The BEST Group
THE BUFFALO ENERGY SCIENCE AND TECHNOLOGY GROUP
-Winter Lecture Series
FUNDAMENTALS OF HARMONICS
Presented by:
Syed Khundmir T
Department of Electrical Engineering
University at Buffalo
khundmir@buffalo.edu
Prime reference: Electrical Power Systems Quality By Roger C. Dugan, Mark F. McGranaghan, Surya Santoso, H. Wayne Beaty Harmonic Distortion
•  It is caused by non-linear devices.
•  In such devices, the current is not proportional to applied voltage.
•  Most of the times, only odd harmonics are present in the system.
•  Vast majority of nonlinearities in the system are found in shunt
elements ie., loads.
Voltage vs Current Distortion
•  Non-linear loads appear to be the source of harmonic current in shunt and
injects harmonic currents into the power system.
•  Voltage distortion is the result of distorted currents passing through the
linear, series impedance of the power delivery system.
•  Harmonic currents passing through the impedance cause a voltage drop for
each harmonic. This results in voltage harmonic appearing at the load bus.
•  While the load current harmonics result in voltage distortion, it should be
noted that load has no control over voltage distortion.
•  The above mentioned phenomenon is shown in the figure present in the next
slide:
Contd…
•  Control over harmonic currents takes place at the end-user application.
•  Control over the voltage distortion, assuming the harmonic current
injection is within reasonable limit, is excercised by the entity having
control over the system impedance, which is often the utility.
Harmonics vs Transients
•  Transients exhibit high frequency only briefly a:er an abrupt change in power system. •  The frequencies of transients have no rela?on with the fundamental frequency. •  Waveform distor?on that produces harmonics is present con?nually, or at least for several seconds. •  Transients are usually dissipated within a few cycles. Effect on Power
•  No real work will result from the por?on where the current is not in phase with the voltage. •  In the non-­‐sinusoidal case, the computa?on of the ac?ve power must include contribu?ons from all harmonics components; thus it is the sum of ac?ve power at each harmonic. •  Apparent power S is a measure of the poten?al impact of the load on the thermal capability of the system. •  The reac?ve power component at fundamental frequency, may be used to size shunt capacitors. Power factor: Displacement and True
•  Power factor is the ra?o of useful power to perform real work to the power supplied by the u?lity. •  In other words, it is the power expended for its intended use. •  In the sinusoidal case, the power factor can be computed as the cosine of the phase angle and is commonly referred as the displacement power factor. •  In the non-­‐sinusoidal case, the power factor which takes into account the contribu?on from all ac?ve power, including both fundamental and harmonic frequencies, is known as true power factor. •  Considering only the displacement power factor will give a false sense of security. Harmonic Phase Sequences
•  The method of symmetrical components allows the three phase system to be transformed to three single phase system that are much simpler to analyze. •  It allows any unbalanced set of phase currents or voltages to be transformed into three balanced sets. •  The posi?ve sequence set contains three sinusoids displaced 120 degrees from each other with normal ABC rota?on. •  The nega?ve sequence will have opposite rota?on. •  The sinusoids of the zero sequence are in phase with each other. Triple Harmonics
•  Triple harmonics are odd mul?ples of the third harmonics. •  System response is o:en considerably different for triplens than for the rest of the harmonics. •  These have become an important issue for grounded-­‐wye systems with current flowing on the neutral. •  Two typical problems are overloading the neutral and telephone interference. •  Transformer winding connec?ons have a significant impact on the flow of triplen harmonic currents from single-­‐phase non-­‐linear loads. Contd…
•  Two cases: 1.  In the wye-­‐delta transformer, the triplen harmonic currents are shown entering the wye side. Since they are in phase, they add in the neutral. They can flow but they remain trapped in the delta and do not show up in the line currents on the delta side. 2.  Using grounded-­‐wye windings on both sides of the transformers allows balanced triplens to flow from the low-­‐voltage system to the high-­‐
voltage system unimpeded. •  Transformers are suscep?ble to overhea?ng when serving single phase loads having high third-­‐harmonic content. •  These rules about triplen harmonics apply only to balanced loading condi?ons.