Effects of harmonic currents
Symptoms of harmonics usually show up in the power distribution equipment that supports the non-linear loads. There are two basic types of non-linear loads: single-phase and threephase.
Single-phase, non-linear loads are prevalent in offices, while three-phase loads are widespread in industrial plants.
Each component of the power distribution system manifests the effects of harmonics a little differently, yet all are subject to damage and inefficient performance if not designed to handle electronic loads.
In a three-phase, four-wire system, neutral conductors can be severely affected by nonlinear loads connected to the 120 V branch circuits. Under normal conditions for a balanced linear load, the fundamental 60 Hz portion of the phase currents will cancel in the neutral conductor.
In a four-wire system with single-phase, non-linear loads, certain odd-numbered harmonics called triplens — odd multiples of the third harmonic: 3rd, 9th, 15th, etc — do not cancel, but rather add together in the neutral conductor. In systems with many single-phase, nonlinear loads, the neutral current can actually exceed the phase current. The danger here is excessive overheating because, unlike phase conductors, there are no circuit breakers in the neutral conductor to limit the current.
Excessive current in the neutral conductor can also cause higher-than-normal voltage drops between the neutral conductor and ground at the 120 V outlet.
Common thermal-magnetic circuit breakers use a bi-metallic trip mechanism that responds to the heating effect of the circuit current. They are designed to respond to the true-rms value of the current waveform and will trip when the trip mechanism gets too hot. This type of breaker has a good chance of protecting against harmonic current overloads.
A peak-sensing, electronic trip circuit breaker responds to the peak of current waveform.
As a result, it won’t always respond properly to harmonic currents.
Since the peak of the harmonic current is usually higher than normal, this type of circuit breaker may trip prematurely at a low current. If the peak is lower than normal, the breaker may fail to trip when it should.
Bus bars and connecting lugs
Neutral bus bars and connecting lugs are sized to carry the full value of the rated phase current. They can become overloaded when the neutral conductors are overloaded with the additional sum of the triplen harmonics.
Panels that are designed to carry 60 Hz currents can become mechanically resonant to the magnetic fields generated by higher frequency harmonic currents. When this happens, the panel vibrates and emits a buzzing sound at the harmonic frequencies.
Telecommunications systems often give you the first clue to a harmonics problem because the cable can be run right next to power cables. To minimize the inductive interference from phase currents, telecommunications cables are run closer to the neutral wire.
Triplens in the neutral conductor commonly cause inductive interference, which can be heard on a phone line. This is often the first indication of a harmonics problem and gives you a head start in detecting the problem before it causes major damage.
Commercial buildings commonly have a 208/120 V transformer in a delta-wye configuration.
These transformers commonly feed receptacles in a commercial building. Single-phase, non-linear loads connected to the receptacles produce triplen harmonics, which add up in the neutral. When this neutral current reaches the transformer, it is reflected into the delta primary winding where it causes overheating and transformer failures.
Another transformer problem results from core loss and copper loss. Transformers are normally rated for a 60 Hz phase current load only. Higher frequency harmonic currents cause increased core loss due to eddy currents and hysteresis, resulting in more heating than would occur at the same 60 Hz current.
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