fundamentals of transformer inrush

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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

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Page 1: 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

Page 2: Fundamentals of transformer inrush

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

Page 3: Fundamentals of transformer inrush

Basics of Differential Relays Very Simple –

Sum of All currents should be zero.

Must Compensate for Phase Shift and Magnitude Difference

Page 4: Fundamentals of transformer inrush

Problems with Transformer Differential Relays

Page 5: Fundamentals of transformer inrush

Inrush Current Impact on Differential Relay

Page 6: Fundamentals of transformer inrush

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

Page 7: Fundamentals of transformer inrush

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

Page 8: Fundamentals of transformer inrush

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

Page 9: Fundamentals of transformer inrush

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

Page 10: Fundamentals of transformer inrush

Impact of Remnant Flux

Remnant Flux can be positive or negative This can lead to increased or decreased magnetizing

inrush current

Page 11: Fundamentals of transformer inrush

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

Page 12: Fundamentals of transformer inrush

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

Page 13: Fundamentals of transformer inrush

Transformer Inrush Waveform No CT Errors – Time Domain

Page 14: Fundamentals of transformer inrush

Transformer Inrush Waveform No CT Error – Freq Domain

Page 15: Fundamentals of transformer inrush

Transformer Inrush Waveform with CT Sat – Time Domain

Page 16: Fundamentals of transformer inrush

Transformer Inrush Waveform with CT Sat – Freq Domain

Page 17: Fundamentals of transformer inrush

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:

Page 18: Fundamentals of transformer 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

Page 19: Fundamentals of transformer inrush

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”

Page 20: Fundamentals of transformer inrush

CT Saturation Waveform

Note that a saturated CT waveform is highly non-linear

Page 21: Fundamentals of transformer inrush

CT Saturation Spectrum

Note the high 2nd harmonic component

Page 22: Fundamentals of transformer inrush

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

Page 23: Fundamentals of transformer inrush

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

Page 24: Fundamentals of transformer inrush

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

Page 25: Fundamentals of transformer inrush

Waveshape Based Method Relies on flat spots

near zero value

CT saturation can compromise security and dependability

Were used widely in solid-state relays

Page 26: Fundamentals of transformer inrush

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

Page 27: Fundamentals of transformer inrush

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

Page 28: Fundamentals of transformer inrush

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

Page 29: Fundamentals of transformer inrush

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

Page 30: Fundamentals of transformer inrush

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

Page 31: Fundamentals of transformer inrush

Transformer Inrush Impact on Generator Differential

Flux balanced CT configuration can be used on smaller Generators

Page 32: Fundamentals of transformer inrush

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

Page 33: Fundamentals of transformer inrush

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

Page 34: Fundamentals of transformer inrush

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

Page 35: Fundamentals of transformer inrush

Thank You

Suhag [email protected]

562-233-1371