(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. Often, 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:

• кВА_sys = Short-Circuit Capacity of the System
• kVAR = Amount of Capacitor
• kVAR on the Line
• h = The Harmonic Number referred to a 60 Hz Base

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.

1. 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.
2. 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.
3. 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.

 

A shunt harmonic filter therefore accomplishes three things:

1. Provides power factor correction.
2. Prevents harmonic overvoltages due to resonance.
3. Reduces voltage harmonic distortion and transformer harmonic loading at frequencies above its tuning point.

In some circumstances, a harmonic resonance condition may accrue gradually over time as capacitors and nonlinear loads are installed in a plant. The replacement of such capacitors with harmonic filters in order to correct a problem may be prohibitively expensive. Custom-designed harmonic filters which are able to eliminate problems associated with resonance at any particular frequency while providing an extremely low amount of power factor correction capacitance. These low kVAR filters are therefore able to provide the same amount of filtering capacity as a much larger conventional filter, but at a lower cost.

Solutions for Systems with High Harmonics

If the plant loads vary, then a switched capacitor/filter bank is recommended. For systems with widely varying loads where harmonic cancellation is the primary goal, a Harmonic Correction Unit (HCU) is recommended.