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Thread: Electrical Imponderable

  1. #1 Electrical Imponderable 
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    Thought I had run out of 'em, but realized today, this has troubled me for years. Story necessary: I took a job right out of college as Plant Engineer for a facility manufacturing gypsum wallboard, which operated 24/7 no matter what. Electric motor intensive, thus highly inductive load overall, the Power Company charged a penalty fee added to the regular electric bill, based on Power Factor; all power supplied with P.F. lower than 80% resulted in a penalty, which in their case ran as high as $1,000 per month. So I was working up the proposition of adding power factor correction to the plant, and got to "snooping" around as needed, and discovered the big transformer outdoors which fed the power to the plant had a temperature indicator which was red-lined!. An ammeter built in indicated about 2200 amperes much of the time, this being 480 volt three-phase, the power level exceeded the transformer rating by quite a bit.

    I explained to my boss, the Works Manager, that this condition could result in a shut-down plant for quite some time. His hard hat (always worn, even in his office, guy was a bit eccentric) popped halfway up to the ceiling! Immediately calling Home Office in New Jersey (plant was in Colorado), he asked for the Corporate Facilities Manager, guy named Tibore Penzes, I think he was something like Hungarian, spoke with a thick, hard to understand accent. He refused to believe, having been present during the design and start-up of the plant, that over the years so much additional load had been added. First thing I had done, before reporting in, was to call the Power Company, which immediately dispatched a crew to the plant to assess the situation; they had brought large electric fans with them, and quickly began setting them up to force air flow over the transformer's cooling radiator. (Now when I think about it, the damn thing likely was filled with PCB cooling fluid!). Working around the Corporate conversation, I told "Ty" about the Power Company's activities, that convincing him of the gravity of the situation.

    If you've read this far, here's the reason for the thread. In working with the Power Company crew, I learned one of them was an Electrical Engineer, so got into the Power Factor discussion with him. I think I asked him to what extent does the lagging current caused by the highly inductive plant load affect them at their end, the supply end. He said, actually, not much. The big transformer, about the size of an average dump truck, had a Delta-wound primary and Wye-wound secondary, and that combination did NOT REFLECT THE LAGGING CURRENT CONDITION BACK INTO THE PRIMARY. (????)

    Finishing the story, a planned shut-down was made, and the old transformer changed-out, quite a feat to behold given the very rugged terrain surrounding the area of it's location. These transformers were property of the Power Company, and they of course had as little desire as the plant to see the old one fail, possibly causing explosion and fire. I worked there only one year, beginning in 1977. The transformer reflected power factor question has never been resolved in my mind; can anyone here explain this?

    Thanks for sticking it out through this whole read! jocular


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  3. #2  
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    Can I confess that I am an Electrical Engineer (Honours) I assume this transformer was a step down transformer. So the high voltage and low current lines from the power company were being stepped down to a lower voltage to be used by the motors. You have not detailed that the lines were 3 phase. A transformer has two huge coils of wire. The two coils are never joined so there is never any current across the transformer. However there is a relationship between the coils to do with the number of loops in each coil. When the motors in the plant do not use all the power supplied it can overheat and melt the transformers. Most power stations have automated or manual load balancing that should account for any power factor issues.


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  4. #3  
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    Quote Originally Posted by uptonryan View Post
    Can I confess that I am an Electrical Engineer (Honours) I assume this transformer was a step down transformer. So the high voltage and low current lines from the power company were being stepped down to a lower voltage to be used by the motors. You have not detailed that the lines were 3 phase. A transformer has two huge coils of wire. The two coils are never joined so there is never any current across the transformer. However there is a relationship between the coils to do with the number of loops in each coil. When the motors in the plant do not use all the power supplied it can overheat and melt the transformers. Most power stations have automated or manual load balancing that should account for any power factor issues.
    Thank you for responding! Sorry, I assumed that mentioning Wye secondary and Delta primary implied three-phase operation. Wye, or Star operation is desired to provide single-phase 277 volt power for lighting purposes. In other facilities I have worked, the main service transformer(s), almost always outside of the building proper, often on the roof, were Delta-connected, with smaller transformers located within the building having Wye-connected secondary(s) to provide for single-phase lighting. As originally-designed, the lighting loads are carefully connected to keep single-phase loading on the three phases as nearly equal as possible. However, as local efforts done by the plant in adding work area also add additional single-phase lighting loads, they are often simply connected to the nearest 277-volt breaker panel, with the result that unbalance of loads can become a serious problem, depending on their extent. In the past, lighting loads tended to be mainly inductive due to use of magnetically-ballasted fluorescent lighting and vapor-arc lighting. Today, electronic ballasts are widely available, though at higher cost. Phase-load imbalance results in unequal leg-to-leg voltages, while the mag ballasts add yet more to the unfavorable power factor condition. Unequal leg-to-leg voltages reduce electric motor efficiency, as well as adding unexpected and undesirable "circulating currents". So, 3-phase operation, though vastly superior to single-phase for most applications, especially industrial, is not all "sugar & honey".

    "When the motors in the plant do not use all the power supplied it can overheat and melt the transformers." Please explain what you mean by this? I am under the impression that the amount of power supplied by the transformer(s) is dependent on the load, that is, the number and size of the motors in this case. Perhaps you mean that as Power Factor becomes very undesirable, much less than unity, the load on the transformer becomes very excessive? This introduces the imponderable of whether poor power factor affects loading of the source, imponderable to me, for having studied this stuff 50 years ago!

    This statement: "Looking at the waveform plot for power, it should be evident that the wave spends more time on the positive side of the center line than on the negative, indicating that there is more power absorbed by the load than there is returned to the circuit. What little returning of power that occurs is due to the reactance; the imbalance of positive versus negative power is due to the resistance as it dissipates energy outside of the circuit (usually in the form of heat). If the source were a mechanical generator, the amount of mechanical energy needed to turn the shaft would be the amount of power averaged between the positive and negative power cycles.

    Mathematically representing power in an AC circuit is a challenge, because the power wave isn't at the same frequency as voltage or current. Furthermore, the phase angle for power means something quite different from the phase angle for either voltage or current. Whereas the angle for voltage or current represents a relative shift in timing between two waves, the phase angle for power represents a ratio between power dissipated and power returned. Because of this way in which AC power differs from AC voltage or current, it is actually easier to arrive at figures for power by calculating with scalar quantities of voltage, current, resistance, and reactance than it is to try to derive it from vector, or complex quantities of voltage, current, and impedance that we've worked with so far."
    I found here: Power in resistive and reactive AC circuits : Power Factor and it refers to this:



    I'm wondering if we might get insight from Harold? jocularity
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  5. #4  
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    I don't know of any reason why a delta-wye transformer would help with power factor. Perhaps the engineer meant third harmonic current or unbalanced phase current.
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  6. #5  
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    The issue is that you want to keep the real current flowing though the transformer a little as possible. Real current will react with the resistance in the transformer and produce heat. In three phase cable the lines are balanced so that the energy is transferred by the imaginary part. If the load is not balanced correctly then this can cause a mismatch and the current to become real. Complex differential equations spring to mind.
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  7. #6  
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    Quote Originally Posted by uptonryan View Post
    The issue is that you want to keep the real current flowing though the transformer a little as possible. Real current will react with the resistance in the transformer and produce heat. In three phase cable the lines are balanced so that the energy is transferred by the imaginary part. If the load is not balanced correctly then this can cause a mismatch and the current to become real. Complex differential equations spring to mind.
    Sorry, no, that's a terribly confused explanation. ANY current flowing through a resistance produces dissipation.

    The issue of real v complex is this: In a reactive system, the magnitudes of the currents can be disproportionately large relative to the average power delivered. That's what produces higher losses, and that's why utilities penalize users who present significantly non-resistive loads -- the utilities still have to pay for transmission lines that can handle the peak currents. For equal current magnitudes the losses will be the same, independently of whether those currents are reactive or real.
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  8. #7  
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    Quote Originally Posted by Harold14370 View Post
    I don't know of any reason why a delta-wye transformer would help with power factor. Perhaps the engineer meant third harmonic current or unbalanced phase current.
    Over the years, I have asked the question of many having technical electrical backgrounds, but never received a definitive answer. Perhaps the Colorado Power Co. Engineer was wrong. Still, having done Engineering work along electrical lines of various types all my life, including of course, Power Factor Correction work, I find the idea daunting. Obviously, power factor is important to some; one often sees P.F. correcting capacitors sitting atop transmission poles even in the middle of "nowhere". joc
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  9. #8  
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    Quote Originally Posted by tk421 View Post
    [
    The issue of real v complex is this: In a reactive system, the magnitudes of the currents can be disproportionately large relative to the average power delivered. That's what produces higher losses, and that's why utilities penalize users who present significantly non-resistive loads -- the utilities still have to pay for transmission lines that can handle the peak currents. For equal current magnitudes the losses will be the same, independently of whether those currents are reactive or real.
    One would expect that some consistency would exists from one geographical area to another, yet, in my experiences with two manufacturing plants in Phoenix, Arizona, I learned that the electric power supplier, Salt River Project, DID NOT impose a billing penalty due to poor power factor. Perhaps it was assimilated in the billing structure? If so, unless such structure excluded residential use, which surely must be less reactive than industrial, homeowners were also paying for power factor considerations. jocular
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  10. #9 A Bit "Deeper"? 
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    I was taught early on that given a device exhibiting Inductance, such as an electric motor, the current flowing through it's windings would appear at a finite time AFTER the voltage was applied, this being termed "lagging current", and caused by Inductive Reactance.

    It was also asserted that given a circuit containing Inductance and pure Resistance, the current present flowing through the resistance would ALWAYS occur in time-synchronization with appearance of voltage, that is, IN PHASE.

    Now, given that windings in an electric motor present Inductance, as well as some small amount of resistance inherent in the material used, that resistance being universally distributed along the length of the conductors, how can we resolve the discrepancy that current through those conductors lags the voltage, but remains in phase with it over the resistance present? Huh?? joc
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  11. #10  
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    Quote Originally Posted by jocular View Post
    Quote Originally Posted by Harold14370 View Post
    I don't know of any reason why a delta-wye transformer would help with power factor. Perhaps the engineer meant third harmonic current or unbalanced phase current.
    Over the years, I have asked the question of many having technical electrical backgrounds, but never received a definitive answer. Perhaps the Colorado Power Co. Engineer was wrong. Still, having done Engineering work along electrical lines of various types all my life, including of course, Power Factor Correction work, I find the idea daunting. Obviously, power factor is important to some; one often sees P.F. correcting capacitors sitting atop transmission poles even in the middle of "nowhere". joc
    If we connect a purely reactive load to the secondary of the delta-wye transformer, there's no real power transmitted, but there's current. So the power factor is zero. Now what is the power factor on the primary? Well, it's not delivering any power to the load, but it has current which is in direct proportion to the secondary current. So the power factor is zero there, too. The delta-wye didn't fix anything.

    I was taught early on that given a device exhibiting Inductance, such as an electric motor, the current flowing through it's windings would appear at a finite time AFTER the voltage was applied, this being termed "lagging current", and caused by Inductive Reactance.

    It was also asserted that given a circuit containing Inductance and pure Resistance, the current present flowing through the resistance would ALWAYS occur in time-synchronization with appearance of voltage, that is, IN PHASE.

    Now, given that windings in an electric motor present Inductance, as well as some small amount of resistance inherent in the material used, that resistance being universally distributed along the length of the conductors, how can we resolve the discrepancy that current through those conductors lags the voltage, but remains in phase with it over the resistance present? Huh?
    Normally we think of the separate parts, resistance and inductance in series. The voltage across the inductor is 90 degrees out of phase with the current, and the voltage across the resistor is in phase with the current. Now break this up into an infinite number of resistor-inductor pairs of infinitesimal length in series. Does that help? Probably not.
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  12. #11  
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    Quote Originally Posted by jocular View Post
    I was taught early on that given a device exhibiting Inductance, such as an electric motor, the current flowing through it's windings would appear at a finite time AFTER the voltage was applied, this being termed "lagging current", and caused by Inductive Reactance. It was also asserted that given a circuit containing Inductance and pure Resistance, the current present flowing through the resistance would ALWAYS occur in time-synchronization with appearance of voltage, that is, IN PHASE.Now, given that windings in an electric motor present Inductance, as well as some small amount of resistance inherent in the material used, that resistance being universally distributed along the length of the conductors, how can we resolve the discrepancy that current through those conductors lags the voltage, but remains in phase with it over the resistance present? Huh?? joc
    I think part of your problem is the vagueness inherent in "the voltage." In your scenario, there are several voltages.

    For a resistance, the voltage across the resistance is in phase with the current through it.

    For an inductance, the voltage across the inductance leads the current through it.

    For any combination of resistance and inductance, the above statements still hold true. So if by "the voltage" you mean the voltage applied to one end of the network, referenced to ground, you will encounter an ambiguity because you have not identified the proper voltages to which the above definitions apply. As long as you identify all the right voltages, there will not be a problem.
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  13. #12  
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    Can I just say "Back EMF". Sorry been dying to say that.
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  14. #13  
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    No, you can't. That will be three days off for trolling.
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  15. #14  
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    Quote Originally Posted by jocular View Post
    Quote Originally Posted by tk421 View Post
    [
    The issue of real v complex is this: In a reactive system, the magnitudes of the currents can be disproportionately large relative to the average power delivered. That's what produces higher losses, and that's why utilities penalize users who present significantly non-resistive loads -- the utilities still have to pay for transmission lines that can handle the peak currents. For equal current magnitudes the losses will be the same, independently of whether those currents are reactive or real.
    One would expect that some consistency would exists from one geographical area to another, yet, in my experiences with two manufacturing plants in Phoenix, Arizona, I learned that the electric power supplier, Salt River Project, DID NOT impose a billing penalty due to poor power factor. Perhaps it was assimilated in the billing structure? If so, unless such structure excluded residential use, which surely must be less reactive than industrial, homeowners were also paying for power factor considerations. jocular
    You are absolutely correct. I should have written "...users who present significant non-resistive loads..." (ah, the damn adverb!). Utilities only care about large deviations from resistance, as viewed from their perspective. Very large plants quite often have power-factor correction capacitors somewhere on-site. Utilities don't care about minor deviations (such as presented by residences and smaller plants), just as you correctly observe. It is also quite possible that they negotiate special rates for certain customers, but I have no direct knowledge of that part of the business.
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    Quote Originally Posted by Harold14370 View Post
    If we connect a purely reactive load to the secondary of the delta-wye transformer, there's no real power transmitted, but there's current. So the power factor is zero. Now what is the power factor on the primary? Well, it's not delivering any power to the load, but it has current which is in direct proportion to the secondary current. So the power factor is zero there, too. The delta-wye didn't fix anything. Understood, and agreed! (I think!). This assumes zero resistance present, does it not? Resistance present would DROP some voltage across it, which would oppose the reactive voltage developed due to field collapse, yes? Then, power MUST be delivered by the primary, but would it not have to be 180` out of phase with that developed in the resistance located in the secondary circuit? Apparently then, (wrong word, apparent!), evidently then, for the special case of pure reactive load, the primary would not be affected by zero power factor. Now, given the non-pure reactive load (with resistance present), the primary MUST deliver some REAL power, the waveform of which which according to the diagram in post #3 which represents an in-phase condition, is of higher frequency and lies MORE than 50% above the datum line. I forget how the "Q" (Xl/R) presents mathematically w/respect to phase angle, but obviously, given the presence of REAL power dissipated by the secondary, then phase angle must also be present in the delivery by the primary.....


    Normally we think of the separate parts, resistance and inductance in series. The voltage across the inductor is 90 degrees out of phase with the current, and the voltage across the resistor is in phase with the current. Now break this up into an infinite number of resistor-inductor pairs of infinitesimal length in series. Does that help? Probably not.
    It definitely DOES! Thank you! A new way of picturing this; along with a graphical representation of "power waveform", which I had never before seen illustrated. Our studies considered that "power" was represented by the area of the A.C. curve above (or below) the datum. Total power required that the sign be neglected, as that portion below would be "negative" power, an undefined concept. joc
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    Quote Originally Posted by tk421 View Post
    You are absolutely correct. I should have written "...users who present significant non-resistive loads..." (ah, the damn adverb!). Utilities only care about large deviations from resistance, as viewed from their perspective. Very large plants quite often have power-factor correction capacitors somewhere on-site. Utilities don't care about minor deviations (such as presented by residences and smaller plants), just as you correctly observe. It is also quite possible that they negotiate special rates for certain customers, but I have no direct knowledge of that part of the business.
    Thank you, as well as Harold: Very enlightening. Mentally, not visibly! I applied for a Facilities Engineering position at a Kerr-McGee Chemical Co. plant way back in the '70s, and upon interviewing there, was amazed to learn they generated their own electric power!. Given the imperative of constant production operation, 24/7, it occurred to me they must have had an option of "signing on" to purchased power, were their own power house to fail. Imagine the price they would pay for such sudden "service"! jocular
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    Quote Originally Posted by jocular View Post
    One would expect that some consistency would exists from one geographical area to another, yet, in my experiences with two manufacturing plants in Phoenix, Arizona, I learned that the electric power supplier, Salt River Project, DID NOT impose a billing penalty due to poor power factor. Perhaps it was assimilated in the billing structure?
    Or they just compensated for it at the substation. A consistently inductive load is pretty easy to compensate for; a bank of capacitors does the trick.

    It was also asserted that given a circuit containing Inductance and pure Resistance, the current present flowing through the resistance would ALWAYS occur in time-synchronization with appearance of voltage, that is, IN PHASE.
    Across the resistor, voltage and current are always in phase - but across the inductor they are not. If they are series connected they must carry the same current but do not have to have the same voltage distribution across them. Thus across the resistor the voltage is always in phase with the current, but across the inductor the voltage always leads the current. (That's the same as saying the current lags the voltage.)
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