This invention relates to a switchgear device.
Power lines in a substation of an electrical transmission network
typically operate at a potential several hundred kilovolts (kV) above ground potential.
The lowest voltage on the British electrical transmission network, for example,
is about 145kV before it is stepped down for electrical distribution.
Switchgear for a substation typically includes circuit breakers of
the kind having a fixed and a moving contact. The moving contact is powered in
response to a trip signal. Typical voltages handled by such circuit breakers are
between 145kV and 300kV. For the higher voltages of around 420kV, a double contact
is currently used. The conventional circuit breaker is usually housed in a porcelain
stack which acts as an insulator.
It is accordingly common to employ voltage transformers between the
power line and ground to step down the very high voltage to a smaller voltage
which is suitable for monitoring and driving protection devices, which in turn
trip circuit breakers.
An example of a known switchgear device is given by FR-A-2 649 245,
which includes a capacitor voltage divider formed from two series of connected
A typical voltage transformer for use at high voltage is most commonly
of the capacitor divider type. A plurality of capacitors is linked in series between
the power line, at several hundred kV, and ground. Each capacitor drops a fraction
of the total potential difference between these two points, and a voltage proportional
to the line voltage is tapped off the capacitor closest to ground.
Typically, capacitor voltage transformers (CVTs) have an isolating
electromagnetic voltage transformer in parallel with the capacitor closest to ground.
This produces an output in the region of 50VA which is used to drive the protection
devices. The internal impedance of the CVT (and hence the size of the capacitors)
necessary to deliver this power is dictated by the magnitude of this output.
It is useful for the transient behaviour of the signal on the power
line to be mirrored accurately at the output of the CVT for monitoring purposes.
However, the electromagnetic voltage transformer has a non-uniform frequency response.
It tends to store energy, thereby distorting transients. It has been proposed to
substitute the electromagnetic voltage transformer with an electronic isolator
circuit with a high input impedance. The voltage drop across the capacitor in the
stack closest to ground is used as the input to the isolator circuit, which in
turn produces an optical output signal proportional to the monitored voltage. The
optical output signal is then transmitted across the substation to a complementary
circuit which reconstitutes the output voltage and power of the CVT.
The advantage of using an electronic isolator rather than an electromagnetic
voltage transformer is that the accuracy is enhanced, since any transient conditions
are more faithfully reproduced.
The installation of each circuit breaker and each CVT is a labour
intensive exercise. Each item has had to be installed separately.
It is an object of the present invention to provide a switchgear
device which at least alleviates these problems of the prior art.
According to the present invention there is provided a switchgear
device as claimed in claim 1. Optional features are set forth in the claims appended
The inventor has appreciated that it is possible to realise significant
savings in the cost of constructing an electricity substation if the number of
devices that have to be installed can be reduced.
Since the electronic isolator has a high input impedance, the series
of capacitors is effectively buffered. The inventor has realised that it is therefore
possible to reduce the size of the plurality of capacitors. Substantial advantages
arise from employing electronic circuitry rather than an electromagnetic voltage
transformer. In particular, the CVT is now small enough to be located along with
a circuit breaker in a standard single-break circuit breaker housing because of
the reduction in size of the capacitors. This substantially reduces the number
of separate switchgear devices that have to be installed in an electricity substation.
Since the installation of the devices themselves is a significantly expensive
part of a substation, the financial benefit is substantial.
Preferably, the housing contains an insulator and a base, the insulator
comprising, for example, first and second stacks of insulator material mounted
on the base and separated by the first terminal, the second terminal being mounted
on the end of the second stack remote from the base.
Preferably, the electronic isolator, which may include a microprocessor,
is located in the base, and substantially all of the capacitors and the circuit
breaker are in the insulator.
In one preferred embodiment, the isolator is connected via a cable
to signal conditioning means, the signal conditioning means being outside of the
housing. The cable may be a coaxial cable or a fibre-optic cable, for example.
Fibre-optic cable is particularly attractive as it overcomes the problem of electromagnetic
noise which tends to affect coaxial cables .
The circuit breaker may comprise first and second contacts between
the first and second electrical terminals, and a contact actuator mechanism. If
the switchgear device has a base, it is preferable that the drive mechanism is
The present invention can be put into practice in various ways one
of which will now be described by way of example with reference to the accompanying
drawings in which:
- Figure 1 shows a switchgear device according to the present invention including
a circuit breaker; and,
- Figure 2 shows a schematic diagram of an isolator circuit and a complementary
circuit for driving protection relays.
Referring to Figure 1, a switchgear device is shown generally at
10. The device 10 comprises a base unit 150, a lower insulator 130 situated on
the base unit, and an upper insulator 140 on top of the lower insulator, the two
insulators being separated by a first, lower terminal 90. A second, upper terminal
100 is positioned on top of the upper insulator 140. The lower insulator 130 provides
mechanical support for the upper insulator 140 as well as insulation for the first
terminal 90. The insulators are typically constructed as a stack of shedded porcelain
collars or other commercially available insulating materials as will be familiar
to those skilled in the art. Each stack is filled with sulphur hexaflouride (SF6)
which has good insulating and arc quenching properties.
A stack of capacitors 30 is connected in series between the first
terminal 90 and a ground terminal. Each of the capacitors is of the polypropylene
type with a total capacitance for the stack 30 of around 1 nanofarad (nF). An
electronic isolator circuit 20 is connected in parallel across the lower capacitor
40 of the stack 30 which is connected to ground.
As shown in Figure 1, a circuit breaker 50 is located between the
second terminal 100 and the first terminal 90. This circuit breaker is of the "single
break" type, and comprises a hydraulic actuator mechanism (not shown) within the
base unit 150. The hydraulic actuator mechanism is attached to a connecting rod
60 located in the lower insulator 130, the end of the rod 60 distal from the hydraulic
actuator mechanism being attached to a first breaker contact 70 in the upper insulator
140. A second breaker contact 80 is also located in the upper insulator 140, and
is attached to the second terminal 100. The lower insulator (130) thus conveniently
provides mechanical coupling of the first breaker contact 70 to the grounded hydraulic
The isolator circuit 20 is connected to a complementary receiver
circuit 120 via a fibre-optic cable 110. The arrangement is shown in schematic
form in Figure 2. Here, the voltage across the lower end capacitor 40 of the capacitor
stack 30 of Figure 1 is used as an input to the isolator circuit 20. The isolator
20 generates a signal suitable for transmission along the fibre-optic cable 110,
using a pulsed laser diode 200. This is preferable as the fibre-optic cable is
immune to the substantial electromagnetic interference in an electricity substation.
The signal generated by the diode 200 is received at the complementary
receiver circuit 120 by a detector 210 comprising a photodetector 220 and suitable
amplifier circuitry 230 to amplify the low power electrical output signal from
the photodiode 200. The output from the detector 210 enters a decoder 240 which
converts the digital transmission signal protocol used by the isolator circuit
20 and detector 210 into a protocol suitable for use with a protection relay. The
protection relay or relays may be of the type that require only minimal input
power, such as is manufactured by A.B.B. Switchgear A.B. of Sweden. Thus, the isolator
circuit 20 only needs to provide a very low level output. The protection relays
quickly switch out faulted sections of the power system, thus preventing any slowdown
in the localised electricity generator from causing a loss in synchronism with
the rest of the power system.
The electronic circuit 20 has a high input impedance and the capacitor
stack 30 is thus buffered. The reduced output power requirement relative to the
electromagnetic voltage transformer, in particular, allows the physical size and
capacitance of each capacitor in the stack 30 of Figure 1 to be reduced significantly
for a similar voltage rating. As the capacitor stack 30 occupies the biggest proportion
of space within the hollow insulator 130, this reduction in size can be used in
the present invention to particular advantage.
In order to protect the components in the substation in case of a
fault, separate high voltage circuit breakers have traditionally been employed
in conjunction with protection relays to switch out faulty sections of the power
system. The more circuit breakers that are provided on the system, the smaller
the part of the system that has to be switched out in order to correct the fault.
The introduction of the isolator circuit 20, which consequentially reduces the
size of the capacitors in the stack 30 and obviates the need for an electromagnetic
voltage transformer, permits both the CVT and circuit breaker components to fit
into a single body designed for a circuit breaker alone, as is shown in Figure
The principle cost of a CVT arises from its insulation. The typical
electrical substation has 50 circuit breakers currently costing around £50,000
each, and 50 CVTs costing around £12,000. In addition, the cost of installing
and commissioning a CVT is currently in the region of £12,000. By locating the
components of the CVT within the same body as a circuit breaker in the present
invention, both the equipment and installation costs are substantially reduced,
leading to potential savings of the order of £1,000,000 for a typical substation.
Although a preferred embodiment has been described, it will be appreciated
that the invention may be put into practice in a number of ways. For example, the
isolator circuit 20 may be connected to the complementary receiver circuit 120
via a less expensive coaxial cable rather than a fibre-optic cable, the former
being suitable where electrical noise is not such a significant problem, for example.
Additionally, the signal generated by the isolator circuit may be an analogue signal
rather than a digital signal, the amplitude of the analogue signal can be much
lower than previously possible which avoids the need for a power amplifier. The
polypropylene-insulated capacitors may be replaced by any suitable capacitance
in order to step down the voltage from the first, lower terminal 90. Additionally,
the circuit breaker may consist of two or more contact assemblies rather than the
one illustrated in Figure 1, and may additionally or alternatively use a spring
or other mechanism rather than a hydraulic mechanism.