1

ABSTRACT: HVDC (High Voltage Direct Current) systems and FACTS (Flexible AC Transmission Systems) 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 of cascading 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 in transmission 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 Power Supply; Smart Grid Technologies

December 04th to 07th 2007 - All-Russian Exhibition Centre (AREC), Moscow, Russia

04 – 07 декабря 2007 г. - Всероссийский выставочный центр (ВВЦ), Москва, Россия

International Exhibition and Seminar

Международная выставка и семинар

December 04th to 07th 2007 - All-Russian Exhibition Centre (AREC), Moscow, Russia

04 – 07 декабря 2007 г. - Всероссийский выставочный центр (ВВЦ), Москва, Россия

International Exhibition and Seminar

Международная выставка и семинар

*dietmar.retzmann@siemens.com

Benefits of Multilevel VSC Technologies for Power Transmission and

System Enhancement

Joerg Dorn, Dirk Ettrich, Joerg Lang, Dietmar Retzmann*

Siemens Germany

2

1. INTRODUCTION Environmental constraints will play an important role in the power system developments [1-2, 18]. However, regarding the system security, specific problems are expected when renewable energies, such as large wind farms, have to be integrated into the system, particularly when the connecting AC links are weak and when sufficient reserve capacity in the neighboring systems is not available [3]. Furthermore, in the future, an increasing part of the installed capacity will be connected to the distribution levels (dispersed generation), which poses additional challenges to the planning and safe operation of the systems. Power electronics is to be used to control load flow, to reduce transmission losses and to avoid congestion, loop flows and voltage problems [4-6, 12].

2. INTEGRATION OF RENEWABLE ENERGY SOURCES – A BIG CHALLENGE Power output of wind generation can vary fast in a wide range [3], depending on the weather conditions. Therefore, a sufficiently large amount of controlling power from the network is required to substitute the positive or negative deviation of actual wind power infeed to the scheduled wind power amount. Fig. 1 shows a typical example of the conditions, as measured in 2003. Wind power infeed and the regional network load during a week of maximum load in the E.ON control area are plotted. The relation between consumption and supply in this control area is illustrated in the figure. In the northern areas of the German grid, the transmission capacity is already at its limits, especially during times with low load and high wind power generation [11].

The prospects of connecting large amounts of regenerative energy sources and dispersed generation into the power systems are depicted in Fig. 2. It can be seen that this will have impact on the whole transmission and distribution network structure. Load flow control will be much more complex, system control and system protection strategies will need to be adapted and reserve generation capacity will be required.

Fig. 1: Network Load and aggregated Wind Power Generation during a Week of maximum Load in the E.ON Grid

Additional Reserve Capacity is requiredAdditional Reserve Capacity is required

This will be a strong Issue in the German Grid Development

Problems with Wind Power Generation:o Wind Generation varies stronglyo It can not follow the Load Requirements

Source: E.ON - 2003

3

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

GGHH

GG

GG

GG

Fig. 2: Regenerative Energy Sources and Dispersed Generation – Impact on the whole T&D Grid Structure

4

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-Sourced 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 capability such 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 & QParallel and/or Series Connection of ConvertersFast 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 “fast”Transition from “slow” to “fast”

Switching Frequency

On-Off Transition 20 - 80 ms

1-2 %1-2 %

The Solution for Bulk Power Transmission The Solution for Bulk Power Transmission

Depending on SolutionDepending on Solution

2-4 %2-4 %

Fig. 3: Power Electronics for HVDC and FACTS – Transient Performance and Losses

5

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 Multilevel Converter (MMC) Approach Both the size of voltage steps and the related voltage gradients can be reduced or minimized if the AC voltage 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

6

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

7

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 which ensures 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

8

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 3

State 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 Voltage Generation

VConv.

- Vd /2

0

+Vd /2

VAC

AC and DC Voltages controlled by Converter Module Voltages:

9

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 HVDC transmission 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 voltage distribution 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 the finite number of available voltage steps.

Phase Unit Submodule

PLUSCONTROL

High-Speed Bypass Switch

Fig. 10: MMC – Redundant Submodule Design

10

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 previously reserved 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

11

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 the

relevant 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

12

- 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 essential differences 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. What’s more, the small and relatively shallow voltage 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 determined

essentially only by the performance of the controls, i.e. the number of submodules that can be

Fig. 13: Main Tasks of PLUSCONTROL TM

SIMATIC TDCC&P System

SIMATIC TDCMeasuring System

Calculation of required Converter Leg Voltages

Selection of Submodules to be switched

Control of Active and Reactive Power

Submodule Voltage Balancing Control

13

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. What’s 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 Hardware and Software

Low Generation of Harmonics

Low Switching Frequency of Semiconductors

Use of well-proven Standard Components

Sinus shaped AC Voltage Waveforms

Easy Scalability

Reduced Number of Primary Components

Low Rate of Rise of Currents even during Faults

High Flexibility, economical from low to high Power Ratings Only small or even no Filters

required

Low Converter Losses

High Availability of State-of-the-Art Components Use of standard AC

TransformersLow Engineering Efforts,

Power Range up to 1000 MWHigh Reliability, low

Maintenance Requirements

Robust System

Fig. 14: Features and Benefits of MMC Topology

14

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 CO2 emissions. 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 tomorrow’s 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”

15

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

16

Fig. 17: Trans Bay Cable, USA – World’s 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 DCCable

2010

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==

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Q = +/- 170-300 MVAr Dynamic Voltage SupportDynamic Voltage Support

No Increase inShort-Circuit PowerNo Increase inShort-Circuit Power

Energy Exchangeby Sea CableEnergy Exchangeby Sea Cable

Elimination of Transmission BottlenecksElimination of Transmission Bottlenecks

a)

PG&E Pittsburg

Substation

PG&E Potrero

Substation

< 1 mile 53 miles1 mile 1 mile < 3 miles

San Francisco Pittsburg

Cables

AC/DCConverter

Station

Cables

AC/DCConverter

Station

San Francisco – San Pablo – 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/DCConverter

Station

Cables

AC/DCConverter

Station

San Francisco – San Pablo – 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)

17

Fig. 18: Results of Computer Simulation – 400 MW with 200 Submodules per Converter Module

VDC + 200 kV

VDC - 200 kV

PLOTS : Graphs

1.000 1.010 1.020

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0 50

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PLOTS : Graphs

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0 50

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PLOTS : Graphs

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AC Terminal Voltages

Currents in the AC Terminals

Six Converter Module Currents

Obviously, no AC Filters requiredObviously, no AC Filters required

Fig. 19: Options a) and b) for Transmission Redundancy

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b) Solution with 3 Cables

HVDC Transformers required

Bipole

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b) Solution with 3 Cables

HVDC Transformers required

BipoleBipole

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G ~G ~G ~ G ~G ~G ~

a) Solution with 4 Cables

Use of Standard AC Transformers

Symmetrical Monopole~

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a) Solution with 4 Cables

Use of Standard AC Transformers

Symmetrical MonopoleSymmetrical Monopole

18

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 and 3-Level Converter Technologies

In Comparison with 2 and 3-Level Converter Technologies

… 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

19

3.4 Benefits of Active AC and DC Filters 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 AC side 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-Site Measurements400 kV AC On-Site Measurements

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 7Harmonic numbers

Only Passive Passive + Active

Fig. 22: Active Filter for AC Side – HVDC Skagerak III, Nordel – Europe

20

- 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-Site Measurements500 kV DC On-Site Measurements

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 Filter and IGBT Converter

b)

a)

21

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 MMC technology 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 Benefitsin Comparison with HVDC PLUS

VSCVSCa)

Fig. 26: SVC PLUS – The Advanced STATCOM a) A View on the Technology – Containerized

Solution b) 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)

22

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 SharingHVDC Classic – for Load & Generation Reserve Sharing

HVDC PLUS – from Offshore to LandHVDC PLUS – from Offshore 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 CO2 reduction 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 Europe’s Electricity Networks of the Future”, 2006, Luxembourg, Belgium

[2] DENA Study Part 1, “Energiewirtschaftliche Planung für die Netzintegration von Windenergie

in Deutschland an Land und Offshore bis zum Jahr 2020”, February 24, 2005, Cologne, Germany

23

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