Question about protective relays in power generation

[sigshane]

Hey all. I am learning about line protection, electromechanical voltage and current relays in particular.

When looking at circuit diagrams of the two, they are strikingly similar; they also look a lot alike physically. So that got me wondering: what is the difference? Could one be used in place of the other?

The major difference I found was the resistance of the operating coils of them - the current relay has very little resistance (0.9 to 0.6 ohms from lowest to highest tap setting), while the voltage relay was 108 to 308 ohms from lowest to highest.

Is the higher resistance in the voltage relay to compensate for the increase in current across the PT, and the much lower resistance in the current relay to compensate for the increase in voltage?

Thanks in advance for help,

Shane

 

[GoldDigger]

Hey all. I am learning about line protection, electromechanical voltage and current relays in particular.

When looking at circuit diagrams of the two, they are strikingly similar; they also look a lot alike physically. So that got me wondering: what is the difference? Could one be used in place of the other?

The major difference I found was the resistance of the operating coils of them - the current relay has very little resistance (0.9 to 0.6 ohms from lowest to highest tap setting), while the voltage relay was 108 to 308 ohms from lowest to highest.

Is the higher resistance in the voltage relay to compensate for the increase in current across the PT, and the much lower resistance in the current relay to compensate for the increase in voltage?

Thanks in advance for help,

Shane

The distinction is very basic. You want to sense something or actuate something and move the relay contacts based on that. The magnetic force to move the contacts can come from a small number of turns at high current or a large number of turns at low current.
The power consumed by the relay will depend on its mechanical design and therefore the magnetic field required.

If what you want to detect is voltage, you use a large number of turns of small wire, producing a high resistance. This minimizes current draw.
If what you want to detect is current, you use a smaller number of turns, producing a lower resistance. This minimizes voltage drop.

For a motor overload, you do not want to sense voltage, you want to sense current.
For a phase loss detector you do not care about current, you want to sense voltage.

You determine the purpose of the relay, the voltage and current the contacts must handle, and the amount of power it can consume without causing problems and select a relay accordingly.

The same difference between high and low resistance is found in voltmeters versus ammeters respectively.

 

[sigshane]

Thanks for that explanation - I think I may have been on the right path, just wanted to be sure.

Shane

 

[SG-1]

You may find it helpful to find the instructions for the following relays on line. They describe the operating principles & show the basic connections. The relays were designed & manufactured by Westinghouse until they were sold to ABB.

CO-2 basic overcurrent protection 50/51 http://www05.abb.com/global/scot/scot229.nsf/veritydisplay/7042f1d45630fca185256eac0053e7d4/$file/41-101U.pdf
CVQ undervoltage & negative sequence voltage 27/47
COV-5 voltage controlled overcurrent protection 51/27
HU transformer protection with harmonic restraint unit 87
KLF field relay 40
CV-1 under & over voltage protection 27/59
KAB bus differential relay 87B

You may have to copy & paste the address.

 

[SG-1]

You may also like this series of instructional videos. Here is lecture 1, Elements of System Protection.

http://www.youtube.com/watch?v=uizZgjY71e0&feature=relmfu

 

[sigshane]

Thank you SG-1, for your helpful links.

Wow! That video narrator is the same guy in the training modules prescribed by our training coordinator - he look so young on youtube, without glasses and gray hair LOL

 

[JoeStillman]

The 50/51 relays need to have a low impedance in order to avoid saturation of the CT under fault conditions. Protection-rated CT's can maintain their accuracy up to around 20X nominal secondary current. If your available SC current is more than 20X the CT primary, you may not be able to get the 50 relay to trip. The higher the relay impedance, the more risk of saturation.