Relay-to-Relay Communication in Smart Grids Yields Robust Protection
By Amin Yazdaninejadi, Sajjad Golshannavaz, Farrokh Aminifar
Realizing sustainable electrification process requires design and implementation of robust protection schemes. In one classification, the distribution network- or microgrid-wide protection strategies enabled by inelegant electronic devices (IEDs) (also known as GPS-connected smart relays or micro phasor measurement units (Micro PMUs) falls under a centralized structure around the phasor data concentrator (PDC). However, the recently developed relay-to-relay communication-based protection scheme shows more promise due to its distributed nature (see Fig .1). In the centralized structure, relays capture synchronous data frames and frequently report the calculated phasor values and its derivatives to the PDC. The data is then refined and handed to the distribution network or microgrid monitoring, protection, and control (MPAC) center for making the final decision. It is evident that, in the case of any failure in the infrastructures of centralized approach, protection system functionality is seriously imperiled resulting in a widespread outage. In the relay-to-relay communication scheme, smart relays share essential data with each other in a given protection zone (PZ), namely immediate neighbors, to discover faulty branches. Based on predefined logics, they reach a consensus on the protection decision of the faulty branch and main/backup relays in charge. The data sharing mechanism of this strategy assures protection scheme robustness and enhances electrification process suitability in smart grids hosting distributed energy resources (DERs) .
Fig. 1. (a) Centralized protection versus (b) relay-to-relay communication-based protection scheme.
In faulty occasions, DERs can contribute to remarkable fault currents which instigates a technical protection bottleneck known as blinding issue for backup relays . For example, the magnitude of fault current increases with the contribution of synchronous generator based DERs. Or, directional elements of protective relays are adversely affected due to the fault behavior of inverter based DERs (IBDERs), especially in the islanding mode. Since directional elements are integral to either primary or backup protection in distribution systems with bi-directional fault currents including microgrids, tackling this issue is a high priority. One effective solution is the communication-based relay-to-relay protection mechanism providing data sharing opportunity for protective relays. As an instance, let assume that a fault occurs in PZ1 of Fig. 1. In this case, immediate relays of each branch cooperatively identify that the fault is on the respective branch or outside. This task can be performed through different methods as illustrated in Fig. 2 . During a fault outside the PZ and on Line B, smart switches A and B detect the fault at their left sides. Consequently, switch A detects the fault outside PZ and shares this data with switch B through the communication link. A fault inside PZ is also discriminated in a similar manner through the associated communication link and data sharing. Additionally, relays R1, R3, and R5 in Fig. 1 (b) talk to each other in a loop to specify the operation priority of relays, i.e. the main relay, the backup relay, and the idle one. Likewise, R2, R4, and R6 communicate in the peer-to-peer structure.
Fig. 2. An illustration of relay-to-relay communication-based protection scheme.
As pointed out earlier, conventional directional elements fail to detect the direction of faults in the presence of IBDERs. Therefore, for relay-to-relay communication-based protection, relays need to employ new, efficient, and consistent logics rather than the conventional ones. Recently, it was inferred that employing fault components can enable fault and fault direction detection -; hence, eliminating the technical bottlenecks of this type of protection. Based on the superposition principle, any power system in a fault condition can be modeled and analyzed as the sum of two systems. The pre-fault system is obtained by eliminating all changes in voltage and current due to the fault and the superimposed system is based on the changes caused by the fault in the basic parameters. Based on the superimposed components in the second system, faults are discriminated from the other phenomena such as overload in distribution networks, etc. These components also facilitate determination of the fault direction within the context of relay-to-relay protection scheme. Based on the obtained results, this study touts deployment of the superimposed component phasors of voltage and current calculated by numeric relays.
Additionally, relay-to-relay communication-based protection requires further investigation on its implementation at all power system levels from transmission systems to microgrids with radial and loop topologies. Based on the data sharing between relays, this protection strategy provides a proper response against the solid/high-impedance faults and fault with lower magnitude in islanding modes. In the context of relay-to-relay communication, not only these issues but also many others still need to be explored deeply to develop mature and well-defined protection strategies.
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This article edited by Mehmet Cintuglu
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