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STS Heat Trace Reactor

09/12/2017 6:03 PM

Hello guys. so I got a STS Heat Tracing system (skin effect) installed on a 15km pipeline carrying refined crude oil residuum. the heat trace itself is single phase 1570V. but it is supplied from a three phase transformer 415/1570V 50Hz. because the system is single phase, it uses a load balancer circuit to distributes and balances the current on all three phases (see poorly drawn circuit).

http://i66.tinypic.com/f9i6np.jpg

so now, the reactor L is damaged, completely burned. the capacitors are ok. my questions are:

- Has anyone seen this kind of circuit before? how does it work exactly?

- Can it be modified in anyway to make it work temporarily until we get the new reactor (the residuum will solidify inside the pipeline which is a huge problem).

- can I supply the load (STS) directly from two phases of the transformer (remove C1 and L)? assuming it can take the load, does using only 2 phases affect the transformer?

- can I use an old transformer as a reactor? connecting only one winding in the place of the reactor?

Thanks.

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#1

Re: STS Heat Trace Reactor.

09/12/2017 7:36 PM

Here is a copy of your circuit. (You can paste pictures in with the little camera gizmo on the panel above.)

The three phases are 120 degrees apart. A capacitor can shift the current ahead by up to 90 degrees (ideally) and an inductor can shift the current back by up to 90 degrees (ideally) with respect to the voltage.

I haven't used this circuit, but I think the idea of this circuit is that the L2 current will be advanced by the capacitor and add to L1 and be retarded by the inductor and add to L3. Note that if the rotation of the source is not L1, L2, L3 this would not work. And, of course, the current is not shifted 120 degrees.

This document might help:

"A Method for Balancing a Single-Phase Loaded Three-Phase Induction Generator"

file:///C:/Users/Richard/Downloads/energies-05-03534.pdf

file:///C:/Users/Richard/Downloads/energies-05-03534.pdf

Here's another approach: If your load can be driven by DC, a 3 phase bridge could rectify the 3 phase AC and drive the load and be fully balanced.

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#4
In reply to #1

Re: STS Heat Trace Reactor.

09/13/2017 12:54 PM

I like the simplicity of the three-phase rectifier. Perfect answer. Simple and maintains phase balance.

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#6
In reply to #1

Re: STS Heat Trace Reactor.

09/13/2017 6:06 PM

I dont think STS work with DC? I'm not sure. I will look it up but if I were to use this I need high voltage and current rectifiers 2000v 200A rated. Are those easy to find?

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#8
In reply to #6

Re: STS Heat Trace Reactor.

09/14/2017 7:52 AM

Only to those with a willingness to look for them.

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#10
In reply to #6

Re: STS Heat Trace Reactor.

09/14/2017 2:03 PM

Apparently, it does have to be AC, is there a preferable frequency, or is 50 or 60 Hz optional for skin-effect heat-tracing?

You could probably make use of "new" GaN transistor technology to make the power requirements easier to achieve.

Now that I think about it, you should re-consider the choices to be made, and probably go with 3-phase supply, two phases shifted w.r.t. the remaining phase, thus arriving at single phase output.

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#11
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Re: STS Heat Trace Reactor.

09/14/2017 9:34 PM

It works on 1570v 50Hz. But 60Hz is okay too.

what if I disconnected the load balancer. I mean remove the two capacitors and the inductor and connect the load to L1 and L3 directly (see drawing). The current drawn by the heater is about 135A. The question here is, can I take only two phases from the 415/1570v transformer and leave the other phase L2 disconnected? What is the effect on the transformer?

The transformer primary is 415v 600A delta

secondary 1570v 150A delta

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#12
In reply to #11

Re: STS Heat Trace Reactor.

09/15/2017 9:06 AM

I think Solar Eagle already addressed the point of disconnecting the middle phase.

It will work, but I would not expect optimal results, as if you were using a transformer set up specifically designed for the task at hand, i.e. with the load balancer operating.

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#7
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Re: STS Heat Trace Reactor.

09/13/2017 9:17 PM

Yeah sts is works only on AC supply because it is based on skin effect which only observed with AC current....

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#9
In reply to #7

Re: STS Heat Trace Reactor.

09/14/2017 10:55 AM

Okaaaay . . .

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#13
In reply to #7

Re: STS Heat Trace Reactor.

09/15/2017 4:44 PM

It will work with 2 phases giving 1570 V AC. Rated transfo current = 150A, Heater 135 A. But check that the balancing is not to protect generators, which do not like unbalanced loads. It may be that the HV load is negligible compared to generator size anyhow.

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#2

Re: STS Heat Trace Reactor.

09/12/2017 8:30 PM

I think I would disconnect the middle lead, then try to add some btu's....possibly you could speed the pump up a bit...maybe add an auxiliary portable heater midway down the pipeline....possibly you could raise the voltage a bit...

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#3

Re: STS Heat Trace Reactor.

09/13/2017 5:39 AM

Your power supply uses a technology more than 70 years old. If it works let it be, but if it gets broken again, it would be better to think of something better. Currently there are power converters way cheaper, smaller, more reliable, and power factor does not depend on the load.

Weather you want the converter to control the amount of energy delivered or you want it to always work at full power is up to you. My company does custom made power converters, but you probably can find something similar in your location.

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#5

Re: STS Heat Trace Reactor

09/13/2017 1:50 PM

As pointed out by Rixter, you could enjoy much better control by opting for a 3-phase bridge rectifier with DC output.

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#14

Re: STS Heat Trace Reactor

09/19/2017 2:13 PM

So, I disconnected the middle phase L2 and connected the load directly to L1 and L3 and started, it was drawing 145~150A from the transformer, which i thought was too much (transformer rated for 150A). Then I connected the PFC Capacitor (C2 in the drawing) and that reduced the current by about 20A. So it started-up at 135A and as the pipeline temperature increased the current dropped to about 120A when it reached the required temperature (60C). It's been working for two days now.

So my question now is, I looked everywhere on the web but I couldn't find a clear answer on what are the effects of using only two phases at almost rated current of a transformer? Now both L1 and L3 current is ~120A and L2 is zero. The transformer secondary is delta connected so no N. I'm planning on running it that way untill the spare inductor comes, so it's only temporarily, but still I may damage the transformer?

I hope someone can provide me with information about unbalancing a transformer..

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#15
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Re: STS Heat Trace Reactor

09/19/2017 3:38 PM

Are you then utilizing this as a middle leg grounded delta? I think that is done in quite a few places, including the oil patch. You should make your team aware of possible safety issues if one of the other legs goes open circuit, etc. That is if any safety issues can be identified. I am really not aware of any major problems with grounded middle leg delta configurations.

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#16
In reply to #15

Re: STS Heat Trace Reactor

09/19/2017 7:26 PM

Are you saying I should connect the middle leg directly to ground? What's the advantage? Is it a must? Remember the secondary voltage is 1570v.

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#18
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Re: STS Heat Trace Reactor

09/20/2017 9:06 AM

I am most definitely telling you what to do. I know there are places that allow for middle leg grounded delta.

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#17
In reply to #14

Re: STS Heat Trace Reactor

09/20/2017 7:26 AM

Hi Durguy,

My first thought was that that transfos do not care about unbalanced loading (not unless you are trying to mitigate overload on one phase with underload on another) & that transfo protection relay lore does not require negative phase sequence [unbalance] protection. But my second was to check in J & P Transfo book - un-balanced load chapter does not flag any problem with 3 phase transfos with zero current on one phase in a dozen winding arrangements.

For the avoidance of doubt,

  1. please advise the transfo type Yd1? (rating plate diagram if possible, any volts, loss watts on plate?) star-delta or what? Nothing unusual like zig-zag winding?
  2. Is it oil or dry?
  3. Is the rating site at ? celsius or standard (40 celsius). Is your ambient less than that with current season?
  4. Is there an oil temperature or winding temperature indicator so you can spot any changes? Is WTI in a phase you are loading(usually a single phase instrument)?
  5. Is it a "shell" type - 50 Hz transfos are usually "core" type.

On a basic level, 120A versus 150 A rating means 64% of full-load resistive loss[I2R, 0.82 = 0.64, and less because winding resistance falls with temperature], that in two windings, third no-loss. That seems a good margin for unquantified losses. What are actual primary voltages and primary rated voltage (i.e. are magnetising losses normal)?

I wonder about harmonic currents, I do not know levels caused by hysteresis and eddy current for pipe heating. I would suggest measuring at transfo secondary, if you can, - fundamental & each harmonic frequency current and then asking transfo maker if it is a problem - to set your mind at rest.

Point by other poster about earthing is relevant - but have you made any change from pre-existing load-side earthing design before reactor failure?

Personally, I can see no benefit for transfo from the reactor, it may help the generator loading.

67model

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#19
In reply to #17

Re: STS Heat Trace Reactor

09/22/2017 8:09 PM

Hi 67model, thanks for replying.

well,

1. I will post a picture of the name tag of the transformer when I get to the site (Sunday or Monday). But I'm pretty sure it's delta-delta.

2.oil type.

3. normal temperature ranges from 0C in winter to about 45C max in summer. this time of the year it's around 32C max.

4. there's an oil temperature indicator and the transformer has a high temperature alarm and trip. indicator was reading 34C last time I looked (more or less ambient temperature). (I know this may be stupid but the transformer is very cool to the touch). I will check but I'm sure there's no WTI.

5. the transformer is a core type.

The transformer is rated 415V 570A primary. but voltage these days is 400V due to maintenance in the main power station at site. current at primary is 522A.

I will check if we have any means of measuring the harmonics. nothing was changed before reactor failure, although there was a problem with the cabinet air conditioning (the reactor and the two capacitors are placed inside a cabinet outdoors) which wasn't working for a while due to a lack of spare parts (I made a report before the reactor failure about high temperature inside the cabinet causing the insulation on the reactor to become soft).

what would happen if L2 was earthed? is it beneficial to our current setup?

we are not powered by generators except at mains power failure. the site is connected to the main power grid so I think it doesn't matter on a large scale.

In the STS heater project documents, it's written that the heater works on single phase but if it's not available a single phase system that can take the load, or at a different voltage (1570V) the system can be connected to three phase using a load balancer to balance he current draw on the three phases (rated current for heater is 135A and for the three phases with the load balancer is 85A).

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#21
In reply to #19

Re: STS Heat Trace Reactor

09/24/2017 3:53 PM

ref 3 & 4 You are obviously considerably below normal temperatures. Any wind makes a lot of difference. The temperature you read may not be maximum, could be warming up from lower temperature due to night-time temp. Full load oil temp rise would be about 50'C. Oil filled transfos have big thermal mass.

With 100% rated current load on two phases & D connection, one winding is at normal current & other two at half normal full load [applies to primary & secondary for D-D transfo]. But you are only at 0.8 rated current, so I estimate losses as 1/4 rated normal, expect oil temp about 15 degrees above mean daily temperature. Additionally you are below max ambient & normal volts.

I would expect the philosophy, with dire result of loss of pipe heater, would be unearthed - live with "earth somewhere" while you locate it & bring in repair team. Having an earth could mean instant trip of power on overcurrent due to second earth somewhere on pipe & then heating zero. I guess the design spec would require 1570 V insulation to minimise risk of earth fault [ 3.3 kV cable - normally 1900V to earth?] & assure an earth on one pole would not overstress insulation on the other & cause consequent failure.

Having a balanced load is a normal requirement for power systems, but your load seems negligible compared system capacity & not a problem pending inductor repair.

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#20
In reply to #17

Re: STS Heat Trace Reactor

09/24/2017 2:43 PM

Can you please take a picture or scan the pages where from j&p transformer book where it explains unbalanced load.

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#22
In reply to #20

Re: STS Heat Trace Reactor

09/24/2017 4:25 PM

Top of page left out as you have D-D type. Winding currents are amps for 100A load.

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#23
In reply to #22

Re: STS Heat Trace Reactor

09/25/2017 4:11 AM

so, some old guy at work is convincing the manager that single phase load on a three phase transformer will damage the transformer and the manager is approving a shut down off the system. the old guy reference is a paper he printed form the internet. see pictures.

what do you think?

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#24
In reply to #23

Re: STS Heat Trace Reactor

09/25/2017 9:35 AM

This is the text from J&P, first bit by character recognition was long winded, just did image for 2nd page - I left out winding configs probably not relevant.

<<

(4) In considering the question of unbalanced loading it is easiest to treat the subject from the extreme standpoint of the supply to one single-phase load only, as any unbalanced three-phase load can be split up into a balanced three-phase one and one or two single-phase loads. As the conditions arising from the balanced three-phase load are those which would normally occur, it is only a question of superposing those arising from the single-phase load upon the normal conditions to obtain the sum total effects. For the purpose of this study it is only necessary to consider the more usual connections adopted for supplying three-phase and two-phase loads, as six-phase transformers which are used mainly for supplying rectifiers seldom supply unbalanced loads. The value of current distribution is based upon the assumption that the single-phase currents are not sufficient to distort the voltage phasor diagrams for the transformers or transformer banks. This assumption would approximate very closely to the truth in all cases where the primary and secondary currents in each
phase are balanced. In those cases, however, where the primary current on the loaded phase or phases has to return through phases unloaded on the secondary side, the distortion may be considerable, even with relatively small loads; this feature is very pronounced where three-phase shell-type transformers and banks of single-phase transformers are employed. Figures 25.11 25.2 and 25.3 show the current distribution on the primary and secondary sides of three- to three-phase transformers or banks with different arrangements of single-phase loading and different transformer connections, and in Scott-connected three- to two-phase groups with different arrangements of single-phase loads.
These diagrams may briefly be explained as follows:

>>

So the "red flag"/"alarm bell" items are "shell type" & 3 ph bank made up of single phase units - which do not apply.

Obviously, Zenatrix in your post are in the business of minimising losses by good system design and monitoring of the actual loading situation (and drumming-up business by making unbalanced load seem a serious problem).

However, the issue here is whether the transfo or its supply system are overloaded, risking failure before plant is restored "as designed". Nothing you have posted suggests overload [rather losses much less than rated] and ambient & voltage are on safe side.

Even if you had an overload situation, you would assess by considering that each hour with 8 'C higher insulation temperature is equal to 2 hours of normal life - about 25 years for a transfo [to be clear 16'C would be 4 hours, halving of life for each 8'C higher] and whether that was significant enough to warrant flagging a reduced life on that transfo. You would take into account that the transformer normal load is less than rated & increase of life over "standard". The overload capacity of oil filled transfos is considerable and is utilised, in distribution, during life due to exception weather or loading where a lot of load is outside control of utility. You would also compare the risk of transfo failure (do you have a spare) & cost with the loss of production and restoration cost for the pipeline (the transfo cost is likely small change compared to pipeline).

As I wrote, if you want to worry, measure the currents/voltages including harmonics in each phase primary & secondary & put figures to transformer maker - or ask for their reaction (or heating system suppliers) to the effect of removing the reactor from the load. There may be an issue of management wanting to give business to Zenatrix here or acquire monitoring equipment/expertise as a long term facility - in which case, they should provide rationale & funding from strategic funds - not expect it from the maintenance/ "fire-fighting" end of operations

One final point - have you looked at the ??/415/240V transfo upstream of the heater transfo? - you have not indicated its rating or actual loading. It is likely star/delta.

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#25
In reply to #24

Re: STS Heat Trace Reactor

09/25/2017 5:51 PM

Thank you for the reply. I have read the book, found it at the university library.

It seems that they insist on shutting down the system, and I'm not concerned about that. I am just kinda mad that they still believe this situation will damage the transformer, I came here for proof because I already knew that, within limits, the system can be operated without a reactor. anyway, it's their decision but I am now continuing to research the subject for my own knowledge.

"Obviously, Zenatrix in your post are in the business of minimising losses by good system design and monitoring of the actual loading situation (and drumming-up business by making unbalanced load seem a serious problem)."

^^ that's exactly what I told them, oh well...

the primary is rated 415V/570A and was reading 522A on two phases. this is is the name tag of the transformer:

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#26
In reply to #25

Re: STS Heat Trace Reactor

10/04/2017 5:27 PM

Post-mortem.....

Thanks for transfo plate image. Note transfo is rated for 50'C ambient [and for specific height above sea level - corresponding to site?] & is to IEC60076 standard, usual for 50Hz.

The vector group [Schaltgruppe] had me puzzled for a while, because Dy6 is not physically possible - according to IEC60076, the capital letter is always the HV winding, regardless of whether it is primary or secondary [ the plate makes it clear 415V is primary].

Number 6 means "6 on clock face" referenced to "12 o'clock" source supply vector or 180 degree phase shift. On checking diagrams only Dz6 can give 180 degree phase shift ( similar Dz0, but polarity swapped on all windings on one side to make 180 degree change) which is a zig-zag winding on "star-like" side.

So the diagram that applies is "l" below with line-line 1 phase load....

Remember transformers are reversible - so source side on right does not alter current relations. Note that current on two lines on one side (zero in third) transfers to 2 line current on the other - the D-Y, D-D connections in post #22 always give current in 3 lines on source side. Load Current in only 2 lines on primary side matches your measurement of 522A on two phases.

The current in all the windings on D side is in phase with the dotted line (67 +33 = 100: note currents in B & C windings are same - zero line current at their join) so for...

Winding B, I lags V. - capacitor needed for compensation.

winding C, I leads V. - inductor needed for compensation.

The sum of the two compensation currents cause a current in the third line - I numbered it 2 to match your diagram, balancing the primary 415 volt load.

The Dz winding seems to be regarded as best for coping with a 2 line single phase load on a 3 phase system.

I am sure that the designers of the heater system made sure that the system would still work, to give maximum availability of pipe heating, if the inductor or/and capacitor connected to "2" terminal were disconnected.

67model

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