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System Compatibility of Adjustable Frequency Drives

Last Revision - April 25, 1997

GENERAL
This paper is an evaluation of the System Compatibility Report (July 1996) released by the Electric Power Research Institute, Power Electronics Application Center as an approach to meet the objectives of the project SC-610: Performance Criteria for System Compatibility of Adjustable Speed Drives Used in Low Voltage AC Power Systems.

The principle objectives defined within the report are: Establish and define baseline squirrel cage induction motor/ASD performance versus electric utility expectations of product capability.

Based on the resulting documentation of ASD capabilities and expectation, the project goal was to facilitate a dialog between electric utilities and ASD product manufacturers which could result in enhanced product system compatibility and therefore improved reliability to end-users.

The tests covered within the context of the report concentrate on ASD characterization relating to that portion of the system more directly related to the interests of the sponsoring electric utilities. This report did not address issues relating to the suitability of motors used with ASDs for topics such as reflected wave and motor bearing issues or for electrical noise interference.

SPECIFICS
The categories for which tests performed within the context of the report consisted of the following:

Initial Characterization
This test measured the following parameters of the AFD test unit(full load at base speed).

  1. Input phase to phase voltages
  2. Input phase currents
  3. Input Voltage % Total Harmonic Distortion
  4. Input Power factor

Input Line Current Harmonics
This test measured the following parameters of the AFD test unit(full load at base speed).

  • Input Current % Total Harmonic Distortion
Line Voltage Unbalance
This test measured the following parameters of the AFD test unit(full load at base speed).

  1. Input phase currents with input voltage unbalances of 0.6%, 1.3% and 2.4% <
  2. Input Current % Total Harmonic Distortion with same unbalanced input voltage

Low Steady-State Input Voltage
This test measured the following parameters of the AFD test unit with the ac motor operating at full load at base speed with 5% and 10% steady state voltage reductions.

ASD:

  1. Input kW.
  2. Input kVA
  3. Input Current
  4. Current THD
  5. DC Bus Voltage

MOTOR:

  1. % Slip Full load
  2. Temperature rise
  3. Efficiency
  4. Power factor
  5. Current
  6. Speed.

Ride-through response for 6 cycles and 30 cycles voltageLL sag
This test measured the following parameters of the AFD test unit(75% load at base speed).

  1. DC Bus voltage
  2. Motor Speed
  3. Input Current

Response to Capacitor Switching Voltage Transients
This test monitored AFD response to transients on the input voltage source caused by the connecting of power factor correction capacitors to the system.

EVALUATION
An evaluation for each test is detailed below.

  1. Initial Characterization
    This test was primarily intended to verify the each AFD operated. The numerical values resulting from the test provided no useful information since no ideal or expected values were defined which could be used as a comparison.
  2. Input Line Current Harmonics
    This test was performed using a soft and stiff voltage source. The numerical values resulting from the test provided no useful information since no ideal or expected values were defined which could be used as a comparison. The results indicated that the source characteristics had a large influence on the shape of the current waveform and that additional series impedance, such as reactors, would reduce the peak value of a current pulse. The test provided little information concerning injected (reactive) current waveforms or forward power producing current waveforms. The test supported the misconception that any distortion of the current waveform classifies as harmonic current distortion whether injected back into the electrical system or not.
  3. Line Voltage Unbalance
    This test showed that a line voltage unbalance will result in an input current unbalanced. It also indicated the importance of series impedance as a method of correcting some level of unbalance. The numerical values resulting from the test provided no useful information since no ideal or expected values were defined which could be used as a comparison.
  4. Low Steady-State Input Voltage
    This test showed that the AFD aids in minimizing the effect on the motor when the electrical system is subjected to input voltage reductions. The numerical values resulting from the test provided some useful information since general knowledge exists for the impact on motor when exposed to a reduced line voltage.
  5. Ride-through response for 6 cycles and 30 cycles voltageLL sag
    This test was primarily intended to verify how the AFD responded when exposed to a temporary reduction in input voltage. The criteria of 50% motor speed was used as the limit or evaluation point. The numerical values resulting from the test provided no useful information since the 50% operating motor speed limit does not relate to the needs of the process or to the behavior of other system components. Using the speed of the motor as the single criteria for evaluation, assumed that motor speed or the lack of it determines a successful process. There was no consideration of safety considerations or other functions within the process. The single judgment point was loss of production due to a significant loss in motor speed. Based on the concerns of sponsoring electric utilities about their potential liability to electrical power users for loss production in the event of a power outages, it is obvious that this test was conducted in a narrowly focused manner.
  6. Response to Capacitor Switching Voltage Transients
    This test showed the AFD response to transients on the input voltage source caused when capacitors are connected to the ac line to correct power factor.The numerical values resulting from the test provided no useful information since no ideal or expected values were defined which could be used as a comparison.

CONCLUSION

The System Compatibility Report (July 1996) provided an excellent example describing the results of many hours of organizing, equipment construction, testing and report generation. However, the report provided little useful information for the electric utilities or for AFD users who may have the occasion to see the report or hear third-hand tales about the report.

It is likely, however, that some unscrupulous individuals may use this report as a qualified resource to promote self interests which will ultimately hurt or at least mislead utilities and AFD users to make inaccurate decisions about equipment selection and installations. There is only one thing worst than bad information, and that is misinformation or technically limited propaganda.

There are always concerns about using new technology in the control process. Provided partial information or recommendations to potential users about what to consider when evaluating the use of new technology makes any decision more difficult when results do not agree with the recommendations provided. Full information is required in order to provide credible recommendations. Anything less is potentially harmful.

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