prospects of multilevel VSC technologies for power transmission noi ve HVDC plus dung MMC

Luận văn, Điện tử công suất, đề tài tốt nghiệp, đồ án, thực tập tốt nghiệp, đề tài

Abstract Deregulation and privatization are posing new

challenges to high voltage transmission and distributions systems

System components are loaded up to their thermal limits, and

power trading with fast varying load patterns is leading to an

increasing congestion In addition to this, the dramatic global

climate developments call for changes in the way electricity is

supplied

Innovative solutions with HVDC (High Voltage Direct

Current) and FACTS (Flexible AC Transmission Systems) have

the potential to cope with the new challenges New power

electronic technologies with self-commutated converters provide

advanced technical features, such as independent control of

active and reactive power, the capability to supply weak or

passive networks and less space requirements In many

applications, the VSC (Voltage-Sourced Converter) has become a

standard for self-commutated converters and will be increasingly

more used in transmission and distribution systems in the future

This kind of converter uses power semiconductors with turn-off

capability

Index Terms Elimination of Bottlenecks in Transmission;

Enhanced Grid Access for Regenerative Energy Sources (RES);

Increase in Transmission Capacity; Security and Environmental

Sustainability of Supply; Smart Grid Technologies

I I NTRODUCTION

NVIROMNMENTAL constraints will play an important

role in the power system developments [1-2] 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]

In this paper, the basic concept and the technical

performance of the new MMC PLUS technology are

discussed in detail and the area of applications is depicted

B Gemmell is with Siemens Power Transmission & Distribution, Inc.,

Wendell, NC 27591 USA (e-mail: brian.gemell@siemens.com)

J Dorn, D Retzmann, D Soerangr are with Siemens AG, PTD High

Voltage Division, Power Transmission Solutions, 91058 Erlangen, Germany

(e-mails: joerg.dorn@siemens.com, dietmar.retzmann@siemens.com,

dag.soerangr@siemens.com)

II I NTEGRATION OF R ENEWABLE E NERGY S OURCES – A

B IG C HALLENGE

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]

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

The prospects of embedding 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

In what follows, the global trends in power markets and the

Prospects of Multilevel VSC Technologies

for Power Transmission

B Gemmell, Siemens USA; J Dorn, D Retzmann, D Soerangr, Siemens Germany

E

„Additional Reserve Capacity is required

„Additional Reserve Capacity is required

This will be a strong Issue in the German Grid Development

Problems with Wind Power Generation:

o Wind Generation varies strongly

o It can not follow the Load Requirements

Source: E.ON - 2003

prospects of system developments are depicted, and the

outlook for VSC technologies for environmental sustainability

and system security is given

Fig 2: Regenerative Energy Sources and Dispersed Generation –

Impact on the whole T&D Grid Structure

III S MART G RID S OLUTIONS WITH P OWER E LECTRONICS

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

A 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 several kHz, however, with 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

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

It is, however, necessary to mention 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

B 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)

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 hundred 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

Use of D ispersed G eneration

Use of D ispersed G eneration

H G

G

G

G

G

G H

G H G

G

G

More Dynamics for better Power Quality :

zUse of Power Electronic Circuits for Controlling P , V & Q

zParallel and/or Series Connection of Converters

zFast AC / DC and DC / AC Conversion

Thyristor

50/60 Hz

Thyristor

50/60 Hz

GTO

< 500 Hz

GTO

< 500 Hz

IGBT / IGCT

Losses

> 1000 Hz IGBT / IGCT

Losses

> 1000 Hz Transition from “slow” to “fast”

Switching Frequency

On-Off Transition 20 - 80 ms

Transition from “slow” to “fast”

Switching Frequency

On-Off Transition 20 - 80 ms

1-2 %

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

Depending

on Solution

Depending

on Solution

2-4 %

voltage steps are applied at the AC converter terminals which

require extensive filtering measures In Fig 4, the principle of

two-level converter technology is depicted From the figure, it

can be seen 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

Fig 4: VSC Technology – a Look back

C 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 called 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 5, a comparison of two, three and multilevel technology is depicted A new and different multilevel approach is the modular multilevel converter (MMC) technology [9]

The principle design of conventional multilevel converter and advanced MMC is shown in Fig 6 and Fig 7 depicts the HVDC PLUS MMC solution in detail

A converter in this context consists of six converter legs, whereas the individual converter legs 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 switching element

- a DC storage capacitor

High harmonic Distortion

High Stresses resulting in HF Noise

)

Desired voltage Realized voltage

- V d /2

0

+V d /2

)

Desired voltage Realized voltage

- V d /2

0

+V d /2

V Conv.

V d /2

V d /2

V Conv.

V d /2

V d /2

Power

Electronic

Devices:

GTO / IGCT

Fig 5: The Evolution of VSC and HVDC PLUS Technology

For the sake of simplicity, the electronics for controlling

the semiconductors, measuring the capacitor voltage and for

communicating with the higher-level control are not shown in

Fig 7 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 to 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

Fig 6: The Multilevel Approach

a) Conventional Solution

b) Advanced MMC Solution

c) Sinus Approximation – and

V d / 2

V d / 2

V Conv.

V d / 2

V d / 2

V d / 2

V d / 2

V Conv.

V Conv.

a)

V d

V Conv.

V d

V Conv.

V Conv.

b)

Small Converter AC Voltage Steps Small Rate of Rise of Voltage Low Generation of Harmonics Low HF Noise

Low Switching Losses

c)

Submodule (SM)

Fig 7: HVDC PLUS – Basic Scheme

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-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 a converter leg So, in principle, the two converter legs of each phase module represent a controllable voltage source In this arrangement, the total voltage of the two converter legs in one phase unit equals the DC voltage, and by adjusting the ratio of the converter leg voltages in one phase module, the desired sinusoidal voltage at the AC terminal can easily be achieved Fig 8 depicts this advanced principle of AC voltage generation with MMC It can be seen that there is almost no or – in the worst case – very small need for AC voltage filtering

to achieve a clean voltage, in comparison with the two-level circuit with PWM in Fig 4

Off Off

Off Off

On Off

On Off

Off On

Off On

TABLE I

S TATES AND C URRENT P ATHS OF A S UBMODULE IN THE MMC T ECHNOLOGY

V Conv .

- V d /2 0

+V d /2

AC and DC Voltages controlled

by Converter Leg Voltages:

- V d /2 0

+V d /2

AC and DC Voltages controlled

by Converter Leg Voltages:

VAC

Fig 8: The Result – MMC, a perfect Voltage Generation

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 leg, 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 leg 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 9 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 leg,

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 leg 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, there will be little difference between the momentary values of the three DC voltages, if for no other reason than that the number of available voltage steps is finite

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 converter legs 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 legs, 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 line through the free-wheeling diodes to the short-circuit, so that the only way this fault can be corrected is by opening the circuit breaker

PLUSCONTROL High-Speed Bypass Switch

Fig 9: MMC – Redundant Submodule Design

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 10

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

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 modules are assembled to form

transportable units that are easy to install with the proper

tools The required number per converter leg 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 11 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 11) 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 12), the central control unit The PLUSCONTROL was developed specifically for HVDC PLUS and has the following functions:

- Calculation of appropriate converter leg 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

Phase Unit

SM electronics

1

2

IGBT2 D2

D1 IGBT1

PLUSCONTROL

Submodule Protective Thyristor Switch

Fig 10: Fully suitable for DC OHL Application – Example Line-to-Line Fault

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

- 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:

Converter Leg with more than 200 Submodules

Typical Converter Arrangement for 400 MW

Optional

Seismic

Reinforcements

Fig 11: HVDC PLUS – The Advanced MMC Technology

Calculation of required Converter Leg Voltages

Selection of Submodules

to be switched

Control of Active and Reactive power

Submodule Voltage Balancing Control

SIMATIC TDC C&P System

SIMATIC TDC Measuring System

Fig 12: Main Tasks of PLUSCONTROL TM

- 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 very flexible 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

leg 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 semicon-

ductors results in very low switching losses Total system

losses are therefore relatively low for VSC PLUS tech-

nology, and the efficiency is consequently higher in com-

parison with existing two and three-level solutions

- HVDC PLUS utilizes industrially proven standard com-

ponents that are very robust and highly reliable, such as

IGBT modules These components have proven their

reliability and performance many times over 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 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 compo-

nents, 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, such

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 13-15 summarize the advantages in a comprehensive way Added to these are the aforementioned advantages that ensue from the use of VSC technology in general 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, because of the capability to manage DC side short-circuits and prompt resumption of system operation

- Back-to-back arrangement, i.e rectifier and inverter in one station

- The implementation of multiterminal systems is relative-

ly 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-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 mega- watts, in which presently only line-commutated converters are used

- Grid access to very weak grids or islanded networks

- 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 and technical performance, HVDC PLUS is tomorrow’s solution for the supply of megacities

To achieve transmission redundancy, HVDC PLUS can be configured in two ways, as depicted in Fig 17 Option a) is the standard solution, providing a full n-1 redundancy for the whole transmission scheme, including cable or line Option b) can be selected, when cost saving for one cable/line conductor

is required

In this case, however, standard AC transformers can not be used, HVDC transformers would be required

HVDC PLUS

HVDC

“Classic”

HVDC

“Classic”

Example 400 MW

Space Saving

b)

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 Transformers Low Engineering Efforts, Power Range up to 1000 MW High Reliability, low Maintenance Requirements Robust System a)

Fig 13: a) Features and Benefits of MMC Topology

b) Space Saving in Comparison with HVDC “Classic”