MOV Failures

[TechGuy]

Here is the device.  The MOV is supposed to protect two diodes & a capacitor.
http://www.electromagnetic.biz/Portals/0/Docs/CDT/CTD1%20&%202%20UL.pdf

Here is the data sheet for the 130V MOV.  The maxium continous voltage is given in RMS.  Or am I reading this incorrectly ??
http://www.alliedelec.com/Images/Products/Datasheets/BM/LITTELFUSE/846-0244.PDF

The manufacturer is using a .6 watt (continuous )

I have sent an e-mail to the manufacturer asking, why not a transil ??
I will pass along your recomendation for not using the MOV in a high reliability devices, that they degrade every time they clamp.

Thanks

OK Looks like its the correct MOV for AC power, I just wasn't sure from your earlier posts. A lot of folks forget to factor in the RMS voltage issue so I thought it was prudent to bring up.

Is there a line filter in front of the surge suppressor (CTD-1 module)? A line filter includes a small RC filter to snub small spikes and to filter out common-mode noise.

What is the device your protecting? Note from the CTD-1 module data sheet the intended application:

Application: This device provides a source of energy for circuit breakers and switch trip coil operation
during a loss of AC control voltage

This device is using a half wave rectifier, and not a full bridge rectifer used in most devices. a half wave recifier will have issues with high current applications.

Also note, this is a single channel shunt mode protector. It will not prevent damage from common mode surges, or other surge protectors that use three way surge protection. Consider if there is a common mode surge where line and neutral experience a common mode spike (ie both line and neutral have a 400V spike at the same time). Since the device has a earth ground, the MOV will do nothing and permit a high voltage differential between L-N and ground. The device will like pass the surge through it using the ground line.
Consider that other devices at the customer premises might include a three way shutmode suppresion circuit that diverts the surge to ground. Now the high side of the surge is the Earth ground and use Line-Neutral and return blowing your device from the ground. Consider that the average household\commerical ground line has about 6 ohms resistance, providing ample room for destruction.

Three way MOV surge protection:

[SG-1]

you should be passing onto your supplier any costs you incur in fixing the issue, if they sell you faulty gear and it causes £££ of damage that is theirs to pay for

In a corporation there is always a lot of politics & you have to watch out that somebodys relative is not involved.  The decision to stay with this vendor will come from several layers up from me.  Trying to collect would be difficult, because the cause of the failures will probably never be known.  We say it's them, they say it is us.  In the end the lawyers get it all anyway.     

[SG-1]

Just my 2 cents but have you checked the mains supply?.High voltage harmonics may be a possible cause.

This is always a possibility.  In which case the replacements will fail too. There is always a lot of other electronic devices present.  The big dollar items are the protective relays. They can cost tens of thousands of dollars each. Usually customers do not place harmonic generating loads on the transformer for the switchgear control power.  Most of these MOVs have failed inside the factory & in different ways. Some died on power up, others worked for a while.  ( I remember one site with bad harmonics, they were burning up 3-Phase 75KVA CPTs. )

[SG-1]

<snip from wikipeadia>While a MOV is designed to conduct significant power for very short durations (? 8/20 microseconds), such as caused by lightning strikes, it typically does not have the capacity to conduct sustained energy. Under normal utility voltage conditions, this is not a problem. However, certain types of faults on the utility power grid can result in sustained over-voltage conditions. Examples include a loss of a neutral conductor or shorted lines on the high voltage system.<snip>
The neutral conductor in modern times cannot be relied on to remain at 0 volts WRT earth.This can cause significant voltage fluctuations for short periods of time causing havoc with voltage sensitive devices, and it's getting worse! see thread in this forum https://www.eevblog.com/forum/index.php?topic=1561.0

 If this were the case a lot of more expensive equipment would be smoking on the primary & secondary side. This device is designed to operate continuously at 140VAC.  The failures inside the factory have occured with multiple testers at different test stations.   If other devices were failing for unknown reasons power quality could be examined.  Much of the gear we supply has power quality instruments built in.  You do bring up a good point and that is a great thread.   

[SG-1]

[/quote]

Is there a line filter in front of the surge suppressor (CTD-1 module)? A line filter includes a small RC filter to snub small spikes and to filter out common-mode noise.

What is the device your protecting? Note from the CTD-1 module data sheet the intended application:

Application: This device provides a source of energy for circuit breakers and switch trip coil operation
during a loss of AC control voltage

This device is using a half wave rectifier, and not a full bridge rectifer used in most devices. a half wave recifier will have issues with high current applications.

Also note, this is a single channel shunt mode protector. It will not prevent damage from common mode surges, or other surge protectors that use three way surge protection. Consider if there is a common mode surge where line and neutral experience a common mode spike (ie both line and neutral have a 400V spike at the same time). Since the device has a earth ground, the MOV will do nothing and permit a high voltage differential between L-N and ground. The device will like pass the surge through it using the ground line.
Consider that other devices at the customer premises might include a three way shutmode suppresion circuit that diverts the surge to ground. Now the high side of the surge is the Earth ground and use Line-Neutral and return blowing your device from the ground. Consider that the average household\commerical ground line has about 6 ohms resistance, providing ample room for destruction.

Three way MOV surge protection:


[/quote]

There is no line filter ahead of this or any other device on the AC supply.  The AC supply typically consists of a stepdown Control Power Transformer.  It's primary side is connected to the switchgear's medium voltage bus ( 2400-38000 volts ), usually Line to Line. The secondary side is wired for 240/120.   The gear is a giant steel box with a .25x2 inch ground bus traveling through. The ground references are attached to the ground bus. Resistance would be very low. There are other devices present that are much more sophisticated, the protective relays like the SEL brand. The CTD-2 does not have a ground reference.  The relays do.  Some relays have a chassis ground, safety ground, & a filter ground. 

The device we are protecting varies greatly.   Medium Voltage Switchgear is used as service entrance equipment for hospitals, large industral plants, etc.
The devices all utilize the primary voltage, say 13800.  Lets say the load is a 12,000 HP air compressor. That section of gear would provide a protective relay ( customer's choice ) for the motor.  The protective relay senses a problem has developed, maybe high temperature on a set of bearings.  The relay then closes a normally open contact.  This completes a trip circuit for the medium voltage circuit breaker,  the CTD-2 supplies the DC voltage.  Customers that are too cheep to maintain a battery rack use a capacitor trip device. Maybe they just think it is more "green".  The trip current is 6 amperes peak. The shunt trip coil is energized for 34 milli-seconds.  With the AC control off, the CTD-2 capacitor is discharged from 179VDC to 80VDC.

What do you consider a high current application ?  For our switchgear that would be about 4000 amperes.

[PetrosA]

I think that last post put things into scale for me . I can see how having a 12,000 HP motor shutting off unnecessarily would be a problem. I also see that recommending you look for answers over on mikeholt would be useless (I thought your nick looked familiar...) Good luck with getting a solution!

[TechGuy]

That section of gear would provide a protective relay ( customer's choice ) for the motor.  The protective relay senses a problem has developed, maybe high temperature on a set of bearings.  The relay then closes a normally open contact.  This completes a trip circuit for the medium voltage circuit breaker,  the CTD-2 supplies the DC voltage.  Customers that are too cheep to maintain a battery rack use a capacitor trip device. Maybe they just think it is more "green".  The trip current is 6 amperes peak. The shunt trip coil is energized for 34 milli-seconds.  With the AC control off, the CTD-2 capacitor is discharged from 179VDC to 80VDC.

What do you consider a high current application ?  For our switchgear that would be about 4000 amperes.

Sounds like the CTD-2 is being used for its intended application as a power source for a trip breaker. I though perhaps the CTD-2 was being used for to supply rectified DC power for an applicance.

Based upon your information, I suspect the problem may be line noise caused by the AC motors, or even the switchgear. I would ask the vendor of the CTD-2 if they have problems with there device at sites that have industrial equipement creating a lot of spikes caused by large AC Motors (induction leakage spikes). Ask them if including a line filter or an RC snubber to attenuate the spikes would solve the problem or if they have a better solution. I suspect the line filter would correct the problem but I can't guarentee it. An AC line monitor test would required to make sure the Line filter was properly attenuating the noise.

That said, I wonder if the MOV is really necessary, since if I understand the application, the CTD-2 is a reserve power supply to trip a relay or breaker during a power loss. Relays and breakers are not senstitive to power surges, unless they contain semicondutors. Most relays and breakers contain no semiconductors.

[SG-1]

I think that last post put things into scale for me . I can see how having a 12,000 HP motor shutting off unnecessarily would be a problem. I also see that recommending you look for answers over on mikeholt would be useless (I thought your nick looked familiar...) Good luck with getting a solution!

In this instance it would not be able to shut down for any reason,  by the time somebody used their finger to open the breaker all the magic smoke would be out of the million dollar+ motor.  Fire Dept in route. Lawers right behind.  All because of a MOV failure.  Now imagine this is a main breaker for a hospital.

See you over at Mikes. Don't tell them I have been whoreing around !! 
I have been communicating with GAR.

[SG-1]

That section of gear would provide a protective relay ( customer's choice ) for the motor.  The protective relay senses a problem has developed, maybe high temperature on a set of bearings.  The relay then closes a normally open contact.  This completes a trip circuit for the medium voltage circuit breaker,  the CTD-2 supplies the DC voltage.  Customers that are too cheep to maintain a battery rack use a capacitor trip device. Maybe they just think it is more "green".  The trip current is 6 amperes peak. The shunt trip coil is energized for 34 milli-seconds.  With the AC control off, the CTD-2 capacitor is discharged from 179VDC to 80VDC.

What do you consider a high current application ?  For our switchgear that would be about 4000 amperes.

Sounds like the CTD-2 is being used for its intended application as a power source for a trip breaker. I though perhaps the CTD-2 was being used for to supply rectified DC power for an applicance.

Based upon your information, I suspect the problem may be line noise caused by the AC motors, or even the switchgear. I would ask the vendor of the CTD-2 if they have problems with there device at sites that have industrial equipement creating a lot of spikes caused by large AC Motors (induction leakage spikes). Ask them if including a line filter or an RC snubber to attenuate the spikes would solve the problem or if they have a better solution. I suspect the line filter would correct the problem but I can't guarentee it. An AC line monitor test would required to make sure the Line filter was properly attenuating the noise.

That said, I wonder if the MOV is really necessary, since if I understand the application, the CTD-2 is a reserve power supply to trip a relay or breaker during a power loss. Relays and breakers are not senstitive to power surges, unless they contain semicondutors. Most relays and breakers contain no semiconductors.

Had an engineer try to use a cap trip device for a power supply once, I was in warrenty then. Customer called said cap trip had failed.  Air express a new unit, not the CTD-2.  Six months later he calls back with the same complaint.  The prints are pulled for his order & he get a real power supply this time.  Engineer gets promotion. 

The vendor says we are the only customer having a problem.

The power circuit breaker that would require a capacitor trip has a full wave bridge rectifier in it's closing circuit ( it is built into the breaker ). I think that recitifier is a 400 volt, 20A model.   The closing coil & the trip coil only come in DC.  The breakers weigh between 250 & 1900 pounds.
http://www.eaton.com/EatonCom/Markets/Electrical/Products/CircuitProtection/MediumVoltagePowerBreakers/index.htm   
The square ones are 36 inches wide.
They have a universal type motor to charge the springs. Basically a drill motor.  Potential spike generator.   We have been using the CTD-2 for years with no problems, then last month...    The vendor claims the MOV is there to protect the 250V capacitor as spikes will charge it above the usual 169 volts.  If that is the case I think he has chosen the wrong device.

The relays are protective relays, that run complex algorithims to analyze the wave forms from the current transformers & potential transformers.  They serve as the brains where as the breaker is the brawn.  I am sure their input protection is more soficsophisticated than the CTD-2.  These devices sell for 10s of thousands of dollars. 

http://www.selinc.com/products/ 

[SG-1]

130V MOV Results

All readings are in Volts AC RMS.
I changed the 10K 1/4 watt resistor to a 4K 12 watt wire wound resistor.
The 1 watt MOV did get slightly warm to the touch.  The voltage was only on long enough to capture the data.

INPUT VOLTAGE           MOV VOLTAGE
129                           129
134                           134
139                           139
144                           143
154                           150
170                           159
180                           164
200                           171
225                           177
250                           182

[Zero999]

I assume you tested it with AC voltage?

If so, did you use a true sine wave meter?

If not the results could be distorted.

An oscilloscope is the best way to test this but you'll need to run it off an isolation transformer for safety reasons.

[SG-1]

As stated the voltages are in AC RMS.  I used a Fluke 196C.  I am just too blonde to post the pictures. I have found additional options, browse, I selected a j-peg, the location appeared in the attach box.  I do not see how to upload it. When I click on preview it does not appear in the preview only the text.

[SG-1]

Lets try some more.  Blue is the input voltage & red is the MOV voltage.  This is the 130V MOV.  I used a point & shoot camera to "capture" the screen. 

[SG-1]

The RMS values themselves are deceiving.  The MOV is holding the peak to about 200 volts & the wave is getting flatter with each increase in voltage. 

[SG-1]

Some new numbers that make more sense. It also seems to make more sense that if you are trying to protect a device from transients, that you actually want to protect it from the peak value, because the RMS wave form may be distorted, as it clearly is here.

 

[Zero999]

The RMS values themselves are deceiving.  The MOV is holding the peak to about 200 volts & the wave is getting flatter with each increase in voltage. 

Yes, that makes sense.

As you can see, the RMS voltage will be equal to the input voltage, until it starts clipping, then the RMS voltage will rise towards the limit of the clipping voltage (200V) but will never actually reach it. The maximum AC voltage you can connect to the MOV is 141VAC but to avoid clipping during normal operating in the nominal maximum operating voltage should be around 10% under the clipping voltage so that's 127V.

[SG-1]

UPDATE:
First thanks to everyone who helped by replying.

The manufacture has change the MOV from a 150 volt to a 175V model.  They have also increased the test voltage from 120 to 150VAC for 6 hours of the 24 hour burn in. Other tests, such as the capacitor discharge time, are also performed.  And like the man falling off the 100 story building & is passing the 50th floor, so far so good !   

[Zero999]

Well it looks like the voltage rating is the peak voltage which is odd for AC MOVs which are normally spevified at the RMS operating voltage,

Anyway, I'm glad it seems to be working.

[saturation]

Same here, and appreciate SG-1 for posting the photos.

Well it looks like the voltage rating is the peak voltage which is odd for AC MOVs which are normally spevified at the RMS operating voltage,

Anyway, I'm glad it seems to be working.