HV Solid State Generation

8/14/2019 HV Solid State Generation

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LIM/EHV/March 2007HV System 1

To Be A World Class Maritime Academy

High Voltages Solid State


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Solid State Switching Principle

The power systems engineers is interested in high voltages primarily forpower transmission, and secondly for testing of his equipment used inpower transmission in laboratory

High voltage can be obtained locally from power generating plant throughthe use of solid state

In many testing laboratories, the primary source of power is at low voltage(400 V three phase or 230 V single phase, at 50 Hz). From which highvoltage can be obtained

On board ship the same technology can be used to use high voltage

Laboratory test are aimed to design the required high voltage Since insulation is usually being tested, the impedances involved are

extremely high (order of M ohm and the currents small (less than anampere).

High voltage testing does not usually require high power.

Thus special methods may be used which are not applicable whengenerating high voltage in high power applications.


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Solid State Switching Principle

In the field of electrical eng. & applied physics, high voltages are required forseveral applications As:

-a power supply (eg. hv dc) for the equipments such as electron microscope andx-ray machine.

-Required for testing power apparatus insulation testing.

-High impulse voltages are required for testing purposes to simulate over

voltages due to lightning and switching. Sometimes, high direct voltages are needed in insulation test on cables and

capacitors. Impulse generator charging units also require high dc voltages ofabout 100-200kV.

Normally for the generation of dc voltages of up to 100kV, electronics valverectifiers are used and the output currents are about 100mA. The rectifier

valves require special construction for cathode and filaments since a highelectrostatic field of several kV/cm exists between the anode and cathode inthe non-conduction period.

The ac supply to the rectifier tubes maybe of power frequency or maybe ofaudo frequency from an oscillator. The latter is used when a ripple of verysmall magnitude is required without the use of costly filters to smoothen the

ripple.


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Half and Full Wave Rectifier

Rectifier circuits for producing high dc voltages from ac sourcesmaybe

b. Half-Wave

c. Full-Wave

o The rectifier can be an electron tube or a solid state devices.Nowadays, single electron tubes are available for peak inversevoltages up to 250kV and semiconductor or solid state diodesup to 250kV.

o For higher voltages, several units are to be used in series. Whena number of units are used in series, transient voltagedistribution along each unit becomes non-uniform and specialcare should be taken to make the distribution uniform.


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V out

Vin

RL

Half Wave Rectifier

VAVG

Vp

0

T

Mean Load Voltage or Average Value of half wave output


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R Lto t1 t2to t1

+

-

D 1

D 2

t o t1 t2

VpVAVG

Full wave Rectifier Circuit

Mean Load Voltage or Average Voltage Full-wave output


7/17LIM/EHV/March 2007HV System 7


Voltage Multiplier Circuits

Both full-wave as well as half-wave circuits canproduce a maximum direct voltage corresponding tothe peak value of the alternating voltage.

When higher voltages are required voltage multipliercircuits are used. The common circuits are the voltagedouble circuit

Used for higher voltages.

Generate very high dc voltage from single supplytransformer by extending the simple voltage doublercircuit.


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Types of high voltages;

High d.c. voltages

High a.c. voltages of power frequency

High a.c. voltages of high frequency

High transient or impulse voltages of very short

duration - lightning overvoltages

Transient voltages of longer duration switching

surges


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The voltage doubler circuit makesuse of the positive and the

negative half cycles to charge twodifferent capacitors. These arethen connected in series aiding toobtain double the direct voltageoutput. Figure shows a voltagedoubler circuit.

In this case, the transformer willbe of small rating that for thesame direct voltage rating withonly simple rectification. Furtherfor the same direct voltage outputthe peak inverse voltage of thediodes will be halved. Voltage doubler circuit


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High Alternating Voltages

Required in laboratories and a.c. tests as well as for the

circuit of high d.c. and impulse voltage.

Test transformer are generally used.

Single transformer test units are made for high alternating voltages

up to about 200 kV. However, for high voltages to reduce the cost (insulation cost

increases rapidly with voltage) and make transportation easier, acascade arrangement of several transformers is used.

For higher voltage requirement, series connection or cascading ofthe several identical units of transformer is applied.


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Cascade arrangement of

transformers


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1600 kV, 9.6 MVA Cascaded Power Transformer


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A typical cascade arrangement of transformers used to obtain up

to 300 kV from three units each rated at 100 kV insulation. Thelow voltage winding is connected to the primary of the firsttransformer, and this is connected to the transformer tank whichis earthed.

One end of the high voltage winding is also earthed through thetank.

The high voltage end and a tapping near this end is taken out atthe top of the transformer through a bushing, and forms the

primary of the second transformer.

One end of this winding is connected to the tank of the secondtransformer to maintain the tank at high voltage.

The secondary of this transformer too has one end connected tothe tank and at the other end the next cascaded transformer is fed.

Cascade arrangement of transformers


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This cascade arrangement can be continued further if a still

higher voltage is required. In the cascade arrangement shown, each transformer needs

only to be insulated for 100 kV, and hence the transformer canbe relatively small. If a 300 kV transformer had to be usedinstead, the size would be massive. High voltage transformers

for testing purposes are designed purposely to have a poorregulation.

This is to ensure that when the secondary of the transformer isshort circuited (as will commonly happen in flash-over tests ofinsulation), the current would not increase to too high a valueand to reduce the cost. In practice, an additional seriesresistance (commonly a water resistance) is also used in suchcases to limit the current and prevent possible damage to thetransformer.



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What is shown in the cascade transformer arrangement is the basic principleinvolved. The actual arrangement could be different for practical reasons.

In the cascade arrangement shown, each transformer needs only to be insulated for

100 kV, and hence the transformer can be relatively small. If a 300 kV transformer

had to be used instead, the size would be massive. High voltage transformers for

testing purposes are designed purposely to have a poor regulation. This is to ensure that when the secondary of the transformer is short circuited (as

will commonly happen in flash-over tests of insulation), the current would not

increase to too high a value and to reduce the cost. In practice, an additional series

resistance (commonly a water resistance) is also used in such cases to limit the

current and prevent possible damage to the transformer. What is shown in the cascade transformer arrangement is the basic principle

involved. The actual arrangement could be different for practical reasons.



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High D.C. Voltages

Generation of high d.c. voltages is mainlyrequired in research work in the areas of pure

and applied physics. Needed in insulation test.

Use rectifier circuit (diode) to convert a.c. to

d.c. voltage. vacuum rectifiers, semiconductor

diodes


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Impulse High Voltage

Impulse voltages (IVs) are required in hv tests to simulate thestresses due to external and internal overvoltages, and also forfundamental investigations of the breakdown mechanisms.

Usually generated by discharging hv capacitors throughswitching gaps onto a network of resistors and capacitors.

In hv technology, a single, unipolar voltage is termed animpulse voltage.

Rectangular and wedge-shaped IVs are normally used for basicexperiments while for testing purposes, double exponential IVsare used.

Standard test of impulse voltages can be represented as doubleexponential wave, and its mathematical equation is defined asfollows;

V = Vo [exp(-t) exp(-t)]

Where and are constants of microsecond values.

HV Solid State Generation

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