HVDC POWER TRANSMISSION SYSTEM

HVDC POWER TRANSMISSION

High voltage direct current (HVDC) power systems use high voltage direct current for transmission of bulk power over long distances. HVDC transmission system is more costly, but it gives improved efficiency, stability, reliability, and transmission capacity. For long-distance power transmission, HVDC lines are less expensive and losses are less compared to AC transmission.

WORKING 

In generating stations, AC power is generated which is converted into DC by a rectifier. So we need two converters one at each side that acts as a rectifier at sending end and acts as an inverter at receiving end.

TYPES OF HVDC LINKS

There are three types of HVDC links for connecting two networks or systems. They are explained below:

1. Monopolar link: It has a single conductor of negative polarity and uses the earth as a return path of current.

2. Bipolar link: The Bipolar link has two conductors one is positive, and the other one is negative to the earth. The link has a converter station at each end. The midpoints of the converter stations are earthed through electrodes.

3. Homopolar link: It has two conductors of the same polarity usually negative polarity, and always operates with earth or metallic return. In the homopolar link, poles are operated in parallel, which reduces the insulation cost.

     

COMPONENTS OF THE HVDC SYSTEM

The essential components in an HVDC transmission system are 6/12/24 pulse converters, converter transformer with suitable ratio and tap changing, filters at both DC and AC sides, smoothing reactor on the DC side, shunt capacitors, and DC transmission lines.

1. Converter unit: HVDC transmission requires a converter at each end of the line. The sending end converter acts as a rectifier which converts AC power to DC power and the receiving end converter acts as an inverter which converts DC power to AC power.

This unit usually consists of two three-phase converters which are connected in series to form a 12 pulse converter. The converter includes 12 thyristor valves and these valves can be packaged as a single valve or double valve or quadrivalve arrangements.

Due to the evaluation of power electronic devices, the thyristor valves have been replaced by high power handling devices such as gate turn-off thyristors (GTOs), IGBTs, and light-triggered thyristors.

2. Converter Transformer: The transformers used before the rectification of AC in an HVDC system are called converter transformers. The different configurations of the converter transformer include three-phase- two winding, single-phase- three winding, and single-phase- two winding transformers.

3. Filter: Due to the repetitive firing of thyristors, harmonics are generated in the HVDC system. These harmonics are transmitted to the AC network and led to the overheating of the equipment and also interference with the communication system. To reduce the harmonics, filters and filtering techniques are used. Types of filters include ac filter and dc filter.

4. Shunt capacitors or Reactive Compensation: Due to the delay in the firing angle of the converter station, reactive volt-amperes are generated in the conversion process. Since the DC system does not require or generate any reactive power, this must be suitably compensated by using shunt capacitors connecting at both ends of the system.

5. Smoothing reactor: It is an extensive series reactor, which is used on the DC side to smooth the DC current as well as for protection purposes. It regulates the DC current to a fixed value by opposing the sudden change in the input current from the converter. It can be connected on the line side, neutral side, or at an intermediate location.  

                 

6. Transmission medium or lines or cables: Overhead lines act as the most frequent transmission medium for bulk power transmission over land. Two conductors with different polarities are used in HVDC systems to transfer the power from sending end to receiving end.

7. DC and AC switchgear: The switchgear equipment provides protection to the entire HVDC system from various electrical faults and also gives the metering indication. The switchgear equipment includes isolator switches, lightning arrestors, DC breakers, AC breakers, etc.

ADVANTAGES OF HVDC

1. The HVDC link is an asynchronous connection between two ac stations i.e. the transmission of power is independent of sending and receiving end frequencies.
2. No skin effect and less corona loss. Lesser number of conductors and insulators are required thereby 
3. Reducing the cost of the overall system.
4. No proximity effect.
5. It doesn’t generate or absorb any reactive power. So there is no need for reactive power compensation.
6. It increases system stability, reliability, transmission capacity, and efficiency.

DISADVANTAGES OF HVDC

1. Converter stations are expensive, complex, and have a small overload capacity.
2. Converter stations generate current and voltage harmonics. So, filters are needed.
3. Designing and operating of multi-terminal HVDC system is complex.
4. Grounding of HVDC transmission involves a complex and difficult installation.

COST ANALYSIS

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From the graph above, it is seen that the initial investment is more in HVDC than HVAC because dc terminal cost is higher than ac terminal cost. As the distance increases, the line losses and the cost of conductors become more in the HVAC system. Finally, the total cost of an HVDC system is lesser than an HVAC system for a longer distance than a critical distance.
Hence HVDC power transmission system is becoming popular for longer distances nowadays as substation equipment is becoming less expensive and improved capacity.