8/7/2019 SVC_Tyll_TD2005-000804

1/6

1

Abstract In the early days of power transmission problems

like voltage deviation during load changes and power transferlimitation were observed due to reactive power requirements of the transmission system. Today these problems have even higherimpact on reliable and secure power supply in the world of globalisation and privatisation of electrical systems and energytransfer. Fast and highly reliable power electronic devices

(thyristor valves) in Static Var Compensators (SVC) and HVDCapplications proved their effectiveness in HV transmissionsystems to reduce energy transfer limitations. Influence of reactive power unbalances on the transmission systems andapplication of dynamic shunt compensation [1,2,3,4] ie SVC isdiscussed. Detailed information about the used components inSVCs is provided.

Index Terms SVC, Reactive Power Compensation, FACTS

I. I NTRODUCTION

OWER transmission based on three phase systems started

in the late 19th

century. The supply of electrical energydeveloped from separated utilities to large interconnected systems. In former times distributed power generationsupplied load centers within a limited supply area. Thesesmaller systems were operated at lower voltage levels.Nowadays there is increased power exchange over larger distances at highest system voltages allowing reserve sharingand competition. Electrical energy shall be made available atmost locations at minimum cost and at highest reliability .

Following problems have been observed in three-phase-systems already at early times of power transfer:

Voltage control at various load conditions Reactive power balance (voltage, transmission losses) Stability problems at energy transfer over long distances Increase of short circuit power in meshed systems Coupling of asynchronous systems Coupling of systems with different system frequencies

The last two problems can be solved using HVDC technologyand the upper ones can be solved by proper use of reactivepower compensation based on FACTS devices.

H. K Tyll is with Siemens AG, Erlangen, GermanyHigh Voltage Division, PTD H16Reactive Power Compensation and FACTS devices(e-mail: heinz.tyll@siemens.com).

II. E FFECTS OF REACTIVE POWER ON SYSTEM OPERATION ANDHOW IT CAN BE INFLUENCED

Reactive power is made available by components which areincluded in the system itself and by other components whichare added to the system for balancing the system reactivepower.

1) Types of Var Sources

a) System components Inductances in electrical machines,

transmission lines, transformers, reactors Capacitances in transmission lines, cables

b) Compensation components Mechanically switched reactors and capacitors Synchronous condensers Thyristor controlled shunt and series compensation [1] Converter controlled shunt and series compensation

2) Influences in steady state system operation

The influence on voltage from reactive power changes can bedescribed simplified by the equation:

PscQ

V

=

V is the voltage change in pu at a point in the system with aneffective short circuit power Psc and a injected reactive power of Q. An inductive power change with neg. sign result in avoltage drop, a capacitive power change results in an increaseof voltage.

Figure 1a shows a simplified transmission line represented byits line constants having a sending end and a receiving end.Figure 1b shows the voltage at the end of the transmission linewith different kinds of load connected. The cos( ) of the load varies from 0.9 lagging to 0.9 leading.

Figure 1a Simplified transmission line

Application of SVCs to SatisfyReactive Power Needs of Power Systems

H. K. Tyll, Senior Member, IEEE

P

Line constants: x, r, c, gVS VR

S=P+jQTransmission Line

Sending end Receiving end

8/7/2019 SVC_Tyll_TD2005-000804

2/6

2

Figure 1 b Voltage at the end of the line at various power transfer levels P/P SIL and different load factors

Using an appropriate compensation the voltage can be kept

const for most load conditions. Above curve shows also thatthere are power transfer limits beyond no stable power transfer will exist. Above curves are based on a 200 km longtransmission line. A safe distance to the "noses" of abovecurves should be used to avoid instability problems attransient system events.

Figure 2 below shows a power plant which is connected to alarger system via a double line transmission system(upper part). The lower part of the figure shows simplified thevoltages at the end of the transmission system for various load cases :

heavy load

light load outage of one line during heavy load condition load rejection at the end of the line

According to the loading conditions voltage decreases and increases will occur with larger deviations at contingencyconditions. An SVC will be typically designed in size to limitvoltage deviations during normal load conditions and a good voltage profile is kept for this operation. At other contingencyconditions larger voltage deviation will occur due to the sizingfor normal conditions.

Figure 2 Voltages at the end of a transmission system under various operating conditions

3) Influences in transient system operation

Figure 3 below shows the case of load rejection. The voltagerises rapidly in will be reduced by the voltage control meansof the system ie voltage controllers in power plants. If a SVC

is connected close the voltage will also rise rapidly but will bereduced in only a few cycles by the fast reaction of a SVC.The first peak cannot be influenced because the SVC controlmust first abserve the increase and can only react afterwards.

Figure 3 Action of a SVC on load rejection

Figure 4 below shows a transmission system with strongactive power oscillations after a severe line fault followed byfault clearing and switch off the faulted line.

Figure 4 Damping of power oscillations by SVC

Without a SVC the oscillations continue at low damping for along time. Using a SVC with voltage control already helps in

damping of the oscillations and reducing the oscillation time.Using an SVC with a specific POD (power oscillationdamping) control function will even damp out the oscillationsquite faster and increase thus the margin in system stability.Providing damping to the system as the primary purposeduring this critical system condition means that voltagecontrol is no longer the main task and deviations from theanticipated system voltage is allowed.

III. S VC TECHNOLOGY

1) Tasks of SVCs

Load

SVC

230 kV - 300 km

V2 V1

Grid

without SVC

with SVC

1.2

1.1

1.0

0.9

0.8

a b c d

V2

V2N

a Heavy loadb Light loadc Outage of 1 line

(at full load)d Load rejection

at bus 2

System Conditions:

Voltage control at steady-state and during transientsystem conditions

8/7/2019 SVC_Tyll_TD2005-000804

3/6

3

Dynamic reactive power control for dynamic loads and Var management

Damping of active power oscillations Improvement of system stability Voltage symmetrization

2) Basic branches of an SVC

The dynamic controllable branches of an SVC are the TCR (Thyristor-controlled-reactor) and the TSC (Thyristor-switched-capacitor).

Figure 5 below shows the basic circuit of a TCR.

Figure 5 Basic circuit of a TCR

Antiparallel connected thyristors are series-connected with areactor of high quality (very low losses). A sinusoidal voltagewill result in a sinusoidal current when the thyristors arecontinuously conducting ( = 90). By delaying the start of the current in each halve cycle by delaying the firing signalfor the thyristors the current will start later and end earlier asseen in the lower part of figure 5 (example for = 120).Such a chopped current wave form contains fundamental and harmonic currents. The advantage of the TCR is the finecontrol of its installed reactive power from full load to zeroand vice versa.

The chopped current waves contain all harmonics of order 3,5, 7, 9, 11, 13 etc within the single branch of a TCR. Byconnecting three TCR branches in delta all triplen harmonicswill be suppressed. In reality the line current does not onlycontain the so-called characteristic six pulse harmonic currentsbut also non-characteristic currents which arise mainly due tonegative sequence voltage content in the system voltage (alltriplen harmonics) and even harmonics which result fromtolerances on the firing pulses in positive and negativedirection.

The figure 6 below shows a set of harmonic currents based on1% neg. seq. voltage content and a firing angle unsymmetry of

0.1.Figure 6 Characteristic and noncharacteristic harmonic

currentsThe x-axis shows the harmonic number, the y-axis shows theamount of harmonic content in % related to the 90-current of the TCR.

Figure 7 below shows the basic circuit of a TSC.

Figure 7 Basic circuit of a TSC

Here the reactor of the TCR is replaced by a capacitor. At fullconduction of the valves the synusoidol voltage Vsys resultsin a 90 phase shifted capacitive current I. If the thyristors areno longer switched by firing pulses the current I through thecapacitor stops and the voltage Vc of the capacitor does nolonger follow the system voltage Vsys and remains at thevoltage at the time of blocking. The best time to reconnect thecapacitor is at the point of time where system voltage and thecapacitor voltage are equal. At that point only minimumtransients due to switching-on will occur. At all other times

I

a = 90a = 120

Vsys

I120

Vsys

I90

I

a = 90a = 120

Vsys

I120

Vsys

I90

Even Harmonics due to Tolerances

Characteristic Harmonics of TCROdd Harmonics due to Negative Sequence

0.01

0.1

1

10

1 3 5 7 9 11 13 15 17 19 21 23 25

Even Harmonics due to Tolerances

Characteristic Harmonics of TCROdd Harmonics due to Negative SequenceEven Harmonics due to Tolerances

Characteristic Harmonics of TCROdd Harmonics due to Negative Sequence

0.01

0.1

1

10

1 3 5 7 9 11 13 15 17 19 21 23 25

Vsys

I

Vsys

Vc

VcI

blocking switch-in

Vsys

I

Vsys

Vc

VcI

blocking switch-in

more or less strong transients will occur and therefore onlystep-wise control of the TSC is allowed.

8/7/2019 SVC_Tyll_TD2005-000804

4/6

4

3) Configurations of SVCs

Using TCR-, TSC- and Filter (or so-called fixed connected:FC) branches result in possible configurations as shownbelow:

Figure 8a Six pulse arrangements

Figure 8b Twelve pulse arrangements

Figure 8a shows a combination of TCR and FC without usinga TSC branch. The TCR branch must be designed tocompensate the FC branch and in addition to absorb reactivepower from the system if required. In the arrangement TCR,TSC and FC the capacitive installed power is sub divided in aTSC branch and the FC branch. The FC branch typically willbe arranged as two filters tuned to 5 th and 7 th harmonic. Theinductive operating point will now be reached together with asmaller rated FC branch . The TCR branch is thereforedesigned on a lower power level. The power of the TCR must

be slightly larger than the TSC branch to avoid hunting atswitch-over points.Figure 8c Direct connection

Direct connection of TCR and Filter branches may also beused for system voltages below 36 kV.

4) V / I Characteristics of SVCs

All SVCs based on the described configurations have a V/Icharacteristic similar to figure 9.

Figure 9 Typical V/I Characteristic as seen from the highvoltage system

The V/I characteristic is limited by a straight line on the leftside according to the installed capacitive power of the SVC.The straight line on the right side is the limitation according tothe required inductive design point. At low voltages the SVCoutput follows the capacitive limitation, at higher voltages theSVC output follows the inductive limitation. The SVC willwork on a controlled basis in a typical system operatingvoltage range between reference voltages of 0.95 to 1.05 pu.

TCR, FC TSR, TSCTCR, TSC, FC

HV

LV1

LTCR12

TCR 1

LTCR12

Filter 1

LF1

CF1

13 kV

LTCR22

TCR 2

LTCR22

Filter 2

LF2

CF2

LV2 13 kV

LTCR2

TCR

LTCR2

Filter 3

LF3

C F3

V

8/7/2019 SVC_Tyll_TD2005-000804

5/6

5

Figure 10 Typical V/I Characteristic as seen from the lowvoltage SVC bus

At capacitive operation the secondary side voltage increasesdue to the leakage inductance of the transformer winding. Atthis operating points any saturation of the transformer must beavoided. Contrary to typical system transformers this results inmagnetization knee point of 1.2 to 1.4 pu depending onspecified operating requirements.

5) Design considerations for SVC main components [5]

The tables below summarizes main tasks of the variouscomponents on the left side and show also some designaspects on the right side.

The decision of three phase or single phase transformers are

typically based on spare part considerations ie availabilityconstraints. Environmental conditions like noise requirementssometimes lead to noise shielding.

Figure11Three phase transformer enclosed by a brick wallbuilding for noise reduction

Figure 12 Single phase transformers separated by fireprotection walls

The decision for single phase reactors (TCR) is based typically on spare part requirements.

Figure 13 Typical arrangement of double-stacked TCR reactors

Figure 13Capacitor bank (externallyfused)

Restrictionto 150 MVar

Capacitivedesign point-150 MVarat 0.95 pu

Ind. RangeCap. Range

Minimumoperatingvoltage

1.3

pu

1.1

1.0

0.5

2 %5 %

10 %

10 %5 %

2 %

VBase = 14 kVIBase = 100 MVA

Continuous OperationRestricted Operation

VLV

Inductivedesign point75 MVarat 1.02 pu

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 ILV [pu]

Restrictionto 150 MVar

Capacitivedesign point-150 MVarat 0.95 pu

Ind. RangeCap. Range

Minimumoperatingvoltage

1.3

pu

1.1

1.0

0.5

2 %5 %

10 %

10 %5 %

2 %

VBase = 14 kVIBase = 100 MVA

Continuous OperationRestricted Operation

VLV

Inductivedesign point75 MVarat 1.02 pu

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 ILV [pu]Reactors Single phase air core reactors

in different 3phase arrangement Iron core reactors with air gaps

as three phase units

Continuous operating range Short time overload Harmonic stresses Spare parts Transportation limitations Environmental conditions

Transformer Matching to the system Single or three phase units

or 3-winding transformers short circuit impedance

Continuous operating range Short time overload Harmonic stresses Spare parts Transportation limitations Environmental conditions

Capacitors Series and parallel connection of

small units Outdoor / indoor installation Internal / external fusing

Continuous operating range Overload requirements Harmonic stresses Protection systems

8/7/2019 SVC_Tyll_TD2005-000804

6/6

6

Figure 14 (below)Thyristor valve phase modul based on8 kV, Light triggered thyristors, integrated overvoltage protection on silicon wafer

Figure 15 Simplified voltage control block diagram

In most cases SVCs are used for system voltage control ie fastcapacitive support in critical system conditions. In some

conditions additional control functions may be included for better system control:

Power oscillation damping control to improve systemstability

Reactive power control direct compensation of largeindustrial plants or in conjunction with Var management schemes

Degraded operating control modes to increases thetotal operating flexibility of the SVC Automatic gain adjustment to provide in all system

situations best control response.

IV. SUMMARY

Major surplus or lack of reactive power in transmissionsystems can result in severe voltage stability and/or power transfer problems. Dynamic shunt compensation devices likethe SVC help to overcome these problems also during criticalsystem conditions. More than 80 % of these dynamic Vars areusing highly reliable thyristor based configurations.

It is most important to specify [6] the required operatingcharacteristics correct together with system conditions onharmonics and fault level to be sure that the devices willfunction as expected.

V. R EFERENCES

Papers Presented at Conferences (Unpublished):[1] H. K. Tyll, "FACTS Technology for Reactive Power Compensation and

System Control", presented at IEEE T&D Conference Sao Paulo, 2004

Books:[2] Y.H. Song and A.T. Johns, "Flexible ac transmission systems (FACTS)",

IEE 1999, ISBN 0-85296-771 3[3] N.G. Hingorani and L. Gyugyi, "Understanding FACTS", IEEE PRESS,

ISBN 0-7803-1196-6[4] T.J.E. Miller, Reactive Power Control in Electric Systems, John Wiley

&Sons, 1982

Technical Reports:[5] H.K.Tyll et all, "Design considerations for the Eddy County Static Var

Compensator", 93-SM-450-7 PWRD

Standards:[6] IEEE Functional Specification sand Application of Static Var

Compensators, IEEE Standard 1031-2002

VI. B IOGRAPHY

Heinz Karl Tyll (M88, SM93) was born inHof, Federal Republic of Germany on May 15,1947. In 1968 he graduated in ElectricalEngineering from Coburg Polytechnikum. In 1974he received the Diplom degree from the TechnicalUniversity of Berlin. After joining Siemens AG, heworked in their High Voltage TransmissionEngineering Department since 1975 in the field of network and SVC system analysis with transientnetwork analyzer and digital programs. In 1988 hetransferred to the System Engineering Group of theHVDC and SVC Sales Department. Since 1996 he

is responsible for Basic Design of SVC, SC and FACTS applications. Hecontributed to CIGRE WG 38 TFs and to relevant IEEE WG. He is member of IEEE and VDE.

Valves TCR, TSR or TSC valves Optimum use of thyristor ratings

Continuous operating range Overload requirements Stresses during system faults and

false firing events Insulation co-ordination

Control and protection ON / OFF control

Local / remote control Control characteristics

Voltage control Reactive power control Power oscillation damping Symmetrization

Use of conventional protection

Type of control

Control requirements Harmonic instability System characteristics

Network impedance System unbalances

Component design ratings

Power SystemVT

Vact

Vref

VSystemVoltageEvaluation

TCR TSCFC BSVC

VoltageController

TSCController

TCR/TSRController

LV

HVPower System

VT

Vact

Vref

VSystemVoltageEvaluation

TCR TSCFC BSVC

VoltageController

TSCController

TCR/TSRController

LV

HV