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Harmonics and Power Quality Analysis webinar questions and answers

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This webinar was arranged by the IET’s Electrician EngTech programme in association with Wiring Matters. Electrician EngTech helps electricians gain recognition of their professional competence.

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Your questions answered.

Are active filters the best solution to tackle the harmonics or would you recommend capacitor banks?

Capacitor banks can create harmonic resonance and therefore I wouldn't use them in systems with a presence of harmonics without first undertaking some power quality analysis. These are more for power factor correction and so if they are needed I'd use the results from the system analysis to ensure you detune any PFC you install. Active filters are the better option although expensive, it's always worth considering the total cost of ownership vs any savings in energy costs.

Where can G5 be accessed?

You can download this free from the Energy Networks Association:

The Network Electrical supplier has a requirement of 5%. In a number cases sites exceed this e.g. 6%. Has any company been taken to task i.e. fined and forced to rectify?

I have done some research and found no evidence of any court cases that existed where a company was taken to task. Perhaps this needs to start?

Why is it mostly the 5th and 11th harmonic that are creating problems in the system?

5th and 11th are negative sequence harmonics, i.e. they go in opposing directions to the fundamental system waves. This can cause counter electromotive force (CEMF) which means a motor has to do more work and subsequently draws more current. This is an inefficiency in the design.

Can we measure 0-100th Voltage and Current harmonics as well as inter and sub harmonics?

Yes with the right instrument and set up.

Is G5/4-1 outdated now? Do we need to perform the measurement and analysis based on g5/5?

Yes, but, there is a period whereby projects commenced under G5/4 will be allowed to be completed under G5/4, but all new works now need to be to G5/5.

What is the unbalance factor (%) as per BS?

If you are referring to the allowable voltage imbalance between phases, the specification given in BS EN 50160:2010+A3:2019, Voltage characteristics of electricity supplied by public electricity networks states:

4.2.4 Supply voltage unbalance
Under normal operating conditions, during each period of one week, 95% of the 10 min mean r.m.s. values of the negative phase sequence component (fundamental) of the supply voltage shall be within the range 0% to 2% of the positive phase sequence component (fundamental).

NOTE 1 In some areas with partly single phase or two phase connected network users' installations, unbalances up to about 3% at three-phase supply terminals occur.

NOTE 2 In this European Standard only values for the negative sequence component are given because this component is the relevant one for the possible interference of appliances connected to the system.

Is ESQCR relevant?

Yes and the application of G5 is how the Network Operators ensure that they comply with the power quality requirements of ESQCR.

Is there any way to justify or calculate the neutral current due to harmonic contents?

Yes if you do not check the level of N current, how can you be sure that you will not overload the N conductor and thus cause the cable to fail, or result in a breach of EAWR.

Are you measuring Fault Level to allow accurate G5/5 Reporting?

You would need to assess the fault levels and all of the other required parameters in G5/5 to generate the correct calculations. We will know more once the G5/5 guidance note is out. I have yet to undertake any jobs covered by G5/5 as it is quite newly published.

What is the reg for Power quality issues in Hospital? Which regs do we need to follow for the limits?

You will have obviously G5/5 and the requirements of the HTM's which are still relevant to that area. Following this you may have customer specific requirements based on the equipment that is being installed and operated within the hospital premises.

WRT to neutral conductor ratings, based on BS EN 61000 single phase devices are not permitted to exceed 30% third harmonic. Why do we not routinely de-rate TP&N cables by say 15% based on the fundamental design current Ib? The industry obsession with 200% neutrals seems unnecessary?

A good question. However, we can't assume design current will always be an accurate reflection of what we see in the applied space. I'd personally rather have an uprated cable cost vs a burnt out cable and any subsequent losses as a result.

The EMC Directive applies to equipment manufacturing, (Emissions and Immunity), not fixed installations?

Not quite true, the EMC directive does apply to products, yes. Clause 37 refers to fixed installations, you might want to satisfy yourself by reference to that such that the EMC regulations can be ignored for all but product. Also, the effects of multiple products connected to the same supply is cumulative, therefore the DNO has to ensure that the limits of ESQCR are not breached by this accumulation, ergo reference to the EMC Regs. I would also refer you to Schedule 1 Clause 2, Fixed Installations.

Do we need to consider harmonics up to (or beyond) 100th harmonic as this seems to be implied by ENA EREC G5/5?

Yes we do for new installations as per the new G5/5. There may be derogations applicable to installations that predate G5/5 as I believe G5/4-1 only looked at orders up to the 50th.

When you are talking about harmonics on the network, and measuring it, and planning levels, is this voltage? is it necessary to also measure harmonics on the current?

That's correct, the planning levels that we look at are on the voltage. It isn't mandatory to look at current for planning, but seeing as current distortion has an impact on the voltage distortion it may help to perform analysis on this as well, (which the modern power analysers will allow).

How do you convert the Fourier transform coefficient into relative intensity of third harmonic over first harmonic?

I would refer you to K. S. Stroud, Engineering Mathematics Rev 6

Do DALI lighting drivers typically cause a leading power factor?

I don't think the DALI technology itself impacts the current in terms of leading/lagging. This is simply the communications protocol that automates the lighting system. It would be specific to the ballast and the characteristics of this device.

Could the DC offset voltage be measured (easily) with a PQA to determine if it's actually there - currently (for type A vs AC RCD) we assume it will be but I cannot find a method to verify this (without say, an oscilloscope).

A PQA will measure DC voltage offset, however, most current clamps for PQA's are the flexible split type, which are Rogowski coils; these coils are not designed to measure DC, though a PQA will detect DC voltage and a certain amount of DC current.

Current transducers work at their optimum in the middle of their range also, therefore to detect currents even in a simple small domestic install up to maybe 100A AC, then also to try and detect imbalance or leakage which is say 6 orders of magnitude below this, and DC also in the mA range is a challenge that as far as I am aware, not been overcome as yet.

Once we can measure RMS AC in the hundreds and mA ranges at the same time then this will be a start, all we need to do then is detect the DC current and its waveform. I know that is a bit of a misnomer, but not all DC is a nice steady level voltage and current.

Once this is detection and measurement is cracked by the sensor people, then it's down to the instrument makers to generate hardware and firmware to extract and extrapolate that data. Once we get to this point then we can start to part with our several £k's to get the instruments which can be used to figure all this out in the field, rather then the £10's of k's on lab level equipment which is not designed to be rugged and lugged around site.

Is it necessary to measure or work out fault-current at PCC to choose appropriate thresholds?

I would recommend this, this may even be required by G5/5, however, as yet, I have not had chance to go through this to get fully up to speed.

I am an Electrical Technician on a CHP plant. We are having issues with power quality on our single phase waveform. There are 6 spikes on the sinewave which we believe are coming from our excitation system of the generators. We believe this is what is damaging some of our sensitive electronic equipment on site. How can we smooth this sinewave?

I would suggest you get the generator controls OEM to verify that there are no faults with the control or firing system and that it is correct for the application and correctly installed and configured, wrt earthing and bonding, cable sizes etc. Then I would monitor the system for a reasonable period and analyse the results to see exactly what is happening and when. I would then base my decision on a logical sequence looking at loads, stability magnitude of "issues" when they occur and decide on the mitigating measures based on that. Perhaps a static system may help, or maybe a dynamic mitigation system may be necessary.

Where in a new system is it best to install or make provision for harmonic controls? At the MCC's, Elevators etc. or at the supply authority intake? Or both?

It is certainly good practice to allow for harmonic mitigation measures at the locations you suggest. Always start with the drive OEM, if they don't understand what is required and how to help you to control the emissions from their drives, then I would suggest finding another drive supplier (This statement is the opinion of the webinar speaker as an ex-employee of a drives OEM, not on behalf of the IET).

What about power lag cos@. How can it be measured and rectified for domestic installations?

It is not normal to worry about power factor in domestic premises. However, it would be measured in the same way as that for a commercial or industrial premises. I would do this with a power quality analyser, monitoring the current and voltage and recording these over a period, then analysing the results. If the PF was steady then it would be simple to fit some PF correction equipment to correct this. If we find that the PF varies, then it might be necessary to fit active or switched PFC which would likely be too expensive for the benefits to be gained.

Some manufacturers are providing PQ analysis option on their Primary LV devices e.g. ACB. What are thoughts on their utility?

If this gives an adequate level of information, I think it is a good idea as this monitoring would be permanent and ongoing, thus it could be trended over time and might highlight changes indicating or example defects in the active front end of a drive which has this kind of system, or in the switching devices providing power to the motor. Either or both could cause a change in the harmonics and other PQ features. This could then be used as a predictive maintenance tool to identify issues perhaps such as dried out electrolytic capacitors, contamination within the drive, or perhaps even in the motor; only analysis would tell.

How do you measure power factor and rectify?

I would monitor the supply with my CA8336 PQA over an "operating period" and provide a report detailing the variations in power factor to the client, and then suggest that my client engage the services of a power factor correction equipment company to design and provide a suitable solution to mitigate the power factor and its variation.

Is the move towards more distributed generation (wind, solar etc.) a significant factor in power quality on the distribution network?

I totally agree with Paul, there have been some really good papers written that are beginning to capture potential grid inertia instabilities from the connection of electrically separated supplies. I.e. solar, which has no moving parts and even wind which has electrical separation between the prime mover and the grid. By having a big spinning lump of motor in a turbine, and the fact the grid is in sync, therefore, all spinning machines are in sync, then if one machine trips out, there is enough inertia in the grid to maintain frequency stability. That is, the rate of change of frequency (ROCOF) is kept to a minimum.

Renewable source does not provide this physical inertia and so thought must be given on how the ROCOF is kept to a minimum.

How can you mitigate the effects of harmonics?

By design, by correct installation, earthing and bonding of the equipment in accordance with the equipment OEM instructions. By passive or active harmonic mitigation such as filters. Fundamentally good design and selection of product is critical.

Is there any measure to avoid the effect of harmonics at stage design? instead of reviewing the quality of the loads once a building is being built?

If it's a brown field site you could do some upfront PQA to see what you're up against, then design any change around that. This could include a need to design filtering into the new kit. For a green field site, as above.

How do you determine the harmonic resonance of a motor for example when commissioning a vsd?

This would be done by careful changing of drive parameters and observation/measurement of motor reaction to these changes. If the motor/drive is part of an engineered system then the system OEM may already have the information. Guidance from the drive & motor OEM will be helpful in ascertaining this.

Is there any information on working out phasor diagram at planning stage?

Yes if you have the power factor and duty cycle of all connected equipment. However, this is not simple as the PF will vary depending on connected equipment and its loading, so whilst possible, it may not be as worthwhile as it first appears due to the lack of definitive information on the system loading at the design stage as opposed to once in real operation.

What are the signs that your system is suffering from harmonics?

Instrumentation misbehaving is a good sign that there is a problem, especially if the 'noise' observed in instrumentation signals aligns with motor operation. Unfortunately, unless there are really visible symptoms you may not catch them, and even when there are symptoms, you will still have to invest in analysing your system to conclude that it is the result of harmonics. Other signs include measurement, increased temperature in N conductors noise, or overheating of supply transformers.

In UPS systems, does it really need to have a 200% sized neutral conductor to address harmonics issues?

Possibly, if the UPS OEM requires it then yes. However, it would be a concern for the installation to have such high levels of harmonics present in the supply. It would also depend on whether this N conductor is in the supply to the UPS or in the output of the UPS, so it is difficult to answer the question.

From harmonics, which side is affected most? Power supplier side or load side?

Current harmonics are generally generated within the installation, voltage harmonics tend to be conducted into the installation from outside. The current harmonics can have an effect on the installation which creates them. For example, large loads being switched on can cause a short term drop in voltage even within the installation resulting in flicker affecting lighting or causing drops in voltage such that other systems will malfunction.

Do domestic smart meters monitor power quality?

I have seen nothing to evidence this, so I doubt this is so.

What is the best measured value of harmonics to be expected at consumer point?

Ideally none, although as long as its compliant with G5/5 we could probably take that as ok.

Is 5% still the maximum tolerance for THD on BUS bars and are harmonic trap filters on BUS bars at the point of common coupling relevant for HV and LV systems?

It's not quite that simple. The question does not distinguish between voltage or current THD. I would recommend that you refer to the relevant standards G5/5 & IEC.

Could there be a conflict of having voltage optimizers on an installation also having power factor correction?

The personal view of the speaker is yes, I am not a fan of voltage optimisation as it is often fitted without thought to the consequences of this to the installation under BS 7671, and, I am not convinced that it really saves energy.

Is there a cut-off value, whereby equipment below a certain kW rating and/or using certain control equipment, doesn't need to be accounted for regarding Power Quality Analysis?

If it cannot generate harmonics, then it does not need consideration. However, lots of small harmonic generating loads will cause harmonics to sum in the N conductor which ultimately can cause issues.

What has happened to the problems of triplens that seem to be most prevalent in the N?

It hasn't gone away, they still do, they summate in the N and they can result in N currents greater than those in the Line(Phase) conductors.

Can the effects of CEFM from the 5th and 11th Harmonic cause vibration within motors?

Yes as they are negative sequence and will fight against the rotating field of the fundamental at a varying frequency.

If voltage is the problem why do we constantly get overvoltages in properties, and should the network operator look into providing voltage optimisers?

It's not steady voltage that is the real problem, whilst excessive voltage is in itself a power quality issue, the phenomena we are talking about here are transient in nature and short term, apart from harmonics. Voltage optimisation is not the panacea of all evils it has been suggested as by some. What needs to happen is the DNO needs to control the voltage supplied to the premises in accordance with ESQCR, G5/5 & IEC 50160.

Does a lot of modern LV equipment conform to the relevant part of IEC 61000 and would it pass a stage 1 A assessment through OEM type test declarations?

Potentially, but there is still the installation of these to consider, efforts and failings in the installation and utilisation of products can cause them to fall outside compliance.

What is the best method to eliminate third harmonics causing vibration on rotating machines i.e. fans?

If these 3rd harmonics are created by the electrical fan drive, then this needs to be resolved by reference to the drive configuration and installation. If the fan is connected directly to the mains supply and these harmonics are coming in from the DNO supply then this needs to be investigated. Without knowing the detail of the origin of the 3rd harmonics it is not possible to determine a satisfactory means to mitigate them.

Do the DNOs monitor power quality at consumer connections on a regular basis?

Domestic consumers, no unless there are complaints, commercial and industrial, I cannot say for definite, but I would have thought it would be based on a case by case basis and complaint or problem dependent, e.g. supply transformer issues caused by harmonics.

Is there any specific, usual property of the 3rd and 7th harmonics? Previously it was mentioned that 5th and 11th are negative sequence. So, will 3rd and 7th have negative effects on power transmission and protection system problems?

3rd harmonics sum in the N conductor, which at sufficient levels these can cause overloading of the N. 7th harmonics have a positive sequence which have the same rotation as the fundamental. These cause a heating affect in supply equipment as the add to the fundamental.

Has anyone seen increases in THD (and thus reduced PQ) in deploying VFD technology? If so what measure have they used to correct this?

This would only be expected. When VSD's are implemented they will generate harmonics. Firstly the VSD's must be installed in accordance with the drive OEM instructions. Secondly cabling to the motors must also meet this requirement. SWA is rarely suitable downstream of a VSD of any sort. If the drive is installed in accordance with the drive OEM data then the next port of call would be to measure the harmonics carefully and note their origin. Discuss this with the drive OEM, and start with mitigations they have in their catalogue. These could be input filtration or filter devices on the output cable to the motor. These harmonics can be addressed and eliminated by a logical fault finding process.

What is The Total Harmonic Distortion and how it is significant, instead of voltage or current distortion?

THD is a measure of the overall voltage or current distortion measured in a system. It is voltage THD that is generally the concern of the DNO, as this is what is transmitted through their systems, whereas current THD which is the measure of how badly loads within the installation are affecting the supply.

What is best or most popular way to reduce current harmonics in facilities that are outside limits?

Identify the source of the harmonics and then implement mitigations to eliminate them in accordance with the findings. Always identify the source and implement engineering measures first, ensure that the fundamental design is correct before implementing mitigation. That is identify and treat the disease, not the symptoms.

I've heard anecdotal stories of VSD fed motor bearings fail due to power quality issues - can you explain why this may happen?

This is a known phenomenon, it results in a bearing defect known as electrical fluting in the bearings. It is created by circulating currents in the rotor of the motor and can be minimised by correct design of circuit. Additionally it might be necessary to add brushes or rubbing strips connected to the protective conductor network and running on the rotor. Another solution is special bearings which are fitted with an insulating ring to interrupt the path. This could be a plastic sleeve, but, this could also be ceramic, as long as it insulates. It is not common, but definitely exists.

Do the new solar power systems cause harmonic issues and how can this be resolved?

We can think about this from a fundamental principles standpoint.

If, you are referring to Solar Photovoltaic generation, then this generates DC current, ad this is then run through an electronic inverter to create AC current.

This process has the potential for the DC to be transferred into the AC waveform as a DC offset by poor design, or fault within the inverter, so that would be one power quality issue that could arise.

This "DC leakage" could also affect type AC and potentially even type A RCD's if enough current leaked to give a significant impact.
The next thing we can look at is the way that the AC waveform is generated, electronically, this may have a poor waveform, by poor design or fault in the inverter.

This would cause both distorted voltage and current waveforms.

Then we have "inertia", when we generated electricity solely by big spinning lumps of metal, it took large currents to put enough load on these to cause them to slow down, and as a result the control loops could be slow acting and nicely damped to smooth out any sudden fluctuations in the voltage and current.

Electronics conversely is fast acting in switching and in failure under load, so we need fast acting control loops which may result in under or overshoot, thus causing short duration sags and spikes in voltage, hopefully these would be smoothed out by the rest of the supplies into the grid, and the inherent inductance of the grid, but, nevertheless sags and swells are PQ issues.

I think finally I am led to believe that the large scale systems not things like 2-3kW domestic SSEG, are having "synthetic inertia" designed into the inverters, now how this is done, I am unsure.

On the inverter and servo drive systems I used to work with we would fit large capacitance to the DC bus to allow the motors to have a voltage "reserve" for sudden short bursts of acceleration, and we would allow these also to be "topped up" when decelerating before switching the power supplies into regeneration or "bleeding off" the Excess DC voltage via resistances across the DC bus.

As LED lighting is becoming more prevalent what are the implications for power quality; is there an argument for low voltage distribution using high quality power supplies within buildings?

LED lighting tends to use crude switched mode power supplies built to a price not to a quality, and whilst they are low power devices, their affect is cumulative, so one solution may well be DC supplies with centralised power supplies with controlled power quality affects.

Would adding overcurrent detection in N (to disconnect corresponding Ls) be a reasonable mitigation of the dangers?

Certainly for detection of excess current flow in the N conductor, I think that could be reasonable mitigation, but that could be complex, mind as yet, the full scale of this issue is not apparent. Also the changes in the ENA requirements should bring this under control, coupled hopefully with greater awareness we should see a cap on this scenario perhaps.


SY Cable

Do you recommend SY cable for EMC even though it does not meet the requirements of BS 7671? Can you do a cable calc on Amtech to verify right size for SY? And what about the fact that regardless of claims of quality, SY cable isn't CE marked?

YY is not suitable for EMC control as it does not contain a braid.

The two cables which are suitable for EMC control are SY & CY.

Note that SY is not suitable for use outdoors unless specified by the cable OEM as it suffers accelerated degradation when subject to sunlight (I suspect UV) and weathering.

SY & CY cables when used must be correctly terminated, compression, or "stuffing", and TRS glands are not suitable means of termination.

"Bonding nipples" the glands with the bar and two screws are completely unsuitable.
One needs to look at CXT, GXD, and EMC control glands such as those made by Wiska, Harting, and many many others.

These EMC glands may look like ordinary compression glands but, they are not.

As far as SY/CY cables and BS 7671 goes, it may be so that these cables are no considered within BS 7671, however, we also have clause 110.2.xi, which excludes machinery from the scope of BS 7671.

If the cables are not used within construction, they will not require CE marking under the construction products directive.

If they are used on a machine, then they are covered by the Declaration of Incorporation or Conformity (DoC/CE mark) by that provided by the equipment OEM, under the Machinery Directive (MD) as they have considered the design and application of the cable and they consider it suitable.

As SY & CY cable has not been withdrawn from the market since the new approach directives have been issued, and bearing in mind that the first LVD was 1975, so it's not new legislation then it must be suitable for something, and there have been no general complaints or reports to get this wiring material withdrawn from sale across Europe.

One also needs to consider how many motors which are connected to VSD's are actually covered by BS 7671, I would suggest not many, most will fall under the definition of machinery, and thus be excluded by 110.2,xi and thus covered by the machinery directive, air conditioning and refrigeration plant are certainly MD, as are lifting equipment, escalators, the aspects of lifts which fall outside the lifts directive fall under the MD, automation of all kinds such as say baggage or cargo handling in airports, sewage treatment systems would fall under the description of machinery, and many others.

I would refer the reader to the definition of machinery as per the HSE website & or the guidance on the EU website.

At this time we are still following EU rules, and little will change short term as the MD is the Supply of Machinery Safety Regulations (SMSR) and is UK law.

The department of Business Enterprise Innovation and Skills (BEIS) has in their last communication to us stated that there will be changes to the current published advice, so their statement was stick with the existing SMSR requirements for now.