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Fundamentals of transformer inrush
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Transcript of Fundamentals of transformer inrush
Fundamentals of Transformer Inrush
Suhag Patel, P.E.GE Digital Energy
Placentia, CA
Texas A&M Protective Relay ConferenceCollege Station, TX April 13, 2010
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
Basics of Differential Relays Very Simple –
Sum of All currents should be zero.
Must Compensate for Phase Shift and Magnitude Difference
Problems with Transformer Differential Relays
Inrush Current Impact on Differential Relay
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
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
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
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
Impact of Remnant Flux
Remnant Flux can be positive or negative This can lead to increased or decreased magnetizing
inrush current
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
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
Transformer Inrush Waveform No CT Errors – Time Domain
Transformer Inrush Waveform No CT Error – Freq Domain
Transformer Inrush Waveform with CT Sat – Time Domain
Transformer Inrush Waveform with CT Sat – Freq Domain
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:
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
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”
CT Saturation Waveform
Note that a saturated CT waveform is highly non-linear
CT Saturation Spectrum
Note the high 2nd harmonic component
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
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
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
Waveshape Based Method Relies on flat spots
near zero value
CT saturation can compromise security and dependability
Were used widely in solid-state relays
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
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
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
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
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
Transformer Inrush Impact on Generator Differential
Flux balanced CT configuration can be used on smaller Generators
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
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
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