Solid State Decoupler (SSD) in Cathodic Protection

Starting from the 90s, the AC interference attracts more attention of corrosion engineers due to the intensive use of very high electrical resistance coating of pipelines as well as sharing the Right of Way (ROW) between power transmission line with pipeline, It has both safety implication (high touching voltage) and pipeline integrity concerns (AC corrosion).  The problem of reducing AC interference on gas pipelines so that AC corrosion could be minimized. Solid State Decoupler (SSD) and solid state Polarisation Cell Replacement (PCR) become more and more popular practice in modern cathodic protection system design. understanding the SSD and solid state PCR principle and their implications are essential on CP operation, testing and survey.

it was reported that there are three different types of SSD

• Type 1
  If V_ac+V_dc <V_Threshold , open circuit to ac and dc,
  if V_ac+V_dc >V_Threshold , short circuit to ac and dc
• Type 2

If V_ac+V_dc <V_Threshold , open circuit to dc, short circuit to ac.
if V_ac+V_dc >V_Threshold , short circuit to ac and dc

• Type 3

If V_ac+V_dc <V_Threshold and I_ac < I_Threshold, open circuit to dc, short circuit to ac
If V_ac+V_dc >V_Threshold or I_ac > I_Threshold, short circuit to ac and dc.

For Cathodic Protection sytem, normally Type 2 SSD is utilised for AC mitigation.

Some key points of SSD during normal operation, testing and survey:

• Fail short or fail open

Determine fail short for either AC or DC current, or fail short for both. This fail mode will impact the CP current demand during the SSD failure, particularly while SSD connects to copper local earthing grid. normally while SSD should fail short for both DC and AC.  while SSD connects to copper earthing grid, high CP current will shunt to local earthing

• Over threshold DC voltage/current

For type 2 SSD, as long as V_ac+V_dc >V_Threshold, SSD will become bi-direction for DC as well. Therefore, selecting correct threshold voltage is important for proper Cathodic protection operation. It is advised that SSD device is not designed to take constant, large DC current. Therefore, precaution should be taken for nearby pipeline welding activities, since escaped welding current may damage SSD devices.  Short connection and /or disconnection these SSD to avoid such damage are recommended to address galvanic corrosion and safety concern. SSD should be inspected before return to normal CP operation

• DCVG and CIPS

SSD will cause interference for both DCVG and CIPS activities.