For a number of years, ABB’s excitation systems have relied on MICO, Murrelektronik's intelligent load circuit controller, to monitor the 24Vdc supply. These modules are now matched to Murrelektronik’s MB Redundancy Balance power supply units, which can be integrated into an existing MICO power distribution system using a bridge with minimal wiring work required.
MICO has become an essential component of ABB’s excitation controller power supplies because it is proven to increase their reliability and fault tolerance. While it may initially be an expensive option, MICO does pay off in the longer term, as ABB’s vice president, Excitation Products and Technology, Tobias Keller explains:
"Since we have been using MICO, 24V earth faults and short circuits no longer cause total system failures. And MICO also does well on the financial side. When changing over from conventional circuit breakers to MICO, product costs rose for a short time; however, thanks to the considerably lower total operating costs, the initial increase was more than compensated.”
Murrelektronik's MB Redundancy Balance power supplies provide two advantages: a significantly lower power loss and an equal load on both units. Controller power supply systems are often designed with dual redundancy so that machines continue to work in the event of failure of one of the supplies. However, when entire systems are redundant, including the load, they become expensive. Partially redundant systems are more cost-effective as the power supply units are the only part designed redundantly.
Partially redundant power supply systems are often designed with two power supply units of the same size, each being capable of supplying the total nominal load current. Such a design requires that both units are decoupled. If the units are not decoupled, there is the risk that if one power supply unit has a short circuit in the output, the other will suffer the same failure.
Conventionally, partially redundant systems are created with diode modules that decouple the power supply units. If one of the two power supply units fails, the other continues to work unimpaired, supplying the load with power. However, when decoupling using diodes, the power loss is significant. With a load current of 20A, a 0.7V diode causes a power loss of 14W. This reduces system efficiency, incurs additional costs for cabinet cooling and potentially reduces the life of other components in the cabinet because of higher operating temperatures.
Another disadvantage of this system is that the two power supply units are unevenly stressed. If the load current is not regulated by a diode module, one of the power supply units delivers the complete load current while the other is idle. This can significantly reduce the life of the thermally stressed unit.
The active redundancy module MB Redundancy Balance offers a solution to these problems, thanks to innovative MOSFET technology which considerably reduces the power loss. With a load of 2 x 10A and a perfect balance between the two power supplies, the power loss is just 1.3W.
This integrated regulation feature makes sure that each of the power supplies delivers half of the load current so that both components are working with equal loads. If, for example, an output current of 10A is required, both units deliver 5A each, prolonging their life and reducing the maintenance burden. If one of the units fails, MB Redundancy Balance ensures that the power required for operation is delivered by the unit that is still working. Moreover, the component sends a message to the controller, facilitating fast troubleshooting.
Manuel Senk is with Murrelektronik GmbH, based in Oppenweiler, Germany