Note: Descriptions are shown in the official language in which they were submitted.
~S~5~ ~
1 489723
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
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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
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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
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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
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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
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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
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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-
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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.
