Transcript E E 6372
E E 6372 High Voltage Engineering 1 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) 2 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 3 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 4 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 5 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 6 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 7 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 8 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 9 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. 10 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 11