HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR THE SUPPLY OF
DC TRACTION RAILWAYS
David Stuart-Smith
BE (Elec) MEM CPEng FIEAust
Arup, NSW Australia
SUMMARY
For an electric railway the loss of traction power supply can instantaneously paralyse part or all of the rail
system with the impacts being felt a long way from the site of the incident that caused the power failure. In
addition to operational disruption, prolonged loss of traction supply can give rise to adverse consequences
such as passengers being stranded in trains without functioning air-conditioning which can be quite serious
on hot days.
This paper discusses the advantages and disadvantages of various high voltage network connection
topologies and busbar configurations for the supply of railway DC traction systems.
The outcomes for the traction system are a function of both the configuration of the rail system, and the
characteristics of the electricity host network. A sub-transmission network dedicated to the railway and
which connects to multiple supply points will usually improve security of supply. However, ultimately the upstream networks will be interconnected and so are vulnerable to common-mode failures which must be
considered. While operating such a network can provide a great improvement in security of supply and
flexibility for maintenance, there is also significant investment required and the whole-of-life integrated
logistics support implications of having an in-house network are significant. These issues are explored in
the first part of this paper along with appropriate incident response and back-up strategies.
The second part of the paper covers possible busbar configuration options appropriate to different voltage
levels and switchgear technologies. Discussion includes the impact of the maintenance requirements of
different technologies on the number of isolators and circuit breakers required. Examples are drawn from
the author’s practical experience in Sydney, Melbourne and Kuala Lumpur, and the paper discusses the
features of examples that are relevant to benchmarking future proposals.
The paper demonstrates that different configurations are appropriate to different circumstances and gives
practical examples.
INTRODUCTION
While it is the nature of railways that the
operational impact of a failure in one part of the
network is soon felt at distant points, few types of
infrastructure failure can have such instantaneous
and widespread impact as the interruption of high
voltage bulk power supply.
Railways with DC traction systems typically have a
traction system voltage between 600 volts and
3000 volts DC and the traction substations are
spaced 2 to 15 kilometres apart with capacities
ranging from less than 1 MW to 10 MW. Electric
railways must draw their bulk power supply from
the electricity grid, and in the case of DC railways
power levels dictate connection at high voltage
distribution or sub-transmission level. Even in the
rare enduring case of a railway with its own
generation capacity the generation will still be
centralised and a high voltage network used to
distribute the power to traction substations.
Figure 1 – Detrained Passengers
(Reproduced with the permission of Cassandra Hill)
The actual voltage level of the connection is
usually dictated by the network voltages used in
the area in question or by other historic factors. 11
or 22kV is most suitable at the power levels
applicable to light rail while 33kV is most suited to
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HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
heavy rail applications. However, over longer
distances the smaller conductors of a 66kV system
can be advantageous and if the host network is
operating at 66kV in that area then it might be
appropriate to adopt this voltage rather than
installing system transformers to provide a 33kV
supply.
The decision by the rail operators to invest in high
voltage network infrastructure, or not, will be
influenced by a number of factors including:
the insulating medium allowing for reduced
clearances, compact footprint, and minimal
maintenance.
High voltage distribution - Electricity network
infrastructure, usually operating at between 5kV
and 33kV and which primarily supplies distribution
substations which feed the low voltage reticulation
systems. In some geographies these voltage
levels are referred to as “medium voltage”.
•
The reliability of the available high voltage
host network;
HV Parallel – The parallel connection of high
voltage two bulk supplies via the railway high
voltage system.
•
The potential safety impacts of a loss of
supply; and
SCADA –
Acquisition.
•
The political
disruptions.
Sub-transmission
–
Electricity
network
infrastructure, usually operating at between 33kV
and 132kV and which supplies high voltage
distribution systems via zone substations and/or
supplies large industrial or railway loads.
acceptability
of
service
The safety impacts can be a function of ambient
conditions, both hot and cold, and the ease with
which passengers might be detrained. Clearly the
presence of tunnels, viaducts and remote locations
are factors which must be considered.
There will be greater sensitivity about disruptions
of systems with high patronage levels and which
provide a longer distance service than for those
serving smaller numbers of customers over shorter
distances where alternative transport might be
more easily arranged.
Supervisory
Control
and
Data
Sub-transmission substation – A substation fed
at transmission voltage level and which supplies
one or more sub-transmission feeders.
In designing the high voltage power supply
arrangements for a railway the main parameters
which can be chosen are:
•
System topology;
•
Form of construction
underground); and
•
Protection schemes.
(overhead
or
System topology and protection schemes are
closely related and this paper explores some of
the options for both.
Note Regarding Scope
The scope of this paper is restricted to DC
railways.
Railways with AC traction systems
generally have the traction substations more
widely spaced (by an order of magnitude) and
consequently the power level at each traction
substation is correspondingly higher.
This,
combined with considerations of power factor and
phase balance makes connection at transmission
voltage level the preferred option. The technology
options and economics are somewhat different at
transmission level and so are beyond the scope of
this paper.
NOTATION
Compact to gas insulated switchgear (GIS) –
High voltage switchgear in which a high dielectric
strength gas such as sulfur hexafluoride is used as
Figure 2 – Symbol Legend
TRACTION SYSTEM CONTINGENCIES
The required reliability of the high voltage supply
must be seen in the context of the contingencies
which the traction system is designed around. It is
normal practice to operate DC rail systems as fully
closed meshes on the DC side with both ends of
each traction feeder being supplied concurrently
from the traction substation. This parallel feeding,
meshed architecture means that for dealing with
contingencies there are a number of potential
strategies available. Some traction systems, such
as the 1500 volt system in Melbourne, are
configured with single unit substations and are
designed to tolerate the complete loss of any one
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substation. The loss of bulk power supply to a
single substation is therefore a tolerable event.
However, in systems where the substations are
configured with two or more rectifiers generally the
loss of an entire substation is not tolerated and so
maintaining high voltage supply to each substation
becomes critical.
HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
Figure 3 and Figure 4 show some possible
configurations for radial feeders.
Zone
substation
HIGH VOLTAGE NETWORK TOPOLOGIES
Common basic network connection arrangements
for DC traction substations include:
•
Radial;
•
Open ring;
•
Closed ring.
Traction
substation
Figure 3 - Simple dedicated radial from
single bulk supply substation.
Greater differentiation is seen when the upstream
arrangements are considered along with the
arrangements for the adjacent traction substations.
Zone
substation
Radial Connections
A radial connection could be a dedicated feeder
from a zone substation or a simple tee-off from a
distribution feeder serving the area around the
traction substation. If adjacent traction substations
are fed from the same distribution feeder or from
the same zone substation busbar the system will
be less resilient than if the adjacent substations
are fed from independent sources. It must also be
noted that zone substation bus bars are not
immune to the outages and where zone
substations are fed from the same subtransmission substation there is always the
possibility of a common outage.
The lower power level required for light rail
systems generally makes more sophisticated
connections appear inappropriately costly and so
radial connections are typically used for light rail
systems.
Radial feeding is not uncommon on heavier
systems. However, normally radial feeders are
only used for single-unit substations – if two
rectifiers are required for n-1 redundancy and
noting that rectifiers are now quite reliable, then it
is unlikely that the required overall substation
reliability could be achieved with only a radial
supply. Systems with single-unit substations are
typically designed to tolerate the loss of any one
traction substation and so the loss of the radial
feeder would normally be tolerable. In the case of
the NSW system, where there is a single-unit
substation between two dual-unit substations the
strategy in the event of the loss of the single-unit
substation is to put both rectifiers on at the two
adjacent substations.
*
Distribution feeder
*
Traction
substation
*
- other distribution loads
Figure 4 - Radial feed from a Tee from a
distribution feeder.
Ring Configurations
Similarly there are a number of possible
alternatives for ring configurations.
Ring
configurations typically provide two incoming
supplies to each traction substation by providing
rail network high voltage connections between
adjacent substations. Only a small number of
traction substations in such arrangements have
direct connections to the bulk supply points.
Where both ends of the ring are connected to the
same bulk supply there is the possibility of
operating the ring as a closed ring. Generally this
is not possible when the ring spans two bulk
supply points as it would likely carry significant
circulating current between the supply points.
Notwithstanding this, it is usually possible to get a
short term HV parallel between supply points to
allow for shifting loads to alternate supply points
on a make-before-break basis. However, in some
cases, differences in phase angle between the two
bulk supply points will be such that even a short
term HV parallel impossible.
The rail corridor itself is an obvious route for aerial
or underground high voltage feeders linking
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traction substations to form a ring. In tunnel
sections 22 or 33kV cables can be erected on the
tunnel wall if there is sufficient clearance to the
kinematic envelope and to communications
cables. If the corridor is circuitous, cross-country
routes might be considered where conditions are
favourable.
Due to the momentum of the trains, interruptions
of less than 1 minute the in high voltage supply for
traction are generally tolerable. With SCADA
control and a ring topology, the interruption
following a single fault can normally be restored
quickly enough to have only minimal impact on
train operations. A greater level of automation
could be employed to carry out the required
switching sequence without unnecessary delay,
thus further reducing the restoration time.
Notwithstanding this, a second fault can have a
very significant impact on networks with a ring
topology. To mitigate this, the number of traction
substations on each ring might be limited, typically
by connecting additional bulk supply points along
the ring. In practice, it is unusual to see more than
five traction substations on a single ring. In
designing the 22kV bulk supply for the Sunbury
Electrification Project on the north-west outskirts of
Melbourne the established arrangements in the
NSW system were carefully analysed and it was
found that there were a number of sequences of
between four and six traction substations in “daisychain” between other nodes in the network. These
cases comprised aerial lines which require regular
maintenance outages and long operational
experience has shown such arrangements to be
acceptable. On the basis of this benchmark it is
reasonable to conclude that sequences of five
substations would be quite practical and larger
sequences might be contemplated with care.
Rings comprising high voltage cables, like the five
substation ring designed for the Sunbury
Electrification
Project,
will
require
less
maintenance outages and are not susceptible to
lightning strikes. Accordingly such rings will have
a higher level of security than the proven aerial
line examples.
HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
Figure 5 through Figure 8 show some possible
configurations for ring feeders.
Bulk Supply
substation
Traction
substations
Figure 5 - Simple open ring from single
bulk supply substation.
Bulk Supply
substation
Traction
substations
Figure 6 - Simple closed ring from single
bulk supply substation.
In environments where bulk supplies are less
secure greater resilience might be achieved by
creating two rings with every second traction
substation supplied from the alternate ring.
Another variant is to have a single ring arranged
out-and-back to the end of the line with the open
point at the end of the line and every second
traction substation supplied from the alternate side
of the ring.
Bulk Supply
substations
Traction
substations
Figure 7 - Simple ring between two bulk
supply substations.
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HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
OWNERSHIP CONSIDERATIONS
The benefits to the rail operator of having its own
high voltage network infrastructure include:
•
Reduced feeder outage rate;
•
Faster restoration of supply;
•
On-going control of the network topology;
and
•
Greater flexibility in planning maintenance
outages.
However, this comes at a cost of a wider asset
base and an extended set of integrated logistics
support arrangements to cover additional asset
classes.
Bulk Supply
substations
Traction
substations
Figure 8 - Alternated rings between two
bulk supply substations (with open points
at opposite ends)
In the event of any single incident affecting a ring
feeder, either the loss of an incoming supply or a
feeder fault, the strategy is the same: quickly
understand the location of the fault and rearrange
the network to restore supply. This requires unit
protection for accurate fault localisation, voltage
transformers at strategic locations (including all
incoming supply points) and total SCADA
coverage, preferably including the status of the
distributor’s supplying circuit breakers.
Good
procedures,
communications,
and
positive
relationship with the network operator are also
important in maximising the advantages of such an
infrastructure solution.
Where a normally open ring connects two different
bulk supply points it is desirable to have an
agreement in place with the electricity distributor to
allow a make-before-break HV parallel for loadshifting within the ring – moving the open point to
facilitate maintenance outages. It is also important
to understand the network circumstances in which
an HV parallel is available.
This might be
unconditional or might depend on specific network
conditions to avoid excessive circulating current
due to voltage or phase differences. If this is
known to be uncertain then voltage transformers
and a relay might be used to block any potentially
problematic close.
The reduced feeder outage rate arises primarily
from having feeders that run direct, point-to-point
and so are shorter and thus less exposed to
incidents and requirements for planned outages.
Faster restoration of supply arises from the rail
operator either being able to restore supply by
remote controlled switching via its own SCADA
system, or through being able to prioritise the work
by field teams in order to restore traction supply –
in the case of an electricity distributor owned
shared feeder, in the event of an incident the
distributor’s priority is likely to be to get as many
consumers as possible back on supply in the least
time, not to restoring traction supply.
In extreme conditions where the host network
operator might implement wide scale load
shedding, an independent network can be very
advantageous. The flexibility that comes from this
means that there can be the possibility of
rearranging the network and continuing to operate
the railway utilising other bulk supply points.
If the rail operator owns the feeders then they can
ensure that they remain dedicated to the traction
supply and do not have their reliability diminished
by being looped and interconnected to all manner
of other loads. Finally, ownership by the rail
operator allows the rail operator to maintain control
over the location in the system of the point of
common coupling for the purpose of assessing
allowable harmonics. If this can be maintained as
the bus at the sub-transmission substation then it
is usual that no mitigation of the ac side harmonics
from the traction rectifiers is required.
PROTECTION CONSIDERATIONS
A protection system is a series of devices whose
main purpose is to protect persons and primary
electric power equipment from the effects of faults.
Without effective protection schemes, a fault on a
high voltage network can result in the uncontrolled
release of significant energy with the potential for
injury to persons and damage to equipment and
property. However, simply disconnecting the fault
from the supply is only part of the role of the
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protection system.
In addition to this, the
protection system must localise the fault as
precisely as possible so that supply can be
restored to the reminder of the network without
undue delay.
In order to ensure discrimination, grading margins
are required between overcurrent schemes on
series connected circuit breakers. There is also a
need to limit the maximum clearing times in order
to ensure the safety of persons and limit damage
to equipment. This therefore imposes an upper
limit on the number of time-graded overcurrent
schemes that can be configured in series.
For radial feeder configurations this does not
usually represent a problem. However, ring feeder
configurations often have a larger number of circuit
breakers between the bulk supply point and the
traction rectifier. In order to provide maximum fault
localisation and high speed operation, unit
protection schemes, in particular line differential
schemes, are often required. With open or closed
ring feeder configurations the power flow could be
in either direction at any time and so the use of the
line differential protection is almost essential in
order to achieve fault localisation.
HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
The following busbar configurations are identified
for further discussion:
•
5-breaker H
•
3-breaker H
•
Dual RMU with tie
•
5-breaker double bus
•
3-breaker double bus
In the author’s experience, breaker-and-a-half
configurations are generally applied only at
transmission voltage level and are therefore not
used in the systems that are the subject of this
paper.
While considerations of safety, in particular the
prevention
of
unauthorised
access,
land
requirements, and aesthetics generally mean that
new outdoor busbars are unlikely to be
constructed at voltages less than 66kV, outdoor
configurations are included in the discussion for
completeness and historical context.
Similar outcomes might possibly be achieved with
directional and distance schemes.
However,
modern numerical relays with optical fibre pilots
are conceptually simple and provide a high degree
of discrimination. Such schemes also provide
absolute certainty about the boundaries of the
protection zones.
In the case where a normally open ring connects
two different bulk supply points it is advisable to
provide differential elements at the rail operator’s
intake substations. These should be arranged to
look back towards the distributor’s network and
should be set relatively low. In the event of a bad
parallel the directional element will trip the relevant
circuit breaker, providing a predictable outcome
and ensuring the integrity of the networks of both
the electricity distributor and the rail operator.
BUSBAR CONFIGURATIONS
Figure 9 - Berowra 66kV switching station,
NSW
Descriptions of Busbar Topologies
5-breaker “H”
5-breaker “H” form busbars have the following
features:
•
One circuit breaker for each incoming
feeder
Busbar arrangements for radially fed traction
substations are necessarily simple and there is
little scope for innovative approaches.
•
One circuit breaker for each rectifier
transformer
•
One bus section circuit breaker
In contrast to the situation with radially fed
arrangements, busbar configurations for ring and
meshed systems are much more interesting. The
following discussion is based on the simplest ring
configuration with two feeders and two rectifier
transformers.
More complex situations are
discussed in the next section.
•
Isolators on the bus side of each circuit
breaker (- in the past, line isolators have
also been provided for the feeder circuit
breakers - outdoor implementations only)
Background
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Figure 10 and Figure 11 show indoor and outdoor
5-breaker “H” form busbars.
HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
Figure 12 and Figure 13 show outdoor 3-breaker
“H” form busbars.
Figure 12 - 3-breaker "H"
Figure 10 - 5-breaker “H” indoor
Figure 11 - 5-breaker “H” outdoor
3-breaker “H”
3-breaker “H” form busbars have the following
features:
Figure 13 - Wyong traction Substation,
NSW – 66kV AC |1500V DC - 3-breaker "H"
(Google Maps ©2013 Google)
•
Typically an outdoor configuration
Dual RMU with tie
•
One circuit breaker for each rectifier
transformer
Busbars comprising two RMUs with a tie cable
have the following features:
•
One bus section circuit breaker
•
Typically an indoor configuration
•
Isolators on the bus side of each circuit
breaker
•
One three unit “Ring Main Unit” (RMU) for
each rectifier transformer
•
Line isolators only for the feeders
•
RMUs linked by a tie cable
•
Each line differential protection zone
includes half the busbar and trips the bus
section circuit breaker
•
Six circuit breakers are required in total
•
Used on the NSW system at 66kV
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Figure Figure 14 shows an indoor busbar
arrangement comprising dual RMUs with a tie
cable.
HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
Figure Figure 15 shows an indoor 5-breaker
double bus configuration.
Figure 14 - Dual RMU with tie
5-breaker double bus
For greater flexibility and resilience double busbar
configurations are sometimes utilised in electricity
networks at transmission and sub-transmission
level. Double busbar arrangements are configured
with two separate busbars and each circuit
breaker is provided with duplicated isolators
allowing it to be connected to either busbar. Such
arrangements have been implemented as outdoor
switchyards. For indoor applications gas insulated
switchboards are often available with a doublebusbar option.
•
•
•
3-breaker double bus
3-breaker double bus configurations have the
following features:
•
One circuit breaker for each rectifier
transformer
•
One circuit breaker for each incoming
feeder
Each transformer circuit breaker has dual
isolators to allow connection to each or the
two busbars.
•
One circuit breaker for each rectifier
transformer
The feeders are connected one to each of
the busbars via isolators only.
•
One bus coupler circuit breaker between
the two busbars
•
Configure the busbar such that the
transformer circuit breakers are connected
to the incoming feeder and the bus coupler
provides protection for the outgoing
feeder.
5-breaker double bus configurations have the
following features:
•
Figure 15 - 5-breaker double bus
Each feeder or transformer circuit breaker
has dual isolators to allow connection to
each or the two busbars.
One bus coupler circuit breaker between
the two busbars
This concept is used at 66kV in the NSW system
at Lawson (commissioned circa 1956), and also
until recently at Meeks Rd, again in NSW (33kV),
both outdoor. In Kuala Lumpur it is used for the
indoor 33kV bus at the Kerinchi bulk supply
substation for the Kelana Jaya LRT Line which
opened in 1998.
This is a proposed innovative solution for
open-ring applications only, and is not suitable for
closed rings.
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HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
Figure 16 and Figure 17 show an indoor
3-breaker double bus arrangement with the
isolators configured for power follow in each
direction.
Power
in
Power
out
The capital cost relates not only to the high voltage
switchgear but also to the building or switchyard to
house it.
Land and building costs can be
significant and often exceed the cost of the power
system kit by a considerable margin.
The choice of high voltage switchgear technology
can have a significant impact on the costs during
the operations and maintenance phase of the
asset lifecycle, with some indoor compact GIS
switchgear requiring very little maintenance.
Notwithstanding this, the busbar topology can also
have an impact, particularly if some elements are
only available for routine maintenance under
power-off conditions at times when both rectifiers
can be taken off line.
Closely related to the question of the availability of
maintenance is the question of redundancy – not
all topologies can tolerate a failure of any
component.
Figure 16 - 3-breaker double bus, feeding
left to right
Power
out
Power
in
While in theory it should be possible to overlay any
protection scheme on any busbar topology, there
are constraints, particularly in relation to the
position of CTs in compact GIS products and to a
lesser extent some outdoor circuit breakers with
integral CTs.
This must be considered in
designing the busbar scheme using commercially
available components.
“Strategic Maintainability” is a term proposed by
the author to refer to maintainability in the context
of the situation where the maintenance task is the
complete replacement of the equipment.
Generally this is of concern in relation to indoor
switchgear. If the switchgear is obsolete at the
time of replacement and the new equipment is not
a direct replacement, the replacement of a single
circuit breaker panel or even half of the busbar
independent of the other equipment is unlikely to
be feasible. On the other hand the replacement of
the entire switchboard is likely to take several
weeks and cannot be accomplished during a
normal outage.
Discussion of Busbar Topologies
5-breaker “H”
Figure 17 - 3-breaker double bus, feeding
right to left
Evaluation Criteria
The choice of a high voltage busbar configuration
for a DC traction substation is based on a number
of criteria including:
•
Capital cost
•
Routine maintenance
•
Redundancy
•
Fit with protection concept
•
Strategic maintainability
This topology was very common for outdoor
busbars and was often adopted for metal-clad,
withdrawable indoor switchboards.
In outdoor implementations the maintainability of
such arrangements is generally good due to the
spacing between equipment and the customary
provision of separate isolators on both sides of the
bus-section circuit breaker.
In the past line
isolators were provided in association with feeder
circuit breakers. This made sense in the case of
old bulk-oil circuit breakers which required regular
maintenance. However, with modern vacuum or
gas circuit breakers the line air-break switch is
likely to require more maintenance than the circuit
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breaker and so nothing is achieved by the
inclusion of the line isolator – it just adds to the
asset base and the maintenance schedule, and
reduces reliability!
Indoor implementations of 5-breaker “H” are
problematic in three ways:
•
In some cases it is necessary to isolate
both busbar sections in order to carry out
some maintenance tasks in the bussection cubicle;
•
A flashover or similar fault in the bussection cubicle will take the whole
switchboard (and therefore the whole
substation) off line;
•
The configuration has poor strategic
maintainability – it would likely be
necessary to assemble and commission
the new switchboard in a new location and
then cut each circuit over to the new
board. This requires considerable spare
space within the substation or space
available nearby (very nearby) to the
substation for the new or temporary
switchboard.
HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
Dual RMU with tie
This configuration has been adopted in recent
times for indoor compact GIS implementations on
the NSW system. While the capital cost is higher
than for a 5-breaker “H” this configuration provides
total immunity to any single failure. It also very
effectively addresses the issue of strategic
maintainability as each of the RMUs can be
replaced independently and there are no
mechanical fit interfaces between them.
While it might be argued that the cubicle at one
end of the tie cable need only be a remote
controlled isolator, the marginal cost of a circuit
breaker can be relatively small and there are
benefits in the uniformity of configuration if both
ends are circuit breakers.
For busbars at larger nodes this concept can be
expanded to a ring bus. NSW implementations
have standardised on 5 circuit breakers per
switchboard – two ties; one rectifier; and either two
feeders or one feeder and a system transformer.
Strategic maintainability is generally not an issue
for outdoor busbars as each element can be
isolated and replaced separately.
Although providing a high degree of flexibility, it
can be argued than with only three outgoing loads
(two rectifiers and one out-going feeder) the
utilisation of five circuit breakers is excessive.
3-breaker “H”
This topology was used in the NSW system at
66kV.
As for the 5-breaker “H” configuration, the
maintainability of outdoor such 3-breaker “H”
arrangements is generally good due to the spacing
between equipment and the customary provision
of separate isolators on both sides of the bussection circuit breaker.
The absence of line circuit breakers does mean
that in the event of a feeder fault, half the busbar,
including one rectifier, is lost until the line isolator
can be opened by remote (SCADA) control. This
is usually only a minor concern as the second
rectifier can usually be put in service immediately
until the fault is understood and sectionalised.
Note that in the event of a busbar fault the inability
to put the feeder back into service has no impact
as without the bus there is nothing for it to feed.
While there is a slight loss of flexibility, for three
outgoing loads (two rectifiers and one out-going
feeder) three circuit breakers would appear to be a
logical equipment count.
Figure 18 - Two of the three sections of the
33kV ring bus at Argyle traction substation,
NSW – 33kV | 1500V DC, 10 MW firm.
5-breaker double bus
The use of a double busbar configuration rather
than simply two bus sections provides a little more
flexibility than is achieved with the 5-breaker “H”
configuration.
Indoor GIS products are typically available in
double bus variants.
While the number of panels for a 5-breaker double
bus is the same as the number of panels for a 5breaker “H” the capital cost will be higher.
The routine maintenance requirements for gas
insulated switchgear are generally quite modest
Conference On Railway Excellence
Adelaide, 5 – 7 May 2014
David Stuart-Smith
Arup
and isolations are not normally required. That
said, the increased number of components, in
particular bus isolators which may be motor
operated will increase the maintenance burden
somewhat over a 5-breaker “H” using the same
series of switchgear.
The vulnerability of the switchboard to a single
failure is probably a little worse than for a 5breaker “H” – more so in the case of 22 or 33kV
equipment, less so in the case of 66kV equipment
which is constructed more in the style of pressure
vessels than welded stainless steel boxes with
internal partitions.
As both sections of busbar are present in all
panels double bus configurations do not facilitate
partial replacement and so are poor from a
strategic maintainability perspective.
Full
replacement in situ is unlikely to be practical and
additional space will be required for the
construction of the new switchboard.
Note that in relation to 66kV GIS, while not the
complete solution for switchboard replacement,
careful consideration should still be given to the
strategic placement of full pressure diaphragms to
allow partial disassembly – something that must
be designed up front in the switchboard
procurement process.
3-breaker double bus
At 66kV the capital cost per GIS bay is
approximately five times that at 33kV. This is
because the 33kV switchgear is basically an upsized version of distribution voltage equipment
with vacuum interrupters and the SF6 gas running
at approximately atmospheric pressure. At 66kV
the equipment is a down-rated version of
transmission voltage equipment with the arc
interrupted in the SF6 gas which runs at
approximately 6 times atmospheric pressure,
hence the containment is more like a series of
pressure vessels and the cost is much higher.
It should also be noted that the transmission type
GIS is very reliable equipment. Against this
background, the author proposes a 3-breaker
double bus configuration for 66kV indoor GIS
implementations. While still not addressing the
issue of strategic maintainability, this configuration
reduces equipment count and so significantly
reducing the capital cost and routine maintenance
cost with little impact on functionality. The more
compact configuration may then allow space to be
reserved for a replacement switchboard in the
future.
HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
Connection of Non-Rectifier Transformers
Typically there are requirements to connect two
types of non-rectifier transformers to the subtransmission busbar:
•
Substation auxiliary transformers; and
•
System transformers, such as those for
step down to high voltage distribution
voltage levels.
The substation auxiliary load is typically a few tens
of kVA and so a dedicated panel on a switchboard
a sub-transmission level could be the major part of
the cost of providing the auxiliary supply,
particularly if the supply is duplicated. The most
cost-effective approach appears to be to supply
the auxiliary transformers from the secondary side
of the rectifier transformers. Power quality may be
an issue due to voltage regulation and harmonics.
However, as the main substation loads are the
battery charger, lighting, and equipment to
manage waste heat, it is still likely to be more
economical to design the equipment to cope with
the power quality issues, or to provide a line
conditioner or similar equipment to fix the power
quality issues rather than deriving the supply direct
from sub-transmission level.
If it is necessary to connect a significant system
transformer, say rated at 1MVA or above then
providing a separate panel would likely be
appropriate. For smaller system transformers as
are sometimes required only to supply signalling
loads, consideration might be given to duplicating
the system transformers and sharing the circuit
breaker with the rectifier transformers. At 66kV
and above this appears to be particularly attractive
option from an economic perspective.
CONCLUSION
There are a number of conclusions that can be
reached from a methodical evaluation of high
voltage system and busbar topologies:
•
The blind replication of topologies
historically used for outdoor busbars in
modern indoor switchboards may lead to
sub-optimal configurations
•
While operating a sub-transmission
network may not be “core business” for a
rail operator, in some circumstances there
are a number of good reasons why this
might be the right solution, including
greater control over maintaining and
restoring supply, increased reliability, and
greater flexibility in relation to harmonics.
•
The choice of network and switchboard
topology must be informed by the
allowable contingencies on the DC system
and the way redundancy on the DC
system is achieved.
Conference On Railway Excellence
Adelaide, 5 – 7 May 2014
David Stuart-Smith
Arup
•
•
•
•
HIGH VOLTAGE AC SYSTEM TOPOLOGIES FOR
THE SUPPLY OF DC TRACTION RAILWAYS
The choice of a high voltage busbar
configuration for a DC traction substation
is based on a number of criteria including:
o
Capital cost
o
Routine maintenance
o
Redundancy
o
Fit with protection concept
o
Strategic maintainability
For indoor GIS applications the much
higher cost of switchgear for 66kV and
above means that different topologies
might be appropriate to those appropriate
at lower voltages.
An innovative configuration for a 66kV
traction substation busbar has been
identified with the potential to save ~40%
on switchgear cost.
Sharing circuit breakers, even if this
means duplicating other equipment, may
be a cost-effective solution.
The fundamental approach must be to identify the
system performance required and design to that,
considering the full extent of the required operating
conditions space and exactly what functionality is
required form each element. When these factors
are understood, and the costs and benefits of the
options evaluated accordingly good investment
decisions can be made to deliver enhanced
customer experiences and reduce the incidence of
events
such
as
that
shown
in
Figure
Figure 1.
REFERENCES
Transport for NSW 33 & 66kV Network Diagrams
Transport for NSW 1500V Sectioning Diagrams
Transport for NSW Substation AC Operating
Diagrams
Metro Trains Melbourne 22kV Network Diagram
for Sunbury Electrification Project
Kelana Jaya Line (Kuala Lumpur, Malaysia) 33kV
Network Diagram
ACKNOWLEDGEMENTS
My thanks to Billy Tan of my Arup team who
painstakingly prepared all the diagrams for this
paper and Richard Arnold who provided valuable
comments and suggestions on the draft.
Conference On Railway Excellence
Adelaide, 5 – 7 May 2014