Industrial Power Transformers -- Special features of transformers for particular purposes [part 4]

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TRANSFORMERS FOR HVDC CONVERTERS

With the increasing number, world-wide, of high-voltage direct current (HVDC) interconnections between high-voltage transmission networks such as, for example, that between the UK and France, the use of HVDC converter transformers is becoming more widespread. HVDC links may simply be back to-back schemes used for the interconnection of AC systems having incompatible characteristics, which usually means having different frequencies, or they may be used for EHV transmission over long distances.

In the former case the DC voltage need not be very high and can be optimized to suit the economics of the converter station. Since 25-30 percent of these substation costs are determined by the cost of the converter transformers, transformer design considerations have an important bearing on the overall design of the interconnection. In the case of long distance transmission, where the requirements of the transmission line represent a significant factor in the economic equation, it is often the case that the highest technically feasible voltage is selected for the DC system. In both cases, however, system interconnections are usually made at those points on the AC systems having the highest voltage level, so that the 'AC windings' of converter transformers are normally of 400 kV or higher.

Since all the windings of any transformer normally operate on AC, it is not very specific to refer to the AC windings of converter transformers. The windings which are directly connected to the AC system are normally termed the line windings. The windings connected to the converter are termed the valve windings. The other parameter unique to converter transformers is the commutating reactance which can usually be taken as the transformer reactance.

IEC 61378 Converter transformers is a three-part document that deals specifically with these equipments. Part 1 has the sub-title Transformers for industrial applications and Part 2 is the one that deals with HVDC converter transformers. Part 3 is an application guide covering both industrial and HVDC converter transformers and sets out in narrative form much useful information concerning the differences between converter transformers and conventional power transformers designed to EN 60076. In early 2007 only Parts 1 and 2 have been accepted as European Norms.

Winding connections


FIG. 9 AC/DC system schematic

The generation of harmonics is an undesirable feature of any converter equipment and in order to minimize these, 12-pulse converters are normally used.

FIG. 9 shows a typical arrangement in which two converters are connected in series on one pole. Each converter has one valve winding connected in delta and the other in star, so that the valve winding voltages are displaced by 30º electrically. It is usual to arrange that both star- and delta-connected valve windings have a common star-connected primary line winding, although a number of alternatives are possible:

• One three-phase transformer having one line (primary) and two valve (secondary) windings.

• Two three-phase transformers each having one line and one valve winding.

• Three single-phase transformers each with one line and two valve windings.

• Six single-phase transformers each with one line and one valve winding.

Since system interconnections frequently have fairly large ratings, converter transformers are almost invariably also fairly large. Transport limitations require that these must frequently be subdivided and the use of single-phase units is the most usual form of subdivision with the third of the options identified above being adopted as the most economic. FIG. 10 shows the core and windings of a single-phase transformer with two wound limbs. One limb has a line winding plus a star valve winding, and the other a line winding plus a delta valve winding. The two line windings are connected in parallel and each has a tapping winding contained in a separate outer layer and controlled by an on-load tapchanger. A reduced depth yoke is used in order to reduce transport height so that the core has external reduced section outer return limbs. FIG. 11(a) shows the arrangement of the windings in section and FIG. 11(b) the connections for the three-phase bank.


FIG. 10 Core and windings of a 234 MVA converter transformer for Chandrapur, India. The transformer has a primary rated voltage of 400 _/3 kV, 50 Hz, and supplies DC system having a rated voltage of -215 kV. The tapping range is +/-27 to -6 percent and the transformer has OFAF cooling (Areva T&D)



FIG. 11(a) Section through core and windings of HVDC converter transformer (Areva T&D) (b) Three-phase bank connections (Areva T&D)

Insulation design

It is in the area of insulation design that HVDC converter transformers differ most significantly from conventional transformers designed solely to withstand AC voltage stresses. The valve windings experience a DC bias voltage which is a function of the DC system voltage and this is superimposed on the AC voltage distribution. The DC voltage experiences a polarity reversal when the direction of power flow is reversed. To further complicate this situation, the behavior of insulating materials, paper, pressboard and oil, differs greatly in its response to DC stress than it does to AC stress. Oil is weakest dielectrically so that areas subject to high levels of DC stress must be suitably barriered and the barriers must be shaped to limit the stress levels at the oil/pressboard interface.

In a system subjected to AC stresses the voltage distribution is determined by the material dimensions and dielectric constants. For a composite insulation structure such as that formed by oil and pressboard, the stress in each component is in inverse proportion to the dielectric constant, so that the oil, having a lower dielectric constant than pressboard, will be subjected to the higher stress.

In the case of a system subjected to a DC stress the distribution is determined by the material dimensions and their resistivity. Pressboard has the higher resistivity and is thus subjected to the highest stress. Generally this is beneficial since the pressboard also has the highest electrical strength. However, unlike the situation for AC conditions, which do not usually create any special problems, very careful consideration must be given to the design of pressboard/oil inter faces and insulation discontinuities. The situation is, of course, made more difficult from a design viewpoint by the fact that a combination of AC and DC stresses occur in practice.


FIG. 12 Electrical stress distribution in insulation before and after voltage reversal.

Because the AC and DC stresses are predominantly determined by different parameters, it may be assumed that any voltage field can be analyzed according to its AC and DC components, the stress due to each field calculated separately and the two components then combined to give a measure of the actual stress.

A critical condition occurs on polarity reversal as can be seen by reference to FIG. 12. This shows the voltage distribution at a pressboard/oil inter face both, at steady state before, and temporarily immediately after a polarity reversal. From this it will be seen that although the oil stress ultimately reaches the same level after reversal as it was originally, except for having the opposite polarity, it is initially very much greater than before the event because immediately after the polarity reversal the charge in the pressboard remains in the opposite direction.

Further factors add to the complexity of the insulation design problems.

For AC stressing purposes the oil and pressboard dielectric constants may be regarded as fixed with respect to level of stress and varying only very slightly with temperature. On the other hand the resistivities of different materials are very much dependent on moisture content, temperature and applied electrical stress. When the DC voltage is applied, a capacitative charging voltage distribution appears initially, changing to the final resistivity governed distribution over a period which might be anything from a few minutes to as long as an hour.

This DC distribution will have superimposed upon it the AC distribution.

Calculation of the resultant voltage distribution is thus a very complex procedure requiring that the stress levels be known in order to determine the resistivity, which, in turn, needs to be known in order to determine the stress levels. This used to demand that a lengthy iterative process be employed. Nowadays the use of computer programs based on finite element modeling (FEM) techniques has simplified the process considerably.

The design of the HV leads and their connections to the DC bushings is an another area requiring particular attention because of the combination of AC and DC stresses as well as the polarity reversal condition. This usually involves the use of a number of pressboard cylinders as well as pre-formed pressboard insulation structures which often provide support as well as an insulation function.

Insulation conditioning

Because of the effect of moisture on the resistivity of insulation material, it is necessary to obtain and maintain a high level of dryness in the insulation of HVDC transformers. This is equally important in service as it is in the factory at the time of testing. In addition, a very high level of cleanliness must be observed involving extensive filtering of the oil. Any particulate contamination of the oil, whether in the form of cellulose fibers or metallic particles can migrate under the influence of the DC stress field so that as they come into contact with electrodes or solid insulation materials they can cause corona discharges or even breakdown. Oil circulation in service is generally maintained continuously regardless of load so as to ensure that the temperature distribution remains as even as possible thus ensuring that the DC stress distribution is not distorted due to thermal effects on resistivity.

Harmonics

Brief mention has already been made of the problems of harmonics and the need to reduce their effect on the system to which the converter is connected.

This is achieved by the use of filters connected as shown in FIG. 9. The harmonics cannot however be eliminated from within the converter windings and it is important to allow for the effects of these in the design of the transformer.

The harmonics add considerably to the stray losses in the transformer windings, core and structural steelwork and due allowance must be made for their effects, not only in carrying out the thermal design but also when testing, to ensure that adequate cooling provision has been made. The harmonics arise because of the circuitry and the mode of operation. FIG. 13(a) shows the conventional idealized waveform, which itself has a high harmonic content, in practice the leading and trailing edges of the current pulses are parts of a sine wave, Fig. 13(b) when rectifying and FIG. 13(c) when inverting. The harmonic con tent is made greater by transient overshoot creating oscillations at the turn-off points of the current pulses. EN 61378-2:2000 gives a method of estimating eddy current losses in windings and in structural steelwork based on two sets of loss measurements: one at operating frequency, 50 or 60 Hz, and one at a frequency of not less than 150 Hz. It is suggested that most manufacturers who are able to manufacture HVDC transformers will have plant capable of supplying the transformer at the higher frequency because this is needed to enable induced overvoltage testing to be carried out.

The method is further explained in Part 3 of EN 61378. It is based on the fact that eddy current loss in windings vary as f^2 , whereas those in high-current bus-bars and structural steel parts vary as f ^0.8.

Suppose then:

P1 is the total load loss measured at 50 Hz

P3 is the total load loss measured at 150 Hz

I^2 LNR is the ohmic loss at rated current based on a measured (or calculated) winding resistance Rf is the frequency.

It is required to establish the following quantities:

PWE1 is the eddy current loss in the windings at 50 Hz PSE1 is the stray loss in the structural parts at 50 Hz.

Then, P1 + I^2 _LN R + P_WE1 + P_SE1 and


The two equations can then be solved for the two unknowns PWE1 and PSE1 thus providing a fairly accurate estimate of the magnitude of loss to be added when carrying out a temperature rise test.


FIG. 13 Valve winding current waveforms

Commutating reactance and short-circuit current

Fault current in the case of converter transformers is likely to contain a very much greater DC component than is the case for normal transformers. Fault current is dependent on the impedance of the valves but also the winding DC resistance, which in all probability will be very low. Furthermore, unlike in the case of faults in conventional AC circuits for which the DC component decays very rapidly, for converter circuits the high DC component will continue until the protection operates. The resulting electromagnetic forces can therefore be very significant, and great importance is placed on high mechanical strength of windings and support structures for busbars and connections. One method of limiting short-circuit forces is to design converter transformers to have a higher impedance than would normally be associated with a similar rating of conventional transformer. High impedance, however, always results in high regulation, which system designers will seek to avoid, and as experience is accumulated with the design and operation of converter transformers the trend is towards lower impedances and closer design and manufacturing tolerances. For converter transformers the tapchangers, which are used in addition to the control of valve ? ring angle to control the power flow, will often have up to 50 percent greater range than for conventional AC power transformers, so that the need to limit the variation of impedance with tap position becomes an important consideration in determining the winding configuration.

Tapping windings and tapchangers

The extent of the tap winding and its location such as to minimize impedance variation results in a high voltage being developed across it under impulse conditions, placing demands on the winding insulation design as well as the impulse withstand capability of the tapchanger itself. In addition, the operation of the thyristor valves results in AC-side current waveforms with a steeper rate of rise than that occurring under normal sinusoidal conditions. This places more severe demands on the switching capability of the tapchanger. The increased dielectric and switching requirements placed on the tapchanger result in it being larger than that required for a conventional transformer of similar rating and voltage class. FIG. 11(b) shows the arrangement of tapchanger connections used for the single-phase converter transformer of FIG. 10. A separate tapchanger is used for each half line winding. There are no current sharing problems since the line winding currents are determined by their respective delta- or star-connected valve windings.

Bushings and connections

The converter valve stack is normally housed within a building to provide weather protection, and connections must be made to this from the transformer which is outdoors. This is normally done by taking the valve winding bushings directly through the wall of the valve hall so that the bushings are usually mounted horizontally on the side of the transformer. This arrangement prevents the connections from transmitting high-frequency radio interference. Another alternative would be the use of gas-insulated busbars. The use of epoxy-resin-impregnated paper (e.r.i.p.) bushings avoids the danger of oil leaks causing contamination of the valve hall, see FIG. 14.


FIG. 14 Transformer of FIG. 10 undergoing works tests (Areva T&D)

The design of bushings for DC operation presents particular problems which do not arise on AC systems. The external insulator surface is very vulnerable to atmospheric deposits (another advantage in housing this indoors) and requires a special design with a long creepage distance. The high harmonic content of the current waveform gives rise to high dielectric losses.

As explained above, because of the DC voltage stresses, internally within the transformer the design of the interface with the valve winding lead requires very careful consideration. Finally, the length of the external portion of the bushing coupled with the fact that this is mounted horizontally creates large cantilever forces which must be taken into account in its design.

Testing

The testing of converter transformers must take account of the special operating requirements and IEC 61378-2 specifies additional tests over and above EN 60076 as well as amending some of these tests. It is perhaps convenient to consider all the testing that is recommended should be carried out.

These include routine tests in accordance with EN 60076; vector group, volt age ratio, winding resistance, no-load losses and currents, impedances and, where appropriate, switching impulse tests. Application of switching impulse tests to the valve windings are made with the winding ends connected together and the non-tested windings connected to ground. Lightning impulse tests are also carried out in accordance with EN 60076 but again with special provisions applying for the valve windings. AC separate source voltage withstand with partial discharge measurements and induced overvoltage withstand with partial discharge measurements are generally in accordance with EN 60076 except that for the valve winding the separate source test voltage is maintained for 1 hour.

Load losses are carried out in accordance with EN 60076 except that measurements are made at two frequencies as explained above. If the test plant is not able to supply rated current for the higher frequency test then EN 61378-2 suggests that a current of between 10 and 50 percent rated current is sufficient.

Temperature rise and sound-power level tests on transformers and coolers are also be carried out in accordance with EN 60076 as type tests. The load loss for the temperature rise test is to include the additional loss components calculated from the two sets of loss measurements as described above and, when the unit is reduced to full-load current for the establishing of gradients, the current is to include a correction factor for harmonics calculated in a similar manner to that derived for additional losses.

The additional testing to check those parameters that are specific to HVDC converter transformers are identified in EN 61738-2 as follows.

DC separate source voltage withstand test

Because of the variation of resistivity with temperature the document says that this test must be performed at 20 +/- 10ºC. The test voltage is to be applied with positive polarity and all terminals of the transformer must be grounded for a period of 2 hours prior to the application of the test voltage. During the application of the test voltage all non-tested terminals are to be connected to ground.

The test voltage must be brought to the test level within 1 minute and maintained for a period of 2 hours, after which it is to be reduced to zero within 1 minute. Partial discharge must be monitored throughout the test. The test is considered acceptable if during the final 30 minutes no more than 30 pulses 2000 pC are noted with no more than 10 of these in the last 10 minutes. If this requirement is not met over the last 30 minutes the test may be extended for a further 30 minutes and if the requirement is met during this extension the test is acceptable, but only one extension to the test time is permitted.

Polarity reversal test

The same limitations with regard to temperature for the test apply as for the DC separate source test and also, as for that test, all terminals are to be connected to ground for 2 hours prior to the test. The test consists of two reversals of polarity. The test voltage is applied with negative polarity and held for 90 minutes, whereupon the polarity is reversed to be followed with 90 minutes of positive polarity. Following the second polarity reversal the test voltage is held with negative polarity for the final 45 minutes. Both reversals must be carried out within 2 minutes. Partial discharge is to be monitored throughout the test.

The test is accepted if no more than 30 pulses 2000 pC are observed during the 30 minutes following each reversal with not more than 10 of these within the last 10 minutes.

Load current run

Finally to verify the current carrying capability of the transformer a load current run is carried out for a minimum of 12 hours with the unit loaded to the corrected full-load loss including harmonics determined as described above. IEC 61378-3 warns however, that supplying total winding losses as sinusoidal current rather than current having a harmonic content as in service, will produce a different distribution of leakage flux, and therefore of losses, in the winding than that which will occur in service. In service the winding eddy current loss, with harmonics, will tend to be concentrated near to the ends of the windings since these are the areas subjected to the highest leakage flux. It warns that in view of this care must be taken at the test stage to prevent thermal stresses arising that are beyond those occurring in service. It is suggested that the use of fiber-optic sensors within the winding should be considered to assist in avoiding such problems.

Oil samples for dissolved gases are taken at the usual stages throughout the testing, that is, before and after dielectric testing and before and after the temperature rise tests and load current runs.

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