D H Drives Ltd

Power quality in building services applications

Harmonic distortion is a form of electrical pollution, which can affect the quality of the electrical power distributed on the public network. Building services applications face particular challenges in preventing their effects - we look at the options.

Excessive harmonic distortion of electrical supplies is a phenomenon that must be carefully guarded against. A harmonic frequency is simply a frequency that is a multiple of the fundamental frequency; for instance, a 250 Hz waveform superimposed on a 50 Hz network is the 5th harmonic. Harmonic currents are produced by non-linear loads such as fluorescent lights, computer power supplies and variable speed drives, and are seen as a voltage distortion when imposed on the supply.

The symptoms of harmonic voltage distortion can be severe and serious problems can result. Transformers may overheat, cables may get too hot and the insulation can break down. Motors may also overheat or become noisy. Capacitors can overheat or form tuned circuits that resonate. Electronic displays and lighting may flicker, circuit breakers can trip, fuses blow spuriously, computers fail and metering give false readings.

Variable speed drives are one source of harmonics and the many drives used in HVAC applications pose a challenge for the building services engineer seeking to provide a clean, reliable power supply.

Most office buildings contain many items sensitive to excessive harmonic distortion, such as computers, and these must be protected from the harmful effects.

Options for cutting harmonics

The options for cutting harmonics depend largely on whether the project is a new build or a refurbishment.

When installing variable speed drives to run a HVAC system in a new building, a solution can be designed that will produce zero harmonics. With a refurbishment, where a new drive system is needed, space may be a problem and this will affect the type of solution that can be applied. Also, with a refurbishment, the existing cabling may not be able to withstand the heating effect caused by the harmonic currents. For this situation, a solution needs to be found which will retain the existing transformer and keep the cable ratings unchanged.

Another consideration for HVAC applications is the size of the drives to be used. While smaller units take up less space, they produce more harmonic currents, which in some cases can reach 50 percent of their normal rated output

Larger drives have lower harmonic levels, but obviously the bigger the drive, the more usable floor space is taken up.

A number of solutions exist for keeping the harmonic currents from causing harm. One of the simplest is to segregate the drives from sensitive equipment, so that harmonics are not generated in their immediate vicinity.

A similar solution is to use drives that are phase shifted in relation to each other, so that any harmonics generated cancel each other out. The disadvantage with this method is that it requires a special transformer. These are physically large components, with associated losses, and the floor area may not be available to accommodate them, particularly in a refurbishment.

A better option for HVAC applications is to use a power quality filter on a standard drive. This is a compact item, which will occupy little valuable space.

Drives based on an Inverter Supply Unit or"active rectifier", can also be used. These are regenerative drives, which can also use the braking torque of the motor to produce energy. These produce very low harmonic current levels.

New regulations

New regulations on harmonics, EA Engineering Recommendation G5/4, affect everybody connected to the public electricity supply. G5/4 limits the overall voltage distortion at each level in a network; the limits are applied at the Point of Common Coupling, or PCC, where the internal supply system meets the public network.

All consumers have an agreement to connect with their Network Operating Company, and part of any new agreement includes a requirement to meet G5/4. Failure to meet these conditions could, as a last resort, result in disconnection from the supply.

G5/4 defines three stages of assessment, which increase in complexity. Stage 1This concerns only 230V single phase and 400V three phase supplies.

Any equipment, or combination of equipment, under 16A aggregate input current, that meets the Harmonised European Standard EN 61000-3-2, may be connected without further assessment.

G5/4 also allows an aggregate total of 12 kVA of 6 pulse converters. Unfortunately, few installations have only single loads or total loads under 16 A.

When this is the case, the total harmonic currents from each item must be added together, ensuring that the total is less than the levels stipulated in Table 7 of the G5/4 document.

`If the connection is not acceptable at Stage 1 or is at medium voltage (6.6 kV to 22 kV), then it is possible to undertake a Stage 2 assessment.

Stage 2

If the total converter load is lower than 130 kVA of 6-pulse or 250 kVA of 12-pulse diode rectifiers, for a medium voltage PCC, then there is no need for further assessment.

For converter loads greater than this, Stage 2 connection may still be permissible, after an assessment to determine whether the existing voltage distortion and total harmonic currents introduced by the one consumer on the MV supply are within the limits set within Stage 2 of G5/4.

In this case, a measurement must be made of the existing distortion. This measurement should be based on a minimum of seven days to ensure that the peak distortion is recorded. G5/4 allows a background level of harmonic distortion that does not exceed the defined planning level for 95 percent of the time.

If, for 95 percent of the time, the value of existing distortion does not exceed 75 percent of the appropriate planning level, then the total harmonic currents defined in G5/4 can be introduced.

If the measured level is above 75% of the planning level, the voltage distortion due to the harmonic currents of the new installation needs to be calculated.

Should the resultant overall level or level of 5th harmonic remain within the planning levels, then connection may be agreed.

Stage 3
If the levels of harmonics exceed those for Stage 2, or if the point of common coupling is at 33 kV or above, then a different and substantially more complex procedure is called for. The supply utility, the consumer and the manufacturer need to agree upon the method to adopt.

It is vitally important that adequate and sufficient information is available at the outset of a project, as any delay in provision of information could be critical to implementation of a project. The calculations can be complex, and the main requirement is that the consumer, the supply utility and the drive supplier should work together to achieve the optimal solution.

The distorted current or voltage waveform is the sum of the fundamental (e.g. 50 Hz) wave and harmonic (250 Hz) wave.

Drives and the Building Management System

Modern Building Management Systems (BMS) provide a wide range of control functions: fire control, security, power monitoring and air conditioning control. The latter of these is perhaps the best known function of a BMS, and one that can be enhanced and improved by using electronic variable speed drives as an integral part of the control system.

Variable speed drives, or VSDs, provide infinite control over the speed of motors driving pumps and fans. This ensures that the BMS achieves maximum controllability over the building's environment, matching the temperature and humidity to the demands of the prevailing weather and the number of occupants. Compared to cruder systems, such as dampers or valves which choke off the flow of air or liquid produced by a fan or pump, a VSD can drive the fan at the exact speed needed to maintain the building's internal conditions, making the most of the capabilities of the BMS' software and sensors.

Energy Saving

The other major advantage of VSDs is energy saving - by controlling the speed of the motor so it runs at only the speed needed, VSDs help the BMS control energy usage. As an example, a 55kW motor in continuous duty costs £24,000 per year to run at a cost of 5 pence/kilowatt hour. If 50% airflow is required all the time, there are various ways to achieve this. The variable speed drive would use 12.5% of the rated motor power to produce this, compared to the least efficient method, the outlet damper, which would use 80%.

The difference between the most and least efficient method in the example above is £16,000 worth of electricity - £16,000 that need not be spent every year if variable speed drives are used.

An example of the benefits possible is an air conditioning project for Coutts & Co, the international private banking arm of Natwest Group. Coutts is now saving some £70,000 per annum with the help of drive technology from ABB, a cut of 90 per cent on its previous energy bill.

One 4kW, one 22kW and two 75kW drives are driving the pumps for chilled, hot and condenser water, as well as medium pressure water for the boiler room. The pumps are now running at about 40% of maximum speed, whereas before, they would be running at 100% continuously.

Sensors in the pipework, connected to the drives, help regulate the speed of the pumps to ensure optimum usage of the water's energy content. The system is co-ordinated with the Building Management System, enabling easy control of the indoor climate. The lower pump speed also reduces mechanical wear, saving maintenance costs.

Building the system

Drives are also relatively straightforward to connect to the BMS. In many applications, they require only three hard-wired I/O connections; a start/stop, an analogue 0-10V input for speed control, and a fault relay output. The other main connection option is to use a field bus system, which has the advantage of requiring only two wires rather than individual hard-wired connections for each drive. Fieldbus is sufficiently fast for building control, yet there is reluctance among BMS specifiers to move from the tried and trusted dedicated I/O method.

Fieldbus can also support an unlimited number of drives - systems incorporating hundreds of VSDs are not uncommon. This gives BMS designers more options, allowing them to use more VSD features.

When choosing a VSD for a BMS, there are various criteria and issues to take into account. If it is intended for use on a Fieldbus system, it should have interfaces to all the major protocols, such as LonWorks, Profibus, Trend IQ and Modbus. Trend IQ is one of the most popular BMS protocols on the UK market, but few drives use it and so an interface is needed, such as the Compass Point developed by North Communications. Used by ABB drives, up to 20 drives can be connected to a single Compass Point unit.

It is also important for the drive to have low harmonics as standard - drives, fitted as standard with internal harmonic chokes do produce less harmonic current and therefore reduce the level of harmonic pollution seen by the supply network. This is particularly important now that the new G5/4 standard has been introduced.

The VSD should also have a good quality EMC filter that meets the requirements of first environment restricted distribution.

Perhaps most important is that the drive should be a dedicated HVAC unit, with software suitable for building applications. HVAC drives are quite different from process control drives and should not be confused. The drive's software suite should be one that can be programmed to meet the particular demands of HVAC applications - for example, the drive should ideally have an dedicated exhaust fan function (sometimes referred to as a fireman's override) to disperse smoke in the event of a fire and one that responds to a loss of input signal from the BMS by switching to top speed or to a speed set by the user.

Another thing to look for is the IP enclosure rating of the drive - an IP54 rating will allow the drive to be sited next to the motor in potentially dusty or wet conditions. Cabling costs can thus be kept to a minimum, equally important whether the drive is being designed into a new building or retrofitted into an older one.

Summary

Variable speed drives bring major advantages, helping the BMS to achieve one of its primary goals of managing the building environment. Combined with the high energy savings they can bring, drives are becoming an integral part of a modern BMS system.

Drives used in building control need to be dedicated HVAC drives, equipped with the appropriate software

Electromagnetic compatibility (EMC)

All ABB drive products fulfill the requirements of the EMC Directive and the EMC Product Standard for variable speed drives. To ensure the EMC compatibility of the entire installation, there are some basic principles that need to be followed.

Conducted disturbances can propagate to other equipment via all conductive parts including cabling, earthing and the metal frame of an enclosure. Conductive emissions can be reduced by RFI filtering for high frequency disturbances, and by using ferrite rings in any power connection points.To avoid disturbance through the air, all parts of the power drive system should form a Faraday cage against radiated emissions. This includes drive modules, cabinets, auxiliary boxes, cabling, motors, etc.

•Unpainted metal to metal contacts must be used throughout, with conductive gaskets, where appropriate.
•Use conductive gaskets in doors and covers. Covers should be secured at no more than 100 mm intervals in sections where radiation can escape.
•Holes in the enclosure should be minimised.
•Use special HF cable entries for high frequency earthing of power and control cable shields.
•Use shielded cables in accordance with the manufacturers' instructions.
•Route power and control cables separately.
•Use twisted pairs to avoid disturbances.
•Selection and installation of accessories should be made in accordance with the manufacturers' instructions.
•Use 360° earthing at motor end. See product specific instructions. ·Special attention must be given to earthing.