**Alienation Coefficient and Wigner Distribution Function Based Protection Scheme for Hybrid Power System Network with Renewable Energy Penetration**

**Sheesh Ram Ola 1, Amit Saraswat 1,\*, Sunil Kumar Goyal 1, Virendra Sharma 2, Baseem Khan 3, Om Prakash Mahela 4, Hassan Haes Alhelou 5,\* and Pierluigi Siano 6,\***


Received: 4 February 2020; Accepted: 25 February 2020; Published: 2 March 2020

**Abstract:** The rapid growth of grid integrated renewable energy (RE) sources resulted in development of the hybrid grids. Variable nature of RE generation resulted in problems related to the power quality (PQ), power system reliability, and adversely affects the protection relay operation. High penetration of RE to the utility grid is achieved using multi-tapped lines for integrating the wind and solar energy and also to supply loads. This created considerable challenges for power system protection. To overcome these challenges, an algorithm is introduced in this paper for providing protection to the hybrid grid with high RE penetration level. All types of fault were identified using a fault index (FI), which is based on both the voltage and current features. This FI is computed using element to element multiplication of current-based Wigner distribution index (WD-index) and voltage-based alienation index (ALN-index). Application of the algorithm is generalized by testing the algorithm for the recognition of faults during different scenarios such as fault at different locations on hybrid grid, different fault incident angles, fault impedances, sampling frequency, hybrid line consisting of overhead (OH) line and underground (UG) cable sections, and presence of noise. The algorithm is successfully tested for discriminating the switching events from the faulty events. Faults were classified using the number of faulty phases recognized using FI. A ground fault index (GFI) computed using the zero sequence current-based WD-index is also introduced for differentiating double phase and double phase to ground faults. The algorithm is validated using IEEE-13 nodes test network modelled as hybrid grid by integrating wind and solar energy plants. Performance of algorithm is effectively established by comparing with the discrete wavelet transform (DWT) and Stockwell transform based protection schemes.

**Keywords:** alienation coefficient; hybrid power system network; protection; power system fault; solar energy; wind energy; Wigner distribution function

#### **1. Introduction**

Adverse environmental impacts of fossil fuel based power plants have forced the utilities to integrate clean energy with the grid in order to meet future energy demands. Fast development of renewable technologies and government incentives to reduce carbon footprints have motivated the utilities to switch from the conventional power plants to the renewable energy (RE) generation sources [1]. This is achieved by forming the hybrid grid with multi-tapped transmission and sub-transmission lines to supply the loads and integrate RE sources such as wind and solar power plants. Formation of hybrid grids using multi-tapped lines provide economic solutions for RE integration to the grid; however it creates protection challenges due to variable nature of RE generation and bidirectional flow of power in the lines. This resulted in the requirement of new protection schemes, which can be deployed in the recent structure of hybrid grids for effective protection. These techniques must be independent of direction of power flow, unbalanced nature of loads, fault current and variable generation [2]. This can be effectively achieved by the use of machine learning and signal processing techniques. Fang et al. [3], proposed an improved distance relay scheme using time delay and zero-sequence impedance for the grid to which RE sources are integrated. This scheme has high reliability compared to the conventional relays and reduced risk of malfunction. A detailed study of challenges associated with the protection of grid integrated distributed generation (DG) and adaptive protection schemes for these systems are presented in [4]. A detailed study related to the application of signal processing techniques and intelligent methods such as artificial neural network (ANN), fuzzy set theory (FST), and expert system (ES) in the field of protection of DG sources integrated power system is presented in [5]. A sensor based fault detection isolation scheme for the grid, interfaced with RE sources and electric vehicles (EV) is introduced by authors in [6]. This can effectively be deployed in eleven order multi-area smart dynamic power system interfacing RE and EV. In [7], authors introduced an algorithm based on the fast recursive discrete Fourier transform (FRDFT) for the protection of distribution system, integrated with DG. This is a novel, fast and adaptive relay technique for relay systems, which is effective for obtaining the optimal protection settings when system conditions are continuously changing. Application of the syntactic methods for identification of power system signals by measuring the parameters is reported in [8,9]. This method has the capability to provide syntactic and semantic information simultaneously. Nieto et al. [10], introduced a detailed study for improvement of quality of power in grid with integration of energy storage systems. This study helps to differentiate the power quality disturbances from the disturbances associated with the faulty events. In [11], a current-based transmission line protection scheme using Wigner distribution function and alienation coefficient is introduced. However, this scheme fails to provided protection to hybrid grids in the presence of both wind and solar energy penetration. Discrete wavelet transform was implemented for the identification of wind and solar energy penetration into the utility grid [12,13]. This method has the disadvantage of generating false tripping signals in the presence of high noise level. Stockwell transform was implemented for the identification of wind and solar energy penetration into the utility grid [14,15]. This method overcomes the demerits of discrete wavelet transform (DWT) scheme; however, it has the disadvantage of slow speed of protection scheme due to large data involved in the extraction of features from voltage and current signals. Therefore, new protection scheme is designed to provide effective protection against various faults to the hybrid grid with high RE penetration level. This protection scheme has merits, such as fast response (recognition of fault in a time duration which is less than quarter cycle) and performance is not affected by presence of high level of noise. Following are main contributions of the paper:


Nine sections are used to arrange the contents in the article. A brief introduction to carry out research on the selected topic and main contributions is included in Section 1. Section 2 describes the hybrid grid test network incorporated with RE generators. The proposed algorithm implemented for the protection of hybrid grid is described in Section 3. Section 4 demonstrates the simulation results related to fault identification in the hybrid grid. This section also includes the simulation results to highlight the requirements of protection scheme for the hybrid grid. Fault classification results are included in Section 5. Section 6 discusses the results of various case studies. Results for the discrimination of switching events from the faulty events are discussed in Section 7. A brief comment on the results and performance comparison of the proposed protection scheme with other existed schemes in the presence of RE is detailed in Section 8, followed by the conclusions (Section 9).
