Electrical protection in mines John Ainsworth

1. ELECTRICAL PROTECTION IN MINES
and HB 119
John Ainsworth
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2. Introduction
This presentation is about Electrical Protection of Power Systems
and Installations.
With particular reference to Mine Installations.
It refers to the forthcoming Australian Standards Handbook HB 119
“Mines and Quarries Electrical Protection”
It aims to give an understanding of some of the key underlying
principles, issues, and drivers relating to electrical protection, and to
provide a framework for study of the Standards, legislation and
documents which apply.
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3. Specialised Engineering Discipline
Education, Training and Qualifications
Protection is a specialised discipline and requires people qualified,
trained, and competent in the discipline to carry out the various
protection functions such as:
1. Protection planning
2. System analysis
3. Design and specification of relay systems
4. Circuit design
5. Relay settings
6. Testing
7. Installation
8. Commissioning.
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4. Key Principles and Requirements
1. A system of Primary protection is required.
It shall have complete coverage of the power system and
sufficient sensitivity to detect all faults, ie, no fault shall be
beyond its reach.
It must not have any blind – spots.
2. A system of Back-up protection is also required.
It also shall have complete coverage and adequate
sensitivity.
Expand on reasons for Back-up
Back-up protection shall be totally independent of Primary
protection and shall cater for failure of any component of
the scheme.
3. The principles apply al all levels of the power system.
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5. Protection and Safety
Protection is the first line of defence with respect to safety. Safety of both
staff and public is dependent on the protection system in 3 ways:
1. Dependability. The safety outcome for people in the vicinity of a fault will
be much worse if the protection fails to operate.
2. Coverage. All faults must be detected and cleared
3. Speed. Fast fault clearance will limit the damage to the human body from
arcing faults and can in many cases prevent splitting of equipment tanks,
prevent or lower the incidence of fire, and reduces the risk of
electrocution from step and touch voltages. There is a large difference in
safety outcomes between clearing times of, say, 0.1 s and 1.0 s.
The Australian work, Health, and Safety Act is relevant here. It requires that
anything that can be done to reduce or eliminate a hazard, shall be done,
unless defensible reasons are documented.
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7. Australian Standards Handbook HB 119
Mines and Quarries Electrical Protection
HB 119 is a forthcoming document to be published as an Australian
Standards handbook. The draft should be available for public
comment within a few months.
It has been prepared by a working group of committee EL 23.
The purpose is to provide a reference document in the nature of an
Application Guide for electrical protection in the mines and quarries
context.
And to provide a document that references, coordinates, and
integrates the various Australian Standards and other relevant
reference documents.
The draft will be available for free download on the Standards
Australia website in a few months time, as a public comment draft.
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8. AS 2067 – 2016 Substations and HV Installations.
AS 3000 – Wiring Rules LV
These two Australian Standards are mandatory and contain basic
requirements for protection. They are not the only standards and documents
containing protection requirements.
1. AS 3000 now applies to LV only – up to 1000 v
2. AS 2067 – 2016 applies to HV and has taken over this role from AS 3000
3. AS 2067 – 2016 contains a detailed Appendix F on protection matters
which is recent, up to date, and should be studied by all involved in
protection work.
4. AS 3000 throughout requires protection of all power circuits against fault
and also against overload, since overloading is a common cause of
failure.
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10. Australian National Electricity Rules
The National Electricity Rules is an important document, is
legislated, and applies to electricity service providers utilities, and
energy companies.
It contains requirements about protection.
In particular, a statement about Back-up protection is relevant:
Clause S 5.1.9 (c) states:
“ ….a Network Service Provider must provide sufficient primary
protection systems and Back-up protection systems (including Breaker
fail protection systems) to ensure that a fault of any fault type anywhere
on its transmission system or distribution system is automatically
disconnected …”
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11. AS/ NZS 4871 and Earth Fault Limitation
The Concept:
1. AS/NZS 4871 ‘Electrical Equipment for Mines’ Clause2.1.2 requires that power
supplies to mines and quarries use IT type earthing to limit earth fault currents to
very low magnitudes.
2. AS/NZS 3007 and AS/NZS 2081 provide further detail of the requirements.
3. The earth fault levels required are in the order of 5 A for systems up to 4 kV including
LV, 25 A for up to 12 kV, and 50 A for above 12 kV.
4. The standards require very sensitive earth fault protection on all circuits set to not
greater than 10 % of the earth fault level, with fast operating times, almost
instantaneous.
5. Neutral-earth impedance, usually resistors are connected from star-point to earth of
all source transformers.
6. Earthed screens are required between phases of equipment, particularly cables, end
boxes, plug connections, etc, to ensure that all faults will be earth faults.
7. The concept requires that all circuits are radials. It will not work in closed rings.
8. Refer HB 119 Sections 4.2 and 4.8
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12. AS/ NZS 4871 and Earth Fault Limitation
The Purpose.
9. To ensure that faults are low current, low energy levels to enable these
faults to be contained in flameproof enclosures and fittings to prevent
ignition of mine gas and dust. That is, to trip the circuit from the sensitive
earth fault protection before the earth fault can develop into a high energy
phase to phase fault.
10. To ensure that Step and Touch voltages are kept low in order to avoid
electrocution and to prevent ignition from sparking of earthed
conductors.
Note however that:
11. In some equipment such as transformers, motors, switchgear, it is not
possible to segregate the phases and that consequently the possibility of
high energy phase to phase faults still exists.
12. The very sensitive AS/NZS 4871 earth fault protection is an additional
protection. A full complement of ‘normal’ protection is still required.
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13. AS/ NZS 4871 and Earth Fault Limitation
Other phase voltages:
13. A consequence of using high impedance earthing is that when a fault on
one phase to earth occurs, that phase goes to near zero volts, the
transformer star-point goes up to phase to neutral volts to earth, and the
other two phases go up to near phase to phase volts with respect to
earth.
14. For example, in an 11 kV system all phases are normally at 6.3 kV to earth
but when a fault occurs on A phase, the voltage on B and C phases rises
to 11 kV to earth.
15. This requires better insulation on all phases of all equipment fed from
that source transformer, cables, OH line insulators, transformers,
switchgear, motors, etc. This is a significant cost and if it is not done, the
other phases are likely to fail.
16. Lightning arresters of higher voltage rating have to be used and this
results in a poorer protective level being provided
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14. AS/ NZS 4871 and Earth Fault Limitation
Cross-country faults.
17. When a phase to phase fault occurs on an impedance earthed system, the volts
on the other two phases rise to near full phase to phase volts.
18. This can trigger a phase to earth fault on another phase
19. The second fault is usually on another feeder, at another location, anywhere in
the impedance earthed network, and can be in different equipment.
20. Thus we have a fault between two phases but the fault current flows through
earth and is no longer limited to the low value by the NER at the source.
21. This negates the purpose of the IT earthing and the AS/NZS 4871 compliant
relaying.
22. It also causes multiple faults which are more difficult to find.
23. Describe the Sydney CBD 11 kV system as an example, and also the Singleton
66 kV system.
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15. Australian Work Health and Safety Act
1. The Australian WHS Act is enacted as individual State WHS Acts
which are copies of a Commonwealth Model Act. There are also
state based Mines WHS Acts. See also AS 5577.
2. The Acts require that all hazards be identified by designers,
owners, and operatives. They require that hazards be eliminated,
or if that is not possible, that they be dealt with by a hierarchy of
measures, and if a lesser measure is adopted, that the reasons
for this be justified and documented.
3. This places great responsibility to not overlook or discard a safer
measure on the basis of cost or inconvenience.
4. All this applies particularly to Protection because it is provided
specifically as the safety mechanism for when primary electrical
failures do occur. Coverage and speed of protection are relevant
here.
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17. Protection Battery Supplies
Protection systems and relays have to use sources of energy for their
operation and tripping which are independent of the power system AC
voltage because:
1. The AC system voltage is depressed or disturbed during power system
faults.
2. The protection has to be operational and ready before the AC system it
protects is energised.
3. Protection has to be operational throughout a supply interruption in
readiness for restoration.
The source of energy will generally be in the form of battery supplies. The DC
supply is a critical function and must always be available.
Note the initialising time of digital relays and devices.
The required DC voltage operating range of relays is problematic.
Refer Section 2.7 of HB 119.
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18. Components of a Protection Scheme
A protection scheme has several component items, each of which
must function correctly for the scheme to work. They are:
1. Relays
2. Circuit – breakers
3. CTs and VTs
4. Battery or equivalent supply
5. Wiring and cabling, including optical fibres
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19. Components of a Protection Scheme
Taking this further:
6. IEDs (intelligent electronic devices) performing protection related
functions, such as Merging Units, Multiplexers, Routers, Switches
7. Software and firmware
8. The relay settings
9. The concept used in the particular scheme.
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20. Back – up Protection Principles
Some points of principle about Back-up protection are:
1. Back-up is necessary because the relays, circuit-breakers, wiring, etc
are complicated and there are many things that can go wrong.
2. Back-up is required at all levels of the power system.
3. Back-up must have complete coverage
4. Back-up must cater for all fault types
5. Back-up must cater for the failure of any of the 5 or 6 components of
a protection scheme
6. Back-up must be fully independent of the Primary protection – no
common items or common failure modes
7. Back-up may be arranged by either the Remote Back-up (RBU) or
Local Back-up (LBU) concepts
8. Refer Section 7 in HB 119
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22. Protection Scheme Categories
The main categories of protection scheme are:
1. Overcurrent
2. Earth fault including Restricted EF and Sensitive EF
3. Differential -- Tx diff, Feeder diff, Busbar diff, Machine diff.
4. Directional schemes for overcurrent and earth fault
5. Distance schemes
6. Frame leakage
7. Pressure and flow sensing schemes such as Buchholz
8. Optical Arc-fault sensing schemes
9. Intertripping and protection communication schemes
10. Undervoltage release schemes
11. Transfer Trip schemes.
These individual schemes are almost invariably used in combinations.
Refer Sections 5 and 6 of HB 119.
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23. Digital / Numeric Relays
Some points about digital / numeric relays:
1. Good accuracy, providing ??
2. Require DC power supply – battery or equivalent
3. Lower cost than electro-mechanical equivalent
4. They have a significant start-up or initialising time
5. Versatile – many setting options and logic options in the one relay
6. Problematic to set because so many setting and logic options
7. Use RMS sensing relays (not average or peak sensing calibrated as fake RMS)
8. Reset rate has to be watched
9. Susceptible to surges and electrical interference
10. Short life of the electronics – 15 y at best, often much less
11. Self supervising – up to a point
12. Have many internal contacts – despite being called solid state
13. Software is problematic – not usually very robust, frequent fixes, software
control is a problem, hidden from the user.
14. Many additional functions available such as fault and event recording, Scada
interface.
15. Back-up considerations require care.
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24. Post Type CT and Bushing Failures
1. Porcelain housing Post Type CTs and bushings have a history of explosive
failure. The failure gives no warning and is very destructive and dangerous.
2. We are talking about paper insulated, oil filled, porcelain housing units
extensively used in the power industry from 33 kV upwards.
3. The failure mechanism is by partial discharge within the papers, leading to a
power arc under oil, creating immediate pressure and explosive bursting.
4. Regular oil sampling and DGA analysis can give an indication that something
is happening, but is not a reliable indicator of how much time before failure.
5. In the case of post type CTs the best approach is to get rid of them in favour of
Dead tank circuit-breakers with CTs over the bushings. See photo
6. In the case of bushings the best approach is to replace them with ERIP (epoxy
resin impregnated paper bushings) with foam filling rather than oil.
7. Refer HB 119 section 9.3.5
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27. Lightning and Overvoltage Protection
1. Lightning protection is a further part of the scene. It has to be co-
ordinated with the fault current type protection.
2. Refer HB 119 sections 8.9 and 8.10
3. Refer AS/NZS 1768 Lightning Protection
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28. Low Voltage Arcing Faults
1. LV faults on large circuits are almost always arcing faults and are very
destructive because of the large arc energy released. See Section 2.12 of
HB 119.
2. Protection for these faults is made more difficult by two factors:
• The current is restricted to 30 % or less of the bolted fault level by the
arc resistance (for 240 / 415 v circuits)
• The current is very erratic. The arc typically blows out in a second or
less and repeatedly re-strikes a second or two later. Even within each
burst of current, it varies dramatically from one cycle to the next.
3. This means that overcurrent protection of these circuits is unsatisfactory
and that much faster and more sensitive types of protection are required.
4. See oscillogram on next slide.
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30. THANK YOU
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