Electric Power Transmission: High Voltage Direct Current

Hector R Ayala Nieves

Currently the most efficient and widely used system for transmission of electric power is a three phase AC method (Tri-phase AC) where power is sent in a three wire configuration (or four wire if a neutral wire is included) with each wire carrying current at the same frequency but with each one’s phase shifted 2/3π (120 degrees) from each other.  Tri-phase AC transmission’s advantages for power distribution include the relative ease at which voltage can be modified using a transformer and the fact that power transferred with this method is constant with a smoother flow.  Houses usually utilize a single phase AC  from those three wires with the neutral wire or two of the three live wires, while industrial scale machinery utilize all three tri-phase wires to operate.

But there is a slight problem to using AC for transferring power over very long distances.  Due to the nature of alternating current this method of transferring power cannot be used because due to capacitance between the phases and between the phases and the medium surrounding it, energy is lost and at certain distances AC methods are practically useless.  Since those cables are at high voltages, and those cables are surrounded by a relatively thin insulator, they act as coaxial capacitors and current  changes, therefore additional current must go through the cable to charge the capacitor as well as carry the power though the cable.  At very long distances all the current could be used up simply to charge the capacitor (underwater the maximum length before AC transmission becomes inefficient is about 30km).  Furthermore, the corona discharge can cause power losses with the creation of ions in the fluid and air surrounding the wires.  How can these problems be solved?  By using High Voltage DC (HVDC)!  The main reason DC is not used for power transmission is because it is expensive to install the necessary equipment for voltage stepping, but for very long distances, DC transmission is actually ideal.  

Capacitance also exists in HVDC, but only initially when the current starts to flow for the first time.  Since direct current does not change, once capacitance is fully filled with the extra necessary current then the rest of the current is fully transferred.  The cases where HVDC is used is when the cost of using AC for very long distances is greater than the equipment needed to be installed at the end of the HVDC lines to transform it into tri-phase AC.  It is also used when AC is impractical or impossible for very long distances such as underwater AC transmission.  Another use for HVDC is the transfer of power from one power grid to another which phases are asynchronous.  Imaging having two tri-phase systems needed to be synchronized and be 1200km apart, each system having their own power sources therefore each power plant has to be producing energy at the same frequency and phase shift for each line simultaneously.  This is a tedious control problem, which could even produce cascading failures if one network fails.  However, having a HVDC line connecting them makes transfer of power between them more manageable and even cheaper at those distances.  In addition, HDVC requires only one cable for transmissions unlike tri-phase AC, reducing costs, and does not need as much insulation therefore could be used in preexisting wiring.  Also, HVDC can carry more power per conductor.  The amount of conducting and insulating material used is determined by the voltage that will be carried.  For DC the voltage is constant, but for AC the peak voltage determines the amount used.  Then again, the standard voltage considered in AC is the RMS voltage which is about 71% of the peak voltage; therefore for the same voltage you need more materials for AC than for DC.

The main factor against the use of HVDC is the conversion, control and maintenance.  Converting HVDC into AC and voltage stepping requires expensive equipment at the ends of each HVDC line.  In addition, setting up multi-terminal HVDC systems is complex and require all terminals to be in good communication under good control to regulate power flow, which in AC is practically self regulated by phase shift properties and impedance.  Nevertheless, it is interesting to see how an unorthodox method had to be created and is being used for this kind of power transmission where a simple property such as capacitance and power losses caused delivering power difficult or impossible.

Power Transmission Records

    * Highest capacity system: 6.3 GW HVDC Itaipu (Brazil) (±600 kV DC)

    * Highest transmission voltage (AC): 1.15 MV on Powerline Ekibastuz-Kokshetau (Kazakhstan)

    * Largest double-circuit transmission, Kita-Iwaki Powerline.

    * Highest pylons: Yangtze River Crossing (height: 345 m)

    * Longest power line: Inga-Shaba (length: 1,700 kilometers)

    * Longest span of power line: 5,376 m at Ameralik Span

    * Longest submarine cables:

          o NorNed, North Sea - (length of submarine cable: 580 kilometers)

          o Basslink, Bass Strait - (length of submarine cable: 290 kilometers, total length: 370.1 kilometers)

          o Baltic-Cable, Baltic Sea - (length of submarine cable: 238 kilometers, HVDC length: 250 kilometers , total length: 262 kilometers)

    * Longest underground cables:

          o Murraylink, Riverland/Sunraysia - (length of underground cable: 180 kilometers)