SWITCHGEAR DEVICE

Abstract: A switchgear device (10) comprises a housing containing a circuit breaker (50) and a capacitor voltage transformer (CVT). The CVT consists of a stack of serially-connected capacitors (30) connected between a first terminal (90) and a ground potential, and a digital electronic isolator (20) connected in parallel between the lower capacitor (40) of the stack (30) and ground. The output of the isolator (20) is connected to a complementary circuit (120) distant from the isolator via a fibre-optic cable (110). The digital electronic isolator (20) is substantially smaller than either an electromagnetic or a power-amplified analogue device, and it has a high input impedance, thereby buffering the capacitor stack (30) which may therefore be reduced in size. The overall reduction in size of the components of the CVT permits them to be fitted into the body of a standard circuit breaker with consequent cost savings.

Description: Switchgear Device 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 m 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, whicn in turr. trip circuit breakers. A typical voltage transformer for use at high voicage 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 outpu . 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 n 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 ob ect 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 comprising a housing, first, second and third electrical terminals, a circuit breaker in the housing connected between the first and second terminals, a plurality of serially connected capacitors also in the housing and connected between the second terminal and the third terminal, and an electronic isolator connected across at least one of the capacitors and the third terminal, the isolator being operable to provide an output signal that is indicative of the voltage across the second and third terminals. 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 second terminal, the first 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 tne switchgear device has a base, t is preferable that the drive mechanism is located there. The present invention can be put into practice n 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 (SF,) 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 cetween 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 actuator mechanism. 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 Dower, such as is manufactured by A.B.B. Switchgear A.B. cf 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 signi icantly 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 1. 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 CVT≤ 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 m 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 l, and may additionally or al t ernatively use a spring or other mechanism rather than a hydraulic mechanism.

Claims: CLAIMS 1. A switchgear device comprising a housing, first, second and third electrical terminals, a circuit breaker in the housing connected between the first and second terminals, a plurality of serially connected capacitors also in the housing and connected between the second terminal and the third terminal, and an electronic isolator connected across at least one of the capacitors and the third terminal, the isolator being operable to provide an output signal that is indicative of the voltage across the second and third terminals. 2. A device as claimed in Claim 1, in which the housing includes an insulator and a base. 3. A device as claimed in Claim 2, in which the insulator comprises first and second stacks of insulator material mounted on the base and separated by the second terminal, the first terminal being mounted on the end of the second stack remote from the base. 4. A device as claimed m Claim 2 or Claim 3, in which the electronic isolator is located in the base. 5. A device as claimed in Claim 2, 3 or 4 , m which substantially all of the capacitors and the circuit breaker are in the insulator. 6. A device as claimed in any one of the preceding claims, in which the electronic isolator is connected via a cable to signal conditioning means which are outside of the housing. SUBST/TUTESHEET(RULE26) 7. A device as claimed in Claim 6, in which the cable is a coaxial cable or a fibre-optic cable. 8. A device as claimed in any one of the preceding claims, in which the digital electronic isolator includes a microprocessor. 9. A device as claimed in any one of the preceding Claims, in which the circuit breaker comprises first and second contacts each respectively electrically connected with the first and second terminals, and an actuator mechanism operable to move the first contact relative to the second contact in response to an actuation signal. 10. A device as claimed in Claim 9 when dependent upon any of Claims 2 to 8, in which the actuator mechanism is in the base.

Inventors: PILLING NEIL, PILLING, NEIL
Applicants: NAT GRID COMPANY PLC, THE NATIONAL GRID COMPANY PLC
Application year: 1997
Main classification: H02B13
IPC codes: H02B13