(source : Eaton Sheet 35007 sept 2011)
A discussion of power system harmonics is incomplete without discussing the effects of power factor correction capacitors. In an industrial plant containing power factor correction capacitors, harmonic currents and voltages can be magniﬁed considerably due to the interaction of the capacitors with the service transformer. This is referred to as harmonic resonance or parallel resonance. For a typical plant containing power factor correction capacitors, the resonant frequency (frequency at which ampliﬁcation occurs) normally falls in the vicinity of the 5th to the 13th harmonic. Because nonlinear loads typically inject currents at the 5th, 7th, 11th and 13th harmonics, a resonant or near-resonant condition will often result if drives and capacitors are installed on the same system, producing the symptoms and problems with blown fuses, damaged capacitors or failures in other portions of the electrical distribution system.
Note: 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, therefore, must be removed. However, 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:
- kVAsys = 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.
For example, 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 kVA, then kVAsys would equal 17,391 kVA.
If 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 (by >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(s). 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 Figure 1) is, 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. Therefore, the ﬁlter provides an inductive impedance path to those currents at harmonic frequencies created by nearly all three-phase non-linear loads (5th, 7th, 11th, 13th, etc.). 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:
- Provides power factor correction.
- Prevents harmonic overvoltages due to resonance.
- 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.