LIFE EXTENSION STRATEGIES FOR MODERN POWER CIRCUIT BREAKERS

Satin American has manufactured retrofit systems for 600V class power circuit breakers since 1988. Back then, most upgrades were performed because aging electromechanical trip devices were no longer serviceable. Since that time, OEMs have standardized on more reliable electronic protection. Now, after decades of dependable performance, these modern systems are beginning to approach end of life.

Circuit breakers make up the backbone of power distribution infrastructure. They are robustly constructed and expected to have a service life spanning several decades.   Industry depends on this equipment to operate safely and reliably long after OEMs have introduced new models.
Modern breakers are controlled by an electronic control unit (ECU) that interfaces with current sensors and a flux trip device.  The ECU determines the magnitude of current flowing in each phase conductor by measuring the output of its corresponding current sensor. It then compares this current to preprogrammed thresholds. If a threshold is exceeded, the ECU executes a predetermined time delay, verifying that current remains above pickup and then sends a pulse to fire the flux device and open the breaker. Drift or failure of any component in this system can result in compromised coordination, nuisance tripping or complete loss of protection.  While the protective system is arguably the most critical subsystem on the breaker, experience has shown that it is also most susceptible to developing performance issues over time.

Like any electronic device that has seen years of service, ECU’s are vulnerable to numerous modes of failure. These include degradation of components (especially capacitors), contact and circuit board failures due to electrical, mechanical or thermal stress, as well as corrosion and development of tin whiskers. Figure 1 depicts a “bathtub” curve that is widely used in reliability engineering to model the lifecycle of a population of products. The left side of the curve shows a high but rapidly decreasing failure rate that occurs when the product is new. These failures are referred to as “burn-in” or “infant mortality” and are frequently caused by improper handling or installation and are usually identified during the commissioning process. After infant mortality issues are eliminated, the population enters its “useful service life” which is a long period of high reliability and low failure. After time, “end of life” failures begin to manifest and the mortality rate across the population increases rapidly.

EXTENDING THE SERVICE LIFE OF POWER CIRCUIT BREAKERS

Industry expects the installed base of power circuit breakers to provide decades of safe and reliable service. These breakers are well designed and mechanically serviceable. Unfortunately, in many instances, their electronic protection units are beginning to approach end of life. Most power circuit breakers produced since the early 1980’s were manufactured with direct-acting electronic protection units (EPU’s).

The EPU has numerous advantages over the electromechanical trip devices that were common on earlier vintage breakers. Specifically, EPU’s allow a more precise selection of pickup and delay settings which permit tighter coordination and selectivity. EPU’s often have advanced features that include the ability to provide
detailed trip reports, pre-trip indication, and communications capability.

Integrity of the protective system
Any electronic device is susceptible to performance degradation and failure after years of service. These failures manifest because of value drift in passive electronic components, failures due to thermal, mechanical or electrical stress, corrosion or the growth of crystalline filiform. Failure of the EPU can affect the integrity of the protective system in one of three manners:
1. Drift of performance characteristics.
2. Failure to trip.
3. Nuisance tripping.

Ideally, an EPU that has reached the end of its service life will be identified during normally scheduled maintenance testing. While it is likely EPU’s that fail to trip or no longer conform to time-current curves will be readily detected, it is more difficult to identify an EPU that is susceptible to nuisance tripping. A single nuisance trip can cause an unexpected power outage that costs millions of dollars in lost productivity. If the breaker fails to clear a fault the cost can be even higher and jeopardize both property and human life. When breakers begin to operate in an unsafe and unpredictable manner, immediate attention is required.
Because power breakers are employed in mission-critical applications where reliable 24/7/365 power is expected, it can be both difficult and expensive to address EPU’s that have reached end of life. This difficulty is further compounded because these failures usually occur years or even decades after the breaker was manufactured and the OEM may no longer provide spare parts support.

Usually far less expensive
One option that is always available is to replace failing breakers with the latest OEM production vintage. This option can be prohibitively expensive both in terms of the capital required for the equipment and the costs associated with the outage while it is installed. Many OEMs are now offering retrofill solutions where new breakers can be fit into existing switchgear. While the cost of this option is not insignificant, it is usually far less expensive than replacing both the breakers and switchgear.

Retrofit the existing breakers
A second option is to retrofit the existing breakers. This can be an attractive solution because the core mechanical structure of the breakers are often highly serviceable and not in need of replacement. Depending on the condition of a breaker and the associated components of the protective system, a retrofit may be as simple as replacing the EPU. In other instances, current sensors and the flux trip device may require change-out.

Reliably extend the service life
Over two decades and thousands of installations world-wide, companies such as Satin American have proven that retrofits of low voltage power breakers can reliably extend the service life of this equipment. Provided that the breakers are maintained and tested at regular intervals, there is ample evidence to suggest that they can safely operate without incident for decades after the OEM has abandoned spare part support.

Masterpact M & MP STR Trip Unit Replacement

ETC-12Q Series: The first Quick Swap Trip Unit for the Masterpact M & MP Breaker.

  • Direct Plug-in Replacement for STR-28D, STR-38S, STR-58U and STR-68U Masterpact Trip Units
  • RMS sensing
  • Adaptable to all frame sizes with no need for ratings plugs
  • LSIG protection with defeatable protection bands
  • Digital display with 3-phase ammeter and detailed trip logging
  • Uses original CT’s and flux trip device

 

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