have lineman working on energized equipment. Majority of
accidents that occur in power utility companies are not from the result of electrocution,
but instead from an arc flash. Why is an arc flash important to understand? It can kill
you!
7
Arc Flash Characteristics
Why perform an arc flash assessment?
The following statement comes from Rule 410A3 of the National Electric Safety
Code: “Effective as of January 1, 2009, the employer shall ensure that an assessment is
performed to determine potential exposure to an electric arc for employees who work on
or near energized lines, parts or equipment.” In the event that an employee exposure is
greater than 2 cal/cm2 exists, the employer shall:
A. Perform a detailed arc flash assessment to determine the effective arc rating of
clothing or clothing system to be worn by employees working on or near
energized lines, parts, or equipment at voltages 50-800,000 volts. The arc
flash assessment shall include a calculation of the available short circuit
current, time it takes for arc to clear, and the distance from arc to employee.
B. Require employees to wear clothing or a clothing system with an effective arc
rating not less than the anticipated level of arc energy. Unless flame resistant,
when exposed to an electric arc or flame, clothing or a clothing system made
from acetate, nylon, polyester or polypropylene shall not be worn.
How might an arc flash occur?
Arcing fault current can be easily created through poor electrical contact, failed
insulation and carelessness. There is no way to eliminate arc fault hazard when working
on an energized line, but we can decrease the amount of energy dissipated from an arc
and the injury it can create to a lineman.
8
What are the results from an arc flash and why is it such a great concern?
Injury is the primary reason we are focused on studying the system analysis of arc
flashes. Temperatures up to 35,000 degrees Fahrenheit are created at the point of contact
(4 times greater than the sun)! Upon contact molten metal is blasted forward from
terminal material resulting in serious burns to victims. A large shockwave is produced
that could throw a human off ladders or against walls. The light developed from a flash
contains UV rays that can cause blindness. The sound pressure from a blast is enough to
cause serious ear injury. If an arc occurs and a worker is in close proximity, the survival
rate of the worker is dependent upon the following:
Dependents of Survivability from Arc Flash
1. Clearing time- time of the over current protective device to operate. It
determines how fast the device will create an open in the circuit and clear arc
fault.
2. Magnitude-amount of arc fault current created.
3. Distance-length worker is away from point of contact (arc).
4. Personal Protective Equipment (PPE)-clothing material worker is wearing to
protect him or herself.
Units of Measurement
There are two different units in measuring an arc flash. A joule is a standard unit
of energy in general scientific application. 1 joule = 1 watt/second of dissipated energy.
A calorie is a unit of heat and is a form of kinetic energy. It takes 1 calorie of heat energy
to raise one gram of pure liquid water 1 degree Celsius.
1 calorie = 4.1868 joules
1 joule = 0.2388 calorie
9
Degree of Burns
1st degree burns: Affects outer layer of skin. It can be painful, but it is usually not
permanent or life threatening.
2nd degree burns: Causes tissue damage. Creates blistering to the skin. It also destroys
outer skin layer.
3rd degree burns: Causes complete destruction of skin. Small areas may be recovered
through skin graphs.
Arc flash protection is aimed to limit the injury to no more than a 2nd degree burn. Even
though this is the goal for a system study, it can still cause a serious burn. 1.2 cal/cm2 is
the threshold of 2nd degree burns.
Steps Necessary to Prepare for an Arc Flash Analysis
1. Review the existing data
a. Existing sectionalizing/fault current study
b. Substation size/impedance/voltage/grounding configurations
c. Source impedance of transformers
d. Breaker/recloser settings
2. Collect new data (per substation area)
a. Update source impedance
b. Update substation size/impedance
c. Update engineering software model
d. Locate large 3 phase pad mounted transformer locations
10
i.
Since pad mount transformers are in an enclosed area they
produce more energy to a lineman compared to an open aired
transformer where the energy is dissipated to open air.
e. Update all recloser/breaker settings, TCC opening/closing times etc
f. Fuse information: manufacturer, type, size/rating, speed
(standard/slow), and current limit of fuse.
g. Relays: Are they electronic or mechanical? Need type, manufacturer,
settings, current transformer rating, and opening/clearing times.
h. Transformer Data: kVA rating, primary/secondary voltage, impedance
percentage, connection configuration (Y-Y or Delta-Y)
i. Engineering model: Need all circuit configurations, wire size/section
lengths, operating voltages, and circuit configuration
3. Determine Personal Protective Equipment Settings (see above Arc-flash
Hazard Category table).
4. Conduct an Analysis Report:
a. Report should contain data collected from utility companies
including: Substation names, transformer sizes, primary/secondary
voltages, working distance, available bolted and arcing fault
current, incident energy level, and risk category for each scenario.
Checklist completed by utility company:
The following is a series of questions that a consulting firm is to ask utility
company. Questions answered are taken into consideration when performing arc flash
11
assessment. The checklist must be thorough and answered as accurate as possible to have
the most precise assessment.
CHECKLIST FOR
ARC FLASH STUDIES FOR
ELECTRIC DISTRIBUTION SYSTEMS
A. Current utility safety procedures and PPE clothing
1. Clothing,
a. What is the calorie rating of the shirt/pants typically worn by employees?
b. When do line workers use eye protection?
i) When do they wear safety glasses?
ii) When do they wear safety glasses and a face shield?
c. Hearing protection. When is it used?
d. When do line workers use special arc flash protection clothing beyond the
normal fire retardant clothing?
2. Hot line work
a. When working on an energized primary line, does the utility set the
substation or closest circuit reclosers/breaker to non-reclose?
b. What are the minimum pickup settings and what curves are used on the
recloser/breaker for the non-reclose operation?
c. Working distances
i) Rubber gloving, 17 inches to chest?
d. Hot sticks, 4 feet?
e. When is line work done energized with
i) Rubber gloves?
ii) Hot sticks?
f. Secondary work, when is the 277/480 or 480 volt work done while the 480
volts is energized:
i) Visual checking, no contacts?
ii) Voltage measurements?
12
iii) Making connections?
iv) Replace equipment?
v) Metering work, change meters?
B. Transmission line data
1. Voltage(s)
2. Source substation transformer base kVA and impedance
3. Conductor size(s)
4. Type of construction
5. Is work done while a transmission line is energized? If yes what type of
work?
a. Installing and removing grounds
b. Phase checking, voltage measurements
c. Inter-setting a new pole
d. Changing insulators
e. Replacing an existing pole
f. Others explain.
C. Substation
1. Primary voltage(s).
2. Secondary voltage(s)
3. Transformer MVA capacities and impedances.
4. Available fault currents
5. Type of transformer and secondary bus protection
a. Fuses (type, speed and size)
b. Relaying.
i) Type and settings,
ii) Differential, does it protect only the transformer or does it also protect
the secondary bus up to the load side of the circuit breakers?
6. Work practices within the substation while substation is energized.
a. Visual inspections, no electrical contacts.
b. Installing and removing grounds.
c. Primary switching, pull or replace primary fuses?
13
d. Low side: change arresters, circuit breaker by-pass, reclosers/breakers and
isolating switches?
e. Energized work practices.
i) Working distances from energized equipment.
ii) Current PPE, calorie rating, face and eye protection
iii) Are changes feasible?
D. Primary distribution lines, voltage(s) i.e. 12.47/7.2 kV or 24.9/14.4 kV?
1. Recloser types and settings. i.e. “L,” “VWVE,” “E”?
2. Primary fuse type and sizes used to protect laterals and underground risers.
3. Does the utility have a policy of setting the reclosers to non-reclose while
working on energized primaries?
a. Substation
b. Line reclosers
c. Alternate fast trip capability?
4. Voltage and phase checking with hot sticks?
5. Energized work procedures
a. Rubber glove?
b. Hot sticks?
c. When is insulated cover up used?
6. Pad-mounted primary switchgear including pad-mounted transformers.
a. Work practices, hot sticks for removing and connecting load break
elbows?
b. Opening and closing pad-mounted load break switches?
c. Working distances?
7. Distribution transformers
a. Pad-mount 277/480 volt.
i) Size(s)
ii) Impedance(s)
iii) If riser fuses protect pad-mount transformers, type & size?
iv) Do the transformers have bayonet fuses, if yes type and size.
b. Overhead three-phase 277/480 banks
14
i) Size(s)
ii) Impedances
iii) Fusing standards, type and size
8. Metering for 277/480 volt services.
a. Work energized or de-energized?
b. To check voltage or make amp measurements?
c. Replace meter.
d. Check CT ratio.
Conductor, type, size and length to meter.
How does an arc flash operate?
Jacob’s Ladder Test
Figure 2: Jacob's Ladder Test
http://sub.allaboutcircuits.com/images/quiz/00561x01.png
The Jacob’s Ladder is a device, which is used to show how electric sparks
operate. The figure above shows a Jacob’s ladder, more formally known as a high
voltage-traveling arc. This is a device designed to produce a continuous train of large
sparks, which rise upwards. The top of the figure shows two wires that start close
together and gradually diverge away from each other to form the “V” shape.
15
The high-voltage transformer creates a potential difference between the vertical
wires. The electrons repel from each other causing them to jump from one wire to try
and get as far apart as possible.
When high enough voltage is applied to the bottom of the wires a spark forms
across them, which quickly changes into an electric arc. Air breaks down around 30
kV/cm factoring in humidity, temperature etc. Besides the anode and cathode voltage
drops, the arc behaves as a short circuit, drawing as much current as the electrical power
supply can deliver, and the heavy load dramatically reduces the voltage across the gap.
Since hot air rises, the spark continues to climb until the voltage provided by the power
source is not enough. It then dies and a new spark starts at the bottom again. A spark can
jump between two conductors, which relative to each other can carry a high voltage. The
gases in the atmosphere are pulled apart, at an atomic level, by the high electric fields
generated between such conductors. The air can conduct, just like a wire, but with a few
side effects like light and sound (like thunder and lightning but on a much smaller scale).
Incident Energy Calculations (IEEE1584):
To determine the incident energy at the worker location:
•
Incident Energy = 4.184 x Cf x Ea[(t/0.2) x (610x/Dx)]
•
Log(Ea) = k1 + k2 + [1.081 x (log(Ia))] + 0.0011G
•
Ea = 10(Log Ea)
•
Ea is normalized for 0.2 sec. and 610 mm gap. For actual Incident energy………..
•
Incident Energy = 4.184 x Cf x Ea[(t/0.2) x (610x/Dx)]
Ia=Arcing fault current in kA
G = conductor gap in millimeters (mm)
K1=Open air or in a box factor
K2 = ground/ungrounded factor
16
Cf = calculation factor
t = arcing time in seconds
X = Distance exponent
Arc Flash Analysis Calculations:
The following illustration shows a Milsoft model being used to calculate the
incident energy that a lineman could be exposed to.
Figure 3: Existing System
The previous example shows the calculations for distribution transformers. The
substation is providing 12.47k volts to a distribution transformer. Here the 12.47k volts
are stepped down to 480 volts, which is supplied to a commercial building. A 30-amp
fuse is currently protecting the transformer since the current carrying capacity of
transformers is 23.2 amps. The bold black circle simulates a lineman working on the
secondary side of transformers. Power remains constant through a transformer. Since
the equation for power is P=volts x amps, if the voltage decreases the amperage must
increase providing more danger to the lineman. The calculated data box shows results of
17
what happens if a lineman creates a short circuit while doing maintenance. With a
distance of 431.8mm (17inches) between point of contact and worker’s face/body and
arcing fault current of 5.027k amps, there are 57.34 joules (13.704 cal/cm2) of energy
exposed to the lineman. The following chart shows the hazard category levels as a
function of incident energy. 13.704 cal/cm2 has a risk factor of 3.
Figure 4: Hazard Category as a Function of Incident Energy
Improvements/Recommendations
After reviewing the chart of “Hazard Category as a Function of Incident Energy”,
we can see that this provides a risk category of 3. If the goal is to try and maintain a
category of 2 or lower some recommendations are applied. The following illustration
shows transformers being protected by a 70-4H hydraulic recloser.
18
Figure 5: Recommended System
It can be seen that the working distance and arc fault current remain constant.
However, the incident energy and clearing time of reclosers have decreased substantially,
providing incident energy levels of 18.59 joules (4.4441cal/cm2). After reviewing the
chart we can see that this puts us at a risk category of 2. Reclosers act differently than
fuses in that they can operate on an instantaneous setting where as a fuse has melting
time. Reclosers are much more precise in reactance time and helps reduce incident
energy levels extremely.
Official Calculation Spreadsheet:
The appendix of this report on page 27 shows a blank calculation spreadsheet
used to insert the incident energy calculations along with necessary information. The
spreadsheet is designed to be a quick reference to locate what the incident energy
calculation is at a particular spot. The following is a brief description of what each
column heading represents 1-18 respectively.
19
1. Sub - Lists substation short circuit calculation is taken from.
2. Item of Interest – Indicates what piece of equipment the calculation is
taken from. Typically load side of transformer indicating size and
location on the system.
3. Dist. From Sub (Miles) – Distance calculation is taken from substation.
4. Protective Device – Lists type and size of protective device that is up-line
from calculation. Generally device will be fuse or recloser.
5. Phase – Generally listed as three phase. However if single phase, which
phase is being concentrated on, A B or C phase?
6. Primary Voltage – What is the source side voltage of transformer
7. Service Voltage – What is the load side voltage of transformer
8. Bolted Fault Current – Short circuit current rating creating shortest path
between two conductive materials.
9. Arc Fault Current – Arcing current formed from bolted fault current but
uses air as the conductor.
10. Gap (mm) – Distance between two terminals of transformer measured in
millimeters. Following table lists the typical distances.
11. Gap (in) – Distance between two terminals of transformer measured in
inches.
Typical bus
distance, in
Distance
Equipment class
mm
X-Factor
Open air
10-40
2.000
Low-V switchgear
32
1.473
15 kV switchgear
152
0.973
5 kV switchgear
104
0.973
Low-V MCCs and panel boards
25
1.641
Cable
13
2.000
Figure 6: Variable factors for different types of equipment
20
12. Distance Factor – Calculation factor for different scenarios. Previous
tables list these as the “x-factor”.
13. % of max – Use 85% of fault current when during calculations on
equipment that is protected by fuses. This is to give more of a worst-case
scenario. The lower the current the longer the fuse will take to clear,
since fuses are dependent on time.
14. Arc Clearing Time – time it takes for up-line protective device to clear
short circuit measured in seconds.
15. Oper. Dist (in) – distance from arc to workers face/body measured in
inches
16. J/cm2 – Incident energy calculated in joules per centimeter squared. This
is 4.168 times greater than calories per centimeter squared.
17. Cal/cm2- Incident energy calculated in calories per centimeter squared
18. NESC level – National Electric Safety Code category level to determine
what type of personal protective equipment to wear. Levels are based on
a 0-4 scale.
Arc Flash Boundary
A perimeter must be set up around an area where maintenance is being done on
energized equipment. The perimeter is measured as distance from energized part. Only
qualified personnel must cross this boundary. Unqualified personnel may not cross
boundary unless escorted by a qualified person. The same PPE is required in this
boundary as if direct contact is made with live part. This is done to maximize safety and
minimize possible arc flash.
21
Arc Flash Boundary Formula
Dc=[2.65 x MVAbf x t]1/2
•
Dc = Distance in feet of person from arc source based on limit of having incident
energy of 1.2 cal/cm2
•
MVAbf = Bolted fault current in Mega volts at location of maintenance.
•
T=time of arc exposure measured in seconds
Example: What is the flash protection boundary for the secondary side of a 3,750kVA
transformer rated at 480 volts line to line with short circuit current of 50,000 amps
and clearing time of 0.1 seconds (6 cycles)? The utility company provides this
information. 6 cycles is the standard instantaneous time of protective device to
operate.
MVAbf = [50,000amps x (1.74)1/3 x 480volts] = 41.57 MVAbf
Dc = [2.65 x 41.57 MVAbf x 0.1sec] ½ = 3.319 feet
Economic Tradeoff
As previously stated, it shows no question that a company might want to install a
recloser into their system to help the efficiency of arc flash hazards. However, is the
economic tradeoff worth the safety? The costs of fuses and reclosers, along with their
labor charges, are listed in the table below.
FUSE
RECLOSER
Material Cost(per phase) Material Cost(all three phases) Cost of Installation
$150
$450
$150
$1,500
$4,500
$450
Figure 7:Fuse vs Recloser Economic Cost
22
Total
$700
$4,950
Fuse vs. Recloser
These values do not include the cost to maintain these protective devices.
Reclosers have the capability to reclose on their own in the event a fault is cleared, such
as a tree branch falls on a power line. A fuse would stay open until a maintenance crew
came out to replace the fuse. Also, a recloser has the capability of being installed further
up line from the transfer and therefore is able to protect possibly 3 or 4 transformers. So
in this scenario we have one recloser protecting 4 transformers vs. 4 transformers being
protected by 4 different fuses. The initial cost of installing three reclosers (one recloser
per phase) with labor is approximately $5000. The cost of installing three fuses (one fuse
per phase) with labor is approximately $700. Since reclosers can reclose on their own, no
replacement or labor charges were affected after initial installation.
Figure 8: Fuse vs Recloser Payoff Chart
It can be seen that in order to have the recloser payoff the economic value of the
fuses, the fuses would have to blow and be replaced after 7 times. Utility companies
23
have to determine if this is feasible. If a company is looking to improve their system
through arc flash incidents, then yes, they might want to invest their money by installing
reclosers. If the company wants to invest their money in other areas of the industry, then
they may want to pass on this opportunity and replace the fuses.
Future Changes
Changes are currently being made to the National Electric Safety Code (NESC)
Rule 410 that effect arc flash assessments. Some future changes are as follows:
1. The original intent of Rule 410 was to limit the possibility of technicians or
the general public to contact two metallic surfaces that may be at different
voltage levels and thus receive an electric shock. It is appropriate that the
proper rule be presented as a mandatory “shall” requirement, and not as a
present “should requirement”.
2. “For work below 1000 volts, applicable rules required by this part and
engineering controls shall be utilized to limit exposure. In lieu of performing
an arc hazard analysis, clothing or a clothing system with a minimum
effective arc rating of 4 cal/cm2 shall be required to limit the likelihood of
ignition.
3. One except to the clothing rule is if the clothing that is required by arc flash
assessment produces additional and greater hazards than the possible exposure
to the heat energy of the electric arc, then clothing with an arc rating or arc
thermal performance value less than that required by the rule can be worn.
24
Conclusion:
An arc flash assessment for a power systems utility company can be a
complicated process. For recommendations, much more detailed analysis must be
provided that is out of the scope of this report. Conducting this report I have learned that
you cannot just jump into a project and focus only on the next step. As an example when
recommending reclosers, the decision can’t be made just on the arc flash assessment side,
but affects on the system as a whole need to be considered. Every protective device in
the system needs to coordinate with each other in relation to timing. You don’t want
protective devices upline clearing before protective devices down line do. This will
cause unnecessary outages and provide no purpose for the down line protective devices.
I also learned that although it may be easy to consider installing reclosers numerously
throughout the system, the economic tradeoff is not worth it.
A majority of the injuries that occur in power utility companies are not the result
of electrocution, but rather from an arc flash! This is a new problem to the electrical
engineering world but something that needs to be resolved or else more injuries will
occur. Results from improvements suggested by an arc flash analysis will not only save
lives, but also save a company from economic tragedy in the event of lawsuit or
equipment damage. Once my report is finally completed, I plan to provide a reliable
method to conduct an arc flash analysis; a benefit not only to myself, but also to the
power industry as a whole.
25
Appendix
26
Figure 9: Blank Calculation Spreadsheet
27
Budget Analysis
Software Budget:
1. Milsoft/Windmil
i. Initial Cost = $15,000 per license
ii. Annual Fee = $1,200 per license
2. ArcPro
i. One time fee = $1,500 per license
28
Definitions
Arch Flash Hazard- Dangerous condition associated with the release of energy caused by
an electric arc.
Arc Fault Current- Fault current flowing through electric arc plasma.
Available Fault Current- The electrical current that can be provided by the serving utility
and facility-owned electrical generating devices and large electric motors, considering the
amount of impedance in the current path.
Bolted fault current- Electrical contact between two conductors at different potential in
which the impedance or resistance between the conductors is essentially zero.
Arc Fault Current- Current that flows from a conductor to either ground or another
conductor when energized, due to an abnormal condition.
Flash Hazard Analysis- Method used to determine the risk of personal injury as a result
of exposure to incident energy released from an electrical arc flash.
Incident Energy- Amount of energy impressed on a surface, a certain distance from the
source, generated during an electrical arc event.
Shock Hazard- Dangerous condition associated with the possible release of energy
caused by contact or approach to energized parts.
Voltage (nominal) - Nominal value assigned to a circuit or system for the purpose of
conveniently identifying its voltage class.
Working Distance- Distance between possible arc points and the head and body of the
worker positioned in place to perform the assigned task
29
Arc Gap- Distance between two contacts where possible arc(s) can arise. When high
voltage is applied to the gap, an arc has the possibility of crossing between two
conductors.
Clearing Time- Amount of time it takes for an over current protection device to clear
once fault occurs and protective device settings are past their settings. An example is
below.
Figure 10: Time Current Curve
The previous graph shows a Time Current Curve (TCC) chart. It denotes the time
(located on “y” axis) in seconds and the current amount (located on “x” axis) in amperes.
30
The graph shows the curve settings of an 85 “T” fuse. Notice that as the fault current
increases, it takes less time for a fuse to clear. The bottom line of the black curve
indicates the time a fuse begins to melt. After a short amount of time, at same fault
current, the top line is reached. This line indicates the complete melting time of fuses for
which circuit is open and arc flash has diminished.
31
Recommendations for Project Continuation
Arc flash events are a serious hazard that can potentially put people in life
threatening situations. National Electric Safety Code and OSHA are in the process of
introducing changes to electrical safety rules that require electric utilities to perform arc
flash hazard analysis of all electric facilities operating at and above 1000 volts.
In 2008, EPRI started a supplemental project to research arc flash issues with over
20 utility members. This effort uncovered new areas where further research could
improve arc flash protection approaches. 2008 unveiled that there is still much concern
for arc flash when network systems were the main topic for worry when most utilities
were sought the need for future improvements.
Open-air medium voltage systems, after recent testing and analysis of monitoring
data revealed several insights on arc characteristics. One of the findings is that realistic
arc lengths under work-type scenarios are longer than the 2 to 6 inch lengths assumed in
the National Electric Safety Code table 410-1.
Better personal protective equipment is still needed and requires testing to help
understand the best assessment approaches and protection options for specific utility
equipment. As arc flash assessments can still be conducted and used as professional
studies, there are still improvements that can be incorporated to achieve improved arc
flash analyses.
32
References
T2G Technical Training Group “www.brainfiller.com” Jim Phillips, P.E.
ESC Engineering, Jerry Hager, P.E.
IEEE1584 Based Arc Flash Calculator and Warning Label Creator,
http://www.arcadvisor.com/arcflash/ieee1584.html
Electrical Solutions with Energy and Integrity “Mayer Electric,”
http://mayerelec.com/arc.htm
Arc Flash Information Resource Center: IEEE 1584 Arc Flash Calculations,
http://www.arcflash.me/ieee-1584-arc-flash-calculations.php
33
Acknowledgements
I would like to acknowledge the following individuals with their help in this
project:
Dr. George Collins
–
Professor Colorado State University, Electrical and Computer Engineering
–
http://www.engr.colostate.edu/ece/facultystaff/facultypage.cfm?pass=9
Gerald H. Hager
-
ESC engineering: Vice President, Senior Engineer P.E.
34
Download