The Future of High Power Electronics in

Transmission and Distribution Power Systems

Colin C Davidson

AREVA T&D UK Ltd – HVDC & FACTS

St Leonard’s Avenue

Stafford, ST17 0JZ, UK

Tel.: +44 / (0) – 17 85 23 87 69

Fax: +44 / (0) – 17 85 23 88 42

E-Mail: colin.davidson@areva-td.com

URL: http://www.areva-td.com

Guillaume de Préville

AREVA T&D – Special Power Supplies

102 rue de Paris

Massy, 91300, France

Tel.: +33 / (0) – 1 64 47 81 30

Fax: +33 / (0) – 1 64 47 82 32

E-Mail: guillaume.de-preville@areva-td.com

URL: http://www.areva-td.com

Keywords

Power transmission, HVDC, FACTS, Static Synchronous Compensator (STATCOM), Voltage Source

Converter (VSC).

Abstract

Although the term "Power Electronics" covers a very wide spectrum of power ratings and applications

(from Watts to Gigawatts), Power Electronics applications for the Transmission and Distribution

(T&D) market tend to be amongst the very highest in terms of voltage and power rating. Moreover,

these applications are part of a very rapidly growing market and one that is closely linked with the

emerging field of “Smart Grids”.

Some T&D applications of Power Electronics, such as High Voltage Direct Current (HVDC) have

been available for many decades, yet are currently enjoying an unprecedented period of market growth

and rapid technical development. Voltage-Sourced Converter technologies (VSC) now sit alongside

conventional Current-Sourced (Line Commutated) HVDC solutions and are opening up new market

segments.

Other "Flexible AC Transmission" (FACTS) systems, such as the STATCOM, are also available in the

Transmission (HV) market and are starting to penetrate the Distribution (MV and LV) markets.

AREVA T&D was a pioneer in the design of transmission STATCOMs, and built the world's first

commercial STATCOM project using its "chain link" technology.

MV applications of power electronics are driven mainly by the challenges imposed by distributed

generation such as wind energy. The STATCOM can be equipped with battery energy storage to

smooth out short-term variability of power generation, or even to connect to other STATCOMs via a

DC cable network, creating an "MVDC" grid. With a new generation of MV or HV DC-DC

converters, it is even possible that one day DC will once again become the preferred medium for

power transmission, just as it was promoted by Thomas Edison in the 1880s.

Introduction

“Power Electronics” is a term which means different things to different audiences. Semiconductors

used to switch powers of a few Watts are often referred to as Power Electronics; however, at the other

end of the power scale are applications for electricity Transmission and Distribution (T&D) where

powers are measured in Gigawatts.

The highest power ratings of any Power Electronics application are those associated with power

transmission by High Voltage Direct Current (HVDC). HVDC is not a new technology – the first

HVDC link was put into service more than 50 years ago – but is re-emerging as a very important

technology for large-scale bulk power transfer over long distances, particularly in emerging economies

such as China, India and Brazil. As a result, the HVDC market is currently enjoying very rapid

growth and technological development. Schemes based on traditional line-commutated HVDC

technology are now being built for transmitting powers of up to 6.4GW at ±800kV, and the newer

Voltage-Sourced Converter HVDC technology is becoming more competitive for intermediate power

levels up to a few hundred MW.

In the related field of Flexible AC Transmission Systems (FACTS), the market growth and

technological change are also fast. The most well-established FACTS technology (the Static Var

Compensator or SVC) is well established but still represents an important technology for T&D

systems. Its Voltage-Sourced Converter equivalent, the STATCOM (Static Synchronous

Compensator) has been available for niche applications on HV transmission networks for over a

decade, and is starting to find many applications on MV networks, particularly because of the growth

of distributed generation such as wind energy.

The STATCOM and VSC-HVDC technologies actually have much in common, and have the potential

to be used in many more applications than they are currently being used in. As will be discussed

below, they have the potential to be an “enabling technology” for a new system of DC grids overlaid

upon (and perhaps one day even replacing) the traditional AC grid that has existed for over a century.

Ultra-High Voltage Line-Commutated HVDC

High Voltage Direct Current (HVDC) has been used in niche power transmission applications for

more than half a century, and represents the highest power of any “power electronics” application. As

is well known [1], it offers advantages over power transmission by AC in two key situations: (a) when

large amounts of power have to be transmitted over long distances, and (b) to interconnect two

different asynchronous AC networks. HVDC has the advantage of allowing additional power to be

injected into a network without increasing the fault level.

Until relatively recently, all HVDC projects have relied on “line commutated” technology, that is, the

switching elements (originally mercury arc valves but using high power thyristors since the 1970s),

had the capability of turning on but not turning off. With this arrangement, the converter is said to be a

“Current-Sourced Converter”. Whilst the lack of turn-off ability imposes certain operational

restrictions that do not exist with the more modern Voltage-Sourced Converter HVDC described in the

next section, Line-Commutated HVDC (LCC-HVDC) remains far more efficient at transmitting large

amounts of power. In short, thyristor technology is certainly not dead yet.

In fact, in recent years there has been a rapid growth in the market for LCC-HVDC, and this is

spurring rapid technological change. The market growth is driven by two main factors:

• The growing need to connect large-scale renewable energy sources, located well away from

population centres, to the grid.

• The rapid development of the electricity infrastructures of large developing countries such as

China, India and Brazil.

Until only a few years ago, a “large” HVDC transmission project involved a power of 2000-2500MW

transmitted at ±500kV over a distance of perhaps 1000km. Today, however, the powers and distances

involved in many of the projects being considered are so great that the transmission voltages have

been increased from the de facto standard ±500kV to ±800kV in just a few years, and even ±1000kV

is under consideration in China. Fig. 1 shows the evolution of typical transmission voltages over time

for HVDC projects.

Operating at higher DC voltages requires development in a number of key areas. Technologically the

most difficult areas are those that have nothing do with power electronics, for example the converter

transformer, its bushings and associated measurement transducers and switchgear.

Fig. 1: Evolution of HVDC transmission voltage, 1954-present day

However, the developments associated with the core of the HVDC converter, the thyristor valve, are

also significant. As is well described in literature such as [1], a thyristor valve for HVDC consists of a

large number of identical “thyristor levels” connected in series. The number of thyristor levels

connected in series depends on the operating voltage of the valve, but to a good approximation the

design is scaleable – to achieve twice the voltage rating, the valve needs twice the number of thyristor

levels. However, there are subtle effects related mainly to the distribution of stray capacitances in the

converter which dictate that care needs to be taken in extrapolating from low voltage to high voltage.

The second main area of development for the thyristor valve is to ensure that the external surface of

the valve is very smooth. Any sharp points or corners would result in very high local electric field

strengths, leading to local ionization of the air. The resulting corona discharge would generate

harmful ozone and UV radiation as well as increase the risk of flashover. Even with a smooth external

profile, an air-insulated component operating at 800kVDC requires a clearance to ground of some 9m,

and this figure can be increased rapidly by sharp points creating local high-field regions.

Figure 2 shows a photograph of an 800kV valve undergoing voltage withstand testing with an impulse

voltage of 2600kV, resulting in an arc more than 8m long, and figure 3 shows an image of a thyristor

valve being built for a 660kV DC project in China. The structure shown contains two thyristor valves

in series, each containing 111, 7.2kV, 5” diameter thyristors connected in series.

19701970 19801980 19901990 2000 20102010200kV400kV600kV800kV1000kV1200kV

1960196019501950 20202020Itaipu 600kV*Cahora-Bassa533kV*

1000kV – When?500kV becomes de factostandard for single 12-pulse bridge per pole

First 800kV projectOrdered 2007* Multiple bridges per pole

660kV being considered in China as new “Standard”Rio-Madeira6500MW at 600kV ordered

Fig. 2: High voltage testing of an 800kV thyristor valve

Fig. 3: A “Double-Valve” structure for the Ningdong-Shandong ±660kVDC project in China

Along with the rapid development of transmission voltage has come an equally rapid development of

DC current capacity. Until just 2 years ago, very few HVDC transmission projects operated with a

DC current above 3000A. Today, however, one ±800kV HVDC project is being built in China with a

rated DC current of 4000A (giving a total bipole power of 6.4GW) and many more are planned, some

at even higher currents. The valves for an HVDC Back to Back converter operating at a nominal DC

current of 4500A have just been tested by AREVA T&D. The key enabling technology for this has

been the development of 6” diameter thyristors. With a voltage rating of 7.2kV per thyristor, these

thyristors are capable of operating at DC currents of 5000A, and even higher currents can be achieved

by using lower-voltage thyristors (which have thinner silicon and hence lower on-state voltage)

although this would obviously come at the expense of requiring more thyristors in series per valve.

LCC-HVDC looks set to remain a very important technology for long-distance transmission of high

powers for at least the next 10-15 years. However, for lower powers in the range of a few hundred

MW, Line-Commutated HVDC is increasingly facing competition from the newer Voltage-Sourced

Converter (VSC) HVDC technology, which has important technical advantages in some applications.

12m?

Voltage-Sourced Converter HVDC

The main disadvantage of classical line-commutated HVDC is its inability to operate on very weak

AC systems at the inverter end without the use of synchronous compensators or other systems to

provide a rotating voltage reference with inertia. This makes it impractical to use LCC-HVDC to feed

small islands which have no generation of their own.

The newer Voltage Sourced Converter technology started to appear in HVDC applications in 1997 and

avoids these problems because the semiconductors have the capability to be turned off as well as on –

that is to say, the converter is Self-Commutated as opposed to Line-Commutated (strictly speaking,

most of the performance advantages of “Voltage Sourced” HVDC arise from the fact that it is Self-

Commutated, rather than from the fact that it is “Voltage-Sourced” per se).

A Self-Commutated converter requires semiconductor devices with both turn-on and turn-off

capability. Several candidate high-power devices are available such as GTOs, IGCTs, IEGTs etc but

the most commonly used device is the Insulated Gate Bipolar Transistor (IGBT). However, since

commercially available IGBTs have rated voltages of only a few kV each, designers are faced with the

same dilemma as for classical HVDC - how to extrapolate to much higher voltages. There are several

different solutions to this problem [2]. The following sections outline the main alternatives.

Series-connected IGBTs with Pulse-Width Modulation

Almost all of the VSC-HVDC installations in service today rely on a relatively simple converter

circuit that follows closely the principles used in classical HVDC. This circuit requires large number

of IGBTs to be connected and switched in series. Figure 4 shows the basic circuit for a “2-level”

converter used for VSC-HVDC.

Fig. 4: Two level converter in 3-phase (Grätz bridge) configuration

Whilst this circuit is superficially simple and bears an apparent resemblance to the circuit used for

LCC-HVDC, there are a number of disadvantages. First, to achieve the required voltage rating, large

numbers of IGBTs need to be connected in series in each valve. Ensuring safe voltage sharing

between the series-connected IGBTs requires either large, heavy, passive snubber circuits (as on LCC-

HVDC valves) or sophisticated active voltage sharing using the transistors in their linear mode.

Also, because this is only a “2-level” circuit, that is to say it has only two possible output voltage

states per phase, the valves need to be switched many times per cycle (Pulse Width Modulation:

PWM) in order to obtain adequate harmonic performance. Whilst PWM is a well-established

technique at lower powers, it dramatically increases the switching losses of the semiconductors, which

can impose a heavy economic penalty in T&D applications. The AC connections to the converters

also experience very fast, large-amplitude, repetitive voltage transients with very high dv/dt, leading to

potential EMC problems and requiring a complex design of phase reactor.

+ V

- V

+ V

- V

Finally, although the low-order (11th, 13

th etc) harmonics that are characteristic of LCC-HVDC

converters are eliminated, there is still a need for AC harmonic filters to deal with the sidebands and

harmonics of the PWM frequency (typically 2kHz and above).

Multi-Level Circuits

Numerous so-called “Multi-Level” circuits have been proposed, such as the 3-level neutral-point-

clamped circuit which produces three possible output voltage levels per phase, instead of the two

obtained from the preceding circuit. Several VSC-HVDC schemes have been built with this 3-level

neutral-point-clamped circuit. However, 3-level circuits used on an HVDC application still need

Pulse-Width Modulation and passive AC filters, and the same would be true of more complex circuits

such as 5-level converters.

Modular Multi-Level Converter (MMLC)

Other Multi-level circuits, however, such as the “Modular Multi-Level Converter” (MMLC) described

in [3] are able to obtain almost perfect voltage and current waveforms on the AC side by virtue of

providing tens or even hundreds of discrete output voltage levels.

The principle of this circuit is based on the series connection of a large number of independent

converter cells in each phase, each comprising a DC link capacitor, two IGBTs and two freewheel

diodes in parallel with the IGBTs: see Figure 5.

Each “Valve”:

Fig. 5: Modular Multi-Level Converter (MMLC)

In the MMLC converter, each “valve” is actually a controllable voltage source, arranged to synthesise

a fully offset-sinusoidal voltage varying from zero to Vdc (where Vdc is the line-to-line DC voltage of

the converter). Each valve carries one third of the DC current superimposed on half of the AC current,

and is therefore a partly-offset sinusoidal current which is positive for 240° of the cycle.

Other Modular Multi-Level circuits

The circuit described in the preceding section is not the only circuit that can be used to obtain high

pulse-numbers with a scaleable, modular approach. A variation is the use of the “full chain link”

circuit shown in Figure 6. This circuit was pioneered by AREVA in STATCOM applications as

described in the following section and in [4]. When used in the HVDC circuit it functions in a very

similar way to the MMLC circuit described in the preceding section but has the added flexibility that

each module can be controlled to give an output voltage of either polarity instead of a unipolar output.

Module Output voltage

Udc_moduleUdc_module

IGBT1

IGBT2

U

Udc_module

½Udc

½Udc

DC Transmission

System

VSC Valve

V1

VSC Valve

V2

AC

U

+Udc

Valve Voltage

0

U(V1) U(V2)

Fig. 6: Multilevel converter using “full chain links”.

Characteristics

One of the major advantages of VSC-HVDC over LCC-HVDC is that, because the semiconductor

devices are fully controllable (they can be turned off as well as on), it is possible to control the

converter to operate at unity power factor. When combined with a circuit that gives sufficiently low

harmonics, this gives the possibility to operate with no AC harmonic filters at all.

However, although it is possible for the circuit to operate at unity power factor, it is not necessary.

The converter has a defined rating, in MVA, which (with certain limitations) can be used entirely for

real power transfer, entirely for reactive power transfer or any combination of the two – as illustrated

on Figure 7. The change of behaviour in the right hand half of the chart corresponds to limitations

imposed by the DC link voltage (which cannot be less than the peak AC voltage produced by the

converter) when the converter is generating reactive power (which requires a converter voltage

exceeding the AC system voltage).

Fig. 7: P-Q operating characteristics of a VSC-HVDC.

This makes the VSC-HVDC converter tremendously useful and versatile. Whilst its primary purpose

is, of course, to transfer real power, in principle it can also be used as a reactive power compensator –

even in the event that the DC cable or the other DC converter is faulty.

Module Output voltage

Udc_module

IGBT1

IGBT2

U

Udc_module

IGBT3

IGBT4

Or..

+Q

(capacitive)

-Q

(inductive)

+P (Inverter)

-P (Rectifier)

Low AC VoltageLow AC Voltage

High AC VoltageHigh AC Voltage

Constant MVA

Constant MVA

Limitation in capacitive

mode

Limitation in capacitive

mode

STATCOM (Static Synchronous Compensator)

Operating Principles

The STATCOM is a type of power electronic converter designed specifically for reactive power

compensation only. It performs the same function as the Static Var Compensator (SVC), a technology

which has been available since the 1950s, but works on a different principle. The STATCOM uses a

Voltage Sourced Converter to synthesise a fundamental-frequency sinusoidal voltage of controllable

amplitude, approximately in phase with the AC system to which it is connected (Figure 8). If the

STATCOM voltage is greater in magnitude than the AC system, reactive power is generated (the

STATCOM behaves like a shunt capacitor) while if it is lower than the AC system, reactive power is

absorbed (like a shunt reactor). These characteristics are the same as for a Synchronous Compensator

apart from the fact that the magnitude of the synthesised voltage can be changed more rapidly.

Fig. 8: Principles of STATCOM

The STATCOM can be regarded as a special case of a VSC-HVDC converter – one in which the real

power is zero and the DC transmission circuit is open.

Demonstrator STATCOMs first appeared on the Utility power systems in the 1980s in Japan and the

mid 1990s in the USA, but the first fully commercial Utility STATCOM was ordered in 1997 by

National Grid in the UK [4].

Overview of Technologies

STATCOM technologies cover a wide range of powers from very large STATCOMs in the hundreds

of MVar range for Utility power systems, all the way down to low-voltage STATCOMs rated at under

1MVar for direct connection to LV (400V) distribution systems.

The power circuit for a STATCOM has to satisfy exactly the same compromises as does the power

circuit for VSC-HVDC – that is to say:

• how to obtain a sufficiently good approximation to a sinusoidal output voltage, and

• for larger STATCOMs for MV and HV applications, how to obtain sufficiently high voltage

and high reactive power when the individual semiconductor devices on which it is based are

only rated at a few kV each

Not surprisingly, the same choices of circuit considered for VSC-HVDC are also considered for

STATCOMs. The STATCOMs that have been built to date fall into one of two basic categories:

• Simple 2-level or 3-level main circuit, using either pulse-width modulation or transformer-

coupling several converters together to achieve adequately low harmonics

• Modular, multi-level approach using series-connected H bridges (“Chain Links”).

UL

I

US UC

U

UC

US

US

UC UL

If UC < US , then I is inductive

I

t

UL

I

US UC

U UC

US

Uc

Us UL

If UC > US , then I is capacitive

I

t

Within each of these two categories, several variations are possible depending on the required rating

and the available types of semiconductor.

2-level and 3-level STATCOM circuits

With a simple converter type such as the basic 2-level or 3-level neutral-point clamped circuits (as

discussed under VSC-HVDC above) there are a number of ways of realising a STATCOM:

• Single converter with a single semiconductor device in series per valve. This is of course the

simplest circuit but is limited to a few MVar per converter. PWM is required in order to

obtain adequate harmonic performance.

• Multiple converters connected in parallel, with a single semiconductor device in series per

valve. PWM is required in order to obtain adequate harmonic performance. For low to

moderate ratings this is quite a flexible option; however the AC connection voltage is

necessarily quite low (typically 1-2kVrms) so a step-down transformer is required and LV

busbar currents can be quite high.

• Single converter with multiple semiconductor devices in series per valve. PWM is required in

order to obtain adequate harmonic performance. This is similar to the approach used for most

of the existing VSC-HVDC installations, and carries the same technical challenges. However,

it does offer the possibility of realizing a high rating (>100MVar) in a single converter.

• Multiple converters connected in parallel, with multiple semiconductor devices in series per

valve and Pulse Width Modulation. The only significant advantage of this option over the

preceding one is the prospect of reducing the operating voltage of the converter, which could

be an advantage with certain types of semiconductor (for example GTOs or IGCTs) where

series connection of very large numbers is more difficult.

• Multiple converters connected in parallel, with multiple semiconductor devices in series per

valve, not using Pulse Width Modulation. In this variation, which has been used in at least

one Utility STATCOM [5], the valves are only switched once per cycle (to minimize

switching losses) and to obtain adequate harmonic performance, many such converters are

combined magnetically using a complex multi-winding transformer. This approach was

chosen because the semiconductor devices available at the time (GTOs) had high switching

losses and were difficult to connect in series strings of more than 5 or 6.

AREVA T&D have a range of converters (the “MaxSine” family) which fall into the first and second

of these groups. They are designed to be used in STATCOM applications for the LV and MV

networks, in addition providing active filtering capability. The highest-power product, “SVC

MaxSine”, uses a three-level topology with an AC connection voltage of 2kV rms and a nominal

MVar output of 2MVar per module. The basic circuit is as Figure 9.

Fig. 9: Three level neutral-point-clamped converter as used in “SVC MaxSine”.

The system is modular and easily expandable by simply connecting modules in parallel. Up to 12

such modules, giving an output of ±24MVar, can be connected in parallel to a single step-down

transformer, and several such transformers can be grouped together under a single master controller

½Vdc

½VdcV

+½Vdc

-½Vdc

+½Vdc

-½Vdc

Line-Neutral voltage

V

Line-Line voltage+Vdc

-Vdc

(Figure 10). An installation of this type, consisting of a ±72MVar STATCOM (36 modules) and

72MVar fixed capacitor has recently been commissioned for an electric arc furnace application.

Fig. 10: A STATCOM installation consisting of multiple “MaxSine” modules in parallel

The SVC MaxSine product is being developed to extend the output voltage and power per module so

that it can be directly connected to Medium Voltage AC networks in the range 10-33kV without a

step-down transformer.

Cascaded H-Bridge STATCOM (Chain Circuit)

The combination of simple 2-level and 3-level converter bridges gives a cost-effective solution for

small STATCOMs. However, for the largest STATCOMs, or where single-phase control is needed,

the “chain circuit” used in the world’s first commercial STATCOM, as described in [4], gives greater

flexibility.

This uses an arrangement of cascaded H-Bridges (the so-called “Chain Circuit”) in each phase, to

synthesize a sinusoidal voltage in each phase (Figure 11). Chain links can be connected either in a

Delta configuration or a star configuration. The system is inherently single-phase and therefore very

suitable for applications where unbalanced loads are present, for example Electric Arc Furnaces (EAF)

and traction load balancers.

Fig. 11: Chain Circuit STATCOM

The first project built using this topology, the East Claydon STATCOM for National Grid in UK, used

16 such chain links in series (2 redundant per phase) to realise a ±75MVar STATCOM, which was

A “Chain” with three “Links”

U+

U+

U+

U+

U+

U+

U+

U+

U+

3U

2U

U

0

-U

-2U

-3U

Output Voltage3U

2U

U

0

-U

-2U

-3U

Output Voltage

T6777

ValveArrester

1

2

3

4

N

-

-

-

Buffer

Reactor

Buffer

Reactor

CDC

+

-

One LinkN typically 15

MV bus

LV bus (2kVrms)

Up to 12 Modules (±24MVAr) in parallel

per transformer

Up to 12 Modules (±24MVAr) in parallel

per transformer

HV

Master Controller

2MVAr2MVAr 2MVAr2MVAr 2MVAr2MVAr 2MVAr2MVAr 2MVAr2MVAr 2MVAr2MVAr 2MVAr2MVAr 2MVAr2MVAr

LV bus (2kVrms)

combined with a 127Mvar TSC and 23 Mvar passive filter. A second project in the USA [6], at

Glenbrook in Connecticut, used two ±75MVar STATCOMs, each with 15 chain links per phase (in

this case, only 1 redundant). In this latter case, no AC harmonic filters were required.

A cascaded H bridge STATCOM for distribution grid (D-STATCOM) is being developed for

industrial and distribution grid applications. The arrangement is based on the same principles

presented above. The control is different as it uses the Pulse Width Modulation technique. The

advantage of such a structure is the minimization of the losses due to the benefit of series connection

of cells and the overlapping of PWM patterns amongst them, thus the switching frequency per

component is low, whereas the resulting frequency on the network is high. For instance, for a +/-

10MVar, consisting of 4 Chain links in series, with PWM at 750Hz, the resulting frequency is 6 kHz

at the output as shown in the picture 12. A small AC filter is then sufficient to get THD <1% at the

common point of coupling and a simple distribution transformer is needed for voltage adaptation to

the grid.

4 cascaded H-bridge - 2L PWM

0 0.01 0.02 0.03 0.04 0.05 0.06 -1

-0.5

0

0.5

1

Ua

b/(

4*U

c)

f1= 50.0 Hz fs=750 Hz m=0.80

0 0.5 1 1.5 2 2.5 3

x 10 4

0

0.01

0.02

0.03

0.04

0.05

Fréquence →

Am

plit

ud

e

Fig. 12: Output voltage and frequency spectrum for a 4 chain link STATCOM

The Chain link D-STATCOM can be proposed in star configuration or in delta configuration for load

balancing considerations. The other main characteristic is the overload capability for voltage support

in case of fault on the grid. This has its main importance in wind energy integration. AREVA is

developing a +/-10MVar with an overload capability of 2 p u for 2s per 10min.

Typical MV network applications

The shunt Static Var System is used in distribution utilities and industrial network for power quality

purposes as fast voltage support and reactive power compensation in flicker mitigation, harmonic

filtering & damping and fault-ride-through capabilities. For instance, in wind farm integration [7],[8]

the reactive power compensation is advantageously managed by a SVC or a D-STATCOM. Figure 13

shows the benefits that a D-STATCOM of +/-10MVA brings for a wind farm of 10MW consisting of

several fixed-speed wind turbine generators, in the case of low voltage: Without any compensation,

the machines recover with some difficulties, causing some perturbations on the voltage at the point of

common coupling. With an appropriately-rated D-STATCOM, the recovery is facilitated.

Fig. 13: Low Voltage Ride-Through (LVRT) with a D-STATCOM

Another typical application is the flicker mitigation with a large Electrical Arc Furnace [9]. The EAF

is generating large reactive power fluctuations causing flicker. To mitigate the flicker, fast reactive

power compensators are needed, such as SVC and D-STATCOM or the combination of both. For

instance, Fig. 14 hereafter presents the flicker reduction performance (expressed in terms if PST –

Perturbation Short-Term) with a SVC, a full D-STATCOM and a combination of both. For large

flicker mitigation performance, a full D-STATCOM or a combination of SVC and D-STATCOM is

preferred to a single SVC solution.

Fig. 14: PST reduction efficiency with shunt SVS.

In the distribution grid evolution, small and dispersed reactive power compensators will be needed to

mitigate Quality issues. It could be done through small sized D-STATCOM products or small sized

Static Var Compensators, preferably directly connected to the grid, without a step-down transformer.

2.5 s

Generator

0.5 s

0.3

-13 MVAR

Voltage at PCC (pu)

Voltage at Generator Busbar (pu)

Reactive power (MVAR)

Active Power (MW)

Voltage at PCC (pu)

Current at PCC (pu/10MVA)

Active Power (MW)

Reactive power (MVAR)

Voltage at Generator Busbar (pu)

Current at PCC (pu/10MVA)

Power Quality issues can be also approached through active and hybrid filters based on VSC

technology.

STATCOM with energy storage

Many of the problems associated with connecting wind farms in the MV grid can be solved by short-

term storage of energy. Short term storage can be provided quite easily using banks of batteries.

Batteries require, of course, a DC charging/discharging supply, but fortunately this is exactly what is

available with a STATCOM.

As noted above, a STATCOM is really just a special case of a Voltage Sourced Converter (VSC)

which operates with reactive power only. This means that a STATCOM can quite easily be augmented

by adding a battery bank in parallel with the DC capacitor, so that in addition to providing reactive

power compensation, it can charge the battery when surplus energy is available and discharge it back

into the grid during short supply interruptions. This concept has already been proven on a converter of

the AREVA “SVC MaxSine” range.

Medium Voltage DC (MVDC) interconnections

MV Distribution systems tend to be radial in nature and (particularly in rural areas) can suffer from

problems of voltage rise or voltage drop at the ends of the radial feeders. These problems are

exacerbated by the connection of Distributed Generation to distribution feeders. The “obvious”

solution to this problem (strengthening the MV AC network) tends to increase the fault level and can

often cause the ratings of already-installed switchgear to be exceeded.

HVDC is a well-established solution to the analogous problem in HV Transmission networks, where it

can bring all the benefits of a highly interconnected network without increasing the fault level.

However HVDC has yet to make any substantial impact on MV systems, mainly because the cost has

been too high. However, with VSC technology, for the smaller ratings applicable to MV networks,

there is the prospect of a family of low-cost, standardised, multi-purpose converters suitable for real

and/or reactive power applications.

Having already proven the suitability of the MaxSine converter family for use with real power

applications it is a small additional step to connect two such converters together via their DC link

capacitors to form a MVDC connector. AREVA T&D foresees that MVDC networks of this type,

overlaid upon the traditional AC grid, may start to become increasingly widespread as large-scale

integration of renewable energy sources becomes more common. They may, one day, even replace

AC networks altogether, bringing history interestingly back to the situation that existed in the 1880s

where AC and DC were fighting for supremacy in the so-called “Battle of the Currents”.

However, two further developments are necessary to realise this goal:

• The development of high voltage, high power DC circuit breakers

• The development of high power, high-efficiency DC-DC converters.

Both of these areas are being actively researched by AREVA T&D.

Conclusion

Power Electronics has, for many decades, been a niche application in T&D systems. Applications

such as HVDC have long had their place, without ever being “mainstream”. However, the situation is

changing and Power Electronic systems are increasingly being seen not just as a niche application but

central to the whole operating principles of the grid.

There is much talk of “Smart Grids” and how this somewhat ill-defined alliance of technologies will

revolutionise the power grid. Smart Grids are widely assumed to be only about smart metering and

wide area energy metering systems, but implicit in the whole concept is the idea that grid operators (be

they human or computerised) need to be able to control where the power flows. With conventional

AC grids this is simply not possible: the flow of power between two points is determined only by the

impedance and phase angle between the two points, neither of which is readily controllable. Only

power electronics has the ability to over-ride these classical laws and make the power flow where it is

needed, rather than where it wants to flow.

A new range of multi-purpose Voltage-Sourced Converters has the potential to create the building

blocks of a future “DC Grid”, both at HV and MV levels, which could see history come full circle

back to the time when Thomas Edison and George Westinghouse were rivals in the “Battle of the

Currents” in the 1880s.

References

[1] Arrillaga J., High Voltage Direct Current Transmission, ISBN 0 85296 941 4.

[2] Flourentzou N., Agelidis G., Demetriades G.D., VSC-Based HVDC Power Tranmission Systems: An

Overview, IEEE Transactions on Power Electronics, Vol 24, No. 3, March 2009.

[3] Lesnicar A. and Marquardt R.: An innovative modular multi-level converter topology for a wide power

range, IEEE Power Tech Conference, Bologna, Italy, June 2003

[4] Knight R.C., Young D.J., Trainer D.R., Relocatable GTO-based Static-Var Compensator for NGC

Substations, CIGRE Session 1998, Paris.

[5] Schauder C., Gernhardt M., Stacey E., Lemak T., Gyugyi L., Cease T.W., Edris A., Development of a

±100MVAR Static Condenser for Voltage Control of Transmission Systems, IEEE 94 SM 479-6 PWRD.

[6] Scarfone A.W., Oberlin B.K., Di Luca J.P.Jr., Hanson D.J., Horwill C., A ±150MVAr STATCOM for

Northeast Utilities’ Glenbrook Substation, IEEE Power Engineering Society General Meeting, 2003.

[7] Courault J., de Preville G., Integration of Offshore wind Farm in power system, CIGRE 2004, Paris.

[8] de Preville G., Wind farm integration in large power system: dimensioning parameters of D-

STATCOM type solutions to meet grid code requirements, CIGRE 2008, Paris.

[9] Michel D., de Preville G., Mixed topology for flicker mitigation, IEE 2004, Edinburgh.