Industrial Power Transformers -- Special features of transformers for particular purposes [part 12]

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TRACTION TRANSFORMERS

Traction transformers are used to provide single phase supplies for train over head catenary pick-up systems and, since the late 1950s in the UK, these have operated at a nominal voltage of 25 kV AC. These transformers have ratings which vary between about 5 and 18 MVA and were initially ONAN cooled with impedances of between 8 percent for the smallest units up to 12 percent for the largest. Later units were provided with mixed cooling, typically ONAN/ OFAN/OFAF to give ratings as, for example, 18/20.5/26.5 MVA. In addition, all sizes of transformers are required to have the capability to provide a cyclic output of 133.3 percent of rated load in an ambient temperature of 30ÂșC for 8 hours followed by 16 hours at 60 percent load. This is to cater for the situation of an outage of an adjacent unit. The transformers normally take their supply from two phases of the 132 or 275 kV transmission networks and feeder stations are located adjacent to the rail tracks at 40 or 50 km intervals.

This section describes the design and constructional features of the transformers used to provide traction supplies in the UK, but the general principles are applicable to transformers used to provide these supplies in many parts of the world.

The duty of traction supplies transformers is a particularly onerous one in that, although their loading may be only intermittent, they are subjected to rapid and repeated load current fluctuations taking them from zero to twice full-load current and with an incidence of system short-circuits which may be as high as 250 per year of varying magnitude up to full fault current. In terms of the electromechanical stresses applied to the transformer windings, this duty is very similar to that described for arc furnace transformers in the previous section and hence, the constructional features of the windings and their clamping arrangements are also very similar to those of arc furnace transformers.

Traction transformers, from the earliest times of the use of AC supplies, have had to withstand the additional duty resulting from the high harmonic content of the load current. This is a problem common to all transformers sup plying rectifier loads and has been described in the earlier sections covering HVDC converters as well as ordinary rectifier transformers.

With the most recent AC traction systems utilizing Insulated Gate Bi-polar Transistor (IGBT) drive mechanisms, the nature of the harmonics problem has become somewhat different. The IGBT system is switched at a high frequency, 4 kHz in some systems used in the UK, so that the harmonic frequencies appearing are in the 70th to 80th range. These harmonics unfortunately excite particularly severe harmonic voltages on the catenary system, typically 5 kV at 4 kHz, so the LV windings of the traction supplies transformers must be designed to withstand these voltages superimposed onto their normal working voltage.

LV system voltage

The 25 kV system voltage must comply with the requirements of the European Standard, EN 60850. This sets down the following voltage requirements:

Nominal voltage 25 000 V r.m.s.

Highest permanent voltage 27 500 V r.m.s.

Highest non-permanent voltage 29 000 V r.m.s. (this is the maximum voltage which may be present for up to 5 minutes) Lowest permanent voltage 19 000 V r.m.s.

Lowest non-permanent voltage 17 500 V r.m.s. (this is the minimum voltage which may be present for a maximum of 10 minutes)

This represents the voltage range which must be available at the locomotive and for efficient operation the no-load voltage must be maintained as close to the upper limit as possible. The drop in supply voltage is dependent upon load, the transformer impedance, the load power factor and the voltage drop in the overhead catenary system. The incoming grid supply voltage is permitted to vary between _10 percent of its nominal value but at any given bulk supply point the variation will normally be no more than _3 percent of the average value prevailing at that particular supply point. It would be possible to provide an on-load tapchanger to compensate for the regulation in the transformer and overhead catenary system, but recognizing that this regulation will occur every time a locomotive draws a large load current in the supply section associated with a particular transformer, it is clear that a tapchanger operation frequency would be very much greater than that normally experienced in a tapchanger associated with a supply network and it has been suggested that based on this usage, the life of a tapchanger could be expected to amount to no longer than a few weeks. The transformers are therefore usually provided with off-circuit taps only, which enable the open-circuit voltage ratio to be optimized to suit the voltage normally occurring at the grid supply point to which the transformer is to be connected. These are usually for a range of 0-12.5 percent in 2.5 percent steps on the LV winding. Experience suggests that after selecting the most suitable tapping when the transformer is put into service, it will remain at this setting unless any major changes are made to the local supply network.

The transformers operate with one pole of the LV winding connected to the rail and solidly bonded to ground. The other pole is thus nominally at a volt age of 25 kV to ground and the insulation requirement equates to that of a three phase system having a nominal voltage of 44 kV. Such a system has a phase voltage of 25.4 kV to ground and dielectric test levels are those for a system having a highest voltage for equipment of 52 kV. For several years traction transformers were designed with this insulation level for the LV winding, however recently consideration, in particular, of the superimposed harmonic voltages on the LV has led to the increasing of the insulation level one step higher to that of a system having a highest voltage for equipment of 72.5 kV.


FIG. 46 Typical traction load-current-demand curve

Rating and impedance

Selection of the most suitable rating for a particular installation is not easy considering the rapidly fluctuating nature of the load and the additional heating effects of harmonics. FIG. 46 shows a typical load-current-demand curve and illustrates the difficulty of relating this to a continuous current rating. The procedure generally adopted is to equate the load-demand cycle to a series of half hourly maximum demand values. If an existing comparable installation is available, these can be obtained from actual meter readings. If no comparable existing installation is available, then estimated values must be used. Because the heating effect of the load peaks is proportional to the square of the load, and in order to provide some allowance for the effect of the harmonic cur rents, it is then considered that the rating of transformer to be used is selected by multiplying the mean half hourly maximum demand value by a factor of between 1.2 and 1.3. The precise value of the factor used will probably be that which results in the selection of an existing standard rating, but clearly it is preferable to err on the conservative side in arriving at this.

The values of impedance quoted above are those which have been used throughout the 1970s and 1980s and are set so as to limit the fault current in the event of a system short circuit to 6 kA. Operating experience up to this time suggests that this is the maximum fault current which can be tolerated returning in the rails without causing interference with signaling circuits. The other effect of impedance is, of course, to cause regulation and, as explained above, it is desirable to obtain as high a voltage as possible at the locomotives, so that, particularly in view of the fact that on-load tapchangers cannot be used, the lowest possible impedance is to be preferred. As more modern signaling systems are installed which are not susceptible to interference by fault current in the rails, it will be possible to reduce the transformer impedance to a value which restricts the fault current to 12 kA, the fault capability of the switchgear on the locomotives.

Impulse withstand

Since the transformers are connected to two phases on the HV side, the HV windings may be subjected to impulse voltages applied to either terminal individually, or to both terminals simultaneously. It has been recognized for some years that when delta-connected three-phase windings are subjected to simultaneous impulse voltages on two line terminals the effect is to pro duce a doubling of the magnitude of the incident waves at the center point of the winding. A single-phase traction supply transformer connected across two phases is entirely equivalent to a delta-connected three-phase unit and can be expected to show exactly the same response. To demonstrate the traction supply transformer's ability to withstand this condition it is usual to specify that they shall be impulse tested by carrying out a full series of shots applied to both HV terminals connected together as well to each terminal individually.

Some supply authorities outside the UK do not subject traction supply transformers to this double terminal impulse test, but studies in the UK in the 1960s and 1970s of the incidence of lightning strikes on overhead transmission net works showed that in more than one-third of the lightning faults more than one phase of the 132 kV system was involved. For the 275 and 400 kV systems the proportions involving more than one phase were very much less, around 5 and 3 percent, respectively, but these figures would appear to justify the test at least for 132 kV transformers.

Construction

Cores may be of two-limbs-wound or single-limb-wound construction. If the latter is used, outer 50 percent cross-section return limbs will be required. The choice is usually determined by factors such as primary current, impulse volt age distribution and whether tappings are specified. For example, at the lowest ratings, say, 5 MVA, a fairly small frame size will be required. The volts per turn will therefore be quite low, necessitating a fairly large number of turns, so that a continuous disc winding will have a large number of turns per section.

A large number of turns per section will result in high inter-section voltages under impulse conditions demanding that an interleaved winding arrangement is adopted, and adoption of an interleaved arrangement will result in the winding being an expensive one. If the winding is expensive, it is more economic to have one of these rather than two so a single-limb-wound arrangement is likely to be adopted. It must be recognized that the windings are effectively of the uniformly insulated type since both ends require to be insulated for line voltage to ground so that even if the cost of the extra strengthening measures is discounted, they will still be more costly than non-uniformly insulated windings of the same voltage class.

If the two-limbs-wound construction is utilized then normally one pair of windings will be connected in series and the other in parallel. This ensures equal load sharing between the limbs. Even at 18 MVA the HV current is only 78.7 A. At a current density of 3 A/mm^2 the strand cross-section need only be about 26 mm^2 , having uncovered dimensions of say, 4 x 7 mm, which is quite small for winding into a stable winding. Certainly a conductor of half this cross-section should be avoided. Hence HV windings would probably be in series, with LVs in parallel.

The off-circuit tappings are likely to be accommodated in a separate layer beneath the LV winding and have a multistart configuration. This ensures that electromagnetic balance is maintained for all tapping positions. The arrangement of windings radially from the core will be: LV tappings, LV, HV. There is no merit in having the HV line lead at the center of the limb, so that the start and finish of the HV winding will be at the top and bottom of the limb. FIG. 47 shows a typical core and windings assembly.


FIG. 47 Core and windings of a single-phase 18 MVA, 132/25 kV, 50 Hz transformer for trackside supplies to an electrified railway system. The LV side is provided with tappings for a range of +10 to - 20 percent.

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