dc converter technology for hvdc

7/22/2019 DC Converter Technology for HVDC

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ContentsINTRODUCTION ...................................................................................................................................................2

TYPES, OPERATIVE PRINCIPLES AND LIMITATIONS .............................................................................................2

A. LCC: Current-Source Converters..............................................................................................................3

B. SCC: Voltage-Source Converters.............................................................................................................. 4

SPECIFICATIONS AND COMPARATIVE ADVANTAGES ..........................................................................................5

FUTURE INDUSTRIAL DEVELOPMENT TRENDS....................................................................................................7

CONCLUSION .......................................................................................................................................................8

APPENDIX I: HVDC Benefits and Further Converter Operative Elaboration ........................................................9

APPENDIX II: HVDC Demonstrative Projects List...............................................................................................12

REFERENCE WORKS ...........................................................................................................................................13

Report Statistics

Main critical text of this report contains a total of[1537]words; this excludes content matter of headings,

figures, data tables, footnotes, appendices and bibliography.

The report additionally contains:

[15]figures

[03]tables

[02]appendices

[28]footnotes [44]bibliography references


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AUTHOR: Muhammad Ali Qaiser STUDENT ID: 26561999

MODULE: Power Electronics ELEC6125 COURSEWORK#2: HVDC Converters

University of Southampton, ECS, 2014 Page 2 of16

INTRODUCTIONThe advent of commercialized electricity saw small DC networks around 1882 [1]; since then however, the

transmission paradigm has always been one based on AC [2]. In recent years the concept of super-grid has

emerged in Europe [3], whereby HVDC1 transmission is being developed to again play a central role in power

connections across the world (refer toAppendix II).

Amongst others, a key component of this infrastructure is the Source Converter at HVDC Stations. This report

investigates major categories, operative principles and limitations of these converters. It then reviews

specifications and comparative advantages2. The work concludes with a view of likely futuristic trends and

industrial topologies.

TYPES, OPERATIVE PRINCIPLES AND LIMITATIONSThis section presents device configurations popular in the industry; for further elaboration and variant

arrangements, the reader is directed toAppendix I.

There are two major categories of DC converters (Figure 1). The classic type3 uses SCR4 valves and is referred

to as line-commutated conversion (LCC). A later version5, introduced commercially in 1997 by ABB [4], uses

IGBT switches6 and is labelled self-commutated conversion (SCC). Whilst the latter improves harmonic

performance and power flow controllability, the former allows greater power transfers and loss efficiency [5].

1 High-Voltage Direct Current2 Example being uni-directionnal power flow optimization vs. bi-directional efficiency3 Till 1970, mercury arc valves were used4

Silicon-Controlled Rectifier (also known as Thyristor)5 Referred to as HVDC Light by ABB, HVDC Plus by Siemens and HVDC MaxSine by Alstom6 Insulated-Gate Bipolar Transistor

ANNOTATION

TxAC transmitting AC network

RxAC receiving AC network

Xfmr transformer

X link control reactanceC link control capacitance

DCL DC link

SCR thyristor converter station

IGBT transistor converter station

Q reactive power flow

P real power flow

CSC current-source converter

VSC voltage-source converterFigure 1: Single-line configuration of LCC vs SCC systems

(adapted from ABB [44])


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A. LCC: Current-Source Converters

The HVDC Classic approach utilizes line-commutation that leads to uni-directional DC current flow7 injection

into the receiving AC network; hence the term CSC8 because the output current is pinned to a constant level.

In case reversal of power flow direction is required, it is achieved by inversion of DC polarity on both stations.

The circuit configuration is known as Graetz bridge, which is a 3-phase version of the single-phase wheatstone

rectifier (Figure 2).

It can be observed that each thyristor conducts for a third (120o) of the full AC cycle, and at any time 2 switches

are in conduction (one from upper and one from lower half of bridge) 9. Every 60o a different VLL combination

arises, resulting in 6 ripples for the DC output, which is a rectification resulting from subtraction of the negative

AC curve from the positive one asVDC= VP- VN or a consecutive addition of 6 possible permutations of VLLphasors (Figure 3).

The DC side current flow can be considered stable and constant due to introduction of smoothing inductance

of DC link (XDC in circuit layout above) [6].

7

Source to sink side8 Current-Source Converter9 Except at commutation, 2 switches from same half (hence total 3) can conduct simultaneously; refer to Appendix I

Figure 2: 6-pulse Graetz thyristor rectifier bridge (adapted from Rashid et al [10])

ANNOTATION

1 primary-side phase ()

2 secondary-side phase (Y)

X link inductance

AK thyristor drop (V)

A,B,C phase sequence (V, I)

n neutral point on Y side

VP +ve common wrt neutral

VN ve common wrt neutral

DC resultant DC

T thyristor

60o

Vcb

Vab

Vac

Vbc

Vba

Vca

Figure 3: Waveform and phasor for 6-pulse converter (adapted from Mohan et al [32])


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In practice, the converters DC power

output (IV) can be decreased by

increasing firing delay () of the valves

through an active pulse generator; this

cuts out portions ofVP but prolongs VN

[7]. Not only does that reduce DCvoltage value, it also makes the

waveform less uniform (hence higher

harmonics) as shown in Figure 4. While

each thyristor still conducts for 120o, its

conduction current now lags behind source AC phase; hence CSC converter always consumes reactive power

whether in rectifier or inverter mode. Beyond = 90o, net DC voltage becomes negative and system becomes

an inverter. While current flows in same direction, inversion of voltage polarity means that the system now

transfers power from DC to AC side.

The 6-pulse bridges at HVDC stations are connected in series using -Y phase-shift 30o against - on

secondary side of transformer. This 12-pulse arrangement10

causes most harmonics to circulate between thetwo bridges and not reflect back on AC side, resulting in pseudo-sinusoidal current waveforms [8]. For

constant DC-side current or voltage regulated by filters and compensators, the resulting reactive-active power

relationship (normalized against rated power output PN) is shown in Figure 5.

B. SCC: Voltage-Source Converters

By replacing thyristors with IGBT-diode valve-sets11, the Graetz bridge can be upgraded to allow self-

commutation12 (i.e. gate turn-off pulsing). Here, the DC-side voltage is held constant by capacitor (Figure 6)

across a two-level bridge, hence the term VSC13. Flow of active power over the transmission link can be

controlled by simply raising the DC output level at the end-converter that is to be made sender-side (refer to

Figure 1) and inverting flow of current; there is no need for polarity inversion as for thyristor converter.

10 Higher pulse arrangements are explained inAppendix I11 Diodes placed anti-parallel to IGBT, GTO or IGCT switches12

Also referred to as forced commutation13 The bridge is again two-level, like LCC converter, with upper valve sets contributing to positive half of DC output and

lower sets to negative half

Figure 5: 12-pulse converter and Q-P control relationship (adapted from Siemens [8])

Figure 4: Relationship of firing angle with DC level (adapted from Williams [7])


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A hypothetical view of VSC arrangement would be to consider it as a 3-phase ideal transformer set with

continually controllable turns-ratio [9]; in practice this is achieved by switch-mode DC conversion using pulse-width modulation14 that follows the same fundamental frequency as AC-side supply (Figure 7). Varying the

amplitude of PWM pattern changes DC-link magnitude; phase-shifting the pulses allows power factor control

[10], which is a significant feature of SCC system. The operative principle rests against a reference voltage that

the DC output at capacitor is compared with; subsequent error causes PWM generation to alter valve duty

cycle [11]. Flow of current towards DC side (rectifier mode) discharges the capacitance, hence drawing power

from AC side; reversal of the current flow (inverter mode) causes supercharging, which then decreases the

firing duty to return power towards AC side.

SPECIFICATIONS AND COMPARATIVE ADVANTAGES

A review of comparative industrial specifications and technological merits of the two converter categoriesdiscussed earlier is undertaken in this section.

The superiority of VSC lies in its ability to self-commutate and control the

power factor fully between inductive and capacitive modes. This means

that it can operate in all 4 quadrants of PQ domain, as illustrated in Figure

8; consequently it does not require a strong AC grid comprising

synchronous machines that provide the reactive power required by CSC

[12].

It can thus feed simple passive systems without installation of reactive

compensators. Also, the high frequency switching characteristics of IGBT

valve-sets allow near-sinusoidal AC to be generated with low harmonic

content; this eliminates the need for extensive AC-side filtering equipment.

All of this translates into smaller HVDC station space requirements by

around 50% [13], beneficial for example in offshore or city-centre

locations. In addition, as the conversion is not dependent on

commutation from line source and has its own PWM controller, VSC stations can be installed in remote

locations where a black-start is be required. Owing to full commutation control, they can be tailored to suit

very low power systems (from tens to 1,200 MW) including 0 operation, whereas LCC arrangements are

effective only at higher levels (usually 1,000 to 3,000 MW) [4].

14 PWM

Figure 6: Voltage-source converter (two-level IGBT-diode valve bridge)

(adapted from Alstom [13])

Figure 7: VSC pulse-width modulation and level control

(adapted from Rashid [10])

Figure 8: PQ zone of VSC

(adapted from [12])


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On the other hand, CSC systems have greater conversion efficiencies owing to

the lower switching frequency (dependent on mains fundamental) compared

to VSC; the former show average losses of around 0.5% compared to around

1.53.5% of the latter (Figure 9). They also have high current surge capability.

However, reactive power control is marginal in that it can only be achieved

indirectly by decreasing DC output voltage while increasing current and firing

delay, using methods of CCA15 and VDCOL16 [14]; since losses increase by i2,

these techniques is supplanted by filtering compensators.

The above technical and performance comparisons between CSC and VSC

systems are summarized in Table 1 and Table 2 respectively.

Table 1: Specification comparison between LCC and SCC systems (collated from ABB [15], Siemens [16], Alstom [17])

Parameter Unit Current-Source Voltage-Source

(line-commutation) (self-commutation)First project: ABB

Siemens

Alstom

year 1970

1975

1993

1997

2010

2011

DC power MW 500 - 6400 100 - 1000

DC voltage per converter kV 150 - 800 80 - 320

Converters per station ea 2 - 8 1 - 3

DC current A 200 - 4500 200 - 1000

Converter loss(1 - ) % 0.5 2.0 - 3.5

Min power operation (vs rated) % 10 0

Q compensation required % up to 50 0

Harmonic factor % 35 15

Site footprint ratio 1 0.6

Table 2: Functional comparison between LCC and SCC systems (collated from IET [18], IEEE [19], Cigre [20])

Feature Current-Source Voltage-Source

Semiconductor valves Thyristor set IGBT-Diode set

Switching control On:

Off:

On: PWM

Off: PWM

Switching frequency Low: mains (50 or 60 Hz) High: Vmod (up to 2,000 Hz)

Control types Constant power

Constant frequency

Damping

Proportional-integral

DQ transformation

Vector control

Active control method Inversion of V polarity Inversion of I direction

Reactive control method Capacitive compensator Modulation shift

Loss origin Valve Vdrop (low) Highfswitching (high)

Black-start capability No Yes

Inherent Q control No Yes

Harmonic content High Low

Filters, compensators required More Less

Project suitability High power link (>1000 MW) Low power link (


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FUTURE INDUSTRIAL DEVELOPMENT TRENDSVSC technology has seen rapid efficiency improvements since introduction of the 1st generation two-level

configuration (Figure 6); these next generation topologies are decreasing losses [12].

The most recent topology that ABB is researching is the 4th

generation18 (Figure 10) cascaded two-level19 system; valve

cells20 are connected in series within modules to increase DC

output whilst providing conversion with very low switching

frequencies per cell; hence reducing losses. This will allow

research emphasis to be placed in future on improving

device conduction losses and blocking levels.

On the other hand, Siemens is basing its VSC research on a

modular multi-level converter21 approach; here the aim is to

break up voltage steps and allied stress gradients on the

valve-sets by increasing the number of series-linked IGBTsub-modules (Figure 11). The result is voltage synthesis in

much smaller increments and lower switching frequency

than those afforded by two or level circuits [16]. As a result,

switching noise, losses and harmonics are reduced, and sinusoidal AC voltage superior to PWM is obtained

[21].

Another interesting future avenue is the

possibility of hybrid converters, which

combine CSC and VSC as STATCOM

system; this carries advantage of

formers low losses and latters superiordynamic characteristics [22].

For the traditional HVDC side, CSC

converter improvements are to be

sought in future by development of

thyristors with lower conduction losses,

smaller extinction angles and higher resistance to commutation failure [23]. This would eventually lead to

higher blocking voltages (and thus kV values) and allow progressive ratings for UHVDC 22 technology. In

addition, light-triggered SCR switches are gaining popularity over pulse-fired ones, to make converter modules

robust against electromagnetic noise [24].

ABB has also explored another CSC optimization known as capacitor commutated converter23 [25], whose

future experimentation shall hold an important key to keeping LCC systems still relevant as SCC technology

catches on. At present, the capacitor-varistor parallel sets are connected between transformer feed and valves

bridge; this precludes the need for VAr compensation through switching of filter or shunt banks at the instant

of active power directional change [26].

18 For an overview of 2nd and 3rd generation technology, refer toAppendix I19 CTL20 Each cell comprises a two-level IGBT and anti-parallel diode21

MMC22 Ultra-High Voltage Direct Cycle, currently rated 800 kV23 CCC

Figure 10: VSC 4th generation NPC topology

(adapted from Energy Procedia [40])

Figure 11: MMC configuration and AC waveform (adapted from IEEE [21])


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CONCLUSIONHVDC transmission is gaining a firm foothold over conventional AC for long distance and network stability

applications since its introduction in late twentieth century. In this review report, it was seen that while the

CSC is still the method of choice for projects with higher power and loss-efficiency considerations,

improvements in circuit configuration and device physics mean that the VSC is fast growing in popularity. Itmay even completely replace the CSC for medium power applications, due to its better harmonic and reactive

power performance.


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APPENDIX I: HVDCBenefitsandFurtherConverterOperativeElaborationThis section elaborates further on converter topologies and operative characteristics discussed in main text.

DC Transmission Advantages

DC transmission carries the superiority of asynchronous interconnectivity, lower losses compared to AC,

unlimited distance due to lack of cable reactance and damping of power swings in allied AC networks [27].

Since its introduction with undersea connections in Gutland24 (1954) [28], the HVDC capacity stood at 100 GW

worldwide in 2013 at an annual market value of 3 billion [29]. Practical applications of HVDC include

interpolating DC links in AC grids for stability, connectivity with offshore 25, remote and island locations,

interconnection of AC grids and feed-in to city centres. HVDC transmission turns out to be more cost-efficient

than AC by a factor of nearly 2 ($12.5 vs $25.0 per MWh at 65% utilization) [30]

CSC-LCC: Graetz Bridge

The reader should refer to Figure 2 in main text for this discussion. Popularity of this configuration over 3-

phase rectifiers (such as double star and interphase) is due to its efficient power conversion precluding

oversizing of converter transformer, and perfect symmetry of line currents on AC side [10]. Connecting oneside of the supply-line transformer as delta has the benefit of reducing higher order harmonics [31].

The arrangement contains 6 thyristor valves, each connected to conduct positive and negative cycle of

respective phases. Thyristors are numbered in the sequence they conduct, necessitating six firing pulses per

360o cycle. Operative principle of the bridge can be understood in conjunction with Figure 3; since the 3 source

lines are combined at neutral point, a thyristor in the upper half of bridge can only be fired at the instant

where its phase becomes most positive compared to others26; the reverse holds true for thyristors in lower

half (in that the VLL becomes most negative) [32].

If firing angle is set to 0o, thyristors fire as soon as the respective VLL is available27 at phase crossover points,

effectively behaving like diodes.

Apart from the concept of voltage level subtraction presented in main text (VP VN), one can alternatively

postulate that the rectified DC is a consecutive addition of 6 possible permutations of VLL (line-line voltage

phasors VAB, VAC, VBC etc. as illustrated in Figure 3) [33]. While conduction current swings direction in thyristors

(IA is plotted as example), the DC side experiences uni-directional current flow.

The effect of firing angle delay can be

explained as follows. Even if a

thyristor is bias-ready it will not

conduct until gate-pulsed; constant

current flow on DC side means that

previous valve is able to experienceconduction in negative voltage zone

until the next one is commutated

(firing instance of each valve is

indicated in Figure 12) [34].

24 Diocese of Sweden25

Mainly wave or wind26 Hence line-line voltage difference (VLL) across one switch become positive-most27 The fact that a small allowance for VAK has to be made is discounted here

Figure 12: Effect of firing delay on VP and VN waveforms (adapted from IET [34])


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In addition to the DC link reactor, another important stabilizing inductance occurs due to secondary

transformer windings (LS) on AC source side. This should form at least 5% of the total AC side impedance for

stability according to German VDE standards [35]. The transformer although not necessary for power

conversion, enables features like on-load tap changing and phase-shift between bridges.

An adverse effect ofLS is time-lapsed commutation (angle )at point of thyristor take-over; it prevents phase current from

switching between valves instantaneously, as the stored

reactive energy has to be transferred from outgoing to

incoming thyristor28 .This overlapping cuts out a further notch

in output waveforms (see shaded area in Figure 13), and

lowers effective power output [36].

In practice, the extent of firing delay is further constrained by

extinction angle (), which is the recovery time for a thyristor

to enter blocking mode after conduction [37]. This is typically

20o and maximum firing delay is thus:

= 180

The AC line currents of a 6-pulse convertor experience high harmonic content due to and delays during

thyristor switching, especially of order 6k 1 (where kis integer) [38].

Higher pulse configurations, practically up to 48, are possible by increasing the number of parallel

transformers and decreasing phase-shift proportionately, or by connecting a multi-step valve-reactor

between two converters and DC-side load [39]. Further, the output voltage can be raised by using double-pole

HVDC configuration; the DC positive and negative poles each have their own 12-pulse converter sets.

VSC-LCC: Two-Level Bridge

The reader should refer to Figure 6 in main text for this discussion. IGBT-diode

valve sets are switched on such that AC current passes through IGBT of one phase,

and returns to mains via diode of another; alteration of phase conduction

sequence is similar that seen earlier in thyristor rectifier [10]. In order to prevent

diodes from conducting at the wrong moment29, the DC-link voltage is kept higher

than that created by diode rectifier bridge alone (Figure 14).

The 2nd generation of VSC was created by ABB using a three-level neutral-point

clamped30 configuration (Figure 15); this introduces 0 V level in addition to negative

and positive half-steps. It therefore improves sinusoidal synthesis withoutincreasing switching frequency [40], but adds cost due to additional IGBT-diode

sets. The 3rd generation reverted to two-level system, but with improved device

physics and a PWM template programmed to decrease harmonic content.

In terms of economy, the converter equipment is cheaper for VSC than CSC but with

higher losses; however, this is offset for special applications such as connectivity to

offshore wind farms.

28

The voltage during commutation is therefore average of incoming and outgoing AC phase29 At the instant when an IGBT should conduct30 NPC

Figure 14: Negative diode blocking

for VSC (adapted from [10])

Figure 13: Notching due to commutation

(adapted from Chalmers [36])

Figure 15: VSC 2nd generation topology

(adapted from Energy Procedia [40]))


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Further Future Trends

The reader should refer to Figure 11; virtually no snubbing or filtering auxiliaries would be required as MMC

this technology is perfected. A trade-off has to be negotiated eventually between cost of increased devices

and waveform superiority of longer series circuits.

Increasing HVDC transmission inter-linkage complexity by multi-terminal converter stations is anothercontentious area of research, especially with geographically scattered and isolated renewable sources such as

wind farms [41]. The main problems in this regard are interoperability of converter modules manufactured by

different OEMs as well as protection coordination. The direction the industry is expected to take for

resolution relates more to technical standardization rather than technology improvement.


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APPENDIX II: HVDCDemonstrativeProjectsListThis section lists a sample of important HVDC projects completed over the last 10 years (collated from IEEE

[42] and Cigre [43]).

Table 3: Demonstrative list of HVDC projects worldwide (2003-2012)

SYSTEM / PROJECT SUPPLIER YEAR POWER

(MW)DC(kV)

LINE(km)

THYRISTOR/TRANSISTOR

LOCATION

EAST-SOUTHINTERCONNECTOR II

SIEMENS 2003 2000 500 1450 THY INDIA

THREE GORGES-CHANGZHOU ABB/SIEMENS 2003 3000 500 860 THY CHINA

THREE GORGES-GUANGDONG ABB 2004 3000 500 940 THY CHINA

GUI-GUANG II SIEMENS 2007 3000 500 1200 THY CHINA

CELILO (VALVE REPLACEMENT) SIEMENS 2004 3100 400 1200 THY U.S.A.

LAMAR SIEMENS 2005 210 64 B-B THY U.S.A.

BASSLINK SIEMENS 2006 500 400 350 THY AUSTRALIA

ESTLINK ABB 2006 350 150 105 TRA ESTONIA-FINLAND

THREE GORGES-SHANGHAI ABB 2006 3000 500 900 THY CHINA

NEPTUNE SIEMENS 2007 660 500 105 THY U.S.A.

SHARYLAND ABB 2007 150 21 B-B THY USA - MEXICO

LEVIS DE-ICER AREVA 2008 250 17.4 27 to

242 THY CANADA

NORNED ABB 2008 700 450 580 THY NORWAY-NETHERLANDS

BALLIA - BHIWADI SIEMENS 2010 2500 500 800 THY INDIA

OUTAOUAIS ABB 2009 2x625 315 B-B THY CANADA

NORDE.ON 1 ABB 2009 400 150 203 TRA GERMANY

AL FADHILI AREVA 2009 3 x 600 3 x222

B-B THY SAUDI ARABIA

CAPRIVI ABB 2010 300 350 950 TRA NAMIBIA

BRITNED SIEMENS 2011 1000 400 260 THY UK - NETHERLANDS

YUNNAN-GUANGDONG SIEMENS 2010 5000 800 1418 THY CHINA

XIANJIABA-SHANGHAI ABB2010

6400 800 1980 THY CHINA

HULUNBEIR-LIAONING HVDCLINK

ABB 2010 3 000 500 920 THY CHINA

LINGBAO II EXTENSIONPROJECT

ABB/ALSTOM 2010 750 168 B-B THY CHINA

JINDO-JEJU ALSTOM 2011 400 250 105 THY KOREA

BORWIN1 ABB 2012 400 150 200 TRA GERMANY


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dc converter technology for hvdc

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