125 Vdc vs 120 Vac for local control stations 2

[cmelguet]

Hi,

Anyone knows an standard or document where it states that in a plant is not a good practice to have some local control stations with 120 Vac and others with 125 Vdc?. At the project where I am, it was proposed to have Medium voltage motors with LCS´s control voltage of 125 Vdc and low voltage motors with LCS voltage of 120 Vac. To me it make sense to have only one voltage going from the MCC to the Motors local control panels, no matter if it is Medium or low voltage motor.


Your opinion is wellcome
Regards

 

[dpc]

In plants with medium-voltage switchgear, what you are describing is very common. I think trying to standardize on one control power voltage everything sounds good, but is not that important in practice. There will be circuits of many different voltages running around, many in the same panels and some in the same conduits. No one can ever ASSUME they know a circuit voltage.

There are good reasons for using 125 V dc for circuit breaker control in critical applications.

 

[mgtrp]

At low voltage, protection of the circuits is probably performed by circuit breakers that don't need any auxiliary power to operate - MCCB's, etc, and the control power is primarily for closing contactors.

At medium voltage, protection of the circuits is probably performed by circuit breakers that are tripped by protection relays using some form of auxiliary power for the trip coil. If AC is used, then in the event of a fault, supply voltage may dip too low to trip the circuit breaker. Therefore, DC is used.

Because of the above, medium voltage circuit breakers are much more readily available with DC trip coils, while MCC's are much more readily available with AC controls.

 

[cranky108]

To us substation guys thinking. The 120 Vac dosen't work, because it tends to fail at the same time as we need it the most. But we don't want super large battery sets, so we use 120 Vac for things that are less important, like the air conditioner, and lights, and sump pumps, cooling fans, etc.

 

[cmelguet]

Thanks, sorry for the late replay. I think my question was wrong understand. The MV MCC have a 125 Vdc internal control voltage to use in the circuit breaker trip circuit, this control voltage is back up by a battery bank. The problem is that the client is asking to use the same 125 Vdc to supply the local control station next to the motor. So in MV motors we will have 125 Vdc at the the local control station, and in LV motor we will have 120 Vac at the local control station.

Regards,
Cmelguet

 

[burnt2x]

In plants, you use AC for motor contactors. In substations where control power is very important, DC is used. Although some switchboards use AC but those are provided by inverters taking power from the battery stations. The key is whether you want uninterruptible power--> which is why DC is desirable.

 

[DTR2011]

A few things to consider / questions.

In the LV do you have LV Circuit Breakers? If yes are they electrically operated (charging / tripping closing)? If yes is it AC or universal (AC/DC).

It used to be that LV Switchgear had AC control as it was easier / less expensive to have a control power transformer to supply AC loads. This all works well until the bus is dead. In light of arc flash concerns, how is the breaker closed remotely / electrically if there is no source of AC control power?

I have been in a few plants that have 125DC, 48DC,24DC and 120AC all in the same room (+primary voltages). Each voltage being for a specific system (protection/control, fire alarms, DCS, 480V MCC, etc).

If the DC system is sized properly, there is no reason not to add it to LV, other than cost and additional equipment. You have to be careful that the LV equipment can accept the DC.