2.8. Harmonic Neutralization
Magna-Power’s Harmonic Neutralizers eliminate families of harmonic components by multiplying the number of input phases with specially wound magnetic components, reducing the total harmonic distortion (THD) drawn from the AC mains. In combination with equally loaded, high-power DC power supplies, these magnetic components offer a cost-effective way to hold power quality at acceptable levels, extending Magna-Power’s high-frequency switch-mode supplies into the multi-megawatts.
A 12-pulse Harmonic Neutralizer is embedded in all 250 kW MT Series models, and its installation is transparent to the end user. For applications demanding a better THD level than a 12-pulse waveform provides, external 24-pulse and 48-pulse Harmonic Neutralizers are available.
2.8.1. Why Neutralize Harmonics?
Input current harmonics are a by-product of nearly all power supplies. Power can only be delivered to the load if the frequency and phase of the voltage and current match. For a three-phase supply using a three-phase input rectifier, the input current has a theoretical spectrum of 6n±1, where n is an integer incrementing from 1; this is known as a 6-pulse waveform. Such a supply draws input currents at 1, 5, 7, 11, 13, 17, 19 and higher multiples of the fundamental frequency, with theoretical magnitude decaying as the reciprocal of the harmonic number. The 5th and 7th harmonic components have magnitudes of 20% and 14% of the fundamental component, respectively.
Harmonic currents in power systems can find unusual paths and can cause problems where the magnitude is significant and other loads are sensitive to harmonic frequencies. Lighting ballasts, for example, have series connected capacitors and inductors that can be excited by harmonic currents. IEEE 519 defines recommended limits, but implementing the standard requires knowledge of the power system and of the other loads producing harmonics, so the same power supply may exceed the limits in one installation and not in another. In the same respect, a supply may or may not cause a harmonic-related problem with or without meeting IEEE 519. The most dependable approach is to eliminate the harmonic current at its source.
2.8.2. Understanding AC Harmonic Waveforms
Standard Magna-Power models from 1.5 kW through 150 kW produce 6-pulse waveforms, while 250 kW models produce 12-pulse waveforms. Harmonic Neutralizers suppress families of harmonics by increasing the number of power phases, and can be used when multiple power supplies are connected in series or parallel and are equally loaded.
Neutralizers produce 12-pulse, 18-pulse, 24-pulse, or 48-pulse waveforms, which carry harmonic current components on the order of 12n±1, 18n±1, 24n±1, or 48n±1 respectively. As the pulse number rises, the lowest-order harmonic present rises with it and the input current waveform approaches a sinusoid:
2.8.3. Embedded 12-Pulse Neutralization
All 250 kW MT Series models include a 12-pulse Harmonic Neutralizer within the power processing hardware, as shown in Fig. 2.9. The neutralizer cancels the 5th and 7th harmonic families, leaving the 11th and 13th as the lowest-order input harmonic currents emanating from the power supply. No installation, configuration, or maintenance is required of the end user, and no additional floor space is needed beyond the cabinet dimensions given in Product Diagrams.
150 kW MT Series models produce a 6-pulse input current waveform. Where a 150 kW installation requires better THD performance, an external Harmonic Neutralizer may be applied.
2.8.4. External Harmonic Neutralizers
External Harmonic Neutralizers are floor-standing cabinets installed upstream of the power supplies they feed. They are designed and manufactured by Magna-Power exclusively for its MT Series products, and are protected with appropriately sized primary-side circuit breakers.
An external neutralizer requires that the supplies it feeds be equally loaded. Harmonic cancellation depends on the phase-displaced windings carrying balanced current; if one supply in the group is operated at a substantially different output than the others, cancellation degrades and the input spectrum moves back toward that of the underlying 6-pulse or 12-pulse arrangement.
Note
Select the neutralizer model whose AC input voltage and frequency match the installation and the power supplies being fed. Contact your Magna-Power sales partner to confirm the neutralizer configuration for a given system, including installations that combine multiple neutralizers.