A certified electrician with a certificate in harmonics can lead the charge

. Ensure the electrician uses a True-RMS meter. Average response ammeters are insufficient because they are accurate only when measuring 60-hertz loads that have sinusoidal current waveforms. They cannot measure the current of nonlinear load accurately.

Nonlinear loads draw current in a nonsinusoidal manner and produce reflective harmonic currents that operate above 60 hertz. If an average response ammeter is used to measure nonlinear load currents, readings below the actual True-RMS current might result. The actual current might exceed equipment ratings.

VFD Input Current

 

Contact your VFD manufacturer to find out what harmonic filters already exist as part of the drive system

. Some VFD manufacturers use DC bus filters (a capacitor between the diodes and the IGBTs) instead of input line reactors to protect the surrounding electrical system from harmonic currents associated with VFDs. Although these filters do a great job protecting the system from harmonics, they do not protect the VFD from power supply problems within the building side of the line.

Traditional input line reactors offer this protection, as well as harmonics filtering. The additional protection by line reactors is worth the investment, which is approximately $9 per drive horsepower. The Institute of Electrical and Electronics Engineers Inc.'s (IEEE) recommended practices and requirements for harmonic control in electrical power systems are outlined in IEEE Standard (Std) 519-1992. Although many drives meet these requirements, the drive will no longer meet IEEE 519 if the VFD is not installed under the same standard.

Plants should use specific distortion limits for total harmonic voltage and current distortion calculation and measurement.

For voltage harmonics, individual or simultaneous operation of the VFDs should not add more than 3 percent total harmonic voltage distortion while operating from the utility source, or more than 5 percent while operating from standby generator, if applicable.

For current harmonics, the maximum allowable total harmonic current distortion limits for each VFD should not exceed 5 percent as calculated and measured at the point of common

coupling (PCC). The PCC with the consumer/utility interface is the closest point on the utility side of the customer's service where another utility customer is or could be supplied. The ownership of any apparatus such as a transformer that the utility might provide in the customer's system is immaterial to the definition of the PCC.

Look for the manufacturer's published data on the allowable motor lead lengths for its VFDs

. Long-run motor leads from the VFD can cause insulation failure. Long wiring runs between the motor and control encourage harmonic currents. To solve unavoidable long runs, line reactors or harmonic filters can be placed between the motor and control. Refer to the IEEE "Guide for Specifying and Selecting Power, Control and Purpose Cable for Petroleum and Chemical Plant," (IEEE 1242-1999).

VFD Input Current

 

Several standards should be used when selecting and using motors with VFDs, including National Electrical Manufacturers Association (NEMA) MG1-1993, Part 31.40.4.2, IEEE 841-2001 (2001), and American Petroleum Institute (API) 541 95 3RD.

NEMA MG1-1993, Part 31.40.4.2 states that an inverter-fed motor should be designed for a peak voltage of 1,600 volts (V) and a minimal rise time of 0.1 microsecond for motors rated less that 600 V. This standard always should be specified when using motors with VFDs that use IGBTs. This is critical because voltage spikes as high as 1,600 V are possible when the transistor switches on from the off in the harmonics switching scheme.

IEEE 841-2001 has been updated to include "premium-efficient" motors and applies to high-efficiency totally enclosed fan-cooled (TEFC), horizontal and vertical, single-speed, squirrel cage polyphase induction motors. Included are motors up to and including 370 kW (500 horsepower [hp]), in NEMA frame sizes 143T and larger, for petroleum, chemical and other severe-duty applications (commonly referred to as severe-duty motors). Excluded from the scope of this standard are motors with sleeve bearings and additional specific features required for explosion-proof motors.

It is important to note that most inverter-ready motors have a Class H insulation system rated for 180 Degrees C operation, and for every 10 Degrees C cooler a motor operates, the insulation life doubles.

API 541 95 3RD, for larger motors, covers the minimum requirements for the production and purchase of all form-wound squirrel cage induction motors above NEMA 440 frame sizes (nominally 250 hp and larger) for use in petroleum industry services. It can also be applied to adjustable-speed motors and induction generators with appropriate attention to the specific requirements of such applications. It also gives information for electrical and mechanical design features, insulation systems, accessories and tests.

Several VFD drive manufacturers have introduced a variety of technologies and designs to minimize harmonics in the AC line

. An important aspect of AC line harmonics to understand is that the total harmonic distortion (THD) is based on a percentage of load, not drive capacity. More importantly, harmonic content, when measured in percent of load, becomes a much higher percentage as load decreases, which is specifically true in variable-torque applications in which the load current changes at the square of the speed. Therefore, it might be more appropriate to refer to total harmonic content in terms of total demand distortion, which considers the system's load capacity, not just the load at the demand point.

Some of the new technologies and designs include: