For the economical operation of a transmission line, high voltage transmitted for long-distance. But there are some limitations to the HVAC transmission system. To overcome these limitations, introduced the concept of an HVDC transmission line.
The generation of electrical power is in the form of AC. For DC generation, there is difficulty in commutation for high voltage bulk power. Therefore, it is not possible to generate electrical power in DC.
After 1990, this transmission system becomes popular because of the development of high voltage power electronics equipment.
The electrical power generated in AC and transmitted in DC. Hence, this system is a hybrid system.
The power generates from the power plant in the form of three-phase AC. This voltage stepped up by the step-up transformer.
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This high voltage AC converted into high voltage DC with the help of a static rectifier. The power transmitted for long distances in the form of DC through the HVDC transmission line.
At the receiving end of the line, the high voltage DC converted into high voltage AC by means of the static inverter.
After that, a stepped down transformer used to reduce the voltage level at the required value. From there, the LT transmission lines are run in different areas.
For the HVDC transmission system, most advanced equipment needed.
Equipment of HVDC Transmission line
1) Terminal substation
The generation and use of electrical power are in the form of AC. Therefore, AC substations required at both ends of the line. These substations are a conventional type which contains AC switchgear, busbars, CT, PT, circuit breaker, etc.
2) Electrode line and earth electrode
At each end, the mid-point of the converter earthed through the earth electrode. The earth mat used to reduce the galvanic corrosion of the station.
The earth electrode kept 5 to 20 km away from the substation. This connection made through the electrode line.
3) Converter transformer
Converter transformer connected between the converter valve and AC bus bars. This transformer is different compared to a conventional transformer. Because it is specially designed as there is the DC component.
There are three single-phase or one three-phase unit in the converter. It used to step-up the voltage at sending end and step-down the voltage at the receiving end.
4) Converter valve
Converter valves contain power electronics switches. It used to convert AC into DC at sending end and DC into AC at receiving end.
Most of the thyristors used as a power electronics switch. These connected in series-parallel to obtain the required voltage and current.
Auxiliaries required for supplying the gate pulse, voltage grading and protection of thyristors. The maintenance of these auxiliaries is difficult.
Rating of valves in voltage and current like 125 kV – 1200 A; 150 kV – 1800 A; etc.
5) Smoothing reactors
When there is a fault in the DC line, the smoothing reactor limits the fault current. It also used to smooth the direct current.
Reactors are magnetically shielded and contain an iron core with air-gap or air core. Their windings are disc type. These designed for high inductance and typical ratings are 1H, 2000A. The size of the reactor is big.
6) Harmonic filter
It made of resistor, inductor, and capacitor. The harmonics produced due to switching operations. The harmonic filter used to suppress the harmonics.
The filter provides the low inductance ground path to the harmonics. In the 12-pulse converter, 11th and 13th harmonics are present.
7) Control and protection system
Tap changing, firing angle and extinction angle β are controlled. There is protection provided for overvoltage, overcurrent, transformer protection, and converter protection.
Types of the HVDC system
There are four types of HVDC systems.
- Mono polar system
- Bipolar system
- Homopolar system
- Back to back system
1) Mono polar system
This system has only one pole. The return path for the current provided from the sea or earth. The polarity of the pole may be positive and negative with respect to the earth.
The cost of this system is very less because it used only a single pole. This system used for low power links.
2) Bipolar system
This system has two poles. One line is positive with respect to the earth and one line is negative with respect to earth. Midpoint is earthed. Some balance current will flow through the earth in normal operation.
If a fault occurs in one line, the rest of the system will work as a mono-polar system with one pole. And continuity of service maintained.
Again, this system comes back to the bipolar mode when the fault cleared.
There are two conductors used in this system. Let say for 500 kV system, one conductor is 500 kV positive with respect to the earth and other conductor is 500 kV negative with respect to earth. Hence, the voltage difference between the two conductors is 1000 kV.
The cost of this system is high compared to the bipolar system but the power transfer capacity of this system is more than the bipolar system.
3) Homo polar system
This system also has two poles. But both poles are at the same voltage level. The return path for current is through the earth.
4) Back to back system
There is no DC transmission line. Two AC systems interconnected through the converter substation. Such type of tie link makes the asynchronous interconnection of two nearby AC system.