Broad or inadequate specifications for an electrical installation can cost a lot more than specialist advice at the planning stage. Phil Kreveld explains.

Writing specifications for large industrial or commercial electrical installations and extensions is a job for consulting engineers.

Electrical contractors can be badly caught out, as demonstrated when specifications are broad or even vague, particularly in relation to harmonics.

Electrical power distribution companies (owners of the poles and wires) can, and frequently do, insist that installations comply to the letter of their requirements for power factor, harmonics, and balance. These may well be variations of the relevant Australian Standards.

Failure to comply results in contractors bearing much of the cost of remedial work, something experienced in recent capital city and regional installations.

The advent of electronic metering has allowed power suppliers to extract data relating to demand, reactive demand and power factor, as well as harmonic distortion. In addition, some power suppliers monitor their sub-distribution system on a 24/7 basis. They are acutely aware of problems that harmonic distortion can cause for their static reactive compensators.

Non-compliance problems can arise because there are at least two agendas:

• Specification writing can be costly, and those costs have to be contained in order to offer a competitive price to the client.
• Contractors are assumed to adhere to the specifications, with any exceptions clearly noted.

The devil is in the detail, and even with exceptions being taken, there is generally no way of ignoring the power distributor’s requirements.

Today’s installations are replete with loads that don’t make their presence felt on the line diagrams. Once connected, installations can cause problems such as excessive harmonics. Lighting, climate control, IT equipment (such as server rooms) and uninterruptible power supply (UPS) equipment in commercial buildings are all candidates.

Industrially, quite apart from the ubiquitous variable-speed drive, there are induction heaters, welders, plating baths, computer numerical control machining centres, electrical heat treatment plant, induction heating, dielectric glueing and forming, and loads similar to those in commercial installations.

Extensions to installations can pose special problems, such as with sub-station transformers, static power factor controllers, existing switchboards with additional protective gear, problems with co-ordination and flicker. These feed through to the primary side and can again cause a power supplier to issue a non-compliance note.

Harmonics mitigation
It is no exaggeration that a minefield awaits the contractor who has not thoroughly analysed the specifications.

Even when that process is thought to have been thorough, there may well be explicit requirements (eg: the installation shall answer to the connection requirements of relevant supply authorities) that override compliance of the contractor’s quotation with the itemised list of line equipment such as harmonics mitigation chokes, filters, etc.

It’s a fact of life that prediction of load effects such as flicker, harmonics, etc – other than the calculation of nominal rms load current – is a task for a specialist.

In commercial installations, where mechanical services such as heating, ventilation and air-conditioning could be up to 30% of the total load, contributions to harmonic and power factor effects at the supply point can bring the contractor a heap of problems.

There may be a lot of detail on the thermodynamic side (relative humidity levels, sensible heat and enthalpy ranges, volumetric flow rates, etc), but the electrical input end is often loosely specified. There is an enormous difference between a six-pulse inverter and a 12 or 18-pulse inverter (the latter two require multiple transformers and a substantial cost increase) for fan drives, chillers, etc.

The commonly used six-pulse inverter will be an economical solution, but its harmonic contributions can be as high as 100% of the ‘useful’ current component.

It needs to be clearly understood that complete designs for a greenfield installation are not often provided because it is a complicated and expensive task requiring many engineering man-hours.

Load descriptions in kVA, kW and brief ‘one-liners’ of the technology employed (ie: Ward-Leonard, UPS, inverter, rotary converter, burst/phase power controller) are a generally a good guide for conductor size basics but not for distortion effects.

Lighting loads also need to be tightly specified, in particular the ballasts. There is great variation in harmonic contribution among the various types available.

As for IT equipment plugged into general power outlets, it is safe to assume a very high third harmonic contribution. The balancing of phases for this type of load is essential, as is effective mitigation of this and higher zero-sequence harmonics.

It cannot be assumed that any active filters will do this job so that there is no net zero sequence contribution on the primary side of a substation transformer.

The prediction of harmonics levels depends greatly on short-circuit impedances, and therefore also on transformer leakage reactance. Sometimes high leakage reactance transformers are specified, but usually there will be little or no information on the transformer.

Calculating harmonics levels – taking conductor and transformer reactances into account, in combination with passive and active filters – requires specialised software and expertise to interpret results. It is unlikely that any meaningful quantitative analysis will have been done as a basis for the typical reticulation specification.

The reader may well think potential problems are being overstated. Unfortunately, that is not the case.

Electronic, non-linear loads are by far in the majority, meaning that supply authorities have to supply lots of reactive power at the fundamental frequency, and an increasing harmonics reactive power. The result is more voltage regulation and harmonic voltage distortion.

This ‘double whammy’ has made it very difficult to meet specifications at the point of common coupling (PCC). The relevant Standard for Australia is AS/NZS 61000.3.6, based on a similarly named IEC Standard. However, individual supply authorities are not obliged to adhere to the standard. MV installations may well be subject to different permissible harmonic levels at the PCC.
 
A ‘gotcha’ is hidden in the high harmonics, whose contributions may not be specified in contractor documents but are part of the total harmonics level at the supply point. In other words, correcting for specified harmonics may still attract a liability depending on how the specifications are worded.

Another vital consideration is total harmonic distortion (THDV). The question is: what is that value composed of and, of equal importance, what was the harmonic contribution before the installation was connected? A new installation may just send the THDV ‘over the edge’.

Harmonic imports are another potential problem, and sorting this out is difficult. Contractors should be aware that power companies imposing restrictions on THDV are not necessarily expert in harmonic analysis and may have to rely on independent experts.

However, determining definitively whether problems are being caused by the new installation or by consumers down the line is not an easy task – even for experts.

PCC analysis and inspection of the line diagram
As shown in a recent large installation, liability was attached primarily to the contractor, with some being accepted by the client and consulting engineers.

This indicates the problematic nature of such clauses as “the installation shall answer to the connection requirements of relevant supply authorities”.

Contractors should not accept clauses stipulating adherence to PCC requirements. If they do accept, the cost of a predictive calculation of harmonic levels – and, in many cases, an analysis of distortion levels at the proposed connection point – should be included in their pricing. This will allow a ‘before and after’ comparison.

Mitigation methods, distortion levels, power factor and equipment at the installation side should be tightly specified. When that is not the case, options should be offered without taking on liability for performance standards at the supply point.

So, when a specification lands on the desk, how should it be examined for problem areas?

Sometimes the hazard of complying with the requirements is stark. In one case the specification stipulated a large number of harmonics loads, was silent on harmonics mitigation but did require the installation to meet the connection requirements of the supply authority.

However, that is a rather glaring inconsistency and more often there will be some level of detail. The traps in specifications are in the detailed study of the installation line diagram.

As an example, consider an installation with a main switchboard, supplying special function switchboards (lifts, HVAC, data centre) with general lighting and power fed from the main switchboard.

The lifts will provide a high level of harmonics, as will the HVAC system with its chillers and fans all driven by inverter-connected motors.

An electrical contractor undertaking to match PCC requirements is not likely to meet the required harmonic pollution maximums if he is sticking only to harmonics mitigation for lighting and general power for the building.

In order to have a reasonable handle on the job, the purpose of general power outlets (IT, other office and shop electronics such as scanners, cash registers) should be established so that a harmonic load take-out can be done on the basis of the line diagram.

A quantitative estimate can than be intelligently done of third and higher harmonics, and ditto for lighting loads. Yet mitigation based on this part of the analysis will usually be insufficient if parties responsible for the special functions have not taken adequate steps for harmonics mitigation.

In some recent large installations where power connections were initially allowed, tough timelines were imposed by the power company on the consumer-client to clean up its act. This resulted in counter claims to make good issued to consulting engineers and contractors, with the client assuming some costs in relation to client-supplied equipment.