Device for protecting medium voltage equipment against voltage surges

Suppression of transient voltage surges in an electrical power-carrying line is achieved by a voltage surge suppressor having a capacitor, varistor, and gas tube connected in parallel between the line and a current carrying conductor. The invention is effective for suppressing transient voltage surges more reliably, efficiently, and safely than is possible using primary arrestors or serially connected varistors in accordance with conventional technology. The invention may be adapted for use on medium voltage lines carrying either single-phase or three-phase electrical power.

FIELD OF THE INVENTION 
The invention relates generally to a method and device for suppressing 
transient voltage surges and, more particularly, to a medium voltage surge 
protection device effective for protecting electrical equipment, operating 
between about 1,500 volts and about 4,160 volts, from voltage surges. 
BACKGROUND OF THE INVENTION 
A number of different tools and types of electrical equipment require 
three-phase electrical power having medium voltage, ranging from about 
1,500 to 4,160 volts, to operate. Such tools and equipment include, for 
example, fans, motors, and submersible pumps. When such tools experience 
failure, they can be difficult and expensive to repair and/or replace. 
Furthermore, while they are being repaired and/or replaced, other systems 
dependent on them may be rendered inoperable, thereby resulting in down 
time, and lost productivity and profits. 
Failure of the foregoing tools and equipment commonly results from 
electrical failure, such as from transient voltage spikes and surges in 
electrical power used to operate such tools and equipment. A spike occurs 
when an amount of voltage which is higher than normal in a line occurs for 
a very short period of time (e.g., less than 50 microseconds). A surge 
occurs when an amount of voltage which is higher than normal in a line 
persists for an extended period of time (e.g., more than 50 microseconds). 
The term "surge" will be used herein to refer to both spikes and surges. 
Surges may be caused by many different factors, such as static 
electricity, (e.g., lightning, dust storms, wind, and the like), tree 
limbs falling on power lines, a car hitting an electric pole, inductive 
load switching (e.g., turning on and off electrical equipment), and the 
like. Because there are a number of possible sources of surges which can 
cause electrical equipment and tools to fail, it is important to be able 
to suppress such voltage surges. 
Conventionally, transient voltage surges to medium voltage electrical 
equipment are controlled by primary arrestors set at primary voltages 
ranging, for example, from about 10,000 volts to about 15,000 volts. Such 
voltage settings are too high for sensitive medium voltage equipment and, 
as a result, such equipment is not reliably protected and is vulnerable to 
electrical damage and failure. 
Accordingly, a continuing search has been directed to the development of 
transient voltage surge suppressors which respond quickly enough and 
absorb sufficient energy to prevent medium voltage electrical equipment 
from being damaged. 
SUMMARY OF THE INVENTION 
According to the present invention, suppression of transient voltage surges 
in an electrical power-carrying line is improved by a voltage surge 
suppressor having a capacitor, varistor, and gas tube connected in 
parallel between the power-carrying line and a common bus. The invention 
is effective for suppressing transient voltage surges more reliably, 
efficiently, and safely than is possible using primary arrestors or 
serially connected varistors in accordance with conventional technology. 
The invention may be adapted for use on medium voltage lines carrying 
either single-phase or three-phase electrical power.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 of the drawings, the reference numeral 10 generally 
designates a medium voltage surge protection device embodying features of 
the present invention. The device 10 includes a first circuit 12, a second 
circuit 14, and a third circuit 16, each of which are identified in dashed 
outline. The first circuit 12 includes a first terminal 22 connected to a 
first line 32 of a three-phase power supply 38, which supplies from about 
1,500 to about 4,160 volts, phase-to-phase typically about 1,500 volts, 
2,500 volts, or 3,600 volts, of potential to electrical equipment 40 such 
as a medium voltage motor and pump, or the like. Similarly, the second 
circuit 14 includes a first terminal 24 connected to a second line 34 of 
the three-phase power supply 38, and the third circuit 16 includes a first 
terminal 26 connected to a third line 36 of the medium voltage, 
three-phase, delta power supply 38. Each of the circuits 12, 14, and 16 
further includes a second terminal 42, 44, and 46, respectively, connected 
together to a common bus 48, i.e., a current carrying conductor (not a 
neutral or a ground). 
The first circuit 12 comprises a conventional fuse 50, such as a dielectric 
dissipative element made by Busman, Part Number HVU, having a first 
terminal 50a connected to the first terminal 22 of the first circuit 12. 
The fuse 50 is rated at about 0.5 amperes and between 2,500 and 5,000 KVA 
(kilo volt amperes). A resistor 52 and indicator light 54 are serially 
connected together and are connected in parallel with the fuse 50 between 
the first terminal 50a of the fuse 50 and a second terminal 50b of the 
fuse 50. The resistor 52 is a conventional resistor sized to provide about 
500 million ohms of resistance on about 10 watts of power. The indicator 
light 54 is preferably a neon light configured to operate on 120 volts, 
though the light 54 may be any of a number of different types of lights or 
alarms such as a buzzer, a horn, or the like. 
A capacitor 60, a varistor 62, and a gas tube 64 are connected in parallel 
between the second terminal 50b of the fuse 50 and the second terminal 42 
of the first circuit 12. The capacitor 60 is preferably a conventional 
metal foil ceramic type of capacitor rated at about 2,500 pF. The varistor 
62 is preferably a metal oxide semi-conductor 60 mm (millimeter) varistor 
rated according to the voltage for which the device 10 is rated. For 
example, if the device 10 is rated for 1,500 volts, then the varistor 62 
is preferably rated for about 1,700 volts, 4,300 volts peak nominal 
clamping voltage, and 3,200 joules; if the device 10 is rated for 2,500 
volts, then the varistor 62 is preferably rated for about 3,484 volts, 
2,800 volts peak nominal clamping voltage, and 7,500 joules; if the device 
10 is rated for 3,600 volts, then the varistor 62 is preferably rated for 
about 5,244 volts, 7,400 volts peak nominal clamping voltage, and 9,600 
joules. Such varistors are available from Harris/RCA at a number of 
different locations such as, for example, Florida. The gas tube 64 is 
rated for 6,500 volts and is hermetically sealed to prevent a gas, such as 
neon, contained therein from escaping. The gas tube 64 is rated to absorb 
a large quantity of electrical energy by exciting the gas contained 
therein, preferably absorbing at least 2.5 million joules of energy. Such 
gas tubes are available from C. P. Clare, Inc. located in Chicago, Ill. 
The foregoing fuse 50, resistor 52, indicator light 54, capacitor 60, 
varistor 62, and gas tube 64 are considered to be well known to a skilled 
artisan based upon a review of the present description of the invention, 
and will therefore not be described in further detail herein. 
The circuits 14 and 16 and components therein are substantially similar to 
the circuit 12 and the components described therein and, therefore, for 
the sake of conciseness, will be described only briefly. Accordingly, the 
circuit 14 comprises a fuse 70 having a first terminal 70a connected to 
the first terminal 24 of the second circuit 14. A resistor 72 and 
indicator light 74 are serially connected together and are connected in 
parallel with the fuse 70 between the first terminal 70a of the fuse 70 
and a second terminal 70b of the fuse 70. A capacitor 80, a varistor 82, 
and a gas tube 84 are connected in parallel between the second terminal 
70b of the fuse 70 and the second terminal 44 of the second circuit 14. 
The circuit 16 comprises a fuse 90 having a first terminal 90a connected to 
the first terminal 26 of the third circuit 16. A resistor 92 and indicator 
light 94 are serially connected together and are connected in parallel 
with the fuse 90 between the first terminal 90a of the fuse 90 and a 
second terminal 90b of the fuse 90. A capacitor 100, a varistor 102, and a 
gas tube 104 are connected in parallel between the second terminal 90b of 
the fuse 90 and the second terminal 46 of the third circuit 16. 
In the operation of the device 10, if the electrical equipment 40 is not 
experiencing any substantial surges in the three phase power supplied by 
the power supply 40 onto the lines 32, 34, and 36, then substantially no 
energy is absorbed by the device 10, none of the circuits 12, 14, or 16 
are activated, no current is reflected, and no voltage is absorbed. If 
there is a surge, it will generally occur on only one of the lines 32, 34, 
or 36. While two or three of the lines 32, 34, and 36 could simultaneously 
experience a surge, such is the exception. Whether one, two, or three of 
the lines 32, 34, and/or 36 experience a surge, each of the circuits 12, 
14, and 16 respond substantially similarly to a surge which occurs on a 
respective line 32, 34, and 36. Therefore, in the interest of conciseness, 
operation of the circuits 12, 14, and 16 which be described 
representatively by the response of the circuit 12 to a surge. 
Accordingly, as soon as a surge appears on the line 32, the circuit 12 is 
activated. The fuse 50 passes the surge in power to the parallel-connected 
capacitor 60, varistor 62, and gas tube 64. Since the resistor 52 has a 
relatively high resistance, current does not initially flow through the 
resistor 52, and the indicator light 54 is, thus, not initially 
illuminated. The capacitor 60 absorbs a small portion of the surge energy 
and is also effective for "dampening" the waveform of the surge received 
from the line 32, thereby enabling the varistor 62 to respond more 
efficiently to the surge. The varistor 62 absorbs a greater amount of the 
energy of the surge than the capacitor 60. Prior to exceeding the energy 
rating of the varistor 62, gas in the gas tube 64 is excited and absorbs 
energy that has not been absorbed by the capacitor 60 and the varistor 62, 
generally, up to about 2,500,000 joules. If the energy of the surge 
exceeds the capacity of the gas tube 64 to absorb the energy, e.g., if it 
exceeds 2,500,000 joules, then the lead current through the circuit 12 
will open the fuse 50, thereby preventing the circuit 12 from exploding, 
and causing current to flow through the resistor 52 and to illuminate the 
light 54 to provide notice that the fuse 50 has opened. When there is a 
surge in the line 32, a portion of the current in the line is also 
reflected through the bus 48 and the second terminals 44 and 46 to the 
other two circuits 14 and 16, respectively. Typically, such current will 
flow to the one of the circuits 14 or 16 having the least resistance, and 
sharing between the circuits 14 and 16 would be atypical. 
It can be appreciated that, as mentioned above, if there is a surge on one 
of the lines other than the line 32, i.e., on either line 34 or 36, then 
the operation of the circuit 14 or 16, respectively, would be similar to 
the operation of the circuit 12 discussed in the foregoing. 
By the use of the present invention shown in FIG. 1, an improved system and 
method is provided for effectively suppressing transient voltage surges 
more reliably, efficiently, and safely than is possible using primary 
arrestors or serially connected varistors in accordance with conventional 
technology. More specifically, with reference to the circuit 12, the 
device 10 is more efficient because the capacitor 60 dampens the waveform 
of the surge entering the varistor 62, thereby enabling the varistor to 
operate more efficiently. The gas tube 64 enables the device 10 to operate 
more reliably because it will absorb up to about 2,500,000 joules of 
energy. The fuse 50 enables the device 10 to operate more safely since it 
prevents the circuit 12 from violently exploding. 
It is understood that the present invention can take many forms and 
embodiments. Accordingly, several variations may be made in the foregoing 
without departing from the spirit or the scope of the invention. For 
example, the device 10 may be adapted for single-phase power by removing 
one circuit, such as the third circuit 16. In another variation, the 
device 10 may be operated without the serially connected resistors 52, 72, 
and 92 and the lights 54, 74, and 94. The device 10 could also be operated 
without the fuses 50, 70, and 90, though with some risk of danger of 
explosion. 
Having thus described the present invention by reference to certain of its 
preferred embodiments, it is noted that the embodiments disclosed are 
illustrative rather than limiting in nature and that a wide range of 
variations, modifications, changes, and substitutions are contemplated in 
the foregoing disclosure and, in some instances, some features of the 
present invention may be employed without a corresponding use of the other 
features. Accordingly, it is appropriate that the appended claims be 
construed broadly and in a manner consistent with the scope of the 
invention.