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TRIPPING OF HARMONIC FILTER
2

TRIPPING OF HARMONIC FILTER

TRIPPING OF HARMONIC FILTER

(OP)
We have a 6,6kV 6MW 6/12 pulse thyristor Mine Winder with a 3,1 MVAr 3rd order damped filter (Resistor and decoupling capacitor across the reactor) tuned at 4,8p.u. connected to the same busbar (no other loads). Acceleration of the winder peaks the load at about 15MVA.

Measurement at approx. 500 ms averaging shows normal harmonics for the winder with the filter out of circuit. With the filter in circuit acceleration of the winder results in immediate tripping of the filter on harmonics (the filter is rated for 90% harmonics at 5th).

Parallel resonance below the tuning frequency occurs at exactly 4p.u. We measured the filter harmonics during the trip and managed to see one set of readings as the winder accelerated before the filter tripped. The 4th harmonic was at around 350A on all phases (it could be and is likely,  based on the levels that the relay saw, that the peak 4th is much higher).

The 4th without the filter in circuit is 4-5A giving an amplification factor of anywhere between 90 and 300.

Simulation of the network shows a max. possible amplification of 25. What on earth is going on?

Is it possible that some interaction is occuring between the filter and the winder and is it related to the lack of damping on the busbar?

In South Africa we have relatively high levels of harmonics due to the many mines and therefore are very experienced in the field of harmonics. However, this is difficult to explain - the max. amlification that we have seen in 20 years is 40x and that situation was theoretically  explained.


RE: TRIPPING OF HARMONIC FILTER


Long-shot possibility from ieee.org
Paul C. Buddingh, Even Harmonic Resonance—An Unusual Problem  IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 4, JULY/AUGUST 2003 p.1181

Abstract—This paper is a case study examining the cause, effect, and solution of an actual “noncharacteristic” even harmonic problem in an electrochemical plant. While issues concerning “characteristic” harmonics are well documented in the literature, limited information is available describing how noncharacteristic harmonic currents create operational problems. The author has not found any papers describing a case of even harmonic resonance and would like to share the experience of resolving a 4th harmonic resonance problem in the hopes it will benefit others.
   

RE: TRIPPING OF HARMONIC FILTER

The tuned frequency of a harmonic filter is related to the kVAR of the capacitance and the reactor rating. In the case of the capacitance, the operting voltage and the rated voltage are different due to the design approach. Therefore, there is a possiblity that the filter tuned frequency is lower than the expected 4.8. If you can provide all the parameters of the filter I can verify this specific issue.

RE: TRIPPING OF HARMONIC FILTER

(OP)
We are specialist consultants. The filter tuning frequency is definitely 4,8.

What we believe now is that the filter parallel resonance (which is spot on 4th)is amplifying the small 4th (4A) 25 to 30 times (100A - 120A), causing synchronising problems with the thyristors. This in turn is causing the winder to generate more uncharactaristic harmonics (4th) and we have a snowball.

Does this sound feasible?

RE: TRIPPING OF HARMONIC FILTER

Suggestion/Questions:
1. How do you perform the filter tuning?
2. Where is the 4th harmonic coming from?

RE: TRIPPING OF HARMONIC FILTER

(OP)
1. V=6,6kV; C=220uF/phase; L=1,92mH (the damping circuit will not affect the tuning, although for interest the resistor is 5ohms and the decoupling cap 8uF).
2. Any thyristor converter will generate some uncharactaristic harmonics. This winder draws some 2000A during acceleration with peak 4th harmonic currents of 4-5A. This is a very small harmonic but we believe that with amplification the resultant voltage distortion is causing some firing problems.

RE: TRIPPING OF HARMONIC FILTER

Suggestion to the previous posting:

1. 4A of 4th harmonic is 5A/2000A x 100%=0.25% which appear to be negligibly small

2. 6-pulse conversion will generate 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th, ... characteristic harmonics

3. 12-pulse conversion will generate 11th, 13th, 23rd, 25th, 35th, 37th, ... characteristic harmonics

If the 5th harmonic is present and filtered then the 6-pulse conversion is applied

4. The untuned harmonic filter can absorb an excessive amount of harmonic current.

5. References:
5.1 Steve Ashmore, Manager, Harmonic Filter Systems, High Voltage Capacitor Division, "Capacitor Banks and Harmonics," 1977 ABB Electric Utility Conference, Subject VII-3

Among other contents, it states that the tuning frequency has to be accurately controlled by design, ....

5.2 Derek A. Paice "Power Electronic Converter Harmonics, Multipulse Solution for Clean Power," IEEE Press, 1995

RE: TRIPPING OF HARMONIC FILTER

(OP)
4A is small untill it is amplified 30x.

All thyristor converters generate some uncharacteristic harmonic.

The filter can handle 90% 5th harmonic. It is however tripping on about 1500% harmonic overload.

2nd harmonic is not being significantly amplified and is therefore negligible.

Filtering out the small 4th is not an option. Avoiding significant amplification is, and if the insertion of some damping load does not sort out the problem we will probably convert the filter to a 2nd order filter.

RE: TRIPPING OF HARMONIC FILTER

Suggestion: Apparently, the present filter design is amplifying the 4th harmonic due to its resonance.

RE: TRIPPING OF HARMONIC FILTER

jbartos,

Redwing stated this in his initial post. Have you just come to this conclusion?

RE: TRIPPING OF HARMONIC FILTER

For the given value of C = 220 MFD/phase and L = 1.92 mH, the calculated resonant frequency is 245 Hz. This is 4.9f for the 50 Hz system and 4.08f for the 60 Hz system.

What is the voltage rating on the capacitor unit?
What is the kVAR of the given unit?
How are they connected (how many in parallel and how many in series). This is required to see how much derating is to be applied on the capacitor unit.
Voltage rating of the system  = 6.6 kV (line-line)
Voltage rating/phase          = 3.81 kV
The available kvar from each unit will be much less than the rating of the capacitor unit.

RE: TRIPPING OF HARMONIC FILTER

(OP)
Yes, 4.9p.u. is correct. 4.8 was a typo but makes no difference.

Filter output is 3,14 MVAr, cap is single phase 44uF - 5000V, 1 series - 5 parallel, reactor is 440A, Vcap at nominal is 3976V.

I am battling to see what this has to do with my problem as I know how to work out bank ratings. It is the interation with the winder that I am interested in.

RE: TRIPPING OF HARMONIC FILTER

Suggestion to ScottyUK (Electrical) Sep 2, 2003 marked ///\\\
jbartos,
Redwing stated this in his initial post. Have you just come to this conclusion?
///No. There appears to be a peculiar situation. The unwanted harmonics are usually filtered out or prevented to happen. To filter out 4th harmonic is not an option. See reactive (Electrical) Sep 1, 2003 for the statement. To prevent 4th harmonic to exist, one would have to stop its generation that is allegedly by thyristor converters. Now, there are not that many options left, are there? It looks like some engineering mission impossible subject to further research. It may or may not be for free then.\\\

RE: TRIPPING OF HARMONIC FILTER

There are five capacitors in parallel in each phase with the following rating. 44  MFD/phase, 200 kVAR/unit,  5 kV/unit, five of them in parallel. The expected reactive per phase is 1,030 kVAR.  The operating voltage of the capacitor is 3.81 kV per phase. The delivered reactive power is 632 kVAR/phase. The corresponding per phase current at fundamental frequency is 165.8 A. The total capacitance per phase is 138.6 MFD. The resonant frequency will be 6.17f. The design resonant frequency is 3.9f.  The winder may or may not have a resonant frequency at 4f. But the dominant harmonic frequency at 5f due to the six pulse converter is able to excite at 4f. This analysis is not taking the resistance in to account.

There are ways to correct this condition. In order to evaluate the overall condition, the harmonic analysis program has to be used at various conditions. You may contact Practical Power Associates using the e-mail practical_power@yahoo.com. The owner of this company has extensive experience with filters for arc furnaces, dc converters and utility applications. He is the author of the book: Ramasamy Natarajan, Computer Aided power system analysis, Marcel Dekker, New York, 2002. In this book there are chapters on load flow, short circuit, stability, harmonic filtering, switching surge, lightning surge etc. written using recent computer programs. You can obtain this book through internet.

RE: TRIPPING OF HARMONIC FILTER

Comment: My suggestion to filter out 4th harmonic that is significantly increased due to resonance implied a modification of the current filtering scheme. Details of the filter parameters have not been posted.

RE: TRIPPING OF HARMONIC FILTER

(OP)
I don't know how you work out that 5x44=138.6. Hopefully you don't design filters.

As I have already stated I don't need help with the filter parameters, I know what they are. I have designed over 200 filters.

Jbartos, there is no way to get rid of the uncharacteristic harmonics i.e. 4th. The problem is that we are amplifying the absolutely normal, almost insignificant 4th.

The solution is to modify the filter to a 2nd order filter by removing the decoupling capacitor. This will increase the damping at lower frequencies. The problem is that the resistor will then be underrated.

My question was really to do with the interaction of the amplified 4th with the winder.

RE: TRIPPING OF HARMONIC FILTER

Suggestion to the previous posting marked ///\\\
reactive (Electrical) Sep 5, 2003

Jbartos, there is no way to get rid of the uncharacteristic harmonics i.e. 4th.
///Yes, I agree that traces of even harmonics are generated by switching scheme and devices that have to be applied.\\\
 The problem is that we are amplifying the absolutely normal, almost insignificant 4th.
///Agree.\\\
The solution is to modify the filter to a 2nd order filter by removing the decoupling capacitor. This will increase the damping at lower frequencies. The problem is that the resistor will then be underrated.
///This is what I also meant by "filter parameters (including design circuit) have not been posted." in my previous posting.\\\
My question was really to do with the interaction of the amplified 4th with the winder.
///What options are left for an engineer and designer of over 200 filters?
1. Prevent 4th harmonic generation - This is not possible due to hardware selection and design.
2. Filter out 4th harmonic - This is not possible as stated by the designer.
3. Inject 4th harmonic of opposite phase.
4. Perhaps, switch to Forum of "Electrical Eng-Tips Magicians"\\\

RE: TRIPPING OF HARMONIC FILTER

JB,

I like your idea of antiphase current injection. I was involved with some development work on active harmonic filters for LV applications quite some time ago, and had not made the connection with this application. Interesting idea, especially at this sort of power level.

RE: TRIPPING OF HARMONIC FILTER

(OP)
I think that the easiest and a relatively low cost solution is to install a higher rated resistor and remove the decoupling capacitor, resulting in much better damping at lower frequencies.

There are literally hundreds of these type of filters installed on winders without problem (although busbars with no other damping loads are rare).

My question wasn't really to do with the solution it was to do with the interaction of the amplified 4th and the winder which results in tripping of the filter.

Even if the 4A 4th is amplified 30x this only gives us 120A 4th which, although not great, the filter can handle (damping loads would decrease this significantly). However, we are seeing 4th harmonic currents in excess of 350A which indicates (my understanding) that there is interaction with the winder (possibly causing problems with the synchronising) which is in turn causing the winder to generate more 4th, in turn being amplified, and so on............

I wanted to know if anyone had had experience with similar interaction due to uncharacteristic harmonics.

Thanks

PS There is no black magic in electricity.

RE: TRIPPING OF HARMONIC FILTER

Reactive,

Is it possible that your voltage source is being modulated by the 4th harmonic i.e. giving you a 4th harmonic voltage component?

Can you measure whether there is 4th harmonic current contribution from the transformer, as well as from the rectifier? It seems like you have an energy source which isn't accounted for within the 4 or 5A originating within the rectifier. The passive type of filter you are describing will have a well defined Q-factor which you appear to have calculated, and this doesn't seem to account for amplification to 350A or more with the Q-factor quoted, so there must be another source of current.

I don't have the simulation tools I had at my previous employer, so I'm casting ideas forward to provoke thought.(or to be shot at!)

RE: TRIPPING OF HARMONIC FILTER

Perhaps you could model the filter and the winder in EMTP to find out whether, as you suspect, the distorted voltage causes an increase in 4th harmonic currents generated by the winder.

RE: TRIPPING OF HARMONIC FILTER

Reactive,

Did you finally solve your problem?

The last post on EMTP modelling is an excellent recommendation, and I also would recommend this. In my earlier years when working under a noted harmonics expert, he always recommended never allowing a first parallel resonant point sitting on the 4th. He never supported his advice, but said there were possible "control" interactions with the rectifier. I have never measured, read or seen this at work, but it appears you are living in the midst of what he spoke about. I would be interested in knowing what your actual conclusions are about your issue.

RE: TRIPPING OF HARMONIC FILTER

(OP)
What we have discovered is that the winder is operating in so called 'sequential' mode. Apparently this is a control system developed by Alstom specifically for the South African market due to relatively weak supplies in some areas. During the winder cycle this sequential control converts the firing from 6 to 12 pulse and back to 6 pulse and it appears that the problems occur during this conversion. Our theory is that in this sequential mode the control synchronising is highly sensitive to 4th harmonic no matter how small.

We have increased the damping on the filter and it now stays in circuit. We have therefore been able to measure the disturbances. As the winder converts from 6 pulse to 12 pulse the characteristic 6 pulse harmonics rapidly diminish and large uncharacteristic even harmonics appear (with the filter in circuit). The voltage THD just before this happens is less than 2%. Strange one!

Alstom are going to site on Monday to try the winder in standard 12 pulse mode to confirm that this will solve the problem. I will let you know.

Seems to us to be a case of a rarely encountered and largely untested control system.

RE: TRIPPING OF HARMONIC FILTER

Reactive,

Great info on a unique problem being debugged.

We have debugged several like this. One involved energizing a backup UPS with filters onto a backup gen. The transformer magnetizing inrush and UPS filter first parallel resonant point on the 3rd created an excessive transient over-voltage. We actually simulated the problem in EMTP to prove cause, and to determine mitigation alternatives.

If you have any waveform data captured (like a stream of three phase V and I samples for say 1 to 10 seconds), let me know. I would be interested in running them through an analysis program I have developed. If you were any closer (Oregon, USA), I would offer to do a free measurement. This is a phenomenon really worth documenting.

I would suggest that you consider (of course you would have to have time), writing a paper, magazine article, or white paper on this phenomenon. Like I said earlier, this has been talked about, but I have never seen any documentation on this interaction problem.

RE: TRIPPING OF HARMONIC FILTER

Suggestion: Even harmonics are generated by half-wave rectifiers (AC-DC converters) or similar devices that use three diodes and three controlled rectifiers not six controlled rectifiers and no diode in the AC-DC conversion part.

RE: TRIPPING OF HARMONIC FILTER

Possible source of even harmonics is transformer in-rush.  Perhaps when the driver starts the winder he closes the convertor transformers whose in-rush current interacts with the filter bank and is the source of 4th.  I am unclear how a 6 or 12 pulse convertor generates 4th harmonic, unless one or more thyristor devices is mis-firing, however for such a big drive which spends so much time in inversion, this would manifest itself in the drive shutting down for other reasons.  
For 12 pulse series convertors, sequence control is simply the control of each 6 pulse series bridge in such a manner as to reduce the kVAR demand from the supply at intermediate speeds.  Both bridges start on the their backstops and as the drive requires voltage, one convertor fires foward, the other stays on its back-stop.  When the volage demands causes one bridge to reach its front-stop, the other convertor bridge starts to fire forward.  Maximum loop voltage is then reached with both convertors on the front stop.  When the winder slows down the sequence reverses, with one bridge retarding its firing angle until it reaches its backstop and then the other bridge retards.  This is as opposed to normal 12 pulse series control where both 6 pulse bridges operate with the same firing angle.  I still do not understand how this gets 4th harmonic.  

Question, has this system just been installed? or has it been working for some years and only just now giving problems.  Also has someone actually checked and measured the capacitors, just in case you have lost some capacitor cans and the effective MVAR capability is much lower than designed?

Kevd

RE: TRIPPING OF HARMONIC FILTER

(OP)
Transformer inrush is not the problem here. We are seeing the problem during the constant speed part of the cycle and not during acceleration or deceleration.

As mentioned before the problem does not occur with the filter out of circuit. However, the only harmonic that the filter can amplify is 4th and we believe that this amplification of an uncharacteristic even harmonic is somehow causing problems with the control system, even though the resultant amplified 4th is relatively small. Voltage THD just prior to the chaos is less than 2%.

Cap bank has been checked (it also has out of balance protection). Winder has been in service for years but filter has just been installed (old filter moved from another shaft).

As mentioned the control system was specifically developed for South Africa by Alstom and I wonder how well it has been tested in service.

More comments would be appreciated.

RE: TRIPPING OF HARMONIC FILTER

Suggestion to the previous posting: Small traces of even harmonics can came from imperfect characteristics of various devices, e.g. switching devices. Have you tried to trace the 4th harmonic origin, e.g. by a power quality analyzer, etc.?

RE: TRIPPING OF HARMONIC FILTER

(OP)
All converters generate some uncharacteristic harmonic.

RE: TRIPPING OF HARMONIC FILTER

Comment on the previous posting: The uncharacteristic harmonics can be even and odd; however, they tend to be noticably smaller than characteristic harmonics.

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