Dave's Technical Tidbits - Information Required for VFD Harmonics Analysis

Six-pulse variable frequency drives (VFDs) have the potential to create problems related to harmonics on the electrical power system within an industrial manufacturing facility or a neighboring facility. “Harmonics” describes power distortion at multiples of the electrical power system fundamental frequency (i.e. 120, 180, 240, and 300 Hz. for a 60 Hz. system). Six-pulse VFDs are those which include a very basic rectifier composed of six diodes for converting three-phase ac voltage to dc voltage. The current draw is in pulses in lieu of being nearly sinusoidal, and this creates current harmonics. Current harmonics along with impedances in the power system distort the supply voltage. Odd-numbered harmonics can cause power system components such as distribution transformers and circuit breakers to overheat. They can also have detrimental effects on instrumentation and sensitive electronics. This Technical Tidbit lists the information required to analyze the magnitude of harmonics expected from a large VFD or several VFDs either added to an existing power system or installed on a new system. Oftentimes, the primary goal of this exercise is to ensure that the installation complies with IEEE 519 guidelines when the point of common coupling (PCC) is where the electric utility company's power distribution equipment meets the end user’s or owner's equipment. It is important that the voltage harmonics at this point be low enough so that a neighboring facility or facilities are not affected by harmonics generated within the owner’s facility.

Obtaining the following information from the electric utility company helps ensure a more accurate assessment:

·        Utility-owned Transformer Nameplate kVA

·        Utility-owned Transformer Nameplate Secondary Voltage

·        Utility-owned Transformer Nameplate Impedance Percentage

·        Power Frequency (usually 60 Hz. or 50 Hz.)

·        Wire Length for Connection Between Utility-owned Transformer with Medium-voltage (MV) Secondary Winding and Facility Transformer with Low-voltage (LV) Secondary Winding

[The LV transformer is oftentimes pad-mounted, located just outside the building, and may be owned by either the electric utility company or the end-user (owner.)]

If the MV transformer information is not available, the available fault current value (ISC) and supply voltage from the electric utility company along with a service-impedance estimate can be used.

The following information from the LV transformer and the related power distribution system directly feeding the VFD system or systems is required:

·        Transformer Nameplate kVA [or short-circuit current (ISC)] and K Factor

·        Transformer Nameplate Secondary Voltage

·        Transformer Nameplate Impedance Percentage

·        Cable Lengths from the Transformer to Power Distribution Equipment (Switchgear, Switchboard, Panelboard, MCC, Industrial Control Panel, Etcetera.) and from this feeder power distribution equipment to the VFDs

The accuracy of the harmonics estimate will increase with the amount of data gathered. For instance, it is imperative to list all of the linear loads (e.g. resistive heaters) and all of the non-linear loads (e.g., VFDs, UPS systems, and the like) connected to the LV transformer secondary winding directly upstream of the VFDs. Power levels can be recorded in horsepower, kilowatts, kVA, or converted using amperes. An electrical distributor or systems integrator will know the loads for the system they are providing. However, they also need to know additional loads connected to the secondary winding of the LV transformer. In addition, it is prudent, but not imperative, to have a list of the loads at all points in the power system all the way back to the utility connection point. Some additional data collected are wire type and size, motor and drive power factors, and motor and drive efficiencies. Finally, armed with sufficient collected data, the electrical distributor or systems integrator can decide which cost-efficient harmonics mitigation techniques should be applied. Some examples of harmonics mitigation techniques are multi-pulse VFD substitutes (e.g., 12-Pulse), line reactors, dc-link chokes, and active techniques such as filters and VFDs with active ac-dc converter sections.

To ensure that the harmonics generated by six-pulse VFDs do not create heat-related power system problems such as transformer, wire, and circuit breaker failures, the harmonics for a new VFD installation need to be estimated. Although conservative estimates are possible without some of the upstream data from the electric utility company and for the entire facility power system, collecting more data allows more accurate estimates.