POWER QUALITY BASICS: HARMONICS

Tuesday, July 26, 2011

Harmonics are described by IEEE as sinusoidal voltages or currents having frequencies that are integer multiples of the fundamental frequency at which the power system is designed to operate. This means that for a 60-Hz system, the harmonic frequencies are 120 Hz (2nd harmonic), 180 Hz (3rd harmonic) and so on. Harmonics combine with the fundamental voltage or current producing a non-sinusoidal shape, thus, a waveform distortion power quality problem. The non-sinusoidal shape corresponds to the sum of different sine waves with different magnitudes and phase angles, having frequencies that are multiples of the system frequency.

Harmonic Waveform Distortion
Harmonic Waveform Distortion
Harmonic distortion levels can be characterized by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component. It is also common to use the Total Harmonic Distortion (THD), as a measure of the effective value of harmonic distortion. It has become an increasing concern for many end-users and for the overall power system because of the growing application of power electronics equipment. Protection from high levels of harmonics includes isolation or modification of the source, phase multiplication, pulse width modulator (PWM) and application of passive or active harmonic filters.

Causes

Harmonics exists due to the nonlinear characteristics loads and devices on the electrical power system. These devices can be modeled as current sources that inject harmonic currents into the electrical system. Consequently, voltage distortion is created as these currents produce nonlinear voltage drops across the system impedance.

Prior to the proliferation of power electronic equipment, harmonics are commonly caused by electric machines working above the knee of the magnetization curve (magnetic saturation), arc furnaces, welding machines, rectifiers, and DC brush motors. Today, all non-linear loads, such as power electronics equipment including Switched Mode Power Supplies (SMPS), Adjustable Speed Drives (ASD),  high efficiency lighting and data processing equipment.

Consequences

Harmonics primarily result to significant overheating of equipment, cables and wires. Other consequences of having a high harmonic level in the system include the following:

  • Neutral overload in 3-phase systems
  • Electromagnetic interference with communication systems
  • Loss of efficiency in electric machines
  • Increased probability in occurrence of resonance
  • Nuisance tripping of thermal protections.
  • Errors in measures when using average reading meters

Total Demand Distortion

Current distortion levels can be characterized by the total harmonic distortion, although sometimes this can be misleading. For example, many ASDs will display high THD values for the input current when they are operating at very light loads. Nonetheless, this is not alarming because the magnitude of harmonic current is low, even though its relative distortion is high.

As a result, the IEEE (Std 519) defines the Total Demand Distortion (TDD), in order to typify harmonic currents in a consistent manner. The TDD is the same as the total harmonic distortion except that the distortion is expressed as a percent of some rated load current rather than as a percentage of the fundamental current magnitude at the instant of measurement.

Synopsis:

Magnitude: 0 to 20% (typical)
Spectral Content: 0 to 100th Harmonic
Duration: Steady-state
Source: Nonlinear Devices (i.e. Power Electronics)
Symptoms: Malfunction and Overheating
Occurrence: Low to Medium

References:
Almeida, A., Delgado J., and Moreira, L. (nd). Power Quality Problems and New Solutions
Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd Ed.). New York: McGraw-Hill.
IEEE 1159-1995. Recommended Practice For Monitoring Electric Power Quality.
Utility Systems Technologies, Inc. (2009). Power Quality Basics

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality and Protective Relaying. I have been working in an electric distribution utility for about seven years now as a Planning & Design Engineer. I handle PQ studies, power system analysis and capital budgeting for company projects.