Phase Shifting Transformers

Prof. S. V. KulkarniDepartment of Electrical Engineering

Indian Institute of Technology Bombay, INDIAsvk@ee.iitb.ac.in

CBIP Conference, New Delhi 28-11-2013

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bay Outline

Introduction

Applications of PSTs

Basic Concepts

Design Considerations

Other Technologies

Concluding Remarks

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Introduction

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bay Need for Phase Shifting Transformers

Power systems are becoming complex

Devices useful in controlling power flow in a complex interconnected network are desirable

Power system deregulation and market mechanism

Power flow from generation centers to load centers in an efficient way

Major disturbances and system blackouts

Stability of a large interconnected system can be improved

Overloading and under-utilization of transmission lines

Lines can be optimally loaded

PSTs would be more relevant when penetration of intermittently available renewable energy sources increases in the system

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bay Functions of Phase Shifting Transformers

Power flow control between the sending and receiving ends of a transmission line

Control of power flow between two parts of a large power system network

Improvement of system stability and reliability

Re-routing of powers through transmission lines leading to their optimum utilization/deferment of investment costs for new lines: Congestion management

Avoid/minimize loop currents and corresponding power flow

Allow efficient evacuation of new generation

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Applications of PSTs

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bay Representative and Notable Applications of PSTs

i.Control and increase the power flow between two large systems: PST option was found better than HVDC and series capacitors

(Ref: Patel, B. K., Smith, H. S., Hewes, T. S., and Marsh, W. J. Application of phase shifting transformers for Daniel-Mcknight 500 KV interconnection, IEEE Transactions on Power Delivery, Vol. PWRD-1, No. 3, July 1986, pp. 167–173)

ii. Kothagudem thermal power station: - 500 MW evacuation – 220 kV lines would be overloaded- One option: 400/220 kV ICT – which would also increase the loading of

the underutilized double-circuit 400 kV line- System studies indicated reverse flow – from 400 kV to 220 kV- Hence the option chosen : ICT with a phase shifter(Information: Courtesy - BHEL)

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bay Representative and Notable Applications of PSTs

iii. Reduction in loop flows

400 MW

300 MW 400 MW

100 MW100 MW“Loop flow”

100 MW

400 MW 300 MW

400 MWArea-1

Area-2

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Ref: Kulkarni, A. M. Power System Operation and Control – NPTEL Web Course, Module 4 : Voltage and Power Flow Control, Lecture 19

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Basic Concepts

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bay Power Flow Through a Transmission Line

The real power transferred to the receiving end is proportional to the angle difference whereas the reactive power is proportional to the magnitude of the voltage drop across the line.

Gsss jQPS

SV 0RV

LoadRRR jQPS

sinS RR

V VP

X

2

cosS R RR

V V VQ

X X ( )R R

S R

V VV V V

X X

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governed by Kirchoff’s circuit laws, irrespective of line loading and thermal limits

In order to control the flow of active power and reactive power, regulating transformers can be used

Magnitude-regulating transformer:

The reactive power flowing through a line can be controlled by adjusting its tap setting

The transformer can also be used to modify the magnitudes of reactive powers flowing through two parallel transmission lines

Phase-shifting transformer

Control of active power flow between two interconnected parts

Redistribution of active powers between two parallel lines

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Source: S. V. Kulkarni and S. A. Khaparde, Transformer Engineering: Design, Technology, and Diagnostics, Second Edition, CRC Press, Taylor & Francis Group, New York, September 2012.

1 1 1

2 2 2

1 1 1

2 2 2

o1 1 1

2 2 2

0.425 0.3 1.000.425 0.3

0.4354 0.1525 1.050.4146 0.4475

0.1817 0.3148 1.0 40.6683 0.2852

S P jQ j aS P jQ j

S P jQ j aS P jQ j

S P jQ j aS P jQ j

G

SourceBus

LoadBus

1I

2

Load

0.15

LI

j

0.15

V

I1

I2

j

=1.0 0

I

LVS

|

|

2

1:1

1:a

1

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Equal area criterion for assessment of stability Without PST Accelerating area: abcda, Decelerating area: defgd, Margin: fghf With PST When the power angle is α + β while swinging, a phase-shift of α is

introduced Accelerating area: mnopm, Decelerating area: pqrstup, Margin: tuvt (> fghf)

Source: S. V. Kulkarni and S. A. Khaparde, Transformer Engineering: Design, Technology, and Diagnostics, Second Edition, CRC Press, Taylor & Francis Group, New York, September 2012.

PSafety margin

c

ePmech

a

b

d g h

f

0 0

Safety margin

m

n

r

su

v

o

p

q

P

t

Pmech

Without PST With PST

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bay Power Flow with Regulating Transformers

Source: S. V. Kulkarni and S. A. Khaparde, Transformer Engineering: Design, Technology, and Diagnostics, Second Edition, CRC Press, Taylor & Francis Group, New York, September 2012.

Ideal Transformer

Inm mI

SSm

nZ1

Y=

1: a

VV a nm nVm

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Off-nominal tap ratio, a may be real (e.g., 1.04) or imaginary (ejπ/60 or 3о shift).

Lossless transformer:

or

Now,

Multiplying both sides by

m n

m m m n

m n m n

n n m m n

S S

V I aV I

I aI I a II V aV Y aY V Y V

a

,m m m n n nS V I S V I

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If a is a real number, and we can draw an equivalent πcircuit for this case:

a a

But if a is imaginary (as in a phase shifting transformer), the Y bus is not symmetrical, and we can’t draw a π equivalent circuit.

m a Y n

(a a )1 Y Vn

m ( 1 a ) YV

22 *Now,

n m m n

m m mm mn m

n n nm nn n

a I I a a Y V a Y V

V I Y Y Va Y a Ya a aV I Y Y VaY Y

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Design Considerations

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V L V S V SV L

V V

S

PST

L

phase rotation

Phase-advanceA leading quadraturevoltage drop is added to the source voltage

Phase-retardA lagging quadraturevoltage drop is added to the source voltage

Phase-advance and phase-retard modes

Source: S. V. Kulkarni and S. A. Khaparde, Transformer Engineering: Design, Technology, and Diagnostics, Second Edition, CRC Press, Taylor & Francis Group, New York, September 2012.

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PSTs are manufactured in different ways depending on the power output, the rated voltage, and the amount of required phase shift. Single-core design: for smaller voltage and power ratings

Disadvantages: taps at line ends – tap changer cost increases, vulnerability to system transients

Phase-advance mode (OLTC without reversing switch)

Source: S. V. Kulkarni and S. A. Khaparde, Transformer Engineering: Design, Technology, and Diagnostics, Second Edition, CRC Press, Taylor & Francis Group, New York, September 2012.

Ve

VSRVLR

VRR2

R1 R2S L

L

LS

S

VR1R

R

R

Y

B B

R

Y

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bay Two-core design: larger ratings

Source: S. V. Kulkarni and S. A. Khaparde, Transformer Engineering: Design, Technology, and Diagnostics, Second Edition, CRC Press, Taylor & Francis Group, New York, September 2012.

VAA’VLRVSR

Series unit

Exciting unit (Main unit)

a’a

A’AS L LS S L

R R Y Y B B

B’

B

VBB’

Advantages:The series and exciting units can be independently designed, reduced tap changer cost

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Other Technologies

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Mechanical: Low cost, slow, wear and tear, step-control

Solid-state: Smooth variation, fast acting, high cost,

complex control

Hybrid: Cost-effective

OLTC based PST: –150 to + 150 (coarse in step of 50)

and Power Electronics based PST –50 to +50 (smooth)

Overall –200 to +200 variation

Mechanical, Solid-State and Hybrid Phase-Shifters22

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Reduction in line reactance through a series compensator

(capacitor):

Thyristor Controlled Series Compensator (TCSC)

Voltage Source Converter (VSC) based series compensator

High Voltage DC Transmission

Other Means of Power Flow Control

VSC

+_

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Concluding Remarks

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bay Challenges

Need for co-ordinated PSTs

Issue: “You work, I enjoy” – PST deployed in one region may benefit an adjacent region

Optimized location is valid for a given operating point

As the base case changes the location of PST changes – use truck mounted PSTs

Expansion of network – optimum location will change

PST design and control strategies can be site-specific – detailed systems studies are necessary

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PST – a cost-effective option with conventional technology

Advantages: optimal line loading, stability improvement, elimination of loop flows, deferment of investment in transmission infrastructure, damping of low frequency oscillations

Potential of PSTs is unexploited in India

Relevance of PSTs is more now in the context of deregulation and penetration of intermittent renewable energy sources

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Reddy, T., Gulati, A., Khan, M. I., and Koul, R. Application of Phase Shifting Transformer in Indian Power System, International Journal of Computer and Electrical Engineering, Vol.4, No.2, April 2012.

Belivanis, M., and Bell, K. R. W. Coordination of Phase-Shifting Transformers to Improve Transmission Network Utilisation, Innovative Smart Grid Technologies Conference Europe (ISGT Europe), 2010 IEEE PES.

IEEE Standard C57.135-2001TM, IEEE guide for the application, specification, and testing of phase-shifting transformers.

Kramer, A. and Ruff, J. Transformers for phase angle regulation considering the selection of on-load tap changers, IEEE Transactions on Power Delivery, Vol. 13, No. 2, April 1998, pp. 518–525.

Iravani, M. R. and Mathur, R. M. Damping subsynchronous oscillations in power systems using a static phase shifter, IEEE Transactions on Power Systems, Vol. PWRS-1, No. 2, May 1986, pp. 76–83.

Iravani, M. R., Dandeno, P. L., Nguyen, K. H., Zhu, D., and Maratukulam, D. Applications of static phase shifters in power systems, IEEE Transactions on Power Delivery, Vol. 9, No. 3, July 1994, pp. 1600–1608.

Del Vecchio, R. M., Poulin, B., Feghali, P. T., Shah, D. M., and Ahuja R. Power transformer design principles: with applications to core-form power transformers, Gordon and Breach Science Publishers, Canada, 2001, pp. 499–542.

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O’ Kelly, D., and Musgrave, G. Improvement of power-system transient stability by phase-shift insertion, Proceedings IEE, Vol. 120, No. 2, February 1973, pp. 247–252.

Hertem, D. V., Verboomen, J., Cole, S., and Belmans, R. Influence of phase shifting transformers and HVDC on power system losses, IEEE Power Engineering Society General Meeting, 2007, pp. 1-8.

Kulkarni, A. M. Power System Operation and Control – NPTEL Course, Module 4 : Voltage and Power Flow Control, Lecture 19.

Sweeney, B. Application of phase-shifting transformers for the enhanced interconnection between Northern Ireland and the Republic of Ireland, Power Engineering Journal, June 2002, pp. 161-167

Verboomen, J., Hertem, D. V., Schavemaker, P.H., Kling, W.L., and Belmans, R. Phase Shifting Transformers: Principles and Applications, International Conference on Future Power Systems, Nov. 2005 , Vol. 6. Belivanis, M. , and Bell, K.R.W. Use of Phase-Shifting Transformers on the

Transmission Network in Great Britain 45th International Universities Power Engineering Conference (UPEC), 2010, pp.1-5.

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Thank You !

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