WHAT fault currents are, WHY they are important in existing electricity distribution networks, and WHY they will become even more important in the future networks required to connect renewable generation.
Occasional faults are inevitable in all electrical networks and can result in currents 10-20 times larger than the normal circuit current. This must be allowed for in the design of the electrical equipment. Sections of the network experiencing short-circuits are rapidly isolated from the rest of the network by the operation of circuit-breakers. This is vital both to limit the damage at the point of the fault and to protect the rest of the network from overload currents. The following diagram shows a simplified electrical circuit with a generator ('G') connected through cable with a load and a circuit breaker ('Switchgear') which opens the circuit in case of a short circuit (lightning):
Short circuits bypass the Load and enormous fault current flows.
Switchgear opens the circuit to avoid damage.
The magnitude of the fault current surge is known as the ‘fault level’. Increasingly distributed generation and loads, and highly parallel interconnection as illustrated in the diagram below, increases the fault level, thereby increasing the energy dissipated at the fault. This can result in switchgear (positioned in substations and designed for a maximum manageable fault current) that can no longer handle the fault current flowing into a short circuit, even though it can still accommodate the normal load current. Existing circuit breakers can therefore become inadequate as the characteristics of the network change. Such changes are increasingly being driven by networks evolving to accommodate increased generation from intermittent renewable power sources such as wind turbines or combined heat and power plants.
Short circuits cause even higher currents to flow when there is substantial parallel interconnection.
A fault current climbing beyond the name plate rating of the circuit breakers can have devastating impact on the component, its environment and network personnel. To allow for the increased fault current levels, network operators conventionally would have the option of increasing the impedance (AC resistance) of the network by splitting it into smaller pieces, installing higher impedance transformers or installing series reactors.
These solutions all have multi-year lead times and drawbacks in terms of high costs, customer interruptions, quality issues and network losses.
The ideal solution which overcomes such issues is a device that has very low resistance when normal load current is flowing and very high resistance when a fault occurs. This can be achieved by a novel design known as a Fault Current Limiter. Applied Superconductor is a global leader in developing commercial Fault Current Limiter solutions.
A Fault Current Limiter has near - zero impedance normally.
This rises rapidly when a fault current flows.
The Fault Current Limiter is a device that is installed in substations in electricity networks. Under normal operating conditions it has no effect on the network. However, when a fault current flows it reacts due to an inherent physical property of the material, introduces a high resistance into the network and reduces those fault currents to a level at which the circuit breakers can operate safely.
There are different types of fault current limiters
There are two fundamental types of Fault Current Limiters - resistive and inductive ones. These operate in different ways and result in different limited fault current characteristics. The optimum choice of technology for a particular substation application will depend on the local electrical network environment.
Resistive limiters pass the line current directly through the superconducting material, which turns resistive as a result of the initial rise in current during the onset of a fault. The figure below shows the characteristic sharp limitation of the first peak followed by a greatly attenuated symmetrical AC fault current, combined with total suppression of any DC component. The length of the superconducting element required is directly related to the voltage rating of the Fault Current Limiter.
This type of fault current limitation is based on the use of a pre-saturated core. Such Fault Current Limiters pass the line current through a few turns wound onto an iron core which is driven into magnetic flux saturation by means of a second winding which depending on the magnetic design will comprise of either a superconducting or copper coil. The figure below shows how the limited current follows the envelope of the unlimited current. In this case, the length of the winding is not directly related to the voltage rating which is why pre-saturated core limiters are used at higher voltages and currents.
Types of Fault Current Limitation - resistive vs. inductive.