Chapter 2 Generation of high voltages A fundamental knowledge about generators and circuits which are in use for the generation of high voltages belongs to the background of work on h.v. technology. Generally commercially available h.v.generators are applied in routine testing laboratories;they are used for testing equipment such as transformers, bushings,cables,capacitors,switchgear,etc.The tests should confirm the effi- ciency and reliability of the products and therefore the h.v.testing equipment is required to study the insulation behaviour under all conditions which the apparatus is likely to encounter.The amplitudes and types of the test voltages, which are always higher than the normal or rated voltages of the apparatus under test,are in general prescribed by national or international standards or recommendations,and therefore there is not much freedom in the selection of the h.v.testing equipment.Quite often,however,routine testing laboratories are also used for the development of new products.Then even higher volt- ages might be necessary to determine the factor of safety over the prospective working conditions and to ensure that the working margin is neither too high nor too low.Most of the h.v.generator circuits can be changed to increase the output voltage levels,if the original circuit was properly designed.There- fore,even the selection of routine testing equipment should always consider a future extension of the testing capabilities. The work carried out in research laboratories varies considerably from one establishment to another,and the type of equipment needed varies accordingly. As there are always some interactions between the h.v.generating circuits used and the test results,the layout of these circuits has to be done very carefully. The classes of tests may differ from the routine tests,and therefore specially designed circuits are often necessary for such laboratories.The knowledge about some fundamental circuits treated in this chapter will also support the development of new test circuits. Finally,high voltages are used in many branches of natural sciences or other technical applications.The generating circuits are often the same or similar to those treated in the following sections.It is not the aim,however,of this introductory text to treat the broad variations of possible circuits,due to space limitation.Not taken into account are also the differing problems of electrical power generation and transmission with high voltages of a.c.or d.c.,or the
Chapter 2 Generation of high voltages A fundamental knowledge about generators and circuits which are in use for the generation of high voltages belongs to the background of work on h.v. technology. Generally commercially available h.v. generators are applied in routine testing laboratories; they are used for testing equipment such as transformers, bushings, cables, capacitors, switchgear, etc. The tests should confirm the effi- ciency and reliability of the products and therefore the h.v. testing equipment is required to study the insulation behaviour under all conditions which the apparatus is likely to encounter. The amplitudes and types of the test voltages, which are always higher than the normal or rated voltages of the apparatus under test, are in general prescribed by national or international standards or recommendations, and therefore there is not much freedom in the selection of the h.v. testing equipment. Quite often, however, routine testing laboratories are also used for the development of new products. Then even higher voltages might be necessary to determine the factor of safety over the prospective working conditions and to ensure that the working margin is neither too high nor too low. Most of the h.v. generator circuits can be changed to increase the output voltage levels, if the original circuit was properly designed. Therefore, even the selection of routine testing equipment should always consider a future extension of the testing capabilities. The work carried out in research laboratories varies considerably from one establishment to another, and the type of equipment needed varies accordingly. As there are always some interactions between the h.v. generating circuits used and the test results, the layout of these circuits has to be done very carefully. The classes of tests may differ from the routine tests, and therefore specially designed circuits are often necessary for such laboratories. The knowledge about some fundamental circuits treated in this chapter will also support the development of new test circuits. Finally, high voltages are used in many branches of natural sciences or other technical applications. The generating circuits are often the same or similar to those treated in the following sections. It is not the aim, however, of this introductory text to treat the broad variations of possible circuits, due to space limitation. Not taken into account are also the differing problems of electrical power generation and transmission with high voltages of a.c. or d.c., or the
Generation of high voltages 9 pure testing technique of h.v.equipment,the procedures of which may be found in relevant standards of the individual equipment.Power generation and transmission problems are treated in many modern books,some of which are listed within the bibliography of an earlier report.(1) This chapter discusses the generation of the following main classes of volt- ages:direct voltages,alternating voltages,and transient voltages. 2.1 Direct voltages In h.v.technology direct voltages are mainly used for pure scientific research work and for testing equipment related to HVDC transmission systems.There is still a main application in tests on HVAC power cables of long length,as the large capacitance of those cables would take too large a current if tested with a.c.voltages (see,however,2.2.2:Series resonant circuits).Although such d.c.tests on a.c.cables are more economical and convenient,the validity of this test suffers from the experimentally obtained stress distribution within the insulating material,which may considerably be different from the normal working conditions where the cable is transmitting power at low-frequency alternating voltages.For the testing of polyethylene h.v.cables,in use now for some time,d.c.tests are no longer used,as such tests may not confirm the quality of the insulation.(50) High d.c.voltages are even more extensively used in applied physics (accelerators,electron microscopy,etc.),electromedical equipment (X-rays), industrial applications(precipitation and filtering of exhaust gases in thermal power stations and the cement industry;electrostatic painting and powder coating,etc.),or communications electronics (TV,broadcasting stations). Therefore,the requirements on voltage shape,voltage level,and current rating, short-or long-term stability for every HVDC generating system may differ strongly from each other.With the knowledge of the fundamental generating principles it will be possible,however,to select proper circuits for a special application. In the International Standard IEC 60-12)or IEEE Standard.4-19953)the value of a direct test voltage is defined by its arithmetic mean value,which will be designated as V.Therefore,this value may be derived from v-vod. (2.1) where T equals a certain period of time if the voltage V(t)is not constant,but periodically oscillating with a frequency of f =1/T.Test voltages as applied to test objects then deviate periodically from the mean value.This means that *Superscript numbers are to References at the end of the chapter
Generation of high voltages 9 pure testing technique of h.v. equipment, the procedures of which may be found in relevant standards of the individual equipment. Power generation and transmission problems are treated in many modern books, some of which are listed within the bibliography of an earlier report.1Ł This chapter discusses the generation of the following main classes of voltages: direct voltages, alternating voltages, and transient voltages. 2.1 Direct voltages In h.v. technology direct voltages are mainly used for pure scientific research work and for testing equipment related to HVDC transmission systems. There is still a main application in tests on HVAC power cables of long length, as the large capacitance of those cables would take too large a current if tested with a.c. voltages (see, however, 2.2.2: Series resonant circuits). Although such d.c. tests on a.c. cables are more economical and convenient, the validity of this test suffers from the experimentally obtained stress distribution within the insulating material, which may considerably be different from the normal working conditions where the cable is transmitting power at low-frequency alternating voltages. For the testing of polyethylene h.v. cables, in use now for some time, d.c. tests are no longer used, as such tests may not confirm the quality of the insulation.50 High d.c. voltages are even more extensively used in applied physics (accelerators, electron microscopy, etc.), electromedical equipment (X-rays), industrial applications (precipitation and filtering of exhaust gases in thermal power stations and the cement industry; electrostatic painting and powder coating, etc.), or communications electronics (TV, broadcasting stations). Therefore, the requirements on voltage shape, voltage level, and current rating, short- or long-term stability for every HVDC generating system may differ strongly from each other. With the knowledge of the fundamental generating principles it will be possible, however, to select proper circuits for a special application. In the International Standard IEC 60-12 or IEEE Standard. 4-19953 the value of a direct test voltage is defined by its arithmetic mean value, which will be designated as V . Therefore, this value may be derived from V D 1 T T 0 Vt dt. 2.1 where T equals a certain period of time if the voltage Vt is not constant, but periodically oscillating with a frequency of f D 1/T. Test voltages as applied to test objects then deviate periodically from the mean value. This means that Ł Superscript numbers are to References at the end of the chapter.
10 High Voltage Engineering:Fundamentals a ripple is present.The amplitude of the ripple,8V,is defined as half the difference between the maximum and minimum values,or V =0.5(Vmax -Vmin). (2.2) The ripple factor is the ratio of the ripple amplitude to the arithmetic mean value,or 8V/V.For test voltages this ripple factor should not exceed 3 per cent unless otherwise specified by the appropriate apparatus standard or be necessary for fundamental investigations. The d.c.voltages are generally obtained by means of rectifying circuits applied to a.c.voltages or by electrostatic generation.A treatment of the generation principles according to this subdivision is appropriate. 2.1.1 A.C.to D.C.conversion The rectification of alternating currents is the most efficient means of obtaining HVDC supplies.Although all circuits in use have been known for a long time, the cheap production and availability of manifold solid state rectifiers has facilitated the production and application of these circuits fundamentally.Since some decades,there is no longer a need to employ valves,hot cathode gas- filled valves,mercury pool or corona rectifiers,or even mechanical rectifiers within the circuits,for which the auxiliary systems for cathode heating,etc., have always aggravated their application.The state of the art of such earlier circuits may be found in the work of Craggs and Meek,(4)which was written in 1954.All rectifier diodes used now adopt the Si type,and although the peak reverse voltage is limited to less than about 2500V,rectifying diode units up to tens and hundreds of kVs can be made by series connections if appropriate means are applied to provide equal voltage distribution during the non-conducting period.One may treat and simulate,therefore,a rectifier within the circuits-independently of the voltage levels-simply by the common symbol for a diode. The theory of rectifier circuits for low voltages and high power output is discussed in many standard handbooks.Having the generation of high d.c. voltages in mind,we will thus restrict the treatment mainly to single-phase a.c.systems providing a high ratio of d.c.output to a.c.input voltage.As, however,the power or d.c.output is always limited by this ratio,and because very simple rectifier circuits are in use,we will treat only selected examples of the many available circuits. Simple rectifier circuits For a clear understanding of all a.c.to d.c.conversion circuits the single-phase half-wave rectifier with voltage smoothing is of basic interest (Fig.2.1(a)). If we neglect the leakage reactance of the transformer and the small internal
10 High Voltage Engineering: Fundamentals a ripple is present. The amplitude of the ripple, υV, is defined as half the difference between the maximum and minimum values, or υV D 0.5Vmax Vmin. 2.2 The ripple factor is the ratio of the ripple amplitude to the arithmetic mean value, or υV/V. For test voltages this ripple factor should not exceed 3 per cent unless otherwise specified by the appropriate apparatus standard or be necessary for fundamental investigations. The d.c. voltages are generally obtained by means of rectifying circuits applied to a.c. voltages or by electrostatic generation. A treatment of the generation principles according to this subdivision is appropriate. 2.1.1 A.C. to D.C. conversion The rectification of alternating currents is the most efficient means of obtaining HVDC supplies. Although all circuits in use have been known for a long time, the cheap production and availability of manifold solid state rectifiers has facilitated the production and application of these circuits fundamentally. Since some decades, there is no longer a need to employ valves, hot cathode gas- filled valves, mercury pool or corona rectifiers, or even mechanical rectifiers within the circuits, for which the auxiliary systems for cathode heating, etc., have always aggravated their application. The state of the art of such earlier circuits may be found in the work of Craggs and Meek,4 which was written in 1954. All rectifier diodes used now adopt the Si type, and although the peak reverse voltage is limited to less than about 2500 V, rectifying diode units up to tens and hundreds of kVs can be made by series connections if appropriate means are applied to provide equal voltage distribution during the non-conducting period. One may treat and simulate, therefore, a rectifier within the circuits – independently of the voltage levels – simply by the common symbol for a diode. The theory of rectifier circuits for low voltages and high power output is discussed in many standard handbooks. Having the generation of high d.c. voltages in mind, we will thus restrict the treatment mainly to single-phase a.c. systems providing a high ratio of d.c. output to a.c. input voltage. As, however, the power or d.c. output is always limited by this ratio, and because very simple rectifier circuits are in use, we will treat only selected examples of the many available circuits. Simple rectifier circuits For a clear understanding of all a.c. to d.c. conversion circuits the single-phase half-wave rectifier with voltage smoothing is of basic interest (Fig. 2.1(a)). If we neglect the leakage reactance of the transformer and the small internal
Generation of high voltages 11 V-(t) RL(load) h.t. transformer (a) F2.8V v(t) (t a.T v_(t) T=1/f ( Figure 2.1 Single-phase half-wave rectifier with reservoir capacitance C. (a)Circuit.(b)Voltages and currents with load RL impedance of the diodes during conduction-and this will be done throughout unless otherwise stated-the reservoir or smoothing capacitor C is charged to the maximum voltage +Vmax of the a.c.voltage V~(t)of the h.t.transformer, when D conducts.This is the case as long as V<V~(t)for the polarity of D assumed.If I=0,i.e.the output load being zero (R=oo),the d.c.voltage across C remains constant (+Vmax),whereas V~(t)oscillates between +Vmax. The diode D must be dimensioned,therefore,to withstand a peak reverse voltage of 2Vmax. The output voltage V does not remain any more constant if the circuit is loaded.During one period,T=1/f of the a.c.voltage a charge o is transferred to the load RL,which is represented as e=/aou=元人o=r=子 (2.3)
Generation of high voltages 11 (a) (b) V~(t) V~(t) V (t) t a.T V max V min D C h.t. transformer V c 2.d V a i L (t) RL i (t) (load) i (t) T = 1/f Figure 2.1 Single-phase half-wave rectifier with reservoir capacitance C. (a) Circuit. (b) Voltages and currents with load RL impedance of the diodes during conduction – and this will be done throughout unless otherwise stated – the reservoir or smoothing capacitor C is charged to the maximum voltage CVmax of the a.c. voltage V¾t of the h.t. transformer, when D conducts. This is the case as long as V<V¾t for the polarity of D assumed. If I D 0, i.e. the output load being zero RL D 1, the d.c. voltage across C remains constant CVmax, whereas V¾t oscillates between šVmax. The diode D must be dimensioned, therefore, to withstand a peak reverse voltage of 2Vmax. The output voltage V does not remain any more constant if the circuit is loaded. During one period, T D 1/f of the a.c. voltage a charge Q is transferred to the load RL, which is represented as Q D T iLt dt D 1 RL T Vt dt D IT D I f. 2.3
12 High Voltage Engineering:Fundamentals I is therefore the mean value of the d.c.output i(t),and V(t)the d.c.voltage which includes a ripple as shown in Fig.2.1(b).If we introduce the ripple factor 8V from egn(2.2),we may easily see that V(t)now varies between Vmax≥V(t)≥Vmin;Vmin=Vmax-2(8V). (2.4) The charge O is also supplied from the transformer within the short conduction time te=aT of the diode D during each cycle.Therefore,O equals also to e-idr=. (2.5) JaT As aT<T,the transformer and diode current i(t)is pulsed as shown idealized in Fig.2.I(b)and is of much bigger amplitudes than the direct current it1. The ripple 8V could be calculated exactly for this circuit based upon the expo- nential decay of V(t)during the discharge period T(1-a).As,however,for practical circuits the neglected voltage drops within transformer and rectifiers must be taken into account,and such calculations are found elsewhere,(3)we may assume that a=0.Then 8V is easily found from the charge O transferred to the load,and therefore IT I Q=28C=1T;8V=2元=2f元· (2.6) This relation shows the interaction between the ripple,the load current and circuit parameter design values f and C.As,according to eqn (2.4),the mean output voltage will also be influenced by 8V,even with a constant a.c.voltage V~(t)and a lossless rectifier D,no load-independent output voltage can be reached.The product fC is therefore an important design factor. For h.v.test circuits,a sudden voltage breakdown at the load (RL0) must always be taken into account.Whenever possible,the rectifiers should be able to carry either the excessive currents,which can be limited by fast, electronically controlled switching devices at the transformer input,or they can be protected by an additional resistance inserted in the h.t.circuit.The last method,however,increases the internal voltage drop. Half-wave rectifier circuits have been built up to voltages in the megavolt range,in general by extending an existing h.v.testing transformer to a d.c. current supply.The largest unit has been presented by Prinz,(5)who used a 1.2- MV cascaded transformer and 60-mA selenium-type solid state rectifiers with an overall reverse voltage of 3.4 MV for the circuit.The voltage distribution of this rectifier,which is about 12 m in length,is controlled by sectionalized parallel capacitor units,which are small in capacitance value in comparison with the smoothing capacitor C (see Fig.2.14).The size of such circuits, however,would be unnecessarily large for pure d.c.supplies. The other disadvantage of the single-phase half-wave rectifier concerns the possible saturation of the h.v.transformer,if the amplitude of the direct current
12 High Voltage Engineering: Fundamentals I is therefore the mean value of the d.c. output iLt, and Vt the d.c. voltage which includes a ripple as shown in Fig. 2.1(b). If we introduce the ripple factor υV from eqn (2.2), we may easily see that Vt now varies between Vmax ½ Vt ½ Vmin; Vmin D Vmax 2υV. 2.4 The charge Q is also supplied from the transformer within the short conduction time tc D ˛T of the diode D during each cycle. Therefore, Q equals also to Q D ˛T it dt D T iLt dt. 2.5 As ˛T − T, the transformer and diode current it is pulsed as shown idealized in Fig. 2.l(b) and is of much bigger amplitudes than the direct current iL ¾D I. The ripple υV could be calculated exactly for this circuit based upon the exponential decay of Vt during the discharge period T1 ˛. As, however, for practical circuits the neglected voltage drops within transformer and rectifiers must be taken into account, and such calculations are found elsewhere,3 we may assume that ˛ D 0. Then υV is easily found from the charge Q transferred to the load, and therefore Q D 2υVC D IT; υV D IT 2C D I 2fC. 2.6 This relation shows the interaction between the ripple, the load current and circuit parameter design values f and C. As, according to eqn (2.4), the mean output voltage will also be influenced by υV, even with a constant a.c. voltage V¾t and a lossless rectifier D, no load-independent output voltage can be reached. The product fC is therefore an important design factor. For h.v. test circuits, a sudden voltage breakdown at the load RL ! 0 must always be taken into account. Whenever possible, the rectifiers should be able to carry either the excessive currents, which can be limited by fast, electronically controlled switching devices at the transformer input, or they can be protected by an additional resistance inserted in the h.t. circuit. The last method, however, increases the internal voltage drop. Half-wave rectifier circuits have been built up to voltages in the megavolt range, in general by extending an existing h.v. testing transformer to a d.c. current supply. The largest unit has been presented by Prinz,5 who used a 1.2- MV cascaded transformer and 60-mA selenium-type solid state rectifiers with an overall reverse voltage of 3.4 MV for the circuit. The voltage distribution of this rectifier, which is about 12 m in length, is controlled by sectionalized parallel capacitor units, which are small in capacitance value in comparison with the smoothing capacitor C (see Fig. 2.14). The size of such circuits, however, would be unnecessarily large for pure d.c. supplies. The other disadvantage of the single-phase half-wave rectifier concerns the possible saturation of the h.v. transformer, if the amplitude of the direct current