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Response of Amptector solid state trip unit to non-sinusoidal current
2

Response of Amptector solid state trip unit to non-sinusoidal current

Response of Amptector solid state trip unit to non-sinusoidal current

(OP)
We have a generator that feeds the input to a half-wave rectifier through DS breakers.

The current waveform on a given phase is expected to be similar to slide 1 attached, normalized to peak magnitude 1.0.  Each phase carries currrent for only 1/3 of cycle... current returns through a neutral:
RMS    0.4855
Avg Absolute Value    0.2760
Average Value    0.2760
Peak positive    1.0000   
Peak Negative    0.0000

The DS breaker has an internal CT which feeds a Westinghouse Amptector solid state trip unit.

I assume the CT will remove the DC and feed a waveform to the trip unit similar to that shown on slide 2:
RMS    0.3995
Avg Absolute Value    0.3676
Average Value    0.0000
Peak positive    0.7240
Peak Negative    -0.2760

The Amptector trip unit time overcurrent trip is calibrated using sinusoidal signal.

Would you expect that the trip unit is responsive to the rms value of the non-sinusoidal signal?  The average absolute value?  The peak?  Or something else?

Is there any basis for understanding how the solid state trip unit would respond to a non-sinusoidal signal?  (how does that unit work).

Or is it something that cannot be determined without test?
 

=====================================
(2B)+(2B)'  ?

RE: Response of Amptector solid state trip unit to non-sinusoidal current

Only the mfr. will know. Published data sheet for trip units always mention if they are "true rms sensing". Most modern ones are.

Rafiq Bulsara
http://www.srengineersct.com

RE: Response of Amptector solid state trip unit to non-sinusoidal current

(OP)
Thanks. These trip units are obsolete, but widely used in our plant.  I assume some others may be familiar with them (?).

We have some instruction manuals and test device documentation, but I haven't found anything to characterize the response.  Since they are solid state electronic vs digital I doubt they are true rms sensing.
 

=====================================
(2B)+(2B)'  ?

RE: Response of Amptector solid state trip unit to non-sinusoidal current

Contact http://www.thyritronics.com  They should be able to answer your question over the phone.

RE: Response of Amptector solid state trip unit to non-sinusoidal current

Amptectors are not true RMS sensing, your best bet is to retrofit these with a more modern trip unit.  

RE: Response of Amptector solid state trip unit to non-sinusoidal current

I agree with Zog - these are old analog "peak sensing" units. The pickup settings are supposed to relate to rms current, but  I think this was just scaled from the peak current.  Harmonic currents will definitely cause them problems.  Dc offset current might also be a problem.  I would expect trouble with the installation you are describing due to the non-sinusoidal current.  

You might contact Eaton/Cutler-Hammer or submit an inquiry via their website.  They still have a few old-time "Circle Bar W" engineers locked away that can probably answer your question.  I have been able to get some very old obsolete data from them on old Westinghouse products, although it can take several days stretching into weeks.  

David Castor
www.cvoes.com

RE: Response of Amptector solid state trip unit to non-sinusoidal current

(OP)
Thanks.  My gut feel is it would be either peak sensing or average-absolute-value sensing (which would be rectified before going to low pass filter).

If it is in fact peak sensing, then
RMS of actual waveform / Peak of sensed waveform  =  0.4855 /  0.7240  = 0.67
that is a bit below rms/pk = 0.707 of sinusoid.... indicating we have more margin between the protective trip and cable thermal capability (less operational margin between normal operation and trip setpoint).

If it were Average Absolute Value. sensing, then:
RMS of actual waveform / A.A.V of sensed waveform  =  0.4855 /  0.3676 = 1.32
That is a bit above rms/AAV of a sinusoid = 1.11.  This error would be in opposite direction from what we saw with peak sensing.

I guess it will be tough to draw any conclusion even about the qualitative direction of the error without better info.
 

=====================================
(2B)+(2B)'  ?

RE: Response of Amptector solid state trip unit to non-sinusoidal current

Are you concerned about the unit tripping under load or for faults on the circuit to the rectifier?  Faults will not be rectified.
 

RE: Response of Amptector solid state trip unit to non-sinusoidal current

(OP)
We are looking at a modification which will increase the dc loading which is why I mentioned the time overcurrent (not instantaneous).  I hadn't given much thought to the instantaneous (which is also controlled through the same electronic unit), but that's a good point... if the subject comes up I am now better prepared to address it.  

=====================================
(2B)+(2B)'  ?

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