This invention relates to a current control apparatus for use in low, medium and high voltage DC and AC applications.

When operating any electrical apparatus or network, the electrical current flowing through the apparatus or network is typically maintained within a predetermined current rating of the electrical apparatus or network. However, fault or other abnormal operating conditions in the electrical apparatus or network may lead to the development of a high fault current exceeding the current rating of the electrical apparatus or network.

The development of high fault current may not only result in damage to components of the electrical apparatus or network, but also result in the electrical apparatus being offline for a period of time. This results in increased cost of repair and maintenance of damaged electrical apparatus or network hardware, and inconvenience to end users relying on the working of the electrical apparatus or network.

According to an aspect of the invention, there is provided a current control apparatus for use in low, medium and high voltage DC and AC applications, the apparatus comprising:

first and second terminals for connection into an electrical network;

a fault current limiter and a circuit breaker connected between the first and second terminals; and

a control unit to measure the voltage across the fault current limiter and to selectively switch the circuit breaker between a current admittance state to permit, in use, current flow in the electrical network, and a current interruption state to interrupt, in use, current flow in the electrical network,

wherein the control unit switches the circuit breaker to the current interruption state in response to a detection of a voltage drop across the fault current limiter that exceeds a predefined voltage level.

For the purposes of this specification, a fault current limiter refers to a device that, when activated, restricts the amount of current that can flow in an electrical network to which the device is applied. The combination of a circuit breaker and a fault current limiter in the current control apparatus provides the electrical network, into which the current control apparatus is connected, with both current limiting and current interruption capabilities. The electrical network may be in the form of a network line or circuit. In the event of a fault or abnormal operating condition in the electrical network resulting in high fault current, the fault current limiter is automatically activated to insert a high impedance into the electrical network so as to limit the resultant fault current to a level which does not exceed the current rating of the electrical network and thereby avoid damage to its components. This results in an increase in voltage drop across the fault current limiter, which is detected and measured by the control unit. If the detected voltage drop across the fault current limiter exceeds a predefined voltage level, the control unit triggers the switching of the circuit breaker to its current interruption state to discontinue current flow in the electrical network.

A circuit breaker is often designed to automatically switch to a current admittance state after being in a current interruption state for a set period of time. As such, after the circuit breaker is switched back to its current admittance state, the control unit will again switch the circuit breaker to its current interruption state if the fault or abnormal operating condition has not been removed or repaired and the voltage drop across the fault current limiter still exceeds the predefined voltage level necessary to trigger the switching of the circuit breaker. Otherwise it will remain in its current admittance state.

In such a current control apparatus, the fault current limiter may be configured to decrease the upper limit of current permitted to flow in the electrical network. This in turn lowers the fault current flowing in the electrical network to reduce power dissipation in the fault current limiter and/or to reduce the current rating of the circuit breaker. The former effect improves the efficiency of the current control apparatus, while the latter permits the use of cheaper, lower-rated circuit breakers instead of more expensive, higher-rated circuit breakers, such as an SF ₆ -based circuit breaker which is costly and uses a greenhouse gas.

In contrast, conventional circuit breakers typically rely on the detection of a high magnitude of fault current to trigger the switching of a circuit breaker to a current interruption state. After the circuit breaker switches back to its current admittance state, it will again switch to its current interruption state if the fault current still exceeds the predefined current level necessary to trigger the switching of the circuit breaker.

Configuring the circuit breaker in this manner could however restrict the ability to use a fault current limiter to decrease the upper limit of current, and thereby the fault current, permitted to flow in the electrical network. This is because, if the fault current is lowered to a level below the predefined current level necessary to trigger the switching of the circuit breaker, the circuit breaker will not be triggered to again switch to its current interruption state even though the fault or abnormal operating condition has not been removed or repaired. Consequently the fault current will continue to flow through the electrical network, fault current limiter and the circuit breaker, which may lead to component damage caused by excessive heat generation and, in the case of the circuit breaker, component damage caused by its rated short time withstand current and peak withstand current being exceeded.

The provision of the control unit in the current control apparatus therefore permits the configuration of the fault current limiter to decrease the fault current flowing in the electrical network. This thereby results in an efficient and reliable apparatus for protecting electrical networks against fault current situations.

The control current apparatus may be applied to, or incorporated in, a wide range of low, medium and high voltage DC and AC applications, such as power transmission and distribution networks, traction engines and industrial plants. Preferably the fault current limiter has a variable impedance so as to selectively exhibit a zero or near-zero impedance in a normal operating mode, or a non-zero impedance to restrict current flow between the first and second terminals in a fault current limiting mode.

The structure of the fault current limiter may be configured to match the current limiting requirements of the electrical network. In embodiments of the invention, the fault current limiter may be formed using one or more elements selected from a group including power electronic components, magnetic materials, superconducting materials, pyrotechnic materials, electron valves and plasma valves.

Preferably the fault current limiter is provided in the form of or may include:

an impedance element connected to a switching element that is operable to selectively switch the impedance element into circuit to cause current to flow therethrough and out of circuit to cause current to bypass the impedance element. Such a fault current limiter may be, for example, a power electronic fault current limiter.

The fault current limiter is preferably provided in the form of or may include a magnetic fault current limiter and/or a superconducting fault current limiter.

A preferred embodiment of the invention will now be described, by way of a non- limiting example, with reference to the accompanying drawings in which:

Figure 1 shows, in schematic form, a current control apparatus according to an embodiment of the invention that is connected into a line of an electrical network in a normal operating state, in which the impedance Z of the superconducting fault current limiter is equal to a negligibly low impedance Zo;

Figure 2 illustrates, in schematic form, an occurrence of a short-circuit fault across the line of the electrical network of Figure 1 ;

Figure 3 illustrates, in schematic form, an increase in impedance Z of a superconducting fault current limiter of the current control apparatus of Figure 1 , whereby Z is equal to a high impedance Z| _imit ed; and

Figure 4 illustrates, in schematic form, the opening of a circuit breaker of the current control apparatus of Figure 1 .

A current control apparatus 10 according to an embodiment of the invention is shown in Figure 1 .

The current control apparatus 10 comprises first and second terminals 12,14, a superconducting fault current limiter 16 and a circuit breaker 18.

In use, the current control apparatus 10 is connected in series into a line of an electrical network having a power supply 20, load 22 and ground 24. In particular, the first terminal 12 is connected to the power supply 20, which may be an AC or

DC power supply, and the second terminal 14 is connected to ground 24 via the load 22.

The superconducting fault current limiter 16 is connected in series with the circuit breaker 18 between the first and second terminals 12,14.

In use, the superconducting fault current limiter 16 provides a virtually zero resistance when it is in a superconducting state, i.e. a low impedance state, and enters a quench state and exhibits a normal resistive state, i.e. a high impedance state, when current flowing through the superconducting fault current limiter 16 exceeds a critical current value. It is envisaged that, in other embodiments of the invention, the superconducting fault current limiter 16 may have a variable impedance that is inductive or both resistive and inductive, whereby the variable impedance varies as a function of current flowing through the superconducting fault current limiter 16.

It is envisaged that, in other embodiments of the invention, the superconducting fault current limiter 16 may be replaced by or used in combination with another type of fault current limiter that is capable of providing a variable impedance in order to exhibit low impedance, i.e. a zero or near-zero impedance, during normal operation of the electrical network and high impedance to restrict current flow in the line of the electrical network during a fault current limiting scenario. The other type of fault current limiter may, for example, be formed using one or more elements selected from a group including power electronic components, magnetic materials, superconducting materials, pyrotechnic materials, electron valves and plasma valves.

It is also envisaged that, in other embodiments, the superconducting fault current limiter 16 may be replaced by or used in combination with a magnetic fault current limiter.

It is further envisaged that, in other embodiments, the superconducting fault current limiter 16 may be replaced by or used in combination with an impedance element connected to a switching element that is operable to selectively switch the impedance element into circuit to cause current to flow therethrough and out of circuit to cause current to bypass the impedance element. Such a fault current limiter may be, for example, a power electronic fault current limiter.

The current control apparatus 10 further includes a control unit 26, which includes voltage detection equipment 28 to measure the voltage drop across the superconducting fault current limiter 16. In use, the control unit 26 selectively switches the circuit breaker 18 between a current admittance state, i.e. closes the circuit breaker 18, to permit current flow through the superconducting fault current limiter 16 and the circuit breaker 18 between the first and second terminals 12,14 and through the line of the electrical network, and a current interruption state, i.e. opens the circuit breaker 18, to interrupt current flow between the first and second terminals 12,14 and through the line of the electrical network. During normal operating conditions of the line of the electrical network, the control unit 26 sends a control signal to the circuit breaker 18 to trigger its switching to its current admittance state, which allows a line current l _N to flow in the line of the electrical network, i.e. between the power supply 20 and ground 24. Meanwhile the superconducting fault current limiter 16 is in its low impedance state. This results in the superconducting fault current limiter 16 inserting a negligible impedance Z ₀ into the line of the electrical network so as to minimally affect the amount of line current l _N flowing in the line of the electrical network. The level of the line current l _N is below the maximum allowable current rating of the line of the electrical network.

Occurrence of a short circuit fault 30 across the line of the electrical network that bypasses the load 22 may lead to a rapid rise in line current l _N and therefore the formation of high fault current l _F in the line of the electrical network, as shown in Figure 2.

In the event of high fault current l _F flowing through the line of the electrical network, the superconducting fault current limiter 16 exhibits its high impedance state, typically within a quarter of a cycle, once the fault current l _F exceeds the critical current value of the superconducting fault current limiter 16. This results in the superconducting fault current limiter 16 inserting a high impedance Z| _imi ting into the line of the electrical network so as to restrict the amount of fault current l _F flowing through the line of the electrical network, as shown in Figure 3. This in turn imposes an upper limit on the fault current l _F and increases the voltage drop across the superconducting fault current limiter 16, which is measured by the control unit 26. If the voltage drop exceeds a predefined voltage level, the control unit 26 sends a control signal to the circuit breaker 18 to trigger its switching to its current interruption state to discontinue current flow through the line of the electrical network. There is a time delay, which is typically five cycles, between the receipt of the control signal by the circuit breaker 18 and the switching of the circuit breaker 18 to its current interruption state due in part to mechanical hysteresis.

The switching of the circuit breaker 18 to its current interruption state causes the line current in the line of the electrical network to drop to zero, which in turn enables the superconducting fault current limiter 16 to return to its low impedance state, as shown in Figure 4. This allows the short-circuit fault 30 in the line of the electrical network to be removed or repaired to enable the line of the electrical network to revert to its normal operating conditions.

After a set period of time, the circuit breaker 18 is automatically triggered to switch back to the current admittance state.

If the short-circuit fault 30 has been removed or repaired and the line of the electrical network has reverted to its normal operating conditions, the resulting drop in line current l _N flowing in the line of the electrical network allows the circuit breaker 18 to remain in its current admittance state and the superconducting fault current limiter 16 to remain in its low impedance state so as to insert a negligble impedance Z ₀ into the line of the electrical network.

If the short-circuit fault 30 has not yet been removed or repaired, the switching of the circuit breaker 18 to its current admittance state will cause the high fault current l _F to resume flowing in the line of the electrical network. This causes the superconducting fault current limiter 16 to rapidly enter its state of high impedance so as to insert a high impedance Z| _im i, _in g into the line of the electrical network. The resulting increase in voltage drop across the superconducting fault current limiter 16 is detected by the control unit 26, which sends a control signal to the circuit breaker 18 to trigger its switching back to its current interruption state. The circuit breaker 18 remains closed until the short-circuit fault 30 is removed or repaired and the line of the electrical network has reverted to its normal operating conditions

Optionally the superconducting fault current limiter 16 may remain in its high impedance state while the circuit breaker 18 is in its current interruption state to allow the superconducting fault current limiter 16 to instantaneously limit the high fault current, l _F , upon switching of the circuit breaker 18 to its current admittance state, if the short-circuit fault 30 has not yet been removed or repaired.

As described earlier, the configuration of the current control apparatus 10 as set out above allows the superconducting fault current limiter 16 to decrease the upper limit of line current permitted to flow through the line of the electrical network. This has the benefit of reducing power dissipation in the superconducting fault current limiter 16 and thereby improving the efficiency of the current control apparatus 10. In addition, this reduces the risk of components of the superconducting fault current limiter 16 and the line of the electrical network being exposed to excessive heat generation, which may damage the components.

Moreover the decrease in the upper limit of current flowing through the line of the electrical network not only lowers the required current rating of the circuit breaker 18, which reduces the size, weight and cost of the current control apparatus 10, but also reduces the risk of arc damage to the circuit breaker 18.

It is envisaged that, in other embodiments, the superconducting fault current limiter 16 and the circuit breaker 18 may be arranged between the first and second terminals 12,14 to form other configurations of the current control apparatus 10 so as to limit and interrupt current flowing through the line of the electrical network.