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HVDC Transmission Substation

HVDC Transmission

Definition: High voltage direct current (HVDC) control frameworks use D.C. for transmission of mass control over long separations. For long-separate power transmission, HVDC lines are more affordable, and misfortunes are less when contrasted with AC transmission. It interconnects the systems that have diverse frequencies and attributes. 
In a transmission, exchanging rushes of voltage and current goes in the line which alters its course every millisecond; because of which misfortunes happen as warmth. Not at all like AC lines, the voltage and current waves don't alter their course in DC. HVDC lines increment the productivity of transmission lines because of which control is quickly exchanged. 
In a joined AC and DC framework, created AC voltage is changed over into DC at the sending end. At that point, the DC voltage is upset to AC at the less than desirable end, for dispersion purposes. In this manner, the change and reversal hardware are likewise required at the two stopping points. HVDC transmission is conservative just for long separation transmission lines having a length more than 600 kms and for underground links of length more than 50 kms. 
How does HVDC transmission framework work? 
In producing substation, AC control is created which can be changed over into DC by utilizing a rectifier. In HVDC substation or converter substation rectifiers and inverters are set at both the finishes of a line. The rectifier terminal changes the AC to DC, while the inverter terminal believers DC to AC. 
The DC is streaming with the overhead lines and at the client end again DC is changed over into AC by utilizing inverters, which are set in converter substation. The power continues as before at the sending and getting stopping points. DC is transmitted over long separations since it diminishes the misfortunes and improves proficiency.


hvdc-layout-compressor
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 multi terminal DC substation. (Input Power = Output Power)

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.
Graph-hvdc-compressorThe point where two curves meet is called the break even distance. Above the break even distance, the HVDC system becomes cheaper. Break even distance changes from 500 to 900 km in overhead transmission lines.


Comparison of both HVAC and HVDC Transmission System

HVDC Transmission SystemHVAC Transmission System
Low losses.Losses are high due to the skin effect and corona discharge
Better Voltage regulation and Control ability.Voltage regulation and Control ability is low.
Transmit more power over a longer distance.Transmit less power compared to a HVDC system.
Less insulation is needed.More insulation is required.
Reliability is high.Low Reliability.
Asynchronous interconnection is possible.Asynchronous interconnection is not possible.
Reduced line cost due to fewer conductors.Line cost is high.
Towers are cheaper, simple and narrow.Towers are bigger compared to HVDC.
Favorable circumstances of HVDC transmissions 

  • A lesser number of conductors and separators are required in this way diminishing the expense of the general framework. 
  • It requires less stage to stage and ground to ground freedom. 
  • Their towers are less expensive and less expensive. 
  • Lesser crown misfortune is less when contrasted with HVAC transmission lines of comparative influence. 
  • Power misfortune is diminished with DC in light of the fact that less quantities of lines are required for influence transmission. 
  • The HVDC framework utilizes earth return. In the event that any blame happens in one post, the other shaft with 'earth returns' carries on like an autonomous circuit. This outcomes in a progressively adaptable framework. 
  • The HVDC has the offbeat association between two AC stations associated through a HVDC interface; i.e., the transmission of intensity is free of sending frequencies to getting end frequencies. Henceforth, it interconnects two substations with various frequencies. 
  • Because of the non attendance of recurrence in the HVDC line, misfortunes like skin impact and vicinity impact does not happen in the framework. 
  • It doesn't produce or retain any responsive power. Along these lines, there is no requirement for responsive power pay. 
  • The exact and lossless power moves through DC connect. 

Burdens of HVDC transmission :-

  • Converter substations are put at both the sending and the less than desirable end of the transmission lines, which bring about expanding the expense. 
  • Inverter and rectifier terminals create sounds which can be diminished by utilizing dynamic channels which are additionally over the top expensive. 
  • On the off chance that a blame happens in the AC substation, it might result in a power disappointment for the HVDC substation put close to it 
  • Inverter utilized in Converter substations have restricted over-burden limit. 
  • Circuit breakers are utilized in HVDC for circuit breaking, which is additionally extravagant. 
  • It doesn't have transformers for changing the voltage levels. 
  • Warmth misfortune happens in converter substation, which must be decreased by utilizing the dynamic cooling framework. 
  • HVDC interface itself is additionally exceptionally convoluted. 

Thinking about every one of the benefits of DC, it appears that HVDC lines are more capable than AC lines. However, the underlying expense of HVDC substation is high and their substation hardware is very complicated.Thus, for long separation transmission, it is best that control is produced in AC, and for transmission, it is changed over into DC and after that again changed over once more into AC for definite use. This framework is affordable and furthermore improves the productivity of the framework.


Application of HVDC Transmission


1). Undersea and underground cables
2). AC network interconnections
3). Interconnecting Asynchronous system
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Location: Aligarh, Uttar Pradesh, India

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