Voltage drop Vs Voltage Sag

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Hameedulla-Ekhlas

Hameedulla-Ekhlas

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Greeting all,
What is difference between the voltage drop and voltage sag. I know both but I need your comments and explainations too.
 
In my opinion voltage drop describes voltage 'lost' to conductor resistance.

Voltage sag indicates the source of supply is not capable of maintaining voltage.
 
Hameedulla-Ekhlas said:
Can you give some farther explanation regarding to "maintaining voltage" meaning?
Click to expand...

I simply mean the source is not capable of maintaining proper voltage with the amount of load being placed on it.

Or in other terms, I can usually do something about voltage drop, I can do nothing about voltage sag.
 
Both describe a voltage that is lower than nominal. Neither, directly, describes the magnitude of the voltage. Rather they are related to the length of time an event occurs.

In many power quality discussions.
A sag (or its opposite 'swell') lasts for a short period of time measured in cycles. A drop (or its opposite 'rise') lasts for a long period of time measured in seconds. The final group of disturbances (like a spike) is measured in mSec.
 
Sag / Voltage Drop

Sag / Voltage Drop

Jim, great response just to add a sag even though only exist for a short period of time may affect equipment that may be voltage sensatative in some applications a UPS system can help maintain voltage stability or at a service entrance sag ride thru equipment can be used to compensate for sags in the utility distribution system.
 
100503-0714 EST

Ham:

Voltage drop --
1. The difference in voltage between two points in a circuit.
2. The change in voltage at a load location resulting from going from no load to loaded. This results from the total source impedance and the load current. And assumes constant source voltage.

Voltage sag --
1. This is a result of a change of the source voltage.
2. In a regulated power supply this results from adjusting the voltage knob.
3. In a small system with a DC generator and one load this results from adjusting the field excitation or the generator RPM.
4. In a large system with many varying loads, distributed impedance, and my concern with voltage at the entry to my facility the common impedance between those other loads and my system produce voltage variations of the equivalent source voltage to my system.
5. Probably means that source voltage drops below an acceptable level.

.
 
iwire said:
I simply mean the source is not capable of maintaining proper voltage with the amount of load being placed on it.

Or in other terms, I can usually do something about voltage drop, I can do nothing about voltage sag.
Click to expand...

Sometimes it happens that sending end supplies proper voltage and receiving end gets proper votlage but still there is voltage sag.
 
Hameedulla-Ekhlas said:
Thanks for reply but sometimes happens that sag voltage increases than the nominal voltage in transmission line.

I will just write the formula

I^2 * w * L > V^2 * w * C
Click to expand...
I have no idea what the formula is about, but by definition a sag is a reduction in voltage. If something happens when there is a sag and the voltage increases, it is no longer a sag.
 
don_resqcapt19 said:
I have no idea what the formula is about, but by definition a sag is a reduction in voltage. If something happens when there is a sag and the voltage increases, it is no longer a sag.
Click to expand...

sometimes it happens, we have at both site ( sending end and receiving end ) 1.0 p.u voltage but there is still sag exist. This sag as below

I^2 * w * L = V^2 * w * C ( zero or flat power flow )

I^2 * w * L < V^2 * w * C ( dip or sag like prabola shape mounted up )

I^2 * w * L > V^2 * w * C ( this one same sag prabola shape but mounted down site and in this case voltage is more than nominal voltage ).
 
SG-1 said:
Are you saying a sag in one part of the system can cause a swell in another part ?

Or that a sag is usually followed by a swell ?
Click to expand...

In long transmission line sometimes it happens, we have at both site ( sending end and receiving end ) 1.0 p.u voltage but there is still sag exist. This sag as below

I^2 * w * L = V^2 * w * C ( zero or flat power flow )

I^2 * w * L < V^2 * w * C ( dip or sag like prabola shape mounted up )

I^2 * w * L > V^2 * w * C ( this one same sag prabola shape but mounted down site and in this case voltage is more than nominal voltage ).

This occurs only in middle transmission line and affects breakers and other transmissionline equipment.
 
gar said:
100503-0714 EST

Ham:

Voltage drop --
1. The difference in voltage between two points in a circuit.
2. The change in voltage at a load location resulting from going from no load to loaded. This results from the total source impedance and the load current. And assumes constant source voltage.

Voltage sag --
1. This is a result of a change of the source voltage.
2. In a regulated power supply this results from adjusting the voltage knob.
3. In a small system with a DC generator and one load this results from adjusting the field excitation or the generator RPM.
4. In a large system with many varying loads, distributed impedance, and my concern with voltage at the entry to my facility the common impedance between those other loads and my system produce voltage variations of the equivalent source voltage to my system.
5. Probably means that source voltage drops below an acceptable level.

.
Click to expand...

Gar:
According to me voltage sag always be between two points. like transmission line conductor.

But when we control at terminal point ( generators excitation ) that is called under voltage and over voltage problem.

Am I right or there is something different between voltage sag and under voltage and over voltage.
 
100504-2054 EST

Ham:

I think the general usage here for the word sag relates to a voltage change in the lower direction at a load of the load voltage and not a result of a change of that load. A surge would be one of an increasing voltage.

It is may opinion that sag or surge relates to a change that extends for a little while. This I might suggest means maybe at least a couple cycles. I would distinguish a sag or surge as different from a transient. A transient is a short time event of anything from a picosecond to a fractional part of a cycle. Some times this might have a longer duration such as a 1000 Hz lightly damped sine wave superimposed on top of a 60 Hz waveform.

These words really have overlapping meanings.

Today I lost power for about 1 to 2 seconds. This could be called a sag, but I would prefer to just call it a momentary power loss. I tend to think of the word sag as something that drops below a certain level but does not go to 0. To me it is something less severe than a total loss.

What I was trying to suggest in an earlier post is that a sag would result from an event not caused by the load. Whereas voltage drop would be something caused by the load of concern.

Obviously the words have sufficiently general meanings to be used in a number of ways that may depend upon the context within which the words are used.

How is your university search coming along?

.
 
gar said:
100504-2054 EST

Ham:

I think the general usage here for the word sag relates to a voltage change in the lower direction at a load of the load voltage and not a result of a change of that load. A surge would be one of an increasing voltage.

It is may opinion that sag or surge relates to a change that extends for a little while. This I might suggest means maybe at least a couple cycles. I would distinguish a sag or surge as different from a transient. A transient is a short time event of anything from a picosecond to a fractional part of a cycle. Some times this might have a longer duration such as a 1000 Hz lightly damped sine wave superimposed on top of a 60 Hz waveform.

These words really have overlapping meanings.

Today I lost power for about 1 to 2 seconds. This could be called a sag, but I would prefer to just call it a momentary power loss. I tend to think of the word sag as something that drops below a certain level but does not go to 0. To me it is something less severe than a total loss.

What I was trying to suggest in an earlier post is that a sag would result from an event not caused by the load. Whereas voltage drop would be something caused by the load of concern.

Obviously the words have sufficiently general meanings to be used in a number of ways that may depend upon the context within which the words are used.

How is your university search coming along?

.
Click to expand...

Thanks for useful information and I have just filled two universities online form and soon will submit them.
 
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