For electrical systems to function in their intended manner without significant loss of performance or life, they require a supply of electricity that is of good quality. Good quality electrical power has the following characteristics:
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The term 'clean power' is used to describe electricity that is considered to be of good quality (see above) with particular reference to a very low harmonic content. Therefore, the term 'dirty power' is used to describe electricity that is considered to be of low quality (opposite to the above) with particular reference to a very high harmonic content.
In Australia, our electricity is supplied in alternating current at a frequency of 50Hz. In alternating current (AC) the movement of electric charge periodically reverses direction. In direct current (DC), the flow of electric charge is only in one direction. 50Hz means that the AC has a frequency of 50 cycles per second. This is also known as the 'fundamental' frequency. AC is the form in which electricity is delivered to businesses and residences. Most electrical devices (such as motors) need clean electricity to function properly. This means that the electrical supply needs to be a clear sinusoidal wave.
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Yes! This is the best way to eliminate harmonic related problems.
Harmonics mitigation is most cost-effective when 'designed in'. Mitigation in existing installations is possible but more than likely to cost significantly more without the performance of an 'ab initio' approach.
Good reasons for designing-in harmonics mitigation relate to economy, performance and reliability. All three of the following reasons have dollar implications.
Harmonic mitigation solutions should be tailored to the type of application. In many ways each installation is unique but broad guidelines apply.
The three-phase loads are generally HVAC, moving walkways and escalators, and elevators. HVAC plant generates significant harmonics.
Single-phase loads usually provide the highest aggregate harmonic load. Primary sources are IT equipment, and lighting ballasts. Load balancing for single-phase circuits is an important design consideration when there is high harmonic content. Two-phase loads are less frequent but might be reserved for some commercial kitchens.
Changes in tenancies, and expansion of facilities (more offices, shops, etc) can overload the reticulation because of increases in harmonic content. The capacity of a sub-station transformer can easily be surpassed unless harmonic mitigation has been installed.
Many loads are three-phase, three-wire. Sub-station transformers are likely for large consumers connected to medium voltage distribution and plant expansion can challenge the sub-station beyond capacity.
Single-phase loads in aggregate are likely to be less than three-phase loads. Careful consideration of circuit and load mitigation will provide high power quality (eg: mitigation of negative sequence harmonics, commutation notching from converters) and allow control gear (PLCs, micro-controllers, industrial bus systems) to operate from clean supply.
Power factor is often low or variable due to loading and unloading of motors. Consideration of power factor control in combination with harmonic mitigation is important and best done at the design phase.
Harmonics mitigation adds cost to electrical installations and for that reason is often ignored. Sometimes a ‘suck it and see’ policy is adopted.
Harmonics are not just an internal problem. They are exported into the supply system in spite of interposing transformers—even the ubiquitous third harmonic, since installations are never phase-balanced.
If the supply authority complains about the degree of voltage distortion at the point of common coupling (pcc) harmonic filtering might be specified. Note that the relevant standard for harmonic voltage distortion in Australia is AS/NZS .3.6. Supply authorities can insist on their own maximum voltage distortion levels.
As energy regulators look more closely at 'gold-plating' of poles and wires, often defended on the basis of having to cater for high demand intervals, consumers can expect considerable additional charges for poor power factor and/or high apparent power demand. Flattening demand peaks is the single most important way of achieving cost-effective distribution systems.
Demand is measured in various ways by:
There can also be combination of the above demand parameters and power factor, this normally being the displacement power factor. Harmonics severely lower power factor and therefore increase the apparent power.
Consumers who buy electrical energy at medium rather than low voltage will have a sub-station transformer which may or may not owned by them but the problems caused by harmonics will be owned by them. Harmonics can severely de-rate the transformer and it is not an exaggeration that in Australia not much attention has been paid to 'harmonics-hardened' transformers. Eddy current losses, proportional to the square of both the amplitude and harmonic frequency will shorten transformer life span.
Proximity and skin effects in conductors due to harmonics will often increase energy losses well in excess of the I2R losses at the fundamental frequency (50 Hz). These additional losses can wreck switchboards, overload conductors including neutrals and cause voltage distortion in the customer’s installation. Reliability of protection is affected. Magnetically tripped breakers can be tripped through severe current distortion (very high peak current) and thermally tripped breakers through the heating effects of harmonic current components. Harmonics can therefore upset coordination of protective gear. Residual current devices (RCDs) can also malfunction.
Energy waste in the reticulation system is best avoided by harmonics mitigation as close to the harmonics generating sources as possible. Economies can be achieved by aggregating some harmonics generating loads for example variable speed drives. Large rating loads are often best connected on separate circuits. Single-phase loads are generally most economically mitigated at sub-distribution board level using three-phase, 4-wire harmonic filters (active filters are recommended for high variability loads).
Neutral current compensation can be a problem in the case of unbalanced phases, and zero sequence (3rd, 9th, etc) harmonic compensation best achieved on a per phase basis. Transformer selection for sub-stations should be on the basis of having a line diagram of the installation with predicted harmonic content available. For installations with stand-by generators, mitigation filters should avoid creating leading power factors. For installations using passive capacitor-bank displacement power factor correction, detuning chokes have to be employed to avoid resonance problems.
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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.
You can download this free from the Energy Networks Association: https://www.ena-eng.org/ena-docs/
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?
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.
Yes with the right instrument and set up.
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.
If you are referring to the allowable voltage imbalance between phases, the specification given in BS EN :+A3:, 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.
Yes and the application of G5 is how the Network Operators ensure that they comply with the power quality requirements of ESQCR.
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.
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.
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.
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.
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.
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.
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).
I would refer you to K. S. Stroud, Engineering Mathematics Rev 6
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.
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.
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 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.
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).
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.
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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.
I would monitor the supply with my CA 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.
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.
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.
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.
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.
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.
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.
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.
I have seen nothing to evidence this, so I doubt this is so.
Ideally none, although as long as its compliant with G5/5 we could probably take that as ok.
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.
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 , and, I am not convinced that it really saves energy.
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.
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.
Yes as they are negative sequence and will fight against the rotating field of the fundamental at a varying frequency.
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 .
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 goes, it may be so that these cables are no considered within BS , however, we also have clause 110.2.xi, which excludes machinery from the scope of BS .
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 , 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 , 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.
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