Power quality issues – Harmonics

Harmonics issues within an electrical installation frequently go overlooked due to a lack of understanding or awareness of them. This then often leads site and facility managers experiencing problems within their installations to focus on the symptoms rather than the underlying cause of those problems. In this Blog on power quality issues we explore the causes, symptoms and some solutions to the problems of harmonics.

Within the last 30 years there has been a big increase in the number of non-linear loads connected to the electrical network, including computers and associated IT equipment, uninterruptable power supplies, variable speed motor drives, electronic lighting ballasts, and LED lighting, to name just a few. The growing use of such equipment and the application of electronics in nearly all electrical loads are beginning to have some worrying effects on the electricity supply. It is estimated that today over 95% of the harmonic interference within an installation is generated by equipment within that installation.

When a linear electrical load is connected to the supply it draws a sinusoidal current at the same frequency as the voltage, however, non-linear loads draw currents that are not necessarily sinusoidal.

The current waveform can become quite complex, depending on the type of load and its interaction with other components in the installation. These non-linear loads increase current, and in severe cases voltage, distortion in the electrical supply, which can lead to significant energy losses, shortened equipment lifespans, and reduced efficiency of devices.

Waveform distortion can be mathematically analysed to show that it is generally equivalent to superimposing additional frequency components onto the original 50Hz sinewave. These frequencies are harmonics of the fundamental frequency, and can sometimes propagate outwards from the non-linear loads causing problems elsewhere on the electrical installation.

Regardless of how complex the current waveform becomes it is possible to deconstruct it into a series of simple sine waves using Fourier analysis.

One of the measures often used to indicate the amount of harmonic distortion present in an electrical installation is total harmonic current distortion or THDi. This is a ratio of the sum of all the harmonic currents to the current at the fundamental frequency described by the equation: –

THDi

Harmonic currents have negative effects on almost all items on the electrical system by upsetting sensitive electronic devices and causing dielectric thermal and mechanical stresses.

The most significant of these include computer and other IT equipment crashes and lockouts, flickering lights, electronic card failures in process control equipment, power factor correction equipment failure, high load switching failure, neutral conductor overheating, unexpected circuit breaker operation and inaccurate metering.

Some of these, such as flickering lights and IT equipment crashes are, at the least, an irritant to businesses. Electronic card failures on production lines or process control equipment can cost businesses in unplanned down-time. Worst of all though, failure of power factor correction and electrical distribution equipment, cables, transformers, motors and standby generators can be catastrophic. At the least the presence of harmonics will cause reduced electrical efficiency within the installation and excessive power consumption which you will be paying for.

The internal resistance of a capacitor reduces as frequency rises, and at high frequencies can appear almost as a short circuit. Power factor correction capacitors are generally designed to operate at the fundamental frequency, and the lower impedance seen by the higher frequency harmonic currents result in an increased amount of capacitor overheating. It is also possible to experience permanent damage to capacitors due to parallel resonance occurring between them and transformers.

Resistive heating is proportional to the square of the harmonic order, and so it follows that the greater the number of higher order harmonics that exist the greater the heating effect.

At the least this will lead to large increases in iron losses, and therefore power consumption, in rotating machines and transformers, as well as increased eddy current losses in transformers. In the worst cases fires in wiring and distribution systems or even catastrophic transformer failure.

Apart from losses due to heating effects, motors in particular can be significantly negatively impacted by harmonics due to torsional oscillation of the motor shaft. Torque in AC motors is produced by the interaction between the air gap magnetic field and induced currents in the rotor. When a motor is supplied non-sinusoidal voltages and currents, the air gap magnetic fields and the rotor currents will obviously contain harmonic frequency components. The harmonics are grouped into positive, negative and zero sequence components. Positive sequence harmonics (1, 4, 7, 10, 13, etc.) produce magnetic fields and currents rotating in the same direction as the fundamental frequency harmonic. Negative sequence harmonics (2, 5, 8, 11, 14, etc.) develop magnetic fields and currents that rotate in a direction opposite to the positive frequency set, and zero sequence harmonics (3, 9, 15, 21, etc.) do not develop usable torque, but produce additional losses in the machine. The interaction between the positive and negative sequence magnetic fields and currents produce torsional oscillations of the motor shaft. These oscillations result in shaft vibrations, and if the frequency of oscillations coincides with the natural mechanical frequency of the shaft, they become amplified and severe damage to the motor shaft may occur. It is sometimes possible to literally hear a transformer or motor “sing or growl” due to these vibrations and this is often one of the first observed indications of a harmonic problem.Transformer failure

Some of the most troublesome harmonics are the 3rd, and odd multiples of the 3rd, i.e. the 9th, 15th etc. These harmonics are called “triplens”. The triplen harmonics on each phase are all in phase with each other which will cause them to add rather than cancel in the neutral conductor of a three phase four wire system. This can overload the neutral if it is not sized to handle this type of load.

Fortunately, the identification and measurement of harmonics is easily achieved using a power quality analyser or power and energy logger (PEL) with harmonic capabilities, and while they cannot be eliminated, since they are generated by the various loads in the installation, they can be confined to an area as close to the polluting load as possible in order to prevent them from reaching the overall network.

The main methods used involve installing passive or active filtering or isolating systems designed to limit the deterioration of energy quality and other harmful effects as well as the use of tuned power factor correction equipment. Once the harmonics are “under control”, the associated problems, power losses, equipment failures and outages, and energy costs will be reduced.

Harmonics can be a major issue in the modern electrical installation, becoming increasingly more important as more switching and smart loads are introduced. Harmonics must be monitored regularly in order to verify their levels and prevent potential failures or high losses.

This article and images was published courtesy of Chauvin Arnoux, and for further advice on instrumentation that can be used for monitoring power quality issues the BSRIA Instrument Solutions team on 01344 459314 or e-mail instruments@bsria.co.uk  Alternatively simply visit http://www.bsria.com/uk/instrument/ to view the range of products available for both Hire and Purchase.

Instrumentation for critical healthcare environments

Today’s hospitals contain many critical environments where building services play an important role in the wellbeing of patients, staff and visitors. Even the best-designed and built facility will need initial commissioning and constant monitoring to ensure peak performance throughout its lifecycle.  Accurate, fit-for-purpose fixed and portable measurement instruments are required in most departments of a hospital, from the boiler room to the pharmacy to ensure that all areas are functioning correctly.

In the wards and operating theatres it is imperative there can be no spread of infections or exposure to potentially hazardous materials.  Providing this effectively in terms of equipment ease of use and efficiently both in terms of the cost of instruments and the cost of staff presents challenges to the building services engineer, laboratory or medical personnel.  In an isolation facility, for example, staff need to monitor the pressure between rooms (positive or negative) to stop the spread of infections either to or from the patient, the temperature within the protected space, supply or extract ventilation rates, the quality of the air in terms of particulate concentration, as well as the flow-rates of medical gasses. Where, all of theses parameters can be measured with fixed (built-in) devices or portable (hand held) instruments.  Measurements of surface contamination may also need to be ascertained for infection control, but these are normally undertaken using standard laboratory techniques.

A number of medical facilities have incinerators on site to dispose of locally generated clinical waste; many different types of fixed measurement instrumentation are used on every aspect of the incineration process from the temperature thermocouples within the primary incinerator, through to the gas and particulate emissions measurement at the end of the process.  To compliment the fixed instrumentation, a selection of portable instruments are also often maintained to crosscheck and temporarily replace the fixed range of measurement equipment should a problem occur.

The boilers that supply steam to the hospital require various types of instrumentation to run correctly, hydrometers for example are used to measure the total dissolved solids (TDS) within the boiler water.  If the TDS level rises too high then this can cause foaming and carryover to the steam main leading to contamination of control valves, heat exchangers and steam traps.

There are also water supplies that have to be considered, and the need to combat the possibility of Legionnaires’ disease by chlorine dosing the systems to ensure all of the water pipes are disinfected.  The water quality then has to be sampled periodically with the appropriate instrumentation to ensure the water is fit for use.

BSRIA has been working on solving building services design, installation, commissioning and operating problems in hospitals for many years and is only too well aware of the importance of correct measurement. Most of the published work however concerns the facilities themselves rather than the instrumentation used for measuring performance.  A well designed facility that has been built and commissioned correctly should be a safe environment to work or visit from day one of its operation.  But, if the wrong instruments are used during commissioning or routine monitoring it could have life and death consequences, as there is the risk of spreading potentially infectious or hazardous agents.  In the field of pressure measurement there is a large array of instruments that measure this physical parameter, but if an instrument is used with an accuracy based on its full scale deflection, not on the indicated value, at low pressures it is impossible to establish if a system is operating correctly.  In a surgical suite it may be required to operate at a pressure differential between rooms of 10 Pa.  If an instrument with a range of 2000 Pa is used with a manufacturer’s claimed accuracy of ± 0.5% fsd (full scale deflection), there can be an error of some ±10 Pa. This errors being almost double the required measurement.  Similarly, when measuring air flow rates in a biological flow cabinet, instruments with a typical accuracy of ± 1.5% mv, +0.2 m/s (measured value) can be used.  But if the target measurement value is only 0.5 m/s, this accuracy equates to a possible reading as low as 0.29 m/s being accepted which could be very problematic in a critical environment and potentially expose an operator to hazardous materials.

Understanding manufacturers’ claimed instrument accuracies is only part of the problem in the correct selection of pieces of instrumentation; the correct calibration of the equipment is equally vital to ensuring reliable data.  For measuring pressures as low as 10 Pa in the surgical suite, the hospital engineer or laboratory staff needs calibrations with an uncertainty of no more than 0.1Pa, which often exceeds what manufacturers are offering.  The building services engineer must look beyond the simple requirement of measuring pressure, and understand the details of the complete process.  Understanding the real technical merit of an instrument therefore must have a greater significance in the future as services in healthcare facilities become more critical.

When buying, or hiring, instruments the engineer now has a global choice as to which product will meet today’s challenging testing environments.  Calibration of this instrumentation is, and always will be, of paramount importance to users, but keeping up-to-date of what is available especially in changes of technology and the scope of instrumentation available must also be considered during the selection process.  Tests that often took hours to conduct can now be undertaken easier, faster, and more accurately than those taken years ago.  For example there are pieces of instrumentation that can fit test N95 respirators and masks to protect workers against airborne biohazards such as TB or even SARS.  Likewise there are new types of ultrafine particle counters that can be used to trace air pollutants in operating theatres, as well as being used for the checking of the integrity of filter seals within laboratory fume cabinets.

With such a wide range of instruments available to today’s healthcare professionals they need to look beyond any procurement source that is tied to an individual manufacturer to obtain the best pieces of instrumentation within the marketplace.   Equally staff at the suppliers have to understand the finer points of the instruments they offering including calibrations at the ranges to be used.  Equipment can, where applicable, include data damping, backlit displays, self calibration check tools, data logging, keypad lock out to unauthorised users and long life battery operation to name just a few options that can also influence a final purchasing decision. BSRIA Instrument Solutions is a leading supplier of specialist test and measurement instruments to Industry since 1990. It has built its reputation by providing the most reliable and advanced test equipment from leading manufacturers supporting it with a high level of customer service and technical support to meet with its client’s requirements and expectations.  They are able to offer a choice of test equipment solutions with products from many leading instrument manufacturers.

 

Face fit testing a nurse to ensure the correct fitting face mask is used.

 

 

 

 

 

 

 

 

Simple to use fume hood controller.

 

 

 

 

 

 

 

 

 

 

Hospital isolation suite room pressure monitor

 

 

 

 

 

 

 

 

Particle counting in a clean room facility

 

 

 

 

 

 

 

 

Calibration of an anemometer in the BSRIA laboratory.

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