1. Introduction
Generally, the interconnection of distributed energy resources (DERs) such as DFIG wind turbines, photovoltaic (PV) panels, diesel generators (DG) and battery storages (BS) into distribution networks is considered a promising solution for supplying cheaper and cleaner electric power to customers. However, the complexities introduced into the protection systems and functioning of protective relays within interconnected distribution networks impose technical challenges to the reliability and efficiency of the distribution system operation. In fact, with the connection of DERs into a distribution network the assumption of unidirectional power flow does not hold as multiple generation sources contribute to supply electrical power under different operation conditions [
1,
2]. Thus, conventional system protection schemes in radial distribution networks such as time over current (TOC) curves with fixed decision boundaries are not reliable to meet the desired reliability and performance requirements for the protection systems within interconnected distribution networks. Many years of practice in developing protection schemes based on conventional protection system strategies and introduction of modern information and communication technologies (ICT) infrastructure embedded into power system operation have provided opportunities for power protection engineers to adopt various protection strategies for reliable and safe interconnection of DERs into distribution networks. In the literature [
3,
4,
5], different protection methods have been proposed to address protection complexities corresponded to interconnection of DERs into distribution network. In its simplest form fault current limiters (FCLs) have been proposed to protect power system component against excessive high currents contributed by DERs connected into distribution networks. This approach has mainly been adopted to prevent undesired thermal stresses and avoid exceeding fault current capacities for circuit breakers (CBs). In [
6] Gokhan et al. investigated the effect of FCLs in different locations to reduce the fault current contribution from an interconnected wind farm in a range of CB ratings within the distribution network. By adopting a similar approach, Sharhriari et al. [
3] proposed solid state FCL (SSFCL) constituting parallel connections of an inductor, GTO thyristor and metal oxide variostor (MOV) to address coordination between TOC protection curves and fuses within a radial distribution network interconnected with DERs. The proposed SSFCL is used at the connection point of each DER where the inductor is devised to prevent high fault current and the MOV suppress overvoltages while the GTO thyristor is the low resistance path during normal operation conditions of the distribution network. Further improvement for protection system based on FCL have been addressed by Bayati et al. [
7], where application of FCL with optimization in DER placements have been considered to improve operation time for the protection relays and increase reliability within interconnected distribution networks. In spite of the low cost and reduced complexities when using FCL methods for protection systems in interconnected distribution networks, it is mainly only effective for specific radial distribution networks with low DER penetrations.
In recent years with the introduction of modern numerical relays known as IEDs for power the concept of adaptive protection systems (APS) have been studied and investigated in many research papers [
8,
9,
10,
11,
12] to address protection challenges in interconnected distribution networks. In its general form APS is devised based on online activity that modifies protection settings for preferred protective response of the protection system to a change in power system condition which is usually automatic. In early applications of APS Rockefeller et al. [
13] proposed a centralized architecture utilizing computational and data storage devices to underscore feasibility of better performance for protection system by adjustment to protection settings in protection IEDs. Further, to that Rockefeller et al. introduced APS for improvements in protection systems at the substation level as variations in operation conditions and topological changes take place. Additionally, application of the APS for adaptive distance protection has been discussed for improvement of the relay performance within multi-terminal transmission lines [
8]. In [
9] Brahma et al., adopted APS to address coordination problems within a distribution network where fault current contributions from DERs can affect selectivity and performance of the protection system. The outline of the proposed protection scheme is based on dividing the distribution network into separated protection zones and applying online adjustment for differential protection function according to the operation condition in each protection zone. For protection systems in microgrids Oudalove et al. [
10] proposed APS to address protection issues related to variations of the fault currents during grid-connected and islanding mode operation. Utilizing modern ICT and protection IEDs Oudalov et al. adopted a “connect and forget” approach in which a centralized architecture for the protection system is devised to automatically adjust the protection settings for TOC curves protection functions depending on the grid-connected or islanded mode operation conditions. Finally, Vasileios et al. [
14] designed and implemented APS for interconnected distribution network in which a non-linear programming (NLP) method such as particle swarm optimization (PSO) is used to adjust the protection settings of the overcurrent protection functions to improve coordination between protection IEDs. Having discussed the advantages for centralized architectures for APS, there have been few research papers which have embarked on the study of decentralized APS and its reliability for protection systems in interconnected distribution networks. In designing a decentralized APS Maleki et al. [
15] proposed a scale down for the centralized architecture into individual protection IEDs where each IED constitutes a central unit for monitoring and adjustment of the protection settings according to changes in topology and operation mode of the DERs connected to the network. Bahadornejad et al. [
16] also proposed a decentralized approach based on estimation of the Thevenin equivalent network upstream and downstream of the DERs’ location where a fault current calculation method is used to adjust the protection settings for the CBs upstream and downstream of DERs. In conclusion, adaptive protection systems are flexible and capable to be utilized for complicated protection systems within interconnected distribution networks; however, given the tightly coupled interactions between protection IEDs the application of APS is developed for specific distribution networks and in large scale interconnected distribution networks the complexities increase.
Another important advancement in developing protection system is associated to the MAS concept, where a combination of ICT and computer science techniques is employed to deal with large-scale, heterogeneous and interdependent problems within protection systems of the interconnected distribution networks [
17]. In fact, the idea of knowledge sharing and communication between protection IEDs within a MAS framework have been an enabling factor to extend the conventional protection philosophy from the component level to the system level where total stability and avoiding cascaded failure are given high priority [
18]. In the literature [
19,
20,
21] various parameters such as architectural arrangements, agent types and agent interactions have been highlighted as important factors to develop a reliable and functional protection system based on MAS. For many protection systems based on MAS hierarchical architectures and layer-based arrangements of the agents have been proposed to fulfill protection tasks within power system networks [
18,
21,
22,
23]. Ming et al. [
22] proposed a three layers hierarchical architecture for wide area protection applications in which not only an agent can have a local view of the power system but agents in various layers can fulfill wide area protection tasks by utilizing cooperation and behavioral interactions inherent to the agent technology paradigm. Although compared to the APS approach decision making in multi-agent-based protection systems is based on local and global information which is achieved through loosely coupled interactions between different agents but with the hierarchical layered-base architectures communication bandwidth and disruption of the communication link can be a main drawback to the performance of the protection system. In this paper a distributed decision making based on MAS and heuristic reasoning have been proposed to adjust protection settings with respect to dynamical changes in the operation conditions of distribution networks interconnected with DERs.
The structure of this paper is explained as follows: In
Section 2, a typical distribution network interconnected with DERs is described to address protection challenges under various operating conditions and topological changes considering real-world operation scenarios. Later in
Section 3, the proposed methodology for establishment and integration of heuristic MAPS are explained where different agent types and their interaction within the system protection are highlighted. In the implementation phase, real-time simulation studies on the performance of the MAPS are developed in
Section 4 where simulation techniques such as hardware-in-the-loop (HIL) and co-simulation are adopted. Simulation results and verification of the MAPS functionalities for various operation scenarios are represented and compared with conventional TOC protection systems. Finally, conclusions are made based on the simulation results which highlight the advantages and significance of the proposed MAPS in dealing with system protection challenges arising due to the connection of DERs.
2. Problem Statement
Typically, in the planning and development of conventional distribution networks, specific operation conditions are determined as normal loading conditions with all other technical aspects related to control and protection systems including size, type and protection settings being gauged for duration of life cycle. In this paper, the core of the study is concerned with the protection system and conflicting operation conditions that prevail upon interconnecting DERs into distribution networks. Technically, conventional protection schemes are well-established and practiced based on fixed decision-making boundaries and local measurement variables such as current, voltage and power flow direction are continuously monitored to decide on fault occurrences within the designated protection zones. Although the aforementioned protection strategy is appropriate with radial distribution networks where single generation source supplies unidirectional power flow from network upstream to downstream consumers, interconnection of DERs at the distribution level can alter the unidirectional power flow paradigm as load-generation balance varies depending on the DER operation scenario. Therefore, with the prevailing changes in operating conditions of the interconnected distribution network, normal loading currents can vary introducing adverse effects in the reliability and dependability of the protection systems. As a matter of fact, system protection in interconnected distribution networks has become a hot topic for many research studies in the field of power system protection engineering where the prospect of future power grids is envisaged with connection to various types of DERs [
24,
25,
26].
In the following, some of the challenges corresponding to system protection with various parameters and operating characteristics have been investigated to represent real-world operation scenarios within interconnected distribution networks. As the first step a typical distribution network interconnected with DERs is described.
2.1. System Description
For the purpose of analysis and investigation of the system protection during any fault incident, modeling and simulation of the interconnected system plays a crucial role in evaluating the functionality and performance of the protection system. Therefore, to address a real-world scenario in the operation of an interconnected distribution network, a distribution network interconnected with DERs such as DFIG wind turbines and DG has been developed.
Figure 1 shows a single line diagram for a distribution network which is connected to the grid through CB
main. The local power generation units at the distribution level consist of a wind farm with DFIG type wind turbines and DG which acts as frequency controller to distribution system during the islanded mode operation when CB
main is open. Also, Load1 (L1) and Load2 (L2) represent local loads supplied by both grid and local generation sources (DFIG and DG). In addition to that, the sizing and capacity of the local generation sources have been selected to be able to supply the local loads (L1 and L2) during islanding operating mode which is a desirable characteristic to form an islanded distribution system. The CBs illustrated in
Figure 1 have been devised to operate and isolate faulty section(s) of the distribution lines with trip signals which are sent by protection IEDs associated to each CB. It is also worth mentioning that there are two fault scenarios, Fault1 and Fault2, shown in
Figure 1, which are considered to evaluate the system protection performance against conventional approaches. Finally, CB1, CB2, CB
main and CB
DFIG are configured to operate with TOC protection function while CB3 (shown in
Figure 1 as a pair of CBs) is considered to operate on a differential protection function.
2.2. Operation Scenarios
In relation to connecting DERs such as DFIG systems into distribution networks, there are operational aspects corresponding to embedded control strategies and sizing of the DFIG systems which can introduce technical complexities in devising protection settings. As a matter of fact, variations in wind speed over a wind farm area can affect the power output from DFIG wind turbines altering the load-generation balance by causing changes in the operating conditions at each CB location. Thus, with respect to conventional approaches, the protection settings for TOC curves have to be adjusted accordingly to ensure reliable and safe operation of the interconnected distribution network. In order to highlight variations in operation conditions at different CB locations within a interconnected distribution network,
Figure 2 and
Figure 3 represent normal loading currents for different wind speed and size of the DFIG wind turbines, respectively.
Figure 2 represents the root mean square (RMS) currents at the point of common coupling (PCC) connection of the DFIG system (CB
DFIG) which can vary depending on the available wind speed and the number of connected DFIG systems in the wind farm.
Similarly,
Figure 3 illustrates variations in loading currents at different CB locations as the number of connected DFIG systems at the PCC changes. Consequently, having represented the effective parameters in variations of load currents within the interconnected distribution network, it is obvious that adjustment for protection settings is imperative to maintain the desired reliability and performance of the protection system.
2.3. Grid Connected/Islanded Mode
One of the important operation characteristics for an interconnected distribution network compared to radial distribution networks is the ability to operate in islanded mode where the distribution system relies only on local DER units to supply electric power for consumer loads. Although the possibility to operate under islanding conditions can introduce improved power quality for customers at the distribution level, there are technical issues corresponding to system protection which are critical and need to be addressed. Basically, unlike grid connected mode, fault current levels under islanded mode are altered due to the limited electric power capacity of the DERs for supplying a sustained fault current during the fault occurrence period. Moreover, with respect to low mass inertia within the islanded part of the distribution network, the fault current characteristics can vary and a detailed time-domain simulation for analysis of transient short circuit current is necessary. Therefore, in order to investigate the effect of transitioning to islanded mode on fault current behavior, a 3-phase fault scenario downstream of the DFIG wind farm at Fault1 location has been applied for both grid-connected and islanded mode. As illustrated in
Figure 4, fault currents under grid-connected operation mode are much higher compared to islanded mode as the operating condition of the DFIG system such as wind speed and number of connected DFIG system are negligible. Moreover, comparing the time-domain response and transient envelope of the fault current within islanded mode they are different as the maximum fault current occur multiple cycles further from the fault initiation time. The labels GC and IS represent grid-connected and islanded mode of the operation for interconnected distribution network while the subscripts point to the operation conditions for the DFIG system indicated by wind speeds. For example GC
wind9 represents grid-connected mode in which the operation condition of the DFIG system is for a wind speed of 9 m/s. In a similar way the IS represents the islanded mode operation where the operation condition of the DFIG is for a wind speed of 9 m/s (IS
wind9). Also the prefix number to the labels represents the number of DFIG systems connected to the grid and 3GC
wind 12.5 represents the operation conditions of the interconnected distribution network under grid-connected (GC) mode when 3DFIG systems are operating and the wind speed is 12.5 m/s.
It is obvious from
Figure 4 that under different operation mode, the requirement for the adjustment of the protection settings within the protection IEDs is inevitable. Also, to further investigate the time domain performance of a TOC curve devised for conventional system protection, comparisons have been made for fault currents at CB1 and CB2 under different modes of operation to highlight the protection complexities within an interconnected distribution network. As seen in
Figure 5, the time domain responses of the fault currents during islanded operation mode are not detected by the TOC curve as the fault current level and fault current duration are not sustained for the minimum time required to trigger a trip for the corresponded protection IEDs. Therefore, it is essential for system protection that the TOC curve settings be adjusted according to the operation mode of the interconnected distribution network. In
Figure 5 the notation for GC
subscript follows the same notation as explained for
Figure 4, however the prefixed number and fault suffix correspondingly represent the number of DFIGs connected to the grid and the fault locations. For example 3GC
wind9 Fault1 represents a fault current curve at CB3 when a 3-phase fault occurs at the Fault1 location and the number of DFIG systems connected to the grid is three. Also the International Electrotechnical Commission (IEC) curves highlight the TOC protection functions based on the fixed pickup current settings at CB3 locations with consideration of the DFIG operation conditions.
6. Conclusions
Given the complexities for fault current behaviour in interconnected distribution networks, development of a reliable and efficient protection system is dependent on many factors such as athe vailability of renewable energy resources (wind, solar radiation, etc.), the number of DER connected into the grid and the types of the DER with the control strategies embedded by the original equipment manufacturer (OEM). In this paper, a new approach based on integrating MAS and distributed decision making has been adopted to deal with system protection challenges within a distribution network interconnected with DFIG wind turbine systems. The significance for the proposed methodology is concerned with distributed decision-making based on knowledge representation of the system protection using domain-specific ontology which is consistent to human mind logic. Further to that, a knowledge engineering process based on generalization of system protection knowledge into human mind cause-effect reasoning process where a rule-based knowledge database established to support decision making similar to an expert system. The main advantages for the proposed MAPS are associated to scalability and interoperability of the proposed protection system which are critical factors for large-scale, heterogeneous and interdependent future power systems. Moreover, using element of distributed artificial intelligence such as MAS has been shown to be effective to enable proposed MAPS for taking into consideration the operation information across neighbouring nodes for decision-making and accordingly adjust the protection settings. Finally, the MAPS performance and its messaging capabilities have been verified using real-time co-simulations and HIL techniques to meet the requirements for real-world operation scenarios.