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Phase Shifting Transformers
Prof. S. V. Kulkarni Department of Electrical Engineering Indian Institute of Technology Bombay, INDIA
[email protected] CBIP Conference, New Delhi
28-11-2013
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Outline
Introduction
Applications of PSTs
Basic Concepts
Design Considerations
Other Technologies
Concluding Remarks
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Introduction
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Need for Phase Shifting Transformers
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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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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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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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Representative and Notable Applications of PSTs iii. Reduction in loop flows 300 MW 400 MW
400 MW
Area-1
100 MW
100 MW “Loop flow”
100 MW
Area-2 400 MW
300 MW
400 MW
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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Power Flow Through a Transmission Line VS
VR 0
G
Load S R PR jQR
S s Ps jQs
PR
QR
VS VR X VS VR
X
sin
cos
VR X
2
VR X
( VS VR )
VR X
V
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.
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Current (and power) flows through low impedance paths, 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 Bus |
I1
G
1 j 0.15
1:1
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Load Bus I1
Load
IL |
I2 VS
j 0.15
I2
1: a 2
S1 P1 jQ1 0.425 j 0.3
VL =1.0 0
a 1.00
S2 P2 jQ2 0.425 j 0.3 S1 P1 jQ1 0.4354 j 0.1525
a 1.05
S2 P2 jQ2 0.4146 j 0.4475 S1 P1 jQ1 0.1817 j 0.3148
a 1.04o
S2 P2 jQ2 0.6683 j 0.2852 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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Improvement in Stability P
Pmech
0
P
Safety margin f
e
a
d
b
c
g
Safety margin
m
Pmech
p
h
0
n
Without PST
r
q
o
t s
v u
With PST
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.
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Power Flow with Regulating Transformers
m
Vm
I
In
m
aVm
Sm
Sn
Y=
1 Z
n
Vn
1: a Ideal Transformer
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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Off-nominal tap ratio, a may be real (e.g., 1.04) or imaginary (ejπ/60 or 3о shift).
Sm Vm I m ,
Sn Vn I n
Lossless transformer:
Sm Sn Vm I m aVm I n I m aI n or I m a I n
Now, I n Vn aVm Y aY Vm Y Vn
Multiplying both sides by a
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a I n I m a a Y Vm a Y Vn
Now, a a a
2
a 2Y aY
a * Y Y
Vm I m Ymm V I Y n n nm
Ymn Ynn
Vm V n
If a is a real number, a a and we can draw an equivalent π circuit for this case: aY
m
Vm
a ( a 1) Y
n
(1 a )Y
Vn
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.
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Design Considerations
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Design of PSTs Phase-advance and phase-retard modes V
V
VL
VS
VL
VS
S
phase rotation
Phase-advance A leading quadrature voltage drop is added to the source voltage
L
PST
Phase-retard A lagging quadrature voltage drop is added to the source voltage
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 SR
R1 R2
LR
R V RR SY
LY
V LR
2
VR R 1
V SR
Ve SB
LB
Phase-advance mode (OLTC without reversing switch) Disadvantages: taps at line ends – tap changer cost increases, vulnerability to system transients 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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Two-core design: larger ratings Series unit
a
a’
SR A
A’ L R
SY
LY S B
LB
B
B’
Exciting unit (Main unit)
Advantages:
V AA’
V V The series and exciting units canV be independently designed, reduced tap changer cost SR
BB’
LR
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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Other Technologies
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Mechanical, Solid-State and Hybrid Phase-Shifters 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
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Other Means of Power Flow Control Reduction in line reactance through a series compensator (capacitor): Thyristor Controlled Series Compensator (TCSC)
Voltage Source Converter (VSC) based series compensator VSC
+ High Voltage DC Transmission
_
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Concluding Remarks
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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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Conclusions
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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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References
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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 !
