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7/21/2019 HVDC-TransSys-Benefits of Multilevel VSC ENR 07-12 v 1a
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ABSTRACT:HVDC (High Voltage Direct Current) systems and FACTS (Flexible AC TransmissionSystems) provide essential features to avoid technical problems in the power systems; they increase
the transmission capacity and system stability in a very efficient way, and assist in prevention ofcascading disturbances. Moreover, they effectively support the access of renewable energy resources
to the grid and help reduce the transmission losses by optimization of power flows.
HVDC systems and FACTS controllers based on line-commutated converter technology have a long
and successful history. Thyristors are the key components of this converter topology and they have
achieved a high degree of maturity due to their robust technology and high reliability. It is, however,
worth mentioning that line-commutated converters have some technical restrictions. Particularly the
fact that the commutation within the converter is driven by the AC voltages requires proper conditions
of the connected AC system, such as a minimum short-circuit power.
Power electronics with self-commutated converters, such as Voltage-Sourced Converters (VSC), can
overcome these limitations and they provide additional technical features. In many applications, VSC
have become a standard of self-commutated converters and will be used increasingly more often intransmission and distribution systems in the future. VSCs do not require any driving system voltage
- they can build up a three-phase AC voltage via the DC voltage (Black-Start capability). So, in the
case of DC transmission, HVDC PLUS with VSCs is the preferred technology for interconnection of
islanded grids, such as offshore wind farms, with the power system.
So far, VSCs for HVDC and FACTS applications are mostly based on two or three-level converters. It
is, however, a fact that multilevel VSCs provide significantly more advantages with respect to the
performance and harmonic impact. For these reasons, a new Modular Multilevel Converter
technology (MMC), referred to as HVDC PLUS and SVC PLUS, has been developed, which provides
tremendous benefits for high voltage applications.
KEY WORDS: Elimination of Bottlenecks in Transmission; Enhanced Grid Access for Regenerative
Energy Sources (RES); Increase in Transmission Capacity; Security and Sustainability of PowerSupply; Smart Grid Technologies
December 04th to 07th 2007 - All-Russian Exhibition Centre (AREC), Moscow, Russia
04 07 2007. - (), ,
International Exhibition and Seminar
December 04th to 07th 2007 - All-Russian Exhibition Centre (AREC), Moscow, Russia
04 07 2007. - (), ,
International Exhibition and Seminar
Benefits of Multilevel VSC Technologies
for Power Transmission andSystem Enhancement
Joerg Dorn, Dirk Ettrich, Joerg Lang, Dietmar Retzmann*
Siemens Germany
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In what follows, the global trends in power markets and the prospects of system developments are
depicted, and the outlook for VSC technologies for environmental sustainability and system security is
given.
3. SMART GRID SOLUTIONS WITH POWER ELECTRONICS
The vision and enhancement strategy for the future electricity networks is depicted in the program of
SmartGrids, which was developed within the European Technology Platform (ETP) of the EU in its
preparation of the 7th Frame Work Program.
Features of a future SmartGrid of this kind can be outlined as follows [1, 18]:
Flexible:fulfilling customers needs whilst responding to the changes and challenges ahead
Accessible: granting connection access to all network users, particularly to RES and highly
efficient local generation with zero or low carbon emissions
Reliable:assuring and improving security and quality of supply
Economic: providing best value through innovation, efficient energy management and level
playing field competition and regulation
It is worthwhile mentioning that the Smart Grid vision is in the same way applicable to the system
developments in other regions of the world. Smart Grids will help achieve a sustainable development.
The key to achieve a Smart Grid performance will be the use of power electronics.
3.1 HVDC and FACTS Technologies
HVDC systems and FACTS controllers based on line-commutated converter technology (LCC) have a
long and successful history. Thyristors have been the key components of this converter topology and
have reached a high degree of maturity due to their robust technology and their high reliability. HVDC
and FACTS with LCC use power electronic components and conventional equipment which can be
combined in different configurations to switch or control reactive power, and to convert the active
power. Conventional equipment (e.g. breakers, tap-changer transformers) has very low losses, but the
switching speed is relatively low. Power electronics can provide high switching frequencies up to
Use of Dispersed GenerationUse of Dispersed Generation
GG GG
HH
GG
GG
GG
GG
Load Flow will be fuzzy
Today:Tomorrow:
GG
GGHH
GG HH
GG
GG
GG
Fig. 2: Regenerative Energy Sources and Dispersed Generation Impact on the whole
T&D Grid Structure
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several kHz which, however, leads to an increase in losses.
Fig. 3 indicates the typical losses depending on the switching frequency [16]. It can be seen that due to
the low losses, line-commuted Thyristor technology is the preferred solution for bulk power
transmission, today and in the future.
It is, however, worth mentioning that line-commutated converters have some technical restrictions.
Particularly the fact that the commutation within the converter is driven by the AC voltages requires
proper conditions of the connected AC system, such as a minimum short-circuit power.
3.2 Voltage-Sour ced Converters
Power electronics with self-commutated converters can cope with the limitations mentioned above and
provide additional technical features. In DC transmission, an independent control of active and
reactive power, the capability to supply weak or even passive networks and lower space requirements
are some of the advantages. In many applications, the VSC has become a standard of self-commutated
converters and will be used more often in transmission and distribution systems in the future. Voltage-
Sourced Converters do not require any driving system voltage; they can build up a 3-phase AC
voltage via the DC voltage. This kind of converter uses power semiconductors with turn-off capabilitysuch as IGBTs (Insulated Gate Bipolar Transistors). The benefits of VSC technology are depicted in
Fig. 4.
Up to now, the implemented VSC converters for HVDC applications have been based on two or three-
level technology which enables switching two or three different voltage levels to the AC terminal of
the converter. To make high voltages in HVDC transmission applications controllable by
semiconductors with a blocking ability of a few kilovolts, multiple semiconductors are connected in
series up to several hundreds per converter leg, depending on the DC voltage. To ensure uniform
voltage distribution not only statically but also dynamically, all devices connected in series in one
converter leg have to switch simultaneously with the accuracy in the microsecond range. As a result,
high and steep voltage steps are applied at the AC converter terminals which require extensive filtering
measures.In Fig. 5, the principle of two-level converter technology is depicted. From the figure, it can be seen
More Dynamics for better Power Quality:
Use of Power Electronic Circuits for Controlling P, V & Q
Parallel and/or Series Connection of Converters
Fast AC/DC and DC/AC Conversion
ThyristorThyristor
50/60 Hz
ThyristorThyristor
50/60 Hz
GTOGTO
< 500 Hz
GTOGTO
< 500 Hz
IGBT / IGCT
Losses
> 1000 Hz
IGBT / IGCT
LossesLosses
> 1000 Hz
Transition from slow to fast
Switching
Frequency
On-Off Transition 20 - 80 ms
Transition from slow to fastTransition from slow to fast
Switching
Frequency
On-Off Transition 20 - 80 ms
1-2 %1-2 %
The Solution forBulk Power TransmissionThe Solution forBulk Power Transmission
Dependingon SolutionDependingon Solution
2-4 %2-4 %
Fig. 3: Power Electronics for HVDC and FACTS Transient Performance and Losses
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that the converter voltage, created by PWM (Pulse-Width Modulation) pulse packages, is far away
from the desired green voltage, it needs extensive filtering to approach a clean sinus waveform.
3.3 The Modular Mul til evel Converter (MMC) Approach
Both the size of voltage steps and the related voltage gradients can be reduced or minimized if the ACvoltage generated by the converter can be selected in smaller increments than at two or three levels
only.
The finer this gradation, the smaller is the proportion of harmonics and the lower is the emitted high-
frequency radiation. Converters with this capability are termed multilevel converters.
Furthermore, the switching frequency of individual semiconductors can be reduced. Since each
switching event creates losses in the semiconductors, converter losses can also be effectively reduced.
Different multilevel topologies [7-10], such as diode clamped converter or converters with what is
termed flying capacitors were proposed in the past and have been discussed in many publications.
In Fig. 6, a comparison of two, three and multilevel technology is depicted. A new and different
multilevel approach is the modular multilevel converter (MMC) technology [9].
High harmonic Distortion
High Stresses resulting in HF Noise
Desired voltage Realized voltage
- Vd/2
0
+Vd/2
Desired voltage Realized voltage
- Vd/2
0
+Vd/2
VConv.
Vd/2
Vd/2VConv.
Vd/2
Vd/2
Fig. 5: VSC Technology a Look back
Fig. 4: General Features of VSC Technology
Grid Access of weak AC Networks
High dynamic Performance
Independent Control of Active and Reactive Power
Supply of passive Networks and Black Start Capability
Low Space Requirements
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Fig. 7: The Multilevel Approach
a) Conventional Solution
b) Advanced MMC Solution
c) Sinus Approximation and Benefits
Vd/ 2
Vd/ 2VConv.
Vd/ 2
Vd/ 2
Vd/ 2
Vd/ 2VConv.VConv.
a)
Vd
VConv.
Vd
VConv.VConv.
b)
Small Converter AC Voltage Steps
Small Rate of Rise of Voltage
Low Generation of Harmonics
Low HF Noise
Low Switching Lossesc)
Power
Electronic
Devices:
Topologies:Two-Level Three-Level Multilevel
IGBT in PP IGBT ModuleGTO / IGCT
Fig. 6: The Evolution of VSC and HVDC PLUS Technology
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The principle design of conventional multilevel converter and advanced MMC is shown in Fig. 7
while Fig. 8 depicts the HVDC PLUS MMC solution in detail.
A converter in this context consists of six converter modules, whereas the individual modules consist
of a number of submodules (SM) connected in series with each other and with one converter reactor.
Each of the submodules contains [9, 16, 17]:
- an IGBT half bridge as a switching element
- a DC storage capacitor
For the sake of simplicity, the electronics for control of semiconductors, measurement of the capacitor
voltage, and for communication with the higher-level control are not shown in Fig. 8. Three different
states are relevant for the proper operation of a submodule, as illustrated in Table I:
1.
Both IGBTs are switched off:
This can be compared with the blocked condition of a two-level converter. Upon charging, i.e.
after closing the AC power switch, all submodules of the converter are in this condition.
Moreover, in the event of a serious failure all submodules of the converter are put in this state.
During normal operation with power transfer, this condition does not occur. If the current flows
from the positive DC pole in the direction of the AC terminal during this state, the flow passes
through the capacitor of the submodule and charges the capacitor. When it flows in the opposite
direction, the freewheeling diode D2 bypasses the capacitor.
2. IGBT1 is switched on, IGBT2 is switched off
Irrespective of the current flow direction, the voltage of the storage capacitor is applied to the
terminals of the submodule. Depending on the direction of flow, the current either flows through
D1 and charges the capacitor, or through IGBT1 and thereby discharges the capacitor.
3.
IGBT1 is switched off, IGBT2 is switched on:
In this case, the current either flows through IGBT2 or D2 depending on its direction whichensures that zero voltage is applied to the terminals of the submodule (except for the conducting-
Fig. 8: HVDC PLUS Basic Scheme
Submodule (SM)
Vd
Converter Module
Phase Arm
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state voltage of the semiconductors). The voltage in the capacitor remains unchanged.
It is thereby possible to separately and selectively control each of the individual submodules in all
phase units. So, in principle, the two converter modules of each phase unit represent a controllable
voltage source. In this arrangement, the total voltage of the two converter modules in each phase unit
equals the DC voltage, and by adjusting the ratio of the converter module voltages in one phase unit,
the desired sinusoidal voltage at the AC terminal can easily be achieved.
State 1 State 2 State 3State 1 State 2 State 3
Off
Off
Off
Off
On
Off
On
Off
Off
On
Off
On
State 1 State 2 State 3
Table I: States and Current Paths of a Submodule in the MMC Technology
Fig. 9: The Result MMC, a perfect VoltageGeneration
VConv.
- Vd/2
0
+Vd/2
VAC
AC and DC Voltages controlledby Converter Module Voltages:
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Fig. 9 depicts this advanced principle of AC voltage generation with MMC. It can be seen that there is
almost no or in the worst case negligible need for AC voltage filtering to achieve a clean voltage,
in comparison with the two-level circuit with PWM in Fig. 5.
As is true in all technical systems, sporadic faults of individual components during operation cannot be
excluded, even with the most meticulous engineering and 100-percent routine test. However, if a fault
occurs, the operation of the system must not be impeded as a result. In the case of an HVDCtransmission system this means that there must be no interruption of the energy transfer and that the
system will actually continue to operate until the next scheduled shut-down for maintenance.
Redundant submodules are therefore integrated into the converter, and, unlike in previous redundancy
concepts, the unit can now be designed so that, upon failure of a submodule in a converter module, the
remaining submodules are not subjected to a higher voltage. The inclusion of the redundant
submodules thus merely results in an increase in the number of submodules in a converter module that
deliver zero voltage at their output during operation. In the event of a submodule failure during
operation this fault is detected and the defective submodule is shorted out by a highly reliable high-
speed bypass switch, ref. to Fig. 10. This provides fail-safe functionality, as the current of the failed
module can continue to flow, and the converter continues to operate, without any interruption.
As in all multilevel topologies it is necessary to ensure, within certain limits, a uniform voltagedistribution across the individual capacitors of the multilevel converter. When using the MMC
topology for HVDC this is achieved by periodic feedback of the current capacitor voltage to a central
control unit. The time intervals between these feedback events are less than 100 microseconds.
Due to the fact that in each line cycle in the converter modules, current flow occurs both in one and in
the other direction and that charging or discharging of the individual capacitors is possible, evaluation
of the feedback and selective switching of the individual submodules can be used to balance the
submodule voltages. With this approach, the capacitor voltages of all submodules of a converter
module in HVDC PLUS are maintained within a defined voltage band.
From the perspective of the DC circuit, the described topology looks like a parallel connection of three
voltage sources the three phase units that generate all desired DC-voltages. In practice, however,
there will be little difference between the momentary values of the three DC voltages, owing to thefinite number of available voltage steps.
Phase Unit Submodule
PLUSCONTROL
High-Speed Bypass Switch
Fig. 10: MMC Redundant Submodule Design
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To dampen the resulting balancing currents between the individual phase units, and to reduce them to
a very low value by means of appropriate control methods, a converter reactor is integrated into the
individual phase arms. In addition to the aforementioned function, these reactors are also used to
substantially reduce the effects of faults arising within 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.
These faults are swiftly detected, and, due to the low current rise rates, the IGBTs can be turned off at
absolutely uncritical current levels. This capability thus provides very effective and reliable protection
of the system.
The following describes a very interesting fault occurrence:
In the event of a short-circuit between the DC terminals of the converter or along the transmission
route, the current rises in excess of a certain threshold value in the converter modules, and, due to the
aforementioned limitation of the speed in the current rise, the IGBTs can be switched off within a few
microseconds before the current can reach a critical level, which provides an effective protective
function. Thereafter as with any VSC topology current flows from the three-phase AC system
through the free-wheeling diodes to the short-circuit, so that the only way this fault can be corrected is
by opening the AC circuit breaker.The free-wheeling diodes used in VSC converters have a low capacity for withstanding surge current
events related to their silicon surface, i.e. only a very limited ability to withstand a surge in current
without sustaining damage. In an actual event, the diodes would have to withstand a surge fault current
without damage until the circuit breaker opens, i.e. in most cases for at least three line cycles. In
HVDC PLUS, a protective function at the submodule level effectively reduces the load of the diodes
until the circuit breaker opens. This protective measure consists of a press-pack thyristor, which is
connected in parallel to the endangered diode and is fired in the event of a fault, ref. to Fig. 11.
As a result, most of the fault current flows through the thyristor and not through the diode it protects.
Press-pack thyristors are known for their high capability to withstand surge currents. This
characteristic is also useful in conventional, line-commutated HVDC transmission technology. This
fact makes HVDC PLUS suitable even for overhead transmission lines, an application previouslyreserved entirely for line-commutated converters with thyristors.
Phase Unit
PLUSCONTROL
Submodule
Protective Thyristor Switch
Fig. 11: Fully suitable for DC OHL Application Example Line-to-Line Fault
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Thanks to its modular construction, the HVDC PLUS converter is extremely well scalable, i.e.
conveniently adaptable to any required power and voltage ratings. The mechanical construction
adheres consistently to the modular design. Sets of six submodules are assembled to form
transportable units that are easy to install with the proper tools. The required number per converter
module can be optimally realized by a horizontal array of such units and if required by assembling
them in a vertical arrangement to meet the specific project requirements.
Fig. 12 depicts a view of the MMC design. In principle, both a standing and a suspended construction
can be readily achieved. However, a standing construction was chosen, since in that case the converter
design imposes less specific requirements on the converter building.
If required in specific projects, highly effective protective measures against severe seismic loads can
also be implemented (ref. to Fig. 12). For such a situation, provisions have been made for diagonal
braces at the individual units that ensure adequate stability of the construction.
The submodules are connected bi-directionally via fiber optics with the PLUSCONTROL (Fig. 13),
the central control unit. The PLUSCONTROL was developed specifically for HVDC PLUS and has
the following functions:
- Calculation of appropriate converter module voltages at time intervals of several microseconds
- Selective actuation of the submodules depending on the direction of current flow and on therelevant Capacitor voltages in the submodules so as to assure reliable balancing of capacitor
voltages
In addition to the current status of each submodule, the momentary voltage of the capacitor is
communicated via the fiber optics to the PLUSCONTROL. Control signals to the submodule, such as
the signals for the switching of the IGBTs, are communicated in the opposite direction from the
PLUSCONTROL to the submodules.
Key features of the PLUSCONTROL are:
- Mechanical construction in standard 19-inch racks
- High modularity and scalability through plug-in modules, and the capability of integrating
different numbers of racks into the system
- Uniform redundancy concept with an active and passive system and the ability to change over on
the fly
Fig. 12: HVDC PLUS The Advanced MMC Technology
Converter Module with more than 200 Submodules
Typical Converter Arrangement for 400 MW
Optional
Seismic
Reinforcements
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- Modules and fans can be replaced during operation
- Sufficient interfaces for communication and control of well over 100 submodules per rack
- High performance with respect to computational power and logic functions
The PLUSCONTROL was integrated into the industry-proven SIMATIC TDC environment, which
provides the platform for the measuring system and the higher-level control and protection.
The MMC topology used in HVDC PLUS differs from other, already familiar VSC topologies in
design, mode of operation, and protection capabilities. The following summarizes the essentialdifferences and related advantages:
- A highly modular construction both in the power section and in control and protection has been
chosen. As a result, the system has excellent scalability and the overall design can be engineered
in a flexible way. Thus, the converter station can be perfectly adapted to the local requirements,
and depending on those requirements, the design can favor a more vertical or more horizontal
construction. The use of HVDC can therefore become technically and economically feasible
starting from transmission rates of several tens of megawatts.
- In normal operation, no more than one level per converter module switches at any given time. As
a result, the AC voltages can be adjusted in very fine increments and a DC voltage with very little
ripple can be achieved, which minimizes the level of generated harmonics and in most cases
completely eliminates the need for AC filters. Whats more, the small and relatively shallowvoltage steps that do occur cause very little radiant or conducted high-frequency interference.
- The low switching frequency of the individual semiconductors results in very low switching
losses. Total system losses are therefore relatively low for VSC PLUS technology, and the
efficiency is consequently higher in comparison with existing two and three-level solutions.
- HVDC PLUS utilizes industrially proven standard components, such as IGBT modules, which are
very robust and highly reliable. These components have proven their reliability and performance
many times under severe environmental and operating conditions in other applications, such
as traction drives. This wide range of applications results in a larger number of manufacturers as
well as long-term availability and continuing development of these standard components.
- The encountered voltage and current loads support the use of standard AC transformers.
- The achievable power range as well as the achievable DC voltage of the converter is determinedessentially only by the performance of the controls, i.e. the number of submodules that can be
Fig. 13: Main Tasks of PLUSCONTROLTM
SIMATIC TDC
C&P System
SIMATIC TDC
Measuring System
Calculation of requiredConverter Leg Voltages
Selection of Submodulesto be switched
Control of Active andReactive Power
Submodule VoltageBalancing Control
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operated. With the current design, transmission rates of 1000 MW or more can be achieved.
- Due to the elimination of additional components such as AC filters and their switchgear, high
reliability and availability can be achieved. Whats more, the elimination of components and the
modular design can shorten project execution times, all the way from project development to
commissioning.
- With respect to later provision of spare-parts, it is easy to replace existing components by state-of-the-art ones, since the switching characteristics of each submodule are determined
independently of the behavior of the other submodules. This is an important difference to the
direct series-connection of semiconductors as in the two-level technology where nearly
identical switching characteristics of the individual semiconductors are mandatory.
- Internal and external faults, such as short-circuit between the two DC poles of the transmission
line, are reliably managed by the system, due to the robust design and the fast response of the
protection functions.
Figs. 14-16 summarize the advantages in a comprehensive way. Added to these are the
aforementioned advantages that ensue from the use of VSC technology in general (see Fig. 4).
With these features, HVDC PLUS is ideally suitable for the following DC systems (Fig. 16):
- Cable transmission systems. Here, the use of modern extruded cables, i.e. XLPE, is possible, since
the voltage polarity in the cable remains the same irrespective of the direction of current flow.
- Overhead transmission lines, due to the capability to manage DC side short-circuits and
prompt resumption of system operation, and
- Back-to-back arrangement, i.e. rectifier and inverter in one station.
- The implementation of multiterminal systems is relatively simple with HVDC PLUS. In these
systems, more than two converter stations are linked to a DC connection. It is even possible to
configure complete DC networks with branches and ring structures. The future use for systems
such as these was addressed in the development of HVDC PLUS by pre-engineering the control
strategies required for them.
- It goes without saying that the converters can also be used as STATCOMs, e.g. when the
transmission line or cable is out of service during maintenance or faults. STATCOM with PLUS
technology is also useful in unbalanced networks, for instance in the presence of large single-
High Modularity in Hardwareand Software
Low Generation of Harmonics
Low Switching Frequency ofSemiconductors
Use of well-proven StandardComponents
Sinus shaped AC VoltageWaveforms
Easy Scalability
Reduced Number of PrimaryComponents
Low Rate of Rise of Currentseven during Faults
High Flexibility, economicalfrom low to high Power Ratings
Only small or even no Filtersrequired
Low Converter Losses
High Availability of State-of-the-Art Components
Use of standard ACTransformers
Low Engineering Efforts,Power Range up to 1000 MW
High Reliability, lowMaintenance Requirements
Robust System
Fig. 14: Features and Benefits of MMC Topology
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phase loads. Symmetry of the three-phase system can to some extent be restored by using load
unbalance control.
This multitude of possibilities in combination with the performance of HVDC PLUS opens up a wide
range of applications for this technology:
- DC connections for a power range of up to 1,000 megawatt, in which presently only line-
commutated converters are used
- Grid access to very weak grids or islanded networks, and
- Grid access of renewable energy sources, such as offshore wind farms, via HVDC PLUS. This
can substantially help reduce CO2emissions. And vice versa, oil platforms can be supplied from
the coast via HVDC PLUS, so that gas turbines or other local power generation on the platform
can be avoided.
Furthermore, with its space-saving design (Fig. 15) and its technical performance (Fig. 16), HVDC
PLUS is tomorrows solution for the supply of megacities.
Due to these technical and economical benefits, in September 2007 Siemens secured the order to
supply two converter stations for a new submarine high-voltage direct-current (HVDC) 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,
ref. to Fig. 17. From March 2010, the 55 mile (88 kilometers) long HVDC PLUS system will transmit
electrical 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. Main advantages of the
new HVDC Plus link are the increased network security and reliability due to network upgrade
including voltage support and reduced system losses. Today, the major electrical supply for the City of
HVDC PLUSHVDC PLUS
HVDC
Classic
HVDC
Classic
Example 400 MWExample 400 MW
Space
Saving
Fig. 15: Space Saving in Comparison with HVDC Classic
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San Francisco is coming from the south side of the San Francisco peninsula. The city relies mainly on
AC grids which run along the lower part of the bay with the new HVDC PLUS interconnection link
power flows directly in the center of San Francisco and closes the loop of the already existing Greater
Bay Area transmission loop. Therefore, the system security is increased, for DC cables will be buried
in a corridor separate from any existing AC lines as well as a reduced power flow on existing
Peninsula and East Bay lines, benefiting the entire Bay Area.
Furthermore, the DC 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 electrical system. The order was placed by Trans Bay Cable LLC,
based in San Francisco, and a wholly-owned subsidiary of the project developer Babcock & Brown.
As consortium leader, Siemens was awarded a turnkey contract which comprises the converter stations
for the HVDC PLUS system, including engineering, design, manufacturing, installation and
commissioning of the HVDC transmission system. Siemens will deliver all high voltage components
including transformers, converter modules, converter reactors and breakers and is responsible for the
control & protection, civil works and building systems. Furthermore, Siemens is to fulfill all major
requirements, which have to be considered for the electrical components as well as for all buildings for
a highly seismic zone such as San Francisco. The HVDC PLUS solution can meet all the needs in
terms of the minimum space available for the converter sites in urban areas as well as in terms of the
less significant environmental impact such as visual implication, audible noise and transport during
construction. The consortium partner Prysmian will supply and install the submarine cables.
Fig. 18 depicts the results of a computer simulation for the 400 MW MMC system, which will be
applied in the case of the Trans Bay Cable project. The figure clearly shows that with 200 submodules
per converter module no additional AC filtering will be required.
When transmission redundancy is required, HVDC PLUS can be configured in two ways, as depicted
in Fig. 19. Option a) is a solution with two symmetrical monopoles. Option b) - with bipole - can be
selected, when cost saving for one cable/line conductor is required instead. In this case, however,
standard AC transformers can not be used, HVDC transformers would be required.
Both options provide full n-1 redundancy for the whole transmission scheme, including cable or line.
Fig. 16: Applications and Features of HVDC PLUS
DC Cable TransmissionDC Cable Transmission
DC Overhead Line TransmissionDC Overhead Line Transmission
Back-to-Back SystemsBack-to-Back Systems
Multiterminal SystemsMultiterminal Systems
STATCOM Features includedSTATCOM Features included
The Advanced MMC TechnologyThe Advanced MMC Technology
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Fig. 17: Trans Bay Cable, USA Worlds 1st VSC HVDC Project with advanced MMC-
Technology and +/- 200 kV XLPE DC Cable
a) Geographic Map and System Requirements
b) Siemens Converter Stations and Prysmian Cable Technologies
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P = 400 MW, 200 kV DC
Cable
2010
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Q = +/- 170-300 MVAr Dynamic Voltage Support
No Increase in
Short-Circuit Power
Energy Exchange
by Sea Cable
Elimination ofTransmission Bottlenecks
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P = 400 MW, 200 kV DC
Cable
2010
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Q = +/- 170-300 MVAr Dynamic Voltage SupportDynamic Voltage Support
No Increase in
Short-Circuit Power
No Increase in
Short-Circuit Power
Energy Exchange
by Sea Cable
Energy Exchange
by Sea Cable
Elimination ofTransmission BottlenecksElimination ofTransmission Bottlenecks
a)
PG&E
Pittsburg
Substation
PG&E
Potrero
Substation
< 1 mile 53 miles1 mile 1 mile < 3 miles
San
Francisco Pittsburg
Cables
AC/DC
Converter
Station
Cables
AC/DC
Converter
Station
San Francisco SanPablo Suisun Bays
ACAC
115 kV
Substation
230 kV
Substation
Converter: Modular Multilevel HVDC PLUS Converter Rated Power: 400MW @ AC Terminal receiving End DC Voltage: 200kV Submarine Cable: Extruded Insulation DC Cable
Converter: Modular Multilevel HVDC PLUS Converter Rated Power: 400MW @ AC Terminal receiving End DC Voltage: 200kV
Submarine Cable: Extruded Insulation DC Cable
SubmarineDC Cables
PG&E
Pittsburg
Substation
PG&E
Potrero
Substation
< 1 mile 53 miles1 mile 1 mile < 3 miles
San
Francisco Pittsburg
Cables
AC/DC
Converter
Station
Cables
AC/DC
Converter
Station
San Francisco SanPablo Suisun Bays
ACAC
115 kV
Substation
230 kV
Substation
Converter: Modular Multilevel HVDC PLUS Converter Rated Power: 400MW @ AC Terminal receiving End DC Voltage: 200kV Submarine Cable: Extruded Insulation DC Cable
Converter: Modular Multilevel HVDC PLUS Converter Rated Power: 400MW @ AC Terminal receiving End DC Voltage: 200kV
Submarine Cable: Extruded Insulation DC Cable
SubmarineDC Cables
b)
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In conclusion, the features and benefits of the HVDC PLUS technology can be summarized as
follows, see Figs. 20-21.
Low Switching Frequency
Reduction in Losses
Less Stresses
In Comparison with 2 and3-Level ConverterTechnologies
In Comparison with 2 and3-Level ConverterTechnologies
with Advanced VSC Technology with Advanced VSC Technology
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Clean Energy to Platforms & Islands Clean Energy to Platforms & Islands
Fig. 20: HVDC PLUS The Power Link Universal System
Compact Modular Design
Less Space Requirements
Advanced VSC Technology
HVDC PLUS One Step ahead
Compact Modular Design
Less Space Requirements
Advanced VSC Technology
HVDC PLUS One Step ahead
Fig. 21: HVDC PLUS with MMC Technology The Smart Way
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3.4 Benefi ts of Active AC and DC Fil ters
Active filters with VSC offer many benefits in comparison with passive filters only. In high voltage
systems, the active filters are used in combination with passive filters. By means of their controls, they
can track the system frequency, and they can filter several harmonics at the same time:
- Excellent performance even in case of detuning of the passive filter or variation of system
frequency
- Superior harmonic performance through the elimination of several harmonics simultaneously
with a single active filter
- Lower resonance frequencies due to interaction with network impedance or other filters,
capacitors, reactors
- Easy adaptation to existing passive filter schemes
- Containerized design allows to test the complete system at the factory and reduces commissioning
works, and
- Active filters meet the highest harmonic performance, which is an important environmental issue
in cities and megacities.
This technology which uses VSC has been successfully applied since long. An example for the ACside application in Europe at HVDC station Skagerak III is shown in Fig. 22.
For DC filtering, Fig. 23 shows the results, measured at Tian-Guang HVDC station in China. The
figures 22-23 show that active filters significantly improve the power quality on the AC and DC side
respectively. In Fig. 22, the containerized active filter (blue box) is positioned close to the associated
passive filters of the HVDC station.
For the Neptune HVDC project in USA [18, 19], a superior harmonic performance on the AC side of
the DC transmission system was needed due to the power quality requirements. Adhering to these very
tight requirements was not possible with passive filters alone. For flexibility reasons, the MMC
concept was also introduced in the new active filter development for the Neptune project. Highlights
of this new design (ref. to Fig. 24), already fully proven in practice, are as follows:- The rating has been increased to 26 kV 600 A (RMS)
400 kV AC On-SiteMeasurements400 kV AC On-SiteMeasurements
Remark: the Output of the Measuring System is proportional to the Frequency
11 13 23 25 35 37 47 49Harmonic numbers
5 7 11 13 23 25 35 37 47 495 7
Harmonic numbers
Only Passive Passive + Active
Fig. 22: Active Filter for AC Side HVDC Skagerak III, Nordel Europe
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- Up to 16 independent harmonic frequencies can be mitigated with either voltage or current
control, and
- Active damping is possible. The energy balance is maintained by the fundamental frequency
component.
- The main circuit is independent of auxiliary power.
Multilevel converter technology renders the power transformer superfluous.
Comparison of DC Currents with passive Filter alone (yellow)and
with active Filter inserted (green).Power: 450 MW (0.5 pu) per Pole.
500 kV DC On-SiteMeasurements500 kV DC On-SiteMeasurements
Fig. 23: Active Filter for DC Side HVDC Tian-Guang, China
Fig. 24: Advanced Active Filter for AC using MMC Technology a) Application
for Neptune HVDC, Site View, b) Topology
Topology: Passive AC Filter
Switchgear
HF Filterand IGBT
Converter
b)
a)
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3.5 STATCOM with MMC Technology SVC PLUS
It is obvious that the advanced MMC technology can also be applied to STATCOM with benefits
similar to those of HVDC PLUS. With respect to technology similarities and synergies, the decision
was made to use the active filter modules for the STATCOM application in combination with a power
transformer.
The concept and the compact, modular design of the new SVC PLUS development with MMCtechnology are summarized in Figs. 25 and 26.
In the figures, synergies with the active filter are highlighted. It can be seen that the SVC PLUS
solution uses the same H-Bridge modules as the active filter.
Fig. 25: From Active Filter - a) to SVC PLUS - b)
b)VSCVSC
Similar Benefits
in Comparison
with HVDC
PLUS
VSCVSCa)
Fig. 26: SVC PLUS The Advanced STATCOM
a) A View on the Technology Containerized
Solutionb) Converter with H-Bridge Modules
Control System Modular Multilevel Converter Cooling System
a)
Modul #1
Modul #2
Modul #3
Modul #4
Modul #8
Modul #7
Modul #6
Modul #5
b)
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Fig. 27: Conclusions Integration of large Offshore Wind Farms into the Main Grid
Prospects of HVDC in Germany
Vattenfall
Europe Transmission~= =
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HVDC Classic for Load &
Generation Reserve Sharing
Vattenfall
Europe Transmission~= =
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HVDC Classic for Load &
Generation Reserve Sharing
HVDC Classic for Load &
Generation Reserve Sharing
HVDC PLUS fromOffshore to LandHVDC PLUS fromOffshore to Land
4. CONCLUSIONS
The new Modular Multilevel Converter technology (MMC) for HVDC PLUS and SVC PLUS
provides tremendous benefits for power transmission. It will help increase sustainability and security
of transmission systems significantly.
In future, a combination of the different transmission technologies may offer additional benefits for
power systems. This idea is outlined in Fig. 27.
Its basis is the widely promoted political intention to install huge amounts of wind energy, most on
offshore platforms, in Europe and in Germany in particular. The transmission scenario, as depicted in
the figure, uses both Bulk Power HVDC Classic and HVDC PLUS each on its place. The goal is a
significant CO2reduction through the replacement of conventional power plants by renewable energy
sources, mainly offshore wind farms [2], however, without jeopardizing the system security [12-15,
18, 19], as indicated in the figure.
5. REFERENCES
[1]
European Technology Platform SmartGrids Vision and Strategy for Europes Electricity
Networks of the Future, 2006, Luxembourg, Belgium
[2] DENA Study Part 1, Energiewirtschaftliche Planung fr die Netzintegration von Windenergie
in Deutschland an Land und Offshore bis zum Jahr 2020, February 24, 2005, Cologne,
Germany
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[3]
M. Luther, U. Radtke, Betrieb und Planung von Netzen mit hoher Windenergieeinspeisung,
ETG Kongress, October 23-24, 2001, Nuremberg, Germany
[4] Economic Assessment of HVDC Links, CIGRE Brochure Nr.186 (Final Report of WG 14-
20)
[5]
N.G. Hingorani, Flexible AC Transmission, IEEE Spectrum, pp. 40-45, April 1993
[6]
FACTS Overview, IEEE and CIGRE, Catalog Nr. 95 TP 108
[7] Working Group B4-WG 37 CIGRE, VSC Transmission, May 2004
[8] F. Schettler, H. Huang, N. Christl, HVDC Transmission Systems using Voltage-sourced
Converters Design and Applications, IEEE Power Engineering Society Summer Meeting,
July 2000
[9] R. Marquardt, A. Lesnicar, New Concept for High Voltage Modular Multilevel Converter,
PESC 2004 Conference, Aachen, Germany
[10]
S. Bernet, T. Meynard, R. Jakob, T. Brckner, B. McGrath, Tutorial Multi-Level
Converters, Proc. IEEE-PESC Tutorials, 2004, Aachen, Germany
[11]
L. Kirschner, D. Retzmann, G. Thumm, Benefits of FACTS for Power System
Enhancement, IEEE/PES T & D Conference, August 14-18, 2005, Dalian, China
[12]
G. Beck, D. Povh, D. Retzmann, E. Teltsch, Global Blackouts Lessons Learned, Power-
Gen Europe, June 28-30, 2005, Milan, Italy
[13]
G. Beck, D. Povh, D. Retzmann, E. Teltsch, Use of HVDC and FACTS for Power System
Interconnection and Grid Enhancement, Power-Gen Middle East, January 30 February 1,2006, Abu Dhabi, United Arab Emirates
[14]
W. Breuer, D. Povh, D. Retzmann, E. Teltsch, Trends for future HVDC Applications, 16th
CEPSI, November 6-10, 2006, Mumbai, India
[15] G. Beck, W. Breuer, D. Povh, D. Retzmann, Use of FACTS for System Performance
Improvement, 16th CEPSI, November 6-10, 2006, Mumbai, India
[16]
J. M. Prez de Andrs, J. Dorn, D. Retzmann, D. Soerangr, A. Zenkner, Prospects of VSC
Converters for Transmission System Enhancement; PowerGrid Europe 2007, June 26-28,
Madrid, Spain
[17] J. Dorn, H. Huang, D. Retzmann, Novel Voltage-Sourced Converters for HVDC and FACTS
Applications, Cigre Symposium, November 1-4, 2007, Osaka, Japan
[18]
W. Breuer, D. Povh, D. Retzmann, Ch. Urbanke, M. Weinhold, Prospects of Smart Grid
Technologies for a Sustainable and Secure Power Supply, The 20TH
World Energy Congress,
November 11-15, 2007, Rome, Italy
[19] D. Ettrich, A. Krummholz, D. Retzmann, K. Uecker, Prospects of HVDC & FACTS for
Bulk Power Transmission and System Security, Electrical Networks of Russia International
Exhibition and Seminar, December 4-7, 2007, Moscow, Russia