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Protection Question No.50 - Protective relays and system harmonics
In our industrial plant, the power electronic convertor devices such as industrial frequency convertor, variable speed drives and ac to dc convertors have used, and we concern to voltage and current distortion and its bad affects on electrical protection systems. Please inform me if there are any limitations for application of protective relays in typical mentioned system.  
Author : Bahula - From: India
 
#1
Wed, September 23rd, 2009 - 12:27
IEEE 519 states, “Distortion factors of 10-20% generally are required to cause problems in relay operation.”  These levels are higher than the recommend limits given in Section 11 of the document.
The document discusses the various devices used to measure voltage and current harmonics and the goals of these measurements.  Below are selected items from a list given in section 9.1 of IEEE 519 that relate to protective relays and their possible use for these purposes.

• Monitoring existing harmonic levels and comparing to recommended or admissible levels.
• Observing/trending existing background harmonic levels (daily, monthly, seasonal trends).
• Measuring harmonic levels to compare to harmonic load flow study results.
• Measuring harmonic current and voltage phase angles with respect to the fundamental.  Such measurements can help determine the harmonic driving point impedance at a location for use in studies.

The document points out the limitation that existing digital protective relays generally have in regards to measuring harmonic quantities:

“The techniques used for harmonics measurements differ from those used for ordinary power system measurement.  The frequency bandwidth of the ordinary measurements of voltage, current, and power can be accomplished with attention to a narrow band of frequencies near the distribution frequency.  Substantially wider bandwidths (up to 3kHz) are required in the study of power system harmonics.”

IEEE 519 specifies requirements that must be met for accurate harmonic measurements:

“Accuracy.  The instrument must perform the measurement of a constant (stead-state) harmonic component with an error compatible with the permissible limits.  It is reasonable to use an instrument with an uncertainty no larger than 5% of the permissible limit.  For example, assume a 480V, three-phase system in which the 11th harmonic should be less than 0.70%.  The line-neutral 11th harmonic, V11, is less than 1.94V.  This indicates that the instrument should have an uncertainty of less than ±(0.05)(1.94) = ±0.097 V.”

“Bandwidth.  The bandwidth of the instrument will strongly affect the reading, especially when harmonics are fluctuating.  It is recommended that instruments with a constant bandwidth for the entire range of frequencies be used.  The bandwidth should be 3 ± 0.5Hz between the –3 dB points with a minimum attenuation of 40 dB at a frequency of fh + 15 Hz.  In situations in which inter-harmonics and transients are present, a larger bandwidth will cause large positive errors.”

“For measurements of harmonic currents in the frequency range up to 10 kHz, the normal current transformers that are used for switchgear metering and relaying have accuracies of better than 3%.  If the CT burden is inductive, there will be a small phase shift in the current.”
 
Author : Hamid - From: Iran
 
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