Border Protection Of Ehv Transmission Lines

It is well known that the transmission line protection could be classified into two categories: the non-unit protections, which rely totally on locally derived fault information, and the unit protections, those which realized with derived information of two terminals. The conventional non-unit transmission line protections, which use the voltage or current data at single end, do not posses the instantly discriminative properties for the whole protected line, for example, the protective zone of the instant distance relay only up to 80% of the transmission line. Therefore, unit protection schemes are widely used in high voltage transmission systems. Naturally, the communication tunnels and equipments are the important parts of the protection relays, which lead to the decreasing of reliability and fast clearance. With the developments of the Extra High Voltage transmission systems, the traditional protections will not meet the system stability and fast clearance. The boundary protection, which relies on derived information at single end with the properties of instant discrimination for the whole transmission line, is proposed in the paper. The transmission line boundaries do exist, because of the bus bar, transformers, line traps and other devices connected at line ends, which lead to the varieties of the surge impedance at line boundaries. Suppose that traveling wave travels in the line boundary, parts of traveling wave will be reflected back and others will go through the line boundary. So, there must exist different characters of fault-induced signals at line boundaries between internal and external faults. With comparison of the expressions of the fault-induced transients we discovered that the frequency characters of transient current are mainly controlled by the reflection and refraction coefficients of the line boundaries, and the frequency characters vary with time due to the reflection of traveling wave from one end to another. Analyzing the frequency response of the reflection, refraction coefficients and the propagation functions discovers that the different characters of transient components are mainly focused in time period of [0,2T], where T is traveling time form one end to another, that is: (i) the ratio of wavelet spectra energy in some frequency band of backward traveling wave to that of forward traveling wave is larger than 1 when positive fault occurs; however, the ratio is zero when reverse fault occurs. (ii) the ratio of wavelet spectra energy of voltage in some frequency band to that of current is the equivalent impedance viewed form line to bus when positive fault occurs, and the ratio is the line surge impedance when reverse fault occurs. (iii) the wavelet spectra energy of transient component in high frequency band induced by an internal fault is much larger than that induced by an external fault. Based on the former analysis, the discriminative criterions of start unit, directional unit and discrimination of internal and external fault unit are constructed by extracting the fault information using wavelet analysis. The phase selection criterion has also been constructed by extracting the information of the different characteristics of different fault types in the mode domain. EMTP simulation results show that these criterions are right and isolated from the affects of fault position, fault types, fault resistance and fault inception angles. The simulation results of lightening strokes and operations of breaks are performed to test the boundary criterions. The test results show that the operations of breaks take on affection to the boundary protection, and the lightening strokes will lead to wrong action of the boundary protection. In order to identify the lightening strokes without fault, the wavelet scale spectra are analyzed for constructing the identification criterion. It is discovered that the wavelet spectra of transient components have no attenuation in a time period when lightening strokes the line without leading to a fault, however that have large attenuation when fault occurs. The identification criterion is constructed by comparison the wavelet spectra energy of different time period in the same scale. In addition, a new fault location algorithm with more accuracy properties is proposed by constructing a fault location function using the propagation equations of transmission line. At last, the boundary protection scheme is carried out, which contains start unit, directional unit, the internal and external discriminative unit, fault phase selection unit and identification of lightening stroke unit. The tests by field recorded data and EMTP simulations show that the criterions of boundary protection are right and efficiency. This paper creatively attempt to study a novel principle of transmission line protections with properties of inherently and fast clearing the fault on the arbitrary point of the whole line. There still have more continue works to be done for realizing the boundary protections. It can be predicted that with the development of the DSP technologies, wavelet analysis and the utilization of the optical transformers, the boundary protection can be improved by taking advantage of these conditions.