Response of Over Voltage & Over Frequency Relay

The way the manufacturer intended them to. It is impossible to answer your question without knowing the most basic facts about your relay such as type, analog/digital, filtering, settings, sensing configuration, etc, etc. When you contact the manufacturer (the only culpable source) be sure to provide relevant information such as the model number and the settings that you are questioning.

In this case, the manufacturer can not possibly test for all conditions out in the field. In a large facility where equipment is constantly turning something on and off, there will be all kinds of pings and boings on the electrical line. Any one of those could be rich in high frequency trash that gets lumped in with the general definition of harmonics.

The point is simply that manufacturers usually to provide a good product. Under certain conditions, that product just won't perform. They can't cover every possible condition and still be competitive. But some will come close. Those products tend to be more expensive.

A line reactor can fix a boat load of sins to an electrical system if it is sized correctly. But to blindly trust your supplier? That is a good way to get burned!

Harmonics generally do very little to the supply voltage since your power transformers must be oversized a little in order to prevent "brown outs" or under-voltage. And even harmonics generally don't have significant impact because they are always in the correct direction (with respect to rotation) and in phase.

Odd harmonics can be additive, subtractive, and/or reversed in rotation. The effect at your relay will really depend on how bad your harmonics issue actually is. You will probably have to research the manufacturers specifications to get the best answer.

And not all products take into account every consideration. For instance, a cheap voltmeter is not very accurate in the presence of high harmonics. It takes a "True RMS meter" to give you accurate results.

I have "educated" more than one stubborn electrical engineer by taking them to a power distribution panel to measure the current on the neutral bus. Most of the time, they can not tell me the correct magnitude of the current. When I show them the frequency reading (usually 3 times the fundamental) they become defensive and accusatory. That's just because they are embarrassed that they don't understand what they are seeing. But, I digress.

Usually, there is enough filtering to exclude the effects of harmonics on your relay. In reality, it is possible that your relay may experience nuisance trips. If that is the case, you may want to install a line reactor to block the high frequency (harmonic) components. A line reactor will also reduce wear and tear on your other equipment in most cases.

Mr.Noturordinaryjoe,

suppose we have scenario in our case that,

(1) Standered Digital over voltage & over frequency relay is installed at line side of the incomin feeder.

(2) No any Harmonic filter is installed at all.

(3) In normal condition when we have clean power followings were the measured

(a) Phase to phase voltage =380vAC

(b) Phase to Neutral Voltage = 220vAC

(c) Frequency =50HZ

(d) Current = 500A

(3) 2nd or 3rd harmonics produced due to x,y reason then what will be the effects or change will come on the above four Quantities a,b,c & d ? ?

Thanks,

Well, by definition, the relay should not be sensitive to current. In order for it to measure the voltage, it must first convert the AC to DC and then compare it to some setting that you pick. The scale on these relays is not calibrated. Therefore you need to determine the point at which it reacts under normal conditions. If you read the actual voltage with a meter and then mark the spot on your dial, you will be able to gage where 10% (or what ever number you like) should be. Then set it to that point.

Since I have no documentation on your device, I can only guess at how to adjust the frequency section. Again, your frequency sensor will be looking at variation of the primary voltage, not the current. With harmonics, the distortion generally begins with non linear consumption of power at the supplied voltage. This is simply because the source of the voltage is many times larger than anything you are doing. Unless, you are running your substation at or near the rated power rating of its transformers. Generally if you are using 500 Amps, your substation needs to be rated (or capable of supplying) for at least 640 Amps. This oversize requirement allows for the harmonics to be absorbed in the transformer core and blown off as heat.

As I was saying, since Power = Voltage times current, (and the voltage tends to be more uniform because it is following the motion from a network of generators) then the thing that will change due to various loads is current.

However, (there is always a "however") anything that absorbs power, such as a power correction capacitor bank, or DC power supplies for various devices may be subject to the effects of harmonic distortion. The second harmonic and other even number harmonics (4th, 6th, .....) will not cause any problem. The odd harmonics (3rd, 5th, 7th, 9th, 11th and 13th) can be responsible for circuit breakers tripping, component burn outs, and motors overheating. The Over Voltage and Over Frequency relay is not likely to be useful to stop or prevent this damage.

The kind of problems from harmonics tend to change very rapidly as other devices are started. If you are using a reasonable range of variation for your relay settings, the way you use power should not make it trip.

What your relay actually is supposed to do is to protect you from changes that occur outside your facility. When a substation suddenly drops out of service, your voltage may go up. When people switch in or out large power factor correction capacitors, you may see a change. If your relay is like the ones I have used, it may take a second or two of change before it reacts. The specifications for the device will be subject to how you set it.

Thanks, for your explanation.

If the relays are numerical, they would have a harmonic filter, which would allow only the fundamental component into the relay.

But, if the relay is digital or without harmonic filter then ?

A device can perform only, that the maker tested for and specifies, and absolutely nothing else, reliably.

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NotUr... led a discourse, that was informative and educational.

But, from a component's (and it's makers) point of view some things are missing.

One example will illustrate, why this is so. A previous note said: AC is rectified to DC, that is compared to whatever, and triggered..........

Well, AC with harmonics is a complicated subiect not only based on voltages, but on their phase relationships.

Taking the harmonics, based only on their phase relationships, you can produce very different waveforms, from nothing remarkable to tall, energetic spikes at any phase of AC, all carrying together the same overall energy. But, the peak to average voltage will in some cases be strikingly different. Any rectifying circuit, depending on its design details will produce (some strikingly) different responses.

You and I cannot know, and the maker will not know, until testing and specifying the details.

Building and running equipment really needs practical knowledge. But, I for one would shy away from poorly specified components in protection circuits. There, I do not like surprises. And don't even start on replacement components.