This document discusses transformer inrush current and its impact on differential relays. Transformer inrush occurs when the flux in the transformer core needs to be established, causing a large magnetizing current to flow. This inrush current appears as a differential current that can cause misoperation of transformer differential relays. The document examines characteristics of inrush current like the switching point, remnant flux, system impedance, and transformer design. It also discusses various harmonic-based methods for restraining differential relays during inrush like percentage of total harmonic, percentage of 2nd harmonic, and adaptive 2nd harmonic methods. The considerations for applying these methods include reliability, security, and speed of operation.

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