| The electrical power energy and load center in China are distributed inversely, therefore the UHV (Ultra High Voltage) synchronous power grid is necessary for the huge power transmission over long corridors, as well as the optimal configuration over wide regions. It is well understood that the protective relaying is always recognized as primary defending line for the security and reliability of power system. As a result, the new issues of protection strategy for UHV synchronous power grid should be reconsidered deeply. Five critical issues of protective relaying in UHV synchronous power grid and two UHV transmission lines are investigated in this paper, which are described as below.Firstly, power swing is always a threaten common and influential for power system, which may become one of the most significant reasons for blackout. Nowadays, the power swing blocking strategy of distance relay in China is that the distance relay zone-Ⅰ and zone-Ⅱ are only available in the initial time quantum for about150ms to300ms, defined as the open-time, based on the operation experiences and statistic datum over years. However, the theoretical analysis for this open-time is still a blind area considering the complicated operating conditions of power system. Therefore the open-time is analyzed theoretically. The reasonable model for power system is chosen, which is utilized to establish a complicated relationship between the rotor characteristics in the first swing period and the equivalent power angle. The equivalent power angle is defined as the angle that makes the measured impedance of distance relay zone-Ⅱ swing into its protective region. The analysis involved is expected to be an important supplement for the theory basis of power swing blocking strategy in China. At the same time, the correct fault phases discrimination during power swing is also difficult, which is the second research aspect in this paper. Nowadays, one of the fault phases selector widely used in China is based on the relative phases between fault current of zero-sequence and negative sequence with additional confirmation by impedance measurements. The measured impedance of distance relay during power swing is comprehensively analyzed in this paper. Then a new fault phases selective principle is proposed based on the fault phases selector mentioned above, appropriate for normal operation state and power swing. A delay of200ms is added for the measured impedance of un-faulted phases to swing out of its protective region. The new fault phases selector proposed in this paper was utilized in real project. The matches with other protective schemes are easy and reliable, making it more valuable in an engineering point of view.Thirdly, the first UHV demonstration project from Jindongnan to Jingmen is of great importance for the future UHV power grid in China. The protection schemes for this transmission line are investigated in details, a special problem was observed on the Jindongnan-Nanyang transmission line. The zero-sequence current measured at remote terminal relay was severely deviated from sinusoid after the occurrence of a single line grounding (SLG) fault near the Jindongnan side, significantly threatening protective relays based on the zero-sequence components. Therefore Laplace transformation and nodal voltage algorithm is applied on equivalent networks of zero sequences. The series capacitor of the Jindongnan side was confirmed to be the main contributor of such distortion. An improved scheme was proposed, which was to spark out the gap according to the signal sent by initial procedure immediately. The correctness and effectiveness of proposed scheme were verified by numerical simulations on RTDS.Fourthly, the parallel transmission lines are applied in the second UHV project for Huainan to Huxi, making the secondary arc current and recovery voltage not accordance with the requirements for successful auto-reclosure because of the inter-phase and inter-line capacitance. Nowadays, the shunt reactors with neutral reactor in the neutral point are always utilized to restrain the secondary arc current, especially in double-circuit model. While one of the parallel transmission lines is in maintenance state, the electrical isolation between line I and II must be maintained at all time for the safety of devices and staffs. Therefore the compensation effect is greatly weakened especially under a cross-country fault. Three optimal schemes are proposed for the double-circuit compensation model mentioned above, which focus on the accuracy, reliability and economy. The proposed schemes are verified by relevant simulations on RTDS, appropriate for normal operating state and maintenance state.At last, UHV power grid is adjacent to load center. The lack of transmission corridor and the flexible operation conditions of power system increases the multiple-circuit transmission lines, especially lines with some shared towers. A new fault locating principle is proposed in this paper, appropriate for different arrangement modes for multiple-circuit transmission lines. The most important advantage of this scheme is that the synchronous sampling is not necessary and hence a synchronous operator is introduced to eliminate the corresponding errors. Meanwhile, it uses the comprehensive positive-negative-sequence components of the voltage and current un-synchronously sampled from two ends of individual lines. The additional communication devices are unnecessary. And the transmission line equations based on distributed parameters are applied to avoid the influence caused by distributed capacitance. RTDS simulation shows the accuracy of this fault locating scheme, involving different single line faults and cross-country faults. The scheme was configured into the protection of1000kV UHV transmission lines for further improvement by engineering experiences.Oriented to project demand, five critical issues for UHV synchronous power grid are investigated comprehensively in this paper. Some valuable solving schemes are proposed and also configured in real operations. It is hopeful that the work is acceptable in the view point of engineering project and scientific research. |
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