25 kV Disconnector Switch Control Circuit Questions 2

[Oblsss]

Dear all,

I'm checking a quote for a 25 kV Disconnector Switch and I noticed that in the attached circuit diagram the manufacturer uses 230 V AC (Europe) for all circuits, namely for the Control box auxiliaries (heating, light bulb, socket), the control circuit, the motor circuit and the signals to the remote terminal unit (Disconnector is located outdoor in an overhead circuit).

1) I can understand that with the above mentioned circuit we reduce cost by one cable that would have been used to carry the auxiliary voltage for the motor circuit , the control circuit and the the signals to the remote terminal unit (eg 48 V DC or 110 V DC ). According to your experience, are there any potential problems in using only 230 V AC instead of 230 V AC & 48 V DC or 110 V DC?

2) The 230 V AC enters the 25 kV Disconnector Switch Control box and directly feeds the auxiliaries, i.e. the heater, without an intervening a power switch, see the attached photo. The circuit breaker -Q11 located inside the control box protects the motor and control circuits. However, in the event of a fault in the heater or light bulb, we lose the 230 V AC of the disconnector (the remote circuit breaker shall trip). Is this configuration generally acceptable? Does it make sense to lose control of the Disconnector Switch for a short circuit in a heater or a light bulb?

Thank you very much!

 

[crshears]

According to your experience, are there any potential problems in using only 230 V AC instead of 230 V AC & 48 V DC or 110 V DC?

It depends.

How critical is it to be able to operate the switch by motor and from a remote control centre in the event of a local blackout that interrupts the station service power supply to the switch?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Oblsss]

@crshears Actually I think there is no big difference in terms of continuity of service, since the 230 V AC are provided from the output of a UPS system installed in the nearest station ...

 

[bacon4life]

Although some substation circuit breakers have separate fusing for heaters versus trip coils versus closing coils, I have found rather poor series coordination between fuses/MCCBs in the breaker cabinets and the upstream feed from the station AC/DC panels. I suspect in some cases the additional component count from adding more overcurrent devices has actually decreased the overall reliability.

Using a UPS introduces a different set of risks including:
1)UPS often produce very little fault current, so a fault in the control box may result in the UPS shutting down rather than an overcurrent device tripping. 2)UPS may need oversizing to handle motor starting.
3)UPS may need oversizing to handle highly inductive trip coil currents

 

[Oblsss]

From the answers provided, it seems that it is acceptable to lose control of a MV disconnector over an electrical fault in the heater or the light of a control box.
This does not make a lot of sense to me!

Any tips or suggestions from the more experienced?

 

[Overvoltage]

I think you're getting the wrong impression (or at least painting with too broad of a brush).

As @crshears said, "it depends". If the impact of potentially losing the ability to operate the device during a fault on the 230V system is acceptable, then it avoids the need for DC components and conversion.

At a higher voltage, I certainly wouldn't do it. But at 25kV, it is likely that you would have other disconnect or protective devices that could be used, even at the expense of causing an outage to customers in the event that the specific failure occurred. The risk may be acceptable if the savings are significant enough. Without knowing more about where it is being used and the impacts, I don't think anyone here could say "yes it's acceptable" or "no absolutely not".

This is where the "art" of engineering comes in. Balancing risk versus cost. You could ensure almost perfect reliability, but no one could afford it. Or, you could cut the costs significantly, but reliability would be poor. Finding an acceptable balance is important.

 

[stevenal]

An RTU is mentioned. Suggest alarming on loss of control voltage.