ON LINE INSULATION TESTER

[petronila]

Good day to all, Here I am posting a new thread. Anyone with experience with this devices? All Comments are wellcome:

The automatic insulation resistance testers monitor the condition of either AC or DC rotating electrical equipment. Testers are connected to the "B" phase at the motor contactor, or generator breaker, and to equipment ground.
Testers apply a fixed dc test voltage to the winding while the motor or generator is idle. Typical test voltages are 500, 1000, 2500 or 5000 V dc. The tester measures current leakage to ground, which is related to the dielectric strength of the equipment insulation.
The tester's fault-alarm trip point can be set from .05 to 30 megohm to suit the application. When leakage exceeds the set point, the tester can trigger an alarm and locks out the monitored equipment. The equipment can not be started until the fault is cleared.

Thanks for your comments.

Petronila

 

[electricpete]

I was browing through ANSI standards the other day and I cam across ANSI F 1134 – 94 (Reapproved 2002) - "Standard Specification for Insulation Resistance Monitor for Shipboard Electrical Motors and Generators"

It does exactly what you describe. I have never seen it in action.

On the plus side
- this would be great for large outdoor motors. We have had a number of them fault shortly after starting during wet weather. A nearby powerplant actually takes a megger test every time they start a circ water motor if it has been idle more than a day. They have an exceptional mist problem there ... I think it is mist from the traveling screens blowing toward the circ water motors.
- the Navy has a pretty large population of motors and some pretty good resources for evaluating technologies. You have to think that most things they invest in are good technologies... at least for their situation.

On the minus side
- I need one per motor. That could start to get expensive when you consider material, installation, wiring.
- The dc voltage would remain applied to not only the motor but the cabling up to the switchgear. Some people get extremely paranoid about dc voltage on cable... especially cable which can be wet (which is the applications I have in mind) and type XLPE insulation with well-known treeing phenomenon due to combination of wet cable and dc voltage. Even though the voltage level is relatively low (not a hi-pot), the duration of application could be very long. I would need to investigate that aspect further before I applied it at our facility.
- max limit 30 megaohms - that is pretty darned low for form-wound equipment for example 4kv and 13kv motors. The IEEE standard specified 100 megaohms minimum and we find all our motors in this category are 1000 megaohms or more (except those connected to wetted cable). I would prefer the capability for earlier warning than that.

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[electricpete]

Also if we applied this, I would connect it for alarm so that action could be taken prior to the time you need the motor. I'm not sure if that's the application you had in mind.

I don't think I would connect it to lock out the equipment. If operators know they have an alarm then they shouldn't start it except in dire circumstances.... I don't want to deny them the capability to start it when those dire circumstances apply.

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[maypot]

We have many on site, the only inconvenience,if any, is that it is effective only before applying power to the motor.

Bob

 

[petronila]

Thanks Electricpete and Maypot for your comments.

The point here is Predictive Maintenance, I am agree with your comments Electricpete, this is very important in Large Generators and Motor or very important small motors, and we can find some advantages like : You don´t have to perform conventional megger test(Dont need persons, equipment and "Time") about the Maypot comments I would like to know your PLUS AND MINUS about this device, how many earliest problems did you find on your applications and if you have some retourn investment analysis related to this OnLine Device.

Thanks again

Petronila.

 

[electricpete]

It seems there might be some small vulnerability of failure of test device components during motor running which become important for critical motors.

Does anyone know how the test device automatically removes itself from the circuit, what type switching device if any, and how much remains connected to the motor while running?

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[jraef]

The one's I have seen are on-line all the time and do not disconnect when applied to low voltage motors, but they must be isolated from a medium voltage motor circuit.

http://www.msegroup.net/prod-ms.htm

One soft starter manufacturer incorporates this as a feature in their products, but fails to mention the issue of needing to have the disconnect switch or circuit breaker open to use it on MV until after you buy it and read their manual. It also means that the circuitry must have a direct connection to the motor leads, and without another isolation contactor to separate them, it means MV potential is in some way present in the control cabinet. I imagine they may use a dropping resistor system, but I for one would not trust that.

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[itsmoked]

Can't the megger feature run through whopping high value resistors providing *some* isolation/protection?

 

[electricpete]

That's probably what jraef means by dropping resistor system but as he says that has it's limiations. If you get an unintended open circuit, you can get full sensed voltage up to the point of the open circuit.

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[electricpete]

Also high voltage on the system side of the resistor even with system operating correctly. As you say a resistor divider is better than nothing but not a common means to isolate high voltage.

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[itsmoked]

Hence the *some* isolation comment. Essentially you have current isolation but decidedly NOT voltage isolation.

electricpete I really have no clue how these things work but I would imagine they look for the motor to be off then briefly run the check. Then run the check occasionally as long as the motor remains off. They probably abort the check instantly on motor actuation. If it runs through a resistor the dc sitting on the motor probably just vanishes when the motor is hooked to the mains. (barring all the apt comments about long lines/capacitance.)

I agree too, as to what happens when the unit is accidently disconnected.. wouldn't it think everything was megging really high, and be happy? Wrong but happy!

 

[petronila]

Thank to all for the interest, I was researching for this devices and I can found a Manufacturer please visit this address:

http://www.megalert.com/wirediag.html

http://www.megalert.com/catalog.html

What do you think about this devices?

Regards

PETRONILA

 

[jraef]

I would look at the fact that it appears to be only connected to one leg of the motor winding. I imagine they have a way of determining some sort of major insualtion failure to ground by looking at combined series resistance values through all other winding configurations, but I find it hard to see how it could detect a phase-phase short. Even if we give them the benefit of the doubt on that issue, how would it pick up a problem in the A-C phase when it is only connected to phase B? The other problem I have is that it appears to create it's megohmeter power from the 120V secondary of the transformer, making it at best a 150V megger. That will only find the most catastrophic insulation failures. Most motor mfgrs recomment 500VDC on a 460V motor as a minimum.

My concern then is along the lines of what itsmoked mentioned; that you will forgo regular testing in favor of these things, and they will not necessarilly give you adequate protection. The value I suppose would be in giving you warning of a problem developing in between schedules tests, but only like an "idiot light" on your car dashboard. By the time it goes off, it's too late anyway. So maybe they should really sell it as a way to cut down on diagnostic time after a failure. Hardly seems worth the cost and installaion.

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[electricpete]

For most motors all three phases are tied together at the neutral, so we only need to apply the test at one phase to monitor ground insulation of all three phases.

You are right that this doesn't test certain fault paths such as phase to phase tracking in the end-turn area.

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