Numerical Relays (SCADA)


Thread Starter

C S Chandrasekhar

One of our power utility customers wanted to use
advanced Numerical Relays which can do protection, CB control as well as metering functions for a power distribution automation project.

As per the conventional scheme, these relays could be interfaced to the RTUs thru RS485 in the substation and linked to the control centre over a comm link.

Can this scheme of using RTUs be dispensed with and only keep Numerical Relays to behave as RTUs, as some of the relays offered by reputed mfrs claim that they are compliant to any protocol? Our
apprehension is that will there be anything that we miss if we "dont go" for the RTU in this scheme?

Thanks in advance for the response.

Chandrasekhar C S
dear chandrasekhar:
I dont think u would miss anything. Many Numeric Relays have facility to interface with modbus.
e-g: GE-Fanuc Powertrak modules which are being used extensively for substation automation.
For further details, please contact me by e-mail

Glen P. Najera

dear chandrasekhar,

I beg to disagree with santhiraj. You do have something to loose. First of all, analog data accuracy will decline. This is due to the fact that relay type CTs are to be used instead of the usual metering type CT that interfaces to the RTU. Some setups utilize numerical relays for its actual function of protection and status indication. But uses separate transducers and metering CTs to get a more accurate analog value. The separate transducers are then fed into an RTU for the MMI system.


APG Fausto

Darold Woodward

As Mr. Najera points out below, there is some accuracy difference between true "metering" systems that may be available through RTUs and relays. However, the accuracy difference in measurements made by my company's relays (and I suspect others) would not be discernable on a SCADA display. Metering for customer billing, etc. is still in the domain of devices other than RTUs or numerical relays and is typically performed by dedicated meters and then reported through the RTU or other means to the SCADA system.

In order to really answer the question for the specific network in question, we need to know a little more about your application.:

What do you mean by metering? Does this mean revenue metering? SCADA monitoring/display/historical data collection? or some other funciton.

Next, are you talking about an MMI or HMI for a single substation? Or do you have multiple substations with multiple relays?

Is the master on site or off site? Does it have the ability to easily configure for multiple slave devices at a single site? This can seriously increase the overhead in configuration and communication?

What protocol are you interested in? Most manufacturer's support several protocols with differing degrees of functionality over each protocol. You will need to look at the actual relays planned and determine if all of the features you plan to access are available.

Lastly, and especially if there are multiple substations, I would suggest using an RTU or other master device (like a communications processor) in each substation as a local master device. I have done extensive testing and written
a technical paper that shows that the typical bus network is the lowest performance network you can build. This is simply because of the communications overhead required to communicate with 10 or 12 devices rather than 1. Star based networks with a central master node offer many enhanced features like transparent connections to the relays, IRIG time sync. distribution, places to dial in for collection of event oscillography and other features.

Darold Woodward PE
SEL Inc.
[email protected]