What is Harmonic Distortion?

A measure of the amount of deviation from a pure sinusoidal wave form that can be caused by a non-linear load because it only draws from the power line as required.

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What is a distorted waveform?
Fundamental (60Hz) sinusoidal waveform and multiples of the fundamental frequency typically the 5th, 7th, 11th, 13th & 17th. 

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How do you minimize total harmonic distortion values?  
Modern technology, incorporating sensitive components, present new challenges for plant managers and engineers. For example, widespread use of SCR Rectifiers and Diode Rectifiers used in the power conversion section of Adjustable Frequency Drives (AFDs), Computers, Copiers, solid state lighting ballasts, etc., have raised concerns regarding power quality and its role in harmonic distortion. When discussing power quality it is important to relate the type of electrical waveforms present in the electrical power system.

Electrical waveforms are either linear (sinusoidal) or nonlinear (non-sinusoidal). Figure 1 illustrates the two waveform types.

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A linear waveform is commonly referred to as a “sinewave”, and is simple to describe mathematically; a non-linear waveform is not. Voltage and current waveforms are closely tied to harmonic distortion. Every wave shape has a Total Harmonic Distortion (THD) value. With this in mind, it is up to the end-user to specify what, if any, harmonic distortion levels can be tolerated at the service entrance of the building.

 

For example, a 1000 kVA building transformer at a voltage of 460 Volts and 5% impedance is used in conjunction with 850 total HP of VFD load. Applying the IEEE519-92 standard to this power system will set the total harmonic current limits to 8% and total harmonic voltage limits to 5%. The objectives of the current limits are to limit the maximum individual frequency voltage harmonics to 3% of the fundamental and the voltage THD to 5% for systems.

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The three commonly used methods to combat power line distortion are: Baseline VFD, Input Line Trap Filter, and 12 Pulse Transformer.

 

12 Pulse Transformer Design Using a 12 Pulse Transformer configuration will meet power quality specifications such as IEEE519-92. It is important to note that the majority of installations do not need cancellation of harmonics in the power system. If the transformer is not fully loaded, the transformer should have enough capacity to provide clean power to all equipment. A 12 Pulse Transformer is able to meet the IEEE519-92 or THD limits because of its unique capability to utilize two input rectifiers in a drive to realize true cancellation of the 5th and 7th harmonic. This is accomplished by phase manipulation - electrically shifting the input power to the drive. Since many Yaskawa ac drives have two six pulse rectifiers as standard, one six pulse rectifier just needs to be shifted by 30° in respect to the second six pulse rectifier. To insure maximum cancellation, it is important to properly balance the impedance of the transformer secondary.

 

One drawback to standard phase shift transformer, is that the delta and wye configurations have different impedance. This in turn causes one of the diode sections to draw more current; the result being inadequate harmonic cancellation. The Yaskawa 12 Pulse Transformer uses a different power transformation technique (see Figure 2) to insure maximum impedance balance, and therefore maximum harmonic cancellation. Using the dual diode input (standard in many GPD drives) with a 12 Pulse Transformer can reduce current distortion levels by 90%. The transformer effectively eliminates the 5th and 7th harmonic in the transformer windings. Since the 5th and 7th harmonic contribute to approximately 90% of the harmonic spectrum, very high density packaging of drives to transformer size can be achieved.

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When discussing power quality it is important to relate the type of electrical waveforms present in the electrical power system. Electrical waveforms are either linear (sinusoidal) or nonlinear (non-sinusoidal). Figure 1 illustrates the two waveform types. A linear waveform is commonly referred to as a “sinewave”, and is simple to describe mathematically; a non-linear waveform is not. Voltage and current waveforms are closely tied to harmonic distortion.

 

Every wave shape has a Total Harmonic Distortion (THD) value. With this in mind, it is up to the end-user to specify what, if any, harmonic distortion levels can be tolerated at the service entrance of the building. For example, a 1000 kVA building transformer at a voltage of 460 Volts and 5% impedance is used in conjunction with 850 total HP of VFD load. Applying the IEEE519-92 standard to this power system will set the total harmonic current limits to 8% and total harmonic voltage limits to 5%. The objectives of the current limits are to limit the maximum individual frequency voltage harmonics to 3% of the fundamental and the voltage THD to 5% for systems.

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