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Electric-power transmissionis the bulk transfer of electrical energy from generating plants toelectrical substations located near demand centres. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to aselectric power distribution.

Transmission lines, when interconnected with each other, become transmission networks.Most transmission lines use high-voltagethree-phase alternating current(AC), althoughsingle phase AC is sometimes used in railway electrification systems.High-voltage direct-current(HVDC) technology is used for greater efficiency in very long distances (typically hundreds of miles (kilometres)), or insubmarine power cables(typically longer than 30 miles (50km)).And as far as the electrical distribution systems are concerned, virtually all public electricity supplies are AC today.HVDC links are also used to stabilize against control problems in large power distribution networks where sudden new loads or blackouts in one part of a network can otherwise result in synchronization problems andcascading failures.

Advantages of DC over AC transmission:It requires only two conductors as compared to three for AC transmission.There is noinductance,capacitance ,phase displacement and surge problems in DC transmission.Due to the absence of inductance, the voltage drop in a DC transmission line is less than the AC line for the same load and sending end voltage. For this reason, a DC transmission line has better voltage regulation.There is noskin effectin a DC system. Therefore, entire cross-section of the line conductor isutilized.

For the same working voltage, the potential stress on the insulation is less in case of DC system than that in AC system. Therefore, a DC line requires less insulation.A DC line has less corona loss and reduced interference with communication circuits.The high voltage DC transmission is free from the dielectric losses, particularly in the case of cables.In DC transmission, there are no stability problems andsynchronizingdifficulties.

PROBLEMS IN EXISTING DC TRANSMISSIONThe use of DC has been limited from point to point due to the unavailability of sufficiently fast DC circuit breakers(CB) i.e, there was no way to create a grid using DC technology. A grid is what is needed to effectively balance supply and demand and ensure reliability.

HYBRID HVDC CIRCUIT BREAKERSConventional DC CBs use a strategy similar to AC CBs in which it imposes fault current by discharge of a capacitor bank. But this requires 10s of milliseconds and additional passive circuits.

A revolutionary CB developed by ABB, which combines very fast mechanics with power electronics, and is capable of interrupting power flows equivalent to the output of a large power station within 5milliseconds- that is thirty times faster than the blink of a human eye.

OPERATIONDuring normal operation the current will only flow through the bypass, and the current in the main breaker is zero. When an HVDC fault occurs, the load commutation switch immediately commutates.This is followed by the opening of the mechanical switch.Then the main CB operates and breaks the current

The mechanical switch isolates the load commutation switch from the primary voltage across the main HVDC breaker during current breaking. Thus, the required voltage rating of the load commutation switch is significantly reduced

Our idea to implement DC grid :Such a DC overlay grid can be implemented in Indian Ocean, Arabian sea, bay of Bengal, Pacific Ocean to interconnect off-shore wind parks, and to transmit the electrical energy to the on-shore load centres which connect several countries such as India, U.A.E, Australia, Bangladesh, Sri Lanka, Malaysia ,China, Maldives and so many.So the day is not so far with Internation hybrid DC grid.

As we just discussed, we know that it is very advantageous to opt for DC transmission.But can we really do away with AC???Or is there a way to combine AC and DC grid.



In the above shown system, there are 2 kinds of sources-1)DC source consisting of Photovoltaic and offshore wind generators and 2)AC source The system consists of AC and DC grids. There are two modes of operation: 1)islanding mode and 2)interconnection modeIslanding mode:- Here there is no transfer of power between the two segments. All sources are connected to AC grid. The supply demand imbalance between power source and load is eliminated by charging and discharging storage

batteries through bidirectional converters.2) Interconnection mode:- Here there is a transfer of power between both segments. The transfer of power between the segments is done on DC grid whereas internally, for AC applications, AC is supplied.

ADVANTAGES OF DC-AC HYBRID GRIDIt combines the advantages of both, DC and AC grids.DC GRIDIn the modern day world, where countries are rapidly opting for non conventional sources of energy generation (offshore wind farms, solar power plants), DC grid becomes very beneficial.

Electric vehicles- future of the transport industry, run on batteries which require DC for charging. Huge data centers like those of Face book and Google have already shifted to using DC supply to reduce power loss and hence reduce electricity bills.DC is widely used for traction purposes, telephone exchanges, for industrial processes like Aluminum smelting.

AC GRIDFor transmission lengths lesser than 400 km, AC is more economicalFor domestic loads, where most of the load consumes AC, it is beneficial to supply AC.

Apart from the previously shown advantages, the hybrid grid has an inbuilt provision for the Islanding mode of operation which is very beneficial in case of occurrence of faults in the grid which may result in cascade tripping.

REFERENCES:www.wikipedia.org/wiki/High-Voltage_direct_current.www.enotes.com Sciencehybrid grid.www.wikipedia.org/wiki/Electric_power_transmission.www.modernizethegrid.com/transmissionanddistribution.www.fixhybrid.com.www.gogrid.com.www.abb.co.in Product GuideHigh Voltage Products.www.smartgridnews.com/.../Discovery-Showcase-ABB-...-United States.www.energy.siemens.com.www.nordicenergy.org/.../grids-for-integration-of-large-scale-wind.www.epsrc.ac.uk/.../Calls/.../CollaborativeResearchChinaSmartGrids.science.howstuffworks.com/environmental/.../power2.htm.