High Voltage Engineering

High Voltage Engineering Md. Alamgir Hossain Assistant Professor Department of Electrical and Electronic Engineering Khulna University of Engineering & Technology 1. [C.L._Wadhwa] High Voltage Engineering 2. [E._Kuffel, W. S. Zaengl, J Kuffel] High Voltage Engineering Fundamentals 3. [M._S._Naidu, V. Kamaraju] High Voltage Engineering 4. [Mazen_Abdel-Salam etc] High Voltage Engineering – Theory and Practice 5. [R.D._Begamudre] Extra High Voltage A.C. Transmission Engineering •Low ≤ 1kV Distribution •Medium 1kV ~ 66kV •High 100kV ~ 230kV •Extra High 275kV ~ 765kV Transmission •Ultra High ≥1MV X- Ray 100 kV Electron Microscope 100 kV ~1MV Electrostatic Precipitator Up to 1MV Insulation Test Half-wave Rectifier During one period, T=1/f of the a.c. voltage a charge Q is transferred to the load RL, which is represented as The ripple voltage, This charge is supplied by the capacitor over the period T when the voltage changes from Vmax to Vmin over approximately period T neglecting the conduction period of the diode. Suppose at any time the voltage of the capacitor is V and it decreases by an amount of dV over the time dt then charge delivered by the capacitor during this time is The single phase half-wave rectifier circuits have the following disadvantages: i. The size of the circuits is very large if high and pure d.c. output voltages are desired. ii. The h.t. transformer may get saturated if the amplitude of direct current is comparable with the nominal alternating current of the transformer. H.V. output at no-load The steady state potentials at all nodes of the circuit are sketched for the circuit for zero load conditions. From this it can be seen, that: . the potentials at the nodes 1’, 2’,…..n’ are oscillating due to the voltage oscillation of V(t); . the potentials at the nodes 1,2...n remain constant with reference to ground potential; . the voltages across all capacitors are of d.c. type, the magnitude of which is 2Vmax across each capacitor stage, except the capacitor Cn’ which is stressed with Vmax only; . every rectifierD1, D1’, ....Dn, Dn’ is stressed with 2Vmax or twice a.c. peak voltage; and . the h.v. output will reach a maximum voltage of 2nVmax. If the generator supplies any load current I, the output voltage will never reach the value 2nVmax. There are two things to deal with: the voltage drop ∆V0 and the peak-to-peak ripple 2δV Now let a charge q be transferred to the load per cycle, which is obviously q=I/f=IT. This charge comes from the smoothing column, the series connection of C1...Cn. If no charge would be transferred during T from this stack via D’1..D’n to the oscillating column, peak-to-peak ripple would merely be Therefore, the total ripple will be I Thus the lowest capacitors are most responsible for the ripple For equal capacitor C1.........Cn q q Load q 2q q 3q 4q q nq The capacitor Cn will be charged to a voltage C’n-1 can only be charged up to a maximum voltage of If all the capacitors within the cascade circuit are equal or then the voltage drops across the individual stages are I Since the lowest capacitors are most responsible for the total ΔV0 as is the case of the ripple, only a doubling of C’n is convenient, because this capacitor has to withstand only half the voltage of the other capacitors; namely Vmax. Therefore, ΔVn decreases by an amount of 0.5nq/c, which reduces ΔV of every stage by the same amount, thus n times. Hence, I For this case and, n≥4 linear term can be neglected and therefore approximate the maximum output voltage by The optimum number of stages assuming a constant Vmax, I, f and C can be obtained for maximum value of V0max by differentiating equation with respect to n and equating it to zero. That is, Using the value of nopt Example 2.1. A ten stage Cockraft-Walton circuit has all capacitors of 0.06µF. The secondary voltage of the supply transformer is 100kV at a frequency of 150Hz. If the load current is 1mA, determine (i) voltage regulation (ii) the ripple (iii) the optimum number of stages for maximum output voltage (iv) the maximum output voltage. - 27.5 =3.05kV 4 3.05 0.15% Robert Jemison Van de Graaff developed belt driven electrostatic generator in 1931 Can generate up to some Mega volt The advantages of the generator are: Very high voltages can be easily generated Ripple free output Precision and flexibility of control The disadvantages are: Low current output Limitations on belt velocity due to its tendency for vibration. The vibrations may make it difficult to have an accurate grading of electric fields High Voltage Cascaded Transformer High voltage is generated in the laboratory for testing power equipments and research purpose. The current required for testing are as follows: Insulators, C.B., bushings, Instrument transformers = 0.1– 0.5 A Power transformers, h.v. capacitors. = 0.5–1 A Cables = 1 A and above For less than 300kV, a single unit transformer is suitable but for more higher voltage cascade transformer is used that reduces the problem with- Cost Transportation Erection and Insulation The differences between testing and single-phase power transformer are related mainly to a smaller flux density within the core to avoid unnecessary high magnetizing currents which would produce higher harmonics in the voltage regulator supplying the transformer, and to a very compact and well insulated h.v. winding for the rated voltage. Therefore, a single-phase testing unit may be compared with the construction of a potential transformer used for the measurement of voltage and power in power transmission systems. The primary winding might often be split up in two or more windings which can be switched in series or parallel to increase the regulation capabilities. The iron core is fixed at earth potential as well as one terminal of each of the two windings. The iron core ‘1’ is as well as one terminal of each of the two windings are grounded. Fig. Single unit testing transformers The primary winding ‘2’ is usually rated for low voltages of ≤1 kV, but might often be split up in two or more windings which can be switched in series or parallel to increase the regulation capabilities. Fig. Simplified cross-sections of two possible constructions of single unit testing transformer Figure (b) shows a grounded metal tank unit, for which an h.v. bushing ‘6’ is necessary to bring the high voltage out of the tank ‘5’. Instead of a bushing, a coaxial cable could also be used if this improves the connection between testing transformer and test object. In Figure (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. The primary winding close to the iron core and surrounded by the h.v. winding ‘3’ that increases the coupling of both windings. 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. 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. Exciter Compensating Secondary winding High Voltage Primary 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. The tertiary winding of stage-I has the same number of turns as the primary winding, and feeds the primary of the stage-II transformer. Through h.t. bushings, the leads from the tertiary winding and the h.v. winding are brought out to be connected to the next stage transformer. The three secondary windings are connected in series so that the output voltage between ground and the third stage transformer secondary is 3V. For the three-stage transformer, the total output VA will be 3VI= 3P and, therefore, each of the secondary winding of the transformer would carry a current of I= P/V. The primary winding of stage-III transformer is loaded with P and so also the tertiary winding of second stage transformer. Therefore, the primary of the second stage transformer would be loaded with 2P. Extending the same logic, it is found that the first stage primary would be loaded with 3P.

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