Overview

The WattNode® meter is an AC electric power meter and cannot measure purely DC circuits. However, there are cases where DC current is present in AC circuits, most commonly due to half-wave rectified loads as shown in the figure below.

Positive-Half-Cycles.png

We have seen these waveforms from flash electric hot water heaters, laser printers, heat guns, and the like, where half-wave rectification is used as a simple method to reduce the power to a heating element.

Concerns

Measuring DC Current

The first concern is that standard magnetic current transformers like ours cannot measure DC current. Hall-effect CTs are available that can measure DC current, but they are very expensive and require a separate power supply. However, this is not a serious concern because of the following principles of power measurement:

  • Normally power is computed by multiplying voltage and current samples and adding the products. However, it can be equivalently computed by converting to the frequency domain with a fast Fourier transform (FFT), and then multiplying each voltage and current component (frequency bin) together and summing the products.
  • If any particular frequency bin (harmonic or DC) has a zero or very low value for the voltage, then the real power and energy associated with that frequency will be zero or very low.
  • So if there is no DC in the voltage waveform, there is no DC power being consumed, regardless of DC in the current waveform. There would be apparent power associated with the DC current.

The inability of standard magnetic CTs to measure DC current does not directly cause errors in the real power or energy. It does mean the WattNode meter can only report AC current, so it will under-report the actual current flowing in the conductor, as will most current clamps. Also, the reported apparent power will be low because of the unmeasured DC current.

Current Transformer Core Saturation

There is a more serious concern with CT magnetic core saturation. In the presence of a DC current, the CT core will acquire a DC magnetization. If this is large enough, the core will start to saturate, resulting in the following:

  • Increased phase angle error
  • Low reported current
  • Distorted current waveform

These effects can be mild for mild saturation, or very severe, sometimes resulting in measured power errors of 25% or higher.

Solid-core or toroid type current transformers are highly susceptible to this problem. Split-core or opening CTs perform much better in the presence of DC current, because the gaps in the core help prevent saturation.

Conclusion

DC current and half-wave rectified currents are a rare occurrence, especially at currents high enough to cause core saturation, but if you know or suspect you are monitoring a load with DC current, especially over 10 amps, contact CCS for assistance selecting current transformers.

See Also