Keep on running (full length version)

Electrical power is perhaps the most essential raw material used by commerce and industry today. However it is an unusual commodity because it cannot be stored conveniently in bulk and it cannot be subject to quality assurance checks before it is used.

Many businesses are increasingly reliant on high quality and secure power supplies for data processing and industrial applications. This article explores the issues surrounding power quality.

What do we mean by Power Quality

The perfect power supply would be one that is always available, within voltage and frequency tolerances and has a pure ‘noise free' sinusoidal waveform. Just how much deviation from perfection can be tolerated depends on a number of factors including: -

• The user's application
• The type of equipment used
• The user's view of his requirements

Power quality defects include the following: -

• Blackouts
• Under or over voltage
• Dips (or sags) and surges
• Transients
• Distortion of the supply as a result of harmonic effects
• Frequency Variation

Examples of operations that are sensitive to power quality include the following:

Continuous process operations - where short interruptions can disrupt the synchronisation of machinery. An example is the paper industry where clean up following a power failure is long and expensive.

Multi-stage batch operations - where an interruption during one process can destroy the value of previous operations. An example is the semi-conductor industry where production of a wafer requires a few dozen processes over several days and the failure of a single process is catastrophic.

Data processing - The value of the transactions will typically be high but the cost of the actual processing is low. An example is share and foreign exchange dealing where the inability to trade can result in large losses that far exceed the cost of the operation.

Hospitals - where power failures could be life threatening to patients undergoing surgery.

These are examples of sensitive industries but it is surprising how many seemingly mundane operations have quite critical power supply requirements. Examples include retail units with computerised point of sale equipment and manufacturing plants with distributed control.

Air Traffic Control Centres - where the facilities that serve aircraft need to operate at the same level of reliability as the aircraft they serve. This also applies to other safety critical installations such as nuclear facilities.

How reliable is the Power Supply in the UK?

The UK power supply industry regularly claims a supply availability of 99.98%. This sounds very impressive but to the average customer it represents unpredictable disconnections totalling 97 minutes per year with 90% of all customers experiencing interruptions of significant duration.

These figures are also misleading because interruptions of less than 1 minute are often not reported. For many commercial operations an interruption of 1 second is just as disruptive as an interruption of say 10 minutes.

What is the Cost of Poor Power Quality?

According to one source (4) £200 million was paid out by insurers for losses associated with power cuts in 1995, the latest information available, and it is likely that the figure is much higher now. It is estimated that the cost to industry in the United States is around $4 billion per annum (4).

Costs to overcome power quality problems include the cost of generators, Uninterruptible Power Supplies (UPS) equipment, active filtration etc and there is a statutory requirement to limit harmonic currents that are connected to the public network under the G5/4 regulations. Costs of not recognising power quality problems can include accelerated ageing of equipment and increased engineering effort to diagnose spurious problems and possible disconnection from the utility supply if harmonics in excess of those allowed by G5/4 are connected to the mains supply.

Power Quality Solutions

The following are examples of solutions that are employed to overcome power quality problems: -

Duplicated incoming supplies - These are ideally derived from separate sub stations and diversely routed in to the site such that if there is a problem with the sub station feeding the client's site there is an alternative supply available, one which is unaffected by a local outage elsewhere on the supply network. Geographical constraints often mean the cost of providing such an arrangement can be prohibitively expensive. It should be noted that changeover from one supply to another is normally undertaken as a manual operation by trained staff which can take several hours so whilst this will provide power in the event of a prolonged outage at the substation that was supplying the site (assuming the standby supply is not affected by the outage) there will be an interruption in the power supply for the time it takes to have a trained HV electrician attend site.

Duplicated Power Distribution - Power distribution is often duplicated and diversely routed within the facility such that the facility can continue to operate if there is a failure of an LV panel and associated cabling. It is difficult to avoid single points of failure at bus couplers within panelling unless the distribution is completely duplicated throughout the installation.

Standby Generators - Standby generators are normally configured to start upon detecting a mains failure and after a period to allow for the standby generator to run up to speed so that it is ready to accept the load (typically around 30 seconds although modern technology can reduce this to less than 10 seconds) the standby generator will cut in and supply the load. It should be noted that unless the standby generator is rated to supply the full site load in a single load step, shedding of non-essential loads would be required to ensure the standby generator does not become overloaded. As with the duplicated incoming supplies there is a break in supply until the standby generator is able to accept the load so this solution will protect against outages provided sufficient fuel supplies are available and the generators are adequately maintained. There will be a break in supply until the standby generator is ready to accept the load albeit this should be shorter that that required to changeover to a standby incoming supply, as the standby generator should start automatically. Unless the standby generator is approved by the supply authorities to parallel with the mains supply there will also be a power interruption upon restoration of the mains supply when load is transferred from the standby generator back to the mains. Generators can be prime or standby rated depending on the amount of use they would be expected to receive. Continual rating which is defined in the latest edition of ISO 8528 can be misleading as this type of can operate continuously but not at 100% of its rated load. The alternator should be oversized and/or checked for acceptable performance if it is expected to be used in the presence of significant power harmonics.

Uninterruptible Power Supplies (UPS) - This is often seen as a solution to power quality problems and this equipment is often seen as an essential feature where business critical equipment is in use. UPS installations are designed to continue supporting the load during a failure of the mains supply without interruption. They will also provide a degree of power conditioning depending on the type of UPS used and how they are configured. However it should be noted that UPS equipment can introduce undesirable effects on to the power supply so that by protecting against power failures power quality can be compromised.

Surge Suppression - These devices can be incorporated at major wiring centres etc to provide protection against surges and spikes in the supply as a result of events such as lightning strikes or switching of major loads on the utility supply.

System Configuration - Major loads should be powered directly from the main distribution to avoid switching transients and voltage drops affecting other parts of the system.

Comparison of UPS technology

Static UPS - For small loads the UPS market is dominated by battery backed "static" UPS sets that keep batteries charged during normal operation. During a mains failure the stored energy in the batteries is used to supply the load without interruption. There is a finite time that a static UPS can support the load depending on the battery autonomy. Where UPS sets are used in conjunction with standby generators this will typically be between 10-20 minutes at full load to allow sufficient time for the standby generator to start and accept the load, making due allowance for a number of failed standby generator starts. Static UPS sets convert AC power to DC via rectifiers and back in to AC power via a double conversion rectifier/inverter path and the output waveform is an approximation to a sinusoidal waveform. The input waveform using 6-pulse equipment is fairly poor and operating efficiency is low but this improves when using 12 or higher pulse equipment, which should be specified for larger loads. True 12 pulse rectifiers should be used in preference to Pseudo 12 pulse equipment. Static UPS systems can be configured in a series or parallel arrangement and are normally arranged in an on line manner.

Diesel Rotary UPS (DRUPS) or "No Break" Generators - Rotary UPS sets utilise a permanently rotating machine to provide uninterruptible power to the load. Rather than utilising batteries to provide the energy store during a loss of supply a flywheel or induction coupling is used to keep the machine rotating until the diesel engine can be run up to speed and connected such that it can support the load. Rotary generators therefore combine the functions of the standby generator and the UPS and consequently space savings can be achieved although it is often argued that by combining these functions reliability is compromised. They have superior load handling characteristics but however they are generally only cost effective for larger loads. For fully duplicated systems problems can arise because DRUPS can introduce phase shifts between the 2 systems which may prevent static transfer switches operating.

Hybrid Rotary UPS - This type of UPS combines the load handling advantages of rotary equipment by producing a high quality sinusoidal waveform but uses batteries to provide energy during a power outage. It should be noted that both static and Hybrid Rotary UPS will be subject to significant battery recharging loads of up to double the rated output of the UPS although more sophisticated UPS equipment can have recharge limiting. These loads should be taken into account in the sizing of the infrastructure supporting the UPS.

A comparison of various UPS systems is given in Table 3.

Reliability/Resilience Considerations

In order to decide on an appropriate power protection strategy it is necessary to undertake a quantative reliability study taking account of system and component Mean Time Between Failure (MTBF) and Mean Time To Repair (MTTR).

It is also necessary to consider resilience of the system to failure or unplanned outages of individual components such that fault tolerance is built in to the system. N+1 is a common equipment arrangement whereby if N items of plant are required to meet the load N+1 units are provided to ensure there is a spare item of plant available to cover for failure of any of the N items of plant. N+N is a higher standard giving a truly redundant configuration without any common points of distribution.

The system should be analysed for single points of failure that could negate other measures built in to the system to improve reliability.

The electrical system should be analysed both with and without mains being available to ensure compliance with wiring regulations under both scenarios, as this is normally harder to achieve when operating with a standby generator.

The uptime institute publish a system of "Tier" definitions in order to categorise systems in terms of system configuration, expected availability and presence of single points of failure. Tier ratings run from 1-4 with Tier 4 currently being the highest standard which is based on double redundancy with a full System + System arrangement, no single points of failure and an expected availability of 99.995%. Statistically this means that a Tier IV facility is likely to be unavailable for around 26 seconds per annum reflecting the probability of both systems failing simultaneously or human error which is recognised as a significant risk. Uptime Institute Tier Definitions are indicated in Tables 1 & 2.

Safety critical industries such as aviation and nuclear often require higher levels of reliability the ultimate being accepted as five nines or 99.99999% reliability. This is normally achieved by employing triple redundancy and addressing human factors that can contribute to failure. Vendors offering 100% reliability should be treated with the utmost scepticism.

Systems should be subject to integrated systems test at handover to demonstrate that the systems operate as designed under all feasible failure modes, and regular testing should be undertaken to ensure reliability is maintained.

Harmonic Distortion

Harmonic frequencies are integral multiples of the fundamental. For example for a fundamental frequency of 50 Hz the 2nd harmonic would be 100 Hz and the 3rd harmonic would be 150 Hz. Most cyclical waveform can be deconstructed in to a sinusoid at the fundamental frequency plus a number of sinusoids at harmonic frequencies.

Fig 1 shows a typical waveform that has been distorted by harmonic effects together with the various terms that are used to describe the distortion.

All ‘non-linear' loads generate harmonic currents. These include: -

• Switched Mode Power Supplies (SMPS) found in most computers and electronic equipment
• Fluorescent and high frequency lighting ballasts
• Variable speed drives
• Uninterruptible Power Supplies (UPS)
• Magnetic cored devices

Problems associated with harmonics include the following: -

• Voltage distortion to the equipment served and equipment in the vicinity
• Zero crossing noise which can create problems with electronic controllers which rely on detecting the point at which the voltage crosses zero.
• Measurement problems. It can be difficult to measure current accurately unless specialist equipment is used.
• Overloading of neutral conductors
• Overheating of transformers and induction motors
• Nuisance tripping of circuit breakers particularly RCCB devices.
• Over stressing of power factor correction capacitors
• Skin effect in conductors where the majority of the current is transmitted in the skin of the cable leading to local overheating of conductors and degradation of the insulation.
• Difficulties in achieving compliance with engineering standard G5/4.

In a series of studies undertaken by BSRIA 8 out of 12 sites investigated revealed significant problems associated with harmonic distortion. These included high neutral currents and elevated earth voltages leading to currents of up to 48 amps.

High Neutral Currents

Whilst harmonic currents cancel in the phase conductors at many frequencies, the so-called ‘triplens' frequencies (the multiples of the 3rd harmonic i.e. the 3rd, 9th, 15th and the less common 6th and 12th) add in the neutral conductor.

Solutions to Power Harmonic Problems

It is possible to design and build electronic equipment that does not introduce harmonics and legislation has been brought in recently to limit the harmonics created by switch mode power supplies. However this has resulted in equipment being fitted with capacitors that result in unity or leading power factor. This can create other problems as many UPS systems need to be derated when operating in the presence of loads in excess of 0.8 lagging.

Specify equipment that reduces harmonic distortion - by specifying higher order (12 or 24 pulse) rectifiers as opposed to 6-pulse equipment harmonic distortion can be significantly reduced and system efficiency improved.

Uninterruptible Power Supplies (UPS) - UPS equipment can provide a degree of isolation between the power source and load and can isolate the user from transients and interruptions in the main supply. However problems arising from equipment supplied by the UPS will still be present and since the supply impedance of a UPS is usually much higher than the mains supply voltage distortion in systems supplied by UPS equipment can be much worse.

Cancellation effects - It is possible that when a number of loads are connected to the same supply differences in phase angle will produce cancellation effects that can reduce the total Harmonic distortion within the system. Careful design of the system can lead to a reduction of harmonic problems without the need for artificial measures being implemented.

Filters - Several different varieties of harmonic filters are available commercially. These generally fall in to the categories of passive filters, switched filters, active filters, line reactors and electronic filters. However consideration should be given to problems that can arise when filters are taken out of service for maintenance etc. The use of filters on the input to UPS systems can overcome some of the problems caused by lower pulse (particularly 6 pulse) rectifiers.

Transformers - Speciality transformers are available that can reduce the impact of harmonic currents. These can take the form of isolation transformers, K rated transformers or zigzag transformers.

Derating - This can often be the simplest and most cost effective method of protecting against harmonic problems particularly in the neutral conductor which can be subjected to currents approaching double those found in the phase conductor.

Power Factor Correction - Harmonic control can sometimes be combined with power factor correction equipment in the form of tuned capacitor banks.

Earthing and Earth Leakage

The primary purpose of the earthing system for a building is to ensure that a safe environment is maintained for occupants and to protect equipment from damage in the event of a fault. This is achieved by: -

• Providing a low resistance path for earth fault currents so that protection equipment can operate quickly
• Establishing an equipotential platform on which equipment can be safely operated
• Bonding exposed metalwork to earth.

There is an increasing trend toward using the earth of the building as a reference for electronic equipment which leads to leakage currents (permitted by regulation to be up to 3.5 milliamps) at the fundamental frequency. Because of the high population of electronic equipment in modern office buildings and data centres earth leakage currents can be significant. Because the earth system is required to carry currents any break in the conductor will mean that the isolated section will rise to a potentially dangerous voltage. For this reason the wiring regulations require that in these situations a high integrity earth be provided.

For areas having high concentrations of electronic equipment such as computer rooms and data centres there is often a requirement to provide a ‘clean' or functional earth in addition to the protective earth. This has commonly been provided in the form of a star connected clean earth running directly back to the main earth terminal. It should be noted that functional earths are often subjected to significant currents/voltages and noise. The so-called ‘clean' earth is often far from clean compared to its ‘dirty' counterpart.

The star connected earth can be subjected to resonance effects when the conductor lengths are long and the length is close to certain fractions of the wavelength of the noise frequency depending on the relative values of the conductor reactance and capacitance. Also it is extremely difficult in practice to separate, and maintain separation of, clean and dirty earth paths and this can lead to currents flowing in loops formed by ground wires of data cables.

This problem can be overcome by the use of a mesh earth arrangement whereby the raised access floor acts as a continuous ground reference plane with multiple earth paths back to the main earthing terminal.

Identification of Problems

Predictive studies are not always undertaken for new facilities. In reality they may only be necessary for facilities housing large amounts of IT equipment, high frequency lighting, inverter drives etc that can generate harmonic currents that need to demonstrate compliance with engineering recommendation G5/4 (15).

The photograph shows a current measurement being taken by 2 separate meters on the same cable. One device shows a current of 59.2 amps being drawn and the other 40.5 Amps because the meter with the lower reading is insensitive to higher frequencies. This illustrates the importance of using the correct equipment when undertaking harmonic studies.

Harmonic analysers are available that can be used to investigate harmonic distortion throughout the distribution network. Prior to undertaking this type of work reference should be made to regulation 14 of the Electricity at Work Regulations, which is a key safety standard when working on or near live apparatus.

Conclusions

Power quality is a major issue affecting a variety of industrial and commercial buildings. Appropriate solutions need to be developed and tailored to meet the needs of the clients operations.

Further Reading

1. BSRIA Application Guide AG2/200 - The BSRIA Power Quality Guide

2. CIBSE Guide K - Electricity in Buildings

3. Co-location and Hosting Good Practice Guide

4. CDA Publication 123, Electrical Design - A Good Practice Guide

5. CDA Power Quality Application Guides 1.1, 1.2, 2.1, 3.1, 3.2.2, 3.3.3, 4.1, 5.1, 5.1.3, 5.2.1, 5.3.2, 6.1.

6. IEE Power Series 14 - Uninterruptible Power Supplies

7. IEE Power Series 24 - Power System Commissioning and Maintenance Practice

8. IEE Power Series 32 - High Voltage Engineering and Testing

9. IEE Commentary on IEE Wiring Regulations 16th Edition BS 7671:2001

10. IEE Wiring Regulations 16th Edition BS 7671:2001

11. Electromagnetic Compatibility Club (Discussion forum for the implementation of EU Directive on Electromagnetic Compatibility) Articles - EMC for Systems and Installation Part 0-6 inclusive.

12. Article from CIBSE Journal November 1999 - Devastating Power Quality.

13. CIBSE Applications Manual AM7 - Information Technology and Buildings

14. BS EN 50160:2000 - Voltage Characteristics of Electricity Supplied by Public Distribution Systems.

15. Electricity Association Engineering Recommendation G5/4 February 2001 - Planning Levels for Harmonic Voltage distortion and the connection of non-linear equipment to transmission systems and distribution networks in the United Kingdom.

16. ETSI Telecommunication Standard ETS 300 253 - Earthing and bonding of telecommunication equipment in telecommunication centres.

17. EA Technology Power Quality Conference Seminar Notes May 2002.

18. Electricity Association Technical Specification 41-24 - Guidelines for the design, installation, testing and maintenance of main earthing systems in substations.

19. Article from CIBSE Journal March 2001 - Back up Power Supplies

20. Article from CIBSE Journal April 2005 - Standby Generation