Getting Your Plant Out of the Twilight Zone

Anyone who has ever watched the "Twilight Zone" knows that the show's common theme centers around strange occurrences that disrupt normal daily activities. Recently, facility engineers at a chemical plant in southern Texas dubbed a recurring ," but seemingly unexplainable ," problem at a control unit the "Twilight Zone."

Problems at the unit included multiple distribution transformer failures, as well as burnout of circuit breakers and electrical connectors. Electric motors were overheating, even while operating within their respective amperage ranges. All of these problems were symptoms of overload. The electrical personnel affectionately called one area the "Bermuda Triangle" because even radios experienced power interference.

When an electrician put away the traditional average response ammeter and traced the problem with a True-RMS (root-mean-square) meter, the investigation began to get somewhere. Initial measurements of phase currents showed values that did not exceed equipment ratings. The neutral conductor was carrying nearly the same amperages as the phase currents, and the phase loads were well balanced.

At this point, attention turned to the recent installation of a medium-voltage variable-frequency drive (VFD) and, as a result, harmonic currents became the "suspects" in this mysterious scenario. Before delving deeper into this situation, however, some background on harmonic currents is in order.

Today's automated systems have increased productivity significantly and lowered operational costs. However, the expanded use of VFDs has given rise to the occurrence and negative consequences of harmonic currents.

During the past 15 years, an explosive increase in the use of solid-state electronic technology in chemical processing plants has occurred, particularly through the growth in popularity of VFDs. Highly efficient VFDs offer power consumption advantages and increased productivity. VFDs convert 60-hertz alternating current to direct current through switching power supplies that contain rectifiers and often capacitors. The current then is converted back to alternating current with a different frequency.

Diodes located in the incoming power side of VFDs convert three-phase alternating current to direct current. The direct current then is switched by insulated gate bipolar transistors (IGBTs) to create a simulated alternating current.

Because VFDs draw current differently than non-solid-state (electronic) equipment, impedance plays a large role in this drama. Impedance is the total opposition to the flow of alternating current. It is the combined opposition of resistance, inductive reactance and capacitive reactance. Impedance is measured in ohms and is expressed by the symbol "Z." It can be shown in complex notation as Z = R +iX, where R is the ohmic resistance and X is the reactance.1

Prior to the introduction of VFDs, non-solid-state applications had a constant impedance drawing current in proportion to the sinusoidal voltage. Electronic equipment such as VFDs changes impedance by switching on and off near the peak of the voltage waveform. The frequencies are modulated to control the motor speed. Switching loads on and off causes quick nonsinusoidal current pulses.

Harmonics are the reflective currents sent back into the power distribution system by switching loads on and off during part of the waveform. Harmonics are described as sinusoidal waveforms operating at frequencies that are multiples of 60 hertz. They are expressed in orders. For example,

second-order harmonics equal 120 hertz (60 hertz x 2); third-order harmonics equal 180 hertz; fourth-order harmonics equal 240 hertz, and so on.

Electronic equipment such as VFDs, computers and copy machines contributes to harmonics. Most solid-state or electronic equipment produces harmonic currents. Just watch the light flicker in your home when you hit the print button on several models of laser printers. It is very important to note that although VFDs are only one contributor to harmonics, they tend to play a greater role in the hydrocarbon processing industry in both low-voltage and medium-voltage applications.

What are the effects of high-frequency harmonic currents? As higher-frequency harmonic currents flow through the power system, they can create problems such as:

Overheating of electrical distribution equipment such as cables, transformers and standby generators.

Overheating of rotating equipment such as electric motors by voltage drop.

High voltages and circulating currents caused by harmonic resonance.

Equipment malfunctions resulting from excessive voltage distortion.

Increased internal power losses in connected equipment. This results in component failure and shortened lifespan by increasing the eddy currents that flow in their laminated cores, through increased leakage currents across insulation, and by producing a skin effect in conductors. (A skin effect is caused when alternating current flows through a conductor, and an inductive action forces the current in the conductor toward its surface. The current density is greater at the surface than at the center, and under certain conditions practically no current flows along the axis of the conductor. The result is voltage drop and energy loss.)2

False motor overload trip.

Metering errors.