Adaptive concentric power swing blocker
© The Author(s) 2016
Received: 26 May 2016
Accepted: 15 November 2016
Published: 1 December 2016
The main purpose of power swing blocking is to distinguish faults from power swings. However, the faults occur during a power swing should be detected and cleared promptly. This paper proposes an adaptive concentric power swing blocker (PSB) to overcome incapability of traditional concentric PSB in detecting symmetrical fault during power swing. Based on proposed method, two pairs of concentric characteristics are anticipated which the first one is placed in a stationary position (outer of zone3) but the position of the second pair is adjustable. In order to find the position of the second pair of characteristic, Static Phasor Estimation Error (SPEE) of current signal is utilized in this paper. The proposed method detects the abrupt change in SPEE and puts the second pair of characteristic in location of impedance trajectory correspondingly. Second concentric characteristic records travelling time of impedance trajectory between outer and inner zones and compares to threshold value to detect symmetrical fault during power swing. If recorded time is lower than threshold, three-phase fault is detected during power swing. Intensive studies have been performed and the merit of the method is demonstrated by some test signals simulations.
KeywordsConcentric PSB Phasor estimation error Power swing Symmetrical fault during power swing
Distance relay malfunction has raised concerns about blackouts in power systems. Distance relays make decisions based on entering of impedance trajectory in protected zones. When a fault occurs in a protected line, the impedance trajectory enters in distance relay zones and the relay operates. However, this impedance penetration may also occur during power swing condition. During a power swing the voltage and current fluctuate simultaneously, causing fluctuation in the measured apparent impedance at the distance relay, which may enter the relay tripping zones. This condition causes relay malfunction and may lead to consecutive events (cascading outages) and even a blackout eventually [1–3 ].
To avoid this malfunction, Power Swing Blocker (PSB) is installed in modern distance relay . The main task of PSB is discriminating power swing from fault and block distance relay from operating during power swing. Moreover, it should detect any fault during power swing and unblock distance relay. However, due to the symmetric nature of power swing, detection of symmetrical faults during power swing is more difficult than unsymmetrical faults. Therefore, this issue attracts attentions of many researchers at the moment.
The purpose of this paper is to modify the traditional concentric PSB to enable it for detecting three-phase fault during power swing. In this paper, Phasor Estimation Error (PEE) is employed as a quantity with high abrupt at fault initiation that helps the proposed method in determining the location of second pair of concentric PSB. According to proposed method, two indices are used in this method for detecting fault during power swing, which these indices complete each other. The first index (IX1) is transient monitor that shows occurrence of transient in signal and determines the location of second pair of characteristic and the second index (IX2) that is the output of second concentric characteristics as final index for detecting three-phase fault during power swing.
Static phasor estimation error
Limitation of traditional concentric PSB
In normal situation, the measured impedance is far away from the distance relay protection zones. However, when a fault initiates, the measured impedance moves in the complex plane (R, X) rapidly from load point to characteristic of line impedance. As a result of the electrical property of a fault, the rate of change of impedance is very high but it is very slow during power swing a result of the mechanical property of power swing. Traditional concentric PSB utilizes this difference to discriminate power swing from fault. To achieve this goal, two concentric impedance characteristics (outer and inner zones) along with a timer are used in traditional concentric PSB. The required time for impedance movement between outer and inner zones during quickest power swing is considered as threshold value. If the recorded time is lower than the threshold value, it is detected as a fault and in contrast, if the recorded time is higher than threshold, it is detected as power swing.
In order to analyze the performance of traditional concentric PSB in discriminating power swing from fault, a series of tests are carried out on a two-machine equivalent system, shown in Fig. 2. The data of the power system, are: E B = 1∟0, E A = 1∠δ(t), Z A = 0.25∠750, Z B = 0.25∠750, Z Line = 0.5∠750. The power system frequency is 50 Hz and simulation time step is 500 μs.
Case2: A second test is programmed to show capability of traditional CPSB in detecting symmetrical fault during power swing in special condition. Similar to the previous case, power swing is simulated by displacement angle of source A as Eq. (2). A three-phase fault is simulated in right end of the protected line at t = 0.85 s during power swing.
The impedance trajectory of this case is shown in Fig. 4. According to the figure, after power swing initiation, impedance moves toward distance zones so that it crosses the CPSB for the first time and then the timer records the traveling time between outer and inner zones. Therefore, power swing can be detected by comparing the recorded time with threshold value and then distance relay is blocked. As a consequence of power swing, impedance trajectory moves back and gets away from the distance zones and so leaves the outer zone of CPSB. Next a three-phase fault occurs at t = 0.85 s during power swing. This causes the impedance trajectory crosses CPSB again during fault and so a new travelling time is recorded by timer, which can be used for detecting fault individually. Hence, traditional CPSB can detect both power swing and fault during power swing in this case.
Although, distance relay can easily detect unsymmetrical faults with various faulted loops by assessing the negative sequence of current signal, it is faced by challenge in symmetrical faults during power swing because of inconsiderable amount of negative sequence during three-phase fault.
Proposed adaptive concentric psb
According to motioned simulations and explanations, traditional concentric PSB has limitation for detecting symmetrical fault during power swing and cannot detect it in special condition. When a symmetrical fault occurs, while impedance trajectory of power swing is inside of inner zone of CPSB, this kind of CPSB cannot detect fault because there is no second cross through zones of the CPSB during fault period.
Another key point of this proposed method is detection of the location of impedances trajectory (the place in complex plane) for placing the second CPSB. In order to achieve this goal, phasor estimation error (PEE) is employed in this paper. By monitoring PEE during power swing, abrupt change of PEE can be used as primary indicator of symmetrical fault initiation and then the second CPSB is set at corresponding impedance in complex plane.
Simulation part of this paper is divided into three parts. In the first part, the proposed method for detecting three-phase fault during power swing is examined in single machine to infinite bus (SMIB) and in the second part; the performance of the proposed method in three-machine power system is verified and in the last section, the performance of the proposed method is examined in IEE 39-Bus power system.
Simulation results of the proposed method in single machine to infinite bus (SMIB)
Simulation results of three-machine power system
Simulation results of IEEE 39-Bus power system
Measured apparent impedance by a distance relay moves into relay operating zones during power swing as a consequence of disturbance in power system that causes malfunction of distance relay. Traditional CPSB is designed inside of distance relay to prevent this malfunction by blocking distance relay during power swing. However, if a fault occurs during power swing, it should be detected and distance relay is blocked. Traditional CPSB is a common method for detecting power swing. However, it has limitation in detecting symmetrical fault during power swing. Therefore, adjustable concept of this method is proposed in this paper to overcome this difficulty. According to the proposed method, two pairs of CPSB are employed; the first CPSB is used for discriminating fault from power swing and the second CPSB is used for detecting symmetrical fault during power swing. According to results, the proposed method demonstrates its ability to unblock distance relay in three-phase fault during power swing.
JK, Ph.D. student, brings up the idea of adaptive procedure, performed the primary simulations and drafted the manuscript. MK, JK Ph.D. supervisor, participated in enriching the manuscript (in theoretical idea and simulation section (IEEE 39-Bus power system)) and carried out the revising the manuscript (response to the reviewers and editing grammatical and lexical mistakes). Both authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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