differences between static, digital, and numerical relays 1

[SMB1]

Hi everyone,

Could you please explain the difference between static, digital and numerical relays in terms of technology ?

thank you very much

 

[davidbeach]

Digital isn't really a class.

Static relays use discrete components, op-amps, comparators, etc. and operate on rectified currents and voltages. They may use microprocessors for some timing functions. The most definitive characteristic is the use of rectified quantities.

Numeric relays take the input analog quantities and convert them to numeric values. All of the relaying functions are performed on these numeric values.

 

[DiscoP]

A simple definition I use is the method of comparision to determine a operation.

With electromechanical, the electrical quantity is turned into a mechanical force, and compared with another mechanical force - be it a spring, magnet etc

With electronic, the the electrical quantity is turned into a much smaller electronic quantity and compared against another electronic quantity

With numerical, the electrical quantity is turned into a number, and compared against another number.

There is a lot more to it than that, but when introducing new people into the world of protection relay jargon it helps with the understanding.

I also got asked this question in the job interview for the job I currently have, the above is the answer I gave - it must have been OK !!

I agree with david about digital, it doesn't mean a great deal, many people say it but mean to say numerical.

 

[SMB1]

thank you,

but i think digital and numerical relays are not the same, digital relays is similar to static relays but with numerical HMI.

For example
MCGG Static
KCGG Digital
P120 numerical

do you agree?

 

[davidbeach]

Sorry, don't recognize any of those.

The Basler single function BE1-xx relays are all solid state relays; some of them use microprocessors for timing functions but the active quantities are never converted to numeric values. Solid state, some with digital capabilities, some without. They then also have a family of multi-function numeric relays. SEL is all numeric relays. Multilin is, I believe, all numeric relays, while some of GE's historic solid state and electro-mechanical relays are still produced. Etc.

The relay manufacturer's use three groupings: electro-mechanical, static, and numeric. Digital does not make a clear distinction between static and numeric.

 

[slavag]

Hi SMB1
Good Q.
We also separate relaus to digital and numerical.
MMM.
P120 for my pinion is also digital, but P139 is numerical.

Isn;t exactly, but digital relays have only EEPROM's and list of function w/o any possibility for the free logic.
Numeriacl, have a library of function, PLC functionalitu, maybe possible say-have processor ( Intel, AMD, Motorola, Samsung)

 

[SMB1]

Hi slavag
i think all MiCOM relays are numerical,

i found definision from Areva protection guide book, here it is:

7.4 DIGITAL RELAYS
Digital protection relays introduced a step change in
technology. Microprocessors and microcontrollers
replaced analogue circuits used in static relays to
implement relay functions. Early examples began to be
introduced into service around 1980, and, with
improvements in processing capacity, can still be regarded
as current technology for many relay applications.
However, such technology will be completely superseded
within the next five years by numerical relays.
Compared to static relays, digital relays introduce A/D
conversion of all measured analogue quantities and use
a microprocessor to implement the protection algorithm.
The microprocessor may use some kind of counting
technique, or use the Discrete Fourier Transform (DFT) to
implement the algorithm.

7.5 NUMERICAL RELAYS
The distinction between digital and numerical relay rests
on points of fine technical detail, and is rarely found in
areas other than Protection. They can be viewed as
natural developments of digital relays as a result of
advances in technology. Typically, they use a specialised
digital signal processor (DSP) as the computational
hardware, together with the associated software tools.
The input analogue signals are converted into a digital
representation and processed according to the appropriate
mathematical algorithm. Processing is carried out using a
specialised microprocessor that is optimised for signal
processing applications, known as a digital signal
processor or DSP for short. Digital processing of signals in
real time requires a very high power microprocessor.

 

[DiscoP]

Following that definition, relays like the KCGG, LFCB and LGPG would be classed as digital - but why can't you also class them as static - there are no moving parts !

Likewise numerical relays use A/D convertors, so why can't you call them digitial !

In my view (and according to the definition given) P120 is definitely numerical, it just has not as many additional features as (say) the P139.

I've worked with an experienced engineer who won't be swayed from his view that MCGG is numerical - his view is anything with a microprocessor is numerical. I don't agree with him, but he knows very well the limitations of this relay compared to a modern relay, so is there any need to argue with him ??

Short hand terms for definitions cause too many misunderstandings & disagreements for no real purpose !!

Why are you asking by the way

 

[lz5pl]

DiscoP, K-series, LFCB, LGPG, Optimho are digital relays.
M-series, like MCGG are static (I personally like them, they don't need 500+ pages User Manual for commissioning ).
MiCOM's are numerical.
Please follow the explanation posted here by SMB1, it is the most comprehensive I have seen on that topic. Generally according to me AREVA (and previously GEC Alstom) documents are the best, with some exclusion for these on MiCOMx30-series.

Regarding this experienced colleague - If you work as commissioning engineer you should already know the First Rule of Commissioning Engineer: Don't trust directly anybody, including experienced engineers and including yourself too! Study the problem first. Everybody makes mistakes (me too).


------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[DiscoP]

Hi lz5pl - I totally agree with you relay classifactions (I thought I said the same thing !!)

I guess the point I was making is they are only names, and without clear definitions are open to different interpretations.

Depending on the context, it may or may not be important.

I agree the definitions given in the Areva NPAG are good, but not everyone uses these.

 

[jghrist]

I vote for davidbeach's explanation over Areva's. It is more concise and more definitive.