What is HVDC(High Voltage Direct Current) Transmission?
The HVDC power system uses a high voltage direct current for transmission of a bulk amount of electrical power over long distances. The HVDC power system has the following advantages
- The HVDC transmission lines are less expensive than HVAC transmission lines
- In an HVDC power system transmission losses are less.
- The HVDC system interconnects the two different frequency and characteristic networks.
- By using an HVDC system, a bulk amount of power is supplied over long distances.
- In an HVDC power system, voltage fluctuation is very less.
- In the HVDC power system, efficient and economic transmission of a bulk amount of electrical power at long distances are possible.
In a High voltage AC system, voltage and current waves change their direction at regular intervals of time. Due to this losses are occurred in the transmission line conductors in form of heat. So the efficiency of the high voltage AC system is less for long distances and less power is transmitted.
In a high voltage DC system, voltage and current waves can not change their direction at regular intervals of time because DC voltage does not have any frequency. The losses in the High voltage DC system are negligible. So the efficiency of the high voltage DC system is more and power are rapidly transferred over long distances.
In a High voltage DC system, AC voltage and DC voltage are combined. In generating stations, high voltage three-phase AC voltage is generated and this three-phase AC voltage is converted into the two-phase DC voltage by using a rectifier at sending end. This two-phase DC voltage is again converted into the three-phase AC voltage by using the inverter at receiving end for distribution purposes.
The High voltage Direct current system is more economical and efficient for transmission lines having a length of more than 600km and cable lines having a length of more than 50 km.
Working of HVDC transmission system
The High voltage DC system consists of two converting stations. One converting station at sending end converts three-phase AC voltage into the two-phase DC voltage. It is called a rectifier station. Second, converting station at receiving end converts the two-phase to three-phase AC voltage. It is called rectifier stations. Between rectifier converting stations and inverter converting stations two-phase high voltage DC lines are used for transmission of a bulk amount of power.
The voltage at sending end and voltage at receiving end remain the same.
The HVDC Substation has the following equipment
1. Terminal substations
2. Earth Electrode line and earth electrode
3. Converter transformer
4. Thyristor valve or converter valves
5. Smoothing reactors
6. Harmonic filter
7. Control and protection system
1. Terminal Substations
The HVDC system consists of two AC substations at both ends. One terminal substation at sending end and a second substation at receiving end. The terminal substations are consists of A.C switchgear, bus bars, current transformers, potential transformers, circuit breakers etc.
2. Earth electrode line and earth electrode
The HVDC substations consist of two converting stations. One at sending end is called the rectifier station and the second at receiving end is called the inverter station. The mid point of each converting station is earthed through the earth electrode. For reducing the galvanising corrosion of the earth mat, earth electrodes are kept away from the HVDC stations at 5 to 25 km. The connection between the mid point of the converting stations and the earth electrode is made through the earth electrode line.
The earth electrode is installed away from the substations and major pipelines to avoid the galvanic corrosion of the substation earthing system, underground pipes, buried cables and structures.
3. Converter transformer
The converter transformer is connected between the a.c terminal substations or a.c bus bars and the thyristor valve or converting valves. The main function of the converter transformer is to transform the ac voltage to a suitable value for feeding the converter. In a converter transformer unit, there are three single phases or one three-phase is used.
The converter transformer steps up the voltage at sending end and steps down the voltage at the receiving end. The converter transformer supplies the reactive power to the converter through tap changing. The other function of the converter transformer is control of fault level by suitable reactance offered by the converter transformers and help in harmonic suppression. The converter transformer is specially designed for direct current.
4. Thyristor valves or converter valves
The thyristor valves or converter valves are made by connecting a number of thyristors in series. The converter valves convert the ac voltage into dc at sending end and convert the dc voltage into the ac voltage at the receiving end of the substations. The thyristor valves are used for conversion and inversion purposes.
The thyristors in the HVDC system are used in the 12-pulse bridge form. To obtain the required voltage and current, the thyristors are connected in a series-parallel configuration. The ratting of the thyristor valves are in voltage and current i.e 125 kV, 1200A.
5. Smoothing reactor
In HVDC transmission line smoothing reactors are connected in series with each pole of the converter. The smoothing reactors prevent the commutation failure in the inverter unit by limiting the rate of rising of direct current during commutation in one bridge when the direct voltage of another bridge collapse. The reactors are mainly used for smoothing out the direct current after conversion from ac to dc.
The smoothing reactor is used to reduce the fault current. In smoothing reactors, windings are disc and core is air core or iron core with air is used. The smoothing reactors are magnetically shielded and designed for high inductance. The rating of the smoothing reactor is 1H 2000A and the size of the smoothing reactor is very big.
6. Harmonic filter
In the HVDC transmission line, three-phase bridge converters are used to convert a pure ac sinusoidal waveform to a pure dc form. In practice, the conversion operation generates the harmonic voltage and harmonic current on the ac side as well as the dc side. These harmonics can not interfere with the operation of the conversion operation but these harmonics flow in ac line and dc lines.
These harmonics in ac lines and dc lines cause harmful effects like overheating of capacitors and generators, overvoltages at points in a network, interference with protective gears, interference with the nearby communication systems, radio interference and tv interference.
To eliminate these effects harmonic filters are used in HVDC systems. The harmonic filters are made of a resistor, inductor or capacitor bank and are used to suppress the alternating current harmonics produced due to the operation of the converter.
7. Control and protection system
The control of firing angle is very important in the HVDC transmission system and is carried out by optic fibre-based hardware circuitry. Other required controls are tap changing for voltage and reactive power control and extinction angle control.
The protection system used in HVDC systems is overvoltage protection, over current protection, transformer protection and converter protection.
Advantages of HVDC system
1. Charging current is absent in the HVDV transmission system. So the dc system operated at a unity power factor.
2. There is no proximity and skin effect because of frequency of dc is zero.
3. There are no stability problems in the HVDV transmission system.
4. The dielectric losses in the dc system are less so the current carrying capacity is higher. So power losses are reduced in the HVDC system.
5. The corona loss and radio interference in the dc system are less as compared to ac system.
6. The insulation required for equipment used in the HVDC system is less.
7. The switching surges in the HVDC system are lesser as compared to the ac system.
8. There is voltage regulation in the dc system that is better.
9. There is no Ferranti effect in the dc system.
10. The only two conductors are required in the dc system as compared to ac system which requires three conductors and also a lesser number of insulator strings are required in the dc system. So the cost of the HVDC system is lesser than compared of the ac system.
11. The ground is used as a return conductor in the HVDC transmission system.
12. By using an HVDC system the bulk amount of power can be transmitted over long distances.
13. In HVDV transmission system synchronous operation is not required.
14. In the dc system short circuit current is very low as compared to ac system.
15. In an HVDC system, tie-line power can easily be controlled.
16. Interconnection of two different frequencies ac system is possible with dc line.
17. The power flow through the HVDC line can be easily controlled.
18. The HVDC system requires less phase to phase and phase to ground clearance. So the size of the tower is reduced as compared to the ac system.
19. The HVDC system towers are cheaper than the ac system.
20. There are two conductors that are used for transmission of power in the bipolar mode so power loss is reduced in the HVDC systems.
21. In an HVDC system, there is no need for reactive power compensation devices.
22. In an HVDC system, accurate and lossless power flow is possible in dc link.
Disadvantages of HVDC system
1. The conveyer substations are placed at sending end and receiving end of the HVDC transmission line due to this cost of the HVDC system is increased.
2. The inverter and rectifier are used in the HVDC system which generates the harmonics. For suppression of these harmonics, the harmonic filter is required which increases the cost of the HVDC system.
3. In an HVDC system, high voltage dc currents are very difficult to break hence HVDC system requires high voltage dc circuit breakers which increases the cost of the system.
4. In an HVDC system, the inverter has limited overloading capacity.
5. The HVDC system does not have any Transformer for changing the voltage levels.
6. The HVDC system is very expensive and complicated as compared to the dc system for short distances.
7. There are heat losses are takes place in the converter Substation which are reduced by the cooling system.
8. If a fault occurred in an ac substation, it causes the power failure in HVDC Substation.
9. The HVDC system is not suitable for primary transmission and distribution. So HVDC systems are not used.
10. The HVDC system has additional losses in thyristor valves and converter transformer and these losses are continuous.
11. The HVDC system can not operate in overloading conditions.