Understanding Solid State Frequency Converters

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

Because SCR rectifiers cannot take in sinusoidal currents, they will distort the utility input current. The negative impacts of non-linear loads on the quality of the electrical energy supply are called system perturbations or more generally electrical pollution. This includes harmonics, flicker, transient faults, asymmetric and voltage outages. For units that are equipped with controlled or uncontrolled rectifiers and capacitors in the DC-Link, the input current waveform is far from sinusoidal. They create high harmonic distortions in the input current waveform. They lower the input power factor taking more reactive current and they induce commutation spikes into the utility power supply.

Any device that draws a pulse of current from the electrical network for less than the entire voltage wave generates harmonics. Harmonics are simply a mathematical representation of these distorted waveforms that allow us to model electrical network response at multiple frequencies, and better understand and predict how the electrical network will react to this high-frequency content — or “electrical pollution.” The worldwide standard IEEE 519-1992 requires that end users limit harmonic levels to ensure network stability for all users. It outlines acceptable levels of harmonic distortion (both voltage and current) and the point of common coupling (PCC) with the utility.

There are two alternative designs to reduce the above effects of the simple “6-pulse” rectifier. The first is to use more SCRs in the rectifier which has led to “12-pulse” and “24-pulse” designs. The second is to use IGBTs in the rectifiers as well as the inverters. IGBTs have several advantages over SCRs. They do not have to be turned on and off at the zero crossing point; they do not change the input power factor or induce commutation spikes into the input power source. They also allow better control of the entire power and frequency conversion process. For these reasons, IGBTs are now being used in high end SSFCs just as they are used in state of the art solid state uninterruptible power supplies (UPS).

To better understand the above points, let’s look at how each of these devices is used in the typical SSFC. A low end SSFC will have a pair of SCRs for each phase of the power source “6-pulse.” One SCR is turned on for the positive portion of the AC waveform and turned off at the next zero crossing. While the opposing SCR of the pair is turned on for the negative portion of the AC waveform and off at the next zero crossing. This on/off pattern is used to transmit full power to the output load.

A 12-pulse SCR system follows the same basic pattern but it splits the utility power into two 6-pulse rectifiers. The second 6-pulse rectifier is typically fed through a 30-degree phase shifting transformer. This provides a smoother DC current which puts less stress on the DC capacitors and shifts the input current harmonic distortion to higher frequencies with less energy content, but the design requires more components.

For both rectifier types (including versions with diodes instead of SCRs) the energy will follow one direction only, which means the energy is only fed to the load. There is no way to absorb energy from the load and transmit it back to the utility.

IGBTs are also used in pairs for each phase of the power source. Instead of turning on and off at the zero crossing point, IGBTs are fully controlled using a pulse width modulation (PWM) control algorithm.

PWM systems typically operate at much higher switching frequencies (6 – 8 kHz instead of 120 Hz for SCRs) which eliminates most of the audible noise. The higher switching frequency allows much finer control of the power conversion process. It also does not produce the audible noise that is often found in low-end systems. The width of each pulse determines the amount of power that is converted to DC in the rectifier. But, because there are thousands of pulses in each cycle the energy being transferred per pulse is small which gives the IGBT system significant advantages over an SCR system.

Since the transistor can conduct the current in both directions, it is possible to feed some power back to the utility in case the load creates some reverse power. This is another significant advantage over an SCR system (simple no break power transfer capability).

Electrical Noise
Whenever loads draw current in a non-linear manner, such as that experienced with rectifier based equipment, harmonic distortion is experienced. Harmonic current generates heat in all of the current carrying components of the electrical distribution system. The system includes not only the converters but also the upstream switchgear, breakers, fuses, cabling, capacitors, bus ducts, bus bars and transformers. Based on the higher frequency, harmonic current generates more heat on a per-Amp basis than current at the fundamental frequency (50 or 60 Hz).

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