Dual-mode chokes teach old inverters new tricks: Page 2 of 3

April 01, 2019 //By Michael Freitag
Dual-mode chokes teach old inverters new tricks
Important changes in the way electricity is generated and used—such as increasing reliance on energy from renewable sources, the change to efficient variable-speed drives in industrial and domestic appliances, and adoption of hybrid or battery-electric vehicles—are increasing demands for electronic inverters that can be controlled to provide ac power at a desired voltage and frequency.

In any IGBT-based inverter for a wind or solar generator, noise signals can exist at frequencies up to 1 MHz or more. These and other sources of noise such as dc-dc converter switching elsewhere in the system, coupled onto the ac output power lines, can impair the output power quality and cause interference. This can affect the system’s own control signals, such as analog feedback signals, as well as nearby equipment.

To prevent such distortion and interference, standards such as IEEE 1547 and UL 1741, which apply to inverters for distributed-power systems like wind or solar generators, impose limits on the harmonic content allowed in the inverter output. Radiated electromagnetic interference (EMI) is also subject to limits imposed by standards such as FCC Part 15 B.


Mitigating switching noise

Fig. 2: The main functional blocks of a solar power-conditioning system, highlighting filtering requirements.

To comply with applicable specifications on noise and electromagnetic compatibility, filters placed throughout the system remove harmonics from voltage and current waveforms, correct the power factor by ensuring voltage and current waveforms are in phase, and minimize distortion. Figure 2 shows the locations of filters for attenuating noise in a solar-energy power-conditioning system. The filter at the output of the inverter is designed to remove switching frequency transients. It contains a combination of X and Y capacitors, inductors, and chokes to remove common-mode and differential-mode noise at the main harmonics of the switching frequency.

Chokes and capacitors at the inverter output attenuate common-mode (blue) and differential (red) noise.
Fig. 3: Chokes and capacitors at the inverter output attenuate
common-mode (blue) and differential (red) noise.

Figure 3 provides more detail on the composition of the filter. In principle, X capacitors and chokes remove differential-mode noise, while Y capacitors and common-mode inductors remove common-mode noise. Common-mode noise appears in the same direction on two conductors, whereas differential-mode noise appears in opposite directions on two conductors.

The common-mode choke coil as shown in figure 3 is a four-terminal device that comprises two conducting wires wrapped in opposite directions around a single magnetic core. Conventionally, this core consists of ferrite material. Since magnetic flux flows inside the core, common-mode choke coils act as inductors that provide high impedance against common-mode (noise) currents while allowing wanted differential currents to pass.

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