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Progresses and Prospects for Fault Processing in Distribution Grids
Liu Jian
Abstract
Keywords
1.1 Introduction
According to statistics, failures in distribution grids cause more than 85% of outages due to faults. Thus, fault processing technologies for distribution grids are of great importance in improving service reliability.
Faults can be divided into two categories: interphase short circuit faults and single-phase grounding faults. These faults can then be further divided into permanent and temporary faults.
As for earth-neutral systems, fault processing technologies for interphase short circuit faults and single-phase grounding faults are the same. However, for neutral ineffective grounding systems, such as those in China, systems are allowed to operate under single-phase grounding fault conditions for no more than 2 hours in order to ensure service reliability. The position of a single-phase grounding fault should be located and repaired in time to avoid causing an interphase short circuit fault. Interphase short circuit faults should be cleared immediately and as many affected healthy regions should be restored as quickly as possible.
The fault processing technologies can be classified into three types: (1) fault processing based on local intelligence, (2) fault processing based on distributed intelligence, and (3) fault processing based on centralized intelligence.
Fault processing approaches based on local intelligence were the earliest technologies in which neither a communication system nor master station is needed. The decision is made based solely on the information collected at the local position. Fault processing approaches based on local intelligence are still used today and include relay protection, automatic reclosing control, and backup automatic switching control. They have the advantage of fast speeds. However, the coordination of over-current protection is rather difficult in some cases, such as the feeder trunk in an urban area. Automatic reclosing control is suitable for feeders with overhead lines. Backup automatic switching control may switch the load to the backup power supplying route in several seconds, but it is only effective for loads with more than one power supplying route.
Feeder Automation (FA) based on recloser and voltage-delay type sectionalizers, reclosing with a fast over-current protection mode, and the fast healing approach based on neighbor communication, are three typical technologies of fault processing approaches based on distribution intelligence. FA based on recloser and voltage-delay type sectionalizers was invented by Japanese engineers in the 1970s and has been successfully used in Asia for several decades, but it needs reclosing twice. Reclosing with the fast over-current protection mode is an improved approach that only needs reclosing once, but requires circuit breakers instead of the former’s load switches. Both FA based on recloser and voltage-delay type sectionalizers and reclosing with a fast over-current protection mode do not require communication systems and the whole feeder must undergo a period of outage. With the fast healing approach based on neighbor communication, the fault area can be located and isolated immediately and the healthy areas are hardly affected by the fault. However, high speed communication and reliability are both needed. Besides, the sectionalizing switches should be circuit breakers.
The typical technology of fault processing based on centralized intelligence is the Distribution Automation System (DAS), which consists of a master station, some sub-working-stations, a large number of Feeder Terminal units (FTU), and the communication system. Since global information can be collected, the fault location area of DAS can be much smaller and the service restoration schemes may be optimized. But DAS based fault processing needs a rather long time period, typically several minutes.
With the increasing of the amount of Distribution Generations (DG) in distribution grids, fault processing technologies coping with such challenges have been achieved.
In this chapter, the progress in fault processing technologies will be overviewed, and included is most of the literature written by the authors, which is also included in the following chapters of this book.
1.2 Progresses in Local Intelligence-Based Fault Processing
Although relay protection technologies have been used in electrical power systems for a long time, the coordination of relay protection is rather difficult in some distribution grids, such as short length urban feeders.
In many utilities, one over-current relay protection is coordinated with one or two fuses. Even on the output circuit breaker of a feeder in the substation only one over-current protection is installed. Coordination and setting of three-section overcurrent protection is investigated in References [1]–[4]. It is pointed out in [5] that interphase short circuit currents along the sectionalizing switches of a short length urban feeder are almost the same, thus the coordination of three-section overcurrent protection is difficult. An approach of time-delay coordination of the over-current relay protection scheme is suggested, in which outage on the trunk can be avoided in case of branch fails and outage on the branch can be avoided in the case of lateral fails. Four modes of hybrid schemes of three section overcurrent protection and time-delay over-current coordination are proposed in [4], which are commonly used in Chinese utilities. The coordination of over-current protection with FA based on recloser and voltage-delay type sectionalizers is described in [6].
Automatic reclosing control and backup automatic switching control also have a rather long history of application. Reference [7] describes a scheme suitable for switches on the branches or laterals of a feeder. Reference [8] describes a coordination scheme of backup automatic switching control with DAS for an area requiring high service reliability.
The local intelligence-based fault processing technology will be detailed in Chapter 2.
1.3 Progresses in Distributed Intelligence-Based Fault Processing
A family of switches with distributed intelligence are described in Reference [9] including FA based on recloser and voltage-delay type sectionalizers, FA based on coordination of reclosers, and FA based on recloser and over-current counting type sectionalizers.
FA based on recloser and voltage-delay type sectionalizers invented by the Toshiba Co. is the most widely used technology. Hai and Chen imported the technology from Japan to China and set up production lines for mass manufacture. The basic principle of FA based on recloser and voltage-delay type sectionalizers is described in References [10]–[12]. The appropriate setting of the recloser and voltage-delay type sectionalizers is the critical application problem, which is investigated based on a hierarchical model in Reference [13] and a program is also used to calculate the setting values for arbitrary grid topologies is developed.
Reclosing with a fast over-current protection mode is another distributed intelligence-based fault processing technology, the basic principle of which is described in Reference [14]. But the method in [14] has some limitations, such as long restoration time for temporary faults and enlargement of fault isolation area due to overload. Improvements are made in [15]. The duration time of temporary fault restoration is considerably reduced by adding a time delay mechanism to the tripping procedure of sectionalizers in the case of out-of-voltage. The drawback of enlarging the outage area due to overload is avoided by introducing an out-of-voltage lock mechanism into sectionalizers and loop switches, respectively. A linear planning approach is also proposed for optimizing the setting values in [15].
The approach of FA based on recloser and voltage-current mode switches is described in [16], which can be regarded as the combination of FA based on recloser and voltage-delay type sectionalizers and reclosing with a fast over-current protection mode.
These distributed intelligence-based fault processing approaches do not need communication systems and have played a great role, but they have some drawbacks, such as setting values should be adjusted in the field when the operation mode is changed.
Some distributed intelligence-based fault processing approaches with communication systems are published in [17]–[20]. A fast healing approach based on communication with GOOSE among the adjacent FTUs is described in [17], which is the typical scheme for distributed intelligence-based fault processing approaches with communication systems. The basic approach in [17] is improved in [18], in which both temporary fault and permanent fault can be located and isolated immediately without override tripping and it works well even in cases where a few switches fail to control. Other progresses also requiring communication systems are described in [19]–[20].
The distributed intelligence-based fault processing technology will be detailed in Chapter 3.
1.4 Progresses in Centralized Intelligence-Based Fault Processing
Centralized intelligence-based fault processing is the core technology of centralized intelligence-based distribution automation systems, which is always a hot topic of research, and there have been many achievements.
1.4.1 Fault Location
A unified matrix based algorithm for fault section detection and isolation in distribution systems is put forward in [21], which is improved in [22]. But the matrix based methods require both space and long calculation times for large scale distribution grids.
A fault location approach based on a directed graph is proposed in [23] without calculating a matrix. In [24], a large scale distribution grid is divided into many small scale connected systems consisting of some connected feeders and a fault may be processed in its corresponding connected system, thus the space and calculation time can be greatly reduced no matter how large scale the distribution grid is.
In [25] a hierarchical model based algorithm of fault section diagnosis for distribution networks is suggested. A fault location method based on pattern identification is described in [26]. A multi-objective distribution network restoration using an heuristic approach and a mixed integer programming method is proposed in [27]. A multi-agent based fault processing approach is described in [28].
In the field of robust fault location in case of insufficient information, there have been many achievements, which make the fault location program more usable in practice. In [29] and [30] a genetic algorithm is introduced into fault location to improve its robustness. A data mining approach based on the combination of a rough set with a neural network is described in [31] to solve the fault section identification problem in cases of insufficient information. A fuzzy reasoning approach for robust fault location in a distribution automation system is introduced in [32] and [33]. Uncertainty reasoning approaches based on Bayes probability theory for fault location in distribution grids are put forward in [34]–[37]. An integrated intelligent service restoration system for a distribution network with an auto-learning fuzzy...
