EXPERT TIP #11: USING PORTABLE INTERRUPTERS FOR CP RECTIFIERS & POWER SUPPLIES

Installation Options & Issues

Interrupters
An “interrupter” is a switch installed on Cathodic Protection (CP) rectifiers to enable the rectifier to be switched OFF and ON in a timed cycle. The interrupter switch can be a conventional mechanical (metallic contact) switch or an electronic, solid-state switch. The purpose of the Interrupter is to enable the CP technician to measure pipe-to-soil potentials in both an ON (CP current applied) & OFF (CP current interrupted) condition.  Referred to as an “interrupted survey”, the results of the survey reveal the effectiveness of the CP system within the influence range of the CP rectifier.

Interrupters can be included into a rectifier during manufacture. This is rare as it is a special option that many people do not know is available.  As a result, most rectifiers do not have a built-in interrupter. Therefore, to conduct an interrupted survey, it is necessary to connect a portable interrupter to the rectifier.

Portable interrupters are available from many manufacturers.  The type, rating and technology of the interrupters vary.  It is important to select an interrupter that has a suitable current rating for the application. Additionally, if multiple interrupters are required along the pipeline, the interrupters must be capable of synchronization.

Regardless of the type of portable interrupter, there are options for how it can be connected to the rectifier to provide the necessary interruption cycle.

Connecting a Portable Interrupter to a CP Rectifier

Following are three options to connect an interrupter to a rectifier.  (Refer to Figure 1 for clarification.)  Each method has advantages and disadvantages as they may have some affect on survey results.  Depending on type of structure protected, some of the conditions noted may not apply.

Option 1 - Interrupter Installed in the AC Input to the Rectifier.

Advantages

• Low switching currents in the primary of the transformer, typically less than 10 amperes. An interrupter with a low current rating would be capable of interrupting a rectifier with a high DC current rating.

Disadvantages

• Requires the technician to manage connections with high AC input voltages
• In-rush AC current can trip the AC input breaker
• In-rush current can be experienced when turning ON the switch to the main panel circuit breaker.  When this happens, you may hear a low pitch “bong” or the breaker will trip immediately. 
• The magnitude of the in-rush current is dependent on when the switch is closed relative to the input AC sine wave. Note that the AC voltage varies from zero to maximum 120 times a second.  If the breaker is closed when the AC voltage is at or near zero, there will be minimal in-rush current as the voltage ramps up on the sine wave.  If the switch is closed at or near maximum voltage on the sine wave, high voltage hits the transformer creating the high in-rush current. 
• Depending on the rating of the breaker, it may or may not trip.  So, it is possible to set up the interrupter on the AC input side and initially, everything may look good.  However, the circuit breaker can trip in the middle of the survey, which is highly inconvenient.
• When the transformer is switched OFF, the collapsing magnetic field will cause a voltage spike on the rectifier output.  Often referred to as “anodic spiking”, this can cause inconsistent pipe-to-soil potential readings.
• If the pipeline under survey has induced AC voltages, these induced voltages will pass through the rectifier stack and be converted to DC during the OFF cycle.  The rectified DC will be impressed onto the pipeline during the OFF cycle, resulting in “false high OFF” pipe-to-soil potential. 

Option 2 – Interrupter Switch is Installed in the AC Secondary of the Main Transformer.
A popular technique to accomplish this is to remove one of the coarse or fine rectifier tap link bars, then connect the interrupter in place of the tap link bar. This technique directs the AC secondary current through the interrupter.

Advantages

• No in-rush current issues as the main transformer is already running. There is no concern regarding the AC input circuit breaker tripping.
• Convenient as it only requires removing a tap link bar and connecting the interrupter to the rectifier tap studs
• AC current will have less arcing on the interrupter contacts compared to DC current
• No anodic spiking

Disadvantages

• Interrupter must have a current rating equal to the output of the rectifier

• As in Option 1, if the pipeline under survey has induced AC voltages, these induced voltages will pass through the rectifier stack and be converted to DC during the OFF cycle.  The rectified DC will be impressed onto the pipeline during the OFF cycle, resulting in “false high OFF” pipe-to-soil potential. 

• Rectifier may indicate a lower DC current output due to added resistance of interrupter equipment

Option 3 – Interrupter is Installed in the DC Output of the Rectifier.

Advantages

• No in-rush current issues
• No false high “OFF” readings due to induced AC
• No anodic spiking

Disadvantages

• Interrupter must have a current rating equal to the output of the rectifier
• Rectifier may indicate a lower DC current output due to added resistance of interrupter equipment
• Managing the large DC output cable may be an inconvenience
  
  

Final Comments

Interrupting a conventional transformer rectifier has its challenges.  There are better ways to interrupt a CP rectifier.  Some conventional transformer rectifiers are manufactured with a “hybrid” rectifier stack - a stack with two conventional diodes and two SCRs (Silicon Controlled Rectifiers). The hybrid stacks are usually found on constant current and constant potential controlled rectifiers. This technology allows the rectifier to be interrupted using a simple, low current relay to interrupt the gate control current to the SCRs. Therefore, it is possible to interrupt a rectifier, even one rated at 500+ amperes, with an interrupter relay that only needs to switch 5 milliamps. 

Figure 1

NOTE:  A new generation of CP power supplies is available that use “switch-mode” technology.  These power supplies can be more user friendly and are designed to be interrupted with a low current external relay.  They can also be interrupted with an external input voltage, i.e., the output of a portable interrupter that was intended to drive a large high current relay. When interrupting a conventional rectifier with a hybrid stack or a switch-mode CP power supplies, all the disadvantages noted in the above Options 1 thru 3 will be eliminated.