Thomas Eriksson, Jesper Magnusson, Johan Nohlert Switching and Systems Västerås, Sweden firstname.lastname@example.org, email@example.com, firstname.lastname@example.org; Bjoern Gottschlich, Peter Jost Control & Protection Solutions Ratingen, Germany, email@example.com, firstname.lastname@example.org
Enabling decentralized energy generation systems to safely connect to the grid without incurring huge additional costs has been a long-standing challenge. Now, thanks to a new emphasis on the energy-saving potential of CHP, authorities in London, UK are testing a fault current limiting circuit breaker (FLCB) device designed by ABB that promises to be exactly what is needed to safely and affordably exploit this low-carbon technology.
The FLCB designed by ABB combines state-of-the-art power semiconductors with extremely fast mechanical switches to detect and limit fault currents in just a few thousandths of a second – 20 times faster than any existing circuit breakers.
In view of this, UK Power Networks, an electrical distribution network operator covering London, the South East and East of England, recently claimed a world first by installing FLCBs in a substation in Tower Hamlets, east London →01. The new circuit breakers are designed to make it easier and cheaper to connect Net-Zero-ready energy technologies such as CHP units to the network .
Faster, smaller, cheaper
The new super-fast circuit breakers are a quarter of the size and half the price of limiters representing competing technologies and are expected to open the door to connecting an additional 460 MW of distributed generation to the city’s network. Their advantages are particularly evident when compared to conventional circuit breakers where the current is interrupted by opening a contact gap, using mechanical contacts, and extinguishing the arc. In such circuit breakers the full fault current flows through the breaker and the rest of the network for 50-100 ms. If all of the downstream substations and network components are designed for the current, the fault is cleared safely. However, if new distributed generation is to be added to a network there is a risk that this design limit will be exceeded, in which case other measures need to be implemented. One possibility is to introduce a device that will limit the fault current below the critical design value of the power grid, ie, a fault current limiter (FCL).
An FCL needs to be so fast that it limits the current within milliseconds, rather than tens of milliseconds. Here, two basic concepts have already been in use for years: dynamic impedance, and fast interruption.
Dynamic impedance devices have a low impact on the grid under normal operating conditions but will rapidly increase their impedance during a fault event, thus limiting the maximum peak current. Examples of this type of device are electromagnetic and superconducting fault current limiters . Their main characteristic is that they limit the maximum peak current but do not interrupt the current completely. After the fault is cleared, the impedance is restored, and nominal operation resumes. A common disadvantage is that these devices are, in most cases, large, heavy and, when based on a superconducting system, require advanced low temperature cooling. The advantage of this technology is that solutions are intrinsically fail-safe because most credible failure modes result in failing to a high impedance state where overcurrents are limited.
The second concept, fast interrupters, limits the fault current by interrupting the current before it reaches the critical design limit. This should be occur in a few ms. One example of this type is the already commercially available Is-limiter, . The IS-limiter consists of a fast detection device and a fast commutation switch in parallel with a fuse. When a fault is detected, a pyro device opens the switch, which commutates the current into the fuse. This melts the fuse and thus interrupts the current.
Another possibility for fast interruption is the introduction of power semiconductors. These devices have the capability to interrupt currents within microseconds and may easily be switched back on remotely after a fault is cleared – an advantage over fuse technology, which needs manual replacement after operation. However, power semiconductors are expensive and require additional cooling, especially at higher nominal currents.
The FLCB developed by ABB offers significant advantages over the above-mentioned technologies. Unlike an IS-limiter where the pyrotechnical device and the fuse have to be replaced after each operation (single shot device), the hybrid FLCB is designed for many operations both with and without current. To achieve this, a hybrid solution has been implemented that combines the fast switching of power semiconductors and the low losses of mechanical switches.
The result is a more compact solution than passive dynamic impedance devices, which has made it possible to implement it in the London area and other densely populated urban areas where other technologies are difficult to adopt.
The hybrid FLCB has three major structures – a fast mechanical switch, power electronics, and a surge arrester →02. In nominal operation, current flows through the mechanical switch with very low losses. When a fault is detected, the switch is opened, and the arc voltage transfers/commutates the current to the power electronics (PE). In a second step the PE is turned off and the current is commutated to the surge arrester. The arrester is designed to create a counter voltage that is greater than the system voltage, thus forcing the current to zero. A second important design parameter for the surge arrester is the ability to dissipate stored inductive energy during a fault interruption →03. To illustrate the interruption in comparison to the full prospective fault current a conceptual graph is shown in →04.
Fast commutation switch
To be able to implement a hybrid concept characterized by interruption times below 1 ms, which would be necessary for a 25 kA prospective fault current, an ultra-fast mechanical switch is essential. Looking at the interruption sequence in →04, it is understood that the contact separation needs to take place in approx. 0.35 ms after a fault is detected or a current is identified as a fault. This is accomplished by combining a tailor-made, light weight contact system with an electro-magnetic drive system. This provides the proper reaction time and acceleration.
For the pilot installation at UK Power Networks’ Tower Hamlets location, a development that has been run as a joint project and pilot by ABB and UK Power Networks under the project name Powerful CB, the basic requirements for the FLCB were:
• Nominal voltage: 12 kV
• Nominal current: 2000 A
• Prospective fault current: 25 kA (RMS)
• Limited current: 13 kA (peak)
• Current limited in less than 1 ms after fault detection
In an earlier project, , it was shown that these requirements are achievable with the suggested hybrid concept described above. During the project’s execution several tests were conducted to verify performance. An example of an interruption test with full prospective fault current is displayed in →05.
To ensure that the maximum allowed fault current in the grid is never exceeded, while taking into account the maximum current derivative dI/dt for a 25 kA prospective fault current of approximately 11 kA/ms, the FLCB needs to limit the current within less than 1 ms after detecting the fault. This puts extremely high demands on both the detection system and the operation of the device. To meet these demands an Is-limiter control unit was used. In the context of this application, this device continuously measures the instantaneous value of the current and, when a pre-set value is exceeded, sends a trip signal to the control system of the FLCB within a few micro-seconds. Since the FLCB consists of several active devices, a fast and accurate control system was implemented for the operation sequence. The time resolution of the control system is in the micro-second range.
Implementation in the grid
The main purpose of the FLCB is to limit the maximum fault current in the grid or in a substation. To accomplish this functionality there are different ways of introducing it onto the grid. Some examples are shown in →06 and →07. In the configuration shown in →06, when one transformer is taken offline due to a fault or planned maintenance, the busbar sections are connected to run solid to support the load without overloading the remaining transformers. This, however, can increase fault levels up to or above the design limit and usually requires generation to be disconnected. However, if an FLCB were connected as a bus coupler instead of a standard circuit breaker, this would reduce the fault level contribution from one bus section to another and allow generation to remain connected during these abnormal running arrangements. The reason for this is that the FLCB will separate the busses in less than 1 ms during a fault, resulting in a very limited fault current being transferred from one bus section to another. This creates maximum operational flexibility while minimizing the risk of over-stressing the substation and allowing generators to remain connected.
In an alternative configuration the FLCB is placed on one or more of the incoming feeders →07. In this arrangement, it disconnects the incoming feeders connected via FLCB during a fault so that the fault contribution is effectively limited based on the high performance of the device. This configuration provides additional fault level headroom, making it possible to connect more distributed generation without exceeding the fault level design limit.
Another option is that the FLCB could be directly connected to a generator feeder, eg, from a newly installed CHP system. Here, in the event of a fault, the current from the generator would be limited within 1 ms, thereby enabling connection of new distributed generation without infringing on available fault-current headroom.
Pilot installation results
ABB and UK Power Networks are currently carrying out a pilot evaluation of the FLCB in a London primary substation. The trial site was chosen because it has historically registered a relatively higher number of faults, space is available for the installation, and the fault level headroom is not far from the design limit. For this specific installation, the device was placed in three standard medium voltage panels, one for each phase. Throughout the trial, the FLCB will be evaluated both as a bus coupler and as a breaker on an incoming feeder, between the transformer and the busbar.
The pilot device will be remotely monitored by ABB throughout the study. The health status of various components together with transient system recordings will be automatically gathered and transmitted to ABB for analysis through the set-up →08. This will make it possible to study the long-term behavior of the device and to analyze the detailed outcome of protection operations. Furthermore, these steps will allow specialized personnel to support the customer by responding rapidly to any possible malfunctions based on detailed diagnostics.
To date, although continuous monitoring has been in effect and the FLCB has been ready to respond to any fault, no faults have been registered in the system since its activation →09 .
 Network, “UK Power Networks pioneers new super-fast circuit breakers,” February 7, 2020. Available: https://networks.online/heat/uk-power-networks-pioneers-new-super-fast-circuit-breakers/” [Accessed January 24, 2021].
 Y. Zhang, R.A. Dougal, 2012, “State of the art of Fault Current Limiters and their applications in smart grid,” 2012 IEEE Power and Energy Society General Meeting, 1 – 6.
 Is-limiter. Available: https://new.abb.com/medium-voltage/apparatus/fault-current-limiters/current-limiter. [Accessed May 19, 2021].
 L. Liljestrand, L. Jonsson, M. Backman, M. Riva, 2016, “A new hybrid medium voltage breaker for DC interruption or AC fault current limitation”, 18th European Conference on Power Electronics and Applications (EPE’16 ECCE Europe), 1 – 10.
 For further information regarding ABB’s fault current limiting solution, please contact: DE-FCL@abb.com