Application and Feasibility of Fault Current Limiters in Power Systems
Members H. Schmitt (DE) - (Convenor) J. Amon Filho (BR), R. Adapa (US), D. Braun (CH), Y. Brissette (CA), G. Buchs (CH), D. Cvoric (NL), F. Darmann (US), K. Edwards (US), P. Fernandez (BR), D. Folts (US), K.H. Hartung (DE), O. Hyun (KO), J. Jäger (DE), D. Iioka (JP), H. Kameda (JP), Y. Kim (KO), M. Kleimaier (DE), F. Lambert (US), L. Martini (IT), M. Noe (DE), K. Park (KO), J.-L. Rasolonjanahary (UK), M. Steurer (US), J. van der Burgt (NL).
Many investigations in the field of Fault Current Limiters (FCL) have been carried out in the past and several demonstrators and prototypes have been installed in medium voltage systems. Superconducting FCLs, as well as other FCL technologies like saturable core solutions, are installed in actual system conditions and solid state devices have been developed. The installations show the technical feasibility of such devices based on different technologies. Nevertheless the FCL community is still waiting on its break through. There is still a need for this technology but up to now, none of the novel approaches have led to an economically viable solution for a fault current limiter for medium voltage or high voltage networks. Taking into account the needs and the benefits of such FCL equipment and considering the system requirements, it will be the task in the future to determine appropriate applications, to convince possible users and increase the acceptance level for potential applications in medium and high voltage systems. This paper introduces the TB of Cigre WG A3.23 which summarizes the current status of work in the field of fault current limiting devices and presents useful information for interested people who are active in the field of FCLs. The TB is also addressed to all those readers who are so far not familiar with fault current limiting and fault current limiters but interested in more information about the fascinating possibility to reduce the amplitude of fault currents with increasing short circuit levels and strengthened electrical systems.
Cigre SC A3 has been engaged in the study of fault current limiting and R&D efforts of the different fault current limiter (FCL) technologies for many years. In consequence, Cigre Working Group WG A3.23 was established in 2008 to continue and summarize the work of two previous Cigre working groups (WG A3.10  and WG A3.16 ) and draws to a close A3’s investigations into FCLs. The working group makes use of the results of the previous working groups and investigates and summarizes the following topics: • location of FCL installation • different types of FCL and their limiting behaviour and drawbacks • experience from former and new pilot projects in order to provide a realistic picture • feasibility of the application of conventional and novel FCL technologies • acceptance issues and how to overcome them • customer system requirements • interactions with protection and other control and power devices Utilities all over the world are feeling the ever increasing need for fault current limitation in MV and HV systems as evident in the responses to surveys carried out by CIGRE and EPRI. An overview of fault current limiting measures is given in Fig. 1. “Passive” measures make use of higher impedance under all the conditions, whereas “Active” measures introduce
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Fig. 1 Overview of fault current limiting measures
higher impedance only under fault conditions. The measures may also be classified as “Topological” and “Apparatus” measures. Further, some measures have been identified as “Novel” depending on the technology used.
General Case Studies The TB “Application and Feasibility of Fault Current Limiters in Power Systems” by WG A3.23 reports a description of the most relevant worldwide applications of FCL devices. For what concerns the distribution level, it is worthy mentioning the 10 kV saturated core FCL developed within the Dutch cooperative research project “Grid-Con” a 10 kV magnetic FCL for single wind parks, the Italian 9 kV SFCL for outgoing and incoming feeder applications, the German 12 kV resistive SFCL installed in Boxberg, the 12 kV iron core type SFCL installed in California and the English DC biased iron core SFCL installed in North Lincolnshire. Some examples are available also at the sub-transmission level and the high voltage level corresponding to the transmission level in the aforesaid TB. Fig. 2 shows the typical wave shape of an unlimited fault current and the typical effect of a FCL device with and without fault current interruption capability on that wave shape. Furthermore, the characterizing data of an FCL, as described in the TB No. 239 by CIGRE WG A3.10 , which can be used to specify a FCL device, are also presented in Fig. 2.
Impact And Interactions Of Fcl The basic physical effect of FCL applications is the increase of impedance in series with the line. Therefore grid impacts and interactions have to be considered when applying FCLs. Generally FCL impacts can be divided into two groups as shown in Fig. 3.
Fig. 2 Typical fault current wave shape and characteristic data: a: FCL without fault current interruption; b: FCL with fault current interruption
FCL impact Fault current management
Fig. 3 Structure of FCL impacts
Fault current management represents primarily the intended impact of an FCL application; implication management comprises physical impacts or site effects of FCL applications. The main subjects of impacts and interactions can be structured as follows: • Transient stability (Rotor angle stability) • Protection system • Transient response (TRV) • Power quality (Voltage drop – fault recovery, Harmonics, Ferroresonance) • Thermal losses Below some basic considerations on transient stability are introduced. The detailed survey about other issues is reported in the TB “Application and Feasibility of Fault Current Limiters in Power Systems” by WG A3.23. Transient stability with respect to the rotor angle stability of generators is a well known subject. In the past it was mostly applicable to transmission and industrial networks. In the course of the future transition to smart grid structures, public distribution systems become also susceptible to transient stability phenomena. Micro turbines as one of the new typical devices of smart grids introduce inertia constants H below one second into the grid and make it highly sensitive against power swing effects. On the other hand the application of FCLs in the smart grid becomes more and more necessary. The permissible short-circuit ratings of these devices are endangered to be exceeded. The application of a FCL is solving this problem mostly more economic as retrofitting the switchgear.
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Fig. 4 Influence of the FCL recovery time on transient stability
A FCL can improve or degrade the transient stability of the network. It depends on the FCL parameters (e.g., type, nominal current, limiting factor, recovery time) or FCL installation site and fault location. The dependency on the recovery time will be illustrated in the following scenario . A block of a generator and a transformer is connected by a single line to an infinite bus IB as shown in Fig. 4. A FCL is assumed to be installed in the generator feeder between transformer and station. In the case of a three-phase fault at A, the feeding power of the generator is nearly zero, the mechanical turbine power is assumed to be constant and the generator speed accelerates consequently. The transient stability is kept if the acceleration area AA, which is determined by the fault inception and dead time period, is equal or less the deceleration area DA, which is determined by the post fault conditions (Fig. 5). The FCL influence on the transient stability is determined by the comparison of the ratio of these areas with and without a FCL. If the FCL recovers, that means it comes back into the initial state before reclosing of circuit breaker S1, both areas AA and DA do not change in comparison with those situation when FCL is not installed in the circuit as shown in Fig. 5. In this case the FCL has no impact on the transient stability. If the FCL recovers after reclosing the breaker S1, the acceleration area AA remains equal but the damping area DA becomes less, as shown in Fig. 5. Between δ2 and δ3 the resulting line impedance X is increased and the feeding power P of the generator is decreased in comparison with the former case. The transient stability is decreased consequently. The application of superconducting inductive FCLs can increase the transient stability in definite cases. In contrast to superconducting FCLs, conventional methods such as linear limiting reactors lead to an
Fig. 5. Power diagram of transient stability at a three-phase fault in A, when FCL recovers at δ3
increase of the interconnecting impedance and therefore to a decrease of the transient stability in general.
Acceptance Issues The need and benefits of FCLs have been investigated for many years. Many different FCL options and types have been developed, but at present there is still no commercial FCL solution. It can be assumed that the main reasons are acceptance issues of the user. From the user point of view important acceptance issues are: • Technical performance • Cost versus benefits • Safety, risks, hazards • Reliability • Availability • Knowledge A compromise has to be found between high shortcircuit capacity at normal operation and low or at least limited short-circuit capacity during fault conditions for power systems. High short-circuit capacity at normal operation results in low voltage drops, high steady state and transient stability and low system perturbations while low short-circuit capacity during fault conditions is favorable in terms of low thermal and mechanical stress on components and devices. FCLs offer an optimal solution for this compromise (Fig. 6) and therefore offer superior technical performance in comparison to other short-circuit limitation methods. There are no other methods that offer negligible impedance at normal operation, fast and effective short-circuit current limitation within the first half cycle and fast and automatic recovery.
Conclusions The field of fault current limiting devices has continued to advance over the past few years
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Fig. 6 Short-circuit capacity in power systems
including new R&D, pilot and field demonstration projects. FCL demonstrators and prototypes have been installed in medium voltage systems to gather field testing experience. Significant efforts have been made and respectable results achieved because of the industrial availability of advanced materials and a new generation of conductors, especially for superconducting fault current limiters. Nevertheless, none of these novel approaches have led to an economical solution for a fault current limiter for either medium voltage or high voltage networks. The tasks for the next few years will be to determine the most profitable FCL applications, to educate potential users and to increase the acceptance levels for potential applications in medium and high voltage systems taking into account the benefits of FCL equipment, rapidly changing technology and electrical system needs. Fault current limiters based on novel technologies, such as solid-state and superconductivity, have the potential to be highly effective and efficient. But these FCL technologies are not sufficiently mature for grid deployment. Once ready for use, these next generation FCLs are expected to find widespread applications in transmission and distribution systems all over the world. The TB of Cigre WG A3.23 summarizes the current status of work in the field of fault current limiting devices and presents useful information for interested people who are active in the field of FCLs.
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Literature  Cigre WG A3.10: “Fault Current Limiters in Electrical Medium and High Voltage Systems”, CIGRE TB, No. 239, 2003.  Cigre WG A3.16: “Guideline on the Impacts of Fault Current Limiting Devices on Protection Systems”, CIGRE TB, No. 339, 2008.  M. Yagami; S. Shibata, T. Murata; J. Tamura, “Improvement of Power System Transient Stability by Superconducting Fault Current Limiters”, IEEE /PES Transmission and Distribution Conference and Exhibition: Asia Pacific, 2002.
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