Combating unseen faults with Harmonics Mitigation

For manufacturing facilities, the real cost of harmonic damage is almost unmeasurable because of its nature.  Some sites have found they have equipment life expectancies of 25 per cent of what is considered normal.  
That is to say a main supply contactor design with a 30 year life, given the installation and load, now requires replacement every 7-8 years.  For other sites the real cost is in the unscheduled maintenance and loss of production, deadline reliability and, in some cases, production quality. 
Replacing equipment because of damage caused by harmonics can increase capital expenses (CAPEX) by as much as 15 per cent and all that additional heat loss and production loss add to operational expenditure (OPEX) by as much as 10 per cent. As a result, Australian companies are increasingly turning to harmonics mitigation strategies to combat the effects of these minor, yet crucial, faults.

Harmonics explained
Harmonics are unwanted currents that create excessive heat and even fire in otherwise lightly loaded wiring and transformers in manufacturing facilities.  For example, a transformer designed for 100amps suppling only 20amps of 50Hz + 20amps of high frequency harmonics can overheat yet appear to be underloaded.  If the transformer is already loaded to the extent it is approaching capacity, the results can be disastrous.
Harmonics are created by non-linear loads which are common in industrial sites and often comprise of equipment such as welding machines, arc and induction furnaces, variable speed drives for AC or DC motors, battery chargers, uninterruptible power supplies and more recently some lighting systems. 
Non-linear loads deviate from regular, or sinusoidal, waveforms and therefore create harmonic current through the distribution system. Voltage distortion is created from this, causing unseen damage to equipment.
While they are often seen to be a small fault, harmonics can significantly weaken the reliability and shorten the life expectancy of equipment. An increase of 10°C of the operating temperature of a motor, for example, can result in a lifetime reduction of up to 50 per cent.
Malfunctions in equipment are possible with a harmonic distortion factor of only five per cent of the voltage. Malfunctions become probable as low as 8 per cent. These minor faults have momentous long-term effects on OPEX including overloading of the electrical system, increased power demand, loss of systems, increased outages and shorter equipment lifetime, oversizing of components and even plant shutdown. 
For this reason, many Australian companies are beginning to use a broad range of solutions for harmonic mitigation.  Four of the most common solutions are input reactance, multi-pulse supply transformer, band-pass filters and finally active injection filters.
Each technology has different ways to offset the unseen damages harmonics cause every day and the best choice is dependent on the nature of the load as well as the power demand of connected equipment. Manufacturers must understand how each of the solutions work and interact to ensure they select the most appropriate combination.

AC line reactance and DC link reactance (chokes) for drives
Both AC line reactors and DC link chokes help to smooth out the flow of current to Variable Speed Drives, such as the Altivar range, and thereby reduce the level of harmonics on the network. AC line reactors are placed in series with the incoming AC power line while DC link chokes are connected after the input diodes in the power circuit. 
These devices are used to reduce the current peaks in a circuit. Without a choke or reactor in place, the inverter produces high current peaks. When the choke or reactor (or both) are added, the current draw is expanded and the amplitude is reduced. This helps to partially mitigate the level of harmonics.
When high quantities of drives are present within an installation, the use of AC line reactors or DC link chokes is recommended for each individual drive as implementing these devices increases the lifetime of the drives. 
The three per cent AC line reactor will protect the drives diodes against network voltage transients and prevent excessive input voltage drop from the drive which can cause nuisance fault trips on network protection devices. 
The DC link choke provides three per cent impedance also but with no voltage dip protection due to diode overlap conduction. However by incorporating both AC line reactors and DC link chokes six per cent impedance is achieved. While six per cent might not seem like a large figure, it can substantially reduce OPEX due to reduced equipment damage over an extended time. These extra funds can then be used as capital in other investment options.

Multi-pulse Transformer Arrangement
Particularly cost effective in new installations or major upgrades of drive systems over 400kW, the multi-pulse system consists of a transformer with more than one secondary winding matched to multiple 6 pulse rectifiers on the VFD.
In order to make a simple 12-pulse option work correctly, a 30° phase shift transformer must be included. In its simplest form the MV primary is usually the site standard of 3.3, 6.6 or 11kV while the two secondary windings are each 415v (or 690v) with one star and one in delta therefore offering 30° phase shift to power two sets of rectifiers, one from the standard part of the transformer and the other from the 30° phase shift part. Additional gains can be made for industrial users by utilising 18-pulse, 24-pulse or even higher with great effect. This limits the cumulative damage caused by of harmonic emission and usually no further mitigation is necessary. Multi-pulse solutions are also the most efficient for reduction of power loss.

Band Pass or Passive Filters
A passive filter consists of a number of separate internal circuits, each tuned to remove a specific harmonic or group of harmonics from the supply network caused by VFD. Unlike line reactance which offer a percentage drop, passive filters offer a known result to the network of typically 5, 10 or 15 per cent THDi compliance at the input.
There are some downsides to the technology though – passive filters are not efficient at partial loads. While expensive compared to the simple choke, they provide a simple way for manufacturers to mitigate the unnoticed effects of harmonics that create costly voltage peaks and drops while improving the network power factor.

Active Filters
Active filters measure and negate the harmonic currents in a different way. Active filters measure harmonics and other network disturbances on a real-time basis and produce a harmonic current spectrum in direct opposition to the original. 
This has the effect of cancelling the harmful harmonic currents. In other words, the active filter can be seen as a generator of harmonics but it produces the opposite harmonics of the measured distortions, much like a set of ‘noise cancelling’ headphones.
Active filters are available in different supply voltages (three-phase with and without neutral) and can be used for filtering networks.  Placement of monitoring CTs and selection of the injection point relative to other equipment is critical. 
The two clashing spectrums cancel each other out which brings the harmonics level to near zero. A 300 amp unit will inject 300amps of inverted current into the system but adds less than 50 amps to the total network load as the energy injected is energy saved.  Typically, on large networks, several filters are paralleled to offer thousands of amps of mitigation to cover numerous network problems including harmonics.

The need for harmonics mitigation
Manufacturing facilities are active places where a lot of large machinery is operating in a very active and visible way. With such loud and dynamic equipment it can be easy to identify when a big error occurs and, as such, a lot of care is devoted to avoiding big faults that can cause shutdown and effect OPEX/CAPEX. 
In the case of harmonics however, the smallest, least obvious faults can affect a plant’s bottom-line in ways often unseen and unnoticed. Harmonics cannot be ignored and a robust harmonics mitigation strategy is necessary to cut operating costs and redirect CAPEX. Input reactance, multi-pulse supply transformer, band-pass filters and active injection filters are just some of the solutions that can help manufactures achieve this. Companies need to understand which of these is best for their equipment and plant overall. Those that ignore the small, yet crucial, elements of equipment maintenance risk squandering the savings from effective OPEX – savings that could otherwise be used in vital CAPEX initiatives.



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