CA1159514A - Static var generator with overvoltage protection for the switched capacitors thereof - Google Patents
Static var generator with overvoltage protection for the switched capacitors thereofInfo
- Publication number
- CA1159514A CA1159514A CA000359939A CA359939A CA1159514A CA 1159514 A CA1159514 A CA 1159514A CA 000359939 A CA000359939 A CA 000359939A CA 359939 A CA359939 A CA 359939A CA 1159514 A CA1159514 A CA 1159514A
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- Prior art keywords
- voltage
- switch
- combination
- reactive
- capacitive reactive
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/30—Reactive power compensation
Abstract
48,723 ABSTRACT OF THE DISCLOSURE
A static VAR generator of the type which in-cludes as part thereof a discretely switched capacitive bank is taught. The aforementioned arrangement includes a thyristor switch across which is connected in parallel circuit relationship a non-linear resistive device or the like which has a voltage breakdown level which allows current to be conducted therethrough when the voltage across the switch reaches an abnormally high value for the switch. Nevertheless, the capacitive device may be dis-connected from the circuit it serves quickly and effi-ciently by using the non-linear resistive device to dis-charge the electrical charge stored therein at an appro-priate subsequent half cycle.
A static VAR generator of the type which in-cludes as part thereof a discretely switched capacitive bank is taught. The aforementioned arrangement includes a thyristor switch across which is connected in parallel circuit relationship a non-linear resistive device or the like which has a voltage breakdown level which allows current to be conducted therethrough when the voltage across the switch reaches an abnormally high value for the switch. Nevertheless, the capacitive device may be dis-connected from the circuit it serves quickly and effi-ciently by using the non-linear resistive device to dis-charge the electrical charge stored therein at an appro-priate subsequent half cycle.
Description
~S~5~ ~
STATIC VAR GEN~R~TOR WITH OVERVOLTAGE
PROTECTION FOR THE S~ITCHED CAP~CITORS THEREOF
me sub~ect matter of this in~ention ~s related to that disclosed and claimed in U9 S ~ Patent No~ 4,307,331, issued December 2~ 9 1~81 to Laszlo Gyugyi, entitled "Hybrid S~tched Capacitor Controlled-Inductor Static VAR Genera-tor and Control Apparatus".
~ The subJect matter o~ this in~ention relates generally to VAR generators and more speclfically to static VAR generators in which switched capacitors are utilized~
It is known to make ~AR generators by connecting a fixed capacitor and a switched lnduc-tor in parallel across two lines o~ an electric power system to be regu-lated or controlled by the VAR generator~ A suitablecontrol system is provided ~or sending an output signal to the switch portion of the switohed inductvr to establish a conduction ~nterval during a prede-termined period o~ timeO
The conductlon interval allows current to ~low ~or a portion o~ the predstermined period o~ time, thus generat-ing an inductively reactive current whlch interact~ with the ~ixed capacitively reacti~e current to pro~uce a net reactlve current which cooperates with the voltage acros~
the lines to produce reactive power. The predetermined interval o~ time ls usuall~ one-hal~ c~cle o~ the line ~ `
~, .
STATIC VAR GEN~R~TOR WITH OVERVOLTAGE
PROTECTION FOR THE S~ITCHED CAP~CITORS THEREOF
me sub~ect matter of this in~ention ~s related to that disclosed and claimed in U9 S ~ Patent No~ 4,307,331, issued December 2~ 9 1~81 to Laszlo Gyugyi, entitled "Hybrid S~tched Capacitor Controlled-Inductor Static VAR Genera-tor and Control Apparatus".
~ The subJect matter o~ this in~ention relates generally to VAR generators and more speclfically to static VAR generators in which switched capacitors are utilized~
It is known to make ~AR generators by connecting a fixed capacitor and a switched lnduc-tor in parallel across two lines o~ an electric power system to be regu-lated or controlled by the VAR generator~ A suitablecontrol system is provided ~or sending an output signal to the switch portion of the switohed inductvr to establish a conduction ~nterval during a prede-termined period o~ timeO
The conductlon interval allows current to ~low ~or a portion o~ the predstermined period o~ time, thus generat-ing an inductively reactive current whlch interact~ with the ~ixed capacitively reacti~e current to pro~uce a net reactlve current which cooperates with the voltage acros~
the lines to produce reactive power. The predetermined interval o~ time ls usuall~ one-hal~ c~cle o~ the line ~ `
~, .
2 4~,723 voltage. Consequently, on a half cycle by hal~ cycle basis the switch1ng lnterval can be chan~ed to provide differing amounts o* reactive power as i5 determined ~s necessary by the calculating or control portion of the 5 system. Systems of the pre~ious type can be ~o~nd in U.S.
Patent 3,936,7279 issued February 3, 1976 to F. W~ Kelly, Jr. and G. R. E. Laison and U.S. Patent 3,999,117, is3ued December 21~ 1976 to L. ~yugyi et al. In ~he related Patents discrete banks of capacitors can be switched into and out o~ a disposition o~ conduction relati~e to the previously described lines to cooperate with the voltage thereacros to provide discrete le~els of capacitive VAR's.
Often this i~ utilized in conjunction wi~h a switched inductor so that the net ouput VAR may be either negati~e or po~itive depend.ing upon which elements are switched or and out o~ a disposition of conduction. The switch is usually a thyristor switch and in practice of~en consists o~ a string of reverse parallel connected devices to provide suffic.ient ~toltage blocking capability. me thyristor switches are 20 normally fired or gated in response to a VAR demand signal at the time when the capacitor voltage and the AC net work ~oltage are equal, that is when the ~oltage across the thyristor switch i~ zero. me disconnection o~ the capacitor bank, though, on the other hand~ usually takes place at the ~nstance when the current ~lowing through the ~h~ri~tor switch crosses zero. At thi3 time the voltage across the capacitor bank is usually equal to the peak or crest o~ the AC network voltage. mus, the capacitor bank remains charged to that voltage a~ter disconnection. mls unfortunately provlde~
a dlsadvantage; namely~ s~nce the capacitor bank remalns charged to the peak of the AC voltage, the thyristor switch has to be rated to b70ck twice thi~ voltage. This is because the voltage across the thyristvr switch i~ the sum oi the applied AC voltage and thecapacltor voltage and thus it reaches a maximum value of twice the peak AC
voltage, once in each cycle~ In the past, to protect the thyristor switch against high voltage stress cau~ed by
Patent 3,936,7279 issued February 3, 1976 to F. W~ Kelly, Jr. and G. R. E. Laison and U.S. Patent 3,999,117, is3ued December 21~ 1976 to L. ~yugyi et al. In ~he related Patents discrete banks of capacitors can be switched into and out o~ a disposition o~ conduction relati~e to the previously described lines to cooperate with the voltage thereacros to provide discrete le~els of capacitive VAR's.
Often this i~ utilized in conjunction wi~h a switched inductor so that the net ouput VAR may be either negati~e or po~itive depend.ing upon which elements are switched or and out o~ a disposition of conduction. The switch is usually a thyristor switch and in practice of~en consists o~ a string of reverse parallel connected devices to provide suffic.ient ~toltage blocking capability. me thyristor switches are 20 normally fired or gated in response to a VAR demand signal at the time when the capacitor voltage and the AC net work ~oltage are equal, that is when the ~oltage across the thyristor switch i~ zero. me disconnection o~ the capacitor bank, though, on the other hand~ usually takes place at the ~nstance when the current ~lowing through the ~h~ri~tor switch crosses zero. At thi3 time the voltage across the capacitor bank is usually equal to the peak or crest o~ the AC network voltage. mus, the capacitor bank remains charged to that voltage a~ter disconnection. mls unfortunately provlde~
a dlsadvantage; namely~ s~nce the capacitor bank remalns charged to the peak of the AC voltage, the thyristor switch has to be rated to b70ck twice thi~ voltage. This is because the voltage across the thyristvr switch i~ the sum oi the applied AC voltage and thecapacltor voltage and thus it reaches a maximum value of twice the peak AC
voltage, once in each cycle~ In the past, to protect the thyristor switch against high voltage stress cau~ed by
3 ~8,723 an overcharged capacitor bank, a protec~ion arrangement that inhibits the disconnection of the capacitor bank under high AC network voltage had been employed. Unfor-tunately, during overvoltage it is disadvantageous to keep the capaci~or banks connected because that situation may further increase the a]ready high network voltage and can even lead to dangerous oscillatory conditions in the network. It would be advantageous therefore if apparatus could be found which would allow the capaci~ors to be ln disconnected at ~he proper time without causing the over-voltage conditions previously described.
SUMMARY OF THE INVENTION
In accordance with the invention an arrangement is provided in which a capacitor is discharged by a non-linear clamping (limiting) device such as a conventionalstation-type surge arrester or a zinc oxide device during the half cycle following the disconnection of the capaci-tor bank. The clamping device is connected in series circuit relationship with the capacitor bank and the surge current limiting inductor, if one is present, and is connected in parallel with the thyristor switch. The characteristic of the clamping device is such that for a voltage below a certai~ level the clamping device exhibits a very high resistance. Above that certain level a very low and ideally zero resistance is present. Thus, during normal voltage levels the current in the clamping device is essentially zero, regardless of whether the thyristor switch is in the conducting or blocking state. However, when a conducting thyristor switch i5 turned off during a high network overvoltage, the voltage across the thyristor switch and clamping device increases. When the breakover voltage is reached in the clamping device, the clamping device begins to conduct current through the series con-nected capacitor bank, while it nevertheless keeps the voltage across the thyristor switch nearly constant. The current conducted is essen~ially proportional to the capacitance of the capacitor bank and the rate of change o~ the applied voltage. This current rapidly discharges `
~s~
48, 723 the capacltor bank to the level a~ l~hich the c~pacltor voltage plu~ the AC network voltage e~uals the breakover voltage of the olamping devlce wh~ch 15 ~1 sai~e value for the thyrixtor ~witch to block.
~
For a better understanding of the invention ref erence may be made to the pre~erred embodilDents thereo~
exe~li*ied by the îollowing drawings in which:
Flgure 1 shows a portion OI a prlor art VA~
10 generator OI the klnd whlch uses barlks o~ discretely controlled capacitors and contimlously controlled induc-tor~ to pro-ride VAR generatlon;
Fig. 2 shows a portlon of th~ capaoiti~e bank of Fig . 1 employing a protectlve clamp~ ng de~Jice and surge suppresirlg inductor; and Flg. 3 curve A is a plot OI llne voltage and VAR
generator current Por a thyrlstor switch turn~off condi-tio~l; Fig. 3 curve B shows a plot of the voltage acros~
the capaclt~e element of Fig. 2 for the co~dltions de-picted in curve A; Fig. 3 c~rve C ~hows a plot of the voltage açross the switch o~ Fig, 2 for the conditions depicted in curve A; and ~ig. D is a plot of the discharge current shown in Fig. 2 for the condltions d~plcted in curve A.
DESCRIPTION OF THE PREFERRE~ EMBODIMENTS
Re~erring now to the drawings, it should be kept in mind that the prior art in the area of stat~c VAR
generators is well explained in the aforementioned U.5, Patents. In Fig. 1 of the drawings~ whlch ~s also labeled prior art, two llnes L1 and L2 are ~hown having a voltage v impres~ed thereacross and currents i1 ~nd is ~lowing therein. In this ca~e, compensat~on i~ provided by ~our capacitl~e elements C1 through C4 and inductive element L.
Each of the capaciti~e elements has a switch TH1 through TH4 connected thereto re~pecti~ely for bringing the capaciti~e elements C1 through C4, respecti~ely, lnto or out o~ clrcuit conduction. In the speci~ic embodiment sho~n in Fig. 1, the ....
.~..l5~ 5~ ~
~,723 thyristor swltches TH1 through TH4 employ reverse parallel connected thyristor elements and ~e there~ore bidirec~
tional. Each o~ the capacitive elements C1 through C4 ls capable of generating capacitlve currents i~C1 through iCC49 respectively9 depending upon the conduction status of the switches TH1 through TH4. It ls possible in this case ~or any or a~l o~ ~he capaci~ive elements C1 through C4 to be lnterconnected ln parallel circuit relationship~
thus producing any of a discrete number of net capacitive currents ~or compensation. m e in~uctiYe element L, on the other h~nd, which is connected ~o a bilateral thyr~s-tor switch TH, produces a current iCL over a continuous range a~ determined by the conductor angle o~ the thyris-tor switch TH. A control system CS is pro~$ded between the lines L1 and L2 ~or sensing circuit variables 9 such as the voltage v and the current i1~ ~or exa~ple, ~or thus producing ~ignals which are ~pplied to ~he gates of the various thyristors ~or causing conduction in a manner deemed appropriate ~or proper VAR compensation. Such a control system ls described ln greater detail ln the aforementloned U.S0 patent No. 4 9 307,331.
Re~erring now to Fig. 2, a replacement c~pac~tor bank for any o~ the capacitors shown in Figo 1 0~ the prior art i5 shown. In th$s case, ~he designations of the various elements, currents and ~oltages have been ohanged sl~ghtly for pu~po~es of clarity o~ lllustratlon. Gener-ally, the lines L1 and L2 have ~i~po~ed therebetween circuit elements similar to that ~hown in ~ny o~ the bank~ o~ Fig.
1. Specifically, there is a capao~t~ve element CN, o~e side o~ which ~ oonnested to l~ne L1 and the other slde of which may be connected to a surge current limiting induc-tor LN. The other side of the surgecurrent limlt$ng indUctor LN may be connected to one side o~ the bipolar thyristor switch SW and to one side of a non-l~near resis~
ti~e element RR . me non-linear resisti~e element R~ may be a conYentional statior~t~pe surge arrester or a zinc-oxide de~ice. The other 3ide o~ the non-linear re~istive element R~ and the other ~lde o~ the bipolar thyristor switch S~J may be connected together ~d to the line L2. At an appropriate time the bipolar thyristor swi-tch or simi-lar gated device i ~ gated on, causin~; reactive current to ~low through the oapacitive element CN and the surge c,urrent limiting irldllctive element LN. ~is reactive current interac ts with the voltage v betweerl ~he lines L1 and L2 to produce approprlate VAR~s ~or the network to be co~pensated, The appropria~e V~? s may be the result OI
the combination of the aIorem~rltioned capacl~ive VAR' s 10 with ~n~uctive VAR' s in a manner simllar to that described pre~riously 1:~ that is desired. The voltage drop across the capacitive element CN i9 designated vcN. The volt~ge drop across the switch SW ls designated vsw. me current thro~lgh the non-linear re~istlve element R,~ is deslgnated 15 ~R.
Referring now to Fig~ 3, curves A, B, C and D;
as well as Fig. 2, the interrelationship of the various vcltages and currents as descrlbed with respect to Fi~o 2 is depicted. To satisfy the requirements oi the AC supply 20 network, as represented by the llnes L1 and L2, a~d to provlde ~he ~ati~facto~y protection o~ the thyri~tor switch SW against overvoltages across the lines L1 and L2, an arrangement in which ~he capacltor CN is discharged by the non~linear clamp~ng or limiting device ~ durlng the 25 hal~ cycle ollowing the disconnection of the ca~acitor bank CN is provided. me clamping device ~ is connected in series circult relationsh~p with the capacltor bank CN
and the surge llmiting inductor LN and i~ ~urthermore connected in parallel with the thyristor ~witch SW, as is 30 shown in Figo 2~ The characteristic o~ the clamping device R~ is such that below a given voltage levelj which shall be called the clamping or breakdown voltaga, the clamping device R~ ~xhibits a very high resistance~ On the other hand, above the aforementioned clamping or 35 breakdown voltage le~el, a very low, ideally zero~ reslst ance i~ exhlbited by the clamping dev.lce RR. Thus, ~uring normal voltage levels, that is voltage levels below the clamping voltage level, the current 1~ ls close to or 5.~
7 ~,723 equal to zero, regardless o~ whether -the thyristor switch SW is in the conducting or blocking state. Howe~er, when the conducting ~hryrlstor switch SW is turned off during a hlgh network oYervoltage, the non-llnear resistive device R~ will begln to conduct if the o~ervoltage is above the breakdown voltage. At time t17 when the thr~stor switch SW turns o~f a~ a current zerot as shown in curv~ A, the capacitor voltage vcN, as shown in curve B, is maximum, the ~oltage ~SW across the parallel co~nected th~ristor switch SW and clamping device R~ is zero~ As the AC
network voltage v changes7 the voltage vsw across the thyristor switch SW and clamping devlce R~ increases.
When the breakdown voltage ~D as shown lncurve C is exceeded, the clamping devics R~ starts to conduct current i~ as is best ~hown in curve D at time t2~ This current ~lows through the serie~ connected capacitor bank CN. Never-theless, the voltage vsw across the thyristor ~witch is kept nearly constant, as ls best shown in curve B. The current i~ is proportional to the capacitanc~ CN o~ the capacitor bank and the rate o~ change o~ the applied ~oltage, that is, i - CN dv/dt. This curren-t i rapidl~ discharges t~e capacitor bank CN to the level at which the capacitor voltage vcN plus the AC network voltage ~ equals the breakover voltage vBD of the clamping de~ice R~ , which is a safe value ~or the thyristor switch SW ~o block.
It i~ to be understood with respect to the embodiments o~ this invention that the presenc~ of the surge limitin~ inductor de~ice LN is optionalO It is also to be under~tood that neither the switch dev~ce SW, the capacitive dev~ce CN nor the non-linear resistive device RR is limited to being co~posed o~ one de~ice. In fact, any or all of the~e devices may compri~e banks o~ de~ices to accomplish necessarr purpo~. It is ~lso to be under-stood that the directions of the currents i and i~ are chosen for simplicity of illustration and are not n~ces-sarily limlting, except ln the extent that the normal principles of electriclty re~ulre~
The apparatus taught with respect to the ~mbodi-~ 3 ~ ~
8 48,723 ments of this invention have many advantages. One advan-tage lies in ~he economy associated with the aforemen-tioned device in tha-t the capacitor CN as connected in series with the clamping device RQ limits the magnitude and duration of the clamping current iQ to practical values. Furthermore, when the thyristor switch SW is off, the clamping device RQ also provides a continuous protec-tion for the thryistor switch SW by limiting the voltage transient which may appear thereacross to the level of the breakover voltage vBD. Another advantage lies in the fact that the aforementioned device makes it possible to meet the AC supply network requirements of disconnected capaci-tive compensation under overvoltages with a thyristor switch of moderate voltage rating using a clamping device of relatively small energy absorption capability.
SUMMARY OF THE INVENTION
In accordance with the invention an arrangement is provided in which a capacitor is discharged by a non-linear clamping (limiting) device such as a conventionalstation-type surge arrester or a zinc oxide device during the half cycle following the disconnection of the capaci-tor bank. The clamping device is connected in series circuit relationship with the capacitor bank and the surge current limiting inductor, if one is present, and is connected in parallel with the thyristor switch. The characteristic of the clamping device is such that for a voltage below a certai~ level the clamping device exhibits a very high resistance. Above that certain level a very low and ideally zero resistance is present. Thus, during normal voltage levels the current in the clamping device is essentially zero, regardless of whether the thyristor switch is in the conducting or blocking state. However, when a conducting thyristor switch i5 turned off during a high network overvoltage, the voltage across the thyristor switch and clamping device increases. When the breakover voltage is reached in the clamping device, the clamping device begins to conduct current through the series con-nected capacitor bank, while it nevertheless keeps the voltage across the thyristor switch nearly constant. The current conducted is essen~ially proportional to the capacitance of the capacitor bank and the rate of change o~ the applied voltage. This current rapidly discharges `
~s~
48, 723 the capacltor bank to the level a~ l~hich the c~pacltor voltage plu~ the AC network voltage e~uals the breakover voltage of the olamping devlce wh~ch 15 ~1 sai~e value for the thyrixtor ~witch to block.
~
For a better understanding of the invention ref erence may be made to the pre~erred embodilDents thereo~
exe~li*ied by the îollowing drawings in which:
Flgure 1 shows a portion OI a prlor art VA~
10 generator OI the klnd whlch uses barlks o~ discretely controlled capacitors and contimlously controlled induc-tor~ to pro-ride VAR generatlon;
Fig. 2 shows a portlon of th~ capaoiti~e bank of Fig . 1 employing a protectlve clamp~ ng de~Jice and surge suppresirlg inductor; and Flg. 3 curve A is a plot OI llne voltage and VAR
generator current Por a thyrlstor switch turn~off condi-tio~l; Fig. 3 curve B shows a plot of the voltage acros~
the capaclt~e element of Fig. 2 for the co~dltions de-picted in curve A; Fig. 3 c~rve C ~hows a plot of the voltage açross the switch o~ Fig, 2 for the conditions depicted in curve A; and ~ig. D is a plot of the discharge current shown in Fig. 2 for the condltions d~plcted in curve A.
DESCRIPTION OF THE PREFERRE~ EMBODIMENTS
Re~erring now to the drawings, it should be kept in mind that the prior art in the area of stat~c VAR
generators is well explained in the aforementioned U.5, Patents. In Fig. 1 of the drawings~ whlch ~s also labeled prior art, two llnes L1 and L2 are ~hown having a voltage v impres~ed thereacross and currents i1 ~nd is ~lowing therein. In this ca~e, compensat~on i~ provided by ~our capacitl~e elements C1 through C4 and inductive element L.
Each of the capaciti~e elements has a switch TH1 through TH4 connected thereto re~pecti~ely for bringing the capaciti~e elements C1 through C4, respecti~ely, lnto or out o~ clrcuit conduction. In the speci~ic embodiment sho~n in Fig. 1, the ....
.~..l5~ 5~ ~
~,723 thyristor swltches TH1 through TH4 employ reverse parallel connected thyristor elements and ~e there~ore bidirec~
tional. Each o~ the capacitive elements C1 through C4 ls capable of generating capacitlve currents i~C1 through iCC49 respectively9 depending upon the conduction status of the switches TH1 through TH4. It ls possible in this case ~or any or a~l o~ ~he capaci~ive elements C1 through C4 to be lnterconnected ln parallel circuit relationship~
thus producing any of a discrete number of net capacitive currents ~or compensation. m e in~uctiYe element L, on the other h~nd, which is connected ~o a bilateral thyr~s-tor switch TH, produces a current iCL over a continuous range a~ determined by the conductor angle o~ the thyris-tor switch TH. A control system CS is pro~$ded between the lines L1 and L2 ~or sensing circuit variables 9 such as the voltage v and the current i1~ ~or exa~ple, ~or thus producing ~ignals which are ~pplied to ~he gates of the various thyristors ~or causing conduction in a manner deemed appropriate ~or proper VAR compensation. Such a control system ls described ln greater detail ln the aforementloned U.S0 patent No. 4 9 307,331.
Re~erring now to Fig. 2, a replacement c~pac~tor bank for any o~ the capacitors shown in Figo 1 0~ the prior art i5 shown. In th$s case, ~he designations of the various elements, currents and ~oltages have been ohanged sl~ghtly for pu~po~es of clarity o~ lllustratlon. Gener-ally, the lines L1 and L2 have ~i~po~ed therebetween circuit elements similar to that ~hown in ~ny o~ the bank~ o~ Fig.
1. Specifically, there is a capao~t~ve element CN, o~e side o~ which ~ oonnested to l~ne L1 and the other slde of which may be connected to a surge current limiting induc-tor LN. The other side of the surgecurrent limlt$ng indUctor LN may be connected to one side o~ the bipolar thyristor switch SW and to one side of a non-l~near resis~
ti~e element RR . me non-linear resisti~e element R~ may be a conYentional statior~t~pe surge arrester or a zinc-oxide de~ice. The other 3ide o~ the non-linear re~istive element R~ and the other ~lde o~ the bipolar thyristor switch S~J may be connected together ~d to the line L2. At an appropriate time the bipolar thyristor swi-tch or simi-lar gated device i ~ gated on, causin~; reactive current to ~low through the oapacitive element CN and the surge c,urrent limiting irldllctive element LN. ~is reactive current interac ts with the voltage v betweerl ~he lines L1 and L2 to produce approprlate VAR~s ~or the network to be co~pensated, The appropria~e V~? s may be the result OI
the combination of the aIorem~rltioned capacl~ive VAR' s 10 with ~n~uctive VAR' s in a manner simllar to that described pre~riously 1:~ that is desired. The voltage drop across the capacitive element CN i9 designated vcN. The volt~ge drop across the switch SW ls designated vsw. me current thro~lgh the non-linear re~istlve element R,~ is deslgnated 15 ~R.
Referring now to Fig~ 3, curves A, B, C and D;
as well as Fig. 2, the interrelationship of the various vcltages and currents as descrlbed with respect to Fi~o 2 is depicted. To satisfy the requirements oi the AC supply 20 network, as represented by the llnes L1 and L2, a~d to provlde ~he ~ati~facto~y protection o~ the thyri~tor switch SW against overvoltages across the lines L1 and L2, an arrangement in which ~he capacltor CN is discharged by the non~linear clamp~ng or limiting device ~ durlng the 25 hal~ cycle ollowing the disconnection of the ca~acitor bank CN is provided. me clamping device ~ is connected in series circult relationsh~p with the capacltor bank CN
and the surge llmiting inductor LN and i~ ~urthermore connected in parallel with the thyristor ~witch SW, as is 30 shown in Figo 2~ The characteristic o~ the clamping device R~ is such that below a given voltage levelj which shall be called the clamping or breakdown voltaga, the clamping device R~ ~xhibits a very high resistance~ On the other hand, above the aforementioned clamping or 35 breakdown voltage le~el, a very low, ideally zero~ reslst ance i~ exhlbited by the clamping dev.lce RR. Thus, ~uring normal voltage levels, that is voltage levels below the clamping voltage level, the current 1~ ls close to or 5.~
7 ~,723 equal to zero, regardless o~ whether -the thyristor switch SW is in the conducting or blocking state. Howe~er, when the conducting ~hryrlstor switch SW is turned off during a hlgh network oYervoltage, the non-llnear resistive device R~ will begln to conduct if the o~ervoltage is above the breakdown voltage. At time t17 when the thr~stor switch SW turns o~f a~ a current zerot as shown in curv~ A, the capacitor voltage vcN, as shown in curve B, is maximum, the ~oltage ~SW across the parallel co~nected th~ristor switch SW and clamping device R~ is zero~ As the AC
network voltage v changes7 the voltage vsw across the thyristor switch SW and clamping devlce R~ increases.
When the breakdown voltage ~D as shown lncurve C is exceeded, the clamping devics R~ starts to conduct current i~ as is best ~hown in curve D at time t2~ This current ~lows through the serie~ connected capacitor bank CN. Never-theless, the voltage vsw across the thyristor ~witch is kept nearly constant, as ls best shown in curve B. The current i~ is proportional to the capacitanc~ CN o~ the capacitor bank and the rate o~ change o~ the applied ~oltage, that is, i - CN dv/dt. This curren-t i rapidl~ discharges t~e capacitor bank CN to the level at which the capacitor voltage vcN plus the AC network voltage ~ equals the breakover voltage vBD of the clamping de~ice R~ , which is a safe value ~or the thyristor switch SW ~o block.
It i~ to be understood with respect to the embodiments o~ this invention that the presenc~ of the surge limitin~ inductor de~ice LN is optionalO It is also to be under~tood that neither the switch dev~ce SW, the capacitive dev~ce CN nor the non-linear resistive device RR is limited to being co~posed o~ one de~ice. In fact, any or all of the~e devices may compri~e banks o~ de~ices to accomplish necessarr purpo~. It is ~lso to be under-stood that the directions of the currents i and i~ are chosen for simplicity of illustration and are not n~ces-sarily limlting, except ln the extent that the normal principles of electriclty re~ulre~
The apparatus taught with respect to the ~mbodi-~ 3 ~ ~
8 48,723 ments of this invention have many advantages. One advan-tage lies in ~he economy associated with the aforemen-tioned device in tha-t the capacitor CN as connected in series with the clamping device RQ limits the magnitude and duration of the clamping current iQ to practical values. Furthermore, when the thyristor switch SW is off, the clamping device RQ also provides a continuous protec-tion for the thryistor switch SW by limiting the voltage transient which may appear thereacross to the level of the breakover voltage vBD. Another advantage lies in the fact that the aforementioned device makes it possible to meet the AC supply network requirements of disconnected capaci-tive compensation under overvoltages with a thyristor switch of moderate voltage rating using a clamping device of relatively small energy absorption capability.
Claims (12)
1. A VAR generator of the type which supplies reactive power to an electrical system for regulation thereof, comprising a) capacitive reactive means interconnectable with said electrical system for supplying said reactive power thereto during a predetermined interval of time;
b) controllable switch means interconnected in series circuit relationship with said capacitive reactive means for connecting said capacitive reactive means in reactive circuit relationship with said electrical system during said interval of time; and c) non linear clamping means connected in series circuit relationship with said capacitive reactive means and in parallel circuit relationship with said switch means for conducting therethrough capacitive reactive means dis-charge current while said switch means is an off state but only when the voltage across said switch means is above a maximum predetermined allowable value and during a period of time which is subsequent to said interval to thus limit the voltage across said capacitive reactive means to a pre-determined said level,
b) controllable switch means interconnected in series circuit relationship with said capacitive reactive means for connecting said capacitive reactive means in reactive circuit relationship with said electrical system during said interval of time; and c) non linear clamping means connected in series circuit relationship with said capacitive reactive means and in parallel circuit relationship with said switch means for conducting therethrough capacitive reactive means dis-charge current while said switch means is an off state but only when the voltage across said switch means is above a maximum predetermined allowable value and during a period of time which is subsequent to said interval to thus limit the voltage across said capacitive reactive means to a pre-determined said level,
2, The combination as claimed in claim 1 wherein said clamping means comprises a zinc-oxide device.
3. The combination as claimed in claim 1 wherein said clamping means comprises surge-arrester means.
48,723
48,723
4. The combination as claimed in claim 1 wherein said switch means comprises thyristor means.
5. The combination as claimed in claim 4 wherein said clamping means comprises a zinc-oxide device
6. The combination as claimed in claim 4 wherein said clamping means comprises surge-arrester means.
7. A VAR generator of the type which absorbs reactive power from an electrical system for regulation thereof, comprising:
a) capacitive reactive means interconnectable with said electrical system for supplying said reactive power thereto during a predetermined interval of time;
b) controllable switch means interconnected in series circuit relationship with said capacitive reactive means for connecting said capacitive reactive means in reactive circuit relationship with said electrical system during said interval of time; and c) non-linear clamping means connected in series circuit relationship with said capacitive reactive means and in parallel circuit relationship with said switch means for conducting therethrough capacitive reactive means discharge current while said switch means is in an off state but only when the voltage across said switch means is above a maximum predetermined allowable value and during a period of time which is subsequent to said interval to thus limit the voltage across said capacitive reactive means to a prede-termined safe level.
a) capacitive reactive means interconnectable with said electrical system for supplying said reactive power thereto during a predetermined interval of time;
b) controllable switch means interconnected in series circuit relationship with said capacitive reactive means for connecting said capacitive reactive means in reactive circuit relationship with said electrical system during said interval of time; and c) non-linear clamping means connected in series circuit relationship with said capacitive reactive means and in parallel circuit relationship with said switch means for conducting therethrough capacitive reactive means discharge current while said switch means is in an off state but only when the voltage across said switch means is above a maximum predetermined allowable value and during a period of time which is subsequent to said interval to thus limit the voltage across said capacitive reactive means to a prede-termined safe level.
8. The combination as claimed in claim 7 wherein said clamping means comprises a zinc-oxide device.
9. The combination as claimed in claim 7 wherein said clamping means comprises surge-arrester means.
10. The combination as claimed in claim 7 wherein said switch means comprises thyristor means.
11. The combination as claimed in claim 10 wherein said clamping means comprises a zinc-oxide device.
11 48,723
11 48,723
12 . The combination as claimed in claim 10 where-in said clamping means comprises surge-arrester means.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US7840679A | 1979-09-24 | 1979-09-24 | |
| US078,406 | 1979-09-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1159514A true CA1159514A (en) | 1983-12-27 |
Family
ID=22143832
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000359939A Expired CA1159514A (en) | 1979-09-24 | 1980-09-09 | Static var generator with overvoltage protection for the switched capacitors thereof |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JPS5656135A (en) |
| CA (1) | CA1159514A (en) |
| ZA (1) | ZA805279B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9294003B2 (en) | 2012-02-24 | 2016-03-22 | Board Of Trustees Of Michigan State University | Transformer-less unified power flow controller |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS49123565A (en) * | 1973-03-30 | 1974-11-26 |
-
1980
- 1980-08-26 ZA ZA00805279A patent/ZA805279B/en unknown
- 1980-09-09 CA CA000359939A patent/CA1159514A/en not_active Expired
- 1980-09-24 JP JP13174380A patent/JPS5656135A/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9294003B2 (en) | 2012-02-24 | 2016-03-22 | Board Of Trustees Of Michigan State University | Transformer-less unified power flow controller |
Also Published As
| Publication number | Publication date |
|---|---|
| ZA805279B (en) | 1981-12-30 |
| JPS5656135A (en) | 1981-05-18 |
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