Industrial Power Transformers -- Operation and maintenance [part 1]

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1. DESIGN AND LAYOUT OF TRANSFORMER INSTALLATIONS

Outdoor substations

In planning a transformer layout there are a number of requirements to be considered.

All power transformers containing BS 148 or IEC 60296 oil are considered to represent a potential fire hazard and awareness of this must be a primary consideration when designing a transformer substation. They should be located in such a way that, should a transformer initiate a fire, this will be limited to the transformer itself and its immediate ancillary equipment and not involve any other unit or equipment, cabling or services associated with any other unit.

This requirement is particularly important if two or more transformers are to be installed in the same substation as standby to each other.

Fire hazard imposed by mineral-oil-filled transformers

In having regard to the above recommendations it should be recognized that mineral oil is less of a fire hazard than is often thought to be the case. The closed flash point is specified as not lower than 140ºC, that is, it shall not be possible to accumulate sufficient vapor in an enclosed space to be ignited upon exposure to a flame or other source of ignition at temperatures below this figure. In non-enclosed spaces the temperature will be proportionally higher. It is generally considered that mineral oil needs a wick in order for it to produce sufficient vapor to enable it to burn freely. The incidence of fires involving transformers is small and continues to confirm work done some time ago when a review of UK electricity supply industry statistics carried out within the Central Electricity General Board (CEGB) (and unpublished) suggested that the likelihood of a fire resulting from an incident involving a transformer below 132 kV is very low. This is probably because at the lower system voltages, fault levels and protection operating times are such that it is not possible to input sufficient energy in a fault to raise bulk oil temperature to the level necessary to support combustion. Provided the sensible precautions identified below are taken, therefore, mineral-oil-filled transformers of 33 kV high voltage HV voltage and below can be installed within reasonable proximity of buildings and other plant without the need to resort to the use of fire-resistant fluids, dry-type or cast resin-insulated transformers. Such measures only become necessary when transformers are installed inside buildings and indoor installations will be discussed separately below.

Where fires have been initiated in the past, it has usually been the case that a fault has occurred which has split the tank resulting in very rapid loss of the oil. If the site of the fault, at which, almost by definition, a high temperature will exist, is, as a result, exposed to the atmosphere ignition will occur and the transformer insulation will then serve as the wick to sustain the combustion. Again this emphasizes that where the fault energy is not so high as to cause rupture of the tank, the fire risk is greatly reduced. Rapid fault clearance times will, of course, also reduce the energy input into the fault and adequate provision of pressure relief devices, that is, more than one on a large tank, will reduce the risk of tank rupture. Consideration may be given to arranging that operation of the pressure relief device trips the transformer, but any resultant risk of spurious tripping will need to be balanced against a possible gain in respect of reduced fire risk.

Any potential low-energy ignition mechanism must also be guarded against.

Typically this can occur where a fault causes a gradual drip or seepage of oil onto a heated surface. Such a situation may arise when an overheating external bushing connection overheats due to a high contact resistance. If this reaches a temperature at which the thermal movement cracks a porcelain insulator so that oil leaks onto the overheated joint, this can be ignited and the continuing slow feed of oil can turn the area into a blowtorch. One of the dangers of incidents of this type is that electrical protection is not initiated and the fault can remain undetected until the fire has reached a very serious level.

Minimizing the fire hazard

The conventional practice for many years has been to provide a surface of chippings in substations containing oil-filled transformers and switchgear with a drainage sump so that any oil spilled will quickly be taken off the surface and thus prevented from feeding any fire resulting from a major fault. However, as a result of the UK Central Electricity Generating Board's investigations in the 1960s into a number of serious generator transformer fires, it became clear that chippings which had become oily over the years and had acquired a coating of grime, tended to provide the wick which, when a fire had been initiated, made this more difficult to extinguish.

Of course, in the case of isolated substations it is not always possible to pro vide an arrangement better than chippings, but the CEGB, following the above investigations, developed a system which proved very effective in preventing major fires following the type of incidents which had on earlier occasions given rise to them. This involves providing each transformer with a fixed water spray fire protection installation. It consists of a system of spray nozzles located around the transformer and directed towards it which provide a total deluge when initiated, usually by the bursting of any one of a series of glass detector bulbs (frangible bulbs) in an air-filled detector pipe placed around and above the transformer. The whole installation is normally empty of water and when a detector bulb initiates, the resultant air pressure drop releases a water control valve allowing water into the projector pipework and thence to the spray nozzles. As the water is normally maintained at a pressure of 8.5 bar it immediately begins to control the fire and back up fire pumps are started to maintain the water supply pressure. An important part of the strategy for rapid extinguishing of a fire is the rapid removal of any spilled oil from the surface of the plinth.

When stone covered sumps were provided this often resulted in any oil which had collected with time, being washed back up to the surface due to the spray water displacing it. To avoid this, instead of chippings, the surface must be smooth concrete. Large drainage trenches are provided and these must have an adequate fall to a transformer oil collection and containment system. Another important consideration in designing any system of fixed waterspray fire protection is to ensure that the installation is not vulnerable to serious damage by the initiating incident. This means that the routing of the fire supply main, in particular, must be very carefully examined to ensure that it cannot be disrupted by an explosion in any of the transformers that it is installed to protect.

Clearly large quantities of oil and water cannot be allowed to enter the normal stormwater drainage system, so the drainage trenches are taken to interceptor chambers which allow settlement and separation of the oil before allowing the water to be admitted to the normal stormwater drainage system.

A typical arrangement is shown in Fig. 1. Although the plinths are designed to drain rapidly, it is important to ensure that any water which might be contaminated with oil is not allowed to flood into neighboring areas, so each plinth must be contained within a bund wall which will hold, as a minimum, the total contents of the transformer tank, plus 5 minute operation of the fire protection, and this after heavy rain has fallen onto the area.

The system is costly in terms of civil works and it requires the availability of the copious quantity of water necessary to support the waterspray fire protection system, so it cannot normally be considered for other than transformers in power stations or important transformers in major transmission substations where such resource can be made available, but in these types of situations it is clearly the most effective method of dealing with the fire risk. Good house keeping in transformer compounds is also of considerable benefit.


FIG. 1 Arrangement of water and oil drains for transformer plinth

Oil containment

Even where the more traditional system of chippings and sump is used as a base for the transformer compound, consideration will need to be given to the possibility of loss of all the oil from the transformer tank and its cooler.

Suitable provision must be made to ensure that this will not enter drains or watercourses. Such provision will normally be by means of a bund wall surrounding the transformer and its cooler which together with any sump must be capable of containing the total oil quantity in addition to the maximum likely rainfall over the area. Since the bunded area will under normal operating conditions need provision for stormwater drainage, then suitable oil interception arrangements must be made for separation and holding any oil released.

Segregation and separation

Where it is not economic to consider the type of elaborate measures described above, then other design features must be incorporated to allow for the possibility of fire. Such features involve segregation or separation of equipment.

Separation involves locating the transformer at a safe distance from its standby, where one is provided, or any other plant and equipment which must be protected from the fire hazard. Ten meters is usually considered to be sufficient distance. This means that not only must the transformer be a minimum of 10m from its standby, but also all connections and auxiliary cabling and services must be separated by at least this distance.

On most sites such an arrangement will be considered too demanding of space, so this leads alternatively to the use of a system of segregation, which relies on the use of fire-resistant barriers between duty and standby plant and all their associated auxiliaries. The integrity of the barrier must be maintained regardless of how severe the fire on one transformer or of how long the fire persists. In addition the barrier must not be breached by an explosion in one of the transformers, so it will normally be necessary to construct it from reinforced concrete and of such an extent that flying debris from one transformer cannot impinge on any equipment, including bushings, cables, cooler and cooler pipework or switchgear associated with its standby. Generally for access reasons transformers should be at least 1m from any wall but this space may need to be increased to allow for cooling air as described below.

Other considerations for substation layout

In addition to the requirements to preserve the integrity of standby from duty plant and vice versa as outlined above, an important consideration when arranging the layout of a transformer substation is that of ensuring correct phase relationships. The need for these to be correct to enable transformers to be paralleled is discussed further in Section 6.4. Every site should have a supply system phasing diagram prepared showing incoming circuits and plant within the site. Although the principles are very simple errors are found during commissioning with surprising regularity. It greatly helps the avoidance of such errors to rigorously adhere to a convention when arranging the layout of a transformer. Low-voltage (LV) cables between transformer and switch gear can be transposed to enable these to appear in the correct sequence at the switchboard, but it is not always easy to transpose HV overhead connections or metal clad phase-isolated busbars, so the transformer should always be positioned in such a way as to allow these to run in the correct sequence and connect directly to its terminals without any requirement for inter changing phases. In the UK, the convention is that the phase sequence when viewed from the HV side of the transformer is A, B, C, left to right. This means that viewed from the LV side the phase sequence will run c, b, a left to right or a, b, c right to left. If there is a neutral on HV or LV, or both, these may be at either end but they must be shown on the transformer nameplate in their correct relationship with the line terminals. Phasor relationships are referred to the HV side of the transformer with A phase taken as the 12 o'clock position.

Phasors are assumed to rotate anticlockwise in the sequence A, B, C.

In the concluding section of the previous section it was explained that movement of a large transformer on site is a difficult process. In designing the sub station layout, therefore, another important factor is that of access for the transformer and its transporter. Small transformers up to, say, 25 tonnes might be lifted from the transporter using a mobile crane and set down in the correct orientation directly onto their foundations. However, most will require to be maneuvered by means of jacks and greased rails into their correct position.

Allowance must therefore be made for positioning of the transporter adjacent to the raft in the best position for carrying out this operation, and appropriately located anchor points must be provided for haulage equipment. Of course, although transformers are extremely reliable items of plant, they do occasionally fail, so that allowance should also be made for possible future removal with minimum disturbance to other equipment in the event of the need for replacement.

In planning the layout of the transformer substation, except where the transformers are water-cooled, consideration should also be given to the need for dissipation of the losses. Whether radiators are tank-mounted or in separate free-standing banks there must be adequate space for circulation of cooling air. If the cooler is too closely confined by blast walls and/or adjacent buildings it is possible that a recirculation system can be set up so that the cooler is drawing in air which has already received some heating from the transformer. Ideally the cooler, or the transformer and its radiators if these are tank mounted, should have a space on all sides equal to its plan dimensions.

FIG. 2 shows a typical two-transformer substation layout having consideration for the above requirements and with the appropriate features identified.


FIG. 2 Typical two-transformer arrangement within 132 kV substation.

Transformers in buildings

Although all the recent experience and evidence emphasize the low fire risk associated with oil-filled power transformers, particularly those having an HV voltage below 33 kV and rating of less than, say, 10 MVA, where a power transformer is to be installed within a building the fire risk is perceived to be such that the use of mineral oil is best avoided. Such a condition is likely to be imposed by insurers even if design engineers or architects were to suggest that this might not be necessary.

The use of all types of electrical equipment in buildings is nowadays extensive and the consequent magnitude of the electrical load has meant that many office blocks and commercial buildings take an electricity supply at least at 3.3 kV so that this must be transformed down to 415V for internal distribution.

There is thus a growing market for fire-resistant transformers. There is also a great diversity of types of transformers available.

As discussed in Section 3.5, until the non-flammable dielectrics of the type based on polychlorinated biphenyls (PCBs) were deemed to be unacceptable in view of their adverse environmental impact, they had little competition as the choice of dielectric for transformers installed in buildings. Possibly some manufacturers and users saw benefit in avoiding the use of liquid dielectric entirely and turning to dry-type transformers, but at this time, class C dry-type materials were unreliable unless provided with a good clean dry environment and cast resin was very expensive as well as having questionable reliability.

There was therefore very little call in textbooks for sections such as this, since the choice was very simple and the installation and operating problems of PCB transformers were few.

PCB was such an excellent dielectric that none of the possible replacements are quite able to match its electrical performance or its fire resistance. In addition, there is now a greater awareness of the need to avoid environmental hazards, not only those resulting from leakage of the dielectric or faults within the transformer but also from the combustion products should the transformer be engulfed in an external fire, so that for any prospective new dielectric there is a very stringent series of obstacles to be overcome. Nowadays the designer of an installation within a building must have satisfactory assurance on the following points:

• The dielectric must be non-toxic, biodegradable and must not present a hazard to the environment.

• The dielectric must have a fire point above 300ºC to be classified as a fire resistant fluid.

• The dielectric must not contribute to or increase the spread of an external fire nor must the products of combustion be toxic.

• Normal operation, electrical discharges or severe arcing within the transformer must not generate fumes or other products which are toxic or corrosive.

The liquid dielectrics identified in Section 3 will meet all of the above requirements. The fire performance of cast resin is dependent on the type of resin and the type and quantity of filler which is used. Cast resin encapsulated transformers supplied by most reputable manufacturers will be satisfactory on these aspects, but, if there is any doubt, the designer of the installation should seek assurance from the supplier of the transformer.

Generally, a liquid-filled transformer will be cheaper and smaller than a resin encapsulated or other dry-type unit but the installation must make provision for a total spillage of the dielectric, that is, a sump or a bunded catchment area must be provided to prevent the fluid entering the building drains. If the transformer is installed at higher than ground floor level, and electrical annexes on the roof are frequently favored by architects, then the installation must prevent leak age of the fluid onto lower floors. The cost of these measures could outweigh the saving on the cost of the transformer and the extra space taken by a bunded enclosure could offset any saving in space resulting from the more compact transformer. Conversely, where cast resin or dry-type transformers are used, other services within the building, particularly water mains, should be located so as to ensure that the transformer and its associated switchgear are not deluged in the event of a pipe leak. Such events unfortunately appear to be common during the finishing phase of a new building. Needless to say the area where the transformer is to be located should be completed and weatherproof before installation of a dry-type transformer. (Whilst manufacturers of cast resin transformers will, no doubt, be keen to stress their ability to withstand onerous conditions such as condensation or dripping water, both the HV and LV connections to the transformer are unlikely to be quite so tolerant of these adverse conditions.) A dry-type or cast resin transformer will probably be housed in a sheet-steel cubicle integral with the switchboard with LV busbars connected directly to the switchboard incoming circuit breaker. The cubicle and transformer will very likely be delivered and installed as separate items, although some manufacturers are now able to supply these as a single unit. The cubicle should be securely bolted to the switchroom floor and, when installed, the transformer should be positively located and fixed within the cubicle. The floor finish (screed) should be smooth and level so that the transformer can easily be rolled into or out from its cubicle and the floor should be capable of withstanding the imposed loading of the complete transformer (see Table 1) at any location within the switchroom. A minimum spacing of 0.75 m should be allowed between the transformer cubicle and the rear of the switchroom and ample space must be provided in front of the cubicle for maneuvering the core and windings in and out. Switchroom doors should be large enough to enable the transformer to enter and also to be removed at some later date should a problem arise in ser vice. This is an aspect which is often overlooked and it is not uncommon for switchroom doors to have to be hastily modified when the transformer arrives on site before it can be taken into the switchroom. FIG. 3 shows a typical arrangement of 415 V switchboard with integral 11/0.415 kV transformer.


Table 1: Typical total weights of oil-filled and cast resin insulated transformers - 3 phase, 11 kV [It should be noted that the above weights are typical only for transformers having average impedance and losses. ]

Significant departures from the above values may be found in specific cases. Losses of up to 30% less are easily obtained but weights would be considerably greater in proportion.


FIG. 3: 415 V switchboard with integral 11/0.415 kV cast resin transformer (Schneider Electric)

Whilst it is desirable that the switchroom should be clean, dry and have some heating in service before the transformer is installed, the heat dissipated by the transformer must also be taken into account in the design of the heating and ventilation system. The iron loss, which could amount to 2 kW for a 1 MVA transformer, will need to be dissipated from the time that the transformer is put into service. Load loss could be up to 10 kW at full load for a 1 MVA unit, so a considerable demand is likely to be imposed on the H and V system. Table 1 gives typical losses for other ratings of transformers.

In order to obtain the full rated output and any overloads, indoor transformers should always be accommodated in a well-ventilated location which at the same time provides the necessary protection against rain and dripping water.

Too great a stress cannot be laid upon the necessity for providing adequate ventilation, since it is principally the thermal conditions which decide the life of a transformer. Badly ventilated and inadequately sized switchrooms undoubtedly shorten the useful life of transformers, and hence should be avoided.

A liquid-filled transformer does not lend itself so conveniently to incorporation into the MV switchgear in the same way as a dry-type, since it will be installed within a bunded area with the switchboard on the outside of this.

Although it is possible to bring out 415 connections via 'monobloc' type bushings suitable for connecting to busbar trunking, this has less flexibility as regards layout than 415 V cables. It is likely, therefore that a cable connection would be the preferred choice. FIG. 4 shows a synthetic liquid-filled 11/0.415 kV transformer suitable for indoor or outdoor installation and designed for connection via 415 V cables to its MV switchboard. Such a transformer has the advantage that it is virtually maintenance free.


FIG. 4 A synthetic liquid-filled 11/0.415 kV transformer suitable for indoor or outdoor installation and designed for connection via 415 V cables to its MV switchboard (Schneider Electric).

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