Solid State De-couplers
The Solid State De-coupler (SSD) functions as a DC voltage level clamping device in buried pipeline and storage tank systems. It can, at the same time, mitigate AC voltage. This feature allows up to 15A AC induced current to pass and prevents the flow of DC current while the DC voltage remains below the predetermined design blocking threshold. When the DC voltage exceeds the aforementioned threshold, the device instantaneously switches on and limits the DC voltage of the device under Cathodic Protection. Immediately, the voltage falls as the surge event passes and decays to below the threshold level, the SSD switches off and reverts to its blocking mode.
Are there differences in Solid State De-Coupler technology?
De- Coupler devices should upon a predetermined voltage threshold, begin to conduct forming a near short circuit across the circuit it is connected to. Once the fault has passed or been subdued and the voltage falls below the pre-determined device threshold, the device should commutate (switch) to the “off” state.
Some types of De-Coupler devices do not comply with NFP 70 as well as Eskom guidelines when they are in conduction mode as both Cathodic Protection applied DC and Induced AC present on the operating circuit (i.e. Insulated & well coated Pipe) are short-circuited (to Ground). Once “on”, the current flowing through the device can maintain these devices in the conducting state even when the voltage fault threshold has fallen below the level set. They can be economical and relatively small in size, but main disadvantage is that their application can result in surging of Cathodic Protection power supplies and as mentioned above and do not fully meet NFP 70 requirements and Eskom guidelines. Typical current discharge limits are 1,2kA and 3,7kA. Greater discharge limits are available but are generally on request.
Where are Solid State De-Couplers applied?
* De-couple dissimilar metals that must, by regulation or galvanic
corrosion be AC bonded but isolated in the DC mode.
* Isolate electrical DC equipment in Cathodic Protection systems.
* AC coupling to earth where DC blocking/isolation is required.
* Over voltage conditions emanating from induced AC surges,
lightning and switching transients and Rectifier failure.
* Protection of insulating joints on pipelines.
* Mitigation of AC induced voltages.
* Decoupling in gradient control (earth/grounding) mats from pipelines
Benefits of Cathects Solid State De-Coupler:
* Higher Blocking voltages over older technologies such as polarization cells
* Technically advanced
*Serialized and individually tested.
* Can handle large steady state clamping currents for longer periods of time
than Metal Oxide Varistor (MOV)
How does a SSD deal with Induced AC?
In AC mitigation applications the De-Coupler should provide a continuous conduction path to ground for the AC whilst preserving the applied Cathodic Protection DC and provide over voltage protection when AC & DC rise above a predetermined level. The Equipment should fully comply with Eskom & NFP 70 guidelines and truly preserve the DC component as required by NFP70 in normal operation.
Typically, the Cathtect SSD-DCDth +ACM and SSD-DCD thex +ACM are short circuit devices which upon reaching overvoltage clamping threshold, short circuit the overvoltage condition and are programmable in their negative range from -2 through to -22V. Positive clamping voltage is factory set at +2V. Maximum fault current ratings are typically 3,7kA and 1,2kA for 30cycles
All SSD-DCD and SSD-DCDth variations are capable of steady state 30V @ 45A 50/60Hz.
SSD-DCD + ACM devices on the other hand, clamp the overvoltage state preserving the DC when an over voltage condition prevails and are typically factory set to the following thresholds
+1 to -11VDC steady state 30V @ 45A 50/60Hz.
+1 to -3VDC steady state 30V @ 45A 50/60Hz.
+2 to -2VDC steady state 30V @ 45A 50/60Hz.
Fault current maximum ratings are 1,2kA; 3,7kA; 5kA & 14kA for 30cycles.
Individual test certificates are provided with each unit supplied and SABS 3rd party impulse type test is available on request.
What about De-Couplers in hazardous applications?
SSD-DCDex + ACM & SSD-DCDthex + ACM units are tested and certified under XPL/17001/15.1726 complying to SANS 60079-0 Ed 5/IEC 60079-0:2011 Ed 6 Explosive atmospheres Part 0: Equipment – General Requirements and SANS 60079-15 Ed 4/IEC 60079-15:2010 Ed 4 Explosive atmospheres Part 0: Equipment – Equipment protection be type of protection “n”.
What is a Voltage Limiting Device?
Are devices which conduct once a predetermined voltage has been attained. Voltage clamping devices will, once a predetermined threshold is reached, the voltage will be clamped whilst the conduction will be at a maximum. Disadvantages of Voltage clamping devices are heat generation (ignition temperatures in Hazardous zones)
Cathtect CEVLD70 and CEVLD70Ex devices are generally applied where additional protection is required to protect general workers that can access electrical connections to pipelines and structures, (such as Test points, link panels, cross bond connection panels etc.) where possibility exists that AC or DC Voltages above the operating voltage may be present from time to time.
The devices are designed to clamp AC between 37-55VAC and DC between 55-70VDC. They display a service life of 20 operations of 8/20uS @ 100kA and 3 operations of 10/350uS @ 50kA. The devices are also purposefully fabricated to fit snugly into the Transnet dead front test and link panel found on the NMPP pipeline as well as the picture insert format.
- DC Voltage Blocking @ 75V
- Lightning impulse clamping voltage @360V
- Very fast clamping response time @25ns
- Lightning current rating Class ll @50 kA for (8/20 µs) +
- Device failure open circuit
- Submersible enclosure up to 2 meters in depth – IP68
Gas Discharge Devices
form an arc in an altered (gas filled) closed environment when a preset voltage threshold is reached. Significant current and voltage relative to the conduction threshold are required to maintain the arc. It, therefore, stands to reason that if the device is not hermetically sealed the characteristics of the device will fluctuate with climatic changes. Typical discharge currents are 50kA & 100kA. Please be advised that Gas Discharge devices cannot accommodate an indefinite number of discharges. Most discharge devices can only cope with 1 (one) 100kA discharge before the characteristics of the device are irreversibly altered through repetitive discharge.
For More Information on our AC Mitigation Products & Equipment emails us at Sales@cathtect.co.za or visit our Webpage www.cathtect.com