Even Order Harmonics as early detection of failure. 3

[waross]

This grows out of a statement made in the thread
thread238-340664

Even-order harmonics are usually a result of a problem - asymmetry in the firing delay angle being a common cause.

Is this effect ever used in preventive maintenance?
I am wondering if monitoring or trending even order harmonics may be used to detect and then locate failing equipment. If an increase in even order harmonics is seen, then the problem may be located by drilling down through Power Distribution Centers, Motor Control Centers and then individual circuits to find the source of the problem.

Bill
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"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes. It has been used to protect DC motor commutators from too much ripple in the armature current. This was way back in the sixties. We didn't have easy access to chip computers then (intel 4004 appeared 1971). So we used an alternate way. Instead, we checked the DC current for 50 Hz (it would be 60 Hz in your quarters) using a twin tee filter in the feed-back path of an opamp. This created a narrow band-pass filter for 50 Hz and when the signal grew above a set limit, an alarm went off. We saved quite a few DC motors with this technique. Then, the firing units became much more reliable and we never used these protections after around 1972.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[prc]

In transformers we are using even harmonic(2 nd harmonic )currenr for another type of protection.With differential protection of transformer, the problem was the inrush current during energisation as seen as an internal fault current by relay and leading to nuisance tripping. Engineers found a solution to this during 1930's ( C D Hayward of GE,1938) by incorporating harmonic restraining feature in differential relay.It was noticed that second harmonic current has appreciable presence (10-30%) in inrush current while internal faults in transformer was without even harmonics. This was used in relay to make it sleep when there is second harmonic in unbalnced current so that with in rush current (5-20 times full load current of transformer) differential relay will not operate.This worked well for nearly 50-60 years,but developments in transformer technology ( improved core material-CRGO- and new type of core joints)resulted in lower second harmonic current % content (some times less tahn 5 %) in inrush current of large transformers causing maltripping during energisation.Engineers developed two to three alternate solutions to counter this problem.

 

[ghostbuster7]

Yes,we have used this technique many times to detect failing SCR control circuitry within large mfg. plants.The last one I recall was failing scr circuitry for a 800 amp electric furnace used for baking black paint onto metal parts.Even current harmonics appeared at the main switchboard indicating the scr's were failing.