Protection System For Dc Drive

[AB2005]

Dear Friends,

 

I have 95KW two old DC drives at our corrugator's single facers. The drives have 6 thyrister protection fuses in series of incoming power. During a problem in machine, fuse are blown which are very costly. I want to install an electronic over current monitoring system. Locally, we have Electronic Over current Relays with CTs for monitoring the AC line current. Is it possible to install that relay for monitoring the incoming 3 phase current?

If not, then what is the recommendation from my friends?

[jOmega]

Dear Friends,

 

I have 95KW two old DC drives at our corrugator's single facers. The drives have 6 thyrister protection fuses in series of incoming power. During a problem in machine, fuse are blown which are very costly. I want to install an electronic over current monitoring system. Locally, we have Electronic Over current Relays with CTs for monitoring the AC line current. Is it possible to install that relay for monitoring the incoming 3 phase current?

If not, then what is the recommendation from my friends?

 

 

The fuses are there to "protect" the thyristers ! As such, if you were to pull the specs on the fuses, you would see that they are very fast acting. IF you can "find" a relay system that would meet or exceed the response time of the fuse(s) ... then theoretically it would be possible to replace the fuses. However, consider, that the mfgr. of the dc drives was/is well aware of cost of the fuses. Now don't you think that if his engineers had been able to find a less costly solution—i.e., current transformer/relay or electronic switch that would protect the thyristers, that he would have provided such a solution ? Ya think ?

 

In the face of a fault condition, the fuses are selected so that their I²t characteristic will allow the fuse to clear before the thyrister junctions melt or fuse open.

 

Having been involved with dc drives in corrugator applications in the 60's, 70's, and 80's I can say without equivocation that if you are experiencing fuse failure, then you have a problem that needs to be investigated and solved. Properly applied it should be an extremely rare occasion that a fuse fails. If they are failing with some frequency of event... you've got a problem that is exceeding the design capability of those drives. You need to find the problem and correct it; not look for a fuse substitute.

 

Single Facer drives are very simple applications with essentially constant loading. If you can be sure that the dc drives are in good working order, then look at the motor, wiring to the motor, a ground fault perhaps? and then look for a mechanical problems. Here's a thought, is the CURRENT LIMIT on the drive working ? If the drive isn't going into CL.... but the fuse(s) are blowing then the thyristers are being asked to supply a very high current with undoubtedly a fast rise time. Could be mechanical i.e. flute ... could be electrical i.e. mtr et.al..... If the drive(s) are going into CL, then you have a load or motor problem that needs to be investigated.

 

Your approach— i.e. looking for a less expensive replacement for the fuses— reminds me of the fella who tried to convince his girlfriend to take Penicillin to cure his kidney infection.

 

Good luck.

 

 

 

[AB2005]

Dear jOmega

 

Welcome back, long time passed to see you here.

Thanks for detailed answering. I don't want to remove the fuses from circuit. I just want to add an equipment for monitoring the current. Normally, our single facers run approx 60% of full load current and want to set 70% value to the current monitoring equipment. The drives are very old and don't have any internal protection like over current.

 

[jOmega]

Dear jOmega

 

Welcome back, long time passed to see you here.

Thanks for detailed answering. I don't want to remove the fuses from circuit. I just want to add an equipment for monitoring the current. Normally, our single facers run approx 60% of full load current and want to set 70% value to the current monitoring equipment. The drives are very old and don't have any internal protection like over current.

 

 

I think it's been about three years—give or take. After a massive system crash and major power problems following a lightening strike about 20 feet from the house, I'm slowly getting things restored and recovering data etc. Also took a left turn in interests and have been pursuing other fields of interest.

 

Looks like I misread your original post a bit. Sorry about that.

 

You said:

Locally, we have Electronic Over current Relays with CTs for monitoring the AC line current. Is it possible to install that relay for monitoring the incoming 3 phase current?

 

The answer is yes, it can be done.

 

First let me seek concurrence about the configuration of the SCR or Thyristor or Thyrister bridge in your drives... you said there are six Thyristers in the power converter bridge of each drive. Here's where I need clarification.... Some configurations used fuses in series with each of the six Thyristers ..... ergo, six fuses. Others only fused the three-phase AC input... ergo, three fuses.

 

Which one is your configuration?

 

(by any chance are the DC Drives old Reliance Electric Stat II's ??... and whose corrugator: Langston ? S&S or SS ?)

 

 

Second, let me ask what your intention is for when the relay operates indicating that a current threshold has been exceeded ?

 

Kind regards,

jΩ

[AB2005]

Dear Mr.jΩ

 

I am sorry.

1) The drives are 4 Quadrant type and having 6 thyristor MODULES and each module has 2 thyristors. There are 6 thyristor protection fuses installed in the series of Power incoming.

 

2) The drives are BMB branded (Italian)

 

3) Corrugator machine is AGNATI brand (Italian)

 

4) Motor is 94Kw 256A. Normally its current remains up to 150A. I want that the relay operate if current has been exceeded from 180A.

 

 

Best Regards

 

 

[jOmega]

Dear Mr.jΩ

 

I am sorry.

1) The drives are 4 Quadrant type and having 6 thyristor MODULES and each module has 2 thyristors. There are 6 thyristor protection fuses installed in the series of Power incoming.

 

2) The drives are BMB branded (Italian)

 

3) Corrugator machine is AGNATI brand (Italian)

 

4) Motor is 94Kw 256A. Normally its current remains up to 150A. I want that the relay operate if current has been exceeded from 180A.

 

 

Well you've piqued my curiosity. Not familiar with either that brand of dc drive or corrugator.

 

Six SCR's (Thyristors) Forward motoring; Six SCR's for Regen, correct? Three fuses for the Motoring SCR's and three fuses for the Regenerating SCR's, correct ?

 

If so, then the fuses that are failing are in which path ..... motoring ? or regenerating ? Both ???

 

Same fuses each time ?

 

How's the plant power ? Ever looked at it with an O'scope ? relatively clean ? or notches and spikes on the a-c sinewaves ?

 

Can you describe the operating mode of the drive when the fuse failure occurs ? (i.e., threading the Flute? accelerating up to operating speed ? running at fixed speed ? decelerating to stop ?, etc. )

 

Is there a large inductor in series with the motor armature ?

 

Are there smaller inductors in sereies with the a-c input of each SCR or in series with the a-c input to the SCR's ?

 

Is the AC supply common to each bridge ? or does each bridge have it's own supply ? from the secondary of a transformer ? (separate secondary windings for each bridge..... or common secondary winding for both bridges)

 

(Trying to get a mental picture of configuration).

 

Motor Shunt field supplied from a fixed, constant supply ? (i.e., no field weakening) ..

 

 

I can't imagine the drives not having Current Limit and Instantaneous Overcurrent Trip (fast rising high peak current like in a shoot-thru or a regen fault occurs) .... We had this at Westinghouse and Reliance and other places back in the 60's ... Relying on just the fuses for protection is insane !!!

 

I should also tell you that you could consider using your current sensing relay to operate a shunt-trip circuit breaker to KILL the a-c input to the SCR bridges.... An aux contact on the circuit breaker could be used to initiate an emergency shutdown of the corrugator line. But what a waste of product !!!!!

 

'twould seem the better solution would be to either upgrade the drives (one-time cost vs ongoing cost of lost production) ... or to find and fix the cause of the problem.

 

(excuse typos.. eyes are tired from squinting trying to read this small font on 46 inch monitor... new glasses soon)

 

 

Have a good weekend.

 

 

 

 

[marke]

The fuses are (should be) selected to provide over current protection for the semiconductors.

Semiconductor fuses are "energy limiting" fuses and will interrupt the current at an energy let through below the failure energy of the device.

If you wish to replace the fuses with an electromechanical switch, there could be problems because the energy let through under fault conditions would be too high.

If you wish to add an electromechanical switch in addition to the semiconductor fuses, that would be acceptable.

 

One of the problems with semiconductor fuses is the wide variation between the "pre-arcing i2t" and the "total clearing i2t".

 

The fuses begin to fail at an energy that is much below the clearing energy resulting in apparent premature fuse failures.

If the pre-arcing i2t is low, then the electromechanical protection may not be fast enough to prevent the fuses from failing.

 

Sometimes, it is possible with careful selection, to select an alternative fuse with the same clearing i2t and a higher pre-arcing i2t.

 

Best regards,

[AB2005]

Dear Mr. jOmega

 

Sorry for late posting the answer. I tried many times to open the forum but IPS error occured. I dont know about this error may be Mr.Mark can explain well. Today i tried and found the forum openned. Here is my answer according to your questions;

 

 

I am not sure that six thyristors are for Forwarding and six for regenerating. I know only that the single facer can be run Fwd and Rev. I think three fuses for Fwd bridge and three for Rev. (Sorry for my lack of knowledge about regenerating drive)

 

Fuses are blown for both Fwd & Rev bridges. Some time 3 fuses and some time 4 or 5 fuses blown.

 

I did not checked the plant power as many other drives of this machine and other machine work without any trouble and many other plants (Paper and Board mill) are also connected at the same line.

 

Fuses are blown some times during Acceleration and some time during Deceleration but most of the time during deceleration.

 

Is there a large inductor in series with the motor armature ? NO

 

There is a three phase line reactor in series with incoming power.

 

AC supply is common to each bridge. There is no isolation transformer.

 

Yes, motor shunt has a separate power supply bridge.

 

Actually, I don’t have manual for this drive for better understanding about its Current Limiting feature.

 

 

Thank you very much for spending your precious time for replying.

 

 

[marke]

Hello AB200

Fuses are blown for both Fwd & Rev bridges. Some time 3 fuses and some time 4 or 5 fuses blown.

 

The forward bridge and reverse bridge are the same, but have a reverse polarity. If SCRs from both bridges are enabled at the same time, you will have fuses from each bridge fail. If the SCRs from only one bridge are enabled,then only fuses for that bridge will fail.

To have fuses from both bridges fail at the same time suggests that there is a fault in the control system that is enabling both bridges at the same time. This is like have both contactors on a reversing contactor closed at the same time. The smoke comes out somewhere.

In this situation, where both bridges are enabled simultaneously, current limiting will not help Effectively there are two SCRs in series across two phases and this effectively is a short circuit across two phases and the fuse will fail.

 

best regards,

Mark.

[AB2005]

Now true pictures has been appearing. If both sides fuses are being blown, it means there is some problem in control circuitry or a very critical situation appears. Good direction to be followed.

Thanks