Note: Descriptions are shown in the official language in which they were submitted.
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1 Field of the Invention
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This invention relates genera]ly to
circuits for AC power line protection and filtering
and more particularly to such circuits using pro-
tected capacitors.
In the past transient voltage suppressors
have been connected across multilead AC lines with
each suppressor connected between each pair of
lines. As a result when a breaker is positioned
in only one o~ those lines, normally the live line~
the failure o~ a suppressor not connected directly
to the fused line does not activate the breaker.
15Further, many line filters prove to be
unreliable because capacitors connected across the
suppressor are subjected to large transient voltages
causing failure of the capacitor.
Further, in applications where AC capaci-
tors are required, polarized capacitors have notbeen found suitable because they are not designed
to operate under reverse voltage conditions without
experiencing failure.
The present invention not only provides
an improved line protection and filtering circuit
arrangement but enables polarized capacitors to be
used in high voltage alternating current applica-
tions.
30It is an object of this invention to
provide an improved filtering and protection circuit.
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It is another object of the present invention to
provide such a circuit using solid state diode elements capable
of withstanding a high voltage environment and useful ~o sup-
press transient voltages and to protect against voltage surges.
It is also an ob~ect of the present invention to
provide fault protection using a single breaker for line,
neutral, and ground leads that is activated in response to a
fault between any of those leads.
It is a further object o~ the present invention to
provide the use of polarized capacltors in AC circuits.
It is another object o~ the present inven~ion to
provide a protection circuit wherein filter capacitors are
protected from voltage transients and surges.
These and other object~ of the present invention
are achieved b~ a power line protection circuit for connection
between a live lead, a neutral lead and a ground lead. The
circuit includes a breaker intended to be located in only the
live lead. At least three transient voltage suppressors are
connected from a common node on one end to each of the leads
so that when any of the suppressors fail the breaker will be
~` activated.
Still other advantages are achieved by a polarized
capacitive device useful to replace non-polarized alternating
current capacitors. The device includes a pair of polarized
2S capacitors and a pair of diodes. One capacitor and one diode
are connected to bypass the half cycle of the voLtage
across the device and the other capacitor and diode are con-
nected to bypass the negative half cycle of the voltage acros~
the device.
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In one arrangement, a gas dlscharge tube is provided
in series with the suppressor that is connected from the common
node to the ground line. In another arrangemlent with or with-
out the gas discharge tube, the suppressors are bidirectional
devices with a rated clamping voltage equal to less than the
peak line voltage and a predetermined amount greater than
one-half of the peak line voltage.
BRIEF DESCRIPTIO~ OF THE DRAWING
These and other objects and advantages of the present
invention will become more apparent by reEerence to the
following description taken in conjunction with the
accompanying drawing wherein:
FIG. 1 is a circuit schematic showing one embodi-
ment of the protection circuit of the present invention;
FIG. 2 is a circuit schematic of an embodiment of
the AC equivalent capacitor of the present invention;
~ IG. 3 is a circuit schematic showing an alternate
embodiment for the circuit shown in FIG. 2;
FIG. 4 is a circuit schematic of another embodiment
of the present invention utilizing a gas discharge tube; and
FIG. 5 is a circuit schematic of yet another embodi-
ment of the present invention.
DESCRIPTION OF T~ PREFER~D EMBODIMENT
Referring to the drawing wherein like reference
charac~ers are used for like parts throughout, there is illus-
trated in FIG. 1 a three phase power line with live lines Ll
through L3, neutral line ~, and ground line G. Connected
across these various lines is a power line filtering and pro-
tection circuit 10. ~ach of ~he live lines Ll through L3
includes a conventional breaker 12a, 12b or l~c but the neutral
and ground lines do not include such a breaker. In acldition,
3~
inductors 14a, 14b, 14c and 14d are serially respectively
located in the live lines Ll, L2 and L3 and on neutral line N
between the AC source (not shown) and the circuit 10. Of course,
it should be understood that in a specific embodiment the
inductors 14a, 14b, 14c, and 14d are omitted.
A filtering and protection circuit 10 includes a
plurality of filtering and protection devices 16a through 16e
enclosed in dotted lines in FIG. 1. The devices 16 are con-
nected in a wye arrangement between a common node 18, indi-
cated by a line in FIG~ 1, and the live lines Ll through L3,
the neutral line N and the ground line G. With this arrange-
ment even if a fault occurs in any of the devices 16a through
16e wherein the device fails short, a breaker 12 connected in
one of the live lines will be activated since each of these
devices is connected by way of common node 18 to one of the
live lines inc.luding a breaker 12.
Each device 16 includes a tr~nsient voltage sup-
pressor 20 and in specific embodiments a parallel capacitor
22. The transient voltage suppressors 20 are preferably
silicon p~n junction devices rated for high voltage and
curre~t applications~ The current-voltage characteristic of
each of the suppressors 20 is preferably similar to that of
zener diodes, clamping at a given rated voltage in one direc-
tion and having a forward diode characteristic in the other
direction~ Suitable transient voltage suppressors are distri-
buted by TRW, 301 West O Street, Ogallala, Nebraska with iden-
tifying numbers TVP1500 through TVP1534. Since the clamping
time from zero volts to the breakdown voltage is on the order
o a nanosecond the response of these devices is essentially
instantaneous. These suppressors are available wi~h clamping
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voltages higher than normal line voltages. Each suppressor
20 is arranged to permit forward current flow from the common
node 18 into a line Ll through L3, a neutral N or a ground G
while allowing reverse current flow only when a clamping
voltage is exceeded. While the transient voltage suppressors
20 are shown in FIG. 1 with the anodes of each suppressor 20
being connected to the common node 18, it should be realized
that the suppressors 20 can be connected in the reverse sense
with the cathodes of each suppressor 20 being connected to
the common node 18.
A suppressor 20 can also be combined with a
p~rallel surge suppressor ~not shown). For example, a shunt
diode with a high forward surge rating increases the forward
surge rating of the combination by shunting high node to line
currents across the suppressor 20. If the clamping voltage
of the suppressor is less than the breakdown volta~e of the
shunt diode the other shunt diode characteristics are not
critical. Similarly a bi-directional clamp or protection
device, such as a gas discnarge tube, a metal oxide varistor,
a bi-directional voltage transient suppressor or a combination
of such devices can be shunted across the suppressor Z0 for
surge protection. As still another alternative embodiment a
bi-directional transient voltage suppressor made up of back
to back p-n junction silicon devices, such as those described
above can be utilized.
The capacitors 22 are protected due to the ability
of the suppressors 20, to quickly conduct to shunt the capa-
citor 22 in response to a surge or transient.
In other specific embodiments it should ~e realized
that the wye arrangement of the protective devices 16 is suit~
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able for single phase operation as well as other line arrange-
ments.
In accordance with important aspects of the present
invention, the capacitors 22 in a specific embodiment are
polarized capacitors such as electrolytic. The polarized
capacitors are connected with the positive terminal connected
to the cathode of a respective suppressor 20. In this way,
the polarized capacitors are capable of safe operation in an
AC circuit without experiencing reverse voltages that could
result in failure of the capacitor 22.
Referring now to FIG. 2, an AC equivalent capacitor
netw~rk 23 results Erom the parallel arrangement in FIG. 1 of
the capacltors 22 and the suppressors 20. The AC equivalent
capacitor network 23 of FIG. 2 includes a pair of diodes 26a,
26b connected in series anode to anode. A polarized capacitor
24a is connected across diode 26a with the positive terminal
of the capacitor 24a connected to the cathode of the diode
26a. Similarly, a polarized capacitor 24b is connected across
the diode 26b with the positive terminal of the capacitor 24b
connected to khe cathode of the diode 26b.
The capacitor 24a on one side of the network and
the diode 26b on the other side of the network form one diode
clamper and the diode 26a together with the capacitor 24b
form another diode clamper. The two diode clamper circuits
are connected together at common node 28 with each diode 25
shunting current flow from common node 28 around their parallel
capacitors 24 and preventing reverse voltages across either
capacitor 24. The s~eady state voltage across the diode 26
of each clamper is therefore the difference between the voltage
across the filter 23 and ~he voltage across clamper's capacitor.
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Thus the voltage across each diode 26 is a non-zero crossing
sinusoid, one positive and one negative. Thus each polarized
electrolytic capacitor 24 is capable of acting in a high energy
alternating current environment without experiencing a reverse
voltage.
Conveniently a very large resistance (not shown~
can be connected across the filter 23 so that the charge
trapped on the capacitors 24 will eventually bleed off after
a long period of non-use. Such a resistance draws negligible
current and does not alter the characteristics o the clrcuit.
The diodes 26a and 26b can be any suitably rated p-
n junction device including a conventional diode or a zener
diode but in order to avoid overvoltage of the capacitors
silicon transient voltage suppressors, such as the silicon
1~ transient suppressor discussed above, commercially distributed
by TR~, are preferred. The forward diode current of the diodes
26 is essentially zero and the reverse breakdown voltage or
the clamp voltage in the case of transient suppressors should
be greater than tha peak applied voltage but le~s than the
breakdown voltage of the capacitors 24.
In an alternative embodiment (not shown) the pola-
rity of the capaci~ors 24 and diodes 26 can be reversed. The
circuit operation i5 otherwise ide~tical to that shown in
FIG. 2 and described above.
The filters 23 shown in FIG. 2 or the alternative
embodiment just d_scribed can be stac~ed as shown in FIG. 3
to increase the voltage capability of the overall device or
to reduce the required ripple current rating of each unit.
The filters 23 can be stacked to form a ladder network of any
desired length with the polarity of the capacitors 24 and the
diodes 26 alternating.
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Considering now another embodiment o~ the present
invention and referring now to FIG. 4, the protection circuit
30 includes a gas discharge tube 32 or similar device that is
connected in series with the suppressor 20c provided between
the common node 18 and the ground line Go With this arrange-
ment, the suppressors 20a and 20b clamp normal mode transients
appearing between the live line L and the neutral line N while
the suppressor 20c and the gas discharge tube 32 clamp common
mode transients such as transients that appear across both
the live line L and the neutral line N with respect to the
ground line G.
Further, the gas tube 32 provides a very high
impedance to the ground line G. The high impedance to ground
is an important consideration to satisfy existing safety stan-
dards as currently defined. When a high voltage transient is.
present, the gas discharge tube 32 is immediately extinguished
when the transient falls below a lev~l approximately equal to
the peak line voltage due to the clamp voltage ratings of the
suppressor 20c and either the suppressor 20a or 20b. Accord-
ingly, in contrast to conventional gas discharge tube protec-
tion circuits, the gas discharge tube 32 does not remain con-
ductive until the next zero crossing of the AC line voltage
and therefore avoid~ discontinuities in the output waveform
and the possibility of activating the line circuit breaker.
. 25 Referring now to FIG. 5 and considering other aspects
of the presPnt invention, the protection circuit 40 includes
transient voltage suppressors 34a, 34b, and 34c and a gas
discharge tube 32 connec~ed in series with the suppressor
34c. The transient voltage suppressors 34a, 34b, and 34c are
bi-directional suppressor devices such as varistors as discussed
31.23(~9~l~9
hereinbefore in connection with an alternative
embodiment of FIG. 1. In addition to varistors t
another specific type of suitable ~i-directional
transient voltage suppressor is availab:Le from
General Semiconductor Industries, Inc. sold under
the Trademark TRANSZORB as dèvice-types 1.5KE7.5C
through 1.5KE400C.
The protection circuit 40 provides the
same degree of transient protection as the protec-
tion circuit of FIG~ 4 and additionally allows theutilization oE bi-directional suppressors 34a,
3~b, and 34c that have a rated bi-directional clamp
voltage that is one-hal the rating of the devices
20a, 20b, and 20c of FIG. 4. For example, the
voltage rating of the suppressors 34a, 34b, and
34c is less than the peak line voltage and a pre-
determined amount greater than one-half of the
peak line voltage. Thus, in a specific application
for 120 VAC line operation, a clamp voltage rating
20 of 100 to 110 volts for each o~ the suppressors is
suitable. Further, and as discussed hereinbefore,
the bi-directional transient suppressor devices
34a, 34b, and 34c are suitable for use without the
gas discharge tu~e 32 in the circuit of FIG. 1 in
place of the suppressors 2G.
Many modifications and variations of the
present invention are possible in light of the
above teaching. Thus, it is to be understood that,
within the scope of the appended claims, the inven-
tion may be practiced otherwise than as speci-
fically described above.
