In this new era of technology, the demand of energy is increasing globally and the use of high voltage, high power, fully controlled semiconductor technology continues to have a significant impact on the development of controlled, stable and reliable power transmission. VSC based HVDC transmission system provides independent control of real and reactive power which improves system stability and ensures optimal flow of power.

The control strategy used for VSC-HVDC transmission to improve the transient and voltage stability of power system. The possibility of interruption in power transmission due to the application of sudden perturbation is prevented for the fast power run back capability of VSC-HVDC transmission. The instant power reversal ability of VSC-HVDC keeps the system free from transient instability. The voltage support capability of VSC helps to protect the system from voltage collapse, hence losing of synchronism can be avoided.

HVDC

A high-voltage, direct current (HVDC) electric power transmission system is also called a power superhighway uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems .For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance each cycle. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be justified, due to other benefits of direct current links. HVDC uses voltages between 100 kV and 1,500 kV.

HVDC allows power transmission between unsynchronized AC transmission systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. HVDC also allows transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks.

Increasing trend of energy demand and its mitigation by use of several conventional and its mitigation by use of several conventional and station to remote nonenergy sources and transportation of energy from generating areas is a great challenge. To serve the above purpose it is needed to have a bulk power transmission over a long distance through overhead transmission line becomes hectic in case of AC transmission due to high charging current and losses caused by capacitance. Problem related to interconnect the unsynchronized grids to the existing grid where and frequency is the main constraint which restricts the interconnection through an AC link. For the eradication of above problem, it solution by using DC transmission is having a where a controlled DC transmission provides the flexibility for a bulk power transmission over a long distance through a DC link Converter stations manner which enables a being used at the generating end for AC/DC conversion in a controlled power flow. A rapid development Andre search electronics switches provide a better, efficient technique for control mechanism hence control over power flow.

HVDC transmission resides a two-basic type of converter technology. Those are classical

line commutated current source converter (CSCs)

self-commutated voltage sourced converters (VSCs)

Classical HVDC technology employs line commutated current source converters with thyristor valve used as a base technology for DC transmission in 1950s. Where thyristors are not fully controlled switches, hence it put limitation to control mechanism used for controlled power flow. Voltage source converter base transmission technology introduces flexibility in power transmission, as it uses fully controllable switches like IGBT which provides one of the efficient control mechanism for control of power flow. Both classical and VSC-HVDC are used for the applications like long distance transmission, underground and undersea cable transmission and interconnection of asynchronous networks. But from control point of view VSC-HVDC having more flexibility and efficient power flow mechanism, as it is capable of controlling both active power and reactive power independently of each other, to keep stable voltage and frequency. Particularly self-commutation, dynamic voltage control and black start capability allows VSC i transmission technology to serve isolated loads on islands over long distance submarine cables. Thyristor based classical HVDC mostly used for point to point large power transmission long distance over land undersea cables. It has certain disadvantage like commutation failure as thyristors can’t be off immediately, and it requires 40 ~ 60 % reactive power supply of the total active power transmission. To have a solution IGBTs are used that can be switched off and on immediately, no commutation problem, active and reactive power control independently, no reactive power compensation required, filter requirement out high frequency signals from PWM, no requirement of telecommunication between two is less as to filter stations of VSC-HVDC system. VSC -HVDC link consist of a back to back voltage sourced converters (VSCs), a common DC link, which includes a large DC capacitors and DC cables. The control strategy is being designed to coordinate the active power control between two station which is realized by controlling the DC side voltage of one converter where other converter controls the active power. Automatic control of power flow between stations is the result of a constant DC voltage source gives “slack bus”. AC voltage control and reactive power control will switch as per the requirement

Motivation:

Power system operates closer to their stability limits, which may affect the damping of electromechanical oscillation and risks the system with a decreased system stability margin .As a solution to the above problem, to increase the power transfer capability and to have a good stability margins the only left option is VSC-HVDC transmission .
VSC- HVDC provides the flexibility in power transmission, and an efficient utilization of efficient control strategy for VSCs yields a faster and independent power networks. An control of active and reactive power efficient faster system recovery after post disturbance. Using VSC based HVDC can improve transient stability, increase the damping to low frequency oscillations Improve voltage stability and provides a bidirectional power flow. Attracting feature of VSC is that. it can have black start capability and interconnection of asynchronous grids with existing power grids.

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