Understanding Solid State Frequency Converters

If solid state is determined to be the solution to your needs, then the rectifier design must be considered.

The current requirements for SSFC units range from tens of amps to several hundred amps. The percent of distortion is thus very large at low current levels and improves as the amount of current rises. This is why most manufacturers only give the amount of harmonic distortion at full load. Typical levels of THID for six pulse rectifiers are listed in Figure 3.

The % THID is representative of systems which do not have any means of filtering included in their input circuit. Filtering will lower these values at the expense of overall system efficiencies.

These THID values are high at lower loads, because the SCRs are turned on later in the waveform. The later in the waveform the SCR is turned on, the greater the harmonic distortion that is reflected back into the utility grid.

The IGBT design produces much lower harmonic distortion (<5%) compared to the SCR design on both the input and output waveforms. Therefore, whenever efficiencies and unit losses are measured, the total system losses should be measured at each load point. This will provide a better measurement of the energy requirements for the system and not just the individual piece of equipment, especially when that piece of equipment generates large losses in the upstream cables and transformers of the system.

Input power factor
A power factor of one or “unity power factor” is the goal of any electric utility company since if the power factor is less than one, they have to supply more current to the user for a given amount of power use. In so doing, they incur more line losses. They also must have larger capacity equipment in place than would otherwise be necessary. As a result, a facility will be charged a penalty if its power factor is much different from one.

SCR rectifiers present a variable power factor to the utility. Typically, the lower the load is, the lower the power factor that is presented to the utility and the more power the utility must supply. Generally, at a 30- percent load, the power factor is 0.6 or less. As the load approaches full load the power factor will approach 0.8. Factors that affect these values are input voltage (the higher the voltage the lower the power factor) and the construction of the output inverter.

IGBT rectifiers present a 1.0 pf to the utility independent of load and input voltage. This means that there is no phase shift between voltage and current and thus no reactive power is required from the utility. This means that there are no additional losses at the utility power distribution center and the utility feeder cables. The required current is as low as physically possible. This results in lower operating costs for the units. This will increase the system efficiency (minimize the losses of the input power feeder run) and will not create any problems with any other loads that use the same feeder subsystem.

Commutating gaps (High spike voltages)
Due to the higher switching frequencies of the IGBTs, there are no switching/commutating notches (see the blue trace below and the oscilloscope screen capture) of the input voltage. The SCR design requires a short time when two phases are both on. This causes a momentary short circuit which tends to collapse the voltage waveform which causes the notch. This “noise” is sent back to any equipment that is being powered by the same utility source. Without proper filtering, this “noise” can damage electronic circuits and shorten the life of the equipment. Because the commutating gap also distorts the incoming power waveform, it requires the utility to provide more energy to compensate for the line losses. This results in higher operating costs for these SCR units.

No break power transfer capability
Most of the newer aircraft, especially the wide body aircraft, require a so-called no break power transfer. During this transition phase, the SSFC and the internal auxiliary power unit (APU) of the aircraft are working in parallel to prevent any interruption of the power while switching over from internal power to external power. Since there is no precise synchronization and no load sharing /load transition regulator in place, the APU could feed some power back into the SSFC (reverse power).

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