Definition of HVDC Transmission:
High voltage direct current (HVDC) power systems uses D.C. for transmission of bulk power over long distances. For long distance power transmission, HVDC lines are less expensive, and losses are less as compared to AC transmission. It interconnects the networks that have different frequencies and characteristics.
In AC transmission, alternating waves of voltage and current travels in the line which change its direction every millisecond; due to which losses occur in the form of heat. Unlike AC lines, the voltage and current waves don’t change their direction in DC. HVDC lines increases the efficiency of transmission lines due to which power are rapidly transferred.
In a combined AC and DC system, generated AC voltage is converted into DC at the sending end. Then, the DC voltage is inverted to AC at the receiving end, for distribution purpose. Thus, the conversion and inversion equipments are also needed at the two ends of the line. HVDC transmission is economical only for long distance transmission lines having a length more than 600kms, and for underground cables of length more than 50kms.
HVDC transmission system working:
In generating substation, AC power is generated which can be converted into DC by using a rectifier. In HVDC substation or converter substation rectifiers and inverters are placed at both the ends of a line. The terminal substation, which converts AC into DC is called a rectifier terminal, while the terminal substation which converts DC into AC is called an inverter terminal.
The DC is flowing with the overhead lines and at the user end again DC is converted into AC by using inverters, which are placed in converter substation. The power remains the same at the sending and receiving ends of the line. DC is transmitted over long distances because it decreases the losses and improves the efficiency.
A system having more than two converter stations and one transmission line is called a ‘two terminal DC system’ or a ‘point-to-point system’. Similarly, if substation has more than two converter stations and interconnecting DC terminal lines, it is called multiterminal DC substation.
Economic Distance For HVDC transmission lines:
DC lines are cheaper than the AC lines, but the cost of DC terminal equipment is very high as compared to AC terminal cables (shown in the graph below). Thus, the initial cost is high in HVDC transmission system, and it is low in the AC system.
The point of intersection of two curves is called the breakeven distance. Above the breakeven distance, the HVDC system becomes cheaper. Breakeven distance varies from 500 to 900 km in overhead transmission lines. HVDC lines are not preferred below the breakeven distance lines except for interconnection.
Advantages of HVDC transmissions:
Disadvantages of HVDC transmission:
High voltage direct current (HVDC) power systems uses D.C. for transmission of bulk power over long distances. For long distance power transmission, HVDC lines are less expensive, and losses are less as compared to AC transmission. It interconnects the networks that have different frequencies and characteristics.
In AC transmission, alternating waves of voltage and current travels in the line which change its direction every millisecond; due to which losses occur in the form of heat. Unlike AC lines, the voltage and current waves don’t change their direction in DC. HVDC lines increases the efficiency of transmission lines due to which power are rapidly transferred.
In a combined AC and DC system, generated AC voltage is converted into DC at the sending end. Then, the DC voltage is inverted to AC at the receiving end, for distribution purpose. Thus, the conversion and inversion equipments are also needed at the two ends of the line. HVDC transmission is economical only for long distance transmission lines having a length more than 600kms, and for underground cables of length more than 50kms.
HVDC transmission system working:
In generating substation, AC power is generated which can be converted into DC by using a rectifier. In HVDC substation or converter substation rectifiers and inverters are placed at both the ends of a line. The terminal substation, which converts AC into DC is called a rectifier terminal, while the terminal substation which converts DC into AC is called an inverter terminal.
The DC is flowing with the overhead lines and at the user end again DC is converted into AC by using inverters, which are placed in converter substation. The power remains the same at the sending and receiving ends of the line. DC is transmitted over long distances because it decreases the losses and improves the efficiency.
A system having more than two converter stations and one transmission line is called a ‘two terminal DC system’ or a ‘point-to-point system’. Similarly, if substation has more than two converter stations and interconnecting DC terminal lines, it is called multiterminal DC substation.
Economic Distance For HVDC transmission lines:
DC lines are cheaper than the AC lines, but the cost of DC terminal equipment is very high as compared to AC terminal cables (shown in the graph below). Thus, the initial cost is high in HVDC transmission system, and it is low in the AC system.
The point of intersection of two curves is called the breakeven distance. Above the breakeven distance, the HVDC system becomes cheaper. Breakeven distance varies from 500 to 900 km in overhead transmission lines. HVDC lines are not preferred below the breakeven distance lines except for interconnection.
Advantages of HVDC transmissions:
- A lesser number of conductors and insulators are required thereby reducing the cost of the overall system.
- Lesser phase-to-phase clearance and phase-to-ground clearance.
- Lighter and cheaper towers.
- Lesser corona loss and reduced radio and television interference as compared to HVAC transmission lines of similar power.
- Power loss is reduced with DC because fewer numbers of lines are required for power transmission.
- Earth returns can be used in HVDC system. If any fault occurs in one pole, the other pole with ‘earth returns’ acts as an independent circuit. This results in a more flexible system.
- It forms an asynchronous connection between two AC stations connected through an HVDC link; i.e., the transmission of power is independent of sending frequencies to receiving end frequencies. Hence, it interconnects two substations with different frequencies.
- Due to the absence of frequency in the HVDC line, losses like skin effect and proximity effect does not occur in the system.
- It does not generate or absorb any reactive power. So, there is no need for reactive power compensation.
- Power flows through DC link can be controlled rapidly and accurately.
Disadvantages of HVDC transmission:
- Converter substations are placed at both the sending and the receiving end of the transmission lines, which result in increasing the cost.
- Inverter and rectifier terminals generate harmonics which can be reduced by using active filters which are also very expensive.
- If a fault occurs in the AC substation, it may result in a power failure for the HVDC substation placed near to it
- Inverter used in Converter substations have limited overload capacity.
- Circuit breakers are used in HVDC for circuit breaking, which is also very expensive.
- It does not have transformers for changing the voltage levels.
- Heat loss occurs in converter substation, which has to be reduced by using the active cooling system.
- HVDC link itself is also very complicated.
What is High voltage direct current(HVDC) Transmission
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