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
L&T CONSTRUCTION
W. ADITHYA KUMAR
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
2
INTRODUCTION
Growing at a rapid pace
Increased
Transformer Capacity
Interaction between
the New & existing
transformers
resulting in
unexpected
saturation in
transformers.
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
3
Inrush Current
Remanence Point-on-
Wave
closing
Total
Impedance
Maximum
Flux Density
Saturation
level of
existing
transformers
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
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.
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
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.
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
6
Methods of mitigating sympathetic interaction between
power transformers
Series Compensators
Tap Changers
Sequential Phase
Energization
Current Limiting Resistors
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
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.
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
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
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
9
Case-1:Sequential Phase Energization
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
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
1
0
Case-1:Sequential Phase Energization
• 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.
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
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
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
1
2
Case-2: Series Voltage Source PWM Converter
• 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
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
1
3
Case-2: Series Voltage Source PWM Converter
• 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)
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
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
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
1
5
Case-3: Tap Changers
• 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.
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
1
6
Case-3: Tap Changers
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
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
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
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
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.
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
1
9