Trial Design - Chapter 7 - Energy and Earth Resources

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7 REFCL Trial design
The REFCL trial was the first research of its type anywhere in the world and it required extensive
custom design activity in all aspects: design of the basis experiment, design of the test rig, design of
the on-site facility, design of the overall test program and design of individual tests. Discussions with
international users of REFCLs and a review of technical literature failed to identify any suitable
standard templates for any of these and the REFCL Trial included extensive design activity (refer
Appendix E for greater detail).
Design of the basis experiment encompassed two separate considerations: selection of the fire
cause and fault geometry; and, definition of clear principles to guide all other design decisions.
7.1 Selection of fire cause and fault geometry
The REFCL tests were based on a ‘wire on ground’ powerline fire cause because it is known to cause
fires on high fire risk days and it better supports test rigour than alternatives.
7.1.1
Fire cause analysis: 2010 national survey
Conductor breakage is a major cause of powerline fires on high fire risk days. A national survey of
owners of rural electricity distribution networks by Nous Group in April 2010 indicated that
nationally, conductor breakage was the largest single cause of powerline fires on high fire risk days1.
Figure 1: 2010 national survey of powerline fire cause
Relative presence in top 5 fire causes
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Fire cause analysis: Victorian data 2004-2009
More detailed fire data for Victorian rural networks indicates that two causes, trees and
conductor/tie/connector failures, cause 42 per cent of powerline fires on total fire ban days2.
1
Separate data for extreme fire risk conditions (Code Red days) was not available for the 2010 survey, so data
for total fire ban days was used as a surrogate. Rural network owners in Victoria, South Australia and Tasmania
also provided data split by fire risk. The data was the response to the question ‘in order of frequency of
occurrence, what are the five main causes of fires started by your assets on TFB days?’
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Figure 2: Powerline fire causes Victorian TFB days five years to 2009
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As well as being a major cause of powerline fires in high fire risk conditions, the selected fire cause
for REFCL tests had to be capable of supporting adequate test rigour. The essential prerequisite for
rigour is repeatability, which is tightly linked to the capability to eliminate non-essential random
factors that could influence individual test outcomes.
7.1.3
Fire cause analysis: Victorian data 2011-2014
More recent data on powerline fire causes was provided by ESV from the regular reports by network
owners under the scheme set up following the Black Saturday fires. Selecting only fire data on Total
Fire Ban days, there were 134 ground fires associated with powerline assets over the period 2011 to
March 2014. External factors caused 39 powerline fires and asset failure caused 95 powerline fires.
The fire causes are shown in Figure 3 and Figure 4.
Figure 3: External causes of 39 ground fires associated with powerlines on TFB days - 2011 to March 2014
2
1
5
Vehicle (Contact, Struck Asset)
Animal (Bird, Possum, Bat, etc)
10
Trees (Fallen, Touch - Human or
Nature)
Other (Unknown, conductor clash,
etc)
Lightning
21
2
Analysis covered 155 Victorian powerline fires that occurred on 69 total fire ban days over the five years to
2009.
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Figure 4: Powerline asset failures that caused 95 ground fires on TFB days - 2011 to March 2014
1
10
23
Underground Cable or UG Assets
(Pillars, CHP, etc)
8
Pole (Exludes pole fire unless it causes a
ground fire)
Crossarm (Exludes crossarm fire unless
it causes a ground fire)
Fuse (Mostly EDOs - exludes hung
up/burnt unless it causes ground fire)
Conductor (Broken, Ties)
17
OH Cable (ABC or services)
Connection (HV or LV - Loose, etc, JBs,
FSDs, etc)
15
Other (Substations, SDs, ACRs, LV Eq't,
etc)
7
14
Review of this information confirmed ‘wire on ground’ faults are still a significant component of
Victoria’s rural powerline fire experience on Total Fire Ban days.
This more recent data is of much higher quality than previous sources. It shows that conductor
failure (this category also includes conductor tie failure which can allow a conductor to come away
from a supporting insulator) remains a significant element of the total range of powerline fire
causes. There is no data for Code Red days as no Code Red declarations have been made in recent
years. Hypotheses raised by some experts postulate a heightened presence of conductor failures on
Code Red day powerline fire causes due to higher mechanical stresses caused by associated high
wind speeds. However, there is not enough data to conclusively prove or disprove such hypotheses.
7.1.4
The 2011 Powerline Bushfire Safety Taskforce arc-ignition research
The PBST arc-ignition research in 2011 demonstrated the value of careful selection of powerline fire
cause and fault geometry to enable rigour in the derivation of quantitative ignition probability
results. For the 2011 tests, conductor breakage was selected as the powerline fire cause to be
studied. For repeatability, the fault configuration chosen was a live wire falling on an earthed
structure, then falling away and drawing an arc in the presence of fuel as it does so. This
configuration was selected from considerations of
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Worst case energy release into the environment at the fault location and
Repeatability (an actuator could precisely control the movement of one electrode while the
other remained fixed).
The primary question answered by the 2011 research was ‘is it within the capabilities of modern
powerline protection systems to detect and clear a conductor breakage fault fast enough to prevent
a fire?’ The test program successfully answered this question by producing precise quantitative data
on ignition probability for different fault clearance times.
7.1.5
Selection of powerline fire cause for the REFCL Trial
The selection of the powerline fire cause for the REFCL test program was based on the same
considerations as those used in the 2011 PBST tests: it must be one of the more frequent causes of
powerline fires on high fire risk days and it must enable rigour by supporting repeatability and
allowing the elimination of non-essential random factors.
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The two candidate powerline fire causes considered were:
1. ‘Wire on ground’: This is known to be a cause of powerline fires on high risk days. Though it
did not support repeatability as well as the configuration used in the 2011 tests as it
demands a larger fuel bed and control of soil conditions, it was still considered feasible
provided worst case conditions of low-height wind speed, soil type and moisture content
were defined (in addition to those already defined in the 2011 tests such as air temperature,
relative humidity, fuel moisture content, etc.).
2. ‘Wire into tree’: This is also known to be a cause of powerline fires on high risk days.
However, it poses severe challenges in repeatability due to a much higher number of
random factors. These include the vegetation species and form (trunk/branch/twigs/leaves)
at point of contact, pressure of wire on vegetation, height of contact above ground,
resistivity of vegetation (which in turn will depend on moisture content and whether it is
living or dead), thickness of bark at point of contact, etc.
Based on the above criteria, the selected powerline fire cause for REFCL tests was ‘wire on ground’.
The ‘wire into vegetation’ configuration is clearly of similarly high potential value, but the challenge
of achieving rigour and repeatability in tests was assessed as much greater.
7.1.6
Selection of fault geometry for the REFCL Trial
The REFCL Trial tests used a ‘side on’ impact geometry in which the conductor hits the ground flat
(rather than ‘end first’) and rebounds before settling on the ground.
There are basically two geometric forms in which a falling conductor can hit the ground:
1. ‘End first’: the severed end of the conductor digs into the ground, probably at an angle and
drags along the ground scratching a furrow. This is considered less likely because the lowest
point of the conductor is mid-span and no matter where in the span a break occurs, the
lowest point will generally reach the ground first. Since most conductor breaks occur near
poles, this is unlikely to be the severed end. This may not be the case where the span is
across a deep gully, but is generally true for most spans of non-SWER powerlines. Further, if
the conductor were to hit the ground ‘end first’, this geometry is likely to give a lower
energy release in a REFCL-protected network than a ‘side on’ impact. The REFCL will limit
current flow after the first few tens of milliseconds. In this short period, an ‘end first’ impact
is likely to result in some penetration of the ground by the conductor with the conductor
remaining in contact with the ground for a longer period than in a ‘side on’ impact. This
limits the possible length of the ‘arc thread3’ during the critical initial period and hence, the
energy release into surrounding vegetation.
2. ‘Side on’: a relatively flat section of conductor hits the ground along its length. Because
powerline conductor is relatively stiff, the initial contact will be spread along some length of
conductor. This was considered the more probable result of a conductor break. It was also
considered possible that in such a case there may even be an initial rebound clear of the
ground which may produce a longer ‘arc thread’ length, leading to a larger energy release
into surrounding vegetation. This behaviour was confirmed in the proof-of-concept ‘line
drop’ tests.
3
The 2011 arc-ignition tests revealed that released energy is the fault current times the arc voltage which in
turn is proportional to arc ‘thread’ length. The ‘conductor falling away’ configuration used in those tests
released the largest amount of energy into the local environment as it involved the possibility of a relatively
long arc thread length close to vegetation fuel. The falling conductor tests in the REFCL trial included
conductor rebound to ensure worst case energy release close to vegetation.
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Of the two alternatives, ‘side on’ impact was considered to be both the more likely and the worst
case for ignition and was selected as the fault geometry for the REFCL Trial.
7.2 Definition of underlying principles
From the start, all aspects of the REFCL trial were shaped by three guiding principles:
1. Direct comparison: The primary objective of the test program should be to measure ignition
probability in two series of tests that differ in only one variable – whether the REFCL is in
service or not. The vicissitudes of the test program meant that this principle was honoured
as often in the breach as the observance. For example, the challenge of managing variations
in soil resistivity over a period of three months sometimes prevented exact replication of
conditions from test series to test series. However, the goal of a direct comparison
continued to guide all activities throughout the project.
2. Realism: The conditions of the test should represent a realistic powerline fault and the
network response to that fault should also be realistic. For example, this principle led to the
adoption of the ‘sandpit’ solution to replace the shunt resistor. Compliance with this
principle was influenced by practicality constraints on test rig design, the nature and location
of the test site and the stringent requirements to protect the network and its connected
customers during test activity.
3. Worst case conditions: To ensure the results are relevant to catastrophic bushfires, the tests
should be carried out in conditions (airflow, air temperature/humidity, and fuel/soil
moisture/resistance) that represent ‘close to worst case’ fire risk days.
These principles were approved by the relevant governance bodies and were adopted by all involved
parties during the design and conduct of the tests.
7.3 Selection of conductor type, conductor impact speed and bounce height
Basic physics theory states that a conductor that falls freely from a five metre height will hit the
ground at a speed of ten metres per second. If it falls from a height of eight metres (estimated
minimum height under high customer load on a very hot day), it will hit the ground moving at 12
metres per second. At these speeds, some rebound could reasonably be expected.
A search of public domain information failed to locate any published video of powerline conductors
falling to the ground. To verify theoretical calculations and gain a better understanding of the
behaviour of conductors that fall to earth, a series of ‘line drop’ tests was carried out at the
Frankston test site using a temporary 50 metre span with conductors tensioned to about two kilonewtons.
Three typical rural powerline conductors were strung, cut with a pump action hand-cutter (both
mid-span and end-span cuts were used) and allowed to drop from a height of about nine metres.
Both normal speed and moderately high speed video records of the conductor falls were analysed.
The arrangements for the line drop tests are shown in Figure 5.
The three conductor types selected for the line drop tests were:
1. 3/12 steel – typical of SWER and remote single phase spur lines
2. 7/3.0AAC (All Aluminium Conductor) – typically used in rural areas with low load density
3. 19/3.25AAC – typical used for 22kV feeder backbones and longer distance three-phase rural
powerlines
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Figure 5: Arrangements for line drop tests February 2014
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The line drop video records revealed that:
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The calculated 12 metres per second speed of impact was correct within the limited
accuracy of the analysis of the video records.
Conductor types with only a single layer of strands (the 3/12 steel and 7/3.0AAC) tended to
untwist once tension was released and formed a coil shape in the air as they fell.
The 19/3.25AAC conductor with two counter-twisted layers of strands tended to fall flatter.
All conductors bounced to some extent, generally about 100 millimetres from the ground,
though this was not uniform along the fallen length with some parts rebounding while
others were in contact with the ground. This continued for a second or two until the
attached conductor hung vertically from the cross-arm.
After hitting the ground, the fallen conductor tended to slide towards the pole to which it
was still attached, at a speed of 1-2 metres per second. This stopped when the attached end
hung approximately vertically from the cross-arm.
Sometimes the falling conductors hit end-first and sometimes side-on. Neither impact type
predominated in the test outcomes.
After discussion of the line drop test results with technical experts and governance bodies, the
impact speed of 12 metres per second was confirmed as the standard for the ignition tests, with a
target bounce height of around 150 millimetres.
After the proof-of-concept test series showed that sliding conductors did not draw arcs through air
but appeared to conduct current directly into the soil, the complex task of replicating the postimpact ‘conductor slide’ in the test rig design was assessed as unwarranted.
The test rig was designed and equipped to perform tests using any of the three conductor types. The
great majority of tests were carried out using 19/3.25AAC conductor with some tests done using
3/12 steel conductor. No major difference was observed in ignition results between these two.
7.4 Selection of soil, fuel and soil/fuel bed geometry
In the proof-of-concept tests conducted in February 2014 at Ausgrid/TCA’s high voltage power
laboratory in Lane Cove West NSW, a range of different soils and fuel bed designs were tested in the
prototype test rig.
7.4.1
Selection of soil
Four soil options were tested in the proof-of-concept tests. The results indicated common
bricklayer’s sand was the most suitable soil for the main test series:
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Dry washed sand – exhibited extremely high resistivity, was assessed as unrepresentative of
real soils in most rural areas.
Sandy loam – showed similar resistivity to bricklayer’s sand and was seen as representative
of soils in many rural areas. Potential test-to-test uncertainties due to organic material
spread throughout the soil, and less predictable response to moisture than bricklayer’s sand
(some steam explosions occurred at higher currents).
Bricklayer’s sand (sand with approximately seven to ten per cent clay and 20 per cent fine
silt) – seen as representative of soils in many rural areas, showed consistent behaviour at
constant moisture content and consistent response (resistivity) to moisture content change.
Clay – exhibited anomalous response to high voltage, with arcs rapidly (almost
instantaneously) propagating across the soil surface to the nearest metal with no
penetration of the soil.
Based on these results, bricklayer’s sand was selected as having the best balanced scorecard on
representation of soils in rural areas and superior manageability in an experimental situation.
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7.4.2
Selection of fuel
Experience in the Powerline Bushfire Safety Taskforce 2011 arc-ignition tests was reviewed. It was
decided that the mix of fine grass and straw used in those tests was also the most appropriate fuel
for the REFCL Trial.
7.4.3
Soil/fuel bed design
The proof-of-concept tests used a galvanised steel half-cylinder as the soil receptacle for the tests.
Different sizes were trialled. After consideration of the test results, a radius of 145 millimetres and a
length of 400 millimetres were selected for the main test series. This reduced the risk of manual
handling injury for the test rig operator as some of the larger soil bed sizes weighed more than 40
kilograms.
The issue of fulgurite formation revealed by the proof-of-concept tests prompted redesign of the soil
bed container to minimise the chance of arc ‘punch-through’ to the metal sides. For the main test
series, plastic sided containers were used with a 100 millimetre wide, three millimetre thick copper
band along the bottom of the soil bed to collect the soil current. This successfully suppressed
formation of fulgurites in most tests. It meant the electric field within the soil surface layers was no
longer completely radial from the conductor. However, this requirement was seen as lower priority.
The finalised soil bed design is shown in Figure 6.
The management of soil moisture content evolved during the test program as the soil supplier
provided batches of differing moisture content depending on weather. Early batches were fully
made up and then dried. Later ones were progressively made up in layers and each layer was dried
before a new layer was added. Drying was done in a controlled atmosphere chamber. Accurate
control of soil moisture content remained a challenge throughout the program.
Figure 6: soil/fuel bed design – empty soil container, soil beds ready for fuel conditioning, soil bed ready for test
Eight bunches of dry fuel each weighing four grams were stood in the soil in a defined zig-zag pattern
along the centreline by pressing the centre of each bunch into the soil with a metal plate to a depth
of 30 millimetres. The fuel was conditioned immediately before the test by exposing it to 45°C air at
low humidity for not less than two hours.
7.5 Selection of worst case fire weather conditions and fuel moisture
content
The analysis performed for the Powerline Bushfire Safety Taskforce arc-ignition tests in 2011
demonstrated that worst case fire conditions included 45°C air at a relative humidity of less than 20
per cent with fuel moisture content at five per cent or less. These same conditions were targeted in
the REFCL Trial tests.
It was not always possible to fully achieve the worst case target temperature and humidity. Frequent
opening of the door on the test rig container and heat loss through the container walls and ceiling
occasionally made it difficult to maintain the internal temperature at 45°C in the cold wet site
conditions. Conditions in the fuel preparation container were held closer to the target levels and on
days when conditions in the test rig container were not quite at the target levels, each soil/fuel bed
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was put into the test rig just before the test was performed. This procedure minimised the effect of
any shortfall in achievement of worst case conditions. As the tests progressed, the test team formed
a view that provided fuel moisture content was close to the target five per cent, other factors were
less critical.
The 2011 PBST arc-ignition tests revealed that wind speed was a critical factor in production of
sustained ignition. In the 2011 tests, the arc-vegetation interaction was postulated to occur at a
height of 500 millimetres above ground and worst case wind speed was determined to be ten
kilometres per hour. For the REFCL Trial, low-height wind speed tests were carried out as research
had failed to find any published data on wind speeds at very low heights. The test rig setup for the
wind speed tests is shown in Figure 7 on the next page.
Measurements were taken for one hour at 1:00 pm on 20 February 2014 which was a relatively
windy day by Melbourne standards.
Air speed measurements were taken at heights of two metres, one metre and at 500, 250, and 60
millimetres. At wind speeds below about 1.8 kilometres per hour, anemometers stop turning, so a
technique to more accurately estimate minimum wind speed at 60 millimetre height was developed.
The minimum/maximum and average/maximum ratios derived from measurements taken at greater
heights (where wind speed was always sufficient to keep the anemometers turning) were applied to
the maximum wind speed recorded at 60 millimetres – which was also sufficient to keep the lowest
height anemometer turning.
This analysis yielded a minimum wind speed at 60 millimetres height of 1.0 kilometre per hour. Using
standard wind speed versus height models, wind speed data for two metres height was confirmed
against the Bureau of Meteorology wind speed data taken at standard BOM measurement height of
ten metres at Moorabbin Airport some kilometres to the South. This was 37 kilometres per hour,
about half the speed recorded there on Black Saturday.
By this logic, the minimum wind speed on Black Saturday at a height of 60 millimetres was likely to
have been two kilometres per hour.
The 2011 arc-ignition tests had demonstrated the difficulty of achieving consistent uniform airflow
at low wind speeds. The possibility was also considered of sheltered pockets with lower wind speed
than the open slope on which the tests were carried out. After consideration of options, the decision
was made to conservatively adopt zero wind speed as the worst case for the REFCL Trial tests.
7.6 Design of test facility
Design of the on-site test facility encompassed site selection and layout, safety architecture, site
infrastructure, high voltage supply, test rig, control and protection systems and data acquisition
system.
7.6.1
Site selection
The field test facility was built on a vacant portion of the Frankston Terminal Station site belonging
to SP AusNet. The selected location was 7.8 route-kilometres from the Frankston South zone
substation along feeder FSH21, as shown in Figure 8 on page 107.
The Frankston South high voltage distribution network totals 179 route-kilometres of overhead and
36 kilometres of underground high voltage powerlines supplying around 24,000 connected
customers. The zone substation is fitted with an eight ohm NER and a GFN which normally tunes its
coil to a position around 144 amps, implying total network capacitance to earth is 36 microfarads or
12 microfarads per phase.
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Figure 7: low height wind speed tests Mulgrave February 2014 (wide view is facing into the prevailing wind)
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Figure 8: test site location
Test site
Zone
substation
Feeder FSH 21 route
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7.6.2
Site concept and construction
The overall site concept is illustrated in Figure 9.
Figure 9: Field test site - high level concept
HV supply
HV supply space
HV supply
Ignition test space
Fuel/soil prep
Optical cables (full electrical isolation)
Data collection & control
space
Includes:
•
HV supply to site (3 phase)
•
HV isolation and earthing (visible to site workers) controlled by optical cable
•
Visible and audible warning of HV presence on site
•
Earthing to limit step and touch to safe levels on site
•
HV current limiting resistor (200 amp max)
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ACR with full protection suite controlled by optical cable
•
Local protection graded with sub FSH protection to preserve supply reliability
Includes:
•
Safety interlock on physical access
•
Climate controlled ignition test space (notional 1.5m cube)
•
Controlled air speed and temperature
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Swappable soil trays with vegetation layer
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Live wire drop mechanism to achieve up to 10m/s impact
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Fire extinguisher
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3 phase wide-band voltage divider and digital transducers
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Wideband digital current transducer
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SD video camera
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Optical transmitters for cables to data collection space
Includes:
•
Independent LV power supply
•
PLC controller to automate test run sequence
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Optical cable controls on HV switches and isolator/earth switch
•
Optical receivers for cables from ignition test space
•
8 track digital data collection sampling at >10kHz
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Video record collection
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Test controller and data controller desks
•
RP and LR desks
01 October 2013
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The test site as built is shown in Figure 10. It was commissioned on 8th April 2014 and its detailed design aligned very closely with the concept shown above
which was developed six months earlier.
Figure 10: the Frankston field test site
The pole-mounted equipment shown on the right is the high voltage supply facility (see 7.6.4 below). The two grey cabinets are high voltage resistors and
the two green shipping containers house the test rig and the fuel preparation space. The high voltage connections between the resistors and the test rig
were via two single core high voltage cables lying on the ground. The Control Hut is the portable building on the left. Separate portable diesel generators
(white boxes on black skid bases) supplied power to the test rig and fuel preparation containers and the Control Hut. The solar power supply shown in the
right foreground kept the batteries in the pole-mounted switches charged when the site was de-energised. The orange poles mark two corners of the
underground earth grid into which the earth fault current was injected.
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7.6.3
Safety architecture
The test site had demanding safety requirements as it was designed to stage multiple earth faults
with test personnel on site. The test program of 259 tests was completed over a three month period
without any injuries. Incidents such as cross-country faults were managed without any risk to on-site
personnel.
The first step was to establish a low resistance earth grid to take the injected fault current without
excessive local earth voltage rise. Soil resistivity at the site was very high, so this was a challenge.
The earth grid was designed to provide four ohms resistance and achieved 2.5 ohms. A lower level of
earth resistance could not be achieved at reasonable cost. This meant the worst case earth voltage
rise inside the site boundary was some kilovolts and the test facility was designed to ensure safety in
this situation.
The safety architecture for the site was centred on the following key principles:
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Tests must create no safety risks to people beyond the site boundaries.
Minimum number of persons on site during test activity.
Control of the location of all persons on site during test activity.
Provision of an electrically isolated equipotential space (the Control Hut) as a safe location
for on-site personnel during tests, i.e. whenever high voltage current could flow to earth.
Clear segregation of high voltage areas (with physical barriers and requiring formal access
procedures) from other areas of the site.
Development and agreement on detailed work procedures for all test activities to meet
Green Book requirements.
The test site perimeter was securely fenced and the only access gates were locked whenever the
test program was active. Site access was managed by the Test Controller who ensured all onsite
personnel were inside the Control Hut prior to removal of safety earths on the high voltage supply
and energisation of the test rig prior to a test. Test personnel were only permitted to leave the
Control Hut after the high voltage supply was again isolated and earthed.
The control hut was galvanically isolated from everything else on the site. It provided an
equipotential space for safe location of the test team during tests. To achieve this isolation,
communication and control pathways between the Control Hut and other areas of the site were
limited to seven fibre optic cables (four measurement transducers, the multi-channel SEL2505
control relay and two high speed cameras), two carbon dioxide pneumatic lines (slot cover actuator
and fire extinguisher) and three radio links (for live monitoring cameras of the internal spaces within
the containers). The Control Hut had its own dedicated diesel generator isolated from other low
voltage supplies on the site.
The Test Controller managed access to all areas of the site:
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The Test Controller had direct line of sight into both containers whenever the container
doors were open. When doors had to remain closed, e.g. to maintain atmospheric
conditions within, surveillance cameras in each container allowed the Test Controller to
monitor activity within them.
Both the test rig container and fuel conditioning container could only be accessed when the
high voltage supply switch was open, the high voltage supply earthed and the
electromagnetic door locks unlocked, as confirmed by a green ‘safe’ status beacon above
the container doors. The left hand door of each container was padlocked during tests and
the right hand door interlocked with the site safety systems. High voltage supply could not
be connected unless doors on both containers returned a ‘locked’ status signal.
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The high voltage apparatus space and the ignition test space inside the test rig container
were isolated from other spaces by transparent, polyacrylate barriers. An Electrical Access
Permit (EAP) was required to remove these barriers to gain access to the high voltage
apparatus space.
The ignition test space could be isolated from the high voltage apparatus space by an
earthed metal pneumatically operated slot cover and from the fuel conditioning container
by an earthed metal hatch. A mechanical interlock extended between the two containers
ensuring that access into the ignition test space via the hatch (e.g. to change the soil bed
between tests) was only possible when the slot cover was closed.
An EAP was not required to access the ignition space provided the rig’s conductor arm was
raised and the slot cover was closed and locked in place by the interlock in the fuel
conditioning container.
The first person to access either container after a test was required to wear an oxygen
monitor. Prior to entering either container following a positive ignition test, an externally
operated exhaust fan was used to purge any smoke, carbon dioxide or other potentially
harmful gases from the ignition space.
Access to the fuel conditioning container did not require an EAP provided that either the
metal hatch remained closed or that the slot cover in the test rig container was closed.
Access to the camera space in the test rig container did not require an EAP.
All on-site personnel signed on to relevant JSA and SWMS documents each morning. Site safety was
discussed by the test team each morning prior to the start of testing. Any guests present during tests
were fully briefed upon arrival and given a site safety induction prior to test commencement.
7.6.4
High voltage supply
The high voltage arrangements for the site are shown in Figure 11 on the next page.
High voltage supply to the test site was from feeder FSH21 located on McClelland Drive outside the
eastern site fence. A fifty metre three-core high voltage cable brought supply onto the site under
three other powerlines. A manually operated gas switch on the cable riser pole on McClelland Drive
was used to isolate the site from the network for periods of decommissioned status.
On the site, the high voltage supply was strung in four six metre spans between five temporarily
erected poles:
Pole 1: Incoming riser for cable connected to feeder FSH21
Pole 2: ACR with full feeder protection suite to protect FSH network from test activities
Pole 3: Three phase capacitive voltage divider for voltage measurements
Pole 4: Remote controlled three phase gas switches to switch supply to the high voltage
resistors and to earth the supply to the resistors (and hence, the test rig)
Pole 5: Supporting a short low slung span to allow the selection of a single phase to supply
the test rig via the high voltage resistor.
A solar power supply provided 230 volt power to charge ACR and gas switch batteries when the site
was de-energised.
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Figure 11: single line diagram of high voltage supply to the test site
The two high voltage resistors were designed and manufactured by Fortress Resistors to be
adjustable over a wide range. The internal construction is illustrated in Figure 12.
Figure 12: internal construction of high voltage resistors (left: 100-400 ohms, right: 400-15,200 ohms)
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7.6.5
Test rig
The test rig and fuel conditioning facility were housed in two heavily modified 40 foot shipping
containers. A schematic of the overall arrangement is shown in Figure 13.
Figure 13: Test rig - high level overview
Soil/fuel bed
earthed with
welding cable, rolls
on rails for changeover
Overall
layout
Barriers
Barrier with slot
opening for arm
HV cable entry
Rig actuator and controls
High voltage apparatus space
Climate
control space
Ignition test
space
Camera space
Camera
space
HV bus (insulated off roof)
Sandpit
Earthed slot cover
Metal rails earthed
each end
Metal hatch
Soil/fuel preparation and storage space
Fuel conditioning shelves
Soil/fuel bed
earthed with
welding cable
Side view of
arm operation:
Barrier
with slot
Conductor
Arm in
‘cocked’
position
HV insulation
The test rig container was divided into a camera space, climate control space, ignition test space and
high voltage apparatus space segregated by removable, transparent barriers. Two ducts connected
the two containers, one to enable soil beds to be changed between tests and the other to allow air
circulation between the two containers.
Conductor-soil impact was provided by a rotating lever arm holding a replaceable length of
conductor strung under an aluminium bow (Figure 14 and Figure 15). An electrically insulating glass
fibre reinforced rod connected the pivoting hub of the mechanism to a servo motor driven linear
actuator which provided controlled rotation of the hub.
A programmable logic controller was used to operate the test rig mechanism with remarkable
reproducibility; control systems confirmed the time delay between sending the initiate signal from
the Control Hut and recording the initial soil current was 2,865 ± 5 milliseconds test-to-test and ± 20
milliseconds over the entire test program.
Conductor velocity just before impact was measured from high speed video records. These
measurements were used to tune rig performance and verify the desired impact speed of 12 metres
per second were achieved. A hydraulic dampener was used to limit conductor rebound to
approximately 150 mm, however there was some variability in rebound height between tests, arising
from slight variations in test bed height, soil compliance and conductor tension, the latter resulting
from deformation of the aluminium lever arm and conductor during repeated impact tests.
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Figure 14: test rig - open view (original arm design, sandpit not installed)
Figure 15: test rig hub detail showing shunt resistor/sandpit current switch (final arm design) and actuator rod
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Figure 16 shows the test rig during ignition tests with cameras in place and the climate control duct
which slowly diffused hot air into the bottom of the ignition test space.
Figure 16: test rig with cameras and high intensity lights in position for a test
The test rig high voltage supply could be configured within the high voltage apparatus space to
enable either of the two high voltage resistors to be connected in series with the supply from the
22kV feeder, or in a shunt arrangement connected in parallel with the rig, or disconnected entirely.
Similarly, the ‘sandpit’ could be connected or disconnected.
Power to the two containers was supplied by a 100 kVA diesel generator. The generator, as well as
both containers, all metal fittings, the soil bed or ‘bolted fault’ circuit, and the shunt circuit (‘sandpit’
or resistor) were connected via copper earth bars running along the internal walls of each container
and earthed to the local earth grid at a single location. The earth bars in each container were
connected using insulated cable passed through the two connecting ducts.
Further test rig setup details and experimental conditions are outlined in Appendix E.
7.6.6
Simulation of additional conductor length in ignition tests
The response of the network earthing arrangement (NER, ASC or GFN) to an earth fault will depend
to an extent on the magnitude of the fault current. The strength of this dependence varies as shown
in Error! Reference source not found.:
Table 1: How fault current level affects response to fault
Network
Influence of fault current on network response to fault
earthing
NER
Slight except with fault current above 300-500 amps.
ASC
Higher fault current leads to more complete and faster collapse of voltage on fallen
conductor.
GFN
Once fault is detected, the same response always occurs (RCC activation). If the fault is
not detected, the response is the same as that of an ASC.
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The response of the NER to faults was relatively unaffected by the fact that the soil current was less
than the equivalent fault current. However in ASC and GFN ignition tests, it was important to
produce a realistic fault current, even if not at precisely the level of current prescribed by Error!
Reference source not found.. This ensured the response of the network in REFCL ignition tests was
realistic.
To do this, a second current path was established in the test cell. The current in this path bypassed
the ignition test rig and had the effect of increasing the fault current in the test while leaving the soil
current unchanged. Using a switch attached to the test rig arm, commencement of the current flow
in this second path was synchronised with the instant the test rig conductor struck the soil bed and
current flow was continuous from that time until the end of the test.
This second current path was provided to simulate an additional length of conductor on the ground
in a real ‘wire on ground’ fault to complement the limited (400 millimetres) length of conductor
impacting the soil bed in ignition tests.
In early tests, the second current path was provided by a shunt resistor of either 600 ohms or 1600
ohms, i.e. current in the second path at the instant of the fault was either 21 amps or eight amps.
However, it was found that this arrangement did not accurately reproduce the non-linear resistance
characteristic of the conductor-soil interface which occurred in the test rig. It was observed in some
tests using the 600 ohm shunt resistor that the residual soil current in the test rig disappeared
entirely while some current continued in the shunt resistor.
To achieve a more realistic simulation of additional length of fallen conductor, from the start of
Tranche 3 the second current path was a 12.5 metre coil of conductor resting on a ‘sandpit’ of soil in
the back of the test cell container as shown in Figure 17. The ‘sandpit’ contained soil of the same
composition (but not necessarily the same moisture content) as that used in the test rig soil beds.
In NER tests, the ‘sandpit’ consistently drew an additional 2.9 amps at the instant of the fault,
settling to 2.5 amps for the remaining test duration. In REFCL tests, sandpit current fell as conductor
voltage collapsed. The additional current was sufficient to ensure the GFN detected the test as a
fault even when the soil bed current was lower than the GFN fault detection limit of one amp.
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Figure 17: 'sandpit' with 12.5 metres of conductor on soil
7.6.7
Control and protection systems
The ACR mounted on Pole 2 (see 7.6.4 above) provided the primary protection of the network from
activities on the test site. It also provided protection for the test site. It was set to trip (disconnect
the site from the network) if the test fault current exceeded five amps for more than two seconds.
The two gas switches that controlled the high voltage supply to the test rig container via the
resistors were controlled by a SEL351S feeder protection system. The protection features of this
device were not used. It provided a convenient set of manual controls and interlock programming
functions that were relied upon extensively during the tests. Multiple back-up timers were
implemented to trip the ACR should any test suffer a control failure that extended the fault duration
to the point where there was a risk that the zone substation GFN might trip the feeder.
When all conditions for a test run were ready, the Test Controller enabled PC control which caused
the SEL351S to cede control of the test rig’s high voltage supply to the Control PC which controlled
the sequence of events during a test to precise timing standards. The Control PC was a laptop
running Labview software connected to a National Instruments USB chassis with an eight channel
high speed digital input/output (I/O) module. Four output channels were connected to the digital I/O
module as listed in Table 2.
Table 2: Control PC channels
Channel Name
Function
Cell open
Open the test rig high voltage supply switch
Cell close
Close the test rig high voltage supply switch
DAQ
Signal to the GEN3i data acquisition system to enable or initiate recording
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Fault
Trigger test rig to cause conductor to impact the soil bed
Each channel was directly linked to its own dedicated hardware counter, so pulses were sent at set
times with better than one millisecond precision. A LabView program allowed the test operator to
customise the test sequence (order and timing of events) and run the test from a simple graphical
user interface, shown in Figure 18. A variation of this program was also created for ‘bolted fault’
tests.
Figure 18: Control PC user interface to control and run tests
7.6.8
Data acquisition systems
A HBM GEN3i data acquisition (DAQ) mainframe running ‘Perception’ software was used, both to
view in real-time and to record the electrical signals associated with tests. Each signal was measured
by a transducer connected to a portable isolating digitiser linked via fibre optic cable to the GEN3i
system in the Control Hut. Figure 19 shows (from left to right) the GEN3i mainframe, a monitor
showing an image from one of the container surveillance cameras, the rig control PC, the high
voltage supply control box incorporating the SEL351S digital controller.
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Figure 19: Control Hut interior
The DAQ system had the capacity to record four high speed channels. In the majority of tests three
voltage channels and one current channel were recording at one million samples per second with 14
bit (0.006 per cent) accuracy. Record duration varied from 500 milliseconds to 60 seconds depending
on the test being performed, with a pre-trigger record of 60 milliseconds to three seconds.
Phase-to-ground voltage measurements were taken directly from the three phases of the high
voltage supply to the site. Capacitive voltage dividers (CVDs) stepped the voltage down by a factor of
approximately 2100 (from 12,700 volts to approximately six volts) for input into the isolating
digitisers. Four stages of over-voltage protection were provided by the various devices in the voltage
measurement chain.
Water ingress into the CVD high voltage coupling capacitors and their low voltage coax cables was a
challenge throughout the test program. For a few tests, when the test schedule did not allow time to
repair a water-affected device, a temporary calibration factor was calculated for it based on the
assumption that the three phase voltages in NER tests were perfectly balanced. In these
circumstances, the DC offset produced by electrolytic action from water ingress was also calculated
and removed before the more complex stages of processing of the test records.
The soil current, or bolted fault current depending on the test configuration, was measured in the
test rig container using a wideband coaxial 0.1 ohm current shunt. In certain test configurations an
additional record was taken of the current in the shunt resistor or ‘sand pit’ paths in parallel with the
test rig, using a Rogowski coil. The four channel limit of the GEN3i DAQ system meant that in these
instances voltage could only be recorded on two phases, so neutral displacement could not be
calculated. The Rogowski coil and current shunt were connected to an isolating digitiser linked to
the GEN3i DAQ system. The Rogowski coil and a non-wideband shunt were trialled as primary
current transducers. However, the coaxial shunt was selected for its superior performance at the
high and low extremes of the current frequency spectrum, both of which were of interest in tests.
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Figure 20: Primary voltage and current transducers
22kV coupling capacitor
0.1 ohm wideband coaxial shunt
Rogowski coil
All transducers were subject to laboratory calibration before the test program. Calibration factors
were input into the GEN3i perception software in order to view and record data in engineering units.
Overall accuracy was of the order of 0.1 per cent in the CVD and coax shunt channels and slightly
less if the Rogowski coil was used.
During the main ignition tests series, GEN3i records were imported into Matlab for immediate
processing on site. Matlab scripts produced charts of 20 millisecond rolling averages and ten
millisecond rolling RMS values of each phase voltage, soil current and neutral displacement. These
helped the test team interpret test outcomes and make decisions on parameters for upcoming tests.
Multiple cameras were used extensively to capture arc and flame behaviour and rig operation. Two
high speed camera systems (Photron FastCam optimised for arcs and a NAC optimised for flame)
were used by MACS Images to capture high definition video at rates up to 4,000 frames per second,
both for immediate playback to investigate issues and to guide progress of the test plan. All videos
were recorded for later analysis and reference. A normal speed camera was used as the reference
record of ignition results from the tests. More than four terabytes of video data was recorded in the
test program. Video records were constantly referred to in the course of the tests and proved
invaluable in investigations of unforeseen effects as well as in measurement of ‘time to ignite’ data.
7.7 Test procedure
Design of test procedures encompassed not only the procedure for a single test run, but also
procedures for access to different parts of the facility as required to safely maintain and adjust its
functions. Documented procedures listed in Table 3 were developed and reviewed with safety
experts in the host network owner organisation.
Table 3: documented test site procedures




Start of day – site preparation
EAP access to test rig high voltage apparatus
Non-EAP access to fuel preparation container
Non-EAP access to CVD termination box




End of day – site closure and security
EAP access to high voltage resistor cabinets
Non-EAP access to test rig container
Conduct of test run
As an example, procedure for a typical ignition test run is outlined in 7.7.1:
7.7.1
Test run procedure
Purpose:
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

Safe conduct of tests
Compliance with test plans and conditions


Time-efficiency and fast test cycles
Repeatability of test outcomes.
Expected frequency: Approximately 200 times over 15-20 days of tests, up to 40 times per day.
Responsible person: Trevor Dixon (Test Controller) without delegation
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
Confirm site gate is locked
Confirm all persons on site are inside Control Hut and advised to remain inside during test
Confirm via video no persons are in test cell container or fuel preparation container
Confirm container doors are closed
Confirm via video that hatch is closed
Open slot cover and confirm via video it is open
Check PC control program and on-screen settings are correct for the test
Unlock Master Control Switch and set to ‘enable’
Lock container doors and confirm they return ‘locked’ status
Open test rig high voltage supply earth switch
Enable PC-control (for ignition tests, first close test rig high voltage supply switch)
Confirm video and DAQ systems are armed and ready for test
Run test sequence via PC-control:
a. Click on ‘Arm’
b. Click on ‘Initiate Fault’
At completion of test duration, disable PC Control
Confirm test cell HV supply switch is open (if not, open it)
Close test cell earth switch
Set master Control Switch to ‘disable’ and lock it
If fire is burning, ask Test Rig Operator to extinguish with CO2
Advise team that test run is complete and they can leave Control Hut and must advise Test
Controller if they wish to approach any apparatus
7.8 Test settings
Settings were documented for each test series and constantly refined and adjusted as new
information emerged. Typical ignition test settings are shown in Table 4.
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Table 4: typical ignition test settings
Test:
NER
GFN
ASC
Fault-confirm
Test conditions
Conductor
Soil/fuel
19/3.25AAC impacting soil bed at 12 m/s
Fresh soil/fuel bed each test: bricklayers’ sand, grass/hay at 5%MC
Environment
≥45°C air, ≤20% RH, zero wind speed
HV supply settings
Series R
100Ω
0Ω
100Ω
100Ω
Shunt R
Disconnect at rig
5000Ω
Current duration
800ms
500ms
2300ms
1300ms
6500ms
Cell open
3655ms 3355ms
5155ms
4155ms
9355ms
Backup timer 1
1500ms
3000ms
1800ms
6700ms
Backup timer 2
4000ms
5500ms
4600ms
9500ms
Most with and a few without 5000Ω
DAC system settings
Voltage span
80 kV
Current span
500 A
Voltage TUM
Calibration data, nominally 2000 v/v, approximately 2100 v/v
Current TUM
10 A/v
Offsets (all)
Zero
Pre-trigger time
Total time
100 ms
1000 ms
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3000 ms
3000 ms
8000 ms
Monday, 4 August 2014
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