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Harmonics prediction on an installation with variable speed drives.

Gareth Whitty

I have been asked by a client to provide a harmonics prediction report on an installation with variable speed drives. I have never done one or been required to look into this before. Could someone assist me with the sort of information i would need.

We are installing around 15 Variable speed drives that will control numerous pumps and agitators around the process plant.  It is a brand new facility, so at the moment I don't know the nature of the supply to the facility although i will be able to find this out soon.

The variable speed drives will range from 0.37kW and 5.5kW, 3 Phase so nothing big.

I basically need to know what sort of information I would need to obtain from the client and the VSD manufacturer, and then how I would compile that information into a report.

I assume i will need to provide a report on each drive rather than an overall report for the installation.

Any help will be greatly appreciated.

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    It may be worthy also to get familiar with relevant standards that address the level of expected harmonics on drives. The North American IEEE-519 is one of them. The Point of Common Coupling (PCC) is an important concept to understand on this standard.

    Drives vendors or specific specialised engineering companies may also be able to provide some sort of basic level harmonic estimation tools based on the information available from the client power distribution load diagram. Expected Total Harmonic Distortion at the PCC can then be estimated this way. 

    It is well known that conventional mature Direct Front End 6 pulse drives generate the more significant level harmonics. Other More sophisticated Active Front End drives offer much lower level of harmonics. There are other harmonic mitigation approaches as well.

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  • 0

    It may be worthy also to get familiar with relevant standards that address the level of expected harmonics on drives. The North American IEEE-519 is one of them. The Point of Common Coupling (PCC) is an important concept to understand on this standard.

    Drives vendors or specific specialised engineering companies may also be able to provide some sort of basic level harmonic estimation tools based on the information available from the client power distribution load diagram. Expected Total Harmonic Distortion at the PCC can then be estimated this way. 

    It is well known that conventional mature Direct Front End 6 pulse drives generate the more significant level harmonics. Other More sophisticated Active Front End drives offer much lower level of harmonics. There are other harmonic mitigation approaches as well.

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  • +1 in reply to JG

    Harmonics Prediction Report for Variable Speed Drives (VSDs) Installation

    Overview

    This report provides a prediction of harmonic distortion levels for the installation of 15 Variable Speed Drives (VSDs) ranging from 0.37 kW to 5.5 kW, 3 Phase, which will control various pumps and agitators throughout the process plant.

    Variable Speed Drives Specifications

    • Quantity: 15 VSDs
    • Power Ratings: 0.37 kW to 5.5 kW
    • Phases: 3 Phase

    Harmonic Distortion Analysis

    1. Harmonic Generation by VSDs: VSDs are known to generate harmonics due to their non-linear operation. The most significant harmonics typically generated by VSDs are the 5th, 7th, 11th, and 13th harmonics.

    2. Total Harmonic Distortion (THD) Calculation: The Total Harmonic Distortion (THD) is an important parameter to assess the overall harmonic distortion in the system. It is the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.

    3. Harmonic Mitigation Techniques: To minimize the impact of harmonics, several techniques can be employed:

      • Passive Filters: Designed to block specific harmonic frequencies.
      • Active Harmonic Filters: More effective and can adapt to changes in the harmonic spectrum.
      • 12-Pulse or 18-Pulse VSDs: These drives inherently produce fewer harmonics.

    Calculation Assumptions

    • Load Distribution: Evenly distributed among all VSDs.
    • Power Factor: Assumed to be 0.95 for each VSD.
    • Impedance of the System: Assumed to be negligible for simplification.

    Step-by-Step Harmonics Calculation

    1. Determine Individual VSD Harmonic Currents: For each VSD rating, determine the harmonic currents generated at each harmonic frequency (5th, 7th, 11th, 13th).

    2. Aggregate Harmonic Currents: Sum the harmonic currents from all VSDs to find the total harmonic current for each harmonic frequency.

    3. Calculate THD: Use the formula:

      THD=∑h=2∞(Ih2)/I1\text{THD} = \sqrt{\sum_{h=2}^{\infty} (I_h^2)} / I_1THD=h=2∑∞​(Ih2​)​/I1​

      where IhI_hIh​ is the current at the hthh^{th}hth harmonic and I1I_1I1​ is the current at the fundamental frequency.

    Predicted Harmonic Distortion Levels

    Harmonic Order Current (A) per VSD Total Current (A)
    5th 20% of fundamental Aggregated Value
    7th 14% of fundamental Aggregated Value
    11th 9% of fundamental Aggregated Value
    13th 7% of fundamental Aggregated Value

    *Note: The exact current values will depend on the specific ratings and loads of each VSD.

    Mitigation Recommendations

    1. Install Passive Harmonic Filters: For each VSD or group of VSDs, passive harmonic filters can be used to attenuate specific harmonic frequencies.

    2. Use Active Harmonic Filters: Consider the installation of active harmonic filters at the main distribution panel to dynamically compensate for harmonics generated by the VSDs.

    3. Consider Multi-Pulse VSDs: Utilize 12-pulse or 18-pulse VSDs to inherently reduce the harmonic generation at the source.

    Conclusion

    The installation of 15 VSDs ranging from 0.37 kW to 5.5 kW is likely to introduce significant harmonic distortion into the electrical system. Proper analysis and mitigation strategies, such as the use of harmonic filters and multi-pulse drives, should be implemented to ensure compliance with harmonic standards and to maintain the reliability and efficiency of the plant's electrical system.

    For precise harmonic analysis, a detailed study with real load conditions and system impedance should be conducted using harmonic analysis software or consulting with a power quality specialist.

     

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