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POWER ENGINEERING JOURNAL JUNE 2000 129

Power flow management

Innovative power flowmanagement and voltagecontrol technologies

Power flow control concepts

Power flows can be influenced bycontrolling the basic electrical

parameters, namely impedance of the

transmission line and system voltages,

as shown in eqn. 1:

PS = Re US

QS = Im US(1)

wherePS = active power across the transmission line

QS = reactive power at the sending end

US = sending end voltage

UR = receiving end voltage

Xl = impedance of the transmission line

To be able to control the flows of active power

P and/or reactive power Q, one or several of

these parameters can be controlled by power

equipment already available or under

development. The control of the basic electrical

parameters can be achieved using a shunt

control device, series control device, shunt

current injection device, series voltage

injection device or a combination of these.

In this article the following model

assumptions are used:

4 lossless transmission lines

4 sending and receiving ends are stiff nodes and

their voltages are equal in magnitude

4 performance characteristics are drawn for

midpoint location of control devices.

Shunt control device

The impact on power flow due to a capacitive

shunt device with a reactance ofXC can be

investigated using the transmission model,

vector diagram and mathematical relationsshown in Fig. 1. Shunt devices basically impact

the voltage at the point of connection. When

connected to weak nodes in the power system,

for example in the midpoint or in the receiving

end of a long transmission line, the power flow

can be influenced substantially by the change of

voltage due to the shunt device.

Series control device

The voltage in series with the line can be

created by the natural voltage drop caused by

the line current across an impedance element

with a capacitive reactance ofXC. The insertion

of a series compensation device in a

transmission line directly impacts the power

Driven by ever increasing energy demands, environmental constraints, deregulation

and privatisation of the power supply industry, existing transmission systems are

often operated and stressed to the limit and occasionally beyond the performance

capability of their original design in order to maximise asset utilisation. To ensurethat under these conditions the economical, reliable and secure operation of the

grid is maintained, the need for various aspects of power flow management within

the power systems is becoming increasingly evident.

by E. Wirth and A. Kara

US URjXl

*

US URjXl

*


2/12

130 POWER ENGINEERING JOURNAL JUNE 2000

1 Transmission-line

model, power flow

equations and vector

diagram of the system

with a shunt controldevice

2 Transmission-line

model, power flow



with a series control

device

flow on the line. The influence of a capacitive

element providing the series voltage can be

investigated using the equations shown in

Fig. 2.

Shunt current injection device

Power flow control devices can utilise the

physical principles described above, or

depending on their construction and operating

mode, can be based on the concepts of

controllable shunt current injection and

controllable series voltage injection. The

concept of a device based on shunt current

injection can be demonstrated using the system

shown in Fig. 3. Ii is the controllable shunt

current injected to the midpoint of the

transmission system.

Series voltage injection device

As mentioned already, the series voltage can be

provided by a controlled voltage source. The

series voltage device can be constructed such

that the injected voltages magnitude UTand/or

phase angle can be varied. The impact onpower flow can be investigated by using the

transmission model, vector diagram and

equations shown in Fig. 4.

Impact of power flow control and reactive

power compensation devices on system

performance

By employing devices that can control the basic

electrical parameters, power system

performance can be significantly improved.

One of the major aims of improving a

transmission systems performance is to

increase its power transfer capability. By usingthe concepts discussed above, it is possible to

quantify the impact that shunt and series

control devices have on power transfer


Um

IcXc

*

US

PS = Re US

US URUm

2Xc

Xl1 UR

UR

Xl/2 Xl/2 IR

IRIS

Ic

IS

( ( jXl 4XcXl(1 )*

QS = Im US

US

2Xc

Xl1 UR( (US

jXl 4Xc

Xl(1 )

Um1

*

Um2US

PS = Re US

US URUm2 Um1

UR

XcXl/2 Xl/2 I

I

I

US URj(Xl Xc)*

QS = Im US US URj(Xl Xc)


3/12


capability and reactive power requirements in

transmission systems. The reactive power

balance is one of the many requirements that

enforces a practical limit on how much activepower P can be transferred over a system. Series

devices providing a specific amount of

compensation in general enable more active

power to be transferred with less sending end

reactive power supply requirements as

compared with a shunt device. Capacitive

series devices increase the transfer capability

(their reactive power output increases also with

line loading) and, in addition to increasing the

stability limit, the voltage regulation

capabilities of the system are significantly

improved.

Shunt compensation and control devices

improving voltage maintenance and power

transfer capability through reactive current

supply

Shunt devices help maintain the system voltage

when transferred power is varied. Shuntreactors are used to compensate for the reactive

power surplus in case of reduced power transfer

or open transmission lines. In case of long

transmission lines, some of the shunt reactors

are permanently connected to the system to

give maximum security against overvoltages in

the event of sudden load rejection or opening of

lines. The conventional shunt capacitor

compensation provides the most economical

reactive power source for voltage control in

cases when additional voltage support is

required.Conventional shunt control devices and

modern shunt current injection devices, e.g.

the STATCOM, can also control the power flow


3 Transmission-line

model, power flowequations and vector


with a shunt current

injection device

4 Transmission-line

model, power flow



with a series voltage

injection device. The

equations are based on

the concept with active

powerPT drawn from

the network andreactive powerQTgenerated locally,

shown in Fig. 6.

Um1 Um2US

Um1= UR +jX2IR UT

Um1*

UR

UTjX1IS

US

jX2IR

Um2

Um1

UR

X1 X2 IRIS UT

US = Um1 +jX1 IR + Re(UTIR*)

Um1*IS = IR +

PS = Re (US IS*)

QS = Im (US IS*)

Re(UTIR*)

Um

*

US

PS = Re US

UR

IR

Ii

IS

US UR IijXl 2*

QS = Im US US UR IijXl 2

Xl/2 Xl/2

US URUm

IS IR

Ii


4/12

through a transmission system in a limited

range by supplying or absorbing reactive

current at the point of connection to the

system.

Series voltage injection devices improving

flexibility and enhancing system performance

In the case of the series voltage injection

devices, further system performance

improvement can be achieved by providing

greater operational flexibility in addition to

increasing power transfer capability. There are

basically two ways of generating this series

voltage. One way is to draw all the active power

PT and reactive power QT requirements needed

to generate this voltage from the network, as

shown in Fig. 5. The other way is to draw only

the active power from the network and provide

the reactive power required locally as in Fig. 6.

The power flow control capabilities of

devices capable of coupling a series voltage

with a variable phase angle are shown in

Fig. 7. The impact on power flow control of theformer concept is shown by the green curve and

of the latter by the purple one. Both the curves

are for series voltages of 20% of the nominal

sending end system voltage and a transmission

angle of 60 between the receiving andsending end voltages. As the phase angle ofthe series injected voltage is varied between 0

and 360, the active power flowing through the

transmission system can theoretically be

controlled for a range from a maximum

through to minimum values. From Fig. 7, it can

also be seen that the locally provided reactive

power concept has a bigger impact on power

flow control compared with obtaining reactive

power from the network.

Fig. 8 shows the improvement and the limits

(0 and 180) in power transfercapability with the different series voltage

injection concepts, for an injected voltage

magnitude of 20% of nominal system voltage,

over a range of transmission angles . Thepurple band shows the operating capability of

the series voltage injection device with locally

supplied reactive power QT whilst the green

meshed band is due to a device drawing activeand reactive power from the power system. The

bands indicate the control ranges of devices for

varying between 0 and 180.



7 Achievable

transmitted active

power for the different

series voltage injection

concepts and a

transmission angle of 60

8 Impact on power

transfer capability using

different series voltage

injection concepts for

transmission angles between 0 and 90

UT

PS

QS

PT,QT

PT,QT

1600

1200

800

PS,MW

400

0 30

, deg

60 90

PTfrom network, QTlocal generated

PT, QTfrom network

no control

180

0

1600

1200

800

PS,MW

400

0 90

PTfrom network, QTlocal generated

180

, deg

270 360

PT, QTfrom network

UT

PS

QS

PT

QT

5 Series voltage injection with Pand Q

from network

6 Series voltage injection with Ptaken

from network, Qgenerated locally


5/12


Solutions to transmission system concerns

using power flow control technologies

Finding the most cost-effective solution to the

various issues limiting transmission

performance is attracting ever growing interest

as utilities deregulate and a competitive

electrical supply environment is becoming a

norm rather than an exception. Power flow

control technologies can provide the key to

these solutions. An overview of the

transmission issues and the possible effective

solutions are summarised in Table 1. These

solutions include both conventional as well as

innovative technologies, though they are by no

means exhaustive. In must be noted that, due to

the wide range of network configurations and


9 (a) TCSC system and

(b) its performance

characteristics

PSXC

a

US UR

X1 X2

thermal line overload issues tripping of parallel circuit

voltage and low voltage at heavy load

reactive power high voltage at light load control issues voltage deviation following outage

power flow parallel line load sharing issues post-fault sharing

power flow control

dynamic and lack synchronising torque stability issues dynamic flow control

and transient stabilitypower oscillations voltage stability

BSC

BSR

TCSC

ASC

SVC

STATCOM

TCPAR

QBT

IPC

UPFC

BDV = breaker switched capacitor

BSC = breaker switched reactor

IPC = interphase power controller

QBT = quadrature boosting transformer

STATCOM = static synchronous compensator

SVC = static VAr compensator

TCPAR = thyristor-controlled phase-angle regulator

TCSC = thyristor-controlled series capacitor

UPFC = unified power flow controller

12

10

08

06

04

02

0

200 40 60 80, deg

b

PS,pu

100 120 140

Xcmax compensation

Xcmincompensation

no compensation

160 180

ASC = advanced series compensator

Table 1 Overview of transmission system limitations and possible solutions using control devices


6/12

system operation procedures, proper corrective

actions to deal with various issues are of

necessity application dependent.2

In steady-state conditions the total power

flow on all lines that connect two power

systems is determined by unbalance betweenpower production and load demand including

losses in the individual systems. On the other

hand, during transients the power flow control

equipment can also have an impact on the total

power exchange between the systems. Power

flow control technologies and equipment can

thus be generally categorised according to their

ability to solve steady-state or dynamic problem

domains.

Thermal issues are generally related to

thermal limits caused by a change in the

network configuration during outages and can

be overcome by rearranging the network or byadding a power flow control equipment.

Voltage and reactive power control issues are

related to voltage constraints in the power

system. Low voltage at heavy load can be a

limiting factor under steady-state conditions.

The corrective actions include correcting the

power factor and compensating the reactive

losses in lines by supplying reactive power.

High voltage at light load is an undesirableoccurrence in the transmission and distribution

systems and may be diminished using

mechanically switched shunt capacitors or

reactors to supplement the action of

tapchangers. Low voltage as well as high

voltage following outages can exceed the

voltage limits so that corrective actions have to

be taken to avoid further equipment damage.

Power flow issues are generally related to

controlling the active power in the power

system for better utilisation of the transmission

assets, minimisation of losses, limit flows to

contract paths, post contingency strategies etc.Dynamic and stability issues are related to

dynamic performance of the power system.

Transient stability describes the ability of the



PS

US UR

X1

UT X2

a

10 (a) ASC system,

(b) its steady-state

operating and

(c) performance

characteristics

inductive capacitive UT, pu

I, pu

b

16

14

12

10

08

06

04

02

0

0

PS,pu

02

20 40 60 80

, deg

c

UT= 05 pucapacitive compensation

UT= 0no compensation

UT= 05 puinductive compensation

100 120 140 160 18004


7/12



11 (a) SVC system,(b) its steady-state

operating and

(c) performance

characteristics

power system to survive the first few seconds

after a major disturbance and can be improved

by extracting energy from the sending end of

the network, supplying energy to the receivingsystem respectively by increasing the

synchronising power between sending and

receiving ends. Power system oscillation

describes sustained or growing power swing

oscillations (generally in range below 1.5 Hz)

between generators or group of generators,

initiated by a disturbance (fault, major load

changes etc.). Solutions to this problem lie in

the use of equipment that permits dynamic

damping of these oscillations. Voltage stability

problem is a slow process caused by progressive

increase in load and can be improved by voltage

support, e.g. by using reserve devices, co-

ordinating system load tapchangers, automatic

undervoltage load shedding or generator

control action.

Power flow control devices and their

performance characteristics

As Table 1 shows, solutions to the transmission

issues can be addressed by various power flowcontrol devices. Their application and

suitability to solve a particular problem depend

on many factors covering technical as well as

economical considerations. This section

provides brief descriptions of the technical

capability, technology and performance of each

of the devices listed in Table 1, allowing a first

estimate of device suitability for an intended

application.

Breaker switched capacitor and reactor (BSC,

BSR)

Shunt-connected equipment of these types

allow the reactive power to be supplied via

capacitor banks or absorbed via reactor banks

and thus have significant influence on the

PS

a

US UR

X1 X2

inductivecapacitive

Ush, pu

Ish, pu

b

12

10

08

06

04

02

0200 40 60 80

, deg

c

PS,pu

100 120 140

SVC on capacitive limit

no control

SVC on inductive limit

160 180


8/12



voltage at the point of connection. Series-

connected equipment allow the impedance

characteristics of the transmission system

where they are installed to be varied and thushave direct impact on the power transfer

capability. These devices can be permanently

connected to a system or are connected through

circuit breakers. Breaker switched devices offer

greater operational flexibility in terms of

allowing the operators to adapt to changing

reactive power requirements of their power

systems. Their performance is limited by their

step-wise control characteristics.

Thyristor-controlled series capacitor (TCSC)

The thyristor-controlled series capacitor system

is shown in Fig. 9 together with its performance

characteristics. The variation of capacitance

can be achieved by varying the thyristor-

controlled reactance that is connected in

parallel to the capacitor. The reactance is

determined by the thyristor valve firing angle.

The controllable parameter influencing the

power flow is the capacitance of the TCSC.

Advanced series compensator (ASC)

In contrast to the TCSC where the reactive

power is produced or consumed by capacitors

and reactors, advanced series compensators usepower electronics elements with turn-off

capability such as integrated gate commutated

thyristors (IGCT). By proper repetitive

switching of the IGCTs, the phases of the

system are connected and/or disconnected

causing reactive power to flow among them.

The main difference from the TCSC is that the

injected series voltage UT of the ASC does not

depend on line current. The controllable

parameter here is the series injected voltage and

is coupled in general to the power system via a

booster transformer. Fig. 10 shows an ASCsystem with its corresponding performance

characteristics.

Static VAr compensator (SVC)

An SVC consists of a combination of fixed

capacitors, thyristor-switched capacitors and

thyristor-controlled reactors connected in

parallel with the power system in most cases via

a step-up transformer. The maximum SVC

reactive currents are dependent on SVC

terminal voltage. The reactive power produced

or consumed by an SVC is generated or

absorbed by passive reactive components. The

controllable parameter in this equipment is the

parallel capacitive or inductive susceptance.

PS

US UR

X1 X2

a

12 (a) STATCOM system, (b) its steady-state operating and

(c) performance characteristics

inductivecapacitive

Ush, pu

Ish, pu

b

16

14

12

10

08

06

04

02

0

0

PS,pu

02

20 40 60

, deg

c

STATCOM on capacitive limit

no control

STATCOM on inductive limit

80 100 120 140 160 18004


9/12



13 (a) TCPAR system,

(b) its steady-state

operating and

(c) performance

characteristics

Within the SVC rating, its susceptance can be

continuously controlled. When the SVC

reaches its capacitive or inductive limit, it then

acts as a parallel capacitor or reactor,

respectively. Fig. 11 shows a SVC system, its

steady-state operating and performance

characteristics.

Static synchronous compensator (STATCOM)

By employing power electronics elements with

turn-off capability as in the case of the ASC, the

SVC system can be similarly improved to

become a static synchronous compensator

(STATCOM). The STATCOM basically consists

of an IGCT converter and a DC circuit. The

reactive power generation or absorption is

performed by the system itself and in balanced

conditions reactive elements are necessary for

energy storage during short periods betweenpower electronic switching. From the

STATCOM operating characteristics in Fig. 12,

it is evident that it can supply constant reactive

current almost over the entire range,

independent of the terminal voltage. The

STATCOM controllable parameter is its reactive

current.

Thyristor-controlled phase angle regulator

(TCPAR)

Phase-shifting transformers (PST) are

transformers with complex turn ratios. The

phase difference between the PST terminal

voltages is achieved by connecting a boosting

transformer in series with the transmission

line, as shown in Fig. 13. The active and

reactive powers that are injected into the

transmission line must be taken from the

network by the shunt transformer and

redirected to the boosting transformer. If losses

are neglected, the PST does not produce or

consume reactive power.The thyristor-controlled phase angle

regulator is one type of PST with equal input

and output voltage magnitudes but with a

PS

UT

a

US URUM

X1

UT

US

UMUR

+

b

10

09

08

07

06

05

04

03

01

02

050 0 50 100

PS,pu

, deg

c

150 200 250

=40

=10

=0


10/12

phase shift between these voltages. The TCPARis controlled extremely quickly by a static

thyristor based on-load tapchanger. The

controllable parameter of the TCPAR is the

voltage phase shift angle . Fig. 13 shows alsothe steady-state operating and performance

characteristics of the TCPAR.

Quadrature booster transformer (QBT)

The quadrature booster transformer is another

type of PST where the phasor of the injected

voltage is shifted by a constant angle withrespect to the input voltage vector. Various

types of QBT enable various angles. Thecontrollable parameter of the QBT is the

magnitude of the injected voltage UT. Fig. 14

shows a QBT system with =90, its steady-

state operating and performancecharacteristics.

Interphase power controller (IPC)

The interphase power controller is a series-

connected device, where the major

components in each of the phases are a reactor

and a capacitor subjected to individually phase-

shifted voltages provided by two phase shifting

transformers PAR1 and PAR2. There are many

IPC configurations, depending on specific

application requirements and on the method

used to implement the internal phase shifts. In

the case where the reactor (XA) and thecapacitor (XA) form a conjugate pair, each

terminal of the IPC will behave as a voltage-

dependent current source and provide the IPC



14 (a

) QBT system,(b) its steady-state

operating and

(c) performance

characteristics

PSUT

a

US URUM

X1

b

UTUS

UM UR

12

10

08

06

04

02

00

UT=05 UT=05

UT=0

PS,pu

50

, deg

c

100 150 20050


11/12


with the unique decoupling effect property, a

feature that is desirable. The controllable

parameters are the phase shift angles 1 and 2

of PAR1 and PAR2, respectively. Fig. 15 showsan IPC system and its performance

characteristics.

Unified power flow controller (UPFC)

The basic structure of the unified power flow

controller and its performance characteristics

are shown in Fig. 16. It consists of shunt

(exciting) and series (boosting) transformers.

Both of these are connected by two IGCT

converters and a DC circuit represented by the

capacitor. One difference between the UPFC

and a PST is that the UPFC reactive powerinjected into the line by the series branch does

not need to be transmitted from the parallel

branch. It is generated by the converter

connected to the series branch. The active

power injected into the system by the series

branch must be taken from the system by the

parallel branch and transmitted to the series

branch over the DC circuit. Additionally, the

reactive power of the parallel branch can be

controlled in the same manner as for the

STATCOM. The voltage UTcan be of any phase

with respect to the input voltage US and can

have any magnitude ranging from 0 to UTmaxcorresponding to the dimension of the UPFC.

The controllable UPFC parameters are phase

and magnitude of the injected voltage UT and

the magnitude of the parallel branch reactive

current.

Innovative system solutions the key to

cost-effective power flow control

Driven by ever increasing energy demands,

environmental constraints, deregulation and

privatisation of the power supply industry,

existing transmission systems are oftenoperated and stressed to the limit of, and

occasionally beyond, the performance

capability of their original design. To ensure

that under these conditions the economical,

reliable and secure operation of the grid is

maintained, power flow management concepts

employing innovative technologies have been

proposed.

Load sharing and loss minimisation,

regulating power flow through transmission

corridor, transient stability enhancement and

rapid power flow management to prevent

overloads as well as controlling power flow

patterns are transmission issues that are of

concern and interest to system operators

worldwide. Technical solutions for these

concerns have been proposed and discussed.

In Fig. 17 a 200MVA phase-shifting

regulating transformer for 240kV/132kV based

on a new compact concept is shown. The two

booster transformers for in-phase control and

quadrature control, normally connected in

series with the main transformer, are replaced

by only few extra windings inside the main

transformer tank. This considerably reduces

not only the investment costs but also the

operating costs. The main saving is in the

transformer cores and the copper windings.

Another key benefit is the significantly smaller

space that is required.

Utilities share many of the common energy

transmission problems yet have differenttechnical, economical and environmental

requirements. In order that their needs are

individually met, and cost-effective solutions


15 (a) IPC system and

(b) its performance

characteristics

PS

URUS

X1

PAR 1

a

XA

XA

PAR 2

1

2

12

10

08

06

04

02

090 0

, deg

PS,pu

XIPCX1

90

XIPC =XA

1 + 22 sin2

IPC =1 + 2

2

b


12/12

are provided, the key lies in the application of

innovative power flow control technologies.

Co-operation between the power industry

partners can develop optimised solutions

capable of meeting the performance

requirements demanded in the new and

evolving electrical utility environment.

References

1 DUNLOP, R. D., GUTMAN, R., AND MARCHENKO,

R. P.: Analytical development of loadability

characteristics for EHV and UHV transmission lines,

IEEE Trans., March/April 1979, PAS-98, pp.606617

2 CIGRE TF 38-01-06: Load flow control in high

voltage power systems using FACTS controllers,

CIGRE, January 1996

3 WIRTH, E., and RAVOT, J. -F.: Regulating

transformers in power systems new concepts and

applicationsABB Review, 4/1997

4 JAUCH, T., KARA, A., KIEBOOM, G., and WIRTH,

E.: Operational aspects and benefits of interphase

power controllers with conventional or electronically

switched phase shifting devices a robust FACTS

application, CIGRE-Session, Paris, August 1998

5 LINDER, S. et al.: A new range of reverse conducting

gate-commutated thyristors for high-voltage

medium-power applications, Proceedings of the 7th

European Conference on Power Electronics and

Applications, Trondheim, Norway, September 1997

IEE: 2000

The authors are with ABB High Voltage Technologies

Ltd., Dept. AET, PO Box 8546, CH8050, Zurich,

Switzerland.



16 (a) UPFC system

and (b) its performance

characteristicsPS UT

US UR

X1

a

17 200 MVA phase-

shifting regulated

transformer for 240

kV/132 kV based on a

new compact concept

12

10

08

06

04

02

00

UT=05 puP maximum

UT=05 puP minimum

UT=0

PS,pu

50

, deg

b

100 150 20050

voltage control technologies

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