generating of high alternating voltage

High Voltage (EPO 630)

Generation of High Voltages & Currents(Assignment 2)

TEAM MEMBERS• Abdiwali Hussein Mohammad• Mohammad Muslim Ramli • Khairudin Bin Hadzli 2010486346• Mohd Jamil Abdullah 2011132445• Muhd Hussaini Bin Che Hashim 2011727477• Muhammad Adib Bin Ma Hussin 2011597201

GENERATION OF HIGH ALTERNATING VOLTAGES

• Cascaded transformers • Resonant transformers• Tesla coil

Methods which can be used to generate high a.c. voltage are

CASCADE TRANSFORMERS

• When test voltage requirements are < 300 kV, a single transformer can be used for test purposes.

• For higher voltage requirements, -A single unit construction becomes difficult and costly due to insulation problems. -Transportation and assembly of large transformers become

difficult. • These drawbacks are overcome by: • Series connection (or cascading) of several identical units of transformers,

in which the high voltage windings of all the units are in series

FIGURE CASCADE TRANSFORMER UNITS.

CASCADE TRANSFORMERS• It consists of:

HV and LV windings Meter winding - to measure the output voltage

• Circuit configuration: – 1st transformer, T1 is at ground potential – 2nd transformer, T2 is kept on insulators and maintained at a potential of V2 (i.e. the

output voltage of the first unit above the ground). – The HV winding of the T1 is connected to the tank of T2. – The LV winding of T2 is supplied from the excitation winding of T1. – Excitation winding of T1 is in series with the HV winding of T1 at its high voltage end. – The rating of the excitation winding is almost identical to that of the primary or the low

voltage winding. – The HV connection from T1 and the excitation winding terminal are taken to T2 through

a bushing. – In a similar manner, the 3rd transformer T3 is kept on insulators above the ground at a

potential of 2V2 and is supplied likewise from T2.


• The numbers of stages in this type of arrangement are usually two to four, but very often, three stages are adopted to facilitate a three-phase operation so that (3V2) can be obtained between the lines.

• In Fig, a second scheme for providing the excitation to the second and the third stages is shown


Cascade TX unit with isolation TX for excitation


• Disadvantages of this scheme: - Expensive and requires more space.

• The advantages of this scheme is that: -Natural cooling is sufficient -The transformers are light and compact. -Transportation and assembly is easy. - The construction is identical for isolating transformers and the HV cascade units.

RESONANT TRANSFORMERS

A high-voltage transformer in which thesecondary circuit is tuned to the frequency of the power supply. • It consists of: – Leakage reactances of the windings – Winding resistances – Magnetizing reactance – Shunt capacitance across the output terminal due to the bushing of the HV terminal and test object.


Figure equivalent circuit

RESONANT TRANSFORMERS• The advantages of this principle are:

– Gives an output of pure sine wave – Power requirements are less (5 to 10% of total kVA required) – No high-power arcing and heavy current surges occur if the test object

fails (since resonance ceases at the failure of the test object) – Cascading is possible for very high voltages – Simple and compact test arrangement – No repeated flashovers occur in case of partial failures of the test

object and insulation recovery. • The disadvantages:

– Require additional variable chokes capable of withstanding the full test voltage & the full current rating.


Figure shown Series resonant a.c. test system


Figure shown Parallel resonant a.c. test system

RESONANT TRANSFORMERS• Parallel resonant a.c. test system

– A voltage regulator is connected to the supply mains – The secondary winding of the exciter transformer is connected across the high voltage reactor L and the capacitive load C.

– L is varied by varying its air gap and operating range is set in the ratio 10:1

– C comprises of the capacitance of the test object, capacitance of the measuring voltage divider, capacitance of the high voltage bushing etc.

• Advantages of parallel resonant circuit: – More stable output voltage – High rate of rise of test voltage (independent of the degree of tuning and the Q-factor)

• Used to generate high frequency a.c. high voltage – Also known as high frequency resonant transformers

• High frequency a.c. high voltages are required for: – Rectifier d.c. power supplies – Testing electrical apparatus for switching surges

• The advantages of these high frequency transformers are: – The absence of iron core in transformers and hence saving in cost and size – Pure sine wave output – Slow build-up of voltage over a few cycles and hence no damage due to

switching surges – Uniform distribution of voltage across the winding coils due to subdivision

of coil stack into a number of units

TESLA COILS

TESLA COILS

• The primary is fed from a d.c. or a.c. supply through condenser C1 • A spark gap G connected across the primary is triggered at the desired voltage

V1 which induces a high self-excitation in the secondary. • The primary and the secondary windings (L1 and L2) are wound on an

insulated transformer with no core (air-cored) and are immersed in oil. • The windings are tuned to a frequency of 10 to 100 kHz by means of

condensers C1 and C2. • The output voltage V2 is a function of L1, L2, C1, C2 and the mutual inductance

M. • Usually, the winding resistances will be small.

TESLA COILS

• Methods which can be used to generate high impulse voltage are: – Single-stage generator circuits –Multistage impulse generators - Marx Circuit

GENERATION OF HIGH IMPULSE VOLTAGES

Standard Impulse Waveshapes

• Transient overvoltages are due to: – Lightning – of very short duration – Switching surges – of longer duration

• They can cause steep build-up of voltages on transmission lines and other electrical appliances

• Lightning overvoltage waveform can be represented as double exponential

• The waveform has a fast rising edge and a longer tail, which is described through the peak value (magnitude), rise time (or front time) and duration (or tail or fall time), as shown in Figure 2.

Standard Impulse Waveshapes

Impulse waveform

SINGLE-STAGE GENERATOR

CIRCUITS• Impulse waveform may be produced by using single-stage generator circuits. • Two basic circuits for single-stage impulse generators are shown in Fig. 2.25. • Capacitor C1 is slowly charged from a d.c. source V • When the spark gap G is triggered, C1 will be discharged into R1, R2, C2 (i.e. the

wave shaping network) • Resistors R1, R2 and the capacitance C2 form the wave shaping network. • R1 will primarily damp the circuit and control the front time T1. • R2 will discharge the capacitors and therefore essentially control the wave tail. • Capacitance C2 represents the full load (i.e. the object under test as well as all

other capacitive elements which are in parallel to the test object) • The discharge voltage V0 (t) will produce the desired double exponential wave

shape. • For the analysis, we may use the Laplace transform circuit sketched in Fig. 2.25(c) • It simulates the boundary condition, that for t ≤ 0, C1 is charged to V and for t > 0

this capacitor is directly connected to the wave shaping network.

Wave shape Control

• Generally, for a given impulse generator of Figure 2.25(a) and (b), the generator capacitance C1 and load capacitance C2 will be fixed depending on the design of the generator and the test object.

• Hence, the desired wave shape is obtained by controlling R1 and R2. • The following approximate analysis is used to calculate the wave front and

wave tail times. – The resistance R2 will be large. – Hence, the simplified circuit shown in Fig. 6.16b is used for wave front

time calculation. – Taking the circuit inductance to be negligible during charging, C1

charges the load capacitance C2 through R1. – The time taken for charging is approximately 3 times the time constant

of the circuit and is given by

MULTISTAGE IMPULSE GENERATORS - MARX CIRCUIT

• In the above discussion, the generator capacitance C1 is to be first charged and then discharged into the wave shaping circuits. – A single capacitor C1 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. • Hence, for producing very high voltages, a bank of capacitors are charged

in parallel and then discharged in series. – Such as circuit is called as multistage impulse generator or Marx

generator • The schematic diagram of Marx circuit and its modification are shown in

Fig. 6.17a and b respectively.

MULTISTAGE IMPULSE GENERATORS - MARX CIRCUIT

GENERATION OF HIGH IMPULSE CURRENTS

• Method which can be used to generate high impulse current is: –Impulse current generator

Circuit for Producing Impulse Current Waves

• The waveshapes used in testing surge diverters are the 4/10 and 8/20 μs waveforms. – The tolerances allowed are ± 10% only.

• Basic circuit to produce impulse current waveform is shown in Fig below:


Figure basic circuit of an impulse current generator


• A bank of capacitors connected in parallel are charged to a specified value and are discharged through a series R-L circuit.

• 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 the shunt L is an air cored high current inductor.

Generation of High Impulse

Currents• For producing large values of impulse currents, a number of capacitors are charged in parallel and discharged in parallel into the circuit

• The arrangement of capacitors is shown in Fig:

Generation of High Impulse Currents

Figure shown arrangement of capacitors for high impulse current generation

Generation of High Impulse Currents

• To minimize the effective inductance, the capacitors are subdivided into smaller units.

• If there are n1 groups of capacitors, each consisting of n2 units and if L0 is the inductance of the common discharge path, L1 is that of each group and L2 is that of each unit, then the effective inductance L is given by:

Thank You!

generating of high alternating voltage

Engineering