Keywords

1 Introduction

There is a significant need to prevent damage of highly sophisticated and widely used equipment from electrical failures, fault, and discontinuities. The protective scheme to fulfill the above need should ensure high degree of continuity with electrical supply. The technique lies in designing a system that monitors the power systems for its failures at times. The design should intuitively respond to the fault characteristics, which help in identifying the same. Protection is needed not only against short circuits but also against any other abnormal conditions which may arise on a power system. Most of the faults on transmission and distribution lines are caused by overvoltage resulting in short circuits [1]. Three terminal and multi-terminal line projects generally have protection complexities and lead to compromises in reliability. In the recent past, there is a huge advancements with used in utilizing renewable energy sources. The hybrid energy system [2] is capable of providing electricity to remotely located areas. Wind turbines and solar panels are some to exemplify such systems. The wind turbines operate only during the winter season where there is a possibility of huge wind. Similarly, the solar panel is functional during summer when there is abundant sunlight. If any fault occur on the transmission line due to bad weather or any other condition, hybrid system are present to continue the supply [3]. The proposed system configuration and control scheme provides integration of PV source and wind energy source as illustrated in Fig. 1. The proposed research work is carried out to protection of multi-terminal transmission system with SVC in presence of hybrid system which is the combination of wind, PV generation using wavelet-based multi-resolution analysis. The system with hybrid generation [4] and three terminal transmission systems are tested with various combinations of faults on the phases with respect to the distance.

Fig. 1
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Block diagram of hybrid PV and wind energy system

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2 Multi-terminal Transmission Line Protection Scheme

Transmission system typically connects the generation plants to the remotely located region. These are further classified based on their individual voltage levels. This paper describes the most common types of three terminal protection scheme [5] for various types of faults. At a point where permanent fault occurs in the lines it takes huge amount of time to detect the fault, meanwhile the reliability of the system diminishes. There is a financial advantage in the development of three terminal transmission systems since it stays away from the cost of all or a segment of a substation and normally decreases the transmission line losses.

The technique of using a differential current relay (DCR) circuits is often considered as the simplest way of detecting faults and protecting the electrical system. The sensed imbalance forces a signal to trip, at the terminals with respect to its preset value and give the trip signal received at the terminals will immediately activate the local breaker. In such protection scheme is proposed which is capable of covering the entire length of multi-terminal transmission line protection (MTL) [6].

3 Wavelet Analysis

Wavelet analysis is a versatile tool which is extensively signal processing applications [7]. In this work such wavelet analysis is employed to detect as well as to discriminate the faults in transmission lines. The wavelet transform is one of the method which involves in sensing the electromagnetic transients which are possibly generated during the power system failures at the time of switching instants. The technique is similar to application of Fourier transform for fault analysis. However, wavelet-based application is more reliable and is capable of analyzing nonperiodic signals.

Wavelet transform allows the decomposition of a signal into different levels of resolution. The signal characterization is better and more reliable, thus discrimination can be obtained by using wavelet transform. It is a unique method for evaluating transient signal [8] at different frequencies with resolutions can be effectively studied by wavelet multi resolution analysis of power system transients.

4 Description of the Proposed System

Figure 2 describes single line diagram of static var compensator compensated multi-terminal transmission system with hybrid energy source with all terminals of the proposed scheme (Table 1).

Fig. 2
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Single line diagram of SVC compensated multi-terminal transmission system with hybrid energy source

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Table 1 Power system specification and corresponding wavelet characteristics
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5 Proposed Scheme for MTL Protection

Synchronized sampling at all the three terminals of three-phase currents is carried out with the help of a global positioning system. The absolute values of the detailed coefficients for three-phase currents of all the three terminals are determined using Bior 1.5 mother wavelet. The detail coefficients are used for discriminations of fault in the MTL system [9]. The detail coefficients generated from phase currents of each terminal and then sum of the detailed coefficients are calculated. The number of faulty phases is discriminated by using the sum of the detailed coefficients [10] with a fault threshold value of three-phase currents. The proposed scheme is tested for various types of faults at different locations and fault inception angles in the range 0–180°. It is possible to state that the technique is effectively working in detecting and discriminating all possible types of faults.

6 Simulation Results

The fault indices of three-phase current signals are obtained by using wavelet first level detail coefficients of Bior 1.5 mother wavelet. The number of faulty phases can be identified by comparing fault index If1 of each phase current with the predetermined threshold value Th1. It is observed that the fault index of phase A is very large compared to that of other phases as illustrated in Fig. 3. Thus, Phase A to ground fault at 40 km distance from terminal-1 of transmission system. Figure 4 shows that three-phase currents and Local D1-coefficients at terminal-1 multi-terminal transmission system with SVC (Fig. 5).

Fig. 3
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Phase A to ground fault at 40 km distance from terminal-1 of transmission system with deviation of effective coefficients

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Fig. 4
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a Three-phase currents at terminal-1 of Phase A to ground fault at 40 km distance of the line with SVC. b Local D1-coefficients of terminal-1 for Phase A to ground fault with SVC

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Fig. 5
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Sum of detailed coefficient index varying at terminal-1 from 40 km distance with a single line to ground b double line

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The Fault Index If1 is calculated with various locations from 10 to 100 km with step size is 10 km up to middle point of the transmission line from terminal-1 is illustrated for SLG, DL of all combinations of phase A, phase B and phase C. It is observed that fault index If1 of all faulty phases is greater than Threshold Th1 value. The fault Index of healthy phases remains less than the threshold value. So that the faulty phases determined effectively which are illustrated in Figs. 6, 7 and 8 and it describes that the Fault index and variation of distance of 3ph Currents for SVC compensated multi-terminal transmission system with hybrid generation of 40° at terminal-1 for SLG, DL. The effect of variation fault index is studied by varying the fault inception angle ranging from 0° to 180° in steps of 20° at various locations. In all the cases If1 is always less than Th1. Figures 9, 10 and 11 illustrates variation in fault index and incidence angle of three-phase currents at terminal-1 for 40% of the transmission line with SVC at line to ground fault and double line fault. It is observed that the proposed algorithm detects the fault less than half cycle using wavelet analysis.

Fig. 6
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Fault index and variation of distance of 3ph currents for fault inception angle of 40° for SVC compensated multi-terminal transmission system with hybrid generation at terminal-1 a SLG fault b DL fault

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Fig. 7
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Fault index and variation of distance of 3ph currents for fault inception angle of 40° for SVC compensated multi-terminal transmission system with hybrid generation at terminal-2 a SLG fault b DL fault

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Fig. 8
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Fault index and variation of distance of 3ph currents for fault inception angle of 40° at terminal-3 a SLG fault b DL fault

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Fig. 9
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Variation in fault index and incidence angle of three-phase currents at terminal-1 for 40% of the transmission line with SVC. a SLG fault b DL fault

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Fig. 10
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Variation in fault index and varying incidence angle of three-phase currents at terminal-2 for 40% of the transmission line with SVC a SLG fault b DL fault

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Fig. 11
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Variation in fault index and varying incidence angle of three-phase currents at terminal-3 for 40% of the transmission line with SVC. a SLG fault b DL fault

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7 Conclusions

The proposed system is developed for multi-terminal transmission system with hybrid PV and wind energy source with static var compensator using simulation software. The paper describes the protection algorithm for detection and classification of short circuit faults using wavelet-based multi-resolution analysis approach. The fault Indices are calculated with various locations at all the three terminals by analyzing the detailed coefficients of the current signals. Detection and discrimination of the fault in the system is analyzed by the wavelet coefficients of current signals which are greater than the threshold value which indicates the type of fault is observed. It is found that the scheme is working reliable and accurate for several types of faults at different locations and at different fault inception angles with and without SVC.