(извор : Еатон лист 35007 седам 2011)

Хармонични Разматрања

Дискусија о електроенергетског система хармоника је непотпун без разматрања ефеката за корекцију фактора снаге кондензатора. У индустријском погону који садржи корекцију фактора снаге кондензатора, хармоника струје и напони се могу значајно увећане због интеракције кондензатора са сервиса трансформатора. Ово се назива хармоника резонанца или паралелно резонанца. За типичну биљке садрже корекцију фактора снаге кондензатора, резонантна фреквенција (фреквенција на којој фактор појачања јавља) нормално спада у близини 5. до 13. хармоника. Јер нелинеарних потрошача обично убризгава струје у 5., 7ог, 11тх и 13. хармоници, резонантна или близу-резонантни услов ће често резултирати ако дискови и кондензатори су инсталирани на истом систему, производњу симптоме и проблеме са дуване осигурача, оштећени кондензатори или неуспеси у другим деловима дистрибутивног система електричне.

Приметити: Capacitors themselves do not cause harmonics, but only aggravate potential harmonic problems. Често, harmonic-related problems do not “show up” until capacitors are applied for power factor correction.

It is a common misconception that the problem of applying capacitors in harmonic environments is limited to problems caused for the capacitor itself—that the capacitor’s lower impedance at higher frequencies causes a current overload into the capacitor and, стога, must be removed. Међутим, the capacitor/harmonics problem must be viewed from a power system standpoint. The capacitor-induced increase of harmonic voltages and currents on a plant’s system may be causing problems while the capacitor itself remains within its acceptable current rating.

Capacitor Banks and Transformers Can Cause Resonance

Capacitors and transformers can create dangerous resonance conditions when capacitor banks are installed at the service entrance. Under these conditions, harmonics produced by nonlinear devices can be ampliﬁed many fold.

Problematic ampliﬁcation of harmonics becomes more likely as more kVAR is added to a system which contains a signiﬁcant amount of nonlinear load.

An estimate of the resonant harmonic frequency is found by using the following formula:

х_{р} = \sqrt{\frac{k У A_{с y с}}{k У A р_{ц a П}}}

h_r = \sqrt{\frac{kVA_{sys}}{kVAr_{cap}}}

Где:

кВА_sys= Short-Circuit Capacity of the System
kVAr_cap = Amount of Capacitor
kVAr on the Line
х_р = Resonant harmonic numberHarmonic

If h is near the values of the major harmonics generated by a nonlinear device—i.e., 3, 5, 7, 11—then the resonance circuit will greatly increase harmonic distortion.

На пример, if a plant has a 1500 kVA transformer with a 5-1/2% impedance and the short-circuit rating of the utility is 48,000 кВА, then kVA_sys would equal 17,391 кВА.

Ако 350 kVAR of capacitors were used to improve power factor, h would be:

Because h falls right on the 7th harmonic, these capacitors could create a harmful resonance condition if nonlinear devices were present in the factory. In this case the capacitors should be applied only as harmonic ﬁltering assemblies.

Diagnosing a Potential Harmonics Related Problem

Negative symptoms of harmonics on plant equipment include blown fuses on capacitors, reduced motor life, false or spurious operations of fuses or circuit breakers, decreased life or increased noise in transformers or mis-operation of electronic or microprocessor controls. If one or more of these symptoms occurs with regularity, then the following steps should be taken.

If the plant contains power factor correction capacitors, the current into the capacitors should be measured using a ‘true rms’ current meter. If this value is higher than the capacitor’s rated current at the system voltage (по >5% or so), the presence of harmonic voltage distortion is likely.
Conduct a paper audit of the plant’s harmonic-producing loads and system conﬁguration. This analysis starts with the gathering of kVA or horsepower data on all the major nonlinear devices in the plant, all capacitors, and rating information on service entrance transformer(с). This data is analyzed to determine whether the conditions are present to create unfavorable levels of harmonics.
If the electrical distribution system is complex—e.g., multiple service entrances, distributed capacitors— or if the paper audit is incomplete or considered to be too burdensome, the most deﬁnitive way to determine whether harmonics are causing a problem is through an on-site plant audit. This audit involves an inspection of the electrical system layout and connected loads, as well as harmonic measurements taken at strategic locations. This data can then be assembled and analyzed to obtain a clear and concise understanding of the power system.

Eliminating Harmonic Problems

When power factor correction is required in the presence of nonlinear loads, or the amount of harmonic distortion must be reduced to solve power quality problems or avoid penalties, the most reliable, lowest cost solution is often realized with the use of harmonic ﬁlters.

Passive and Switched Harmonic Filters

A shunt harmonic ﬁlter (see Слика 1) је, essentially, a power factor correction capacitor combined with a series iron core reactor. A ﬁlter provides power factor correction at the fundamental frequency and becomes an inductance (like a motor) at frequencies higher than its “tuning point.” Most harmonic ﬁlters are tuned below the 5th harmonic. Дакле, the ﬁlter provides an inductive impedance path to those currents at harmonic frequencies created by nearly all three-phase non-linear loads (5ог, 7ог, 11ог, 13ог, итд). Because the ﬁlter is not capacitive at these frequencies, the plant electrical system can no longer resonate at these frequencies and can not magnify the harmonic voltages and currents.

Схунт хармонички филтер стога постиже три ствари:

Омогућава корекцију фактора снаге.
Спречава хармонијске пренапоне услед резонанције.
Смањује хармонијско изобличење напона и хармонијско оптерећење трансформатора на фреквенцијама изнад тачке подешавања.