a case study of mitigation of sympathetic … · a case study of mitigation of sympathetic...

February 22nd - 23rd , 2017, Scope Complex, Lodhi Road, New Delhi, India

6th International Conference on Large Power Transformers – Modern Trends in

Application, Installation, Operation & Maintenance

by

and

A CASE STUDY OF MITIGATION OF

SYMPATHETIC INTERACTION BETWEEN

POWER TRANSFORMERS FED BY LONG

OVER HEAD LINES

K.K.JEMBU KAILAS

1

L&T CONSTRUCTION

[email protected]

L&T CONSTRUCTION

[email protected]

W. ADITHYA KUMAR




2

INTRODUCTION

Growing at a rapid pace

Increased

Transformer Capacity

Interaction between

the New & existing

transformers

resulting in

unexpected

saturation in

transformers.




3

Inrush Current

Remanence Point-on-

Wave

closing

Total

Impedance

Maximum

Flux Density

Saturation

level of

existing

transformers




4

Sympathetic Inrush Current

Sympathetic inrush is a phenomenon that occurs when a

transformer is driven into saturation by large and prolonged

inrush currents as a result of a nearby transformer being re-

energized into the system.




5

Sympathetic Interaction

• Sympathetic interaction can be greatly reduced if:-

– Inrush current is reduced

– Resistance of the supply system is small

– Resistance of transformer windings or between transformers is

large.




6

Methods of mitigating sympathetic interaction between

power transformers

Series Compensators

Tap Changers

Sequential Phase

Energization

Current Limiting Resistors




7

CASE STUDY

• 350MVA, 400/132kV, 3-phase Power Transformer is considered for

the case studies.

• Inrush currents are highly unbalanced among three phases.

• If a transformer is Y grounded at the energization side, its neutral

current will also contain the inrush current.

• If a resistor is inserted into the transformer neutral, it may reduce the

magnitude of the inrush current in a way similar to that of the series-

inserted resistor.




8

Case-1:Sequential Phase Energization

• If one closes each phase of the breaker in sequence with some

delays between them, the neutral resistance could behave as a

series resistor and improve the results.

• This simple improvement has proven to be very effective.

• In fact, the idea of sequential energization of three-phase

equipment can lead to a new class of techniques to reduce

switching transients.

1.0 [ohm]

C

B

A

BRK 350.0 [MVA]400.0 [kV] / 132.0 [kV]

Re

sis

tor

Load

Sequential SwitchingLogic with Neutral Resistor




9


Magnetizing inrush current with Sequential Phase energization

Magnetizing Inrush current without sequential phase energization

R-Phase

Y-Phase

B-Phase

R-Phase

Y-Phase

B-Phase




1

0


• Observations:-

The inrush current occurs in all phases that are electrically energized.

For example, all three phases experience inrush currents when the

3rd phase is switched in. The highest inrush current does not

necessarily occur in the switched phase.

It takes about 5 to 10 cycles for the inrush current to reach steady

state. As a result, there is no need to wait for a long time to energize

the next phase.

The instants of energization have an impact on the magnitude of the

inrush current. However, the inrush currents are small when the

neutral resistance is in certain range. There is no need to control the

instant of switching closing.




1

1

Case-2: Series Voltage Source PWM Converter

Voltage Source Pulse Width Modulation Converter Circuit Topology

1.0 [ohm]

C

B

A

BRK 350.0 [MVA]400.0 [kV] / 132.0 [kV]

Tline9 Tline9

TLine9

T

180KM

350.0 [MVA]400.0 [kV] / 132.0 [kV]

350.0 [MVA]400.0 [kV] / 132.0 [kV]

GG

GG

GG

DC

DC

DC

Capacitor

Capacitor

Capacitor

PWM Signal

Generator

P Controller

Triangular Wave

1

1

1




1

2


• The filters used to limit

switching ripple are connected

to the output side of the voltage

source PWM converter.

• T2 & T3 are existing

Transformers in the Substation.

Transformer T1 is added.

• T1 is energized while T2 & T3

are already connected to loads.

• Closing a transformer without

load will lead to the emergence

of sympathetic inrush current in

adjacent transformer.

Sympathetic inrush current without any mitigation

- with T1 energized at 0.5 sec and T2, T3 already

energized at 0.0sec




1

3


• When second transformer is energized in t=1(s) with voltage source

PWM Converter doesn’t produce inrush current in first transformer that

energized in t=0.1(s).

Main : Graphs

0.00 0.20 0.40 0.60 0.80 1.00 1.20 ...

...

...

-400

-300

-200

-100

0

100

200

300

400

y (A

)

Ia (A)




1

4

Case-3: Tap Changers

Mitigation of Sympathetic inrush currents by Tap Changers

0.001 [ohm]

BRK

Tline1 Tline1

TLine1

T

180KM

Tline2 Tline2

TLine2

T

180KM

BR

K

#1 #2

350.0 [MVA]400.0 [kV] / 132.0 [kV]

#1 #2

350.0 [MVA]400.0 [kV] / 132.0 [kV]

#1 #2

350.0 [MVA]400.0 [kV] / 132.0 [kV]

600.0 [MW]

250.0 [MW]

Tap1

Tap2

Tap3




1

5


• Simulation results offer a superior performance and inrush current

reduction if the least turn number is used in the power transformer

under energization.

• In this technique, sympathetic inrush current induced on the other

transformers operational in parallel is also reduced.

• If inrush current is reduced, the voltage at the secondary side,

experiences a lower variation, accomplishing safe operation of

equipment connected to the transformers.

• Taps are changed from the lowest to the central tap and the most

economic tap is selected which not only reduce inrush current but

also the voltage regulations on the secondary winding of

transformer.




1

6


Inrush Current of Trafo-1 at the lowest tap

Main : Graphs

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 ...

...

...

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

y (k

A)

Ia (kA)

Sympathetic inrush current on Transformer-2 at lowest tap Main : Graphs

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 ...

...

...

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

y (k

A)

Ia (kA)

Inrush Current of Trafo-1 at the Centre tap

Sympathetic Inrush Current on Trafo-2

Sympathetic inrush current on Transformer-2 at centre tap




1

7

Results

Switching sequence

will lead to 20% to

30% reductions on

the breaker contact

voltages and 80%

to 90% reductions

on the inrush

currents. Time

delay between the

switching events is

in the range of 5 to

60 cycles.

With the introduction

of PWM Voltage

source converter in

series with

Transformer,

sympathetic inrush

current is completely

reduced. However,

this method is proved

to be expensive

considering the no. of

transformers in the

substation

It is evident that,

from the point of

view of the

system operation,

the most favorable

condition to

energize a

transformer is

positioning the

tap to give the

highest number of

turns to be

excited




1

8

Conclusion

The above analysis

and results are done

with analytical /

numerical examples

to illustrate the

purpose / use of the

mitigation techniques

to reduce the

Sympathetic inrush

current.

Depending upon the

system requirement

and usage, the

appropriate and most

suitable methodology,

can be adopted which

doesn’t affect the

system and hamper

the main objective of

the use of

Transformer.




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a case study of mitigation of sympathetic … · a case study of mitigation of sympathetic...

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