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AbstractFlexible AC transmission systems (FACTS) devices

can control power flow in the transmission system to improve

asset utilization, relieve congestion, and limit loop flows. High

costs and reliability concerns have restricted their use in these

applications. The concept of distributed FACTS (D-FACTS) is

introduced as a way to remove these barriers. A new device, the

distributed static series compensator (DSSC), attaches directly to

existing HV or EHV conductors and so does not require HV

insulation. It can be manufactured at low cost from conventional

industrial-grade components. The DSSC modules are distributed,

a few per conductor mile, to achieve the desired power flow con-

trol functionality by effectively changing the line reactance. Ex-

perimental results from a prototype module are presented, along

with examples of the benefits deriving from a system of DSSC

devices.

Index TermsDistributed FACTS, Flexible AC Transmission

Systems (FACTS), Interconnected power systems, Phase shifters,

Power system economics, Power system planning, Power system

reliability, Power transmission congestion, Power transmission

control, Power transmission lines

I. INTRODUCTION

HE US transmission system was developed to serve a ver-

tically integrated regulated utility structure. Substantialchanges and capital investment are required to modify it forderegulated market needs. Thus it presents a major infrastruc-tural obstacle for the continuing growth of the US$224 billionUS electricity market [1]. Electricity demand has increased25% over the last decade and continues to increase. Overall,adequate generation capacity now exists, or is planned, tomeet projected needs in the US. At the same time, annualinvestment in transmission facilities has declined over the lastdecade [1]. As a result of load growth, deregulation, andlimited investment in new facilities, transmission congestionhas rapidly increased. Over 50 transmission corridors in theUS are routinely congested, causing high economic impact.

According to the New York Independent System Operator(NYISO), congestion on the T&D system cost over $1 billionper year [2].

Unfortunately, ac power flow follows Ohms Law, not con-

This work was supported in part by the Tennessee Valley Authority and Soft

Switching Technologies Corporation..

D. M. Divan, W. E. Brumsickle, R. S. Schneider, W. Kranz and R.

Gascoigne are with Soft Switching Technologies, Middleton, WI 56562 USA (e-

mail: ddivan@softswitch.com, brumsickle@ieee.org).

D. T. Bradshaw, M. R. Ingram, and I. S. Grant are with the Tennessee

Valley Authority, Chattanooga, TN 37402 USA (e-mail: dtbradshaw@tva.gov,

mringram@tva.gov, isgrant@tva.gov).

tract law. Uncontrolled loop flow causes congestion and reli-ability problems, and reduces the ability to fulfill energy con-tracts. Loop flows also impact the ability to fully utilizecertain transmission lines, even as other lines suffercongestion, further limiting available transfer capacity undernormal and contingency conditions.

New transmission lines could relieve congestion, but areexpensive to build (US$0.52 million/mile typically, but costscan exceed $10M/mile) and require several years for approvaland construction. One solution to the problem of managingpower flow on transmission lines has been through the use ofFlexible AC Transmission Systems (FACTS). FACTS devicesallow control of power flows on ac power systems through theuse of large power converters (10300 MW) [3]. While sev-eral FACTS installations are operating worldwide, wide scaledeployment has not occurred. FACTS typically costs $120$150 per kVAr, compared to $15$20/kVAr for static capaci-tors.

This paper introduces the concept of a distributed staticseries compensator(DSSC) that uses multiple low-power sin-gle-phase inverters that attach to the transmission conductorand dynamically control the impedance of the transmission

line, allowing control of active power flow on the line [4]. TheDSSC inverters are self-powered by induction from the lineitself, float electrically on the transmission conductors, andare controlled using wireless or power line communicationtechniques. Implementation of system level control uses alarge number of DSSC modules controlled as a group torealize active control of power flow. The DSSC can be used toeither increase or decrease the effective line impedance, al-lowing current to be pushed away from or pulled into atransmission line. The DSSC concept overcomes some of themost serious limitations of FACTS devices, and points theway to a new approach for achieving power flow controltheuse of Distributed FACTS or D-FACTS devices. This paper

details the principles of operation of the DSSC, showsoperating results from a prototype device, and presents ananalysis of its possible impact on typical power systems.

II. FEATURES AND LIMITATIONS OF FACTS DEVICES

FACTS devices are typically high-power high-voltagepower converters, operating at 138500 kV and 10300MVA, that are used to control power flow in the transmissionand distribution network. Three basic types of FACTS devicescan be identified as shown in Table I [5].

A Distributed Static Series Compensator System

for Realizing Active Power Flow Control onExisting Power Lines

Deepak Divan, Fellow, IEEE, William Brumsickle,Member, IEEE, Robert Schneider,Member, IEEE,

Bill Kranz, Randal Gascoigne, Dale Bradshaw,Member, IEEE, Michael Ingram, Senior Member,IEEE, and Ian Grant, Fellow, IEEE

T

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TABLE I

TYPES OF FACTS DEVICE

TypeGrid Parameter

ControlledFACTS Device

Series and Shunt (Type A) P and Q UPFC

Series (Type B) P

TCSC, Static

phase shifter,

SSSC

Shunt (Type C) Q SVC, Statcom

Shunt devices such as the static VAR compensator (SVC)and static synchronous compensator (Statcom), have beenmost widely applied, and are typically used for reactive VARcompensation and voltage support. Series devices such as thethyristor controlled series capacitor (TCSC) and the staticsynchronous series compensator (SSSC) can be used forcontrolling active power flow on transmission lines. Series-shunt devices such as the universal power flow controller(UPFC) can be used for accomplishing both functions withmaximum flexibility, and higher cost.

For controlling power flow on transmission lines, the series

elements clearly have the highest potential and impact. The

real and reactive power flow, P and Q, along a transmissionline connecting two voltage buses is governed by the two volt-

age magnitudes 1V and 2V and the voltage phase angle differ-

ence, = ( 1 2 ), as

LX

VVP

sin2112 = , (1)

LX

VVVQ

cos212

112

= , (2)

where LX is the impedance of the line, assumed to be purelyinductive. A series compensator is typically used to increaseor decrease the effective reactive impedance LX of the line,

thus allowing control of real power flow between the twobuses. The impedance change can be effected by seriesinjection of a passive capacitive or inductive element in theline. Alternatively, a static inverter can be used to realize acontrollable active loss-less element such as a negative orpositive inductor or a synchronous fundamental voltage that isorthogonal to the line current [6, 7]. In the latter case, thepower flow depends on the injected quadrature voltage qV as

( ) ( )

( )

+

=

2cos2

2sin

2cossin

2

2

1

2

2

21

12112

V

V

V

VVX

VV

X

VVP

L

q

L

, (3)

and the bracketed term is unity if VVV == 21 . Figure 1

shows, for equal bus voltage magnitudes, the variation ofpower flow along a transmission line that can be achieved by

injecting a passive impedance injX or an active impedance

[7].

0 90 1800

0.25

0.5

0.75

1

1.25X

inj= 0.2

0.1

0

0.1

Transmission Angle ()

TransmittedPower(p.u.)

0 90 1800.25

0

0.25

0.5

0.75

1

1.25V

q=0.334

0.155

00.13

0.241

Transmission Angle ()

TransmittedPower(p.u.)

Fig. 1. Variation of transmission line power flow by impedance injection, (a)

passive impedance injection, as p.u. of XL (TCSC), (b) quadrature voltage

injection to achieve active impedance injection (SSSC, DSSC)

Significant barriers remain to the widespread commercialdeployment of FACTS. Ratings of FACTS devices are oftenin the 100 MW range, with system voltages of 138 to 500 kV.Further, series injection devices such as TCSC and SSSC, re-quire platforms or custom transformers for isolation, and needto handle fault voltages and currents. This approach to system

implementation has resulted in large and complex converterinstallations and barriers that have, so far, limited thecommercial success of FACTS technology. These include High cost resulting from device complexity and

component requirements, Single point of failure can cause the entire system to shut

down, Maintenance and on-site repair requirements for a

complex custom-engineered system adds significantly tosystem operating cost and increases mean time to repair(MTTR);

Lumped nature of system and initial over-rating of devicesto accommodate future growth provides poor return on in-

vestment (ROI); Custom engineered nature of system results in long design

and build cycles, resulting in high system cost that will noteasily scale down with volume.

III. THE DISTRIBUTED STATIC SERIES COMPENSATOR(DSSC)

A controlled transmission system can be made up of alarge number, e.g., hundreds or thousands, of DSSC modules,each module containing a small rated (120 kW) single phaseinverter, a communications link and a single turn transformer

(a)

injL XX

VP

+=

sin2

( )L

q

L X

VV

X

VP 2

cossin2

=

(b)

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(STT) that is mechanically clamped on toand suspendedfromthe transmission line conductor (or insulator). TheSTT uses the transmission conductor as a secondary winding,directly injecting the desired voltage into the cable itself. Theinverter is self-powered by induction from the line, and can becontrolled to inject a voltage that is orthogonal to the linecurrent directly into the conductor. The module can either besuspended from the conductor or configured as a replacement

for the conductor support clamp on an insulator. Further,since it does not require supporting phaseground insulation,the module can easily be applied at any transmission voltagelevel. Figure 2 shows an electro-mechanical concept diagramfor a typical DSSC module, while Fig. 3 shows the powercircuit schematic. The mechanical form of the module mayeither clip-on to the conductor, as shown in Fig. 2, or may beincorporated into the insulator suspension clamp, avoidingany concern about weight and conductor vibration damage.

Fig. 2. DSSC concept showing clamp-on capability

Fig. 3. DSSC Circuit schematic

When the transmission line is not powered up, the STT isbypassed by a normally closed relay contact (R1) that opensonce control power is available. A current transformer is usedto generate control power, allowing the DSSC module to oper-ate as long as the line current is greater than a minimum level,say 150 A. The line appears to the inverter as an inductivecurrent source. The single phase inverter uses four IGBT de-vices along with an output LC filter and a dc bus capacitance.The inverter output voltage is controlled using pulse widthmodulation techniques, and has two components. The first isin quadrature with the line current, and represents the desiredimpedance to be injected. The second is in phase with the linecurrent, and allows compensation of power losses in the in-verter, and regulation of the dc bus of the inverter. Systemcommands for gradual changes are received from a centralcontrol center using a wireless or power line communication(PLC) technique [8]. In the event of rapid transients or faultsthe DSSC modules can be programmed to operate autono-mously. With the DSSC attached and operating at the con-ductor potential, conductor temperature measurement capabil-

ity is easily added and actual temperature readings can becommunicated to the central system controller.The STT is a key component of the DSSC module. It is de-

signed such that the module can be clamped onto an existingtransmission line. The STT is designed with a high turns ratio,say 75:1. This implies that under a normal line current of say1500 A, the inverter would only handle 20 amperes.Designing the inverter for 500 volts rms output would thenallow the DSSC module to inject 7 V rms leading or lagging,corresponding to 10 kVAr in series with the line undernormal operating conditions. It is anticipated that such amodule could be designed to weigh less than 45 kg (100 lb),making the module suitable for direct clamp-on mounting on

the transmission conductor.The STT also allows the inverter to possibly continue oper-

ating under fault conditions. For instance, at a fault current of50,000 A, the inverter current is still only 667 A, well withinthe capability of commercially used IGBT devices. This raisesthe interesting possibility that unlike TCSCs, the DSSC couldregulate line impedance under normal conditions, switchinginto a maximum inductance injection mode within microsec-onds to prevent an increase in the fault current levels.

The inverter ratings clearly demonstrate that the semicon-ductors and components used are commercially available invery high volumes for the motor drives, UPS, and automotiveindustries, thus validating the potential for realizing low cost.

IV. OPERATION OF DSSC IN POWER SYSTEMS

A controlled transmission line implemented with multipleDSSC modules can realize significant benefits at a systemlevel. At the highest level, it can enhance asset utilization, reduce system congestion, increase available transfer capacity (ATC) of the system, enhance system reliability and capacity under contingen-

cies, and enhance system stability

P o w e r

s u p p l y

S i n g l e - T u r n

t r a n s f o r m e r

F i l t e r

P W M

I n v e r t e r

D C C a p a c i t o r

V

C o n t r o l s

C o m m u n i c a t i o n

M o d u l e

C u r r e n t

f e e d b a c k ( C T )

L i n e C u r r e n t

R 1

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and can do so with lower capital and operating cost thanmost conventional single-point lumped solutions, such asFACTS devices.

An overall meshed network system implementation isshown schematically in Fig. 4. DSSC modules can bedeployed on select lines, or on all lines. Overall operation ofthe modules is coordinated using a single (or redundant)communication channel (RF or PLC), with a built-in fail-safe

operating mode in case of fast transients (such as systemfaults) or communication link failure. A typical system levelcommand could be for the DSSC modules to emulate adesired impedance or voltage as a function of line current. Forinstance, the line impedance could automatically increaseabove a current set point, causing current to be preferentiallysteered to other lines that are lightly loaded.

Fig. 4. Meshed network system DSSC implementation.

Operation of the DSSC modules can be coordinated from thesystem control center to realize a variety of optimization func-

tions or operating conditions including system optimization, e.g., loss or VAr minimization; maintaining lines out of congestion or below thermal limit; reconfiguring current flows to compensate for tripped

lines; operating lines above steady-state thermal limit under con-

tingency conditions; forcing power to flow along contract paths; controlling power flow through flow-gates; decreasing susceptibility to sub-synchronous resonance; marginal reduction of fault currents; and providing damping of system oscillations.

The DSSC modules are insensitive to the cable voltage

rating, and are targeted for 138 kV to 500 kV systems. Themaximum level of impedance control for specific lines is pro-

jected at up to 1020% of the actual line impedance underrated current conditions. At lower current levels, the range ofimpedance control can be correspondingly increased. For in-stance, at half the nominal current, the impedance controlrange can be doubled. The distributed nature of the proposedsystem also provides fine granularity in the system rating,along with the ability to expand the system with demand.System planners can thus plan on increasing the ATC of a lineby 2% every year for 10 years to meet projected growth

needs, without having to invest all the capital at project start.Table II shows the characteristics of typical lines at 138 to

765 kV, and examines the applicability of DSSC technologyto realistic applications. Transmission lines are considered tobe typical and representative.

Taking the 138 kV line as an example, it is seen that the re-active voltage drop is 608 V/mile at rated current (corre-sponding to 0.79 ohms/mile). A 1% change in the impedance

thus requires an injection of 6.08 V/mile, corresponding to acombined DSSC rating of 14 kVA/mile based on three phaseinjection. A variation of 20% in line impedance would thusneed 280 kVA or 28 of the 10 kVA DSSC modules/mile orapproximately 9 modules per conductor per mile.

TABLE II: CALCULATION OF NUMBER OF DSSC MODULES TO CHANGE LINE

IMPEDANCE BY 1%

Line voltage 138 kV 345 kV 765 kV

Thermal Line Capacity 184 MVA 1195 MVA 6625 MVA

Current carrying capacity 770 A 2000 A 5000 A

# of conductors/diameter

(inches)

1/1.0 2/1.2 4/1.45

Reactance ohms/mile 0.79 0.60 0.54

Reactive voltage drop/mile 608 V 1200 V 2700 V

1% Compensation/mile 6.1 V 12 V 27 V

DSSC kVA/mile- 1% Comp. 14 kVA 72 kVA 400 kVA

Total 10 kVA DSSC

modules/mile/1% comp.

1.4 7.2 40

A. Examples

Figure 5 shows a simple example of how the DSSC can becontrolled so as to relieve transmission congestion. Two linesof unequal lengths are used to transfer power from bus 1 tobus 2. The assumed line parameters are listed in Table III.

TABLE III. LINE PARAMETERS FOR EXAMPLE OF FIG. 5

V1 = V2 = V 79.7 kV (138 kV L-L)

Line 1 impedance Z1=R1+jX1 20 miles x (0.17+j 0.8) ohms/mile

Line 2 impedance Z2=R2+jX2 30 miles x (0.17 + j0.8) ohms/mileLine 1 thermal rating, I1 max 750 A

Line 2 thermal rating, I2 max 750 A

Line 1 current I1 2 V sin(/2)/Z1

Line 2 current I2 2 V sin(/2)/Z2

Fig. 5. DSSC control of power flow over parallel lines connecting voltage

buses: uncompensated (top) and with DSSC compensation (bottom).

Setting = 7.95 yields line currents of 1I = 675 A, and 2I =450 A, and power transferred along the lines is 1P = 161 MWand 2P = 107 MW, for a total power transfer of 268 MW be-

)( 121212 injXXjR ++

6V

4V 5V

2525 jXR +

11 V

3636 jXR +

3434 jXR +

)( 454545 injXXjR ++

S y s t e m

C o n t r o l s

2626 jXR +

3.4+j16

5.1+j24

138kV

0

138kV

7.95675A

450A

138kV

0

138kV

9.48675A

663A

DSSC

+j3.2

-j4.8

3.4+j16

5.1+j24

3V

DSSC

Module

22 V

)( 131313 injXXjR ++

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tween the buses.Assuming a DSSC in each line, increasing 1X by 20% and

decreasing 2X by 20% allows use of a larger phase angle =9.48, which results in 1I = 675 A giving 1P = 161 MW, and

2I = 663 A and 2P = 158 MW, for a total power transfer of319 MW between the buses. These results are summarized inTable IV. The power transferred through line 2 has increasedby almost 50%, while line 1 has been controlled to well within

its thermal limit. Approximately 10.9 MVA of control action,achieved with 1092 DSSC modules spread over 50 miles oftransmission lines7.3 modules per mile per conductor, re-sulted in 51 MW of additional power flow between buses.This number would be even more advantageous at highersystem voltages. This simple example confirms the ability ofthe DSSC to control loop flows, to manage congestion, and toincrease the power handling capacity of transmission lines.

TABLE IV. POWER TRANSFER BETWEEN TWO BUSES

V

kV

XL

ohm

Xinj

ohm

Xeff

ohm

deg

IL

Arms

P

MW

DSSC

MVA

Line 1initial

138 16 0 16 7.95 675 161 0

Line 2initial 138 24 0 24 7.95 450 107 0

Line 1 +DSSC 138 16 +3.2 19.2 9.48 675 161 4.4

Line 2 +DSSC

138 24 -4.8 19.2 9.48 663 158 6.5

V. LUMPED VERSUS DISTRIBUTED SOLUTIONS

The distributed nature of the DSSC system providesseveral benefits. A significant benefit accrues from the abilityto stage capital investments over a longer time, and being ableto match the investment with the actual need or demand. Forinstance, system planners exploring the addition of a FACTS

system such as an SSSC or a phase shifting transformer to in-crease line ATC would typically plan on a 30 year system op-erating life, and have to design the system to handle thegrowth in demand expected over that period. Since only aportion of the system capacity would be usefully deployed inthe first year, this suggests that the return on capital employed(ROCE), a key financial metric for most organizations, wouldbe low. By way of contrast, a system implemented with DSSCcould be expanded every year in tandem with actual loadgrowth, and would maximize the utilization of capital. Asimple example is used to show the difference.

The 30 mile transmission line shown in Figure 5 isassumed to operate with an average line current of 340 A

(45% of thermal limit), growing at 2.5% per year to 713 A(95% of thermal limit) in 30 years. Lumped and distributedseries compensators (or phase shifting transformer) are to becompared as solutions to control power flow on the line.Ratings for the two solutions are: (a) Lumped: a 19.13 MVASSSC installed beginning of year 1; (b) Distributed: 0.64MVA of DSSCs installed annually for 30 years. Deploymentof either solution allows power flow to increase from 81.2MW in year one to 170.3 MW by year 30. Significantdifferences are seen to occur in capital cost and operating cost(energy losses) for the two solutions. Table V summarizes the

overall impact on cost of ownership.

TABLE V: COST COMPARISON FOR LUMPED VERSUS DISTRIBUTED SOLUTIONS IN

A 138kV / 750A LINE (US DOLLARS)

Lumped Distributed

Equipment Cost including

Cost of Capital over 30 yr.$4.17 M $1.91 M

Energy Costs over 30 yr. $2.69 M $1.46M

Total Cost of Ownership $6.86 M $3.37 M

Assumptionsfor the purposes of the example summarizedin Table Vinclude $100/kVA first cost for either solution, a6% annual cost of capital over 30 years, 2.5% per annum loadgrowth, nominal energy losses of 2.5% (under full voltage andcurrent conditions) and an energy cost of $25/MWh. Withtodays FACTS capital costs at $120$150/kVA, and DSSCcosts expected to be well below $100/kVA, the advantages ofthe distributed system appear even more compelling.

Another important issue relates to the inaccuracy of the es-timated growth rates that are the basis of the economic deci-

sion. If the achieved rate of demand growth is lower than theprojected 2.5%, the distributed solution can be deployed onlyas required, with minimal impact on the overall cost recoveryand return on investment. On the other hand, for lumped solu-tions, a slower rate of growth would dramatically reduce thereturn on investment.

VI. OPERATIONAL AND ECONOMIC BENEFITS

From an operational perspective, the following characteris-tics may be realized for DSSC systems: Ability to increase or decrease steady state line current un-

der system controller command, or autonomously Ability to monitor actual conductor temperature and

manually or automatically limit currents as a function ofconductor temperature

High system reliability due to massive redundancy, singleunit failure has negligible impact on system performance

Zero footprint solution Robust and rugged under typical fault conditions Can be used with conventional or advanced conductors Mass produced modules can be stocked on the shelf, and

repaired in the factorydoes not require skilled staff onsite

Easy and rapid installation (may be possible on live line)

These operational benefits in turn lead to significant economic

benefits. These include Simple scalable tool for congestion managementIn-

crease ATC and revenues Improves contingency management capabilityReduce

TLR calls and meet contingency operating requirements

without building new lines Minimize loop flows and wheeling lossesImproved

asset utilization and lower operating cost Reduce trapped capital in assets sized for future projected

growthImproves return on capital employed (ROCE) Improves flexibility of locating new generation and allows

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power flow along contract path Enables bulk energytrading and reduces overall energy costs

Incrementally increase line capacity and defer new lineconstruction Minimize environmental impact, Right ofWay and NIMBY issues

VII. EXPERIMENTAL VERIFICATION

A prototype module was designed, built, and tested to vali-

date the DSSC concept. The DSSC module, shown in Fig. 6,is completely enclosed and designed for suspension from atransmission line. It operates in a self-exciting mode, drawinga small real power component from the line for control powerand internal losses. This prototype was not designed for ex-tended operation suspended on HV transmission systems, norfor minimization of corona effects.

Fig. 6. DSSC prototype with suspension clamps.

Key module specifications are summarized in Table VI.The power losses in the DSSC module were measured using a

Voltech model PM 3000A power analyzer.

A. Module testing results

Laboratory testing was done at 480 Vrms with a 3000 kVAtransformer (5.75% impedance) source and resistive loads.Figure 7 shows operating waveforms as the module starts upat 1500 Arms. The inverter voltage is in phase with the linecurrent as real power is extracted from the line to charge theinverter DC bus capacitors. Figure 8 shows steady-stateoperating waveforms; note that the injected voltage isorthogonal to the line current, and represents a positiveinductance injection.

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

2500

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Time (s)

Line current (A) Inverter voltage (V) DC bus (V)

Fig. 7. DSSC start-up waveforms at 1500 Arms

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

2500

0 0.02 0.04 0.06 0.08

Time (s )

Line current DSSC voltage x100

Fig. 8. DSSC steady-state waveforms at 1500 Arms and maximum positive

injection. Note: voltage amplitude is scaled by a factor of 100.

In case of overload current levels, e.g., with transmissionfaults, the DSSC protects itself by quickly shorting the trans-former primary winding, first using the inverter IGBTs, thenwith a normally closed mechanical relay. Unlike solutionssuch as the TCSC, the DSSC modules do not contribute to thesystem fault current, and actually operate to reduce it. Theprotection features were demonstrated under simulated

15,000 Arms current surge conditions by temporarilyswitching 1,500 A into a ten-turn coil that was placed inparallel with the single-turn line conductor carrying 350 Armsinitially. Figure 9 shows the resulting voltage and combinedeffective current waveforms at full negative insertion output.The DSSC inverter voltage can be seen to quickly collapse tozero during the current surge. The unit automatically restartedat its previous command point after the fault current conditioncleared.

B. Current steering in parallel transmission lines

A laboratory simulation of parallel transmission lines be-

TABLE VI

PROTOTYPE DSSC MODULE SPECIFICATIONS

Rated line current 3301500 A(rms), 60 Hz

Conductor allowance 1.762 in. OD Bluebird ACSR, or smaller

System voltage 69161 kV target

Overload line current

(fault condition)

15,000 A(rms) for 5 cycles (83 ms)

Injected voltage 04.3 Vrms, lagging or leading line current by

approximately 90

Module VA output 6.5 kVA

Insertion impedance,

Standby mode0.8 H (300 at 60 Hz)

Power loss at 1000 A a. Standby (bypass) mode: 80 watts

b. Maximum negative insertion: 492 watts

c. Maximum positive insertion: 364 watts

Power loss at 1500 A a. Standby (bypass) mode: 150 watts

b. Maximum negative insertion: 640 watts

c. Maximum positive insertion: 750 watts

Weight 74 kg (163 lb)

Dimensions 0.99 m wide x 0.36 m high x 0.37 m deep

Transformer ratio 1 : 105

Transformer core 12 mil silicon steel, cut core

Inverter IGBTs 300 A, 1200 V, 3rd-generation standard speed

Switching frequency 12.5 kHz

DC bus capacitance 900 V, 2.52 mF nominal

Inverter output filter L-C with active damping

Line current feedback Split-core current transformer, 7500:1 ratio

Communications Future (now pre-programmed voltage trajectory)

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tween a voltage bus and a load bus was configured as shownin Fig. 10. Identical inductors were placed in series with eachof two 500 kcmil cables to provide a representative per unitline impedance. The DSSC, represented as a variableinductance

-25000

-20000

-15000

-10000

-5000

0

5000

10000

15000

20000

25000

0 0.05 0.1 0.15 0.2

Time (s)

Total line current Inverter voltage x10

Fig. 9. DSSC operation under 7-cycle 15k Arms fault current. Note: voltage

amplitude is scaled by a factor of 10.

injX in Fig. 10, injected quadrature voltage to effectively

change the line currents while keeping the DSSC currentabove the operational minimum of 330 Arms. The resultingrms current profiles, as the DSSC output is varied, are shownin Fig. 11. The effects of pushing current away and pullingcurrent into the controlled line are evident. By design, thisprototype unit operates with line currents down to 330 A.DSSC designs operational to 100 A or less are possible.

Fig. 10. Simulated parallel transmission lines lab setup. Values are per unit on

a 480 V, 333 kW base. Line 1 is on top, Line 2 on the bottom.

0 10 20 30 40 50200

300

400

500

600

700

800

Time (s)

Line2

Line1 (DSSC)

Load Current

0 10 20 30 40 50200

300

400

500

600

700

800

Time (s)

Line2

Line1 (DSSC)

Load Current

Fig. 11. Current steering effect of DSSC quadrature voltage injection as injected

voltage is varied from maximum negative injection at time zero, to maximum

positive injection at time 10s, to zero injection at time 25s.

C. Extension to transmission system usage

The module test data allow us to calculate the impact ofdeploying the DSSC modules a power system, such as that de-

picted in Fig. 5. Assuming 10 modules per conductor mile,one sees that the energy losses when the system is in bypassmode total 800 watts/mile at 1,000 A. The insertion

inductance in bypass mode is 8 H/mile or 0.003 ohms, ascompared with a typical transmission conductor impedance ofapproximately 0.8 ohms/mile, representing a 0.37% change inline impedance. When the system is operating at full output,the total DSSC system losses are estimated at 14.7 kW/mile

(for three phases) to achieve a 5.1% increase in current andMVA, compared with 239 MVA of power flowing throughthe transmission system at the rated operating point (138kV/1000 A). It is anticipated that these losses can beconsiderably reduced in a final production version of theDSSC, through optimization of component materials.

VIII. CONCLUSIONS

This paper has presented the novel concept of a distributedstatic series compensator (DSSC) that uses multiple low-power single-phase inverters that clip-on to the transmissionconductor to dynamically control the impedance of thetransmission line allowing control of active power flow on the

line. The DSSC inverters are self-powered by induction fromthe line itself, float electrically on the transmissionconductors, and are controlled using wireless or power linecommunication techniques. Implementation of system levelcontrol uses a large number of DSSC modules that aredeployed and controlled as a group to realize active control ofpower flow. The DSSC can be used to either increase ordecrease the line impedance, allowing current to be pushedaway from or pulled into a transmission line in a networkedsystem. The DSSC concept overcomes some of the mostserious limitations of FACTS devices, and points the way to anew approach for achieving power flow controlthe use ofDistributed FACTS or D-FACTS devices. This paper detailsthe principles of operation of the DSSC, shows operatingresults from a prototype device, and presents a preliminaryanalysis of its possible impact on typical power systems.Although many aspects of implementation have yet to beexplored, this paper has demonstrated the fundamentalconcept is sound and the economics are compelling.

IX. ACKNOWLEDGMENT

The authors gratefully acknowledge the contributions ofRick Mills for his work on the DSSC prototype packaging.

X. REFERENCES

[1] S. Abraham, National Transmission Grid Study, U.S. Dept. of Energy,May 2002. Accessed April 2004 at http://www.eh.doe.gov/ntgs/.

[2] W. J. Museler, President and CEO of the New York Independent System

Operator (NYISO), presentation May 22, 2003, New York City. Ac-

cessed April 2004 at http://www.nyiso.com/topics/articles/news_

releases/2003/pa3_presentation.pdf

[3] N. Hingorani, Flexible AC Transmission,IEEE Spectrum, v. 30, No. 4,

pp. 4045, Apr. 1993.

[4] D.M. Divan, W. Brumsickle, and R. Schneider, Distributed Floating

Series Active Impedance for Power Transmission Systems, U.S. Patent

Application #10/679.966.

[5] D.J. Gotham and G.T. Heydt, Power Flow Control and Power Flow

Studies for Systems with FACTS Devices, IEEE Trans. Power Systems,

Vol. 13, No. 1, Feb. 1998.

8/7/2019 01397481

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[6] D.M. Divan, Non-Dissipative Switched Networks for High Power

Applications, IEE Electronics Letters, Mar. 20, 1984, pp. 277-279.

[7] L. Gyugyi, C.D. Schauder, and K.K. Sen, Static Synchronous Series

Compensator: a Solid-State Approach to the Series Compensation of

Transmission Lines, IEEE Trans. Power Delivery, Vol. 12, No. 1, Jan.

1997, pp. 406-417.

[8] D.J Marihart, Communications technology guidelines for EMS/SCADA

systems, IEEE Trans. Power Delivery, Vol.16, No..2, Apr 2001,

pp.:181-188.

XI. BIOGRAPHIES

Deepak M. Divan (S78, M78, SM91, F98),

Chairman, Founder & CTO of SoftSwitching Tech-

nologies, is a world recognized authority in power

electronics converters and controls. He was a Profes-

sor at the University of Wisconsin for 13 years, and an

Associate Director of the renowned WEMPEC

industrial consortium program. He is a Fellow of the

Institute of Electrical and Electronics Engineers

(IEEE), has 30 patents including the fundamental

patent on the DySC product, and over 200 technical

papers in refereed technical journals and conferences. He has worked on

research projects and product development for NASA, US Air Force, Oakridge

National Laboratories, Ford and ABB. He has also served as a consultant to

companies including General Electric, Sundstrand, EPRI, Ansaldo and Eaton.He started SoftSwitching Technologies as a high-tech spin-off from UW

Madison in 1995 and has provided the strategic and technical direction to the

company from its inception. At SST, he has managed an entrepreneurial start-up,

has developed high profile sales channels, has forged alliances with global

industrial leaders, has been deeply involved in raising capital, and has

transitioned the company to a seasoned operations team.

William E. Brumsickle (AM 89, S92, M98)

received the B.S. degree in physics from the University

of Washington, Seattle, in 1982, and the M.S. and

Ph.D. degrees in electrical engineering (Power and

Energy Systems) from the University of Wisconsin-

Madison in 1995 and 1998. He was an applications

engineer for static power conversion products at

Enerpro, Inc., Goleta, California from 1985 to 1992.

From 1992 to 1998 he participated in WEMPECresearch projects in soft switching converters, active

filters, power electronics building blocks (PEBB), large-scale UPS, and traction

drive applications of high voltage IGBTs. In 1998 he joined SoftSwitching

Technologies Corp., Middleton, Wisconsin, where he presently works as

Director, Power Monitoring Group. His professional interests include power

electronics conversion for utility and industrial applications, soft switching

inverter design and control, and the physics, monitoring, and mitigation of power

quality disturbances.

Robert S. Schneider (M 90) received a B.S. degree

in Physics from the University of Wisconsin-Eau

Claire in 1982 and an M.S. degree in Electrical

Engineering from the University of Wisconsin-

Madison in 1986. He is presently a Senior Electrical

Engineer at SoftSwitching Technologies Corp., Mid-

dleton, Wisconsin. He was the project leader andprinciple hardware designer for the first generation of

the DySC line of products that spanned a range of

1.5kVA through 1.33 MVA. Robert has over 18 years

of industrial experience in designing and field testing power electronic

converters and systems including implementation of photo-voltaic, battery

storage, motor drive, servo motor, mini-hydro-electric, distributed generation and

power quality systems. Other projects led by Robert at SoftSwitching include

automatic voltage regulators, single-to-three phase motor drives and the

introduction of demand flow manufacturing technology for DySC products.

Robert is a co-inventor of three of SoftSwitching's patented product technologies

and is presently pursuing a second M.S. degree in Controls from UW-Madison.

He is a licensed Professional Engineer in the State of Wisconsin.

Bill Kranz received the B.S. degree in electrical

engineering from the University of Wisconsin-

Madison in 1999. After graduation he joined

SoftSwitching Technologies Corp., Middleton,

Wisconsin, where he presently works as a Senior

Design Engineer. His professional interests include

real-time software and analog controls for industrial

and utility power protection systems, power quality

and voltage monitoring equipment, and fuel cell based

inverters.

Randal W. Gascoigne received his BSEE degree from the University of

Wisconsin-Madison in 1982. A native of Wisconsin, he specialized in analog

and integrated electronics while at the University, and after graduating he was

employed by the University engineering custom instrumentation and controls for

a wide variety of applications. From 1987 to 1995 he was on the staff of the

Wisconsin Electric Machines and Power Electronics Consortium (WEMPEC),

serving as the lab manager. From 1995 to 2004 he worked as a Senior Engineer

for SoftSwitching Technologies Corp. of Middleton, Wisconsin, where he

continued to apply his expertise in circuit design, component

selection/specification, and packaging/system design. Randy also designs and

builds electrical fish barrier controllers for the Wisconsin Department of Natural

Resources, and enjoys applying electronics to provide unique solutions to

problems in other fields of endeavor.

Dale T. Bradshaw (M) is currently a senior manager at the Tennessee Valley

Authoritys (TVA) Energy Research & Technology Applications organizationmanaging Power Delivery Technologies specializing in research, development,

demonstration, and deployment (RDD&D) of new, or first-of-a-kind

technologies like the SuperVAR project, chemical vapor deposition of

diamond tips and edges in a vacuum

field effect transistor, by improving

transmission lines and substation

equipment, upgrading power system

communications, enhancing TVAs grid

operations, and evaluating the use of

energy storage technologies, etc.; that

seek to increase revenues, reduce

operating and capital costs, improve

reliability, and increase the system

security for TVAs Transmission system. He is a member of the IEEE PES. He

has been with TVA for 28 years, and has been involved in RDD&D of advanced

solid oxide fuel cells, coal gasification, alternate lower cost fuels for fossil powerplants, coal refineries, biomass conversion technologies and other renewable

fuels, advanced combustion turbines technologies, etc. Before coming to TVA,

he spent two years at Public Service of New Mexico. He is a retired LTC in the

Army Reserves, is married and has three children and four grand children. He

holds a BS in Engineering Physics from the University of Oklahoma, MS in

mechanical engineering from the University of Oklahoma, ABD in Nuclear

Engineering from the University of New Mexico, and an MBA in finance from

the University of Tennessee at Chattanooga, TN.

Michael R. Ingram (S) received the BEE degree, with Honor, from Auburn

University, in 1987; and the MS, Engineering Management, from the University

of Tennessee Chattanooga, in 1992. He is a licensed PE in the state of

Tennessee. He is employed at TVA as Program Manager, Transmission Per-

formance Technologies, Energy Research & Technology Applications. He is also

an active member of the IEEE Chattanooga Section.

Ian S. Grant (F) is presently Manager, Special

Studies Unit in TVAs Transmission Planning

Department. He received a B.E. from the University

of New Zealand in 1962 and an M.E. from the Uni-

versity of NSW in 1967. After graduation he joined

the Electricity Commission of NSW, followed by

General Electric, Power Technologies, Inc., Evonyx

Inc., and the Tennessee Valley Authority. He is the

author of over 40 papers and books on transmission

design, lightning and transient studies, and insulation coordination.