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1. Fundamentals of Transformer
Inrush
Suhag Patel, P.E.
GE Digital Energy
Placentia, CA
Texas A&M Protective Relay Conference
College Station, TX April 13, 2010

2. Objective
 Understand why transformer inrush
occurs
 Understand the characteristics of an
inrush waveform
 Understand the impact transformer
inrush can have on differential relays
 Discuss various methods to reliably
restrain differential relay operation

3. Basics of Differential Relays
 Very Simple –
Sum of All
currents should
be zero.
 Must Compensate
for Phase Shift and
Magnitude
Difference

4. Problems with Transformer
Differential Relays

5. Inrush Current Impact on
Differential Relay

6. What is Inrush Current
 All transformers must establish flux in the transformer core
 This flux causes a current to flow known as the
magnetizing current
 Magnetizing current appears as differential current

7. Steady State Magnetization
Current
 Non-Linearity of the core results in a non-linear
magnetizing current waveform
 Notice flux lags excitation voltage by 90 degrees
 Steady State Magnetizing current is in the range of 1-3% of
XFMR FLA

8. Magnetizing Current Under
Non-Steady State Conditions
 When an abrupt change in excitation voltage occurs, a
large magnetizing current can flow.
 The Magnetizing Inrush Current is dependant on several
factors, which will be discussed on the following slides

9. Impact of Switching Point
 Highest magnitude inrush occurs when excitation voltage is
applied at zero crossing.
 Lowest magnitude inrush occurs when excitation voltage is
applied at –90 degrees.
Time
e
ϕ
Start of event
Time
e ϕ
Start of event

10. Impact of Remnant Flux
 Remnant Flux can be positive or negative
 This can lead to increased or decreased magnetizing
inrush current

11. Impact of Power System
Impedance
 The peak magnitude of the inrush current is
dictated by the strength of the power system
source
 The duration of an inrush event is dictated by the
resistive losses in the circuit

12. Impact of Transformer Design
 Electrical steel has remained fairly
constant over the years
 Laminated core designs lead to lower
reluctance cores
 Lower reluctance cores are more
efficient leading to lower magnetizing
current

13. Transformer Inrush Waveform
No CT Errors – Time Domain

14. Transformer Inrush Waveform
No CT Error – Freq Domain

15. Transformer Inrush Waveform
with CT Sat – Time Domain

16. Transformer Inrush Waveform
with CT Sat – Freq Domain

17. When Does Inrush Occur?
 During Transformer Energization:
 Typically the most severe case, because excitation voltage is going
from zero to maximum value
 During Post Fault Voltage Recovery:
 During a fault the system voltage is depressed, and then returns to
full value
 Not typically as sever as Energization because Flux won’t be fully
offset from excitation voltage
 Sympathetic Inrush:

18. Inrush Restraint Methods
 As shown earlier, high levels of inrush
current can cause differential relay
misoperation
 We need to identify this condition and stop
the differential relay from operating while
inrush condition is present
 Many methods exist, all rely on the
characteristics of the inrush waveform

19. Percentage of Total Harmonic
 This method utilizes the fact the inrush
waveform is rich in harmonics.
 EM relays applied this per phase.
 Problems
 More efficient core designs produce less harmonic
content
 CT Saturation essentially creates a setting “floor”

20. CT Saturation Waveform
 Note that a saturated CT waveform is
highly non-linear

21. CT Saturation Spectrum
 Note the high 2nd harmonic component

22. Typical 2nd Harmonic Ratios
 Typical values of 2nd
harmonic to
fundamental ratios in
the range of 10%-
60%
 Can be much lower as
shown
 Microprocessor relays
have introduced
methods to deal with
this problem

23. Percentage of 2nd Harmonic
 This method utilizes the fact the inrush
waveform has a dominant second
harmonic component.
 EM relays applied this per phase.
 CT Saturation still a problem

24. Percentage of 4th Harmonic
 This method utilizes the fact the inrush
waveform is not symmetric, leading to even
harmonics
 Used in some microprocessor relays
 CT Saturation still a problem
 No significant benefit over 2nd harmonic
methods

25. Waveshape Based Method
 Relies on flat spots
near zero value
 CT saturation can
compromise
security and
dependability
 Were used widely
in solid-state
relays

26. Adaptive 2nd Harmonic Method
 Method utilizes 2nd
Harmonic
Magnitude and
Angle
 Dynamically
restrains over a
maximum of 6
cycles
 May slow
operation by a few
cycles if 2nd
harmonic current
is present

27. How to Apply Various
Methods?
 EM relays typically used either % total
harmonic or % 2nd harmonic methods
 EM relays applied them on a per-phase
basis
 Microprocessor relays can apply many
methods on a per-phase, 1 out of 3
(cross blocking), 2 out of 3, or average
basis
 Pros and Cons to each

28. Considerations When Applying
Harmonic Restraint
 Reliability – Ability for the differential
relay to operate on all internal faults
 Security – Ability for the differential relay
to restrain for all transformer inrush
events
 Speed – How quickly internal faults are
cleared
 No method is best, depends on user
requirements

29. Considerations When Applying
Harmonic Restraint
 1 out of 3 (Cross Blocking)– Very secure,
but can reduce reliability or speed:
 Consider fault during energization
 Per Phase – Less secure, very reliable:
 Consider low 2nd harmonic inrush
 2 out of 3 – More secure then Per Phase,
potentially less reliable
 Averaging Method – More secure then
Per Phase or 2 out of 3, no compromise
on reliability

30. Transformer Inrush Impact on
Generator Differential
 High DC component of Inrush may saturate
Gen CT’s.
 Using harmonic restraint is not a good
solution, adds too much delay
87G

31. Transformer Inrush Impact on
Generator Differential
 Flux balanced CT configuration can be used on
smaller Generators

32. Transformer Inrush Impact on
Generator Differential
 For problem installations, transformer CB close
can be used to delay 87G
87G
Transformer Close CB Command
Delays 87G Relay

33. Importance of Good
Waveform Capture
 Depending on specific system conditions and transformer
design, varying levels of 2nd harmonic content may be
present
 It is in the users best interest to capture inrush
waveforms whenever possible
 If a fairly complete library of actual waveform data is
available, this can be used to fine tune settings and
evaluate new methods

34. Conclusion
 Transformer Inrush will occur anytime a change to
the transformer excitation voltage occurs
 Transformer Inrush appears as differential current to
the transformer differential relay
 2nd harmonic based methods should not be set lower
then 15% otherwise dependability is put at risk
 Many blocking methods exist, however, they pose
various compromises to security, reliability, and
speed.
 The right choice of blocking method depends on the
individual user
 Generator Differential relays can also be impacted by
transformer inrush

35. Thank You
Suhag Patel
suhag.patel@ge.com
562-233-1371