User:OregonEEretired/sandbox

DC systems do not have displacement VARS, but in non-liner circuits and particularly some DC:DC converters and single phase DC:AC inverters there can be considerable AC ripple in the current even when the source is at a steady DC voltage. DC:AC inverters typically have little or no energy storage or low frequency passive filters on the DC side. The result is that the current waveform must follow the power transfer and when the AC output current goes to zero the DC input current also goes to zero. The peak to peak ripple current then is twice the DC average current and the frequency is twice the frequency of the output AC. Higher or lower P-P values have been observed, but this value is representative in cases where the load on the AC side is resistive. Studies may be needed to characterize the effects of displacement power factor or distortion power factor on the AC side and how it may be passed to the DC side currents feeding to inverters.

The effect of the ripple current superimposed on the DC is to increase the heating of elements in the DC circuit such as the wiring, the circuit protection, switches and batteries. Typical instruments, such as the DC ammeters ignore the ripple current and understate the heating that can occur. AC ammeters may read the ripple current accurately, but some do not if the DC current causes partial saturation of a current transformer used for the measurement. When a good AC measurement is obtained, the effective RMS value of the composite current will be found to be the square root of the sum of the squares of the DC measured current and AC measured current. For the typical example the result is 120% of the DC measured current. This value should be taken into account for wire and circuit protection selection.

The math describing this phenomenon closely parallels the discussions given for non-linear loads in AC systems. In this case true power transfer is the product of the DC component of voltage and the DC component of current. Where ripple current components interact with the DC source, no average power is delivered. If ripple appears on the DC voltage the product of those ripple components times the DC component of current does not result in average power transfer either.

In some ways this is a more dangerous issue than power factor in AC systems where the instrumentation shows the full effective current. Because the ripple on DC systems is not as readily measured, wires, fuses and switches are frequently under sized. Regulatory and standards agencies should be providing guidance for proper installation.