Auxiliary Tripping Relay: Use of parts with reduced coil DC ratings for faster operation. Typical Circuit Doubt.

[Distribution73]

Hello,
While reviewing drawings from an (old) existing substation, I have come across a number of tripping circuits where auxiliary relays (normally GE HFA51A type) are used in an arrangement I cannot quite understand.
These auxiliary relays are used in Busbar Diff and CBF circuits, where contacts of protection relays energize these auxiliary relays coils. The substation DC is 125Vdc, but the auxiliary tripping relay coils are 12Vdc rated. I understand this is something actually recommended by the aux relay manufacturer to increase speed of operation of the aux relay. What I dont understand is the use made of a resistor is series with the aux relay coils. I attach a circuit sample. I have seen IEEE recommendations (https://www.pes-psrc.org/kb/report/017.pdf) and this type of series resistor appears shunted by a NC contact of the auxiliary relay or a capacitor. I understand this being intended to allow full 125VDC excitation of the aux relay coil during a few milliseconds and then protect the coil once the aux relay has operated. In the substation however the resistor is permanently in series with the coil. Any feedback on the rationale of such arrangement (with a resistor permanently in series) would be greatly appreciated. It appears to me to be defeating the purpose of using a reduced DC rating coil.

On a similar note I have seen the coils been protected with diodes in parallel. These have resistor in series. Any feedback/source where I could learn what are the basis for the dimensioning of these parallel resistances would also be much appreciated.
Thank you in advance for the help.

 

[brucesilver]

Hello,
While reviewing drawings from an (old) existing substation, I have come across a number of tripping circuits where auxiliary relays (normally GE HFA51A type) are used in an arrangement I cannot quite understand.
These auxiliary relays are used in Busbar Diff and CBF circuits, where contacts of protection relays energize these auxiliary relays coils. The substation DC is 125Vdc, but the auxiliary tripping relay coils are 12Vdc rated. I understand this is something actually recommended by the aux relay manufacturer to increase speed of operation of the aux relay. What I dont understand is the use made of a resistor is series with the aux relay coils. I attach a circuit sample. I have seen IEEE recommendations (https://www.pes-psrc.org/kb/report/017.pdf) and this type of series resistor appears shunted by a NC contact of the auxiliary relay or a capacitor. I understand this being intended to allow full 125VDC excitation of the aux relay coil during a few milliseconds and then protect the coil once the aux relay has operated. In the substation however the resistor is permanently in series with the coil. Any feedback on the rationale of such arrangement (with a resistor permanently in series) would be greatly appreciated. It appears to me to be defeating the purpose of using a reduced DC rating coil.

On a similar note I have seen the coils been protected with diodes in parallel. These have resistor in series. Any feedback/source where I could learn what are the basis for the dimensioning of these parallel resistances would also be much appreciated.
Thank you in advance for the help.

Hey!

What you're describing makes sense — using a lower voltage coil with a series resistor on a higher DC voltage can speed up relay operation while limiting current afterward. Even if the resistor is always in series (not just during pickup), it still limits the steady-state current, preventing coil damage. Maybe the design prioritizes faster operation with some current limiting, without the added complexity of a bypass circuit.

As for the diode with a series resistor, that’s usually for coil transient suppression (like a flyback diode). The resistor slows the diode response a bit, reducing the relay's dropout time. Sizing depends on coil inductance and desired suppression level — IEEE and relay manufacturer docs are great for guidelines.

Let me know if you want to dig into the math!

 

[dpc]

I would get a copy of the HFA instruction book and read that. This is a pretty standard configuration.

HFA Multicontact Auxiliary Relay - Manuals

HFA Multicontact Auxiliary Relay - Manuals

www.gevernova.com

 

[Distribution73]

Thank you very much Brucesilver and DPC.
@brucesilver,
I agree the circuit appears to seek the type of compromise you mention. However I am not sure what is the actual procedure to seize the resistors (series and transient reduction). From the substation samples I got the impression that the series resistor is calculated based on the DC resistance of the relays coils, so that during energized steady state the voltage drop is that for which the coil is rated (12Vdc in my case). This makes sense somehow but again it appears to me to defeat the original purpose of using a reduced DC rated coil if you are preventing it from being subject to the full circuit Vdc. With regards to the sizing of the transient resistor I haven´t found much info on how to calculate it. Best reference found https://www.te.com/commerce/Documen...v&DocNm=13C3311_AppNote&DocType=CS&DocLang=EN, however it appears to me that the sizing is much down to the manuf. recommendations.

@dpc,
I read the instructions prior to the submittal of the thread, but I dont think the series resistor arrangement is contemplated in the HFA literature (at least I am not able to find it). Note that one arrangement with a series resistor is proposed for the HFA51S, but the series resistor is shunted with a NC contact as I mentioned above.

 

[dpc]

You could contact GE-Multilin tech support. I think these are still made and sold. It's been a long time since I've thought about HFA relays and I don't have a simple answer for you right now.

 

[Distribution73]

Thank you DPC. I will do. If I manage to get some feedback I will upload to the thread.