TRANSFORMER WITH PROTECTION DEVICE

Abstract: Transformers in electrical power distribution systems are subject to failure during lightning activity even when the high voltage terminations of the transformer (1) are protected by surge diverter devices (7, 8) coupled to ground. The mechanism responsible for these previously unexplained failures is described, and a solution to the problem is described which requires the provision of a selected transient voltage clamping device (10) coupling the neutral termination on the low voltage side (3) of the transformer to ground. Selection of the transient voltage clamping device is effected in dependence upon the withstand voltage of the low voltage windings (3) of the transformer and to ensure that it can withstand the effects of a ground fault at the high voltage side (2) of the transformer for at least the auto-reclose period of a circuit-breaker connected to the system.

Description: TRANSFORMERWITHPROTECTIONDEVICE Field of the Invention: This invention relates generally to improvements in or relating to electrical power distribution and more particularly concerns the protection of power distribution transformers, as are used in the UK and elsewhere for transforming the high voltages employed for power transmission to the low voltages utilized at domestic and industrial installations, against the adverse effects of lightning strikes. The invention is particularly, but not exclusively, concerned with pole mounted electrical power distribution transformers. Background of the Invention: There are upwards of 1 million power distribution transformers in the UK for transforming the llkV power distribution voltage to the 400V phase-to-phase domestic/industrial service voltage, and there are many more such transformers in continental Europe where typically the distribution voltage is 20kV. In addition to three phase transformers as are commonly used in industrial situations, there are single phase transformers for domestic or light industrial situations. The majority of such transformers are pole mounted. Each year in the UK there are many thousands of failures of pole mounted transformers during lightning activity. These have been attributed to lightning strikes either on or near to overhead high voltage power distribution lines that are coupled to the transformers or on the metal casings of the transformers themselves. To protect the transformer against such occurrences it has long been conventional to provide an earth (ground) connection to the transformer casing and to provide surge diverter devices connected between the high voltage terminations of the transformer and its grounded casing. As is well known, a surge diverter is a device such as an arc gap for example which is normally very high resistance but which provides a low resistance through path under high voltage. The mechanism whereby transformers can suffer damage by lightning strikes despite the provision of high voltage surge diverter protection has not been understood and has been the subject of much discussion over a considerable number of years. Transformers are relatively expensive devices and their failure gives rise to significant expense not only in replacement costs, but also arising out of the disruption that is caused to electricity users by the failure of their electricity supply. GB-A-390 102 describes a number of measures for protecting distribution transformers from lightning strikes. In accordance with one prior art proposal described in GB-A-390 102 a discharge gap having a breakdown voltage less than the breakdown strength of the transformer insulation is placed in the circuit to protect the transformer and drain off surge voltages, the arrangement for distribution transformers comprising the provision of a flashover point, in the form of stud type bushings, between the transformer winding and the transformer casing. For pole mounted equipment, this arrangement, which necessitates that the transformer casing be connected to ground, is said in GB-A-390 102 to be undesirable since it exposes workmen to hazard and does not provide for quenching of the arc when flashover does occur. In accordance with another prior art proposal, mentioned in GB-A-390 102, lightning arresters are connected to each electrical lead connected to the transformer, the lightning arresters being grounded in order to drain off surge potentials seeking to enter the transformer. The teaching of GB-A-390 102 is that this method of protection is inadequate unless the resistance of the ground connection is relatively low and the electrical length of the grounded conductor relatively short, since otherwise a high surge potential will be built up upon the winding resulting in insulation damage. The invention of GB-A-390 102 provides for high tension electrical discharge devices to be connected between each of the terminals on the high tension winding of the transformer and the transformer tank, and a low tension electrical discharge device to be connected between the tank and the grounded neutral of the low tension winding. Resistance elements are proposed to be provided in series with each discharge device for limiting follow current, and a self-quenching arc gap type discharge device combined with a resistor element is described. The high-tension discharge devices are said to have a breakdown voltage sufficiently high that an arc discharge will not be initiated by the customary dynamic voltage applied to the high tension line, and the low tension discharge device is said to have a cut-off voltage higher than the normal crest voltage of the low tension winding. For various reasons, some of which will be explained in the following, none of the measures described in GB-A-390 102 are utilized in present day distribution transformer installations. Summary of the Invention: We have investigated the problem of lightning induced transformer failure in collaboration with East Midlands Electricity PLC who, in common with other electricity companies in the UK, have suffered an unacceptable level of transformer failure. Our investigations have exonerated the transformer manufacturers from any responsibility and have led us to what we believe is the failure mechanism and to a relatively simple solution. A conventional distribution transformer installation in the UK will typically be pole mounted with a casing earth connected to the metal transformer casing and running down the pole to where it is connected to a number of metal ground rods which are knocked into the ground at the base of the pole. Each high voltage transformer termination, there being one such termination for each phase of a 3-phase transformer and there being two such terminations for a single phase transformer, has connected thereto a respective surge diverter or surge arrester device providing a path to ground via the transformer casing and the casing earth for lightning strike transients. On the low voltage side of the transformer there are corresponding terminations and there is additionally a neutral termination, and the neutral termination is connected to a neutral earth conductor which runs down the pole and is connected to a neutral ground constituted by one or more metal rods knocked into the ground. Conventional practice dictates that this neutral earth should be located between 3 and 5 metres distance from the transformer casing earth. We have found that the resistance to ground of the transformer casing earth is typically of the order of 10 ohms. In the event of a lightning strike on one or more of the high voltage power lines connected to the transformer, the associated surge diverter(s) operate(s) to protect the high voltage winding of the transformer. This results in, typically, a lOKAmp 8/20 μsecs current flowing through the surge diverter(s) to ground via the typically 10 ohms transformer casing earth. An 8/20 μsecs transient current is one which peaks after 8 μsecs and decreases to half its peak value in 20 μsecs. This 10KA transient flowing through the 10 ohm transformer casing earth resistance results in the transformer casing transiently rising to a voltage of lOOkV, and this high voltage transient appears between the transformer casing and the low voltage winding of the transformer where it is connected to the neutral earth. Now, the high voltage windings of an llkV transformer will typically have an impulse voltage withstand of 75kV or 95kV, whereas the low voltage windings typically have only a 25kV or 30kV impulse voltage withstand, this being a feature of the transformer construction. The application of a lOOkV transient to the transformer casing thus, we have found, can result in breakdown of the insulation of the transformer low voltage winding at its closest point to the casing with the result that a high transient current flows from the transformer casing to and through the low voltage winding and to ground via the neutral ground conductor. Our investigations have established that this mechanism is responsible for the failure of transformers during lightning activity, namely lightning currents passing to the transformer casing through surge diverters connected to the high voltage terminations of the transformer can give rise to surge voltages at the transformer casing which exceed the voltage withstand level of the low voltage winding. Additionally, the imposition of the lOOkV transient upon the low voltage winding of the transformer can result in much higher transient voltages on the high tension windings as a result of the transformer action and this too can cause damage. Having ascertained that the above-described mechanism is responsible for transformer failures during lightning activity, the solution to the problem of how to avoid such failures clearly is to provide overvoltage protection for the low voltage winding and this can be achieved by provision of a transient voltage clamping device, for example a suitably high voltage rated surge arrester device as described in GB-B-2 188 199, connected between the low voltage neutral and the transformer casing. The surge arrester device must have a high voltage rating because the use of a surge arrester with a continuous operating voltage equal to or not substantially greater than the low voltage supply, this being the teaching of GB-A-390 102, would not be satisfactory as premature failure of the arrester would occur in the event of a phase-to-earth fault arising on the high voltage side of the transformer. Electric power distribution systems incorporate auto-reclosing circuit breaker devices which, in response to a sensed ground (earth) fault current on a power line caused for example by a squirrel shorting out a termination on the high voltage side of a transformer, will initially break the respective circuit for a period of typically 10 seconds (dead time) and will then automatically re-close and re¬ energize the circuit. Should the circuit still have a permanent earth fault on it the protection may take up to 10 seconds before opening the circuit breaker. The transient voltage clamping device that is provided for protection of the low voltage winding must be able to withstand, without failure, the voltage to which it would be subjected in such an event. Accordingly, in order for a transient voltage clamping device coupled between the low voltage neutral termination of the transformer winding and the transformer casing to be capable of providing the requisite protection, the device must fulfil the following requirements, namely: (a) it must be able to withstand, for the re-make period of the auto-reclosing circuit breaker device, the voltage to which it will be subjected when a ground fault condition exists on the high voltage side of the transformer; and (b) it must be capable of clamping the voltage at the transformer casing relative to the neutral termination of the low voltage transformer winding to below the withstand voltage of the low voltage winding in the event of a lightning strike on one or more of the high voltage lines connected to the transformer. For an llkV power distribution system as used in the UK, these requirements would be satisfied by a transient voltage clamping device able to withstand without failure a voltage of 7kV for 10 seconds and able to clamp the voltage at the neutral termination of the low voltage winding of the transformer to not greater than 20kV for lightning currents of 10KA. A standard 6kV rated metal oxide surge arrester as currently manufactured by Bowthorpe EMP Limited of Stevenson Road, Brighton, Sussex, England as Type No. EGA6 satisfies these requirements. The features of the present invention for which an exclusive property or privilege are claimed are set forth in the appended claims. The invention will be further explained in the following by reference to the accompanying drawings. Description of the Drawings: Figure 1 is a schematic single phase diagram showing a prior art transformer installation; and Figure 2 shows how the transformer installation according to the present invention differs from the prior art installation. Detailed Description of the Invention: Referring first to Figure l, this represents a single phase of a typical -phase transformer installation as presently employed in the UK for transforming the llkV power transmission voltage to a phase-to-phase service voltage of 400V. Other phases of the installation have an equivalent structure. The three phase windings on the high voltage side of the transformer will conventionally be connected in delta configuration, and the three phase windings on the low voltage side will conventionally be star connected with the neutral point grounded. The transformer 1 consists of high voltage and low voltage windings 2 and 3, respectively, upon a core structure 4, these being contained in an oil-filled or otherwise insulated metal casing or tank 5 which carries shedded insulating terminations 6 for the windings. Transformers of this kind are manufactured to British Electrical Supply Standards Institute Standard 35-1 and are available from South Wales Transformers, Treforest, Mid Glamorgan, Wales for example. High voltage surge arresters or diverters 7 and 8 are conventionally coupled between the high voltage terminations of the transformer and the transformer casing and may for example comprise duplex arc gaps. In the field, the transformer casing is grounded as aforementioned and a ground resistance R c . e of the order of 10 ohms is typical. It is conventional furthermore to provide a separate ground for the neutral termination of the low voltage transformer winding and this gives rise to a resistance R,,^ of the same order of magnitude as R c< . In the event of a lightning strike on a power line connected to the high voltage side of the transformer, the surge arrester 7 for example will operate to pass the lightning transient to ground. A current typically of 10KA flows through the surge arrester 7 for a period of 8 to 20 μsecs and, given the ground resistance R c.c of 10 ohms, the transformer casing transiently is elevated to a voltage of lOOkV. This lOOkV casing voltage appears between the transformer casing and the grounded low voltage winding 3 of the transformer and, as mentioned hereinbefore, exceeds the typically 25kV withstand voltage of the low voltage windings. Transformer failure then results as a flashover occurs between the transformer casing and the low voltage winding. The high voltage winding of the transformer can also be damaged as the step-up ratio of the transformer causes a much higher voltage transient to be imposed on the high voltage windings as a result of the lOOkV transient on the low voltage winding. Figure 2, which otherwise is the same as Figure 1, shows the provision of a transient voltage clamping device constituted by a high voltage surge arrester 10 coupled between the neutral termination of the low voltage winding 3 of the transformer and the transformer casing 5. In the event of a lightning strike, as described above, giving rise to a high voltage transient between the transformer casing and the neutral termination of the low voltage transformer winding, the high voltage surge arrester 10 will provide a preferred discharge path as compared for example to a path from the transformer casing through the insulating medium or over the low voltage terminations of the transformer to the grounded low voltage winding. Provision of the high voltage surge arrester 10 thus protects the low voltage side of the transformer. As previously explained, the high voltage surge arrester 10 must clamp the transformer casing to a voltage not exceeding the voltage withstand of the low voltage winding. Typically, the low voltage winding of an E.S.I. Standard 35-1 distribution class transformer has a voltage withstanding rating of 25kV, which means that the surge arrester 10 should preferably be selected to clamp the transformer casing to no greater than say 20kV under lightning strike conditions. A further consideration, as previously explained, is that the arrester 10 should be able to withstand without failure a ground fault condition on the high voltage side of the transformer for a predetermined time period, eg. 10 seconds. Such a ground fault condition, caused for example by shorting out of one of the transformer high voltage terminations to the transformer casing, puts the high tension line voltage onto the transformer casing and gives rise to a corresponding voltage across the surge arrester 10. Circuit breakers in the distribution system will not permit this condition to prevail for more than a predetermined time period, 10 seconds in the case of the llkV power distribution system utilized in the UK, and the surge arrester 10 must be able to withstand the corresponding voltage loading of 7kV (llkV ÷ J ) for this period. A standard 6kV rated surge arrester Type No. EGA6 manufactured by Bowthorpe EMP Limited of Brighton, Sussex, England in accordance with the teachings of GB-B-2 188 199 meets these requirements. It is preferred that in installations such as that represented by Figure 2, the surge diverters 7 and 8 are constituted by metal oxide surge arresters such as are described in GB-B-2 188 199 for example. Furthermore, spark gaps may be provided in parallel with the high voltage surge arresters 7, 8 for the purpose of ensuring that a path to ground remains for fault current in the event of surge arrester failure. The provision of such spark gaps has substantially no affect upon the present invention. In areas which are highly susceptible to electrical storms, additional protection is obtainable by installation of low voltage surge arresters on the low voltage side of the transformer on the next pole downstream of the transformer. In the practice of the present invention, it is necessary to ensure that the values of the ground path resistances R c.e and R^ are kept low. Higher resistance values will exacerbate the stressing of the transformer windings. Having thus described the present invention by reference to specific arrangements, it is to be well understood that modifications and variations to the described arrangements are possible without departure from the spirit and scope of the invention as set forth in the appended claims. For example, the Bowthorpe EMP Limited surge arrester Type No. EGA6 may be considered to be "over engineered" for the application of the present invention and an alternative, more simple surge arrester configuration may be preferred for the surge arrester 10 shown in Figure 2. Furthermore, whilst it is preferred that the surge arresters 7 and 8 shown in Figure 2 are of the metal oxide varistor type exemplified by the surge arrester described in GB-B-2 188 199, they could simply be constituted by arc gap devices, for example duplex gaps.

Claims: CLAIMS : 1. A transformer in or for an electrical power distribution system for transforming the high distribution voltage of the system into a low service voltage, the transformer having a ground connection and the high voltage side of the transformer including a surge diverter connected between each high voltage termination of the transformer and said ground connection, and the transformer having a neutral termination on its low voltage side and being provided with a transient voltage clamping device coupled between said neutral termination and said ground connection, said transient voltage clamping device being selected so as to be capable of clamping the voltage at said neutral termination to below the withstand voltage of the low voltage transformer winding when, in use of the transformer, a lightning strike on the high voltage side of the transformer causes one or more of said surge diverters to operate so as to discharge the lightning strike to ground and thereby subject the transformer ground connection to a transient high voltage. 2. A transformer as claimed in claim l wherein the withstand voltage of the low voltage transformer winding is of the order of 25kV, and the transient voltage clamping device that is coupled between said neutral termination and said ground connection is adapted to clamp the voltage at said neutral termination to a voltage of the order of 20kV. 3. A transformer as claimed in claim 1 or 2 wherein the transient voltage clamping device that is coupled between said neutral termination and said ground connection is selected so as to be capable of withstanding for a predetermined time period the voltage to which it would be subjected under a ground fault condition on the high voltage side of the transformer. 4. A transformer as claimed in claim 3 wherein the high distribution voltage of the system is of the order of llkV and the transient voltage clamping device that is coupled between said neutral termination and said ground connection is selected so as to be capable of withstanding a voltage of the order of 7kV. 5. A transformer as claimed in claim 3 or 4 wherein said predetermined time period is about 10 seconds. 6. A single-phase transformer as claimed in any of the preceding claims. 7. A three-phase transformer as claimed in any of claims 1 to 5. 8. A transformer as claimed in any of the preceding claims wherein said neutral terminal is connected to ground potential. 9. A transformer as claimed in claim 9 wherein the connection of said neutral terminal to ground is by way of a conductor separate from the connection of said ground connection of the transformer to ground. 10. A transformer as claimed in any of the preceding claims wherein said surge diverters comprise arc gap devices. 11. A transformer as claimed in any of claims 1 to 9 wherein said surge diverters comprise metal oxide varistor surge arresters. 12. A transformer as claimed in any of the preceding claims wherein said transient voltage clamping device comprises a metal oxide varistor surge arrester. 13. An electrical power distribution system comprising a pole mounted transformer according to any of the preceding claims and wherein low voltage surge arresters are provided on a next adjacent pole downstream of the low voltage side of the transformer, said low voltage surge arresters coupling the low voltage lines to ground. 14. A method of protecting a distribution transformer against the effect of a lightning strike, said method comprising connecting a transient voltage clamping device between the neutral of the low voltage winding of the transformer and the grounded transformer casing, said transient voltage clamping device being selected (a) to limit the transient voltage at the transformer casing to below the withstand voltage of the low voltage transformer winding in the event of a lightning strike and (b) to be capable of withstanding for a predetermined time period a ground fault condition at the high voltage side of the transformer. 15. A transformer in or for an electrical power distribution system for transforming the high distribution voltage of the system into a low service voltage, the transformer having a ground connection and the high voltage side of the transformer including a surge arrester connected between each high voltage termination of the transformer and said ground connection, and the transformer having a neutral termination on its low voltage side and being provided with a transient voltage clamping device coupled between said neutral termination and said ground connection.

Inventors: DOONE RODNEY MEREDITH, WILLIAMS STUART, DOONE, RODNEY, MEREDITH, WILLIAMS, STUART
Applicants: BOWTHORPE IND LTD, DOONE RODNEY MEREDITH, WILLIAMS STUART, BOWTHORPE INDUSTRIES LIMITED, DOONE, RODNEY, MEREDITH, WILLIAMS, STUART
Application year: 1997
Main classification: H02H7
IPC codes: H02H7