Designing Harmonic Filters for Adjustable Speed Drives to Comply with New Ieee-519 Harmonic Limits

This paper discuses the application of the revised IEEE-519 Harmonics standards to typical industrial facilities employing adjustable speed drives (ASDs). The harmonic generation characteristics of ASDs are described. Requirements for control of the harmonic currents are developed as a function of the ASD characteristics, overall plant loading level, power system characteristics, and power factor correction requirements. Filter design procedures are presented for controlling the harmonic currents injected onto the power system. INTRODUCTION The increasing application of power electronic equipment (especially adjustable speed motor drives, or ASDs) in the industrial environment has led to a growing concern for harmonic distortion and the resulting impacts on system equipment and operations. Possible problems include transformer overheating, motor failures, fuse blowing, capacitor failures, and misoperation of controls. [1] Harmonic currents are generated by the operation of nonlinear loads and equipment on the power system. These include ASDs, other power converter equipment, voltage controllers, transformers, and arcing loads. Voltage distortion results from the interaction of these currents with the system impedance vs. frequency characteristics. A revised version of IEEE Standard 519, "Recommended Practice for Harmonic Control in Electric Power Systems", provides recommended limits for harmonics in two categories [2]: 1. Harmonic current limits are specified for individual customers. These are evaluated at the point of common coupling between the customer and the power system. 2. Harmonic voltage limits are specified for the overall power system and provide an indication of the power quality that a customer can expect. This paper discusses these limits and their application to typical industrial facilities employing ASDs. The harmonic generation characteristics of ASDs are developed and then the expected harmonic levels are evaluated for different power system characteristics and power factor correction practices. Finally, filter design procedures are developed for controlling the harmonic currents. ASD CHARACTERISTICS The characteristics of the input current for ASDs depend on the drive type, drive loading, and the characteristics of the system supplying the drive. The harmonic distortion in these currents can vary over a wide range. However, it is possible to identify two basic waveform types that can be used for analysis purposes: TYPE 1: High Distortion Current Waveform This is characteristic of virtually all ASDs that have voltage source inverters (either stepped wave or pulse width modulated) that do not have additional choke inductance for current smoothing. The total harmonic distortion for the selected waveform is 80%. Actually, it can be higher for small drives but this waveform is a good representation for larger drives or groups of smaller drives. TYPE 2: Normal Distortion Current Waveform This waveform represents dc drives, large ac drives with current source inverters, and smaller ac drives with voltage source inverters and added inductance for current smoothing. The selected waveform has a distortion level of 38%, which is obtained from a 100 hp PWM drive with a 3% choke inductor. DC drives and larger ac drives could have somewhat lower distortion levels (e.g. 25-30%). Example waveforms and harmonic spectrums for the two waveform types are given in Figure 1. These waveform characteristics are used throughout the paper to evaluate harmonic control requirements.