voltage dip or frequency dip? 1

[powering2002]

Dear all,
I have a little question.
Could you explain, which is more dangerous to our electrical equipment (electric motors, TV, refrigerator, etc), Voltage Dip or Frequency Dip?
Thanks for your attention..

 

[itsmoked]

In a short term less than a minute or two - voltage. In a long term frequency can present problems.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

How large a dip? 1% 5% 10% 20% 50%

Frequency dip means islanding or "National Situation"

A large frequency dip saturates transformer cores within tens of milliseconds. A 10% voltage dip can overload an asynchronous motor in five or ten minutes.

Most other loads are not very sensitive to voltage dips. Some electronic devices, VFDs are one case, may get a problem with either excessive ripple in DC supplies or alarms from grid supervision. Thyristor controlled devices will definitely be influenced because most of them expect either a 60 or 50 Hz supply with very small deviations.

Older switchers could kill themslves during beown-outs. The reason is that they were supposed to deliver a constant voltage at a constant load. Hence, they were taking constant power from grid. When voltage went down, current went up. Sometimes the current blew the rectifiers and the switch transistors. Modern supplies have input voltage supervision that switch off before destruction occurs. See

http://www.gke.org/presentationer/files/SICK PSU overcurrent when grid drops out 1.jpg

for an example.

The question is too broad, I think. What loads are you thinking about?

CBMA and other trade organizations have published charts on this kind of deviations. Mostly voltage dips. Can't remember that I've seen much on frequency dips.



Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

The larger the grid the less likely you are to see a damaging frequency dip. That said, I have seen a very large industrial plant install under-frequency relays mandated by the utility. there was a possibility that a large part of the grid could become islanded. In an island event, if the frequency dropped 4 Hz, the main 140,000 Volt feed breaker was tripped to avoid the rest of the grid crashing. This may not have been a normal combination of large load and vulnerable inter-ties.
Frequency dip is much more likely with standby and emergency generator supplies.
However almost all generators above about 15 KVA have a feature called Under Frequency Roll Off (UFRO) incorporated in the Automatic Voltage Regulator (AVR).
If a heavy load pulls the frequency down more than 3 Hz the UFRO reduces the voltage in proportion to the frequency dip. Inductive loads such as motors and transformers want to see a particular Volts per Hertz ratio. The UFRO maintains the V/Hz ratio close to the optimum and the worst effects of both under voltage and under frequency are avoided provided the dip is of short duration.
That said, if you are running construction or domestic equipment on a generator small enough for two men to load it into a pick-up truck, it probably will not have the UFRO feature and it will be best to avoid pulling the frequency down with prolonged overloads.

Bill
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"Why not the best?"
Jimmy Carter