fluctuations in voltage level feeding cement plant 1

[usamaegypt]

Hi,
Our problem in a cement plant is a lot of shut downs due to power fluctuations. We are facing a voltage drop in the main feeder that feed the plant and we would like to know how to mitigate the effect of this problem to aviode kiln shut down.
Best regards.

 

[ScottyUK]

Given the variety of equipment in such a plant, and the variety of voltages at which it is fed, could you possibly give us a clue which bits of equipment are affected by the fluctuations? The solutions will be slightly different if, for example, the problem was a PLC shutting down as compared to the problem being an 11,000V drive tripping on U/V.



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[jraef]

I think the first order of business is to determine the cause of the voltage fluctuations. Is it incoming from the utility, or are you causing them when turning on large loads? If the latter, there are a number of ways to mitigate that in your own facility to help prevent it in the first place. If the utility is fluctuatng on its own without your influence, the solution lies with them. All you can do is mitigate the effects.

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[usamaegypt]

actually the voltage drop coming from utility and we need to know a way to mitigate its effect to aviode shuting down our kilns

 

[edison123]

Transformer with on-load tap changer with automatic voltage control ?

* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[ScottyUK]

A few options:

Lower impedance source transformer - be sure to consider the corresponding increase in fault level.

Stengthen the infeed from the utility - larger conductors, double circuit, etc.

Take supply at next voltage level up, e.g. at 33kV or 66kV instead of 11kV, or whatever applies in you locality.

You haven't detailed what equipment is affected by the drop in utility voltage. Also, is this a transient effect during motor starting, or a longer-term problem with low voltage being caused by high load from the kilns?



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[rcw retired EE]

ScottyUK is suggesting the correct approach. Find out what is tripping your kilns. That will give the information needed to determine a solution.

A voltage disturbance study by a Detroit, Michigan (USA) utility with a lot of auto industry plants in its service area, found most plants were loosing production on voltage drops because they were using 120VAC sensing voltages on their PLC inputs. During a voltage dip, the motors and process were OK but the PLC would register false loss of inputs and dump the process. Switching to 24 VDC inputs or adding small constant voltage transformers in the PLC panels solved the majority of the problems.

A refinery I worked at tripped on voltage dips and faults. Some proposed a new 150 MW power plant as a solution. My boss had me investigate the root cause. A vibration monitor was tripping a 2500 HP instrument air compressor. Then the plant tripped as pneumatic control systems went down. Adding a constant voltage transformer, and a minor change in the tripping circuit cured the problem.

At another facility, we switched to DC control power on some critical motors. At a kiln facility we used dual feeds and transfer switches and a PLC with an auto-restart program that automatically restarted the motors quickly after a power loss to keep the kiln in operation.

You need to know the problem before you design the solution or argue with the utility.

Install some power quality monitoring equipment to determine the duration and extent of the voltage dips at the utility feed and other critical bus locations in your plant. The solution to a fast dip is different than the solution to a 5 second outage or a breaker reclose.

Good luck.

 

[PowerfulStuff]

rcwilson,
I wholeheartedly agree. I also worked on a cement plant with tripping issues and again a number of large, expensive options were being considered. By actually identifying the problem, it was possible to remove 90% of power related trips. Solutions were as simple as ridethrough settings on some VSD's and ensuring that no control circuits were accidentally connected to non-battery backed supplies.

As a side note, we complained bitterly to the power supplier who werent't very interested. Recently the cement plant (competition) down the road closed, and there hasn't been a dip since!

Regards,
PowerfulStuff

 

[usamaegypt]

we have metering instruments on the incoming voltage which is 35 KV and we have the following voltage drops:

duration:
(s) voltagr Magn. %
0,1900 55
0,1700 63
0,1000 79
1,7500 73
0,1500 46
0,1790 45
0,7500 37
0,0112 155
according to that big MV induction motors trip by undervoltage protections. We are thinking to use dynamic voltage regulator , dynamic UPS , or transformer with thyristor tap switchs , please advise

 

[stevenal]

Delay your undervoltage trip time.

 

[itsmoked]

usamaegypt those nice metering results are over what period of time??

 

[usamaegypt]

itsmoked , please find attached hereinafter the voltage dips measurements associated with dates

date: duration(s) voltage magnitude %
16.6.05. 0,1900 55
8.7.05. 0,1700 63
9.7.05. 0,1000 79
21.8.05. 1,7500 73
24.8.05. 0,1500 46
10.9.05. 0,1790 45
30.9.05. 0,7500 37
4.10.05. 0,0112 155

 

[itsmoked]

Thanks usamaegypt.

Looks like the nasties happen about twice a month..

stevenal may be correct. If you set your under voltage trip time to 1 second you'd have avoided all of these but one.

Would that work?

I guess it would be important to know "What" is shutting down your plant during these fluctuations. Is it just the plant central control computer? Or are other things bailing like contactors that must be reset, etc.?

 

[usamaegypt]

itsmoked,
what happen as follows:
ID fan motor , 2 MW slip ring motor , 6 KV , trip by under voltage protection , and some of the contactors lose holding , also VSD trip , but not so frequently by a fault , so please advise how to mitigate the effects
Regards.

 

[itsmoked]

I see your problem now.. Ideas Guys?

 

[PowerfulStuff]

usamaegypt,
I go along with the suggestion of adjusted UV time and propose -
* UPS or batteries depending on the control supply. This should be quite inexpensive as you only have short periods of backup required.
* Study of VSD parameters. Most newer VSD's have a ridethrough system where they suck control power back from the motor if the power is interrupted for a brief period and ramp back up to speed once the power returns. It often defaults to disabled. A look at where the control supply comes from may also help - if it's off the AC incoming supply, connecting this through to the control UPS may remove problems. A more obscure solution that I've seen is to add additional capacitors on the DC bus, but a first step could be to check that the existing capacitors are still OK.

Regards,
Martin

 

[RalphChristie]

usamaegypt:

Something you can look into for the contactors are voltage-dip proofing inverters. (

http://www.dipproof.com)

We are using some of it in some of our plants, but I can't really comment on how good they are, because I am not much involved with it. However, there are mixed feelings between the different sections using it, some say it helped a lot in their areas, while others want to get rid of it.

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[stevenal]

The undervoltage tripping device is to protect the motors, but the heating effect takes time to do damage. That's why I suggest increasing the delay. Control circuits can be fed by DC from a charger/battery system, or a UPS, or a constant voltage transformer. If you use a UPS, I suggest the on line type. I have heard that switching style UPSs can drop a contactor upon transition. Before scattering a bunch of UPSs around the plant, make sure you think about how they will be tested and maintained. I believe the dynamic voltage regulators are more of a whole plant type of device. You motors and heaters should be able to ride through most of these events, so I believe that would be overkill.

These events are caused by faults on the utility lines. If utility has a good right of way maintenance program, there's not a whole lot more they can do without huge amounts of money. The most cost effective solution is to address the ride through of your equipment. Make sure you have something in your specs for purchasing new stuff.

 

[RRaghunath]

From what I have seen, generally the voltage dips are caused by large motors starting on weak power source.

If this is the problem, you need to examine appropriate options including making sure that the starting currents are low by use of VSDs or FCMA drives whatsoever.

In case of resistance starters for slip ring motors, it is important to see that the starter mechanism is working well in controlling the starting currents.

In one cement plant, I have come across the complaint that the incoming utility line is old and is not maintained properly by the grid authorities resulting in frequent faults and excessive voltage dips, some times lasting for many seconds as the protection system also fails to work properly.
If that is the case, I think the solution could be getting another line or if you have in-plant generation, going in for effective islanding scheme to isolate the important drives from the utility power supplies and put them on the in-house generation as and when needed.