Date Published: March 21, 2019
Publisher: Public Library of Science
Author(s): Hao Wu, Xingxing Dong, Qiaomei Wang, Jie Zhang.
To overcome the contradiction between speed and reliability in existing busbar protection schemes, a new busbar protection algorithm based on a polarity comparison of fundamental frequency currents is proposed. The algorithm extracts the fundamental frequency components of the fault reference current and the virtual current through a wavelet transform. The angle between the two currents is used to characterize the polarity relationship. The polarities of the virtual current and the reference current are the same when an internal fault occurs, and the angle will be small. The polarities of the two currents are opposite for an external fault, in which case the angle is larger. By analysing the variation characteristics of the angle between faults inside and outside busbar, a protection criterion is established, and the fault area is determined. In simulation results based on PSCAD/EMTDC, the algorithm can quickly and reliably identify the faults inside and outside the busbar area, and its performance is not affected by the initial fault angle, fault resistance, fault type or capacitor voltage transformer (CVT) transmission characteristics.
Accurate identification of the fault area after a busbar fault will help quickly remove the fault and improve the stability of the power system . There are two kinds of busbar protection: fundamental frequency protection and transient protection . Busbar protection at the fundamental frequency is mainly used to distinguish the fault region by the combination of a polarity comparison and an amplitude comparison. The polarity relationship is mainly expressed by comparing the phase relation between the voltage of the fundamental frequency fault component and the current and correlation degree of the current sampling value of each connection branch of the busbar [3,4]. In , fault discrimination was realized by analysing the ratio of the voltage phasor and the sum of the current phase of the busbar branch. The ratio for an external fault is large, and the phase angle is close to 90. The ratio for an internal fault is small, and the phase angle is close to 0. Accordingly, the fault area of the busbar can be distinguished. In , the degree of correlation between the current sampling value of each branch of a busbar was calculated, and a criterion for busbar protection was constructed by using the waveform correlation of the fault in the busbar area and the saturation of the current transformer (CT). However, to accurately obtain the fundamental frequency phase, a strict filtering measure is needed. Filtering delay greatly reduces the speed of the protection action and does not meet the ultra-high-speed requirements of a smart grid. Due to the lack of anti-CT saturation in traditional power frequency busbar protection, in , a digital differential busbar protection scheme based on the generalized α-plane method was proposed. The algorithm mapped a periodic CT secondary current signal to the plane for fault zone identification, which could effectively address the insufficient anti-CT saturation ability of a traditional differential busbar. However, for reliable protection, a filtering process was added, and the protection response took a long time. In , a busbar differential protection principle with adaptive characteristics was proposed, which used the principles of alienation protection and differential protection to achieve better adaptive characteristics. Although this approach led to better relay performance than traditional differential protection, the time for fault diagnosis when CT saturation occurs was longer. Although the above reference solved the problem of an insufficient anti-CT saturation capability in traditional power frequency protection, the reaction speed of the protection was slightly slow in a super/extra-high voltage power grid.
From the above analysis, we can accurately determine the fault area. If the reference current is the fault component current of line L5, the virtual current is the sum of the fault components of lines L1~L4. Then,
In this paper, based on the analysis of the polarity relationship between the associated lines of the busbar, the reference current and the virtual current are defined. By analysing the angle of the two currents, the polarity relationship is determined. A busbar protection algorithm based on the comparison of the fundamental frequency currents is proposed, which solves the problem associated with the slow speed of traditional busbar protection. The reliability of the fault initial angle and the high impedance fault is low, and the transient characteristics of the CVT will be greatly affected. Through theoretical analysis and simulation analysis, the following conclusions can be drawn: