Download E E 6372

E E 6372
High Voltage Engineering
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Generation and Transmission
o Started with DC
o Edison in NYC in 1882
o Holburn in London in 1882
o Low voltage DC cannot deliver much power
due to voltage drop
o Utilization had to be at low voltage
o No effective method to alter voltage levels (until
solid-state switching developed a century later)
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Generation and Transmission
o AC Systems developed later
o Nikola Tesla filed 7 patents in 1887 for inventions
including the transformer, AC generator and
motor.
o Tesla backed financially by George Westinghouse
o 10 kV system at Depford transmitted power 28
miles to London in 1890
o Tesla/Westinghouse exhibit at World’s Fair in 1893
in Chicago led the USA development
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Generation and Transmission
o Power transmitted is proportional to V2/ZL
where V is the voltage and ZL is the surge
impedance.
Z L  60ln
2h
Rc
o Typical ZL = 250 
o A voltage of 100 kV would allow 40 MW
o A voltage of 1000 kV would allow 4 GW
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Generation and Transmission
o This incentive led to higher voltages such as
current levels of 345 kV, 500 kV and 765 kV.
o New technologies (solid-state high voltage and
high power switches) allow better utilization
of existing ROW’s:
o Static VAR compensators
o DC transmission lines
o Underground cable systems better with DC
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Voltage Stresses
• Normal operating voltages not a threat unless
– Pollutants/contaminants compromise insulation
– Structural damage to insulation
• External voltages (mainly lightning)
– Less important at higher voltages
• Internal voltages
– Mainly switching and faults
– Load & generator fluctuations
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Voltage Stresses
• The balance between stress on insulation and
dielectric strength is called insulation
coordination.
• Voper_level < Vprot_action < Vinsul_rating
• In classical approach,
Safety Margin = Vinsul_rating – Vprot_action
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Testing Voltages
• Must test for over-voltages as well as rated ac
and dc levels
• Simulate switching surges & lightning strikes
• AC voltage tests: 1 minute at rated frequency
– Dry test only for indoor equipment
– Wet and dry tests for outdoor equipment
– “Standard rain” used for wet tests
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Lightning Impulses
• Short rise time: 1 ms – 10’s of ms
• Standard: 1.2 ms rise with half peak at 50 ms
1.2 ms
50 ms
t
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Switching Impulses
• Equipment vulnerable to slow rise times at
around 100 – 300 ms.
• Standard test pulse is 250 ms rise time to half
peak at 2.5 ms.
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DC Test Voltages
• Hard to test cables due to large capacitance
• Use DC test voltages
– Some premature failures due to dc testing
– Now use very low frequency (VLF) at about 1 Hz
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