a. Describe 5 major advantages associated with High Voltage Direct Current (HVDC) distribution systems when considered alongside alternative solutions. [5 marks]
i) HVDC have lower losses when compared to the equivalent AC cable distribution. The DC power transmission lines run at losses of approximately 0.3-0.4% per 100km. Converter stations also need to be considered which have losses of around 0.6%. Overall HVDC transmission losses are nearly always lower than AC transmission.
ii) HVDC can have a lower investment cost. Although HVDC stations are more expensive due to the additional cost of converter stations, if the length of the HVDC line goes past the ‘breakeven point’ which is around 600-800km, the overall cost will be cheaper due to having to run less lines than an AC one.
iii) …show more content…
This is because AC systems transmit in three phases whereas DC systems only need two conductors and HVDC can carry more power per conductor of a certain size.
iv) HVDC systems allow the asynchronous interconnection of grids that operate at different frequencies, or are otherwise incompatible. This allows them to exchange power without the need of the coordination of a synchronous network. This is done by the use of a ‘Back to Back HVDC Link’.
v) In a HVDC system the distribution of current over the cross-sectional area of the conductor is equal. When it comes to an AC system there is an uneven distribution of current over the cross sectional area of the conductor. This effect is known as the ‘Skin Effect’ and its effect increases as the length of the conductor increases.
b. Describe 5 major inherent disadvantages associated with High Voltage Direct Current (HVDC) distribution systems. Illustrate your answers using relevant, practical examples where appropriate. [5
i) HVDC have lower losses when compared to the equivalent AC cable distribution. The DC power transmission lines run at losses of approximately 0.3-0.4% per 100km. Converter stations also need to be considered which have losses of around 0.6%. Overall HVDC transmission losses are nearly always lower than AC transmission.
ii) HVDC can have a lower investment cost. Although HVDC stations are more expensive due to the additional cost of converter stations, if the length of the HVDC line goes past the ‘breakeven point’ which is around 600-800km, the overall cost will be cheaper due to having to run less lines than an AC one.
iii) …show more content…
This is because AC systems transmit in three phases whereas DC systems only need two conductors and HVDC can carry more power per conductor of a certain size.
iv) HVDC systems allow the asynchronous interconnection of grids that operate at different frequencies, or are otherwise incompatible. This allows them to exchange power without the need of the coordination of a synchronous network. This is done by the use of a ‘Back to Back HVDC Link’.
v) In a HVDC system the distribution of current over the cross-sectional area of the conductor is equal. When it comes to an AC system there is an uneven distribution of current over the cross sectional area of the conductor. This effect is known as the ‘Skin Effect’ and its effect increases as the length of the conductor increases.
b. Describe 5 major inherent disadvantages associated with High Voltage Direct Current (HVDC) distribution systems. Illustrate your answers using relevant, practical examples where appropriate. [5