Harmonics Testing

[GoldDigger]

Just an impedance between two points will not reduce the effect of the V THD caused by one as seen at the other. The harmonic content will however be reduced to the extent that the impedance back to the grid source, the generators, is low.
But as the harmonic load current increases the ability of the POCO infrastructure to handle it remains fixed. Without upgrades within POCO (whether in the form of reduced impedance or harmonics specific filtering or other measures) the V THD will continue to increase.

 

[Open Neutral]

To expand on this concept a little more (if I may)...
..... and the total percentage of the power we use is actually now shifted to being mostly non-linear.

Gulp...
I think I want to buy stock in a capacitor manufacturer.....and not the small ones....

{Maybe we should be using twisted trio primary cable.}

 

[PQ Man]

Harmonics - the "Chicken" or the "Egg" - Voltage Distortion vs. Current Distortion

Harmonics - the "Chicken" or the "Egg" - Voltage Distortion vs. Current Distortion

? Harmonics are always a question of ?the chicken and the egg? ? what causes what (current or voltage distortion)?
? Generally speaking for power systems from large industrials to residential, Current distortion causes Voltage distortion
? Voltage distortion is created by pulling distorted current through an impedance
? Amount of voltage distortion depends on:
? System impedance ? the impedance is made up of the resistance and reactance of the wires, transformers and other power system equipment between the source of power and the loads. (local generators have 3X the impedance of utility service transformers and therefore voltage distortion on a generator is much worse than on the utility as a rule of thumb unless you severely oversize the generator)
? Amount of distorted current pulled through the impedance (i.e. non-linear loads ? electronic loads like VFD, modern lighting (LED/CFL), and computers
? If either (system impedance or harmonic current) increases, V_THD will increase
? Other situations could cause high voltage distortion such as your neighbor drawing a significant amount of non-linear current and distorting the source voltage to your facility OR there may be a harmonic resonance condition where capacitors on the power system are amplifying the harmonics from the load ? this will severely distort the voltage

So, to answer your initial question, looking from the perspective of the 24 pulse drive (and all other downstream loads from that point into the system if you monitoring on the main breaker, for example), the voltage distortion is affected by the current flowing ?into? the drive or other loads ? your assumption is correct that you should measure on the input to compare. In addition, you should compare the harmonic orders (i.e. 23^rd, 25^th or other to the voltage distortion you are measuring) at that point and upstream since current harmonics from that source will cause harmonic voltages of that same order. The comment that the characteristic harmonics should be 23/25, 47/49, etc is true but you may also see some (significantly less) of other ?non-characteristic? harmonics including 5^th, 7^th, 11^th, 13^th, 17^th, 19^th, etc.

Since you are seeing quite high voltage distortion (>8%) at the 4160V level, I would be curious to see if you either have one of the following:

1. A weak source ? i.e. on-site generation (generators have 3X impedance of normal utility source impedance) or long feeder from the utility
2. A resonance condition where a capacitor on the power system is amplifying the harmonic currents and distorting the voltage

Bottom line is this, harmonic distortion causes one of two problems:

• Excessive current distortion causes heating, nuisance tripping of some devices and other current related issues
• Voltage distortion (caused by current distortion) causes misoperation of some loads and nuisance issues

Just because you have harmonics, doesn't mean you have a "problem". There is no ?magic number? where every power system will have issues but you are doing the right thing in identifying the levels with baseline measurements and looking for the ?source?. Many solutions exist to make the system better but all are dependent on the cost/benefit of the solution. IEEE-519-2014 (new revision this year) helps to identify levels as a starting point for most power systems.
Sorry for the long-winded and late response?

 

[ATSman]

? Harmonics are always a question of ?the chicken and the egg? ? what causes what (current or voltage distortion)?
? Generally speaking for power systems from large industrials to residential, Current distortion causes Voltage distortion
? Voltage distortion is created by pulling distorted current through an impedance
? Amount of voltage distortion depends on:
? System impedance ? the impedance is made up of the resistance and reactance of the wires, transformers and other power system equipment between the source of power and the loads. (local generators have 3X the impedance of utility service transformers and therefore voltage distortion on a generator is much worse than on the utility as a rule of thumb unless you severely oversize the generator)
? Amount of distorted current pulled through the impedance (i.e. non-linear loads ? electronic loads like VFD, modern lighting (LED/CFL), and computers
? If either (system impedance or harmonic current) increases, V_THD will increase
? Other situations could cause high voltage distortion such as your neighbor drawing a significant amount of non-linear current and distorting the source voltage to your facility OR there may be a harmonic resonance condition where capacitors on the power system are amplifying the harmonics from the load ? this will severely distort the voltage

So, to answer your initial question, looking from the perspective of the 24 pulse drive (and all other downstream loads from that point into the system if you monitoring on the main breaker, for example), the voltage distortion is affected by the current flowing ?into? the drive or other loads ? your assumption is correct that you should measure on the input to compare. In addition, you should compare the harmonic orders (i.e. 23^rd, 25^th or other to the voltage distortion you are measuring) at that point and upstream since current harmonics from that source will cause harmonic voltages of that same order. The comment that the characteristic harmonics should be 23/25, 47/49, etc is true but you may also see some (significantly less) of other ?non-characteristic? harmonics including 5^th, 7^th, 11^th, 13^th, 17^th, 19^th, etc.

Since you are seeing quite high voltage distortion (>8%) at the 4160V level, I would be curious to see if you either have one of the following:

1. A weak source ? i.e. on-site generation (generators have 3X impedance of normal utility source impedance) or long feeder from the utility
2. A resonance condition where a capacitor on the power system is amplifying the harmonic currents and distorting the voltage

Bottom line is this, harmonic distortion causes one of two problems:

• Excessive current distortion causes heating, nuisance tripping of some devices and other current related issues
• Voltage distortion (caused by current distortion) causes misoperation of some loads and nuisance issues

Just because you have harmonics, doesn't mean you have a "problem". There is no ?magic number? where every power system will have issues but you are doing the right thing in identifying the levels with baseline measurements and looking for the ?source?. Many solutions exist to make the system better but all are dependent on the cost/benefit of the solution. IEEE-519-2014 (new revision this year) helps to identify levels as a starting point for most power systems.
Sorry for the long-winded and late response?

Thanks for the input.
I have always been under the asumption that without voltage (EMF), current cannot flow. So wouldn't the chicken
be voltage THD? If not then pls explain the error in my thinking.

 

[GoldDigger]

Thanks for the input.
I have always been under the asumption that without voltage (EMF), current cannot flow. So wouldn't the chicken
be voltage THD? If not then pls explain the error in my thinking.

If the load is non-linear, there be current harmonics without any voltage harmonics driving them. As PQ Man pointed out, there may be resulting voltage harmonics too as long as the supply is not infinitely stiff.
With linear (not necessarily resistive) loads any current harmonics would be directly proportional to the voltage harmonics. (Although for reactive loads, the proportion between the voltage and current at that harmonic will very with frequency.

 

[aelectricalman]

Just mentioning the 24 pulse drive says you have found the harmonic source.
PQA at input to the TR will show the harmonics generated.

My guess is the 17th or 19th will be the biggest.

With a 24 pulse drive, the only harmonics that "should" be measurable are the 23rd and 25th harmonics along with multiples of the 24th harmonic. For example, the 47th and 49th (48th) and so on.

The reason form my original post here is that I am finding (via my fluke 1750 PQ analyzer), that the harmonics from a Toshiba 24 pulse drive are all around 1%-2% with 5% on the 49th order harmonic. While I still don't know what is causing it, I am pretty sure it is a parallel resonance tank condition from an adjacent factory dumping 5th or 7th order harmonic back into the system. If anyone else can explain this mystery 5% 49th order harmonic, I would greatly appreciate it.

I agree with PQ man in that there could be some other harmonic orders but nothing to get too worried about.

 

[aelectricalman]

? Harmonics are always a question of ?the chicken and the egg? ? what causes what (current or voltage distortion)?
? Generally speaking for power systems from large industrials to residential, Current distortion causes Voltage distortion
? Voltage distortion is created by pulling distorted current through an impedance
? Amount of voltage distortion depends on:
? System impedance ? the impedance is made up of the resistance and reactance of the wires, transformers and other power system equipment between the source of power and the loads. (local generators have 3X the impedance of utility service transformers and therefore voltage distortion on a generator is much worse than on the utility as a rule of thumb unless you severely oversize the generator)
? Amount of distorted current pulled through the impedance (i.e. non-linear loads ? electronic loads like VFD, modern lighting (LED/CFL), and computers
? If either (system impedance or harmonic current) increases, V_THD will increase
? Other situations could cause high voltage distortion such as your neighbor drawing a significant amount of non-linear current and distorting the source voltage to your facility OR there may be a harmonic resonance condition where capacitors on the power system are amplifying the harmonics from the load ? this will severely distort the voltage

So, to answer your initial question, looking from the perspective of the 24 pulse drive (and all other downstream loads from that point into the system if you monitoring on the main breaker, for example), the voltage distortion is affected by the current flowing ?into? the drive or other loads ? your assumption is correct that you should measure on the input to compare. In addition, you should compare the harmonic orders (i.e. 23^rd, 25^th or other to the voltage distortion you are measuring) at that point and upstream since current harmonics from that source will cause harmonic voltages of that same order. The comment that the characteristic harmonics should be 23/25, 47/49, etc is true but you may also see some (significantly less) of other ?non-characteristic? harmonics including 5^th, 7^th, 11^th, 13^th, 17^th, 19^th, etc.

Since you are seeing quite high voltage distortion (>8%) at the 4160V level, I would be curious to see if you either have one of the following:

1. A weak source ? i.e. on-site generation (generators have 3X impedance of normal utility source impedance) or long feeder from the utility
2. A resonance condition where a capacitor on the power system is amplifying the harmonic currents and distorting the voltage

Bottom line is this, harmonic distortion causes one of two problems:

• Excessive current distortion causes heating, nuisance tripping of some devices and other current related issues
• Voltage distortion (caused by current distortion) causes misoperation of some loads and nuisance issues

Just because you have harmonics, doesn't mean you have a "problem". There is no ?magic number? where every power system will have issues but you are doing the right thing in identifying the levels with baseline measurements and looking for the ?source?. Many solutions exist to make the system better but all are dependent on the cost/benefit of the solution. IEEE-519-2014 (new revision this year) helps to identify levels as a starting point for most power systems.
Sorry for the long-winded and late response?

You are the MAN! Thank you.

I suspect due to an elevated 49th order harmonic (that I previously did not mention), there is a series or parallel resonance tank condition as well (in an adjacent building). Although this 4160V transformer only has one motor, this comapny has 3 other transformers that are fed from the utility(metered). The 4 total transformers are all fed from the same node. Also, it is known that there are resonance issues in one of the buildings, so we will have to dig deeper and look at some likely equipment. These other three transformers have 500hp, 600hp and 400hp motors and DC drives on them (respectively). Any input on this would be great?

I am still wet behind the ears a bit but I think I follow what you said. Thanks for your insight!

 

[GoldDigger]

Are you metering only voltage harmonics, or are you reading current?
If the drive is turned off, voltage harmonics seen must be externally driven.
If the drive is pulling 49th harmonic current, compare the amplitude of that current to the amplitude of the corresponding voltage harmonic to see whether the impedance looking back toward POCO shows signs of a resonant condition.
If the harmonic current and harmonic voltage are in phase, you would be looking at a high DC resistance (i.e. VD) at that frequency.

 

[petersonra]

so you do the PQ study. then what?

does it even matter? a lot of nasty looking power out there that is harming no one.

if it is coming from off site is there anything that you even can do about it?

 

[PQ Man]

Harmonic voltage distortion

Harmonic voltage distortion

If there is really 5% voltage distortion at the 49th, then there is likely one of two things:

1) Parallel (or series resonance) but you need a capapcitor somewhere on the system to elevate a 4160 V system at this level. With resonance, the higher the harmonic order, the smaller the (involved) capacitor - and with that amount of 49th, I'd be suspicious. I would see if you can find the offending capacitor and if it isn't resonant at the 49th or near the 49th, then there is likely another issue. If a capacitor is in resonance with the source impedance (inductance) and excited by the harmonic producing load(s), it will have a high harmonic current at the resonant frequency (i.e. in this case, the 49th).

2) Could be a measurement issue with the 1750 on the PT's you are using - since you can't measure at 4160V directly with that Fluke, you are likely using PT's. I suspect you can't tranform the 49th harmonic well through a PT (usually transformers will reproduce (accurately) up to about the 25th harmonic then all bet are off especially on a PT).

 

[PQ Man]

Voltage and Current

Voltage and Current

Thanks for the input.
I have always been under the asumption that without voltage (EMF), current cannot flow. So wouldn't the chicken
be voltage THD? If not then pls explain the error in my thinking.

You are right in the sense that power flows from the source and so you need a driving voltage to produce current (linear or non-linear) but from the standpoint of harmonics, it is easiest to explain and model a power system where the loads are considered constant current sources (i.e. the current is "produced" by the load and sent back up through the system so it then produces harmonic voltage drop at the same frequencies (5th, 7th, etc) as the current). With an infinite source, meaning that the source impedance is zero and could provide infinite fault current, the harmonic voltage drop would be zero but the more impedance you have, the higher the voltage drop at the given frequencies and therefore, the higher voltage distortion.

 

[aelectricalman]

so you do the PQ study. then what?

does it even matter? a lot of nasty looking power out there that is harming no one.

if it is coming from off site is there anything that you even can do about it?

Considering they likely blew up a $400,000 drive system by causing capacitors in the old 18 pulse drive to swell and burst, causing a 4 alarm fire, they want to get to root of the problems!

 

[aelectricalman]

Are you metering only voltage harmonics, or are you reading current?
If the drive is turned off, voltage harmonics seen must be externally driven.
If the drive is pulling 49th harmonic current, compare the amplitude of that current to the amplitude of the corresponding voltage harmonic to see whether the impedance looking back toward POCO shows signs of a resonant condition.
If the harmonic current and harmonic voltage are in phase, you would be looking at a high DC resistance (i.e. VD) at that frequency.

Metering all power quality information, including currents.

Here is how the two analyzers are connected.

Analyzer 1

Located on the load side of the 24 pulse drive (represented by the top graph). There are no reactors located between the drive and the analyzer. There was a 35:1 PT used for the 4160V drive system. The CT's were connect directly to the shielded cable. Manufacturer approved but not sure about the 49th order harmonic issue as pertains to the PT. Will investigate.

The drive system (as you can see) was in operation some and out of operation some.

Analyzer 2

Located on the line side at the Point of common connection (PCC) by the customers electrical meter. That is represented by the middle graph!

The bottom graph is a representation of the on off times from the customer account. You can see that it matches the top graph to a T. As a result I was able to sync the customers account with the top graph which is from my Fluke 1750. the two PQ analyzers are synced from the same handheld, so I know all data is in sync.

My plan was to view the data graphically to see if there was a correlation of data between the two analyzers. Looking at this graph I can say that i do not feel the drive system is contribution substantially to the PCC. Although there are times that are close, this data is exactly snyced. There are some instances that are not even close.

Of course this is only V(thd) data but I would expect to see some smoking gun if it were contributing.

 

[aelectricalman]

If there is really 5% voltage distortion at the 49th, then there is likely one of two things:

1) Parallel (or series resonance) but you need a capacitor somewhere on the system to elevate a 4160 V system at this level. With resonance, the higher the harmonic order, the smaller the (involved) capacitor - and with that amount of 49th, I'd be suspicious. I would see if you can find the offending capacitor and if it isn't resonant at the 49th or near the 49th, then there is likely another issue. If a capacitor is in resonance with the source impedance (inductance) and excited by the harmonic producing load(s), it will have a high harmonic current at the resonant frequency (i.e. in this case, the 49th).

2) Could be a measurement issue with the 1750 on the PT's you are using - since you can't measure at 4160V directly with that Fluke, you are likely using PT's. I suspect you can't transform the 49th harmonic well through a PT (usually transformers will reproduce (accurately) up to about the 25th harmonic then all bet are off especially on a PT).

The 49th order harmonic elevation is seen at the load side of the drive but is not found at the PCC. I'm interested in what you said about the +25th harmonic orders not reproduction properly. Do you have any literature on this phenomenon?

 

[aelectricalman]

The 49th order harmonic elevation is seen at the load side of the drive but is not found at the PCC. I'm interested in what you said about the +25th harmonic orders not reproduction properly. Do you have any literature on this phenomenon?

here are all the JPG's of the current and voltage thd for both meters.

 

[GoldDigger]

Harmonics on the load side of the customer's drive are only of interest to the operation of the motor attached to that drive and should not have any direct side effects on any other equipment in the plant. They will be mostly a function of the switching frequency of the drive itself and not the fundamental power frequency. There should not be any kind of capacitance deliberately connected to the drive output, unless maybe on the motor side of load reactors.
With a harmonic order that high (49th), I would not expect much if anything to appear on the line side of the drive as a result, given any decent filtering on the DC bus of the drive.

If a 5% 49th harmonic of the input frequency shows up on the output of a VFD, I would suspect a defect in the drive.

 

[aelectricalman]

Harmonics on the load side of the customer's drive are only of interest to the operation of the motor attached to that drive and should not have any direct side effects on any other equipment in the plant. They will be mostly a function of the switching frequency of the drive itself and not the fundamental power frequency. There should not be any kind of capacitance deliberately connected to the drive output, unless maybe on the motor side of load reactors.
With a harmonic order that high (49th), I would not expect much if anything to appear on the line side of the drive as a result, given any decent filtering on the DC bus of the drive.

If a 5% 49th harmonic of the input frequency shows up on the output of a VFD, I would suspect a defect in the drive.

This is a new drive so I know there is some filtering but no line or load reactors. I agree with what you are saying in regards to the motor operation. That's the only reason we placed it there. So are you saying that I can not use the on off pattern of the motor to syncronized and determine if the harmonics coming from the drive system are affecting the PCC harmonics?

By the way, this motor is the only piece of equipment on this service. Nothing else can contribute to the harmonics except for perhaps harmonics from poor grounding. However since its a delta 4160, those harmonics would just circulate in the windings of the next upstream transformer, correct?

I understand what you mean by drive correcting the harmonics issues and the harmonics not showing up on the line side of the drive. There are plenty of filters built into these guys to protect their equipment. I apparently lost my bearings on that one.

 

[GoldDigger]

This is a new drive so I know there is some filtering but no line or load reactors. I agree with what you are saying in regards to the motor operation. That's the only reason we placed it there. So are you saying that I can not use the on off pattern of the motor to syncronized and determine if the harmonics coming from the drive system are affecting the PCC harmonics?

By the way, this motor is the only piece of equipment on this service. Nothing else can contribute to the harmonics except for perhaps harmonics from poor grounding. However since its a delta 4160, those harmonics would just circulate in the windings of the next upstream transformer, correct?

I understand what you mean by drive correcting the harmonics issues and the harmonics not showing up on the line side of the drive. There are plenty of filters built into these guys to protect their equipment. I apparently lost my bearings on that one.

I am saying that the load side and line side of the drive are isolated by the DC bus and drive electronics. Your idea that you need to look on the line side of the drive was correct if you suspect that the drive and motor are causing problems elsewhere in the plant.

 

[PQ Man]

here are all the JPG's of the current and voltage thd for both meters.

That explains more if it is on the output - the output of the drive is likely a PWM voltage waveform - see the attached and spectrum and waveform - it should look familiar. The way 1750 will do OK with that waveform but it will likely not reproduce it perfectly - perhaps filtering it or smoothing off the edges of the pulses. The switching frequency of these pulses is pretty high - usually 2-10kHz so the higher order harmonics are not a suprise.

Regarding the 25th harmonic, I don't have a particular reference short of experience but generally, you can assume that PT's and CT's are not very accurate above 3 kHZ and they start to tail off below that (i.e. at or above 25th).

 

[PQ Man]

here are all the JPG's of the current and voltage thd for both meters.

Your THD for voltage and current at the input of the drive are not too unusual and show about 2-6% THD voltage distortion and honestly are not that suprising - the predominant harmonics are 5th and 7th (again, not a huge suprise) but elevated for a 24 pulse drive - not achieving perfect cancellation of lower order harmonics.

After looking at the trending and spectrums, I don't think these are the smoking gun but would have to look more closely at the whole system/model. It is likely that the situation you are monitoring is not the level or event/issue that caused damage.

The failure of the filters on the 18 pulse drive are curious. A filter is generally applied at the "lowest expected harmonic" otherwise, you could AMPLIFY the harmonics that are there. So, here is a scenario - the new 24 pulse drive produces a decent amount of 5, 7, 11, 13 and the 18 pulse drive has an 11th or 17th filter - the filters could amplify the lower order harmonics and fail. Just speculation - lots to understand - I don't even know if both are on at the same time or one replaced the other.