Impressed Current Cathodic Protection in Presence of an AC Interference


A brief and generic introduction to rectifiers taken straight from the book.

Cathodic protection (CP) rectifiers have the following major components. These typically include a transformer to step down AC line voltage to low voltage AC on the secondary with a tap arrangement to permit selecting a range of voltage, a rectifying element (usually full wave silicon diodes for rectification), and a housing for outdoor mounting. These components are supplemented by an AC circuit breaker and DC output meters. Both single-phase and three-phase units are in common use. Figure shown below illustrates diagrammatically single-phase and three-phase units of the full-wave bridge-connected type.

A pipe under CP an image for your reference.

Starting the project with the simulation model for the Impressed Current Cathodic Protection with Presence of an AC Interference

The diode bridge converts AC voltage into a rectified DC output.

Shown below is the simulation model of a pipe under the Impressed Current Cathodic Protection with a 15V AC interference. What I am trying to do here is to represent a pipe with CP protection as a simulation model. If I come up with an accurate simulation model for the SOIL and PIPE I can change rectifier and AC interference electrical parameters to see what effect they cause. The model was created and simulated with the MULTISIM BLUE simulation software. Download the CP Similation File Here. The download file is the actual simulation file where you can change simulation parameters and add/remove circuit components.
The circuit inside the yellow box represents the rectifier. The rectifier is represented as an 10VAC sinusoidal voltage source connected to a diode bridge. The circuit inside the blue box represents the AC interference. The AC interference is simulated as an AC voltage source. Outputs of the rectifier and the AC interference are connected together inside the SOIL box, shown in grey on the image below. The soil is simulated as a series connection of a resistor and a capacitor.

XSC1 and XSC2 are waveform oscilloscopes connected to the pipe simulation model, the rectifier output, and AC interference. The waveforms are measured with respect to the GROUND BED.

The output waveform of the simulated rectifier looks as follows, it is measured on Channel_A with 5V per division setting. It is a rectified DC voltage.

The output waveform of the simulated AC interference looks as follows, it is measured on Channel_B with 10V per division setting.

Rectifier output is the white color waveform. The rectifier output do not cross the 0V axis. It is a DC waveform.

The grey rectangle labeled "SOIL" on the schematics diagram is the simulation model of the soil around a pipe. In this model the soil has resistive RSOIL and capacitive properties CSOIL.

Taking voltage reads with DC voltmeters, XMM1 and XMM2, on the rectifier output, XMM2, and the pipe under CP, XMM1, gives me the following results:

As you can see the -7V applied by the rectifier turns into -0.322V on the pipe.

The image below clearly shows a change in CP polarity under influence of the AC interference (white waveform crosses the 0V axes).

The next step is to measure the current that is supplied by the rectifier:

The XMM2 multimeter now is connected in series with the output of the rectifier and set to measure current.

Now I am measuring AC current flowing from the AC interference source:

The XMM2 multimeter now is connected in series with the output of the rectifier and set to measure current.