Fault detection during power swing in thyristor-controlled series capacitor-compensated transmission lines

https://doi.org/10.1016/j.epsr.2020.106481Get rights and content

Highlights

  • Challenges associated with the protection of power networks equipped with TCSCs are summarized.

  • The proposed fault detection method based on the Mathematical Morphology is introduced in detail.

  • It is shown that the proposed method provides high speed and accuracy for all types of faults.

  • It is shown that the proposed method is not sensitive to nonlinear changes of TCSC

Abstract

Fault detection during power swing in compensated transmission lines by Thyristor Controlled Series Capacitors (TCSCs) is a challenging problem and conventional detection methods might not be accurate. In this paper, a high speed fault detection approach based on Mathematical Morphology is proposed. The proposed method uses voltage and current signals for fault and swing detections, respectively. The performance of the proposed method is evaluated using two simulation case studies simulated in PSCAD/EMTDC: (i) a single machine infinite bus network and (ii) IEEE standard 9 bus three machine network. It is shown that the proposed approach is not sensitive to nonlinear behavior of TCSCs and it is capable of detecting all types of faults, including three-phase ones, during power swing accurately. In addition, due to low computational complexity, the proposed method provides significantly high detection speed.

Introduction

Recently, the use of series compensators to optimally utilize transmission lines capacity has attracted more attention. Due to the vulnerability of series compensators to overvoltage, there is a need for suitable protection mechanisms such as a Metal Oxide Varistor (MOV) and an air gap. Nonetheless, these protection elements and their nonlinear behavior can introduce challenges to existing transmission lines protection schemes [1,2]. As a TCSC can provide various advantages for transmission systems like continuous control of compensation level, it has been increasingly used in power networks. Besides its advantages, because of its nonlinear behavior during faults, the TCSC can make the fault detection more challenging during power swing.

A disturbance in power systems such as line switching due to a fault, generator disconnection, or switching of large loads results in the fluctuation of generators rotor angles. This phenomenon can cause malfunctioning of distance relays due to the presence of power swings. Thus, power swing blocking (PSB) techniques are used to prevent faulty operation of distance relays during power swing events [3,4]. The effect of series compensators on the performance of distance relays have been extensively studied, however, limited studies have been done to analyze the effect of series compensators on power swing and in particular, fault detection during power swing [5]. Three methods for detecting power swing, impedance reduction, swing center voltage (SCV), and power derivative have been investigated for series capacitor-compensated lines in [6], however, balanced three phase faults are not considered. In [1], a method based on negative current sequence has been presented for fault detection during power swing in compensated lines with fixed series capacitors. However, the impact of other kinds of series compensators such as Thyristor Controlled Series Capacitors, which makes the fault detection more challenging, has not been studied. In [7], a sample estimation and error calculation method based on Taylor Series expansion is proposed for improving the power swing detection performance of protection relays. A negative-sequence current-based technique to detect the fault occurrence, its classification and the location, and the fault inception time during power swing conditions in a series compensated line is presented in [8]. Another method based on exponential change of admittance to detect all types of faults (symmetrical/ asymmetrical) during power swing is proposed in [9]. Overall, most of the proposed methods are focused on detecting faults during power swings without the presence of series compensation (e.g., [10], [11], [12], [13], [14]).

Within this context, one of the promising methods that could alleviate the limitations of existing approaches is the Mathematical Morphology [15]. This method has been recently applied to power system applications such as fault detection and classification and the least squares method for estimating the fault location [15]. Mathematical Morphology has been used in [16] to differentiate power swing events from faults. However, the effect of line compensation with series compensators is not considered, especially TCSCs. Therefore in this paper we have improved this method to make it applicable to TCSC compensated lines. The main advantages of the Mathematical Morphology compared to other methods are: (i) it is not affected by the power swing frequency and (ii) the fault impedance does not affect the performance of this method.

In this paper, a new approach based on Mathematical Morphology for fault detection during power swing is proposed. The advantages of the proposed method are: i) high speed and accuracy of the method in detecting a variety of faults including high-impedance three-phase faults during power swing in compensated lines, ii) robustness of the method to nonlinear changes of the reactance values and TCSC modes as well as nonlinear behavior of MOV during faults, and iii) straightforward calculations and low computational burden of the algorithm.

The rest of this paper is organized as follows: Section II presents the effect of series compensators (in particular Thyristor Controlled Series Capacitors) on the fault detection during power swing. A brief introduction of Mathematical Morphology is presented in section III. In section IV, the proposed algorithm is introduced. Section V presents the performance evaluation of the proposed method.

Section snippets

Thyristor controlled series capacitors

Flexible AC Transmission Systems devices (FACTS) can be effectively used to control power circulation, voltage regulation, transient stability enhancement and damping system oscillations. Among them, Thyristor Controlled Series Capacitor (TCSC) is used to improve power transmission capacity on long transmission lines [17].

The basic design of the TCSC is shown in Fig. 1. It includes a series capacitor in parallel with a Thyristor Controlled Reactor (TCR). In practical implementations, several

Mathematical morphology

The basis of Mathematical Morphology consists of two basic operators called Dilation and Erosion. Based on these two operators, other operators can be produced according to the requirements. The most widely used Mathematical Morphology operators in the field of power systems include Dilation, Erosion, Opening and Closing [20]. The definition of these operators is summarized in the following.

If f(n) is considered as a signal and g(m) as a structure element (SE), where m and n are integers,

Power swing detection

Power swings generally occur after a disturbance in power systems. An example of the current waveform during a power swing is shown in Fig. 2. This waveform is obtained from the simulation case study described in section V.

As shown in Fig. 2, during a power swing, the peak current fluctuates in each cycle, so the SE for detecting the power swing must have at least one cycle of samples. The choice of the SE for this filter has been made based on the typical swing frequency (i.e., 1–7 Hz). The

Single Machine Infinite bus (SMIB)

The schematic of this network is shown in Fig. 5. The specifications of this system are given in Appendix A. The PI model is used to simulate the transmission line. The signals are measured at the beginning of the line (B1 and B3).

The simulated TCSC is based on the model designed in [24] and a PI controller is used to control it. The flowchart of control block is depicted in Fig. 6 the compensation level for this TCSC is between 40 and 70 percent [6]. It should be noted that the protective

Comparison with the prior art

Recently, several techniques have been proposed for fault detection during power swing. A summary of these methods is presented in Section 1. In this section a comparative assessment of the proposed scheme with the Cumulative Sum of the negative sequence current (CUSUM (Δ|I2|)) [1], Transient monitor index of three phase current phasors [28], the zero frequency filter based fault detector index (ZFFO) [26] and Teager-Kaiser Energy Operator based algorithm [27] is presented. In what follows, the

Conclusion

In this paper, a method for fault detection during power swing in compensated transmission lines by TCSC is presented. The proposed method is based on the Mathematical Morphology and uses current and voltage signals to detect power swing and fault during power swing, respectively. The performance of the proposed algorithm has been tested for a variety of symmetrical and unsymmetrical faults in two test systems, and the accuracy and speed of fault detection during swing have been evaluated. It

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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