Dr. Rainer Gruber
Siemens AG
Use of MMC Technology in Rail Electrification
Use of Modular Multilevel Converter (MMC) Technology in Rail
Electrification
Dr. Rainer Gruber, Siemens AG and Darin O’Brien, Siemens Ltd. (Australia)
SUMMARY
Until recently, power converters for application to the electricity grid were mainly based on Thyristor based
technology. Today high power Voltage Source Converters (VSC) are becoming more advantageous and are
practical with the advent of fast digital control systems able to handle complex paralleling of converters..
Innovations in technology involving new Modular Multilevel Converter (MMC) technology allow VSCs in the
range of hundreds of MegaWatts.
1.
INTRODUCTION
This paper describes the application of new
Modular Multilevel Converter (MMC) technology in
the field of railway electrification power supply, as
found in Static Frequency Converters (SFC) and in
Static Var Compensators (SVC).
2.
NOTATION
Acronym
PCI
IGBT
IGCT
HV
HV DC
TCR
FFC
RFC
SC
MMC
SVC
Classic SVC
Active SVC
SFC
MMDC
PCR
BC
3
Definition
Pulse Controlled Inverter
Insulated-Gate Bipolar
Transistor
Integrated Gate Commutated
Thyristor
High Voltage
High Voltage Direct Current Link
Thyristor Controlled Reactor
Fixed Filter Circuit
Rotating Frequency Converter
Short Circuiter
Modular Multilink Converter
Static Var Compensator
An older Thyristor based SVC
An IGBT based SVC
Static Frequency Converter
Modular Multilevel Direct
Converter
Pulse Controlled Rectifier
Braking Chopper
as shown in Figure 1. With this technology it is
possible to set up various configurations such as a
Static
Frequency
Converter,
Static
Var
Compensator and HV DC link converter.
The submodules have a simple interface
consisting of two power, cooling water and two
fibre optic connections. They need no external
auxiliary power. The electronics are fed from the
DC-link capacitor of the submodule. The
submodules are connected directly in series and
can be switched individually as required. Each
submodule is able to switch positive, zero or
negative voltage to the output terminals. If one
submodule fails, the bypass-switch will be closed
providing continuous converter operation without
interruption. The failed sub module could then be
replaced at a future scheduled maintenance date.
SM electronics
IGBT11
IGBT21
D11
D21
+
uC
u
–
IGBT12
IGBT22
D12
D22
MODULAR MULTILEVEL
CONVERTER (MMC)
The innovation responsible for the new advances
in MMC technology is in the submodules. The
submodules consist primarily of 4 IGBTs, a
capacitor, a bypass-switch and electronic controls
Figure 1: Principle diagram for a MMC
submodule
AusRAIL 2014
11 – 12 November 2014, Perth
Dr. Rainer Gruber
Siemens AG
4
Use of MMC Technology in Rail Electrification
MMC APPLICATION: STATIC
FREQUENCY CONVERTER (SFC)
Modular Multilevel Direct Converters are an
innovative type of Static Frequency Converter for
Traction Power Supplies. They are characterized
by their simple and modular design as well as by
their system compatibility and wide applicability.
They can be easily expanded to the required
power and are especially suited for high power.
Compared to existing designs audible noise
emissions and required space are reduced.
4.1
frequency differing from the public electricity grid.
At the commencement of rail electrification, seriescharacteristic motors were used. This was
originally a DC motor with modifications so that it
could also be used for single phase with low
frequency. Central Europe used 16.7Hz and in the
US 25 Hz was introduced.
The energy for these early railways was produced
in dedicated power plants. Later, energy from the
public grid was utilised and converted via Rotating
Frequency Converters.
Over the last 40 years, SFC (Static Frequency
Converters) have been used for the power
conversion from three phase to single phase.
HISTORY OF SFC’s
Many single phase railway lines in Central Europe
and in the eastern part of the US are fed with a
Figure 2: Feeding scheme
4.2
Cyclo Converter
The first SFCs were cycloconverters with line
commutating Thyristors. Figure 3 shows a
simplified diagram of a cycloconverter. This was a
very robust and easy design. The Thyristors could
be easily connected in series and with that high
power level could be reached.
Unfortunately, the cycloconverter cannot store the
pulsating power of a 1AC line. In the
cycloconverter, there is no decoupling of the three
phase from the single phase grid. Also the power
quality of the cycloconverter is poor and requires
considerable filtering. These units are now
superseded with the last unit commissioned in
1992 in Baltimore (US) [1].
Filter
Filter
Figure 3: Principle diagram of a cycloconverter
AusRAIL 2014
11 – 12 November 2014, Perth
Dr. Rainer Gruber
Siemens AG
Use of MMC Technology in Rail Electrification
Figure 4. Voltage (green) and current (red) of a cycloconverter
L1
L2
L3
L
PCI
PCR
SC
BC
PCI
SC
BC
PCR
PCI
PCI
N
Figure 5: Principle diagram DC-Link converter
4.3
DC-Link Converter
The development of the DC-Link converter
commenced in the 1980’s. Equipped with
Thyristors (GTO, IGCT) and Transistors (IGBTs),
this type of converter is state of the art. Figure 5
shows the principles of a DC-Link converter
commissioned in the second half of 1994.
The energy conversion is carried out in two steps:
1) In the first step, the three phase AC is rectified
to DC by a line commutated Thyristor rectifier
or as shown in figure 5 by a Pulse Controlled
Rectifier (PCR). With the PCR, energy
exchange is possible in both directions.
2) The second step is the conversion of DC to
one phase AC. This is undertaken by one or
more Pulse Controlled Inverters (PCI). The
PCI voltages are summed up via the output
transformer and the higher the switching
frequency of the semiconductor, the better the
output voltage that can be approximated to a
sine wave (see Figure 6). Depending on the
power quality limits / requirements, input
and/or output filters are necessary. The DC-
AusRAIL 2014
11 – 12 November 2014, Perth
Dr. Rainer Gruber
Siemens AG
Use of MMC Technology in Rail Electrification
Link decouples the two grids. The pulsating
power is delivered by a DC-Link filter which is
tuned to double the frequency of the one
phase network.
U
t
phases, with each phase consisting of one branch
reactor and two branches. The number of “in
series” connected submodules per branch is
defined by the desired power level. For SFC
applications, the number of submodules per
branch is typically between 10 and 30, which leads
to a power level between 20 and 60 MVA. If the
level of the output voltage is equivalent to the
overhead contact line voltage, the converter can
be connected via an outgoing reactor. If the
converter is to be fed into a HV network then an
output transformer is used.
Function
Figure 6: Output voltage of 1 three level PCI
Compared to a cycloconverter, the structure of the
DC-Link converter with its different components
(Complex transformers, PCR BC, SC, DC-Filter,
PCI and AC-Filter) makes the units complex.
4.4
The New Innovation - Modular
Multilevel Direct Converter (MMDC)
Combining
the simple structure of
the
cycloconverter with the decoupling and the energy
storage capability of the DC-Link converter leads
to the MMDC.
Compared to the cycloconverter (where all
thyristors connected in series have to be switched
at the same time (line commutated)), the MMDC
submodules are switched individually as required.
Each submodule can supply positive, zero or
negative voltage. Hence one branch could be seen
as an “in step switchable voltage source”. The
reference value for one branch is the line-to-earth
voltage of the connected phase (of the three phase
side) and one half of the railway voltage. The
reference voltages for a 50 Hz to 16.7 Hz SFC of
all six branches are shown in Figure 8.
30
20
10
1
1
1
2
2
2
...
...
...
n
n
n
U [kV]
Submodule
0
10
20
16 ⅔ Hz
50 Hz
Branch
reactors
30
0
10
20
30
40
50
t [ms]
50 Hz
Transformer
Outgoing
reactor
1
1
1
2
2
2
...
...
...
n
n
n
Figure 7: Structure of a MMDC
Structure
The structure of the MMDC looks like a B6-bridge
as shown in Figure 7. The converter is built from 3
Figure 8: Reference values branch voltages
(red phase 1, green phase 2 and blue phase 3,
solid line upper branch and dotted line lower
branch respectively, black half of the single
phase voltage)
The branches also have to carry three-phase
current and one-phase current simultaneously.
During a short circuit (SC) on the one phase side,
the 3AC current normally is low and the 1AC
current could be increased. A typical value for the
AusRAIL 2014
11 – 12 November 2014, Perth
Dr. Rainer Gruber
Siemens AG
Use of MMC Technology in Rail Electrification
short circuit current is 20% more than nominal
current.
Compared to feeding the load with a RFC or direct
feed via transformer from the public grid, supplying
with a SFC requires a redesign or certification of
protection schemes to assure adequate protection
operation considering lower fault current levels [3].
Reference Sites
SFC with the new Multilevel Technology already in
operation include:
•
•
•
•
•
Nürnberg
Häggvik
Eskilstuna
Rostock
Adamsdorf
2 x 38 MVA
4 x 24 MVA
1 x 17 MVA
2 x 19 MVA
2 x 19 MVA
SFC with the new Multilevel Technology in the
project realization phase include:
•
•
•
•
•
•
•
•
Cottbus
Frankfurt.
Winkeln
Lund
Älvängen
Åstorp
Ystad
Uttendorf
2 x 19 MVA
2 x 19 MVA
2 x 60 MVA
2 x 27 MVA
2 x 27 MVA
3 x 19 MVA
1 x 19 MVA
1 x 50 MVA
5.
MMC APPLICATION: STATIC VAR
COMPENSATOR (SVC)
5.1
Thyristor based SVC (SVC Classic)
For many years thyristor based Static Var
Compensators have been on the market. They are
in use for balancing purposes, for example on the
British side of the Channel tunnel. In Figure 9, a
configuration with a Thyristor Controlled Reactor
(TCR) and a Fixed Filter Circuit (FFC) is shown.
The current through the reactor can be controlled
by phase angle control. The current of the
capacitor can’t be controlled easily; this is why the
inductive current has to compensate the capacitive
current more or less depending on the operation
point. This is why phase angle control harmonics
are produced by the TCR. The capacitor and a
small reactor are tuned to compensate for these
harmonics (single-, double- or triple-tuned filters
may be necessary). The high reactive power and
the filtering lead to a large space requirement
Figure 9: SVC consisting of TCR and FC
Reference Sites
There are many Thyristor based classic SVCs in
operation worldwide including Australia and in
particular in Queensland, with classic SVCs for
Rail application including:
•
•
•
•
•
•
•
•
•
Blackwater,
Coppabella,
Dingo,
Dysart,
Grantleigh,
Gregory,
Moranbah,
Oonooie,
Mt McLaren.
All of these were originally built by a competitor
and were re-furbished by Siemens with new
thyristors & control systems in 2006.
Oonooie SVC is typical of the above nine SVCs:
•
•
•
A-B: -14 to +17 MVAr
B-C: -12 to +19 MVAr
C-A: -15 to +16 MVAr
Siemens Classic SVC’s are also installed for
Transmission Utility application at
Nebo,
Strathmore, Alligator Creek, Greenbank and
Southpine.
Strathmore was commissioned in 2007.
These units enhance the power transfer in the
interconnection between Central and Northern
parts of the transmission system in Queensland.
They have a part of their swing range that can
operate in single phase mode to balance single
phase railway loads in the area
AusRAIL 2014
11 – 12 November 2014, Perth
Dr. Rainer Gruber
Siemens AG
5.2
Use of MMC Technology in Rail Electrification
IGBT based SVC (SVC plus / Rail
Active Balancer)
SVC PLUS
For the IGBT based SVC, the same MMC
submodules are used as in the SFC. Figure 10
shows a SVC in delta configuration. Each phase
consists of a smoothing reactor, the series
connected submodules and a small high frequency
filter.
n
.
2
.
1
n
...
2
n
1
These substations each have a +/-100MVAr swing
range. A fourth SVC PLUS also with a swing
range of +/-100MVAr is currently in construction at
Wotonga.
Rail Active Balancer
A Rail Active Bbalancer, implemented on the
traction power side of the point of common
coupling, has been installed on the Adelaide Rail
Electrification project as illustrated in Figures 12
and 13.
2
..
Wycarbah,
Duaringa, and
Bluff.
A SVC controls the voltage of a three phase grid
by delivering reactive current to the phases
depending on their voltage level. An R rail Aactive
Bbalancer is using the same hardware, but
different control strategy. Depending on the load
current of the rail way line, it feeds reactive current
into the three phases in such a way, that the load
current of the three phase grid is symmetrical.
..
1
•
•
•
Branch
reactor
Submodule
HFB Filter
Figure 10 Principle diagram SVC plus
The benefits include a small footprint, minimised
parts, containerised design as illustrated in Figure
11 and high availability (RAMS).
In this application, the overall AC traction system
has been designed to allow for continuous
operation with or without the SVC in operation.
While in operation, the system addresses all power
quality requirements of the network services
agreement between the rail operator and the
supply authority. The network services agreement
also permits a specified active balancer outage
period per year (eg. to allow for scheduled
maintenance.)
There are three SVC PLUS units installed and in
operation in Queensland for Rail application since
2012 located at:
AusRAIL 2014
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Dr. Rainer Gruber
Siemens AG
Use of MMC Technology in Rail Electrification
Figure 11 Active Balancer Container Components
Figure 12 Active Balancer SVC with connection on Traction Power Side
AusRAIL 2014
11 – 12 November 2014, Perth
Dr. Rainer Gruber
Siemens AG
Use of MMC Technology in Rail Electrification
Figure 13 Adelaide Rail Electrification Active Balancer
6
CONCLUSION
The newer Siemens IGBT based MMC technology
provides a high degree of flexibility in DC Link,
SFC and SVC converter design and facilitates an
optimal substation layout in an Active Balancer
configuration.
The benefits include a small footprint, minimised
parts, containerised design and high availability
(RAMS).
7
[1]
REFERENCES
[2]
Fieber, E.; Gruber, R.; Lehmann, V.;
Schuster,
R.:
Statischer
180-MVAFrequenzumrichter für den North-EastCorridor von Amtrak. In: Elektrische Bahnen
101 (2003), H. 8, S. 363–370.
[3]
Bagnal,
T.;
Siliézar,
F.
POWER
ELECTRONICS BASED TRACTION POWER
SUPPLY
FOR
50Hz
RAILWAYS
CORE2014
[4]
N., N.: Discover the World of
Technology, Siemens AG 2011
FACTS
Halfmann, U.; Recker, W.: Modularer
Multilevel Bahnumrichter. In: Elektrische
Bahnen 109 (2011), H. 4-5, S. 174–179.
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