Handling Harmonics

Active harmonic filters (AHF) monitor and correct harmonics, eliminating disturbances before they can cause problems.

The metalworking industry is a very capital-intensive and globalized market. The keys to success are productivity improvement with the highest operational performance, combined with the highest possible energy efficiency and lowest impact on the environment.

Because most of the power supplied to end users comes from a utility source, the majority of power problems occurring at the factory level are really derived from within the operation of the facility itself.

Harmonics commonly are present within a manufacturing facility’s power system, which can damage sensitive electronic components. Harmonic filtering, which can be accomplished actively or through capacitors tuned to a specific order, can eliminate the problem, enhance efficiency, and improve overall plant performance.

When machinery and equipment are electronically controlled and need to meet exacting production standards, sensitive electronic components need to be protected from harmonics driven by the facility’s electrical power distribution system. The use of active harmonic filters can achieve the reliable functioning of the machinery and ensure the process quality, while helping to support the financial bottom line.

Harmonics Explained

In general, harmonic currents are the result of the nonlinear behavior of electrical devices

The sources of harmonic currents, and thus harmonic voltage in power systems, are multiple and vary in size from a few kilovolt-amps (kVA) up to several megavolt-amps (MVA). Typically, devices with magnetic iron cores, like transformers and generators, have been a key area of harmonic concern, as are arc furnaces and arc welders.

With the demand for energy efficiency of power electronic equipment, such as uninterruptible power supply (UPS) and variable-frequency drives, mitigation or reduction of harmonics continues to be a priority for many industrial customers, including those in metalworking and fabricating facilities.

Frequency inverters are among the most widely used equipment for AC motor control. Such components are found in virtually every area of industry, in applications as diverse as machine tools, conveyors, pumps, and even HVAC equipment. In the quest for ultracompact, efficient power conversion, inverter manufacturers employ high-speed semiconductor switching and pulse width modulation (PWM) techniques, which can create harmonic problems.

Harmonics and Arcs

Arc furnaces and welding power sources, including welding robots, consume a lot of power. This high power, combined with a highly nonlinear voltage current, produces substantial amounts of harmonic distortion. From a technical standpoint, arc furnaces operate in different phases (melting, air refining, refining) with different levels of harmonics. Additionally, the equipment has a combination of ignition delays and rapid voltage changes caused by random variations of the arc. This leads to an unusual harmonic spectrum with even and odd multiples of the fundamental frequency.

Harmonics and CNC Machines

The electronic components within a CNC machine are particularly sensitive to electrical imperfections in the power distribution system. Problems here can include malfunction of the equipment or program, along with damage of the parts and material. This can lead to missed deliveries and potential quality issues, unsatisfied customers, and financial problems for a company.

Harmonics can wreak havoc on the electrical power network, causing failure of PLC circuit boards, machine tools, and robotic systems. They also can trip circuit breakers, blow fuses, overheat motors and transformers, cause insulation to break down, and reduce the service life of equipment.

Furthermore, production downtime/ restart time and shipment (revenue) loss, along with repair costs, may result in reduced company profits.

A harmonic site survey using meters and analyzers or a full engineering study may be necessary to completely determine existing harmonics and other power quality issues in a facility. In many cases, IEEE 519-2014, Recommended Practice and Requirements for Harmonic Control in Electric Power Systems is followed for the acceptable level of distortion. A complete review of the existing system, new or planned system, and retrofitted equipment, along with any plant expansions, should be considered.

Active Harmonic Filtering

Active harmonic filters (AHF) are power quality devices that permanently monitor the nonlinear load and dynamically provide precisely controlled current, helping to prevent distortion in a power network.

This current has the same amplitude of the harmonic current but is injected in the opposite phase-shift, which cancels out the harmonic currents in the electrical system. As a result, the current supplied by the power source remains sinusoidal because the harmonics will negate each other, and the harmonic distortion is reduced to less than 5 percent THDi (total harmonic distortion of current), meeting all standards.

In addition, the AHF power electronics platform is designed to operate at levels that continuously adapt to rapid load variations. With load conditions creating harmonics up to the 50th order, active filters operate in a wide frequency range, adapting their operation to the resultant harmonic spectrum.

Active harmonic filters also can correct poor displacement power factor by compensating the system’s reactive current.

The filter also balances the loads of the phases. Currently, these devices are equipped with insulated gate bipolar transistors (IGBT) and digital signal processing (DSP) components. Generally, active harmonic filters can be installed at any point in a lowvoltage AC network (parallel device), and they usually offer much more functionality than their passive filter counterparts.

Combining these features with a small physical size and efficient operation, active harmonic filters are suitable for numerous applications. They can be provided for three-wire or four-wire connections (three-wire is the most common in North America).

Current transformers deliver a signal to the filter, which can be applied to either the line or load side of the power network.

Active harmonics filters can be applied to a single or a group of nonlinear loads. Possible AHF installations include power factor correction in harmonic-rich environments in which filtering cannot be suitably achieved by the use of capacitors; where both power factor and harmonic correction are required; and where emergency power or distributed generation are present in the electrical network.

They also are available in several ratings. This includes individual units for 50, 100, 200, 250, and 300 amps. Different configurations such as open type, various National Electrical Manufacturers Association (NEMA) enclosure protection ratings, and the ability to parallel multiple active filters for higher-current applications is typical.

Installation voltages are mostly 480 VAC and 600 VAC, but 600 VAC typically requires a step-down (600/480 VAC) transformer with the active harmonic filter. A transformer is not necessary for a purpose-built active filter rated for 600/690 VAC.

Active filters include keypad controls and operator display, communications such as RS485 and TCP/IP Ethernet, and software for communications and monitoring through a Windows-based product.

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