Engineering360: "HVDC vs HVAC power transmission systems"

I see no mention of the special circuit breakers needed to ensure the opening arc is quenched because there is no Zero point transition to assist the arc quenching not the fact that using DC vacuum breakers can lead to the generation of Xrays.

I remember using old DC circuit breakers on medium voltage equipment and how the contacts were connected to coils to produce a reverse magnetic arc quenching field and the added cost of the devices was quite large.

Still you can save on one transmission line if you run DC but what about a flashover from lightning?

Would like more substance in the post.

A simple station-to-station HVDC line does not need breakers to clear a fault. The rectifier station can instantly can to inverter operation which drives the current and voltage to zero. Typically for a DC fault a system called Line Protection will drive the current and voltage to zero and then re-energize the line to test if the fault has been cleared. Most systems will try a number of times, the ones I know will try three times. After the third try the line will go into a reduced voltage operating mode, usually at 70% line voltage. Then a attempt to go back to full voltage can be tried at the control agency request. If the line has a hard fault and can not operate at the 70% level the line is de-energized and line crews are sent out to physically inspect the area where the fault is detected. Another of the control systems in a HVDC transmission line is the ability to locate where the fault occurred within one to two towers.

Multiple HVDC transmission scheme can and do use DC breakers. There has been work on different types of DC breakers. One investigation utilized "off the shelf" AC breaker parts. I think that was an ABB/EPRI project.

tom

I forgot to mention that HVDC transmission line consist of two separate transmission lines, one at a positive voltage and the other negative. When a fault occurs it almost universally on only one of the two conductors. If the fault is bolted and can not be cleared the other line still remains in service and and transmit to power.

A bipolar transmission system has an earth ground at each station and in normal Bipolar operation there is no ground current. Then a pole is tripped out the full current flows through the ground until either the power is transmission is stopped or switching occurs. Each system has different operational procedures regarding ground current operation. There may be limits on the length of time and amperage due to preventing cathodic erosion of various other pipeline, railroad tracks, etc. If the stations have built excellent ground electrodes the current flows towards the mantle and not at the surface of the earth.

tom

Would DC be more efficient than AC in powering railroads?

That is a good question! I do not know enough about railroad locomotive engines to be able to express an opinion. Although I'm pretty sure that the voltages for a train are subcutaneously lower than those used on electric power transmission lines. The stations I worked at had DC line voltages of +-500kv. I would think a train might be a few hundred of volts, in the range of 2-3 hundred. That is only a guess.

Lined HVDC systems (above ground transmission lines) really only make sense if the length of the line is greater than 450 miles. Other practical uses are when to line is buried or underwater which usually means a shorted transmission line length.

The efficiency of the DC conductor is better than an AC is because dc current flows through the entire cross-section. AC only flows at the outer skin of the conductor.

The station equipment for a HVDC transmission is more expensive to build, the right of way is narrower than an equivalent AC line for the same amount of power flow.

Skin effect at 50hz appears to be minimal and most eHV conductors are multiple strands each strand < 10mm diameter. Calculation shows skin effect at 50hz in copper at 9000µm or 9mm.