A traveling-wave-based line protection strategy against single-line-to-ground faults in active distribution networks

https://doi.org/10.1016/j.ijepes.2018.11.032Get rights and content

Highlights

  • A protection which isolates the SLG fault in neutral non-effectively grounded distribution with DG.

  • The time setting based on the grading coordination is added to the traveling-wave-protection.

  • Based on the coordination of relays the fault can be removed from both sides without communication.

Abstract

Electricity networks are in the era of major transition from stable passive distribution networks with unidirectional electricity transportation to active distribution, and the fault on the distribution lines should be cleared from both sides accordingly. This paper proposes a traveling-wave-based line protection scheme which can isolate the Single-Line-to-Ground (SLG) fault from both sides of the faulted line in neutral non-effectively grounded distribution network including Distributed Generation (DG) sources. In the proposed scheme, when a SLG fault occurs, first, relays distinguish the fault direction by comparing the polarities of the initial voltage and current traveling waves; and then the relays that detect the fault on the downstream and upstream sides operate based on the conventional time grading coordination, and the electrical changes of three-phase in a specified time window, respectively; finally, after operation of the relay upstream of the fault on the faulted line with the fastest time setting, the relay downstream of the fault operates by detecting the tripping of the opposite circuit breaker. In order to validate the efficiency of the proposed scheme, extensive simulations have been performed on a typical active distribution system using ATP-EMTP software. The simulation results indicate that the proposed line protection scheme can accurately and quickly isolate the SLG fault from both sides.

Introduction

Around the world, conventional power system is facing the problems of gradual depletion of fossil fuel resources, poor energy efficiency and environmental pollution. These problems have led to a new trend of generating power locally at distribution voltage level by using non-conventional/renewable energy sources and their integration into the utility distribution network. Installation of DG sources may change the power flow in some feeders from single-directional to bidirectional. Distribution networks without any DG units are passive since the electrical power is supplied by the national grid system to the customers embedded in the distribution networks. The distribution system becomes active when DG units are added to it leading to bidirectional power flows in the networks.

In spite of numerous advantages provided by active distribution networks, there are some technical challenges, which need to be addressed by the engineers. One of them is protection and its entities [1], [2], [3], [4]. In order to overcome the protection challenge, various techniques have been proposed which take advantage of multi-point information using communication links [5], [6], [7]. However, since such protection is highly dependent on the communication, the protection will be invalid if communication problems arise. To solve this problem, a fast current protection scheme without communication is proposed in [8] for distribution lines containing DG, but it is not effective for the SLG faults in neutral non-effectively grounded distribution systems.

Even though there are different types of faults in distribution systems, the majority of the faults are of SLG type, which are more than 70% of all faults [9]. In China, most distribution systems are either isolated from ground or grounded using high impedance. In such systems, when a SLG fault occurs, the magnitude of fault current which flows from busbar through protection point to the faulted line is low, and therefore the fault cannot be recognized using traditional over-current protection. During a SLG fault, voltage of sound phases increases, which may result in dielectric breakdown of equipment, thereby causing a phase-to-phase fault [10]. When DG is connected to the distribution system, one rule is that the grounding mode of DG should follow that of the main network. Thus, in the neutral non-effectively grounded distribution, the neutral of DG sources is always isolated from the ground [11]. As discussed in [12], when a SLG fault occurs in the neutral non-effectively distribution with DG sources, current output of DG sources may change slightly because of the control strategy of DG sources in several cycles, which is also difficult to be identified, and the voltage after the fault is similar to that in the distribution without connection of DG sources. Hence, there is no significant difference between the fault features of the neutral non-effectively grounded distribution with and without DG sources.

There are usually two methods for protection of SLG faults in neutral non-effectively grounded distribution systems, i.e. issuing an alarm signal and removing the fault. However, the fault removal is usually needed in high-reliability distribution systems. In order to identify the SLG fault from feeders in neutral non-effectively grounded distribution systems, a faulty feeder selection technique is presented in [13]. Then, authors of [9], [14] propose a traveling-wave-based faulty feeder selection method which significantly improves the accuracy of the faulty feeder selection technique. In this method, in case of a SLG fault incident, only an alarm signal is issued, and therefore it is not suitable for the distribution systems which need to remove the faulted line quickly. To overcome this problem, a traveling-wave-based protection method is proposed in [15]. But this method cannot identify the faulted section. To be more precise, when a fault occurs on a feeder, the whole feeder is removed, which results in expanding the scope of blackout, hence it cannot be suitable for multi-section distribution lines. Moreover, it does not have the ability to isolate the fault from both sides, which is necessary for double- or multi-end distribution systems including DG sources. Therefore, reliable and selective protection of lines against SLG faults in neutral non-effectively grounded distribution networks including DG sources is still a significant challenge.

Based on further analysis of SLG faults, this paper proposes a fast and selective traveling-wave-based line protection scheme which can identify and remove the faulted section from both sides without the need for communication infrastructure. In the proposed scheme, when a SLG fault occurs, relays distinguish the fault direction by comparing the polarities of the initial voltage and current traveling waves; relays which detect the fault on the downstream side operate based on the conventional time grading coordination, and the relays which detect the fault on the upstream side operate based on electrical changes of three-phase in a specified time window; after the relay upstream of the fault on the faulted feeder with the fastest time setting operates, the relay downstream of the fault operates by detecting the tripping of the opposite circuit breaker so that the fault is removed from both sides.

The remainder of this paper is organized as follows: The basic principle, protection configuration and criteria of the proposed protection scheme are introduced in Section 2; the test network for validation of the proposed scheme is introduced in Section 3; the simulation results are presented in Section 4; and finally, Section 5 concludes the paper.

Section snippets

Protection configuration for the distribution with DG

With connection of DG sources to the distribution system, the structure of network is changed from the radial network with a single-end to a double- or multi-end one. Previously, DG sources were not usually allowed to operate in islanding operation mode, and the anti-islanding protection were always adopted, in which DG sources had to be removed after the fault occurrence and no longer provided the current to the fault point [16]. Therefore, only the relays upstream of the fault need to operate

Test network

In order to validate the effectiveness of the proposed protection scheme in this paper, extensive simulations are carried out by the electromagnetic transient program ATP-EMTP. Referring to Fig. 1, a 10 kV distribution system including two 10 MVA DG sources is performed in Fig. 3. As shown, this is a typical radial distribution system which is applied in many areas in China, and the neutral of the system is isolated from the ground. There are six feeders connected to bus A. The three-phase

Simulation and evaluation

To evaluate the performance of the proposed protection method, extensive fault events under different fault resistances, fault inception angles and fault locations were simulated on the test network, as shown in Fig. 3. The fault distance is defined as the distance from the fault point to the closest relay upstream of the fault. The threshold Uset and s are set to 3 kV and 10 A, respectively. In order to investigate the impact of DG connection on the protection criteria, a comparison is

Conclusion

In this paper, a novel protection scheme based on traveling waves against SLG faults for distribution lines in the neutral non-effectively grounded systems with DG is proposed. It can effectively distinguish the fault section without relying on the communication channel. In the scheme, the SLG fault can be removed from the upstream and downstream sides of the faulted line successively, which ensures the selectivity and rapidity of the protection, and it is beneficial to the improvement of the

Acknowledgment

This work is supported by the National Key Research and Development Plan of China (Grant No. 2016YFB0900600).

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