Recently, there has much discussion and post on the subject of power quality. Whereas in the past, power received from the electric utility and use by an industry plant was generally perfect sinusoidal waveform. Nowaday, industrial plants must deal with the problem of distortion power. Its refer to express variety of voltage and current not been pure sinusoidal waveform. The reason can come in the form of short-term transients or steady-state, continous distortions. Beside that, the sources of distortions can be come from reside within the plant itself.
The total harmonic distortion is a most popular that effect bad to power quantity, Its usually associated with an industrial plant increased use of adjustable speed drives, power supplies and other devices that use solid-state switching. However, harmonics distortion majorly generated by variety of nonlinear electrical devices existing a plant or within nearby plants. Because harmonics distortion can cause serious operating problems in plants, it is important that the operating engineer and facilities persionnel understand the fundamentals of harmonics distortion know how to recognize the symptoms of this problem and what we can do to solve this problems.
I. What are harmonics?
A harmonics is a periodic wave having a frequency that is an integral multiple of the fundamental power line frequency of 50Hz. For example, 250hz (5x50Hz) is a 5th order harmonic of the fundamental frequency (Figure 1). Figure 2 shows the resultant wave when combine th fundamental and 5th harmonic cause harmonic distortion of the power waveform.
Harmonics are a steady-state phenomenon and should not confuse with short-term phenomena that exits less than a few cycles. Transients, electrical disturbances, overvoltage surges, and undervoltage sags in the supplied voltage are not harmonics. Some of these short-term disturbances in voltage or current will mitigate by transient voltage surge suppressors, line reactors or isolation transformers. However, these devices usually effect on harmonic currents or voltages.
The level of voltage or current harmonic distortion existing at any point on a power system can measure in terms of the total harmonic distortion (THD) of both current and voltage. The THD for voltage waveform is given by following formula:\[{V_{THD}} = \frac{{\sqrt {{V_2} + {V_3} + … + {V_n}} }}{{{V_1}}}\] Where ${{V_1}}$ = fundamental voltage value, ${{V_n}}$ (n=2,3,4,etc…)=harmonic voltage values
II. What is the consequences of high harmonic distortion?
The high levels of harmonic distortion can create stress and resultant problems for the plants, as well as all of the electrical equipment, the result can shutdown important single machine or entire line or process.
Table 1 summarizes some of the negative consequences that harmonics can effect to typical equipment found in the plant. These effects are categorized by problems create by both current and voltage harmonics.
III. Acceptable harmonic distortion levels on a power system
As IEEE Standard 519-1992 (IEEE recommended practices and requirements for harmonic control in electrical power systems, represents the most recent standard of acceptable harmonic distortion levels on a power system.
Table 2 and 3 summarize the voltage and current harmonic distortion limits.
Note: The current distortion limits are dependent upon the size of the load related to the available short-circuit capacity of the utility.
Where: ${I_{sc}}$=maximum short-circuit current
IL=maximum demand load current (fundamental frequency component)
IV. How are harmonics generated?
Harmonics will generate by nonlinear loads. A nonlinear load is a circuit element that draws current in a nonsinusoidal.
4.1. Electronic power converters
Electronic power converters—for example, adjustable speed drives and power supplies—are by far the largest contributors to harmonic distortion in today’s plant environment An electronic power converter changes electrical energy from one form to another, typically by rectifying the AC voltage into DC and utilizing the DC voltage directly or synthesizing a new AC voltage This change is accomplished by using solid-state devices—silicon control rectifiers (SCRs), diodes, transistors—to periodically switch in the conducting circuits of the converter.
Some names given to electronic power converters:
– Adjustable speed drives
– Variable frequency drives
– SCR drives
– AC motor drives (AC/DC/AC)
– DC motor drives (AC/DC)
– Three-phase full wave rectifiers
– Three-phase full wave converters
– Six-pulse converters
4.2. Arcing devices
Arc furnaces and welders are the two types of arcing devices that cause the most harmonic distortion, although arc lighting (fluorescent, mercury vapor) will also cause small degrees of harmonic distortion.
4.3. Other devices
Motors, generators, transformers and arc lighting also have small nonlinear components, although the contribution of these devices to total harmonic distortion in a plant tends to be relatively small.
4.4. What is the relationship between power factor correction capacitors and harmonics?
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 will magnifies 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 outlined in the previous section
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.
V. How to define a problem related to harmonics and solve it?
5.1.Analysis a problem
If a plant engineer suspects that he might have a harmonics problem, the following steps can easily be performed as an initial investigation into potential problems:
– Look for symptoms of harmonics as listed in Table 4. If one or more of these symptoms occurs with regularity, then the following steps should be taken .
– In case the plant contains power factor correction capacitors, the current into the capacitors will measure 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).
5.2. What is an active harmonic filter?
Active harmonic filters provide dynamic harmonic mitigation and power factor correction by actively injecting reactive currents into an electrical distribution system to cancel damaging harmonic currents and supporting power factor requirements at the point of connection
• Can size to meet specific levels of harmonic correction, providing compliance with IEEE 519 recommended levels
• Broad spectrum of cancellation for robust protection (2nd to 51st harmonic)
• Excess capacity helps to improve power factor to maximize efficiency
Care must be taken when installing an active harmonic filter on a system They can have a negative effect on capacitors and drives that do not have the proper impedance
5.3. What is a passive harmonic filter?
A reactor harmonic filter (see Figure 4) 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 turning point. The filter provides an inductive impedance path to those currents at harmonic frequencies created by nearly all three-phase nonlinear loads (5th, 7th, 11th, 13th and so on). Because the filter is not capacitive at these frequencies, the plant electrical system can no longer resonate at these frequencies and cannot magnify the harmonic voltages and currents.
One reactor harmonic filter therefore accomplishes three things:
• Provides power factor correction
• Prevents harmonic overvoltages due to parallel resonance
• Reduces voltage harmonic distortion and transformer harmonic loading at frequencies above its turning point.
In some circumstances, a harmonic resonance condition may accrue gradually over time as capacitors and nonlinear loads will install in a plant In those instances, replacement of such capacitors with harmonic filters is in order to correct the problem.
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