Generation of high voltages 33 3 High voltage Low voltage (a) XX☒ (b) (c) Figure 2.12 Single unit testing transformers.(a)Diagram.(b c)different construction units.(1)Iron core.(2)Primary Lv.or exciting winding. (3)Secondary h.v.winding.(4)Field grading shield.(5)Grounded metal tank and base.(6)H.V.bushing.(7)Insulating shell or tank.(8)H.V.electrode the connection between testing transformer and test object.In Fig.2.12(c) the active part of the transformer is housed within an isolating cylinder'7' avoiding the use of the bushing.This construction reduces the height,although the heat transfer from inside to outside is aggravated.In both cases the vessels
Generation of high voltages 33 1 3 2 Low voltage High voltage (a) 8 6 4 3 2 1 5 (b) (c) 8 7 5 Figure 2.12 Single unit testing transformers. (a) Diagram. (b & c) different construction units. (1) Iron core. (2) Primary l.v. or exciting winding. (3) Secondary h.v. winding. (4) Field grading shield. (5) Grounded metal tank and base. (6) H.V. bushing. (7) Insulating shell or tank. (8) H.V. electrode the connection between testing transformer and test object. In Fig. 2.12(c) the active part of the transformer is housed within an isolating cylinder ‘7’ avoiding the use of the bushing. This construction reduces the height, although the heat transfer from inside to outside is aggravated. In both cases the vessels
34 High Voltage Engineering:Fundamentals would be filled with high-quality transformer oil,as most of the windings are oil-paper insulated. The sectional view of the windings shows the primary winding close to the iron core and surrounded by the h.v.winding '3'.This coaxial arrange- ment reduces the magnetic stray flux and increases,therefore,the coupling of both windings.The shape of the cross-sectional view of winding no.3 is a hint to the usual layout of this coil:the beginning(grounded end)of the h.v.winding is located at the side close to the core,and the end close to a sliced metal shield,which prevents too high field intensities at h.v.potential. Between both ends the single turns are arranged in layers,which are carefully insulated from each other by solid materials(kraft paper sheets for instance). Adjacent layers,therefore,form coaxial capacitors of high values,and if those capacitances are equal-produced by the reduced width of the single layers with increasing diameters-the potential distribution for transient voltages can be kept constant.By this procedure,the trapezoidal shape of the cross-section is originated. It may well be understood that the design of the h.v.winding becomes difficult if voltages of more than some 100kV must be produced within one coil.Better constructions are available by specialized techniques,mainly by cascading'transformers. The first step in this technique is to place two h.v.windings on one iron core,to join both windings in series and to connect this junction with the core.20)For illustration,the circuit diagram is shown in Fig.2.13 in combi- nation with a simplified cross-section of the active part.The arrangement could still be treated as a single unit transformer,as only one core exists. The mid-point of the h.v.winding is connected to the core and to a metal tank,if such a tank is used as a vessel.The cross-section shows that the primary winding '2'is,however,placed now around the first part'3a'of the whole h.t.winding,whose inner layer,which is at half-potential of the full output voltage,is connected to the core.There are two additional windings, '4a'and 4b',rated for low voltages,which act as compensating windings. These are placed close to the core and reduce the high leakage reactance between 3b'and the primary 2'.Often an exciting winding '5',again a winding rated for low voltages as the primary winding,is also available.This exciting winding is introduced here as it will be needed for the cascading of transformers.Note that this winding is at the full output potential of the transformer. Although no vessel is shown in which such a unit would be immersed,it can easily be understood that for metal tank construction (see Fig.2.12(b)) two h.v.bushings are now necessary.The tank itself must be insulated from earth for half-output voltage.This typical view for testing transformers can be seen in Fig.2.14.If,however,insulating tanks are employed,this internal layout may not necessarily be recognized from outside
34 High Voltage Engineering: Fundamentals would be filled with high-quality transformer oil, as most of the windings are oil-paper insulated. The sectional view of the windings shows the primary winding close to the iron core and surrounded by the h.v. winding ‘3’. This coaxial arrangement reduces the magnetic stray flux and increases, therefore, the coupling of both windings. The shape of the cross-sectional view of winding no. 3 is a hint to the usual layout of this coil: the beginning (grounded end) of the h.v. winding is located at the side close to the core, and the end close to a sliced metal shield, which prevents too high field intensities at h.v. potential. Between both ends the single turns are arranged in layers, which are carefully insulated from each other by solid materials (kraft paper sheets for instance). Adjacent layers, therefore, form coaxial capacitors of high values, and if those capacitances are equal – produced by the reduced width of the single layers with increasing diameters – the potential distribution for transient voltages can be kept constant. By this procedure, the trapezoidal shape of the cross-section is originated. It may well be understood that the design of the h.v. winding becomes difficult if voltages of more than some 100 kV must be produced within one coil. Better constructions are available by specialized techniques, mainly by ‘cascading’ transformers. The first step in this technique is to place two h.v. windings on one iron core, to join both windings in series and to connect this junction with the core.20 For illustration, the circuit diagram is shown in Fig. 2.13 in combination with a simplified cross-section of the active part. The arrangement could still be treated as a single unit transformer, as only one core exists. The mid-point of the h.v. winding is connected to the core and to a metal tank, if such a tank is used as a vessel. The cross-section shows that the primary winding ‘2’ is, however, placed now around the first part ‘3a’ of the whole h.t. winding, whose inner layer, which is at half-potential of the full output voltage, is connected to the core. There are two additional windings, ‘4a’ and ‘4b’, rated for low voltages, which act as compensating windings. These are placed close to the core and reduce the high leakage reactance between ‘3b’ and the primary ‘2’. Often an exciting winding ‘5’, again a winding rated for low voltages as the primary winding, is also available. This exciting winding is introduced here as it will be needed for the cascading of transformers. Note that this winding is at the full output potential of the transformer. Although no vessel is shown in which such a unit would be immersed, it can easily be understood that for metal tank construction (see Fig. 2.12(b)) two h.v. bushings are now necessary. The tank itself must be insulated from earth for half-output voltage. This typical view for testing transformers can be seen in Fig. 2.14. If, however, insulating tanks are employed, this internal layout may not necessarily be recognized from outside.
Generation of high voltages 35 High voltage 48 (a) 4a 4b 3b High voltage (b) Figure 2.13 Single unit testing transformer with mid-point potential at core:Diagram (a)and cross-section (b).(1)Iron core.(2)Primary winding. (3a b)High-voltage windings.(4a b)compensating windings. (5)Exciting winding Cascaded transformers For voltages higher than about 300 to 500kV,the cascading of transformers is a big advantage,as the weight of a whole testing set can be subdivided into single units and therefore transport and erection becomes easier.A review of earlier constructions is given in reference 4. A prerequisite to apply this technique is an exciting winding within each transformer unit as already shown in Fig.2.13.The cascading principle will be illustrated with the basic scheme shown in Fig.2.15.The 1.v.supply is connected to the primary winding 'I'of transformer I,designed for an h.v.output of V as are the other two transformers.The exciting winding
Generation of high voltages 35 5 1 2 (a) 3a 3b 5 High voltage 1 4a 4b 3a 3b 4a 4b 2 High voltage (b) Figure 2.13 Single unit testing transformer with mid-point potential at core: Diagram (a) and cross-section (b). (1) Iron core. (2) Primary winding. (3a & b) High-voltage windings. (4a & b) compensating windings. (5) Exciting winding Cascaded transformers For voltages higher than about 300 to 500 kV, the cascading of transformers is a big advantage, as the weight of a whole testing set can be subdivided into single units and therefore transport and erection becomes easier. A review of earlier constructions is given in reference 4. A prerequisite to apply this technique is an exciting winding within each transformer unit as already shown in Fig. 2.13. The cascading principle will be illustrated with the basic scheme shown in Fig. 2.15. The l.v. supply is connected to the primary winding ‘l’ of transformer I, designed for an h.v. output of V as are the other two transformers. The exciting winding
36 High Voltage Engineering:Fundamentals Figure 2.14 Testing transformer for 1200 kV r.m.s.comprising three single unit transformers according to Fig.2.13,with metallic tanks and bushings (High Voltage Laboratory,Technical University of Munich,Germany).(Note Suspended at ceiling and connected with transformer is a selenium-type rectifier with a reverse voltage of 3.4 MV,see ref.5.) '3'supplies the primary of the second transformer unit II;both windings are dimensioned for the same low voltage,and the potential is fixed to the high potential V.The h.v.or secondary windings '2'of both units are series connected,so that a voltage of 2V is produced hereby.The addition of the stage III needs no further explanation.The tanks or vessels containing the
36 High Voltage Engineering: Fundamentals Figure 2.14 Testing transformer for 1200 kV r.m.s. comprising three single unit transformers according to Fig. 2.13, with metallic tanks and bushings (High Voltage Laboratory, Technical University of Munich, Germany). (Note. Suspended at ceiling and connected with transformer is a selenium-type rectifier with a reverse voltage of 3.4 MV, see ref. 5.) ‘3’ supplies the primary of the second transformer unit II; both windings are dimensioned for the same low voltage, and the potential is fixed to the high potential V. The h.v. or secondary windings ‘2’ of both units are series connected, so that a voltage of 2 V is produced hereby. The addition of the stage III needs no further explanation. The tanks or vessels containing the
Generation of high voltages 37 Transf.Ill I=P/V Transf.Il 3- Transf.I 3V Figure 2.15 Basic circuit of cascaded transformers.(1)Primary windings. (2)Secondary h.t.windings.(3)Tertiary exciting windings active parts (core and windings)are indicated by dashed lines only.For a metal tank construction and the non-subdivided h.v.winding assumed in this basic scheme,the core and tank of each unit would be tapped to the l.v. terminal of each secondary winding as indicated.Then the tank of transformer I can be earthed;the tanks of transformers II and III are at high potentials, namely V and 2 V above earth,and must be suitably insulated.Through h.t. bushings the leads from the exciting coils '3'as well as the tappings of the h.v.windings are brought up to the next transformer.If the h.v.windings of each transformer are of mid-point potential type (see Fig.2.13),the tanks are at potentials of 0.5 V,1.5 V and 2.5 V respectively,as shown in Fig.2.14. Again,an insulating shell according to Fig.2.12 could avoid the h.t.bushings, rendering possible the stacking of the transformer units as shown in Fig.2.16 The disadvantage of transformer cascading is the heavy loading of primary windings for the lower stages.In Fig.2.15 this is indicated by the letter P,the product of current and voltage for each of the coils.For this three-stage cascade the output kVA rating would be 3P,and therefore each of the h.t.windings'2' would carry a current of I =P/V.Also,only the primary winding of trans- former III is loaded with P,but this power is drawn from the exciting winding
Generation of high voltages 37 Transf. III Transf. II Transf. I 3 3 3 P P P P 2P 2P V 2V 3V 3P 1 1 1 2 2 2 I = P/V Figure 2.15 Basic circuit of cascaded transformers. (1) Primary windings. (2) Secondary h.t. windings. (3) Tertiary exciting windings active parts (core and windings) are indicated by dashed lines only. For a metal tank construction and the non-subdivided h.v. winding assumed in this basic scheme, the core and tank of each unit would be tapped to the l.v. terminal of each secondary winding as indicated. Then the tank of transformer I can be earthed; the tanks of transformers II and III are at high potentials, namely V and 2 V above earth, and must be suitably insulated. Through h.t. bushings the leads from the exciting coils ‘3’ as well as the tappings of the h.v. windings are brought up to the next transformer. If the h.v. windings of each transformer are of mid-point potential type (see Fig. 2.13), the tanks are at potentials of 0.5 V, 1.5 V and 2.5 V respectively, as shown in Fig. 2.14. Again, an insulating shell according to Fig. 2.12 could avoid the h.t. bushings, rendering possible the stacking of the transformer units as shown in Fig. 2.16. The disadvantage of transformer cascading is the heavy loading of primary windings for the lower stages. In Fig. 2.15 this is indicated by the letter P, the product of current and voltage for each of the coils. For this three-stage cascade the output kVA rating would be 3P, and therefore each of the h.t. windings ‘2’ would carry a current of I D P/V. Also, only the primary winding of transformer III is loaded with P, but this power is drawn from the exciting winding