Introduction

The Solid-State Decoupler (SSD) series continues the DEI tradition of offering  innovative protection products to the corrosion prevention industry, while building off of proven solid-state technology developed by DEI. With a lightweight, non-metallic housing and lower cost, the SSD can be economically applied throughout a cathodically protected system.

The SSD functions as a DC isolation and AC coupling device (a "decoupler"), preventing the flow of CP current up to a predetermined voltage threshold, while passing any induced AC current. For voltage that attempts to exceed the threshold, the device instantly switches to the shorted mode, providing overvoltage protection. After the event is over, the device automatically switches back to the DC blocking mode. This operation can occur an unlimited number of times, and is typically due to AC faults and lightning, which the SSD is rated for. While the standard threshold is -2V/+2V, the SSD can be supplied with up to a -3V/+1V threshold and several lower threshold combinations.

The threshold is the absolute, or peak, voltage at which switching occurs, and is the sum of the DC and peak AC voltage across the terminals of the device. This results in a very low, and safe, clamping voltage across the SSD terminals. Similar to most DEI products, the SSD has been certified by independent laboratories for compliance to all known U.S., Canadian, and European standards and codes. Testing and certification was performed by Underwriters Laboratories and Demko, with resulting UL, CUL (Canadian UL to CSA requirements) listings, and CE marking. The SSD is certified for use in hazardous locations (Div. 2 and Zone 2).

Applications

The SSD is designed for:

• Decoupling gradient control mats (grounding mats) from pipelines.
• Over-voltage protection of equipment from AC faults, lightning, and switching transients (e.g. insulated joints).
• Decoupling dissimilar metals that must otherwise be AC bonded for safety.
• AC grounding and DC isolation of electrical equipment integral to a cathodically protected system.
• Mitigation of induced AC voltage.

With the introduction of the lower cost SSD, decoupling gradient control mats is now an affordable and attractive option. With a decoupled gradient control mat: (a) the potential of the mat material is irrelevant, (b) the mat can be made from less costly materials than pure zinc, (c) interaction between the mat and CP system is eliminated, and (d) decoupling allows CP readings can be taken on the pipeline in the vicinity of the mat.

DEI also offers cost effective gradient control mats designed for worker safety from both power frequency and lightning / switching transients. Other "ground mat" designs do not address the requirements necessary to provide low touch and step potentials due to lightning and similar transients.

Insulated joints often need over-voltage protection against lightning and AC fault current, and in some cases, steady-state induced AC voltage. Due to the small clearance between opposite sides of the insulated flange, a protective device must provide a low clamping voltage, including the voltage effects of the conductors or bus bars used to connect the product. DEI offers superior designs that address voltage clamping issues and also provide secure mounting methods that aid in limiting voltage to low levels. As an AC mitigation device, the SSD can also keep the steady-state voltage across the flange to a negligible level.

In decoupling dissimilar metals, the SD can be used between two grounding systems, or other structures that require AC safety bonding while preventing galvanic corrosion. As grounding codes may apply, the SSD is listed by UL for meeting the requirements of an effective AC grounding path per U.S. and Canadian electric codes.

Product Capability of the SSD

The key parameters of the SSD are:

• Blocking voltage or threshold voltage.
• DC leakage current for a given blocking voltage.
• AC fault current rating.
• Lightning surge current rating.
• Steady-state AC current rating

Blocking Voltage

At a voltage below the blocking voltage selected, the SSD blocks the flow of DC current and allows AC current to pass. At a voltage above the blocking voltage selected, the SSD is a bi-directional conducting device that readily allows all current to flow, thereby limiting the voltage on the structure. The blocking voltage choices are designated as "A/B" in the model number structure where "A" is the (-) blocking voltage and "B" is the (+) blocking voltage.

Blocking Voltage Ratings

The choices for A/B are:
-A/+B in volts peak

Recommended for most applications:
A/B = -2/+2 (standard)

Other blocking voltage options include -3/+1 and other lower blocking voltage combinations. Contact DEI for other options.

The blocking voltage of -2/+2 is usually adequate for most applications, since the voltage difference between the two connected points is usually much less than 2V. For example, an insulated joint on a cathodically protected pipeline either has cathodic protection on both sides of the joint, leaving the voltage difference near zero, or one side has CP and the other is unprotected, with a typical difference of about 1V. For cases where a higher blocking voltage is needed, the model with a -3/+1 threshold is usually adequate. In the model number structure the polarity signs are not shown, but the polarity described above is implied. Polarity marks (+ and -) are provided on the SSD only when an asymmetrical blocking voltage is specified. Polarity is irrelevant with symmetrical blocking voltage.

DC Leakage Current versus Blocking Voltage

The DC leakage current at the maximum blocking voltage for any SSD model is normally less than 10 milliamperes at 20°C and less than 100 milliamperes at 65°C. With normal cathodic protection voltage across the SSD, the leakage current is typically well under 1 milliampere under either temperature condition, a value that is insignificant to a cathodic protection system.

Steady-State AC Current Rating

This value represents the maximum allowable steady-state AC through the SSD while the device is blocking DC current. The source of this current would be induced from overhead power lines. Measure or otherwise determine the available steady-state current in this intended connection and compare to the SSD rating of 45A AC-rms at 50/60 Hz, leaving margin for varying system conditions.

AC Fault Current Rating

There are applications where an overvoltage protective device may be subject to fault current, even though no induced AC voltage is present. For this reason the SSD was designed to have AC fault current carrying capability. The SSD will limit the voltage between its connection points to less than 10 volts AC under the maximum fault current ratings listed in the following table. The ratings are amperes rms symmetrical.

Lightning Surge Current Rating

The lightning surge current rating should not be confused with the AC fault current rating. Lightning has a very different waveform, with a faster rise time, a shorter duration, and much less energy than an AC current waveform of the same peak current. Lightning current ratings are established by subjecting the over-voltage protective device to representative lightning current in a high power test laboratory. The waveforms most commonly used are the 8 x 20 microsecond waveform and the 4 x 10 microsecond waveform. The first number represents the time it takes the lightning surge to reach its crest value and the second number represents the time it takes for the current to decrease to 1/2 its crest value. The SSD was tested with a 4x10 waveform.

Voltage Between Connection Points Due to Lightning

The SSD is designed to keep the voltage between the device terminals to a limited value. During lightning conditions, a much more important factor than the SSD voltage clamping capability is the voltage developed in the leads or bus used to attach the device. Although the SSD solid-state design limits voltage to a lower level better than any other technology, it is challenging to keep the voltage due to lightning to a low level between the two connection points due to the voltage drop in the leads. This is due to the electrical property of inductance, which is only of importance for fast-rising waveforms such as lightning, and is not a concern for AC fault current. Voltage due to inductance relates mainly to the total conductor length that has lightning current flowing through it, therefore the conductor length should be kept as short as possible to limit this voltage. This phenomena applies to all technologies used to limit voltage due to lightning, and is relatively independent of the conductor diameter. The SSD (or any other device) should be connected between the two attachment points with low inductance bus bars or with conductors ideally less than 6 inches (150 mm) long for optimal results.