The challenge of traditional relaying protection in modern power system
As the primary barrier of the power system, the relaying protection takes the responsibility of the fault clearing. However, with the development of the modern power system, the traditional relaying protection cannot satisfy the requirement for the safe and stable operation of the power system. The main problems are as follows:
The difficulty in calculation and cooperation of the setting value
The relaying protection system is mainly constituted by main protection and backup protection. The multi-stage backup protection mainly consists of the local backup protection and the remote backup protection of the power system, of which the typical mode is three stage protections. Because the grid structure and operation mode of the modern power system is becoming more and more complex and variable, the coordination among the related backup protection is very complex. Under such condition, the backup protection that only utilizes the local measurement and coordinate through the different time delay cannot satisfy the selectivity requirement of the power system protection. Because of the above problem, the backup protection trend to be simplified in the actual engineering application, and what’s worse, the zone II backup protection is suggested to be cancelled and the zone III backup protection should be simplified in some situation. It is worth attention that when high resistance fault occurs in the large power system, the dual-configuration main protection still cannot detect the fault sensitively, which will prolong the fault clearing time and expand the scope of tripping. This will undoubtedly increase the risk of the local grid disaster under emergency state.
Long time trip of remote backup protection
The multi-stage cooperation mode will lead to a long delay time of the backup protection, easily resulting in commutation failure in power grid at receiving side, which is harmful to the security of AC/DC hybrid power grid.
The lack of self-adaptive ability
The setting coordination of traditional backup protection is based on the fixed operation mode of the power system. When the grid structure and operation mode of power grid changes, the backup protection may be hard to corporate and coordinate, which may cause the misoperation of the protection and expansion of the fault range.
The potential risk of misoperation
When the grid structure or running condition changes unexpectedly, the large load power flow transferring may occur in a large scale. The zone III distance protection may cascade trip unexpectedly, resulting in the system splitting or large scale blackout.
The major cause of these problems is that the relaying protection operates only based on the local information at the installation position. If more comprehensive information about the present power system can be acquired, for example, if the backup protection can get the information of the present system and the related information from the wider area, the performance of the protection will be greatly improved. For example, if the remote backup protection can get the relevant information from the remote protected device, the fault can be identified more accurately and cleared more quickly. The problems of traditional protection may be eliminated through the wide area information.
Wide area protection and the wide area relaying protection
Wide area protection (WAP) is the protective relay and the security and the stability control of the power system based on the wide area information, which consists of WARP and wide area security control (WASC) [7]. The wide area information is the information from the local and the remote power grid, rather than the information only from the protected device.
The narrow definition for the wide area protection is only related to the security and stability control, emphasizing the protection for the whole power system [3, 8, 9]. There are also several literatures regarding the security control among 0.1 ~ 100 s as wide area protection [10, 11], in order to emphasize the protection for the wide area power system general safety. From the perspective of the history and applications, the stable control function is achieved by using the local information or wide area information. In recent years, the real-time wide area information are more and more used to improve the stable control function [10, 11], which has enriched the concept of wide area stable protection from the aspect of wide area information. In addition, several literatures, call the WARP as wide area back-up protection [3], which indicates that using the wide area information to improve the performance of backup protection is more concerned at present.
The relaying protection and security and stability control can achieve the security emergency control together for the power system. The relaying protection is called as equipment protection, and the security and stability control is often taken as system protection or SPS (Special Protection System or System Protection Scheme), RAS (Remedial Action Scheme) [3, 8, 9]. In the view of above, the relaying protection can be called as fault protection, and the security and stability control can be called as security and stability protection.
The essence of the security emergency control in Chinese power system is the three lines of defense [12, 13]. In the process of emergency control, relaying protection and security control operate independently, perform their own functions and their function cannot replace each other. It is worth attention that there are two basic principles in the security emergency control: the necessity for failure clearing and the priority of failure clearing. This is the duty and responsibilities of relaying protection and WARP. The fault clearing is the premise of effective security control. So the function of backup protection cannot be replaced by security control [3].
The function and characteristics of wide area relaying protection
The important difference between WARP and traditional relaying protection is using the wide area measured information. The traditional relaying protection mainly uses the local information, and at most uses the remote information which is extended from local protected equipment. Please note that the protected line of the remote backup protection is adjacent to the neighbor element and the state of this line, the previous operation result of the other protection (main protection) and the relevant remote backup protection are all related with the remote information. However, the tradition protection only can make judgments just according to the local measurement information. Hence, the information used for the traditional protection is not sufficient for the power system protection, which is the main reason for the existing deficiency of the protection. Therefore, the studies on WARP at present focus on the backup protection, especially the remote backup protection.
The wide area information can effectively improve the performance of the backup protection. The protective information in the faulty region can be acquired by the wide area communication systems. The fault element can be determined by the multi-information fusion. Then the backup protection can make the protection decision based on the fault element, and the coordination between the protections can be simplified. Meanwhile, the real time system structure and the operation mode can be acquired through the wide area measurement and the communication technology. The setting value of the protection can be up-dated based on the current status of the power system. Hence, the sensitivity and the selectivity can be improved.
The farthest protection scope of relaying protection is the next adjacent equipment, which is the region of backup protection. Therefore, the protection scope of the relaying protection is limited. Hence the information of the WARP is limited, which introduces the concept of limited WARP. The most important characteristic of limited WARP is that it only needs the measurement information from the adjacent limited area, which can lighten the burden of wide area communication or information processing, which is beneficial to the realization of WARP. In practical engineering, it is possible to realize the limited-wide-area relaying protection by using the existing optical fiber communication channel of the pilot protection. On the other hand, the system protection is oriented for the safe and stable operation of the entire power system, which needs the information from the wide-range of the whole system. This is another important difference between the protective relay and the system protection.
Besides, in a substation, if not only the information of local protected equipment can be used, but also the information of relevant equipment in the station can be used, the performance of relaying protection can be improved effectively. This is called substation area relaying protection, which uses the substation area measurement information. The substation area protection does not need the remote communication, which is feasible for the industrial application, especially for the digital substation. The sharing and the utilization of the substation area information can improve the performance of the relaying protection and control functions in the substation, integrate the substation relaying protection and simplify the relaying protection configuration, extent the new relaying protection function flexibly, such as the bus-bar protection and the breaker failure protection in the medium or low voltage substation, implement the coordination between the relaying protection and the control and the unified substation protection system. Meanwhile, the substation protection system can access to the wide area protection system as the child-station, and corporate with the wide area protection to achieve the protection and control for the regional power grid. Therefore, some scholars have proposed the concept of the hierarchical protection, which is constructed by the bay layer relaying protection (traditional primary relaying protection), substation area layer relaying protection (substation area protection) and wide area layer relaying protection (WARP), of which the main design is shown in Fig. 1.
In Fig. 1, the bay layer relaying protection is only aimed at single equipment bay. With the information in bay unit, it achieves the protection function of devices in bay unit. It also collects information and executing orders for WARP. Substation area layer relaying protection is aimed at substation. With synthesized information from multiple equipments in the substation, it judges, decides uniformly and achieves protection and controlling function in substation area. It also preprocesses information and provides communication service for WARP. Wide area layer relaying protection is aimed at limited area grid. With synthesized information from multiple substations, it judges, decides uniformly and achieves wide area protection function in wide area.
The basic approach to achieve the wide area relaying protection
At present, there are mainly two different approaches to realize the basic function of WARP. One approach is based on the principle of online adaptive setting (OAS); another is based on fault element identification (FEI) to achieve WARP [7].
The wide area relaying protection based on on-line adaptive settings
The study on on-line adaptive setting (OAS) started at 1980s and the foreign scholars have defined it as following [14]: An on-line activity that modifies the preferred protective response to a change in system conditions or requirements. It is usually automatic, but can include timely human intervention. And several Chinese scholars express it as following [15]: Taking the event-triggered mode, tracking the change of power system operation mode in time, calculating and on-line regulating the setting value of protection to prevent the protection from losing coordination and improve its sensitivity.
In recent 20 years, the researches on OAS setting approach are mainly around the fault disturbance domain identification, the search of minimum cut point and quick short circuit calculation and so on [15–18]. In China, the protection information system for the setting values online check has been constructed in the power system. Based on the wide area communication system, the protection information system can acquire the in-time operation condition of the power grid, check and reset the setting value, and transfer the updated setting value to each protection device [19–22].
WARP based on OAS has been studied for a long time. There are several progresses achieved. But its application is limited. This is because, although this approach improves the sensitivity and selectivity by adjusting the fixed value online, it still cannot overcome the shortcomings of the traditional backup protection, including the difficulty of the setting value coordination, the long operation time delay and so on. This shortcoming is also the important reason for hidden failure of the backup protection, which leads to the cascading trip and threats the system safety. In addition, how to ensure the timeliness and reliability of the wide area information communication, and how to calculate the setting value quickly after the change of the operation mode are still the questions to be solved.
The wide area relaying protection based on fault element identification
The study on WAPR based on FEI begins at the end of 1990s. For example, Yoshizumi Serizawa came up with the wide area current differential backup protection [23] at 1998, which form the scope of differential protection according to the area of backup protection and it can identify the fault element accurately and determine the operation scope of backup protection.
The WARP based on FEI can be described as following: By using the wide area multi-point measurement information in the power system and taking several fault discriminate mechanism, the position of fault element and the state of fault clearing can be determined. And then, the cutting point of WARP can be determined. The breakers at the cutting point operate to isolate the fault through bay layer’s equipment. Its superiority is that there is no need for setting calculation, the selectivity of backup protection can be guaranteed only by simple coordination of time and logic and the operation time of back up protection can be shorted effectively. For example, no matter where the remote backup protection locates, because it only coordinates with near backup protection (or main protection), the coordination between protection is simplified. Meanwhile, the backup protection can be prevented from cascading tripping during large scale load transferring.
The WARP based on FEI doesn’t require the information changes in time of entire power system. Even for the remote back up protection, it only needs the relevant fault information of the nearby equipment which is in the rim of adjacent substation groups (the scope may be expanded appropriately from the perspective of information fault tolerance). Therefore, this is a limited wide area protection and it is beneficial to the achievement of its engineering.
Recently, the WARP based on FEI has drawn the research concern. There are many achievements in the aspect of identification of fault elements, system constitution and the industrial test. However, according to the requirement of engineering application, there are many technical problems remain to be solved. The following will discuss the topic.
Aiming at the potential risk problems that the unexpected cascading operation of backup protection which is probably caused by large scale load transfer, it can use the wide area information to analyze and distinguish the state of the load transferring in power system and take the measures [24, 25] such as, blocking or changing action characteristics for the relevant backup protection and so on, which can avoid the cascading trip of backup protection and guarantee the system security.