Title of Invention

"AN IMPROVED CONTROLLED SHUNT REACTOR"

Abstract

The inveation relates to an improved controlled shunt reactor comprising thyristor valves connected across the secondary of a 100% impedance transformer. A controller is provided for issuing switchiing commands to the thyristor valves through a base electronics and thyristor monitoring unit. A bypass circuit breaker is provided in parallel to the thyristor valves, and a bypass choke is provided in series with the circuit breaker. The controller (20) is provided with & triggering meansi(25) for keeping the thyristor valves (10) m conducting mode for a short duration only for transferring current from said thyristor valves (10) to the circuit breaker (7).

Full Text

HELD OF APPLICATION The present invention relates to an improved controlled shunt reactor. BACKGROUND OF THE INVENTION Fixed shunt reactors used in 400 KV AC trarsmission lines are now replaced by controlled shunt reactors (CSR) used in ON/OFF mode for improving the voltage profile, enhancing the transferable power and for reducing the losses. Fast acting thyrtstor valves and digital control systems can be used in the controlled shunt reactor for accomplishing transient free soft switching of the CSR. The controlled shunt reactor used as a controlled static compensator of reactive capacity for increasing of transfer capability of electric networtc and also as an arc interrupting device has been described in co-pending patent application no. 1658/CAL/97 filed on 09,09.1997. Indian Patent Application No. 1092/DEL/98 filed on 27.04.98 relates to a controlled shunt reactor of transformer type in the field of electro technique and electro power technique which can be used as a fast regulated inductive impedance for the restated compensation of surplus reactive capacity of traromission lines and for the regulation of condenser batteries capacity. A complete control device for controlled shunt reactor (CSR) consisting of thyristor valve unit has been described in co-pendhg pwent application no. 1481/DEL/98 filed on 01.06.98. Co-pending patent application no.920/0EL/03 Filed on 22.07.03 describes a controlled shmt reactor with a bypass arrangement for providing syndvonization with respect to the line voyage and availability of full reactive capacity right from the first cycle of energization. The prior art thyristor controittd shunt reactor, is allowed to remain 'ON' and the thyristor valves are albwed to remain conducting during the entire period, that is for as long as the controlled shunt rector is required by the system. This prolonged conduction results in higher losses in the thyristOT valve requiring elaborate forced deionized water cooling system. Such a cooling system is to be provided with a water circulation unit and a heat exchanger unit. Both these units have rotetmg parts like circulatv^ pumps and heat exchanger fans, requiring increased attentkm from maintenance point of view and resultant increase in cost. SUMMARY OF THE INVENTION: A need therefore existe for keeping the thyristor valves HI the corakKting mode only for a short duratkm of say 10 cycles (200 mS). For the remaining *0N' perk)d, the current is transferred to the parallel circuit breaker resulting in considerable reduction of thyristor valve tosses and elimirMtion of elaborate forced water cooling requirements of thyristor valves. for the improved controlled shunt reactor (CSR) of the present inventkm, the thyristor valves are brought into comkiction only for a short time to make use of the fast acting capability of the thyristors compared to breakers and also to bring the reactor at the right point on the AC voltage waveform to ensure transient free soft switdiing. During this short corKkictkig period of the thyristor valves, the breaker provided in parallel with the thyristor valve cbses, transfen-ing the current from the thyristor valve to the breaker. As the loss in the breaker is very low compwed to the thyristor valve, the present kivention utilizses this property of the breaker during normal *0N' perriod adn the capabilyty of thyristws for fast and soft switehkig ckving switch on resulting in overall low loss operaticHi without sacrificing speed. The present invention results m a compact thyristor valve system in the absence of elaborate forced cooling circuits for the thyristors and associated snubber circuits. The deionized water cooling system comprising water circulation unit and heat exchanger can be eliminated. The controller provided with a base electronics and thyristor monitoring panel is used fcM' issurtg switching cmmands to the thyristor valves. In the prior art system, a single base electronics and thyristor monitoring panel cittfs to all the three phases and is provided wih elaborate annunciation system for faults and fault locations. In the present invention independent base electronics and thyristor monitoring panels for each phase are used. These ara integrated with the phase thyristor valve as a part of ttw valve structure to form a single self standing unit. The controller is provided with a triggering circuit and an electricaHy programmable device used for keepnig the thyristor valve in the concbctmg mode only for a short duraticrt. The present invention thus provides an improved shunt reactor comprising thyristor valves connected across the secondary of a 100% mipedaice transformer, a controller for issuing switching commands to said thyristor valves through a base elcc&onks and thyristor monitoring unit, a bypass crcuit breaker provided in pa-allei to the thyristor valves and a bypass chdce ri series with said circuit breaker, characterized in that saki controller is provided with a triggering circuit for keeping saki thyristor valves m conducting mode for a short duration only for transferring current frcnm sakI thyristor valves to said circuit breaker. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The invention will now be explained with the help of the accompanying drawings where : Fig. 1 shows a typical controlled shunt reactor system based on the prior art. Fig. 2 shows the controlled shunt reactor system of the present invention. Fig. 1 in schematic form shows a typical controlled shunt reactor installation operating in ON/OFF mode. Thyristor valves 1 are connected across a secondary of a 100% impedance trar»former T. Shorting or opening of the secondary terminats of b-ansfbrmers T results in full rated MVAR or nil MVAR reactive power being drawn from the 400 kV system. The switching commands are issued by the controller 2 to the thyristor valve 1 through a base electronics and thyristor monitoring unit 3. The controller 2 decides the *0N' and *OFF' duration depending on the transmission line voltage. In the prior dst, the thyristor valves remaki conductrig, short circuiting the secondary of the 100% Mnpedar»:e traisformer *T' for the complete 'ON' period. This prolonged conduction gives rise to losses in the thyristor valve and herwe the rec^irement of providing suitable de ionized water cooling system 4 to mamtain the operatrig pnd^on temperafciN-e of the thyr^tors at the desired level. This water cooling system 4 may comprise of a water circulation unit 5 and a heat exchanger 6. A bypass circuit breaker 7 is provkied parallel to the thyristor valve 1. A bypass choke 8 is connected in series with the bypass circuit breaker 7 to maintein a small voltage acroi» the thyristor valve whenever the bypass circuit breaker 7 is closed in order to keep the electrorHcs in the thyristor valve sufficiently charged and ready. This chdce is in the form of a small reactor. An knproved controlled shunt reactw of the present invention is shown in block diagram form In Rg. 2. Items like the 100% inpedance reactor transformer, the bypass circuit breaker and bypass chdce are represented by slmilw references as shown in Fig. 1. An improved controlled shunt reactor of Fig. 2 comprises a controller 20 with a base etectrontcs md thyristor monitoring unit 30 for ctmtrollrtg thyristor valves 10. The controller 20 is further provided with a triggering circuit 25 comprising for example, an electrKatly programmable device for keeprig the thyristor valves 10 in conducting mode only for short duration of stout 10 cycles (200 mS). The base etectronics and thyristw monitor 30 comprises ^dependent pwiels provided in each phase. These are inte^ated wMi the phase thyristcM- valve 10 accommodated as a ptft of the valve striK:ture to form a single self standing unit 30. This arrwgement of the base electronics m-td thyristor monitoring unit Wi ackiition to savkig space results in easier testing and instaliatkHi and consumes less time. The working principle of tiie controlled shunt reactor of the present inventwn will now be described. It is assumed fw the ease of explanation that the circuit breaker takes about 100 mS to open or close after the respective command is given. During line charging the bypass breaker is kept ckised. After line duM-ging, if the controller, based on system requrement, deckJes to tirice out the CSR fi^om the line, a short burst of trigger pulses for a fixed duraticm of 200 mS is released to the thyrstor valve and simultanecMJsly an 'open' command is given to the bypass brearicer. Once the valve conckKts, a major portbn of the secondary current flows through the thyristor valve. Once the bypass tn^eaker opens (after about 100 mS of issuing command, the full current is diverted through \ht thyristor valve tyi the trigger pulses are withdrawn. Hence the valve conducts full current for only about 100 mS. The bypass chdce is a small reactor connected in ser»s wtth the bypass bredcer to makitain a smalt voltage across the thyr»t(M- valve whenever the bypass breaker is closed in order to keep the electrorwcs in the thyristor valve sufTiciently chiMrged and ready. IMder steady state condition, a coolmg time of about 10 minutes is allowed before the CSR can be brought in again (kie to system requrement as decided by the controller. However, in case of emergency (fault or sudden bad throw-off) the CSR can be brought in immed»tely. Once the circuit brewer is ON and the thyristor valve is blocked, the losses in the valve are very low wid the valve can cool down even when the CSR is on. FOR inserting the CSR, the thyristor valve is triggered for a fixed time of 200 mS and simultaneously bypass breaker ^ck>se'comm«id is given. The thyristor valve carries the full secondary ci^rent of the CSR till the bypass breaker closes after about 100 mS and most of the current is diverted tivough the breaker. At the end of 200 mS tlw valve stops conckKting and the breaker carries the full current. Each time the valve conducts, either for removal or insertkm of CSR, a cooling time of 10 minutes is alkiwed before triggering tfw thyristor valve again. However, if a fault occurs or there is sudden load throw-off immediately after removal of CSR, which requires ynmediate reirMMtrtkm, the thyristor valve can be used for reinsertion. In such cases, the next removal can be only after 15 minutes. In most systems, where the CSR is switched on and off between 2 to 30 times per day^ the 10 minutes coofing time will not have any adverse effect on the perfbnmar^e of the new CSR sclwme us compared to the existing scheme. At the same time there will bt sii>stantiai reduction in valve bsses and auxiliwy power consumption of the cooling system (water circulating pump and cooling fans). Added tntnefit of the invention is that the design of control ccH^ipment comprising the thyristor valve BETM and controller can be standardised by keeping the secondary ciH-rent of the transformer same for various MVAR ratings. Only the number of thyristors in series can be chmged to cater to a wide range of rating. Even the mechanical ctesign of the thyristor valve odm be standardised for a wide r«ige of rating (say 50MVAR to lODMVAR) with provision to replace real thyrist(M's with dummy thyristors in case the required number of thyristors in series is less than the design maximum. Another benefit is the posstt)ility of housing the entire conb-ol equipment inside a weather proof container due to its reduced size, and placing the container next to the reactor transformer in the yard. This is an added advantage in cases where existkig Fixed Shi^t Reactors are replaced by ON/OFF type CSRs and where the control room is far from the yard. Arwther benefit is the einmination of all rotating ibems from the scheme such as water circulating pumps and heat exchan(^r fans which require maxknimi attention from maintenance point of view due to wear and tear. Noise generated is drastically reduced diw to the absence of rotatirH) machines. Reliability of the total scheme increases due to lesser number of component. (Table Removed) The number of thyristors shown above are for a secondary current of 2500A. The standard thyristor valve can be designed to accommodate 8 thyristor levels. For lesser ratings, the real thyristors can be replaced with dimfimy thyristors without chtfiging the basic design. The short time stress on the thyristors due to temperature rise have been studied and the above design figures have been arrived at after allowing adequate safety margins. The invention as described is only illustrative of the possible arrangements. Changes and modifications in accordance with these principles may readily occur to those skilled in the art without departing from the scope and spirit of the invention. WE CLAIM; An improved controlled shunt reactor comprising: thyristor valves connected across the secondary of a 100% impedance transformer; a controller for issuing switching commands to said thyristor valves through a base electronics and thyristor monitoring unit; a bypass circuit breaker provided in parallel to the thyristor valves; and A bypass choke in series with said circuit breaker; Characterized in that Said controller (20) is provided with a triggering means (25) for keeping said thyristor valves (10) in conducting mode for a short duration only for transferring current from said thyristor valves (10) to said circuit breaker (7). The controlled shunt reactor as claimed in claims 1 or 2, wherein said triggering circuit comprises electrically programmable device (25). The controlled shunt reactor as claimed in claims 1 or 2, wherein said base electronics and monitoring unit comprises independent panels for each phase, said panels being integrated with the phase thyristor valve (10) forming a single standiag unit. The controlled shunt reactor as claimed in the preceding claims, wherein said th3rristor valves (10) are kept in conducting mode for about 10 cycles (200mS).

Documents:

664-del-2004-abstract.pdf

664-del-2004-claims.pdf

664-del-2004-complete specification (granted).pdf

664-del-2004-correspondence-others.pdf

664-del-2004-correspondence-po.pdf

664-del-2004-description (complete).pdf

664-del-2004-drawings.pdf

664-del-2004-form-1.pdf

664-del-2004-form-19.pdf

664-del-2004-form-2.pdf

664-del-2004-form-3.pdf

664-del-2004-gpa.pdf

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