480 Volt 3 PH Soft Starter By Pass 5

[jburn]

Hi, All

I just read the 12 or so, different post about soft starters, and I have a different question reguarding the use of these devices.

We have installed several SMC soft starters over the past six months. They are on a 25 HP motor, 480 volts 3 PH. The 480 volt power system is an ungrounded system. IE the unit sub is a 2500 KVA delta - delta transformer. They had been running OK for several months. The motor FLA is 31, but the normal running load is only about 12 amps. Very lightly loaded.

Last week, we had several of the soft starters shut down, within seconds of each other. We noticed, that at about the same time we had a single phase ground, coming and going. Several hits every few seconds, lasting for up to 20 min.

When we looked at the phase to ground voltages, using a Dranetz analyzer, we see transients up to 900 volts, when these grounds occure. Apparently the soft starters don't like the "Noise" on the line, and shut down.

My plan is to install a 3 PH contactor in parallel with the soft starter, and have it by pass the soft starter aftr the motor is up to speed.

Question, do I need to size the by pass contactor, based on the motor HP, or can I use a smaller unit, sense it will only see runing current and not starting currents?
The soft starter unit itself, has 3 small relay contacts, used to by pass the SCR's, after the motor gets up to speed.

The bypass contactor will be used to disconnect the motor from the line when the motor is stopped, and / or when the motor overloads take the motor off line.

Thanks for your response.

 

[busbar]

Please search this site for extensive discussion and references on ungrounded and [high] resistance-grounded electrical systems. Transient-overvoltage problems cannot be fixed with bypass contactors.

 

[jburn]

busbar
Thanks. I have read most of the post about ungrounded 480 volt systems. I have studied the subject in great detail and understand the issues with unground systems.

I have ordered a high resistance grounding system to be installed on this unit sub. However, I have about 40 unit substations, and all are delta - delta. This plant has operated for 60+ years with unground systems.

It's these new soft starters, that is this weeks problem. I would like to install the HRG system on all of the unit subs, but need the $$$. This problem may now help me get the $$$, as it did for this one substation.

The by pass contactor will keep the machines runing. As a electrical reliability/power quality engineer, my first need is to keep the production rolling, and second, improve the over all power quality.

Question; Do you think the HRG unit will clean up the "Noise" to the point I would not need the by pass contactors? I'm not sure it will.

I have read reports that indicate, by adding the HRG units, the number of motor failures and grounds on the system will go down greatly. Has anyone seen published data of studies on this?

Thanks

 

[busbar]

1. jburn, if I understand your description, ‘bypass’ contactors leave the VFD line-side rectifiers and load-side transistors still exposed to ø-g transients.
2. High-resistance grounding {and conversions of ungrounded systems} for low-voltage distribution have been discussed in IEEE Transactions on Industry Applications for at least 30 years. [Old habits die hard, I guess.] Search on author J P Nelson, for one.
3. High-resistance grounding is not an absolute panacea, but is so fundamental for low voltage systems that it should be close to the top of the list where ground-overvoltage problems are encountered. Ungrounded-system problems are well recognized and have been publicized since at least 1955 [Beeman].
4. Use your “analyzer” for before-and-after comparisons. Process of elimination.

 

[jburn]

busbar,

Thanks. I have worked with ungrounded systems for over 30 years myself, in several different industrial plants. (23 years in this one) I purchased a copy of Beeman's book years ago, and it is well used.

 

[dpc]

Actually, use of bypass contactors on soft starters is a good idea, even without transient problems. Some manufacturers (Cutler Hammer, I think) provide bypass contactor as a standard feature.

The soft starter semiconductors have voltage drop and heat loss even when continuously conducting and the bypass contactor eliminates most of this.

If you are installing your own bypass contactors, I think they need to be fully rated for the motor horsepower. You also have to make sure that you don't bypass your overload protection, obviously.

If you're trying to reduce contactor size/cost, you could go with IEC-rated contactors instead of NEMA. I wouldn't do it, but it is an option if you have physical space constraints.

 

[Marke]

Hello jburn

Bypass contactors can be used with soft starters and will provide extra protection to the SCRs against current transients while running and reduce the power dissipated by the starter during run also. Additionally, because the SCRs are shorted during run, they will be cool for the next start. This can sometimes allow a larger motor to be connected to a given starter.
The bypass contactor is only going to carry rated line current and will not be in circuit during start, therefore it does not require any overload capacity. Effectivley, the current carried by the bypass contactor looks like the current carried when controlling a resistive load. An AC1 rated contactor with a thermal current equal to or greater than the rated current of the motor is what is needed as a bypass contactor. This will be smaller that the typical motor rated (AC3) contactor for this motor.
Best regards, Mark Empson

http://www.lmphotonics.com

 

[jburn]

dpc & Mark

Thanks for the replies. You have been a big help. In my case, the SMC I am using has internal bypass contact around the SCRs. The SCRs are not being damaged, the SMC units just drop off line. The additional bypass contactor I plan to install will be used to get the SMC electronic trip circuit out of the picture. The additional bypass contactor will also have a set of motor overloads. The duty cycle for this application is only one start every two or three weeks, or less.

 

[jraef]

jburn

The only comment I want to add to Marke's is that using a contactor that is rated for Across-The-Line starting gives you the added redundancy of being able to start the motor should the soft starter fail. Compare the extra contactor cost with the cost / hour of downtime of your machine. Otherwise he is correct in selecting the contactor for the running current only. This method is called a Shunt Bypass by the way.

Something you need to consider very carefully. You did not mention which version of SMC is being used. Some include overload protection and some do not. If yours is the kind with OL built in, you will be bypassing the OL when you bypass the soft start. Check it carefully before implementing.

In addition, I am curious as to which SMC you do have. I was not aware of any that include SCR bypass.

Quando Omni Flunkus Moritati

 

[jburn]

jraef

Both Allen Bradley, and Cutler Hammer have units with an internal bypass. The AB SMC-3 unit has an internal bypass. The Cutler Hammer models S751, and S801 both have an internal bypass.

 

[jraef]

Thank you. I know about the CH unit, but I have not yet seen the SMC3. Quando Omni Flunkus Moritati

 

[tmahan]

Jburn,

I am curious as to how you have determined that the transients are what is causing the problem. The SCRs should just be fully gated and the any transient voltage should just be passed through to the motor (not good for the motor maybe but no problem for the SCRs).

I would suspect the problem is related to one of two things. Since the softstarter is on such a lighly loaded motor 12A vs 25A. I suspect that you have the "energy saver/Solid state heater option" on. If this is the case you may want to try and shut this off. The SMC is looking at PF and may interpet the sudden change caused by the transient as a "Line fault" as described in the AB manual.

The other possiblity revolves around the control power for the softstarters. During the fault the control voltage could be going either high or low. This could also be causing the problem. Try a seperate 120V supply for a while if possible and see what happens, if it solves the problem use a UPS or ther line voltage conditioner on the 120V side

 

[jburn]

tmahan

I have a system in place that capatures, time stamps, and stores every ground event in a database, on all of my substations. I can look at the past history of ground activy on any substation. Every time I had an SMC drop off line, I had recorded ground activity on the substation, that the SMC was on. The corralation left no doubt that was the problem. After the first loss, we installed a Dranetz analyzer, and we recorded the transients on the system, phase to ground, with each ground strike.

 

[jbartos]

Suggestion to jburn (Electrical) Jan 30, 2003 marked ///\\busbar
Thanks. I have read most of the post about ungrounded 480 volt systems. I have studied the subject in great detail and understand the issues with unground systems.
///This is great.\\ I have ordered a high resistance grounding system to be installed on this unit sub. However, I have about 40 unit substations, and all are delta - delta. This plant has operated for 60+ years with unground systems.
///Please, have you performed high-resistance system grounding calculation? What are the results, e.g. for the ground fault current? What is your cables Insulation Class Level?\\ It's these new soft starters, that is this weeks problem. I would like to install the HRG system on all of the unit subs, but need the $$$. This problem may now help me get the $$$, as it did for this one substation.
///Yes, indeed. Especially, the arcing problem with soft starters being bypassed could make money available very quickly, considering that many motors may burn simultaneously by high overvoltages.\\ The by pass contactor will keep the machines runing.
///Yes, however, in very risky operation that you are very well aware of as a reliability/power quality engineer.\\ As a electrical reliability/power quality engineer, my first need is to keep the production rolling,
///Yes, however, this will be at very high risk to various loads especially to motors.\\ and second, improve the over all power quality.
///Sometimes, risks pay off handsomely.\\\
Question; Do you think the HRG unit will clean up the "Noise" to the point I would not need the by pass contactors?
///HRG does not mitigate harmonics. It only allows one fault to ground for a period of time determined by the cable Insulation Class, 100%, 133%, or 173%.
I'm not sure it will.
///Yeas, you are justifiably not sure.\\ I have read reports that indicate, by adding the HRG units, the number of motor failures and grounds on the system will go down greatly.
///Yes, especially when voltage Total Harmonic Distortion (THD) is less or equal 5%.\\ Has anyone seen published data of studies on this?
///There have been some articles published in EE Journals, when the ungrounded systems were gradually commuted to grounded systems using system groundings.\\

 

[peebee]

Regarding your original posting, I definitely agree with Marke and would recommend fully-rated bypass contactors. At the very least, this will maximize your uptime by enabling you to easily start motors regardless of whatever other problems you're having with the SMC's.

 

[busbar]

High-resistance grounding modification could be implemented as a small site-assembled transformer set with a resistor. This is worth examination towards minimizing drive misoperation. With in-place monitoring to observe ground-overvoltage incidents, it should not be overlooked.

…very roughly—three 1kVA transformers—2kW resistor—class-J fuse block…

 

[RJF]

Comments to various posts:
We have a large number of ungrounded Wye 480 volt subs and a number of HRG equipped subs.
Question:
When you say 900 volts on the grounds - is this RMS volts to ground on the ungrouded phases? I have seen three instances (2 on a HRG Wye and one on a floating Wye) where the phase to ground went above 480 Volts to ground. In all cases the fault was a inductance limited ground fault downstream of a 480V lighting ballast. We saw some damage to electronic components (including the voltage meter relay connected across the HRG resistor). We made measurements with an RMS meter - not a scope or Dranetz.

Notes: On one ungrounded sub once I saw 550V DC on multiple phases of a sub - it was being generated by some sort of through fault on an 480V SCR feeding an oil heater.

As busbar notes, you can build your own HRG with some transformers and a resistor. We have built some portable units that we temporarily install on the ungrounded subs so we can use the pulser to track grounds. If you are going to leave it on for a long time, you will need to make sure the components are rated for that duty.

We have seen a number of problems with Drives on ungrounded and HRG grounded subs - mainly with them not liking the fact that the ungrounded phases can go 480 to ground. Some have mov's that are sized for a solidly grounded system and they blow taking the drive down. I'm not close enough to the group that works on these to understand why the drives are shutting down, but can ask. Does the SMC that you have give a fault code? Some of the drive manufacturers seem clueless about how to handle HRG subs's or ungrounded subs. If you carefully read the application manual, they often say thet you need an isolation 480Delta/480 Wye isolation trasformer in front of the drive.

 

[busbar]

A classic description of ungrounded-system damage is 67.115.161.42/dat/beemaIPSH6.doc

http://www.ab.com/manuals/dr/1336/1336-5.0.pdf

page 29 “Ungrounded Distribution Systems” suggests an interesting MOV configuration using four devices.

Applications papers at:

http://www.neiengineering.com/papers/paper1JN.pdf

http://www.ieconsult.fr/ect178.pdf

 

[jbartos]

Comment on the first link in the previous posting. The MOVs can dissipate so much of energy only, e.g. 160 Joules/phase conductor as included in schematics therein. If the energy in overvoltages, e.g. transients due to arcing are more lasting, there is a good chance that the MOV will burn, if not protected by thermal sensors implementing the system shutdown and by an overvoltage relay. The high-resistance or medium resistance system grounding is a safe solution.

 

[jburn]

RJF

The SMC units do have a fault code. We are getting "Phase Loss/Loss of Load" and sometimes "Phase Imbalance". In talking the some of the design engineers from two companies who make SMC's, isolation transformers, or reactors in front of the SMC,s will not help. One of the problems I think we are having with the model we are using, is that the faults (listed above) respond in 16 to 20 ms, and they can not be disabled. Other manufacturers build in a 10 sec delay, plus you can disable these trips functions.

We always use isolation transformers in front of VFD. We have two VFD with isolation transformers on these same machines. They are not dropping out. Thers is a big difference between VFD and SMC's.

The 900 volts I referred to are tranisits. You will only see them with a fast scope or units like a Drantex.

Because the transits are phase to ground, and not phase to phase, I almost have to beleive the "Noise" is some how getting into the microprocessor of the SMC, and it goes bonkers, and shuts down the unit. The only connection to ground the SMC's have is on the 120 volt control power to the unit.

I agree with your statement "Some of the drive manufacturers seem clueless about how to handle HRG subs's or ungrounded subs." I did finialy get the applacation engineer at the factory to say that the model SMC I am using will not work and they did not have anything to offer to resolve the problem. It was my idea to just bypass the whold SMC and turn it off after the motor is up to speed. This is a new model, and has not been on the market very long. We may have been one of the first sites to use these in large numbers. This same manufacture a few years ago also told me I did not need isolation transsformer in fron of their VFD. From my experience Not true, with an ungrounded systems.

There has been a lot of discussion on this forum about ungrounded 480 volt systems and the need to use HRG. I wish I could install HRG units on all of my 480 volt substations.

Just as a finial note. We found the ground. It was an arcing ground, that was coming and going. We found it on one wire, just before it entered a motor pothead. Vibration of the equipment was causing it to come and go. If you reed Beemer's book, this is the worst kind of ground to have. If it had beed a solid ground that stayed on, it would not have caused the problems with the SMC's and we could have found it very quickly.

The way the electricians found this one, was by camping out in the substation. When the arcing ground returned, they quickley turned on our "Tracking Equipment" and when they did it welded the wire to ground. Now with a permenet ground they were able to track it down in about 5 min. Took another 15 to get the production department to shut down the pump, which them confermed that it was the bad guy.

One other note. We had just installed a ground alarm system on all of our 480 volt substations. It had only been on a few weeks. I get a a notice of all grounds on the substations, on my desk computer. With out this arlarm system, we would today still not know that we had the ground we did, and it would still be there. The cost of the ground alarm system has more that paid for itself.

Only problem now is that we are keeping two electricians busy tracing and fixing grounds. Which is good. Before the alarm system we did not know these ground existed.

If we had HRG units, we would not have as many interminit grounds in the first place!!!

Hope this answered you questions.