Basic Extra High Voltage DC (EHVDC) System | Advantages

Basic Extra High Voltage DC (EHVDC) System :

~ The basic EHVDC system block diagram is as shown in Fig 16.1.
~ The major difference between this system and the EHVAC system is that converter units are included in this system at both the sending end and the receiving end.
~ That means thus system connects two AC systems (sending and receiving) by first AC to DC conversion at the sending end and then again DC to AC conversion at the receiving end.
~ The AC to DC conversion at the sending end is achieved by operating the converter station I in rectfier mode.
~ The DC power at the output of converter I then travels over the DC transmission line.
~ The EHVDC transmission system requires only two conductors as compared to three in EHVAC system.
~ This DC voltage is then converted back to AC by converter station 2, which operates in the inverter mode.
~ The DC voltages at which the transmission takes place range from ± 200 kV to ± 600 kV to transmit power over a two cable (two pole) line.
~ In order to reach this voltage level the three phase AC voltage at the sending end is stepped up by using a step up transformer.
~ The converter stations use many high power thynistors connected in series to give the required voltage rating, and many such series units are connected in parallel to meet the current requirement.
~ This high DC voltage is first converted to 3 phase AC voltage by converter 2 and then stepped down to a suitable low AC voltage by the step down transformer on the receiving end for distnbution.
~ The DC filters are added on the DC side of converter stations in order to generate a ripple free DC voltage on the DC transmission lines and to filter out harmonics. i
~ The DC filter design is simplified and becomes economical if each converter is a 12 pulse converter (instead of 6 pulse converter) due to the increase in the ripple frequency at the converter output.
~ The inclusion of converters makes the EHVDC system bidirectional. If for example converterl works as inverter and converter 2 as rectifier the direction of power flow will be reversed. i
~ A similar system can be used where there will be two or more AC generating stations driving a common load.
~ Such an interconnection is always easier through DC transmission system rather than an AC system, as then the AC systems, which are interconnected can work at different i frequencies.

Advantages of DC Transmission:

1. The line construction is simple. Hence, the line is cheaper as compared to AC.
2. The power per conductor of DC is more as compared with AC.
3. There is no charging current required because of which the length of transmission is not limited and the cable need not be derated.
4. The resistance offered by the DC conductors is less as compared to AC conductors.
5. Due to which in AC the current density is higher on the outer section of conductor as compared to the inner section, which is called as skin effect.
6. This is absent in case of DC transmission. i
7. The DC line is cheaper and simpler as it requires two conductors instead of three.
For long distance AC transmission, series and shunt compensation as to be provided, where the shunt load compensation is required to absorb the line charging during light load conditions and the series compensation is for stability. All this is not needed for DC transmission as it operates on unity power factor and zero charging current.
7. The corona loss and radio interference are directly related which is further dependent on the frequency of supply. As for DC the frequency is zero these losses are very less as compared to AC.
8. Higher operating voltages possible. i No stability problem.