20141217092537answers.docx 

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1) How HVDC is created and converted back to HVAC

According to the subject of power network systems these days the world engineering is expecting to utilize the High Voltage Direct current (HVDC) for the replacement of the High Voltage Alternating current (HVAC) , as the previous has numerous negative points than the one which comes now. Some of these favorable circumstances are:

1.      HVDC has high transmission power than HVAC for the same conductor cross area region.

2.      It gives offbeat interconnection between two AC frameworks.

3.      It has boundless for the transmission line separations.

4.      HVDC is more dependable framework than HVAC.

5.      It is more efficient in the long haul.

when we move toward construction the principle parts of the HVDC power framework are the HVDC transmission line which is interfacing the sending end (era region) with the less than desirable end (usage range) converter stations ,one of these stations comprises of the rectifier circuit (AC-DC converter) and the other comprise of the inverter circuit (DC-AC converter) , Both converters have either thyristors or transistor (framework valves) as indicated by the converter sort as clarified underneath, lastly is the transformer which is the association between the AC framework and the converters.

2) Problems with Harmonics and solutions
As harmonics is a signal whose frequency is vital component of the major framework, which is created by the generator. In the HVDC power framework some system modules, converters, transformers, retain the non-direct characteristics which produces framework harmonics. The converters are the primary consonant sources of a steady state operation. They produce harmonics on both the AC and DC sides of their valve bunch. These consonant impacts are not restricted to a converter station; they can also spread to vast separations inside the system. The force in the HVDC power framework is conveyed to both the direct and non-linear system segments. The force conveyed to the non-linear components is changed over to the harmonic force which becomes the consonant sources in the system. Since the system parts are interconnected, the symphonic sources deliver their energy to the diverse parts of the system; this will result in consonant in the infiltration framework. The voltage and current harmonics have a negative consequences for the HVDC transmission systems, this is a direct result of the non linearity of the force electronic gears, these expansive number of music at the operation mode of the HVDC transmission system will influence the force nature of the framework furthermore bother the specialized gadgets which are near to the force framework, this step even prompts undesirable force framework swaying.

3) Flow control and stability issues
The regularly utilized sort for the HVDC framework now a day is the voltage source converters (VSC), so some issues are there in the VSC control of the dynamic and receptive power. Moreover, one of the real concerns in HVDC power framework is the framework dependability and recuperation after the framework aggravations; likewise, how to control the framework and keep up steadiness will be considered.

The HVDC control framework has numerous capacities; some of these are expressed underneath:

1.      Framework beginning and off streaming, this capacity controls the framework amid typical and irregular conditions, for example, ordinary begin, typical off, stop framework in crisis cases, and restart the framework when it is prepared to.

2.      Pattern opposite control, this capacity switches the stream of the force in the transmission line when it is required.

3.      Framework strength work, this is for remaining the framework dependability and ensures the framework supplies.

4.      Framework checking capacity, its target is to check the framework operation conditions and take measures for the control capacity when finding any framework irregularity.

5.      Regulation capacities, this is to control the framework dynamic and receptive force and AC system recurrence.

6.      Framework security change work, this capacity controls the voltage abundance and decreases the deficiency momentums amid shortcircuits or whatever other strange cases to remain the framework in the dependability condition.

Power framework security is grouped into three primary sorts as underneath:

1.      Rotor edge dependability.

2.      Voltage solidness.

3.      Recurrence solidness.

 

Framework solidness normally has two sorts of limits, operational confinements and security restrictions. For the operation confinements, the voltage and the current are fundamental figures which control the dynamic and touchy force stream. The current ought to be restricted to the converter rating and the voltage is constrained to the dielectric protection level of the converter. For the strength constraint is considered in the AC side of the converter and some presumption is considered when examining the framework dependability. The DC voltage is consistent and the AC capacitor is disregarded.

4) Switching and circuit breaker issues
As the HVDC is broadly utilized as a part of the new power frameworks, one of the essential issues in this field that ought to be considered is the exchanging and circuit breakers. Straight forward HVDC electrical switch design is expressed in the accompanying figure.

One of the exchanging means was talked about in a paper amid the universal meeting on the power framework engineering. it is said that "Vacuum exchanging innovation enhances the exchanging limit of the on-burden tap changers in HVDC applications" It incorporated the converter transformer and the applications in the converter transformer and its capacities, for example, keeping up the voltage with a specific end goal to repay the variety in the AC voltage essential side, besides, to keep up the terminating point or shutting edge on both cases rectifier or inverter at the ideal quality, and thirdly is to decrease the DC voltage amid the operation.

There are numerous sorts of HVDC circuit breakers which are characterized as per their obligations. Some of these sorts are impartial transport breakers, unbiased transport ground breaker, metal return exchange breaker, ground return exchange breaker, fast sidestep breaker, and detachment breakers. These breakers are utilized to disengage the framework by refortifying the converters in the stations and not on the HVDC transmission line itself.

5) A typical look at an application of HVDC actual link either in operation or just being completed
There are numerous activities in the fields of HVDC transmission frameworks, some of them have been done while others being executed or proposed. One of these is the north-west Sweden venture; this will help the national AC power network in Sweden furthermore interconnect Norway system to Sweden with an aggregate force of 1440 MW separated into two indistinguishable circuits each one evaluated 720 MW and voltage of ± 300 kv. This task is relied upon administrations of this current year "2014". The possibility, for example, flaw event or support issues are considered in this undertaking by applying two symmetrical monopole setups. The framework outline is readied for any future developments.

An alternate substantial extend around there is the Kii-channal HVDC connect in Japan which was appointed in 2000 and put in administrations to transmit a bilk control between the western parts of Japan. In this undertaking a large number of HVDC new innovations were connected, as illustrations, DCGIS were used. This connection is evaluated 1,400 MW, ± 250 kV, 2,800 A and is intended to be moved up to 2,800 MW, ± 500 kv, for the same current rating later on. The bipolar, metallic return plan arrangement to stay away from the electrolytic erosion in the adjacent territories is utilized when planning the system. The high voltage substantial limit light-activated thyristors valves are one of the gimmicks utilized as a part of this framework; they were amassed in 6 modules keeping in mind the end goal to empower substantially more reduced valves and great structure. Additionally DC gas protected switch gear is utilized which incorporated all the energized conductors in a tank. Also Converter transformer, smoothing reactor submarine cables, control and insurance, Environmental consideration commissioning tests and System operation are considered in this venture.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References:
1. Flourentzou, N., Agelidis, V., and Demetriades, G., (2009). ‘VSC-Based HVDC Power Transmission Systems:An Overview’. IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 3, MARCH 2009.

2. Setr´eus, J.,and Bertling, L.,‘Introduction to HVDC Technology for Reliable
Electrical Power Systems’, IEEE.

3. Louie, K., P. Wilson, P., Wachal, P.,Wang, A., and Buchanan, P., ‘HVDC Power System Harmonic Analysis in the Time and Frequency Domains’. 2006 International Conference on Power System Technology.

4. Kaul, N., and Mathur, R., ‘SOLUTION TO THE PROBLEM OF LOW ORDER HARMONIC RESONANCE FROM HVDC CONVERTERS’. IEEE Transactions on Power Systems, Vol. 5, No. 4, November 1990.

5. Yuan, Z., and Wang, H., ‘The Research on the VSC-HVDC Control System Structure’. 978-1-4577-0547-2/12/$31.00 ©2012 IEEE.

6. Kundur, P., Paserba, J., Ajjarapu, V., Andersson G., Bose A., Canizares, C., Hatziargyriou, N., Hill, D.,Stankovic, A., Taylor, C., Cutsem , T., and Vittal, V. ‘Definition and Classification of Power System Stability’. IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 19, NO. 2, MAY 2004.

7. Shen, D., Kraemer, A., and Dohnal, D. ‘Vacuum Switching Technology Improves theSwitching Capacity of On-Load Tap-Changers in HVDC Applications’. 2006 International Conference on Power System Technology.

8. Aredes, M.,Portela, C.,and Machado, F. A 25-MW Soft-Switching HVDC Tapfor 500-kV Transmission Lines.IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 19, NO. 4, OCTOBER 2004.

9. Ingemansson, D., Wheeler, J., MacLeod, N., Gallon, N., and Ruiton, O., The South – West Scheme: a new HVAC and HVDC transmission system in Sweden. IEEE.

10. Makino, Y., Hara, S., Hirose, M. COMMISSIONING OF THE KII-CHANNEL HVDC LINK-APPLICATION OF NEW HVDC TECHNOLOGY AND OPERATING EXPERIENCE.