Download STATIC FREQUENCY CONVERTERS – A FLEXIBLE AND COST

Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
Static Frequency Converters
STATIC FREQUENCY CONVERTERS – A FLEXIBLE AND COST
EFFICIENT METHOD TO SUPPLY SINGLE PHASE RAILWAY GRIDS
IN AUSTRALIA
Chuanhong Zhao, Senior Engineer, ABB Switzerland Ltd
Cosmin Banceanu, MSc. Power System Engineer, Control SW Engineer, ABB Switzerland Ltd
Thomas Schaad, Senior Engineer, Product Manager for Rail Converters, ABB Switzerland Ltd
Philippe Maibach, Senior Principal Engineer, ABB Switzerland Ltd
Steve Aubert, Electrical Engineer, Product Manager for Hydro Converters, ABB Switzerland Ltd
Summary
This paper presents the results of a study showing the potential benefits of installing static frequency
converter systems instead of the more traditional single phase transformer solution for 25 kV 50Hz
traction power supply.
The static frequency converter (SFC) solution is already well established for railway systems with lower
frequencies like 16.7 Hz and 25 Hz. A detailed analysis will emphasize the technical setup possibilities
and benefits of this solution compared with the conventional transformer supply solution for 50 Hz
substations. Although the power supply by means of single phase transformers connected to the threephase domestic grid seems to be obvious and simple at a first glance, several disadvantages (e.g.,
unbalanced loading of the three-phase grid, harmonic current injection from the railway into the feeding
grid and the need for neutral sections in the catenary system) can be eliminated by using SFC. Results
will show that a traction power supply concept based on SFC solution reduces installation, operation
and maintenance costs, while increasing system efficiency and reliability in addition to generating extra
revenue in the form of reactive power compensation on the public grid.
The paper will also introduce the reference project, Wulkuraka, one of the new static frequency converter
feeder stations of Queensland Rail near Brisbane.
INTRODUCTION
Traction substations for the 25 kV 50 Hz railway
power supply are traditionally based on single
phase transformers connected across pairs of
phases of the three-phase public grid, usually at
very high voltage. Nowadays, this concept can
be very efficiently replaced by a solution using
power electronics static frequency converter
systems.
established transformer approach as observed
in several simulations.
Chapter 3 will introduce the Queensland Rail
Wulkuraka reference project. In addition to
describing the 20 MW static frequency
converter system, the simulation results of
several operational and fault behaviours
demonstrated by the ABB real time hardware
simulator will be discussed.
In the first section, a history of railway
electrification development will be given along
with an overview of solutions present in Europe.
Chapter 2 will provide an overview of the SFC
solution, showing its functional capabilities as
well as its operational and commercial benefits.
Results from an ABB-driven comparative case
study will also be presented contrasting the
benefits of this solution versus the more
AusRAIL 2015
24 – 26 November, Melbourne
Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
Static Frequency Converters
Notations
1.
AC
Alternating Current
1.1
DC
Direct Current
kV
Kilovolt
km
Kilometre
MW
Megawatt
MVA
Megavolt ampere
Engineers first started experimenting with
electrified railway vehicles in the middle of the
19th century. These initial attempts were based
on battery powered DC supplies. For practical
reasons, the supply voltage level was quite low
making the rated power of the vehicles
correspondingly moderate. With the advent of
increasing power requirements, two main
different development routes were chosen:
Hz
Hertz
p.u.
Per unit (scaling of any present
measurement from SI units to per unit)
SFC
Static
frequency
converter
–
semiconductor-based
system
that
can
interconnect two electrical networks
STATCOM
Static
synchronous
compensator – semiconductor-based system
that can supply high dynamic reactive power
towards a grid and stabilize voltage and
improve power quality at the connection point
RPC
Railway static power conditioner –
semiconductor-based system mitigating any
unbalance on a three-phase public grid caused
by rail feeder stations transferring active power
from one secondary winding to another traction
transformer and compensating reactive power
3ph
Three-phase system – designation
used to refer to the supplying public grid side of
a SFC
1ph
Single-phase system – designation
used to refer to the catenary side of a SFC
AT
Auto transformer – simple three-tap
transformer used to create a negative feeder
along a catenary system
BT
Booster transformer – transformer
arranged along the catenary to provide EMC
immunisation
SINGLE-PHASE RAILWAY GRIDS
History Of Railway Electrification
1) Higher DC voltage supply
2) AC voltage supply
Specifically, in the early 20th century, railway
system designers started using low-frequency
AC voltage to supply their railway lines. There
were two key reasons for choosing 16.7 Hz in
central Europe and Scandinavia and 25 Hz in
the USA. On the one hand there was possibility
of re-using the DC motor concept to develop AC
universal motors. And, on another hand, it was
likely to allow the more efficient electric
transmission over long distances by adapting
voltages to higher levels by means of
transformers. Drawbacks, however, of lowfrequency AC systems were bigger and bulkier
parts (e.g. magnetic components and protection
systems) specifically designed for this
application. Once the low-frequency AC supply
grid was established in Switzerland, Germany,
Austria, Sweden, Norway (all 16.7 Hz) and the
USA (25 Hz), it became the standard approach
and all subsequent infrastructure and vehicles
were adapted to this concept.
Countries
which started their railway
electrification after that point were able to
benefit from later technological advancements
achieved with respect to electrical traction
conversion on rolling stocks. Consequently they
introduced the same frequency of 50 Hz for the
single-phase railway system to be used in
supplying the three-phase public grid. A higher
voltage of 25 kV was chosen in order to reduce
transmission losses and minimize overhead
lines sections. Disadvantages to this solution
were mainly load unbalance and interference on
the three-phase feeding public grid as well as
the need for neutral sections to separate the
phase shifted supplied sections. The solution
will be described more in details in the next
sections.
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Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
1.2
Electrical Catenary Concepts
Railway overhead lines for AC supply distribute
electricity from the feeder substations to the
trains. In general there are three catenary
concepts [1]:
1) The single-phase catenary concept where
the feeder station’s transformer secondary side
is directly connected to the contact line and the
rail at each substation.
2) An improvement of the single-phase catenary
concept is the implementation of booster
transformers (BT). The primary side of the BT is
connected in the contact line. The secondary
side of the BT is connected to the rail section,
collecting the return current from the rails and
the earth to the return conductor. Booster
transformers are used to eliminate the stray
currents and the disturbances, obliging the
return current to flow to the return conductor.
3) In the two-phase catenary concept, autotransformers (AT) are used such that the AT
winding is connected between the contact line
and the negative feeder with the rail being tied
to the intermediate point.
The single-phase catenary concept is the least
capital-intensive option. The two-phase
catenary concept with ATs is the most complex
one with the highest investment; however, it
reduces the impedance and increases feeder
substation spacing.
1.3
25 kV 50 Hz AC Power Supply
The 25 kV 50 Hz system was the most adopted
supply system over long distances for electrified
railways after the 1950s, when three-phase 50
Hz public grids were widely available for traction
feeding purposes given that a technical solution
for power conversion on rolling stocks material
was available.
Single-phase traction transformers are mainly
used in feeder substations due to their simple
structure and low cost [1]. A single-phase
traction transformer is connected to two of the
public grid’s three phases. This leads to an
asymmetrical loading on the public grid, which
may have a negative impact on other customers
and potentially on generation facilities. One
commonly used solution to mitigate this
unbalance is to alter the feeding phases for
each consecutive traction transformer. Thus,
neutral sections between two adjacent rail
sections are necessary because of the 120
degree phase angle difference between the
terminal voltages of the two adjacent rail
Static Frequency Converters
sections [2]. Automatic switches and associated
controllers are required to cut off power when
electric vehicles pass these neutral sections.
However, public grids still suffer from singlephase loads at the connection points between
public and railway grids. Therefore, transformer
substations are usually connected to the highvoltage grid, typically 110 to 400 kV, showing a
very high short circuit capacity, in order to
mitigate the impact of the load unbalance and
the harmonics distortion from the railway grid
into the public grid.
Relatively high transformer impedance is used
to limit the grid fault current levels being
transferred to the railway grid, which negatively
influences voltage performance vs load.
1.4
Alternative Supply Concepts
As alternative way to mitigate the load
unbalance and the harmonic impact on the
public grid, is to install power electronic
equipment.
STATCOMs are used as load balancers [3]. The
STATCOM is coupled to the public grid via a
component with an inductive impedance.
Essentially, the STATCOM is a fully controllable
voltage source, with full independence between
the three phases. By matching the public grid
frequency and appropriately controlling the
amplitude and phase angle of its output voltage,
the STATCOM can compensate the negativephase sequence current components in the
current drawn from the public grid and thereby
fulfil the public grid owners’ requirements
regarding phase unbalance. Moreover, it helps
meet requirements regarding voltage fluctuations and harmonic distortions. However, the
fact that most STATCOMs are coupled to the
public grid (which can be as high as 110 kV, 220
kV, or even 400 kV) can lead to a high voltage
rating requirement on the coupling transformer.
In the recent years, RPCs with balanced
transformers have been used to compensate
negative-phase
sequence
currents
for
electrified traction power supply systems [4]-[6].
The RPC is essentially a single phase back-toback power electronics converter including a
dc-link. The RPC is connected between two
secondary windings of balanced transformer
(Figure 1). Only one half of the active current
difference of two secondary windings needs to
be transferred from one winding to the other in
order to have balanced currents on both
secondary windings, resulting in balanced
AusRAIL 2015
24 – 26 November, Melbourne
Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
Static Frequency Converters
currents and zero negative-phase sequence
current on the primary (public grid) side.
Power flow
Public grid
Feeder substation, including
traction transformers and
switching circuits
RPC
HRPC
Railway power conditioner
Railway grid, including
contact line, neutral sections
and maybe return conductor
Figure 1. Simplified representation of RPC
The approach with RPCs can be extended to a
general case, without the use of a balanced
transformer. The RPC can not only supply
active power from one secondary winding to
another one of a traction transformer, but also it
can compensate for reactive power and mitigate
harmonics. The extension theory and possible
implementation is discussed in [7], where a
RPC with Δ/Y transformers is explained as an
example. Special attention has been paid to the
RPC with a three-phase V/V traction
transformer [8], which is widely used in highspeed train railway traction systems.
1.5
Field Experience Of SFC For 16.7 Hz
Countries using 16.7 Hz railway supply feed
their railway network either by means of owned
single-phase generation units, or frequency
converter stations using three-phase AC from
the public grid for the supply of the railway
network at 16.7 Hz and single phase (see
Figure 2).
Originally, the conversion equipment was
realized with rotary converter systems. Those
consisted of motor-generator systems with a
three-phase synchronous motor mechanically
coupled with an asynchronous single phase
generator for 16.7 Hz power supply. Several
such systems are still in operation. Nowadays,
frequency converters based on power
electronics have replaced rotary converters.
Reduced capital investment cost, streamlined
operational expenses, improved availability and
higher efficiency are key properties in favour of
power electronic SFC. ABB can draw on more
than 20 years’ SFC systems experience and an
installed base of more than 1800 MW converter
power in operation for 16.7 Hz railway supply
(see [9]).
Figure 2. Block diagram of traction power
supply with static frequency converters
Basically, a frequency converter can be thought
of as two separated voltage sources: one on the
public grid side (grid side converter) and
another on the railway side (railway side
converter). Both are electrically connected
together by the DC-link. The main characteristic
of such a frequency converter is that both sides
are electrically decoupled by means of the DClink. The active power cannot be stored and has
to be continuously and fully transferred through
the converter. Active power control can be
achieved by controlling the converter active
current flow. Additionally, each side of the
converter can independently control the
voltage, reactive power, and frequency. This is
represented schematically in Figure 3.
Figure 3. Principle diagram of a frequency
converter and its control possibilities
(transformers omitted)
2.
2.1
WHY USE SFC IN AUSTRALIA?
Technical Benefits
The 50 Hz railway traction power supply system
with SFCs, as illustrated in Figure 2, is gaining
more and more attention [10]-[12]. Many new
features are being introduced by using SFCs in
feeder substations to interconnect the three-
AusRAIL 2015
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Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
phase public grid and the same frequency
single-phase railway grid, even though SFCs
often require a higher capital cost than
traditional connections:
SFCs are designed to draw symmetrical loads
from the public grid, i.e. no separated load
balancer system is needed. Hence, a
connection to a lower voltage node in the public
grid with lower short-circuit capability can be
realized.
SFC can also freely control voltages, angles
and frequencies on both public and railway grid
sides. Hence, the overhead line can be
synchronized over a long distance. Adjacent
railway sections can be connected together and
neutral sections can be theoretically eliminated.
A meshed grid concept with higher efficiency
and lower maintenance can be achieved. In
addition, the SFC will control the traction
voltage to a higher level, independent of the
public grid thereby increasing the efficiency of
circulating trains.
Long railway tracks (without neutral sections)
and the SFC active power control permit the
improved use of regenerative energy. Active
power consumption will decrease and global
system efficiency will increase. In addition,
reactive power control on the public grid side
can be provided, offering new potential revenue
sources.
Static Frequency Converters
tunnels and hilly ground profiles were not
considered.
The default setup for supplying the system was
based on three feeding points along a track. The
simulation considered an n-1 redundancy. Thus
the middle feeder station was put out of
operation and appropriated breakers/isolators
were switched accordingly for continuous
operation.
Two kinds of substations were selected. The
first one with single phase transformers (Figure
4), and the second with SFCs (Figure 5).
Figure 4. Transformer-based feeder setup
with n-1 redundancy
The two-sided feeding system does not only
reduce the peak load consumed at the
individual connection points to the public grid
but also it reduces the effective railway grid
impedance. It can even provide some degree of
redundancy.
SFCs have a fixed harmonic spectrum towards
the public 3ph grid. Harmonic distortions from
the rolling stock are eliminated.
2.2
2.2.1
Operational Benefits
Rail system simulation
With simulations of a rail supply system, the
differences
between
operation
with
transformers and SFCs were investigated.
Rolling stock typical commuter arrangements
with rated maximum power of 4.2 MW and a
maximum speed of 110 km/h including
regenerative braking possibility were defined.
The trains were put on a reference rail system
specified as a double track corridor with
passenger stops every 10 km. A headway of
10 minutes was defined. Special conditions like
Figure 5. SFC-based feeder setup 1x 25 kV
AC with n-1 redundancy
The simulation was performed for two catenary
setups: 1x 25 kV AC system without AT and
2x 25 kV AC system including AT along the
track. As general simulation rule, the catenary
voltage was set to be kept within 22.5 kV and
27.5 kV under all circumstances.
2.2.2
Simulations results
The simulations show that the distance between
the feeding points increases significantly by
using SFCs instead of simple transformers as
illustrated in Figure 6. The possible track length
rises from 90 km to 150 km for the 1x 25 kV AC
system. By considering the 2x 25 kV supply
AusRAIL 2015
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Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
system, the distance can even be increased
from 110 km to 230 km.
30km
SFC
SFC
SFC
SFC
50km
SFC
SFC
1*25kV & Transformer
(90km)
1*25kV & SFC
(150km)
SFC
SFC
SFC
SFC
SFC
78km
SFC
40km
2*25kV & Transformer
(110km)
2*25kV & SFC
(230km)
Figure 6. Track length results
Improvement here can mainly be explained by
the voltage control capability of the SFC, which
will supplies the catenary system at a higher
voltage level, independent of the public grid
voltage. It facilitates longer distances between
the feeder stations.
Static Frequency Converters
For faults in the 3ph grid, the SFC provides very
good fault ride through capabilities if the voltage
drop is not below the pulse blocking limit.
Especially with respect to single-phase faults it
is possible to keep operations going, as long as
the public grid is still able to provide the
demanded power on the remaining phases. A
load balancer functionality for the public grid
assists in such fault cases by injecting negative
sequence current. It is particularly useful that,
even when the public grid fails, the SFC can act
as a single phase STATCOM which controls the
catenary voltage by injecting reactive power.
This improves track reliability as compared to a
simple transformer feeding setup. There, any
voltage dips on the 3ph grid will immediately
impact the 1ph grid as well potentially causing
fault currents.
2.2.5
2.2.3
Regenerative braking
As the study shows, the catenary system
operates fully synchronized (no 120° phase
shifting) over very long distances and without
neutral sections. The regenerative energy
gathered from braking trains continues to
circulate over the whole track length and can be
absorbed by other trains. In the meantime, the
regenerative energy can be blocked by the
converter, reducing the active power demand
and the feeding transformer losses.
2.2.4
Improved fault behaviours
The decoupling by means of the SFC means
that the short circuit fault level is decoupled from
the public grid too. Public grids often show a
very high fault current level. With the SFC as the
current source for the catenary system, the
short circuit level is set by the SFC’s maximal
current capability. Depending on the site, it can
be set up to 120% of the nominal substation
rated current. This allows reduced current
design for the rail equipment, which results in
smaller dimensioning of groundings and other
equipment for fault cases. The SFC itself is able
to inject short circuit current continuously which
ensures proper detection of faults by the
corresponding protection system. After short
circuit clearance, the SFC re-establishes its
normal operation conditions. This happens in a
controlled manner, whereby the SFC ramps up
the voltage and avoids saturation effects on
transformers. As a result, the SFC provides a
smooth fault ride through behaviour.
Test and control functionalities
A SFC is an intelligent device compared to the
simple transformer. This controllable device
permits enhanced control and testing
functionalities.
The catenary voltage can be managed
depending on the active power flow. Adaptive
voltage control is possible: where trains
demand active power, the voltage can easily be
increased or, vice versa, be lowered when
facing regenerating power. Due to the line
impedance, active power on the catenary lines
affects the catenary voltage. The adaptive
control facilitated by the SFC cancels out this
effect and presets the voltage depending on the
actual active power. This results improved
catenary power capabilities.
The SFC makes it possible for braking energy
to be redirected to a dedicated feeding point. By
controlling the phase angle of the catenary
system using a characteristic curve deduced
from rotary converters, the active power can be
rejected by the SFC. Instead of accepting the
active power, the SFC can adjust its phase
angle to a condition where no power flows.
Under these circumstances, another SFC faced
with an increased load angle, will act according
the set characteristic, and accept the
recuperated power.
An isolated section of the catenary can be used
in combination with an SFC as a test track for
new trains that are going to be put into service.
New rolling stock can pass homologation tests
and can be proofed in its low frequency stability.
The behaviour of trains in fault conditions can
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Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
be checked. The SFC can simulate voltage dips
or supply the catenary with high or low voltage
levels.
If a catenary section needs to be checked, the
SFC can act in Line Check Mode (similar to an
ordinary laboratory voltage source). The level of
voltage and current can be set independently in
an open loop controlled manner. This allows
any voltage to be supplied to the overhead line
for test purpose. In combination with an
installed grounding on the track, the catenary
can also be loaded with rated current for heat
runs.
2.3
2.3.1
systems by considering the distance between
the feeder stations. An additional benefit is a
lower dimensioning factor per track kilometre
when using the SFC solution, as shown in
Figure 7.
Commercial Benefits
Public grid interaction
With SFC, the power supply substation
operates at a unity power factor on the 3ph grid
side (cosφ = 1). No distortion or unbalance is
present at the connection point. This simplifies
negotiations with power suppliers since the SFC
acts like a fully balanced load.
A simple comparison of transformer versus SFC
costs will, on the surfaces of things, show a
higher price for the SFC solution. However, a
truer representation of costs would be to
compare total solution costs. With the distance
increase between the two feeding points with
SFC, as described earlier, fewer connecting
points along a track will be needed. Additionally
expenditure related to neutral sections can be
avoided. Furthermore, SFC feeding stations
can be connected to medium voltage grid; this
reduces the feeding station costs on the public
grid side, as these will now be designed for a
lower voltage level. Such medium voltage grids
are often widely spread and available in a close
proximity to the railway track. Lower voltage
infrastructure (e.g., 33 kV) is less expensive
than high voltage infrastructure (e.g., 110 kV),
and requires less space. The effort to secure an
erection permit of a new medium voltage line, if
needed, will be much lower than that for a high
voltage line. When total overall costs are
considered, the SFC solution can easily be
cheaper than the transformer option. And, in
some circumstances, it may prove to be the only
possible solution.
2.3.2
Static Frequency Converters
Reduced dimensioning
The study mentioned in chapter 2.2.1 compared
the power ratings at the feeding points for both
Figure 7. Power demand factor per km track
length for substation
2.3.3
Mitigation of catenary sections
The SFC solution reduces the need for section
breakers since the catenary branch is fully
synchronized by the SFCs. The section
breaking concept can therefore be limited to
protection zones where a line, or distance
protection, ensures switch off in fault conditions.
3.
3.1
REFERENCE PROJECT WULKURAKA
Project Overview
In
Australia,
Queensland’s
government
identified the need to upgrade and increase the
existing train fleet in the southeast of the
province. Part of the mandate for the New
Generation Rollingstock (NGR) project was that
the railway network in the existing Ipswich to
Rosewood rail line needed to be strengthened.
This required the maintenance centre at
Wulkuraka to consider what impact the
additional load would have on the railway
corridor performance and the constraints on the
three-phase AC supply grid (see Figure 8).
Queensland Rail awarded ABB the contract to
deliver a turnkey solution based on a PCS6000
rail static frequency converter rated at 20 MVA.
The SFC efficiently converts electricity from
three-phase national grid, with a rated
frequency of 50 Hz to the 50 Hz required by the
single-phase 25 kV railway grid. The system
was delivered in May 2015 and is expected to
AusRAIL 2015
24 – 26 November, Melbourne
Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
Static Frequency Converters
~
~
~
~
SPRINGFIELD
(13.974 KM from Darra)
~
~
=
=
=
=
=
=
=
TO NGR SERVICE
CENTRE
T1
ABB SFC
WULKURAKA
41.223KM
BOOVAL
34.578KM
GOODNA
23.480KM
DARRA
15.926KM
~
~
I
I
go into operation according to customer’s
schedule until end of 2015.
~
~
~
~
~
~
Figure 9. Overview of the proposed 20 MVA
PCS6000 Rail SFC for Wulkuraka
CORINDA
10.44KM
Figure 8. Simplified railway grid topology
3.3
3.2
Highlights Of SFC
Figure 9 gives a simplified overview of the
proposed solution. The three-phase grid voltage
is first transformed down to the converter input
voltage level and then the voltage is converted
for the DC link. Two three-phase, three-level
voltage source converters are used, each of
which are connected to one side of the
transformer secondary winding. This minimizes
harmonic distortion during operations. During
the tendering stage, it was expected that on the
three-phase side no grid filter would be
necessary to reduce the harmonic distortions
even further.
From the DC-link the voltage is again converted
to AC, this time 50 Hz single phase before finally
being transformed up to the voltage for the
railway grid. Four single-phase, five-level
voltage source converters are used to convert
the power from DC to AC. The four converters
are connected to the individual secondary
windings of the railway side transformer. A
dedicated AC filter for the railway grid is used to
cope with the requirements of voltage and
current harmonics. The filter is connected
directly to the high-voltage side of the railway
side transformer.
For Wulkuraka a digital DSPACE® real time
simulator is used to validate the dimensioning
and functionality before the commissioning. The
simulator is used for optimization of control
behaviour and for testing the software before
commissioning. Moreover, the tested software
is used directly on the plant.
The
optimization
and
verification
of
requirements (often difficult to implement onsite), can be done easily with this type of
simulator. This includes, among other things
evaluating voltage dips, grid asymmetries and
harmonic emissions. All this is thoroughly tested
on the simulator to ensure an efficient
commissioning.
Customers
value
this
simulation as it helps keep their secondary
costs low and predictable. There is an excellent
match between the simulator and SFC’s
behaviour in the real world thereby giving
significant flexibility to cope with customer
requirements.
Some of the tests performed for Wulkuraka are
explained more in detail below. Tests were
divided in two categories allocated to the 3ph or
1ph side of the SFC.
3.3.1
Coupling the two grids via a DC-link gives a very
high operational flexibility. The voltage and
power factor can be regulated completely
independently on both sides. The SFC concept
makes it possible to switch from delivering
reversing active power flow instantaneously to
controlling the reactive power on both sides. All
these features offer a high flexibility to match
control behaviour to the customer’s needs.
Operational Cases
Three-phase side tests
Reactive power capability
As highlighted above, the SFC is capable of fast
reactive power control on both railway and
national grid sides. For a better understanding
of the SFC’s response time in terms of reactive
power control, the following test was performed
in the simulator: step from 0 to 0.5 p.u. inductive
power (see Figure 10).
Regarding the full reactive power capability of
the SFC, it is worth mentioning that the following
requirements were set for Wulkuraka during
tendering:
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Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
At 121 kV, the converter should be able to
deliver 16 MW into railway grid at a unity
power factor
At 110 kV (nominal voltage), 0.95
capacitive power factor should be possible
at a 16 MW power flow into railway grid
Primary currents [p.u]
-
3-ph reactive power step
Primary currents [p.u]
1
0.5
0
-0.5
Reactive power / current [p.u]
-1
0
0.02
0.04
0.06
Time [s]
0.08
3-ph short circuit to 0.7 p.u
Primary voltages [p.u]
-
Static Frequency Converters
0.1
1
0
-1
0
0.02
0.04
0.06
0.08
0.12
0.1
Time [s]
0.14
0.16
0.18
0.2
0
0.02
0.04
0.06
0.08
0.12
0.1
Time [s]
0.14
0.16
0.18
0.2
1
0
-1
0.12
Figure 11. Three phase voltage dip to 0.7
p.u.
1
0.5
0
qRef
q
iq
-0.5
-1
0
0.02
0.04
0.06
Time [s]
0.08
0.1
0.12
Figure 10. Three phase reactive power step
to 0.5 p.u. (inductive)
When the voltage recovers on the three-phase
side and the inrush is over, the firing pulses are
released again. Later on the SFC starts
transferring active power according to the actual
set point (i.e., as it was before the dip).
Primary currents [p.u]
Primary voltages [p.u]
1-ph short circuit to 0.05 p.u
Short Circuit
The SFC copes with single, double and threephase disturbances with a fast closed loop
control. Depending on the voltage dip
magnitude the closed loop control reacts in
different ways. If the voltage dips to maximum
0.7 p.u. (see Figure 11), the SFC doesn’t block
the firing pulses to the three-phase side
integrated gate-commutated thyristors (IGCTs).
If the voltage drops below 0.7 p.u. (phase A to
0.05 p.u in Figure 12), the firing pulses are
blocked immediately. However, on the railway
side, the SFC keeps running in phase
compensation mode producing reactive power.
1
0
-1
0
1
2
3
Time [s]
4
5
6
0
1
2
3
Time [s]
4
5
6
1
0
-1
Figure 12. One phase voltage dip to 0.05
p.u.
If the under-voltage protection trips the threephase breaker, the SFC would change its status
to STATCOM on railway side. It is worth
mentioning that both short circuit tests were
performed with 1 p.u. active power flow.
3.3.2
Single-phase side tests
Reactive power capability
Similar to the three-phase side a reactive power
step was performed on the railway side. As
seen in Figure 13 the reactive current
component does not follow the reference. This
is because the railway side control does not
AusRAIL 2015
24 – 26 November, Melbourne
Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
1-ph reactive power step
0
-1
0
0.1
0.2
0.3
0.4
Time [s]
0.5
0.6
0.7
0.8
0
0.1
0.2
0.3
0.4
Time [s]
0.5
0.6
0.7
0.8
0
-0.5
-1
0
1
2
4
3
5
7
6
Time [s]
Reactive power / current [p.u]
1
0.5
1
0.5
0
qRef
q
iq
-0.5
-1
0
1
2
4
3
5
6
7
Time [s]
Figure 13. One phase reactive power step
to 0.5 p.u. (inductive)
For the single-phase side the following design
points were considered for the reactive power
capability during tendering:
- At 27.5 kV the converter should be able to
deliver 16 MW into railway grid at a 0.99
capacitive power factor
- At 25 kV (nominal voltage), a 0.8 capacitive
power factor should be possible at 16 MW
power flow into railway grid
Short Circuit
A simplified version of Wulkuraka’s topology
(towards Corinda Feeder Station only) is shown
in Figure 8. The grid model implemented in the
simulator allows ABB to do tests for a specific
location in the grid. Thus, for the short-circuit
test, the following conditions were considered:
a voltage dip occurred in Wulkuraka and a
rolling stock demanded 0.25 p.u. power in
Darra.
The resulting voltage (see Figure 14) was less
than 0.6 p.u. and the disturbance was classified
as short circuit. The SFC immediately supplied
nominal reactive current (0.9 p.u. in this case) in
order to support the grid protection in detecting
the short circuit properly.
Primary current [p.u]
Primary current [p.u]
1
1-ph short circuit in Wulkuraka
Primary voltage [p.u]
have a current closed loop control. Moreover,
the response time of the reactive power control
is considerably higher in comparison with the
three-phase side.
Static Frequency Converters
1
0
-1
Figure 14. Short circuit to 0.25 p.u. on
railway grid side
As soon as the voltage recovered, the old set
points for active and reactive power were
activated again. For the three-phase side a
short circuit on the railway side is seen as an
active power step since the power flow is
reduced depending on the voltage dip
magnitude. If the under-voltage protection trips
the single-phase breaker, the SFC goes to off
mode.
Island Operation
As can be seen in Figure 8, the SFC in
Wulkuraka runs in parallel with the Corinda
transformer. If the transformer is disconnected,
then the SFC will continue to work.
For the normal operation mode (synchronous
coupling between the two grids) there is no
island grid detection. The SFC will continue to
operate since it runs synchronous with the
three-phase side. For a better understanding, a
test was performed with rolling stock
demanding power in Springfield (0.5 p.u.) and
Corinda (0.2 p.u.) when the parallel transformer
disconnects suddenly.
As can be seen in Figure 15 the control in such
situations translates into a power step on the
railway side (from 0.3 p.u. to 0.7 p.u.). As soon
as the parallel transformer disconnects, the
entire power demand is taken by the SFC in
Wulkuraka. Since the control on railway side is
coupled with the three-phase side, the island
grid conditions are not acknowledged (see
island detection signal in Figure 15). However,
island detection is possible in synchronous
mode if the SFC is parameterised to reject
export of power to the national grid.
AusRAIL 2015
24 – 26 November, Melbourne
Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
demonstrate the immunity of the three-phase
side to harmonics emissions on the railway
side.
1
0
Harmonics emissions
0.08
0
0.05
0.1
0.15
0.2
Time [s]
0.25
0.4
0.35
0.3
1
Amplitude [p.u]
-1
0.04
0.02
0
-1
iG
0
0.05
0.1
0.15
0.2
Time [s]
0.25
0.4
0.35
200
400
600
800
1000
Frequency [Hz]
1200
1400
0.06
island detection
0.3
1-ph primary voltage
3-ph primary voltage
0.06
0
Figure 15. Active power step due to island
grid conditions
Amplitude [p.u]
Primary voltage [p.u]
Active power step due to island grid conditions
Primary current [p.u]
Static Frequency Converters
0.02
0
Black-Start
If the parallel transformer is disconnected, the
SFC can start an island grid. Once started,
further sources (in this case Corinda
transformer) or loads can be connected as in a
normal connected grid. In Figure 16, different
steps for the black-start sequence are shown.
1-ph primary current
3-ph primary current
0.04
200
400
600
800
1000
Frequency [Hz]
1200
1400
Figure 17. Harmonics emissions on both
sides
DC voltages [p.u]
SFC black start
1.1
1
0.9
0
2
6
4
8
Time [s]
10
16
14
12
Grid voltages [p.u]
2
1
0
-1
uG(BC) 3ph
-2
23.4
23.45
23.5
23.55
Time [s]
23.6
uG 1ph
23.65
23.7
Figure 16. Black-start when Corinda
transformer is disconnected
First the three-phase side circuit breaker was
closed and the DC link voltage controlled to 1
p.u. once the three phase side firing pulses
were released. Then the circuit breaker on the
railway side was closed and the voltage was
ramped and synchronized with the three-phase
side. As a final step, the power references was
released.
Harmonics Emissions
For the rolling stock on the railway grid side,
different harmonic profiles can be enabled in the
simulator. An example is shown in Figure 17, to
AusRAIL 2015
24 – 26 November, Melbourne
Ch. Zhao, C. Banceanu, Th. Schaad, Ph. Maibach, S. Aubert
ABB Switzerland Ltd
4.
CONCLUSIONS AND/OR
RECOMMENDATIONS
Using SFCs for supplying 50 Hz AC operated
railways offer a wide range of technical,
operational and commercial benefits to a rail
operator. For example, effective responses
against three-phase faults and the ability to run
the catenary as a fully decoupled grid are major
arguments for an SFC approach.
SFCs can appear more expensive when they
are compared to a simple transformer.
However, when considering total overall costs
and local conditions on a case by case basis,
SFCs can often work out cheaper as there are
fewer adjunct investments required when SFCs
are used instead of transformers alone.
The installation for Queensland Rail is a good
example of the value in following a systematic
approach and taking into account the multifactorial benefits offered by SFC. This case
study demonstrates how a SFC solution can
lead to a more innovative and more cost
efficient solution than just installing traditional
equipment.
5.
ACKNOWLEDGEMENTS
[a] Trevor Bagnall, CPEng MIEAust. RPEQ,
Principal Engineer – High Voltage Systems,
Queensland Rail, Australia
6.
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