Power Quality is our AIM 

Power Quality

It is estimated that power quality problems cost industry and commerce in the EU about £10 billion per annum while expenditure on preventative measures is less than 5 % of this* . The question is the obvious one: "How much money should be invested in prevention to balance the risk of failure?" and the answer depends on the nature of the business. The first step is to understand the nature of the problems and assess how each of them relates to the business and what losses might result.
* Source data: Copper Development Association
According to a study performed by European Copper Institute in 2001, covering 1,400 sites in 8 countries, any given site in Europe has a 5-20 % probability that it will suffer from one or more power quality related problems.  Typically, half of sites in energy-intensive industries or mission-critical office buildings will suffer from two or more problems. Very few sites are trouble free. PQ problems are complex, and often an expert team needs to be assembled for their diagnosis and solution.  Similar symptoms, such as equipment overheating, can have different causes (harmonics, unbalance, overloading), and each needs a different solution.

Typical Power Quality Problems

Computer lockups
Earth current originating in equipment, results in a voltage drop between the equipment and earth. Although small, this noise voltage may be significant when compared with the signal voltages (of a few volts) on which IT equipment operates. PC hardware is designed to minimise sensitivity to this kind of disturbance but it cannot be eliminated entirely, especially as the noise frequency rises. Modern communications protocols have error detection and correction algorithms built in, requiring retransmission of erroneously received data - and consequently reducing the data throughput.

Flickering lights (P28)
Flicker is caused by load switching within electronic apparatus and is commonly produced by devices such as arc-furnaces, rolling-mills and multiple welder loads, electronic ballasts, light dimmer switches.  When the supply cannot fulfil the current demand, the ac voltage will temporarily dip and the effect on a 60W incandescent light bulb connected to the same supply point would be a temporary reduction in light, which if repeated would constitute flicker. The amplitude and frequency of these deviations can cause incandescent lamps to flicker. This is not only potentially annoying, but it can trigger seizures, especially in people with epilepsy and is responsible for the so called "sick building syndrome".

Overheating of transformers
Harmonics cause additional losses in the transformer. When the transformer is close to maximum load, these losses can lead to early failure due to overheating and hot spots in the winding. With the current trend to push equipment harder to its limits, and the increasing harmonic pollution in low-voltage networks, this problem is occurring more frequently. Losses in transformers are due to stray magnetic losses in the core, and eddy current and resistive losses in the windings. Of these, eddy current losses are of most concern when harmonics are present, because they increase approximately with the square of the frequency. In a typical mixed load building the transformer eddy current losses will be about 9 times higher than would be expected, approximately doubling the total load losses. Before the excess losses can be determined, the harmonic spectrum of the load current must be known.

Induction motors
Voltage harmonics cause extra losses in direct line-connected induction motors. The 5th harmonic creates a counter-rotating field, whereas the 7th harmonic creates a rotating field beyond the motor's synchronous speed. The resulting torque pulsing causes wear and tear on couplings and bearings. Since the speed is fixed, the energy contained in these harmonics is dissipated as extra heat, resulting in premature ageing. Harmonic currents are also induced into the rotor causing further excess heating. The additional heat reduces the rotor/stator air gap, reducing efficiency even further.
Variable speed devices cause their own range of problems. They tend to be sensitive to dips, causing disruption of synchronised manufacturing lines. They are often installed some distance from the motor and cause voltage spikes due to the sharp voltage rise times. Special care has to be taken at start-up of motors after a voltage dip when the motor is normally operating at close to full load. The extra heat from the inrush current at start-up may cause the motor to fail.

Skin effect
Individual conductors (or grouped conductors) carrying alternating current will be surrounded by an alternating magnetic field, which will induce opposing eddy currents within the conductor itself.  Near the axis of the conductor those currents will tend to reduce the main current, but near the surface they will increase it.  The result is a non-uniform distribution of the current across the conductor with more current at the surface than the middle. For a.c. the induced e.m.f.'s increase with frequency until the current in flowing in a thin layer only (or skin). The effect is more pronounced with higher order harmonic currents, but a load with a 3rd harmonic current will increase the effect. For example, a conductor with 20 mm diameter has 60 % more apparent resistance at 350 Hz than its dc-resistance. The increased resistance, and even more, the increased reactance (due to higher frequency), will result in an increased voltage drop and an increased voltage distortion.

PLC problems
Severe harmonic distortion can create additional zero-crossings within a cycle of the sine wave. Microprocessors in process control equipment use the zero crossing as a reference point for synchronisation and therefore manufacturing may be disturbed and PLC devices may lock up.

Data network congestion
Earth leakage currents cause small voltage drops along the earthing conductor. In a TN-C network, the combined earth-neutral conductor will constantly carry significant current, dominated by triple-n harmonics. Due to the increasing use of low-voltage signals in IT equipment, bit error rate increases, up to the point that the entire network locks up. How many large and small, privately owned networks enjoy this phenomenon? For an unexplained reason, the network locks up, e-mail services fail, it is no longer possible to print...

Misapplied Power Factor Correction Equipment

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Harmonic frequencies may coincide with resonant frequencies of the combined stray inductance and power factor correction (PFC) equipment, creating excessive voltage or current and leading to premature failure. This problem can be easily overcome by applying the correct type of power factor correction equipment through the use of tuned reactors connected in series with the capacitors. The above voltage distortion levels show a marked reduction when switching the PFC on and off!
In the last 2-3 years the majority of customer requests to attend site for power quality related issues have been traced to incorrectly applied power factor correction equipment.

Voltage flat topping
SMPS as found in IT equipment only draw current at the peak of the voltage waveform. A site with a significant amount of single-phase SMPS will produce voltage distortion in the form of flat-topping of the supply Voltage.  Since current is consumed only at the peak of the voltage waveform (to charge the smoothing capacitor), voltage drop due to system impedance will also occur only at the peak of the voltage waveform.  This reduced peak voltage will translate to a lower DC bus voltage in the SMPS.  Input current to the SMPS will increase because the computer or other electronic load still requires the same amount of power. These increased I2R losses in the SMPS will not only accelerate the aging of its components they will in effect increase the kWh consumption. Also of concern is that the power disturbance ride-through capability will also be reduced since the reduced peak voltage means the large filter capacitor on the DC bus of the SMPS will not be able to store as much energy.

Overloaded Neutrals

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In a 3-phase circuit, there are 3 active conductors, and a return conductor, which carries the unbalance between the 3 phases. However, with the triple-n harmonics summating, significant currents can flow in the neutral conductor. As many neutral conductors have been, in the past, half-sized, this situation can become critical, even when the phase conductors are operating well below full load. In some instances the current flowing in the neutral can actually exceed the current flowing in the individual phases.

IEC 61000-2-4

IEC 61000-2-4 - Compatability levels in industrial plants for low-frequency conducted disturbances. This part of IEC 61000 is concerned with conducted disturbances in the frequency range from 0 kHz to 9 kHz. It gives numerical compatibility levels for industrial and non-public power distribution systems at nominal voltages up to 35 kV and a nominal frequency of 50 Hz or 60 Hz. The compatibility levels specified in this standard apply at the in-plant point of coupling. At the power input terminals of equipment receiving its supply from the above systems, the severity levels of the disturbances can, for the most part, be taken to be the same as the levels at the in-plant point of coupling. In some situations this is not so, particularly in the case of a long feeder dedicated to the supply of a particular load, or in the case of a disturbance generated or amplified within the installation of which the equipment forms a part.
Compatibility levels are specified for electromagnetic disturbances of the types which can be expected at any in-plant point of coupling (IPC) within industrial plants or other non-public networks, for guidance in
a) limits to be set for disturbance emission into industrial power supply systems (including the
planning levels defined in 3.1.5);
NOTE 1 A very wide range of conditions is possible in the electromagnetic environments of industrial and other non-public networks. These are approximated in this standard by the three classes described in Clause 4. However, it is the responsibility of the operator of such a network to take account of the particular electromagnetic and economic conditions, including equipment characteristics, in setting the above-mentioned limits.
b) the choice of immunity levels for the equipment within these systems.
The disturbance phenomena considered are:


BS EN 50160

BS EN 50160 - gives the main voltage parameters and their permissible deviation ranges at the customer's point of common coupling in public low voltage (LV) and medium voltage (MV) electricity distribution systems, under normal operating conditions. In this context, LV means that the phase to phase nominal rms voltage does not exceed 1000 V and MV means that the phase-to-phase nominal rms value is between 1 kV and 35 kV.

ENA G5/4 -1 Recommendation

Electricity Association Recommendation G5/4-1 - Planning Levels for Harmonic Voltage Distortion and Connection of Non Linear Equipment to Transmission Systems and Distribution Networks in the UK.
Copies can be obtained from the Publications Department:
Energy Networks Association
18 Stanhope Place
Marble Arch
London
W2 2HH
+44 (0) 20 7706 5100
www.energynetworks.org.uk

Services

Electrical Motor Services Limited are independent specialists with over thirty years experience and provide a competitive and professional service to assist companies in the on-going challenge to reduce energy consumption and combat the negative effect of poor power quality.

Advice Service

Quite often it only takes a phone call to quickly identify any potential financial savings to be gained. For free impartial advice please call us on any of the numbers listed.


Power Factor Survey

Using snapshot data to produce a report on the site power factor level. This survey will potentially lead to the identification of poor power factor levels and high supply voltage levels.


Power Factor/Load Profile Surveys

Week long, detailed and in-depth report on the site load profile. Used for identifying kW usage, power factor levels, maximum kW and kVA demands, harmonic distortion levels and any irregularities with the supply (i.e. high/low voltages). In 90% of cases this survey will lead to the following.

Specialist Power Quality Surveys

As with Power factor/Load profile surveys but harmonic data is used to confirm  compliance or non-compliance with ENA G5/4 -1, BS EN50160, P28 flicker etc.


Voltage Reduction/Optimisation

In the UK the average L-N voltage is 242 Volts which although within BS EN 50160 regulatory levels is in fact quiet high when considering that the majority of equipment is designed to operate at 220-230V +/- 10%. This higher voltage is actually having two negative effects;

* Extracts from IEE 16th edition guide BS7671.
By reducing the supply voltage it is possible to achieve typical kWh savings in the region of 10-15% (dependent on the types of load applied). There are two ways to achieve this voltage reduction either through the installation of voltage optimisation equipment or simply reducing the output voltage from your supply transformer. However prior to utilising either of these options it is strongly recommended that an electrical survey be conducted.

Power Factor Correction Equipment Inspection, Service & Repair

LESL can service, inspect and repair PFC equipment from all manufacturers.
Our Power Factor Correction Equipment inspection and report service provides;   

Equipment Supply

Supply and commission all types of power factor correction equipment, voltage optimisers and harmonic mitigation equipment.

Managing the monthly electric bill has always been an ongoing challenge. However, with the decline in fuel resources, concerns over Global Warming and spiralling costs, it is not only a financial necessity but also our corporate and personal responsibility to reduce energy wastage and kWh consumption. If these reasons were not enough, the present global economic situation has further increased the need for action now.

How do I reduce our energy costs?
This is often a difficult question to answer as there are many routes to a company operating more efficiently. It could be as simple as changing operational procedures, increasing staff awareness, limiting peak power demands or if commercially viable, installing energy-saving equipment.
However, it has been proven that by simply reducing/optimising the input voltage to a site or the installation of Power Factor Correction equipment, often in the past over-looked, can produce a significant reduction to the electric bill for many plants.
LESL are independent specialists with over thirty years experience in the field of Power Quality and Power Factor Correction who can provide a competitive and professional service to assist companies in this on-going challenge.

POWER FACTOR

Power Factor is a measure of how efficiently a site is utilising the power supplied.
Power Factor
Power Factor is a measure of how efficiently a site is utilising the power supplied and expressed as a decimal figure. i.e. 0.95 which equates to 95% efficient.
Power factor = efficiency (Output/Input)

POWER QUALITY

Electrical load like motors, transformers, florescent lighting etc require REACTIVE power to set up the magnetic field while the ACTIVE power produces the useful work (shaft horsepower).  Total Power is the vector sum of the two and represents what you pay for, ie kVA service capacity charge.
By applying Power Factor Correction Capacitors as shown below, it can be seen that the reactive energy (kVAr) required by inductive loads will be supplied by the capacitors. By applying these capacitors the overall kVA demand will be reduced and obviously the angle ? will reduce and the power factor improved.


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Why Improve your Power Factor?

By improving the power factor the site KVA demand will reduce (total power demand) and consequently the kVA service capacity can be reduced. (L saving).
Utility companies can, and invariably do, impose a reactive sine-wave charge for users with a power factor of less than 0.95 lagging. If correctly specified the new power factor will be above 0.95 lagging and therefore no reactive charges will be incurred. (L saving)
Reduces I2R losses in conductors and consequently will reduce kWh consumption (L saving) As current flows through conductors, the conductors heat.  This heating is power loss which is proportional to current squared (PLoss=I2 R) and the current is proportional to P.F.
Reduces loading on transformers freeing up availability by reducing KVA (total power demand). This can often lead to financial saving as the requirement to increase the site supply to meet future loads is reduced.
Reduction in CO2 emissions produced in the generation of reactive current.

EsType & Correct Application Of Power Factor Correction Equipment

Like all other loads, Power Factor Capacitors are also susceptible to the negative effects of harmonic currents and voltages. Capacitors are a low impedance path to harmonic currents and will absorb a high percentage of the harmonic currents flowing in the distribution network. This leads to overheating of the capacitor elements and/or dielectric failure, causing greatly reduced product life.  In extreme cases, the capacitors can interact with the distribution transformer to create a resonant circuit which can actually cause an amplification of the harmonic currents on the network.
In order to ensure longevity of the product and years of trouble-free operation, the following guidelines are presented.  These guidelines are based upon the percentage of harmonic generating load on the same transformer as the capacitors. To calculate the percentage of harmonic generating load on the transformer, add up the kVA of the harmonic generating loads and divide these by the total connected load.  For example, if you found you had 250 kW of motors controlled by variable speed drives, plus 300kW of high frequency high-bay lighting, by rough estimate then you have 550 kW of non-linear load. If then, your total load was 2000 kVA on a transformer which was sized at 2500 kVA, you would have 550/2000 = 27% non-linear load.  Using this value, you would select Detuned PFC from the description below: 

Less than 15% non-linear load:
Standard automatic power factor capacitors are suitable.  Because harmonic currents can be imported into your facility from the utility network, always specify +460V dielectric capacitors. These capacitors can absorb some harmonic current without damage. Note: If the amount of kVAr exceeds 0.33 x kW, detuned capacitors are recommended to avoid resonance with any harmonics which may be present on the network.

15% to 50% non-linear load:
Detuned automatic power factor capacitors are suitable.  Detuned capacitors employ reactors in series with each capacitor stage to limit the amount of harmonic current that is absorbed and to prevent resonance between the supply transformer and the newly installed capacitor bank.  It is important however to remove any existing capacitors prior to installation of a detuned bank as their presence on the network can change the tuning characteristics of the detuned bank.

Over 50% non-linear load:
Filtered automatic power factor capacitors can be used but only after complete analysis of the network to ensure correct application. Filtered capacitors employ filter reactors in series with each capacitor step to produce a tuned resonant circuit which will offer a low impedance path to absorb a particular problematic harmonic (such as the 5th harmonic produced by variable speed drives).  A filtered automatic bank will provide the necessary power factor correction and reduce the voltage distortion on the network.  It is important however, to remove any existing capacitors prior to installation of a filtered capacitor bank as their presence on the network can change the tuning characteristics of the filtered bank.