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FIELD OF INVENTION
The present invention relates to voltage sensor for a switchgear protection. More
particularly, the invention relates to a low-output voltage sensor for switchgear
protection which merely gives a voltage signal but not power to drive any current in
a circuit.
BACKGROUND OF THE INVENTION
Switchgear is basically an electrical switch of modular and cubicle type construction.
It is used for connecting a power source to a cable that transmits power to a load.
Switchgear performs the basic function of the power circuit switching by means of a
circuit breaker and also has protection circuits with necessary controls that can
cause tripping of circuit breaker during fault condition. The protection circuit receives
power from the power circuit, which is normally at a higher voltage, through
instrument transformers. There are two types of instrument transformers viz., i)
voltage transformer which provides the voltage signal and ii) current transformer
that supplies current signal to the protection unit.
In recent years the micro-processor based relays have been developed, which are
very compact, economic, accurate and reliable. They do not require any power to
operate in contrast to the present electro-mechanical relays. They need only voltage
and current signals for its operation. Hence, instrument transformers are
superfluous for this purpose. The present invention relates to a voltage sensor which
replaces a voltage transformer when a micro- processor based protection circuit is
employed for a switchgear.
A combine voltage and current sensor has been reported (Patent EP 0678950-
1995). This device employs a high ohm resistance for measuring the high voltage
and a Rogowski coil i.e., an air core current transformer to measure the current in
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the high voltage bus. The said sensor is directly connected to the circuit breaker and
the signals are used as inputs for the micro-processor based relay.
However, in order to optimize the size of the switchgear unit, it is preferable to
isolate both current and voltage sensors so that the voltage sensor can alone be
mounted at a convenient location while the current sensor can be connected just
after the breaker output. Further, it is necessary to optimise the size of the voltage
sensor by reducing the size. Also, instead of measuring the current in the high ohm
resistance to obtain the high voltage level, it is preferred to drop this current across
a low voltage measuring resistance, which is connected between the high ohm
resistance and ground along with an over-voltage protection circuit.
OBJECTS OF INVENTION
It is therefore an object of the invention to propose a low-output voltage sensor for
switchgear protection which eliminates the requirement of a voltage transformer.
Another object of the invention is to propose a low-output voltage sensor for
switchgear protection which is lighter in weight and smaller in size.
Yet, another object of the invention is to propose a low-output voltage sensor for
switchgear protection which has a shorter length high-ohm resistor provided with
corona shields.
A still another object of the invention is to propose a low-output voltage sensor for
switchgear protection which measures the current at a junction of it's high-ohm and
low-ohm resistors.
A further object of the invention is to propose a low-output voltage sensor for
switchgear protection which has means to bypass any unexpected high voltage
transients.
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SUMMARY OF THE INVENTION
In the present invention, current through a high wattage resistor of very high
resistance is (HV resistor) used for the measurement of voltage. This resistor is
connected directly to the high voltage circuit at one end and other end of the
resister is connected to the ground through a comparatively low ohm- resistor (LV
resistor). The high resistance value of the resistor, when connected to the high
voltage circuit, allows only a very small current of the order of few micro-amperes so
that the sensor does not load the system. The high wattage rating of the resistor
does not allow the resistor to deteriorate and also change in resistance value due to
heat. Further, inductance is reduced as the HV resistor is basically an insulating tube
over which a resistive film is coated and this film conducts the current. The current
signal is measured as a voltage signal across the LV resistor. This arrangement gives
a small voltage signal of the order of few volts as directly required by the micro-
processor relay. Since no power is being drawn from the sensor which is very
compact, heat generation inside the senor is very less.
According to the invention both current and voltage sensors are isolated so that the
voltage sensor can alone be mounted at a convenient location. Further, in order to
optimize the size of the voltage sensor, the length of the high ohm resistor can be
reduced by providing suitable end corona shields so that it can withstand the
specified voltage level without a surface flashover. Also, instead of measuring the
current in the high ohm resistance to obtain the high voltage level, it is preferred to
drop this current across a low voltage measuring resistance, which is connected
between the high ohm resistance and ground along with an over-voltage protection
circuit.
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BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
The invention can be described in detail with the help of the accompanying drawing
as indicated in Fig 1
Fig.1: shows the details of the total sensor assembly according to the present
invention.
PREFERRED EMBODIMENTS OF INVENTION
The present invention deals with the development of a voltage sensor which can
replace a voltage transformer, where micro-processor based circuit is used for the
switchgear protection and metering.
As shown in figure-1, a main HV resistor is provided which has high resistance and
high power handling capacity and practically no inductance. It is basically a ceramic
tube (1) over which carbon has been coated as strips along the length. A metal
coating has been provided at both ends for fixing at least two (first) metal caps (2)
that can be connected to the external circuit. In order to improve the electrical
surface flashover strength as the surface forms an interface between the insulating
tube (1) and the environment i.e. the epoxy resin, the (first) metal caps (2) have
been designed in the form of a smoothly profiled shields (2). This can help the
resistor (1) to withstand the prescribed high frequency test voltage (standard
impulse voltage test). The shields (2) have a threaded stem with nuts so that fitting
of the resistor assembly (1 and 2) to any other part is possible. The entire resistor
assembly is encapsulated in an epoxy resin casting (3) and it has fixing
arrangements. The top stem (1A) is fixed with a second metal cap (L1) with smooth
profile . In addition to its basic function of fixing connection, this second cap (4)
reduces the electric field so that the external electrical strength is increased. While
the top end (1A) is connected to the power circuit which is at high voltage, the lower
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end (1B) is connected to an insulation chamber (5) in which a LV resistor (6) is
housed. The bottom stem (1B) of the HV (terminal) resistor (1) is taken through a
hole at the top lid (5A) of the chamber (5) and fixed by means of a washer and a
bolt. The LV resistor (6) is also a film coated insulation block with a comparatively
less power handling capacity and has almost no inductance. The LV resistor (6) is
connected between the bottom end (1B) of the HV resistor (1) and a ground
terminal (7) so that when the resistor assembly (1,2) is connected to the power
circuit, a constant current based on the system voltage flows into them. The junction
of HV and LV resistors (1,6) is the measuring point of voltage, which is
corresponding to the system voltage to be measured. This junction is terminated
into a BNC female connector (8) and through which the signal is taken for
measurement and protection purposes. Also, a metal oxide voltage arrester (9) is
connected across the LV resistor (6) to by pass the unexpected high voltage
transients and thereby protects the measurement circuit. The LV resistor (6) and the
metal oxide arrester (9) have been fixed over an insulation plate (11) which is again
fixed to the bottom end (1B) of the HV resistor (1). The ground terminal (7) is
always connected to the reference ground for accurate operation of the sensor. The
bottom (5B) of the chamber (5) is fitted with a lid (10) that protects the components
inside the chamber (5).
Since the HV and LV resistors (1,6) are able to pass the small current without any
change in their characteristics, the input -output voltages show a liner relationship
up to the designed voltage level, i.e., 6.6 kV (system voltage). Further, the voltage
sensor has been tested with the high voltage power circuit and with a micro-
processor based relay and its function is conformed to the basic design parameters.
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WE CLAIM:
1. A low-output voltage sensor for switchgear protection, comprising:
- an insulated HV resistor (1) having high resistance and high-power handling
capacity with substantially no inductance, and being connectable to an external
power circuit, the HV resistor (1) provided with at least two first metal caps (2), one
each at a top-end (1A) and a bottom end (1B), the top end (1A) of the H.V. resistor
(1) further provided with a second metal cap (4), and connected to the high-voltage
circuit;
- a low-voltage insulated resistor (6) connected between the top end (1A) of the HV
resistor (1) and a ground terminal (7) such that a constant current corresponding to
the system voltage flows into the resistor assembly (1,6) when connected to the
power circuit; and
- a BNC female connector (08) for termination of a junction of the H.V. and L.V.
resistors (1,6), the junction constituting the measuring point of the system voltage.
2. The sensor as claimed in claim 1, wherein the lower end (1B) of the H.V. resistor
(1) is connected to an insulation chamber (5) having a top lid (5A) and a bottom
(5B).
3. The sensor as claimed in claim 1 wherein the top lid (5A) of the chamber (5)
comprising a hole, and wherein the lower end (1B) of the H.V. resistor (1) is
connected to the top lid (5A) of the insulated chamber (5) being led through said
hole by means of fastening members.
4. The sensor as claimed in any of the preceding claims, wherein the L.V. resistor (6)
is housed in the insulating chamber (5).
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5. The sensor as claimed in any of the preceding claims, wherein the first and
second metal caps (2,4) are configured with smooth profile.
6. The sensor as claimed in claim 1, comprising a voltage arrestor (9) connected
across the L.V. resistor (6) enabling to by-pass any unexpected high voltage
transients.
7. The sensor as claimed in any of the preceeding claims, wherein the voltage
arrester (9) and the L.V. resistor (6) are fixed over an insulation plate (11).
8. The sensor as claimed in any of the preceeding claims, wherein the bottom end
(5B) of the insulated chamber (5) is provided with a lid (10) to protect the
components inside the chamber (5).
9. The sensor as claimed in claim 1, wherein the resistor assembly (1,6) is
encapsulated in an epoxy resin casting (3) having fixing arrangements.
10. A low-output voltage sensor for switchgear protection as substantially described
and illustrated herein with reference to the accompanying drawings.

The invention relates to a low-output voltage sensor for switchgear protection,
comprising an insulated HV resistor (1) having high resistance and high-power
handling capacity with substantially no inductance, and being connectable to an
external power circuit, the HV resistor (1) provided with at least two first metal caps
(2), one each at a top-end (1A) and a bottom end (1B), the top end (1A) of the H.V.
resistor (1) further provided with a second metal cap (4), and connected to the high-voltage
circuit, a low-voltage insulated resistor (6) connected between the top
end (1A) of the HV resistor (1) and a ground terminal (7) such that a constant
current corresponding to the system voltage flows into the resistor assembly (1,6)
when connected to the power circuit, and a BNC female connector (08) for
termination of a junction of the H.V. and L.V. resistors (1,6), the junction
constituting the measuring point of the system voltage.