Advantages of HVDC over HVAC transmission | EEP

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Advantages of HVDC over HVACtransmission

Advantages of HVDC over HVAC transmission | EEP http://electrical-engineering-portal.com/advantages-of-hvdc-o...

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Advantages of HVDC over HVAC transmission (on photo: The valve hall of the HVDCsystem is where the power is made ready for transmission; by Siemens)

AC as preferred optionDespite alternating current being the dominant mode for electric power transmission, in anumber of applications, the advantages of HVDC makes it the preferred option over ACtransmission.

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Undersea cables where high capacitance causes additional AC losses (e.g., the 250-km BalticCable between Sweden and Germany).

1.

Endpoint-to-endpoint long-haul bulk powertransmission without intermediate taps, forexample, in remote areas.

2.

Increasing the capacity of an existing power grid in situations where additional wires aredifficult or expensive to install.

3.

Allowing power transmission between uunnssyynncchhrroonniizzeedd AACC ddiissttrriibbuuttiioonn ssyysstteemmss.4.Reducing the profile of wiring and pylons for a given power transmission capacity, asHVDC can carry more power per conductor of a given size.

5.

Connecting a remote generating plant to the distribution grid; for example, the Nelson RiverBipole line in Canada (IEEE 2005).

6.

Stabilizing a predominantly AC power grid without increasing the maximum prospectiveshort-circuit current.

7.

RReedduucciinngg ccoorroonnaa lloosssseess (due to highervoltage peaks) compared to HVAC transmission lines ofsimilar power.

8.

Reducing line cost, since HVDC transmission requires fewer conductors; for example, twofor a typical bipolar HVDC line compared to three for three-phase HVAC.

9.

HVDC transmission is particularly advantageous in uunnddeerrsseeaa ppoowweerr ttrraannssmmiissssiioonn. Long underseaAC cables have a high capacitance.

Example (VIDEO)

500 MW HVDC Light transmission interconnection

ABB has commissioned a 500-megawatt HVDC Light (VSC) transmission interconnection thatlinks the IIrriisshh aanndd UU..KK.. ggrriiddss, enabling cross-border power flows and enhancing grid reliability


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and security of electricity supplies.

The East West Interconnector includes a 262 km high voltage cable link of which 186 km runssubsea.

Cant see this video? Click here to watch it on Youtube.

Consequently, the current required to charge and discharge the capacitance of the cable causesadditional power losses when the cable is carrying AC, wwhhiillee tthhiiss hhaass mmiinniimmaall eeffffeecctt ffoorr DDCCttrraannssmmiissssiioonn. In addition, AC poweris lost to dielectric losses.

In general applications, HHVVDDCC ccaann ccaarrrryy mmoorree ppoowweerr ppeerr ccoonndduuccttoorr than AC, because for a givenpower rating, the constant voltage in a DC line is lower than the peak voltage in an AC line.

This voltage determines the insulation thickness and conductor spacing. This reduces tthhee ccoosstt ooffHHVVDDCC ttrraannssmmiissssiioonn lliinneess as compared to AC transmission and allows transmissionline corridors to carry a higher power density.

A HVDC transmission line would not produce the same sort of extremely low frequency (ELF)electromagnetic field as would an equivalent AC line. While there has been some concern in thepast regarding possible harmful effects of such fields, including the suspicion ofincreasing leukemia rates, the current scientific consensus does not consider ELF sources andtheir associated fields to be harmful.

Deployment of HVDC equipment wwoouulldd nnoott ccoommpplleetteellyy eelliimmiinnaattee eelleeccttrriicc ffiieellddss, as there wouldstill be DC electric field gradients between the conductors and ground. Such fields are notassociated with health effects.


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Because HVDC allows power transmission between unsynchronized AC systems, it can helpincrease system stability. It does so by preventing cascading failures from propagating from onepart of a wider power transmission grid to another, while still allowing power to be imported orexported in the event of smaller failures.

This feature has encouraged wider use of HVDC technology for its stability benefitsalone. Power flow on an HVDC transmission line is set using the control systems ofconverter stations. Power flow does not depend on the operating mode of connected powersystems.

Thus, unlike HVAC ties, HVDC intersystem ties can be of arbitrarily low transfer capacity,eliminating the wweeaakk ttiiee pprroobblleemm,, and lines can be designed on the basis of optimal powerflows.

Similarly, the difficulties of synchronizing different operational control systems at different powersystems are eliminated. Fast-acting emergency control systems on HVDC transmission lines canfurther increase the stability and reliability of the power system as a whole. Further, power flowregulation can be used for damping oscillations in powersystems or in parallel HVAC lines.

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Direct-Current (HVDC) Transmission Lines


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For example, the rapidly growing Indian power system is being constructed as several regionalpower systems interconnected with HVDC transmission lines and back-to-back converters withcentralized control of these HVDC elements (Koshcheev 2001).

Likewise, in China, 800-kV HVDC will be the main mode used to transmit large capacity oververy long distances from large hydropower and thermal power bases. Other applications involvelong-distance transmission projects with few tie-ins of power supplies along the line (Yinbiao2005).

RReeffeerreennccee:: Argonne National Laboratory The design, construction and operation of long-distance high voltageelectricity transmission technologies

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Advantages of HVDC over HVAC transmission | EEP