Assessing the performance of harmonic mitigation alternatives

Looking at the theory of operation for various harmonic mitigation techniques takes the guesswork out of harmonic reduction.

By John Streicher MTE Corporation August 1, 2014

Variable frequency drive users often have strict demands placed on them to mitigate harmonic distortion caused by nonlinear loads. Many methods are available including line reactors, harmonic traps, 12- or 18-pulse rectifiers, and low pass filters. Some of these solutions offer guaranteed results and have no adverse effect on the power system, while the performance of others is largely dependent on system conditions.

Certain techniques require extensive system analysis to prevent resonance problems and capacitor failures, while others can be applied with virtually no analysis whatsoever. In some cases harmonic mitigation technique decisions were based on a technical misunderstanding, lack of information, theoretical data, or on invalid assumptions.

Looking at the theory of operation for various harmonic mitigation techniques and their typical real-life performance takes the guesswork out of harmonic reduction by demonstrating the typical performance of various harmonic mitigation techniques and offering a quantitative analysis of various methods in real-life VFD operating conditions.

Source reactance

The magnitude of harmonic currents in a nonlinear load depends greatly on the total effective input reactance, which is composed of the source reactance plus added line reactance. Given a 6-pulse rectifier with dc bus capacitor, one can predict the resultant input current harmonic spectrum based on this input reactance. The lower the source reactance (the more stiff the power source), the higher the harmonic content will be.

Since power distribution transformers frequently have impedance ratings between 1.5% and 5.75%, one would expect that source impedance is often relatively high and that harmonics should therefore be quite low. However, transformer impedance ratings are based on transformer rated KVA, so when the transformer is partially loaded, the effective impedance of the transformer, relative to the actual load, is proportionately lower (i.e., 1.5% impedance at 30% load = 0.5% effective impedance).

Line reactors

Use of ac line reactors is a common and economical means of increasing the source impedance relative to an individual load. Line reactors are connected in series with the 6-pulse rectifier diodes at the input to the VFD.

Typical harmonic performance of reactors

The typical total harmonic current distortion (THID) spectrum data for a 6-pulse VFD load fed by a power supply with an effective source reactance of 3%, 5%, and 8% appears as follows:

This data represents the harmonics measured at the input to the 6-pulse rectifier and will reduce to lower percentages when measured further upstream, provided there are other linear loads operating on the system. If 20% of the system load is composed of VFDs with 5% input impedance, and 80% linear loads, the harmonic current distortion at the VFD input will be 35% THID, but only 7% at the supply transformer secondary.

Typically costing less than 3% of the motor drive system, line reactors are the most economical means of reducing harmonics. Practical ratings can achieve 29% to 44% THID at the input to the 6-pulse rectifier (usually lower THID at the transformer secondary), at full load operation. Their typical watts losses are less than 1% of the load. A reactor is particularly effective where no dc link choke is present.

The harmonic mitigation performance of reactors varies with load because their effective impedance reduces proportionately as the current through them is decreased. At full load, a 5% effective impedance reactor achieves harmonic distortion of 35% THID; however, at 60% load its effective impedance is only 3% {0.6 x 5% = 3%}, and harmonics will be 44% THID. Although THID increased as a percentage, the total rms magnitude of harmonic current actually decreased by nearly 25% {1 – ((.6 x 44%) / 35%) = 24.5%}.

Since voltage distortion at the transformer secondary is dependent upon the magnitude and frequency of current harmonics that cause harmonic voltage drops across the transformer’s internal reactance, the voltage distortion (THVD), at the transformer secondary, actually decreases as this load is reduced.

Tuned harmonic trap filters

Harmonic trap performance

Tuned harmonic filters (traps) involve the series connection of an inductance and capacitance to form a low impedance path for a specific (tuned) harmonic frequency. The filter is connected in parallel (shunt) with the power system to divert the tuned frequency currents away from the power source. Unlike line reactors, harmonic traps do not attenuate all harmonic frequencies. Most often they are tuned for 5th harmonic or 300 Hz.

If applied to a low impedance power source, the harmonic mitigation performance of this filter is quite limited and the benefit of this filter may be unrecognizable. To improve the performance of a trap filter, a 5% impedance line reactor may be connected in series with the input to the filter. If the VFD has internal line reactance, harmonic trap performance may improve slightly. The typical residual THID for a 6-pulse rectifier with a tuned 5th harmonic trap is between 20% to 30% at full load, provided there is significant source impedance.

The watts loss of this type of filter can be 2% to 3% of the load, and it can cost ten times the price of a line reactor. Tuned harmonic traps can alter the natural resonant frequency of the power system and may cause system resonance. They may attract harmonics from other nonlinear loads sharing the same power source and must be increased in capacity to accommodate the addition of new loads. For best results, a power system study should be performed to determine the magnitude of harmonics to be filtered (from all loads), the power system resonant frequency, and the impact of future addition of loads.

Harmonic traps at light load conditions

Harmonic trap filters traditionally achieve their best attenuation of harmonics at full load conditions. However, advancements in filter technology allow some filter designs to adapt to varying load by changing impedance with the load. This allows the adaptive filters to perform well even at lightly loaded conditions.

12-pulse rectifiers

Theory of performance

The 12-pulse rectifier configurations have been used for lowering harmonic levels. The theoretical benefits of 12-pulse rectification include cancellation of 5th and 7th harmonic elements. However, real-life harmonic mitigation resulting from the use of 12-pulse rectifiers can be quite different than the theoretical expectations.

The most common method of 12-pulse rectification involves the parallel connection of two bridge rectifiers, each fed by a 30-degree phase shifted transformer winding. Often the transformer has a single primary winding and dual secondary windings. One secondary winding is a delta and the other is connected in wye configuration to achieve 30 degrees of phase shift between secondary voltages.

One of the major design goals in multi-pulse operation is to get the converter semiconductor devices to share current equally. If this is achieved, then maximum power and minimum harmonic currents can be obtained. To achieve cancellation of harmonics, the two individual bridge rectifiers must share current equally. This can only be achieved if the output voltages of both transformer secondary windings are exactly equal.

12-pulse rectifier drawbacks

Because of differences in the transformer secondary impedances and open circuit output voltages, this can be practically accomplished for a given load (typically rated load) but not over a range in loads. The performance of 12-pulse systems does not hold up well under a line imbalance. Typical losses of a 12-pulse transformer are 3% to 5% of the transformer KVA rating. Note that the extra diodes and transformer windings will add significant cost to the system.

18-pulse rectifiers
Theory of operation

The 18-pulse configurations use a very specialized transformer with three sets of 3-phase outputs that are phase shifted by 20 degrees seach to supply three sets of full wave bridge rectifiers. Theoretically, this configuration cancels the 5th, 7th, 11th, and 13th harmonics. It may be quite optimistic to expect the nine supply voltages, feeding three bridge rectifiers, to be balanced under all operating conditions.

Maintaining equal dc current through three bridges is more difficult than with 12-pulse systems simply because the number of variables increases by 50%. As with 12-pulse systems, the 18-pulse rectifier’s ability to reduce harmonic currents is best when operating at full load conditions and when all of the nine voltages are equal.

Performance at full load with balanced line voltages

In a laboratory exercise it is possible to control the three line voltages that supply the 18-pulse transformer primary winding; however, in real-life applications this may be quite difficult to achieve. Even when the primary voltages are balanced, maximum attenuation of harmonics with 18-pulse rectifiers requires that all nine secondary voltages be balanced.

This allows dc current to be shared equally by each of the three bridge rectifiers, provided the semiconductor and circuit resistances are identical for all phases. Due to the large number of variables, the likelihood of achieving theoretical harmonic performance is rather poor; nonetheless, an acceptable level of harmonic reduction is quite possible.

The 18-pulse rectifiers also experience diminishing performance when line voltages are not balanced, and when operating at less than full load. An 18-pulse drive may offer guaranteed harmonic distortion levels, but typically only at full load and full speed conditions, with voltages that are balanced within 1%. The effect of unbalanced voltages is that as the load is decreased, the magnitude harmonic distortion increases significantly. While THID at full load may be fairly low, at 40% load, harmonic current distortion can be over 20% THID, when the line voltages were only 1% unbalanced. With 3% imbalance the harmonic current distortion increases to over 40% THID. To enhance the performance of 18-pulse drives, line reactors can be added in series with the individual bridge rectifiers.

Electrical system reliability and normal life expectancy of electrical equipment rely heavily upon a clean and reliable power supply. Those wishing to maximize productivity through utilization of clean power technologies have several harmonic mitigation techniques available.

Reactors are low cost and provide a significant reduction in harmonics for drives with no link choke or other harmonic mitigation employed. Reactors provide the biggest reduction in harmonics for the lowest cost and can reduce the harmonic current content from 100% to about 30% THID but will not go much farther.

Traditionally thought of as the bulletproof solution, multi-pulse converters provide good harmonic performance of about 4% to 6% THID under a controlled range of conditions for imbalance and loading. However, multi-pulse solutions are often very costly and take up significant real estate. They are also generally the least efficient, adding as much as 1.5% to 2% losses to the system.

Passive filters are readily available and can be fitted to standard 6-pulse drives yielding harmonic mitigation levels of less than 5% THID. Some newer passive filter designs now employ technology that allows the filters to perform well under imbalanced conditions and at much lighter loads than multi-pulse solutions. The passive filter will have lower power losses and typically be much smaller and lower cost than the multi-pulse.

One should consider the merits of each technique regarding cost, power loss, and harmonic distortion effectiveness, and carefully weigh the pros and cons of each before making a choice.

John Streicher is an application engineering manager for MTE Corporation.