ABSTRACT

A design feasibility study has been conducted for a 240 MVA, high temperature superconducting grid auto-transformer. Conclusions are relevant to superconducting power transformers in general. Economic benefits may be achieved subject to assumptions about achievable conductor properties, cost of components and power system operating requirements. Liquid nitrogen cryogenics is relatively cheap and simple and refrigeration power demand is reduced by a factor of the order of 20 compared to the low temperature case (LTS). Attention is drawn to the importance of AC losses in the superconductor and the difficulty of keeping these sufficiently low. Various technical problem areas and their likely influence on the overall design concept are reviewed. Three particularly important influences are identified: insulating properties of nitrogen coolant, required transformer performance in the through-fault conditions and mechanical strength to withstand electromagnetic forces. Typical conductor design and winding configuration result in an efficiency of the superconducting transformer of 99.85 % .The savings in the cost of losses over the life of transformer provide the economic-feasibility of this design approach.

INTRODUCTION:

From the moment of discovery of the high magnetic field , high current density superconductors, engineers have been challenged to incorporate this " loss-less" conductor in power equipment . These attempts have met with some success in the case of DC applications such as in DC field windings of generators, because of the loss behavior of these materials in time varying magnetic fields ,AC applications have been hindered . One such application is in power transformers . Work from 1961 to 1977 generally indicated that superconducting transformers were not feasible .

It was earlier examined what was necessary to make a superconducting transformer viable and concluded that to minimize flux jumps, the magnetic field intensity at the windings must be kept low ; therefore inter leaving the high voltage windings was proposed and the iron core should be operated large volume for the device . After that 570 MVA transformer made with superconducting tapes and interleaved windings was studied .This effort was followed by examining a number of configurations including the elimination of iron. These approaches proved to higher loss. Later it was rechecked the question of whether interleaving was necessary. Then 600 MVA superconducting transformer made up of Nb₃Sn tapes was examined. A unique concept, later examined called for a set of superconducting windings enclosed by a set of 77 K auxiliary copper windings. This concept also proved uneconomical because of large volume .In 1977 superconducting transformers using conductors operating in the low AC loss "Meissner" region .This design proved uneconomical because of the overall transformer volume .

These failures to devise viable techniques for applying superconductors to transformers were due to large volume configurations, and high AC losses due to large AC magnetic fields or large superconductor volumes.

In 1981, a 1000 MVA ,500/22 KV line-to-line ,1300 BIL low temperature superconducting power transformer (LTS) which uses superconducting windings . The design uses a multiple winding configuration which is current limiting under fault conditions .This conductor design and winding configuration resulted in an efficiency of superconducting transformer of 99.85 % . The savings in the cost of losses over the life of the transformer provide the economic feasibility of this design approach.

The advent of high temperature superconducting (HTS) materials has renewed interest in the possibilities for superconducting power apparatus offering real economic benefits, within power ratings typical of present system practice. Previously developed low temperature superconductors (LTS) require cooling by liquid helium about 4.2K, with advanced cryogenic technology that is expensive both in terms of cost and refrigeration power expended per unit of heat power removed from the cryostat. The technology for the new materials, which is based on liquid nitrogen (LN₂) at temperatures up-to about 79K, is simpler and cheaper. The ratio of refrigeration power used to heat removed is also reduced from around 500 to about 25.

PROPERTIES OF HTS MATERIALS:

The most promising known HTS materials for high current applications are given below:

Yttrium compounds ( YBCO)

Y₁Ba₂Cu₃O_7-x T_c=92

Y₂Ba₄Cu₇O_15-y T_c=95K

Bismuth compounds (BISCCO)

Bi₂Sr₂Ca₁Cu₂O_y T_c=80K

Bi_2-xPbxSr₂Ca₂Cu₃O_y T_c=110K

Thallium compounds

(TlPb)₁Sr₂Ca₂Cu₃O₉ T_c=120K

Tl₂Ba₂Ca₂Cu₃O₁₀ T_c=125K

Mercury compounds

Hg₁Ba₂Ca₂Cu₃O₁₀ T_c=153K

Since the discovery of HTS , the development of the processing technology has resulted in a steady increase in the critical current density in short and long lengths of tapes and wires based on BISCCO and YBCO superconductors . Critical currents in the region of 400-to-800A/mm² and upto 200A/mm² at 77K and zero fields have been obtained in short KM lengths of Bismuth tapes, respectively. For use at lower temperatures and very high fields, Bismuth tapes have shown to be able to carry in excess of 20000A/mm² at 4.2K in a field of 20T.

Recent developments at Los Alamos and Oak Ridge on the processing of YBCO tapes (IBAD and RABIT techniques) have demonstrated the potential of this superconductor achieving critical currents of the order of 1000A/mm² in short lengths of tapes. Twisted multifilamentry tapes are also being developed by several manufactures to reduce the hysteretic losses in AC power applications.

Specification for required HTS :

The critical current density is 80A/mm² RMS (referred to the total conductor cross section), achievable at a maximum temperature of 79K in a magnetic field of 0.8T (parallel to the tape surface), with acceptable AC conductor losses. These form the basis of the characteristics that are specified in the design. It is to be noted that the working RMS current density in a conventional transformer winding is around 3A/mm² and about 12A/mm² in direct water cooled winding.

However, the important advantage of superconductivity for a power transformer lies not so much with the increase in working current density as with the saving of ohmic losses.

AC loss characteristics:

One of the most important issues concerning power application of superconductors is the AC loss at 50Hz and 60Hz. Although the resistive loss is virtually zero in superconductors, hysteretic loss arises in an alternating magnetic field. In real application, the losses are produced by both the self-field as in a cable and the external field as in a coil. Compared with LTS, the operation of HTS at LN₂ temperatures means a great reduction in the refrigeration cost and a significant improvement in the cryogenic stability for the device. Nevertheless, the problem of losses in the superconducting devices remains the dominating factor for the viability and the success of the HTS in power engineering application.

The calculation of AC losses for the design assumes two components: the hysteretic loss in 5µm thick superconducting sheets, and the coupling losses due to currents in the silver matrix. For this design, using the above conductor specification, the hysteretic and coupling current losses are of the same order. Above Fig. shows the AC loss in the superconducting core of a particular tape as a function of the peak AC field, with the field dependence of the critical current taken into consideration. It can be seen that the loss increases with B₀³ at fields below 10mT, where the tape is fully penetrated. Above this field, the loss increases linearly with B₀.

Design aspects of HTS transformer:

Overall concept and principle parameters:

The principal features of the design are the removable of the copper windings and their replacement by HTS equivalents. These are only a fraction (less than 10%) of the bulk of conventional windings. The inevitable result is windings of reduced mechanical strength, which will stand neither the radial bursting forces nor the axial compressive forces that occur during through- fault conditions without special strengthening structural features. The tap winding is kept outside the cryostat to avoid the heat which could otherwise flow into liquid nitrogen through several connections.

With bulk of ohmic losses (in common and series windings ) removed , it is possible to cope with the core loss and remaining ohmic loss(in the tap windings) by forced gas cooling, leading to an oil less design of transformer. This has a great advantage of reducing fire risk and environmental hazard from oil spillage. The superconducting winding have small thermal mass and are cryogenically stable (i.e. capable of returning to normal operation after a period of abnormal heating without disconnecting and cooling down) over only a small range of temperature rise. In consequence, the HTS design has very little capability to recover from a through-fault condition without disconnection, in contrast to a conventional transformer. This is the chief weakness of the HTS design.

Note, however, that the HTS design can survive a through fault, although it subsequently needs disconnecting for a period, and there is also good overload capability.

To ensure efficient cooling, it is necessary to have direct contact between the LN₂ coolant and the conductor. However, this contact is only required over a small percentage of the total surface area and take place on a part of the edges of the conductor strip forms the winding discs. This requirement has implications for the manner in which the inter-turn insulation is arranged which must leave the edges bare.

Balance of electric and magnetic loading (i.e. total ampere turns v/s core flux) is one fundamental freedom in the design process. With a superconducting transformer, a natural tendency would be to increase electric loading as far as possible, since it is lossless and economic in space requirement, and correspondingly reduce magnetic loading. Thus, the tendency would be to minimize radial clearances between windings, consistent with the necessary voltage withstand capability , in order to minimize the permeance of leakage flux paths, and so permit the greatest electric loading consistent with the reactance level.

However, in basic HTS design, another consideration takes precedence. This is the capability to withstand mechanical forces, which are proportional to the square of electric loading and also tend to increase as size is reduced. The overall consequence is that the total ampere turns have been kept the same as in a typical conventional design. The radial space between common and series windings has been deliberately increased beyond the minimum demanded by voltage considerations, in order to adjust the leakage reactance.

The dielectric strength of LN₂ is very good, but that of gaseous nitrogen is less. In HTS design, the nitrogen in the cryostat is in the near saturated state and boiling may occur with the release of gas bubbles into the liquid.

The design features of the HTS transformer are described below.

Core:

This is similar to a conventional design, but with leg length and window spans reduced and consequent saving in core weight and overall size.

Tap change arrangements:

The copper tap windings are retained, adjacent to the core leg, outside the cryostat connected to a tap-changer, both of conventional design.

Cooling of core and tap windings:

Forced-convection gas cooling in which nitrogen is used. Gas is forced through axial ducts (parallel with legs) to cool the core legs and tap windings. Top and bottom yokes are fitted with fiberglass cowling to contain fanned gas and direct it over yoke.

Common and series windings:

Windings are constructed of rectangular section composite conductor, probably comprising about 33% superconducting fibers and 67% matrix metal (silver or silver alloy).The inter-turn insulating tape is applied to one face of the conductor. Individual tapes making up the conductor will be transposed in a normal way. Discs at the high voltage ends of the series windings are interleaved in pairs.

Winding reinforcement:

The outer diameter of the series (high voltage) winding is reinforced with fiberglass hoops (possibly pre-stressed) or a continuous cylinder. A substantial inner cylinder with multiple spacing sticks supports the inner diameter of the common winding.There are annular clamping plates at the top and bottom of the complete in-cryostat winding assembly, pulled together through bolts or studs, all constructed of insulating material.

Cryostat:

One cryostat per leg is used, with cryostat comprising a vacuum vessel constructed double-skinned fiberglass, with the vacuum continuously pumped. Cryostat pressure will be slightly in excess of 1 bar, containing nitrogen at 77K. The intermediate-voltage leads pass through the top lead. The high voltage lead passes centrally through the cryostat wall.

Fault and overload capability:

For any substantial through fault, disconnection is required and a period of minutes is needed before reconnecting. The transformer can survive the most severe through-fault for about 170 ms, within which time disconnection must be achieved. Internal faults are sensed by in-built monitors or terminal voltage and current sensors, which initiate disconnection. Over load capability is expected.

Housing:

Oil-less design obviates the need for a tank, which is replaced by a steel structure carrying the load exerted by bushings. External housing is required for weather-proofing and a gas-seal for the forced-convection nitrogen coolant and acoustic noise reduction.

The principal parameters of the transformer design are listed below.

kVA: 240 000

Normal volts: 400/132kV

Tappings: 132kV+15%-5% in steps (+/-neutral end)

Line currents at normal volts: 346/1054A

Diagram no: Yy0 Auto

Guaranteed reactance: 20%

Rated current densities: series windings=39.6A/mm², common

Winding=36.9A/mm²(39.6A/mm², minimum tap setting)

Tap winding=3.0 A/mm²(conventional)

Between the tapes there are thin insulation strips. The edges are partially exposed to the coolant. The area of the Superconducting exposed to the coolant is 17% of the total edge area for the common winding and 33% for the series winding (limits for both values are due to the required mechanical strength) both windings are supported by uniformly spaced axial sticks covering approximately 25% of the inner surface area.

Thermodynamic stability:

HTS material is inherently much more stable against local temporary effects than LTS material; higher specific heat makes it far less sensitive to transient temperature excursion; and the size of the region that is adiabatically stable is much greater. If, due to malfunction, quenching occurs during normal operation, there is ample time (over 15s) to disconnect the transformer before irreversible damage is caused. However, thermal capacity of the windings is much lower than in a conventional transformer.

Dielectric breakdown:

By using LN₂ as a coolant it automatically becomes a part of insulation system. Further, because this coolant can be in intimate contact with the conductor, it becomes the principal insulant for the main windings. The choice of gas cooling for the core, tap winding and connection leads means that the conductor insulation in the area must rely on solid material. It is proposed that the gas is dry nitrogen.

As a gas, nitrogen obeys Paschen’s law and increases in break-down strength as the temperature is decreased towards its liquid point. For gaps of 10mm and over, gaseous nitrogen has breakdown strength of 6.4kV/mm RMS at 50 Hz AC, with temperature of 77K and pressure of 1 bar. The voltage withstands properties LN₂ mixed with nitrogen gas bubbles may be assumed to be determined by the properties of the gas. It is concluded that all insulating parts of the transformer solid , liquid or gas have a breakdown strength equal to or greater than 6.4kV/mm RMS AC .

Through-fault condition:

Through-fault condition poses a serious problem. The basic design will recover ( without disconnection ) from a through fault of 2 p.u. current plus maximum offset transient , provided that the fault is cleared within 64ms ; this is just on the edge of possibility for fast acting protection in favorable circumstances.

Table:

Recovery time and survival time, as a function of fault level

Higher fault level call for disconnection of the transformer, and will then need to remain disconnected for substantial fraction of one hour in order to cool down ready for resumption of normal working. This disconnection must be achieved in a certain maximum time, if the transformer is to survive the fault and be fit for reconnection in due course. With highest possible fault level (5 p.u.current) , the survival time in the fault condition is about 170ms . These times could be improved using sub-cooled LN₂.

Electromagnetic Forces:

It is well known that the forces acting on transformer windings in the through-fault condition are very high and can cause structural failure. There are three principal forces to consider: an axial compressive force S_c; an axial displacement force S_d which, if there is an initial axial displacement between the center of symmetry of two windings, acts to increase that displacement; and a radial force S_w, which is a bursting force on the outer winding (that must be withstood by tensile hoop force S_h , where S_w = 2S_h), and is a buckling force on the inner winding .

S_c tends to produce axial compressive failure of the winding structure, with the individual conductor sections of the winding disc yielding and zigzagging out of their axial/radial alignment. S_d can overpower the clamping arrangements that hold the primary and secondary windings in axial relation to each other, and in extreme cases can lead to progressive collapse. It is modern to construct the transformer with sufficient attention to symmetry to ensure that the initial displacement is minimized so that S_d is not a problem .S_w can cause the outer winding structure to yield in hoop tension and the inner winding to deform into a clover -leaf pattern with the winding collapsing radially inwards between adjacent axial spacing sticks.

Therefore in this design with increased current density in the superconductor ,the axial forces S_c and S_d have to be sustained on a much smaller area of the windings.

Cryostat and cooling:

To provide the required working temperature, three independent toroidal cryostats are designed to contain the three phases. The cryostat must be non metallic because of eddy currents that will otherwise be induced in it by surrounding magnetic fields. Total heat leaking from the room temperature environment should be kept to a fraction of winding losses. Owing to the limitation imposed on thermal insulation thickness by window dimensions, solid insulation is ruled out by the excessive leak losses through the wall. Another source of heat leak is the conductor entry ports (or thermal bushings), which connects the low temperature superconducting windings with the room temperature leads. One HV port and three LV ports per cryostat are needed. For the high voltage connection, cable technology will be used to design a link through the cryostat wall to the bushing at a point half-way up the cryostat, As shown in fig.(this is also convenient because the series winding is formed of two parallel circuits which can meet at the centre of the winding height).This will leave the top clear for the other connections to exit the cryostat and for the tap winding connections to be brought out of the tap changer with the large clearances necessary to satisfactorily insulate them in dry nitrogen gas.

Protection requirements:

The normal main protection would be required substantially without change, but there are three important aspects that require special consideration. First, the HTS transformer does not have conventional capability to withstand any through-fault for upto 3s. Means must therefore be provided to initiate disconnection when through-faults exceed a permissible level of severity, which is a combined function of current magnitude and time. A suitable form of function can be built into a simple intelligent controller, which is continuously sampling the currents in all three phases and which triggers disconnection if the maximum permissible integrated value of the function is exceeded.

Secondly, the possibility will always exist of a local part of the winding quenching due to unforeseen circumstances, such as local structural looseness leading to vibration and over heating. An acoustic sensor is also employed to detect noise in the LN₂.

Finally, steps must be taken to reduce in-rush current to acceptable levels, e.g. by synchronized switching or by using external resistors or fault current limiters.

Loss analysis:

Total loss of conventional design=100%

Comparative assessment of HTS and conventional transformer:

Above table summarizes the total losses of the HTS transformer design and compares them with the conventional ‘reference’ design. Losses are expressed in percentage form, with the total loss in conventional design taken as 100%. Table below shows the significant global features, covering size and construction as well as performance. On the construction side, the main points of interest are the reduction in core size and weight for the superconducting transformer, the elimination of a large weight of copper, and oil less cooling. It is interesting to observe that the reduction of winding size enables the overall size of the transformer to be reduced, despite needing room for the cryostats around each core leg. Thus the design concept has the effect of easing transport problems and makes a significant contribution to the reduction of environmental risks and fire hazards.

Table: Comparison of technical features

On the performance side, conductor current density is greatly increased and total rated power loss is significantly reduced. Overload capability is somewhat improved. Tolerance of through fault is severely restricted and is has been seen that the medium term prospects for overcoming this without onerous increases in winding AC losses and conductor cost are, at present, speculation. The grid transformers are installed in parallel pairs, in order to improve system security; there may be an argument for allowing one of the pair to be superconducting. This is on the basis that, if one of the transformers has to be disconnected after a through-fault for up to 1 hr before it can be reconnected, the statistical reduction in system security may be tolerable. Table below shows estimates of savings/expenditure components based on continuous operation in rated conditions.

It is clear that expenditure on extra equipments and materials are offset by the enormous value of the saved losses (taken over a 10 year period and discounted at 9.5% per year). However because of the redundancy built into the system for the security, the load factor of a grid transformer is remarkably low and may be taken as 0.225 average and 0.26 RMS. For such load conditions, a pessimistic estimate suggests that the total equivalent first cost saving may now become a net increase of first cost equivalent expenditure of about 20%.

Table: Cost savings on continuous full load

All values referred to the overall first cost of the conventional transformer , which is taken as 100%.

On the other hand, current UK practice is to have 2 transformers fully rated, normally connected in parallel. It is thus worth considering an arrangement of an HTS transformer normally connecter in parallel with a conventional transformer normally disconnected but capable of being switched on quickly when required (e.g. during a through-fault). Hence saving on the losses of the conventional transformer will be very significant.

EXISTING WORK ON SUPERCONDUCTING TRANSFORMERS:

This superconducting technology in power transformers has encouraged several groups to build large transformers with HTS windings. ABB has built three phase 630 kVA 18.7/.42 kV transformer using BiSCCO windings cooled and insulated by liquid nitrogen at 77 K, and have connected it to the grid. A Japanese team has built a 500 kVA 6.6/3.3 kV single phase transformer also using BiSCCO windings cooled and insulated by liquid nitrogen at 77 K and at 66 K. an American team led by Waukesha Electric Systems has taken a different approach and is building a single phase 1 MVA 13.8/6.9 kV transformer, using the cheaper BiSCCO windings and cooling them to 20-35 K using a cryocooler.

Recently a step up and step down coreless superconducting transformer have also been developed in Japan.

It has been found that the coreless superconducting transformer has the potential to offer many advantages, namely reduced power loss; weight reduction; application as a fault current limiter; no necessity of tertiary winding at the receiving side of the transformer and for the substitution of the shunt reactor if considered in the total power system. All these advantages reduce the total life cycle cost compared to that of conventional transformer.

CONCLUSION:

An HTS transformer will be substantially cheaper in first cost, less complex and more reliable in operation than an LTS transformer. The requirement for supply power to the refrigeration plant is much lower per KW of removed heat and cost saving achieved by eliminating ohmic losses from the windings is not substantially re-expended as refrigerator supply power.

The most difficult technical design problem for a HTS transformer concerns the through-fault conditions. This design assuming superconductor properties cannot recover without disconnection after a through-fault of even medium size; it can, however, survive a maximum through-fault, as long as disconnection is achieved in 170 ms. For grid transformers, it is worth considering an operating mode in which HTS and conventional transformers work in parallel pairs; with the HTS transformer normally connected and other normally disconnected but capable of being quickly connected when required. This would replace the present UK arrangement of parallel pairs of conventional transformers (each fully rated), both being normally connected. The proposal would avoid the need for recovery capability in the HTS transformer and also maximize savings on the power loss in normal operation.

There are advantages for a grid transformer in having a conventional design of tap winding, outside the cryostat because of the constructional complexities and total heat leak associated with an in-cryostat design. With oil-less design, the core losses are removed by gas cooling.

A coreless superconducting transformer has also been developed which incorporates many advantages which reduces the overall total cost.

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