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I agree with dpc. Also High side lightning surges may be transfered to the low side by windings capacitive coupling. Utilities usually do not take a chance and put a low side arrester on power transformers. Bahram7 www.Simtech-Intl.com

Hello, I agree with you about the ceiling. Also I do not believe taht the excitation system can do much in this very severe case. The most important parameter is the generating system inertia which limits the machine acceleration during the fault duration. Bahram7

Kingtutley, 0.12 pu of X'd for Hydro units is low, I will go more for 0.20 to 0.25 pu. This is a fairly simple problem that can be solved by a standard short-circuit software or by hand calculation. Bahram7 www.Simtech-Intl.com

What type of short-circuit is involved here? What is the inertia value (H) of the turbine-Generator? How strong is the network connected at the HV ide? 300 ms three-phase-to-ground fault at the HV side is quite severe to save the network synchronism! Concerning the voltage at the low side where...

Grading capacitors on multi-breaks power circuit-breakers are a necessity to distribute the recovery voltage amoung breaks. Values are in the range of 800 to 2200 pF. Even with them, the first break (on the network side) will see 15-20% more voltage during a three-phase-to-ground fault. This is...

Field tests have shown that a couple of miles long HV Pipe-type cable attenuate rapidly fast-front overvoltages coming from the line by a factor of 50%. Do you have any feedback in this regard in the case of HV single-core cables? Thanks, Bahram7 www.simtech-Intl.com

Hydro units have usually H values between 3 to 4.5. While higher values of H will certainly help the transient stability of the network, they are not usually adjusted to meet network requirements. Factors such as the type of the excitation, system stabilizers, circuit-breakers reclosings, etc...

It all depends on the desired level of L-G fault currents. I've seen 6.75 and 7.5 Ohms at 25 kV. Bahram7 www.Simtech-Intl.com

The other two are Cooper and Joslyn I believe. Bahram7 www.Simtech-Intl.com

It all depends on the type of the rotor of the induction generator. For example if of doubly-fed type, the generator can produce reactive power so there is no need for shunt capacitors. For squirrel-cage type, the manufacturer should determine the size of the capacitor banks. Using SVC's (static...

The idea behind using capacitor banks to decrease transmission losses is to avoid transporting MVARS over long transmission lines and thus reduce the line current and consequently RI¨2 losses. Normally these capacitors are located at the receiving-end of the long line. I've seen up to 500 MVARS...

I would recommend EMTP-RV (www.emtp.com) which offers atate-of-the-art modeling of electrical machines. EMTP-RV is currently used in may wind farm simulations around the world in studies related to starting, faults, stability, LVRT, etc. Bahram7 www.Simtech-Intl.com

Large induction motors in the range of several thousand HPs draw between 8-10 pu starting currents. You need to use a motor starting software to evaluate the situation correctly. Bahram7 www.simtech-intl.com

The Delta will let the circulation of zero sequence currents so you will get a line to ground fault current. You can represent the autotransformer by a 3 windings YgYgD by calculation ZH, ZL and ZT from ZHL,ZHT and ZBT. In EMTP-RV (www.emtp.com) you can directly enter the nameplate imprdances...

The DC voltage trapped on the line will decay through the line insulators. In good weather it may stay there for few minutes. Bahram7

We can provide this service in EMTP-RV (www.emtp.com). Bahram7 www.simtech-Intl.com

Van Giang, Fast reclosing means that the breaker will reclose on the line with almost full trap charges. This may dissipate a lot of energy in the arrester. Utilities buy normally 2 columns line arresters at 500 & 800 kV to make sure that the line arresters will not fail during line discharges...

Hi, Can you help me find a good industrial analyis software modeling harmonic currents generated by various power-electronics-based converters used in induction heating, AC/DC ASD's etc? Bahram7 www.simtech-intl.com

Hi Tuan, It is true that by using MOV line-entrance arresters you can eliminate the need of closing resistors on a majority of transmission lines from 400 kV up to 800 kV. If you do fast reclosing (0.5 s) on long lines you need to make sure about the arresters energy capability. Bahram7...