HIGH VOLTAGE DC TRANSMISSION

HVDC stands for High Voltage DC Transmission. The need for transmission of electricity arises from the fact that the generation stations are located far from the load centers. Load centers are the actual consumers of electrical energy. Thus, the power is to be transmitted over very long distances so as to be distributed to the consumers.

The power generated in the electrical generating stations is alternating in nature. The generated power is about 11 KV and 22 KV. This voltage when transmitted through the transmission lines over long distances undergoes a significant loss mainly due to the copper losses occurring in the conductors used for transmission. So as to overcome these losses it is not possible to increase the generation voltages but an increase the voltage level poses to be an effective   method. Hence the voltages are raised from 11 KV or 22 KV to 220 KV or 440 KV or more for primary transmission, then stepped down for secondary transmission and again stepped down at the distribution level so as to meet the power requirements at the consumers end.

As seen the transmission is carried out at extremely high voltages. And to carry out transmission both AC as well as DC can be employed. The deciding factors as what system is to be chosen for transmission, viz., AC or DC for transmission are as follows: 

1. Cost of transmission

2. Technical performance 

3. Reliability

Through an overview of the above considerations, it is more advantageous to choose HVDC for long distance greater than 800 Km bulk power transmission and underground cable transmission. But for shorter distances AC transmission is more advantageous due to economic considerations. The advantages of HVDC transmission are:

1. Bulk power transmission over Long distances.

2. Underground and Under water transmission.

3. Power transmission and stabilization between unsynchronised AC networks. 

4. To integrate renewable resources such as wind into the main transmission grid.

HVDC  Transmission Principle:

In HVDC System, the three phase AC power is taken for transmission. It is then converted to DC through a DC Converter, and is then transmitted through transmission lines till the consumer point and is converted back to AC through AC converter and finally injected into the AC network as the loads are operating on AC. 

Therefore the following are the essential components of HVDC Transmission system:

1. Converter station: The converter station carries out the conversions viz., AC to DC i.e., Rectifier operation for transmission and DC to AC i.e., Inverter operation at the load side. 

Line-commuted converters are usually employed. The converter unit in a converter station comprises of two three phase bridge converters connected in series so as to form a 12 pulse converter unit as shown below:

2. Converter transformer: The converter is fed by converter transformers which are three phase transformers. The converter transformers can be Star/ Delta and Star/Star arrangements. 

3. Filters: The prominent problem encountered in converter transformers is the harmonic currents and the magnetization of the core due to the firing of the valves of the control unit unsymmetrically. These harmonics are eliminated by the use of Filters. The filters can be AC Filters and DC Filters which play a role in eliminating AC and DC harmonics.  The harmonic filters designed so as to deal with the harmonics of 11th and 13th  degree on the AC side, and 12th harmonic on the DC side.

4. Reactive power source: The current drawn by the Line Commuted Converter( LCC) lags behind the voltage. Thus a reactive power source is required to compensate this lag in the current. The AC harmonic filters employed in the application also perform the function of provision of the Reactive power. Shunt capacitors, Static VAR systems like  STATCOM or SVC also can be used as reactive power sources. The choice of the required compensating element depends on the switching speed requirement.

5. Smoothing reactor: The role of Smoothing reactors in HVDC systems is to smooth the current waveform by removing the harmonics and to increase the dynamic stability of the system. A typical reactor layout is shown in the figure below:

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Minnie Florence V

Asst.Professor

EEE Department 

Sphoorthy Engineering College

Sphoorthy Engineering College