EBB 4043 ELECTRICAL AND ENERGY SYSTEM LECTURE 3 GENERATION OF HIGH VOLTAGES AND CURRENTS

Objectives To understand the different method of generation dc voltages, ac voltages and impulse currents. 2

Introduction Electrical engineering and applied physics required high voltages in several applications. For example in X-ray units require high dc voltages, high ac voltages are required for testing power apparatus rated for EHV. High impulse voltages are required for testing purposes to simulate overvoltages that occur in power systems due to lightning or switching action. 3

Introduction 4 The insulating testing for various components in power systems for different types of voltages, such as power frequency ac, high frequency, switching or lightning impulses also required high voltages. Thus, generation of high voltages for testing purposes. High generation of amperes also needed for the short circuit testing of switchgear and surge diverters testing.

Introduction Therefore, test facilities require high voltage and high current generator. High impulse current generation is also required along with voltage generation for testing purposes. 5

Generation of high direct current voltages High dc voltages are needed in insulation tests on cables and capacitors. Impulse generator charging units also require high dc voltages of about 100 to 200 kv. Normally, for the generation of dc voltages of up to 100 kv, electronic valve rectifiers are used and the output currents are about 100 ma. 6

Generation of high direct current voltages Half and Full Wave Rectifier Circuit. Three types rectifier circuits. Half wave Full wave Voltage doubler rectifiers Rectifier may be an electron tube or solid state device (ssd). For voltages up to 250 kv, electron tube is used and for voltages up to 20 kv ssd is used. 7

Generation of high direct current voltages For higher voltages, several units are to be used in series. Hence, transient voltage distribution along each unit becomes non-uniform and action must be taken so that the distribution become uniform. 8

Generation of high direct current voltages Half wave Full wave 9

Generation of high direct current voltages 10

Generation of high direct current voltages Voltage Doubler Circuits. Both full wave and half wave rectifier circuits produce a dc voltage less than the ac max voltage. When higher dc voltages are needed, a voltage doubler or cascaded rectifier doubler circuits are used. 11

Generation of high direct current voltages 12

Generation of high alternating voltages Transformer can be used for test purposes if test voltage requirements are less than 200 kv. However for higher test voltage requirements, a single unit construction become difficult and costly due to insulation problem. To overcome, cascade or connect in series several unit of transformers. 13

Generation of high alternating Cascade Transformer voltages 14

Generation of high alternating Cascade Transformer voltages 1 st transformer at the ground potential. 2 nd transformer is kept on insulators and maintained at a potential V. The output voltage of the 1 st unit above the ground. 15

Generation of high alternating Cascade Transformer voltages The HV supply is connected to the primary winding 1 of 1 st unit, designed for HV output of V. The excitation winding 2 of 1 st unit supplies the primary voltage for the 2 nd unit, where both windings are dimensioned for the same low voltage. The HV or secondary windings 2 for both units are connected in series, so that a voltage of 2V is produced at the 2 nd unit. The 3 rd unit is added in the same way. 16

Generation of high alternating Cascade Transformer voltages The tanks containing the active parts (core and windings) are indicated by dashed lines. The tank of 2 nd and 3 rd transformer are at high potentials, namely V and 2V above earth, and must therefore be suitably insulated, hence raised above the ground on solid post insulators. Through HV bushings the leads from the excitation winding 3, as well as the tapings of the HV windings 2, are brought to the next transformer. 17

Generation of high alternating Cascade Transformer voltages The tanks containing the active parts (core and windings) are indicated by dashed lines. The tank of 2 nd and 3 rd transformer are at high potentials, namely V and 2V above earth, and must therefore be suitably insulated, hence raised above the ground on solid post insulators. Through HV bushings the leads from the excitation winding 3, as well as the tapings of the HV windings 2, are brought to the next transformer. 18

Generation of high alternating voltages 19

Generation of high alternating voltages Series and parallel resonant sets Design to overcome the problem occurred in cascaded transformer. Resonance is firstly used to reduce the demand from the low voltage supply. It also used to ensure that a pure 50 Hz waveform is delivered from a transformer. 20

Generation of high alternating voltages Series and parallel resonant sets The development of a fundamental frequency resonance condition ensures a waveform low in total harmonics distortion and reduces the maximum power requirement from the low voltage winding of the test supply. 21

Generation Impulse voltages Standard impulse Waveshapes Transient overvoltages due to lightning and switching surges cause steep build-up of voltage on transmission lines and other electrical apparatus. These waves have a rise time of 0.5 to 10 s and decay time to 50 % of the peak value of the order of 30 to 200 s. 22

Generation Impulse voltages Standard impulse Waveshapes The wave can be represented as double exponential waves defined by the equation below: V = V o [exp (- t) exp (- t)] where and are constants of ms values. 23

Generation Impulse voltages Standard impulse Waveshapes 24

Generation Impulse voltages 25 Standard impulse Waveshapes Impulse waves are specified by defining their rise of front time, fall or tail time to 50 % peak value, and the value of the peak voltage. 1.2/50 s, 1000 kv wave represents an impulse voltage wave with a front time of 1.2 s, fall time to 50 % peak value of 50 s and a peak value of 1000 kv normally used for lightning impulse testing.

Generation Impulse voltages 26 Standard impulse Waveshapes For switching impulse testing is the 250/2000 s wave. A lightning stroke to an overhead line may generate transient voltages in order of some hundreds of kilovolts regardless of the power system voltage level. A switching surge will be proportional to the power system voltage level, normally between 2.0 to 3.0 pu voltage.

Generation Impulse voltages The impulse voltage generator An impulse voltage generator should be capable of delivering the standard impulse voltage waveshapes into a given test object. The load may be a simple capacitance present in equipment such as a cable or a complex network of resistance, inductance and capacitance such as a transformer. 27

Generation Impulse voltages The impulse voltage generator The peak voltage output of the generator must be variable to accommodate the range of equipment test levels and the varying generator efficiency which changes with the impedance of the test equipment and the required waveshape. 28

Generation Impulse voltages The impulse voltage generator In a simplistic manner, the impulse waveform consists of two individual components, one charging component causing the wavefront and one discharging component causing the wavetail. The circuit is combination of R-C circuits. 29

Generation Impulse voltages The impulse voltage generator 30 A capacitor C 1, the stage capacitance, is initially charged to a predetermined voltage and is then discharged via a switch or sparkgap, G.

Generation Impulse voltages The impulse voltage generator Once the switch is closed, C 1 and C 2 is connected in parallel which both of them simultaneously discharge into R 2. 31

Generation Impulse voltages The impulse voltage generator where C s = C 1 + C 2 R T = R 1 + R 2 Wavetail circuit, where t 2 = 0.7C S R T 32

Generation Impulse voltages The impulse voltage generator 33 When G is flashes over, C 2 receives charge through the series circuit consisting of C 1 in series with R 1 and C 2, R 2 is ignored since >> R 1.

Generation Impulse voltages The impulse voltage generator Wavefront time, normally approximately three times the time constant of the circuit 34

Generation Impulse voltages Multistage Impulse Generator The generator capacitance C 1 is to be first charged and then discharged into the wave shaping circuits. A single capacitor C 1 may be used for voltages up to 200 kv. Beyond this voltage, a single capacitor and its charging unit may be too costly, and the size becomes very large. 35

Generation Impulse voltages 36 Multistage Impulse Generator The cost and size of the impulse generator increases at a rate of the square or cube of the voltage rating. Thus, for producing very high voltages, a bank capacitors are charged in parallel and then discharged in series. The arrangement for charging the capacitors in parallel and then connecting them in series for discharging was proposed by Marx.

Generation Impulse voltages Multistage Impulse Generator Nowadays modified Marx circuit are used for the multistage impulse generators. 37

Generation Impulse voltages Multistage Impulse Generator Usually the charging resistance R S is chosen to limit the charging current about 50 to 100 ma, and the generator capacitance C is chosen such that the product CR S is about 10 s to 1 min. The gap spacing is chosen such that the breakdown voltage of the gap G is greater than the charging voltage V. 38

Generation Impulse voltages Multistage Impulse Generator When the impulse generator is to be discharged, the gaps G are made to spark over simultaneously by some external means. Thus, all the capacitors C get connected in series and discharge into the load capacitance or test object. 39

Generation Impulse voltages Multistage Impulse Generator 40

Generation Impulse voltages Multistage Impulse Generator 41

42 Generation Impulse voltages

Generation of Impulse Currents Lightning discharges involve both high voltage impulses and high current impulse. Protective gear like surge diverters/collectors have to discharge the lightning currents without damage. Thus, generation of impulse current waveforms of high magnitude (~ 100 ka peak) is necessary. 46

Generation of Impulse Currents A typical impulse current wave is shown in Figure (4/10 and 8/20 µs) 47

Generation of Impulse Currents Circuit for Producing Impulse Current Waves A bank of capacitors connected in parallel are charged to a specified value and are discharged through a series R-L circuit 48

Generation of Impulse Currents Circuit for Producing Impulse Current Waves C represents a bank of capacitors connected in parallel which are charged from a d.c. source to a voltage up to 200 kv. R represents the dynamic resistance of the test object and the resistance of the circuit and shunt. L is an air cored high current inductor. 49

Generation of Impulse Currents Circuit for Producing Impulse Current Waves 8/20 µs (α = 0.0535 x 10 6, β = 0.113x 10 6, LC = 65 and I m = VC/14) 50

Generation of Impulse Currents Example: An impulse current generator has a total capacitance of 8 µf. The charging voltage is 25 kv. If the generator has to give an output current of 10 ka with 8/20 µs waveform, calculate (a) The circuit inductance (b) The dynamic resistance in the circuit. 51

Solutions For an 8/20 µs : α = 0.0535 x 10 6, β = 0.113x 10 6, LC = 65 and I m = VC/14 Therefore, a) α = R/2L, where L= 65/C = 65/8 = 8.125 µh b) R = α(2l)= 0.0535 x 10 6 (2 x 8.125) = 0.8694 ohms 52

What about the charging voltage needed if output current is 10 ka? I m = VC/14 Therefore, V = I m (14)/8 = 10 (14)/8 = 17.5 kv 53