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CIGRE 2008
A new Multilevel Voltage-Sourced Converter Topology for HVDC Applications
J. DORN, H. HUANG, D. RETZMANN
Siemens AG
Germany
SUMMARY
Since the Seventies line commutated converters based on thyristor technology have been the
standard for all HVDC applications. This converter topology is, however, marked by its loworder harmonics and high reactive power consumption. Such measures as the use of filter
circuits and switchable capacitor banks have been taken to reduce these effects. On the other
hand, these converters are marked by low operational losses, robustness of the thyristors
against surge currents as well as their overload capability. Therefore, line-commutated
thyristor technology will remain best suited to bulk power transmission in future HVDC
projects.
Compared with the line-commutated technology, additional inherent advantages are obtained
through the use of self-commutated converters. In this kind of converter topology, current
commutation is independent of the line voltage. This is the case because commutation is
purely based on the turn-off capability of the power semiconductors such as IGBTs (Insulated
Gate Bipolar Transistor). These semiconductors have gained in importance over the years
and, due to their reliability, they are most commonly used in such challenging applications as
traction and industrial drives.
The VSC (Voltage-Sourced Converter) is the most common type of self-commutated
converters and has therefore become a standard in the applications mentioned above. The
possibility to control active and reactive power independently within weak or even passive
networks makes them attractive to power transmission and distribution applications.
Most VSCs are based on two-level technology along with PWM (Pulse-Width Modulation)
controls. This implies that the AC connection voltage can only be switched between two
possible levels which are defined by the common DC link capacitor connected to the DC
terminals. Through pulse-width modulation with high switching frequencies (up to kHz
range), the desired RMS voltage can be controlled and even some discrete low order
harmonics can be reduced or eliminated. The use in HVDC applications requires high voltage
ratings of the converter which can be achieved by a series connection of powerhartmut.huang@siemens.com
semiconductors only. Therefore, all semiconductors within one valve have to switch
simultaneously at the mentioned frequencies. This results in high and steep voltage steps at
the AC connection terminal which, in its turn, generates a high level of electromagnetic
interference. Therefore, special measures, such as shielding and filtering, are necessary to
cope with these adverse effects.
Some attempts have been made and introduced in literature to reduce these effects by use of
multilevel converters, which results in the reduction in switching frequencies as well as height
and steepness of voltage steps. These multilevel converters are featured by a higher number of
different possible voltage levels which can be applied at the AC terminals to achieve a
waveform close to the sinus one.
A particular new type of multilevel converter technology is referred to as MMC (Modular
Multilevel Converter). It consists of a series connection of submodules. The output voltage of
each submodule can be switched either to zero or to the voltage of the integrated storage
capacitor. A very smooth and nearly ideal sinus waveform can thus be generated with MMC
converters, for each submodule is switched individually. Owing to this, the requirements to
filter circuits are much less severe. Additionally, the submodules can be switched at a
significantly lower frequency which, in its turn, leads to lower operational losses of the
converter. The MMC converter has a modular design resulting in its high flexibility.
This paper gives an insight into this innovative topology; the principles of its functioning are
explained in detail and information on the results of simulation is provided.
KEYWORDS
HVDC – Voltage-Sourced Converter (VSC) – Multilevel Converter – Modular Multilevel
Converter (MMC)
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INTRODUCTION
Over decades, thyristors as semiconductor switches have held much favour in the field of the HVDC
technology. The main aspect in further development was to increase the transmission power. 3,000
MW transmission systems have been in service for many years and the trend goes towards higher
transmission currents and DC voltages. Schemes with a transmission capability of more than 6,000
MW are on the way to implementation. Next to the high power capability, the thyristor technology is
featured by further outstanding characteristics, such as low losses, high reliability, high overload
capability and robustness against high surge currents. Therefore, thyristor-based technology will
remain best suited to bulk power transmission.
However, thyristor-based systems rest upon a line-commutated converter topology which results in the
generation of low-order harmonics and consumption of reactive power, which, in its turn, calls for
countermeasures. That is, in order to avoid negative impact on the whole system, filter circuits and
switchable capacitor banks have to be connected to the AC side of the converter stations to reduce the
effects mentioned above to an acceptable level.
This kind of behaviour does not occur in self-commutated voltage-sourced converter technology. In
this case, the commutation processes within the converter can be controlled regardless the line voltage.
VSC-based HVDC technology brings about the following technical benefits:
− Footprint requirements are lower.
− The active and reactive power can be controlled independently.
− An excellent dynamic response can be achieved, which is important to comply with the grid
code requirements in the event of AC system faults.
− A collapsed network can be restored by means of what is referred to as the black-start
capability.
− Passive loads without generation can also be supplied.
− Very weak power grids can be connected to the HVDC transmission system with little effort.
CONVERTER TECHNOLOGY
Nowadays, VSC-based HVDC systems in most cases rest upon a two-level technology. The main
principle of control is quite simple: either the positive or negative voltage potential of the common DC
capacitor can be switched to each of the three AC terminals (see Figure 1). Since the desired sine
waveform at the AC terminal cannot be adjusted in terms of magnitude, special measures, such as
PWM, are used to approximate the desired waveform. However, the difference between the
implemented and the desired voltage waveform is an unwanted distortion which has to be filtered.
+Ud/2
Ud / 2
)
Uconv
-Ud/2
Desired voltage
Ud / 2
Realized voltage
Figure 1 : Principle of control of converters based on two-level technology
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Furthermore, high voltage gradients occur in the switching instances, since some hundred kilovolts are
switched within microseconds. A great amount of radiated high frequency is the result as well.
These drawbacks of the two-level topology can be eliminated by applying a much better approximated
sine waveform in terms of adjustable magnitude of the voltage to the AC terminal. This can be done
with what is termed as a modular multilevel converter (MMC) topology. In this topology, the six
converter arms act as a controllable voltage source with a high number of possible discrete voltage
steps. This principle is shown in Figure 2.
+Ud/2
-Ud/2
Desired voltage
Realized voltage
Figure 2 : Overview of MMC topology
The figure shows the electrical equivalent of the MMC converter. During steady-state operation, the
voltages are controlled in order to achieve one third of the total DC current in each phase unit and to
achieve an equal sharing of the AC current in the upper and lower part of each phase unit, as shown in
the figure above. Each of the 6 variable voltage sources are designed with a number of identical but
individually controllable submodules, as shown in Figure 3.
Each submodule is a two-terminal component which can be switched between a state with full module
voltage (IGBT 1 = ON, IGBT 2 = OFF) and a state with zero module voltage (IGBT 1 = OFF,
IGBT 2 = ON) in both current directions. Next to auxiliary components and electronics, each
submodule consists of an IGBT half bridge and a capacitor unit. The capacitor voltage of each
submodule is subject to monitoring and control.
The main components of the submodule are of a well-proven industrial standard type which have been
used in different applications in the course of many years and have shown their reliability and maturity
under rough environmental conditions.
It is possible to individually and selectively control each of the individual submodules in a converter
arm. So, in principle, a converter arm represents a controllable voltage source as described above. The
total voltage of the two converter arms in one phase unit equals the DC voltage, and by adjusting the
ratio of the converter arm voltages in one phase module, the desired sinusoidal voltage at the AC
terminal can be achieved.
Dependent on the current direction, the capacitor can be charged or discharged if IGBT 1 is turned on.
In the event of a submodule failure during operation, this fault is detected and the defective submodule
is short-circuited by a highly reliable high-speed bypass switch, which is connected in parallel to
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IGBT 2 (not depicted in Figure 3). This provides the fail-safe functioning, as the current of the failed
module can continue flowing, and the converter operates without any interruption.
IGBT1
D1
+
D2
IGBT2
Figure 3 : Design of MMC, principle design of a submodule and its electrical equivalent
Waveforms of a steady-state operation of a 400 MW converter with 200 submodules per phase arm
and without any filter equipment are shown in Figure 4. The upper plot shows the DC voltages
(+/- 200 kV) and the AC terminal voltages with respect to a virtual reference point. The plot in the
middle illustrates the current in the AC terminals of the converter. The plot at the foot of the figure
shows the six phase arm currents in the converter.
Obviously, the harmonic distortion is reduced to a minimum.
The converter reactors shown in Figure 3 have two functions:
− The three-phase modules of the converter are connected in parallel at the DC side. Since the
three generated DC voltages of the phase modules cannot be exactly equal, balancing currents
occur between the individual phase units. The converter reactors damp these balancing
currents to a very low level and make them controllable by means of appropriate methods.
− The reactors substantially reduce the effects of faults arising inside or outside the converter.
As a result, unlike in previous VSC topologies, current rise rates of only a few tens of amperes
per microsecond are encountered even in so far very critical faults, such as a short circuit
between the DC terminals of the converter. These faults are swiftly detected, and, due to the
low current rise rates, the IGBTs can be turned off at absolutely uncritical current levels.
Consequently, this feature provides very effective and reliable protection of the system.
Next to excellent on-state characteristics, the MMC topology also offers a strong dynamic behavior,
which is essential to comply with the grid code requirements including fault ride-through capability in
case of faults in the AC network.
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PLOTS : Graphs
250
+Ud
-Ud
US1
US2
US3
200
150
100
U [kV]
50
0
-50
-100
-150
-200
-250
2.00
is1
is2
is3
1.50
1.00
I [kA]
0.50
0.00
-0.50
-1.00
-1.50
-2.00
0.75
i1p
i2p
i3p
i1n
i2n
i3n
0.50
0.25
I [kA]
0.00
-0.25
-0.50
-0.75
-1.00
-1.25
-1.50
1.000
1.010
1.020
Figure 4 : Typical waveforms of a 400 MW VSC based on MMC topology - AC line-to-line
converter voltages; AC converter currents; converter arm currents
Figure 5 gives a picture of the dynamic capability. For example, the figure shows the dynamic
response of a 400 MW system to an AC line-to-ground fault. The converters of this system are
connected to the AC network via star-delta transformers. It can be seen that this kind of fault can be
handled without blocking the converter.
Even in case of a DC pole-to-pole short circuit, the MMC topology offers advantageous features. In
this case, the IGBTs are blocked within microseconds, and a command to open the AC circuit breaker
is sent. Due to the converter reactors, the IGBTs are turned off within their nominal current range. In
comparison with other VSC topologies, DC circuit breakers are not required to avoid a discharge of
the capacitors. Subsequently, a fast re-closure of the AC circuit breaker can be commanded and energy
transfer can continue.
In September, 2007, Siemens secured an order to supply two converter stations for a new submarine
high-voltage direct-current transmission link in the Bay of San Francisco. The HVDC PLUS system
will transmit up to 400 megawatts at a DC voltage of +/- 200 kV and is the first order for the
innovative HVDC PLUS technology of Siemens. From March, 2010, the 55 mile (88 kilometers) long
HVDC PLUS system will transmit electric power from the converter station in Pittsburg to the
converter station in San Francisco, providing a dedicated connection between the East Bay and San
Francisco. The main advantage of the new HVDC PLUS link is a higher level of network security and
reliability due to its enhancement, including voltage support and reduced system losses. Furthermore,
the “Trans Bay Cable” project will save the trouble of building additional new power plants in the City
of San Francisco, decrease transmission grid congestion in the East Bay and it will also boost the
overall security and reliability of the power system.
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CONVERTER B
UL1
UL2
UL3
u_prim (kV)
100
50
0
-50
-100
US1
US2
US3
UdHP
UdHN
i_conv (kA)
u_conv, u_dc (kV)
200
100
0
-100
-200
2.50
2.00
1.50
1.00
0.50
0.00
-0.50
-1.00
-1.50
-2.00
-2.50
1.50
IS1
I1P
IS2
I2P
IS3
I3P
I1N
I2N
I3N
i_cvm (kA)
1.00
0.50
0.00
-0.50
-1.00
-1.50
IdHP
IdHN
i_dc (kA)
1.50
0.0
1.750
1.800
1.850
1.900
1.950
2.000
Figure 5 : Dynamic response to an AC line-to-ground fault of a 400 MW system based on
MMC - AC line-to-ground voltages; AC phase-to-phase voltages and DC voltages;
AC converter currents; converter arm currents; DC current
SUMMARY
Owing to the MMC topology, the power range as well as the achievable DC voltage of the converter is
essentially determined by the performance of the control system. With the current technology,
transmission ratings of 1000 MW and above are feasible. In addition to this, the following advantages
are particularly worth mentioning:
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−
−
−
−
−
The switching frequency of the individual semiconductors is approximately ten times lower
than that of 2 or 3-level topologies. This results in lower switching losses and, consequently,
lower total losses of the transmission system.
Very smooth AC voltage waveforms can be achieved resulting in a low amount of generated
harmonics and radiated high frequency. In most cases, no AC filters are necessary, that is, the
required footprint for the converter station is small.
The topology is capable of handling internal and external faults more effectively than the other
VSC topologies. For instance, a short circuit between the two DC poles of the transmission
line does not lead to a discharge of the capacitors.
Very robust and well-proven industrial standard components can be used to achieve high
robustness and reliability. Such components as medium voltage DC capacitors or IGBTs can
be supplied by a number of manufacturers which guarantees their long-term-availability. They
have often proven their reliability and performance under severe environmental and operating
conditions in other applications such as traction drives.
One of the main objectives of the development was a highly modular design in hardware and
software to achieve an excellent scalability. Furthermore, the system can be adapted to the
specific project requirements due to the high flexibility of the topology. Little engineering
effort and short project duration are the result.
CONCLUSION
The MMC technology is ideally suitable for HVDC cable transmission and back-to-back
arrangements. It is also possible to connect overhead transmission lines without DC circuit breakers.
Multi-terminal systems with more than two converter stations on one DC link are also available with
the MMC topology. Even complete DC networks with radiant, meshed and ring structures will be
feasible in the future. The topology is easy to scale with little engineering effort in the specific projects
due to its modular design and mechanical construction.
This technology holds good for a wide range of different applications. Next to DC cable connections
with a power range of up to 1,000 megawatts, it is also possible to connect very weak grids (e.g.
islanded networks) and renewable energy sources (e.g. offshore wind farms). Oil platforms can be
supplied from the coast via HVDC, so that environmentally detrimental diesel generators or other
local power generation on the platform can be avoided.
The MMC topology was developed for the application to HVDC transmission systems. Compared
with other VSC technologies, MMC offers additional benefits, particularly with respect to operational
losses, EMC and suitability for HV applications.
The technology referred to as HVDC PLUS is now available with a high degree of modularity and
scalability for power ranges of up to 1,000 MW or even higher.
BIBLIOGRAPHY
[1]
Working Group B4- WG 37 CIGRE, “VSC Transmission”, May 2004
[2]
Schettler F., Huang H., Christl N. “HVDC transmission systems using voltage source converters
– design and applications,” IEEE Power Engineering Society Summer Meeting, July 2000
[3]
Marquardt R., Lesnicar A. “New Concept for High Voltage – Modular Multilevel Converter”,
PESC 2004 Conference Aachen, Germany
[4]
Dorn J., Huang H., Retzmann D. “Novel Voltage-Sourced Converters for HVDC and FACTS
Applications”, Cigré Symposium Osaka 2008, Japan
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