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shekhar28280
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Join Date: Sep 2008
Posts: 8

Why control votages are DC only?

09/30/2008 4:26 AM

In case of substation application control voltages provided are normally 30V,110V 220V DC only .Why DC instead of AC supply?

Regards,

Chandrashekhar.
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V.Ambarani.
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Engineering Fields - Power Engineering - New Member

Join Date: Nov 2007
Location: India
Posts: 42
Good Answers: 3
#1

Re: Why control votages are DC only?

09/30/2008 5:12 AM

Mr.Chandrasekar,

In substation application, say 110VDC or 220Vdc is used as control voltage of Protective relays.If these protective relays are energised by Ac Control voltage it is fine in normal condition when Ac power is available.However think of the situation of power failure situation .At that time the AC control voltage for the protective relays will not be available for operation of relays.During that time if the situation in the substation warrants to trip the breakers by the operation of protective relays, then protective relays will not be able to operate due to non availability of (AC) control voltage due to power failure.If Dc power (Voltage)is available at that time from DC Station Battery , then the protective relay will operate and prevent the damage to equipment &personnel and catastrophy.Hence DC Voltage which is always available from Battery (not dependent on Ac Source) is always used as control Voltage for operation of Protective circuits not only in Substation application but also in other areas.

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shekhar28280
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Join Date: Sep 2008
Posts: 8
#2
In reply to #1

Re: Why control votages are DC only?

09/30/2008 6:06 AM

Thanks a lot ..
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martin-electrical
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Join Date: Jul 2007
Posts: 41
#3

Re: Why control votages are DC only?

09/30/2008 10:47 PM

Dc is the current of choice because batteries provide the supply and can be easily charged by a bridge rectified Ac power. A battery is a handy standby power supply that doesn't need much equipment to maintain voltage......
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nesubra
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Join Date: Nov 2006
Posts: 729
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#4

Re: Why control votages are DC only?

09/30/2008 11:14 PM

Reliability.This question has been discussed earlier in the forum

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