Download - Inrush Presentation
-
8/12/2019 Inrush Presentation
1/35
Fundamentals of TransformerInrush
Suhag Patel, P.E.GE Digital Energy
Placentia, CA
Texas A&M Protective Relay Conference
College Station, TX April 13, 2010
-
8/12/2019 Inrush Presentation
2/35
Objective Understand why transformer inrush
occurs
Understand the characteristics of aninrush waveform
Understand the impact transformer
inrush can have on differential relays Discuss various methods to reliably
restrain differential relay operation
-
8/12/2019 Inrush Presentation
3/35
Basics of Differential Relays Very Simple
Sum of Allcurrents shouldbe zero.
Must Compensatefor Phase Shift andMagnitudeDifference
-
8/12/2019 Inrush Presentation
4/35
Problems with Transformer
Differential Relays
-
8/12/2019 Inrush Presentation
5/35
Inrush Current Impact on
Differential Relay
-
8/12/2019 Inrush Presentation
6/35
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
-
8/12/2019 Inrush Presentation
7/35
Steady State Magnetization
Current
Non-Linearity of the core results in a non-linearmagnetizing current waveform
Notice flux lags excitation voltage by 90 degrees Steady State Magnetizing current is in the range of 1-3% of
XFMR FLA
-
8/12/2019 Inrush Presentation
8/35
Magnetizing Current Under
Non-Steady State Conditions
When an abrupt change in excitation voltage occurs, alarge magnetizing current can flow.
The Magnetizing Inrush Current is dependant on severalfactors, which will be discussed on the following slides
-
8/12/2019 Inrush Presentation
9/35
Impact of Switching Point
Highest magnitude inrush occurs when excitation voltage isapplied at zero crossing.
Lowest magnitude inrush occurs when excitation voltage is
applied at 90 degrees.
Time
e
tar t of event
Time
e
tar t of event
-
8/12/2019 Inrush Presentation
10/35
Impact of Remnant Flux
Remnant Flux can be positive or negative
This can lead to increased or decreased magnetizing
inrush current
-
8/12/2019 Inrush Presentation
11/35
Impact of Power System
Impedance
The peak magnitude of the inrush current isdictated by the strength of the power systemsource
The duration of an inrush event is dictated by the
resistive losses in the circuit
-
8/12/2019 Inrush Presentation
12/35
Impact of Transformer Design Electrical steel has remained fairly
constant over the years
Laminated core designs lead to lowerreluctance cores
Lower reluctance cores are more
efficient leading to lower magnetizingcurrent
-
8/12/2019 Inrush Presentation
13/35
Transformer Inrush Waveform
No CT Errors Time Domain
-
8/12/2019 Inrush Presentation
14/35
Transformer Inrush Waveform
No CT Error Freq Domain
-
8/12/2019 Inrush Presentation
15/35
Transformer Inrush Waveformwith CT Sat Time Domain
-
8/12/2019 Inrush Presentation
16/35
Transformer Inrush Waveformwith CT Sat Freq Domain
-
8/12/2019 Inrush Presentation
17/35
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 wont be fully
offset from excitation voltage
Sympathetic Inrush:
-
8/12/2019 Inrush Presentation
18/35
Inrush Restraint Methods
As shown earlier, high levels of inrushcurrent can cause differential relay
misoperation
We need to identify this condition and stopthe differential relay from operating while
inrush condition is present
Many methods exist, all rely on thecharacteristics of the inrush waveform
-
8/12/2019 Inrush Presentation
19/35
Percentage of Total Harmonic
This method utilizes the fact the inrushwaveform is rich in harmonics.
EM relays applied this per phase. Problems
More efficient core designs produce less harmoniccontent
CT Saturation essentially creates a setting floor
-
8/12/2019 Inrush Presentation
20/35
CT Saturation Waveform
Note that a saturated CT waveform ishighly non-linear
-
8/12/2019 Inrush Presentation
21/35
CT Saturation Spectrum
Note the high 2nd
harmonic component
-
8/12/2019 Inrush Presentation
22/35
Typical 2nd Harmonic Ratios
Typical values of 2nd
harmonic tofundamental ratios in
the range of 10%-60% Can be much lower as
shown
Microprocessor relayshave introducedmethods to deal withthis problem
-
8/12/2019 Inrush Presentation
23/35
Percentage of 2nd Harmonic
This method utilizes the fact the inrushwaveform has a dominant second
harmonic component. EM relays applied this per phase. CT Saturation still a problem
-
8/12/2019 Inrush Presentation
24/35
Percentage of 4th Harmonic
This method utilizes the fact the inrushwaveform is not symmetric, leading to evenharmonics
Used in some microprocessor relays CT Saturation still a problem No significant benefit over 2nd harmonic
methods
-
8/12/2019 Inrush Presentation
25/35
Waveshape Based Method
Relies on flat spotsnear zero value
CT saturation cancompromisesecurity anddependability
Were used widelyin solid-staterelays
-
8/12/2019 Inrush Presentation
26/35
Adaptive 2nd Harmonic Method
Method utilizes 2nd
HarmonicMagnitude and
Angle Dynamically
restrains over amaximum of 6cycles
May slowoperation by a fewcycles if 2nd
harmonic current
is present
-
8/12/2019 Inrush Presentation
27/35
How to Apply VariousMethods?
EM relays typically used either % totalharmonic or % 2nd harmonic methods
EM relays applied them on a per-phasebasis Microprocessor relays can apply many
methods on a per-phase, 1 out of 3
(cross blocking), 2 out of 3, or averagebasis Pros and Cons to each
-
8/12/2019 Inrush Presentation
28/35
Considerations When ApplyingHarmonic Restraint
Reliability Ability for the differentialrelay to operate on all internal faults
Security Ability for the differential relayto restrain for all transformer inrushevents
Speed How quickly internal faults are
cleared No method is best, depends on user
requirements
-
8/12/2019 Inrush Presentation
29/35
Considerations When ApplyingHarmonic 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 compromiseon reliability
-
8/12/2019 Inrush Presentation
30/35
Transformer Inrush Impact onGenerator Differential
High DC component of Inrush may saturateGen CTs.
Using harmonic restraint is not a good
solution, adds too much delay
87G
-
8/12/2019 Inrush Presentation
31/35
Transformer Inrush Impact onGenerator Differential
Flux balanced CT configuration can be used onsmaller Generators
-
8/12/2019 Inrush Presentation
32/35
Transformer Inrush Impact onGenerator Differential
For problem installations, transformer CB closecan be used to delay 87G
87G
Transformer Close CB Command
Delays 87G Relay
-
8/12/2019 Inrush Presentation
33/35
Importance of GoodWaveform Capture
Depending on specific system conditions and transformerdesign, varying levels of 2nd harmonic content may bepresent
It is in the users best interest to capture inrushwaveforms whenever possible
If a fairly complete library of actual waveform data isavailable, this can be used to fine tune settings andevaluate new methods
-
8/12/2019 Inrush Presentation
34/35
Conclusion
Transformer Inrush will occur anytime a change tothe 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, andspeed.
The right choice of blocking method depends on theindividual user
Generator Differential relays can also be impacted by
transformer inrush
-
8/12/2019 Inrush Presentation
35/35
Thank You
Suhag Patel
562-233-1371