محاضرات جميلة في الضغط العالي

High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

By Dr. Ali Albakry

HIGH VOLTAGE ENGINEERING By

Dr. Ali A. Albakry Al - Mussaib Technical College of Engineering For

Fourth Class Department of Electrical Power Techniques Engineering

Main References 1- (High Voltage Engineering),by E. Kuffel and M. Abdullah.

2- (Electric Power Systems), by B. M. Weedy.

3- (High Voltage Technology), by B. Alston.

4- (Electric Power Systems), vol.2, by A. E. Guil and Peterson.

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High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

By Dr. Ali Albakry

Lecture (1) 1.1 Introduction In the initial stages D.C was used for electric power transmission. This was due to two important reasons: 1- Ease of control 2- D.C motors were the only electro-mechanical energy converters.

However with the introduction of transformers in 1881 and the three phase systems in 1888, the obvious advantages of alternating current combined with the successful application of associated equipments soon appears to be more economical.

Electrical power may be transmitted using high and extra voltages. The term extra high voltage (EHV) has generally accepted to describe the systems of 230 KV up to 765 KV and for the voltage above 765 KV, the term Ultra High Voltages (UHV) is applied, while the voltage the below 230 KV named High Voltage (HV).

1.2 Advantage and Disadvantage of using HV 1.2.1 Advantage of using High Voltage in transmission and distribution systems : 1- To transmit certain power over certain distance, the conductor volume will be less under high voltage condition as follow: Vc = (P1 L2 Ď / P2 ) . (1/ (V2 cos 2θ)) Where: L: length of the line.

V: phase voltage.

P1: power transmitted / phase.

P2 : power loss / phase.

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By Dr. Ali Albakry

cos θ = power factor.

2- Regulation: The regulation may be defied as: the percentage variation of receiving end voltage when the sending end voltage constant. Transmission lines suffer from inherent voltage variations at the receiving end with changes in loads; however the voltage variations are undesirable, where the regulation of the line may be given by: Reg % = ((P.R + X.Q) / V 2res ) % Where: R and X are resistance and reactance of line, Vres is the receiving voltage of the line.

P and Q are the real and imaginary power.

From the above equation one can see that; the regulation is inversely proportional with the square of the receiving end voltage.

3- The power transmission capacity of a transmission line is proportional to the square of the operation voltage, but the transmission line and terminal equipments costs are also increases with any increase of voltage. As a consequence the overall capital cost of transmission decreases as a voltage increases.

1.2.2 Disadvantages of using H.V in transmission and distribution electrical systems: 1- Due to the voltage stress at the surface of a conductor in air, discharges occur if this stress exceeds a critical value.

These discharges give rise to power loss from the conductor and also provide a source of interference with radio

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By Dr. Ali Albakry

communication. It can be shown that the voltage stress at the surface conductor is given (approximately) as: Emax = V / ( r. log (D/r))

rms KV / m

Where: r: radius of conductors in meters.

D: distance between conductors.

By increasing r, the value of Emax is reduced. For this purpose the hollow conductors can be used but both expensive in manufacture and difficult to erect. Consequently the usual solution is to increase the number of the conductors per phase (bundle conductors). A further advantage of bundle conductors is that; they are more flexible and easier to handle than a single conductor or equivalent cross section.

2) Insulation Level The level of insulation required on a H.V Transmission Line(TL) is determined by the magnitude of the voltage surges which are likely to occur. These surges can be internally generated by switching, or they can be externally induced due to atmospheric cause (lighting).

To withstand these surges the insulation must be sufficient, but it is obviously uneconomic to provide against all contingencies, consequently, equipment is designed to withstand voltages up to a certain values called the Impulse voltage level or Basic Insulation Level (BIL), and surge diverters or spark gaps are employed to protect the normal insulation against abnormal voltages.

3) Stability The expression for power transmitted between two ac systems is given by: 4

High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

By Dr. Ali Albakry

P = (VS . Vr / X) sin δ Where δ is the power angle which is the angle between vectors VS and Vr, and it is also called (load angle).

X: Series reactance of the line (neglecting resistance of that line) in Ω / phase.

And also, we have that: P = Pmax when δ =90o

(Relation between P and δ) It is the usual practice to limit the load angle to about 30o under steady state condition. Hence: The steady state power transfer (PS) will be: PS = (VS .Vr / X) sin 30 = Pmax sin 30 = Pmax / 2 So to maintain system stability an a.c tie line can easily be operated at the half of its theoretical maximum power transfer that the theoretical length in km of a transmitted line that can be operated on its natural load and power angle of 30o without loss of stability is limited by using capacitors in series with the 5

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line, the natural value of (X) can be reduced. These additional equipments are costly.

Lecture (2) High Voltage Insulating Engineering 2.1 Introduction: Electrical insulating materials (dielectrics) are understood as the materials that offer a very high resistance to the passage of the electric current under the action of the applied direct current, voltage and therefore sharply differ in their basic electrical properties from conductive materials where the resistivity of electrical insulating material should be infinitely high.

Insulators must often survive reliably under high electrical stress for very long periods of time, say, for sixty years or more. Many insulating materials may suffer a gradual deterioration, due to small electrical discharges. We can define breakdown of an insulator as: (The transition from a stable state of low conduction to state of low conductivity, for which the current is largely determined by the source impedance and not the insulator).

The working voltages of electrical machines and apparatus, overhead and cable power transmission lines have been increased. This increase in voltage has caused an essential increase in the geometrical dimensions of insulators, the design of which should be capable of withstanding such working conditions.

Solid insulation dose, in general, not recover after electrical breakdown, i.e., breakdown through the material destroys permanently its insulating properties or at least its usefulness as an insulator, while gaseous insulators will recover their original strength after current removing. It is 6

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thus often desirable to protect the solid insulator from voltage transients.

2.2 Insulators and Breakdown types 2.2.1 Solid Insulator: Breakdown of Solid Insulator: The relation between the breakdown strength of a solid insulator with respect to time may be given as in the following figure:

(Log time of Breakdown with Electric Strength)

The time axis ranges from fractions of microsecond to several decades of years. The working stress of equipment must be low enough to prevent breakdown by any mechanism during the expected life of the equipment but high enough to ensure manufacturing costs are economic.

There are many types of breakdowns in solid insulator as follow: 7

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A) Electrical Breakdown From the band theory of solids it is seen that in a perfect crystal at absolute zero of temperature the bands are completely filled up to a certain level and empty thereafter. The upper full one (Valance band) and the first empty one (Conduction band) are separated by a (forbidden energy gap).

As temperature is increased, electrons may gain sufficient energy to cross this gap after which they are free to migrate through the crystal and will be accelerated by the electric field applied in their movement.

Electrical breakdown is characterized by a short time of development in order of a microsecond or less.

B) Electromechanical Breakdown When an electric field is applied to a dielectric (insulator) between two electrodes, mechanical forces will be exerted on the insulator due to the force of attraction between the surface charges. Therefore, as the voltage is increased, the thickness of the insulator will decrease.

Consider a plane electrodes with a solid insulator between them, let (do ) is the initial thickness, then from the

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High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

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general electrostatic and mechanical strain theories, the field strength at breakdown is given by : Eb = V / do = 0.6 [ Y / (Єo Єr) ]0.5 Where : Y = Youngs Modulus Єo Єr = Absolute and Relative permittivity.

C) Electrothermal Breakdown A dielectric to which a voltage is applied liberates heat, the temperature of the dielectric rises and the losses ae therefore increased still more. The process is intensified until the dielectric is heated so much that it gets damaged, and the breakdown of the dielectric occurs at so low voltage at which it would never develop at a low temperature and in an undamaged material.

D) Chemical Breakdown Insulation can degrade chemically even in the absence of an electric field due to one or more of the following causes: 1- Chemical instability: The chemical instability structure of most insulators break down if they subjected to the excessive temperatures. This causes deterioration of insulating properties and loss of mechanical strength.

2- Oxidation : Some insulators oxidize in the presence of air. This causes loss of mechanical strength and cracking. Materials which suffer oxidation include rubbers and polyethylene.

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High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

By Dr. Ali Albakry

2.2.2 Gaseous Insulation 2.2.2.1 Introduction In high voltage apparatus, gases are used for insulation. A gas in it normal state is an almost perfect insulator. However, when an electric field (E) of sufficient intensity is established in the gas for instant between two electrodes, the gas can become a conductor, and the transition from insulating to an almost completely conducting state is called the electrical breakdown.

The term breakdown is used to describe the flow of electric current through the insulator gaseous medium. The requirements for the breakdown case are that some of the gas molecule should be ionized.

There are many types of B.D that are happening in gaseous insulators as follow:

2.2.2.2 Electrical discharges in Gases: The behavior of gas under the application of the electric field between two electrodes could in general be studied for the two following cases: A) Homogenous field (Uniform field): Such field is normally established using the following electrodes:

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High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

By Dr. Ali Albakry

B) Non – Homogenous fields ( Non – Uniform fields) Such field is normally established using the following electrodes:

2.2.2.2.1 Electrical Discharge in Uniform Fields The electrical discharge in uniform fields need four processes to be happened consequently in the simple circuit below which represent a homogenous field gap ( say plane – plane)

And these process are: a) Excitation b) Ionization c) Electron Avalanche 11

High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

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d) Secondary Process

2.2.2.2.2 Electrical Discharge in Non Uniform Fields In this type of fields the distribution of the field is non uniform due to the difference of the electrodes shape which effects on the field distribution in this case the relation between the maximum and the average fields will be: K = Emax / Eave Where K must be equal 1 (K = 1) for a Uniform field while it is not for a non Uniform. The value for this factor must not to be exceeding 5 where when this happen (k < 5) the corona will be appeared and the breakdown also happen.

2.2.2.3 Factors effecting on B.D Voltages 1- Non Uniform fields due to shape and geometry of the electrodes.

2- The waveform of the applied voltage.

3- The pressure or the density of the gas between electrodes.

4- Effect of insulators support of solid dielectric.

5- Solid particles contamination, pollutions and other atmospheric variations.

6- Temperature rising.

2.2.2.4 Design of Insulation in the H.V Equipments There are many factors that must be taken into consideration when we design the insulation for the H.V equipment and can be summarized as follow: 1- Electric Field (E).

2- Thermal loading effect. a) I2 R loss due to the current in the conductors. b) Dielectric loss.

3- Mechanical duty.

4- Availability of the required materials.

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5- Ambient temperature.

Lecture 3 Breakdown in Electronegative Gases Electronegative gas is defied as: the gas which has an external orbital that need one or two electron to be completed, this process named (attachment).

Attachment coefficient ( ): it represent the number of the randomly collision (crashed) electrons per unit length.

Breakdown condition:

where : Îł = The number of secondary electrons produced at cathode per electron produced in the gap by the primary collision process. Îą = The number of ionizing collision made by one electron per unit length drift in the direction of electric field.

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d = distance between electrodes.

Paschens Law : This law describes the factors and the relation that explains how the B.D of gas happen, by the scientist named paschens in 1889. where , at a constant temperature in uniform field the sparking potential depends only on the product of pressure & electrodes spacing.

Vs = F (p . d)

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High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

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By Dr. Ali Albakry

High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

By Dr. Ali Albakry

Lecture (4) High Voltage Generation 2.1 Introduction: We shall not consider methods of high voltage generation for power transmission or high power application, but only methods to generate high voltages for testing purposes and power supplies, with which the efficiency of generation is not of prime importance. Three types of high voltage can be generated:

  

Alternating voltages Direct voltages

Impulse (Transient) voltages The aim of generating these voltages is mainly to test electrical equipment under various working conditions. The main purposes for testing may be defined as : 1- To specify the reliability and efficiency of equipment (cables, circuit breakers, transformer, ….. etc).

2- To study insulation behavior under different working conditions.

3- To determine the safety factor over the working conditions.

 Alternating voltages

The following methods are used to generate high alternating voltages: 1) Staight Generation ( Single Transformer) The simplest circuit is the straight circuit in which single transformer is fed directly from the supply. Voltages up to (1.5 MV) can be obtained using single transformer. The limitations 17

High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

By Dr. Ali Albakry

imposed on using this method are: the cost of transformer for higher voltages increase rapidly. Also we have the problems of transport. In general this method is used for voltages up to ( 220 KV) and relatively low power loads. However, at higher voltages, a higher current is usually required, as the capacitance of the test object is higher at higher voltages.

1)Cascaded Transformer The difficulties of using single transformer for generating very high voltages may be encountered using several units ( small units) in series. The low voltages windings wound over the high voltage winding and connected to it. This is used to feed the primary of the second unit. Schematically, the winding arrangement is shown in fig (1).

Fig (1) (5, 6) primary winding of transformer 2 is supplied from the tapping (3, 4) of transformer 1. Voltages (3, 4) is equal to voltage (1, 2). Point (10) is insulated from the earth, the 18

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voltage between high voltage terminal (9) and earth (E) equals the sum of the three transformers [(9,10), (7,8), (3,e)]. The voltage (9,10) is simply controlled by the primary voltage (1,2).

Direct Voltage Generation Due to increasing difficulties in using A.C in large power systems, and continuing development of high voltage rectifying and inverting equipment, it has become economic proposition to use high voltage D.C for the transmission of power.

To fulfill this need there is at the present time a demand for testing equipments to the test components used in the ever expanding D.C systems. Another wide field for the use of high voltage D.C is the need for testing of H.V , highly capacitive loads such as cables. If these loads were to be tested on A.C , a tremendous reactive power will need be supplied and the testing equipment would be extremely cumbersome.

By using D.C, however, these highly capacitive loads may be tested using only a relatively small equipment since after the initial charging current has been supplied to the load, only a very small leakage current, possibly only of the order of a few milliampers, will need to be supplied.

Basic Circuits of the Generation of H.V D.C 3)Transformer- Rectifier ( H.W Rectifier) The simplest mains- operated sources of high voltage D.C comprises merely of a high voltage transformer and a single half- wave rectifier, possibly feeding a capacitor such simple arrangements are quite acceptable for charging sets, where the will eventually charge the capacitor I to voltage (Vo) as in

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fig (2), i.e. to the peak value of the voltage of the secondary winding.

The rectifier will have to withstand a peak inverse voltage of maximum (2Vo). The circuit is not very suitable for high continuous loads, as the ripple will be quite high when the load resistance is not very high compared with the source impedance.

1)The Bridge Rectifier This arrangement offers reduced ripple and higher efficiency fig (3) . The peak inverse voltage across any rectifier under no-load conditions (worse-case) is Vo.

Fig (3) With a normal load, the peak voltage across any rectifier is typically (0.8 Vo). Compared the bridge rectifier with the half wave rectifier, we have merely to double the rectifier 20

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elements ( 4 rectifiers with peak (Vo) instead of one with peak voltage (2Vo).

The two circuits considered had an output voltage equal to or below the peak voltage of the transformer secondary. As the transformer may will be the most expensive part of an unstabilized circuit, and as its cost and bulk increase considerably with output voltage, it proves very useful to employ doubler circuit

3)Cockcroft â&#x20AC;&#x201C; Walton Voltage Doubler Using this circuit it is possible to produce a D.C voltage of approximately peak to peak value of input A.C voltage.

Fig (4) Considering the circuit of fig (4) with point (a) positive with respect to point (b) the rectifier (D2) will conduct, and the capacitor (C1) will charge up to the peak voltage of (V). In the next half cycle the point (a) will be negative with respect to point (b). In this ca1se the transformer voltage adds on to the voltage appearing on (C), and the polarity of the combination will be such as to cause rectifier (D1) to conduct and the capacitor will change up to a voltage V+V = 2V .

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Cockcroft â&#x20AC;&#x201C; Walton Multiplier Circuits This is a development of the basic circuit as even with a doubler circuit the production of say 1MV D.C at a current of only a few milliampers entails an uneconomic input transformer design. By this we mean a transformer of very high step up ratio, with a secondary winding which from electrical considerations may only need to be of small cross sectional area wire. From purely mechanical considerations it becomes necessary to increase the cross section of the wire to ensure that the transformer winding can made sufficiently rigid.

A circuit which overcome these difficulties is the CrockCroft â&#x20AC;&#x201C;Walton voltage multiplier circuit fig (5).

In this circuit no one component has to withstand the full D.C output voltage and the capacitors help to ensure that the voltage distribution along the rectifier chain is fairly linear. In addition on economic transformer can be produced sice the input transformer needs to be for a reasonable voltage at a higher current.

This circuit is used to generate H.V up to (1.4 MV at 15 mA). A model to explain the basic arrangement of such multiplier is given in fig (5) .

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Fig (5) The section (123) is a half wave rectifier. The sequence of charging the multiplier is as follow: When rectifier (D1) conducts, (C1) charges up to a voltage of +Vmax, and as we had with circuit of fig (2) the voltage of point (1) with respect to earth (3) oscillates between (0) and 23

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(2Vmax) and we have (C2) charged to (2 Vmax) through rectifier (D2). Point (4) then attains a steady voltage of (2 Vmax) and the voltage applied to (C3) via rectifier (D3) therefore varies between (+2Vmax) and (0).

Thus (C3) is charged up to (+Vmax). The voltage of point (5) therefore oscillates between (+2Vmax) and (4Vmax), and (C4) is charged to (+4Vmax) through rectifier (D4). The voltage of point (6) with respect to earth reaches a value of (+4Vmax). The potential of the different point of the multiplier is the doubled as we go upwards attaing a maximum value for the circuit shown of (+8Vmax).

It should be noticed that each rectifier and capacitor has to withstand only twice the output voltage of transformer whatever the number of stage are used.

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Lecture Five

Impulse voltage generation Introduction: Impulse generators are required to study the behaviour of systems or components when they are subjected to high voltage transients, as these insulation may lead to breakdown of insulation of electrical equipment as used in power system.

Such transients may have quite a range of origin: 1)Line Switching 2)Lighting Strokes 3)Component Failure (Fault conditions) As the pulse shapes of these transients vary greatly, it is also necessary to produce different wave shapes with generator to simulate the surges( transients).

Impulse generators are not only employed to test insulation, they are also used to test the transient response of systems or to study fundamental processes.

The wave shapes most frequently encountered with surges on transmission lines is of the double exponential shape: We find usually a relatively fast rise time and a slower decay. To characterize such pulse shapes, we have to introduce the terms , Front and Wave tail time as in Figure below:

The impulse voltage has the characteristics: 25

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1- Rise rapidly to a max value.

2- Decay slowly to zero.

3- Starting time (to): time taken by the pulse to reach 10% of the peak value.

4- Rise time (tr): time for the voltage pulse to rise from 10% to 90% of the peak value. tr = t0.9 - t0.1 5- Wave tail time ( twt): time at which the tail has reached 50% of the peak value on both side referred to (to).

The wave normally defined in terms of the nominal wavefront and wavetail. Durations wavefront is given by: 1.0 tr, 1.2 tr ( as in International Standard), and 1.25 tr ( as in British Standard).

Wave tail duration is defined as 50 µ sec. So impulse voltage is given normally as 1.2 / 50 µ s ( Inter. Standard) Or as 1.25 / 50 µ s ( British Standard).

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The impedance ratio case we have:

By Dr. Ali Albakry

equals the voltage ratio

, in this

This equation can now easily be transformed back into time domain: The highest output voltage will be achieved when C2 is small compared with C1, while the fastest rise will be achieved when:

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High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

By Dr. Ali Albakry

Following the same analysis as used for the original circuit, we arrive at the following expression for the output voltage (Vt):

Vt = E ( e

-

e

)

For practical case R2 is much greater than R1 and C1 much greater than C2. Components of equation may be drawn as in figure below:

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High Voltage Engineering Al-Mussaib Technical College / Elec. Depart. / Fourth Class

By Dr. Ali Albakry

Multistage Impulse Generator Simple circuits like those discussed in previous section can be used for the generation of double-exponential impulses with peak voltage values of about 200KV . Charging sets, high energy storage capacitors, resistors and spark gaps have quite manageable dimensions and properties up to this voltage.

The most convenient methods to generate higher voltages (impulses) use parallel arrangements of charging lines (sets) or capacitor, which are then switched (after charging) into a series configuration to produce an output pulse the amplitude of which is a multiple of the original charging voltage.

Practical impulse generators based on this principle were first introduced by MARX. A typical circuit is shown in the following figure, which show as a five stage generator:

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The stage capacitors C are charged in parallel through high value charging resistors R2. At the end of charging period, the points 1,2,3,4, and 5 will be at the potential of the D.C supply. While points 6,7,8,9,10, and 11 will remain at the earth potential.

The discharge of the generator is initiated by the breakdown of the spark gap (1,6) which is followed simultaneous breakdowns of the all remaining gaps. When firing an impulse generator it is essential that the low sphere gaps (1,6) fires first, this induces 2V across the second gap and an avalanche firing occurs. This may be ensured by having the first gap smaller than the succeeding gaps, but more normally, this is achieved by using a triggered gap in the first stage.

The impulse generators are characterized by the total output voltage, the number of stages, and the stored energy.

Vout = Vmax x number of stages.

Vmax = highest charging voltage.

Normally, the voltage across the test object is lower than the nominal output generated voltage due to the resistance and inductance in series with generator.

The energy of generator is given by: Eg = 1/2 Cg V 2max Where : Cg = discharge capacitance of the generator.

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