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 of the current drawn from the AC mains (THD-I). 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 neutralizer is a passive, phase-shifting autotransformer designed and wound by Magna-Power exclusively for its own products. Where one is embedded in the power supply, its operation is transparent to the end user: it requires no installation, configuration, or maintenance, and it does not alter output regulation.
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
Harmonic Neutralizers suppress families of harmonics by increasing the number of power phases. They 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:
Harmonic cancellation depends on the phase-displaced windings carrying balanced current. Where a single neutralizer feeds more than one power supply, those supplies must be equally loaded. 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.
2.8.3. Harmonic Neutralization in the ML Series
Every ML Series model includes a Harmonic Neutralizer. It is the first element the incoming three-phase mains meets, and it is where the user AC connection and the system circuit breaker are located. The pulse number is set by the power level:
In both cases the neutralizer’s phase-shifted outputs are distributed to the input sections of the individual 250 kW power modules, and the input power factor is greater than 0.96 at maximum power. For a 24-pulse system the lowest-order input harmonic present is the 23rd; for a 12-pulse system it is the 11th. Refer to Lowest-order input current harmonics by pulse number above for the full harmonic spectrum by pulse number.
Note
When modelling the ML Series as part of a wider power system study, represent the supply as a 12-pulse rectifier for 500 kW models and a 24-pulse rectifier for 1000 kW models. The THD-I figures above are at full rated output. THD-I rises at partial load, and it also depends on source impedance and on the other loads present, so treat the values as typical rather than guaranteed.
The neutralizer operates upstream of the module input protection and control circuitry. Because all of the power modules within a single ML Series system are driven from common feedback loops and are therefore equally loaded, the balanced-loading requirement described above is satisfied by the product itself and requires nothing of the user.
Note
1000 kW models are shipped as two cabinets that must be joined at installation. The neutralizer is not an optional accessory and is not interchangeable between power levels. See Harmonic Neutralizer Interconnection for the interconnection procedure.