Machine and Process Management

Redundancy puts backbone into the ring topology

 redundancy in distribution and protection switchboards

What is redundancy anyway?

We all want more for less. And nowhere is that truer than in industry’s constant quest for energy efficiency. But in the big energy-intensive process and infrastructure industries, electric motors account for 70% of their electricity usage – mostly in processes where continuous availability is what counts. So companies are waging competitiveness war on the continuity of service front. Their weapon? Redundancy.
But just what is “redundancy”? I sometimes think it is whatever you want it to be: duplication, fault tolerance, producing the same result in alternative ways, etc. Businesses need to get it straight in their minds what they mean by “redundancy”. Otherwise they impact their process with no improved reliability to show for it.

Ethernet: tops for topology configuring

What I’m interested in here is redundancy in distribution and protection switchboards. There’s huge potential there in Ethernet-based networks. They bring flexibility and scalability and high-speed communications. And you can configure them into different topologies and build in levels of redundancy. That’s the second big question you should ask yourself: What level of redundancy yields the dependability that meets your needs at the right cost?

Meaning and level of redundancy

So what do I mean by redundancy here? Applied to switchboards, I mean addressing a specific motor or motor starter subsystem via two different paths with no manual intervention.
What level of redundancy do I want? Let’s take two:

  1. Redundancy at the level of a network’s backbone so it can address sub-networks of motor starters via two different paths.
  2. Redundancy at the sub-network level, so that sub-networks can reroute when they talk to individual motor starters (or other devices).

Ring topology for network backbone

Traditionally, big companies and industrial facilities use Ethernet tree topology for their electricity distribution. More and more, however, are switching to fault-tolerant ring topologies as their network backbone. Personally, I think they’re right. With their twisted pair cables they’re heavily wired – with all the wiring costs that entails. You can’t connect very many devices to them, either. Anything over seven and the computation and test packets required balloon out of all proportion.
Ring topology is much simpler, which makes calculation simple. And on industrial plant’s shop floor, there is no practical limit to the number of devices that can be connected.

Redundancy in the ring

Ring topology lends itself to redundancy. In many networks you’ll find an additional counter-rotating ring. In the event of a switch failure, data automatically flows across to the other ring. And failover recovery times are extremely fast.
The only disadvantage? No international standard. So failover methods and times depend on the switch manufacturer. You should, therefore, try to use switches produced by the same manufacturer to be sure of homogeneous, continuous service.
To sum up, then, the ring constitutes a backbone of managed switches that all sub-systems connect to. The ring tolerates any single point of failure and automatically reconfigures to reroute subsystem traffic.
Failover is automatic, transparent – and very fast. The typical recovery time is 500ms but can be as fast as 3ms.
Let’s take a look at redundancy in the subsystem level.

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3 Responses
    • Daniel Barstz Daniel Barstz

      It depends on several factors like the real requirement for availabilty of the application concerned, the failure modes of the complete architecture, etc. Most of the time the weak element is not the protective device (when used under the right conditions). Experience show that the weakest factors are auxilliary power supply, bad cabling and wiring, and ignorance of EMC basic rules.
      In my opinion, redundancy of protective relays, even if it is sometimes required by the market, is not the central point. In some cases, it is a good way to sell more expensive “boxes” or just more “boxes”.

  1. Didier Giarratano Didier Giarratano

    In Fact it is depending on the voltage level and the communication technology that you’re using. As soon as fast automations are required or if IEC61850 is used then the need to get seamless Ethernet link appears. Today a standard is existing (IEC 62439) which is describing 2 technologies: HSR and PRP. HSR is covering the ring topology while PRP is covering the tree topology. The general principle is to send the Ethernet packet to 2 directions. The receiver is canceling the “second” packet. This technology will be available on Schneider Electric protection relays (Px40, Px30, etc)